Projection welding is a variation of the spot welding, usually impact bumps on one sheet among two sheets, then weld. Due to the current concentration so it overcomes the shortcoming of molten core offset in spot welding, therefore in projection welding workpiece thickness ratio may exceed 6: 1.
When projection welding electrodes must be fallen rapidly with bumps crushing, otherwise it will splash due to pressure lossing, so we shall use projection welding machine with good follow motion electrodes.
During multiple spots projection welding, if the welding condition is improper, then will cause bumps shifting, then lead to strength decreasing of joint. Experiments show that the shift is caused by the electromagnetic force generated by current passing through.
In actual welding, since bumps height are inconsistent, poor parallel upper and lower electrodes, 1 point fixed and 1 point moved is more than 2 points moved simutaneously.
In order to prevent bumps displacement, to ensure normal molten core, except using a larger electrode force & small welding current, it should try to improve mobility for the pressurized system. The Method to improve mobility is to reduce the mobile part weight of the pressurized system, and the guide section uses rolling friction.
During projection welding, to overcome the imbalance pressure between each bumps, additional preheating pulse or rotatable electrode can be used.
The main process parameters is projection welding:
electrode force, welding time and welding current.
1. electrode force
Projection welding electrode force depends on the performance of the metal being welded, the bump size and bump numbers welded at one time. Electrode pressure should be sufficient to squash bumps before bumps reaching soldering temperature, and make two workpiece tight fitting. Excessive pressure of electrode will squash bumps too early to lose the use of projection welding, also will decrease joint strength due to current density decreased. Also too small pressure will cause serious splash.
2. welding time
For a given material and thickness of the workpiece, welding time decided by welding current and bump stiffness. When projection welding low carbon and low alloy steel, compared with the electrode force and welding current, welding time is secondary. After determining the proper electrode force and welding current, then adjust welding time to obtain satisfactory solder joints. If you want to shorten the welding time, welding current will increase accordingly, but the excessive increasing of welding current may cause metal overheating and splashing, usually projection welding is longer time than spot welding & smaller current than spot welding.
3. the welding current
Each solder welding current of projection welding is less than spot welding. But before completely squashing bump the current must can melt the bump.Recommended current should be max current which will not extrude excessive metal. For a given size of the bump, the extruding amount of metal is increased with current increasing. The incremental increasing of current can reduce extruded metal. As spot welding, as the welded metal thickness and performance are still the main basis for selecting the welding current.For multiple spots projection welding, the total welding current is required approximately equals every bump's current multiply the number of bumps. However, considering the bump tolerance, workpiece shape, and the secondary loop impedance, may need to make some adjustments.
When projection welding should reach welding heat balance between the two plates, otherwise, the plate will be molten before reaching soldering temperature, and therefore the same metal welding should impact bumps on the thicker workpiece, welding dissimilar metals should impact bumps on the higher conductivity workpiece. But when it is difficult to impact bumps on thick plate, then also can impact bumps on thin plate.
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