Research Status and Development of Resistance Welding Electrode Materials for Galvanized Steel Sheets
Text / Zou Jiasheng
Due to its good corrosion resistance, galvanized steel sheets have been used in large quantities in recent years in the steel drum manufacturing industry in place of cold rolled steel sheets. As an important production process, resistance welding has received more and more attention and research in the steel drum manufacturing industry. The most critical electrode joint part of the resistance welding machine, when welding galvanized steel sheet, the low melting point galvanized layer on the surface reduces the contact resistance and current density, compared with the ordinary steel plate (non-coated steel plate) resistance welding Increasing the welding current or prolonging the welding time further promotes the formation of alloying of the electrode head. Moreover, it is frequently contacted with the workpiece under high temperature and high pressure, which accelerates the wear of the electrode. Compared with ordinary steel plate welding, the life of the electrode of the resistance welded galvanized steel plate is generally short.
In order to improve the service life of electrode materials during resistance welding, the development of new resistance welding electrode materials has been highly valued by people. In recent years, welding workers and materials scientists in various countries have conducted extensive research on galvanized steel sheet resistance welding electrode materials. It has been desired to find an electrode material that overcomes the various performance problems present in resistance welding while reducing manufacturing costs.
1 Failure mechanism of resistance welding electrode material
In the resistance welding process of galvanized steel sheet, since the melting point of zinc in the coating layer is very low (about 692K), the hardness is also low (about HB91), and it has strong affinity with copper, and the temperature at the electrode/workpiece interface during welding is relatively high. High, under the action of electrode pressure, adhesion of zinc to the end of the copper alloy electrode occurs, and alloying occurs to form a hard and brittle brass layer with electrical conductivity and thermal conductivity, so that the electrode/workpiece during the welding process The temperature of the interface is higher, the electrode end becomes mushroom-like and becomes thicker, which reduces the welding current density, reduces the solder joint strength, and even the electrode fails and is scrapped due to the pit erosion. Therefore, it is necessary to repair or replace the electrode in time, otherwise the quality of the solder and the appearance quality of the solder joint cannot be guaranteed.
Compared with ordinary low-carbon cold-rolled steel sheets, due to the low melting point of zinc, the galvanized layer will melt before the steel sheet. Under the action of the electrode pressure, it is extruded into the welded area to form a zinc ring. Due to its shunting effect, resistance welding The welding process parameters must be adjusted when welding galvanized steel sheets. Both the laboratory and the actual welding production show that in the resistance welding of galvanized steel sheets, the electrode pressure of the uncoated steel sheet of the same thickness needs to be increased by 10% to 25%, the welding time is extended by 25% to 50%, and the welding current is increased. 25% to 35%, and the thicker the galvanized layer, the greater the welding current required. However, long soldering time will exacerbate adhesion and alloying, resulting in a significant reduction in electrode life.
In addition, when the galvanized steel sheet is welded by the electric resistance welding electrode, a low melting point alloy is generated on the surface of the electrode due to the high temperature. When the electrode leaves the workpiece, some low melting point alloys leave the end face of the electrode under the action of splashing, and one end is formed on the end face of the electrode. Small arc pits, the process of joining together many small arc pits is pit erosion, and as a result, pit erosion is formed on the electrodes. When the temperature of a certain region of the electrode is higher than the recrystallization temperature of the electrode material, recrystallization and grain growth occur in the electrode, the strength is lowered, the electrical resistivity is increased, and the plastic deformation is accelerated. In addition, the resistance welding electrode is not only subjected to high temperature under the working process, but also subjected to the circulation action of heating and cooling, and is subjected to heat and force to generate thermal fatigue. These will speed up electrode failure.
2 Electrode material classification and characteristics
As a resistance welding electrode material for galvanized steel sheets, high temperature hardness is required, and thermal conductivity is also high. Therefore, many electrode materials of various properties have been manufactured according to the use requirements.
2.1 Copper alloy electrode
As early as the 1970s, people added w(Cd) 0.6% to 1.0% to form a solid solution in Cu. The electrode material with higher hardness was hardened by high-temperature cold work, and the conductivity and thermal conductivity were not much lower than that of Cu. The high temperature hardness is higher than that of Cu. In addition, an alloy material which can be heat-treated and strengthened by adding w(Cr) 0.5% to 1.0% in Cu is also formed. It is kept at a temperature below the melting point (1000 ° C) for 1 h, quenched, and then tempered at a temperature of 400 to 470 ° C for 1 to 2 h (precipitation hardening treatment) to precipitate Cr. Since chromium copper is a heat-treated alloy, high temperature hardness and durability are obtained. Also, the electrical conductivity and the thermal conductivity are not low, thereby obtaining an electrode material having high hardness and good electrical conductivity. However, it has been found that the electrode life for galvanized steel sheet resistance welding is very short.
Later, people conducted trial research on various alloys and found that the electrode materials made by adding a small amount of Zr to chromium and copper alloys have greatly improved various properties, and the lifespan is also greatly improved compared with the first two kinds of electrode materials. However, when galvanized steel sheet is used for resistance welding, the Cr-Zr-Cu alloy electrode has two main disadvantages: one is that the electrode will be heavily bonded to the steel plate, which greatly reduces the production efficiency; the other is the resistance welding electrode (including the electrode tip, the electrode cap, and the electrode). The service life of the plates and electrode wheels is significantly shorter than when welding uncoated steel sheets. When welding uncoated steel sheets, the service life of resistance welding electrodes (such as Cr-Zr-Cu alloy electrodes) can usually reach more than 1000 solder joints. When welding galvanized steel sheets, the service life of Cr-Zr-Cu alloy electrodes is not even Up to 250 solder joints.
In order to solve the above problems, Professor Wu Zhisheng of Tianjin University, for the first time applied the cryogenic treatment technology to improve the life of the galvanized steel sheet resistance welding electrode, conducted the life test of the resistance welding electrode before and after cryogenic treatment, and studied the resistance of the chilled treatment to the galvanized steel sheet. The effect of solder electrode life. It was found that when the galvanized steel sheet was welded by Cr-Zr-Cu alloy electrode without cryogenic treatment, the nugget diameter of the first 200 solder joints did not change much, and the diameter of the nugget changed greatly after welding to the 300th solder joint. And quickly reduced, the electrode life is only 550 solder joints. When the Cr-Zr-Cu alloy electrode is used for resistance welding of galvanized steel sheet, the diameter of the nugget of the first 1500 solder joints is relatively stable and the fluctuation is small. The lifetime of the cryo-electrode at -150 °C for 2 h and 4 h for 4 h is 1687 solder joints and 1743 solder joints, respectively, which can be considered to have the same lifetime. The resistance welding process of the cryo-electrode kept at -170 °C for 2 h is more stable. The diameter of the nugget does not fluctuate greatly within the range of 1800 solder joints. The average life of the electrode reaches 2234 solder joints, and the life is further improved. In addition, when the galvanized steel sheet is welded by the electrode which is not cryogenically treated, the sticking electrode phenomenon is severe and splashed, and the surface color of the solder joint is brass color, and when the galvanized steel sheet is treated by the cryogenic treatment electrode, the welding process is less splashed. The surface of the solder joint has a very light brass color, indicating that the copper-zinc alloying tendency is small.
However, Cr-Zr-Cu alloy is still the most commonly used resistance welding electrode material, which is determined by its excellent physical and chemical properties and good cost performance.
(1) The Cr-Zr-Cu electrode achieves four properties of the welding electrode, which makes the index well balanced. The excellent conductivity ensures the minimum impedance of the welding circuit and can obtain excellent welding quality. The high temperature mechanical properties can guarantee the welding high temperature environment. The performance and life of the lower electrode material; the wear resistance makes the electrode not easy to wear, at the same time prolongs the life and reduces the cost; the higher hardness and strength can ensure that the electrode head works under a certain pressure, is not easy to deform and crush, and ensures the welding quality.
(2) The electrode is a kind of consumable in industrial production, and its dosage is relatively large, so its price cost is also an important factor to be considered. The Cr-Zr-Cu electrode is relatively inexpensive compared to its excellent performance and can meet the production. Need. For the above reasons, Cr-Zr-Cu alloy electrodes are still widely used.
2.2 Composite electrode
These composite materials are obtained by mixing and sintering W with high thermal conductivity at a high temperature and Cu having good thermal conductivity, and a W-Cu alloy is typical. The W-based high-density alloy is sintered by adding a small amount of nickel-iron or nickel-copper in W. The W-Cu composite has w(Cu) of 10% to 40%, and is also widely used for resistance welding of galvanized steel sheets.
2.3 Alumina copper electrode
When welding galvanized steel sheets, Cr-Zr-Cu copper electrodes are often bonded to the workpiece. After welding a few points, the welding quality is rapidly reduced, which seriously affects the consistency of the quality of the solder joints. For the above reasons, welding equipment manufacturers hope to have better electrode materials. In 1973, American SCM Company began to introduce aluminum oxide dispersion strengthened copper materials. Thanks to the new manufacturing process, the overall performance has been greatly improved. Compared with Cr-Zr-Cu, it has excellent high temperature mechanical properties (softening temperature 900 ° C) and good electrical conductivity (conductivity 80% to 85% IACS) and wear resistance. More importantly, the dispersed and strengthened copper electrode is dispersed with fine alumina particles, which can prevent the diffusion of Zn into the electrode, thereby greatly reducing the electrode loss caused by the bonding, and prolonging the life of the electrode compared with the ordinary Cu-Cr electrode. 2 to 2.7 times. Alumina copper electrodes have been successfully used in resistance welding electrodes and are now officially incorporated into the ASTM standard. Subsequently, Japan also developed and manufactured and sold a dispersion-strengthened copper electrode material under the trade name DEMIRAS, and also incorporated the Japanese JIS standard. In 2005, Wang Mingpu and Li Zhou of Central South University prepared a zero-burn hydrogen expansion nano-dispersion-enhanced CuAl2O3 alloy. Compared with oxygen-free copper, its RP0.2 is 3-11 times higher than that of oxygen-free copper. Up to 900 ° C, and conductivity up to 75% ~ 96% IACS, with zero hydrogen burning expansion characteristics.
Therefore, the alumina copper electrode made of the above materials is an excellent electrode material, which is excellent in strength, softening temperature and electrical conductivity, and is particularly prominent for welding galvanized sheets, unlike Cr-. The Zr-Cu electrode causes the electrode to stick to the workpiece. It does not need to be sanded frequently, which can effectively solve the problem of welding the galvanized sheet, improve the efficiency, and increase the service life by more than 4 times.
Although the alumina copper electrode has excellent welding performance, its current cost is very expensive, and its price is about 2 times higher than that of the Cu-Cr copper electrode. Therefore, it is not common at present, but it has excellent welding performance and plating on the galvanized sheet. The widespread use of zinc sheets makes them a promising market.
2.4 coated electrode
The coated electrode is a hot trend in electrode research at home and abroad. Its good process performance and low price have attracted many enterprises and universities to actively invest in coating electrode research.
At present, a layer of TiC is deposited on the surface of a Cr-Zr-Cu electrode by an electric spark in China. The electrode life of resistance welded galvanized steel sheet reaches more than 1000 solder joints, which is 2.5 times that of the general Cr-Zr-Cu electrode (400 solder joints). The main reason is that the TiC coating retards and prevents the diffusion of Zn, delaying the alloying with the electrode matrix Cu. However, due to some defects in the TiC coating, such as cracks or poor adhesion to the electrode substrate, Zn diffuses under the TiC coating by penetration during the soldering process and reacts with the electrode matrix Cu. As the solder joint increases, a Cu-Zn alloy with a large brittleness may be formed, which will fall off under the action of heat and force, and the TiC coating which is not firmly bonded to the electrode substrate also falls off, resulting in a larger area and Zn. Contact further accelerates electrode wear. At present, the surface treatment process in North America has been coated with a Ti-based reinforced Ni-based alloy at the end of the electrode to act as a barrier to prevent the copper alloy from reacting with molten Zn, which can improve the use of resistance welded galvanized steel. life. It has been reported that such coated electrodes are currently used in the welding production lines of the three major North American automotive companies. However, in order to obtain better coated electrodes, Professor Chen Wei of our country has carried out detailed analysis and experiments on current products in North America. Based on the above work, a new idea of ​​using double-layer and multi-layer coatings is proposed for the existing problems. A Ni-based alloy is deposited on the head of the Cr-Zr-Cu copper alloy electrode, and then a Ti-based reinforced Ni-based alloy (double-layer coating) is deposited. As a result, it was found that the density, toughness, and adhesion to the matrix of the two-layer coating were greatly improved as compared with the single-layer TiC particle-reinforced Ni-based alloy (current technology in the United States and Canada), and it was not easily cracked during the welding process. The double-coated electrode was tested in the laboratory and production line of Nanjing Automobile Group Fiat Sedan, General Motors and Volkswagen. It was found that the welding current required for the coated electrode was much smaller than that of the uncoated electrode, and the life was normal. More than 2 times the coated electrode.
In order to systematically study the composition of the coating, the deposition process and the metallurgical behavior during welding to develop coating electrodes with better performance and longer life, Professor Chen Yu also proposed a new idea of ​​multilayer electrodes. First, a very thin Ni-based alloy coating (layer 1) is deposited at the end of the electrode, which serves to isolate and prevent the copper alloy from entering the second coating. Then, a layer of conductive cermet coating (layer 2) is deposited on the first layer of coating, which is a TiC or TiB2 reinforced Ni-based or Co-based alloy. Since Ni is a good binder for TiC and TiB2 (and Cu is not), the cermet coating can form a good bond with the first layer of Ni-based alloy coating. Since the adhesion between the TiC or TiB2 ceramic and the molten Zn hardly occurs, the adhesion between the coating and Zn can be greatly reduced. In addition, the hardness of the cermet is high, and the deformation of the end portion of the electrode is effectively prevented to cause an increase in the cross-sectional area of ​​the electrode. In order to further reduce the adhesion between the coating and the molten Zn, the second layer coating is finally subjected to a surface solid lubrication treatment, that is, a dry lubricating film (layer 3) is deposited. For the first time, it is proposed to deposit a solid dry lubricating film on the top of the coating, which can significantly reduce the adhesion between the coating and the molten Zn at the initial stage of welding, delay the alloying of the substrate with Cu, and greatly improve the electrode life.
3 Outlook
At present, the electrode materials used for electric resistance welding of galvanized steel sheets are generally copper or copper alloys. Cu-based composite materials studied at home and abroad are relatively numerous, but they are expensive and expensive, which limits their wide application in production. This shows that to improve the service life of electrode materials, you can start from the following three aspects:
(1) Reducing the manufacturing cost of Cu-based composite materials.
(2) Appropriate treatment of the electrode surface. Since the electrode is easily alloyed with Zn when the galvanized steel sheet is resistance-welded, a coating or a multilayer coating may be deposited on the surface of the electrode to avoid alloying or slow alloying. Find the optimal process parameters of coating deposition, thereby improving the quality of the surface coating, reducing defects such as cracks, voids, and coating peeling, and improving the bonding strength between the surface coating and the electrode substrate, thereby improving the service life of the electrode.
(3) In-depth study of the electrode failure mechanism of galvanized steel sheet during resistance welding, and the development of electrode materials for galvanized steel sheets with better performance and longer life.
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