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1. Thermal redistribution of compressive residual stress introduced by interlayer laser shock peening in hybrid additive manufacturing

   https://doi.org/10.1016/j.procir.2023.10.001  

   Klein, George H; Karunakaran, Rakeshkumar; Sealy, Michael P (January 2024, Procedia CIRP)

   The objective of this research was to quantify the change in magnitude and depth of compressive residual stress (CRS) retained in the subsurface by interlayer coldworking when subjected to localized annealing that superimposed tensile stress. The approach was to hybridize additive manufacturing of AlSi10Mg alloy by coupling powder bed fusion (PBF) with laser shock peening (LSP) and characterize the resultant residual stress state by the hole-drilling method. The research found localized annealing from layer deposition formed two distinct regions in the subsurface, which was driven by localized and bulk stress redistribution. The experiments also showed that residual stress redistribution from LSP reached 550 µm into the subsurface, whereas local annealing from the deposition of layers extended only to a depth of 160 µm. Hence, compressive stress imparted by LSP was not entirely canceled by local annealing from PBF. This work provides the first quantification of the stress state response of hybrid additively manufactured parts to thermal loads and is fundamental to improving part performance through increased functional reliability, fatigue life, and corrosion resistance. 

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2. Modeling thermal and mechanical cancellation of residual stress from hybrid additive manufacturing by laser peening

   https://doi.org/10.1016/j.npe.2019.07.001  

   Madireddy, Guru; Li, Chao; Liu, Jingfu; Sealy, Michael_P (July 2019, Nanotechnology and Precision Engineering)

   Additive manufacturing (AM) of metals often results in parts with unfavorable mechanical properties. Laser peening (LP) is a high strain rate mechanical surface treatment that hammers a workpiece and induces favorable mechanical properties. Peening strain hardens a surface and imparts compressive residual stresses improving the mechanical properties of a material. This work investigates the role of LP on layer-by-layer processing of 3D printed metals using finite element analysis. The objective is to understand temporal and spatial residual stress development after thermal and mechanical cancellation caused by cyclically coupling printing and peening. Results indicate layer peening frequency is a critical process parameter affecting residual stress redistribution and highly interdependent on the heat generated by the printing process. Optimum hybrid process conditions were found to exists that favorably enhance mechanical properties. With this study, hybrid-AM has ushered in the next evolutionary step in AM and has the potential to profoundly change the way high value metal goods are manufactured. 

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3. The Effect of Laser Shock Peening on Back Stress of Additively Manufactured Stainless Steel Parts

   https://doi.org/10.1115/1.4056571  

   Over, Veronica; Donovan, Justin; Lawrence Yao, Y. (April 2023, Journal of Manufacturing Science and Engineering)

   Abstract This work studies the use of laser shock peening (LSP) to improve back stress in additively manufactured (AM) 316L parts. Unusual hardening behavior in AM metal due to tortuous microstructure and strong texture poses additional design challenges. Anisotropic mechanical behavior complicates application for mechanical design because 3D printed parts will behave differently than traditionally manufactured parts under the same loading conditions. The prevalence of back-stress hardening or the Bauschinger effect causes reduced fatigue life under random loading and dissipates beneficial compressive residual stresses that prevent crack propagation. LSP is known to improve fatigue life by inducing compressive residual stress and has been applied with promising results to AM metal parts. It is here demonstrated that LSP may also be used as a tool for mitigating tensile back-stress hardening in AM parts, thereby reducing anisotropic hardening behavior and improving design use. It is also shown that the method of application of LSP to additively manufactured parts is key for achieving effective back-stress reduction. Back stress is extracted from additively manufactured dog bone samples built in both XY and XZ directions using hysteresis tensile. Both LSPed and as-built conditions are tested and compared, showing that LSPed samples exhibit a significant reduction to back stress when the laser processing is applied to the sample along the build direction. Electron backscatter diffraction (EBSD) performed under these conditions elucidates how grain morphologies and texture contribute to the observed improvement. Crystal plasticity finite element (CPFE) modeling develops insights as to the mechanisms by which this reduction is achieved in comparison with EBSD results. In particular, the difference in plastic behavior across build orientations of identified crystal planes and grain families are shown to impact the degree of LSP-induced back-stress reduction that is sustained through tensile loading. 

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4. Analytical modeling of residual stress formation in hybrid additive manufacturing

   https://doi.org/10.1016/j.cirp.2024.04.078  

   Karunakaran, Rakeshkumar; Klein, George H; Sealy, Michael P (January 2024, CIRP Annals)

   Current methods for modeling hybrid additive manufacturing are computationally inefficient for use in optimization algorithms. An analytical tool is needed to understand how cycling thermal and mechanical loads via 3D printing and cold working reshapes cumulative residual stress within a build volume. A novel analytical model was developed that couples beam theory and superposition to rapidly predict cumulative residual stress. Modeling results were experimentally validated on AlSi10Mg after laser shock peening prescribed layers during powder bed fusion. Results demonstrated a vertically translating heat-affected zone, and the use of beam-based superposition accurately accounted for residual stress redistribution from cyclic printing and peening. 

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5. Influence of chemical heterogeneity and microstructure on the corrosion resistance of biodegradable WE43 magnesium alloys

   https://doi.org/10.1039/c9tb00388f  

   Mraied, Hesham; Wang, Wenbo; Cai, Wenjun (October 2019, Journal of Materials Chemistry B)

   null (Ed.)

   Magnesium–yttrium-rare earth element alloys such as WE43 are potential candidates for future bioabsorbable orthopedic implant materials due to their biocompatibility, mechanical properties similar to human bone, and the ability to completely degrade in vivo . Unfortunately, the high corrosion rate of WE43 Mg alloys in physiological environments and subsequent loss of structural integrity limit the wide applications of these materials. In this study, the effect of chemical heterogeneity and microstructure on the corrosion resistance of two alloys with different metallurgical states was investigated: cast (as in traditional preparation) and as-deposited produced by magnetron sputtering. The corrosion behavior was studied by potentiodynamic polarization and electrochemical impedance spectroscopy tests in blood bank buffered saline solution. It was found that the as-deposited alloy showed more than one order of magnitude reduction in corrosion current density compared to the cast alloy, owing to the elimination of micro-galvanic coupling between the Mg matrix and the precipitates. The microstructure and formation mechanism of corrosion products formed on both alloys were discussed based on immersion tests and direct measurements of X-ray photoelectron spectrometry (XPS) and cross-sectional transmission electron microscopy (TEM) analysis. 

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