3.3 Extrusion, Drawing, and Machining

3.3 Extrusion, Drawing, and Machining

Quick Answer: Extrusion pushes billet through a die; drawing pulls rod, wire, or tube through dies to reduce section. Machining and grinding remove metal to final dimensions. Each step can introduce or reveal discontinuities—and each leaves a surface state that helps or hurts PT, MT, and VT. Level III inspectors must recognize product forms and process signatures.

This section completes the major non-weld manufacturing routes that feed NDT: what the part is (extruded shape, drawn tube, machined forging) tells you what to look for and how to prepare the surface.

Extrusion

Extrusion forces a heated (hot extrusion) or room-temperature (cold extrusion) billet through a die orifice. Aluminum architectural shapes, copper bus bars, magnesium profiles, and some steel sections are common products. Hollow shapes use mandrels or porthole/bridge dies that split and reweld metal streams inside the die.

Extrusion Seams

When metal is split around a die bridge or mandrel support and rejoined, a longitudinal seam (weld line) forms along the product. Under good temperature and pressure, this pressure-weld is sound. Under poor conditions—oxides, inadequate pressure, contamination—the seam is incomplete and becomes a linear discontinuity parallel to the extrusion axis. These seams can open in service (especially under hoop stress in tubes or pressure applications) or show as linear indications in ET, UT, PT, or MT depending on alloy and access.

Extrusion Pipe (Central Defect)

Extrusion pipe (also called extrusion defect or funnel) is a central cavity that can form toward the end of a billet extrusion when oxide-rich skin or backend material is drawn into the centerline. Mills crop the back end to remove pipe; inadequate crop leaves central voids in bar or shape stock. Unlike gas porosity, extrusion pipe is process-geometry related and elongated along the axis after further working.

Other Extrusion Issues

• Surface scoring / die lines from rough or damaged dies—VT and may act as stress risers.
• Blistering from subsurface gas or contaminants expanding near the surface.
• Coarse grain or peripheral coarse grain in some aluminum extrusions after heat treatment—affects mechanical properties and UT noise more than classical “crack” NDT.
• Internal inclusions elongated into stringers along the length.

Drawing: Wire, Rod, and Tube

Drawing pulls material through a die (or series of dies) to reduce diameter or wall thickness and improve dimensional control and surface finish. Wire drawing, rod drawing, and tube drawing (over a mandrel or plug) are high-volume processes.

Defect Behavior in Drawing

Drawing elongates existing discontinuities along the product axis. Seams, inclusions, and central pipe become longer and thinner. Surface defects can be ironed smoother (harder to see) or can crack further if lubrication fails.

Common drawing-related issues:

• Cupping / central burst in severe reductions with poor workability—internal chevron cracks detectable by UT on bar/wire lines.
• Surface checks and galling from inadequate lubricant, worn dies, or dirty stock—VT, ET for high-speed wire inspection.
• Seam enhancement—prior billet seams become continuous linear surface flaws.
• Dimensional out-of-round / eccentricity in tube—more dimensional than NDT, but wall variation affects UT and RT interpretation.

Tube mills often combine eddy current or flux leakage (for ferromagnetic tube) for online seam and hole detection with sample UT for wall and laminations.

Machining

Machining (turning, milling, drilling, boring, broaching, etc.) cuts away metal. It does not create “solidification” defects, but it can:

1. Reveal subsurface discontinuities that were buried under cast or forged skin.
2. Introduce surface damage: tears, laps of smeared metal, chatter marks, and sharp tool marks that act as crack starters.
3. Leave residual stresses—especially aggressive cuts, dull tools, and abusive parameters.

Machining Tears and Smeared Metal

Built-up edge, dull tools, or machining gummy alloys (some stainless, aluminum, titanium under wrong conditions) can tear the surface or smear metal over openings. Smeared metal is dangerous for PT and MT: it can bridge a crack mouth so penetrant cannot enter and magnetic leakage is reduced. Procedures may require etch, light abrasive cleaning, or a manufacturing note forbidding peening/smearing before final inspection.

Grinding Cracks

Grinding with excessive heat input (hard wheels, heavy feeds, inadequate coolant) causes localized heating and rapid quench by the bulk metal or coolant. The result is grinding cracks—typically shallow, fine, often network or parallel patterns on the ground surface—and possible grinding burn (temper colors, rehardening, soft spots). Grinding cracks are classic MT and PT findings on hardened steels (shafts, bearing races, gears after grind).

Because cracks are shallow and tight, surface method sensitivity and cleanliness are critical. Etching for burn (nital etch on steels) is a complementary process control check, not a substitute for crack NDT when required.

Residual Stress from Aggressive Machining

Heavy cuts and abusive grinding leave residual tensile stress at the surface in many cases, reducing fatigue life and promoting delayed cracking—especially in high-strength steels and after heat treatment. Stress-relief heat treatment or controlled finishing passes may be specified. NDT after final machining (not only after heat treat) is often required because grinding cracks form at the last operation.

Surface Condition Effects on PT, MT, and VT

Surface state is not cosmetic for Level III procedure writers—it is a sensitivity variable.

PT requires clean, dry, nonporous surfaces with openings not sealed by smeared metal. Roughness limits appear in procedures and standards (excessively rough surfaces are unacceptable without machining/blending).

MT needs magnetic coupling and particle mobility; heavy coatings are removed or limited by procedure thickness allowances. Field direction still follows expected crack orientation (often circumferential cracks on shafts from grinding → longitudinal magnetization).

VT is first and last: lighting, angle, and surface prep standards (e.g., SSPC/NACE cleanliness analogs in industrial work, or shop-specific criteria) belong in the inspection plan.

ET on drawn wire/tube and machined bores is highly surface-condition dependent—lift-off from roughness changes signal amplitude.

Product Forms Level III Should Recognize

Connect the name on a drawing or traveler to the discontinuity family:

• Extruded aluminum profile → longitudinal die seams, surface die lines, possible coarse grain after heat treat.
• Drawn seamless tube → elongated inclusions, residual extrusion/piercing defects, ID/OD scoring, eccentricity.
• Cold-drawn wire → seams, cupping, surface checks; high-speed ET common.
• Machined forging (e.g., turned shaft from open-die forge) → remaining forge laps if not fully machined away; new grinding cracks on journals.
• Ground hardened gear teeth → grinding cracks and burns on flanks.
• Deep-drilled gun-drilled holes → surface finish inside bore; PT/ET of ID when accessible.

Process → Flaw → Method Scenarios

1. Porthole-die aluminum extrusion for structural rail, incomplete pressure weld → longitudinal seam → ET/PT along seam lines; mechanical test of weld quality in process control.
2. Backend of steel billet extruded without adequate crop → central extrusion pipe → UT end or immersion testing of bar.
3. Hardened steel shaft finish-ground with burned temper colors → suspect grinding cracks → clean + wet fluorescent MT with field across expected crack direction.
4. Stainless fitting, heavy lathe smear over a forging lap mouth → false clean PT risk → etch or light abrasive to reopen, then re-PT; or machine away smear stock.
5. High-speed copper wire line → surface checks/seams → eddy current online with VT sample audit.

Integration with Heat Treatment and Welding

Machining often occurs before and after heat treatment. Grinding cracks are most associated with post-harden grind. Extruded aluminum may be solution treated and aged after extrusion—quench from solution treatment can crack thin sections (next section). Drawn products may be annealed between drafts; incomplete anneal leaves residual stress that promotes splitting in later draws.

Level III procedure sequences should specify when in the manufacturing route NDT occurs: after extrusion crop, after draw, after rough machine, after heat treat, after final grind. Finding a grind crack only after plating or assembly is a process failure, not just an NDT miss.

Study Focus

Map extrusion seams and pipe, drawing elongation of defects, machining tears/smear, and grinding cracks to methods. Always couple surface method choice with surface preparation requirements in the written procedure.