Abnormal Grain Coarsening of Laser Surface Remelting Rapidly Solidified TA15 Titanium Alloy

Abstract:

Rapidly solidified microstructure of a laser surface remelted TA15 titanium alloy as a function of laser scanning speed and solidification cooling rate was investigated. Abnormal coarsening of b grains in the rapidly solidified remelted zone was readily observed for the laser glazed TA15 titanium alloy irrespective of the rapid solidification cooling rate. The b grains in laser remelted zone of TA15 titanium alloy were abnormally coarsened and their sizes were up to 10 to 20 times larger than that of the a grains or 32 to 62 times larger than that of the b grains in the original material. The rapid solidification and grain growth of molten bath of laser surface remelting TA15 titanium alloy was a process of directly epitaxial growth of the b grains of the unmelted substrate at the contact of the melt-pool bottom. The rapidly solidified b grain size in the laser remelted zone was determined by the b grain size of unmelted base metal at the bottom of molten bath. During laser heating and molten bath forming in the surface of the TA15 titanium alloy, b grains of the unmelted base metal at the top of heat-affected zone nucleated, grew in an extremely short time owing to the extremely high temperature. Solid-state abnormal coarsening of b grains on the unmelted base metal at the bottom of molten bath resulted in the abnormal coarsening of rapidly solidification structure in laser remelted zone of the TA15 titanium alloy.