Molybdenum (Mo), known for its high melting point, excellent thermal conductivity, and corrosion resistance, is widely used in various industries such as aerospace, electronics, and chemical processing. However, cracks in molybdenum plates during processing and application pose significant challenges. This article provides an in-depth analysis of the causes of cracks in molybdenum plates and offers corresponding coping strategies.
Causes of Cracks in Molybdenum Plates
Stress Concentration and Material Properties
During the deep drawing process of molybdenum, stress concentration often occurs at the port end of the workpiece, forming a small zigzag shape. When residual stress accumulates to a certain extent and exceeds the material’s strength limit, cracks will appear at its weakest point. The direct cause of longitudinal cracks is tensile stress during the drawing process.
Moreover, molybdenum has relatively poor plasticity. When subjected to excessive rolling force during cold rolling, serious delamination may occur within the molybdenum layer, leading to the rapid accumulation of micro-cracks at the grain boundaries and eventual fracture.
Improper Heating Temperature
During the heating and rolling process, if the local heating temperature is too high, recrystallization may occur, causing grain growth and coarser fiber structure, which affects the mechanical properties of the material and increases the risk of cracks during cold rolling. Conversely, if the heating temperature is too low while the processing rate is high, the hardening speed of the molybdenum plate may exceed the plastic deformation speed, leading to cracks.
Rolling Deformation Rate
The cold rolling deformation rate significantly affects material properties. Excessive deformation can lead to severe layering and cracking due to molybdenum’s poor plasticity.
Equipment Precision and Stability
The precision and stability of cold rolling equipment are crucial. Inadequate equipment precision or stability can result in uneven stress distribution during rolling, leading to cracks.
Environmental Factors and Operational Errors
Environmental factors such as temperature and humidity can also affect the cold rolling process. Excessive or insufficient ambient temperatures may alter the material’s physical properties and increase the risk of cracks. Operational errors, such as excessive rolling speed or force, can also cause cracks.
Coping Strategies
Stress Reduction and Material Improvement
To address longitudinal cracks, a turning process can be adopted to remove the zigzag shape at the port end and eliminate stress concentration points. For molybdenum pieces that crack horizontally during deep drawing, increasing the gap between the convex and concave dies by 10% to 15% can effectively reduce cracks. Additionally, annealing processes, including high-temperature annealing (heating to 875-900°C, holding for 30 minutes, and cooling in the furnace for 40 minutes) followed by low-temperature annealing (heating to about 570°C in a muffle furnace, holding for 2 minutes, and hot punching), can soften the material, reduce tensile stress, and improve plasticity, thereby reducing horizontal cracks.
Optimizing Process Parameters
Properly adjusting the heating temperature, rolling deformation rate, and other process parameters can significantly reduce the risk of cracks. Ensuring that the heating temperature is within an appropriate range and controlling the deformation rate can help maintain the material’s mechanical properties and reduce the likelihood of cracks.
Enhancing Equipment Precision and Stability
Regular maintenance and calibration of cold rolling equipment are essential to ensure its precision and stability. This can reduce uneven stress distribution during rolling and prevent cracks.
Strict Control of Environmental Factors and Operational Procedures
Maintaining appropriate ambient temperatures and humidity during the cold rolling process can help maintain the material’s physical properties and reduce the risk of cracks. Additionally, strict adherence to operational procedures, such as controlling rolling speed and force, can further reduce the occurrence of cracks.
In conclusion, cracks in molybdenum plates are a complex issue involving multiple factors. By conducting an in-depth analysis of the causes and adopting corresponding coping strategies, the risk of cracks can be significantly reduced, ensuring the quality and reliability of molybdenum products.