Influence of interrupted quenching on artificial aging of Al–Mg–Si alloys (original) (raw)
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Mechanisms controlling the artificial aging of Al–Mg–Si Alloys
Acta Materialia, 2011
In this study the artificial aging behavior of the Al-Mg-Si alloy AA 6061 was investigated in the temperature range 150-250°C using atom probe tomography, hardness and resistivity measurements for various thermal histories. It was found that the precipitation kinetics and age-hardening response of artificial aging at temperatures below 210°C are lowered by prior natural aging but enhanced above this temperature. An analysis of hardness data was used to evaluate the temperature dependence of precipitation kinetics and dissolution processes. Supported by theoretical considerations, it is assumed that artificial aging of Al-Mg-Si alloys is controlled via the concentration of mobile vacancies. The "vacancy-prison mechanism" proposed determines the mobile vacancy concentration in the case of natural pre-aging by temperature-dependent dissolution of co-clusters and solute-vacancy interactions.
Materials Science and Engineering: A, 2019
In this study we demonstrate the complementarity of transmission electron microscopy (TEM) and atom probe tomography (APT) in studying the early stages of phase decomposition in the age-hardening Al-Mg-Si alloy AA 6016. Samples are subjected to natural ageing at ambient temperature or artificial pre-ageing at elevated temperature in order to produce different types of atomic clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their number density, size and compositions, whereas TEM is applied to analyse and quantify number density, sizes and types of the hardening precipitates during artificial ageing. Finally, the particle statistics derived by APT and/or TEM were utilized to predict the mechanical properties of the various samples and conditions analysed.
Positive effect of natural pre-ageing on precipitation hardening in Al-0.44 at% Mg-0.38 at% Si alloy
Ultramicroscopy, 2009
Al-Mg-Si alloy Positive strengthening effect Three-dimensional atom probe Micro-hardness a b s t r a c t Age hardening in a purely ternary Al-Mg0.4-Si0.4 (0.44 at% Mg, 0.38at%Si) alloy that is similar to AA6060 was investigated by hardness measurement, TEM and three-dimensional atom probe (3D-AP). In particular, the effect of natural pre-ageing before artificial ageing, which is known to have a positive effect in this alloy, was studied by comparing three different conditions: natural ageing only, artificial ageing for 1.5 h at 180 1C only and combined natural pre-ageing and subsequent artificial ageing for 1.5 h at 180 1C. Natural ageing influences the mechanical properties significantly. Naturally aged alloys exhibit a hardening response that is governed by the presence of small clusters. Subsequent artificial ageing of naturally aged specimens increases the value of peak hardness, which is attributed to the increase of the number density of needle-shaped precipitates as compared to the samples without natural ageing. It is assumed that besides these precipitates, the small Si clusters formed at room-temperature storage remain stable during artificial ageing.
Reversal of the negative natural aging effect in Al–Mg–Si alloys
Acta Materialia, 2012
The effect of solute clustering on subsequent precipitation has been investigated for three different Al-Mg-Si alloys with similar Mg/Si ratios but different Mg + Si contents. Hardness measurements were carried out (i) during various natural aging (NA) times at room temperature, and (ii) after a fixed artificial aging (AA) heat treatment preceded by various NA times. Comparison between the two hardness plots stressed an intimate connection between solute clusters and precipitates: for the NA times investigated (between a few seconds and several months), transient phenomena with short (less than 10 h) lifetimes were observed, both occurring at earlier times and with higher relative importance for increasing solute concentration. Upon NA + AA, these transient phenomena were found to be responsible for the well-known "negative NA effect", as evidenced by a close correlation between the respective lifetimes. In particular, the negative NA effect itself was found to be transient, and was reversed upon long storage times by the dominance of a process beneficial to precipitation. Transmission electron microscopy studies of selected alloy conditions ensured that the recorded hardness variations reflected variations in the alloy microstructure. Attempts were made to couple the transient clustering phenomena with solute strain field interactions. Preliminary calculations following this line of thought suggested a competitive nature of both Mg-Mg and Mg-Si interactions, in contrast with the usual view of Mg-Mg repulsion. An alloy hardness optimization procedure as a function of storage time, temperature and alloy composition has been suggested.
A systematic comparison of static and dynamic ageing of two Al–Mg–Si alloys
Aluminium alloys containing Mg and Si as the major solutes are strengthened by precipitation of the metastable precursors (β ) of the equilibrium β (Mg 2 Si) phase. In this study, both static and dynamic ageing of two Al-Mg-Si alloys-the 6061 (Al-1.34% Mg 2 Si) and 6069 (Al-2.25% Mg 2 Si) alloys-were aged at 170 • C and compared to evaluate the potential for improving the mechanical properties. Dynamic ageing was conducted through the procedure of equal channel angular extrusion (ECAE). It is shown that the ageing time scale is reduced from ∼1000 min for conventional static peak-ageing to ∼10 min by using ECAE-aided dynamic ageing. Compared to the significant strengthening effect in static peak-ageing treatment, a notable further increase in ultimate tensile strength (UTS) is achieved by dynamic ageing: over 40 MPa for the 6061 alloy and 100 MPa for the 6069 alloy. Ductility of dynamically aged alloys is found to be comparable to that of the statically peak-aged samples. Microstructures of both statically and dynamically aged alloys were characterised using transmission electron microscopy; dislocation-assisted precipitation was observed to be the primary precipitate nucleation and growth mechanism during dynamic ageing process. It is concluded that dynamic ageing using ECAE is efficient in executing ageing treatment that results in superior mechanical properties of the Al-Mg-Si alloys.
Study on Quenching and Artificial Ageing on Al-Si Alloy
Aluminum alloys have special advantages on the structural changes induced by thermal treatments. Thus, for proper machining a solution quenching is applied, when the hardness and the tensile strength of the alloy decreases. In order to restore the properties natural or artificial ageing is necessary.
Impact of the Direct Ageing Procedure on the Age Hardening Response of Al-Mg-Si 6101 Alloy
Materials, 2018
Al-Mg-Si alloys are used not only as construction material, but also as a material for electrical conductors. For this application, it is crucial for the alloy to achieve a balance between strength and electrical properties. This is achieved in practice by a combination of strain and precipitation hardening. The current paper focuses on a heat treatment procedure in which the EN AW 6101 alloy is cooled by a flowing air stream from the solutionizing temperature down to the artificial ageing temperature. The proposed procedure, unlike the common heat treatment leading to the T6 temper, allowed for the precipitation of the coarser β” phase with the presence of relatively wide precipitate-free zones. The age hardening response was investigated by Brinell hardness measurements, eddy current testing and microstructural observations using transmission electron microscopy (TEM). The applied heat treatment resulted in slightly lower strength (compared to the T6 temper), but improved electric...
Quench sensitivity of Al-Mg-Si alloys: A model for linear cooling and strengthening
This work studies the quench-induced precipitation during continuous cooling of five Al-Mg-Si alloys over a wide range of cooling rates of 0.05-2 • 10 4 K/min using Differential Scanning Calorimetry (DSC), X-ray diffraction, optical-(OM), transmission electron-(TEM) and scanning electron microscopy (SEM) plus hardness testing. The DSC data shows that the cooling reactions are dominated by a high temperature reaction (typically 500 °C down to 380 °C) and a lower temperature reaction (380 °C down to 250 °C), and the microstructural analysis shows they are Mg2Si phase formation and B' phase precipitation, respectively. A new, physically-based model is designed to model the precipitation during the quenching as well as the strength after cooling and after subsequent age hardening. After fitting of parameters, the highly efficient model allows to predict accurately the measured quench sensitivity, the volume fractions of quench induced precipitates, enthalpy changes in the quenched sample and hardness values. Thereby the model can be used to optimise alloy and/or process design by exploiting the full age hardening potential of the alloys choosing the appropriate alloy composition and/ or cooling process. Moreover, the model can be implemented in FEM tools to predict the mechanical properties of complex parts after cooling.
Influence of the thermal route on the peak-aged microstructures in an Al–Mg–Si aluminum alloy
Scripta Materialia, 2013
The influence of the thermal route on the peak-aged microstructures in an Al-Mg-Si aluminum alloy was investigated by atom probe tomography and transmission electron microscopy. Direct artificial aging induced a monomodal size distribution of b 00precipitates, while for artificial aging after natural pre-aging a bimodal size distribution was found. This difference can be explained by the impact of lattice defects on the b 00 -nucleation. b 00 is supposed to contain Al and exhibits a high Mg/Si ratio in the studied alloy.