Sigrid Benfer

The Influence of Ultrasound Enhancement during Friction Stir Welding of Aluminum to Steel

The innovative joining process of friction stir welding (FSW) offers a wide range of advantages for welding similar as well as dissimilar materials. Even for the field of poorly weldable material combinations like aluminum to steel with their strongly differing physical properties the method of FSW proved its capability for realizing dissimilar joints with tensile strengths up to more than 80 % of the aluminum base material. Trying to improve this value and other properties of the joints several approaches for hybrid friction stir welding processes were tested in the scientific community, whereas the ultrasound enhancement of FSW (USE-FSW) looked as one of the most promising reaching good results. To gain a deeper knowledge of the influence of the ultrasound on the friction stir welds different investigations were carried out in this paper. Therefore the method of USE-FSW was applied on two dissimilar aluminum/steel-joints with varying carbon content of the steel in this work. The material combinations AA6061/SAE1006 and AA6061/SAE1045 were welded successfully with and without additional power ultrasound. Afterwards a comparison between FSW-and USE-FSW-joints was carried out regarding the microstructure of the nugget and interface (IF) by light-microscopy as well as scanning electron microscopy. Furthermore the mechanical properties were characterized in a first step.

Realization of Ultrasound Enhanced Friction Stir Welded Al/Mg- and Al/Steel-Joints: Process and Robustness, Mechanical and Corrosion Properties

As an innovative hybrid joining process ultrasound enhanced friction stir welding (USE-FSW) was successfully applied on Al/Mg-joints and offers a beneficial impact on the resulting microstructure and mechanical properties. Whereas in conventional FSW of Al to Mg continuous band-shaped intermetallic layers of Al3Mg2 and Mg17Al12 are always present, the formation of these layers can be influenced positively by the ultrasound energy that is transferred into the stirred zone in USE-FSW. In this case, the intermetallic phases are spread over the complete nugget zone. Consequently, the tensile strength increases about 25% and the fatigue strength rises by up to 3.5 times. Based on this USE-FSW was investigated with regard to possible advantages for Al/steel-hybrid-joints. Preliminary investigations have shown significant differences in the microstructure of ultrasound enhanced and non-ultrasound enhanced EN AW-6061/DC04-joints. While conventional FSW led to hooks and larger particles of steel in the aluminum, USE-FSW-joints show remarkably smaller and more homogenous distributed steel particles in the nugget. Beside mechanical investigations, non-destructive examinations and corrosion tests were carried out.

Recent Developments for Ultrasonic-Assisted Friction Stir Welding: Joining, Testing, Corrosion - an Overview

Due to the steadily increasing demand on innovative manufacturing processes, modern lightweight construction concepts become more and more important. Especially joints of dissimilar metals offer a variety of advantages due to their high potential for lightweight construction. The focus of the investigations was Al/Mg-joints. Friction Stir Welding (FSW) is an efficient process to realize high strength joints between these materials in ductile condition. Furthermore, for a simultaneous transmission of power ultrasound during the FSW-process (US-FSW) a positive effect on the achievable tensile strength of the Al/Mg-joints was proven. In the present work the industrial used die cast alloys EN AC-48000 (AlSi12CuNiMg) and AZ80 (MgAl8Zn) were joined by a machining center modified especially for Ultrasound Supported Friction Stir Welding. The appearing welding zone and the formation of intermetallic phases under the influence of power ultrasound were examined in particular. In order to identify optimal process parameters extensive preliminary process analyzes have been carried out. Following this, an ultrasound-induced more intensive stirring of the joining zone and as a result of this a considerably modified intermetallic zone was detected. At the same time an increase of the tensile strength of about 25% for US-FSW-joints and for fatigue an up to three times higher number of cycles to failure in comparison to a conventional welding process was observed. Moreover, detailed corrosion analyzes have shown that especially the welding zone was influenced by the corrosive attack. To expand and deepen the knowledge of the US-FSW-process further material combinations such as Ti/Steel and Al/Steel will be considered in future.