1. Crowing Glory
2. Resistance Welding
3. Weld Overlap
A filler wire is commonly used in arc welding processes to fill gaps/groove between parts in butt welding, or to provide reinforcement in fillet welds. In addition to providing proper fusion with the base metal and controlling weld chemistry, the filler metal is also expected to achieve the required profile which, in addition to a minimum size, usually requires that the weld have a positive reinforcement on the face of the weld, often referred to as weld crown. Crown is the material above the dashed lines in the schematics shown below. The welding process has to be controlled such that height of the crown is within limits, which for high-quality welds usually ranges from a minimum of practically zero (minor deviations allowed) to a maximum specified on the drawing. Welding processes such as laser welding and electron beam welding usually do not use a filler wire but can incorporate suitable design features such as a raised lip or a pre-placed filler to produce a crown.
A weld crown provides multiple benefits. If the properties of the filler are the same as the base metal, a weld with a crown will be stronger that the base metal for a butt weld configuration. But a greater benefit is in the stresses induced in the weld as it cools from a molten state. The weld metal starts solidifying at the interface between the base metal and the molten metal since the base metal acts like a heat sink. The remaining molten metal continues to solidify and shrink as the weld cools down to room temperature. If there is positive reinforcement to form a crown as the weld cools, the cooling process induces compressive stresses on the weld surface. Presence of compressive stress helps close any cracks that could have formed during cooling due to presence of low-melting phases at grain boundaries. The compressive stresses could also be transferred to the heat affected zone (HAZ) adjacent to the weld and may help in healing cracks in that region as well. However, if the reinforcement is insufficient, the weld surface will be below the dashed line and cooling/shrinking process will produce tensile stress at the weld surface. As opposed to compressive stresses, tensile stresses can pull on weak grain boundaries and can lead to crack formation ( Summer 2008 Newsletter), a no-no in the welding world. With proper training for manual welders and proper process parameters for automated welds, a crown can be easily formed and your weld will heave a sigh of relief, because as they say - easy lies the weld that wears a crown.
As the name implies, resistance welding is a process that utilizes resistance to flow of current to generate heat required for welding. The parts to be welded are pinched between two electrodes while the current flows from one electrode through the parts and then goes back to the transformer through the other electrode. The welding electrodes provide multiple functions including applying welding force, providing path for current flow, and help remove excess heat from the weld. The electrodes are not consumed during the weld, but usually do require resurfacing or grinding to keep them in shape. The overall welding machine is usually composed of the welding head (that holds the electrodes), welding transformer (provides low voltage high current pulse) and the controller (for setting weld parameters). Resistance welding machines come in all shapes and sizes from bench top units to weld wires under 0.001" diameter to behemoths which can fill up a small room and send current up to 100 kA.
As the current passes through the parts being welded, heat generated due to resistance to current flow raises part temperatures. At the low end of temperature range, the process is used to melt solder plating on the part surfaces which then forms a solder bond on cooling. At higher temperatures, the process can be used to perform a brazing operation where the braze alloy in the form of a foil is placed between the parts to be joined; solder and braze alloys can also be added in paste form. At even higher energy levels, the temperature reached can be high enough to soften the parts being welded at the interface, and then under the forging action provided by the welding force, the parts can form a solid-state bond (no fusion) at the weld interface. Such a solid-state bond is quite common when welding conductive metals/alloys such as copper, aluminum, platinum, etc.. When using this process to weld resistive alloys, it is much easier to melt a small volume of metal on either side of the weld interface that forms a fusion nugget at the interface. Resistance welding is the only process that is routinely used to produce all four types of metallic bonds: solder, braze, solid-state, and fusion; quite an achievement indeed.
One of the risks of producing a weld crown is the possibility of spilling the molten metal over the edge of the weld from where it rolls onto the base metal; kind of looks like lava flow from a volcano. This overflow is a defect called weld overlap, another no-no in the welding business. Weld overlap will produce a sharp junction between the overflowing weld metal and the base metal that will create stress concentration and must be avoided. Use of suitable process parameters should eliminate the likelihood of weld overlap; but if you do see some localized overlap you can blend that in with a TIG torch with a process called toe dressing ( Fall 2012 Newsletter). Such toe dressing can be performed with or without a filler metal.