Precipitation Hardening at Elevated Temperatures above 400 °C and Subsequent Natural Age Hardening of Commercial Al–Si–Cu Alloy

Abstract

The precipitation of intermetallic phases and the associated hardening by artificial aging treatments at elevated temperatures above 400 °C were systematically investigated in the commercially available AC2B alloy with a nominal composition of Al–6Si–3Cu (mass%). The natural age hardening of the artificially aged samples at various temperatures was also examined. A slight increase in hardness (approximately 5 HV) of the AC2B alloy was observed at an elevated temperature of 480 °C. The hardness change is attributed to the precipitation of metastable phases associated with the α-Al15(Fe, Mn)3Si2 phase containing a large amount of impurity elements (Fe and Mn). At a lower temperature of 400 °C, a slight artificial-age hardening appeared. Subsequently, the hardness decreased moderately. This phenomenon was attributed to the precipitation of stable θ-Al2Cu and Q-Al4Cu2Mg8Si6 phases and their coarsening after a long duration. The precipitation sequence was rationalized by thermodynamic calculations for the Al–Si–Cu–Fe–Mn–Mg system. The natural age-hardening behavior significantly varied depending on the prior artificial aging temperatures ranging from 400 °C to 500 °C. The natural age-hardening was found to strongly depend on the solute contents of Cu and Si in the Al matrix. This study provides fundamental insights into controlling the strength level of commercial Al–Si–Cu cast alloys with impurity elements using the cooling process after solution treatment at elevated temperatures above 400 °C.

Evolution of Microstructure and Elevated-Temperature Properties during Thermal Exposure with Transition Elements (V, Zr and Mo) in Al–Si 356 Type Cast Alloys

“…6(a). Meanwhile, TEM elemental distribution maps were applied in the present work to differentiate the various particles, 22) and the elemental distribution maps in base-T7A is shown as one example in Fig. 6(b)6(f ).…”

Optimization of Heat Treatment Parameters of AlSi7Mg Alloy

“…However, very often, they require improvement of their mechanical properties due to high operational requirements for heavily loaded components that must feature a high degree of reliability. An effective method of hardening materials, enabling considerable improvement of their mechanical properties, uses the phenomena of decreasing solubility of alloying elements in the solid-state as the limit temperature decreases [ 1 , 2 , 3 , 4 ]. The treatment to which the material is subjected consists of two stages.…”

“…During the second one (aging), precipitation of the supersaturating component in the form of finely dispersed precipitates occurs. In the case of alloys containing Cu or Mg, the hardening occurs due to the precipitation of Al 2 Cu Mg 2 Si and Al 2 CuMg phases [ 1 , 5 , 6 , 7 , 8 ].…”

“…The form of the eutectics during the heat treatment has a significant effect on the susceptibility of eutectic silicon to coalescence and rounding. The phenomena of coalescence and rounding of the eutectic silicon occur more easily and faster when the particles of eutectic silicon are more dispersed prior to the heat treatment, thus enabling shortened time of heat treatment of the alloy [ 1 , 5 ]. To this end, modifications in a furnace are performed, where micro-additives (modifiers), which impact the morphology of silicon, are added to the liquid alloy.…”