Hot Isostatic Pressing (HIP) Capsule & Sheet Metal Vector Guide
Formulating near-net-shape powder compaction capsules, thermal degassing vacuum prepress, and unfolded sheet metal blank geometry.
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1. Powder Metallurgy Hot Isostatic Pressing (PM-HIP) Principles
Hot Isostatic Pressing (PM-HIP) consolidates pre-alloyed gas-atomized metal, intermetallic, or technical ceramic powders into 100% dense solid structures without the segregation, coarse dendritic grain structures, or porosity common in large ingot castings. In a typical HIP cycle, powder is packed into a custom-fabricated sheet metal canister, hermetically sealed under high vacuum ($< 10^{-4}\text{ mbar}$), and loaded into an internally heated pressure vessel subjected to pure argon gas at pressures between $100 - 175\text{ MPa}$ ($1,000 - 1,750\text{ bar}$) and temperatures up to $1200^\circ\text{C}$.
2. Canister Shrinkage Modeling & Near-Net-Shape (NNS) Tooling
Because loose gas-atomized powder occupies only $65\% - 72\%$ of solid theoretical volume (tap density $\rho_{\text{tap}}$), the canister undergoes significant volumetric shrinkage during consolidation. Assuming isotropic densification, the linear dimensional scale factor $S_L$ required when designing the pre-HIP canister DXF cavity is:
S_L = (1 / rho_rel)^(1/3) - 1| Alloy Category | Standard Powder Tap Density | Linear Shrinkage Factor ($S_L$) | Canister Material |
|---|---|---|---|
| Inconel 718 / 625 Superalloys | $68.0\% - 70.0\%$ | $+12.6\% \text{ to } +13.7\%$ | Mild Steel (AISI 1018 / DC01) |
| Ti-6Al-4V Grade 5 Titanium | $65.0\% - 67.5\%$ | $+14.0\% \text{ to } +15.4\%$ | Commercially Pure Grade 1 Titanium or Mild Steel with stop-off barrier |
| 316L / Duplex Stainless Steels | $69.0\% - 72.0\%$ | $+11.5\% \text{ to } +13.1\%$ | Mild Steel (AISI 1018) |
| M2 / CPM-10V Tool Steels | $67.0\% - 70.0\%$ | $+12.6\% \text{ to } +14.3\%$ | Low Carbon Deep Draw Steel |
3. Sheet Metal Canister CAM Prepress & Fabrication Guidelines
- Corner Fillets & Hourglass Distortion: Rigid flat canister endcaps resist radial contraction, resulting in an "hourglass" taper at cylinder centers. To ensure uniform shrinkage, design convex domed endcaps with large transition blend radii ($R \ge 25\text{ mm}$).
- Weld Seam Integrity (TIG / Plasma): All canister longitudinal and circumferential seam welds must achieve 100% full-penetration with zero porosity or undercut. Even a single pinhole leak causes pressurized argon gas to saturate the powder bed, preventing densification and scrapping the entire charge. Helium mass spectrometer leak test all canisters to $< 10^{-9}\text{ mbar}\cdot\text{L/s}$ prior to powder filling.
- Evacuation Stem & Crimp Sealing: Weld a seamless $12.7\text{ mm}$ OD stainless or mild steel evacuation tube to the canister top cap. Following hot vacuum outgassing (350°C for 8 hours), mechanically pinch-crimp the tube with hydraulic guillotine jaws and GTAW seal weld the severed tip.
- Sacrificial Canister Removal: After HIP consolidation, remove the external mild steel canister shell by immersion in hot nitric acid / pickling bath ($20\%\text{ HNO}_3$ dissolves mild steel while passivating stainless steel or superalloys) or by CNC face turning.
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