forging-design

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).