Quench sensitivity of 7xxx series aluminium alloys

by Yong Zhang

Institution: Monash University
Department: Department of Materials Engineering
Year: 2014
Keywords: Quench sensitivity; Continuous cooling precipitation (CCP) diagram; 7xxx series aluminium alloys; Phase transformation
Record ID: 1062671
Full text PDF: http://arrow.monash.edu.au/hdl/1959.1/982881


The quench sensitivity of high strength 7xxx series aluminium alloys becomes an increasingly important issue as the product thickness increases. Due to the nature of thermal conduction, the centre layer of a thick aluminium plate experiences a slower cooling rate than the surface. This can lead to variations in properties such as strength, corrosion resistance and fracture toughness across the thickness of a plate. The objective of this work was to study the quench-induced precipitation behaviour in different 7xxx alloys with systematic changes of alloy composition and processing history. Detailed microstructural analysis has been carried out using SEM, EBSD, TEM and HAADF-HRTEM. Vickers hardness values, electrical conductivities and the precipitation heats determined from differential scanning calorimetry (DSC) were measured for a wide range of different cooling rate conditions to evaluate the different quench sensitivity behaviours. The main findings from this work are as follows: 1. The results show that the Al3Zr dispersoids in recrystallised grains can become preferential nucleation sites for quench-induced precipitates in air cooling condition. The microstructural characterisation shows that the Al3Zr dispersoids tend to maintain their metastable structure and orientation when recrystallisation occurs. This causes them to lose coherency with the matrix after recrystallisation and thereby causes them to become more potent heterogeneous nucleation sites for quench-induced precipitation. It should be noted that incoherent Al3Zr dispersoids are not the only preferential sites for quench-induced precipitates. They can precipitate out at grain/subgrain boundaries in air cooling condition. However, the current research still demonstrates that materials containing a higher fraction of recrystallisation will lead to a significant increase of quench sensitivity. 2. The precipitation heats for quench-induced precipitates have been studied using a specialised DSC technique over a wide range of cooling rate conditions. A continuous cooling precipitation (CCP) diagram for commercially produced alloy 7150 has been developed based on a combination of DSC measurements, microstructure analysis and hardness testing. The results show that there are three main quench-induced precipitation located in different temperature ranges. It is demonstrated that the high temperature reaction from about 350 to 470 °C corresponds to S-phase (Al2CuMg), the medium temperature reaction from about 200 to 400 °C corresponds to M-phase (MgZn2) and the low temperature reaction from about 250 to 150 °C corresponds to a unique platelet phase containing both Cu and Zn. The critical cooling rates were determined to be 3 K/s for the S-phase, 10 K/s for the M-phase, and 300 K/s for the unique platelet phase in alloy 7150.The platelet phase has not been previously reported for alloy 7150. The platelets precipitate with a high aspect ratio and have a hexagonal structure (a=0.429 nm, c=1.385 nm) according to HAADF-STEM images. It is shown that this platelet…