Warm forming of aluminium/magnesium alloy sheet

Abstract

At this time, when energy conservation is gaining popularity, the increased use of aluminium sheets for motor-car body construction to reduce the overall weight of the car, and so use less fuel, is appropriate. Pure aluminium, as used in the days of hand-made bodies, is now quite out of the question for the mass-produced car, due to inherent softness, and currently a number of medium-strength alloys are used. In the 1950's an aluminium alloy containing 3–4% magnesium was generally employed for press-forming panels for the more expensive range of cars. This was a non-heat-treatable strain-hardening alloy, but inevitably was of reduced cold ductility, because of the alloying addition. In the 1970's, an interest in warm forming emerged and it was discovered that an aluminium/6% magnesium alloy could give a total elongation of about 300% at about 250°C at the appropriate strain-rate. The work here reported covers the whole range of commercial aluminium/magnesium alloys — 0 to 6.6% — at temperatures from 20 to 300°C over a range of strain rates, and tests were carried out in both uniaxial and biaxial tension. In uniaxial tension, (i) the higher the magnesium content, (ii) the lower the strain-rate, and (iii) the higher the temperature, the greater is the elongation. Similarly to superplasticity, this elongation is m-value dependent, i.e. the higher the value of m the greater the available elongation. Another interesting finding is that prior cold rolling of the sheet before warm forming is beneficial to total elongation. The limited number of biaxial tests carried out indicate that in this deformation mode the material is less sensitive to temperature and strain rate than in uniaxial tension, suggesting that practical exploitation may be possible without sophisticated temperature and press-speed control.