Welding knowledge

At present, the main welding method is resistance welding. Resistance welding is to press the workpiece to be welded between two electrodes, and apply a current to heat it to a molten or plastic state by using the resistance heating effect generated by the current flowing through the contact surface of the workpiece and the adjacent area to form a A method of metal bonding.

Elements for judging spot welding weldability of metal materials:

① The electrical conductivity and thermal conductivity of the material, that is, the metal material with small resistivity and high thermal conductivity, its weldability is poor;

② High-temperature plasticity and plastic temperature range of materials, that is, materials with high high-temperature yield strength (such as heat-resistant alloys) and materials with narrow plastic temperature ranges (such as aluminum alloys) have poor weldability;

③ The sensitivity of the material to thermal cycling, that is, materials (such as 65Mn) that are prone to generate defects (cracks, hardened structures, etc.) related to thermal cycling, and their weldability is poor;

④ Metal materials with high melting point, high coefficient of linear expansion and high hardness generally have poor weldability.


Of course, when evaluating the spot weldability of a certain metal material, the above factors should be comprehensively and comprehensively considered.

Copper and copper alloys can be divided into pure copper, brass, bronze and cupronickel, among which pure copper, oxygen-free copper and phosphorus deoxidized copper have poor spot welding (not recommended). excellent. Pure copper has extremely high electrical conductivity, making spot welding difficult and poor bonding strength.

The quality of resistance welding is determined by the following 4 factors:


1. The influence of welding current. Current has a greater effect on heat generation than both resistance and time. Therefore, in the welding process, it is a parameter that must be strictly controlled. The main causes of current changes are grid voltage fluctuations and changes in the impedance of the secondary circuit of the AC welding machine. Impedance changes are due to changes in the geometry of the loop or due to the introduction of different amounts of magnetic metal in the secondary loop. For DC welding machines, the change of the secondary circuit impedance has no obvious effect on the current. In addition to the total amount of welding current, the current density also has a significant effect on heating. By shunting the soldered joints, and increasing the electrode contact area or bump size in projection welding, the current density and welding heat are reduced, resulting in a significant decrease in joint strength.

2. The influence of welding time. In order to ensure the size of the nugget and the strength of the welding spot, the welding time and the welding current can complement each other within a certain range. In order to obtain a solder joint of a certain strength, high current and short time (strong condition, also known as strong specification) can be used, or small current and long time (weak condition, also known as weak specification) can be used. The choice of strong or weak conditions depends on the properties of the metal, the thickness and the power of the welding machine used. However, for the current and time required for metals with different properties and thicknesses, there are still upper and lower limits, beyond which a qualified nugget cannot be formed.

3. The effect of electrode pressure. The electrode pressure has a significant effect on the total resistance R between the two electrodes, and R decreases significantly with the increase of the electrode pressure. At this time, although the welding current increases slightly, it cannot affect the reduction of heat generation caused by the decrease of R. Therefore, the joint strength always decreases with increasing electrode pressure. While increasing the electrode pressure, increase the welding current or prolong the welding time to make up for the effect of the decrease in resistance, which can keep the strength of the welding spot unchanged. Using this welding condition is beneficial to improve the stability of the strength of the solder joints. If the electrode pressure is too small, it will cause spatter and reduce the strength of the solder joint.

4. The influence of electrode shape and material properties. Since the contact area of the electrode determines the current density, and the resistivity and thermal conductivity of the electrode material are related to the generation and dissipation of heat, the shape and material of the electrode have a significant impact on the formation of the nugget. As the electrode tip deforms and wears, the contact area will increase and the strength of the solder joint will decrease.

5. The influence of the surface condition of the workpiece. Oxides, dirt, oil and other impurities on the workpiece surface increase the contact resistance. An overly thick oxide layer can even prevent current flow. Local conduction, due to excessive current density, will produce splashes and surface burns. The inhomogeneity of the oxide layer also affects the inconsistency of the heating of each solder joint, causing fluctuations in the solder quality. Therefore, thorough cleaning of the workpiece surface is a necessary condition to ensure high-quality joints.


Advantages of resistance welding:


1. When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.

2. The heating time is short and the heat is concentrated, so the heat-affected zone is small, and the deformation and stress are also small. Usually, it is not necessary to arrange the calibration and heat treatment process after welding.

3. Filler metals such as welding wires and electrodes, and welding materials such as oxygen, acetylene, and hydrogen are not required, and the welding cost is low.

4. Simple operation, easy to realize mechanization and automation, and improve working conditions.

5. High productivity, no noise and no harmful gas. In mass production, it can be compiled into the assembly line together with other manufacturing processes. However, flash butt welding requires isolation due to sparks.


Disadvantages of resistance welding:

1. At present, there is still a lack of reliable non-destructive testing methods. The welding quality can only be checked by destructive tests of process samples and workpieces, as well as by various monitoring techniques to ensure.

2. The power of the equipment is high, and the degree of mechanization and automation is high, which makes the equipment cost high and maintenance more difficult, and the commonly used high-power single-phase AC welding machine is not conducive to the balanced operation of the power grid.


Application status of resistance welding in my country:

With the development of aerospace, electronics, automotive, household appliances and other industries, resistance welding has received more and more attention. At the same time, higher requirements are also placed on the quality of resistance welding. Fortunately, the development of my country's microelectronics technology and the development of high-power thyristors and rectifiers have provided conditions for the improvement of resistance welding technology. At present, my country has produced secondary rectifier welding machines with excellent performance. The control box composed of integrated circuits and microcomputers has been used for the matching of new welding machines and the transformation of old welding machines. Advanced closed-loop monitoring technologies such as constant current, dynamic resistance, and thermal expansion have begun to be popularized and applied in production. All of this will help improve the quality of resistance welding and expand its application fields