Study On The Mechanism Of Microstructure Evolution And Defect Suppression Of W-Nb Alloy By Selective Laser Melting

Traditionally,tungsten alloy parts are manufactured using powder metallurgy or powder injection molding technology.These two methods are quite difficult to produce parts with complex three-dimensional structures.Selective laser melting additive manufacturing technology can directly realize the near-net forming of complex three-dimensional structured refractory metal parts,providing a broad application prospect for tungsten alloys.However,due to the high melting point and high tough-brittle transition temperature of tungsten itself,defects such as microcracks and holes are highly likely to occur during the preparation of tungsten heavy alloys(WHA)by selective laser melting,and the appearance of defects inevitably leads to a reduction in the application range of the material.Therefore,the effective suppression of defects is still a challenge.In this paper,the powder ball milling and preparation process parameters of selected area laser melting(SLM)W-5Nb were investigated,and highly dense W-5Nb and W-5Nb/ZrH₂ samples were successfully manufactured by the optimal ball milling parameters and sample preparation process.The influence of process parameters on the densification,microstructure and defect generation was analyzed,focusing on the densification process of W-5Nb,the suppression mechanism of microcracking by Nb alloying and the effect of doping with 0.5 wt.%ZrH₂ on the microstructure of W-5Nb.For the study of the ball milling parameters of the powder and the sample preparation process parameters,it was found that the optimal ball milling parameters for the preparation of the W-5Nb composite powder were:ball milling speed of 100 rpm,ball milling size ball ratio of 1:2,and ball milling time of 10 hours.The optimal W-5Nb selective laser melting and forming process parameters were:laser power 275 W,scanning speed 390 mm/s,hatching distance 0.08 mm.For W-5Nb,it was found that the densities of the samples showed a variation pattern of increasing and then decreasing with the increase of energy density(E).When E was293 J/mm³,the densities were as high as 98%,the microstructure was mainly composed of a single W-5Nb solid solution phase,and the microcracking was basically suppressed.Nb alloying resulted in a refinement of grain size(36%,relative to pure W)and a significant reduction in the percentage of large angular grain boundaries(>15°)(67%,for E=293 J/mm³ and E=556 J/mm³).Meanwhile,Nb alloying discrete the nanopore distribution formed by W_xO_y gasification segregation,reducing the contribution of nanopore segregation to microcrack eruption.The solid solution strengthening effect of Nb further increases the bond strength between grain boundaries.The increased bond strength at grain boundaries makes the possibility of cracking between grain boundaries much less likely.For W-5Nb/0.5 wt.%ZrH₂,it was found that the microscopic phase composition was a continuous and single solid solution phase of W-5Nb/ZrH₂,which did not produce significant microcracking defects and still had pores.The doping of ZrH₂ reduces the grain size of W-5Nb,but the reduction is not significant and there is no obvious weave orientation.Compared with W-5Nb,the doping of ZrH₂ makes the residual stress distribution more uniform and not concentrated in a certain region.This makes the possibility of cracking due to concentrated distribution of residual stresses greatly reduced.