Molybdenum, renowned for its high melting point (2,623°C), excellent thermal conductivity, and low thermal expansion coefficient, is widely used in semiconductor manufacturing, vacuum furnaces, and high-temperature structural components. However, its high strength, brittleness at room temperature, and susceptibility to work hardening pose significant challenges in precision bending. This study investigates a production case where a 0.3mm-thick TZM (Ti-Zr-C-doped Mo) plate was bent into a U-shaped component with 0.1mm verticality tolerance and Ra ≤0.8μm surface roughness, meeting requirements for vacuum furnace heat shields.
2. Material Characteristics and Challenges
- Material Grade: TZM alloy (Mo + 0.40–0.55% Ti + 0.06–0.12% Zr + 0.01–0.043% C)
- Key Properties:
- Ultimate tensile strength: 620–720 MPa
- Yield strength: 480–550 MPa
- Elastic modulus: 330 GPa
- Work hardening rate: 1.2–1.5× higher than pure Mo
- Process Challenges:
- Springback: Elastic recovery after bending exceeds 1.5mm for a 50mm-radius bend.
- Surface Cracking: Micro-cracks form at bending radii <3× plate thickness.
- Oxidation: Surface discoloration occurs at temperatures above 400°C during forming.
3. Process Optimization Strategies
3.1 Mold Design and Tooling
- Die Geometry:
- Die radius: 1.2mm (4× plate thickness) to minimize cracking risk.
- Clearance Adjustment: Reduced single-side clearance from 2.8mm to 2.65mm (88% of nominal thickness) to counteract springback.
- Anti-Deflection Features: Added ribbed reinforcements to the die base to prevent deformation under 300-ton hydraulic press loads.
- Tooling Materials:
- Punch/Die Coating: Hard chrome-plated tool steel (HRC 62–64) with a 0.02mm TiN coating to reduce friction and wear.
- Top Platen Modification: Integrated a 0.5mm-thick copper mesh layer to distribute pressure uniformly and prevent localized indentation.
3.2 Adaptive Bending Parameters
- Multi-Stage Bending:
- Pre-Bending: 30° preliminary bend at 150°C to align grain flow.
- Final Bending: 85° cold forming at 0.5mm/s ram speed to achieve 90° final angle.
- Stress Relief Annealing:
- Performed at 1,100°C for 30 minutes in a vacuum furnace to reduce residual stresses by 75%.
- Cooling Rate: Controlled at 5°C/min to prevent recrystallization-induced distortion.
3.3 Surface Protection Measures
- Lubrication:
- Applied MoS₂-based dry film lubricant with a 3μm coating thickness to reduce friction coefficient from 0.18 to 0.08.
- Inert Atmosphere Forming:
- Conducted bending in a nitrogen-purged chamber (O₂ <50 ppm) to suppress oxide formation.
- Post-Bending Surface Treatment:
- Electropolishing: Removed 5μm of surface layer to eliminate work-hardened zones and reduce roughness from Ra 1.2μm to Ra 0.6μm.
- Laser Shock Peening: Applied 2GW/cm² pulse intensity to induce compressive residual stresses (−250 MPa) within 50μm of the surface, enhancing fatigue resistance by 40%.
4. Quality Control and Validation
- Dimensional Inspection:
- CMM Measurement: Confirmed 0.1mm verticality tolerance across 200mm-long legs using a 0.001mm-resolution coordinate measuring machine.
- Surface Integrity Assessment:
- SEM Analysis: Revealed no cracks or delamination at bending radii.
- XRD Phase Analysis: Detected no α-MoO₃ oxide peaks, confirming surface purity.
- Mechanical Testing:
- Four-Point Bending: Demonstrated a 15% reduction in springback after optimization, with final angles averaging 90.1°±0.05°.
This case study establishes a methodology for achieving 0.1mm-level precision in molybdenum plate bending while maintaining surface integrity. Key innovations include:
- Die clearance optimization to counteract springback.
- Multi-stage forming with stress relief annealing to minimize distortion.
- Inert atmosphere processing and electropolishing to prevent oxidation and surface defects.
The validated process enables manufacturers to produce high-value components for semiconductor equipment, aerospace, and nuclear industries, where dimensional accuracy and surface quality are critical.