High-Resolution Residual Stress Mapping of Magnesium AZ80 Friction Stir Welds for Three Processing Conditions

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, and A.P. GERLICH

Low-angle synchrotron transmission diffraction has been used to create high-resolution 2D residual strain maps of friction stir welds made with three processing conditions. These spatial maps of residual strain reveal local concentrations not detectable by line scans, and confirm that the asymmetric material flow known to produce asymmetric temperature and texture distributions also results in asymmetric residual strain distributions. The experimental set-up permitted simultaneous measurement of both texture and strain, which provides strong evidence against the correlation of these features in magnesium friction stir welds. Mapping diffraction peak width across the weld provides insight into the spatial distribution of dislocations and microstrains, and indicates locations of interest for higher resolution research such as TEM. A diffraction method is presented to determine the solute content of a ternary system using the lower symmetry of a non-cubic system, which can be extended to detecting the onset of precipitation among other applications. Comparison of three friction stir-welding conditions shows how the residual strains at the interface can reverse from compressive to tensile with decrease in the heat input, explaining a significant disparity in the literature results. Lower residual stress values were found to be well-correlated with improved transverse tensile behavior. https://doi.org/10.1007/s11661-019-05585-3 Ó The Minerals, Metals & Materials Society and ASM International 2019



FRICTION stir welding is a solid-state joining technique offering multiple advantages for use with materials that are challenging or impossible to weld successfully by fusion methods.[1] The non-consumable tool used consists of a wider shoulder and narrower-threaded pin, rotated and progressed along the joint line in the welding direction (WD). Material is heated by the tool action at the advancing side (AS) and sheared or extruded around the pin under the shoulder towards the retreating side (RS). Although aluminum-rich magnesium alloys such as AZ80–AZ91 are unsuitable for fusion welding due to the brittle intermetallic formed along the grain boundaries[2] during fusion, friction stir welds of good quality in this lightweight material have been successful.[3,4] For parts with the same yield strength as AA6061, AZ80 offers a J. HISCOCKS, M.R. DAYMOND, and B.J. DIAK are with the Department of Mechanical and Materials Engineering, Queena˜ s University, Kingston, ONK7L 3N6, Canada. Contact e-mail: [email protected] A.P. GERLICH is with the Department of Mechanical and Mechatronics Engineering, Waterloo University, Waterloo, ON N2L 3G1. Manuscript submitted July 8, 2019.


weight reduction of about 26 pct at the expense of a reduction in ductility[5,6] and higher cost. Residual stresses are the self-equilibrating stresses that exist within a material body on a range of length scales in the absence of any applied stresses.[7] T