Direct Laser Deposition Of High-strength Austenitic Stainless Steel

Improving mechanical property of direct laser deposited metallic components is the key problem that needs to be resolved before this technique is applied into practical use.How to improve mechanical property and eliminate anisotropy without decreasing efficiency and accuracy of deposition process is well worth studying.In this research,330 powder which is a brand of self-developed high-strength Fe-base alloy powder was applied to build single-wall and block samples by using direct laser deposition(DLD)technique,and the manufacturing process and mechanical property were systematically studied.The laser and material parameters of single wall samples were optimized by using Taguchi-method,and these optimized parameters were applied to produce single-wall samples with 330 powder and 316 L powder respectively.Low defocus length of powder,low scanning speed or high laser power was beneficial to improve the utilization rate of powder and the deposited high.Dendritic structure was formed in 330,but columnar structure was formed in 316 L.The dendritic structure of block sample was obviously thinner than single-wall sample,and the dendritic structure became coarse with the rising of laser power.The microstructure of block samples could be divided into three parts,fine grained zone,heat affected zone,white band zone.The microhardness of white band zone was significant lower than other parts.A strong anisotropy was found in direct laser deposited 330 alloy,the tensile strength and the ductility of horizontal sample was significant higher than that of vertical sample.The tensile strength of 330 alloy was twice as high as 316 L alloy,but the ductility of 330 alloy was only a quarter of 316 L alloy.With the rising of laser power,the microhardness and the tensile strength of block samples decreased,but the ductility increased.The tensile strength of block sample was about 60 MPa higher than that of single-wall sample,but the ductility was about 1.5% lower.Dealing with block samples by solution treatment at 1150℃,the microhardness and the tensile strength decreased to 310 HV and 880 MPa respectively,the ductility increased to about 19%.Etching the block samples and single-wall samples with Groesbeck solution,the white band zone became black band.Analyzing by scanning electron microscope(SEM)and transmission electron microscope(TEM),a large amount of fine needle-like σ phase was found in the interdendritic zone,and the σ phase became coarse in the black zone.The dispersed distribution of σ phase brought strong strengthening effect to 330 alloy,but also caused the brittleness of 330 alloy to some extent.After solution treatment,the σ phase dissolved,and the ductility improved a lot.