Improvements in quench factor modelling

In this contribution, the validity of a number of key quench factor analysis (QFA) assumptions is discussed. It is shown that the incorporation of a square root dependency of yield strength on precipitate volume fraction provides a sounder physical basis for quench factor modelling. Peak-aged strength/hardness prediction accuracies are not affected, but C-curve positions are. It is also demonstrated that transformation kinetics are described more correctly by a modified Starink–Zahra equation than by a Johnson–Mehl–Avrami–Kolmogorov type equation, yielding better prediction accuracies when a physically realistic Avrami exponent of 1.5 or greater is used. Finally, a regular solution model is introduced to quantify the influence of the solute solubility temperature-dependency on the minimum strength. These improvements are all implemented within the framework of classical QFA

The effect of homogenizing on the quench sensitivity of 6082

Hardness data for cast, homogenised, solutionised and subsequently peak aged 6082 samples that had undergone a range of homogenizing and quenching treatments can be modelled well using a recently derived model. Hardness increases with homogenizing temperature and time for all quenched conditions. In contrast with extruded 6082 alloys, the homogenizing condition has little effect on quench sensitivity, even though the density of dispersoids, which act as nucleation sites for non-hardening precipitates, decreases markedly on increasing homogenizing temperature

An age hardening model for Al–7Si–Mg casting alloys

Yield strength (YS) ageing curves have been modelled for A356 and A357 aluminium casting alloys below the solvus temperature of the main hardening precipitate. Predictions are based on the Shercliff and Ashby methodology (Acta Metall. Mater. 38 (1990) 1789) for wrought alloys. Differences between strengthening in wrought and cast Al–Si–Mg alloys are considered. A Brinell hardness to YS conversion incorporating strain hardening has been established to enable YS ageing curves to be predicted with reduced experimental effort

An integrated hardening model for the heat treatment of extruded 6082

Hardening in extruded aluminium alloy 6082 has been modelled as a function of solution treatment temperature, quench rate, ageing temperature and ageing time. Solution treatment and ageing temperature effects are treated using a regular solution model with reference to the Al-Mg2Si quasi-binary phase diagram. Quench factor analysis is applied to approximate time-temperature-transformation (TTT) curves, enabling the prediction of hardness values for both step quenching and continuous cooling treatments. Age hardening is modelled using concepts from existing approaches (such as the Shercliff and Ashby age hardening model [H. R. Shercliff and M. F. Ashby, Acta Metall. Mater., 1990, 38, 1789]), as well as novel elements taking account of the effects of solution treatment temperature and quench rate on ageing kinetics. The scope, accuracy and limitations of the model are discussed

Predicting the quench sensitivity of Al-Zn-Mg-Cu alloys: a model for linear cooling and strengthening

In this work the quench sensitivity of Al-Zn-Mg-Cu alloys is studied through continuous cooling at constant rates of a range of alloys using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and hardness testing. The DSC, TEM and SEM data show that the cooling reactions are dominated by a high temperature reaction (typically ~450 °C down to ~350 °C) due mostly to S-Al2CuMg phase formation, a medium temperature reaction (~350 °C down to ~250 °C) due predominantly to ?-Mg(Al,Cu,Zn)2 phase formation and a lower temperature reaction (~250 °C down to ~150 °C) due to a Zn-Cu rich thin plate phase. A new, physically-based model is constructed to predict rates of all reactions, enthalpy changes and resulting yield strength in the artificially aged condition. The model incorporates a recently derived model for diffusion-controlled reactions based on the extended volume fraction concept as well as recent findings from first principles modelling of enthalpies of the relevant phases. The model shows a near perfect correspondence with data on all 6 alloys studied extensively by cooling DSC and hardness testing, and allows prediction of the influence of the 3 major elements and 3 dispersoid forming elements on quench sensitivity

A model for the thermodynamics of and strengthening due to co-clusters in Al-Mg-Si based alloys

An expanded model for the thermodynamics of co-clusters and their strengthening is presented, and the model is applied to predict co-cluster formation and strengthening in Al-Mg-Si alloys. The models are tested against data on a wide range of Al-Mg-Si alloys aged at room temperature. The strengthening due to co-clusters is predicted well. The formation of the co-clusters is studied in an Al-0.5at%Mg-1at%Si alloy using three-dimensional atom probe analysis. The results correspond well with the model. It is shown that in general, (short-range) order strengthening due to co-clusters will be the main strengthening mechanism in these alloys. Apart from the co-clusters, Si clusters also form, but due to their low enthalpy of formation, they contribute little to the strength