The Influence of Laser Defocusing in Selective Laser Melted IN 625

Abstract

Laser defocusing was investigated to assess the influence on the surface quality, melt pool shape, tensile properties, and densification of selective laser melted (SLMed) IN 625. Negative (−0.5 mm, −0.3 mm), positive (+0.3 mm, +0.5 mm), and 0 mm defocusing distances were used to produce specimens, while the other process parameters remained unchanged. The scanning electron microscopy (SEM) images of the melt pools generated by different defocusing amounts were used to assess the influence on the morphology and melt pool size. The mechanical properties were evaluated by tensile testing, and the bulk density of the parts was measured by Archimedes’ method. It was observed that the melt pool morphology and melting mode are directly related to the defocusing distances. The melting height increases while the melting depth decreases from positive to negative defocusing. The use of negative defocusing distances generates the conduction melting mode of the SLMed IN 625, and the alloy (as-built) has the maximum density and ultimate tensile strength. Conversely, the use of positive distances generates keyhole mode melting accompanied by a decrease of density and mechanical strength due to the increase in porosity and is therefore not suitable for the SLM process.

Operando X-Ray imaging of stochastic inter-regime instabilities in laser melting processes: direct evidence of acoustic emission signatures

“…Supplementary method 3) measurements were performed with similar beam sizes having an opposite defocusing position (positive and negative as divergent and convergent beams, respectively) and different laser intensity distribution. In conduction mode, the melt pool dimensions proved to be similar under identical laser processing parameters with a slight increase in melt pool depth and length in positive defocus, in agreement with previous observations 52,53 . Upon transition from conduction to keyhole, a signi cant difference in the melt pool length is observed with 45% higher values in RM2 compared to RM6.…”

Effects of Laser Spot Size on the Mechanical Properties of AISI 420 Stainless Steel Fabricated by Selective Laser Melting

“…The appropriate laser spot size should provide sufficient energy density to melt the current powder layer and fuse to the previously melted one, but also to be small enough to meet requirements of the dimensional precision and surface roughness [ 17 ]. The effects of laser spot size, on various characteristics of SLM builds, have been investigated by a limited number of studies [ 17 , 18 , 19 , 20 , 21 , 22 ]. Change of laser spot size, through focal shift, would affect the laser energy density, porosity, and microstructure of the Inconel 718 alloy; a finer microstructure is found for a smaller laser spot [ 17 , 18 ].…”

“…It is found that a good surface finish in SLM builds of AlSi12 alloy can be obtained with an optimal combination of defocus distance and laser power [ 21 ]. In the study of SLM builds of IN 625 alloy [ 22 ], the surface roughness is increased, with increasing laser spot diameter, but the tensile strength is barely affected by the laser spot size.…”

“…Saeidi et al [ 24 ] and Krakhmalev et al [ 25 ] have investigated the effects of heat treatment on the microstructure and mechanical properties of SLM builds of AISI 420. As described above, laser spot size plays in important role in determining the density, surface, microstructure, and performance of metallic SLM builds [ 17 , 18 , 19 , 20 , 21 , 22 ]. However, there is lack of study on the effects of laser spot size on the microstructure, surface finish, and mechanical properties of AISI 420 fabricated via SLM.…”

Special Issue “Design and Application of Additive Manufacturing”

“…In the research by Paraschiv et al [2], laser defocusing in the selective laser melting (SLM) process (powder bed fusion, PBF) was investigated to assess the influence on the surface quality, melt pool shape, tensile properties, and densification of the produced samples (IN 625 material). It was observed that the melting height increased while the melting depth decreased from positive to negative defocusing.…”