PDF《Hybrid Al/steel-joints》

Hybrid Al/steel-joints manufactured by ultrasound enhanced friction stir welding (USE-FSW): Process comparison, nondestructive testing and microscopic analysis M Thom?1,4 , G Wagner1 , B Stra?2 , B Wolter2 , S Benfer3 and W Fürbeth3

1 Institute of Materials Science and Engineering, Chemnitz University of Technology,

09125 Chemnitz, Germany

2 Fraunhofer Institute for Nondestructive Testing IZFP, Campus E3 1,

66123 Saarbrücken, Germany

3 DECHEMA-Forschungsinstitut, Theodor-Heuss-Allee 25,

60486 Frankfurt am Main, Germany

4 Corresponding author: marco.

[email protected] Abstract. The process of friction stir welding (FSW) is an innovative joining technique, which proved its potential in joining dissimilar metals that are poorly fusion weldable. This ability opens a wide range for applications in industrial fields, where weight reduction by partial substitution of conventional materials through lightweight materials is a current central aim. As a consequence of this, the realization of aluminum / steel-joints is of great interest. For this material compound, several friction stir welds were carried out by different researchers for varying Al/steel-joints, whereas the definition of optimal process parameters as well as the increase of mechanical properties was in the focus of the studies. To achieve further improved properties for this dissimilar joint a newly developed hybrid process named ultrasound enhanced friction stir welding (USE-FSW) was applied. In this paper the resulting properties of Al/steel-joints using FSW and USE-FSW will be presented and compared. Furthermore, first results by using the nondestructive testing method computer laminography to analyze the developed joining area will be shown supplemented by detailed light-microscopic investigations, scanning electron microscopic analysis, and EDX. 1. Introduction Recent developments in many industrial sectors regarding lightweight constructions to save energy in different ways lead to the necessary realization of components out of dissimilar materials like aluminum or aluminum alloys and steel. But in this case, established fusion welding methods struggle with the development of brittle intermetallic (IM) phases in the welding area, which results in a significant reduction of the strength of the joint [1, 2]. In comparison to fusion welding friction stir welding (FSW) as a low temperature joining process is a more appropriate method, because of the reduced development of intermetallic phases. FSW is characterized by a rotating cylindrical tool, which gets plunged in a butt or overlap joint of two materials and then runs along contact area with a defined feed rate [3]. Several research work has been performed on the field of Al/steel-joints concerning the realization of high-strength welds [4C7]. Watanabe et al. reached a maximum tensile strength for their FSW-joints of

86 % of their aluminum base material [4]. But in the most cases in literature the presence of intermetallic phases in the nugget and at the interface of the joints after the friction stir welding process is described [4, 6C8]. This verified that IM-phases also cannot be completely reduced by using FSW. Furthermore, the intermetallics appear as brittle separating layers between the base materials. This weakens the joint strength additionally. To reduce these detrimental effects, different approaches were investigated for FSW of Al/steel-joints. Liu et al. and Ferrando worked with an electrically assisted friction stir welding system, which enables a local electrical current density field to run synchronous to the FSW tool. The occurring electro-plastic effect results in a material softening due to the elevated process temperatures and also in the formation of a thin layer of IM-phase, which was believed to increase the joint strength by micro-interlocking [8, 9]. This additional heating approach for the contact area was also applied by Merklein et al. who used laser assistance for their friction stir welding process. They further observed a reduced tool wear and a higher drawing ratio for their joints [10]. Another method for extra heating was the gas tungsten arc welding (GTAW) supported version of FSW introduced by Bang et al., where a GTAW torch runs in front of the friction stir welding tool and also leads to joints with higher tensile strength. This is attributed to an improved material plastic flow and partial annealing effects [11]. A further concept was realized by Klag et al. and Stra? et al., who investigated Al/Mg-joints by using additional power ultrasound during the friction stir welding process [12, 13]. This hybrid joining method named ultrasound enhanced friction stir welding (USE-FSW) improves the joint strength by the mechanical breakup and fine dispersion of the continuous brittle intermetallic layers. With this method an increase of the tensile strength of