| Recently,much attention has been paid to TiAl-based alloys,which are extensively used in the field of aerospace engine,gas turbine and vehicle industry due to their low density,high specific strength,high specific stiffness and high specific modulus,good high-temperature and corrosion resistance,excellent oxidation and creep resistance.However,the cutting of materials is difficult due to its high hot strength,low thermal conductivity,and room temperature brittleness,which severely hinders the practical application of TiAl-based alloys in industry.In this paper,high-performance TiAl alloy was prepared by the atomized Ti-48Al-2Cr-8Nb powder and solidified by spark plasma sintering technology.A group of cutting parameters with good surface quality is selected based on the integrity of the processed surface.The densification process,phase transformation process,microstructure and mechanical properties of the sintered body before and after mechanical milling,in-situ heat treatment process and surface integrity of fine-grained ?-phase TiAl based alloy were systematically studied.The effects of sintering temperature and sintering pressure on the densification of Ti-48Al-2Cr-8Nb pre-alloy powder were systematically studied.Firstly,the densification mechanism of the powder materials at different sintering temperatures was investigated.At the beginning of the sintering process(T?880?),the powder particles were closely rearranged under pressure.In the middle of the sintering process,the plastic deformation of the powder particles and the material migration between the adjacent particles promoted the densification into the high-speed stage.In the late stages of sintering(T?1250?),the unique equilibrium phase transition of the TiAl alloy allowed the densification to be completed quickly.The influence of sintering pressure on the densification behavior of powder was analyzed.Increasing the pressure can accelerate the process of powder densification,but it has little effect on the density of sintered body.Scanning electron microscopy(SEM)and optical microscopy(OM)were used to observe the micro structure of Ti-48Al-2Cr-8Nb sintered specimen at different sintering temperatures,and the phase composition analysis was carried out by X-ray diffraction(XRD)and electron backscatter diffraction(EBSD)techniques to study the formation process and phase transition mechanism of the near gamma structure(NG),the duplex structure(DP),the full-lamellar structure(FL)and X structure in Ti-48Al-2Cr-8Nb alloy.The mechanical properties were characterized by testing the Vickers hardness,room temperature bending and compressive properties of the sintered specimens.The results showed that the fine-grained DP structure(lamellar ratio of 10%)at the sintering temperature of 1300? had the highest Vickers hardness and room temperature resistance.The Vickers hardness and the bending strength were 290 HV and 675 MPa respectively,and the DP structure(lamellar ratio of 70%)at the sintering temperature of 1350? had the best room temperature compressive properties.The room temperature compressive strength and strain rate were 2544 MPa and 35.44%,respectively.The Ti-48Al-2Cr-8Nb pre-alloy powder was pretreatment before sintering by mechanical milling process,and the effects of powder preparation on properties of spark plasma sintered Ti-48Al-2Cr-8Nb alloy were studied.This paper analyzed the variation of density,microstructure,Vickers hardness,and fracture strength of TiAl alloys prepared with spherical pre-alloyed powder(SP powder)and pre-alloyed powder after 12 hours of ball milling(MP powder)at different sintering temperatures.The results indicate that the density,Vickers hardness,and room temperature(25?)bending strength of Ti-48Al-2Cr-8Nb alloy sintered using MP powder,are significantly higher than that of TiAl alloy sintered using SP powder.Specifically,the densification temperature of the MP powder sintered specimen is reduced by 100?,the Vickers hardness is increased by 15%,and the room temperature bending strength is increased by 51.9%at a sintering temperature of 1250?.The microstructure analysis shows that the Ti-48Al-2Cr-8Nb alloy has the best bending strength when it has a fine grain phase structure.In the experiment,it was found that during the sintering of Ti-48Al-2Cr-8Nb pre-alloyed powder,when the temperature exceeds T?(1250?),the ? phase grains in the sintered specimens have a significant refining process.Through systematic analysis of the ? phase refinement mechanism,the in-situ multiple sintering process was developed to further refine the ? phase grain size.The experimental results show that after the in-situ secondary sintering process,the ? phase grain size in the alloy is only 7-10 ?m,which is only half of the grain size in a single sintering.In addition,the in-situ multiple sintering process was extended to the TiAl-based alloy with a lamellar structure,so that the size of the lamellar colonies grew from 100-150 ?m to 2-3mm,which improved the mechanical properties of the TiAl-based alloy with a FL structure.The influence of different milling parameters on the surface integrity of fine-grained ?-phase TiAl based alloy was studied by orthogonal design experiment(L9(33)),and the cutting parameters of milling fine-grained ?-phase TiAl-based alloys were determined:vc=80m/min,Ap=0.05mm,f=0.005mm/tooth.The results were discussed in terms of cutting force,three-dimensional topography,surface roughness,macroscopic topography,surface defects,and chip morphology.The results showed that under this group of milling parameters,the roughness of machined surface is small(Ra,Sa<0.3 ?m),the three-dimensional topography of the surface is relatively smooth,and the milling speed had the greatest effect on the surface roughness and three-dimensional topography.The typical defects on the machined surface are ?-phase grains being pulled out and edges shattering.The cutting speed and cutting depth had great influence on the surface defects.The surface defects increase with the increase of cutting depth and feed,and decrease first and then increase with the increase of cutting speed.Chip morphology is closely related to the quality of the machined surface.With the decrease of cutting depth and feed,the curl degree of chip increases,the processing plasticity of material increases,and the surface quality will be improved obviously. |
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