The manufacturing process of strand tungsten filaments involves several key steps, each of which significantly influences the filament's final properties, such as its strength, conductivity, and heat resistance. Below is a typical process for manufacturing tungsten filaments and how each stage impacts the properties of the filaments:
1. Tungsten Powder Production
The first step in the process involves producing tungsten powder, which serves as the raw material for filament production. Tungsten is typically extracted from ores such as scheelite (CaWO₄) or wolframite (Fe, MnWO₄). These ores are chemically processed to produce ammonium paratungstate (APT), which is then reduced to pure tungsten powder. The purity and size of the tungsten powder are critical because finer powders allow for more uniform processing and a h.igher-quality finished filament.
2. Powder Compaction and Shaping
The tungsten powder is then compacted into a billet, usually by cold pressing. The powder is pressed into a mold under high pressure to form a solid shape. This is often followed by a sintering process, where the compacted powder is heated at a high temperature in a controlled atmosphere to increase its density and bond the powder particles together without reaching the melting point of tungsten.
3. Wire Drawing
After sintering, the tungsten material is heated and drawn into fine wires through a wire-drawing process. Tungsten is drawn through a series of progressively smaller dies to reduce its diameter to the desired thickness. This process requires high temperatures to reduce the material’s resistance to deformation.
4. Heat Treatment (Annealing)
Once the filament has been drawn into its final shape, it undergoes annealing, a heat treatment process where the filament is heated to a high temperature in a controlled atmosphere. This process is used to relieve internal stresses, improve the mechanical properties, and ensure that the filament is free from defects such as cracks or brittleness.
5. Surface Treatment
The tungsten filaments are often coated or treated to enhance their performance. Common surface treatments include coating the filaments with hafnium or tantalum to improve their oxidation resistance and electron emission properties.
6. Final Inspection and Quality Control
After the filaments are manufactured, they undergo rigorous quality control to ensure they meet the required specifications. This may include testing for tensile strength, resistance to oxidation, and uniformity of diameter.
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