Influencing Factors of Low Temperature Ductility of Tungsten Wire

The low-temperature ductility of the tungsten wire is the main factor affecting the winding performance. The main influencing factors are the purity of raw materials, additives and the surface condition of the wire.

1 Purity of raw materials

The higher the purity of wire, the more excellent low-temperature ductility. The low temperature brittleness is mainly caused by harmful impurity elements. The content of Fe and Ni in the impurity elements is too high, and the plastic-brittle transition temperature of the W wire will rise above 700°C, which will result in the failure of processing. More harmful impurities are mainly reflected in oxygen, nitrogen, and carbon. The presence of these elements will also increase the plastic-brittle transition temperature and reduce the winding performance. These impurity elements can be purified by chemical methods, such as repeated crystallization of APT, nitric acid treatment, and ion exchange of sodium tungstate solution. Among them, ammonia solution purification of tungstic acid to remove impurities is the lowest cost method.

2 Additives

Doping a proper amount of silicon (SiO2≤0.4%), aluminum (Al2O3≤0.03%), potassium (K2O≤0.45%) in W wire can improve its high temperature anti-sag property to a certain extent, but not all for low temperature ductility benefit. Therefore, it is necessary to add other chemical elements to reduce the influence of these additives. For example, adding a small amount of ruthenium or cobalt can improve the winding performance and elongation.

3 Surface Condition

During processing, tungsten wires are prone to defects, such as tiny delaminations, cracks, bumps, and surface stains. Among them, if the surface stain is a tungsten-nickel alloy, the brittleness of the wire can be increased, and the superposition of these defects is likely to cause the wire to break during the winding process. Therefore, it is necessary to continuously use electrolytic cleaning for surface treatment of semi-finished products in the process of swaging to increase the surface finish, reduce defects, and improve low-temperature ductility.

In addition to the above factors, the low temperature ductility of wire can also be improved by adjusting the vertical melting system, annealing, improving the mold and graphite emulsion, etc.

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