(Poster Session)FRICTION STIR WELDING OF THIN 7075 T6 SHEETS COATED WITH ANTICORROSIVE LAYER

摘要

The goal of the work is to develop friction stir welding process for joining aluminum 7075 T6 sheets. The thickness of the sheets is 0.8 mm. The sheets are coated with anticorrosive layer. The purpose of the welding process is to produce components of airplanes and helicopters in PZL Mielec. Currently, riveting and resistance spot welding technologies are used for joining of aluminum components. The main challenge in obtaining defect free joints using FSW technology is the anticorrosive layer at the interface between top and bottom sheet. The anticorrosive layer has thickness of about 0.03 mm and is made of aluminum. Additionally, since aluminum has tendency to oxidize, the anticorrosive layer is covered with passive layer of aluminum oxide. The anticorrosive layer decreases the degree of material stir in the bottom sheet and acts as insulation blocking the flow of heat to the bottom sheet. Both these factors contribute to occurrence of wormholes and kissing bond defects in the bottom sheet. Additionally, the anticorrosive layer has tendency to form hooking and thinning defects. Hooking and thinning defects initiate cracks and severely decrease joint strength.The anticorrosive layers can be removed using chemical etching. However, this approach requires developing alternative technology for protecting joints against corrosion. Aluminum joints in aerospace applications are exposed to humidity and thermal cycles which contribute to rapid corrosion development, especially at the interface between joined sheets. The ability to weld sheets covered with anticorrosive layer would greatly facilitate the overall production process.Aluminum sheets are joined using 3-axial CNC milling machines. Monolithic tools made of hot work tool steel are applied. The welding trajectory has the length of 350 mm. In order to assess the quality of a joint 3 sets of tests are performed: microstructure analysis, uni-axial tensile tests and microhardness measurements.The number of tool geometries were tested with respect to their ability to produce defectfree joints. In this paper three types of tools were investigated. The following conclusion can be drawn. The application of tools that don't have axis of symmetry improves material stir in the bottom sheet. The application of double weld pass allows for pushing anticorrosive layer into the joint center, removing hooking and thinning defects and thus increasing joint strength. At the current stage of the research the challenge is to produce joints that have both good material stir in the bottom sheet and are free of thinning and hooking defects.