Friction stir welding of aluminum - copper
Insinööritieteiden korkeakoulu | Master's thesis
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Master’s Programme in Mechanical Engineering (MEC)
AbstractThe joining of dissimilar metals is one major challenge for welding technology. There are not many feasible welding techniques able to overcome the different physical properties and deliver a sound structural joint. The application of solid-state welding techniques, although challenging, is one common solution. This work details the development of welding conditions, tools and parameters for the Friction Stir Welding (FSW) of Aluminum-Copper (Al-Cu) butt joint. The application of FSW to these dissimilar joints has proved in the past to be feasible, but several technological and joint performance features, still demand further investigation and development. The motivation and support of this project was a cooperation between Aalto University and Promeco Oy, with ABB Drives Oy as a strategic partner, aiming at providing Promeco Oy with the ability to manufacture high-value dissimilar components, such as, Al-Cu bus bars. Multiple conventional FSW tools were designed and tested. Parameters were developed to optimize 6 mm thick AA1050-H14/24 – CU-OF-04 butt joints. After preliminary tests, FSW tool was selected for optimization of the process parameters. The tool has ØD3 concave shoulder and ØB threaded taper N/A long probe. The Taguchi method for design of experiments was used for the optimization of three process parameters: travel speed, weld position and offset position of the tool in relation to the joint line between the base materials. The weld was then thoroughly characterized. Tensile, bending and microhardness tests were used to establish the mechanical properties. Optical microscope and scanning electron microscope were used to investigate the microstructure. Joining mechanisms and intermetallic compounds in the weld were investigated using an X-ray diffraction analysis. The electrical resistance of the weld was assessed using a microhmmeter. The optimized parameters were found to be N/A travel speed; N/A tool plunge and N/A offset into the aluminum. The properties of this optimized weld resulted in 84.8 % Global Efficiency to Tensile Strength (GETS) and 40.8 % Global Efficiency to Bending (GEB) compared to AA1050-H14/24, and 97.2 % electrical conductivity efficiency compared to an ideal bimetallic component made of the same materials with no contact resistance. To understand the benefits of having an Al bus bar with Cu bolted end, compared with one made of monolithic Al material, a dedicated experimental test setup and protocol was designed and implemented. This experimental test enables to monitor the force relaxation of a pre-loaded bolt joint, under cyclic thermal loading, simulating real operational conditions. The test was applied to both Al base material and Cu base material components. The experimental results show that the force relaxation in the Cu bolted joint was about 50 % lower compared with the Al. Thus, bus bars with Cu ends are more stable and will need less maintenance while in operation than bus bars with Al ends. An advanced Stationary Shoulder FSW tool was designed, produced and is ready for testing, envisaging a weld joint with improved top surface finishing and good overall engineering performance.
Thesis advisorVilaҫa, Pedro
friction stir welding, dissimilar joint, aluminum, copper, tool design, stationary shoulder