On the extraordinary low quench sensitivity of an AlZnMg alloy

The scope of this work was to investigate the quench sensitivity of a high-purity wrought aluminum alloy Al6Zn0.75 Mg (in this work called 7003(pure)). This is compared to a similar alloy with the additions of Fe, Si, and Zr at a sum less than 0.3 at.% (in this work called 7003(Fe,Si,Zr)). Differential scanning calorimetry (DSC) was used for an in situ analysis of quench induced precipitation in a wide range of cooling rates varying between 0.0003 and 3 K/s. In 7003(pure), three main precipitation reactions were observed during cooling, a medium temperature reaction with a distinct double peak between 325 and 175 degrees C and a very low temperature reaction starting at about 100 degrees C. An additional high temperature reaction related to the precipitation of Mg2Si starting at 425 degrees C has been observed for 7003(Fe,Si,Zr). In terms of hardness after natural as well as artificial aging, alloy 7003(pure) shows a very low quench sensitivity. Hardness values on the saturation level of about 120 HV1 are seen down to cooling rates of 0.003 K/s. The as-quenched hardness (5 min of natural aging) shows a maximum at a cooling rate of 0.003 K/s, while slower and faster cooling results in a lower hardness. In terms of hardness after aging, 0.003 K/s could be defined as the technological critical cooling rate, which is much higher for 7003(Fe,Si,Zr) (0.3-1 K/s). The physical critical cooling rates for the suppression of any precipitation during cooling were found to be about 10 K/s for both variants.