From micron to spiral: What kind of tungsten filament can break through the limit and perfectly realize spring winding processing?
In the fields of precision electronics, electric light sources, high-frequency heating, and medical minimally invasive devices, Tungsten Wire Coiling/Filament Winding is a core process with high artistic sense and industrial difficulty. Metal tungsten, which has a melting point of up to 3422 °C and is naturally brittle, is precisely bent into dense coil springs or stranded wires at micron tolerances, which puts forward almost harsh requirements on the microstructure and mechanical properties of the tungsten wire material itself.
What kind of tungsten wire can smoothly complete the spring winding processing without "bending but not folding"? What is the metallurgical principle and process control logic behind it?
I. What kind of tungsten wire is a "perfect spring-wound" tungsten wire?
Not all pure tungsten filaments can be directly used for spring winding. Ordinary pure tungsten wires without special treatment have extremely poor ductility at room temperature, and direct winding of springs is prone to Wire Breakage or Delamination. For high-quality tungsten wires that can be smoothly processed by spring winding, the following three key indicators must be achieved: Doped Non-Sag Potassium Bubble Structure (Doped Non-Sag Potassium Bubble Structure): WAL (W-Al-K) doped anti-sagging tungsten wire is the first choice in the industry. By doping potassium (K) in powder metallurgy, "microscopic potassium bubble chains" are formed in grain boundaries after drawing. This not only improves the room temperature plasticity of the material, but also ensures that the spring will not sag and deform under ultra-high temperature electrification after forming.
Up to 70%-90% High Cold Work Degree: Tungsten wires for spring winding need to undergo multi-pass precision drawing of Diamond Dies (Diamond Dies), so that the original equiaxed grains are strongly elongated along the drawing direction, forming extremely fine fibrous fibrous structure (Fibrous Microstructure). This unique longitudinal fiber structure endows tungsten wires with extremely high Tensile Strength and bending toughness.
Excellent Surface and Crack Control (Defect-Free Surface): The spring winding process exerts extreme tensile stress on the outer surface of the tungsten wire. Any micron-scale scratches, graphite lubricant residues, or micro-cracks present on the surface will become stress concentration points at the moment of bending and lead to fracture. Therefore, the qualified rate of the tungsten wire having been electropolished or Chemically Polished White Tungsten Wire is significantly higher than that of the ordinary black tungsten wire.
II. Working principle and whole process analysis of tungsten wire wound spring
1. Processing principle
Plastic rheology using the "anisotropy" of fiber structure Tungsten metal has a high plastic-brittle transition temperature (DBTT) and is prone to brittle fracture at room temperature. The essence of spring winding processing is to use the anisotropic fiber structure formed by cold drawing to make the longitudinal fiber grains slip staggered without breaking under the action of winding torque. For coarse wire diameter or difficult winding, Warm Coiling is usually used, and the tungsten wire is locally heated above the DBTT temperature (about 200 °C-500 °C), so that it has excellent plastic deformation ability for a short time.
2. Core processing flow (Workflow)
Unwinding & Tension Control: Tungsten wires are released smoothly from the high-precision plastic Spool (Spool), and through multi-level tension controllers, the tension fluctuation is within ± 1%.
Mandrel Winding: The tungsten wire is tightly wound on a high-speed rotating high-hardness mandrel (such as molybdenum mandrel or carbide mandrel) at a predetermined pitch under the guidance of the wire nozzle.
Stress Relief Annealing (Stress Relief Annealing): The coiled spring together with the mandrel is sent into a hydrogen (H ₂) protected high-temperature vacuum annealing furnace (about 800 °C-1000 °C) for Stress Relief Annealing, locking the spiral shape, Eliminate residual internal stress.
Mandrel Dissolution: If molybdenum wire is used as the inner core shaft, mixed acids (nitric acid and sulfuric acid) will be used to dissolve the molybdenum core after annealing, and the final high-precision hollow pure tungsten coil spring.
III. Key matters needing attention in the process of winding spring processing
Strictly matched bend ratio (d/d Ratio): The ratio (d/d) of the mandrel diameter (d) to the tungsten wire diameter (d) is generally recommended to be greater than 3. If the d/d is too small, the tensile strain of the outer fiber exceeds the limit, which can easily lead to surface cracking or wire breaking.
Precise control of heating temperature: When using hot winding process, the temperature should not be too high. If the recrystallization temperature is exceeded (> 1200 ℃), the fibrous structure will be transformed into equiaxed grains, making the tungsten wire instantaneously lose its toughness and completely embrittle.
Excellent wire diameter tolerance consistency: Winding springs have extremely high wire diameter tolerance requirements (usually ≤ ± 1%). The small fluctuation of wire diameter will lead to uneven pitch or resistance deviation, which will directly affect the thermal field distribution of terminal components.
Summary and Customization
Support The tungsten wire that can perfectly realize spring winding processing is the crystallization of high-purity chemical formula, diamond drawing process and strict surface defect removal treatment.
We are committed to providing global customers with high-quality WAL-doped anti-sagging white tungsten wires and special filaments for spring winding, providing full-process customization support from micron-level wire diameter control to high-precision length cutting, helping your high-end electronics and Thermal field equipment achieves extremely stable operating performance.
