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1. Introduction

Molybdenum (Mo), a refractory metal renowned for its 2620°C melting point, 138 W/(m·K) thermal conductivity, and 4.8×10⁻⁶/K coefficient of thermal expansion, plays a pivotal role in high-temperature applications. However, transforming Mo powder into dense, defect-free components presents contrasting challenges in powder metallurgy (PM) and forging processes. This article dissects the technical trade-offs between PM-driven densification and forging-based refinement, emphasizing material evolution, process optimization, and industrial implications.

2. Fundamentals of Molybdenum Powder Densification

2.1 PM Processing Chain

• Cold Isostatic Pressing (CIP): 325-mesh Mo powder (44 μm average particle size) is pressed under 200–300 MPa using rubber molds, achieving 60–65% theoretical density (TD).
• Sintering:
  • Low-Temperature Presintering: 1373–1573 K in hydrogen atmosphere for 20–40 minutes enhances green strength by eliminating lubricants and promoting neck formation.
  • High-Temperature Sintering: 1973–2373 K in vacuum or hydrogen reduces porosity to 2–10%, yielding 90–98% TD.
• Advanced Densification:
  • Hot Isostatic Pressing (HIP): Combines 1200–1500°C heating with 100–200 MPa gas pressure, achieving ≥99% TD by eliminating residual pores.
  • Spark Plasma Sintering (SPS): Uses pulsed electric currents to sinter at 1600°C under 50 MPa, reducing processing time by 80% while preserving nanocrystalline structures.

2.2 Critical Challenges

• Porosity Control: Large-diameter (Φ150 mm) rods exhibit radial density gradients (9.78 g/cm³ core vs. 9.85 g/cm³ edge) due to uneven pore closure during sintering.
• Grain Coarsening: Extended sintering above 1800°C triggers abnormal grain growth, degrading mechanical properties.
• Oxygen Contamination: PM-derived Mo contains 500–1000 ppm O, forming brittle MoO₂ phases at grain boundaries.

3. Evolution of Molybdenum Rod Forging

3.1 Forging Process Variants

• Hydraulic Fast Forging (HFF):
  • Mechanism: 1000–3000 kN hydraulic presses apply static loads at 0.01–0.1 s⁻¹ strain rates.
  • Case Study: Φ150 mm sintered rods forged to Φ105 mm achieve 10.11 g/cm³ density (99.1% TD) and 203.9 HV1.0 hardness, with <5% radial variability.
• Air Hammer Forging (AHF):
  • Mechanism: Pneumatic hammers deliver 5–10 Hz impacts at 1–5 MPa peak pressures.
  • Limitations: 10.00 g/cm³ density (98.0% TD) and 194.7 HV1.0 hardness, with 12% radial variability due to uneven stress distribution.

3.2 Microstructural Refinement

• Fiberization: HFF-processed rods exhibit elongated grains (aspect ratio >5:1) aligned with the forging direction, enhancing tensile strength by 30% vs. AHF.
• Dynamic Recrystallization (DRX): At 1200–1400°C, 15–30% strain induces DRX, refining grains to 5–10 μm (vs. 20–50 μm in AHF).
• Defect Mitigation: HFF reduces edge cracking by 75% through controlled strain rates, while AHF’s cyclic loading causes 20–30% higher residual stresses.

4. Process-Microstructure-Property Correlations

4.1 Density and Hardness Gradients

4.2 Fatigue Performance

• HFF Rods: S-N curves indicate 2×10⁶ cycles at 450 MPa stress amplitude, attributed to uniform grain orientation and defect-free microstructures.
• AHF Rods: Fail at 1.2×10⁶ cycles under identical conditions due to edge microcracks.

5. Industrial Implications and Cost-Benefit Analysis

5.1 Process Economics

• PM Route:
  • Pros: Near-net-shape capability, 20–30% material savings, and suitability for complex geometries.
  • Cons: $15–20/kg production cost, 4–6 week lead times, and 1–2% scrap rates.
• Forging Route:
  • Pros: $8–12/kg cost, 2–3 day lead times, and 0.5% scrap rates for simple geometries.
  • Cons: 10–15% material loss during machining and limitations on part complexity.

5.2 Application-Specific Selection

• Aerospace Nozzles: Prefer HFF-processed Mo-La rods (0.4–1.2% La) for 1600°C recrystallization resistance.
• Electron Beam Melting Crucibles: Use PM+HIP Mo for 99.99% purity and 20–30 mm single-crystal grain sizes.

6. Future Directions

• Hybrid Manufacturing: Integrate SPS densification with incremental HFF for 99.5% TD and 1–2 μm grain sizes.
• AI-Driven Process Control: Machine learning models optimize HFF strain rates and temperatures, reducing trial-and-error iterations by 50%.
• Additive-Subtractive Synergy: Combine laser powder bed fusion with ultrasonic-assisted forging to produce intricate lattice structures with 98% TD.

The choice between Mo powder densification and forging hinges on density requirements, cost constraints, and part complexity. While PM excels in purity and near-net-shape capability, forging delivers superior mechanical properties and scalability. Emerging hybrid techniques promise to bridge these gaps, enabling the production of defect-free, high-performance Mo components for next-generation technologies.

Keywords: Molybdenum powder metallurgy, hydraulic fast forging, microstructural refinement, cost-benefit analysis, hybrid manufacturing