Abstract
In the aerospace industry, the refill friction stir spot welding (RFSSW) method is increasingly used to join thin sheets, especially those made of 2xxx and 7xxx series aluminium alloys. This paper presents the results of spot welding lap joints of 7075-T6 aluminium alloy sheets. The load capacity of joints was determined by tensile/shear tests using a universal testing machine. The effect of tool plunge depth, tool rotational speed and duration of welding on joint load capacity and failure mode was investigated. The polyoptimisation of the values of welding process parameters was carried out to ensure the highest load capacity of the joint at a minimum variance of the results obtained. The selection of the optimal parameters of the RFSSW process was carried out using an adequate mathematical model obtained on the basis of Weierstrass’ theorem. It was found that tool rotational speed had the greatest impact on the load capacity of the joints. It was possible to increase the load capacity of the joint by increasing the duration of welding but only to a limited extent. The selection of optimal welding parameters requires a compromise solution, i.e. the selection of a tool rotational speed that ensured adequate plasticisation of the material with a welding duration that ensured that an appropriate joint microstructure was obtained and assured the required load capacity of the joint. The methodology of mathematical modelling of polyoptimisation presented can be useful in optimising similar joining processes.
Highlights
Aluminium alloys are an intensively investigated and widely used group of materials in the aircraft industry due to their high strength and low density
The method presented required experimental determination of the influence of refill friction stir spot welding (RFSSW) parameters on the load-bearing capacity of single lap joints of the 7075-T6 Alclad aluminium alloy sheets commonly used in aircraft applications
Regardless of the assumed tool rotational speed, the highest load capacity of the joint is observed for a tool plunge depth of x2 = 1.5 mm
Summary
Aluminium alloys are an intensively investigated and widely used group of materials in the aircraft industry due to their high strength and low density. The presence of an oxide layer on the surface of aluminium alloys, which is characterised by high contact resistance, causes rapid degradation of the electrode tip [7] Another problem of RSW is the warpage deformation of the asymmetric sheet structures which results from the changed residual plastic strain due to thermal deformations [8, 9]. The effect of the RFSSW process parameters on the joint microstructure and the load capacity of the joint has become a focus of the investigations of Shen et al [13] who found that the overlap shear strength increases with increase in weld time and plunge depth due to increasing nugget diameter. The method presented required experimental determination of the influence of RFSSW parameters on the load-bearing capacity of single lap joints of the 7075-T6 Alclad aluminium alloy sheets commonly used in aircraft applications
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