Investment Casting vs CNC Machining Titanium: Cost and Quality Decision Framework
Executive summary: Investment casting and CNC machining are the two dominant processes for titanium components, and the choice between them depends on part geometry, production quantity, mechanical property requirements, and cost target. Investment casting is preferred for complex net-shape or near-net-shape geometries, for production quantities above about 50 to 200 units (where the tooling amortizes), and for internal features that cannot be machined. CNC machining is preferred for simpler geometries that can be produced from bar or billet stock, for low quantities (where tooling cost is not amortized), and for parts that require the highest mechanical properties (because wrought bar stock has fewer internal defects than cast material). The break-even quantity depends on the part size, the machining time, and the casting tooling cost, but typically falls in the 25 to 100 unit range for aerospace components. The procurement mistake is to default to one process or the other without analyzing the specific part; the right approach is a process selection matrix applied to each part individually.
Process capability comparison
The two processes have fundamentally different capability envelopes. The part geometry and the required features determine which process is capable.
Table 1: Process capability comparison
| Capability | Investment casting | CNC machining |
|---|---|---|
| Minimum wall thickness | 1.5 to 2.0 mm (thin walls difficult) | 0.5 mm (limited by tooling) |
| Internal passages | Yes (with ceramic cores) | Yes (limited by tool access) |
| Undercuts | Yes (multi-piece die) | Limited (5-axis may reach) |
| Tolerances (as-cast) | +/- 0.3 to 0.5 mm typical | +/- 0.025 mm typical |
| Tolerances (machined) | +/- 0.05 to 0.1 mm after secondary | +/- 0.025 mm direct |
| Surface finish (as-cast) | Ra 1.6 to 6.3 µm | Ra 0.4 to 1.6 µm direct |
| Surface finish (machined) | Ra 0.4 to 1.6 µm | Ra 0.4 to 1.6 µm |
| Minimum draft angle | 1 to 3° required | Not applicable |
| Tooling cost (typical) | USD 5,000 to 50,000 | USD 100 to 5,000 (fixturing) |
| Unit cost (typical, Ti bracket) | USD 50 to 500 | USD 100 to 2,000 |
| Lead time (tooling + first article) | 8 to 16 weeks | 2 to 6 weeks |
| Break-even quantity (typical) | 50 to 200 units | 1 to 50 units |
The investment casting envelope favors complex internal geometry, near-net-shape external features, and moderate tolerance requirements. The CNC machining envelope favors tight tolerances, simple external geometry, and low production quantities. The break-even quantity depends on the specific part, but is typically in the 25 to 100 unit range. See the titanium investment casting net shape solutions guide for the casting-side discussion.
Mechanical property differences
The mechanical properties of investment cast titanium differ from wrought bar stock in ways that affect design.
Table 2: Mechanical property comparison, Ti-6Al-4V
| Property | Investment cast (typical) | Wrought bar, annealed (typical) |
|---|---|---|
| Tensile strength, ultimate | 895 MPa (matches wrought minimum) | 925 MPa (typical) |
| Yield strength | 820 MPa (matches wrought minimum) | 860 MPa (typical) |
| Elongation | 8 to 12% | 12 to 18% |
| Fatigue endurance (10^7 cycles) | 300 to 400 MPa | 500 to 550 MPa |
| Fracture toughness | 50 to 65 MPa√m | 65 to 75 MPa√m |
| Internal defects (typical) | Shrinkage porosity, gas porosity (hot isostatic pressing can close) | Centerline segregation (limited) |
The tensile and yield properties are similar because the specification (ASTM B348 or AMS 4928 for wrought, ASTM B367 or AMS 4990 for casting) sets minimum values that both processes meet. The differences are in elongation, fatigue endurance, and fracture toughness, where the wrought material outperforms cast. The mechanism is the internal defect population — cast material has shrinkage and gas porosity that act as fatigue crack initiation sites; wrought bar has fewer defects because the thermomechanical processing breaks up the as-solidified structure. Hot isostatic pressing (HIP) closes the cast porosity and recovers much of the fatigue and toughness gap, but does not eliminate it. The procurement specification for fatigue-critical cast components should specify HIP.
Cost structure comparison
The cost structure of the two processes is fundamentally different. Investment casting has high fixed cost (tooling) and low variable cost (per part); CNC machining has low fixed cost and high variable cost. The crossover depends on the part complexity and the machining time.
Table 3: Cost structure comparison (typical Ti aerospace bracket)
| Cost element | Investment casting | CNC machining |
|---|---|---|
| Tooling / fixturing | USD 15,000 (one-time) | USD 2,000 (one-time) |
| Material (per part) | USD 30 to 80 (cast blank) | USD 50 to 200 (bar stock) |
| Process labor (per part) | USD 30 to 60 (casting + finishing) | USD 80 to 300 (machining) |
| Inspection (per part) | USD 20 to 50 (RT or UT) | USD 20 to 50 (dimensional) |
| Total cost at 10 units | USD 24,500 (mostly tooling) | USD 3,500 |
| Total cost at 100 units | USD 28,000 | USD 23,000 |
| Total cost at 500 units | USD 50,000 | USD 110,000 |
| Total cost at 1,000 units | USD 80,000 | USD 220,000 |
The crossover is around 100 units in this example; below 100 units, CNC machining is cheaper because the tooling amortization is small; above 100 units, casting is cheaper because the per-part cost is lower. For complex parts where the casting replaces 10 or more machining operations, the crossover drops to 25 units or lower. For simple parts where the machining is one or two operations, the crossover can be above 500 units. The procurement decision should be based on the specific part cost analysis, not on a generic rule of thumb.
Process selection matrix
The decision between investment casting and CNC machining reduces to a small number of engineering and economic questions. The matrix below summarizes the recommended process for each combination.
Table 4: Process selection matrix
| Driver | Casting preferred | Machining preferred |
|---|---|---|
| Quantity | Above 100 units typical | Below 50 units typical |
| Complexity | Internal passages, undercuts, near-net-shape | Simple prismatic, prismatic with holes |
| Tolerance | +/- 0.1 mm acceptable | +/- 0.025 mm required |
| Fatigue life | HIP required for fatigue-critical | Wrought properties preferred |
| Material utilization | Net shape (10 to 30% machining) | Subtractive (10 to 30% utilization) |
| Lead time | 8 to 16 weeks (tooling) | 2 to 6 weeks (no tooling) |
| Unit cost target | USD 50 to 500 | USD 100 to 2,000 |
| Surface finish | Ra 0.8 to 6.3 µm | Ra 0.4 to 1.6 µm |
The right answer for a specific part depends on the intersection of the part requirements. A complex internal passage structure at 500 units per year is a strong casting application. A tight-tolerance fatigue-critical bracket at 10 units per year is a strong machining application. A medium-complexity bracket at 50 units per year is a judgment call that depends on the unit cost target and the lead time tolerance.
Procurement rules for process selection
Rule 1 — Analyze the cost crossover for each part. Do not default to one process or the other. The crossover depends on the specific part, the specific tooling cost, and the specific machining time. The crossover can be calculated with a spreadsheet and the supplier quotes for tooling and unit cost.
Rule 2 — Specify HIP for fatigue-critical cast components. HIP closes the internal porosity and recovers most of the fatigue and toughness gap between cast and wrought. The procurement specification should state “HIP per AMS 2774 or equivalent” for any cast component with a fatigue requirement.
Rule 3 — Verify the casting supplier’s inspection capability. Investment cast titanium parts require radiographic (RT) or ultrasonic (UT) inspection for internal defects. The supplier must have the inspection capability and the qualified personnel.
Rule 4 — Match the machining supplier to the material form. A shop qualified on bar stock may not be qualified on cast blank; the machining parameters and the fixturing are different. The supplier qualification should cover the specific material form.
Rule 5 — Engineer contradiction — net shape does not mean no machining. Investment cast titanium parts typically require machining on the critical interfaces (mating faces, bolt holes, bearing surfaces). The casting produces the near-net shape; the machining produces the functional surfaces. The supplier scope should include both.
For the casting-side discussion, see the titanium investment casting net shape solutions guide. For the machining-side discussion, see the titanium CNC machining services guide. To request a process selection analysis for a specific titanium component, request a manufacturing review with the engineering team.