Study On Microstructure And Crack Mechanism Of TiAlCrNb Alloy Formed By Selective Laser Melting

TiAl-based alloy is a new type of lightweight high-temperature structural material with a series of advantages,such as high specific strength,good wear resistance,high temperature resistance,good corrosion resistance and excellent oxidation resistance,which has broad application prospects in the aerospace field.However,TiAl alloys have poor ductility and low fracture toughness,and it is difficult to form practical parts with complicated structures by conventional techniques.Selective laser melting?SLM?is one of the latest and most promising forms of Metal Additive Manufacturing.Because of the advantages such as shorten process cycle,high material utilization rate and can form arbitrarily complex structure,it has become one of the best technologies for the fabrication of complex high-performance metal parts.At present,there have been attempts to form TiAl alloys by SLM.However,the overall stage is at a preliminary stage,and the microstructure and metallurgical defects under the new process need to be further studied.In this paper,Ti-45Al-2Cr-5Nb and Ti-47Al-2Cr-2Nb were used as forming materials to study the process optimization,microstructure and defect mechanism of TiAl alloy formed by SLM,in order to form the accumulation mechanism and technical basis of SLM forming complex high performance TiAl alloy parts.The main research contents are as follows:The process parameters of SLM forming Ti-45Al-2Cr-5Nb alloy were optimized.The optimized parameters were:laser power 250 W,laser scanning speed 700 mm/s,laser scanning space 100?m and layer thickness 50?m,the relatively density of fabricated samples reached to 92.51%.The samles were dominated by?₂,?and B₂ phases,the upper surface of the SLM sample exhibited equiaxed crystals whose grain size decreases from the edge of the melton tracks to the center,and the side surface of the sample exhibited cell crystals grown in the manufacturing direction.Since the grain size was finer than the conventional forming method,the nanohardness of the formed part reaches 7.74±0.39 GPa,which is higher than the conventional centrifugal casting?4.98±0.25 GPa?and hot rolling forming?6.73±0.34 GPa?.The effect of SLM laser scanning space on the microstructure of the formed Ti-45Al-2Cr-5Nb/TiB₂ alloy was studied.With the laser scanning space increases from 80?m to 140?m,the cooling rate increases.Therefore,the average grain size gradually decreases from4.89?m to 4.18?m,and orientation gradually transform from?0001?into?011?1?and?112?1?orientations.Besides,because of the decrease of the liquid phase residence time and proportion of the remelted zone,the ratio of the large angle grain boundary decreases from 91.58%to 87.60%,and the average Schmidt factor decreases from 0.395 to 0.354,the sample's ability to resist deformation is enhanced.The effect of SLM laser scanning space on the phase composition and mechanical properties of the formed Ti-45Al-2Cr-5Nb/TiB₂ alloy were studied.The SLM-processed Ti-45Al-2Cr-5Nb/TiB₂ alloy are dominated by?₂,?,B₂,TiB₂ and TiB phases.With the laser scanning line spacing increases from 80?m to 140?m,the L+???peritectic reaction is inhibited,and the???₂+?eutectoid reaction is promoted.Therefor,the content of?₂ phase decreases gradually and the content of?phase and B₂ phase increases gradually.Because of the mechanical properties of the alloy samples are affected by many factors such as phase composition,grain size and relative density,the nanohardness of the sample increases first and then decreases as the laser scanning space increased.When the scanning space is 100?m,a maximum of 9.41±0.47 GPa is obtained.Furthermore,due to the effect of TiB₂refining grains,the alloy nanohardness of the sample is higher than that of the SLM formed Ti-45Al-2Cr-5Nb matrix material?7.74±0.39 GPa?.Cracking Mechanism and Inhibiting Process of Ti-47Al-2Cr-2Nb alloy fabricated by Selective laser melting were studied.The cracks in the samples are a cold crack,which originate from the existence of defects such as powder sticking and notch on the side edge of the samples.The root cause of cracks formation is that the residual stress inside the molded part is higher than the tensile strength of the material.In addition,the alloy samples contain a large amount of?₂ brittle phase and pores,which are favorable for crack generation and expansion.On the one hand,the preheating causes the decrease of internal temperature gradient,and the residual stress decreases from 267.2±13.4 MPa to 172.6±8.6MPa.On the other hand,preheating leads to an increase in?phase stability and a decrease in cooling rate,both L+???and???₂+?are inhibited,resulting in the decreases of?₂phase.When the preheating temperature reaches 300°C,the number and size of cracks in the sample are obviously controlled,and the relative density of the sample increases from87.64%to 93.84%.