Casting alloys 

Casting alloys of aluminum-zinc-magnesium

Aluminum alloys are wrought and cast. The starting ingot wrought alloys treated by methods of metal forming – rolling, pressing (extrusion) or forging.

and wrought, and cast aluminum alloys have, basically, the same alloying elements. However, their content in wrought alloys is much lower., than in foundries. The main alloying elements are magnesium,, copper, silicon and zinc. Other alloying elements – auxiliaries and modifying – used to improve the physical and mechanical properties of castings from cast aluminum alloys.

Casting alloys of aluminum-zinc-magnesium

Aluminum casting alloy, alloyed with zinc and magnesium, thermally hardened during normal aging, that is, natural aging. In this case, maximum strength is achieved through 20-30 days after casting and at room temperature. This hardening process can be accelerated by artificial aging..

In principle, to achieve optimum properties, these alloys do not require a high temperature for quenching and rapid cooling like other thermally hardened alloys, for example, aluminum-copper and aluminum-silicon-magnesium alloy. However, due to problems in these alloys with microsegregation of the magnesium-zinc phase, they usually undergo rapid solidification..

Aluminum Cast Alloys Series 7xxx.x

According to the American classification, these alloys belong to the 7xx.x series. They have medium to high strength properties.. Annealing ensures good dimensional stability.. The eutectic point of the alloys of this group is high., which is good for details, which are soldered. 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 hardening by the mechanism of separation of the secondary phase. This process continues after several weeks., but with decreasing speed. To alloys 707.0, 771.0 and 772.0 heat treatments of type T6 and T7 can be applied.

casting properties

The casting properties of cast aluminum alloys of the aluminum-zinc-magnesium system are very mediocre. Therefore, to obtain good castings, strict control of the solidification conditions is required.. To prevent shrinkage defects, cooling with moderate and high temperature gradients is used.. Sand casting is most commonly used for these alloys., and when casting in constant forms, you can encounter significant difficulties.

A casting aluminum alloy 771.0

Chemical composition

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 %;
  • lead: 3,5 % Max.;
  • titanium: 0,10-0,20 %;
  • others: 0,05 % each, 0,15 % in total max .;
  • aluminum: rest.

Properties: mechanical and physical

Typical mechanical properties (T5):

  • tensile strength: 290 MPa;
  • yield point: 260 MPa;
  • relative extension: 1,5 %;
  • elastic modulus: 71,0 GPa.

Physical properties:

  • density: 2,823 g / cm3;
  • liquidus temperature: 645 ºS;
  • solidus temperature: 605 oC.

Heat treatment

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

Heat treatment for T5 condition:

  • shutter speed at 180 ° C within 3-5 hours;
  • cooling outside the furnace in calm air.

Heat treatment for condition T51:

  • shutter speed at 205 ° C within 6 hours;
  • cooling outside the furnace in calm air.

Heat treatment for condition T6:

  • shutter speed at 580-595 ° C within 6 hours;
  • cooling outside the oven in calm air to room temperature;
  • aging by aging for 3 hours at temperature 130 ºS;
  • in still air cooling.

Machining by cutting

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

Welding

It can be welded by gas arc welding with a tungsten or metal electrode using an aluminum welding alloy 5356. Depending on the thermal condition of the casting after welding, special heat treatment may be necessary..

In state T5, welding is not performed. For subsequent welding, heat treatment is performed for the state of T51.

fazovaya-diagramma-alyuminiy-cink

Figure – Aluminum-zinc phase diagram

Sources:

Aluminum and Aluminum Alloys, ASM International, 1996

Aluminum Alloy Castings: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy