Resistance welding is the process of joining metals using pressure and passing electric current for any period across the area of the metal to be joined. Since resistance welding is among the most inexpensive welding processes currently available because 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 accustomed to conduct the electrode and apply pressure. The electrodes are mostly created from copper alloys and therefore are cooled by water which flows through conductive tooling from the machine and also the cavities inside of the electrodes.
Machines made for resistance welding were created and manufactured for an array of industrial, automotive, and aerospace industry applications. And also, since many are automated, the working of such machines is well-controlled and will be repeated, at least according to -. Therefore, 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 by utilizing 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. The 2 are applied for a defined period for the weld to be complete. It’s worth noting that spot welds make use of the face geometries of whichever welding electrode is now being accustomed to apply force in the workpiece while focusing the existing on the weld location. After the resistance generated is sufficient, the metals “set down” and combine, and a nugget of weld forms.
Seam welding is basically a subcategory of spot welding. However, under this approach, wheel-shaped electrodes are used to deliver welding current and force to the parts. Under seam welding, workpieces roll between wheel-shaped electrodes being a current is now being applied. According to 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
Projection welding applies exactly the same principles as other resistance welding processes. However, with projection welding, the welds are localized at preset points by utilizing intersections, projections, or embossments, all of these focus the generated heat to some specific point. After the current has generated enough resistance on the POC, the projections fall off, developing a weld nugget.
Flash Welding
The flash welding process leverages exactly the same principle as other resistance welding processes. However, with flash welding, resistance is generated using flashing action which is achieved by applying a higher current to some small contact point between the workpiece. After the flashing process has begun, force is used to some specific point in the workpiece. It’s worth noting that this workpieces are slowly moved in a determined rate.
Upset Welding
Upset welding uses exactly the same principle as other resistance welding processes. And even though it is comparable to flash welding, the main difference between the two is the fact that pieces being welded already are in contact, meaning no flashing occurs. Here, pressure in the workpieces is used just before the current starts and is also sustained until the welding process is complete.
Resistance Welding Parameters
Resistance welding works based on the principle of Joule Heating, where heat, denoted Q, is generated determined by three factors:
Q=I(squared)Rt
- I – represents the existing that’s passing throughout the metal combination
- R – represents the base metals’ and contact interfaces’ resistance
- t – represents some time/time of current flow
Whilst the principle may appear simple, you will find a huge selection of parameters that influence the outcomes of actual welding processes. To comprehend resistance welding technology better, here’s a glance at a number of the parameters that influence welding results:
Welding Current
The welding current is a crucial parameter in relation to resistance welding as it determines the volume of heat generated by way of a power of square. It’s worth noting that weld nugget sizes increase with all the increasing welding current however, when the current is just too high, it could result in electrode deterioration and expulsions. To avert this, welding engineers need to look for the optimal welding time as well as current for every single welding application.
Welding Time
Heat generation in resistance welding is directly proportional to welding time. Due to heat transfer from weld zones to electrodes and also the base metals, as well as a loss in heat from surrounding surfaces, a minimum welding time as well as current are needed to develop a weld. In case the current is not sufficient, increasing welding times alone won’t create a weld. However, if the current is sufficiently strong enough, the weld nugget size will increase as weld time increases until it’s exactly the same size as being the contact portion of the electrode tip. If welding time is protracted, expulsion might occur or perhaps the electrode could end up staying on the workpiece.
Welding Force
Welding force impacts the resistance welding process through its result on contact resistance in the contact area and also at the interfaces because of material deformation. The pieces being worked on needs to be compressed by way of a certain force along the weld zones to enable current to pass. In case the welding force isn’t sufficiently strong enough, expulsion might occur as soon as the current starts flowing because of high contact resistance – which might result in rapid heat generation. On the flip side, when the force is more than normal, which may make the contact area to be larger than normal, something which might result in low current density. At the same time, it might lead to low contact resistance, which could reduce the size of the weld nugget and also heat generation. In projection welding, it’s the force applied which induces projections inside the workpiece to collapse. That triggers changes to the contact area and, consequently, contact resistance and density. Furthermore, in addition, it influences welding results and also heat development.
Contact Resistance
Contact resistance at weld interfaces is among the most influential parameters in relation to materials. However, it’s worth noting that this has highly-dynamic interactions with process parameters. When welding forces increase, contact pressure also increases, meaning the actual contact area on the interface also increases due to the rough surface asperities deforming. That triggers contact resistance on the interface to reduce, reducing heat generation and weld nugget size. Since most metal surfaces have contaminants like oil, dirt, water vapor, and oxides on them, when temperatures rise, a number of them – mainly oil-based and water contaminants – dissipate in the initial cycles, while metals soften due to high temperatures. Consequently, contact resistance will generally decrease as temperatures rise. While contact resistance only affects results during the initial cycles, it’s a decisive effect on how heat is distributed due to how heat is generated and distributed initially.
Material Properties
Almost all material properties alter as temperatures fluctuate, something which boosts 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 higher thermal conductivity and low resistivities like copper and silver, minimal heat is produced despite very high welding currents and is also quickly dissipated. Such metals are difficult to weld using resistance welding. However, they are excellent electrodes. When unlike metals are welded together, metal with higher resistivity will generally generate more heat. This can be something that needs to be thought of in advance 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 can be harder (have higher-yield stress) will generally have higher contact resistance at similar welding forces his or her surface asperities are harder to deform. That brings about smaller contact areas. It’s worth noting that this materials used as electrodes were also accustomed to influence how heat is balanced in resistance welding, specially when joining non-ferrous and light metals.
Resistance Welding Advantages
- Resistance welding has no consumables, including welding rods, brazing materials, or solder.
- Its processing time is quite short.
- It provides operator-safety as being the machines only require 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 should be able to reach either side from the metal pieces they can be bonding. Some spot-welding machines are only able to hold a particular metal thickness, usually 5-50 inches. And although the electrode’s position might be adjusted, most electrode holders have only limited movement.
- The shapes and size of electrodes used determines the strength and size of the weld. The joints only form on the spots where electrodes touch the metal. when the current isn’t hot or sufficiently strong enough, or perhaps the metal isn’t held together well enough, the spot weld might become weak or small.
- A loss in strength and warping may appear around points where workpiece continues to be spot-welded. The joint’s appearance is usually quite ugly and can have cracks upon it. The metal could also turn out becoming less resistant to corrosion.