Study On Selective Laser Melting Forming Of Ti48Al2Cr2Nb Alloy

With the rapid development of aerospace,transportation,petrochemical and other technologies in China,traditional superalloys have been unable to meet the increasingly high requirements for use.The Ti48Al2Cr2Nb alloy has become one of the most promising materials among the new lightweight superalloys due to its low density,high strength,good creep resistance and good oxidation resistance.However,the Ti48Al2Cr2Nb alloy has low ductility,which makes it difficult to be processed plastically.Traditional forming technology faces great difficulties in forming Ti48Al2Cr2Nb alloy,which has always restricted its development.Selective laser melting(SLM)is a unique forming method based on the melting of powder and stacking of layers into three-dimensional parts.It has the advantages of low cost,short cycle and simple process,and is commonly used in the primary forming of precise and complex structural parts.Therefore,it has great application prospects in the fields of medical,military and transportation.SLM has become the key to promote the development of the alloy because it overcomes the problem that Ti48Al2Cr2Nb alloy cannot be formed into precise and complicated parts.However,SLM technology is prone to defects such as cracks,pores,and spheroidization when forming Ti48Al2Cr2Nb alloys.Therefore,in this paper,Ti48Al2Cr2Nb alloy powder is used as the preparation material and the alloy is formed by SLM technology to study the typical defect generation mechanism,forming process and microstructure,and to investigate the effect of remelting and hot isostatic pressing methods on the defects.The main research contents are as follows:(1)The effect of different process parameters on the typical defects of SLM fabricated Ti48Al2Cr2Nb alloy was studied.It was found that the laser energy density is too large and too small will have an effect on the defects.Laser energy density is too small to make the powder melting incomplete,in the powder gap is not fused to form pores;too large to make the powder in the elements evaporate to form pores,and the powder gap gas can not escape in time will also form pores.Cracks are mainly generated around unfused holes,bonded powders,and incomplete fusion of powder gaps.Surface spheroidization is mainly due to the adhesion of unmelted powder and splash particles on the surface of liquid metal.On the other hand,it is due to excessive energy absorption of liquid metal,which tends to reduce the surface energy and form spheroidization.In addition,the optimal forming process after optimization is the laser power of 120W and the scanning speed of 900mm/s.At this time,the formed sample is completely as a whole,without warpage deformation,with formability,and the holes are basically eliminated,but there are still a large number of non-process cracks.(2)The microstructure of SLM-formed Ti48Al2Cr2Nb alloy was studied.It was found that the microstructure of Ti48Al2Cr2Nb alloy formed by SLM is?₂/?lamellar structure composed of?-Ti Al phase and?₂-Ti₃Al phase,and the lamellar structure accounts for 99%,which belongs to the fully lamellar structure.Compared with the powder,the content of?₂ phase in the SLM-formed Ti48Al2Cr2Nb alloy is increased,which is because the?phase will be ordered and transformed into the?₂ phase during the solidification process,and then a small amount of?phase will be precipitated.The grains of the Ti48Al2Cr2Nb alloy after SLM forming are refined,and 90%of the grains are distributed in the range of 5-20?m.It is much smaller than the grains formed by the traditional process(100?m-300?m),which is due to the rapid melting and cooling rate of SLM forming.In addition,the SLM-formed Ti48Al2Cr2Nb alloy contains a large number of brittle cracks inside,which is mainly due to the huge thermal stress formed during the SLM forming process,and the?₂ brittle phase content is more difficult to resist the huge thermal stress to produce a large number of cracks,and cause grain deformation to produce a large number of dislocations inside the alloy.(3)The effect of the number of remelts on surface spheroidization and cracking during SLM forming was studied.The results show that the SLM-formed Ti48Al2Cr2Nb alloy has serious surface spheroidization and large surface roughness,obvious melt pool boundary,and more internal micro-cracks.After one laser remelting,the surface adhesion spheres are melted,the surface roughness is reduced.There is no obvious molten pool boundary,the surface spheroidization is basically eliminated and flatness is increased,and the cracks begin to fuse.After the second laser remelting,the surface adhesion spheres basically melted,but the splash particles increased.Compared with the one remelting,the surface roughness slightly increased,the surface was slightly deformed,and depressions and bulges appeared.The initial cracks were completely fused,but the secondary cracks increased.(4)The effects of hot isostatic pressing(HIP)on the microstructure,mechanical properties and defects of SLM-formed Ti48Al2Cr2Nb alloy was studied.The results show that the microstructure of the SLM-formed Ti48Al2Cr2Nb alloy after HIP is a duplex-phase structure composed of?phase and?₂/?lamellae,with the?phase content increasing to 92.95%and the?₂/?lamellae content decreasing to 7.05%,which is due to the promotion of?phase recrystallization by HIP.The grain of Ti48Al2Cr2Nb alloy is refined after HIP,and 90%of the grain size is less than 5?m,which is mainly due to the fact that dislocations hinder the growth of grain boundaries and the pinning effect of?phase.After HIP,the cracks begin to close,the dislocation content decreases,the?recrystallized grains increase,and the proportion of high-angle grain boundaries increases,which is because the generation of?crystals consumes dislocations.After HIP,the plasticity of the alloy is greatly improved,the hardness is decreased,the compressibility is significantly increased,and the elastic yield stage appears,which is mainly due to the increase in the content of?phase.The high temperature promotes dynamic softening and the flow stress decreases,which is mainly due to the high temperature promoting dynamic recrystallization and dislocation movement.