(1) The definition of resistance welding.
Resistance welding is a welding method in which the workpiece to be welded is pressed between two electrodes, and a current is passed through. The current flows through the contact surface of the workpiece and the adjacent area to generate resistance heat to heat it to a molten state, forming a metal bond . There are mainly four common resistance welding methods, namely spot welding, seam welding, projection welding, and butt welding. In module welding, resistance spot welding and projection welding are generally used. Relatively speaking, resistance spot welding accounts for more. In practical applications, parallel welding in spot welding is the majority. The following analysis is mainly based on resistance spot welding .
(2) Resistance spot welding.
① Principle of spot welding.
In actual production, the proportion of cylindrical battery cell modules using resistance spot welding is the largest. The principle of spot welding is based on the principle of resistance heating. Therefore, it conforms to Joule’s law:
Among them, Q is the heat generated, J; I is the welding current, A; R is the resistance between electrodes, Ω; T is the welding time, so it can be seen from the formula that the current has the greatest impact on the welding effect. In the selection of welding equipment modes, there are generally constant current, constant voltage and comprehensive modes. In order to obtain a stable welding effect, it is recommended to use the constant current mode, of course, it also needs to be determined according to the actual situation of the product and process design.
②Basic characteristics of spot welding.
In order to obtain a stable welding effect and facilitate the design of the welding process, some basic characteristics of welding are analyzed through the welding principle: a. The form of the joint is overlap, that is, the electrode and the welded product must contact; b. Always during the welding process It is necessary to maintain the pressing force, and maintain good contact of the workpiece through the pressing force, which is conducive to welding; c. The energy of resistance spot welding is welded together by thermal melting, that is, the method of resistance heating; d. Thermal melting during welding The temperature should be higher than the melting point of the welding material, otherwise false welding will occur during welding, resulting in welding failure; e. When designing the welding process, try to set the weld nugget hot melt zone in the middle of the weld.
③Analysis of spot welding cycle process.
The spot welding process is generally divided into four basic stages: the pre-compression stage, the welding stage, the maintenance stage and the rest stage. If it is divided according to whether current is applied or not, it can be divided into three stages: pre-pressure stage, energized heating stage and cooling crystallization stage. When analyzing the welding process, a four-stage method is generally used.
④ Factors affecting welding.
From the perspective of Joule’s law and actual production, there are seven main factors that affect the welding effect (Figure 9-17). In order to obtain a good welding effect, these factors must be considered in product and process design. In actual production, welding current and welding time can basically be guaranteed by welding equipment, and the biggest variables are caused by electrodes and workpieces. To obtain good welding results, conditions need to be optimized.
Electrodes are a very important part of resistance spot welding. At the same time, electrodes are also consumables and require strict management throughout the production process. When choosing the electrode material, you need to pay attention to the following four points:
a. High conductivity: it is not easy to generate heat when passing large current;
b. High heat conduction: it can be cooled immediately even if it generates heat;
c. Good mechanical strength: It can maintain the hardness even at high temperature, and it can not be deformed even if it is subjected to pressure impact or heating during welding;
d. It is not easy to form an alloy with the welded object (workpiece): the electrode and the work piece are not easy to stick.
The welding process is a key process in the cylindrical battery cell module. If it is not well controlled, it will cause serious defects in the product. In order to control the welding process, from product design, process to production control, risk analysis must be done in advance, and uncontrollable factors are minimized as much as possible.