Eutectic aluminium–silicon alloys are used for pistons, cylinder heads and valve lifters, parts that need both strength and stability at high temperature. A study published in the Founding Research Journal of the Iranian Foundrymen's Society examined how different amounts of nickel affect the microstructure and tensile strength of Al-12Si-1Mg-1Cu alloy.
Why nickel?
Adding nickel to aluminium alloys is a well-known way to improve high-temperature behaviour, and nickel has a long history in piston alloys. It forms stable intermetallic compounds with aluminium and copper that do not coarsen at high temperature and help retain strength. But their amount and shape matter: if they become coarse and brittle, they concentrate stress and start cracks. The aim was to find the nickel level that gives the most benefit.
How the study was done
The base alloy Al-12Si-1Mg-1Cu was made with 0.1, 0.3, 0.5, 1 and 3 wt% nickel and cast in a preheated metal mould. The microstructure was studied by optical and scanning electron microscopy with EDS and by X-ray diffraction, and tensile tests were run in the as-cast and heat-treated (T6) conditions.
Findings
- Finer silicon: nickel effectively refined the eutectic silicon particles.
- Nickel phases: depending on composition, phases such as γ-Al7Cu4Ni, δ-Al3CuNi, T-Al9FeNi and ε-Al3Ni, as well as Q-Al5Cu2Mg8Si6 and α-Al15(Mn,Fe)3Si2, were found between the dendrites. As nickel increased, some Al3Ni changed to Al3CuNi or Al7Cu4Ni and their shape changed from short strips to "Chinese script". In the 3% Ni alloy, Al3CuNi and Al3Ni were confirmed by X-ray diffraction.
- Tensile strength: as cast, ultimate tensile strength rose from 157 MPa in the base alloy to 220 MPa at 1% nickel and fell to about 195 MPa at 3% nickel.
- Heat treatment: after heat treatment all samples were stronger, and the 1% nickel alloy was again the best (about 295 MPa versus about 187 MPa for the base alloy).
Why more nickel is not better
At high nickel levels the intermetallics become coarser and more continuous. These brittle particles easily concentrate stress and cancel part of nickel's benefit, so the strength-versus-nickel curve has a peak, which in this study was at about 1% nickel.
The role of T6 heat treatment
Magnesium and copper make this alloy precipitation-hardenable. T6 treatment consists of solution treatment at high temperature, quenching and artificial ageing. The results showed that nickel's benefit survives heat treatment and the gap between the optimum alloy and the base alloy even widens, so nickel and heat treatment complement each other.
Industrial use
These results match the composition of commercial piston alloys, which usually contain about 1–3% nickel, and show that precise nickel control together with suitable heat treatment matters. For the foundry, a base ingot of accurate composition is the starting point; special elements such as nickel are usually added through master alloys. Read about silumin and aluminium–silicon alloys and T6 heat treatment.
If you need alloy ingot of a specific composition for automotive parts, talk to the Almas Aluminium team.
Frequently asked questions
Which commercial grade is Al-12Si-1Mg-1Cu comparable to?
It is close to piston alloys of the A332 / LM13 family, with about 11–13% silicon, about 1% copper and magnesium, and some nickel.
How is nickel added to the melt?
Usually as an aluminium–nickel master alloy, so that it dissolves evenly and in a controlled way.
Source
Gholamreza Ashuri, "Modification Effect of Nickel on Tensile Properties and Microstructural Characterization of Al-12Si-1Mg-1Cu Alloy" (in Persian), Founding Research Journal, Iranian Foundrymen's Society, 3(2), 2019, 113–121. DOI: 10.22034/FRJ.2019.128668.1035. Journal website
This article is the Almas Aluminium editorial team's summary of a published research paper. For the full experiments, tables and figures, please refer to the original paper.