Abstract
The manufacturing route primarily determines the properties of materials prepared by additive manufacturing methods. In this work, the microstructural features and mechanical properties of 316 L stainless steel prepared by the selective laser method have been determined. Three types of samples, (i) selective laser melted (SLM), (ii) selective laser melted and hot isostatic pressed (HIP) and (iii) selective laser melted and heat treated (HT), were characterized. Microstructural analysis revealed that SLM samples were formed by melt pool boundaries with fine cellular–dendritic-type microstructure. This type of microstructure disappeared after HT or HIP and material were formed by larger grains and sharply defined grain boundaries. The SLM-prepared samples contained different levels of porosity depending on the preparation conditions. The open interconnected LOF (lack of fusion) pores were observed in the samples, which were prepared with using of scanning speed 1200 mm/s. The blowhole and keyhole type of porosity were observed in the samples prepared by lower scanning speeds. The HIP caused a significant decrease in internal closed porosity to 0.1%, and a higher pressure of 190 MPa was more effective than the usually used pressure of 140 MPa, but for samples with open porosity, HIP was not effective. The relatively high yield strength of 570 MPa, tensile strength of 650 MPa and low ductility of 30–34% were determined for SLM samples with the lower porosity content than 1.3%. The samples after HIP showed lower yield strengths than after SLM (from 290 to 325 MPa) and relatively high ductility of 47.8–48.5%, regardless of the used SLM conditions.
Highlights
Additive manufacturing (AM) methods, which work on the principle of layer by layer, represent powerful freeform fabrication techniques which can fabricate direct deployable components without the necessity of special machining, and are highly efficient when only small quantities are required [1,2,3,4,5].Selective laser melting (SLM) is the most commonly used AM method for forming metal parts, where the Materials 2020, 13, 4377; doi:10.3390/ma13194377 www.mdpi.com/journal/materialsMaterials 2020, 13, 4377 starting material is in pre-alloyed powder form [2]
This method reduces the occurrence of porosity and is recommended for components with smaller XY cross sections
Figure (a) Geometry of used tensile test samples and (b) SLM sample prepared for tensile testing
Summary
Additive manufacturing (AM) methods, which work on the principle of layer by layer, represent powerful freeform fabrication techniques which can fabricate direct deployable components without the necessity of special machining, and are highly efficient when only small quantities are required [1,2,3,4,5].Selective laser melting (SLM) is the most commonly used AM method for forming metal parts, where the Materials 2020, 13, 4377; doi:10.3390/ma13194377 www.mdpi.com/journal/materialsMaterials 2020, 13, 4377 starting material is in pre-alloyed powder form [2]. Selective laser melting (SLM) is the most commonly used AM method for forming metal parts, where the Materials 2020, 13, 4377; doi:10.3390/ma13194377 www.mdpi.com/journal/materials. SLM is able to manufacture almost fully dense parts (98–99%), the presence, as with all powder-based processes (sintering, hot isostatic pressing) and as with other net-shape manufacturing methods (casting), is an inherent porosity associated with the process. The first type of porosity is caused by insufficient or imperfect melting of particles (lack of fusion (LOF) porosity), exhibits angular and random morphologies with a large size of up to 500 μm and usually contains unmelted powder [6]. LOF porosity arises due to the small variance in the energy density of the laser across the surface of the layer and can be partially corrected by changing the process parameters
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