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Main Factors Affecting Weld Quality
Resistance welding utilizes the thermal effect of electric current. When current passes through two metal plates to be welded, the high heat generated by the resistance in the welding circuit melts the plates, which are then joined under pressure and cooled. The welding resistance consists of two parts: the contact resistance between the metal plates and the body resistance of the plates themselves. Therefore, to achieve a good weld, it is necessary to reduce the contact resistance while increasing the body resistance of the material.
To control weld quality, the following five basic parameters should be adjusted: welding resistance, welding pressure, overlap, welding speed, and another variable factor—tinplate. These factors determine the weld nugget spacing, degree of melting, shape, and microstructure of the weld nuggets. These parameters are interrelated; when one parameter changes, the welding conditions must be reset.
(1) Relationship Between Welding Speed and Welding CurrentWhen other conditions remain constant, to obtain a good weld, the set welding speed and welding current should ensure that the tinplate melts properly and the weld nuggets connect. When the welding speed increases, the current must be relatively increased. If the welding speed is too low, the tinplate may overheat, causing the weld nuggets to cool slower than the tinplate contracts, resulting in large holes at the weld points. Conversely, if the welding speed is too high, it may lead to unconnected weld nuggets. Additionally, insufficient heating of the tinplate can create elongated holes or tin soldering between the plates.
(2) Relationship Between Welding Pressure and Welding CurrentThe tin layer on the tinplate surface is a good conductive metal with low resistance, and its low hardness makes it easily deformable under pressure, significantly reducing the surface resistance and facilitating welding. The welding current increases with welding pressure because higher pressure increases the contact area of the tinplate, reducing the surface contact resistance, thus requiring a relative increase in welding current. The welding pressure should be adjusted within an appropriate range. If the pressure is too low, the weld bead will be higher, complicating the repair coating. Conversely, high welding pressure easily achieves a flat weld seam.
(3) Relationship Between Overlap and Welding CurrentA larger overlap requires more welding heat, so the welding current increases with the overlap. Under set welding conditions, if the overlap is larger than normal, the area under the same welding pressure increases, reducing the welding current density and slightly increasing the contact resistance, resulting in insufficient welding heat and cold welds. Conversely, reducing the overlap can lead to overwelding and increased extrusion.
(4) Impact of Tinplate Properties on Welding
1. Tin Coating WeightThe tin coating weight on tinplate affects weld quality. Although the tin layer has low contact resistance and is a good conductor, if the tin coating weight is too low (less than 0.5 g/m²), and the alloy layer is relatively high, the surface contact resistance of the alloy layer is larger, which is detrimental to welding quality. Especially for the same batch of tinplate, if the alloy layer varies widely or the alloy tin content is too high, cold welding can easily occur under the same settings. For tinplate with high tin coating weight, the weld nugget spacing obtained with the same welding current is smaller than that with low tin coating weight, so a good weld requires reducing the welding speed. Additionally, if the welding current is too high, the tin may penetrate along the iron grain boundaries during melting, potentially causing intergranular corrosion in some food cans.
2. ThicknessThe thickness of the tinplate affects the adjustment of welding parameters, especially in high-speed welding machines. As the tinplate thickness increases, the required welding current increases, and the upper and lower limits of welding conditions decrease with increasing thickness.
3. HardnessThe setting of welding current is related to the hardness of the tinplate. When the hardness increases, the welding current should be reduced accordingly.Under set welding conditions, variations in tinplate thickness and hardness within normal ranges do not affect welding. However, if the thickness and hardness vary significantly within the same batch, it will cause unstable welding quality, leading to cold welding or overwelding issues. For example, under set pressure, if the hardness of the tinplate increases excessively, the surface contact resistance between the two plates increases, requiring a reduction in welding current.
4. Base Steel QualityWhen the base steel has a high carbon content, the welding current needs to be increased. Additionally, if there are many inclusions in the base steel, the resistance increases during welding, easily causing spatter.In summary, when producing different types of empty cans or changing the type of tinplate, new welding conditions must be reset.
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Post time: Jul-14-2025