Materials and Processes
Materials questions cluster around identifying alloys and tempers, choosing the right inspection method, and knowing which processes change strength.
Alloys. 2024 (copper) is strong and fatigue resistant but corrodes easily, so it is usually Alclad, a thin pure-aluminum coating. 5052 is non-heat-treatable, very formable, and corrosion resistant. 6061 is heat-treatable, weldable, and reasonably corrosion resistant. 7075 (zinc) is the strongest common aircraft aluminum, is not weldable, and is susceptible to stress corrosion cracking. Temper designations: O annealed, T3 solution-treated and cold worked, T6 solution-treated and artificially aged, H strain hardened.
Heat treatment is solution, quench, and age. Overheating a hardened part destroys its temper; a part heated above the manufacturer's limit must be re-treated or scrapped.
Inspection. Dye penetrant finds surface defects in nonporous materials. Magnetic particle finds surface and near-surface defects in ferromagnetic metals only, never in aluminum. Eddy current finds surface and near-surface defects in conductors. Ultrasonic and radiographic methods find internal defects.
Traps. Questions mix up which NDI method works on which material, treat heat-treated and annealed as interchangeable, and assume all aluminum is weldable.
Rule of thumb. If the alloy or temper is unknown, treat the part as unairworthy until the maintenance manual, structural repair manual, or part marking confirms it.