| Selective Laser Melting(SLM)technology with the important feature of powder bed can directly manufacture precision and complex metal parts without tooling and machining,which is the most concerned and promising technology of laser additive manufacturing at present.SLM can form a variety of metal materials including titanium alloy,nickel-based alloy,stainless steel,aluminum alloy and so on.However,SLM started late in forming high performance low alloy steel,and there is still a lack of systematic research on forming control and post-heat treatment technology.In this paper,a 12CrNi2 alloy structural steel with excellent strength and toughness was fabricated by SLM technology,and the influence of process parameters on microstructure and mechanical properties of the alloy steel was summarized.The formability of 12CrNi2 alloy steel formed by SLM and laser melting deposition(LMD)was studied.In order to find a suitable post-heat treatment process,the change of microstructure and properties of LAM formed 12CrNi2 alloy steel was investigated.The main conclusions are as follows:The macrostructure of the SLM-formed alloy steel can be divided into two parts:molten pool zone and heat affected zone.The microstructure is tempered martensite with a small amount of retained austenite.When the laser energy density(Ev)increasing,the pore defects in the SLM-formed alloy steel are gradually reduced,and the density is gradually increased,which can reach 99.87%.In the meanwhile,the molten pool volume and lifetime increase and the cooling rate decreases,resulting in the widening of the tempered martensite lath and the heat-affected zone;further,the microhardness and strength of the alloy steel are decreased,and the plasticity is increased.When Ev is 81.34 J/mm3,the SLM-formed 12CrNi2 alloy steel exhibits optimal tensile properties,its tensile strength can reach up to 1098 MPa,the yield strength is 882 MPa,and its elongation is 20.07%.The comprehensive mechanical properties of 12CrNi2 alloy steel formed by SLM technology are better than those formed by laser melting deposition(LMD)and casting technology.The forming accuracy of SLM-formed 12CrNi2 alloy steel is higher than that of LMD forming,but the forming efficiency is only 1/4 of that of LMD.There are molten pool zone and heat affected zone in the macrostructure of alloy steels formed by the two processes.The microstructure of LMD formed alloy steel is ferrite and austenite,and the microstructure is thicker.The hardness of formed steel is as follows:SLM>LMD>casting.SLM formed alloy steels have high strength,tensile strength can reach up to 1098 MPa,while LMD formed alloy steels have good plasticity and elongation can reach up to 22%.After quenching at different austenitizing temperatures,the microstructure of SLM formed 12CrNi2 alloy steel is mainly martensite,the microstructure gradually coarsened with the increase of austenitizing temperature.After quenching,the hardness,tensile strength and yield strength of the formed alloy steel all decrease with the increase of heating temperature,but they are obviously higher than those of the formed alloy steel.Compared with LMD formed and cast alloy steels,the hardness and strength of SLM formed alloy steels after quenching are higher.After being held at the austenitizing temperature of 775℃ and 900℃ for 30 min,and isothermal quenched at 200℃ for 20 s,the microstructures of SLM formed 12CrNi2 alloy steel contain bainite,martensite and proeutectoid ferrite(existed at the austenitizing temperature of 775℃),and residual austenite among the laths.The width of bainite lath increases with the increase of austenitizing temperature,and the residual austenitic volume increases.When the heating temperature increasing,the hardness,tensile strength and yield strength decreased gradually,while the elongation increased.The tensile strength of SLM and LMD formed alloy steels are similar and both of them are higher than the as-cast alloy steel.The hardness of the three processes were compared,the result is SLM>LMD>as-cast. |
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