Microstructure, corrosion behavior and mechanical properties of a non-isothermal ageing treated cast Al–4.5Cu–3.5Zn–0.5Mg alloy

Abstract

Al–Cu, being a high-strength aluminium alloy, is used to prepared castings which are widely used in the automotive and aerospace industries as lightweight parts. However, corrosion is an issue. The Al–Cu alloys show improved properties when other elements are added to them. One such alloy is Al–4.5Cu–3.5Zn–0.5Mg. In order to improve the corrosion resistance and strength of this Al–4.5Cu–3.5Zn–0.5Mg cast alloy, a novel non-isothermal ageing (NIA) treatment was developed that comprised a heating stage up to 250 °C, followed by a cooling stage down to room temperature at the rate of 60 °C·h ^−1 . Specimens were removed throughout the process and immediately quenched for morphological, mechanical, and electrochemical testing. The hardness continuously increased up to 124 HV with ageing time. The alloy exhibited optimal properties after ageing for ~340 min (with the aging temperature reaching 130 °C during the cooling stage of the NIA treatment), with a tensile strength and maximum corrosion depth of 395 MPa and 165 μ m, respectively. Fine precipitates discontinuously appeared at the grain boundaries during the cooling stage. Some new fine Ω phases were precipitated in the grains, thereby narrowing the precipitation-free zone. Thus, high strength and good corrosion resistance of the alloy can be obtained via the NIA treatment. Notably, NIA treatments are less time-consuming than isothermal ageing treatments, thereby expanding the applications of high-strength cast aluminium alloys in the manufacturing industry.

Keywords

• Cast Al–4.5Cu–3.5Zn–0.5Mg alloy
• non-isothermal ageing
• tensile strength
• corrosion resistance
• Ω phase
• precipitation-free zone