Influence of quenching temperature on peak aging time and hardness of Al-Mg-Si-Cu alloys strengthened by nano-sized precipitates

Abstract

The quenching temperature plays a rather important role in the subsequent artificial aging behavior and resulting hardness of the Al-Mg-Si (or Al-Mg-Si-Cu) alloys, but related reports are limited. This paper presents a detailed investigation about the influence of the quenching temperature (ranging from 490 °C to 570 °C) on the precipitate microstructure, precipitation rate and aging hardness of a typical Al-Mg-Si-Cu alloy 6061. Methods such as TEM microscopy, real-time electrical resistivity monitoring and Vickers hardness testing were employed. The TEM results show that the main precipitate in peak aged status is needle shaped β″ with sizes of several nanometers in cross section and ten to twenty nanometers in length. It gradually grows up in size and transforms to Q′ phase in the over aged status. Hardness and resistivity measurement results show that the hardness increases while the resistivity decreases as the artificial aging proceeds. With higher quenching temperature, the size of the β″ phase gets finer and its total amount becomes more. Its precipitation kinetics also turns out to be much higher. The time for reaching the peak aging hardness prolongs from about 2–10 h as quenching temperature increases from 490 °C to 570 °C. But these effects becomes not so obvious when quenching temperature exceeds 550 °C, pointing out that it is the critical quenching temperature for this kind of alloy. Related mechanisms are explained from the aspects of super saturation solid solution level, quenched-in vacancy concentration, precipitate nucleation, growth rate etc. These results establish a significant heat-treatment guideline for mass production of these alloys.