Non-Ferrous Metals and Their NDT Behaviour

Learning outcomes

The learner should be able to compare aluminium, copper, nickel, cobalt and titanium alloy families, relate strengthening routes to discontinuities, recognise material-specific service risks and choose NDT with appropriate compatibility controls.

7.1 Aluminium alloys

Aluminium combines low density, useful conductivity and a protective oxide film. Wrought series use alloying systems that may be non-heat-treatable, strengthened by work hardening, or heat treatable through solution treatment and ageing. Cast aluminium alloys use chemistries suited to fluidity and casting performance.

The oxide film melts at a much higher temperature than the underlying metal and can become trapped during casting or welding. Hydrogen porosity, hot cracking, oxide inclusions and lack of fusion are important manufacturing concerns. Some high-strength alloys are susceptible to stress-corrosion cracking or exfoliation in unfavourable tempers and environments.

Aluminium is non-ferromagnetic, so MT is unsuitable. ET is effective for many surface, near-surface and conductivity applications. UT is useful but velocity, attenuation and geometry must be verified. PT materials require compatibility and complete post-cleaning where service demands it.

7.2 Copper alloys

Copper has high electrical and thermal conductivity. Brass principally combines copper and zinc; bronze names several copper alloy groups, commonly including tin, aluminium, silicon or other additions. Copper-nickel alloys are used where marine corrosion resistance matters.

Hot working, casting and welding can create cracking, porosity or segregation. Brass can suffer dezincification and stress-corrosion cracking in susceptible environments. High conductivity affects ET penetration and coil response; calibration must represent grade, thickness and temperature.

7.3 Nickel and cobalt alloys

Nickel alloys serve corrosive or high-temperature environments because they retain strength and resist oxidation or chemical attack. They can be solid-solution or precipitation strengthened. Welding may produce hot cracking, liquation cracking or lack of fusion, and service can produce creep damage, fatigue, oxidation or phase instability.

Cobalt alloys are used for hot strength, wear and corrosion performance. Cast structures and complex geometry can challenge UT. Some nickel and cobalt alloys are magnetic while others are not; the exact condition must be established rather than inferred from the base element.

7.4 Titanium alloys

Titanium offers high specific strength and strong corrosion resistance from a stable oxide film. Alpha, beta and alpha-beta alloy families respond differently to heat treatment. Titanium readily reacts with oxygen, nitrogen and hydrogen when hot, so welding requires effective shielding. Contamination can form a brittle surface layer, while lack of fusion and porosity remain inspection concerns.

Titanium is non-ferromagnetic. It can be examined by PT, ET, UT and RT when technique and chemistry are suitable. Halogen and other contaminant limits can be critical for penetrant products and cleaners; use approved low-contaminant systems where specified.

7.5 Practical selection exercise

For an aged aluminium forging, copper-nickel tube, nickel-alloy weld and titanium fabrication, state the likely manufacturing and service conditions, choose primary and complementary NDT, identify calibration-material needs and list chemical restrictions. Confirm surface treatments and coatings before applying any method.

Common errors and misconceptions

Several common errors recur in this topic: calling all aluminium "soft" or all nickel alloys "non-magnetic"; using ET calibration from one conductivity condition for another without verification; assuming a protective oxide prevents every form of corrosion; applying chloride-bearing products to restricted alloys; and treating cast and wrought forms of the same nominal alloy as structurally equivalent.

Chapter summary

Non-ferrous alloys gain properties through composition, work and precipitation. Their conductivity, magnetism, oxide behaviour, grain structure and chemical sensitivity affect NDT. Exact grade, product form, heat treatment and service environment are required for defensible method selection.