Study On The Powder Characteristics And Forming Properties In Selective Laser Melting Of IN738 Alloy

In recent years,Selective Laser Melting?SLM?has been rapidly developed globally due to its short processing cycle,high material utilization rate,and the ability to form complex parts and structures that are difficult to achieve with traditional processes.At present,the research on SLM forming technology at home and abroad mainly focuses on the development of forming equipment,the optimization of forming process and the post-processing of formed parts.There are relatively few studies on the characteristics of metal powder for SLM forming and its influence on the quality of formed parts.In particular,many domestic high-quality raw materials for forming still need to be imported from abroad,which greatly increases the research and development costs and use costs of SLM forming technology,and restricts the further development of domestic add itive manufacturing technology.Therefore,starting from the powder characteristics,SLM forming tests are carried out on different batches of IN738 alloy powder.The effects of powder properties on the surface quality,density and mechanical properties of the formed parts are compared and analyzed.At the same time,the internal microstructure of SLM forming parts and the distribution characteristics and causes of defects such as splashes,pores and cracks were studied.The performance characteristics of IN738 alloy powder for SLM forming were fully characterized by scanning electron microscope,laser particle size analyzer and laser confocal microscope.The results showed that most of the powder particles were spherical,with a small amount of satellite particles and hollow powder.The powder particle size was single peak and normal distribution,when the amount of coarse powder or fine powder in the powder particles was large,the fluidity,apparent density and tap density of the powder were poor.It is found that the absorption rate of the powder to the laser increases as the particle size increases.SLM forming tests were carried out on different batches of IN738 alloy powder.The results showed that the particle size distribution was d₁₀=15.0?m,d₅₀=28.7?m,d₉₀=52.9?m,fluidity was 23.4 s/50g,the apparent density was 4.20 g/cm³ and the tap density was5.04 g/cm³,the corresponding powder batch SLM parts have the best quality:the density was 99.3%,microhardness was 430 HV,tensile strength was 1110 MPa,and the mechanical properties were higher than that of precision castings.The analysis of the microstructure of SLM forming parts shows that the XOY surface has a"checkerboard"shape distribution,and the XOZ surface has a"fish scal e"shape distribution.The forming structure was composed of cell crystals and columnar crystals.The boundary of the molten pool was a cell crystal structure with a size of about 1?m.The middle of the molten pool is a neatly arranged columnar crystal structure,and the columnar crystals has a vertical molten pool boundary and penetrates through the plurality of cladding layers,exhibiting epitaxial growth characteristics.White MC carbide particles and?-?'eutectic phase are distributed between dendrites.Irregular bulk MC carbide particles and?-?'eutectic phase were distributed between the crystal grains.SLM forming parts have defects such as particle spatter,pores and crack.The unmelted metal powder or molten metal was lifted under the impact of high-speed steam in the molten pool to form spherical or flocculent spattered particles.According to EDS analysis,the flocculent spatter particles were mostly carbides and borides.The pores can be divided into two types according to their shape characteristics,one is irregular pores caused by insufficient uniformity of paving powder,and the other is spherical pores caused by hollow powder and powder splash.The XOY surface cracks of the formed parts were saw-toothed and grow along the boundary of the molten pool.The XOZ surface cracks were short and straight,extending parallel to the deposition direction and penetrating multiple deposition layers.The cracks were characterized by cracking along the crystal.The grain at the fracture was neatly arranged and the interface was smooth,which was a liquefaction crack.Through the EDS analysis of the crack fracture,it was found that there was a film-like?-?'low-melting eutectic phase in the middle of the crack,and the liquid film was torn along the solid-liquid interface under the action of thermal stress,thereby forming a liquefaction crack.It has also been found that Si elements segregated at grain boundaries increased crack sensitivity.