Macro-micro residual stress prediction and hot cracks formation mechanism during laser melt injection processing of ZrO2 particles into Ti6Al4V substrate

Laser melt injection (LMI) is a promising technology to produce ceramic particles reinforced metal matrix composites (MMCs) to achieve surface modification and performance upgrading. In this paper, the LMI process was conducted on Ti6Al4V substrate with ZrO 2 particles (ZrO 2p ) to produce functionally graded material for heat insulation. The track dimensions, particle distribution, and crack morphology of the MMC layer were studied, corresponding to varying laser power, and scanning velocity. The optical morphologies showed all outlines of MMC were in the shape of a lower lip, and there were cracks in the MMC layer. When the linear energy was lower than 6.0×10 3 J/m, the thickness of the MMC layer was under 0.4 mm, and the zirconia particles filled up the entire MMC layer. Then, a novel finite element model was proposed to calculate the thermal-mechanical characteristics during the LMI process, in both macro and micro scales, to expound on the crack formation mechanism. The simulated results showed that the distribution and volume fraction of ZrO 2p greatly influenced the magnitude and distribution of macro and micro residual stress. For the three LMI macro models, the transversal tensile stress was mainly located in the middle segment with 350 MPa. The longitudinal stress at the start and end of the top surface was close to 0 MPa, and the entire MMC layer was almost tensile stress. Besides, the most considerable transversal stress was spread over the heat affect zone, but the most significant longitudinal stress was distributed on the MMC top surface. The two-dimension micro-scale models revealed the residual stress distributed remarkably uniformly in the ZrO 2 particles, and the stress values were more significant than those of the Ti6Al4V substrate. Moreover, the particle spacing was more extensive, and the stress between particles got more significant with the volume fraction of ZrO 2p . In this study, the largest micro longitudinal stress was 891 MPa between particles C1 and C2, the location with the minor transverse stress with 561 MPa. Further, due to the solidification shrinkage of the metal, the ZrO 2p prevented the displacement of Ti6Al4V metal dislocation, which contributed to generating stress concentration points on the surface of ZrO 2p . At the same time, the gaps caused between the Ti6Al4V matrix and the ZrO 2p surface also provided the conditions for the formation of microcracks. Larger cracks would be generated after lots of microcracks were gathered.