Controlling the diameter of black tungsten wire during production is a critical step to ensure product quality and consistency. Below are the main methods and key process points for controlling the diameter of black tungsten wire:
1. Raw Material Selection and Preparation
- Tungsten Powder Quality Control: Select high-purity tungsten powder with uniform particle size as the raw material. Particle size distribution affects the uniformity of subsequent compaction and wire drawing.
- Doping Process: Add appropriate amounts of dopants (e.g., potassium, silicon, or aluminum) to the tungsten powder to improve the ductility and mechanical properties of the wire. The dopant ratio must be precisely controlled to avoid impacting diameter stability.
2. Pressing and Sintering
- Pressing and Forming: Use isostatic pressing or die pressing to form tungsten powder into billets. Pressing pressure and mold dimensions must be strictly controlled to ensure uniform billet density, preventing diameter deviations during subsequent drawing.
- Sintering Process: During high-temperature sintering, control the temperature and duration to ensure the density and structural uniformity of the tungsten billets. Sintering temperatures typically range between 2500–3000°C, avoiding over-sintering or under-sintering.
3. Wire Drawing Process
Wire drawing is the core step in controlling the diameter of black tungsten wire. Specific measures include:
- Die Selection: Use high-precision wire drawing dies (e.g., diamond dies), with die apertures regularly inspected and calibrated to ensure consistent wire diameter after drawing.
- Multiple Drawing Passes: Employ multiple drawing passes to gradually reduce the diameter, controlling the reduction rate per pass (typically 10–20%). Excessive reduction rates can cause surface defects or wire breakage, while insufficient rates reduce efficiency.
- Lubrication and Cooling: Apply appropriate lubricants (e.g., graphite emulsion or oil-based lubricants) during drawing to reduce friction and prevent surface scratches. Maintain proper cooling to control wire temperature, avoiding diameter inconsistencies due to overheating.
- Drawing Speed: Strictly control the drawing speed, as excessive speed may lead to uneven stretching, while overly slow speeds reduce production efficiency. Optimize speed based on equipment performance and wire specifications.
4. Online Monitoring and Feedback
- Real-Time Measurement: Use laser or optical diameter measurement devices to monitor the wire diameter in real time during drawing. Adjust dies or drawing parameters promptly if deviations are detected.
- Closed-Loop Control System: Modern production lines often incorporate automated control systems with sensors and feedback mechanisms to dynamically adjust drawing parameters, ensuring diameter accuracy.
5. Annealing Treatment
- Intermediate Annealing: Conduct periodic annealing during multiple drawing passes (typically in a hydrogen atmosphere at 1000–1500°C) to eliminate internal stresses and prevent wire brittleness or diameter inconsistencies.
- Annealing Process Control: Precisely control annealing temperature, duration, and atmosphere to avoid excessive grain growth or surface oxidation, which could affect subsequent drawing and diameter consistency.
6. Quality Control and Inspection
- Final Inspection: Perform sampling inspections on finished black tungsten wire using high-precision measurement tools (e.g., microscopes or laser diameter gauges) to verify that diameter tolerances meet standards (e.g., ±1% or stricter).
- Surface Quality: Inspect the wire surface for cracks, scratches, or oxidation, ensuring surface quality complements diameter control.
7. Equipment and Process Stability
- Equipment Maintenance: Regularly maintain wire drawing machines, dies, and diameter measurement equipment to ensure precision and stability.
- Standardized Process Parameters: Establish a standardized database of process parameters (e.g., drawing speed, reduction rate, lubricant ratio) and optimize them for different wire specifications.
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