Heat-Treatable Wrought Aluminum Alloys and Applications

Heat-Treatable Wrought Aluminum Alloys

Heat-treatable wrought aluminum alloys possess medium-high mechanical strength, which is significantly higher than that of non-heat-treatable alloys.

2xxx Series: High-Strength Al-Cu Alloys

The 2xxx series (Al-Cu) utilizes copper as the primary alloying element, often with additions of Mg, Mn, Si, and Ni (as well as Ti, Cr, or Zr). Ternary Al-Cu-Mg alloys, known as "duralumin," exhibit high solubility at high temperatures.

• High-Temperature Aging: This process results in superior mechanical properties, with the maximum temperature limited by the solidus temperature (T-solidus).
• Overheating: Excessive heat leads to low Ultimate Tensile Strength (UTS) and reduced ductility due to intergranular cracks and high intergranular corrosion.

Properties of 2xxx Alloys

• High tensile strength at room and elevated temperatures.
• Low corrosion resistance (specifically intergranular corrosion); often requires coating with high-purity aluminum or 6xxx series alloys.
• Limited weldability.

Applications

These alloys are used for structural parts requiring high specific strength, particularly in aircraft for fuselages, wings, and other structural components.

6xxx Series: Versatile Al-Mg-Si Alloys

Known as "Avial Alloys," the 6xxx series contains Mg (0.6-1.2%) and Si (0.4-1.3%). The primary hardening phase is Mg2Si.

Properties of 6xxx Alloys

• Medium tensile strength (lower than the 2xxx and 7xxx series).
• Excellent corrosion resistance and good thermal conductivity.
• Good formability and excellent extrudability.
• Good weldability (superior to the 2xxx and 7xxx series).
• By adding low-melting-phase elements such as lead, bismuth, and/or tin, 6xxx series alloys show very good machinability.

Applications

Used in structural, architectural, and building applications, including heat exchanger tubes, doors, and bars. They are also used in transport for automotive outer body panels and railcars.

7xxx Series: High-Stress Al-Zn-Mg-Cu Alloys

The 7xxx series contains Zn (4-8%), Mg (1-3%), and Cu (1-2%). The high solubility of Zn and Mg in aluminum leads to the formation of MgZn2 precipitates, which significantly increases yield strength (YS).

Applications

These alloys are ideal for structures submitted to high stress, including aircraft structural parts, space exploration, military, and nuclear applications. They are also used in building applications and sports equipment like ski poles and tennis rackets.

8xxx Series: Specialized Aluminum Alloys

The 8xxx series includes alloys with various other elements, encompassing both heat-treatable and non-heat-treatable varieties.

• Al-Ni-Fe Alloys: Provide good corrosion resistance at high temperatures and good electrical conductivity.
• Al-Sn-Ni-Cu Alloys: Offer antifriction properties, making them suitable for bearings.
• Al-Li Alloys: Lithium (density = 0.53 g/cm³) provides extreme lightness for the aerospace industry. These offer higher fatigue strength than the 2xxx series but are more expensive.

Applications

The beams that support loads in the passenger floor of aircraft are frequently made of extruded Al-Li.