Aluminium casting alloys

Aluminium-zinc-magnesium casting alloys

Aluminum-zinc-magnesium alloys

Aluminum alloys alloyed with zinc and magnesium are

  • Thermally strengthened during the process of normal aging, that is, natural aging.
  • In this case, maximum strength is achieved 20-30 days after casting and when kept at room temperature.
  • This hardening process can be accelerated by artificial aging.
  • These alloys do not require high heating temperatures for quenching and rapid cooling as other thermally hardenable alloys, for example, aluminum-copper and aluminum-silicon-magnesium alloys.
  • However, due to problems with microsegregation of the magnesium-zinc phase in these alloys, they are usually subjected to rapid solidification.

Aluminum casting alloys series 7хх.х

According to the American classification, these alloys belong to the 7xxx.x series. They have medium to high strength properties. Annealing ensures good dimensional stability. The eutectic point of alloys of this group is high, which is favorable for parts that are subject to soldering. These alloys have good machinability. They are characterized by high corrosion resistance with some tendency to stress corrosion. They are not recommended for use at elevated temperatures. The strength properties of these alloys increase at room temperature for several weeks after casting as a result of strengthening through the mechanism of release of the secondary phase. This process continues after several weeks, but at a decreasing speed. Alloys 707.0, 771.0 and 772.0 can be treated with T6 and T7 heat treatments.

Foundry properties

The casting properties of cast aluminum alloys of the aluminum-zinc-magnesium system are very mediocre. Therefore, to obtain good castings, tight control of solidification conditions is required. To prevent shrinkage defects, cooling with moderate and high temperature gradients is used. Sand casting is most often used for these alloys, but significant difficulties can be encountered when casting into permanent molds.

Cast aluminum alloy 771.0

Alloy formula:

7Zn-0.9Mg-0.13Cr

Chemical composition:

copper: 0.10% max;
magnesium: 0.8-1.0%;
manganese: 0.10% max;
silicon: 0.15% max;
iron: 0.15% max;
chromium: 0.06-0.20%;
zinc: 6.5-7.5%;
Tin: 3.5% max;
titanium: 0.10-0.20%;
others: 0.05% each, 0.15% total max;
aluminum: the rest.

Typical mechanical properties (T5):

tensile strength: 290 MPa;
yield strength: 260 MPa;
relative elongation: 1.5%;
modulus of elasticity: 71.0 GPa.

Physical properties:

density: 2.823 g/cm3;
liquidus temperature: 645 ºС;
solidus temperature: 605 ºС.

Heat treatment

This alloy can be heat treated to T2, T5, T51, T52 and T71 states

Heat treatment for T5 condition:

  • exposure at 180 ºС for 3-5 hours;
  • cooling outside the oven in still air.

Heat treatment for condition T51:

  • exposure at 205 ºС for 6 hours;
  • cooling outside the oven in still air.

Heat treatment for T6 condition:

  • exposure at 580-595 ºС for 6 hours;
  • cooling outside the oven in still air to room temperature;
  • aging by exposure for 3 hours at a temperature of 130 ºС;
  • cooling in still air.

Machinability

Alloy 771.0 in T5 condition has good stability and machinability. It can be milled 5 times faster and drilled 2 times faster than alloys such as 356.0 and 319.0.

Welding

Can be welded by gas tungsten arc welding or metal arc welding using 5356 aluminum welding alloy. Depending on the thermal state of the casting after welding, special heat treatment may be necessary.

In T5 condition, welding is not performed. For subsequent welding, heat treatment is performed to condition T51.

fazovaya-diagramma-alyuminiy-cink

Figure – Aluminum-zinc phase diagram [2]

 Resources:

  1. Aluminum and Aluminum Alloys, ASM International, 1996
  2. Aluminum Alloy Castings: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy