A Weighting Grade-Based Optimization Method for Determining Refill Friction Stir Spot Welding Process Parameters

Abstract

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.

Investigation into the Effect of RFSSW Parameters on Tensile Shear Fracture Load of 7075-T6 Alclad Aluminium Alloy Joints

“…Based on recent papers by the authors [ 23 , 24 ], RFSSW joints were fabricated at a constant tool plunge depth of 1.5 mm. Various values of the welding time were adopted in the research plan.…”

“…Penetration of the upper sheet by the pin to a depth of 1.5 mm ensures optimal mixing of the materials of both sheets and the highest load capacity [ 23 , 24 ]. The decrease in the strength of the joint with the increase in tool penetration up to the value of 1.5 mm is caused by the weakening of the lower part of the joint by the operation of the sleeve [ 23 , 24 ].…”

“…These regions are the designated stir zone (SZ), heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and base metal (BM). A detailed description of their characteristics can be found in previous papers by the authors [ 23 , 27 ]. It is possible to easily identify the approximate boundary between the BM, HAZ, SZ and TMAZ in the macrographs, as shown in Figure 3 .…”

“…[ 22 ] indicated that tool rotational speed directly affects the process temperature and, as a consequence, the metallurgical reaction taking place at the joint interface. Kluz et al [ 23 ] found that it was possible to choose the optimal welding parameters, taking into account the maximisation of load capacity and the minimisation of dispersion of joint strength. The analysis of the fatigue fracture of the RFSSW joint was carried out by Kubit et al [ 24 ].…”

Analogue Modelling of Flow Patterns in Bobbin Friction Stir Welding by the Dark-Field/Bright-Field Illumination Method

“…By the advancing movement of the rotating tool through the joint interface, the softened and heated material of workpiece plates flows around the pin [11][12][13]. The plasticized mass at by the dark-field/bright-field illumination method the leading edge of the rotating tool transports to the trailing edge of the tool, where consolidates in form of the solid-phase weld [14][15][16][17].…”