Choosing a Process for Complex Parts

“Complex geometry” usually means at least one of the following: features that are hard to access with tools, tight requirements that can’t tolerate distortion, or an assembly that could be simplified into a single part. In defense and aerospace, the stakes are higher because complexity is paired with traceability, qualification, and repeatability. The “best manufacturing process for complex parts” is rarely a single technology—it’s a workflow that reliably produces conforming hardware with the documentation your customer expects.

This article lays out a practical selection method used by successful programs: start with geometry, then confirm material availability and properties, then decide how you’ll hit tolerance and finish, then evaluate rate/volume, and finally confirm compliance. The goal is to help engineers, procurement teams, and program managers build RFQs that match reality and avoid late-stage redesigns.

Geometry assessment

Complex geometry is where process selection begins. Before comparing quotes or lead times, define what “complex” means for your specific part, and which features are mission critical versus “nice to have.”

1) Identify access-limited features. Internal channels, deep pockets, undercuts, re-entrant features, and intersections that can’t be reached by standard cutters are red flags for conventional machining. If the feature cannot be reached by a tool, you either redesign for access, split the part into multiple components, or choose a process that creates geometry without tool access—most commonly additive manufacturing (AM) such as powder bed fusion (PBF) (DMLS/SLM), or a near-net approach like PM-HIP.

2) Separate “complex to make” from “complex to inspect.” Features like internal manifolds, lattice structures, and small internal radii may be manufacturable with AM, but they can be challenging to verify without CT scanning or destructive sectioning. If your acceptance plan requires verifying internal geometry, make sure the inspection method is feasible, repeatable, and available at the supplier (or at a qualified subcontractor).

3) Consider part consolidation and assembly risk. If the current design is an assembly of brazed, welded, or fastened parts, consolidation into a single component can reduce leak paths and improve reliability. However, it also concentrates risk into one part: scrap and rework become more expensive. In practice, teams often run a “consolidation trade study” that weighs reduced assembly labor and improved performance against increased unit cost and inspection burden.

4) Think in terms of “net shape” versus “finish shape.” Most complex parts are not delivered as-built. Even when AM is used, critical interfaces often require post-processing: CNC machining, 5-axis machining for datums and sealing surfaces, reaming/boring for precision holes, and surface treatments. A realistic geometry assessment includes a machining strategy: where will you place machining stock, how will you fixture, and what datums will control the build and the machine operations?

5) Evaluate thin walls, overhangs, and thermal distortion risk. For PBF, thin walls, long unsupported spans, and large cross-section changes can drive distortion, residual stress, and support removal challenges. For machining, thin walls may chatter or deflect during cutting. For castings and forgings, complex thin sections can affect fill, porosity, and distortion after heat treat. If the geometry is both thin and highly constrained by tight GD&T, you may need a process plan that includes stress relief, intermediate machining steps, and controlled heat treat.

Practical takeaway: If the geometry requires internal features that cannot be machined or cast reliably (e.g., conformal cooling, internal manifolds, complex lattice energy absorption structures), PBF AM or PM-HIP tends to move up the list. If the geometry is “complex but accessible” with modern toolpaths and fixturing, 5-axis CNC machining from billet or forging often remains the lowest risk option.

Material requirements

Material selection can eliminate otherwise attractive processes. Defense and aerospace programs usually specify alloy, condition, and often a material specification family (e.g., AMS) with mechanical property requirements. The right question is not “Can the process make this alloy?” but “Can the process deliver the required properties with documented control and traceability?”

1) Confirm material form availability and pedigree. Machining from billet assumes plate/bar availability in the required chemistry and heat lot. Casting and forging assume qualified foundry/forge routes and availability of tooling. PBF AM requires powder with controlled chemistry, particle size distribution, flowability, and moisture/oxygen limits. A robust RFQ calls out requirements for material traceability, heat lots, and certificates of conformance (CoC) for feedstock (bar/forging/casting/powder).

2) Understand how process affects microstructure and properties. PBF builds can exhibit anisotropy, porosity, lack of fusion, and residual stress if parameters and post-processing are not controlled. Castings can show shrink porosity and segregation; forgings can improve grain flow and toughness but may limit geometry. Machining from billet generally offers predictable properties but can be inefficient for high buy-to-fly parts.

3) Use densification and heat treatment strategically. For critical rotating or pressure-containing hardware, teams often combine PBF with Hot Isostatic Pressing (HIP) to close internal porosity and improve fatigue performance. A common aerospace workflow is: PBF build → stress relief → support removal → HIP → solution/age or other heat treat → finish machining → NDE → final inspection. HIP is not a magic fix; it must be integrated with the alloy’s heat treatment and the part’s dimensional strategy because HIP can change dimensions slightly and can affect surface-connected defects differently than internal porosity.

4) Consider PM-HIP for near-net complex solids. PM-HIP (powder metallurgy consolidated via HIP in a sealed container) can produce fully dense near-net shapes without the layer-by-layer constraints of PBF. It can be attractive when you need robust bulk properties and complex external geometry, especially when internal features are limited or can be created with sacrificial cores/containers. PM-HIP is also used for alloys that are difficult to forge or when you want fine, uniform microstructure. The trade is that PM-HIP requires container design, powder loading controls, and often additional machining to reach final tolerance.

5) Match material and process to qualification expectations. If the program expects a stable, well-understood metallurgical route with extensive historical data, forging or billet machining may be favored. If performance gains from part consolidation or internal features are substantial, AM + HIP can be justified—provided the supplier can produce a repeatable parameter set, maintain powder controls, and generate the documentation needed for aerospace acceptance (including build records, post-process logs, and inspection results).

Practical takeaway: The “best manufacturing process for complex parts” is the one that can consistently achieve the required properties in the specified alloy while maintaining traceability from raw material through final part, including all post-processing steps.

Tolerance/finish needs

Complex parts usually fail late due to tolerance stack-up, surface finish issues, or inspection gaps—not because the initial geometry was impossible. The right approach is to plan tolerance achievement and verification as a closed loop: design intent → process capability → inspection method → corrective action.

1) Classify features by function. Separate features into (a) primary datums and interfaces, (b) sealing/flow surfaces, (c) fatigue-critical features, and (d) non-critical geometry. This drives where you need machining and where as-built (AM or cast) surfaces are acceptable. Many successful designs intentionally place critical features on accessible pads that can be machined regardless of the net-shape process.

2) Expect hybrid manufacturing for precision. For PBF AM (DMLS/SLM), as-built surfaces and dimensional variation typically require finishing. A common, low-risk strategy is to add controlled machining stock (for example, on flange faces, bores, and bolt patterns), define datums that survive post-processing, and use 5-axis CNC to finish critical geometry. For castings and PM-HIP, machining is also usually required for tight tolerances and surface finishes.

3) Account for post-processing effects on dimensions. Stress relief, HIP, and heat treatment can cause measurable distortion, especially in thin-walled or asymmetric parts. High-performing suppliers manage this with a combination of build orientation choices (for AM), fixturing strategy, intermediate machining, and empirically validated offsets. Procurement teams should ask for the supplier’s dimensional control approach: what dimensions are held net-shape, what are machined, and what is the expected variation after each thermal step?

4) Specify inspection methods that match feature accessibility. For external geometry, CMM inspection is standard. For internal channels or complex internal geometry, consider CT scanning as an acceptance or process-monitoring method, depending on customer requirements. For aerospace hardware, NDE such as dye penetrant or fluorescent penetrant inspection, radiography, or ultrasonic inspection may be required depending on material and defect risk. If special processes are used, ensure the supplier’s inspection and NDE capabilities align with your flowdowns and that the results can be included in the certification pack.

5) Define surface finish and edge condition realistically. Surface finish is not only an aesthetic requirement; it can affect fatigue life, sealing performance, and flow. AM as-built surfaces are typically rougher than machined finishes and may require abrasive finishing, machining, or other controlled post-processing. If the part is fatigue-critical, specify how surface condition will be controlled (e.g., machining, controlled polishing) and how it will be verified. For tight edge break requirements, ensure the process plan includes deburring and verification steps.

Practical takeaway: If your part has tight GD&T and critical surfaces, assume a hybrid route (net-shape process + machining + inspection). The best process is the one that can repeatedly hit tolerance after all heat cycles and finish steps, with inspection methods that prove it.

Volume considerations

Volume drives not just unit cost, but also risk, lead time stability, and how much process development you can afford. In regulated environments, a process change can trigger requalification, so it’s worth choosing a scalable path early.

1) Prototype and low-rate production. For one-off prototypes or low-rate initial production (LRIP), PBF AM can reduce lead time by avoiding hard tooling and enabling rapid design iterations. Machining from billet is also common for LRIP when geometry is accessible, because it offers predictable properties and straightforward inspection. A practical rule is to choose the process that minimizes nonrecurring engineering (NRE) while still representing the final material condition and post-processing route you intend to qualify.

2) Medium-rate production. At moderate volumes, cost drivers shift to cycle time, yield, and post-processing capacity (HIP, heat treat, machining, and inspection throughput). For AM, build packing strategy, support removal labor, and powder handling controls become critical. For machining, fixture optimization and tool life matter. For castings/forgings, tooling amortization becomes attractive if design is stable and suppliers are qualified.

3) High-rate production. At higher volumes, casting, forging, or metal injection molding (MIM) may offer the lowest per-part cost, but require tooling and stable demand. AM can still win at high rates when the part is highly consolidated, buy-to-fly is extreme, or performance benefits justify cost—but only if the supply chain can support consistent machine capacity, powder supply, HIP and heat treat availability, and automated inspection workflows.

4) Supply chain robustness and second sources. Defense and aerospace programs should consider second sourcing early. If the chosen process requires a narrow set of qualified machines, powders, or special process providers, program risk increases. A procurement-ready RFQ should ask about capacity, lead time stability, and whether the supplier has qualified alternate machines, alternate powder lots, and qualified subcontractors for HIP, heat treat, NDE, and machining.

5) Cost realism: buy-to-fly and scrap risk. Complex titanium and nickel alloy parts can have high buy-to-fly when machined from billet. AM and PM-HIP can improve material utilization, but may introduce different scrap modes (build failures, trapped powder, internal defects, distortion after HIP). The best decision weighs total cost: material, processing, post-processing, inspection, yield, and rework—plus the cost of schedule slips.

Practical takeaway: Choose a process that can meet today’s volume and tomorrow’s ramp without forcing a major requalification. If you expect to scale, ask suppliers to describe how the process will be controlled at rate, not just how the first article will be made.

Compliance requirements

For defense and aerospace buyers, compliance is not a paperwork afterthought—it is part of the manufacturing process. A supplier can produce a beautiful part that is unusable if the documentation, traceability, or controlled workflow is missing.

1) Flow down regulatory and contractual requirements upfront. If the work is export-controlled, ensure the supplier can support ITAR handling, controlled access, and compliant data management. If the program requires domestic sourcing, ensure compliance with DFARS clauses (including specialty metals requirements where applicable) and verify how the supplier manages material sourcing and records.

2) Quality system expectations. Aerospace programs commonly expect an AS9100 quality management system. Even when not explicitly required, suppliers with AS9100 typically have more mature document control, nonconformance management, and traceability practices. For special processes (including many NDE methods and heat treat processes), programs may require NADCAP accreditation or equivalent customer approval. If the part requires HIP, heat treat, or NDE, confirm whether those steps are performed in-house or subcontracted, and whether the subcontractors are approved for your program.

3) Manufacturing records and traceability for AM and PM-HIP. For PBF AM, compliance-ready records often include: powder lot traceability, machine and parameter set identification, build job traveler, environmental controls, recoater and filter maintenance logs, post-processing travelers (stress relief, HIP, heat treat), and inspection records. For PM-HIP, expect container design control, powder handling records, HIP cycle records, and machining/inspection documentation. Procurement should request a clear description of how the supplier maintains traceability from raw powder or bar stock through final serialization.

4) Inspection and acceptance documentation. Many programs require a first article inspection report (FAIR) for initial qualification and then ongoing lot acceptance records. A procurement-ready approach is to specify what must be in the certification pack: CoC, material certs, heat treat certs, HIP charts, NDE reports, CMM reports, CT scan reports (if applicable), and any deviation/waiver documentation. Also confirm data formats, retention requirements, and how nonconformances are handled.

5) Change control and configuration management. Complex parts are sensitive to “small” process changes: powder lot changes, machine changes, parameter updates, HIP cycle adjustments, or alternate heat treat furnaces. A mature supplier will have configuration control for parameter sets and controlled change processes. Program managers should align on what constitutes a major change requiring customer notification or requalification, and ensure it is captured in the purchase order and quality clauses.

Practical takeaway: In regulated manufacturing, the best process is one you can audit, reproduce, and document. Build your process choice around a supplier’s ability to deliver a complete, compliant certification pack with traceability and controlled special processes.

Decision tree

Use the decision tree below to translate requirements into an RFQ-ready process choice. This is written as a set of practical gates; at each step, the “best manufacturing process for complex parts” is the one that passes the gate with the lowest overall risk (technical + schedule + compliance).

Gate 1: Can all functional geometry be produced with subtractive access?
If YES: Start with CNC, including 5-axis machining and advanced fixturing. Consider billet for speed and simplicity, or forging for better material utilization and properties if geometry and lead time support it.
If NO: Proceed to Gate 2.

Gate 2: Are internal features required (channels/manifolds) that must be integral?
If YES: Strong candidates are PBF AM (DMLS/SLM) with a defined post-processing plan (stress relief, HIP as needed, machining, NDE). Ensure internal feature verification is feasible (often CT scanning or validated process controls).
If NO: Proceed to Gate 3; PM-HIP, casting, or fabrication may still be viable.

Gate 3: Is the material and property requirement compatible with the candidate process?
If candidate is PBF AM: Confirm powder availability, parameter maturity for the alloy, and whether HIP/heat treat can meet mechanical property requirements. Ask for typical properties, build orientation considerations, and evidence of repeatability (process capability, not just one test).
If candidate is casting/forging: Confirm qualified sources, tooling feasibility, and whether defect risk can be managed with NDE and process controls.
If candidate is PM-HIP: Confirm container design approach, HIP cycle control, density expectations, and machining plan.

Gate 4: Can tolerance and finish be achieved with a realistic post-processing plan?
Define which surfaces will be machined and how datums will be controlled across thermal steps. For AM and PM-HIP, assume finish machining for interfaces. Confirm inspection methods: CMM for external geometry; CT scanning or other NDE for internal features as required. Ensure the plan includes deburring, surface finish control, and verification.

Gate 5: Does the process scale to your expected volume and schedule?
For LRIP, prioritize low NRE and fast iteration (often billet machining or AM). For sustained production, evaluate throughput constraints: machine time, HIP capacity, heat treat availability, machining hours, and inspection bottlenecks. Ask suppliers how they will maintain lead time at rate and how they handle capacity contingencies.

Gate 6: Can the supplier deliver compliance and documentation?
Confirm ITAR handling if applicable, DFARS/specialty metals sourcing controls as required, AS9100 quality system expectations, and NADCAP requirements for special processes and NDE. Specify certification pack contents in the RFQ and align on change control.

Putting it into an RFQ (a practical template):
Step 1: Provide controlled drawing/model with GD&T and identify critical features and inspection requirements.
Step 2: State material specification, condition, and any required property minima; require full traceability and CoCs.
Step 3: Define the intended process route (e.g., “PBF AM + stress relief + HIP + heat treat + 5-axis finish machining”) or ask the supplier to propose the route with justification.
Step 4: Call out required post-processing and special processes; specify whether NADCAP is required for heat treat/NDE and whether subcontractors must be approved.
Step 5: Define acceptance criteria and certification pack contents (material certs, HIP/heat treat records, NDE reports, CMM/CT reports, FAIR requirements).
Step 6: Request capacity, lead time, and change control commitments, including handling of machine/powder/parameter changes for AM.

Final guidance: When in doubt, choose the process that reduces the number of “unknowns” in your program—especially unknown inspection methods, unknown post-processing distortion, and unknown compliance gaps. Complex geometry is achievable across multiple manufacturing routes, but predictable delivery requires aligning design, process, inspection, and quality documentation from the start.