Machining, Composites and Additive Manufacturing

Learning outcomes

The learner should be able to identify machining damage, explain composite architecture and failure, outline additive process chains, relate post-processing to examination timing and adapt NDT to anisotropic or layered products.

12.1 Material removal and surface integrity

Turning, milling, drilling, grinding, electrical-discharge machining, laser cutting and abrasive cutting remove material by different thermal and mechanical mechanisms. Tool wear, vibration and incorrect parameters can create chatter, tears, burrs, residual stress, local overheating, recast layers or cracking. Grinding burn can soften a tempered steel or re-harden a surface, depending on the thermal cycle. EDM can leave a recast layer with microcracks.

Machining may reveal internal discontinuities or smear ductile metal across them. Final NDT timing must consider whether a finishing operation can open, close, remove or create the target condition. PT before final machining and PT after final machining answer different questions.

12.2 Composite architecture

A composite combines reinforcement with a matrix and often includes an interface or interphase. Fibre-reinforced polymers can be unidirectional, woven, chopped or laminated. Sandwich structures add lightweight cores and bonded skins. Properties are strongly directional; load transfer depends on fibre direction, resin quality, cure and bond integrity.

Manufacturing conditions include porosity, voids, inclusions, resin-rich or resin-starved zones, wrinkles, waviness, misaligned plies, delamination, disbond and core damage. Service adds impact damage, moisture ingress, fatigue, crushing and thermal degradation. Barely visible impact damage can spread beneath a small surface mark.

UT, thermography, shearography, radiography and tap testing each address particular composite structures. Technique qualification must represent lay-up, thickness, curvature, core, adhesive and damage type. Metallic assumptions about isotropic velocity are unsafe.

12.3 Additive manufacturing

Additive manufacturing builds geometry layer by layer from powder, wire, sheet, resin or another feedstock. Metal powder-bed fusion uses a scanned energy source; directed-energy deposition feeds material into a melt pool; binder jetting requires later consolidation. Polymer routes include material extrusion and photopolymerisation.

Quality depends on feedstock, atmosphere, energy input, scan strategy, layer thickness, thermal history, support, build orientation and post-processing. Credible conditions include lack of fusion, keyhole or gas porosity, inclusions, cracking, distortion, roughness and anisotropic texture. Hot-isostatic pressing may close pores but does not automatically remove inclusions or all lack-of-fusion interfaces.

The digital chain is part of traceability: design file, build orientation, machine, parameter set, powder batch, monitoring data, heat treatment and machining state all matter.

12.4 Practical checkpoint

Build an examination plan for a ground hardened shaft, carbon-fibre sandwich panel and powder-bed-fused bracket. For each, state the inspection stage, expected conditions, primary method, reference standard, surface preparation and important limitation. Identify what post-processing may hide or transform.

Common errors and misconceptions

Several common errors recur in this topic: assuming final machining only removes discontinuities; using an isotropic UT model for a laminate; treating a clean additive surface as evidence of a sound build interior; assuming hot-isostatic pressing repairs every planar interface; and ignoring digital build and post-process traceability.

Chapter summary

Machining controls surface integrity and can create or conceal cracks. Composite performance comes from architecture and interfaces, making anisotropy central to NDT. Additive parts carry a layered thermal and digital history. Examination stage and representative references are essential for all three product groups.