Properties of Friction Stir Welded Blanks Made from DC04 Mild Steel and Aluminum AA6016

Abstract

Due to increasing demands for lightweight structures in automotive applications the use of sheet metal components made from aluminium alloys is a promising approach for weight reduction. The combination of steel and aluminium in car bodies may be an interesting alternative compared to a monolithic material design. The weight of structural parts of a car body shell can be reduced if dedicated parts consist of aluminium instead of steel. This approach allows for an optimal exploitation of the material properties of both materials, bringing high strength into highly loaded areas while areas subject to lower loads are equipped with lower strength and weight. However, a multi-material design combining steel and aluminium demands for suitable joining methods, especially if a forming operation is applied to the welded sheets. In conventional fusion welding processes the formation of intermetallic phases due to the metallurgical affinity of aluminium and iron is a serious problem. Recent developments in regulated cold metal transfer (CMT) welding technologies at the Institute of Welding Technology and Joining Technology (ISF) at the RWTH Aachen promise an appropriate solution to this problem. Due to a digitally regulated arc technology, the heat input in CMT is reduced to a minimum. However, the inevitable formation of a welding bead in arc processes with filler material is a criterion of exclusion in the case of production of welds for car body shells. To achieve an optimal appearance of the body shell, the welding beads need to be removed from both sides of the sheet in a second manufacturing step. Hence, to avoid further costs, it seems expedient to search for alternative welding technologies. Friction stir welded (FSW) joints show relatively even welding beads. Furthermore, this joining method is characterised by a low process temperature, which is considerably below the melting temperature of the base materials. Hence, FSW is a promising joining technique to produce tailored blanks out of aluminium and steel. The main objective of the present paper is the evaluation of suitable process parameters for the production of FSW butt joints with a thickness of 1 mm made from the aluminium alloy AA6016-T4 and the mild steel DC04. Welding experiments using a varying rotational speed, tool offset, tool velocity, tool plunge depth and tool tilt angle were carried out. To identify the best parameters in terms of the strength of the joint, tensile tests were performed. It is shown, that an amount of approximately 85% of the tensile strength of the base material AA6016 can be achieved. Using SEM the formation of the fracture surfaces was analysed. Different fracture types were identified and the possible reasons for their occurrence are discussed. It is shown that in the case of optimal joining procedure the failure occurs in the thermomechanically affected zone in the aluminium sheet, were the plastic deformation is low. Additionally, thermography has been employed to evaluate the temperature distribution during the process. In metallographic investigations it was found that during welding the microstructure of the aluminium base material changes due to plastic deformation and temperature increase in the area of the weld seam. Using hardness measurements the change of the mechanical properties in the contact zone of both base materials and in the heat affected zone was examined. Finally, an outlook is given with respect to the possibilities of producing FSW welded sheets that can be formed using conventional deep-drawing.