Residual Stress-induced Distortions in Laser Powder Bed Additive Manufacturing of Nickel-based Superalloys

Laser powder bed fusion (LPBF) is a powder bed additive manufacturing (AM) process used for the production of three dimensional (3D) parts from a wide range of powder materials; it is known with different commercial names, such as selective laser melting (SLM), selective laser sintering (SLS) or direct metal laser sintering (DMLS) [1]. It expands its sectorial utilization by offering good mechanical properties, design freedom [2], easy transition from design to manufacturing, post-processing ability, and low material consumption [3]. Regardless of the various advantages offered by LPBF, remaining challenges such as surface quality [3], residual stresses, distortions [4], or the use of support structures prevent the wider acceptance of this process. LPBF AM uses a laser beam to selectively melt the metal powders by scanning cross sections on the surface of a powder bed layer by layer into an object that has a desired 3D shape based on a computer-aided design (CAD) model [5]. After each cross-section is scanned through a mirror system and solidifies with the help of blowing inert gas, the powder bed is lowered by one layer of thickness, a new layer of material is spread on top with a re-coater, and the process is repeated until the part is complete [4]. In this respect, heat transfer mechanisms include phase change, conduction, convection, and radiation (Fig. 1). As a result of the above-mentioned phenomenon, occurring thermal gradients cause cyclic thermal expansions, which exceed the elastic strain of the material and generate plastic strains, especially at high temperatures. These cumulated strains can generate internal stresses (also known as residual stresses) in the part [6] and cause distortions, which may lead failure by cracking [7] or layer delamination [8]. To eliminate the risk of cyclic thermal expansions and the resultant problems of residual stresses and distortions, several measures can be taken. In this regard, to check residual stresses, inspection techniques can be employed such as neutron diffractometry [9] or X-ray diffraction [10], and distortions with dimensional measurement systems [11] can be used. However, due to the rapid nature of additive manufacturing, these techniques are not Residual Stress-induced Distortions in Laser Powder Bed Additive Manufacturing of Nickel-based Superalloys Poyraz, Ö. − Kuşhan, M.C. Özgür Poyraz1,* Melih Cemal Kuşhan2 1TEI, Tusaş Engine Industries Inc., Turkey 2Eskişehir Osmangazi University, Faculty of Engineering, Department of Mechanical Engineering, Turkey