Study On The Properties Of TiAl Composites Manufactured By Selective Laser Melting

The continuous increase of thrust-weight ratio of aero-engine puts forward higher requirements for the development of new generation gas turbine blade superalloy.Ti Al alloy,as a new structural material of metal compounds,has high specific strength and excellent high temperature performance.By replacing traditional nickel-based superalloy or even some titanium alloys,tial alloy can effectively achieve significant lightweight and comprehensive performance improvement.Due to the poor machinability of Ti Al alloy and the small size but high precision of SLM forming technology,it has a clear demand response for the prototyping of complex precision components in aerospace.In this study,Nb and B nanoparticles modified Ti Al4822 composite powders with nominal composition of Ti-47.53Al-1.98Cr-2.81Nb-0.15B are prepared to form in situ nano-complex phase reinforced Ti Al alloy under the laser induced reaction,so as to further improve the forming quality of Ti Al alloy.At the same time,the structure stability and mechanical properties are enhanced.In this case,the main research points of this paper are as follows:（1）Nano-Nb and nano-B particles modified Ti Al4822 composite powder with nominal composition of Ti-47.53Al-1.98Cr-2.81Nb0.15B are prepared by ball milling process.Ball milling parameters will seriously affect the physical properties of Ti Al composite powder,such as sphericity,stacking angle,particle size distribution,and composition uniformity and oxygen content were quantitatively researched.The composition distribution and oxygen content have a significant impact.Under the optimized grinding time（2 h）,the change of ball-to-powder ratio and grinding speed has a limited impact on the physical properties and oxygen content of the composite powder,but has a considerable impact on the composition uniformity;In addition,the relationship between ball milling energy E_tand particle size distribution width（PSDW）,stacking angle,composition nonuniformity and oxygen content is established.When E_t>1544 J/g and≤3016 J/g and total rotation numberλ>24×10³r and≤36×10³r,or E_t>4014 J/g andλ>36×10³r and≤60×10³r,the best composite powder can be obtained.（2）Single melt channel is the most basic unit of SLM forming.The analysis of the effect of laser process parameters on the quality of single melt channel forming is the basis of subsequent research.Firstly,we use ANSYS to simulate the influence of laser power and scanning speed on the temperature field and stress field of the single melt channel of forming Ti Al-based alloy.The results show that under all parameter combinations,The stress in Y direction is greater than that in X direction.When the scanning speed is changed,the corresponding stress value has a similar trend.The change trend is that it rises rapidly at the beginning and stabilizes after a period of time.Changing the laser power has little effect on the curve of stress versus time.But increasing the laser power,the maximum equivalent stress value decreased significantly.Taking the simulation results as a reference,the forming experiment further refined the process parameter range.The results show that when the laser power is lower than 150 W,the spheroidization of fuse is obvious.When the scanning speed is lower（400 mm/s）or higher（700 mm/s）,the cracks on the sample surface become more and more perpendicular to the scanning direction.The experimental results verify the simulation results.（3）On the basis of the previous research results,the work of optimizing the laser process parameters is carried out.And the effect of laser power and scanning speed on defects such as pores and cracks in Ti Al-based alloy forming parts was further studied.And the mechanism of crack initiation and propagation is analyzed from two-dimensional and three-dimensional perspectives.The results show when the scanning speed was low（400 mm/s and 500 mm/s）and the scanning speed was too high（800 mm/s）,the density of samples in this region was generally low.When the scanning speed is 500 mm/s,the number of holes in the sample is the largest;There are not only macroscopic penetrating cracks,but also small dendritic cracks in the sample;The existence of residual stress produces a large number of secondary and tertiary cracks,and the number and size of cracks in the sample with 500 mm/s is the largest.（4）This chapter uses hot isostatic pressing technology post-processing.The density,forming quality and microstructure of the samples before and after hot isostatic pressing were compared and analyzed.In addition,the mechanical properties of the samples after heat treatment were characterized.The results show that the densification of the three samples after hot isostatic pressing treatment has been improved and maintained above 98%,and the trend of densification has been well consistent with that of morphology.After HIP treatment,the density,microstructure and morphology of the sample changed greatly.The volume fraction and size of theα₂phase increase,and the Nb element is roughly evenly distributed in theα₂andγphases,while the Cr element shows obvious segregation in theα₂phase.The hardness of the sample after HIP treatment is 461.72 HV at the lowest and 476.47 HV at the highest,which is higher than that of the Ti Al alloy sample.