A Weighting Grade-Based Optimization Method for Determining Refill Friction Stir Spot Welding Process Parameters
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A Weighting Grade-Based Optimization Method for Determining Refill Friction Stir Spot Welding Process Parameters Rafał Kluz, Andrzej Kubit, Tomasz Trzepiecinski
, Koen Faes, and Wojciech Bochnowski
(Submitted March 1, 2019; in revised form August 11, 2019; published online October 9, 2019) The welding process used in fabricating thin-walled structures by refill friction stir spot welding (RFSSW) should be characterized by a high strength of welds and high process repeatability which is demonstrated by a small dispersion of the load capacity of the joints. The present work is designed to optimize RFSSW process parameters for 7075-T6 Alclad aluminum alloy sheets used to fabricate aircraft structures. The optimization was performed by scalarization of the objective function using the weighting grades method. The study considers the effect of process parameters, i.e., tool plunge depth, duration of welding, tool rotational speed, on the tensile/shear strength of the joints, and dispersion of the load capacity. It was found that it was possible to choose the optimal welding parameters taking into account maximization of the load capacity and minimization of the dispersion of the joint strength via a best compromise between the tool rotational speed ensuring adequate plasticization of the base material and the duration of welding ensuring that a fine-grained joint microstructure is obtained. Keywords
aluminum alloy, friction stir spot welding, optimization, welding
1. Introduction Friction stir welding is a solid-state joining technique that is finding greater use in aircraft applications for manufacturing thin-walled aircraft components, especially stiffened panels. The FSW process is capable of welding some previously unweldable precipitation-strengthened aluminum alloy sheets. In a classical FSW process, a rotating cylindrical tool composed of a probe and a shoulder is plunged into the materials to be joined. Mechanical deformation of the materials is possible via the heat generated through friction (Ref 1, 2). FSW joints have higher strengths than riveted joints and much lower residual stresses than a typical fusion welded joint, like resistance spot welding (RSW) (Ref 3). Refill friction stir spot welding (RFSSW) is a modern solidstate friction welding process which was developed by Helmholtz-Zentrum Geesthacht (Ref 4) for welding two or more similar or dissimilar sheets in a lap configuration. RFSSW is a solid-state joining technology which connects two similar or dissimilar materials together with minimal heat input or Rafał Kluz and Andrzej Kubit, Department of Manufacturing and Production Engineering, Rzeszow University of Technology, Al. Powst. Warszawy 8, 35-959 Rzeszo´w, Poland; Tomasz Trzepiecinski, Department of Materials Forming and Processing, Rzeszow University of Technology, Al. Powst. Warsaw 8, 35-959 Rzeszo´w, Poland; Koen Faes, Belgian Welding Institute, Technologiepark-Zwijnaarde 935, 9052 Ghent, Belgium; and Wojciech Bochnowski, Department of Mathematics and Natural Sc
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