Influence Of Post-treatment Process On The Microstructure And Properties Of Selective Laser Melted TC4 Titanium Alloy

Ti-6Al-4V is one of the most common aerospace metals and is widely used in industrial applications such as aerospace,biomedical,marine equipment and automotive manufacturing.It is a relatively expensive alloy,and in traditional subtractive manufacturing,more than 80%of the raw material must be machined and consumed in order to achieve the final part geometry,resulting in great waste.Additive manufacturing(Additive Manufacturing)creates solids directly layer by layer through3D models,a process that allows for fast and accurate manufacturing of complex shaped parts while also addressing the waste of raw materials in traditional manufacturing methods.These unique advantages make this technology more and more widely used,but the 3D printed parts have problems such as uneven organization and insufficient comprehensive mechanical properties,which seriously limit the future development of additive manufacturing technology.This paper focuses on the effects of process parameters,Heat Treatment and Ultrasonic Surface Rolling Treatment on the microstructure,material hardness,tensile properties and fatigue crack extension resistance of Ti-6Al-4V alloy fabricated by selected area laser melting.The objective of the research project is to improve the forming quality of the parts by optimizing the process parameters to reduce the porosity and crack defects,and to improve the mechanical properties of the parts by optimizing the post-treatment process to improve the organization,refine the grains,and generate the surface layer with gradient structure.Firstly,we explore the suitable equipment processing parameters to produce specimens with good molding quality.Then the effect of different heat treatment temperatures,times,and cooling methods on the improvement of the microstructure and mechanical properties of the material is investigated.Finally,the ultrasonic surface tumbling technique is combined to further refine the grains on the material surface and create gradient deformation layers to eliminate surface defects,thus improving the surface mechanical properties of the material.Optical Microscope,Scanning Electron Microscope,Transmission Electron Microscope and other material science analysis methods were used to study the effects of different parameters and processes on The effects of different parameters and processes on the macroscopic morphology and microstructure of TC4 alloy have been investigated,and the results of mechanical properties testing equipment(Vickers hardness tester,nanoindentation tester,tensile testing machine,fatigue crack extension machine)have been combined to clarify the intrinsic relationship between the organization and properties.The results of the study provide positive guidance for the production of high-performance TC4 alloy additive manufacturing parts.The main findings are as follows.(1)Cracks formed during the machining of a part mainly have four stages:primary crack emergence,crack expansion,secondary crack emergence,and crack termination.Adjusting the processing processes such as support design,substrate preheating,and scanning strategy to reduce the residual stress during workpiece forming can effectively reduce and eliminate the generation of crack defects.Meanwhile,lower laser energy density will lead to poor denseness of the part,and higher energy density will lead to excessive melting of the part surface morphology and molding difficulties,which will also lead to large residual stresses inside the part and thus cracking phenomenon.Optimization of laser power,scanning speed,scanning spacing and other processing parameters can improve the internal organization of the part,thus effectively reducing the porosity of the part.(2)The microstructure of the selected area laser melted TC4 alloy presents at different magnifications as:columnar?grains of 200-300?m parallel to the deposition direction;needle-like?'tissue of less than 1?m in width tilted fine with respect to each other at specific angles of 0,30,60 and 90°;sub-grains that subdivide the martensitic slats.The XRD patterns of the samples did not show?-phase peaks with uniform distribution of the elements and no significant elemental segregation occurred.The Vickers hardness of the samples tended to stabilize at pressures greater than 3 k N,and the Vickers hardness of the samples was 360 HV with an error interval of 350 HV-370HV,the nanohardness fluctuated in the range of 445 Gpa-495 Gpa,and the Young's modulus varied in the range of 120 Gpa-140 Gpa.The yield strength of the specimens was 1280 MPa,the ultimate tensile strength was 1430 MPa,and the fracture strain was3.8%,which exhibited obvious high strength and low plasticity characteristics.The fatigue crack expansion threshold value?K₀=3.5343 Mpa.m^0.5(mean error±0.6).The fatigue crack extension rate equation for the constant load experimental method is:da/d N=6.9307×10^-11(?K)^2.8745.Under the conditions of constant amplitude 2k N,loading frequency 10HZ,and stress ratio 0.1,the crack extension length is 0.7a/w,and the number of stress cycles LC=138300.(3)During the post-SLM heat treatment,the growth orientation and the fine initial?'structure have a significant limiting effect on the evolution of the microstructure.A slow increase in grain size was observed when the low transition point heat treatment was performed at different temperatures,and the?/?'slat size was closely related to the heating temperature.The increase in?lamella width became significant(>4?m)only when the heating temperature exceeded 900°C.The cooling rate did not significantly affect the mechanical properties of the heat-treated SLM Ti-6Al-4V alloy.For both furnace-cooled and air-cooled samples,the best tensile properties were obtained after heat treatment at 950°C/2 h/AC.The samples have high yield strength(over 1 GPa),high ultimate compressive strength(about 1.1 GPa)and good ductility(12.5%).The short aging heat treatment also improved the fatigue life of the material throughout the fatigue cycle,with the number of stress cycles LC=202755,which is 46.6%higher than that of the SLM specimen.(4)Purposefully designed thermal cycling induced oscillations in the volume fractions of?and?phases,which acted synergistically with the slow cooling phase of the cycle to spheroidize the?phase through epitaxial growth.The resulting bimorphic microstructure resulted in a significant increase in ductility(fracture elongation of 16%±0.6)while maintaining high strength(yield strength of 908±10 MPa),which led to a significant enhancement in the fatigue crack extension performance of Ti-6Al-4V as well(threshold value?K₀=8.5632 Mpa.m^0.5,number of stress cycles LC=396139).(5)Ultrasonic rolling was able to refine the surface organization of SLM-molded TC4 parts,and the deformation layer of the material gradually thickened(from 200?m to 540?m)as the rolling pressure and number of rolls increased,and broke the elongated needle-like?martensite,allowing the formation of high-density dislocations along the boundary of the laminar?structure.This brought about an increase in surface Vickers hardness(173%higher than that of the SLM specimen)and a gradient of plastic deformation layers along the longitudinal section.The yield strength of the material increased from 1270 MPa to 1510 MPa(18.90%compared to the untreated sample),while improving the resistance to fatigue crack extension(threshold value?K₀=7.7526Mpa.m^0.5,number of stress cycles LC=284898),but also brought about a decrease in plasticity(only 72.36%of that of the SLM sample).(6)The USRP treatment had limited effect on the tensile and ultimate yield strength of the HTed SLM Ti-6Al-4V alloy,but the elongation of the HTed+USRPed material was significantly lower than that of the HTed material.It is noteworthy that the yield strength and elongation of the 975/2h/AC HTed+USRPed specimens exceeded those of the forged alloy.After HT+USRP treatment,the fatigue crack extension resistance of the specimens was better than that of the forged alloys.From the experimental data,it can be found that HT and USRP can improve the fatigue life of the material,and the HT+USRP process can even make the fatigue life of the material exceed the fatigue life of the forging alloy.