Influence of Quench Rate on the Artificial Ageing Response of an Al-8Si-0.4Mg Cast Alloy

Abstract

The aim of the study is to present the influence of quench rate on the artificial ageing response of Al-8%Si-0.4%Mg cast alloy in terms of Brinell hardness and yield strength. The investigated material was produced by a gradient solidification technique and exhibited a microstructure that corresponds to the one of gravity die castings, with a dendrite arm spacing of approximately 25 µm. The study comprises two solution treatment temperatures, five quench rates and artificial ageing times exceeding 100 hours at 170 and 220 ⁰C. The microstructure and concentration profiles of Mg and Si were evaluated using energy and wavelength dispersive spectroscopy. Microstructural examination reveals an increment of solutes in the Al-matrix when higher solution treatment temperatures accompanied with high quench rates are applied and shows how both Si and Mg atoms have diffused towards the eutectic during quenching. Consequently, i.e. by increasing the levels of solutes and vacancies, the highest strength levels were realized. The study confirmed that quench rates above 2 ⁰C /s do not offer substantial strength improvement while quenching at lower rates resulted in a lower peak hardness and longer times to peak. IntroductionA T6 heat treatment is often conducted to obtain an increase in strength of cast Al-Si-Mg alloys. A T6 heat treatment consists of a solution treatment, a quench and an artificial ageing. The objective of quenching is to suppress precipitation upon cooling of the casting from the high solution treatment temperature to room temperature. A fast quench results in a high supersaturation of solute and vacancies and a high strength after ageing. If the cooling is slow, precipitates or second phases will form heterogeneously on dislocations or grain boundaries during cooling, which results in a reduction in supersaturation and concomitantly a lower strength after ageing. A high supersaturation as well as a high diffusion rate are needed for the precipitation to take place. These two properties have opposite temperature dependence and the highest nucleation and growth rates thereby occur at intermediate temperatures between 450 °C and 200 °C for most . The time spent in this temperature region during quenching should therefore be as short as possible to avoid precipitation.

Review of the Quench Sensitivity of Aluminium Alloys: Analysis of the Kinetics and Nature of Quench-Induced Precipitation

“…The quench sensitivity of AlSiMg cast alloys has been investigated by a range of researchers, and ex situ methods have generally been used to analyse quench-induced precipitation or its detrimental effects on the resulting properties e.g., [45,47,49,53,56]. None of these previous reports used in situ cooling DSC analysis.…”

“…In 2010 Shuey and Tiryakioglu published a reviewing chapter on "quenching of aluminium alloys" including experimental results on a broad range of alloys as well as theoretical options for their evaluation [44]. Different quenching paths have therefore been frequently used for samples, for instance by using different quenching media [45][46][47][48][49] or by applying Jominy end-quench tests [50][51][52][53]. Subsequent post-quench analysis of certain properties has led to the assessment of the quench sensitivity.…”