Given there are reported 27 variable in resistance welding, the key is to identify each of your variables and minimize their change. When changing any one variable you must consider the influence on the remaining variables. A reduction in time can increase demand for current, and a reduction in force will decrease demand for current to reach welding temperatures. Document all changes and develop history of tests made in developing your weld schedule.
Precise control over welding temperature is vital in resistance welding operations. The primary variables affecting weld formation are heat, time, and pressure. Resistance welding equipment is designed to, 1) provide current, 2) exercise precise control over time and current and 3) supply a forging pressure with rapid follow-up. The following information addresses important variables and steps to follow in making a resistance weld.
Note, the information below does not address material variables. There is plenty to say about material variables, but for now just remember variables left un-monitored can shut your production down today. Any change in vendor or material selection should be closely monitored to ensure optimum welding results.
1) Select the proper electrode material or materials for metals being welded (see Electrode Selection).
2) Determine electrode geometry. If a standard shape electrode is not suitable for your operation an electrode should be designed to properly address the part/electrode interface The final electrode design should be easy to change with sufficient to access for maintenance. (See Electrode Design).
3) Install the electrodes into electrode holders, and align. Electrode holders are typically adjustable for both vertical and rotational positioning. Careful attention should be taken to ensure that the alignment is correct and . Electrode alignment should produce an even footprint, allow for the easy insertion and removal of the parts, plus provide access to clean the electrode on the machine. In some cases electrode geometry in critical and will require cleaning off site. (See Electrode Alignment).
Setting Weld Force
4) Make sure the power source is shutdown and set electrode force using an electrode force gauge. Most weld heads have an indicator that typical displays some nonlinear reference to the force and should not be mistaken as actual force values. The use of a electrode force gauge is recommended
Place the parts between the electrodes, actuate the top electrode to meet the parts. Carefully check to see that proper contact is being made. Determine if the effects of the pressure alone causes any serious deflection of the electrodes and or deformation to the parts (see Pressure Setting).
5) Turn the power on and set the heat control to low amplitude and time settings. Begin by gradually increasing the values to produce the required temperatures to form the weld (see Setting Heat and Time). If the maximum power settings are reached before a proper weld is obtained, gradually decrease the pressure between successive trials. This procedure will increase the contact resistance between the parts and increase the temperatures generated at the interface. (Note: A minimum amount of electrode force is required to stabilize the contact resistance between the electrodes and the parts). If the force is to low electrode sticking and expulsion of molten material will result.
The settings you begin your study with are not as important as the documentation of the changes made and their respective results. To minimize the complications in developing a weld schedule do not attempt to change more than one variable at the same time. Evaluate each weld for the strength and requirements of your particular application (see Testing the Weld). Once the parameters have been determined fully document the process variables and publish in your manufacturing records.
Electrode Selection top
Electrodes can be in the form of a rod, plate or incorporated into a special fixture. Electrode materials are specific alloys designed for a range of conductivity and hardness. RWMA stands for, Resistance Welders Manufacturing Association and they have identified a range of alloys. RWMA2, 3... can be also be referred to as Class 2, 3... Also, some materials are identified with a "Mallory" number. Your selection is based on the alloys ability to withstand the specific demands of you application.
Electrodes simultaneously, conduct electrical current to the weld site, and conduct thermal energy away. In welding resistive materials a conductive electrode pulls heat away from electrode / part interface, ensuring welding temperatures are only achieved at the parts interface. Welding temperatures develop at different rates depending on a materials mass and/or composition. Keep in mind, when welding dissimilar materials, heat generates at different rates, and two different electrode alloys may be required. To control the even heat generation in both parts, electrodes of varying conductivity and or shape are used. For example, a larger diameter or more conductive electrode allows surplus heat to exit from a thinner material while a heavier part reaches welding temperatures. This is referred to as Heat Balance. Changes in electrode material and geometry are used to ensure good heat balance.
Fundamental electrode selection calls for a conductive electrode when welding resistive materials; and resistive electrodes when welding conductive materials. . The parts mass and composition are used as guidelines in the proper selection of the electrode material.
Electrode Material top
Copper is a commonly used electrode material due to its high electrical and thermal conductivity. To withstand the welding environment, copper is alloyed with other elements. For example, copper strengthened by the addition of aluminum oxide particles (commonly referred to as Dispersion Strengthened Copper, and bearing the Trade name GlidCop®) offers higher wear resistance than traditional Copper-Chromium welding alloys. Oxygen-free copper should be avoided, due to their low tensile and yield strength at elevated temperatures electrode deform quickly.
Established standards for welding alloys by the Resistance Welders Manufacturers' Association (RWMA) include a range of copper and refractory alloys. Some of the most common alloys used are: RWMA Class 2 - Copper-Chromium Alloy used to weld resistive materials; RWMA Class 3 - Copper-Cobalt-Beryllium Alloy has the same general applications as the RWMA Class 2, but this material sacrifices conductivity for increased hardness; RWMA Class 11 - Copper-Tungsten Alloy is used in applications where high electrode force and conductivity are required; RWMA Class 13 - Tungsten. Class 13 materials are used for conductive alloys; RWMA Class 11 or Molybdenum electrodes are similar in conductivity to RWMA 13 materials, but Class 11 materials are easier to machine. In summary, always fabricate the electrodes using a resistance welding alloy.
Electrode Design top
When designing electrodes, consider ease of manufacturing, replacement, and maintenance. Electrode design is governed by part thickness, composition, shape, size and the required weld size. Electrode geometry establishes current density in the weld site. The force applied by the weld head on the electrode influences current density in the weld site. The electrodes also maintain intimate contact between the materials and provide a forging action during the formation of the weld. Concurrently, they dissipate heat from the weld site, preventing surface fusion between the electrode and the parts.
The electrode face is directly above the point of fusion, and this area is subject to repeated exposure to high temperature and pressure. If electrodes overheats and begins to fuse with the materials being welded, consider increasing the electrode force, diameter, conductivity or use water cooled electrodes.
Another important factor in determining electrode shape is accessibility to the weld site. The weld site does not always allow for a basic straight shank electrode design. Limited access may require offset, or angled electrodes. With offset electrodes, ensure the electrode diameter is sufficient to transmit the required pressure and current without distortion.
When changes to electrode design and material fail to produce desired results projection welding techniques may offer a solution. A projection or dimple formed, typically in the thinner part, focuses welding current to assist in issues with heat balance or great difference in parts mass.
Electrode Alignment top
The preferred electrode alignment is vertically opposed because is force exerted is one of simple compression. Careful attention to electrode alignment is important; if the centers are not aligned, the net effective tip area is reduced. When electrode tips are not parallel the pressure and current is confined to a fraction of the designed area, and destruction to both the electrodes and the parts is likely.
Electrode Maintenance top
Electrode tips will eventually become pitted or spattered with weld material. They require periodic maintenance to reshape and/or clean the welding surface. Often, files are used as a maintenance tool for cleaning. This not recommended, files easily alter shape and surface finish of the electrode. A preferred electrode maintenance material is emery paper. Specially shaped electrodes must be cleaned and dressed with consideration given to retaining original contours. Standard flat-tip electrodes of equal dimension can be reshaped very simply as follows:
1) Choose an emery paper of #400 to #600 grit. Fold the paper so that abrasive surfaces are exposed. Hold the emery paper between the electrodes and carefully bring the electrodes together so that both tips are in light contact with the emery paper.
2) Rotate emery paper in circles between the tips, or pull it in alternating directions. Continue this polishing operation until both tips are clean and smooth and are in good contact with each other. A small dental mirror provides easy inspection. Care should be taken to keep the plane of the emery paper horizontal and perpendicular to the long axis of the electrodes. Commercially available emery cleaning disk adhere emery paper on heavy paper disk. Keep in mind the paper backing is soft, and electrodes will tend to form a radius by simply spinning these disk between electrodes. If you electrode are of different size, a ridged substrate between the emery paper is recommended. In some cases ceramic is used to clean electrodes.
3) Remove emery paper and metal by products from the tips with either a brush, or flush with alcohol. Adding a small amount of alcohol to emery paper before cleaning operation can minimize the accumulation of cleaning by products on the tips and weld station.
Pressure Settings top
Electrode pressure forces the materials to be welded into good contact with each other before, during, and after welding. The effect on the weld is a forging action. The electrode pressure is indicated on the weld head and graduated in relative units. For precise work, a force gauge indicating actual pressure obtained is recommended. The pressure should be high enough to render the surfaces of the materials uniform in electrical resistance. Forces set to low can cause dramatic changes in current density and may be evident by pitting of the electrode surfaces.
Pressure should not be confused with "follow-up." Follow-up is the ability of the welding head to follow the minute expansion and contraction of the weld during heating and cooling. This action maintains a constant forging action during the manufacture of the weld and insures consistent weld uniformity. Low-inertia and friction are important to providing rapid follow-up during a weld, and are feature of a good weld head design.
Heat Settings top
Always start with low weld current and time settings and work up to the optimum weld results. Power supplies provide several output options including: multiple pulses, off time, upslope, down slope and polarity. A standard technique used to displace plating and or oxides is to use a dual pulse weld where a lower amplitude first pulse is followed by a higher amplitude second pulse. Off time settings are used to cool or quench the weld between pulses. Upslope and downslope features gradually increase or decrease current into the parts, and is beneficial when welding materials that are sensitive to heating and cooling rates. Polarity settings, positive and negative are used to appoint a specific direction to current flow through the parts.
Requirements of the materials being welded determine what methods of heating are necessary. Keep in mind, heat loses by conduction into surrounding parts and electrodes, as well as radiation into the surrounding air. These losses are essentially non-controllable, increasing with increases in total time. At some period during an extended welding interval, the radiation losses will equal the heat input, thus stopping further temperature rise.
Factors affecting the amount of heat being generated by a given weld current for a unit of time are:
1. The electrical resistance of the materials being welded
2. The electrical resistance of the electrode materials
3. The contact resistance between the parts as determined by surface conditions, scale, welding pressure, etc.
4. The contact resistance between the electrodes and parts as determined by surface conditions, area of electrode contact and welding pressure.
Time Settings top
Generated welding temperatures is a linear function of time. Heat may be affected either by a change of current or by a change of time. Finite values of time are required before any weld is formed. The exact minimum length depends upon current magnitudes, material thickness and composition. If weld time is to short now weld will form, regardless of the increase in current.
Testing the Weld top
The most reliable test of weld quality is its strength compared with the strength of the materials joined. A very simple and practical test is described below:
After joining two pieces of sheet metal with one spot weld, peel them apart. If one of the two pieces fails or has a hole in it the weld should have adequate strength. If a hole pulled in one of the materials has a diameter at least twice the thickness of the thinner material the weld is probably as strong as can be obtained.
You now have some basic information to assist you in your welding operations. If this was helpful please let us know.
Prototype & Production Welding | Thermode Repair | Process Development | Applications Support | About Wire Works | Contact
Copyright © Wire Works 2013. All Rights Reserved