Investigation of the formation mechanisms of the bonding zone in impact welding

Investigation of the formation mechanisms of the bonding zone in impact welding

 

Motivation

Material lightweight construction and load oriented design often demand joints between dissimilar metals. Hybrid structures of aluminium and steel in the vehicle sector can be mentioned as one example. Besides conventional form- and force-closed joints, material-closed joints are possible as well. Here especially impact welding has to be mentioned, which bases upon a defined impact between two joining partners at high velocities (above 200 m/s). Thus no heat affected zone exists, which does not lead to any negative effects on the microstructure. The joint area barely contains any intermetallic phases and hence exhibits good (fatigue) strength. During the impact, oxide layers and superficial impurities are removed due to the plastic deformation. Afterwards, the cleaned and highly reactive surfaces are brought together under the immense pressure of the impact so that metallic bonding is enabled. Applied processes are explosive welding and electromagnetic pulse welding. The latter has a very short cycle time, is harmless and capable equipment is available, which makes it suitable for mass production.

[1] Iteration between experiments at the test rig (left), their observation (top right) and their numerical simulation (bottom right)
[1] Iteration between experiments at the test rig (left), their observation (top right) and their numerical simulation (bottom right)

Aim

Its broad application is still hindered by the poor understanding of the process principles, because both explosive welding and electromagnetic pulse welding have disadvantages concerning their observability. The aim of the research work is to make the process predictable by identifying the governing mechanism s and process parameters. This knowledge is the applied in numerical simulations which can be used for any joining geometry.

[2] Unetched microsection of the joint area
[2] Unetched microsection of the joint area

Methodical approach

A novel test rig has been developed at the PtU, which allows the straightforward investigation of the process principles by an aimed variation of parameters. The required impact velocity is generated purely mechanically by two rotors, which carry one joining specimen each at one outer end. The joining process is captured by a special high speed camera to observe the particular mechanisms. The necessary insights are to be gained in comparison with additional numerical simulations. The metallographic analysis of the samples is carried out by the Chemnitz University of Technology. The future aim is to be able to predict the process of impact welding and therewith design future joints reliably.

 

Acknowledgment

The research project is funded by the German Research Foundation (DFG).