Quench sensitivity of 7xxx series aluminium alloys

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 phase can contribute to strengthening. 3. It was shown that the surface layer of a 7150 thick plate is more sensitive to cooling rate than the centre layer. This can be ascribed to a relatively higher degree of recrystallisation in the surface layer. Therefore there are more nucleation sites for quench-induced precipitates in the same cooling condition. It was confirmed with DSC analysis that the precipitation heat values for the surface layer are higher than for the centre layer, demonstrating that there is more quench-induced precipitation occuring in the surface layer. 4. A number of CCP diagrams were developed for alloys 7085, 7037, 7020 and 7055 with systematic changes in alloy composition. It is demonstrated that the quench sensitivity generally increases with increasing total alloy content. Therefore alloy 7020 exhibits the lowest quench sensitivity when compared with the other studied alloys. It is noted that the amount of M-phase increases with increasing Zn content. Therefore alloys 7085, 7037 and 7055 can precipitate more M-phase during slow cooling than alloys 7150 and 7020. However, the precipitation of S-phase during continuous cooling was found to be significantly suppressed with increasing Zn/Mg ratio. Quench factor analysis (QFA) also indicates that higher k3 values can be found for the higher Zn/Mg ratio alloys 7085 and 7037, suggesting that the energy required to form a nucleus is higher in these two alloys. This is in agreement with experimental observations that these two alloys contained less S-phase and exhibited relatively low quench sensitivities. It was shown that the CCP diagrams provide a useful method of quantifying the quench sensitivities of different alloys. The CCP diagrams can also facilitate engineers to design and optimise industrial cooling process more effectively. However, it requires a deep understating of the quench-induced precipitation to develop a reliable CCP diagram for a given alloy, i.e. what types of quench-induced precipitation reactions happen, where they preferentially nucleate and how many much precipitation occurs during the cooling process. The current research therefore shows that the quench sensitivity of a 7xxx alloy can be decreased by decreasing the degree of deformation, decreasing the fraction of recrystallisition, decreasing the total grain boundary area, decreasing the number of incoherent dispersoids, decreasing the total alloying content, increasing the Zn/Mg ratio and decreasing the Cu content