Influence of base material plastic behaviour and process parameters on friction stir weldability

Abstract

In current work the weldability in friction stir welding (FSW) of two aluminium alloys currently used in welding construction, the Al-Mg-Si (AA6082-T6) and the Al-Mg (AA5083-H111) alloys, is analysed and related with base materials plastic properties at high temperatures and strain rates. The weldability of both alloys was evaluated by performing a deep morphological, microestrutural and mechanical characterization of a large number of welds produced under a wide range of welding conditions. Simultaneously, the plastic behaviour of both base materials was characterized by accomplishing an extensive mechanical characterization program, which included performing uniaxial tests, under different temperatures and loading conditions, as well as a deep literature review on microestrutural and phenomenological modelling of aluminium alloys plasticity. The hardening behaviour of the aluminium alloys, at increasing temperatures and strain rates, was then related to base material weldability. It was found that the AA6082 alloy, which displays intense flow softening during tensile loading at high temperatures, and is sensitive to dynamic precipitation and overageing under intense non-uniform deformation, displays good weldability in FSW. Under the same welding conditions, the AA5083 alloy, which in quasi-static conditions displays steady flow behaviour at increasing temperatures, and is sensitive to moderate hardening at high strain rates, displays poor weldability. The relations between FSW parameters, classified as independent variables, and base material properties and weld inspection results, classified as dependent variables, were also explored based on torque sensitivity analysis. It was possible to depict a strong influence of base material properties and plate’s thickness on the torque recorded during welding. Torque sensitivity analysis also showed the suitability of this output process parameter for capturing the different sensitivity of both base materials to varying tool rotation and traverse speeds. In spite of this, the evolution of torque with tool traverse and rotation speed was found to be satisfactorily described by a unique power law equation, for both base materials, indicating that the torque registered during welding is a suitable output parameter to be used in FSW process control. Finally, an extensive mechanical characterization of the welds produced for both alloys was accomplished using testing procedures, and strain data analysis techniques, developed under the scope of current work. From this work it was possible to conclude that the global strength of the welds was relatively independent of welding conditions, for both alloys tested in current work. In this way, the most suitable welding parameters for each alloy should be the ones that conduct to the best performance in terms of welding productivity.