SPP1640 / A6: Mechanisms in joining of dissimilar materials using Friction Stir Welding

Friction Stir Welding (FSW) is an innovative solid state welding technology, which is suitable for joining dissimilar materials. This project aims to identify the joining mechanims of friction stir werlded dissimilar metal joints.

Initial situation

Modern lightweight structures play a key role in the automotive and the aeronautics industry to enable the efficient use of resources and energy. One promising approach is the load-adapted application of materials. This often requires the joining of dissimilar materials, which imposes a challenge to traditional fusion welding technologies. Hence, innovative welding technologies, such as FSW, are used to join dissimilar materials, since the materials remain in the solid state during the process.

Objectives

The research project aims to gain fundamental understanding of the metallographical processes at the interface layer of FSW joints of dissimilar materials. Therefore, cause-effect relations have to be identified and used to create models, which allow a prediction and adaptation of the joining mechanisms of friction stir welded dissimilar material joints. Thus, a method will be provided that allows a stress-related design of the weld seam.

Approach

In the first phase of the project, an analysis of the joining mechanisms of aluminum-titanium joints was performed. These joints were welded in an overlap- and a butt-joint configuration with varying welding parameters. Extensive process data of the FSW process was collected and analyzed. Furthermore, the material was examined prior and after welding and analyses of the structures at the interface, which result from welding, were carried out to nanometer scale. The joining mechanisms could then be identified.

The previous data was enhanced by overlap welded aluminum-copper joints in the second project phase. This guaranteed a fundamental knowledge of the joining mechanisms of friction stir welded dissimilar materials. The research was focused on the cause-effect relations between the welding parameters, the material combination, the process parameters, the type, and development of each joining mechanism, as well as the strength of the joints.

This is the basis to model the (joining-) processes at the interface, which should allow the prediction and adaptation of the joining mechanisms and provide a method for a stress-related design of the weld seam. The effective joining mechanisms as well as the joint properties should now be made predictable using the gathered knowledge. By implementing a temperature control for the FSW of dissimilar joints, a stationary process behavior can be maintained. Hereby, the limits for a robust FSW process for dissimilar materials can be defined.

Results

One essential result of the first project phase is that a metallic layer in nanometer scale exists at the interface of the joining partners. This was observed using transmission electron microscopy (TEM). Hence, the dominating joining mechanism seems to be a substance-to-substance bond. This was the basis to model and predict the cause-effect relations during FSW of dissimilar metal joints.

Acknowledgment

We would like to thank the German Research Foundation (DFG) for the funding of the project A6 "Mechanisms in joining of dissimilar materials using Friction Stir Welding" as part of the priotity program 1640 (SPP 1640) "Joining by plastic deformation".