Factors Affecting the Drawing of Titanium Alloy Wires
Titanium and titanium alloy wires are widely used in important fields such as aerospace fasteners, 3C products, eyeglass frames, automotive parts, medical instruments, and welding rods. Generally, when the diameter of titanium and titanium alloy wires is 30-40% larger than the final product size, cold drawing is used to obtain wire products with high dimensional accuracy.
The cold drawing process and microstructure control of the final product have a significant impact on the performance of titanium and titanium alloy wires. The main factors affecting wire drawing performance, besides drawing temperature and drawing speed, include the quality of the raw material, die parameters, lubrication conditions, and the drawing process route.
1. Raw Material Quality
Chemical Composition: The content of major chemical elements and impurity elements must not exceed the allowable range. Elements such as hydrogen (H), oxygen (O), nitrogen (N), iron (Fe), and silicon (Si) can have a significant impact on titanium. For example, hydrogen can cause hydrogen embrittlement in titanium alloys, so strict control is required during production.
Surface Quality: The wire surface must not have defects such as cracks, folds, scars, ears, or delamination. Surface defects such as cracks and folds may appear in the raw material to varying degrees. These defects can form cracks on the surface, subsurface, or inside the metal, which may further develop during the drawing process, leading to a sharp decrease in strength or even breakage. Unlike cracks, folds are not easily detected as they are often covered by surface oxidation layers and may persist during drawing.
2. Heat Treatment Process
The heat treatment process during cold drawing mainly involves annealing of the wire, which includes pre-treatment annealing of the raw material, intermediate annealing after deformation, and final annealing. The purpose of pre-treatment and intermediate annealing is to reduce the effects of work hardening, increase ductility, and optimize plasticity, making the material more suitable for the next stage of the drawing process.
3. Drawing Dies
Metal drawing dies are generally made from cemented carbide (YK6, YK8) or diamond materials. Cemented carbide consists of tungsten carbide and cobalt, with tungsten carbide being hard and wear-resistant, serving as the skeletal material, while cobalt increases the toughness of the alloy. Cemented carbide dies are widely used in the drawing of various metals and alloy wires. Diamond dies, with high hardness and wear resistance, are more expensive and difficult to process, thus are only used for drawing fine and ultra-fine wires.
Depending on the longitudinal cross-sectional shape of the die hole, standard drawing dies can be divided into two forms: arc-shaped dies and conical dies. The former is typically used for fine wires, while conical dies are commonly used for tubes, rods, and coarse wires. Depending on their function during drawing, die holes are generally divided into four sections: the entrance cone (feeding zone + lubrication zone), working cone, sizing zone, and exit cone.
4. Drawing Process
Reduction per Pass: Titanium alloys have low room temperature ductility, with yield strength close to tensile strength, resulting in a high yield ratio. When drawing metallic materials, the strength of the material after exiting the die must be higher than the yield strength of the material inside the die to prevent wire breakage. Therefore, blindly pursuing excessive reductions per pass in drawing should be avoided.
Total Reduction: The strength of titanium alloy wires increases with the total reduction rate. This is mainly because as the amount of cold deformation increases, dislocation multiplication occurs within the metal grains, increasing the material's resistance to plastic deformation. This leads to work hardening, which increases the wire's breaking force and tensile strength. However, excessive work hardening reduces the wire's toughness, bending, and twisting values, and in severe cases, it becomes brittle, with very low bending performance.
Drawing Speed: Drawing speed is a crucial factor in the metal processing production process and has a significant impact on the performance of deformed metal. Strain rate refers to the rate of change in deformation or the relative displacement volume per unit of time. Titanium alloys are sensitive to strain rate, and different deformation speeds significantly affect their plasticity and deformation performance. Under the same drawing conditions, increasing the drawing speed can improve labor productivity and save energy, but the quality of the wire and the smoothness of the drawing process must be ensured.
