Resistance welding is the method of joining metals using pressure and passing electric current for the period over the area of the metal to become joined. Since resistance welding is probably the most cost-effective welding processes available today since it doesn’t require 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 types of weld electrodes employed to conduct the electrode and apply the pressure. The electrodes are mainly created from copper alloys and they are cooled by water which flows through conductive tooling in the machine along with the cavities inside the electrodes.
Machines manufactured for resistance welding are made and manufactured for an array of industrial, automotive, and aerospace industry applications. And because nearly all are automated, the working of these machines is well-controlled and will be repeated, at least according to this site. Consequently, they make it easier for companies to staff their production easily.
Resistance Welding Types
Seam and Spot Welding
Like resistance welding processes, spot welding creates welds by making use of heat that’s been generated by welding current flow resistance between the surface in touch with a joint and force, which pushes the workpieces together. Both the are applied for a defined period for that weld to become complete. It’s worth noting that spot welds use the face geometries of whichever welding electrode has been employed to apply force around the workpiece and concentration the actual with the weld location. As soon as the resistance generated is enough, the metals “set down” and combine, as well as a nugget of weld forms.
Seam welding is essentially a subcategory of spot welding. However, under this alternative, wheel-shaped electrodes are utilized to deliver welding current and force to the parts. Under seam welding, workpieces roll between wheel-shaped electrodes as a current has been applied. Dependent upon the time settings and current, the resulting welds might 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 by making use of intersections, projections, or embossments, all of which focus the generated heat to some specific point. As soon as the current has generated enough resistance with the POC, the projections fall off, building a weld nugget.
Flash welding applies the same principle as other resistance welding processes. However, with flash welding, resistance is generated using flashing action which can be achieved by making use of a higher current to some small contact point between the workpiece. As soon as the flashing process has started, force is used to some specific point around the workpiece. It’s worth noting how 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 between the two is that the pieces being welded are already in contact, which means no flashing occurs. Here, pressure around the workpieces is used prior to the current starts and is also sustained up until the welding process is finished.
Resistance Welding Parameters
Resistance welding works depending on the principle of Joule Heating, where heat, denoted Q, is generated influenced by three factors:
- I – represents the actual that’s passing from the metal combination
- R – represents the base metals’ and contact interfaces’ resistance
- t – represents the time/duration of current flow
Whilst the principle might appear simple, you can find hundreds of parameters that influence the results of actual welding processes. To comprehend resistance welding technology better, here’s a short look at some of the parameters that influence welding results:
The welding current is a crucial parameter when it comes to resistance welding since it determines the volume of heat generated with a power of square. It’s worth noting that weld nugget sizes increase together with the increasing welding current however, when the current is simply too high, it could cause electrode deterioration and expulsions. To prevent this, welding engineers need to ascertain the optimal welding 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 along with the base metals, in addition to a loss in heat from surrounding surfaces, a minimum welding time and current are necessary to produce a weld. In case the current is just not sufficient, increasing welding times alone won’t develop a weld. However, if the current is sufficiently strong enough, the weld nugget size will increase as weld time increases until it’s the same size as being the contact portion of the electrode tip. If welding time is protracted, expulsion might occur or maybe the electrode might end up staying on the workpiece.
Welding force impacts the resistance welding process through its impact on contact resistance around the contact area and also at the interfaces due to material deformation. The pieces being worked tirelessly on must be compressed with a certain force over the weld zones to permit current to pass through. In case the welding force isn’t sufficiently strong enough, expulsion might occur as soon as the current starts flowing due to high contact resistance – which might result in rapid heat generation. However, when the force is greater than normal, that might result in the contact area to become greater than normal, an issue that might result in low current density. Concurrently, it could lead to low contact resistance, which could reduce how big the weld nugget and heat generation. In projection welding, it’s the force applied which causes projections within the workpiece to collapse. That triggers changes to the contact area and, consequently, contact resistance and density. Furthermore, additionally, it influences welding results and heat development.
Contact resistance at weld interfaces is probably the most influential parameters when it comes to materials. However, it’s worth noting which it has highly-dynamic interactions with process parameters. When welding forces increase, contact pressure also increases, meaning the true contact area with the interface also increases due to the rough surface asperities deforming. That triggers contact resistance with the interface to lower, reducing heat generation and weld nugget size. Since most metal surfaces have contaminants like oil, dirt, water vapor, and oxides to them, when temperatures rise, a few of them – mainly oil-based and water contaminants – dissipate in the initial few cycles, while metals soften as a result of high temperatures. As a result, contact resistance will generally decrease as temperatures rise. While contact resistance only affects results during the initial few cycles, it’s a decisive impact on how heat is distributed due to how heat is generated and distributed initially.
Almost all material properties alter as temperatures fluctuate, an issue that adds to 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 good thermal conductivity and low resistivities like copper and silver, almost no heat is produced in spite of extremely high welding currents and is also quickly dissipated. Such metals take time and effort to weld using resistance welding. However, they can make very good electrodes. When unlike metals are welded together, metal with higher resistivity will generally generate more heat. This really is something which should be considered ahead of time when making welding parts for projection welding and once choosing the kind of electrodes to use in spot welding. Material hardness also affects contact resistance. Metals which are harder (have higher-yield stress) will generally have higher contact resistance at similar welding forces as their surface asperities are not as easy to deform. That results in much smaller contact areas. It’s worth noting how the materials used as electrodes have been employed to influence how heat is balanced in resistance welding, specially when joining non-ferrous and light-weight metals.
Resistance Welding Advantages
- Resistance welding has no consumables, such as welding rods, brazing materials, or solder.
- Its processing time is quite short.
- It gives you operator-safety as being the machines only require low voltage to work.
- Resistance welding doesn’t create messes like other welding techniques it’s neat and environmentally-friendly.
- It creates reliable electro-mechanical joints.
Resistance Welding Shortcomings
- The electrodes used must be able to reach either sides in the metal pieces they can be bonding. Some spot-welding machines is only able to hold a particular metal thickness, usually 5-50 inches. And even though the electrode’s position could be adjusted, most electrode holders only have limited movement.
- The shapes and scale of electrodes used will determine the strength and scale of the weld. The joints only form with the spots in which the electrodes touch the metal. when the current isn’t hot or sufficiently strong enough, or maybe the metal isn’t held together sufficiently, the spot weld might turn out to be weak or small.
- A loss 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 also have cracks onto it. The metal could also end up becoming less resistant against corrosion.