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1. What is resistance welding? Resistance welding applies pressure through the electrodes of the welding equipment and when the power is turned on, resistance heat is generated at the workpiece contact point and adjacent areas to heat the workpiece, and the workpieces are connected under the action of external force. Therefore, resistance heat and external mechanical force during welding are the most important factors affecting the quality of resistance welding. 2. How is the power requirement of resistance welding calculated? According to Joule's law of material resistance heat calculation, the heat generated in the workpiece area is calculated by the following formula: Q=I2Rt Among them: Q--the total heat during welding in the workpiece area; I--the output current of the welding machine; R--the contact resistance of the workpiece area (the resistance of the material itself is not included); t--the power-on time of the welding machine; From the above formula, it can be seen that when the external force (F) is certain (the surface of the workpiece is consistent), the contact resistance R is a fixed value, and the factor that affects the welding energy is mainly the welding current (power); when the power is constant, increasing the welding time can increase the welding capacity. 3. What are the process parameters that affect spot welding quality? The parameters of the basic spot welding cycle are A. Preload time; B. Power-on welding time; C. Power-on welding current; D. Maintaining pressure time. The meaning of each parameter and its impact on welding quality are as follows: A. Preloading time (F>0, I=0), this stage includes the rise and stabilization of the electrode pressure. In order to ensure that the electrode pressure is constant during power-on, the preloading time must be guaranteed. Especially during continuous spot welding, the time required for the action of the welding machine's motion mechanism must be fully considered and cannot be shortened infinitely. ※ When multi-point welding, in order to ensure that the pressure of each solder joint is uniform and the passing welding current is consistent, the preloading time must be correspondingly longer. ※During the welding process, in order to improve the welding efficiency, the opening distance between the upper and lower electrodes should be as small as possible while ensuring easy access to the welding mold, thereby reducing the movement time of the upper electrode and shortening the preloading time. ※The purpose of preloading is to establish a stable current channel to ensure that the welding process obtains a repeatable current density. Therefore, the setting of the preloading time directly affects the welding quality of the workpiece. B. Current welding time (F=Fw.I=Iw), this stage is the stage when the weldment is heated and melted to form a nugget. The welding current can be basically unchanged (referring to the effective value), or it can be gradually increased or stepped up. When the welding parameters are appropriate, a molten core with a size fluctuation of less than 15% can be obtained. During this period, the following phenomena can occur: a. Stirring effect of liquid metal: If the power-on time is too short, insufficient stirring will produce a vortex-shaped heterogeneous nugget, resulting in desoldering. b. Spatter: Spatter can be divided into early stage and late stage according to the generation period. According to the generation location, it can be divided into two types: internal spatter (between the two weldments) and external spatter (the side in contact between the weldment and the electrode). The causes of early spatter are generally: poor surface cleaning of the weldment or serious uneven pressure distribution on the contact surface, resulting in excessive local current density and early melting. At this time, spatter will occur because there is no plastic ring protection. Measures to prevent early spatter generally include: ① Strengthen and ensure the clean quality of the surface of the weldment; ② Have appropriate pre-pressure time and welding pressure. When the pre-pressure time is short and the welding pressure is insufficient, it will cause larger welding spatter, which will seriously burn the surface of the workpiece; ③ Pay attention to the centering before pre-pressure. You can use gradually increasing current or increase the preheating current to slow down the heating speed to avoid early melting and causing spatter. ④ The surface of the welding electrode must be kept clean. The causes of later spatter are: The molten core grows too much and exceeds the effective range of the electrode pressure, thereby breaking through the plastic ring to cause internal spatter in the radial direction, and breaking through the workpiece surface in the axial direction to cause external spatter. This situation generally occurs when the welding current is large and the power-on time is too long. It can be prevented by shortening the power-on time and reducing the current. C. Power-on welding current (F=Fw, I=Iw, T=TW): In order to ensure the heat input Q that makes the workpiece reach the welding strength, it can be known from Joule's law: Q=I2Rt that the welding current I and the welding time T are the two most important process parameters that affect the welding heat input. Especially the welding current I, it can be seen from the formula that the heat input Q is proportional to the square of the current I, so: ※Increasing the welding current is the most effective means to improve the welding ability. ※When the welding workpiece is certain, selecting the shortest welding energization time and increasing the welding capacity by increasing the welding current can improve production efficiency. ※When the welding machine power is constant and the welding nugget formation is satisfied, selecting a shorter welding energization time and a larger welding energization current are priority process methods to improve the surface thermal damage of the weldment and reduce the deformation of the weldment. D. Maintain pressure (F>0, I=0): At this stage, the welding current stops and the welding point is in the cooling stage. Since the liquid metal is in a closed plastic space, if there is no external force, three-dimensional tensile stress will be generated when the metal cools and shrinks, resulting in shrinkage holes, cracks and other welding defects. Especially for workpieces with thicker plates, thicker wires or higher carbon content, these welding defects are particularly likely to occur and cause desoldering. 4. What are the general characteristics of the selection method of spot welding parameters? When using power frequency AC power supply, the spot welding parameters mainly include welding current, welding (energization) time, electrode pressure and electrode size. ①Welding current Iw: The heat deposited by the weldment is proportional to the square of the current, so the welding current has the most sensitive effect on the performance of the solder joint. When other parameters remain unchanged, when the current is less than the corresponding value, the nugget cannot be formed, causing desoldering. When it exceeds this value, the nugget increases rapidly as the current increases, and the strength of the solder joint increases. Then due to the increase in heat dissipation, the growth rate of the nugget slows down, and the strength of the solder joint increases slowly. If the current is further increased, spatter will be generated, and the strength of the solder joint will decrease instead. Therefore, it is generally recommended to use a moderate current that is not sensitive to changes in nugget diameter for welding. In actual production, the fluctuations in welding current are sometimes very large. The reasons are: A. The grid voltage itself fluctuates or multiple welding machines are powered on at the same time; B. The change of the ferromagnetic weldment extending into the welding circuit; C. The shunting of the front point to the rear point, etc.; D. The contact between the conductive welding tool and the electrode of the welding machine causes shunting. ②Welding time tw: The length of the power-on time directly affects the amount of input heat. On currently widely used synchronized control spot welding machines, the power-on time is an integral multiple of the number of cycles (one cycle in my country is 0.02s. Some welding machine manufacturers use computer controllers, and the power-on time is measured in half a cycle). When other parameters are fixed, the nugget will only begin to appear when the energization time exceeds a certain minimum value, thereby realizing the welding connection of the workpieces. As the energization time increases, the nugget will first increase rapidly, and the tensile and shear force will also increase. When the selected current is larger, the nugget will grow to a certain limit and spatter will occur. ※Selecting the shortest possible welding time is the priority in the welding process. However, depending on the different welding machine power, welding workpiece form, welding workpiece material, welding spot number and other factors, the welding time must meet the nugget formation conditions. ③Electrode pressure F: The size of the electrode pressure affects the value of the workpiece contact resistance on the one hand, the amount of heat dissipation on the other hand, and the heat dissipation of the weldment to the electrode on the other hand. From the perspective of energy saving, the minimum electrode pressure that does not produce spatter should be selected. ※When multiple welding machines are welding continuously, special attention should be paid to the stability of the compressed air flow and pressure output of the gas source. When the flow and pressure output are unstable, it is easy to produce spatter or desoldering. ④Electrode working surface size: When the welding current is constant, a smaller electrode working size increases the current density and enhances the welding capability. Therefore, the welding machine electrode must be repaired in time after a certain period of welding to ensure the consistency of the welding current density, thereby ensuring the stability of the welding quality. ※The size of the electrode working surface has an important impact on the appearance of the weldment surface and the stability of the weld nugget size, so special attention must be paid. ※It should be noted that the parameters during spot (row) welding affect each other. According to different welding materials and working conditions, a variety of combinations of parameters can be selected for most occasions. 5. Projection welding process of metal materials A. Low carbon steel wire, bar or pipe cross-shaped projection welding: Cross-shaped projection welding uses the convex welding point state formed when the outer circles of wires, bars or pipes are connected to each other, forming a local current concentration and becoming an ideal projection welding. In order to prevent the weldment (especially thin wires) from being flattened and causing a reduction in strength, the electrode end surface should be cut into a V-shape or U-shape Groove. Table 1 is the welding parameters of low carbon steel wire cross joints. When cross-shaped projection welding of pipes, pipes of different diameters can be used, but the wall thickness must be consistent. Similar to steel wire cross joints, a larger reduction leads to a larger joint strength (expressed here as torque). For example, 22mm * When welding a 1.5mm pipe, when the reduction is 5%, the torque of the joint is 1500N.m; when the reduction is 15%, the torque of the joint is as high as 2500N.m. B. Projection welding of stainless steel and high-temperature alloys When projecting welding stainless steel, attention must be paid to the displacement phenomenon of the nugget. This displacement phenomenon is mainly caused by the attraction of currents in the same direction when current passes between multiple bumps. The displacement phenomenon leads to a reduction in the strength of the nugget. In order to overcome the above shortcomings, the distance between bumps should not be too small, and a higher electrode pressure should be used. However, excessive electrode pressure should be avoided to avoid crushing the bumps. Table 1 Welding parameters of low carbon steel cross joint projection welding Source: Japan Welding Society: Welding Technology. 1982 (9) Note: The amount of reduction refers to the amount of one steel wire pressed into another steel wire in resistance welding. (I don’t understand, it should be the diameter before pressing in minus the diameter after pressing in, divided by the diameter before pressing in, Darcy Lee remarks) 6. Resistance welding machine ① Classification and main technical requirements of resistance welding machine Resistance welding machine, also known as contact welding machine, is a welding machine that uses electric current to generate heat through the resistance between the welding piece and the contact surface, and at the same time applies pressure to the welding joint to weld. Resistance welding machine mainly includes four types: spot (row) welding machine, projection welding machine, seam welding machine and butt welding machine. Its control equipment can be included in the corresponding welding machine or can be classified separately. A. Spot welding machine and projection welding machine: According to the process requirements of spot welding and projection welding, the spot welding machine or projection welding machine must have two basic functions: pressing the welding piece with a certain pressure and feeding welding current to the welding piece. B. Seam welding machine: In addition to having the same pressurizing and feeding current functions as the spot welding machine, the seam welding machine must also drive the welding piece to move, that is, it must have a transmission mechanism to rotate the welding wheel. C. Butt welding machine: According to the requirements of the butt welding process, the butt welding machine must have basic functions such as clamping the welding piece, feeding the welding piece until upsetting, and feeding the welding current to the welding piece. ② Main components of the resistance welding machine: The resistance welding machine is mainly composed of the following three main parts: 1. Welding main power supply: including welding resist transformer, power adjustment mechanism and secondary circuit, etc.; 2. Control device: can synchronously energize and pressurize, so that the entire welding process can be carried out automatically, and some also have welding quality monitoring functions; 3. Mechanical device: including body, pressurizing (clamping) mechanism, feeding mechanism (butt welding machine), transmission mechanism (seam welding machine), etc. A. Characteristics, structure and output power adjustment of the welding resist transformer: The transformer is the core part of the welding machine, and its performance indicators are: rated capacity (Sn); rated welding current (I2n), rated load duration ( * %), primary voltage (U1n), secondary no-load voltage (U20), etc. For the operator, it is absolutely safe electrically to contact the secondary circuit during the operation of the welding machine. Since the output voltage of the transformer is low and the output current is required to be large, the leakage reactance of the transformer is required to be small, and the number of turns of the secondary winding of the transformer is very small, usually only one turn. The load duration rate of the welding resistance transformer is lower than that of the arc welding transformer. According to the current national standards, the rating of the main power supply of a general resistance welding machine is designed with a load duration rate of 50%. B. Output power adjustment: For common industrial frequency AC resistance welding machines, the adjustment of welding current (i.e. power adjustment) is achieved by adjusting the conduction angle of the thyristor of the main circuit switching device. According to the transformer theory, the following relationship can be listed: U1/U20=N1/N2=K In the formula: K--transformation ratio; U1--primary voltage (V); U20--secondary no-load voltage (V); N1--number of primary winding turns; N2--number of secondary winding turns. Usually N2=1, U20=U1/N1 welding current I=U20/Z=U1/Z·N1 In the formula: Z--Welding circuit impedance. When the grid voltage U1 and the welding circuit impedance Z remain unchanged, changing the number of turns of the primary winding of the welding resistor transformer can change the secondary no-load voltage, thereby changing the welding current and achieving the purpose of power regulation. The fewer the number of turns of the primary winding of the transformer, the greater its output power. In order to minimize energy loss and reduce the impact on the quality of the power grid, when designing and manufacturing welding machines, the short-circuit impedance of the secondary circuit can be reduced by reducing the leakage reactance of the transformer. ※When the design power of the welding transformer is constant, the greater the distance between the welding electrode and the secondary circuit of the transformer, the smaller the useful power output from the transformer to the welding workpiece.