Mechanical And Microstructural Investigation Of 18Ni-300 Maraging Steel Manufactured By Selective Laser Melting

Selective Laser Melting(SLM)technology has its unique advantages in the processing of high performance parts based on the idea of layer by layer superposition.SLM is one of the most promising technologies for rapid prototyping,which can be widely used in aerospace,biomedical and other fields,without subsequent excessive machining.18Ni-300 maraging steel has been widely used in aerospace and other fields because of its high strength and excellent plasticity.In this paper,SLM technology is combined with 18Ni-300 maraging steel,and the basic research of 18Ni-300 SLM forming and heat treatment technology is carried out.On the one hand,the influence of different process parameters on the forming characteristics of 18Ni-300 maraging steel was investigated,and then the optimum process parameters were obtained.On the other hand,the effects of solution temperature and aging temperature on the microstructure and mechanical properties of the products were studied by orthogonal test.The main results are as follows:(1)Based on the study of the forming of 18Ni-300 maraging steel by SLM,the mechanism and preventive measures of possible defects in SLM forming process are analyzed and studied,such as spheroidization,porosity,crack and residual stress.It is found that the defects can be reduced by preheating the substrate,optimizing the process parameters,heat treatment,high purity inert gas protection,adding alloying elements and compacting powder and so on.(2)The morphology,density,roughness,hardness and microstructure of 18Ni-300 samples were analyzed with different energy densities.The results show that the surface morphology,density,roughness and hardness of the workpiece are quite different with different energy densities.Optimal process parameters were determined,laser power 170 W,scanning speed 1250mm/s,the layer thickness 30?m,scanning spacing 100?m,print spacing 0.05 mm and the content of oxygen is less than or equal 1.3%.Under the condition of the optimum process parameters,the parts with no obvious defects and uniform internal structure were prepared.Through metallographic analysis,it was found that the deposited structure of SLM formed 18Ni-300 was composed of cellular dendrites with a large number of lath martensite and a small amount of retained austenite.(3)Based on the orthogonal test,the effects of solution temperature,solution time,aging temperature and aging time on the mechanical properties of SLM 18Ni-300 were investigated.The results show that the aging temperature and aging time have significant influence on the forming of 18Ni-300 SLM parts.The optimal heat treatment process of 900?/20min(4 times)+480?/5h is determined by the level factor diagram.Under the heat treatment condition,The maximum tensile strength is 2109 MPa and the yield strength is 1374 MPa.(4)The effects of solution temperature and aging temperature on the microstructure and mechanical properties of SLM 18Ni-300 parts were studied by single variable method.The results showed that when the solution was dissolved at 860?~1000?,the solid solution temperature is closely related to the grain refinement,but it is almost independent of the mechanical properties of the workpiece.The tensile strength is 2000-2100 MPa,and the fracture surface is characterized by dimple fracture.The effect of aging temperature on the mechanical properties of the parts is very different.It has a poor mechanical properties at Too high or too low aging temperature and fracture morphology is quasi cleavage fracture.The mechanical properties are the best when the aging time is 480?,and the fracture surface is uniformly distributed.