Wrought Aluminum Alloys for General and Special Applications: Part One

Aluminum is introduced into use in all industrial fields. Demands for lighter and stronger semi products and products are driving force for continuous improvement of standard alloys in regard of composition, properties, and manufacturing technologies. The competition with non-metallic materials demands further development of new aluminum alloys.

Characteristic properties of aluminum and its alloys are low specific density, wide interval of strength properties, good workability, corrosion resistance, electrical and thermal conductivity, harmlessness for human health, and recyclability, and therefore aluminum is ranked among the most important materials today and in the future. It is used in all the essential engineering fields, as transport, civil engineering, electrical engineering, electronics, machinery, equipment and for packing.

Aluminum constitutes numerous alloys with various elements like magnesium, manganese, silicon, zinc, copper, iron and lithium. With various composition and suitable combinations of heat treatments and mechanical working, semi-finished products and products alloys with a wide range of mechanical and chemical properties can be produced.

The main groups of aluminum alloys which are the most often used in practice besides technically pure aluminum are AlMn, AlMg, AlMgMn, AlMgSi, AlZnMg, and AlZnMgCu alloys. These are wrought alloys which are shaped into products by rolling, extrusion, and forging. Each of the mentioned groups consists of numerous subgroups, depending on amounts of main and additional alloying elements, and they have tensile strength values varying in a wide range from 70 to 600 MPa.

The tendency with standard wrought aluminum alloys is to achieve better strength values, which imposes a great challenge for metallurgists. In this group, there are mainly alloys of AlCuMg and AlZnMgCu type. With the later, the strength values of over 600 MPa at plane-strain fracture toughness of 30 MPa√m have been achieved. These alloys are used for the most demanding purposes like vehicles and airplanes, due to their high strength/weight ratio.

Further improvements are in progress for alloys for sections, rods, tubes (AlMgSi, AlCuMg, free-cutting alloys), for deep drawing purposes, for heat exchangers, and for wrapping materials. Research efforts to optimize production processes and properties of alloys will be continued in future, since many of existent alloys are completely suitable for general use.

7000 Series

Aluminum alloys of 7000-series are known for their high strength which renders them suitable for applications such as structural components for aircraft or for tooling plates; they have a combination of improved strength and toughness properties, reduced hot crack sensitivity during welding and, when in an artificially aged condition, a hardness of more than 180 HB. Alloys AA7075 and AA7055 are examples of this type of alloy and have achieved widespread use in aerospace applications because of their high strength and other good characteristics.

Alloy AA7055 comprises 7.6-8.4% of Zn, 1.8 to 2.3% of Mg, 2.0 to 2.6% of Cu, 0.08-0.25% of Zr, below 0.10% Si and below 0.15% Fe, the balance being aluminum together with incidental elements and impurities. Alloy AA7075 comprises 5.1 to 6.1% of Zn, 2.1 to 2.9% of Mg, 1.2 to 2.0 % of Cu, 0.18 to 0.28 % Cr, below 0.40% Si, below 0.50% Fe and below 0.30% Mn, the balance being aluminum together with incidental elements and impurities.

When artificially aged to its highest strength, which usually involves a treatment period of 20 hours or more at a relatively low ageing temperature of between 100 and 150°C, the alloy is obtained in a condition which is commonly referred to as T6 temper condition. In this condition however, alloys AA7075 and similar alloys are susceptible to stress corrosion cracking ("SCC"), exfoliation corrosion ("EXCO") and intergranular corrosion ("IGC").

Aluminum Alloys for Car Body Sheets

Low density, good workability, strength, and corrosion resistance of aluminum alloys correspond well to demands for the materials for bodies of cars and other road vehicles. These properties also fit to ever stricter environmental regulations regarding pollution and simple and cheap recycling.

Aluminum in cars presents mainly the reduction of weight, and thus reduces environmental problems related to the consumption of fuel. Ten percent lighter vehicle saves 5% of fuel. Weight reduction of only 1 kg reduces CO₂ emission for 20 kg for 170 000 passed kilometers.

Chemical compositions and properties of some alloys are given in tables 1 and 2.

Replacement of steel sheet with an aluminum one means 40 to 50% lower weight of a car body. Lower stiffness of aluminum sheets is one of the rare disadvantages in comparison with steel, but it can be compensated with up to 50% thicker sheet which simultaneously increases the safety in cars. Strength values of aluminum materials for car bodies are slightly lower than those of deep-drawing steel sheets. Regarding higher thickness of aluminum sheets to achieve stiffness and due to age-hardening which can occur during lacquering, the obtained strength values are comparable with those of steel.

Table 1: Chemical compositions of some aluminum alloys for car body sheets (mass %).

Manufacturing of aluminum alloy sheets for car bodies is more costly than manufacturing steel sheets, because of lower ability for deep drawing, greater number of working steps, and higher needed quality of tools. From the viewpoint of weight reduction, energy balance, and recycling, the usage of aluminum has already today a great advantage in the lifecycle of cars.

Table 2: Mechanical properties of some alloys for car-body sheets.

Various manufacturers developed numerous alloys for car body sheet. Their origin is based on standard wrought alloys.

Alloys of the AlMg group (AA5xxx) are not-age-hardenable, and they belong to materials with medium strength values. Workability and strength values depend on the magnesium content: increased magnesium content increases strength and reduces workability. Sheets in soft temper can be better stretch formed and deep drawn in comparison to AlMgSi alloys. Most frequently the AlMg2.5, AlMg3, and AlMg5Mn alloys are used. Some alloys of the AA5xxx group contain also copper. Copper increases strength values due to age-hardening effect. Simultaneously copper reduces corrosion resistance because of Al₂CuMg precipitates.

Alcoa has developed a frame-space concept for car bodies manufacturing, which is based on hollow sections. The sections are made of AlMgSi (AlMgSi0.5) alloys and manufactured at 20:1 extrusion ratio. Sections are joined with the nodes made of AlSi (AlSi10Mg) alloys by pressure die casting. The frame has weight of 130 to 150 kg.

Aluminum car body manufactured this way consists of 20% of castings, 25% of extruded parts, and 55% of sheets. Further development of aluminum materials for car bodies will be oriented towards the improved workability and increased strength properties, the yield stress being at the first place. The aim of further development is to downgauge and to reduce dimensions of extruded parts in order to obtain further reduction of weight and costs.