Effect of thermal behavior on the grain morphology and dimension of 80-mm-thick Ti6Al4V plates joined by laser melting deposition.

As the material experiences non-uniform thermal expansion and contraction under different thermal behavior during laser melting deposition (LMD), the microstructure of the components will be affected, which also causes changes in mechanical properties. It is essential to understand the thermal behavior of the melt pool during LMD to improve the microstructure and joint quality. In this study, two 80-mm-thick Ti6Al4V plates are successfully joined using three different scanning speeds (10, 15, and 20 mm/s). The effects of thermal behavior on the grain morphology and dimension of deposition area, equiaxed crystal zone (EQZ), and the substrate at different scanning speeds are quantitatively analyzed. The results show that the fine equiaxed crystals are formed near the joint boundary due to the high-temperature gradient and cooling rate. The growth direction of fine columnar crystals continuously changes from the joint boundary to the center of the deposition area due to the change of heat-extraction direction. However, the epitaxial growth direction of the coarse columnar crystal is the same as the previous deposition layer at the center of the deposition area. Given the effect of high heat accumulation and low-temperature gradient during LMD, the dimension of columnar crystal is coarsen significantly at the center of the deposition area. [ABSTRACT FROM AUTHOR]