Realization of Al/Mg-Hybrid-Joints by Ultrasound Supported Friction Stir Welding

Abstract

To realize modern light weight constructions it is more and more necessary to combine the advantages of dissimilar materials. Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points of the materials and the development of undesirable brittle intermetallic phases in the welding zone. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) as a pressure welding method, it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly. But as calculated phase diagrams and high resolution microscopic SEM-investigations have shown it is not feasible to avoid them completely. The intermetallic phases form in the contact area very small continuous layers between the joining partners. On the other side it is known that ultrasonic energy can crack oxide layers. Hence a hybrid welding system at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for US-FSW-joints between Al wrought alloys and Mg cast alloys the strength can be increased up to 30% in comparison to conventional friction stir welds. Currently, further investigations are carried out with joints between AC-48000 and AZ80.

Application of the hybrid process ultrasound enhanced friction stir welding on dissimilar aluminum/dual‐phase steel and aluminum/magnesium joints

“…In comparison the friction stir welded joint reaches only 64 MPa, which corresponds to 31 % of the weaker base material EN AC-48000. This can be explained mainly by the brittle intermetallic β-and γ-phases which reduce the tensile strength by acting as a discontinuity and could be detected before and are in accordance with the literature [7,16]. Regarding the fracture area of the specimen the failure always occurred at the interface between magnesium and nugget, where the two IM-phases were present.…”

“…Regarding aluminum/magnesium joints the optimal joint properties were achieved with the aluminum placed on the advancing side and magnesium on the retreating side [5][6][7]. Furthermore, offsets of the pin into the magnesium varied from 0 mm to 1 mm [5,8].…”

“…The presence of intermetallic phases was detected for several investigations regarding dissimilar aluminum/magnesium joints. Thereby two continuous intermetallic layers consisting of the aluminum-rich β-phase (Al 3 Mg 2 ) and the magnesium-rich γ-phase (Mg 17 Al 12 ) at the interface nugget/magnesium were found [5,7].…”

“…Further investigations proved that a reduction of brittle intermetallic phases at the interface of dissimilar metal joints like aluminum/magnesium and aluminum/steel lead to higher tensile strengths [8,14]. To achieve such a reduction in the thickness of the intermetallics the process temperature can be reduced, which is limited due to the pressure welding method itself, or alternatively can be influenced by the additional transmission of power ultrasound into the friction stir welding [5,7,15]. By using this hybrid joining method of ultrasound enhanced friction stir welding (USE-FSW) compared to conventional friction stir welding for aluminum/magnesium joints the Al 3 Mg 2 -phase could be broken and distributed as fine particles over the nugget, which led to an increase in tensile strength [8,16,17].…”

“…Regarding the size and amount of steel particles in the nugget a similar behavior to 225 W of power ultrasound can be determined with a slightly bigger amount of the smallest ASTM 15 class of 33 %. This led to the assumption that an increase of power ultrasound results in a more activated steel base material and joint interface as well as a reduction of steel particle size in the nugget similar to the breakup of continuous intermetallic layers and their fine dispersed distribution over the whole nugget of aluminum/magnesium joints [7,16]. This behavior is also in accordance with the more pronounced pushing of the base material in the nugget to the non-introduction side of the power ultrasound of the EN AC-48000/AZ91 joints.…”