The effect of fusion zone oxygen content and weld bead geometry on hardness and porosity of laser-welded commercially pure titanium sheets

Abstract The current study aims to understand the governing mechanisms behind weld hardness and porosity in laser-welded titanium sheets. Hardness measurements indicate increasing hardness with increased heat input and grain size, which is inconsistent with the general knowledge on hardness behavior. Since hardness is governed by the weld microstructure, morphology and grain size are studied based on the heat input. An increase in weld hardness and transformation of serrated to Widmanstätten structure is observed with increased heat input (or reduced cooling rate). To understand this phenomenon, the oxygen content is measured in the fusion zone using EDS analysis. A greater amount of oxygen is measured in the fusion zone for samples with higher heat input. XRD analysis, also reveals increased Ti2O3 and TiO2 secondary phases in samples with higher oxygen contamination that contribute to weld embrittlement. Results shows that while increased heat input increases the grain size, it also causes a greater chance of oxygen contamination and Widmanstätten formation in the fusion zone by increasing the melt pool size. Increased melt pool width and size increases the chances of oxygen absorption from the environment under similar shielding conditions. Consequently, it is revealed that the fusion zone oxygen content has a more significant effect on titanium weld hardness compared to the grain size. Quantitative measurements of the weld porosity indicate that titanium porosity is controlled by the weld depth. The results of the present research can be applied to reduce embrittlement and porosity formation in laser welding of titanium sheets.