A vision come true: welding the unweldable

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This crash-test gives an impressive demonstration of the interplay and effect of the aluminium-steel component; the aluminium absorbs the deformation energy while the steel maintains the desired clearance. 

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.

Fronius International GmbH

Fronius International is an Austrian company with headquarters in Pettenbach and other sites in Wels, Thalheim and Sattledt. With over 3,250 employees worldwide, the company is active in the fields of battery charging systems, welding technology and solar electronics. Around 95% of its products are exported through 17 international Fronius subsidiaries and sales partners/representatives in over 60 countries. In financial year 2010, the company generated a total turnover of 499 million euros. With its outstanding products and services and 737 active patents, Fronius is world technology leader. 392 employees work in research and development.

Marion Rauch
Fronius International GmbH
Froniusplatz 1, A-4600 Wels
Tel. +43 / (0)7242 / 241 2590
Fax +43 / (0)7242 / 241 95 3940
E-Mail: rauch.marion@fronius.com