Study on Quality Control of Resistance Welding of Galvanized Sheet
Text / 耿化è”
1 Introduction
In the past ten years, China's automobile industry has developed rapidly, and the problem of car body corrosion prevention has become increasingly prominent. According to relevant investigations, the corrosion of domestically-produced new cars in one year will occur, and corrosion penetration will occur in 3 to 4 years. For this reason, the annual maintenance cost in the country amounts to several billion yuan. Therefore, in order to improve the service life of the car body and enhance the corrosion resistance of the car body material, the low-carbon steel plate with the surface coating is widely used, but the most used one is the galvanized steel plate, and the galvanized steel plate for automobiles in Europe and the United States accounts for the entire car. About 60% of the body material, and Japan is more. China has also long established relevant policies to encourage the automobile manufacturing industry to promote the use of domestic galvanized steel sheets. The welding of galvanized sheets has also been included in the national fund project.
Although galvanized steel sheets have good corrosion resistance, in the manufacture of automobile bodies at home and abroad, a large number of resistance spot welding methods are used. Compared with uncoated steel sheets, the following problems exist in the spot welding process of galvanized steel sheets. : The zinc layer melted before the steel plate forms a zinc ring and is shunted, resulting in a decrease in the welding current density; the surface of the zinc layer is burned, stuck, contaminated with the electrode and the electrode life is reduced; the zinc layer has a low resistivity and a small contact resistance; Defects such as splashes, cracks, pores or tissue softening. It is precisely because of the above problems in spot welding of galvanized steel sheets that it has attracted extensive attention in related fields at home and abroad, and has done a lot of research work. At present, the research on spot welding of galvanized steel mainly starts from four aspects: one is the spot welding property of galvanized sheet, the other is the correct choice of electrode material and shape, and the third is the influence of main process parameters on spot welding quality. The fourth is the mode and method of spot welding quality control.
2 Effect of galvanizing method and thickness on spot welding
According to the galvanizing process, galvanized steel sheets can be roughly divided into electrogalvanized steel sheets and hot-dip galvanized steel sheets, and an alloyed galvanized steel sheet is alloyed at 450oC or higher after hot dip galvanizing. Compared with the three, the electrogalvanized sheet is thin, the weldability is relatively good, but the cost is high; the hot-dip galvanized steel sheet is thick, the corrosion resistance is good, but the weldability is poor; the weldability of the alloyed galvanized steel sheet is relatively hot. The galvanized steel sheet is improved; in the case of the same thickness of the zinc layer, the hot-dip galvanized sheet has better weldability than the electro-galvanized sheet. Although the thickness of the zinc layer is very small for the steel sheet, the influence on the weldability is large. It is generally believed that as the thickness of the zinc layer increases, the required welding current is larger, but Cheng Xuanting et al. found through comparison with the test that the thicker the zinc layer is, the larger the required current is in a certain thickness range; however, when the zinc layer reaches a certain level At the thickness, the opposite side of the current is reduced. The relevant information edited by the American Institute of Metals clearly states that when the thickness of the galvanized steel sheet increases (the thickness of the zinc layer is in the range of 0.005 to 0.025 mm), the weldability is lowered, but when the thickness of the zinc layer is above 1.52 mm, the weldability is not Affected by the thickness of the coating. In the manufacture of automobile bodies at home and abroad, the thickness of the electroplated zinc plate is generally 20-90g/m2 (0.003~0.013mm), and the thickness of the hot-dip galvanized steel plate is 40-180g/m2 (0.006-0.025mm). Spot welding of galvanized steel sheets for automobiles requires reasonable selection of process parameters to ensure welding quality. However, the author's factory practice proves that despite the existence of zinc layer, as long as the appropriate equipment, process, etc. are selected, the quality of resistance spot welding of galvanized steel sheets can fully meet the technical requirements of vehicle body manufacturing. Therefore, the galvanizing method or the thickness of the zinc layer of the galvanized steel sheet itself is not a decisive factor affecting the quality of the spot welding of the vehicle body.
3 electrode material and shape selection and design
In the spot welding study of galvanized sheet, the electrode material is a focus of attention. The research mainly focuses on the analysis of the interaction characteristics between the existing electrode and the coating, and the development of new electrode materials. The electrode materials for spot welding of galvanized sheet abroad mainly include Cu-Cr (0.8%Cr), Cu-Zr (0.15%Zr), Cu-Cr-Zr, and dispersion-strengthened copper (DSC) containing Al3O2 particles. In the United States, a composite electrode with a tungsten electrode embedded with a tungsten electrode was used for experiments. The results show that the electrode has a long service life. However, due to the poor thermal conductivity of the tungsten electrode, only low current welding specifications can be used, so it is difficult to manufacture in the vehicle body. Promote use. It has also been proposed to spray a layer of high-melting material such as cobalt, titanium oxide, tantalum, etc. on the surface of the electrode, or to preset alumina powder between the galvanized sheets to increase the contact resistance and shorten the soldering time, thereby improving the electrode life. Most of the domestic researches have been through experiments to study the performance of the above-mentioned several kinds of material electrodes. A large number of experiments have proved that in the case of less favorable welding conditions, whether it is a cheaper Cu-Cr, Cu-Zr alloy electrode, it is more expensive. DSC electrodes are used in similar applications, but factory experience has shown that in some cases of actual production, electrode adhesion is reduced when using DSC20 grade alloy electrodes.
As for the shape of the electrode and the size of the end face, the truncated cone shape is considered to be the best. At the same time, the literature points out that when using moving welding tongs, it is recommended to use spherical electrodes with a radius of 1 ~ 2in, but the literature emphasizes the use of such spherical electrodes.
Foreign researchers have long recognized that for spot welding of galvanized steel sheets, the electrode life depends on how easy it is to alloy the electrode tip with the molten metal. Some alloy electrodes can be completed in 5 cycles (60 Hz). In the alloying process, the electrode which is more prone to alloying has a shorter life. In addition, a large number of spot welding tests have shown that the surface temperature of the electrode during welding is significantly higher than that of the ordinary steel plate. Therefore, the cooling of the electrode is very important, the cooling water flow must be sufficient, and the electrode is kept close to room temperature, so that the alloy reaction between the electrode and the plating layer can be minimized, and the "bump" or the expansion of the electrode due to the softening of the electrode can be prevented. However, the speed of the cooling water flow is high, and the high temperature does not cause the electrode end to overheat. There is no conclusion yet. The literature recommends that for a cone-shaped electrode with a diameter of 16 mm, the water flow rate is not less than 1 gallon/min, and the maximum influent temperature is 32oC.
4 Influence of process parameters on spot welding process and quality of galvanized sheet
In the current lack of reliable and practical detection or adaptive control methods, it is particularly difficult to manually select (input) the appropriate process parameters, because for the most commonly used constant current control spot welder, galvanized sheet Spot welding can be used with a very small adjustment range of welding current, and spot welding quality also involves network pressure fluctuations, equipment characteristics, welder (welder) loops, various shunts, electrode wear, electrode pressure fluctuations, material properties and characteristics. More than 20 kinds of influencing factors, such as material thickness combination, joint form, cooling condition, operation mode, etc., and these factors interact, and any fluctuation or abnormality of the factor will lead to the setting of the welding specification value and the actual value of the welding point. The difference, which in turn affects the optimal matching of process parameters. For enterprise production, the reasonable choice of process parameters is the most important way to control the quality of solder joints, so the research work in this area is of great practical significance.
Some scholars have studied the spot welding process of galvanized steel sheets. Wang Jing, from Shanghai Jiaotong University, compared the results of spot welding process and mechanical properties of ordinary cold-rolled steel sheets, electro-galvanized steel sheets and two different zinc-coated hot-dip galvanized steel sheets, and analyzed that the nugget size and joint strength are In terms of the law of current change, the electro-galvanized steel sheet is basically the same as the ordinary steel sheet, and is parabolic, while the hot-dip galvanized steel sheet has a logarithmic rising logarithmic curve; when the electrode pressure is greater than 2.0 kN during spot welding of the galvanized steel sheet, the electrode pressure The improvement has no obvious effect on reducing the splash; the variation of the strength of the galvanized steel joint with the energization time is basically the same as that of the ordinary steel plate, and it is also increased first and then tends to be gentle or decreased. This is consistent with the conclusions obtained from the spot welding process experiment of the hot-dip galvanized sheet of Baosteel Research Institute.
Some scholars have also analyzed the morphology of the nugget during spot welding of galvanized steel sheets. The influence of the process parameters of galvanized sheet on the quality of spot welding was discussed from the microscopic characteristics. According to the literature, the crystal morphology of nugget in spot welding of galvanized steel sheet is coarse columnar crystal with strong directionality; increasing or decreasing the welding current has no obvious effect on improving the crystal morphology of spot welding nugget of galvanized sheet, but increasing the current. On the contrary, the columnar crystals will be coarser; increasing the energization time and increasing the welding pressure can improve the crystal morphology of the spot welding nugget of the galvanized sheet, the former has the most obvious effect, and the equiaxed crystal structure can be basically formed. However, some scholars believe that even in the case of spot welding of ordinary steel plates with good weldability, it is difficult to obtain equiaxed crystal structures under normal conditions. The literature also points out that even for high-quality ordinary steel plate spot welding heads, the nugget is composed of "columnar + isometric". Therefore, for a galvanized steel sheet with poor weldability, it is quite doubtful to obtain an "isometric + columnar" crystal or a large amount of equiaxed crystal structure by merely adjusting the process parameters. Moreover, some people have used orthogonal experimental methods to study the 08Al galvanized steel sheet. The conclusion is that the welding current is the first influencing factor of joint strength, and the second is the energization time, which is consistent with the spot welding characteristics of ordinary steel sheets, but But it contradicts the literature.
In addition, although most of the authoritative welding institutions at home and abroad have recommended the spot welding specification table for galvanized steel sheets, the relevant data is not uniform. The author also used the same material to verify the welding specifications recommended by IIW and JIW under the same spot welding conditions, but the spot welding effect is not satisfactory. Moreover, the best specifications obtained from the above-mentioned literatures are also inconsistent under the same welding conditions.
Perhaps, for the galvanized steel spot welding with complicated welding process and many influencing factors, these contradictions or differences are reasonable under certain experimental conditions, which is the difficulty of artificial optimization of process parameters. At present, many researchers have proposed artificial fuzzy systems integrated with fuzzy systems, expert systems (ES) and artificial neural networks (ANN) and other integrated systems to optimize the selection of spot welding process parameters, some of which are already in the laboratory. It has achieved success, which will promote the in-depth development of galvanized steel sheet spot welding process parameter optimization research, and even fundamentally solve the drawbacks of traditional methods of process parameter optimization.
Research status of 5 spot welding quality control methods
At present, in the production of automobile body, the quality of spot welding is basically checked by process experiment, visual inspection and destructive test, but there is no convenient and effective quality control means. However, due to the widespread application of resistance spot welding and the urgent need for quality stability, spot welding quality control has always been the focus of scholars and equipment manufacturers in various countries, and has done the most extensive research.
The quality control methods that have been proposed now include electrical parameter method, thermal expansion method, infrared radiation method, ultrasonic method, acoustic emission method and comprehensive control method, among which infrared radiation, ultrasonic method and acoustic emission method are limited by various conditions. It is difficult to promote the application. The electrical parameter method includes constant current, dynamic resistance, energy integration, and interelectrode voltage method. Although many research progresses have been made in the research of these control methods, there is still no way to compensate for the influence of various factors in actual production, and because of their respective limitations, they have not been popularized and applied in production. So more than a decade ago, some scholars pointed out that further research should be the development of multi-parameter integrated control.
At the AWS International Welding Expo, a foreign company has exhibited a resistance welding temperature measurement that can detect multiple parameters (current, voltage, dynamic resistance, electrode displacement, electrode pressure, etc.) of each solder joint during the spot welding process. The effect is good. Northwest Electric University's STD industrial control microcomputer developed the resistance welding multi-parameter monitor, which can also achieve the detection of multiple parameters, and has obtained satisfactory results under the laboratory conditions. Cao Yu et al. used the dynamic parameter test system of spot welding process established by IBM-80286 to reliably obtain various dynamic parameters of the spot welding process. The accuracy of these control methods is high, but the disadvantage is that the detection system is complex, and the author believes that it is difficult to promote it in production.
It is worth mentioning that the inverter resistance spot welding machine has been promoted and used in domestic automobile and electrical appliance manufacturing industries. Its working principle is to rectify and filter the input three-phase AC bridge into DC power, and then invert the inverter. The device generates intermediate frequency alternating current (f=600~1000Hz), and then feeds back to the welding transformer to output low voltage alternating current, and finally outputs direct current with small ripple after single phase rectification. This kind of control method originally used pulse width modulation (PWM). By the 1990s, it had adopted the "zero-conversion-PWM" or "zero-switch-PWM" converter, which not only economical and energy saving, but also the welding specification adjustment. The range is expanded by 3 to 4 times, which is undoubtedly a boon for spot welding of galvanized steel sheets with a small selection of welding parameters.
In general, the development trend of spot welding quality control is the integration of control modes and control methods, the quantitative description of process and quality control laws, and the intelligentization of control decisions. Of course, any control method must have a good market prospect. First, it must be effective and reliable, simple to operate, and cost-effective.
It is new latest design 1500W Hair Dryer. it comes with hot and cool function and also comes with hi and low function . so it have dual switch control. that is not comes with other.in today's day and age, it comes with good quality and low price. so all can afford it. From room occupying appliances and pocket-fitting devices, to meters and meters of cables, no home is stranger to any of these.
Hair Dryer 1500W,nanoe hair dryer,nano titanium hair dryer,Mini Hair Dryer
Ningbo Meirou Electric Appliance Co.,Ltd. , https://www.mrhairdryer.com