High-frequency Welding Integrated With Roll Forming
In addition to this mature tube forming process, high-frequency welding is increasingly being integrated with roll forming to produce more complex, asymmetrical shapes with prepunched holes, notches, and forms.
When compared with other processes, high-frequency welding produces a high-quality butt seam at relatively high speeds. When integrated with a roll forming system, however, the high weld speed may not be able to be realized because of the slower speed of other processes integrated into the line, such as prepunching. These slower line speeds are justified by a savings in the time and equipment that would be required for secondary operations if they were not integrated into the roll forming system.
Production volumes need to be relatively high to justify the capital equipment cost, which typically is much higher than for gas tungsten arc welding, but somewhat less than laser welding.
The orientation of the weld joint is most often on the top of the part from an end view, a cross-sectional perspective in other words. This orientation offers the easiest way to monitor weld quality visually.
However, good quality welds also can be produced on the bottom of the part. From this same perspective, a weld position on the side of the part allows acceptable weld quality to be produced, but requires an unbalanced rollform progression and is not recommended.
Weld orientation of the joint most often is up, as this is the easiest position at which to monitor weld quality visually, but good-quality welds also can be produced upside down. Welding on the side, while acceptable for the high-frequency weld process, overly complicates the roll forming tool design and is not recommended.
The roll forming process requires that the material begin in flat coil stock form (no reforming of rounds, as is common in the tube forming process) to allow prepunched or formed features to be produced efficiently and inexpensively. From the uncoiling and prepunch area, the material proceeds into the preweld form rolls that roll-form the part to the cross-sectional shape required. As the material approaches the weld zone, a fin roll, fit between the two adjacent edges to be joined, accurately controls the required V angle as the edges are brought together to produce a good weld.
Fit inside the part in the weld zone is a device called an impeder. The purpose of the impeder is to direct the current flow where it is required. Unlike in a simpler tube forming application, the size and placement of the impeder is custom-fit and developed for the roll forming/high-frequency welding line because the variance in the magnetic field of the weld current flow created by irregular cross section, holes, and other features as they pass through the weld area must be accommodated.
After the weld is created, the now-tubular part proceeds to the weld, or squeeze, box. This fixture is equipped with rolls that squeeze the part together with enough force to forge and upset the edges of the part together, forcing out impurities in the form of slag.
Subsequent to the weld squeeze operation, the slag is removed, or scarfed, with a shaper-type cutter. The scarf can be extremely sharp, so care should be taken with its handling and disposal. Depending on the application, the slag on the inside of the part also might have to be removed or rolled into the weld joint.
The part then is given a continuous coolant bath. The cooling area must be sized in consideration of the material, material thickness, line speed, and part shape. The amount of thermal distortion put into the part is beyond the capability of a common straightener block (used in a conventional, nonwelded roll forming line) to correct; two to four passes of roll tools usually are required for resizing so the part can be kept under dimensional control as it cools. Stands called cluster rolls often are used for this purpose, as they contain the part with rolls on the sides, top, and bottom.
To remove twist in the part, the last pass often is adjustable in a rotary axis. This fixture is called a Turk's head, probably because of its similarity in appearance to the decorative four-strand knot of the same name. The roll formed and high-frequency-welded part then is cut to length and exits the system.
