1、 The influence of stretching process
Stretching is the process of extruding metal wire through a mold to reduce its cross-section and increase its length. Moderate stretching can refine metal grains and reduce electrical resistivity to a certain extent; However, excessive stretching can lead to a large number of crystal defects (such as dislocations and cracks) inside the wire, which can hinder the directional movement of free electrons and increase the resistivity. For example, if the tensile strength of copper leads is too high, internal stress accumulation will disrupt the regularity of atomic arrangement, resulting in a 5% -10% increase in resistivity compared to the unstretched state (specific values vary depending on the material and tensile parameters).
2、 The role of annealing process
Annealing is a process of slowly cooling metal wires by heating them at high temperatures for a period of time. It can eliminate internal stress generated by stretching and other processes, rearrange disordered grains and restore ordered structure, thereby reducing obstacles to electron transfer and lowering electrical resistivity. For example, copper leads that have been excessively stretched can have their internal defects repaired and their resistivity reduced to near its original state after annealing at 300 ℃ -500 ℃. If the annealing temperature is insufficient or the time is too short, the stress relief is not thorough, and the effect of reducing resistivity is limited; However, if the temperature is too high, it may lead to coarse grains and slightly increase the resistivity.
3、 The impact of twisting process
Carbon brush leads often adopt a multi strand thin wire twisting structure, and the tension control and twisting method during the twisting process will affect the electrical resistivity. If the tension is uneven during twisting, some single wires may undergo plastic deformation due to excessive stress, resulting in an increase in local electrical resistivity; Reasonable twisting tension can ensure a tight fit of the single wire, reduce contact resistance, and overall lower the equivalent resistivity of the lead wire. In addition, the twisting pitch (the length of one rotation per strand) can also have an impact: a pitch that is too small can increase friction and stress between the wires, while a pitch that is too large may cause the single wires to loosen, both of which can cause small fluctuations in electrical resistivity.
4、 The influence of surface treatment process
Surface treatment (such as plating, cleaning, polishing, etc.) mainly affects the resistivity by changing the surface state of the leads. For example, tin plating or silver plating can form a conductive layer on the surface of the lead. If the coating is uniform and dense, it can reduce the surface contact resistance (especially at the connection between the lead and the carbon brush or terminal block), thereby reducing the overall resistivity; But if there are pinholes, bubbles or detachment in the coating, it will form an oxide layer or insulation area, leading to an increase in resistivity. In addition, if impurities such as residual oil stains and oxide scales on the surface are not removed through cleaning processes, these substances will become obstacles to electron transfer, increasing the surface resistance and affecting the overall resistivity.
5、 Indirect effects of molding process
Some leads need to be formed through bending, cutting, and other molding processes to meet installation requirements. If the bending radius is too small, it may cause local wire deformation under stress, resulting in micro cracks and an increase in electrical resistivity in that area; If burrs or uneven sections are generated during cutting, it will increase the contact resistance between the lead and the connecting components, indirectly manifested as an increase in overall resistivity.
In summary, the processing technology has a significant impact on the electrical resistivity by changing the internal structure, stress state, and surface quality of the carbon brush lead. Reasonably controlling various process parameters (such as stretching amount, annealing temperature, twisting tension, etc.) can keep the electrical resistivity of the lead in the optimal range, ensuring its efficient conduction of current in the carbon brush system.
