Aluminium casting alloys

Aluminium-copper casting alloys

 

These aluminum casting alloys, the main alloying element is copper.

They contain:

  • copper  of 4 to 5%
  • the usual impurities of iron and silicon, and
  • sometimes also small amounts of manganese.

The effect of copper

  • Copper substantially improves strength and hardness in the ascast and heat treated conditions of aluminium castings.
  • Alloys containing 4 to 5.5% Cu respond most strongly to thermal treatment and display relatively improved casting properties.
  • Copper generally reduces resistance to general corrosion.
  • Conversely, low concentrations of copper in aluminum-zinc alloys inhibit stress corrosion.
  • Copper reduces hot tear resistance and increases the potential for interdendritic shrinkage [2].

Aluminum-copper phase diagram

These alloys are thermally hardenable and can achieve fairly high strength and ductility, especially if they are obtained from ingots with an iron content of no more than 0.15%.


Figure 1 – Aluminum-copper phase diagram [1]


fazovaya-diagramma-alyuminiy-med

Figure 2 – Aluminium-rich fragment of aluminum-copper phase diagram

Single-phase alloys

  • Aluminum-copper alloys are single-phase.
  • Unlike aluminum-silicon alloys, there is no secondary phase with high fluidity, which is so useful in the final stages of solidification of castings.
  • When such a phase is present, it helps fill the voids that occur during shrinkage with metal, and also compensates for the stresses that arise in the casting during its solidification.

Difficult to cast

These alloys are more difficult to cast than, say, aluminum-silicon alloys. When working with them, it is necessary to take special measures to ensure the solidification of the metal from distant areas of the casting to hotter and more liquid areas, to the profits and then to the feeders.

When such due care is taken, these aluminum-copper alloys can be successfully used to produce castings with high strength and ductility. Note that more complex casting technology is also typical for other single-phase aluminum casting alloys.

Aluminum-copper alloys exhibit very poor casting properties and require more careful design of casting molds to obtain a good casting. These alloys are used mainly for sand casting. If there is a need to cast them into metal molds, then silicon is added to them to increase fluidity and reduce hot cracking. However, silicon additives significantly reduce the ductility of the casting material.

Copper content 7-8%

Aluminum-copper alloys with higher copper content (7-8%) were once the most popular. Currently, they have been almost completely replaced by aluminum-copper-silicon alloys. The only advantage of high copper content aluminum-copper alloys is their insensitivity to impurities. However, they have very low strength and very mediocre casting properties.

Copper content 9-11%

Aluminum-copper alloys, which contain 9-11% copper, have very limited use. They retain high strength at elevated temperatures and have high wear resistance, which is very attractive for use in aircraft cylinder heads and automotive cylinder blocks.

Very good strength at high temperatures is a characteristic property of aluminum alloys that contain copper, nickel and magnesium and sometimes also iron.

Sources:

  1. TALAT Lecture 1203 – Phase Diagrams / M H Jacobs – The University of Birmingham, UK
  2. Aluminum and Aluminum Alloys, ASM International, 1996
  3. Aluminum Alloy Castings: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy