The present invention relates to a hot and cold rolling process for manufacturing aluminum strip or sheet characterized by improved weldability, in particular aluminum strip about 0.1 to 0.7 mm thick for use in motor car bodies.
The use of aluminum sheet for manufacturing car body parts such as doors, engine bonnets, boot lids and wings is well known. Because of planned measures to save energy, in particular in view of the existing laws in the United States of America, vehicles will have to be made lighter. The increased use of strong and readily formable aluminum alloys for car body parts is therefore strongly anticipated.
Spot welding is widely used for joining components in car manufacturing, the lifetime of the electrodes used for welding steel sheet being about 10,000-15,000 welds. The corresponding number of welds with aluminum sheet is at present around 100-150 welds. After that, the copper electrodes have to be cleaned to remove the aluminum which has alloyed itself with the copper.
The reason for the greater degree of alloying between aluminum and copper than between copper and steel is, besides the physical and thermodynamic properties of copper and aluminum which cannot be changed, the relatively marked variation in contact resistance between the copper electrode and the aluminum sheet. This is due mainly to the thickness of the oxide layer and the composition of the surface layer. It is well known that the contact resistance of aluminum alloys has a great influence on the ease of spot welding, the main reason for irregularity in welding and the short life-times of the electrodes used with untreated aluminum sheet being the high and non-uniform contact resistance at the point of current transfer. These affect energy conversion in the secondary circuit and cause marked variation in the quality of the joint.
Surface treatments such as brushing, sand blasting, wet jetting and caustic etching of the aluminum before spot welding markedly reduce the contrast resistance. However, in the case of aluminum sheet, the rolled surface, when subjected to those treatments, becomes very rough. This affects the uniformity of the surface and detracts from the appearance of the part after lacquering.
It is therefore the principal object of the present invention to develop a process for producing readily formable aluminum strip and sheet characterized by superior spot welding properties and furthermore housing a surface which is suitable for shaping and lacquering.