Spot welding technology of three-layer metal plate

In recent ten years, China's automobile industry has developed rapidly, and the anti-corrosion problem of automobile body has become increasingly prominent. According to relevant surveys, corrosion spots will appear in the first year of domestic new cars, and corrosion penetration will occur in three to four years, so the annual maintenance cost in China will reach several billion yuan [1]. Therefore, in order to improve the service life of car body and enhance the corrosion resistance of car body materials, coated low-carbon steel plates are widely used, but galvanized steel plates are the most widely used at present. Galvanized steel plates for automobiles in Europe and America account for about 60% of the whole body material, and more in Japan [2,3]. China has long formulated relevant policies to encourage the automobile manufacturing industry to promote the use of domestic galvanized steel plates, and the welding of galvanized steel plates has also been included in the national fund project.

Although galvanized steel sheet has good corrosion resistance, resistance spot welding is widely used in automobile body manufacturing at home and abroad. Compared with uncoated steel plate, there are the following problems in spot welding of galvanized steel plate: the zinc ring is formed before the zinc layer is melted on the steel plate, which leads to the decrease of welding current density; The surface of zinc layer burns, adheres and pollutes the electrode, which reduces the service life of the electrode; Zinc layer has low resistivity and low contact resistance; It is easy to produce welding spatter, cracks, pores or tissue softening and other defects.

It is precisely because of the above-mentioned problems in spot welding of galvanized steel plate that it has aroused widespread concern in related fields at home and abroad, and a lot of research work has been done. At present, the research on spot welding of galvanized steel plate mainly starts from four aspects: first, the spot welding weldability of galvanized steel plate; Second, choose the electrode material and shape correctly; Thirdly, the influence of main process parameters on spot welding quality; Fourthly, the mode and method of spot welding quality control.

2. Influence of galvanizing method and thickness on spot welding.

According to the different galvanizing processes, galvanized steel plates can be roughly divided into galvanized steel plates and hot-dip galvanized steel plates, and there is also an alloyed galvanized steel plate, which is kept at 450? Alloying treatment above c. Compared with the three, galvanized sheet has thin coating and good weldability, but the cost is high; Hot-dip galvanized steel sheet has thick coating and good corrosion resistance, but poor weldability; Compared with hot-dip galvanized steel sheet, the weldability of alloyed galvanized steel sheet is improved. Under the same zinc layer thickness, hot dip galvanized sheet has better weldability than galvanized sheet.

Although the thickness of zinc layer is small for steel plate, it has great influence on weldability. It is generally believed that the welding current increases with the increase of zinc layer thickness, but Cheng Xuanting and others [4] found that in a certain range of coating thickness, the thicker the zinc layer, the greater the required current; However, when the zinc layer reaches a certain thickness, the required current decreases. However, the relevant materials edited by American Metallurgy Institute [5] clearly point out that the weldability of galvanized steel plate decreases with the increase of coating thickness (the thickness of zinc layer is in the range of 0.005~0.025mm), but when the thickness of zinc layer is above 1.52 mm, the weldability is not affected by the coating thickness. In automobile body manufacturing at home and abroad, the coating thickness of galvanized steel plate is generally 20 ~ 90g/m2 (. However, the author's factory practice proves that the quality of resistance spot welding of galvanized steel plate can meet the technical requirements of body manufacturing, although there is zinc layer, as long as appropriate equipment and technology are selected. Therefore, the galvanizing method of galvanized steel sheet itself or the thickness of zinc layer is not the decisive factor affecting the spot welding quality of car body.

Selection and design of electrode materials and shapes

In the research of spot welding of galvanized sheet, electrode material is a noticeable focus, and the research mainly focuses on analyzing the interaction characteristics between existing electrodes and coatings and developing new electrode materials. The electrode materials used 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. Someone in the United States experimented with a composite electrode with tungsten electrode tip embedded in chromium and copper, and the results proved that the service life of this electrode was very long. However, due to the poor thermal conductivity of tungsten electrode, it is difficult to popularize it in the automobile body manufacturing industry [6]. Some people also advocate spraying a high melting point material on the electrode surface, such as cobalt, titanium oxide, rhodium, etc. Or aluminum oxide powder is preset between galvanized plates to increase contact resistance, shorten welding time and improve electrode life. Most of the domestic research is to study the performance of electrodes made of the above materials through experiments. A large number of experiments have proved that under the unsatisfactory welding conditions, the use conditions of both cheaper Cu— Cr, Cu— Zr alloy electrodes and more expensive DSC electrodes are similar, but factory experience shows that in some cases, when using DSC 20 alloy electrodes, the electrode adhesion decreases.

As for the shape and end face size of the electrode, the frustum shape is considered to be the best. At the same time, reference [5] points out that the spherical electrode with the end radius of 1 ~ 2in is recommended when using the mobile electrode holder, while reference [7] emphasizes that the spherical electrode should be avoided.

Foreign researchers have long recognized that for spot welding of galvanized steel sheet, the electrode life depends on the difficulty of alloying reaction between electrode tip and molten metal. Some alloy electrodes can complete the alloying process within 5 cycles (60Hz), and the electrode that is more prone to alloying reaction has a shorter life. In addition, a large number of spot welding tests show that the surface temperature of electrode during welding is obviously higher than that of ordinary steel plate [8]. Therefore, the cooling of the electrode is very important, and the flow rate of cooling water must be sufficient to ensure that the electrode is close to room temperature, so as to minimize the alloy reaction between the electrode and the coating and prevent the electrode from expanding due to electrode "bulging" or electrode softening. However, at present, there is no conclusion about how fast and how high the cooling water flow rate will not cause overheating of the electrode tip. Reference [7] suggests that the water velocity should be no less than 1 gal/min and the maximum inlet temperature should be 32? C.

4 The influence of process parameters on the spot welding process and quality of galvanized sheet

In the absence of reliable and practical detection or adaptive control means at present, it is particularly difficult to manually select (input) appropriate process parameters, because for the most commonly used spot welder with constant current control, the available welding current adjustment range for galvanized sheet spot welding is very small. Moreover, the quality of spot welding involves more than twenty factors, such as net pressure fluctuation, equipment characteristics, welding machine (electrode holder) circuit, various shunt, electrode wear, electrode pressure fluctuation, material state and characteristics, material thickness combination, joint form, cooling conditions, operation mode and so on. These factors interact. The fluctuation or abnormality of any factor will lead to the difference between the set welding specification value and the actually obtained solder joint value, thus affecting the optimal matching of process parameters. For enterprise production, reasonable selection of process parameters is the most important way to control the quality of solder joints, so the research work in this area has important practical significance.

Some scholars have studied the spot welding technology of galvanized steel plate. Jing Wong et al. of Shanghai Jiaotong University [9] compared the spot welding process and mechanical properties of common cold-rolled steel plate, galvanized steel plate and two kinds of galvanized steel plates with different zinc layer thickness. The analysis shows that, as far as the change law of nugget size and joint strength with current is concerned, galvanized steel plate is basically the same as common steel plate, showing a parabolic shape, while galvanized steel plate shows a monotonically rising logarithmic curve; When spot welding galvanized steel plate, when the electrode pressure is greater than 2.0kN, the increase of electrode pressure has no obvious effect on reducing splash; The variation law of joint strength of galvanized steel plate with the electrifying time is basically the same as that of ordinary steel plate, and it also increases obviously at first and then tends to be flat or decrease. This is consistent with the conclusion drawn by Yan Qi [10] of Baosteel Research Institute from the spot welding process experiment of hot-dip galvanized sheet.

Some scholars have also analyzed the morphology of nugget during spot welding of galvanized steel sheet, and discussed the influence of process parameters of galvanized steel sheet on spot welding quality from microscopic characteristics. Literature [1 1] holds that the crystal morphology of spot welding nugget of galvanized steel sheet is coarse columnar crystal with strong orientation; Increasing or decreasing welding current has no obvious effect on improving the crystal morphology of spot welding nugget of galvanized sheet, but increasing current will make columnar crystals thicker. Increasing electrifying time and welding pressure can improve the crystal morphology of spot welding nugget of galvanized sheet. The former has the most obvious effect and can basically form equiaxed crystal structure. However, some scholars believe that it is difficult to obtain equiaxed grain structure [12] even if ordinary steel plates with good weldability are spot welded. The document [13] also points out that even the spot welding joint of high-quality ordinary steel plate is composed of "column+equidistant". Therefore, it is doubtful that "equiaxed+columnar" crystal or a large number of equiaxed crystal structures can be obtained only by adjusting process parameters for galvanized steel plates with poor weldability. In addition, 08Al galvanized steel plate was studied by orthogonal test, and it was concluded that welding current was the first influencing factor of joint strength, followed by electrifying time [14].

This is consistent with the spot welding characteristics of ordinary steel plates, but it is contradictory to the document [1 1].

In addition, although most authoritative welding institutions at home and abroad have recommended the specification table for spot welding of galvanized steel plates, the relevant data are not uniform. The author also verified the welding specifications recommended by IIW and JIW with the same material under the same spot welding conditions, but the spot welding effect was not ideal. In addition, under the same welding conditions, the best specifications obtained from experiments in some of the above documents are also inconsistent.

Perhaps, for spot welding of galvanized steel plate with complex welding process and many influencing factors, these contradictions or differences are reasonable under certain experimental conditions, which is the difficulty of manual optimization and selection of process parameters. At present, many researchers have proposed an artificial intelligence system integrating fuzzy system, expert system (ES) and artificial neural network (ANN) to optimize the selection of spot welding process parameters, and some of them have been successful in the laboratory, which will promote the in-depth study of spot welding process parameter optimization of galvanized steel sheet, and even fundamentally solve the shortcomings of traditional process parameter optimization methods.

5 Research status of spot welding quality control methods

At present, in the production of automobile body, the quality of spot welding is basically checked by process test, appearance inspection and destructive test, but there is no convenient, effective and reliable quality control means. However, due to the wide application of resistance spot welding and the urgent need of quality stability, the quality control of spot welding has always been the focus of attention of scholars and equipment manufacturers all over the world, and the most extensive research has been carried out.

At present, the proposed quality control methods include electric parameter method, thermal expansion method, infrared radiation method, ultrasonic wave method, acoustic emission method and comprehensive control method, among which infrared radiation method, ultrasonic wave method and acoustic emission method are difficult to popularize and apply due to conditions. Electrical parameter methods include constant current method, dynamic resistance method, energy integration method, inter-electrode voltage method and so on. Although many gratifying progress has been made in the research of these control methods, so far none of them can compensate the influence of various factors in actual production, and due to their respective limitations, they have not been popularized and applied in production. So more than a decade ago, some scholars pointed out that the further research should be to develop multi-parameter integrated control.

At AWS International Welding Expo, a foreign company exhibited a resistance welding thermometer that can detect multiple parameters (current, voltage, dynamic resistance, electrode displacement, electrode pressure, etc.). ), the application effect is good [15]. The resistance welding multi-parameter monitor developed by Northwestern Polytechnical University with STD industrial control microcomputer can also realize multi-parameter detection, and satisfactory results have been obtained under laboratory conditions [16]. Cao Biao et al. [17] used the dynamic parameter testing system of spot welding process established by IBM—80286, which can also obtain various dynamic parameters of spot welding process reliably. Although these control methods have high precision, the disadvantage is that the detection system is complex, and the author thinks it is difficult to popularize and use them in production.

It is worth mentioning that inverter resistance spot welding machine has been widely used in domestic automobile, electrical appliances and other manufacturing industries. Its working principle is to rectify and filter the input three-phase AC bridge into DC, and then generate intermediate frequency AC (f = 600 ~ 1000 Hz) through the inverter, and then feed it back to the welding transformer to output low-voltage AC, and finally output DC with little pulsation after single-phase rectification. At first, this control method adopted pulse width modulation (PWM), but in the 1990s, it has tended to adopt "zero-switch -PWM" or "zero-switch -PWM" converter, which is not only economical and energy-saving, but also expands the adjustment range of welding specifications by 3-4 times [18], which is undoubtedly a boon for spot welding of galvanized steel plates with small welding parameters.

Generally speaking, the development trend of spot welding quality control is the integration of control mode and control method, the quantification of process and quality control law description and the intelligence of control decision. Of course, in order to have a good market prospect, any control method must be effective, reliable and simple to operate.

The cost is reasonable.