ABSTRACT7xxx (Al-Zn-Mg-Cu) alloys develop their strength, through the controlled decomposition (ageing) of a supersaturated solid solution to produce a precipitation hardened microstructure. The supersaturated solid solution is normally produced by water quenching from the solution heat treatment temperature. The kinetics of phase transformations occurring during cooling from the solution heat treatment temperature are limited at small undercoolings by the small chemical free energy, and as the undercooling increases, by reduced rates of substitutional solid state diffusion. This gives rise to the characteristic C shape of time-temperature-property (TTP) curves. The TTP curve for 7010 has been evaluated by a interrupted quench method into a salt bath at temperatures ranging from 450 to 210°C. A TTP curve for 7050T76 sheet data has then been used to predict tensile property inhomogeneity in large 7010 forgings with known cooling rates.
The Jominy End Quench has been used extensively to measure the hardenability of steels as it provides a method for studying a large number of different quenching conditions within a single test specimen, but has not been extensively used with non-ferrous alloys. Quench factor analysis of aluminium alloys usually involves recording the cooling curves of a large number of specimens as they are quenched into a salt bath set at various temperatures for varying lengths of time, to determine a range of constants for the time temperature property C-curve equation. This can be laborious and time consuming. To minimize the amount of effort and time required for Quench Factor Analysis, the authors applied the Jominy End Quench test to an aluminium 7000 series alloy and measured the Vickers hardness along the length of the specimen. Finite element analysis was used to accurately predict the cooling curves at regular intervals along the length of the Jominy specimen. Quench Factor Analysis was conducted and was found to accurately predict the Vickers hardness of the alloy with a standard error of 0.6%. Keywords: Aluminium alloys, 7000, Quench Factor Analysis, Jominy End Quench IntroductionThe Jominy End Quench test has been extensively used to determine the hardenability of steels and has an associated ASTM standard: ASTM-255. The test involves heating a standard cylindrical bar (25.4mm in diameter and 102mm in length) to the proper austenitizing temperature and then transferring it to a quenching fixture so that the specimen is held vertically 12.7mm above an opening through which a column of water is directed against the bottom of the specimen. This results in a progressive decrease in the rate of cooling along the length of the bar. After the specimen has been a quenched, parallel flats 180° apart are ground on the specimen surface and hardness measurements are taken along the length of the specimen. (Ref Steels Honeycombe). While this test has seen widespread use in the steel industry there was limited work conducted on aluminium alloys and other non-ferrous alloys. T'Hart et al used the Jominy test to study the effect of the cooling rate on the Vickers hardness, electrical conductivity, corrosion and microstructural properties of several high strength aluminium alloys (ref x2). More recent publications have promoted the use of the Jominy End Quench test for the aluminium alloys as a simple test that can provide a wealth of information regarding quench sensitivity, microstructural characterization and alloy development (MacKenzie publications). Mackenzie also uses the Jominy end quench and quench factor analysis for predicting properties and how process variables, such as delay time before ageing and the ramp rates during ageing can affect the final properties of the alloy (ref).
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