Study On Microstructure And Properties Of 17-4PH High Strength Steel By Selective Laser Melting And Heat Treatment

17-4PH high strength steel,as the most common material in martensitic precipitation hardening stainless steel,relies on the precipitation of the second relative material to produce strengthening effect.As a stainless steel material with high strength,high hardness and excellent weldability,it is widely used in aerospace,biomedicine,machinery manufacturing,petrochemical industry,nuclear industry and other fields.With the increasingly stringent requirements for materials in today's era,the problems of low efficiency,complex process,high cost and low material utilization of traditional processing technology can not meet the demand for parts.Selective laser melting(SLM)has become a research hotspot in the field of material forming because of its advantages.This paper takes 17-4PH high strength steel as the research object.Aiming at the problems that 17-4PH high strength steel is easy to produce defects in the SLM forming process and is difficult to meet the performance requirements of nuclear power and national defense science and technology,17-4PH high strength steel is prepared by SLM technology.The effects of different forming process parameters and heat treatment systems on the microstructure and properties of 17-4PH high strength steel formed by SLM are systematically studied.The main research contents and results are as follows:(1)The changes of temperature field and stress field during SLM forming of 17-4PH high strength steel were studied.The results show that the size of the molten pool will increase with the increase of laser power,and the depth width ratio of the molten pool will first increase and then decrease.With the increase of laser power,the temperature difference will become larger and larger,resulting in uneven heating of the molten pool,and the residual stress will continue to increase with the increase of laser power,leading to the formation of cracks.Considering that the laser power is 185w,it is the best laser power.(2)The effects of laser power and scanning distance on the internal microstructure and mechanical properties of 17-4PH high strength steel specimens formed by SLM were studied.The results show that when the scanning distance is constant,when the laser power increases from 165 W to 185 W,the laser energy density increases,so that the absorption of laser by powder increases continuously,the molten pool gradually becomes obvious,the width of molten pool increases,and the hardness increases from 367 HV_0.5 to 375 HV_0.5;When the laser power continues to increase to 205 W,the absorbed energy of the powder to the laser continues to increase,the size of the molten pool becomes larger,the sample appears spheroidization,the pores of the sample increase,and the hardness is 357 HV_0.5.When the laser power is constant,the scanning spacing increases from 90?m,increased to130?m,the absorption of laser by powder decreases continuously,and the width and size of molten pool decrease.Finally,it is determined that when the laser power is 185 W and the scanning spacing is 110?m,the surface porosity of the sample is the least,the hardness is the largest,and the formability is the best.The sample of 17-4PH high strength steel formed by SLM is mainly composed of martensite and a small amount of retained austenite,with high-density dislocations and a small amount of residual austenite?-Cu precipitation hardening phase and Si O₂ inclusions.(3)The effect of solution temperature(940°C-1140°C×2 h(water cooling)+480°C aging temperature×4 h(air cooling))on the microstructure and properties of 17-4PH high strength steel samples formed by SLM was studied.The results show that with the increase of solution temperature,the martensite structure becomes larger and the residual austenite transforms into martensite.The typical inclusion block martensite is gradually formed in the sample structure to obtain single-phase martensite structure.The precipitation of precipitation phase is promoted in the aging process.The precipitation of precipitation phase is conducive to improving the strength and hardness of steel.When the solution temperature rises to1140°C,the martensite structure is uneven,the retained austenite increases and the hardness decreases,because the MS point will decrease if the solution temperature is too high.Solid solution 1040°C×2 h(water cooling)+480°C aging×under the condition of 4 h(air cooling),the Vickers hardness of 17-4PH high strength steel sample is from 376 HV_0.5 to 392HV_0.5.The friction coefficient is about 0.6,which is 5%higher than that of untreated samples.Therefore,the appropriate heat treatment system can improve the mechanical properties of the samples.(4)The effect of aging temperature(not aged,aged 480°C×4 h(air cooling),aged620°C×4 h(air cooling))on the corrosion properties of 17-4PH high strength steel samples formed by SLM after solid solution treatment(solid solution 1040°C×2 h(water cooling))was studied.The conclusion shows that the corrosion resistance of the sample is the best under 1040°C solid solution+620°C aging temperature,the microstructure becomes smaller,there are more grain boundaries,the grain boundary density is larger,and there are fewer pores on the surface,while the corrosion pits caused by pores are relatively small,and the charge transfer resistance Rct value increases significantly.Therefore,the passivation film formed on the electrode surface has higher protection and the sample has better corrosion resistance.