Effects of Processing Parameters on Friction Stir Welded Lap Joints of AA7075-T6 and AA6022-T4

Abstract

Friction stir welding (FSW) is a solid-state welding process that has a number of advantages over traditional fusion welding techniques when attempting to join aluminum or dissimilar material workpieces. It is expected to play a large role in the automotive industry, where aluminum alloys are becoming more prevalent in mass-production vehicles.

The research in this thesis evaluates overlap FSW joints between thin sheets of AA7075-T6 and AA6022-T4 when the welding parameters of tool geometry and welding speed are varied. The resulting joints are characterized by optical microscopy, overlap shear tests, microhardness tests, and temperature measurements. The effect of a post-weld heat treatment is also examined. The main objective of the research are to determine a tool geometry that can produce good quality welds over a wide range of operating conditions, for use in an industrial setting.

Friction stir welds of good quality are made successfully at speeds of up to 500mm/min, and it is found that weld microhardness and joint strength are greater at faster welding speeds; whereas temperatures in the weld area are lower at faster welding speeds. Five different tool geometries are tested, and the tool design that delivers the best performance is a one that uses a concave shoulder shape, and a pin with a tapered profile, threads, and 3 flats. A post-weld heat treatment at 180°C for 30 minutes is found to increase joint strength by approximately 10%.

Future studies involving transmission electron microscopy, corrosion testing, and fatigue testing are recommended in order to supplement the results presented in this thesis.