Additive manufacturing by selective laser melting (SLM) was used to investigate the effect of laser energy density on 316L stainless steel properties. Point distance and exposure time were varied and their impact on porosity, surface finish, microstructure, density and hardness, was evaluated. The surface roughness was primarily affected by point distance with increased point distance resulting in increased surface roughness, R
 a, from 10 to 16 μm. Material hardness reached a maximum of 225 HV at 125 J/mm3 and was related to the material porosity; with increased porosity leading to decreased material hardness. Different types of particle coalescence leading to convex surface features were observed (sometimes referred to as balling); from small ball features at low laser energy density to a mixture of both small and large ball features at high laser energy density. Laser energy density was shown to affect total porosity. The minimum amount of porosity, 0.38 %, was observed at an energy density of 104.52 J/mm3.

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Investigation into the effect of process parameters on microstructural and physical properties of 316L stainless steel parts by selective laser melting

The mechanical and microstructural properties of 316L stainless steel (SS) fabricated via Direct Laser Deposition (DLD), a laser-based additive manufacturing method, are presented and compared with those of conventionally-built counterparts. Using a Laser Engineered Net Shaping (LENS ® ) DLD system, the time interval between successive layer deposits, or inter-layer/idle time, for fabricating cylindrical specimens vertically-upward was varied by building either one or nine samples per build plate – thus increasing total assembly volume per build. Subsequently, the effect of thermal history, as well as heat treatment, on microstructural (i.e. grain size and morphology) and mechanical (i.e. tensile, compression, and microhardness) properties of DLD parts were investigated. Results indicate that the DLD 316L SS samples produced herein have a higher yield and ultimate tensile strength relative to their cast and wrought forms. Furthermore, the thermal history, microstructural evolution, and mechanical properties of DLD 316L SS are shown to be dependent on the time interval between deposits. Longer local time intervals result in higher cooling rates, leading to finer microstructures, higher/uniform strength and lower elongation to failure. In addition, porosity and less integral metallurgical bonds are found to be more prevalent in locations further upward from the build plate due to reduced laser penetration depths (e.g. previous-layer remelting decreases). Conversely, parts manufactured with shorter time intervals were found to possess a coarser microstructure, lower strength and higher elongation to failure – attributable to lower cooling rates caused by an increased bulk temperature in the part. These results may aid in future design and control of more efficient, constant-power DLD processes – especially with regard to building multiple and/or larger parts; an approach desirable for minimizing small-to-medium lot production times.

Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316L stainless steel

In-situ characterization of laser-powder interaction and cooling rates through high-speed imaging of powder bed fusion additive manufacturing

https://dr.ntu.edu.sg/bitstream/10356/102525/1/Manuscript.pdf

A numerical investigation on the physical mechanisms of single track defects in selective laser melting

Effects of laser energy density (LED) on densities and surface roughness of AlSi10Mg samples processed by selective laser melting were studied. The densification behaviors of the SLM manufactured AlSi10Mg samples at different LEDs were characterized by a solid densitometer, an industrial X-ray and CT detection system. A field emission scanning electron microscope, an automatic optical measuring system, and a surface profiler were used for measurements of surface roughness. The results show that relatively high density can be obtained with the point distance of 80–105 μm and the exposure time of 140–160 μs. The LED has an important influence on the surface morphology of the forming part, too high LED may lead to balling effect, while too low LED tends to produce defects, such as porosity and microcrack, and then affect surface roughness and porosities of the parts finally.

Experimental investigation on densification behavior and surface roughness of AlSi10Mg powders produced by selective laser melting

The surface tensions (σ) and temperature dependencies (dσ/dT) of several commercial 4-series ferritic stainless steels have been measured using the sessile drop technique on an Al2O3 plate over the temperature range 1789 to 1883 K in an atmosphere of high purity (P
 O
 2 < 10−19 MPa) argon gas. Precise densities of liquid stainless steels have also been obtained using the modified sessile drop method in order to calculate accurate values of the surface tension. The surface tensions of liquid stainless steels decreased markedly with increasing sulphur concentration in the steels. The variation of surface tensions of liquid stainless steels can be described by the following equation σ = 1790 − 182 ln (1 + 260a
 S) (mN/m) when only S is considered or σ = 1820 − 304 ln (1 + 383a
 O) − 182ln (1 + 260a
 S) (mN/m) when both S and O are considered. The equations apply to the following compositional ranges: mass%O = 0.0022–0.0064, mass%S = 0.0008–0.05. The temperature coefficient of the surface tension (dσ/dT) of liquid stainless steel was found to change from negative to positive at a sulphur concentration of about 30 mass ppm in the steel. Nitrogen was found to have little effect on the surface tension of liquid stainless steel.

M. Zeze

Papers

Determination of the surface tension of liquid stainless steel