Effect Of Heat Treatment On Microstructures And Properties Of Ti-6Al-4V Alloy By Wire Arc Additive Manufacturing

As one of the commonly used titanium alloys,Ti-6Al-4V alloy is widely used in aerospace and other fields.However,the traditional manufacturing technology of titanium alloy has some problems,such as long production cycle,low material utilization,relatively high manufacturing cost and being difficult to manufacture complex structural parts,which limit the application of Ti-6Al-4V alloy to some extent.In this paper,the application range of Ti-6Al-4V alloy is further broadened through the study of low-cost and high-efficiency TIG wire arc additive manufacturing(TIG WAAM)technology.TIG WAAM is a near-net forming technology which uses TIG arc as the heat source and directly manufactures the solid parts from the digital model in a bottom-up way through layer-by-layer stacking.TIG WAAM can effectively improve the proplems which is the high production cost and low efficiency,existed in laser additive manufacturing(LAM)technology.In this paper,Ti-6Al-4V samples were prepared by TIG WAAM technology,and the effects of different heat treatment processes on the microstructure and properties of Ti-6Al-4V alloy samples by TIG WAAM were studied.The microstructure,fracture morphology and phase structure of the deposited samples and the samples after different heat treatment were analyzed by optical microscope(OM),scanning electron microscope(SEM)and X-ray diffraction(XRD).And the tensile properties and hardness distribution of the samples were tested by electronic universal tester and Vickers hardness tester.The experimental results show that the microstructure of deposited samples is composed of acicular martensite,net basket structure and lamellar structure.The microstructure of heat-treated samples is composed of a large number of primary α phase(containing a certain amount of lath martensite α’)and α + β between the primary α phase;and the primary α phase gradually coarsens with the increase of the height of the deposition layer.Under the condition of this heat treatment,the microstructure state of the deposited sample can not be completely changed,and there is a phenomenon of structural heredity.With the decrease of cooling rate,the tensile strength(UTS)and yield strength(YS)gradually decreased,while the elongation(EL)and the reduction area(RA)gradually increased under the heat treatment conditions of 950℃/2h/WC(water cooling),950℃/2h/AC(air cooling)and 950℃/2h/FC(furnace cooling).The best mechanical properties(UTS~850MPa,YS~810MPa,EL~5% and RA~6%)obtained by the heat treatment condition of 950℃/2h/AC exceed the casting national standards(UTS~835MPa,YS~765MPa,EL~5%).The as-deposited samples were normalized at 850℃×1h/AC,850℃×1.5h/AC,850℃×2h/AC and 850℃×2.5h/AC,respectively.With the increase of normalizing time,the UTS,YS,EL and RA of the samples increased at first and then decreased,and the comprehensive mechanical properties reached the best state when the normalizing holding time was 2h.With the increase of normalizing temperature,the UTS,YS,EL and RA of the samples increased at first and then decreased.The comprehensive mechanical properties of the samples reached the best states under the condition of850℃/2h/AC,and the tensile properties of UTS~940MPa,YS~926MPa,EL~10.5% and RA~13.7%,exceeded the national standard of Ti-6Al-4V alloy casting.The effects of aging temperature and aging time on the microstructure and properties of deposited samples were studied after solution treatment at 950℃/2h/WC.With the increase of aging temperature,UTS and YS gradually decreased,EL and RA increased at first and then decreased.And the best strength-plastic match was obtained at the heat treatment of950℃/2h/WC+600℃/6h/AC.With the increase of aging time,UTS and YS gradually decreased,EL and RA increased at first and then decreased,the best strength-plastic match was obtained at the heat treatment of 950℃/2h/WC+600℃/6h/AC.And the best mechanical properties at950℃/2h/WC+600℃/6h/AC exceeded the national standard of Ti-Al-4V casting.