An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel
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ADDITIVE manufacturing (AM) refers to a family of layer-by-layer building methodologies capable of producing three-dimensional structures from a precursor material.[1,2] While the focus of this work is on metal-based powder bed fusion AM, AM methods have found applications in metals,[3] polymers, and ceramics[4,5] due to their ability to create complex, net-shape parts with little waste material using a single automated process. Despite over two decades of AM technology, significant challenges for large-scale incorporation of AM methodologies remain. Inherent trade-offs between achieving fast processing times, full density, relatively stress-free structures, and good microstructural and mechanical integrity have created a demand for process optimization. In powder bed fusion AM, a very thin powder layer is distributed and selectively melted by a controlled laser; this procedure is repeated until a complete part is built. AMANDA S. WU, Postdoctoral Researcher, MUKUL KUMAR, Mechanics of Materials Group Leader, and GILBERT F. GALLEGOS, Deputy Division Leader, are with the Materials Engineering Division, Lawrence Livermore National Laboratory, 7000 East Avenue Livermore CA 94550. Contact e-mail: [email protected] DONALD W. BROWN, Scientist, is with the Materials Science & Technology Division, Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545. WAYNE E. KING, Director, Accelerated Certification of Additively Manufactured Metals Initiative, is with the Condensed Matter and Materials Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550. Manuscript submitted May 8, 2014. Article published online September 16, 2014 6260—VOLUME 45A, DECEMBER 2014
Powder size and packing, material, laser settings (power, spot size, and speed), and scanning parameters (pattern, orientation angle, and overlaps) must be selected such that the powder layer is fully melted locally and bonded to the substrate. However, the development of large thermal gradients near the laser spot,[6] rapid cooling, and repetition of this process gives rise to localized compression and tension, resulting in AM parts with significant residual stresses. The thermal gradients present during building are affected by many process parameters (part size, build time, build plate/powder bed temperature, atmosphere, powder thermal characteristics, melt pool size, etc.). Aside from their potential impact on the mechanical performance and structural integrity of AM parts, residual stresses developed during processing may cause localized deformations resulting in a loss of net shape, detachment from support structures, or failure of the AM part. A. Parametric Effects on Residual Stress The growing importance of AM technologies in industry calls for a concentrated, systematic effort to understand the effect of and coupling between each component in the powder bed fusion AM parameter space with residual stress development. However, the confounding behavior inherent in parameter optimization for ful
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