Molybdenum, as a refractory metal, is widely used in high-temperature furnace components, electronic semiconductors, aerospace, and other fields due to its high melting point, high strength, high thermal and electrical conductivity, low coefficient of thermal expansion, and excellent corrosion and wear resistance. However, during the rolling process of molybdenum sheet blanks into thin sheets, lamination is a common and challenging issue. The following provides a detailed analysis from three aspects: causes of lamination, preventive measures, and repair methods.
I. Analysis of Lamination Causes
1. Raw Materials and sheet Blank Preparation
Raw Material Quality: The particle size, distribution, and purity of molybdenum powder have a direct impact on the quality of sintered sheet blanks. Uneven particle size or insufficient purity may lead to defects such as pores and looseness during the sintering process, which in turn can cause lamination.
Sintering Process: The control of parameters such as sintering temperature, holding time, and sintering atmosphere is crucial for the density and uniformity of sheet blanks. Insufficient sintering may result in unbonded areas within the sheet blank, while excessive sintering can lead to coarse grain growth, both of which increase the risk of lamination.
Internal Defects in sheet Blanks: Pores, looseness, and cracks are common internal defects that can cause lamination in molybdenum sheets. These defects can easily become crack sources during the rolling process, leading to lamination.
2. Rolling Process
Reduction Control: Insufficient reduction can result in surface deformation while the central layer remains undeformed, forming an undeformed cold layer and increasing the risk of lamination. Conversely, excessive reduction can cause edge cracking or internal cracks, also leading to lamination. During the rolling process, if the reduction is less than 20%, surface deformation occurs while the central layer remains undeformed, creating differences in mechanical properties between different layers of the metal, which can subsequently lead to lamination during further rolling.
Rolling Temperature: When the rolling temperature is too low, the rate of work hardening exceeds the rate of plastic deformation, causing the sum of internal residual stresses and external tensile stresses to exceed the tensile strength of the molybdenum sheet, resulting in cracking and lamination. Additionally, low-temperature rolling may generate banded cellular structures, causing anisotropy in the microstructure and reducing the plasticity in the vertical direction and strength in the thickness direction of the sheet. On the other hand, high rolling temperatures can trigger recrystallization, causing grain growth through mutual engulfment, resulting in coarse and short fibrous structures, which are also detrimental to sheet performance.
Rolling Method: Unidirectional rolling (full longitudinal or full transverse rolling) can easily lead to anisotropy, reducing the strength and plasticity of the sheet in the vertical direction and thus causing lamination. Cross-rolling can improve this issue, but it requires proper control of rolling parameters.
Rolling Speed and Tension: Excessive rolling speed or improper tension control can lead to uneven deformation of the sheet, generating internal stresses and subsequently causing lamination.
3. Equipment and Operation
Rolling Mill Capacity: The torque and rolling force parameters of the rolling mill need to match the size and material of the molybdenum sheet blank. Insufficient rolling mill capacity may result in stalling during the breakdown process, causing uneven deformation of the sheet and leading to lamination.
Roll Condition: The wear, surface quality, and roll profile design of the rolls all affect the deformation and stress distribution of the sheet during the rolling process. Poor roll condition can increase the risk of lamination.
Operational Procedures: The skill level and safety awareness of operators are crucial for quality control during the rolling process. Failure to follow process requirements for multi-pass rolling, turning, and grinding can lead to surface scratches, pits, peeling, and lamination on the sheet.
4. Post-Processing and Inspection
Annealing Treatment: Annealing treatment can eliminate internal stresses and work hardening generated during the rolling process, improving the plasticity and toughness of the sheet. Improper annealing treatment (e.g., insufficient temperature or time) may fail to effectively eliminate internal stresses, increasing the risk of lamination.
Quality Inspection: The lack of effective quality inspection methods or lax inspection can result in laminated molybdenum sheets entering the market or the next process. Non-destructive testing methods such as ultrasonic testing and X-ray testing can detect lamination defects in a timely manner and prevent the issue from escalating.
5. Environmental Factors
Rolling Environment: The ambient temperature, humidity, and cleanliness during the rolling process can also affect lamination in molybdenum sheets. For example, high humidity can cause surface oxidation or hydrogen-induced cracking on the sheet, leading to lamination.
Storage and Transportation: Molybdenum sheets subjected to impact, compression, or corrosion during storage and transportation may also experience lamination or exacerbate existing lamination.
II. Preventive Measures
1. Optimize the Rolling Process
Proper Reduction Control: The reduction in the first pass should generally be greater than 20% to ensure deformation penetrates to the center of the rolled piece and eliminate the undeformed cold layer. At the same time, the reduction per pass should not be excessive and should follow the principle of decreasing from large to small, generally controlled between 20% and 30%.
Select an Appropriate Rolling Temperature: Choose a suitable rolling temperature based on the material and thickness of the molybdenum sheet. Generally, the rolling temperature should be controlled above the recrystallization temperature to avoid work hardening and the formation of banded cellular structures. At the same time, pay attention to controlling temperature drop to ensure uniform temperature during the rolling process.
Adopt Cross-Rolling Method: Cross-rolling can improve the anisotropy of the sheet and enhance its overall performance. By changing the rolling direction, grains can be stretched and broken in multiple directions, reducing the risk of lamination.
2. Improve sheet Blank Quality
Select High-Quality Raw Materials: Using medium-grained molybdenum powder with uniform particle size, loose distribution, and a reasonable coarse-to-fine ratio as raw materials can improve the density and uniformity of sintered sheet blanks.
Strengthen Sintering Process Control: Measures such as extending the holding time and increasing the sintering temperature can improve the microstructure of sintered sheet blanks, reducing internal defects and coarse grain growth.
Perform Pretreatment: For molten molybdenum alloy ingots containing coarse columnar grains, extrusion breakdown can be performed first to refine the grains before rolling.
3. Improve Equipment and Operation
Select Suitable Rolling Mills: Choose rolling mills with sufficient torque and appropriate rolling force based on the size and material requirements of the molybdenum sheet blank.
Strengthen Operator Training: Provide professional training to operators to improve their operational skills and safety awareness. Ensure that process requirements such as multi-pass rolling, turning, and grinding are strictly followed during the rolling process.
III. Repair Methods
1. Annealing Treatment
Purpose: Annealing treatment can eliminate internal stresses and work hardening generated during the rolling process, improving the plasticity and toughness of the sheet.
Method: Perform annealing treatment on the laminated molybdenum sheet under vacuum or high-purity hydrogen protection. The annealing temperature should be selected based on the material and thickness of the molybdenum sheet, generally controlled below or near the recrystallization temperature. The holding time should be long enough to ensure full elimination of internal stresses.
Effect: After annealing treatment, the lamination phenomenon in the molybdenum sheet can be significantly improved or even eliminated. At the same time, the plasticity and toughness of the sheet will also be improved, facilitating subsequent processing and use.
2. Mechanical Processing Repair
Purpose: For molybdenum sheets with severe local lamination, mechanical processing methods can be used to remove the laminated parts and then re-roll or process them.
Method: Use machine tools such as lathes and milling machines to cut or grind away the laminated parts. During the processing, pay attention to controlling processing accuracy and surface quality to avoid generating new defects.
Effect: Mechanical processing repair can effectively remove the laminated parts and improve the overall quality of the sheet. However, it should be noted that mechanical processing will reduce the dimensional accuracy and surface quality of the sheet, so compensation and correction are required during subsequent processing.
3. Composite Repair
Purpose: For laminated molybdenum sheets that cannot be repaired by annealing treatment or mechanical processing, composite repair methods can be considered. That is, composite the laminated parts with other materials through welding, brazing, etc., to form new composite sheets.
Method: Select appropriate welding or brazing methods based on the material and thickness of the laminated molybdenum sheet. During the welding or brazing process, pay attention to controlling heat input and welding parameters to avoid generating new defects and cracks.
Effect: Composite repair can effectively solve the lamination problem and improve the overall performance and service life of the sheet. However, it should be noted that composite repair increases the manufacturing cost and processing difficulty of the sheet.