Abstract
Aluminum alloys are widely used in transportation industries because of the increasing need to reduce the environmental impact. With advances in technology, the demand for complex parts and components that must be produced using several processing methods has increased. During fabrication and service, a wide range of defects can appear in aluminum components and structures, which could be repaired using a suitable through hole closure method. The search for a friction-based solid-state keyhole repair technique that fulfills the requirements for high-quality repair welds has become an important research topic because conventional fusion welding is difficult to apply in many aluminum alloys. However, many commonly available friction-based welding methods are complex and multistage processes that require specially designed equipment and are not suitable for sealing through holes. The development of an adequate keyhole repair process is thus actual necessity. The present study addresses the development of a suitable keyhole repair procedure of structural aluminum parts using the refill friction stir spot welding process (RFSSW). For this newly developed repair method, a plug made of a similar material is applied as a filler element into the keyhole and RFSSW is used to weld the plug to the surrounding workpiece. To cover a wide range of alloys and potential applications, the repair method was investigated in different precipitation hardening aluminum alloys as well as different keyhole diameters and workpiece thicknesses. A fundamental analysis of the process and resulting material properties considering the alloy-dependent metallurgical transformations was conducted. Moreover, a knowledge-based process analysis approach was chosen to study the behavior of the base material during high-shear-rate plastic deformation and exposure to typical thermal cycles, which are both associated with the conditions found during friction welding. The influence of the base material composition and properties on the energy input during friction welding was investigated and a comprehensive analysis of the friction condition and flow stress development was conducted. The developed keyhole repair process using RFSSW is a universal through-hole closure method with advantages such as defect-free welds, high weld efficiencies and superior surface appearance on both sides of the weld. Within the scope of the present work, processing conditions were defined that lead to defect free repair welds for all investigated materials and workpiece dimensions. The area of lowest strength was in all cases found outside of the weld spot, mainly in the heat affected zone. For all welded precipitation hardening aluminum alloys, metallurgical analysis revealed that the evolution of the strengthening precipitates during and after the weld primarily determines the final mechanical properties. Significant differences observed in the response of the base materials to the process were found to be caused by the alloy composition, specifically by the characteristics of the present precipitates. The fundamental process analysis revealed that in precipitation hardening aluminum alloys, the mechanical properties obtained under quasi-static testing conditions are not adequate to describe or predict the base material properties at the high strain rates and thermal cycles associated with friction welding operations. The alloy composition, initial temper condition and general precipitation evolution during the specific thermal cycles resulting from the friction welding operations were found to determine the material properties at the tested rates of deformation. The knowledge gained by this fundamental process analysis is key to enabling rapid process optimization by guiding the appropriate choice of process parameters for a given alloy.
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