| name | forging-design |
| description | Forging design — die design, flash design, draft angles, parting line, forging allowances, grain flow, closed/open die, isothermal forging, forging pressure calculation, defects, AMS 2750 pyrometry, Nadai/Slab analysis. |
| metadata | {"priority":7,"promptSignals":{"phrases":["forging design","die forging","forging draft angle","closed die forging","forging pressure","grain flow forging"],"minScore":3}} |
Forging Design — Complete Skill
Forging Process Classification
Open-die (smith/slab) forging: dies wider than workpiece; no side constraint; large simple shapes (shafts, discs)
Closed-die (impression) forging: workpiece confined in die impressions; near-net shape; flash forms at parting line
Precision (flashless) forging: billet volume equals cavity volume; no flash waste; close tolerances
Isothermal forging: dies and billet at same temperature; slow strain rate; near-net shape for titanium/nickel
Cold forging: room temperature; higher forces; near-net; good surface finish; limited to simple shapes
Warm forging: 0.3–0.5 × T_melting; balance between cold and hot forging properties
Die Design Fundamentals
Draft Angles
External draft (surfaces moving away from die):
Standard: 5–7° for steel; 3–5° for aluminum; allows part extraction
Precision forging: 3° minimum; isothermal: 1–2°
Internal draft (pockets/holes):
Deeper than external: 7–10° (more friction and spring-back in cavity)
Minimum draft for ejection (rough guide):
Depth/width ratio < 2: 5° draft acceptable
Depth/width ratio 2–4: 7° draft
Depth/width ratio > 4: 10° draft; consider split die
Parting Line
Parting line design principles:
Locate at maximum cross-section of forging → reduces die depth and draft length
Straight parting line: simplest; for symmetric parts
Stepped/broken parting: allows one-sided draft; for asymmetric parts with long cuts
Flash land: narrow land at parting line where flash forms (controlled thickness)
Flash design:
Flash thickness t_f: 2–5 mm for steel; determines forging pressure
Flash ratio: t_f / w_land = 0.15–0.25 (w_land = flash land width 5–20 mm)
Thinner flash → higher pressure → fills cavity better → more tool load
Flash gutter: overflow reservoir for excess material beyond land
Fillet and Corner Radii
Fillet radius (concave): r_fillet ≥ 3 mm (steel); ≥ 4 mm (Ti); prevents die stress concentration
Corner radius (convex): r_corner ≥ 1.5–3 mm; prevents folding defects in workpiece
Fillet vs. corner:
Insufficient fillet → cold shuts (laps), cracks, die stress concentration
Excessive corner radius → flash penetrates die, tool damage
Die radius recommendation:
r_fillet ≥ max(2 mm, 0.05 × D) where D = adjacent feature dimension
r_corner ≥ 0.5 × r_fillet
Forging Allowances and Tolerances
Machining allowance:
Per-side: 1.5–5 mm depending on forging size and surface finish requirements
Larger forgings: more scale, less accuracy → larger allowance
Dimensional tolerances (closed-die forging, steel, ASTM A291/SAE MA 4009):
| Forging Weight | Linear Tolerance |
|---|
| < 1 kg | ±0.5 mm |
| 1–10 kg | ±1.0 mm |
| 10–100 kg | ±2.0 mm |
| > 100 kg | ±3.0–5.0 mm |
Flash tolerance (mismatch): 0.5–2 mm depending on die setup accuracy
Weight tolerance: ±5% for most closed-die forgings (billet volume control)
Forging Pressure Calculation
Slab Analysis (Simple Estimate)
Forging pressure for disk:
p_avg = σ_0 × (1 + 2μR/(3h)) [MPa; σ_0 = flow stress; μ = friction coefficient; R = disk radius; h = height]
Thin flat disk (Siebel equation):
p_avg = σ_0 × e^(2μR/h) [exponential; more accurate for thin disks]
Peak pressure at center:
p_center = σ_0 × e^(2μR/h) [maximum die pressure occurs at center]
Total forging force:
F = p_avg × A_projected [N; A_projected = plan area of forging including flash land]
Typical: 300–800 MPa forging pressure for steel at hot-forging conditions
Flow Stress
Zener-Hollomon parameter:
Z = ε̇ × exp(Q/(RT)) [Z = Zener-Hollomon; ε̇ = strain rate; Q = activation energy; R = 8.314 J/(mol·K); T = temperature [K]]
Flow stress (hyperbolic sine law):
Z = A × [sinh(ασ₀)]^n
σ₀ = (1/α) × arsinh((Z/A)^(1/n)) [α, A, n = material constants from hot compression tests]
Typical flow stress (steel at forging T):
Carbon steel at 1200°C, ε̇ = 10 s⁻¹: σ_0 ≈ 80–120 MPa
Titanium at 950°C (β forging): σ_0 ≈ 100–200 MPa
Nickel superalloy at 1050°C: σ_0 ≈ 200–400 MPa
Grain Flow and Mechanical Properties
Grain flow principle: elongated grains align with material flow during forging → anisotropic properties
Best properties: load axis parallel to grain flow (transverse to forging plane)
Fiber flow design:
Avoid cutting through grain flow in final machining: fibers should follow contour of critical surfaces
Gear: forged to near-shape → grain flow around teeth → maximum fatigue life vs. machined from bar
Grain refinement:
Dynamic recrystallization (DRX): occurs during hot forging → fine grain (ASTM grain size 6–8)
Condition: Zener-Hollomon Z > Z_critical; hot rolling or forging typically achieves this
Typical properties (closed-die vs. machined from bar):
Fatigue life: 30–50% improvement in forged grain flow direction
Impact toughness: 20–40% higher due to continuous grain flow
Common Forging Defects
Cold shuts (laps): two metal streams meet but don't bond; caused by insufficient die fill or improper metal flow
Prevention: adequate fillet radii; correct billet shape; proper flash design
Underfill: incomplete die fill; insufficient billet volume or too little forging pressure
Prevention: verify billet volume = die cavity volume + flash volume
Die shift: top and bottom dies not aligned; causes off-center forging
Tolerance: ≤ 1–2 mm for precision; die lock or alignment pins
Scale pits: oxide scale pressed into surface during forging; remove by descaling (shot blast)
Internal cracking: strain exceeds ductility; caused by low ductility (wrong forging T or alloy)
Detection: ultrasonic testing per ASTM A388
Forging burst: fracture along shear band if internal pressure too high (center burst)
Prevention: pre-heat uniformly; use lower ram speed
Heating and Temperature Control
AMS 2750 (Pyrometry): standard for furnace calibration in aerospace forging
- Type A (±3°C): critical aerospace processes
- Type B (±6°C): standard aerospace
- Temperature uniformity survey (TUS) required every 6–12 months
Forging temperature ranges:
| Alloy | Forging Temperature [°C] |
|---|
| Carbon steel (0.3% C) | 1100–1250 |
| Stainless 304 | 950–1200 |
| Ti-6Al-4V (α+β) | 900–970 (below β-transus) |
| Ti-6Al-4V (β forge) | 980–1050 (above β-transus) |
| IN718 (superalloy) | 970–1080 |
| Al 7075 | 370–450 |
Heating rate: ≤ 150°C/hr to 600°C for large steel forgings (thermal shock prevention)
Soak time: 1 hour per 25 mm of section thickness (rule of thumb)
Standards
| Standard | Scope |
|---|
| ASTM A668 | Steel forgings, carbon and alloy, general requirements |
| ASTM A291 | Carbon and alloy steel gear forgings |
| ASTM A388 | Ultrasonic examination of heavy steel forgings |
| AMS 2750 | Pyrometry (temperature control for forging furnaces) |
| AMS 4928 | Ti-6Al-4V billet and bar for forging |
| NADCA STD-207 | Forging process design standards |
Output
Provide: forging process type (closed-die/open-die/isothermal), material and forging temperature [°C], flow stress σ_0 [MPa], draft angles (external/internal [°]), fillet radius [mm] and corner radius [mm], parting line location description, flash thickness [mm] and land width [mm], billet weight [kg] and volume [cm³] (including flash), total forging force [kN], die material (H13/H21 for steel; Inconel for isothermal), grain flow orientation vs. primary load direction, machining allowance [mm per side], dimensional tolerances [mm], defect prevention measures, AMS 2750 pyrometry class (if aerospace), and applicable standard (ASTM A668, AMS 2750, ASTM A388).
