2.2 Non-Ferrous Metals and Alloys

Non-Ferrous Metals in the Level III Scope

Non-ferrous metals are metals and alloys whose primary base is not iron. On the Basic exam and in plant practice, the families that matter most are aluminum, copper and copper alloys, titanium, and nickel-base alloys, with magnesium and zinc appearing in lighter structural or coating roles. These materials show up in aerospace structures, heat exchangers, marine hardware, chemical process equipment, and high-temperature gas-path components.

A Level III does not need every temper designation memorized, but must answer three questions for any non-ferrous job:

1. How was strength developed (cold work, solid solution, precipitation, or combination)?
2. What physical properties constrain NDT (magnetism, conductivity, density, grain size, surface oxide)?
3. What discontinuities are likely from manufacturing and from service corrosion or fatigue?

Major Alloy Families

Aluminum alloys

Aluminum is low density (~2.7 g/cm³), corrosion resistant in many atmospheres due to a passive oxide, and highly workable. Pure aluminum is soft; engineering strength comes from alloying and temper:

• Non-heat-treatable alloys (e.g., many 3xxx, 5xxx series): strengthened mainly by solid solution and cold work (strain hardening).
• Heat-treatable alloys (e.g., many 2xxx, 6xxx, 7xxx series): strengthened by solution heat treatment + quenching + aging (precipitation hardening).

Typical product forms: plate, sheet, extrusions, forgings, castings, weldments. Typical discontinuities: porosity and shrinkage in castings; lack of fusion and porosity in welds (especially if oxide is not cleaned); stringers/inclusions from rolling; fatigue cracks at fastener holes and stress raisers; stress-corrosion cracking in susceptible tempers and environments (classic concern in some high-strength 7xxx products).

NDT notes: Non-magnetic → no conventional MT. PT for surface cracks and porosity openings. ET widely used on thin sheet, tubing, and aerospace structure (conductivity also used for heat-treat verification). UT for thicker plate and forgings; coarse cast structure may limit UT. RT effective for volume defects in castings and welds; low atomic number means lower absorption than steel—technique charts differ.

Copper and copper alloys

Copper offers excellent electrical and thermal conductivity and good corrosion resistance in many waters. Alloys include brasses (Cu-Zn), bronzes (e.g., Cu-Sn, aluminum bronze), and copper-nickels.

Strengthening: cold work is common; some alloys age harden. Product forms: tube, pipe, plate, castings, electrical bus. Discontinuities: porosity in castings; seams and inclusions in wrought product; dezincification and stress-corrosion cracking (ammonia environments on some brasses); erosion-corrosion in high-velocity water; fatigue in vibrating heat-exchanger tubes.

NDT notes: Non-magnetic. Very high electrical conductivity of pure copper reduces eddy-current penetration (small skin depth at a given frequency)—ET is still used on tubing but coil design, frequency, and fill-factor matter. RT works well; copper’s higher density/atomic number vs aluminum increases absorption. UT is routine on thicker sections; PT for surface-breaking flaws.

Titanium alloys

Titanium combines moderate density, high strength-to-weight, and outstanding corrosion resistance (especially to chloride environments when the passive film is intact). Aerospace airframes, landing-gear components, medical implants, and chemical equipment are common homes.

Crystal structure note: pure titanium is HCP (α) at room temperature and becomes BCC (β) at high temperature; alloys are α, near-α, α+β, or β types. Limited slip systems in α make deformation more directional and raise sensitivity to texture.

Discontinuities: hard-alpha inclusions and high-density inclusions (historic aerospace concern); porosity in castings; welding contamination from oxygen/nitrogen/hydrogen (embrittlement); fatigue cracks; crevice corrosion or hydriding under specific process conditions.

NDT notes: Non-magnetic. UT and ET are workhorses for aerospace forms; PT for surface; RT for castings and welds. Surface cleanliness and oxide/scale can affect PT and ET; Level III procedures must control surface condition.

Nickel and nickel-base alloys

Nickel alloys (including many high-temperature superalloys and corrosion-resistant alloys such as various Ni-Cr-Mo grades) serve where heat, oxidation, or aggressive chemistry defeats steel and aluminum. Strengthening mechanisms include solid solution, cold work, and precipitation hardening (e.g., γ′ in some superalloys).

Discontinuities: solidification segregation and freckles in large castings/ingots; hot tears; weld solidification cracking or heat-affected liquation cracking in susceptible alloys; creep voids and thermal-fatigue cracks in service; carburization or sulfidation damage in process environments.

NDT notes: Generally non-magnetic (some alloys can show slight response depending on composition and cold work). PT, UT, RT, ET dominate. High-temperature parts often need careful surface prep after scale removal; acceptance criteria may be driven by creep-fatigue design, not just fabrication codes.

Strengthening Mechanisms Level III Must Distinguish

Work hardening (strain hardening)

Work hardening raises strength and hardness by plastic deformation that multiplies dislocations. No composition change is required. Effects:

• Strength ↑, ductility ↓, residual stress often ↑
• Can induce preferred orientation (texture) and anisotropy
• In some austenitic stainless and other alloys, severe cold work can form strain-induced martensite and create local magnetism—an exam-relevant exception to “austenitic = always non-magnetic”
• Overworked surfaces may show orange peel, cracking, or reduced fatigue life

Annealing after cold work restores ductility by recovery, recrystallization, and grain growth—and resets NDT baselines (conductivity for ET hardness/temper checks on aluminum is a classic example).

Precipitation hardening (age hardening)

Precipitation hardening applies to alloys with temperature-dependent solid solubility (many Al, Ni, Cu-Be, some stainless grades):

1. Solution treat — dissolve alloying elements into a single solid solution.
2. Quench — trap a supersaturated solid solution.
3. Age (naturally or artificially) — fine precipitates form and impede dislocations.

Underaging / peak aging / overaging change strength and toughness. For NDT, improper heat treatment may not create a “crack,” but it changes conductivity, hardness, and residual stress, which can be monitored with ET conductivity or hardness methods and which alter fatigue and SCC susceptibility.

Physical Properties That Drive RT, UT, and ET

• RT: Higher density and higher atomic number absorb more radiation. Aluminum may need softer techniques or longer exposures than steel for equivalent thickness contrast; copper and nickel alloys absorb more strongly.
• UT: Use correct velocity and expect different attenuation. Coarse cast aluminum or titanium structures can scatter like coarse steel castings.
• ET: Conductivity sets standard depth of penetration. High-conductivity copper needs different frequency/coil strategy than lower-conductivity titanium alloys. Lift-off and edge effects still apply.

Non-Magnetic Reality: MT Is Off the Table

For the large majority of aluminum, copper, titanium, and nickel components, conventional magnetic particle testing is not applicable because the materials cannot be usefully magnetized. Level III procedure review should reject MT callouts on non-magnetic non-ferrous parts and substitute:

• PT for surface-breaking flaws (watch chemical compatibility with aluminum and titanium)
• ET for surface/near-surface cracks and thinning in conductive parts
• UT for internal volume and thickness
• RT for volumetric manufacturing flaws
• VT always as the first and continuous method

Corrosion Behaviors That Create Service Flaws

Non-ferrous equipment often fails by corrosion-assisted mechanisms rather than simple overload:

• Pitting and crevice corrosion — localized attack under deposits, gaskets, or stagnant zones; pits act as fatigue starters; PT/VT/ET detect open pits; deep subsurface pits may need UT thickness or RT.
• Stress-corrosion cracking (SCC) — tensile stress + specific environment + susceptible alloy/temper (e.g., some brasses in ammonia, some Al alloys in moist chloride-bearing conditions). Cracks are tight and branched; surface methods and UT crack sizing strategies matter.
• Galvanic corrosion — dissimilar metal couples (Al fastened to steel, Cu in contact with Al) accelerate attack of the anodic member; look for wall loss at joints.
• Erosion-corrosion / fretting — high-velocity fluids or vibration remove protective films; common in copper-alloy heat-exchanger tubes.
• Hot corrosion / oxidation — nickel alloys in turbines; scale and subsurface attack complicate surface NDT until cleaning is controlled.

Level III takeaway: When writing a technique for non-ferrous service inspection, start from alloy + environment + stress state, then pick methods that can find the expected crack morphology and wall-loss pattern, not from a steel-based default checklist.