Study On Foundational Process Of Ni 625 Alloy Formed By Selective Laser Melting

Aerospace products not only need long life, high reliability and strong adaptability, but also meet the requirements of strength and lightweight. Structures of them are always more complex and requirements for processing technology of metal and alloy are usually very high. Selective laser melting can directly form complex structure metal parts with high manufacturing speed, short product development cycle. Nickel-based superalloy in the aerospace fields can be fabricated directly by SLM. Ni 625 nickel-based superalloy is taken as research material in this paper. The main research is as following:(1) By studying the forming process of SLM, we obtained the optimum process window:laser power P = 150 w, scanning speed V=500 mm/s, scanning spacing D=0.07 mm and laser energy density E=267.85 J/mm3.Relative density of parts can reach 99.4% under the optimization of parameters. Mathematical relationship between laser energy density and relative density is fitted. Scanning speed has a greater effect on relative density through range analysis.(2)Characteristics of micro crack, causes and inhibition process of Ni 625 alloy formed by SLM are studied.The results show that micro cracks are easily forming in Ni 625 alloy in SLM and the longest crack is about 100 um. In SLM, rapid solidification results in segregation of Nb and Mo elements, forming(?+Laves) eutectic. At the same time, due to high temperature gradient leads to thermal stresses. Stress concentration forming around the brittle phase Laves leads to cracking along the grain boundary. Tests of residual stresses demonstrated that substrate preheating could effectively reduce the residual stresses.It was easy to inhabit the generation and extension of cracks. With the increase of temperature, the number of cracks gradually reduced. When preheated at 300?, the number of cracks was the smallest.Microscopic and macroscopic properties of Ni 625 alloy formed by SLM are studied.Microstructure of Ni 625 alloy formed by SLM is narrow columnar crystal. Different characteristics of microstructure show in different two directions: slender columnar structure forms in vertical section, cellular organization forms horizontal section. Average size is at about 0.5 um. Because of high cooling rate and temperature gradient, the trunks of crystallization parallelly grow along the opposite direction of heat losses and lateral growth are inhabited, forming columnar crystal through the layers of boundary Eventually.XRD results show that Ni 625 alloy consisted of gamma matrix and brittle Laves phase.Mechanical properties and anisotropic of Ni 625 alloy are also studied in this paper. The tensile mechanical properties reflect the characteristics of high strength and low plasticity.Tensile strength of sample(936.34MPa),almostly reaches homogeneous state annealing forgings standard(827MPa) at room temperature. But elongation(7.22%) is far below forgings standard(30%). As the preheating temperature rises, mechanical properties of the alloy had a tendency to improve. It achieves the best performance preheated at 300 ?:the tensile strength increased by nearly 21.2%, elongation increased by nearly 33.6% than at room temperature. Results prove that the preheating process is an effective way to improve the mechanical properties of the alloy. In addition, mechanics anisotropic problem is studied.The tensile strength along the horizontal direction is higher than the vertical direction about5.2%, and elongation is higher about 4.3%.The boundary between layer and layer in SLM process performances weak area and is easy to crack. Cracks extend along the layer-layer boundary. This is the main reason why SLM samples performance with poor mechanical properties along the vertical direction.All in all, Ni 625 alloy is taken as forming materials in this study. The work contains optimizing forming process parameters, analyzing microstructure and mechanical properties,eliminating defects and improving the mechanical properties through preheating process.The results provide good ideas and suggestions on practical application for SLMforming superalloy parts.