Study On Friction Welding And Wear Properties Of Additive Manufacturing TC4 Titanium Alloy

Ti-6Al-4V(TC4)alloy has been widely used in aerospace,chemical and biomedical fields,because of its high specific strength,good corrosion resistance and good biocompatibility.The additive manufacturing technology has been emerged in recent years,which can not only greatly improve the utilization rate of titanium alloy materials,but also produce complex structural parts.However,poor wear resistance of TC4 titanium alloy seriously affects the service life and limits its application and development.The study founds that the additive manufacturing TC4 titanium alloy has higher hardness and strength due to its fine grain structure,which may correspondingly improve its friction and wear resistance.However,there are few studies on the friction and wear performance of additive manufacturing TC4.In addition,the parts manufactured by additive manufacturing are limited by the volume of equipment,especially for metal materials produced by electron beam melting(EBM)and selective laser melting(SLM)technology,the size of parts depends on the size of the chamber.Increasing the chamber will further lead to problems such as powder removal,inert gas flushing chamber cost and increased sample residual stress and other defects.Using welding technology to connect additive manufacturing parts provides a way to solve the size limitation.Therefore,it is very important to study the friction and wear performance and welding performance of additive manufacturing TC4 titanium alloy.This work takes the TC4 titanium alloy prepared by EBM(EBM TC4 titanium alloy),the TC4 titanium alloy prepared by SLM(SLM TC4 titanium alloy)and the forged TC4 titanium alloy(forging TC4)as the research objects,using ring-block dry friction and wear experiment to study its friction and wear performance and mechanism;using friction welding technology to study the structure,hardness,modulus change and deformation mechanism of TC4 titanium alloy prepared by different additive manufacturing processes before and after welding,and the tensile property changes of samples before and after welding.The main results are as follows:(1)The microstructure of the forged sample is an equiaxed ? crystal with irregular ?phases distributed at the ? grain boundaries.The microstructure of the SLM TC4 titanium alloy is mainly composed of the original ? columnar crystals grown epitaxially,and the graincontains a large amount of needle-shaped martensite ?' phase and ? phase between ?' bodies.The microstructure of EBM TC4 titanium alloy is characterized by a Widmanstatten structure,the main phase is lamellar HCP structure ?,and a small amount of bcc structure ? distributed among them.The results show that the order of friction and wear resistance under dry friction conditions is: EBM TC4 titanium alloy > SLM TC4 titanium alloy > forged TC4 titanium alloy.The reason why additive manufacturing has better friction and wear resistance than forged TC4 titanium alloy is that the additive manufacturing TC4 titanium alloy's higher hardness and less lateral crack in wear scar structure.(2)The microstructure of the EBM-TC4 titanium alloy show three different regions after welding: base material(BM),thermal mechanically affected zone(TMAZ),and welding center zone(WCZ,width ? 1165 ?m).These three regions are composed of ? and ? phases,but their shapes are different.The most significant change is that the columnar crystals in the matrix metal are decomposed into irregular ? phases and equiaxed ? phases,and some of columnar crystals in the thermal mechanically affected zone become coarser.The results show that weld center zone(hardness: 5.65 ± 2.40 GPa)get higher hardness than base metal(hardness: 3.64 ± 1.47 GPa),and weld center zone has a higher strain recovery rate and higher work hardening ability and ductility.Compared with the as-prepared EBM TC4 titanium alloy,the yield strength and tensile strength of the welded specimen decreased slightly,but the elongation increased slightly by ? 13%.Fracture analysis showed that the welded specimen's fracture mode was ductile fracture.The results show that friction welding technology can successfully weld EBM TC4 titanium alloy without deteriorating the mechanical properties of the material.(3)The microstructure of SLM-EBM TC4 welding sample also has five different regions: SLM base metal,SLM thermal mechanically affected zone,weld center zone,EBM thermal mechanically affected zone,EBM base metal.In the weld center zone,the ? initial grains dynamically recrystallized to form fine equiaxed(? + ?)lamellar symbiotic structure,which are softened in the thermal mechanically affected zone due to shearing forces and high temperature during friction,so each phase of the organization undergoes severe plastic deformation.The results show that the hardness of weld center zone(5.93 ± 0.40 GPa)is between EBM TC4 base metal(hardness: 5.61 ± 0.67 GPa)and SLM TC4 base metal(hardness: 6.55 ± 0.31 GPa).The strain recovery rate,work hardening ability and ductility of weld center zone are greater than that of the EBM TC4 titanium alloy base metal but smaller than SLM TC4 base metal.The activation volume and strain recovery rate indicate that the plastic deformation resistance of weld center zone is higher than EBM TC4 base metal and lower than SLM TC4 base metal.Compared with the as-prepared EBM TC4 titanium alloy,the welded specimens have similar yield strength,tensile strength but increased elongation.Failure fracture occurred in the EBM TC4 titanium alloy matrix instead of weld center zone.Fracture analysis showed that the fracture mode of the welded specimen was ductile fracture.So friction welding technology can successfully weld the heterogeneous TC4 titanium alloy by additive manufacturing.