Determination of Bulk Residual Stresses in Electron Beam Additive-Manufactured Aluminum
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TRODUCTION
ADDITIVE manufacturing encompasses a broad range of fabrication techniques that utilize incremental deposition of material to create fully dense, threedimensional structures. Many of these metallic-based techniques use fusion-based, welding-like approaches that continuously melt and resolidify a feedstock material. Similar to welding, additive-manufacturing techniques are characterized by steep thermal gradients that typically lead to very high residual stresses and/or distortion. Residual stresses can influence subsequent manufacturing steps (e.g., stress-induced distortion during machining) and can introduce variability into the material itself (e.g., crack-related strain field interactions). Understanding how these process-induced residual stresses form can assist in developing methods for controlling and mitigating their effects. This is essential for the maturation and industrial application of additive-manufacturing techniques. A number of previous studies have measured and/or modeled residual stresses for both welding[1–8] and additive-manufacturing[9–17] techniques. The majority of the research done in additive-manufactured materials has focused on the LENS process, initially developed at Sandia National Laboratories. In this process, a laser is used to melt metal powder that is blown onto a substrate or previous layer. The laser spot size is on the order of 1 mm; thus, the overall heat input is low, and CRAIG A. BRICE, Materials Research Engineer, is with the National Aeronautics and Space Administration, Langley Research Center, Hampton, VA 23681. Contact e-mail: [email protected] WILLIAM H. HOFMEISTER, Research Professor, is with the Department of Materials Science and Engineering, University of Tennessee Space Institute, Tullahoma, TN 37388. Manuscript submitted February 13, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
the thermal gradient at the molten pool boundary is high. Rangaswamy et al.[9,12] found peak stresses between 50 and 80 pct of the material yield stress, and the bulk internal stresses in the deposited material were primarily compressive. Likewise, Wang et al.[10] also determined the bulk internal stresses in the deposit to be compressive. These studies focused primarily on stresses within the deposit and did not examine the stress distribution within the substrate plate itself. Stresses in the substrate are important for hybrid deposition approaches where the substrate is incorporated into the final part, such as a grid-stiffened plate. Prior research of residual stress in welded structures provides insight into the effect of metal deposition on the underlying material. Kohandehghan et al.[1] demonstrated that, for arc welding, the thermal conditions within the substrate plate change from start to finish, directly influencing the residual stress. In addition, the sequencing of the weld beads also affects the final residual stress distribution. Nickel et al.[13] showed a similar sequencing effect in layer-deposited structures. Paradowska et al.[5] showed that in a stacked bead c
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