Module 6.2 Aircraft materials - non-ferrous

Knowledge levels

Item	A	B1 / B3	B2 / B2L
6.2(a) Characteristics, properties, identification and heat treatment of non-ferrous materials	1	2	1
6.2(b) Testing for hardness, tensile strength, fatigue strength, impact resistance	—	1	1
6.2(c) Repair and inspection of non-ferrous materials, structures and airframes	—	2	1

Aluminium and its alloys

• Pure aluminium: light (density 2.7 g/cm³, about one third of steel), good conductor, very corrosion-resistant, but weak.
• Alloyed with copper, zinc, magnesium, manganese and silicon for strength.

Four-digit designation (wrought alloys)

Series	Main alloying element	Heat-treatable?	Typical aircraft use
1xxx	≥ 99 % aluminium	no	cladding, fairings
2xxx	copper	yes	2024 - fuselage skins, tension structure, rivets (2117)
3xxx	manganese	no	fuel tanks, cowlings
5xxx	magnesium	no	5052 - fuel/oil lines, tanks
6xxx	magnesium + silicon	yes	6061 - fittings, tubing; weldable
7xxx	zinc	yes	7075 - highest strength: wing spars, stringers, upper wing skins

Key fact: 2024 = copper (good fatigue resistance, damage tolerant); 7075 = zinc (highest strength, more notch-sensitive and less corrosion-resistant).

Temper designations

Code	Meaning
F	as fabricated
O	annealed (softest)
H	strain-hardened (cold worked) - non-heat-treatable alloys, e.g. H32
W	solution heat-treated, unstable (still ageing)
T3	solution heat-treated, cold worked, naturally aged
T4	solution heat-treated, naturally aged
T6	solution heat-treated, artificially aged
T7x	over-aged for better stress-corrosion resistance (e.g. 7075-T73)

Heat treatment of aluminium alloys

1. Solution heat treatment: heat to about 490-500 °C (alloy dependent), soak so alloying elements dissolve, then quench rapidly in water.
2. Ageing (precipitation hardening): strength rises as particles precipitate - naturally at room temperature (days) or artificially in an oven (hours).

• Ice-box rivets (2017, 2024 "D" and "DD" rivets) are solution treated and kept refrigerated to delay age hardening until they are driven.
• Delay in quenching causes intergranular corrosion risk.

Alclad

A thin layer of pure aluminium bonded to each side of a strong alloy sheet (e.g. Alclad 2024). The pure layer protects the core (it is anodic to the core and corrodes sacrificially). Do not damage it with scratches or abrasive cleaning.

Magnesium alloys

• Very light (about 1.74 g/cm³) - wheels, gearbox housings, some castings.
• Very corrosion-prone: highly anodic; needs chromate/conversion coatings and paint.
• Fire hazard: fine chips and swarf burn intensely; never use water on a magnesium fire - use dry sand or class D extinguishant.