How To Successfully Projection Weld Nuts & Studs
(The Secrets To Resistance Fastener Welding)
By Stephen M. Andrassy, Dengensha America, National Sales Manager
For the most part, the projection welding process is very similar to the spot welding process. In spot welding, the size of the contact surface of the electrode cap determines the current flow, while in projection welding the current flow is directed by the projection of the particular fastener. While both AC and DC power sources are suitable for projection welding fasteners, the choice of AC or DC is dependent on material make-up along with the projection size and torque requirements.
Common Projection Welding Problems
While the basic process for the projection welding of nuts and studs may be simple, when a few standard guidelines are not followed, a number of problems may occur. For instance, you might develop weld spatter in the threads, or realize hole misalignment. Cold-welds and thread distortion are common problems as well. Undetected, these problems can easily result in lost production time and scrapped parts, which will end up costing manufacturers money that cannot be recouped until the issues are corrected. The good news, by following just a few simple recommendations, these problems can be easily avoided and successful projection welding can be achieved.
Follow Simple Set-Up Guidelines
The projection welding of a nut or stud involves three distinct phases. First, fastener projections are held in place with the metal stamping by the upper electrode. Current is then applied and starts to heat the projections to the prescribed welding temperature. Finally, the force of the electrode causes the heated projections to collapse rapidly and fusion of the two takes place. In order to achieve a successfully projection welded part, it’s important to adhere to the following guidelines.
First, you need to make sure that the welder is sized correctly for the particular fastener being welded and that the required welding conditions are within the normal operating range of the welder. Next, the frame of the welder and the electrode assembly needs to be rigid so that flexure and misalignment are not present under load. The electrode force must able to hold each projection firmly against the stamping during the initial period of the weld time and must have good follow up characteristics when the projections collapse during the welding process. Slow follow up will result in metal expulsion before the parts are brought together.
You need to make sure that the surfaces of the electrodes are flat and well aligned. They must have clean contact points and be free of any oil, dirt, or film. The upper electrode must also be on the same centerline. Class 2 copper alloys are commonly used but harder tungsten-copper inserts are sometimes incorporated in order to keep electrode wear to a minimum. You then need to make sure that the weld pin of the lower electrode is made of a properly insulated material, either ceramics or a material with a non-conductive coating. And finally, the weld pin should be either spring-loaded or air loaded. Incorporating an air supported pin will not only help cool the weld, it will also expel any weld spatter that might occur.
Know Optimum Welding Parameters
Welding parameters are dependent on several factors. First you need to take into consideration the type of nut or stud to be welded along with the size and location of the projections. The sheet thickness of the stamping, along with the type of material that the stamping is made of must also be considered. Once this information is taken into consideration, the welding parameters of electrode force, weld time in cycles, and welding current can be established for the projection welding application. You can also easily establish a typical minimum torque to failure test for the settings selected.
In order to verify that a minimum specified torque is achieved with the parameters selected, you probably need to conduct a destructive test. This can be done by a push-off or a peel test to check that a piece of the material can be pulled from the sheet at each projection point. If this cannot be accomplished, the welding parameters of electrode force, weld cycle time, and current can be adjusted to reach the desired torque requirements. Typically, a short weld time in cycles requires more current and a longer weld time in cycles requires less current.
For most applications, the current required for projection welding is less than what is required for corresponding spot welding. For projection welding, the current must be at least high enough to create the fusion before the projections are completely collapsed. While shorter welding time is desirable from a production standpoint, higher current is necessary to make a successful weld.
To prevent overheating and metal expulsion, welding parameters need to be optimized. When welding fasteners to high-strength steels, impulse welding may have an advantage in controlling the heating rate. You need to make sure that the electrode force is high enough to flatten the projections completely when they reach welding temperature and the fastener is brought into contact with the metal stamping. On the other hand, excessive force will prematurely collapse the projections and the weld will have incomplete fusion.
Other Problem Causes
Excessive weld spatter in or on the threads of the fastener can be caused by a variety of situations, including low electrode force, a short squeeze time, poor head follow up, or too high of a current. Thread distortion, or the collapse of the nut body can be caused by too long of a weld time. A shorter weld time with higher current and higher force may elevate this condition. And hole misalignment or poor location of the fastener on the stamping is often the result of an inadequate locating weld pin or electrode misalignment.
To ensure successful projection welds, you need to always consider the following:
• Fastener type and size of projections
• Type of material and thickness
• Electrode alignment and material
• Welder size and the ability to meet welding parameters
• Weld time in cycles, electrode force and welding current
Single-Point Projection Welding Machines
Simple, reliable and standardized single point projection welding machines provide an answer and solve many problems. Once the process parameters of current, time and pressure are optimized to meet required test results, the weld control automatically adjusts for normal variations in material as well as for standard electrode wear. The most effective weld controls for projection nut welding are user-friendly, easy-to-understand and intuitive, containing only a few functions required for a repeatable and successful weld.
In addition, due to their small footprint and thin profile, single point projection welding machines can be easily combined in cells to accomplish many welds in the same set-up or used as standalone weld stations. They can also be readily integrated with automatic nut or stud feeders for a simplified and consistent weld system, and can be quickly converted to spot welders when desired. The power supply and weld controls are the same for both projection weld and spot-weld requirements. With standardized designs, replacement parts are also standardized, preventing other problems as well as lowering costs.
Dengensha America, Bedford, OH