Less residual stresses in laser beam melting by investigating on supports, powder layer properties, powder preheating and scanning strategies: Presentation held at International Symposium on Additive Manufacturing, February 25th-26th, 2015, Dresden; ISAM 2015

Laser Beam Melting (LBM) is an emerging additive manufacturing technology for the fabrication of highly complex metal components. With LBM, components are manufactured layer by layer directly on the basis of 3D CAD models. Using a focused laser beam, fine grained metal powders are selectively melted layer upon layer to create a physical part. One of the main characteristics of LBM parts is the high level of residual stresses resulting from high temperature gradients and solidification effects in the melt pool area. Depending on the material-specific thermal conductivity, the cooling rates reach up to 3.5 x 106 K/s [1]. The resulting internal stresses often lead to negative effects like distortions, cracks and total part or process failures. The scientific aim of this project is to develop a concept for the reduction of residual stresses and its consequences in LBM manufactured components, utilizable by end-users. The project’s scope is to work on a holistic approach addressing the following four factors of influence and variables on residual stresses in LBM: scanning strategy (focusing on variations of chessboard and stripe scanning strategies); support (comparing different versions of block- and tree-/cone supports), powder preheating (preheating of the powder by a novel radiation heating system) and powder layer properties (influence of layer thickness and grain size distribution). One part of the approach is to develop concepts, applicable for different materials. Therefore, a survey has been conducted among 13 industrial end-users of LBM machines in 2011 to select materials of high interest. As a result of this survey, specific materials of interest are AlSi10Mg, a widely-used casting alloy, and the nickel base alloy Hastelloy X (NiCr22Fe18Mo) which is commonly used for aircraft, furnace and chemical process components. In addition to the industry-driven material choice, all works within the project are conducted on two commercially available LBM machines – a CONCEPT Laser m2 Cusing and an EOS EOSINT M 270. The presentation centers around the influence of the factors described above on residual stresses and distortions of LBM parts. For experimental trials, a cantilever specimen was designed and used. In order to obtain comparable test results on both LBM machines, the cantilever specimens are placed identically in relation to the centre of the LBM machine build chamber. This work is supported by the Research Association on Welding and Allied Processes