| name | powder-metallurgy |
| description | Powder metallurgy — powder production (atomization, reduction, electrolysis), green compact pressing (Heckel equation, density-pressure relationship), sintering mechanisms (solid-state, liquid phase), hot isostatic pressing (HIP), metal injection molding (MIM), PM steel grades (MPIF 35), dimensional control and tolerances, porosity and mechanical properties, and MPIF/ASTM standards for PM parts. |
| metadata | {"priority":7,"promptSignals":{"phrases":["powder metallurgy","sintering","hot isostatic pressing","HIP process","metal injection molding","MIM process"],"minScore":3}} |
Powder Metallurgy — Complete Skill
Powder Production
Atomization
Gas atomization:
Molten metal stream broken by high-velocity gas jets (N₂ or Ar) → spherical or near-spherical particles
Particle size: 10–150 μm (d₅₀ typically 30–60 μm for pressing-grade powders)
Used for: tool steel, superalloy, stainless steel, aluminum, copper powders
Sphericity → good flowability → consistent die filling
Water atomization:
Water jets break melt → irregular, spongy particles (higher specific surface)
Particle size: 50–200 μm; lower cost than gas atomization
Oxide content higher; often requires reduction annealing; used for PM iron/steel
Cooling rate: gas: 10³–10⁴ K/s; water: 10⁴–10⁶ K/s (finer microstructure; metastable phases possible)
Chemical Reduction
Iron powder by reduction:
Fe₂O₃ + 3H₂ → 2Fe + 3H₂O [sponge iron; irregular particle; low cost; Höganäs process]
Sponge iron: d₅₀ = 50–100 μm; apparent density 2.5–3.0 g/cm³; standard for PM iron parts
Carbonyl decomposition:
Fe(CO)₅ → Fe + 5CO at ~250°C; Ni(CO)₄ → Ni + 4CO at ~40°C
Product: very fine, pure spherical particles (1–10 μm); used for soft magnetic and MIM applications
Key Powder Characteristics
Apparent density (AD): mass/volume in freely poured state; affects die fill volume
Tap density: AD after vibration; ratio tap/AD = Hausner ratio (HR = 1.0–1.25 good flowability; HR > 1.4 poor)
Hall flowability: time for 50 g through Ø2.54 mm orifice per ASTM B213; ≤ 30 s good for pressing
Green strength: minimum compressive strength of pressed but unsintered compact
Compaction (Die Pressing)
Heckel Equation (Density-Pressure Relationship)
Heckel equation:
ln(1/(1−ρ_rel)) = K × p + A [ρ_rel = relative density = actual/theoretical; p = compaction pressure [MPa]]
K = 1/(3×σ_y) [slope; σ_y = yield strength of powder particle material]
A = ln(1/(1−ρ₀)) + B [intercept; ρ₀ = initial relative density after die fill; B = rearrangement constant]
Key compaction parameters:
Iron powder: K ≈ 0.005–0.007 MPa⁻¹; typical σ_y_particle ≈ 100–200 MPa
Compaction pressure: 400–800 MPa for iron-base PM; 150–300 MPa for aluminum
Green density target: 6.4–7.1 g/cm³ for iron (theoretical 7.87 g/cm³; 81–90% density)
Die wall friction:
Friction reduces lateral-to-axial pressure ratio; need double-action pressing for thick/complex parts
Radial pressure ratio: p_lateral/p_axial ≈ ν/(1−ν) ≈ 0.35–0.45 for powder compact
Lubricant: 0.5–1.0 wt% zinc stearate or lithium stearate admixed to reduce die wear and ejection force
Ejection force:
F_eject = μ × p_lateral × A_die_wall [μ = friction coefficient ≈ 0.1–0.2 with lubricant]
Sintering
Solid-State Sintering Mechanisms
Driving force: reduction in total surface energy (particle surface area → grain boundary area)
Primary mechanisms (with material transport paths):
- Surface diffusion (D_s): neck growth, no shrinkage
- Lattice diffusion from surface (D_v): neck growth, no shrinkage
- Grain boundary diffusion (D_gb): neck growth + densification
- Volume diffusion from grain boundary: densification
- Plastic flow (dislocation): rapid at high pressure; HIP
Neck growth (early stage):
(x/R)² = A × t^(n) [x = neck radius; R = particle radius; A, n = mechanism constants]
n = 7/3 for surface diffusion; n = 5/3 for grain boundary diffusion; n = 2 for volume diffusion
Densification (intermediate stage):
ρ_rel(t) = ρ_rel,0 + (1−ρ_rel,0) × (1 − exp(−k_s × t)) [simplified Coble model; k_s ∝ D × γ / (RT × d⁴)]
Rate ∝ T (exponential via D = D₀ × exp(−Q/RT)); finer particle → faster sintering (d^−4 dependence)
Typical sintering conditions (iron-base PM):
Temperature: 1120°C (standard Fe); 1250–1300°C (high-temperature sintering for higher density/strength)
Time: 20–40 min at temperature; atmosphere: endogas (N₂-H₂) or H₂; dew point < −30°C
Density achievable: 6.8–7.3 g/cm³ (86–93% of theoretical) by conventional pressing + sintering
Re-pressing (re-strike) after sintering → density to 7.4 g/cm³
Liquid Phase Sintering (LPS)
Liquid phase sintering: second phase melts → wets solid particles → accelerates densification via solution-reprecipitation
Examples: WC-Co (Co melts; WC dissolves and reprecipitates → full density >99%); Fe-Cu (Cu liquid at 1085°C)
LPS condition: contact angle θ < 90° between liquid and solid
WC-Co hardmetal:
WC-12Co: HV = 1400–1500; TRS (transverse rupture strength) = 2700 MPa; K_Ic = 12–14 MPa√m
Sintering: 1380–1420°C in vacuum or H₂; density >99.8% theoretical (14.0–14.4 g/cm³)
Hot Isostatic Pressing (HIP)
Process Parameters
HIP conditions:
Pressure: 100–200 MPa (argon gas); Temperature: 0.6–0.9 T_m [K]; Time: 2–4 hours
Result: 100% density; eliminates all closed porosity; no interconnected porosity remains
HIP capsule design:
Sheet metal (mild steel) capsule welded around powder compact or pre-sintered compact
Capsule evacuated and sealed before HIP; collapses under isostatic pressure → transmits uniformly
For castings: HIP direct (closed porosity must not have interconnected path to surface)
Densification mechanism:
P_applied >> σ_y (at temperature) → plastic creep flow closes pores
Effective stress on pore: σ_pore = P_applied × f(geometry)
Fully dense below 0.01% porosity (any porosity < detection by Archimedes / CT)
Properties after HIP:
Fatigue life increase: 2–10× vs. conventional PM (elimination of crack-initiating pores)
Uniform properties: no density gradient (unlike pressed compact which has density gradient from die friction)
Ti-6Al-4V HIP: σ_UTS = 930 MPa; σ_y = 860 MPa; RA = 18% (vs. conventional powder: 830/750 MPa)
Metal Injection Molding (MIM)
Process Steps
- Feedstock: fine powder (d₅₀ = 5–20 μm) + binder (paraffin wax + polyethylene + stearic acid); powder loading: 55–65 vol%
- Injection molding: identical to plastic injection molding; mold at 150–200°C; pressure 50–150 MPa
- Debinding: solvent debinding (hexane removes wax) + thermal debinding (400–600°C burnout of polymer)
- Sintering: 1200–1400°C; HV atmosphere; shrinkage: 15–20% linear (must be accounted in mold design)
MIM materials:
316L SS: σ_y = 170 MPa; σ_UTS = 485 MPa; density 7.9 g/cm³ (>96%); MPIF 35 MIM-316L
17-4 PH SS (H900): σ_y = 1100 MPa; σ_UTS = 1180 MPa
Fe-2Ni (MIM iron): σ_y = 400 MPa; σ_UTS = 550 MPa
Dimensional tolerance: ±0.3–0.5% of dimension (better than casting, poorer than machining)
Minimum wall thickness: 0.5 mm; maximum part mass: 100–200 g (economic); mass production: > 1000 pcs/batch
PM Steel Grades (MPIF 35)
MPIF Designation System
Format: Material-Density-Heat treatment
F-0005-20: Fe + 0.5% graphite; density 6.0 g/cm³; 20 ksi (138 MPa) TRS
FC-0205-40: Fe-Cu-C (2% Cu, 0.5% C); 40 ksi TRS
Common PM steels:
| MPIF Grade | Composition | Min UTS [MPa] | Min HRC |
|---|
| F-0000 | Pure iron | 140 | — |
| F-0008 | Fe-0.8C | 310 | 60 (hardened) |
| FC-0205 | Fe-2Cu-0.5C | 310 | — |
| FN-0205 | Fe-2Ni-0.5C | 380 | — |
| FLC-4608 | Fe-Mo-Ni-Cu-C | 620 | — |
Porosity effect on fatigue:
σ_fatigue_PM = σ_fatigue_wrought × (ρ_rel)^n [n ≈ 3–5; porosity degrades fatigue disproportionately]
At 90% density: σ_fatigue ≈ (0.90)^4 × σ_wrought = 0.66 × σ_wrought
Standards and References
| Standard | Scope |
|---|
| MPIF Standard 35 | Material standards for PM structural parts |
| ASTM B212 | Apparent density of free-flowing metal powder |
| ASTM B213 | Flow rate of metal powder |
| ASTM B328 | Sintered bronze bearing dimensions |
| ISO 4490 | Metallic powders — flow rate |
| ASTM E384 | Microhardness of PM parts |
| MPIF Standard 62 | Material standards for MIM parts |
Output
Provide: application and production volume (part geometry, mass [g], annual volume), powder selection (material: Fe/SS/Ti/WC-Co; production method: water atomized/gas atomized/reduced/carbonyl; d₅₀ [μm]; apparent density [g/cm³]; flowability [s/50g]), compaction (target green density [g/cm³]; relative density [%]; compaction pressure [MPa] from Heckel: p = (ln(1/(1-ρ_rel)) - A)/K; die friction coefficient; lubricant type), sintering (temperature [°C]; atmosphere: endogas/H₂/vacuum; time at temperature [min]; density after sintering [g/cm³]; primary mechanism: solid-state/liquid phase), optional HIP (if required: 150 MPa / 1000°C / 3h; final density ≥ 99.9%; property improvement), MIM (if complex geometry < 100 g: feedstock loading [vol%]; binder type; shrinkage [%] for mold compensation; tolerance ±[%]), mechanical properties (σ_UTS [MPa]; σ_y [MPa]; elongation [%]; HRB/HRC; fatigue limit [MPa] corrected for porosity), MPIF grade (designation: e.g., FN-0205-40; or MIM-316L), dimensional tolerances (MPIF Class A/B/C; ±[mm] at each dimension), and applicable standard (MPIF 35 for structural PM; MPIF 62 for MIM; ASTM B212/B213 for powder characterization).
