Abstract:
Magnesium based alloys are finding increasing applications even in structural components of automobiles due to their light weight, 36 % lighter than even aluminum. The AZ91 alloy with containing 9.5% Al, 1.4% Zn and 0.26 % Mn, has been cast and homogenized initially at 375°C for 2 hrs and then at 415°C for 14 hrs. (The ageing behaviour of homogenized AZ 91 alloy is studied at 185°C). The thermomechanical ageing behaviour of homogenized AZ 91 alloy has been studied by pre-ageing the alloy at 185 °C, followed by warm working at the same temperature before ageing it further. The ageing behaviour of the homogenized alloy has also been determined for comparison by simply ageing the alloy at 185 °C. The pre-ageing has been carried out by ageing up to 25% and 50% of increase in hardness at peak of hardness by simple ageing. Pre-ageing has been followed by warm working at 185 °C to introduce 10% and 20% deformations.
The microstructure of the cast AZ 91 alloy contains massive 13-phase (Mg, 7A112) originating from the divorced eutectic. The extent of homogenization is not adequate as the microstructure of the homogenized alloy contains some undissolved 13-phase. Ageing of the AZ 91 alloy shows that hardness increases with time till it attains a peak hardness of 98 VHN, which has been attributed to progressive dispersion hardening contributed by continuous precipitation of 13-phase. The overageing may be due to Oswald ripening. Pre-ageing during thermomechanical ageing is expected to follow similar continuous precipitation as observed during ageing but warm working results in discontinuous precipitation of 13-phase near grain boundary. Further ageing following pre-ageing and warm working results in increasing hardness with increasing time till the peak hardness is attained. Beyond the peak hardness, the hardness decreases. Thermomechanical ageing has resulted in maximum peak hardness of 106 VHN for an alloy subjected to 50 % pre-ageing and 20 % deformation. Thus, there is only marginal increase in hardness by thermomechanical ageing. But there is significant decrease in time required to attain peak hardness, which decreases from 14 hrs in normal ageing to 12 hrs for the alloy subjected to 50 % pre-ageing and 20 % deformation by warm working.
