Metal forming production of semi-finished aluminium-magnesium compounds

Abstract

The increasing demand for efficiency and energy savings can be satisfied by innovative lightweight construction, especially in the automotive and mechanical engineering sectors. One approach lies in the use and development of magnesium alloys for industrial applications, something which is made more difficult due to magnesium's high affinity for oxygen and its subsequent low corrosion resistance. In order to ensure corrosion protection appropriate to the requirements, thicker coatings which cannot be created using conventional coating processes need to be produced and the current preference is to fabricate these from aluminium. An additional aim is to create a protective layer to resist mechanical effects. This, based on its dimensioning and consequent higher percentage of mass, is required to contribute towards absorbing the bending and torsion load in the material bond. In addition, a substance to substance bond needs to be created between the cover and the core material. A bond with the required strength can only be expected if the transportation of material across the interface is made possible between the partners for bonding. Under specific technological conditions, it has proved possible to provide evidence of the formation of a distinctive diffusion zone between a wrought magnesium alloy AZ31 and a standard aluminium alloy EN AW-6060 [1]. Together with the chemical and contact conditions, the following restrictions, among others, are also important when it comes to triggering a solid body reaction of this nature, namely, almost "ideal" contact between the reactants, a high level of disorder in the lattice structure as well as the supply of a certain amount of energy (the so-called threshold energy) [2]. Consequently, a fundamental principle for the creation of a bond using metal forming is the targeted interaction of technological parameters which make the maximum increase in the interface surface possible under high standard compressive stresses. Another important aspect is the forming temperature which not only has a decisive influence on the flow behaviour of the partners for bonding but also supplies the necessary amount of energy to activate transport of the material. Under these conditions it was also possible to identify appropriate basic evidence of very rapid diffusion processes during forming, including the forming processes of extrusion moulding and lateral extrusion [3; 4]. The current results as far as indirect extrusion moulding is concerned show that a peripheral bond forms in the interface but that this is weakened by cracks. By applying hydrostatic extrusion moulding it proved possible to prevent inhomogeneities of this nature; however, it may be assumed that, as far as fault-free bonding is concerned, the process window is highly susceptible to error. For this reason provision has been made for tests with intermediate zinc layers for indirect extrusion moulding to enable their influence to be assessed under conditions that are less than ideal when it comes to achieving a perfect bond. Of major interest is the development of phases using an intermediate zinc layer in the interface, as well as their characteristics in the bonding system described