March 18, 2021

Resistance Welding – An Inside View

Resistance welding is the process of joining metals using pressure and passing electric current for any period on the portion of the metal to get joined. Since resistance welding is among the most cost-effective welding processes now available because it doesn’t require other materials to make a bond.

It’s worth noting that resistance welding is available in several forms which differ primarily with the shapes and types of weld electrodes accustomed to conduct the electrode and apply the strain. The electrodes are mostly made from copper alloys and they are cooled by water which flows through conductive tooling in the machine and the cavities inside of the electrodes.

Machines created for resistance welding are designed and manufactured for a wide range of industrial, automotive, and aerospace industry applications. And because nearly all are automated, the working of the machines is well-controlled and will be repeated, based on information found at this blog. As such, they make it simpler for companies to staff their production easily.

Resistance Welding Types

Seam and Spot Welding

Like resistance welding applications, spot welding creates welds by making use of heat that’s been generated by welding current flow resistance involving the surface in touch with a joint and force, which pushes the workpieces together. The two are applied for a defined period for the weld to get complete. It’s worth noting that spot welds take advantage of the face geometries of whichever welding electrode has been accustomed to apply force about the workpiece and concentrate the current with the weld location. After the resistance generated is plenty, 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 used to deliver welding current and force on the parts. Under seam welding, workpieces roll between wheel-shaped electrodes like a current has been 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

Projection welding applies a similar 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, which all focus the generated heat into a specific point. After the current has generated enough resistance with the POC, the projections fall off, developing a weld nugget.

Flash Welding

The flash welding process leverages a similar principle as other resistance welding processes. However, with flash welding, resistance is generated using flashing action which can be achieved by utilizing a very high current into a small contact point involving the workpiece. After the flashing process has started, force is used into a specific point about the workpiece. It’s worth noting the workpieces are slowly moved at a determined rate.

Upset Welding

Upset welding uses a similar principle as other resistance welding processes. And though it may be just like flash welding, the main difference involving the two is the pieces being welded are already in contact, meaning no flashing occurs. Here, pressure about the workpieces is used before the current starts and it is sustained till the welding process is complete.

Resistance Welding Parameters

Resistance welding works based on the principle of Joule Heating, where heat, denoted Q, is generated dependent on three factors:

Q=I(squared)Rt

  • I – represents the current that’s passing throughout the metal combination
  • R – represents the base metals’ and contact interfaces’ resistance
  • t – represents the time/length of current flow

While the principle might appear simple, you will find countless parameters that influence the outcome of actual welding processes. To learn resistance welding technology better, here’s a short look at some of the parameters that influence welding results:

Welding Current

The welding current is a crucial parameter when it comes to resistance welding as it determines the quantity of heat generated from a power of square. It’s worth noting that weld nugget sizes increase using the increasing welding current however, when the current is way too high, it could cause electrode deterioration and expulsions. To avert this, welding engineers need to ascertain the optimal welding some time and current for every single welding application.

Welding Time

Heat generation in resistance welding is directly proportional to welding time. On account of heat transfer from weld zones to electrodes and the base metals, together with a loss of heat from surrounding surfaces, a minimum welding some time and current are necessary to develop a weld. In the event the current will not be sufficient, increasing welding times alone won’t develop a weld. However, if the current is sufficiently strong, the weld nugget size will increase as weld time increases until it’s a similar size since 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 influence on contact resistance about the contact area as well as the interfaces due to material deformation. The pieces being worked tirelessly on needs to be compressed from a certain force over the weld zones to allow current to pass. In the event the welding force isn’t sufficiently strong, expulsion might occur once the current starts flowing due to high contact resistance – and that might lead to rapid heat generation. On the flip side, when the force is higher than normal, which may result in the contact area to get larger than normal, something that might lead to low current density. Concurrently, it may possibly bring about low contact resistance, and that could reduce the actual size of the weld nugget and also heat generation. In projection welding, it’s the force applied which causes projections within the workpiece to collapse. Which induces changes on the contact area and, consequently, contact resistance and density. Furthermore, it also influences welding results and also heat development.

Contact Resistance

Contact resistance at weld interfaces is among 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, which means the true contact area with the interface also increases as a result of the rough surface asperities deforming. Which induces contact resistance with the interface to reduce, reducing heat generation and weld nugget size. Because most metal surfaces have contaminants like oil, dirt, water vapor, and oxides about them, when temperatures rise, many of them – mainly oil-based and water contaminants – dissipate in the initial cycles, while metals soften due to the high temperatures. Because of this, 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 as a result of how heat is generated and distributed initially.

Material Properties

Virtually all material properties alter as temperatures fluctuate, something 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 higher thermal conductivity and low resistivities like copper and silver, hardly any heat is produced even with high welding currents and it is quickly dissipated. Such metals are hard to weld using resistance welding. However, they can make excellent electrodes. When unlike metals are welded together, metal with higher resistivity will generally generate more heat. This really is something which should be looked at ahead of time when making welding parts for projection welding and whenever choosing the particular electrodes to be utilized in spot welding. The hardness of your welding material also affects contact resistance. Metals that are harder (have higher-yield stress) will generally have higher contact resistance at similar welding forces as their surface asperities are harder to deform. That results in smaller contact areas. It’s worth noting the materials used as electrodes have been accustomed to influence how heat is balanced in resistance welding, especially when joining non-ferrous and light-weight metals.

Resistance Welding Advantages

  • Resistance welding has no consumables, for example welding rods, brazing materials, or solder.
  • Its processing time is extremely short.
  • It offers operator-safety since 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 produces reliable electro-mechanical joints.

Resistance Welding Shortcomings

  • The electrodes used will be able to reach either side in the metal pieces they can be bonding. Some spot-welding machines can only hold a certain metal thickness, usually 5-50 inches. And while the electrode’s position could be adjusted, most electrode holders only have limited movement.
  • The shapes and size of electrodes used determines the strength and size of the weld. The joints only form with the spots the location where the electrodes touch the metal. when the current isn’t hot or sufficiently strong, or perhaps the metal isn’t held together well enough, the spot weld might wind up being weak or small.
  • A loss of strength and warping can take place around points the location where the workpiece continues to be spot-welded. The joint’s appearance is usually quite ugly and might have cracks into it. The metal may also find yourself becoming less resistant against corrosion.