Molybdenum (Mo) plates, valued for their high melting point, excellent thermal conductivity, and corrosion resistance, are widely used in aerospace, electronics, and nuclear industries. However, cracks often occur during the bending process, compromising product quality and reliability. This article systematically analyzes the root causes of Mo plate bending cracks from the perspectives of stress distribution, material properties, and processing parameters, and proposes targeted solutions through process optimization and material pretreatment.
1. Mechanism of Bending Cracks in Molybdenum Plates
1.1 Stress Concentration at the Edge
During bending, the outer surface of the Mo plate experiences tensile stress, while the inner surface experiences compressive stress. The edge region, where stress gradients are significant, is prone to crack initiation. Microscopic defects such as scratches or oxide layers on the plate surface exacerbate stress concentration, reducing the material’s effective bearing capacity.
1.2 Anisotropy and Work Hardening
Molybdenum exhibits pronounced anisotropic deformation behavior due to its body-centered cubic (BCC) crystal structure. Cold-rolled Mo plates often have directional grain structures, increasing the risk of cracking when bending parallel to the rolling direction. Additionally, work hardening during cold forming reduces plasticity, making the material more susceptible to brittle fracture.
1.3 Insufficient Bending Radius
A small bending radius increases the tensile and compressive strains on the outer and inner surfaces, respectively. When the strain exceeds the material’s elongation limit, cracks form. Studies show that for 0.5mm-thick Mo plates, a bending radius below 3mm significantly increases cracking risk.
2. Process Optimization Solutions
2.1 Pre-Bending Heat Treatment
Annealing at 850–900°C for 60 minutes can effectively eliminate residual stresses and soften the material, improving plasticity. For example, Mo plates subjected to this treatment exhibit a 15% increase in elongation, reducing cracking susceptibility during bending.
2.2 Surface Quality Enhancement
Electropolishing with a solution of 120:5:2 industrial alcohol:hydrochloric acid:sulfuric acid at 1.4 A/dm² for 2–8 minutes removes surface defects and reduces the plastic-brittle transition temperature by 12°C. This treatment enhances the material’s resistance to crack initiation.
2.3 Tooling and Process Parameter Adjustments
- Increased Bending Radius: Using a bending radius ≥3 times the plate thickness reduces strain concentration.
- Roll-Type Lower Die: Replacing traditional V-dies with roll-type lower dies reduces friction and prevents surface damage.
- Lubrication: Applying MoS₂ or graphite-based lubricants during bending lowers the coefficient of friction by 30%, minimizing tearing risks.
2.4 Multi-Step Bending Process
For complex geometries, adopting a multi-step bending approach with intermediate annealing (e.g., 800°C for 30 minutes) between steps prevents stress accumulation and cracking. This method has been successfully applied to Mo parts with a thickness-to-width ratio exceeding 1:10.
3. Material Selection and Design Optimization
3.1 Cross-Rolling Technology
Cross-rolling during plate production disrupts the directional grain structure, improving isotropic deformation behavior. Compared to unidirectional rolling, cross-rolled Mo plates exhibit a 20% higher elongation and 30% lower anisotropy coefficient.
3.2 Grain Refinement
Adding 0.5% Ti to Mo forms fine TiC precipitates, refining the grain size from 50μm to 15μm. This refinement increases the material’s fracture toughness by 40%, significantly enhancing bending resistance.
3.3 Structural Design Optimization
Adding fillets with radii ≥0.5mm at sharp corners or introducing process holes near bending lines reduces stress concentration. Finite element analysis (FEA) shows that these modifications can lower peak stresses by 25%.
4. Case Study: Aerospace Component Manufacturing
In a batch production of Mo satellite brackets, initial trials using 1mm-thick cold-rolled Mo plates with a 2mm bending radius resulted in a 40% cracking rate. After implementing the following measures:
- Pre-bending annealing at 880°C for 45 minutes.
- Electropolishing for 5 minutes.
- Increasing the bending radius to 4mm.
- Using a roll-type lower die with MoS₂ lubrication.
The cracking rate dropped to 2%, and the yield strength remained above 900 MPa, meeting the component’s performance requirements.
5. Conclusion
Molybdenum plate bending cracks stem from complex interactions among stress distribution, material properties, and processing parameters. Through systematic process optimization—including heat treatment, surface enhancement, tooling adjustments, and material design improvements—manufacturers can effectively control cracking risks. Future research should focus on developing high-plasticity Mo alloys and refining multi-scale simulation models to predict and prevent deformation defects.