3D Residual Stresses in Selective Laser Melted Hastelloy X

Abstract

Abstract. 3D residual stresses in as manufactured EOS NickelAlloy HX, produced by laser powder bed additive manufacturing, are analysed on the surface closest to the build-plate. Due to the severe thermal gradient produced during the melting and solidification process, profound amounts of thermal strains are generated. Which can result in unwanted geometrical distortion and effect the mechanical properties of the manufactured component. Measurements were performed using a four-circle goniometer Seifert X-ray machine, equipped with a linear sensitive detector and a Crtube. Evaluation of the residual stresses was conducted using sin 2 ψ method of the Ni {220} diffraction peak, together with material removal technique to obtain in-depth profiles. An analysis of the material is reported. The analysis reveals unwanted residual stresses, and a complicated non-uniform grain structure containing large grains with multiple low angle grain boundaries together with nano-sized grains. Grains are to a large extent, not equiaxed, but rather elongated. IntroductionAdditive manufacturing, free form fabrication, rapid prototyping and 3D-printing are some of the different designations for processes where components can be built to finished or near-finished shape without machining a block of material or casting material in a mould [1][2][3]. The processes were primarily developed for simpler materials, such as thermoset plastics and plaster. The lasers equipment originally used could only melt materials with low melting points, for instance brass, and was not powerful enough to completely melt steel. Therefore, this manufacturing method could not meet the requirements for parts subjected to high stress levels or elevated temperatures, e.g., superalloys [4]. With time, the process control was improved and more powerful lasers were developed. With the higher input possible from a more powerful laser it is possible to create a microstructure with a low amount of porosity and no internal defects such as solidification cracks or poor bonding [5].Free-form fabrication of superalloys is gaining increased interest from the industry, since the available range of alloys is growing. Today, alloys for selective laser melting (SLM) include aluminium, titanium, tool steel, stainless steel and heat resistant materials of cobalt-and nickel-base. In the case of melting of metal powders, the dominating manufacturing process is laser melting, often denoted selective laser melting, direct laser metal sintering (DMLS) or LaserCUSING. All of these names are trademarks for different companies manufacturing equipment for laser melting.The laser melting manufacturing process can briefly be described as a layer-by layer process, where powder is distributed on a powder bed, see Fig. 1. Firstly, a powder distributer travels over the powder bed cavity contained by the build chamber walls and build plate. Molten and solidified