| This paper intends to explore the printability of 17-4PH stainless steel,through the study of the shape control and performance control of its laser additive manufacturing,to understand the law of its forming,to understand the solidification process of this steel in additive manufacturing and its influence on phase transformation and performance,so as to provide theoretical and technical guidance for additive manufacturing of 17-4PH stainless steel.In this paper,the materiallic forming law of 17-4PH stainless steel was studied by changing the printing parameters,and the best printing parameters were obtained.According to the best parameters,15 layers bulk specimens were printed,with good external and internal quality.In order to improve the mechanical properties of printed specimens,heat treatment was carried out.Metallography,XRD,EBSD,TEM,room temperature tensile and electrochemical methods were used to analyze the related specimens.The influence of solidification cooling rate simulated by copper mold suction casting on the printing solidification behavior,phase and phase transformation was studied.Many valuable research results were obtained.17-4PH stainless steel has good 3D printability.In a wide range,the combination of different printing power and different scanning velocity can obtain the specimens without cracks,pores and other defects,with uniform structure and smooth surface.Optimal printing parameters are scanning velocity v=480mm/min,power P=2600w,scanning velocity v=600mm/min,power P=2800W.The mechanical and corrosion properties of the printed specimens are not sensitive to the printing parameters,and there is no significant differences in the properties of the specimens from different printing parameters.The best heat treatment process is 1040? solution for 2h + 550? aging for 0.5h.The mechanical properties are improved.The tensile strength is 1480 MPa,the Rockwell hardness is 45.1HRC,the high strength still maintains high plasticity,and the elongation is 36.8%.The corrosion resistance of the specimens aged in a short time is better than that aged in long time.The strength and hardness of aged specimens increased with the precipitation of copper rich phase,and then decreased with the coarsening of copper rich phase.The solidification cooling rate in the process of additive manufacturing has a great influence on the solidification behavior,phase and structure of the metals.In the early stage of printing,the residual austenite existed in the specimens because of the fast solidification cooling rate of the specimens.With the increase of the print cladding layer,the solidification cooling rate of the specimens became slower and no residual austenite was found.The influence of solidification cooling rate on the solidification behavior,phase composition and phase transformation of 17-4PH stainless steel was obtained by copper mold suction casting method.With the solidification cooling rate from fast to slow,the phase composition of the alloy changes into austenite and bcc two-phase bcc phase.When the cooling rate of solidification is very fast,17-4PH stainless steel is cooled to room temperature to obtain all austenite structure,that is,austenite does not change in the cooling process,even when it is cooled to-196?,and does not change in aging at <550?.When the cooling rate slows down,the martensite will be formed when the specimen is cooled from solidification to room temperature,and the martensite transformation will occur.When the cooling rate is very slow,the microstructure of the specimen is all martensite.The reason for this phenomenon is that the solidification cooling rate has changed the diffusion of alloy elements in liquid metals,and the high content of alloying elements is favorable for stabilizing the austenite phase.The hardness test shows that the hardness of all austenite phase is 408 HV,which increases with the increase of bcc content.The corrosion resistance of steel with single component phase is the best,and the coexistence of two phases reduces the corrosion resistance.The corrosion resistance of the specimens made by laser additive manufacturing method and suction casting method is preferable to that of the conventional ingot. |
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