M. Thomä, G. Wagner, B. Wolter, C. Conrad, B. Straß, W. Fürbeth, S. Benfer
Proc. 11th Int. Symposium on FSW, TWI Ltd. Cambridge/UK (2016)
The approach of resource-saving product designs leads to a great interest in light weight constructions for nearly every industrial field. Regarding this dissimilar material combinations like aluminum/magnesium are very advantageous. But as fusion welding is not suitable to join Al alloys with Mg alloys, especially in case of cast alloys, there is a need for appropriate metallurgical joining methods. Friction Stir Welding (FSW), as a pressure welding method, enables to realize such dissimilar light metal alloy joints by plastic deformation in solid state.
A very advantageous variant for joining dissimilar metal sheets through FSW is the Ultrasound Assisted Friction Stir Welding (UA-FSW). Comparative studies on FSW and UA-FSW were carried out for Al/Al-joints and Al/Mg-joints of wrought alloys like EN AW-5454 or AZ80 and cast alloys like EN AC-48000 or AZ91. In this regard a modified machining center was used to realize butt joints between metal sheets. The central objective was the evaluation of the influence of ultrasonic oscillations on the resulting microstructure of the welding zone.
Microstructural investigations of the welding zone, the so-called Nugget, of conventional Al/Al- and Al/Mg-FSW-joints showed the existence of band-shaped-layers, which run through the Nugget itself or separate the Nugget from the adjacent area of magnesium base material. Comparative studies were performed for these joints with ultrasound support. The UA-FSW-joints generally have shown an intensified stirring of the welding zone, which lead to a better connection of the respective materials. Furthermore the oxide layers or intermetallic phases layers got cracked by the ultrasound and dispersed over the whole Nugget area.
To determine the resulting quasi-static behavior of the joints strain-controlled tensile tests were performed. These tests resulted in tensile strengths for conventional FSW-joints that were significantly lower than the tensile strength of the weaker joining partner. Due to the transmission of ultrasound during welding an increase in tensile strength could be observed for EN AW-5454/AZ91- and EN AC-48000/AZ80-UA-FSW-joints.
Stepwise Load Increase Tests as well as constant amplitude tests with the aim to achieve a fatigue lifetime prediction by S-N diagrams for EN AW-5454/EN AW-5454 and EN AC-48000/AZ80-joints were carried out. These investigations revealed a tripled fatigue lifetime for EN AW-5454/EN AW-5454 and a more than tripled fatigue lifetime for EN AC-48000/AZ80 as a result of Ultrasound Assisted Friction Stir Welding.
As corrosion properties always have to be considered in joining of dissimilar metals a detailed characterization of the corrosion properties of the Al/Mg-joints were carried out. It was determined that as a result of the electrochemical ignoble character of the magnesium alloys the corrosive attack in the material system is controlled by the magnesium amount in the analyzed sample volume. Therefore the increased magnesium content in the Nugget area of UA-FSW-joints of EN AC-48000/AZ80 leads to a decrease of the corrosion stability in the welding zone.