Resistance welding is the method of joining metals using pressure and passing electric current for the period on the part of the metal to become joined. Since resistance welding is probably the most inexpensive welding processes now available simply because it doesn’t require many other materials to generate a bond.
It’s worth noting that resistance welding will come in several forms which differ primarily through the shapes and kinds of weld electrodes utilized to conduct the electrode and apply the pressure. The electrodes are generally made of copper alloys and are cooled by water which flows through conductive tooling in the machine and also the cavities within the electrodes.
Machines manufactured for resistance welding are designed and manufactured for a wide array of industrial, automotive, and aerospace industry applications. And since nearly all are automated, the working of those machines is well-controlled and can be repeated, based on information found at a guide for seam welds. As such, they make it simpler for companies to staff their production with ease.
Resistance Welding Types
Seam and Spot Welding
Like resistance welding applications, spot welding creates welds through the use of heat that’s been generated by welding current flow resistance involving the surface in contact with a joint and force, which pushes the workpieces together. Both are applied for a defined period for the weld to become complete. It’s worth noting that spot welds use the face geometries of whichever welding electrode is now being utilized to apply force on the workpiece and focus the actual in the weld location. Once the resistance generated is sufficient, the metals “set down” and combine, and a nugget of weld forms.
Seam welding is simply a subcategory of spot welding. However, under this alternative, wheel-shaped electrodes are widely used to deliver welding current and force on the parts. Under seam welding, workpieces roll between wheel-shaped electrodes as being a current is now being applied. Dependent upon the time settings and current, the resulting welds could end up forming complete welded seams, overlapping seams, or individual welds at specified intervals.
Projection welding applies the same principles as other resistance welding processes. However, with projection welding, the welds are localized at preset points through the use of intersections, projections, or embossments, all of which focus the generated heat into a specific point. Once the current has generated enough resistance in the POC, the projections fall off, developing a weld nugget.
Flash (or butt) welding actually uses the same principle as other resistance welding processes. However, with flash welding, resistance is generated using flashing action that is achieved by utilizing a higher current into a small contact point involving the workpiece. Once the flashing process has started, force is applied into a specific point on the workpiece. It’s worth noting the workpieces are slowly moved in a determined rate.
Upset welding uses the same principle as other resistance welding processes. And even though it is similar to flash welding, the difference involving the two is that the pieces being welded already are in contact, meaning no flashing occurs. Here, pressure on the workpieces is applied ahead of the current starts which is sustained before the welding process is finished.
Resistance Welding Parameters
Resistance welding works in line with the principle of Joule Heating, where heat, denoted Q, is generated influenced by three factors:
- I – represents the actual that’s passing with the metal combination
- R – represents the base metals’ and make contact with interfaces’ resistance
- t – represents enough time/time period of current flow
Even though the principle might seem simple, you will find hundreds of parameters that influence the final results of actual welding processes. To comprehend resistance welding technology better, here’s a peek at some of the parameters that influence welding results:
The welding current is a vital parameter in relation to resistance welding since it determines the quantity of heat generated by a power of square. It’s worth noting that weld nugget sizes increase together with the increasing welding current however, in case the current is too high, it could cause electrode deterioration and expulsions. To avoid this, welding engineers need to determine the optimal welding some time and current for every single welding application.
Heat generation in resistance welding is directly proportional to welding time. On account of heat transfer from weld zones to electrodes and also the base metals, as well as a reduction in heat from surrounding surfaces, the absolute minimum welding some time and current are needed to develop a weld. In case the current will not be sufficient, increasing welding times alone won’t generate a weld. However, as soon as the current is sufficiently strong, the weld nugget size improves as weld time increases until it’s the same size as the contact portion of the electrode tip. If welding time is protracted, expulsion might occur or perhaps the electrode could end up sticking with the workpiece.
Welding force impacts the resistance welding process through its impact on contact resistance on the contact area as well as the interfaces because of material deformation. The pieces being worked tirelessly on must be compressed by a certain force along the weld zones allowing current to pass through. In case the welding force isn’t sufficiently strong, expulsion might occur when the current starts flowing because of high contact resistance – and this might lead to rapid heat generation. Alternatively, in case the force is beyond normal, that may cause the contact area to become bigger than normal, an issue that might lead to low current density. As well, it may possibly bring about low contact resistance, and this could reduce how big the weld nugget and also heat generation. In projection welding, it’s the force applied that causes projections from the workpiece to collapse. That triggers changes on the contact area and, consequently, contact resistance and density. Furthermore, furthermore, it influences welding results and also heat development.
Contact resistance at weld interfaces is probably the most influential parameters in relation to materials. However, it’s worth noting it has highly-dynamic interactions with process parameters. When welding forces increase, contact pressure also increases, which means the true contact area in the interface also increases as a result of the rough surface asperities deforming. That triggers contact resistance in the interface to reduce, reducing heat generation and weld nugget size. Since the majority metal surfaces have contaminants like oil, dirt, water vapor, and oxides upon them, when temperatures rise, a number of them – mainly oil-based and water contaminants – dissipate in the initial few cycles, while metals soften due to high temperatures. For that reason, contact resistance will normally decrease as temperatures rise. While contact resistance only affects results during the initial few cycles, it’s a decisive affect on how heat is distributed as a result of how heat is generated and distributed initially.
Virtually all material properties alter as temperatures fluctuate, an issue that enhances the dynamics of resistance welding. It’s worth noting that material resistivity influences how heat is generated. Heat capacity and thermal conductivity influence how heat is transferred. In metals with high thermal conductivity and low resistivities like copper and silver, almost no heat is produced despite having very high welding currents which is quickly dissipated. Such metals take time and effort to weld using resistance welding. However, they are excellent electrodes. When unlike metals are welded together, metal with higher resistivity will normally generate more heat. This is certainly something which should be thought of in advance when designing welding parts for projection welding so when choosing the kind of electrodes to use in spot welding. Welding material hardness also affects contact resistance. Metals which are harder (have higher-yield stress) will normally have higher contact resistance at similar welding forces as his or her surface asperities are not as easy to deform. That leads to much smaller contact areas. It’s worth noting the materials used as electrodes were also utilized to influence how heat is balanced in resistance welding, particularly if joining non-ferrous and light metals.
Resistance Welding Advantages
- Resistance welding has no consumables, such as welding rods, brazing materials, or solder.
- Its processing time is very short.
- It provides operator-safety as the machines only need low voltage to use.
- Resistance welding doesn’t create messes like other welding techniques it’s neat and environmentally-friendly.
- It produces reliable electro-mechanical joints.
Resistance Welding Shortcomings
- The electrodes used will be able to reach either side in the metal pieces they are bonding. Some spot-welding machines is only able to hold a certain metal thickness, usually 5-50 inches. And even though the electrode’s position might be adjusted, most electrode holders just have limited movement.
- The shapes and scale of electrodes used determines the strength and scale of the weld. The joints only form in the spots in which the electrodes touch the metal. in case the current isn’t hot or sufficiently strong, or perhaps the metal isn’t held together well enough, the spot weld might end up being weak or small.
- A reduction in strength and warping can happen around points in which the workpiece is spot-welded. The joint’s appearance is frequently quite ugly and may have cracks onto it. The metal might also turn out becoming less proof against corrosion.