Study On Process Microstructure And Properties Of 17-4PH Stainless Steel By Wire Wire Arc Additive Manufacturing

17-4PH stainless steel has the advantages of high strength,good corrosion resistance and good weldability.It is widely used in aerospace,nuclear power,petrochemical and other fields.However,using traditional forging and cutting methods,17-4PH stainless steel components have proble ms such as easy cracking,long manufacturing cycle,high cost,and low material utilization.Wire arc additive manufacturing technology is an advanced manufacturing technology that uses arc as heat source and metal wire as filling material to form metal components layer by layer.It has the characteristics of low production cost and high deposition rate.It has obvious advantages in the manufacture and repair of large and complex 17-4PH stainless steel components.In this paper,based on the cold metal transition（CMT）+pulse（P）and cold metal transition（CMT）arc modes,the Wire arc additive manufacturing deposition process of 17-4PH stainless steel was studied,and the heat treatment process was explored.The macroscopic morphology,microstructure characteristics,mechanical properties and corrosion resistance of the deposited and heat treated components were systematically analyzed.The conclusions are as follows:（1）The single-pass forming process of 17-4PH stainless steel was studied,and the optimum process parameters of Wire arc additive manufacturing were obtained:with the increase of wire feeding speed,the reinforcement coefficient increases gradually;with the increase of welding speed,the reinforcement coefficient increases first and then decreases.Based on the comprehensive deposition efficiency and forming quality,the optimum deposition process parameters are obtained as follows:wire feeding speed 6 m/min,welding speed 0.36 m/min,current 175 A,voltage 18.3 V,shielding gas Ar+2.5%CO₂,gas flow rate 20 L/min.The high-speed camera and welding quality analyzer were used to observe the stability of droplet transfer and short-circuit transfer during the whole welding process.Through the optimal process parameters,single-channel multi-layer thin-walled wall and multi-layer multi-channel block deposition manufacturing are carried out,and the macroscopic forming of the deposited components is good without obvious defects.（2）The microstructure and properties of the thin-walled wall were studied.The as-deposited thin-walled wall was composed of Body-centered cubic structure and face-centered cubic structure,and the microstructure was martensite,ferrite and a small amount of retained austenite.By adding pulse mode,the size of lath martensite is finer,and a large number of spherical and short rod-mountedε-Cu phases with a size of 5～10 nm are dispersed.The mechanical properties of C+P mode are better,reaching393 HV_0.2,and the tensile strength is 1153 MPa.Compared with CMT mode,the yield strength is 55%higher,and the impact energy at 0°C is not much different,42 J.（3）The best heat treatment system suitable for Wire arc additive manufacturing of 17-4PH stainless steel was obtained:1040°C×1 h+water quenching,480°C×1h+air cooling.The microstructure and properties of the thin-walled wall after heat treatment were studied:the microstructure was composed of lath martensite,and there was a small amount of retained austenite at the martensite boundary.TEM observation shows that the strengthening phases such asε-Cu and Nb C are fully precipitated,andε-Cu is about 5 nm,which is coherent with the matrix.The microhardness is increased to 445 HV_0.2,the tensile strength is 1383 MPa,the impact energy is decreased by 31%,and the corrosion resistance is improved.（4）The microstructure and properties of the as-deposited and heat-treated 17-4PH stainless steel were analyzed:The X-Ray Diffraction results of the as-deposited state are BCC phase and FCC phase,and more black dot-like precipitates appear in the structure.The tensile strength reaches 1246 MPa,and there is a certain anisotropy in the yield strength and impact toughness,reaching 22%and 38%.After heat treatment,more（Fe,Cr）carbides and Nb（C,N）were observed by EDS.The microhardness and tensile strength were slightly better than those of thin-walled wall samples,reaching 1422 MPa,and the corrosion resistance was basically the same.The impact toughness dropped to 18 J,and the impact fracture showed brittle fracture.Many carbides were observed near the crack.