Aluminium Alloys for Structural Analysis

Structural Aluminium in Aerospace, Motorsport and Advanced Engineering

Aluminium alloys are among the most widely used structural materials in aerospace, motorsport and advanced engineering. They offer a favourable combination of moderate density, good stiffness, high achievable strength, good fatigue behaviour, mature manufacturing processes and well-established certification bases. But "aluminium" as a word is not a material definition — it is a class of materials that spans a wide range of alloys, tempers, product forms and conditions, each with different structural properties. An aerospace-grade 7xxx-series forging in the T6 temper has very different properties from a 6xxx-series extrusion in the T5 temper, and both differ from a cast alloy. The structural analyst must define the alloy, the temper, the product form, the orientation, the thickness, the temperature and the applicable material specification before any material data is meaningful. This article discusses the factors that determine aluminium structural properties and the discipline required to define the material correctly for structural substantiation.

Alloy Families — A Conceptual Overview

Wrought aluminium alloys are classified by a four-digit designation based on the primary alloying elements. The 1xxx series are essentially pure aluminium — low strength, high corrosion resistance, good conductivity, used for electrical and chemical applications rather than primary structure. The 2xxx series are alloyed with copper — high strength, good fatigue behaviour, used extensively in aerospace but with lower corrosion resistance. The 5xxx series are alloyed with magnesium — moderate strength, excellent corrosion resistance, good weldability, used in marine and pressure vessel applications. The 6xxx series are alloyed with magnesium and silicon — moderate strength, excellent extrudability, good corrosion resistance, used in structural extrusions. The 7xxx series are alloyed with zinc — the highest strength wrought alloys, used in highly-stressed aerospace structure, but with more complex heat treatment and some corrosion considerations. The 8xxx series are alloyed with other elements (lithium, tin) for specialised applications. These are conceptual distinctions; the analyst must refer to the specific alloy and temper for any structural application, and must not use generic class-level properties as design data.

Temper and Heat Treatment

The temper designation defines the metallurgical condition of the alloy — whether it has been solution heat-treated, artificially aged, strain-hardened, or annealed. The temper has a profound effect on the strength: the same alloy in different tempers can have very different yield and ultimate strengths. A T6 temper (solution heat-treated and artificially aged to peak strength) is the common high-strength condition for aerospace alloys. A T7 temper (over-aged) sacrifices some strength for improved corrosion resistance and dimensional stability. A T3 temper (solution heat-treated, cold-worked, and naturally aged) is used for some 2xxx-series alloys. An O temper (annealed) is the soft, low-strength condition. The temper is part of the material definition — an alloy without a temper specification is incomplete, and the structural properties cannot be determined without it. The temper also affects the microstructure and the response to welding: welded joints in heat-treatable alloys have a heat-affected zone with locally reduced properties, which must be accounted for in the analysis.

Product Form — Forging, Plate, Extrusion, Sheet, Cast

The product form affects the properties through the processing history and the resulting microstructure. A forging is processed by plastic deformation under high pressure, which produces a refined grain structure and favourable grain flow oriented to the part geometry; forgings typically have the best combination of strength and toughness in the directions aligned with the grain flow. A plate is rolled from an ingot, producing a directional grain structure with different properties in the longitudinal (L), long-transverse (LT) and short-transverse (ST) directions. An extrusion is forced through a die, producing a grain structure oriented along the extrusion direction with good properties in that direction. A sheet is rolled thin, with properties that may differ between the rolling and transverse directions. A casting is solidified from the melt, producing a generally isotropic but potentially porous structure with lower and more variable properties than wrought products. The product form is part of the material definition, and the properties — particularly the directional allowables — depend on it.

Grain Direction and Directional Allowables

Wrought aluminium products have a directional grain structure, and the properties differ in the principal grain directions. The longitudinal direction (L) is the primary working direction — the forging principal direction, the rolling direction, the extrusion direction. The long-transverse direction (LT) is perpendicular to L in the plane of the product. The short-transverse direction (ST) is through the thickness. The strength, ductility, fracture toughness and fatigue properties can differ between these directions, with the ST direction typically having the lowest fracture toughness and the lowest fatigue crack growth resistance. For thick products (plate, forging), directional allowables are published for each orientation, and the analyst must use the allowable appropriate to the direction of the critical stress. Using a longitudinal allowable for a stress that acts in the short-transverse direction is unconservative and can be a significant error. The orientation is part of the material definition.

WROUGHT ALUMINIUM PROPERTIES ARE DIRECTIONAL. The L, LT and ST orientations can have different strengths, ductilities and fracture toughnesses — with ST typically the lowest in fracture and fatigue. For thick products, the analyst must use the directional allowable appropriate to the stress direction. Using a longitudinal allowable for a short-transverse stress is unconservative.

Thickness Effects

The thickness of the product affects the properties through the processing history and the resulting microstructure. Thin sheet may have different properties from thick plate of the same alloy and temper, because the rolling reduction, the cooling rate during heat treatment, and the grain structure all vary with thickness. Allowables are often published as a function of thickness range — the allowable for a thin sheet may differ from that for a thick plate even in the same alloy and temper. The analyst must use the allowable for the thickness range that contains the actual product, and must not extrapolate allowables to thicknesses outside the published range without justification. Thickness also affects the fracture behaviour: thick sections are more prone to plane-strain fracture (lower toughness) while thin sections may be in plane stress (higher apparent toughness). The thickness is part of the material definition.

Properties to Characterise for Structural Aluminium

A complete characterisation of structural aluminium for analysis includes several property categories. The elastic properties (modulus, Poisson's ratio, density) are response properties that enter the stiffness and mass matrices. The yield and ultimate strengths are allowables (when statistically derived) that enter the margin calculation. The stress-strain curve (true stress vs plastic strain) is a response property that enters the plasticity model for non-linear analysis. The fatigue properties (S-N curves or strain-life data) are allowables that drive the fatigue assessment. The fracture toughness is an allowable that drives the damage tolerance assessment. The bearing properties (bearing yield and ultimate) are allowables for fastener-hole analysis. The temperature dependence of all these properties must be characterised if the structure operates away from room temperature. The corrosion behaviour and the environmental effects (including stress corrosion cracking susceptibility) must be considered for the service environment. Each of these properties is specific to the alloy, temper, product form, orientation, thickness and condition.

• Elastic modulus, Poisson's ratio, density — response properties for the stiffness and mass matrices
• Yield and ultimate strengths (directional) — allowables for the static strength margin
• Stress-strain curve (true stress vs plastic strain) — response property for plasticity models
• Fatigue properties (S-N or strain-life) — allowables for the fatigue assessment
• Fracture toughness (directional) — allowable for the damage tolerance assessment
• Bearing yield and ultimate — allowables for fastener-hole bearing analysis
• Temperature-dependent properties — response and allowable variation with operating temperature
• Corrosion and stress corrosion cracking susceptibility — environmental durability considerations

Factors Affecting Aluminium Structural Properties

The following table summarises the factors that affect the structural properties of aluminium and the significance of each for the analysis. Each factor must be defined before the properties can be determined, and each is part of the complete material definition.

Do Not Publish Generic Property Values as Authoritative Design Data

It is tempting to quote typical or nominal property values for aluminium alloys from textbooks or databases. These values are useful for conceptual design and material comparison, but they are not authoritative design data. Authoritative design data — the values that enter a margin of safety calculation for a certificated structure — must be taken from qualified material specifications and statistically-derived allowables (such as MMPDS or equivalent) for the specific alloy, temper, product form, orientation, thickness and condition. Typical values may be higher or lower than the design allowable, and using a typical value where a statistical allowable is required can produce an unconservative margin. The analyst must always establish the source and the statistical basis of the data, and must not use generic values for structural substantiation. This is the two-job principle in action: the response properties and the allowables must both be correctly sourced, and the allowables in particular must be statistically derived for the specific material condition.

'ALUMINIUM' IS NOT A MATERIAL DEFINITION SUFFICIENT FOR STRUCTURAL SUBSTANTIATION. The analyst must define the alloy, temper, product form, orientation, thickness, temperature, environment and applicable material specification. Generic or typical property values are for conceptual comparison only — design data must come from qualified specifications and statistically-derived allowables for the specific material condition.

Key Takeaways

• Aluminium is a class of materials, not a single material — alloy, temper, product form, orientation, thickness and condition all change the properties
• Alloy families (1xxx through 8xxx) differ in alloying elements, strength, corrosion resistance and weldability
• Temper defines the metallurgical condition and has a profound effect on strength
• Product form (forging, plate, extrusion, sheet, cast) drives the grain structure and the directional properties
• Directional allowables (L, LT, ST) must match the stress direction — ST is typically lowest in fracture and fatigue
• Thickness affects properties through processing, cooling rate and fracture constraint — allowables may vary with thickness range
• Design data must come from qualified specifications and statistically-derived allowables, not generic textbook values