Improved quench sensitivity in modified aluminum alloy 7175 for thick forging applications

A commercial high-strength aluminum alloy 7175 is modified as an attempt to retain the higher hardening capability in heavy forgings. The effects of alloying modification, primarily Zn:Mg ratio, on the age-hardening response and the quench sensitivity are investigated using differential scanning calorimetry, transmission electron microscopy, and measurements of mechanical properties and electrical conductivity. As a results, the modified alloy (lean composition but with higher Zn:Mg ratio) is less quench-sensitive compared to the conventional alloys (usual alloying but with a lower Zn:Mg ratio). In the fast quench, the higher Zn:Mg ratio tends to accelerate the decomposition rate to form more stable phases, giving a lower strength with higher electrical conductivity. In the slow quench, the higher Zn:Mg ratio promotes homogeneous precipitation, while the lower Zn:Mg ratio leads to the greater amount of heterogeneous η precipitation on the Cr-dispersoids. The enhanced natural aging plays a crucial role in the subsequent artificial aging in the higher Zn:Mg ratio alloy. Results of the mechanical properties and electrical conductivity tests also ensure the microstructural homogeneity of the modified alloy, which is uniquely applicable in thick 7175 forgings.