Aluminium casting alloys

Aluminium-magnesium casting alloys

All aluminum alloys are divided into wrought and cast alloys. The alloying elements in both wrought and cast alloys are the same, but in wrought alloys their content is much lower.

Casting aluminium alloys

The main alloying elements of cast aluminum alloys are

They provide a qualitative change in the nature of aluminum alloys.

  • In Al-Cu, Al-Mg and Al-Mg-Si alloys, intermetallic compounds are formed, and
  • in Al-Si alloys, eutectics are formed.

Intermetallic compounds, especially in combination with eutectics, make it possible to use various methods of thermal hardening. Other alloying elements – auxiliary and modifying – are used in much smaller quantities to improve the specified mechanical and physical properties of alloys.

Aluminum-magnesium alloys

Single-phase binary alloys

  • Aluminum-magnesium alloys are single-phase binary alloys with medium to high strength levels and good toughness properties.
  • The fact that they are single-phase means that they are not able to increase their strength as a result of heat treatment.

The main feature of these Al-Mg alloys is their high corrosion resistance, including in sea water and the marine atmosphere. The highest corrosion resistance is achieved with a minimum of impurities – both solid and gaseous. Therefore, these alloys are made from high-quality metals and with special care when melting and casting. These alloys are highly weldable and are often used in construction for decorative finishing. Aluminum-magnesium alloys are easy to cut and have an attractive appearance after anodizing.

Common compositions range from 4 to 10% Mg, and compositions containing more than 7% Mg are heat treatable. Instability and long-term room-temperature aging at higher magnesium concentrations can be avoided by heat treatment [2].

Difficult to cast

Compared to aluminum-silicon alloys, all aluminum-magnesium alloys have significantly more problems during casting. They require more careful mold design and higher solidification temperature gradients to produce good castings.

When casting these alloys, one must take into account their increased tendency to oxidize during melting. This is also important because many products made from these alloys require high surface quality and oxide defects are highly undesirable.

Impact of impurities:

  • Copper and nickel reduce corrosion resistance as well as ductility.
  • Iron, silicon and manganese reduce strength and ductility.
  • Tin reduces corrosion resistance.

Casting alloys series 5хх.х

In the American and international classification, aluminum-magnesium cast alloys form a series of alloys 5хх.х. Three of them are presented below.

Cast aluminum alloy 514.0

Alloy formula:

4Mg

Chemical composition:

copper: 0.15% max;
magnesium: 3.5-4.5%;
manganese: 0.35% max;
silicon: 0.35% max;
iron: 0.50% max.
zinc: 0.15% max;
titanium: 0.25% max;
others: each 0.05%, sum 0.15% max.;
aluminum: the rest.

Typical mechanical properties:

tensile strength: 145 MPa;
yield strength: 95 MPa;
relative elongation: 3%;
Poisson’s ratio: 0.33;
modulus of elasticity: 71.0 GPa.

Physical properties:

density: 2.65 g/cm3;
liquid temperature: 630 ºS;
solidus temperature: 585 oC.

Technological properties:

melting point: from 675 to 815 ºS;
casting temperature: from 675 to 790 ºS;
welding alloy – 4043.

Cast aluminium alloy 518.0

Alloy formula:

8Mg

Chemical composition:

copper: 0.25% max;
magnesium: 7.5-8.5%;
manganese: 0.35% max;
silicon: 0.35% max;
iron: 1.8% max;
nickel: 0.15% max;
zinc: 0.15% max;
tin: 0.15% max;
others: amount 0.25% max.;
aluminum: the rest.

Typical mechanical properties:

tensile strength: 310 MPa;
yield strength: 190 MPa;
relative elongation: 5-8%.

Physical properties:

density: 2.57 g/cm3;
liquid temperature: 620 ºS;
solidus temperature: 535 oC.

Cast aluminium alloy 520.0

Alloy formula:

10Mg

Chemical composition:

copper: 0.25% max;
magnesium: 9.5-10.6%;
manganese: 0.15% max;
silicon: 0.25% max;
iron: 0.30% max;
zinc: 0.15% max;
titanium: 0.25% max;
others: each 0.05%, sum 0.15% max.;
aluminium: the rest.

Typical mechanical properties:

tensile strength: 330 MPa;
yield strength: 180 MPa;
relative elongation: 16%.

Physical properties:

density: 2.57 g/cm3;
liquid temperature: 605 ºS;
solidus temperature: 450 oC.

magniy-metallMagnesium ingots for alloying aluminium alloys

Magnesium. Aluminium’s brother

Magnesium is similar to aluminium in many ways.

  • The density of magnesium at 20°C is 1.74 g/cm³ – it floats in liquid aluminium (density of liquid aluminium is 2.4 g/cm³).
  • The melting points of aluminium and magnesium are almost the same: for magnesium – 650 °C, for aluminium 99.5% – 657 °C. Therefore, magnesium is directly loaded into the melting furnace, unlike, for example, silicon.
  • Pure silicon has a high melting point, 1415 °C. For this reason, silicon is introduced into the aluminium melt, usually in the composition of silumin with a silicon content of about 12%.
  • This eutectic aluminium alloy Al-Si melts at a temperature of only about 577 °C.

Aluminum-magnesium phase diagram [2]

Sources:

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