Additive manufacturing (AM) allows for the building up of bulk structures through the fusion of powder particles layer by layer using a high power laser. This novel technique is practical for the fabrication of complex structures without any mold or tool, but this flexibility is accompanied by the delicate balance of AM processing and post-processing parameters which are to be optimized to ensure the desired microstructure in order to obtain the required mechanical properties [1-4]. The current study is aimed at understanding the effect of the AM processing parameters used for the preparation of AM samples on the resulting microstructure, its anisotropy according to growth direction, as well as its implications on the mechanical properties.

Bulk structures of conventional 316L grade stainless steel (with Cr-16.5-18.5%, Ni-10-13%, Mo 2-2.5%, traces of Mn, Si, P, S, C)  were produced by AM through selective laser melting (SLM), followed by an annealing heat treatment for material internal stress release. Two sets of processing conditions with variations in parameters such as the incident laser energy, the laser spot size and the direction of growth were varied, and the resulting effect on the microstructures both before and after the heat treatment are studied through diverse microscopy techniques such as SEM, EBSD and TEM. The variation in the phase distributions, grain size and porosity are analyzed as a function of the process parameters. Mechanical properties are measured by tensile tests completed by microhardness. Additionally, XRay Diffraction (XRD) has been used for identifying the phases present in the alloy and also for measuring residual stress and crystallographic texture.

The presence of the pores with unmolten particles has been observed in all samples produced with lower laser energy (Fig. 1). Alongside, the layers formed immediately on top of such pores show different microstructure, with the new phase observed consistently on the same side of the porosity in all samples (Fig. 2). The origin of this phase separation is expected to be from the differential heat dissipation (during fabrication) through the pore as opposed to the bulk material. The mechanical properties of the bulk structures (tensile stress, microhardness) showed that increasing the laser power results to an increase in the yield point. Fracture surface (Fig. 3) shows clearly presence of the pores which influences largely the mechanical properties such as ductility.

Overall, through the variation of the process parameters, the trend in evolution of the microstructural and mechanical properties is elucidated aimed at optimizing the parameters for AM products of 316L stainless steel.

1. H. D. Carlton, A. Haboub, G. F. Gallegos, D.Y. Parkinson, A. A. MacDowell, Materials Science & Engineering A, 651 (2016) 406–414

2. J. A. Cherry, H. M. Davies, S.Mehmood, N. P. Lavery, S. G. R. Brown, J. Sienz, International Journal of Advanced Manufacturing Technology, 76 (2015) 869–879

3. T. M. Mower, M. J. Long, Materials Science & Engineering A, 651 (2016) 198–213

4. A. Yadollahi, N. Shamsaei, S.M.Thompson, D.W.Seely, Materials Science&EngineeringA, 644 (2015) 171–183

Figures:

Figure 1: Scanning Electron Microscopy micrographs from samples produced with lower laser energy showing (a) heavy porosity (b) presence of unmolten powder particles in the porosity

Fig 2. Optical microscopy micrographs showing the presence of phase differentiation solely on the upper side (in the direction of growth) of the macro-sized pores in samples prepared using lower laser energy.

Fig 3. Scanning Electron Microscopy micrographes of the fracture surfaces after tensile tests. Images (a) and (b) are obtained from samples prepared with the same laser power but with different orientations. The role of anisotropy is brought to evidence in these images. Image (c) is obtained from a sample prepared with higher laser power than the former two but with the same orientation as (b). The heavy role of the porosity in the fracture mechanism is observed

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EMC Abstracts - https://emc-proceedings.com/abstract/effect-of-processing-parameters-on-microstructure-and-mechanical-properties-of-additively-manufactured-stainless-steel/