Welded structures made of cast materials or mixed structures made of cast and wrought materials are state of the art, e.g. for the construction of light-weight chassis or large-scale structures in power installations. These structures utilize the combined benefits of casting technology, notably the freedom in design and the reliable production with welding.
The physical, chemical and mechanical properties of aluminum result in a special welding behavior different from that of ferrous materials. This particularly includes the stable oxide layer at the surface which needs to be removed or broken up in order to obtain a perfect weld joint. With gas-shielded electric arc welding (see Gas-shielded welding processes), this oxide layer is removed by the cleaning effect of the ionized shielding gas in combination with the effect of the electrons flowing from the workpiece to the electrode. Today, the MIG method is often used in the large-scale welding of cast aluminum parts because of the high obtainable feed rate (up to 6m/min).
Despite the low melting range when compared to ferrous materials, aluminum welding requires about the same amount of heat due to the high thermal conductivity and melting heat. The shrinking of the aluminum upon solidification and cooling leads to a high occurrence of welding cracks, distortion and internal stresses. Basically, the material properties in the vicinity of the weld change (Fig. 1, Fig. 2).
The stresses can be reduced slightly by avoiding clusters of seams, by workpiece preheating or by subsequent shot peening. A significant reduction can, however, only be achieved by means of stress-relief annealing.
In artificially aged alloys, this results in a reduction in strength; if followed by artificial ageing, strength values can only be returned to those of state T5 (Table 1). Appropriate welding and design measures should be taken to avoid straightening welded structures wherever possible.
The liquid aluminum may absorb hydrogen from its environment which then precipitates during solidification. This results in pores of different sizes in the weld seams depending on the solidification rate (Fig. 3 and Fig. 4).
Shielding gas for welding
Welding of die cast parts
Hüttenaluminium-Gusslegierungen, product catalog, Aluminium Rheinfelden Alloys GmbH, 2011.
Aluminium Taschenbuch, Aluminium-Verlag, Düsseldorf, 2002.
DVS - Deutscher Verband für Schweißen und verwandte Verfahren e. V., DVS technical codes 0913-1 to -3, www.dvs-aft.de
DVS 0913-1 MIG welding of aluminium – Material-specific basics
DVS 0913-2 MIG welding of aluminium – Devices, processes, auxiliary materials
DVS 0913-2 MIG welding of aluminium – application-oriented notes