Chapter 4: Magnesium, Titanium, Copper and Nickel — Testing and Repair

Magnesium and its alloys

• The lightest structural metal (density 1.74 — about two thirds of aluminium). Used mainly as castings: wheels, gearbox and accessory housings, some engine parts and older helicopter components.
• Magnesium is the most anodic structural metal, so it corrodes readily — especially in contact with other metals. It is protected by chromate conversion (dichromate) treatment and paint; any damage to the finish must be treated quickly. Its corrosion product is a white, powdery, snow-like deposit.
• Fire hazard: magnesium chips, shavings and dust ignite easily and burn intensely. Never use water on a magnesium fire — use dry sand or a Class D (dry powder) extinguisher. Keep filings clean and separate.
• Heat-treatable: solution and precipitation heat treatment as for aluminium, in a protective atmosphere.

Titanium and its alloys

• Density 4.5 — about 60% of steel — with strength similar to alloy steel, so an excellent strength-to-weight ratio, especially at temperatures up to about 500 °C where aluminium is useless.
• Outstanding corrosion resistance; titanium is cathodic, so it is compatible with carbon-fibre composites (titanium fasteners are used in CFRP structure).
• Uses: firewalls, engine nacelle and exhaust areas, compressor discs and blades, landing gear beams, fasteners, hydraulic tubing (Ti-3Al-2.5V). The workhorse alloy is Ti-6Al-4V.
• Alloy types: alpha (weldable, good at high temperature), alpha-beta (most common, heat-treatable — Ti-6Al-4V) and beta (high strength, formable).
• Precautions: hard to machine and prone to galling; work-hardens; never use cadmium-plated tools or fasteners on titanium (cadmium embrittlement) or chlorinated solvents and markers (stress-corrosion cracking). When ground it gives a brilliant white spark — one way to identify it.

Copper and copper alloys

• Copper — excellent electrical and thermal conductor: wiring, bonding straps, some fluid lines.
• Brass (copper + zinc) — fittings, turnbuckle barrels, instrument parts. Bronze (copper + tin; also aluminium bronze and phosphor bronze) — bushings, bearings, valve guides.
• Beryllium copper — heat-treatable to high strength; springs, diaphragms, non-sparking tools, instrument parts. Beryllium dust is toxic — no grinding without controls.

Nickel alloys

• Monel (about two thirds nickel, one third copper) — strong, tough and very corrosion resistant; it can replace mild steel where corrosion is a problem. Used for rivets in some installations, safety wire, oil cooler and exhaust parts.
• Inconel (nickel-chromium-iron) — keeps its strength and resists oxidation at high temperature: exhaust collectors, turbine and combustion parts, high-temperature safety wire.

Testing non-ferrous materials

• Hardness — Rockwell B scale (1/16 in ball) and Brinell with 500 kg load for softer metals; the portable Barcol impressor for aluminium alloys and composites. A hardness reading confirms heat-treatment condition.
• Electrical conductivity — an eddy-current conductivity meter (% IACS) distinguishes alloys and tempers and reveals heat (fire) damage to aluminium structure.
• NDT — penetrant (surface cracks), eddy current (surface and near-surface cracks, around fastener holes, corrosion thinning), ultrasonic (internal flaws, thickness), radiography. Magnetic particle inspection cannot be used on non-ferrous metals.

Repair and inspection procedures

• Inspect visually first (light, mirror, magnifier), then use NDT where cracks are suspected. Measure dents, scratches, nicks and corrosion depth and compare with the structural repair manual (SRM).
• Negligible damage is cleaned up, blended and protected. Repairable damage receives an SRM repair — external or internal doubler, insertion or splice, using the same alloy and temper (or an approved substitute) and the same or stronger fasteners. Damage beyond limits requires part replacement or manufacturer-approved data.
• Blend-outs are smooth, shallow and polished (no sharp corners — stress raisers), followed by NDT to confirm no crack remains, then conversion coating and primer.
• Cracks may be stop-drilled at the tip (a small hole that removes the sharp crack tip) — only as part of an approved repair, never as a permanent fix by itself.
• Heat-treated aluminium alloys are not welded in structural repairs; avoid heating them (it destroys temper). Prevent dissimilar-metal contact with sealant, primer or barrier tape.