TA15钛合金激光表面重熔快速凝固晶粒异常粗化

研究了热轧退火近aTA15钛合金激光表面重熔快速凝固组织及激光扫描速度对快速凝固组织的影响.结果发现:无论激光扫描速度如何加快,采用激光表面重熔快速凝固技术都不能细化TA15钛合金重熔区晶粒组织,TA15钛合金激光表面重熔区快速凝固β晶粒组织"异常"粗化,重熔区中β晶粒尺度分别是原始基材中a晶粒的10～20倍和β晶粒的32～62倍.分析表明,TA15钛合金激光表面重熔熔池的快速凝固是一个以熔池底部热影响区项部与熔体接触的未熔母材β晶粒为衬底的直接外延生长(无形核)过程,因此热影响区顶部与熔体接触的未熔母材β晶粒大小决定了重熔区快速凝固β晶粒的大小.激光加热表面重熔熔池形成过程中,熔池底部与熔体接触的未熔母材β晶粒在短时超高温作用下发生了严重长大,是导致钛合金激光表面重熔区β晶粒组织"异常"粗化的直接原因.

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

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.