Study on spot welding process of galvanized sheet
Text / Huang Chaoqun
Abstract: This paper describes the spot welding characteristics of galvanized steel sheets and the selection points of welding current, welding time and electrode pressure during spot welding of galvanized steel sheets. The test shows that the welding current, welding time and electrode pressure of the hot-dip galvanized steel sheet are improved to different degrees compared with the low-carbon cold-rolled steel sheet of the same thickness, and the spot welding specification of the hot-dip galvanized steel sheet is narrow. Solder joint quality is sensitive to welding specifications, especially current changes.
Key words: galvanized sheet; spot welding process; welding parameters
0 Preface
Galvanized steel sheets have excellent corrosion resistance, but their spot welding performance is significantly deteriorated compared to ordinary low carbon cold rolled sheets. In general, the welding performance of cold-rolled sheets is often described by the range of welding currents that are suitable for use. If the range is large, the welding performance is good, and vice versa [1]. Since the zinc melting point on the surface of the galvanized sheet is low, the zinc layer will melt before the steel sheet, and under the action of the electrode pressure, it is extruded into the welded region to form a zinc ring. Due to its shunting effect, the zinc layer has low resistivity and small contact resistance. It is not conducive to the formation of the core, and the welding process parameters (welding current, electrode pressure, energization time) need to be adjusted when spot welding galvanized steel sheet; in addition, since the molten zinc reacts with the electrode, an alloy layer is formed on the electrode. At the same time, some oxidized zinc is attached, which changes the physical properties and shape of the electrode, accelerates electrode wear, and greatly reduces electrode life. Therefore, in order to obtain a high quality core and consider the protection of the electrode, it is necessary to select the appropriate spot welding parameters.
Table 1 Specification for spot welding of galvanized steel sheets and low carbon steel
materialThickness mm
Electrode material
Welding current ka
Power-on time s
Network pressure V
Electrode pressure ka
Electrode diameter mm
Nugget diameter mm
Low-carbon steel
0.8+0.8
Cd-Cu
7.3
0.14
380±10
160
5
4.2-5.0
Domestic galvanized sheet
0.75+0.75
Ca-Cu
9.8
0.26-0.30
380±10
180
5
4.2-5.0
Domestic galvanized sheet
0.75+0.75
W composite electrode
8.95
0.26-0.30
380±10
180
6
4.2-5.0
Domestic galvanized sheet
0.5+0.5
Cd-Cu
11.10
0.26-0.30
380±10
140
5
4.2-5.0
Domestic galvanized sheet
0.5+0.5
W composite electrode
7.5
0.26-0.30
380±10
140
6
4.2-5.0
Japanese galvanized sheet
0.5+0.5
Cd-Cu
8.95
0.26-0.30
380±10
140
5
4.2-5.0
Japanese galvanized sheet
0.5+0.5
W composite electrode
7.5
0.26-0.30
380±10
140
6
4.2-5.0
1 welding current
Since the galvanized layer reduces the resistance of the solder contact surface, the contact surface increases, and the contact resistance is small due to the small resistance of the metal zinc, the same nugget size is obtained for spot welding with the low carbon steel plate, and the galvanized steel spot spot welding station The required welding current is generally increased by 25% to 50%, and it is generally believed that the thicker the galvanized layer, the greater the welding current required. However, Cheng Xuanting et al. [2] found through comparison of experiments that the thicker the zinc layer is, the larger the required current is in a certain range of coating thickness. However, when the zinc layer reaches a certain thickness, the required current decreases. However, the welding current should not be too large, otherwise it will cause splashing.
2 power-on time
It can be seen from Table 1 [3] that the welding time required for spot welding of galvanized steel sheets is about twice that of low carbon steel. The purpose is to have enough time when the welding is energized, so that the molten zinc layer between the two weldments can be extruded as much as possible, and evenly distributed around the solder joint to make the nugget evenly stirred; but the welding time should not be too long, otherwise Will increase the temperature of the contact surface between the weldment and the electrode, destroy the corrosion-resistant coating on the surface, and reduce the service life of the electrode [4]. Secondly, since the spot welding welding current of the galvanized steel sheet is large, in order to avoid splashing, it is also necessary to increase the welding pressure.
3 electrode pressure
When spot welding of galvanized steel sheet, the electrode pressure needs to be increased by about 10% to 25% compared with the low carbon steel sheet. The purpose is to squeeze the molten zinc layer out of the weld zone as soon as possible, and reduce the amount of zinc remaining inside the nugget. The possibility of defects such as cracks. Secondly, since the spot welding welding current of the galvanized steel sheet is large, in order to avoid splashing, it is also necessary to increase the welding pressure. On the other hand, too high electrode pressure will also lead to excessive indentation, and the zinc on the outer surface will be completely extruded, which will become the source of corrosion pits in the future [5].
4 Comprehensive review
(1) When hot-dip galvanized steel sheet is spot-welded, the parameters of welding current, welding time and electrode pressure are improved to different extents than low-carbon cold-rolled steel sheets of the same thickness.
(2) Compared with low carbon steel sheets, when spot welding of galvanized steel sheets, the contact resistance is small, the current density of the weldments is small, cracks, pores or softened structures are prone to occur, and the weldments and electrodes are easily stained or alloyed. Its spot welding weldability is poor.
(3) When spot welding of hot-dip galvanized steel sheets, the spot welding quality is sensitive to welding specifications, especially current changes.
(4) When spot welding of hot-dip galvanized steel sheet, the welding current, welding time and electrode pressure parameter values ​​should be considered comprehensively. Under the premise of ensuring the quality of the solder joint, the electrode wear is reduced and the electrode life is prolonged.
references
[1] Wei Hongtao. Progress in spot welding of galvanized steel sheets [J]. Shanghai Metals, 1994, 16:251
[2] Cheng Tingxuan. Effect of coating thickness on spot welding quality of galvanized steel sheet. Welding machine, 2001. (ll): 28-30.
[3] Zhang Yish. Research on spot welding of galvanized steel sheet. Welding Communication, 1985. (2): 18-70.
[4] Wang Min. Research on spot welding process of galvanized steel sheet. Journal of Welding, 1998. (4): 34-36.
[5] Qi Qi. Research on spot welding process of hot-dip galvanized steel sheet, 2000.
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