There is increasingly more activity in the field of aluminum bumpers for automobiles and other vehicles. The advantages of aluminum are immediately obvious in that it offers relatively high strength at a substantial weight savings, the weight savings being important both from the standpoint of performance and ecology considerations not to mention substantial fuel savings.
An aluminum product to serve properly as a bumper needs to possess substantial strength, resistance to denting and resistance to stress corrosion cracking. Additionally, it should lend itself to relatively easy fabricating procedures. The significance of the first and second is immediately obvious and the significance of the third revolves around the inevitable exposure of the bumper to salt or other corrosive media which can cooperate with mechanical loads to initiate cracks in an aluminum member, which cracks can later lead to failures. For instance, in employing a bumper jack to raise an automobile as in replacing a tire, a stress corrosion cracking failure can lead to disastrous results and is therefore considered a highly critical aspect in selecting a bumper product. With respect to the fabricating procedures, the aluminum alloy should not require extremely high press loads in the case of extrusion, or have unusually slow extrusion rates in order to be commercially feasible. It would also be useful if the material could be chemically brightened so as to alleviate the need for chrome plating, although this is a less significant aspect since chrome plating is neither excessively expensive nor does it introduce other problems which are difficult to cope with. The major problem to date has been to develop an alloy and fabrication sequence which will produce a bumper having relatively high strength, preferably 56,000 psi tensile yield strength or higher and the requisite resistance to stress corrosion cracking. Some materials satisfy either but not both of these requirements and some even satisfy both, but at substantial expense or difficulty in fabrication so as to either create an undesirable rejection rate problem or other economically undesirable aspects.