Investigation of Residual Stresses Induced during the Selective Laser Melting Process

The selective laser melting process (SLM), belonging to the family of additive manufacturing processes, can create complex geometry parts from a CAD file. Previously, only prototypes were created by SLM, but now this process is used to manufacture quickly and directly functional parts. For example, in the PEP (Pôle Européen de la Plasturgie), this process is used to fabricate tooling parts or injection molds with cooling channels that can’t be obtained by conventional routes. During the process, the laser beam generates violent heating and cooling cycles in the material inducing important thermal gradients in the consolidated part. The cyclic thermal expansions and contractions exceeding the maximum elastic strain of the material induce heterogeneous plastic strains and generate internal stresses the level of which can reaches the yield stress of the material and cracks may appear during the process. This paper deals with the measurement and analysis of residual stresses during the selective laser melting of a simple part in maraging steel. The objective of this study is the analysis of experimental results to validate the numerical model previously presented in [1]. Some authors have investigated the residual stresses produced in SLM parts using different experimental measurement methods such as the incremental hole drilling method in [2], the layer removal method see in [3] and [4] or the non-destructive method, by neutron diffraction in [5]. A new method is proposed to evaluate the residual stresses induced during the SLM process, a rosette is fixed on the bottom face of the support. The residual stresses in the created part are calculated from strain and temperature variations when the fused layer is consolidating during the cooling between two layers. Process parameters like the powder thickness or the time cooling between successive layers are studied in this paper. [1] L. Van Belle, G. Vansteenkiste, J.C. Boyer, Comparisons of numerical modeling of the selective laser melting, Key Engineering Materials Vols. 504-506 (2012) pp 1067-1072 [2] C. Casavola, S.L. Campanelli, C. Pappalettere, Experimental analysis of residual stresses in the selective laser melting process, Proceedings of the XIth International Congress and Exposition, June 2-5, 2008 Orlando, Florida USA [3] M. Shiomi, K. Osakada, K. Nakamura, T. Yamashita, F. Abe, Residual stress within metallic model made by selective laser melting process, CIRP Annals - Manufacturing Technology, Vol. 53, No. 1. (2004), pp. 195-198 [4] T. Furumoto, T. Ueda, M.S. Abdul Aziz, A. Hosokawa and R. Tanaka, Study on reduction of residual stress induced during rapid tooling process, influence of heating conditions on residual stress, Key Engineering Materials Vols. 447-448 (2010) pp 785-789 [5] M. Zaeh, G. Branner, Investigation on residual stresses and deformation in selective laser melting, Production Engineering, Volume 4, Number 1 (2010)