Fundamental Research On Melt Hydrogenation Of Titanium Alloys With TiH₂ As The Additives

Hydrogenation can refine microstructure and improve processing properties of titanium alloys, using hydrogen as a temporary element by its characteristics in titanium alloys. We put forward a new method of hydrogenation in this paper, named melt hydrogenation, which combines hydrogenation and induction skull melting technology, added titanium dihydride into titanium alloys melt during induction skull melting process, and gains casting or ingot of hydrogenated titanium alloys. The influence of processing parameter, including vacuum chamber pressure, heating power, adding ways and the amount of titanium dihydride and cooling way, on melt hydrogenation process was analyzed. We studied the influence of melt hydrogenation on solidification structure of Ti6Al4V alloy, analyzed the defects in ingot and its formation reasons, and researched the influence of melt hydrogenation on the processing properties of Ti6Al4V alloy.Melt hydrogenation process of titanium alloys were divided into three stages: (1) the decomposition of titanium dihydride, namely the falling process of feeding bar of titanium dihydride from the upper of water cooling copper crucible until contacting melt; (2) the dissolution of titanium dihydride, namely the falling process of feeding bar from contacting melt until dissolved totally; (3) the solidification process of hydrogenated Ti6Al4V alloy melt. Based on the theoretical analysis of melt hydrogenation process, the amount of hydrogen in Ti6Al4V alloy after melt hydrogenation was determined by the weight percent of titanium dihydride added into melt and holding time during melt hydrogenation process.After melt hydrogenation with the feeding bar of titanium dihydride added into melt in the form of powder, the evolution of solidification structure of Ti6Al4V alloy and its mechanism were studied. The composition of Ti6Al4V alloy after melt hydrogenation was almost the same with the one without melt hydrogenation. There wereαphase,βphase andδphase in the solidification structure of Ti6Al4V alloy after melt hydrogenation. The macrostructure of Ti6Al4V alloy after melt hydrogenation was equiaxed grain, and refined with the weight percent of titanium dihydride when it was less than 1.25wt.%. And the average grain size decreased almost 50% and theα/βlamellar refined with 1.25wt.% titanium dihydride added into melt. When it was more than 3.75wt.%, the grain boundary become broader. Hydrideδwas discovered needle like precipitated inαlamellar, and increased with the weight percent of titanium dihydride. After melt hydrogenation with the feeding bar of titanium dihydride added into melt in the form of compacted bar, the evolution of solidification structure of Ti6Al4V alloy was studied. The diffraction peaks amount ofβphase andδphase increased, resulted by the increasing ofβphase andδphase amount in microstructure of Ti6Al4V alloy after melt hydrogenation. The solidification macrostructure of Ti6Al4V alloy after melt hydrogenation with this method was irregular granular structure, and there were needle likeαphase existed in the microstructure, which was refined with the increasing weight percent of titanium dihydride.There were shrinkage cavity and pore existed in the ingot of Ti6Al4V alloy after melt hydrogenation and its formation reasons were studied. After melt hydrogenation with the feeding bar of titanium dihydride added into melt in the form of powder, morphology like ripples appeared on the shrinkage cavity surface resulted by gas evaporation in Ti6Al4V alloy, but there were not pores exsited. After melt hydrogenation with the feeding bar of titanium dihydride added into melt in the form of compacted bar, there were pores distributed dispersing on the shrinkage cavity surface. And there were pores distributed dispersing in the microstructure and increased with the weight percent of titanium dihydride.After melt hydrogenation with the feeding bar of titanium dihydride added into melt in the form of powder, the changes of hardness, flow stress and impact fracture morphology of Ti6Al4V alloy were studied. The macrohardness after melt hydrogenation decreased compared with the one without melt hydrogenation. The microhardness increased with the weight percent of titanium dihydride when it is less than 1.25wt.%, and it is lower at the grain boundry than in the grain. The microhardness decreased when the holding time increased during melt hydrogenation process, and it can decrease almost 40% at grain boundary. The flow stress decreased with the increasing weight percent of titanium dihydride when strain rate and deformation temperature were constant. The flow stress rised when increasing the strain rate, and decreased when increasing the deformation temperature. Impaction fracture morphology of Ti6Al4V alloy consisted of dimples and tearing ridges. And dimples increased and refined with the increasing weight percent of titanium dihydride added into melt during melt hydrogenation process.