Metallurgical Arc-joining of Steel and Aluminium to Make Hybrid Sheets

An impressive innovation has shifted the limits of welding still further: Whereas steel and aluminium were once thought to be incapable of thermal joining, this is now perfectly feasible in actual practice. The revolutionary solution has made the visions of material experts, design engineers and production engineers come true. Not to mention the long-held wish of automobile industry managers for dependable high-load thermal joins between solid steel and lightweight aluminium, as required by the loading sequence in the component. This is particularly advantageous for the automobile industry, faced as it is with the need to reduce vehicle weight (and thus fuel consumption and CO2 emissions) and to meet the EU emissions limit of 130 g/km CO2 by 2012. In other sectors, too - ranging from designer objects to factory roofs to wind-power installations - lightweight constructions also deliver huge benefits.

Welding textbook opinion has it that metals which differ so greatly in terms of physical properties such as their fusing temperatures (1500°C versus 590°C), coefficients of expansion (1.2 mm versus 2.3 mm/100°C) or electrochemical potential cannot be joined by arc welding. For over 20 years, Fronius had been rising to the challenge presented by this welding-engineering bottleneck, making the first reproducible joints even before it arrived at the CMT (Cold Metal Transfer) solution. From 2003 onwards, a viable technology crystallised out of Fronius' collaboration with voestalpine. The many trials conducted since then have proved the industrial suitability of this process and of its applications. On the aluminium side there is a welded join, while the steel sheet exhibits a brazed join. Fronius and voestalpine have been granted several patents for this solution. Together, they have developed high-formability hybrid sheets. Used as semi-finished products, these can be formed into profiles or crash-absorbers in processes such as folding, rolling and deep-drawing.

Some technical boundary conditions: The steel sheets must be galvanised (as is usual in automotive manufacturing), the aluminium sheets must be made from materials of the AW5xxx or 6xxx series, and AlSi3Mn1 must be used as the welding/brazing filler metal. The zinc layer on the steel sheet acts as a flux, wetting the steel. The crucial factor is the intermetallic phase (IMP). This should be as thin as possible, and no more than 10 µm. The strength of the joint is then so great that in tensile tests, the seam itself remains intact and the break takes place in the aluminium sheet. Since then, multi-year tests on shaped hybrid sheets have proved that neither intercrystalline nor stress or crevice corrosion occur; only minor surface corrosion has been detected. The ideal welding systems for the hybrid sheets have proved to be ones that use the CMT process to vertically braze-weld both sides 'in sync' from top to bottom. The know-how from voest- alpine relates to weld-preparation in terms of the seam-geometry, while the patented filler metal and the modifications to the (also patented) CMT system are the fruits of Fronius' R&D effort.

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