High strength aluminium alloys of the 7xxx Series

Selection criteria

To the 7xxx series belong aluminium alloys whose main, but in general not only, alloying element is Zinc (Zn).
This series includes two different and well defined families of alloys:

• Alloys with mechanical properties ranging from medium to high, whose chemistry includes, besides zinc, other elements, among which Copper (Cu), but in small amounts, never over 1% weight; some alloys of this family can be readily fusion welded and do not show any particular problem with Stress Corrosion Cracking (SCC). These alloys are used typically in constructions, frames for bikes and motorbikes, welded structures in general.
• Alloys with very high mechanical properties, whose chemistry includes, besides Zinc, other elements, among which Copper, in amounts over 1%; These alloys were in general originally developed for aerospace, and deploy the maximum mechanical properties that can be achieved in the field of aluminium alloys. They are not fusion weldable, and in some tempers show important problem with SCC. They are used typically in those applications, for which the strength to weight ratio is of paramount importance. This paper refers to this family of alloys.

Driving parameters for the selection

The selection of the proper material should be done in order to ensure that all the design requirements are met at the lowest overall cost; the technical parameters determining the selection are many; in the following, three of them will be discussed:

• Mechanical strength (static, fatigue);
• Resistance to Stress Corrosion Cracking;
• Fracture toughness.

Three alloys are considered, whose chemistry is shown in the following table:  

Alloy	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti
EN-AW-7075	0.40	0.50	1.2 2.0	0.30	2.1 2.9	0.18 0.28	5.1 6.1	0.20
EN-AW-7475	0.10	0.12	1.2 1.9	0.06	1.9 2.6	0.18 0.25	5.2 6.2	0.06
EN-AW-7050	0.12	0.15	2.0 2.6	0.10	1.9 2.6	0.04	5.7 6.7	0.06

EN-AW-7075

High strength Al-Zn-Mg-Cu, available in the form of sheet, plate, rod and bar, in heat treatment tempers type T6, T73 and T76.
It is the most popular among the high strength alloys of the 7xxx series, specially in the field of general machinery.
T6 tempers show the maximum mechanical strength, but on the other hand the lowest fracture toughness and, with the exception of thin sections, poor resistance to Stress Corrosion Cracking.
T73 tempers show lower mechanical strength, but resistance to SCC is much higher than with T6 tempers. T76 tempers show intermediate mechanical strength and limited (but in any case better respect to T6 tempers) resistance to SCC.

EN-AW-7475

High strength Al-Zn-Mg-Cu alloy, developed for applications requiring mechanical strength not lower than 7075, but better properties in terms of fracture toughness.
Used originally in aerospace, specially for rotating wing machines, where high vibrations levels are typical, found recently applications in the field of top level racing cars.
This alloy is currently produced in form of sheet and plate only. The typical heat treatment tempers are the same as 7075 alloys; the same considerations are applicable.

EN-AW-7050

High strength Al-Zn-Mg-Cu-Zr alloy, developed to achieve optimum trade-off between high mechanical strength, good resistance to SCC and elevated fracture toughness, specially in the field of thick sections, where the two former alloys show an important decay of the properties.
The alloy is currently produced in the form of plates and extruded rods and bars, although extrusions are not so usual in the european market.
Used in aerospace for highly stressed and critical parts, at the moment the alloy is not used for general machinery (although byproducts of this alloy, in T6-type tempers, are used for the production of moulding dies). Heat treatment tempers are of T7-type; The most popular is T74.

Mechanical strength

To perform the stress analysis of a component, the following parameters are of interest:

• Ultimate tensile stress Rm (MPa), at room temperature and at elevated temperatures
• Tensile yield stress Rp0.2 (Mpa)
• Elastic Modulus in tension E (Mpa)

The values of these properties come out from:

• Chemical analysis of the alloy
• Type of product (extruded bar, hot rolled plate, etc.)
• Thickness of semi-finished product
• Heat treatment temper
• Metallurgical direction. (*)

The following tables show the design mechanical properties at room temperature for the three alloys, for three different thicknesses of the plate.

7075 - Plates
Thickness mm	50			100			150
Temper	T651	T7351i	T7651	T651	T7351	T7651	T651	T7351	T7651
Rm         L	514	446	480	452	412
LT	521	453	487	459	419		360
ST	480	425	446	418	391
Rp0.2    L	452	357	405	384	329		240
LT	439	357	412	370	329
ST	407	336	384	343	315
E	70650

Resistance to Stress Corrosion Cracking

The Stress Corrosion Cracking (SCC ) is the build-up of intergranular cracks due to the combined influence of tensile stress and corrosive environment (for high strength aluminium alloys the usual outdoor environment are a corrosive environment).

Tensile stress can be originated by sustained (persistent) external loads or by internal phenomena (stress due to heat treatment, machining, forming, assembly).

The consequences of SCC are generally catastrophic, since it produces sudden failures caused by static overload due to the reduction of the load bearing section and to notch effect, or it starts fatigue failures.
This phenomenon is highly hazardous since most of the surface of the component under attack is perfectly undamaged, and cracks propagate locally, with very small production of corrosion products.

The measures to undertake in order to prevent the onset of the phenomenon are:

• Select materials with no or low sensitivity to SCC
• Reduce internal stresses
• Eliminate o reduce the corrosive attack by the use of suitable geometric design of the component (avoid cavities and crevices in which the aggressive medium could pick up) and adopt suitable surface protection systems.

 

The table on the right sows a very practical classification of the SCC resistance of the three alloys, referred to hot rolled plates.

The meaning of the letters in the table is the following:

1. Very good. No SCC expected for global persistent tensile stress up to 75% of tensile yield stress.
2. Good. No SCC expected for global persistent tensile stress up to 50% of tensile yield stress.
3. Intermediate. No SCC expected for global persistent tensile stress up to 25% of tensile yield stress.
4. Poor. Fails to meet the requirement of previous letter C. In presence of persistent tensile stress SCC failures will occur.

In the case of plates, SCC behaviour gets worse with increasing thickness.

Fracture toughness

Fracture toughness of a material refers to the stress level required to propagate a flaw within the material.

It is a quite important property of the material, since the presence of flaws and discontinuities, coming from the metallurgical production cycle of the product and from the following manufacturing operations (cracks, porosity, non metallic inclusions, scratches, indentations, local corrosion etc.) can never be excluded.

For critical and highly stressed components (safety components) design methods based on Linear Elastic Fracture Mechanics (LEFM) are nowadays usual. These methods start from the geometry of the component, load conditions, size of flaws and a specific material property called "fracture toughness" to assess the capability of a component in which discontinuities are present to go on working without failures.
The most widely used index that defines such property is the "plain strain fracture toughness" (K1c - Mpa x m 1/2); this index is drawn experimentally from suitable test pieces, in which a mechanical notch is machined. First, the test pieces are loaded in fatigue, so that a fatigue crack is generated and propagates under controlled conditions from the tip of the mechanical notch. Then they are statically loaded until failure occurs.
Fracture toughness of aluminium alloys is a strongly directional property, depending on the main metallurgical directions, so it is usual to refer this property to two metallurgical directions of the semi-finished product, defined conventionally with the letters L (longitudinal), T (transverse), S (short transverse).
The first direction defines the metallurgical direction orthogonal to the plane of the fatigue crack, the second defines the metallurgical direction along which the fatigue crack propagates.
The former figure shows the designation used for rolled plates.

Besides the metallurgical orientation, alloy, heat treatment temper, type of product and thickness have an influence on fracture toughness.
For aerospace applications very high purity alloys (e.g. 7475) have been developed; these show high levels of fracture toughness, that shall be verified and guaranteed  by the manufacturer on each batch of material. The table at the right shows the typical K1c values for rolled plates in the three alloys, in different tempers.

 

 

To summarize

This figure shows the behaviour of the three alloys as far as yield stress and SCC resistance are involved, for plates with thickness between 50 and 100 mm, in the three main metallurgical directions.

It should be noted the strong scattering of properties of the alloy 7075-T651 depending on thickness and metallurgical direction; therefore this material should be considered as valuable for particular applications, but dangerous or badly performing for other applications.

The other materials, developed originally for aerospace to overcome the limits of 7075-T651, show much lower scattering, and therefore can be strongly proposed for applications where the demand for safety is strong, and the arise of SCC phenomena can not be absolutely excluded.

For thick sections (over 80 mm) the overall most performing material is 7050-T7451.

Where the SCC problem is of importance, the selection should be limited to 7075-T7351, 7475-T7351 and 7050-T7451.

Where fracture toughness matters, the selection should address 7475-T7351 and 7050-T7451.

In any case, for safety critical parts, it would be convenient to select products manufactured in conformance with aerospace specifications, instead than with commercial specifications (EN); they are maybe a little bit more expensive, not so widespread on the market, but guarantee a much better quality level, as far as possible internal defects and batch uniformity are involved; as a matter of fact, aerospace specifications:

• Require well defined and detailed process control, specially as far as special processes are involved;
• Require or suggest to require that the production shop performs non destructive testing, in order to screen and put aside products with internal discontinuities exceeding specified limits;
• Screen the manufacturing shops, setting asides the less advanced from the stand point of technology, production equipment, metallurgical culture;
• Require for a strict system of quality management and certification and of product traceability along the entire supply chain.

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