During selective laser melting, the irradiated material experiences large temperature fluctuations in a short time which causes unwanted thermal stresses. In order to assess thermal stresses in a simple and fast way, a new pragmatic method is developed, namely the bridge curvature method. The bridge curvature method is used to assess and qualitatively compare the influence of different laser scan patterns, laser parameter settings and more fundamental process changes on residual stresses. The results from the experiments, as well as the findings from literature, lead to two general conclusions: changes that reduce the high temperature gradient, like using short scan vectors and preheating of the base plate, reduce the thermal stresses. And, thermal stresses in a particular direction can be reduced by optimal choice of the orientation of scan vectors. The experiments indicate the reliability of the bridge curvature method. Statistical analysis is used to check the repeatability of the method and to quantify the uncertainties during measurement.

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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.

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