The heat treatment of Al–Si–Cu–Mg casting alloys

Abstract

Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter. Increasing the knowledge about the heat treatment process is one task in the direction towards this goal. The aim of this work is to investigate and model the heat treatment process for Al-Si casting alloys. Three alloys containing Mg and/or Cu were cast using the gradient solidification technique to achieve three different coarsenesses of the microstructure and a low amount of defects.Solution treatment was studied by measuring the concentration of Mg, Cu and Si in the α-Al matrix using wavelength dispersive spectroscopy (WDS) after various times at a solution treatment temperature. A diffusion based model was developed which estimates the time needed to obtain a high and homogenous concentration of alloying elements for different alloys, temperatures and coarsenesses of the microstructure. It was shown that the yield strength after artificial ageing is weakly dependent on the coarseness of the microstructure when the solution treatment time is adjusted to achieve complete dissolution and homogenisation.The shape and position of ageing curves (yield strength versus ageing time) was investigated empirically in this work and by studying the literature in order to differentiate the mechanisms involved. A diffusion based model for prediction of the yield strength after different ageing times was developed for Al-Si-Mg alloys. The model was validated using data available in the literature. For Al-Si-Cu-Mg alloys further studies regarding the mechanisms involved need to be performed.Changes in the microstructure during a heat treatment process influence the plastic deformation behaviour. The Hollomon equation describes the plastic deformation of alloys containing shearable precipitates well, while the Ludwigson equation is needed when a supersaturated solid solution is present. When non-coherent precipitates are present, none of the equations describe the plastic deformation well. The evolution of the storage rate and recovery rate of dislocations was studied and coupled to the evolution of the microstructure using the Kocks-Mecking strain hardening theory.

High temperature tensile deformation behavior and failure mechanisms of an Al–Si–Cu–Mg cast alloy — The microstructural scale effect

“…The increase in Cu concentration in the dendrite center, illustrated in Figure 3(a) could be a consequence of dissolution of Al2Cu phase (the most abundant intermetallic) or fragmentation of other Cu-bearing phases such as Q-Al5Mg8Si6Cu2, which is noted in other studies [7,26]. A large fraction of the Al2Cu phases in the 10 μm microstructure dissolved at 500 ºC, and the solute concentration reaches 2.22 wt.…”

“…Si precipitates from the as-cast super saturated solid solution during aging at temperatures range of 200-250 °C [6]. High temperature exposure of Al-Si-Cu-Mg alloys may lead to changes in the concentrations of alloying elements and of vacancies in solid solution, and to spheroidization of Si particles [7]. The dissolution rate of the various secondary phases is dependent on time and temperature of the exposure, the chemical composition and coarseness of the microstructure.…”

Evolution of Microstructure in Al-Si-Cu System Modified with a Transition Element Addition and its Effect on Hardness

“…The Figure 7c illustrates a close up of these precipitates which have been reported partially coherent and metastable phases with body-centered tetragonal crystal structure. The lattice parameters are a=b=0.406 nm, c=0.58 nm and space group I4/mcm 2,24,26,27 . The Figure 7b illustrates EDS elemental microanalyses of aluminum matrix and acicular precipitates and confirms the presence of Cu and Al in these precipitated phases.…”

“…The effects of heat treatments in Al-Si-Cu alloys have been studied by different techniques from different authors 1,2,6,7 . The authors reported that β-Mg 2 Si, θ-Al 2 Cu, Al 2 CuMg, and Al 4 CuMg 5 Si 4 phases are dissolved during the SHT and are highly responsible of hardening of Al-Si-Cu-Mg, e.g.…”

Effect of Different Variants of Heat Treatment on Mechanical Properties of the AlSi17CuNiMg Alloy

“…Standard heat treatment of the silumins, comprising copper and magnesium additions, is precipitation hardening, consisting of two operations: solutioning and artificial or natural ageing [6], what enables much better mechanical properties of the castings with simultaneous decrease of the elongation and ductility [7,8,9].…”