Effect of oxygen contents in shielding gas on GTAW joints of commercially pure titanium

Effect of oxygen contents in shielding gas on GTAW joints of commercially pure titanium | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of oxygen contents in shielding gas on GTAW joints of commercially pure titanium Tae Jun Park, Kimin Noh This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7016495/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 5 You are reading this latest preprint version Abstract This work investigates the impact of oxygen content in shielding gas on GTAW joints of commercially pure titanium. Commercially pure titanium GTAW welds exhibit unique color characteristics, with silver, straw, and white/grey indicating varying degrees of quality. The study finds that a 100% argon shielding gas results in silver-colored, acceptable welds. However, increasing oxygen content of shielding gas leads to color changes, indicating oxidation and potential defects. Liquid penetrant testing confirms that higher oxygen levels in the shielding gas can result in cracks and reduced quality. Microstructure analysis reveals the thickening of surface oxide layers with increased oxygen content. XRD analysis identifies oxide phases, such as Ti 6 O and TiO 2 . Weld penetration remains consistent, but oxygen-rich environments lead to narrower melt pools and irregular bead shapes. Microstructural analysis shows the formation of beta-Ti phases and subgrains in the fusion zone, influenced by oxygen content. Oxygen concentration analysis corroborates the presence of oxygen-induced beta-Ti phases. Hardness increases with higher oxygen content due to precipitation and subgrain formation. Subgrain formation is attributed to residual stresses during welding. Overall, maintaining oxygen content below 5% in the shielding gas during GTAW welding of pure titanium positively affects weld hardness. Titanium GTAW Weld Shielding gas Oxygen Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction Titanium (Ti) is one of the most desirable structural materials for many applications such as aerospace, chemical processing industries, medical implants, marine engineering, and power generations. It is because of its excellent mechanical properties, lightweight, corrosion resistance, and biocompatibility [ 1 – 4 ]. Many welding processes have been successfully developed to join Ti such as gas tungsten arc welding, plasma arc welding, electron beam welding, laser welding, and so on [ 5 – 8 ]. Unfortunately, it is also a very difficult material to weld due to high reactivity with interstitial elements such as oxygen, nitrogen, hydrogen at high temperatures. Especially, the reactivity of the Ti with oxygen during fusion welding is a critical consideration in the welding process, as it can affect the integrity and performance of the welded joint [ 6 , 8 ]. The formation of titanium oxides is the most critical problem. When the Ti is heated during fusion welding, it readily reacts with oxygen from the surrounding atmosphere to form primarily TiO 2 . These oxides are solid and have a higher melting point than the base metal. The formation of TiO 2 at the weld interface can trap gas, leading to the creation of porosity within the weldment. This results in increase of tensile strength and hardness in the joints, while the ductility is significantly decreased, leading to a weaker weldment that is more prone to cracking [ 9 , 10 ]. Therefore, novel choice of a shielding gas used in the Ti welding is very important and Ti requires high purity inert gas atmosphere to prevent the contaminate of the weld during the fusion welding. The most common shielding gas is argon, but helium can also be used [ 11 ]. The gas tungsten arc welding (GTAW) is the most popular process for welding Ti, as it produces a high-quality weld with good penetration. The GTAW are commonly used to minimize contamination and achieve high-quality welds in Ti. Strict control of welding parameters and atmosphere conditions are essential to ensure the integrity of Ti welds and the performance of the final components. Previous studies have qualitatively reported that an increase in oxygen concentration during welding leads to an increase in the hardness and a decrease in the tensile strength of the weld zone. However, most of these studies have been limited to microstructural observations using optical microscopy (OM) or scanning electron microscopy (SEM), with insufficient analysis of phase transformations or crystal structure changes associated with varying oxygen levels. In particular, there has been a lack of systematic investigation into the formation mechanism of β‑Ti phase in commercially pure titanium (CP‑Ti) under controlled oxygen environments [ 10 – 14 ]. In this study, commercially pure titanium plates were butt-welded using gas tungsten arc welding (GTAW) under controlled shielding gas environments with oxygen concentrations ranging from 0–10%. The resulting welds were comprehensively evaluated in terms of microstructure, hardness distribution, tensile strength, and ductile behavior. Transmission electron microscopy (TEM) and electron diffraction pattern analysis were employed to identify the formation of β‑Ti phase (BCC structure) within the fusion zone, and the correlation between oxygen concentration and phase formation was examined. Furthermore, based on the Ti–O binary phase diagram and quantitative oxygen analysis, the precipitation mechanism of the β‑Ti phase was elucidated. This study provides an expanded understanding of the oxygen sensitivity in CP‑Ti welding and proposes scientific insights into the formation behavior of oxygen-induced phases. These findings contribute to establishing optimal shielding gas conditions for the production of high-integrity welds in commercially pure titanium applications. 2. Materials and Methods In this study, a 3 mm thick hot rolled plate of pure Ti of ASTM Grade 2 was prepared and its chemical composition and basic physical properties are shown in Table. 1, and the representative microstructure is shown in Fig. 1 . The welding process was GTAW, using a DC welder with a maximum welding current of 350A, and the shape of the welded joint was butt. The welding conditions were fixed at 210A welding current and 0.22m/min welding speed. The detailed welding conditions are shown in Table 2 . To prevent post-weld oxidation by external air, a torch structure was designed and its form is shown in Fig. 2 . Protective gas at 20 l/min was injected first at the torch and Ar gas was injected second at 40 l/min along the welding direction directly behind the torch. In addition, a backup shielding gas of Ar gas was injected at 10 l/min at the bottom of the welding jig to prevent oxidation of the lower part of the weld. To investigate the weld characteristics according to the oxygen content in the shielding gas, the concentration of oxygen in the primary shielding gas was varied by 3 ~ 15 wt%, and the internal and external effects were analyzed. The welds were checked for defects firstly by visual inspection of color and shape and In order to investigate defects that are difficult to see, a second non-destructive test, liquid penetrant test, was performed to check for microcracks in the welds. The liquid penetrant test was performed according to a standardized procedure, where the penetrant MR-Chemie GmbH Mr 68C, the solvent MR-Chemie GmbH MR 70 and the cleaning agent MR-Chemie GmbH MR 79 were used. The tests were performed in accordance with the requirements of the LST EN standard.[ 14 ] The degree of surface oxidation due to the change of oxygen content in the shielding gas was checked using an optical microscope and a scanning electron microscope, and the oxide phase was analyzed using XRD (Rigaku, SmartLab). The weld microstructure was corroded with Kroll's (100 ml H 2 O + 3 ml HF + 5 ml HNO 3 ) solution after polishing, and the weld morphology was observed through an optical microscope. The phase distinction of the precipitates generated in the weld was performed using transmission electron microscopy (Hitachi, HT7800), and the oxygen content in the weld was quantified and analyzed using an Oxygen Determinator (Leco, TCH600). To investigate the change in hardness due to the change in oxygen content in the shielding gas, a Vickers hardness tester was used to measure the hardness with a load of 300g and a holding time of 10 seconds. Table 1 Chemical composition and of commercially pure titanium (Grade 2) Chemical Composition (wt%) Ti N C H O Fe balanced 0.01 0.10 0.01 0.11 0.30 Table 2. The condition of welding in this study 3. Results and Discussion 3.1 The Surface Quality of Welded Joints Titanium welds differ from other welds by the possibility to be assessed by color. Different colors of weld surface are not an issue for steel welds, but titanium color indicates the defects and problems of the welding technique. Usage of shielding gas might influence color change when titanium weld is induced by surroundings at the elevated temperatures[ 15 ]. With full protection or minimal oxidation, the weld color will be silver or light or dark straw; when newly welded part is not protected its color appears to be white or grey[ 16 ]. Figure 3 (a) shows the surface color inspection results of the weld. When the shielding gas is 100% Ar, the upper surface of the weld is silver in color and has a uniform bead shape. When the oxygen content in the shielding gas was increased to 5%, some parts of the weld surface were oxidized, showing blue and brown color, but the overall color was silver, and the shape of the weld was consistent, which was a good indicator of achieving the acceptance criteria. However, when the oxygen content in the shielding gas was increased to 10% and 15%, the color of the weld changed to gray and white, and when the oxygen content in the shielding gas was 15%, the weld was not welded due to excessive oxidation of the weld. Figure 3 (b) shows the results of liquid penetrant testing of the weld. PT has been chosen as an effective way to detect the quality of weld samples surface [ 17 ]. Before penetrant test, complete set of surface cleaning process was done to obtain satisfactory results. The surfaces of weld samples were dry and free of any dirt and grease, weld spatters remains, oils that could hide surface defects and openings. When the shielding gas was 100% Ar and the oxygen content in the shielding gas was 3% and 5%, no defects were found in the weld. However, when the oxygen content in the shielding gas was 10% or more, the weld cracked and did not meet the weld quality standards. From the above results, if the oxygen content in the shielding gas was 15%, it was excluded from further analysis. To further investigate the degree of oxidation on the surface of the weld, the microstructure of the cross-section of the weld was observed by optical microscopy and SEM, and the results are shown in Fig. 4 (a). It can be seen that as the oxygen content in the shielding gas increased, the oxide layer on the surface became thicker. To further investigate the morphology of the oxide layer on the surface, XRD analysis of the surface is shown in Fig. 4 (b). When the shielding gas was 100% Ar, the surface of the weld was not oxidized and analyzed as pure Ti. Oxides of Ti 6 O were produced starting at 3% oxygen content in the shielding gas. When the oxygen content in the shielding gas was increased to 10%, TiO 2 oxide was produced. This surface oxide formation causes a change in the surface color of the weld. 3.2 The Microstructure of Welded Joints according to shielding gas Figure 5 shows a cross section of the weld. It can be seen that full penetration was achieved in all conditions. Furthermore, no defects such as bubble formation in the weld were observed. As the oxygen content in the shielding gas increases, the shape of the melt pool becomes narrower than that of 100% Ar. In addition, the shape of the upper bead becomes irregular and the deflection of the lower bead increases. In order to investigate the influence of the microstructure of the weld on the oxygen content in the shielding gas, a comparison SEM image of the areas of the weld separated by □ in Fig. 5 is shown in Fig. 6 . The comparison areas were classified as follows. 1) Center of fusion zone, 2) Fusion zone near bond line, 3) HAZ near bond line, 4) HAZ near base metal, 5) Base metal. The microstructure change due to the change of shielding gas was almost no change from base metal to HAZ near bond line. From the fusion zone near bond line, a precipitate phase was generated within the grain regardless of the composition of the shielding gas. In addition, fine sub-grains were generated along the precipitate phase. Toward the fusion zone, more precipitates were generated along the sub-grains. Figure 7 shows the results of TEM images, diffraction patterns, and EDS analysis to identify the precipitates in the fusion zone. It can be seen that a different type of organization has been formed along the grain from the matrix part. EDS analysis of the matrix and precipitate sections showed the same analysis of Ti. However, diffraction pattern analysis showed that the precipitate form along the sub-grain was analyzed as β-Ti with a BCC structure. The behavior of β -Ti phase formation in the BCC structure of the fusion zone is illustrated in Fig. 8 by the Ti - O binary phase diagram. At high temperatures above 1600℃, the maximum oxygen solid-solution into Ti is about 2.5%. Using an oxygen analyzer, the oxygen concentration in the fusion zone where the β -Ti phase was produced was checked and is shown in Fig. 9 . The oxygen concentration of the base metal was about 0.1 wt%, and it increased slightly to 0.19 wt% when the shielding gas was 100% Ar. As the oxygen concentration in the shielding gas increased, the oxygen concentration in the fusion zone also increased. And when the oxygen concentration in the shielding gas was 10%, the oxygen concentration in the fusion zone increased to 1 wt%. From the above oxygen concentration analysis results, the mechanism of β -Ti phase generation in the fusion zone was confirmed[ 18 ], because the maximum oxygen solid-solution into Ti is about 2.5%, as can be seen from the Ti-O binary system state in Fig. 8 . 3.3 The Hardness of Welded Joints according to shielding gas Figure. 10 shows the hardness variation of the weld with the change of shielding gas. When the shielding gas was 100% Ar, the hardness of the fusion zone averaged 204 Hv, which was slightly higher than the average 172 Hv of the base metal. In general, there was no trend of decreasing hardness in the HAZ part of the weld. As the oxygen content in the shielding gas increased to 3%, 5%, and 10%, the average hardness of the fusion zone increased to 251 Hv, 278 Hv, and 384 Hv. The large variation in hardness within the fusion zone is attributed to the different distribution of the beta-Ti phase. Despite the fact that the grain size of the fusion zone in the heavily heated GTAW weld is larger than the base metal grain size, the increase in hardness is attributed to the following reasons. The precipitation enhancement effect caused by the generation of beta-Ti phase in the grain and the presence of fine sub-grains along the beta-Ti phase in the fusion zone shown in the SEM image. The formation of subgrains can be explained by the presence of residual stresses introduced during the rapid melting and cooling of the GTA weld. These stresses resulted in dynamic recrystallization and the formation of subgrains as demonstrated by Zainulabdane[ 21 ] and Park[ 22 ]. The reason for the formation of many subgrains in the fusion zone is believed to be due to the relatively wide fusion zone formation in GTAW welds, which increased the stresses due to shrinkage during solidification, and the residual stresses due to heat input during welding stimulated dynamic recrystallization. Based on the results of this study, it can be seen that the presence of oxygen content of 5% or less in the shielding gas during GTAW welding of commercial pure titanium has a positive effect on the hardness of the weld. Figure 11 illustrates the variation in tensile behavior of the welded joints with respect to the oxygen content in the shielding gas. According to Fig. 11 (a), the tensile strength of the weld under a pure argon shielding gas (Ar 100%) was approximately 430 MPa, which is comparable to that of the base metal. However, with incremental increases in the oxygen concentration in the shielding gas to 3%, 5%, and 10%, the tensile strength progressively decreased to approximately 420 MPa, 410 MPa, and 370 MPa, respectively. Notably, the weld under the 10% O₂ condition exhibited a reduction in tensile strength exceeding 14% compared to the Ar 100% condition. This decline in tensile strength is closely associated with microstructural changes in the FZ and HAZ, as shown in Fig. 6 . As the oxygen content increased, distinct acicular or plate-like α′ martensitic structures were observed at the center and boundary of the fusion zone. These structures are attributed to rapid solidification and the solid-solution strengthening effect of dissolved oxygen. Particularly under the 10% O₂ condition, the microstructure became highly dense and directionally oriented, suggesting a pronounced tendency toward brittleness. Such microstructural evolution is also reflected in the hardness distribution presented in Fig. 10 . As the oxygen content increased, the average hardness at the weld center showed a marked rise. While the hardness under the Ar 100% condition remained near 200 Hv, it approached 400 Hv under the 10% O₂ condition. This increase is attributed to a combination of solid-solution strengthening and transformation-induced hardening associated with the oxygen-enriched microstructure. However, excessive hardness may impair plastic deformability and increase the risk of crack initiation, negatively affecting the tensile behavior. The load–displacement curves in Fig. 11 (b) further support these trends. In the Ar 100% and O₂ 3% conditions, ductile deformation persisted beyond the peak load. In contrast, under O₂ concentrations of 5% or higher, both elongation and plastic deformation regions decreased significantly, resulting in early fracture. The 10% O₂ welds exhibited abrupt load drops immediately after reaching peak load, indicative of brittle fracture behavior, likely due to decreased ductility and localized stress concentrations. These findings confirm that increased oxygen levels in the shielding gas promote oxidation reactions in the weld pool, leading to the formation of beta-Ti phase and elevated hardness. Consequently, both tensile strength and ductility degrade with higher oxygen concentrations. The results demonstrate that shielding gas oxygen concentration is a critical factor influencing the mechanical integrity of commercially pure titanium welds, as it governs microstructural uniformity and fracture behavior. Therefore, for structural applications requiring high reliability, the oxygen concentration in the shielding gas should be maintained below 3%. In cases where ductility and fracture resistance are critical, an ultra-high-purity argon environment containing less than 1% oxygen is strongly recommended. 4. Conclusions The effect of oxygen content in the shielding gas on weld quality in GTAW of commercial pure titanium has been studied, with key findings including The type of shielding gas used during commercially pure titanium GTAW plays a significant role in the weld's color and quality. A high percentage of argon (Ar) gas provides good protection and results in a silver-colored, uniform bead shape. However, as the oxygen content in the shielding gas increases, it can lead to oxidation, affecting the weld's color and quality. It is found that when the oxygen content in the shielding gas is 10% or more, the weld can develop cracks and fail to meet quality standards. Welds achieved full penetration in all conditions, but increasing oxygen content narrowed the shape of the melt pool and led to irregular upper bead shapes. Oxygen's rapid reaction with titanium increased the viscosity of the melt pool, affecting its flow. The hardness of the fusion zone increased as the oxygen content in the shielding gas increased. This increase in hardness was attributed to the presence of beta-Ti phase and the formation of subgrains in the fusion zone, likely due to residual stresses introduced during welding. an oxygen content of 5% or less in the shielding gas during gas tungsten arc welding (GTAW) of commercial pure titanium has a positive effect on weld hardness. Declarations Author contributions Taejun Park: Writing – original draft, Investigation, review & editing, Project administration,. Kimin Noh: Writing – original draft, Investigation, review & editing, Funding acquisition, Supervision. Acknowledgement This research was conducted as part of the major project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), titled “Development of High-Purity Natural Graphite / Clean Hydrogen Production and Utilization Technology of By-products for Carbon Neutrality (25-3222),” On behalf of all authors, the corresponding author states that there is no conflict of interest. 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Technol 211:415-423 https://doi.org/10.1016/j.jmatprotec.2010.10.01 Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Major Revisions Needed 14 Nov, 2025 Reviewers agreed at journal 10 Sep, 2025 Reviewers invited by journal 10 Jul, 2025 Editor assigned by journal 03 Jul, 2025 First submitted to journal 02 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7016495","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":483692421,"identity":"81e4b4ac-c2c7-4e63-b3f0-029c0ae3d62f","order_by":0,"name":"Tae Jun Park","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA10lEQVRIiWNgGAWjYFCCAyBsY8AG40sQ0sAD0ZJGkhawTYcN4CIEtdgznj34ueLMeWM+/sVPNzDusWGQnH2AkC3nkiXP3LhtxibxzOwGw7M0Bmm+BEJazhhINny4bcMmcQCo5cBhBjkegn45Y/yz4cM5oJbj34Ba/hOlxUyy4cYBMzb+HpAtBxikCWo5cC7NsuFMsjGbBE/ZjYQDyTySPQS0sM84e/hmwzE7w/n9x7fd+HDATk7iDAEtDHAVEgkMDAmwiMIL+GHu4D9AWPEoGAWjYBSMTAAA/eJFLNq8BlsAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-5830-1039","institution":"Korea Institute of Geoscience and Mineral Resources","correspondingAuthor":true,"prefix":"","firstName":"Tae","middleName":"Jun","lastName":"Park","suffix":""},{"id":483692422,"identity":"b50e93d4-8b7b-43f7-8049-48b431dfa35b","order_by":1,"name":"Kimin Noh","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Kimin","middleName":"","lastName":"Noh","suffix":""}],"badges":[],"createdAt":"2025-07-01 06:10:56","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7016495/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7016495/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":86712674,"identity":"f4557474-ffee-46a8-bf98-804cd6b1ee27","added_by":"auto","created_at":"2025-07-14 19:15:45","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":431999,"visible":true,"origin":"","legend":"\u003cp\u003eOptical microstructure of commercially pure itanium\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/49f19a10ebed7ac7803ad365.jpg"},{"id":86712164,"identity":"1564f83f-51d2-45d4-91cb-aa2301b3112d","added_by":"auto","created_at":"2025-07-14 19:07:44","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":88213,"visible":true,"origin":"","legend":"\u003cp\u003eShape of welding torch\u003c/p\u003e","description":"","filename":"Figure2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/31f69dfe61ad4f83aa0b6ed2.jpg"},{"id":86712675,"identity":"00d20e21-ffb4-4b5d-aea1-b5024b00e720","added_by":"auto","created_at":"2025-07-14 19:15:45","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":184162,"visible":true,"origin":"","legend":"\u003cp\u003eSurface inspection of commercially pure titanium welds: a - Visual inspection; b – PT Test\u003c/p\u003e","description":"","filename":"Figure3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/24d17c437264ce646cad503d.jpg"},{"id":86712169,"identity":"c7fde6a3-759e-47db-8b34-1ee1239ded73","added_by":"auto","created_at":"2025-07-14 19:07:45","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":156213,"visible":true,"origin":"","legend":"\u003cp\u003eSurface microstructure and XRD analysis of commercially pure titanium welds: a - Microstructure image; b – XRD results\u003c/p\u003e","description":"","filename":"Figure4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/ff4ed24948f4f5356aae6c81.jpg"},{"id":86713369,"identity":"2dbc088d-2d53-4440-b852-5b8a36a7cc99","added_by":"auto","created_at":"2025-07-14 19:31:45","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":612592,"visible":true,"origin":"","legend":"\u003cp\u003eOptical microscope image in the weld according to shielding gas: a – Ar 100%; b - Ar 97% - O\u003csub\u003e2\u003c/sub\u003e 3%; c - Ar 95% - O\u003csub\u003e2\u003c/sub\u003e 5%; d - Ar 90% - O\u003csub\u003e2\u003c/sub\u003e 10%\u003c/p\u003e","description":"","filename":"Figure5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/6897609b1be8577ce53f33d3.jpg"},{"id":86713368,"identity":"372217fa-92f2-4c4d-82fa-250ca9893de3","added_by":"auto","created_at":"2025-07-14 19:31:45","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":422882,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of the areas of the weldaccording to shielding gas\u003c/p\u003e","description":"","filename":"Figure6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/b3dad7e527b12d69cf0f5b54.jpg"},{"id":86712681,"identity":"a6319c19-af48-4aea-93df-f26bc6558fdd","added_by":"auto","created_at":"2025-07-14 19:15:45","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":192141,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images, diffraction patterns, and EDS analysis in the fusion zone\u003c/p\u003e","description":"","filename":"Figure7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/23395e3401efcf372520a299.jpg"},{"id":86712174,"identity":"a8f62080-190b-4ddd-aed8-8a91ff36a82a","added_by":"auto","created_at":"2025-07-14 19:07:45","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":52224,"visible":true,"origin":"","legend":"\u003cp\u003ePhase diagram of the Ti–O binary system [20]\u003c/p\u003e","description":"","filename":"Figure8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/8ba74f172e2c75a57479070b.jpg"},{"id":86713117,"identity":"293436db-e0de-4eae-88aa-7a8bfba4699f","added_by":"auto","created_at":"2025-07-14 19:23:45","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":35209,"visible":true,"origin":"","legend":"\u003cp\u003eOxygen Analysis in fusion zone according to shielding gas\u003c/p\u003e","description":"","filename":"Figure9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/66f5144d79695a36e98f53dc.jpg"},{"id":86713683,"identity":"304801a9-4e90-4275-9e43-2818fbb9a4c5","added_by":"auto","created_at":"2025-07-14 19:39:45","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":182859,"visible":true,"origin":"","legend":"\u003cp\u003eHardness of weld in the weld according to shielding gas: a – Ar 100%; b - Ar 97% - O\u003csub\u003e2\u003c/sub\u003e 3%; c - Ar 95% - O\u003csub\u003e2\u003c/sub\u003e 5%; d - Ar 90% - O\u003csub\u003e2\u003c/sub\u003e 10%\u003c/p\u003e","description":"","filename":"Figure10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/126ef1a142a388346c794bdf.jpg"},{"id":86712679,"identity":"51b4813a-7c54-4247-b934-450110916547","added_by":"auto","created_at":"2025-07-14 19:15:45","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":77842,"visible":true,"origin":"","legend":"\u003cp\u003eTensile strength and load–displacement behavior of welds according to shielding gas: a – Tensile strength; b - load–displacement behavior\u003c/p\u003e","description":"","filename":"Figure11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/38f5c37ea863d0239adfb7ff.jpg"},{"id":86713688,"identity":"b215bbf5-9cd3-4502-ba1d-2eccc52cf253","added_by":"auto","created_at":"2025-07-14 19:39:53","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2881174,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7016495/v1/2d2b11b4-5c93-447c-b1c3-4935e34c26bf.pdf"}],"financialInterests":"","formattedTitle":"Effect of oxygen contents in shielding gas on GTAW joints of commercially pure titanium","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eTitanium (Ti) is one of the most desirable structural materials for many applications such as aerospace, chemical processing industries, medical implants, marine engineering, and power generations. It is because of its excellent mechanical properties, lightweight, corrosion resistance, and biocompatibility [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Many welding processes have been successfully developed to join Ti such as gas tungsten arc welding, plasma arc welding, electron beam welding, laser welding, and so on [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Unfortunately, it is also a very difficult material to weld due to high reactivity with interstitial elements such as oxygen, nitrogen, hydrogen at high temperatures. Especially, the reactivity of the Ti with oxygen during fusion welding is a critical consideration in the welding process, as it can affect the integrity and performance of the welded joint [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The formation of titanium oxides is the most critical problem. When the Ti is heated during fusion welding, it readily reacts with oxygen from the surrounding atmosphere to form primarily TiO\u003csub\u003e2\u003c/sub\u003e. These oxides are solid and have a higher melting point than the base metal. The formation of TiO\u003csub\u003e2\u003c/sub\u003e at the weld interface can trap gas, leading to the creation of porosity within the weldment. This results in increase of tensile strength and hardness in the joints, while the ductility is significantly decreased, leading to a weaker weldment that is more prone to cracking [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Therefore, novel choice of a shielding gas used in the Ti welding is very important and Ti requires high purity inert gas atmosphere to prevent the contaminate of the weld during the fusion welding. The most common shielding gas is argon, but helium can also be used [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe gas tungsten arc welding (GTAW) is the most popular process for welding Ti, as it produces a high-quality weld with good penetration. The GTAW are commonly used to minimize contamination and achieve high-quality welds in Ti. Strict control of welding parameters and atmosphere conditions are essential to ensure the integrity of Ti welds and the performance of the final components. Previous studies have qualitatively reported that an increase in oxygen concentration during welding leads to an increase in the hardness and a decrease in the tensile strength of the weld zone. However, most of these studies have been limited to microstructural observations using optical microscopy (OM) or scanning electron microscopy (SEM), with insufficient analysis of phase transformations or crystal structure changes associated with varying oxygen levels. In particular, there has been a lack of systematic investigation into the formation mechanism of β‑Ti phase in commercially pure titanium (CP‑Ti) under controlled oxygen environments [\u003cspan additionalcitationids=\"CR11 CR12 CR13\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this study, commercially pure titanium plates were butt-welded using gas tungsten arc welding (GTAW) under controlled shielding gas environments with oxygen concentrations ranging from 0\u0026ndash;10%. The resulting welds were comprehensively evaluated in terms of microstructure, hardness distribution, tensile strength, and ductile behavior. Transmission electron microscopy (TEM) and electron diffraction pattern analysis were employed to identify the formation of β‑Ti phase (BCC structure) within the fusion zone, and the correlation between oxygen concentration and phase formation was examined. Furthermore, based on the Ti\u0026ndash;O binary phase diagram and quantitative oxygen analysis, the precipitation mechanism of the β‑Ti phase was elucidated. This study provides an expanded understanding of the oxygen sensitivity in CP‑Ti welding and proposes scientific insights into the formation behavior of oxygen-induced phases. These findings contribute to establishing optimal shielding gas conditions for the production of high-integrity welds in commercially pure titanium applications.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eIn this study, a 3 mm thick hot rolled plate of pure Ti of ASTM Grade 2 was prepared and its chemical composition and basic physical properties are shown in Table. 1, and the representative microstructure is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The welding process was GTAW, using a DC welder with a maximum welding current of 350A, and the shape of the welded joint was butt. The welding conditions were fixed at 210A welding current and 0.22m/min welding speed. The detailed welding conditions are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eTo prevent post-weld oxidation by external air, a torch structure was designed and its form is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Protective gas at 20 l/min was injected first at the torch and Ar gas was injected second at 40 l/min along the welding direction directly behind the torch. In addition, a backup shielding gas of Ar gas was injected at 10 l/min at the bottom of the welding jig to prevent oxidation of the lower part of the weld. To investigate the weld characteristics according to the oxygen content in the shielding gas, the concentration of oxygen in the primary shielding gas was varied by 3\u0026thinsp;~\u0026thinsp;15 wt%, and the internal and external effects were analyzed.\u003c/p\u003e\u003cp\u003eThe welds were checked for defects firstly by visual inspection of color and shape and In order to investigate defects that are difficult to see, a second non-destructive test, liquid penetrant test, was performed to check for microcracks in the welds. The liquid penetrant test was performed according to a standardized procedure, where the penetrant MR-Chemie GmbH Mr 68C, the solvent MR-Chemie GmbH MR 70 and the cleaning agent MR-Chemie GmbH MR 79 were used. The tests were performed in accordance with the requirements of the LST EN standard.[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eThe degree of surface oxidation due to the change of oxygen content in the shielding gas was checked using an optical microscope and a scanning electron microscope, and the oxide phase was analyzed using XRD (Rigaku, SmartLab). The weld microstructure was corroded with Kroll's (100 ml H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;3 ml HF\u0026thinsp;+\u0026thinsp;5 ml HNO\u003csub\u003e3\u003c/sub\u003e) solution after polishing, and the weld morphology was observed through an optical microscope. The phase distinction of the precipitates generated in the weld was performed using transmission electron microscopy (Hitachi, HT7800), and the oxygen content in the weld was quantified and analyzed using an Oxygen Determinator (Leco, TCH600). To investigate the change in hardness due to the change in oxygen content in the shielding gas, a Vickers hardness tester was used to measure the hardness with a load of 300g and a holding time of 10 seconds.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition and of commercially pure titanium (Grade 2)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"6\" nameend=\"c6\" namest=\"c1\"\u003e\u003cp\u003eChemical\u0026nbsp;Composition\u0026nbsp;(wt%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eN\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eFe\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ebalanced\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.30\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTable 2. The condition of welding in this study\u003c/p\u003e\n\u003cp\u003e\u003cimg 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+WUtpN95553+l/R9R85zJEIxhAXDPN5HH33UH6O11SG0tg1mP3a0wdZ2wGMw/K389NNPG7+OwpMOw2bSR9NESJJXF6LnSoRiCAvm+++/X8/NIRzrEFpCat+93A9B85DadZO8otH6x5lz/NgSoRjCQkH4ffXVV2sBw0cPqEPoQ5Cm6fCl23+dVnhgV05pqR9//HE/B/rq1av++NyIUAxhofDBQ19+5VpD6EP44IMPrn744YfV2Vtt78GDB/0xv5wLwhG+wrCZL9OCPN+5c6efZ+TYNduzoKvIEMICefjw4erof3RCht0533Ra2MY5v9sgvidPnqzO3kI8nfbXH9e4dC48XUGYTlCvzt5CfJ2muBH3ORGhGMLCQJgggHBV4Mhf1/x8TDB6uBonAkzXqtDzvBCu0hJ62+65abLvcwghGJlTDCEEI0IxhBCMCMUQQjAiFEMIwYhQDCEEI0IxhBCMCMUQQjAiFEM4ETKEsC8yA4ZjLbHDua7JyRK2zIbhKsRZ45oDTxO3q8kyNxpR7+Uc/9baa+7bNa1r9H/hDiEcHa0w2XcVB/dqVQpL8rTyBL+60oTr+PlSPMLX1Sr1fA5Iu65kafkNQZ4JL7zOKJPiIUxdljhHeaIphnBCuo68l8EGtL6uw68tWWNgAQs5wk37o/1h6xA/LF53QqT3f++99zaML6DNyRbjFKZoYOQT82XV1iKGZTthN1mLc0O01BkGJOD169drDZG6wE6kIO45TKRFKIZwAhAWCCBMZSE0Klyns2toqKGvwIq1IyHIcFGCUmA9h60BQJZoAAGisBKOc++TgrBGiLVAqLsgH8LLQz1wrnwiHCVwKQOCHqgHDNjWutiLlcYYQjgiPqyj2/kQeoqBBIaJPqQEwtdhMxDO/blX8Ysa1xSmDE1JYyic8qHhvvI0VGaGyVyveSV+/H04vk95hohQDOHIIAR87otOXTsxgmFszk1CRAKEODnn1+F8LB4gL7qPfLQEj+DakGsJM/ynCEXVB+Uay6/K6fVX8fQkSIfyMIUMn0M4Mgxn2a9EX1MZQraG0GMwfOwEQ7/dKHF0nb8fpo4NnVswzATuY2jKULSTA71fHbID1+Q6QbNx3tqRj3z5vKXD8J3rpK0tCrYN3wlLubVZVoU8a98X5kiZgiBvbKClsu5KhGIIR0abRrmDlhAaA0Gi+xGIrU2oEB6++16FLUglkJin9I8WbkV7XxDIQ1utIqiqwEZwseXpGL5VakX7voDPmVIutxi+CxGKIRwR12QctB830b8LaIrcWwUFgkhfnVvUr7MIG/9oMcdOewhcBHbdlkD/hZRAFgjpbXtAU9bWV3Lf9wX86zrlGns5jNK9dUIIR4B5LboYzj98MBcnf9yXX365Ph6aX/N7hmDebWjuDf/WHKDmFIfS3RfSU35xrTk+0mbOkLoZqx+/Joiv5U85uIe49yWWt0MIJ4fhbScQV2edFDsjMRShGEIIRuYUQwjBiFAMIQQjQjGEEIwIxRBCMCIUQwjBiFAM4USweoM/Xu8Lf0zWUsGWYVj+KK7rns4SjcyCx6GVNyBLQu4nqON90nIiFEM4EazegF2X9wnWDfPfPv2LrnZ+lrxxTQ4Qeqy75py1y/Wep0+fbtgunAOE7PPnzzfyQj5aQmwMXx6plTcIPZYL4kd8CE6HOm4tf9yFCMUQTghL4I5hZBZh0VoKt1Qjs8TTMhgbI7MhXAh08mMZmQU0pPv37/f3Oks0Mgu8OBDmVbuMkdkQLgRf+0u383XIrOHFr/o7rCWu63kJr/W/rCEG0nF/8HXIYp+1wV6GIUhjKJzyQV63lVnlIZ+6R6iMvl57n/IMEaEYwpGhQyMQBJ26dmIEg3fyioSIBAhxVmEhCDMmJMhLFTpD4bk25FrCDP8pQlH1QbnGyg2EHRPIfo24xvIwhQjFEI6MhEF1zjahCDWeMcFX4xcIIQkkT5O4WkLOmSJoiG8oX9zfKuO2ciNEh8KQZ+Wbciltwru2vAuZUwzhyJzSyKzgeoslGpmFap9RxMhsCAsDwXcqI7OCjzWtuOvX2aUYmR36qhwjsyEsDIaLdDGcD+UY7skfN4eRWV3DtYbBDC1b/gw3uWfbEHZX6lC/NfQmbYbG1I3XD35+b2sYTHwtf8rBPcS9L7GnGEI4OQxvO+G3Ouuk2BmJoQjFEEIwMqcYQghGhGIIIRgRiiGEYEQohhCCEaEYQghGhGIIIRhrocj/hvinvLtqtqcF/zbHnTvb8klZpyy7GotH5p9EPQ+3j6ntagjamvqjzIQJ4m1dY1WH/FtpsxKktQzvEDxNuX2gvlr9a6isIP9qcBaIq4bfCv9TFL6gGrg89z/dbwL9u37KgvYxxuLh3/WXUl9hHtQm9l1dQXtTW1NcrPYArfoQfo30WL2iexz5z4lW23i8Lb9tEB5X+9dYWQlLPSmM/IG0W6t4tjE6fO4S6/91Pvdb5dSw3rKrvNXZ/ozFg7HO7gGszkJ4u66XNjFkIGEbmPTXml/aVyfsrr7++uv+nG0E6J+CdqlrrPvFGCsGZkFp84sBhamGZQnf0r4qH374YV9Oj5f1zOQJw7dT6eRRs3+NlZU8soZb68CxNC4wNLFtXXWLUaFIYo5UVH61wJtfVRxqKtfIqFRa/Fz1dVCV5e8qs+LUNTmlqWtDD0zhSbeqzrq3Dmvx87Ck5fEM4WWt1pFJS3kGpaFye/49Hjm/NywTdcoff/yx/3XUBoaeNQqJ8+DBg6uXL1/2x1iccYMHGHPQNdoXZvslICQwEC6H7l9SURt+//33+19HWx2M9Z8pjJWVstHv6D+gFwHyZO+9Z94qjG9BDR0aPrcWWncSu/dz9dXv4TrH/ALHhAXUWsUlNZ84FCfO4V5XjRVnhfilshOHjj2v4NdUNp0TRmq38iaIx9Ou8eCA+7mmMioNHOXQdcF1pUn8Xs9hefAsve3V56n2MPacua44gHap8PUa6anteV9Um/J+MRXiUX8ZQv2qhfKhOMg758pni9q/gHuGygpc93S4rnLvwzVNEVVfmgrItJB+3SQRb52uoP0xErvLeH+ssJjy4a2lt5NrZ7xBJckl3UFxdgVc+bwFO2xSmXnzjG24I/tsXfk2VPqustemi7pK7d+moPwKhi0MCagDqf9V4wTeksSpNNwuHOXzMigN6oi6YnjjdA1o7UfZap7CsvBhKs+zDqFb/alCP2BPF+H2AscgDG0fRztUuuTHR22eHyHNFUcfYSdAndfR1S7QfygreSKeQ7VHR2VV35aNRdJU3lv9d4hrQpEHoURwx4aHQOVvgwLzgGBsXoRwNLhDHiICCuHl9dBKz3cSOxSEq4xi+o5lYXkgbNigSR1SL9bWEHoM7YCneIhTykCrzwy1GYaSEhi87GnbnSbV/MpLOLV50ubFrvPWi1obR7UErIbvDH3pP+pDTAPswi5lZRpCShgvFMpJef3lso3ROcVjIikOVP4UEBy8YfQghuDh8RCh9eC34VrkNlqNYR+YUNdbmYY791684XQwoqFNS5jgaLv7bG2q++nYPirBiKtblkZRaAkblA4JCbVVRmaMSlq7Cu7Ko0eP+l+N4hzyR1+qCgXKhOZapzC1rMgG17x5BvrgxPHUvnpjQpEvczxkvcGmwFsSgTFmUZdGIFV5ipnzFjwE0hFUdkv91rBIQwEeFpW/zwcS0vROFJYLHwHqMJeXHm1ll2GcoE3TliXcgLbHNA9IM5WAEkpLedEvGhwvfU19HQJxoo3xQvey0Sfwqy8C/KXtTmVKWfGvI0gpN9r7uj6TQboO2NPdtJ6wNO81fr17a/V+XaWu/Z49e7Y+xjHRqWPCdQJwfc6EKHHo3OP+05/+tD7WxKlDPGNwj+dLfjrnfr/u+cR1Qq2/x8NwDLUM4HETjrJUf+7zMtY0wdOTI1xYDt6mazv151rbg/pTRWHU1irejmoctGO1W2fsnkMgPcUrVyE91UurTF4nNe/b8q1+5xBO9+zSl67n/Iyh4ocayNJpPbRLLWu4fbhQw51z276x4fM+MG9R1eZLgOERqr/DUKT+TzSEpcIcYidv1m6XabNTswihyJcmfZSZPC+wIGgg/EFVXxlx+/4bP4RwGNmjJYQQjEUNn0MI4dhEKIYQghGhGEIIRoRiCCEYEYohHBH+OaEVT/vA6ij9I4G/blX8HwtKh9Ud1c8hzqlL3qbiacrtA/U1tDTX4/Zyya9VP8S18yoivj6HEOZHKypaK0umwB+cfaUGcflqDq5rBZZDevxZWuk78p8T/THb4235bYPwuLoaCIbKSlhd414PQ9rkY1eiKYZwJA61vI3hBNbEi05AXjNU0vrfLgZRLsnytmiVlTye1PJ2COEw1Cn3sbyNhWmMH9DxJdgUH0NCWVWqQ2SGoDKEABIYS7W8PVZWynZUy9shhHlg2KahI0O8OoSW8YNtQ2vuJVw1eKBhImlwnXBCQ0mcho8MMXcZygLxcN8Yyl8L5UNxUFbOW8YbBPF5WWCsrICfp0OZVe59iFAM4Qh4x1VnVucW+G0TVHRudfqxjt6KX7hw8/ha4Qmn69W1hNlUoUg5VVbNebZoCcUKcQ6VFXS/6h23rZ6dDJ9DmBmGcnNY3maoyBxg10/7OTvm7oa+pHaCYDB+hpJLtrxdGSvroi1vh3CpzGV5m/ASOvpw4RaoK615Peb8lm55u0WrrLxEFm15O4RLZS7L28Qhi9OAJaWWOTnFWdOs/vpdmuVtZ6isCLzZLW+HEA7D57A0ryXkr2vMz+l8bL7Lw3mcHMu/9bGGObeWP3N5um+XebZtkJ7ilauQnsqgOU7Hy+p531ZW4N6KPw/KPZWYDgshHB00RN/3iLnKczU0G6EYQghG5hRDCMGIUAwhBCNCMYQQjAjFEEIwIhRDCMGIUAxhgbihWG0BjKurNjjXtW1mwAhT/1zOefWbAvmbYnaMvJ8bEYohLAwECcvZWKHB+mVWuvDPuk8//bRfueFgj/HVq1f9dVba8H/BFkPCiaV6dZneFDDbNeV/iKx4QRifExGKISwIhCDGUyWoWDKn5Wta4yzNDgGIsFNYjCIMrb9uGXsA1ikfE/KG0G7Zk7wpIhRDWAgMhdEK3ZhC1eJcU2QtsFukwYjCLkYgEK5ap4wwxskorjRLhBnn1eIO16WVSjgTH2FxPsxXGfYZph+DCMUQFgKWaHx7giEkZOr8ohjyr2jojCDEFBoOGIpjdQZBxzAeTU8WwgFByXVA0LG8j3M0XO7FEAVWwB2EN9s3nAMRiiEsBIayMiXWAuGF4JkLDZ2ZG2S+Eqd5QjRSDeO1BYBgPlEaK9exosO5tgdoWavB+g9a8DkQoRjCQhjT8HTNh9ZDprKmmNBiyHuIia8lE6EYwkIYE2YMq+vXXja+4ouzwFo1Q9cpsBmUhuG3jQjFEBYCQq71NZi5Pf96rP8HojX69qrM/+1ilv+UIISnzJeehDchhEUgQ66iZdgV5wZVPUzLsKvw+589e7ZhgJb7dA2Drxh61bkbrcWR3l0zFvvll1+uj/GvcQni9DRvkthTDGFB8NcXPracq4HWfeALNR9t9t6neWYiFENYGAyXESKHbv50DjC058s0f9mZ8gHoFEQohrBA+C8gK1jORZDsC3/kPjcRFKEYQghGvj6HEIIRoRhCCEaEYgghGBGKIYRgbAhFPo/LtA9ftzifYj332Mjk0E0gs0dDUD9TbMGNxaN618oD4PxcTCmFaci81iFwf6udeN9s9Unu4Vpti8SHfytO2tfU/Lbu596aF/VVnEyHtWi1eeCe6jcF8jeWHhDvlL66saKFf5zr3/D8u5zLY/+CPwX+j/xTozqgXg5hWzwqH2UNy0XPcV+0UqTVTvDTig/C+aoVVoaon/KrlSKEV1zcU/uyrygZo9U2h+SDh/U8VwjXivfY8oZ6G+qHYq0pIkW7DPaGKIHF4JzfNPwPq6vY1dlpkdmjQ9kWzznUczgMtBQ9420ayxCs6Gi1E+JDE5KBBregTb/FlqFWuDx69Ght9/D169drDY9+5Oum0fIeP368OhsGzYr+V/8PyZ/HO0G7OnsLWmMnaNdhP/nkk0Ebia02v4+GuCv84Z28j2nIa6FIQToJurEoGz9fTuSqcVWnqbyW2jx0DxWAn37rfSB/jF06VL6u4YSGssrLH/7wh3UY0lFaNe9CQw3yUSvNy+EPj3Ae1vM2VvFAPgiH8VDH6wYoD/Eqf5w7XvdynsdwfLByTYdDUHBc0TPU89mFMQva1fKN+jHt9c6dO2tDEbQH2WLkGsYlqqCrEA6qtRzaYWtJHhZ5iFfcu3dvJxuJlAWhDkNtXn2Gaw5+yi/34FTftb/znMjXYB9ZaYw9qLJ44Vqqtaudro5X9V3hpGIL7tE1paPrfh9wjKoL3OfxcExegXDcS1jF5yo715Q3GBoyEJ+u8atjxUsewMvNL9cUlnQ9fs8L8Xj5av0RVmXiWOe6hiMOPSPFSxzKm+pb8YTTQH3rWaq9OP4Mt1HbCXhbAbUBpavnL7zvKO3a9qbAvZ4u0MYUd82Xpwtqjy28DEJxDbV54ucYP48Xf85Jjzh0r+Lj2PMFrbKJZo49U+p8yog7VS7HXjhBPDihwhGnjnUf8evBcewPbqxy/cHwWx848XuD8Pw4NT/C8wW1TH7OL3G4U948Hsrjcda063ltfFzTc6n5IV5dC6eBZ+PthufD83bqMxyitjeo93r7wL+2ee6v6YvaVojH/RzibZVD1HzVdMf6bW3jKouocRNW7boVr19v9Ylajla9ieZfclhT2V3r1XLZX8PeGef4y6FCSwVtqeKussM2dV2Q1jaIG9V420Y8pMmUAENi3JA1YcJ1FdUPPVwV3wXyRBxeRy1rJsz1zAXl8SFK17iumYcPx+X58+frdoOD1hB6X4b6Df6Y8W/B0LlC+9fwlqElw0raKO2n1d7rDn/cT39TOTn+7LPPNoa2FeplCkwhaeh802wIRcbhDkYp6WRA5be2QdQDa1Uq19zyr5jSaYe2XAQqH0HHA0VQbwPBwcT0NmvCCDDiRLDdv39/5Tsdyjt1S0jV66FQHs2z4Lo34mAnCvNDu2fiXi9BHM9AHzvmYMyCdt2hjxczbavVzjXvCbRTtRPaT523hyrkuNfLSR7oK5pfZN7TXwbEuYvh2HNptxtCkQfJ20Dw5cgrn8r2CU4JUcLQMARheDgIIx6Y4uSXsNsKzxvD09ID4yHRCLk2JjQrNBDCjwks8qvy7GurjsZLHeoFQZxeX0INU+nRyGHqW9UhfrR5NVTFHU5Dy4QX5zzL1rPfB+KjH9GewC1o05foU+pj7JLXDR37YwdtTvtCAyM4xUff4KNIBSE3ZdQm/Ms3oEVO2eeFfAxpvMcCWeAfrzboOtIaxtmMxfHGdQ92deUtGsvLaQwPhJW/j+cZy8vf45MfrhXG/bqH3v8KT0vH7kf4CvF5fiudoF2ng1N4nRM/9aNzzVvonGvgYXBQywIet9IlDzj542oaXk7i8LjlWuUP86P65pk43o641monLTwczvF20YpD7aL17Im31fbH7gHuqWVzuK/mxds1bbNFbePEgZ8Ya/O1vYOfd4J4fUw8/iw8P8TpaTq3xnQYWpm/KS8FNISqqeCHZp9hdDgUNEy00ksagdA/GOYPyYPmh5ZLA/V8bL/cpUK52Gi8wgOPQAxzwHwhbaz1zWCJUA7KM6YgXbRQZE6Hjw9M9u47T3jOIPi64cX6I4vclJUKIUyFwaTmMJcMSoTm38eI5e0QQjBuxfA5hBCmEqEYQghGhGIIIRgRiiGEYEQohhCCEaEYQghGhGIIIRhroeiWVtzJLNAhEMdci+MPgTyMlYelgDLSMMZYPPxjnnoT/GGUc37DZTO1/YzB/dU6Dagd4Vp9Sf23tkviw78VJ211an7r/Zy37lX7x8lIRYuhfsE9+/QV8jOWHhDvJHnGn7cFi6/r4nAWTg8tGJ+CFnfXheOnRgvJDykLjMXjC93D7ePQZy/jBS0jDPjJqAPh3LgBfUz9i1/OwQ06cE/tgwq3DfLkxhNUztb9HtbzXFEc1SjDseUE9daqX2fr8Bk7hG6vbVdYY9g9kNXZzcGC9q7CV2f7MxYPy+66RrA6C7cJtJSuw62P92EpG1d1cqNpngwNFn+FXfzGVUMQgQs1V49xnAu/VtVtQSUPXcdPcVJBiktwr4au+I+pwgpDg6rDDfxq3KD4hYYeuNaQxVG4aqxTdSIoI+krf/XhqA7chfNHBlzpK25oVaht7fNMie+cNq4a4lI2rromFKlsIpLjYfjOXQhJ3mZ6W8i4LAlgrRp/HG+C1huT+LnWMhLrfjww17ooJPdiuBIjm6TBeUtYUTnkmzB6owruoZFxjcajPCp+gT+NSGUh3aFKpNJVJ24hWXUieLDEhZUOykD5eKsrXvIgi+LSGDgO5w3PTwIHLc4NrYK3XZ7nrs90qN3hP2QEFi0RYUZ7oh/Tt6RNtgzjtkDLGzTE2qDVp1sa4RCUhX7v9YWQJQ7O6WcIM/oG14QLPeQB9a9nQF1zf5VF1EvdRVNcE4q8dfTgcCROpQoEpCrUzXGRgKvU3OsVj+pOPPhve0O10DCcoaveXBy3tjsAVRr5dQs5xCGzQVSmGlUd5iNMVXaEJ8hCtkNlE4/KirATVbCTF+LioVKGui2Ddy7FN9Qhwvng+4vouXkn9LZ7akibPieBhcCRkkO7pX3j14K2dyqL2N7Ovb6oT8kL+iTHdf8ZF8b0deQQTv1e/dehnw3Jjq3DZ3XqqpFRoS6tEXouJCuS3FJx52DogSFwqBQeOG4fwUJFS/uTcyEvdjHXvg20DD0o5XmfF0g4Lbdl46ptEF+lJZBanO3GVS2qNsNbhQdPRfqbb0zyAmERMvtsCDWE1O0WekMiHHfZPEfwgKcKvF0bzxAIXeKifmlMrmWG8wRhwnDUX5608zqEPgQEmfetm9q4ahsMtf1lcBEbV7VQoSTFedh01loAJlV5OP7GqWo5D4SHOaSuA8JAW4Dqy5U/HG8cvKFbG+OQB2m2+xpc5QGjCUtjI846LwHUC41Q6fkmW7tCvTBEUOdqNexwXrTm5zinHdfR1b4QH31LbTEbVx3OpI2ruoe4/u9QdV2nX4Xa3FRG93QPpb/WvSHX13Dc5+E7bXH9XyxdrxBG13Wv4F6/n7AtOqG9EY5zzxvXxvJFWKhlhZo/qHHzC6Qrf+6vaegYR1143HJKI5wfekZqG8KfM9dqWxvCw+Ec2sdYHLUvOsRLW6yM3QPcU8sGuq91r7d59aOKlwVHeVwW1Pry9Gq/AT/vBPH6mHhqnxPE2ZI/sFnzZw4FHGtUS6fViC65vOH8oc8NCbelQnkQmENsHT6H08Dw3L9cC//DbQinhi/Vt23jqsVoilWlvkS8jHIhnAOtYfTSYLg8pRzZuCqEEIwMn0MIwYhQDCEEYz181j/xQwjhNpM5xRBCMDJ8DiEEI0IxhBCMCMUQQjAiFEMIwYhQDCGENVdX/w9DZ54b/Rz0sgAAAABJRU5ErkJggg==\"\u003e\u003c/p\u003e\n"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 The Surface Quality of Welded Joints\u003c/h2\u003e\u003cp\u003eTitanium welds differ from other welds by the possibility to be assessed by color. Different colors of weld surface are not an issue for steel welds, but titanium color indicates the defects and problems of the welding technique. Usage of shielding gas might influence color change when titanium weld is induced by surroundings at the elevated temperatures[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. With full protection or minimal oxidation, the weld color will be silver or light or dark straw; when newly welded part is not protected its color appears to be white or grey[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows the surface color inspection results of the weld. When the shielding gas is 100% Ar, the upper surface of the weld is silver in color and has a uniform bead shape. When the oxygen content in the shielding gas was increased to 5%, some parts of the weld surface were oxidized, showing blue and brown color, but the overall color was silver, and the shape of the weld was consistent, which was a good indicator of achieving the acceptance criteria. However, when the oxygen content in the shielding gas was increased to 10% and 15%, the color of the weld changed to gray and white, and when the oxygen content in the shielding gas was 15%, the weld was not welded due to excessive oxidation of the weld.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) shows the results of liquid penetrant testing of the weld. PT has been chosen as an effective way to detect the quality of weld samples surface [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Before penetrant test, complete set of surface cleaning process was done to obtain satisfactory results. The surfaces of weld samples were dry and free of any dirt and grease, weld spatters remains, oils that could hide surface defects and openings. When the shielding gas was 100% Ar and the oxygen content in the shielding gas was 3% and 5%, no defects were found in the weld. However, when the oxygen content in the shielding gas was 10% or more, the weld cracked and did not meet the weld quality standards. From the above results, if the oxygen content in the shielding gas was 15%, it was excluded from further analysis.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo further investigate the degree of oxidation on the surface of the weld, the microstructure of the cross-section of the weld was observed by optical microscopy and SEM, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a). It can be seen that as the oxygen content in the shielding gas increased, the oxide layer on the surface became thicker. To further investigate the morphology of the oxide layer on the surface, XRD analysis of the surface is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b). When the shielding gas was 100% Ar, the surface of the weld was not oxidized and analyzed as pure Ti. Oxides of Ti\u003csub\u003e6\u003c/sub\u003eO were produced starting at 3% oxygen content in the shielding gas. When the oxygen content in the shielding gas was increased to 10%, TiO\u003csub\u003e2\u003c/sub\u003e oxide was produced. This surface oxide formation causes a change in the surface color of the weld.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 The Microstructure of Welded Joints according to shielding gas\u003c/h2\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows a cross section of the weld. It can be seen that full penetration was achieved in all conditions. Furthermore, no defects such as bubble formation in the weld were observed. As the oxygen content in the shielding gas increases, the shape of the melt pool becomes narrower than that of 100% Ar. In addition, the shape of the upper bead becomes irregular and the deflection of the lower bead increases.\u003c/p\u003e\u003cp\u003eIn order to investigate the influence of the microstructure of the weld on the oxygen content in the shielding gas, a comparison SEM image of the areas of the weld separated by □ in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The comparison areas were classified as follows. 1) Center of fusion zone, 2) Fusion zone near bond line, 3) HAZ near bond line, 4) HAZ near base metal, 5) Base metal. The microstructure change due to the change of shielding gas was almost no change from base metal to HAZ near bond line. From the fusion zone near bond line, a precipitate phase was generated within the grain regardless of the composition of the shielding gas. In addition, fine sub-grains were generated along the precipitate phase. Toward the fusion zone, more precipitates were generated along the sub-grains.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the results of TEM images, diffraction patterns, and EDS analysis to identify the precipitates in the fusion zone. It can be seen that a different type of organization has been formed along the grain from the matrix part. EDS analysis of the matrix and precipitate sections showed the same analysis of Ti. However, diffraction pattern analysis showed that the precipitate form along the sub-grain was analyzed as β-Ti with a BCC structure. The behavior of β -Ti phase formation in the BCC structure of the fusion zone is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e by the Ti - O binary phase diagram. At high temperatures above 1600℃, the maximum oxygen solid-solution into Ti is about 2.5%. Using an oxygen analyzer, the oxygen concentration in the fusion zone where the β -Ti phase was produced was checked and is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. The oxygen concentration of the base metal was about 0.1 wt%, and it increased slightly to 0.19 wt% when the shielding gas was 100% Ar. As the oxygen concentration in the shielding gas increased, the oxygen concentration in the fusion zone also increased. And when the oxygen concentration in the shielding gas was 10%, the oxygen concentration in the fusion zone increased to 1 wt%. From the above oxygen concentration analysis results, the mechanism of β -Ti phase generation in the fusion zone was confirmed[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], because the maximum oxygen solid-solution into Ti is about 2.5%, as can be seen from the Ti-O binary system state in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 The Hardness of Welded Joints according to shielding gas\u003c/h2\u003e\u003cp\u003eFigure. 10 shows the hardness variation of the weld with the change of shielding gas. When the shielding gas was 100% Ar, the hardness of the fusion zone averaged 204 Hv, which was slightly higher than the average 172 Hv of the base metal. In general, there was no trend of decreasing hardness in the HAZ part of the weld. As the oxygen content in the shielding gas increased to 3%, 5%, and 10%, the average hardness of the fusion zone increased to 251 Hv, 278 Hv, and 384 Hv. The large variation in hardness within the fusion zone is attributed to the different distribution of the beta-Ti phase. Despite the fact that the grain size of the fusion zone in the heavily heated GTAW weld is larger than the base metal grain size, the increase in hardness is attributed to the following reasons. The precipitation enhancement effect caused by the generation of beta-Ti phase in the grain and the presence of fine sub-grains along the beta-Ti phase in the fusion zone shown in the SEM image.\u003c/p\u003e\u003cp\u003eThe formation of subgrains can be explained by the presence of residual stresses introduced during the rapid melting and cooling of the GTA weld. These stresses resulted in dynamic recrystallization and the formation of subgrains as demonstrated by Zainulabdane[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] and Park[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The reason for the formation of many subgrains in the fusion zone is believed to be due to the relatively wide fusion zone formation in GTAW welds, which increased the stresses due to shrinkage during solidification, and the residual stresses due to heat input during welding stimulated dynamic recrystallization.\u003c/p\u003e\u003cp\u003eBased on the results of this study, it can be seen that the presence of oxygen content of 5% or less in the shielding gas during GTAW welding of commercial pure titanium has a positive effect on the hardness of the weld.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e illustrates the variation in tensile behavior of the welded joints with respect to the oxygen content in the shielding gas. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(a), the tensile strength of the weld under a pure argon shielding gas (Ar 100%) was approximately 430 MPa, which is comparable to that of the base metal. However, with incremental increases in the oxygen concentration in the shielding gas to 3%, 5%, and 10%, the tensile strength progressively decreased to approximately 420 MPa, 410 MPa, and 370 MPa, respectively. Notably, the weld under the 10% O₂ condition exhibited a reduction in tensile strength exceeding 14% compared to the Ar 100% condition. This decline in tensile strength is closely associated with microstructural changes in the FZ and HAZ, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. As the oxygen content increased, distinct acicular or plate-like α\u0026prime; martensitic structures were observed at the center and boundary of the fusion zone. These structures are attributed to rapid solidification and the solid-solution strengthening effect of dissolved oxygen. Particularly under the 10% O₂ condition, the microstructure became highly dense and directionally oriented, suggesting a pronounced tendency toward brittleness. Such microstructural evolution is also reflected in the hardness distribution presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. As the oxygen content increased, the average hardness at the weld center showed a marked rise. While the hardness under the Ar 100% condition remained near 200 Hv, it approached 400 Hv under the 10% O₂ condition. This increase is attributed to a combination of solid-solution strengthening and transformation-induced hardening associated with the oxygen-enriched microstructure. However, excessive hardness may impair plastic deformability and increase the risk of crack initiation, negatively affecting the tensile behavior. The load\u0026ndash;displacement curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(b) further support these trends. In the Ar 100% and O₂ 3% conditions, ductile deformation persisted beyond the peak load. In contrast, under O₂ concentrations of 5% or higher, both elongation and plastic deformation regions decreased significantly, resulting in early fracture. The 10% O₂ welds exhibited abrupt load drops immediately after reaching peak load, indicative of brittle fracture behavior, likely due to decreased ductility and localized stress concentrations.\u003c/p\u003e\u003cp\u003eThese findings confirm that increased oxygen levels in the shielding gas promote oxidation reactions in the weld pool, leading to the formation of beta-Ti phase and elevated hardness. Consequently, both tensile strength and ductility degrade with higher oxygen concentrations. The results demonstrate that shielding gas oxygen concentration is a critical factor influencing the mechanical integrity of commercially pure titanium welds, as it governs microstructural uniformity and fracture behavior. Therefore, for structural applications requiring high reliability, the oxygen concentration in the shielding gas should be maintained below 3%. In cases where ductility and fracture resistance are critical, an ultra-high-purity argon environment containing less than 1% oxygen is strongly recommended.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe effect of oxygen content in the shielding gas on weld quality in GTAW of commercial pure titanium has been studied, with key findings including\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe type of shielding gas used during commercially pure titanium GTAW plays a significant role in the weld's color and quality. A high percentage of argon (Ar) gas provides good protection and results in a silver-colored, uniform bead shape. However, as the oxygen content in the shielding gas increases, it can lead to oxidation, affecting the weld's color and quality. It is found that when the oxygen content in the shielding gas is 10% or more, the weld can develop cracks and fail to meet quality standards.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eWelds achieved full penetration in all conditions, but increasing oxygen content narrowed the shape of the melt pool and led to irregular upper bead shapes. Oxygen's rapid reaction with titanium increased the viscosity of the melt pool, affecting its flow.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe hardness of the fusion zone increased as the oxygen content in the shielding gas increased. This increase in hardness was attributed to the presence of beta-Ti phase and the formation of subgrains in the fusion zone, likely due to residual stresses introduced during welding.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003ean oxygen content of 5% or less in the shielding gas during gas tungsten arc welding (GTAW) of commercial pure titanium has a positive effect on weld hardness.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTaejun Park: Writing \u0026ndash; original draft, Investigation, review \u0026amp; editing, Project administration,. Kimin Noh: Writing \u0026ndash; original draft, Investigation, review \u0026amp; editing, Funding acquisition, Supervision.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was conducted as part of the major project of the Korea Institute of Geoscience and Mineral Resources (KIGAM), titled \u0026ldquo;Development of High-Purity Natural Graphite / Clean Hydrogen Production and Utilization Technology of By-products for Carbon Neutrality (25-3222),\u0026rdquo;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;On behalf of all authors, the corresponding author states that there is no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eNiinomi M (2003) Recent research and development in titanium alloys for biomedical applications and healthcare goods, Science and Technology of Advanced Materials 5: 445-454. http://doi.org/10.1016/j.stam.2003.09.002\u003c/li\u003e\n\u003cli\u003eZhao Q, Sun Q, Xin S, Chen Y, Wu C, Wang H, Xu J, Wan M, Zeng W, Zhao Y (2022) High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process, Materials Science and Engineering: A 845: 143260. http://doi.org/10.1016/j.msea.2022.143260\u003c/li\u003e\n\u003cli\u003eMakurat-Kasprolewicz B, Ossowska A (2023) Recent advances in electrochemically surface treated titanium and its alloys for biomedical applications: A review of anodic and plasma electrolytic oxidation methods, Materials Today Communications 34:105425. http://doi.org/10.1016/j.mtcomm.2023.105425\u003c/li\u003e\n\u003cli\u003eHoque M.E, Showva N.-N, Ahmed M, Rashid A.B, Sadique S.E, El-Bialy T, Xu H (2022) Titanium and titanium alloys in dentistry: current trends, recent developments, and future prospects. Heliyon 11:e11300. http://doi.org/10.1016/j.heliyon.2022. e11300\u003c/li\u003e\n\u003cli\u003eGiri S.R, Khamari B.K, Moharana B.R (2022) Joining of titanium and stainless steel by using different welding processes. A review, Materials Today: Proceedings 66(pt 2):505-508. https://doi.org/10.1016/j.matpr.2022.05.590\u003c/li\u003e\n\u003cli\u003eGupta R.K, Anilkumar V, Xavier X.R, Ramkumar P (2018) Electron beam welding study of grade 1 CP Titanium. Materials Today: Proceedings 5(Pt 2): 2018. https://doi.org/10.1016/ j.matpr.2017.11.542\u003c/li\u003e\n\u003cli\u003eDhinakaran V, Shriragav S.V, Fahmidha A.F.Y, Ravichandran M (2020) A review on the categorization of the welding process of pure titanium and its characterization. Materials Today: Proceedings 27(Pt 2):742-747. https://doi.org/10.1016/j.matpr. 2019.12.034\u003c/li\u003e\n\u003cli\u003eWang Z, Jiang D, Wu J, Xu M (2020) A review on high-frequency pulsed arc welding. Journal of Manufacturing Processes 60:503-519. https://doi.org/10.1016/j.jmapro.2020.10.054\u003c/li\u003e\n\u003cli\u003eWang R.R, Welsch G.E (1995) Joining titanium materials with tungsten inert gas welding, laser welding, and infrared brazing. The Journal of Prosthetic Dentistry 74:521-530. https://doi.org/10.1016/S0022-3913(05)80356-7\u003c/li\u003e\n\u003cli\u003eLi X, Xie J, Zhou Y (2005) Effects of oxygen contamination in the argon shielding gas in laser welding of commercially pure titanium thin sheet. J. Mater. Sci 40:3437\u0026ndash;3443. https://doi.org/10.1007/s10853-005-0447-8\u003c/li\u003e\n\u003cli\u003eVinoth A, Prabu L, Gokul B, Ajith Kumar K (2016) Effect of shielding gas on titanium CP (Gr-2) by using gas tungsten arc welding. Int. J. Eng. Res. Technol 5(5), 2016. https://doi.org/10.4149/km_2019_4_247\u003c/li\u003e\n\u003cli\u003eHalisch, C, Milcke, B, Radel T. et al. (2023) Influence of oxygen content in the shielding gas chamber on mechanical properties and macroscopic structure of Ti-6Al-4V during wire arc additive manufacturing. Int. J. Adv. Manuf. Technol 124:1065-1076. https://doi.org/10.1007/ s00170-022-10214-2\u003c/li\u003e\n\u003cli\u003eKarpagaraj A, Siva Shanmugam N, Sankaranarayanasamy K (2019) Experimental investigations and numerical prediction on the effect of shielding area and post flow time in the GTAW of CP Ti sheets. Int. J. Adv. Manuf. Technol 101:2933-2945. https://doi.org/10.1007/s00170-018-3135-y\u003c/li\u003e\n\u003cli\u003eLST EN ISO 3452-1:2013, Non-destructive testing - Penetrant testing. Part 1: General principles (ISO 3452-1:2013), 2013. \u003c/li\u003e\n\u003cli\u003eBendikiene R, Baskutis S, Baskutiene J, Ciuplys A, Kacinskas T. (2018) Comparative study of TIG welded commercially pure titanium. J. Manuf. Process 36:155-163. https://doi.org/10.1016/j.jmapro.2018.10.007\u003c/li\u003e\n\u003cli\u003eLST EN ISO 5817:2014. Welding \u0026ndash; Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) - Quality levels for imperfections (ISO 5817:2014). 2014.\u003c/li\u003e\n\u003cli\u003eGuirong G, Xuesong G, Yuliang Q, Yan G (2015) Analysis and innovation for penetrant testing for airplane parts. Procedia Eng 99:1438-1442. https://doi.org/10.1016/j.proeng. 2014.12.681\u003c/li\u003e\n\u003cli\u003eHarwig D.D, Fountain C, Ittiwattana W, Castner H (2000) Oxygen equivalent effects on the mechanical properties of titanium welds. Weld. J 79:305s-316s.\u003c/li\u003e\n\u003cli\u003eDanielson P, Wilson R.D, Alman D.E (2003) Microstructure of Titanium Welds, U.S. Department of Energy Report, https://www.osti.gov/servlets/purl/807847 \u003c/li\u003e\n\u003cli\u003eMassalski T.B (1990) Binary Alloy Phase Diagrams, 2nd edn, ASM International, Materials Park, OH, USA.\u003c/li\u003e\n\u003cli\u003eZainulabdein M, Neliasa D, Jullien J.F, Deloison D (2010) Experimental investigation and finite element simulation of laser beam welding induced residual stresses and distortions in thin sheets of AA 6056-T Mater. Sci. Eng. A, 527:3031-3038. https://doi.org/ 10.1016/j.msea.2010.01.054\u003c/li\u003e\n\u003cli\u003ePark T.J, Kong J.P, Uhm S.H, Woo I.S, Lee J.S, Kang C.Y (2011) Effect of Al\u0026ndash;Si coating layer on the penetration and microstructures of ferritic stainless steel, 409L GTA welds. J. Mater. Process. Technol 211:415-423 https://doi.org/10.1016/j.jmatprotec.2010.10.01\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Titanium, GTAW, Weld, Shielding gas, Oxygen","lastPublishedDoi":"10.21203/rs.3.rs-7016495/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7016495/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis work investigates the impact of oxygen content in shielding gas on GTAW joints of commercially pure titanium. Commercially pure titanium GTAW welds exhibit unique color characteristics, with silver, straw, and white/grey indicating varying degrees of quality. The study finds that a 100% argon shielding gas results in silver-colored, acceptable welds. However, increasing oxygen content of shielding gas leads to color changes, indicating oxidation and potential defects. Liquid penetrant testing confirms that higher oxygen levels in the shielding gas can result in cracks and reduced quality. Microstructure analysis reveals the thickening of surface oxide layers with increased oxygen content. XRD analysis identifies oxide phases, such as Ti\u003csub\u003e6\u003c/sub\u003eO and TiO\u003csub\u003e2\u003c/sub\u003e. Weld penetration remains consistent, but oxygen-rich environments lead to narrower melt pools and irregular bead shapes. Microstructural analysis shows the formation of beta-Ti phases and subgrains in the fusion zone, influenced by oxygen content. Oxygen concentration analysis corroborates the presence of oxygen-induced beta-Ti phases. Hardness increases with higher oxygen content due to precipitation and subgrain formation. Subgrain formation is attributed to residual stresses during welding. Overall, maintaining oxygen content below 5% in the shielding gas during GTAW welding of pure titanium positively affects weld hardness.\u003c/p\u003e","manuscriptTitle":"Effect of oxygen contents in shielding gas on GTAW joints of commercially pure titanium","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-14 19:07:40","doi":"10.21203/rs.3.rs-7016495/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2025-11-14T12:55:48+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-09-10T17:41:15+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-10T20:10:18+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-04T00:22:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"The International Journal of Advanced Manufacturing Technology","date":"2025-07-02T23:08:21+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"3cfbe60d-7ec0-4c7f-b976-40bc8d9be669","owner":[],"postedDate":"July 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2025-11-14T17:56:41+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-14 19:07:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7016495","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7016495","identity":"rs-7016495","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}