The investigation of the influence of laser re‐melting on density, surface quality and microstructure of selective laser melting parts

Abstract

Purpose -Selective laser melting (SLM) is a powder metallurgical (PM) additive manufacturing process whereby a three-dimensional part is built in a layer-wise manner. During the process, a high intensity laser beam selectively scans a powder bed according to the computer-aided design data of the part to be produced and the powder metal particles are completely molten. The process is capable of producing near full density (,98-99 per cent relative density) and functional metallic parts with a high geometrical freedom. However, insufficient surface quality of produced parts is one of the important limitations of the process. The purpose of this study is to apply laser re-melting using a continuous wave laser during SLM production of 316L stainless steel and Ti6Al4V parts to overcome this limitation. Design/methodology/approach -After each layer is fully molten, the same slice data are used to re-expose the layer for laser re-melting. In this manner, laser re-melting does not only improve the surface quality on the top surfaces, but also has the potential to change the microstructure and to improve the obtained density. The influence of laser re-melting on the surface quality, density and microstructure is studied varying the operating parameters for re-melting such as scan speed, laser power and scan spacing. Findings -It is concluded that laser re-melting is a promising method to enhance the density and surface quality of SLM parts at a cost of longer production times. Laser re-melting improves the density to almost 100 per cent whereas 90 per cent enhancement is achieved in the surface quality of SLM parts after laser re-melting. The microhardness is improved in the laser re-molten zone if sufficiently high-energy densities are provided, probably due to a fine-cell size encountered in the microstructure. Originality/value -There has been extensive research in the field of laser surface modification techniques, e.g. laser polishing, laser hardening and laser surface melting, applied to bulk materials produced by conventional manufacturing processes. However, those studies only relate to laser enhancement of surface or sub-surface properties of parts produced using bulk material. They do not aim at enhancement of core material properties, nor surface enhancement of (rough) surfaces produced in a PM way by SLM. This study is carried out to cover the gap and analyze the advantages of laser re-melting in the field of additive manufacturing.

Electrical conductivity and porosity in stainless steel 316L scaffolds for electrochemical devices fabricated using selective laser sintering

“…Laser re-melting (outer contour and inner contour parameters) were used in this study so that sample shell strength could be improved, better surface quality achieved, while at the same time retaining a high internal porosity. This result agrees with the findings of other studies, in which laser re-melting can improve the density and enhance the surface roughness of printed parts [35,40,41]. From these findings, the high porosity parts, which were generally printed using an energy density less than 2.00 J/mm 2 , might not print well if their shells were too weak.…”

Surface Quality Research for Selective Laser Melting of Ti-6Al-4V Alloy

“…3). This similar geometry was used in previous works [16,19], because it allows the surface roughness for each inclination angle to be easily measured. Figure 4 illustrates the characteristic surface morphologies of SLM processed titanium alloy representative parts observed at a relatively low magnification produced under the first set of parameters given in Table 2.…”

“…Also, the roughness of top surface differs strongly from the roughness of side surfaces [15,17,18]. To overcome this problem, a variety of surface modification techniques are available in the market including mechanical processes, chemical processes, and thermal processes [1,16,19,20]. In this study, experimentally selected SLM process conditions and orientation of models made by Ti-6Al-4V alloy to ensure a minimum average and total roughness values.…”

“…parameters, scan strategy, the orientation of the component in the building chamber and surface post-processing [15,16]. Laser power defines the severity of the temperature gradient, and due to this temperature rise, melting of the powder takes place.…”

Generation and detection of defects in metallic parts fabricated by selective laser melting and electron beam melting and their effects on mechanical properties.

“…Dividing a part into shell and core sections with an overlap between them may not only reduce the edge heights, but also address the contour scanning issue. Moreover, Yasa et al (2011) also found that laser re-melting is a promising method to increase the density to almost 100% and to enhance the surface quality of SLM parts.…”