By Mathies Kraetzsch

Electric car model.
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Copper is known for its excellent electrical properties that make it predestined for the production of cables and electrical connections. Aluminum, which is lighter and cheaper than copper, also provides very good electrical properties. The Fraunhofer IWS Dresden has set the task to join both materials in a professional and long-lasting way. Some approaches could be developed. They provide solutions for a multitude of joining tasks which are demanded in the field of electromobility.

LASER BEAM WELDING
Laser beam welding stands for efficient joints with optimized weight and performance properties. A multitude of different materials and their combinations can already be welded by laser, e.g., aluminum/steel or cast iron/case-hardened steel. The researchers of the Fraunhofer Institute for Material and Beam Technology IWS Dresden have now developed a technology that enables the laser welding of mixed materials such as aluminum/copper, aluminum/magnesium or stainless steel/copper with clearly better qualities.

Laser beam welding of Al-Cu compounds (process).
© Fraunhofer IWS Dresden

The improvement results by the benefits of brilliant laser beam sources and using a highly dynamic 2D scanner with high scanning frequencies (up to 2.5 kHz max.). The system and a number of technological parameters have been developed within the BMBF funded joint project WELDIMA.

Brilliant laser beam sources in connection with a high frequency beam oscillation make it now possible to join metallic material combinations, which have been conventionally non-laser weldable up to now. It concerns especially such combinations like Al-Cu, where brittle intermetallic phases occur. Extreme small weld seam with high aspect ratio leads to very short melt pool life time. These allow an extensive reduction of the heat input. On the other side, the melting behavior at metallic mixed joint, seam geometry, chemical composition, melt pool turbulence and solidification behavior can be influenced by a high frequency time-, position- and power-controlled laser beam oscillation.

An additional lateral beam shifting in the welding joint region allows the control of the mixing ratio of both material partners. In combination with structural analysis, the width of the intermetallic phase seam with unwanted intermetallic joints, generated in the weld, can be specifically set. The smaller the intermetallic phase seam, the lower the tensile strength of the welded joints. For the aluminum/copper compounds welded at IWS with a highly dynamic beam scanner, phase seam values less then 10 µm have been measured. The tensile strength of the compound achieves the same values as the similar joint aluminum/aluminum and, after all, 70 percent of the non-affected basis material.

LASER INDUCTION ROLL PLATING
The generation of Al-Cu interconnections for the packaging of lithium ion cells is a focal point of the research project DeLIZ. The researchers inDresdenare developing a new laser induction roll process for industrial use. This process combines the advantages of cold and hot roll cladding and enables large-area material joints of bands made from aluminum and copper.

During the process, a laser beam heats the inner surfaces of the metal bands immediately after induction preheating. Thereby, the deformation in the roll gap is largely localized to the (very limited) highly heated volume. Under the influence of the roll pressure, permanent, precise and interface-free transition joints can be generated between the metal bands. Analytical investigations have shown that the formation of the welding zone can be substantially influenced by the choice of the process parameters. The intermetallic phases which are typical for aluminum/copper interconnections can be avoided completely. The band plated at rolling speeds up to 8 m min-1 can be well deformed in the roll-plated state and directly finished. The measured shearing strengths of the compound were on the order of 30 to 40 MPa.

Roll plated and twisted band made from aluminum and copper.
© Fraunhofer IWS Dresden

Mathies Kraetzsch is an engineer for Fraunhofer IWS Dresden in the Welding Group.