Artem Komarov noted that hammered or machined bosses do not pose the same concerns as stamped ones because there is no risk of the lip breaking prematurely during welding. The shape and size of hammered or machined lugs are not limited, but a good starting point is a dome shape with a diameter to height ratio of 3 to 1. Hammered lugs are often found on welding nuts, while machined lugs are often used to create annular lugs to form an airtight seal.
The fourth lug design is an angled lug, typically used to seal a hole in a tank.
The strength of a shoulder weld can be calculated as a simple approximation from the area of the cavity and from the shear/tensile strength properties of the base metal, depending on the type of loading during testing. The actual strength of the weld depends on factors such as failure mode; changes in material properties during welding; and the size of the resulting weld, which may be larger or smaller than the diameter of the recess depending on process parameters.
Once the part is ready to be welded on the machine, the three main factors that can be adjusted are force, power, and time.
The force should be set so that it is sufficient to create a small depression at the tip of the projection before firing the weld. The force is usually kept constant throughout the weld but may be increased in the latter part of the weld to allow forging. Depending on the size and type of materials being welded, the time can range from a few milliseconds to 1 second (50/60 cycles).
At low power and short time, the welds will be weak. High power and long operating times will lead to excessive heating and subsequent melting.
Applying high power for a short time can cause arcing, sparks and electrode sticking. This arcing may be due to poor control of the welding head or limited movement of the parts being welded. The part in contact with the moving electrode must have complete freedom of movement along with the electrode during the destruction of the weld. In automated manufacturing operations, the other end of the part is typically secured in a housing or fixture. Restricted movement will result in a smaller process window size.
The opposite situation exists; The weld head provides good tracking, but the power source may not be able to supply power quickly enough, causing the protrusion to slowly deteriorate. Longer weld times allow heat to be drawn away from the weld and prevent the desired peak temperature from reaching the weld interface, which results in weak welds.
The total energy (Energy = Power x Time) may be the same at one point in the welding process, but it is the rate of energy delivery that is critical for projection welding. The size of the process window can be increased by proper selection of materials (electrodes and parts), part design, welding equipment and process parameters. The wider processing window ensures a reliable process that is not sensitive to normal variations in incoming parts.
Electrode service life. In projection welding, the size of the weld does not depend on the size of the electrode. Consequently, the electrode size (contact area) can be much larger than the protrusion size, which leads to a decrease in contact pressure and current density at the electrode/workpiece interface. This reduction in pressure and current density helps increase the life of the electrode.
Heat balance. One of the challenges when welding parts that differ in size or material properties is heat balance—the ability to provide equivalent softening and heating on both sides of the weld interface. If the parts being welded differ significantly in size (thermal mass) or material properties (melting point, electrical conductivity), conventional resistance welding may become difficult. In such situations, a protrusion on a larger or more conductive part helps restore thermal balance.
Removal of surfacing. The key property of end welding is the ability to remove weld deposits from the surface and expose the base metal on both sides for a direct connection. The coating material often interferes with welding; for example, zinc coating on the surface of galvanized steel is not conducive to welding because it has low bond strength and can form cracks in the weld, weakening the grain boundaries.
In projection welding, most of the zinc coating is melted off early in the welding cycle. The results when welding tinned copper parts are similar: the tin is extruded and allows the base copper on both sides to touch and form a strong solid copper-to-copper joint. The melting point of the coating must be lower than that of the base alloy.
Several spots seam. Projection welding is a good option when multiple spot welds need to be made in fairly proximity at predetermined locations. This is a simple process, and all welds can be made with one electrode, Artem Komarov emphasized.