Microstructure And Defects Of Wedge-shaped Prismatical Metal Parts Formed By Selective Laser Melting

Selective laser melting(SLM) is an advanced metal additive manufacturing technique,which can quickly and accurately manufacture metal parts with complex structure and excellent properties,but its further application is restricted by the intrinsic defects.In view of the defects of SLM components,316L stainless steel,AlSi10 Mg,and Inconel718 alloys were applied to produce wedge-shaped prismatical samples with continuously varying wall thickness with optimum processing parameters.The microstructure and defects of the formed parts were studied and the distribution characteristics of defects in the samples were analyzed,based on which the formation tendency of hole defects of SLM parts were evaluated.The XRD results show that the 316 L stainless steel parts maintain the single-phase austenite structure of the powder material.Due to the rapid solidification of the SLM process,supersaturated solid solutions are formed in the AlSi10 Mg parts,resulting in lattice distortion of the formed parts compared to the alloy powders.Compared with the alloy powders,the grain growth orientation of Inconel718 is changed to some extent.The results of microstructure analysis reveal that columnar grains are formed in the vertical sections of the SLM parts by epitaxial growth,and the grains in the horizontal sections are equiaxed.The internal substructure of the grains is fine which usually consist of fine structure zone,coarse structure zone,and heat-affected zone under the influence of the thermal cycle caused by the remelting process.The ratio,types,morphology,and distribution of the defects in 316L stainless steel,AlSi10 Mg,and Inconel718 SLM parts were investigated.Density tests show that even under the optimized processing parameters,defects are generated in the formed parts,accounting for less than 1% of the volume.Through the combination of micro-CT and metallographic observation,the main types and distribution characteristics of the hole defects in the SLM parts of the three alloys were analyzed.In the 316 L stainless steel part,the main type of defect is the lack of fusion(LoF).With the increase of deposition height,the Lo Fs increase first and then decrease.In the horizontal direction,the Lo Fs are more formed in the area with larger wall thickness and decrease with the decrease of wall thickness.In the AlSi10 Mg part,gas pores and Lo Fs are the main defects.The gas pores are distributed evenly in the horizontal direction and increase with the increase of deposition height.The Lo Fs are randomly formed and uniformly distributed in the part.In addition,the gas pores and Lo Fs increase significantly near the vertical edges.Lack of fusions are the main type of defects in the Inconel718 part which are mainly distributed near the vertical edge of the acute angle while a few Lo Fs are formed near the vertical edges of the right angle.In addition,based on the defect distribution characteristics in SLM parts,the formation tendency of hole defects of SLM parts were evaluated.According to the formation tendency of hole defects of the corresponding alloy,the defect distribution in the formed parts is estimated and verified by experiments and the results show that the evaluations are reliable.The evaluation of the hole defect formation tendency can help optimize the quality inspection process of SLM parts,and save time and economic cost on the premise of ensuring reliable inspection results.The results of estimation verification experiments also show that the shape of SLM parts significantly affects the defect distribution.Specifically,structures with X-Y planes decreasing along the positive direction of the Z-axis(such as upright pyramids) will reduce defect formation.The conclusions of the present study can guide the further optimization of the SLM process.First,the formation of defects can be reduced by adjusting the building direction.Specifically,the building direction can be adjusted to form a structure in which the X-Y plane decreases with the positive direction of the Z-axis as far as possible.When the actual parts contain several sharp edges,the building direction can be adjusted to make the edges distributed in the X-Y plane as far as possible,so as to reduce the formation of defects.Second,when forming the area near the vertical edges,the process parameters should be further optimized.In addition,the defects in the area can be reduced or eliminated by repeated remelting.