Aluminium Alloys

The typical alloying elements are

Copper

Magnesium

Manganese

Silicon

Tin

Zinc.

Aluminium Alloys Are alloys in which

aluminium (Al) is the predominant metal.

Performance Criteria for Primary Structures

Necessary material properties:

- Tensile strength

- Compressive strength

- Shear strength

- Modulus of Elasticity

- Fracture toughness

Fatigue initiation

Fatigue crack growth rate

- Corrosion resistance

Performance Criteria for Primary Structure

If failure of a structural component endangers the aircraft, the component is referred to as primary structure, The requirements for primary structure for subsonic aircraft are shown below

Al-Alloy Product Forms for Aircraft

Wrought Products

Wrought aluminium is when the metal is worked in the solid form with the help of specific tools

Historically used for primary structure

Starting stock is cast ingot

Rolled products

- Plate and sheet material (including Milled Plank)

- Used in range <1 to 10mm say for fuselage skin and stringers

- Range of say 25 to 50mm used for wing skins

- Thickest plate, up to say 150mm used for bulkheads, spars

Wrought Products - Sheet and Plate (Milled Plank Wing)

https://www.youtube.com/watch?v=LpSgj-tKelY

Extrusions

Classified by 'circle size' and thickness of segments

Mainly used for stringers

Can be J, Z, T, 'top-hat' and other shapes

Skin and stringers can be extruded integrally

Forgings

Near-net shapes which may require only a light final machining

Parts pressed into shape using open-die or closed-die methods

Good (grain) directional properties

Replaces thick plate for some components, e.g. bulkheads

Castings

Sand, Lost Wax (Investment), permanent mould casting processes

Process used is based on component size, weight, dimensional accuracy, quality, cost

Mechanical properties considered less consistent than wrought products, so casting factor is used

Benefits include cost reductions due to replacing multi-part assemblies or parts requiring complex machining/fabricating operations

Sand Mould Casting

Investment Castings (Lost wax process)

Permanent Mould Casting

Aircraft Al-Alloys

Wrought alloy designations:

- Developed by Aluminium Association

- 1st digit indicates alloy group or major alloy addition

- 2nd digit indicates modifications of the original alloy or impurity limits

- 3rd and 4th digits identify the minimum aluminium content

Wrought alloy designations:

1XXX Pure Aluminium

2XXX Copper

3XXX Manganese

4XXX Silicon

5XXX Magnesium

6XXX Magnesium and Silicon

7XXX Zinc

8XXX Other

Cast alloy designations:

1st digit refers to the major alloying element

2nd and 3rd digits identify a particular composition

The zero after the decimal point identifies the product as a casting

Letter prefix may be used to denote an impurity level or the presence of a secondary alloying element

E.g. A201.0 has higher purity than 201.0

Cast alloy designations:

1XX.X > 99.0% Al (Pure Aluminium)

2XX.X Copper

3XX.X Silicon and Magnesium, Silicon and Copper, Silicon, Copper and Magnesium

4XX.X Silicon

5XX.X Magnesium

7XX.X Zinc

8XX.X Tin

9XX.X Other

Temper designation

Aluminium alloy properties are determined by the alloy content and method of fabrication

Some alloys are then strengthened by cold working, Other alloys are strengthened by heat treatment

Whether Cold Working or Heat Treating, these processes bring out some of the alloying elements from the grain and are deposited between the grain boundaries. This provides keying between the grain boundaries and makes the Aluminium stronger.

Temper designation

Process used is indicated by an alpha-numeric code which appears after the alloy designation, e.g. XXXX- T3

- Other numbers may be added to the code to identify specific practices, e.g. XXXX-T33

- The number 5 has been specifically allocated to indicate stress relieving of wrought products, - - e.g. 7150-T77511

- The '5' indicates stress relief by deformation (stretching or compressing)

- The first '1' indicates stretching

- A '0' in place of the first '1' would indicate compression

- The second '1' (for extrusions only) indicates mechanical straightening after stress relief

- A '0' in place of the second '1' indicates that no mechanical straightening has taken place

Wrought alloys

2024 and 7075 have been the principle alloys for many years

2xxx alloys predominate in areas where damage tolerance is the primary requirement

7xxx alloys predominate in areas where strength is the primary requirement (not as malleable)

Many variations have been developed to improve strength, durability, etc.

Drivers for selection and development of materials for aircraft have changed considerably over the years

They include, but are not limited to:

Low structural weight

Safety factors

Cost

Availability

Manufacturability

Reliability

Maintainability

Repairability

Recent Developments

Aluminium-Lithium alloys

- Big push for reduced weight

- Alloys 8090, 2091 developed

- Not overly successful due to various problems - Micro cracking during manufacture, Low fracture toughness, Crack deviation, poor corrosion resistance

Weldable alloys

- Uses high-speed production methods, e.g. laser beam welding

- 6013, 6056, 1100, 1464, 2XXX

Large scale castings

Hybrid materials (GLARE) - Glass Aluminium Reinforce Epoxy

CORROSION

Stop or minimize corrosion by Aircraft Washing

Stop or minimize corrosion by Hangaring

Keeps it dry, lets it dry, prevents atmospheric contaminates settling on it and prevents Sun damage

Stop or minimize corrosion by Care of Painted Surfaces

Periodic repainting may be required as part of a company's corrosion programme

Stop or minimise corrosion by Moisture Excluding Compounds

Stop or minimize corrosion by Contamination

Other substances, such as acid, mercury and gallium accelerate corrosion or start ripping apart grain boundaries