Aluminum 7475 - AL-P7475 - EN AW-7475 - EN AW-AlZn5,5MgCu(A) - AL6

An Al-Zn-Mg-Cu composition registered in 1969

7475 is a wrought aluminium alloy first registered in 1969. Zinc, magnesium and copper are its main alloying elements.

Its composition specifies lower iron and silicon limits than those of 7075: 0.12% and 0.10%, respectively, compared with 0.50% and 0.40% for 7075.

Why limit iron and silicon?

ASM classifies 7175 and 7475 as higher-purity versions of 7075, developed in particular to improve transverse ductility in thick products. Reducing iron and silicon content is part of this approach.

In high-strength aluminium alloys, coarse intermetallic particles can provide preferential paths for damage. Reducing iron and silicon content can limit this effect and improve toughness.

This relationship is also apparent in the material itself. In 7475-T7351 plate, coarse intermetallic particles are associated with fatigue crack initiation. The low iron and silicon limits specified for 7475 therefore contribute to the alloy’s toughness. However, they are not sufficient to predict a plate’s behaviour: its microstructure, manufacturing process and temper must also be considered.

7175 and 7050: other developments from 7075

7475 is not the only alloy developed from 7075. 7175 also follows a higher-purity approach. 7050 belongs to the same Al-Zn-Mg-Cu family, but its design notably replaces chromium and manganese with zirconium to reduce quench sensitivity.

Comparison with 7075 highlights one of 7475’s main advantages: in equivalent tempers, 7475 plate has typical fracture toughness values around 40% higher.

This is achieved without unduly compromising corrosion resistance or fatigue performance. According to Alcoa, 7475’s corrosion resistance and fatigue performance are comparable to, and in some cases better than, those of other high-strength aerospace alloys, including 7075, 7050 and 2024.

T651, T7351, T7651: distinct properties

Alcoa provides data for plate in the T651, T7351 and T7651 tempers, as well as bare and Alclad sheet in T61 and T761. The producer recommends T7351 for its resistance to stress corrosion cracking. T7651 is intended to provide exfoliation resistance and improved resistance to stress corrosion cracking, with slightly lower strength than T651.

A choice of trade-offs, not a ranking

These tempers do not all aim for the same balance of properties. A study of 7475 in the T651, T7651 and T7351 tempers shows that the more heavily overaged T7351 has lower strength but greater resistance to fatigue crack growth and stress corrosion cracking than T7651 under the conditions studied.

The choice of temper therefore depends on the required balance between static strength, toughness, crack growth and corrosion behaviour.

Plate strength and fracture toughness

L-T and T-L identify the test orientations and are part of the conditions associated with these values.

What the T651/T7351 comparison shows

This comparison quantifies the trade-off between strength and toughness. Over the common thickness range of 25.43 to 38.10 mm, T7351 has a minimum tensile strength around 9% lower than T651 and a minimum yield strength around 12% lower.

The trend reverses for toughness. Between 31.75 and 38.10 mm, the minimum K_IC increases from 33.0 to 44.0 MPa√m in the L-T orientation, an increase of around 33%. In T-L, it rises from 30.8 to 36.3 MPa√m, an increase of nearly 18%.

These figures illustrate a trade-off: some static strength is sacrificed in exchange for greater resistance to unstable crack growth. From a damage-tolerance perspective, maximum strength is not the only parameter considered.

Physical and thermal properties

Temper affects more than mechanical properties. T7351 also has higher thermal and electrical conductivity than T651.

The same nominal composition can therefore exhibit different physical properties depending on its heat-treatment condition.

Machining and forming by temper

For 7175/7475 flat products in the T651 and T7351 tempers, Kaiser assigns a B rating for machinability and a D rating for cold working. D is the lowest rating on Kaiser’s cold-working scale, while B places machinability towards the top of its relative ranking.

Ohnistova’s study focuses more specifically on milling specimens taken from a 70 mm T7351 plate. Fatigue tests were conducted under cyclic tensile loading at 10 Hz, with a load ratio of R = 0.1. The authors found that cutting conditions and tool inclination influenced the results, but coarse intermetallic particles remained the main sites of crack initiation.

The behaviour of a 7475 component after machining depends on the material’s microstructure and the conditions used to produce its surface.

Fatigue and corrosion: different routes to crack initiation

Studies of 7475 identify several routes to crack initiation that should not be conflated. Under mechanical loading, the study cited above observed crack initiation at coarse, brittle intermetallic particles.

These particles can also play a role in localised corrosion. A study of 7475-T7351 observed dissolution of the matrix around Al₇Cu₂Fe particles, associated with pit formation.

In other tests conducted in an aqueous sodium chloride solution, pits themselves became crack initiation sites under low alternating stress. The authors also reported shorter fatigue life and faster crack growth in the corrosive environment studied.

Composition, microstructure and environment can therefore act at different stages of the damage process. However, these studies use different specimens and test procedures. Their results do not establish a single causal chain applicable to every 7475 component, nor can they be used directly to predict the life of a component in service.

Welding: distinguishing processes and test results

In Kaiser’s 7175/7475 datasheet, T7351 receives a D rating for gas and arc welding: according to this classification, no commonly used method has been developed. Spot welding receives a B rating, which implies the need for special techniques or applications that justify preliminary testing.

Why distinguish FSW from fusion welding?

Friction stir welding, or FSW, raises different considerations from gas or arc welding. It is a solid-state joining process: the material is heated and stirred without fully melting. TWI notes that this avoids several defects associated with melting and solidification, and enables the joining of 2xxx and 7xxx alloys that are difficult to fusion weld.

FSW trials have been carried out on 2 mm 7475-T76 sheets joined in a butt configuration. In this study, the tool rotated at 950 rpm and travelled at 110 mm/min.

Tests on four specimens taken from defect-free regions gave an as-welded tensile strength of 465 MPa, or 92% of that of the base metal. However, the study also reported internal defects at the start of some welds.

Why do these properties matter in aerospace?

Fuselage and wing skins, spars and fuselage bulkheads are among the applications for 7475 sheet and plate.

These applications explain the relevance of the properties discussed above. In critical aircraft structures, static strength is not the only criterion considered. Assessments of fatigue and damage tolerance also take into account repeated loading, fracture mechanics and environmental effects on materials and structures.

This helps explain the overall profile of 7475. It combines high strength with controlled toughness and offers several tempers that shift the balance towards resistance to stress corrosion cracking, exfoliation resistance or higher strength. Its value therefore lies in the combination of these characteristics rather than in any single property.