Development of Superplastic AlCuAgMgZr Alloys for Easy Forming

Abstract

Two aluminium alloys containing up to 6% Cu, 0.8% Ag, 0.7 % Mg and 0.4 % Zr (all in wt %) were continuously cast and hot rolled at 673 K. Thermomechanical treatment involving particle stimulated nucleation (PSN) process was used in order to obtain fine grain size. The microstructure studies after this treatment revealed the presence of subgrains of 0.5-2 μm in size with a misorientation of 3 - 15°. In spite of the presence of zirconium rich and copper rich particles the grain growth up to 10-20 μm was observed after annealing at temperatures above 673K. This was also observed in samples deformed at the temperature of 673K. It affected the superplastic deformation properties of the investigated alloys at higher temperatures in a negative way. The superplastic behaviour was characterised by the m value, which was determined by step strain - rate change tests. The tests were conducted at a slow initial strain rate, which increased stepwise to high constant strain rates. The highest m value obtained approached 0.7 for the lowest temperature (623K) and the strain rates (10 -3 - 10 -4 s -1 ). A value close to 0.5 was observed for temperatures 623-673K and the same strain rates. The lowest m values were found for temperatures T> 623K and higher strain rates. Structure studies of the superplastically deformed samples revealed grain growth and presence of cavities after elongation of a few hundreds percent. Presence of dislocations within grains indicates the participation of the crystallographic slip in the superplastic deformation at 673K.