Stability and shaping accuracy of aluminium alloy sheets during stretch forming

: Bräunlich, H.; Lailach, A.

Abstract
The more widespread use of aluminium and its alloys offers the possibility of increasing the intrinsic value of end-products and at the same time benefiting from the advantages of reduced weight. However, thin aluminium alloys sheets have deformation properties different from those of steel, which make special demands on the technology involved and the design of the tools and equipment used. Decisive parameters for the quality characterization of a workpiece are its stability, in the sense of resistance to bending or buckling, and its accuracy of shape. Thus, the aim of the technological investigations carried out was to explore the process variant of stretch forming as an alternative to conventional deep drawing, with a view to improving product quality in articles formed from sheets of medium and large area. Because of the strict demands on accuracy of shape and stability for such a component, an offset mirror for satellite antennae was chosen as the test object. The experimental res ults indicated that technological possibilities but also material- and tool-related special technical requirements exist in the deep drawing and stretch forming of aluminium alloys. It was demonstrated that the use of stretch forming in contrast to deep drawing can achieve clearly better shaping accuracy with lower tooling costs. For high-precision components the difference Delta r = r max - r min in the planar anisotropy values is decisively important for a forming result of acceptable quality. Apart from a confirmation of the effects of sheet thickness and yield strength, the main result of the stability investigations was the finding that conversion from steel to aluminium necessitates the multiplication of sheet thickness by a factor of 1.3 to 1.4 if a comparable bending resistance is to be attained, whereas with deep drawing the corresponding factor is 1.4 to 1.5. This conversion factor can be reduced still further by modifying the properties of the aluminium alloy used.