Surveying suitability of resistance welding for roll formed parts
Taking a look at high-frequency and rotary spot processes
From hair dryers to clothes dryers, electrical devices we use every day require different electrical amperages or voltages, but the electrical frequency, be it the 60-hertz (60 cycles per second) frequency used in the U.S. or the 50-Hz (50 cycles per second) used in Europe, usually is left as is.
However, when the incoming line frequency is increased to a level of 100,000 to 800,000 Hz, some interesting phenomena occur. At these high frequency levels, current flows only at or near the surface of a conductive material, rather than through the entire thickness. The higher the frequency, the less "skin depth," or penetration, the current has.
When two high-frequency, current-carrying surfaces are brought together, the current concentrates on the two adjacent edges, allowing the direction and location of the current flow to be controlled precisely. If the current is controlled by the location of ground-return conductors so that it follows a V pattern, a weld between the two adjacent edges is formed at the intersection of the V. If the two edges are forged together with enough pressure, the impurities are squeezed out of the weld joint as slag, producing a strong weld with a good appearance and narrow heat-affected zone (HAZ).
The high-frequency welding process can fuse any material that can be welded, including steel, copper, brass, aluminum, stainless steel, titanium, and gold. Even unusual material applications, such as joining copper and steel together, are feasible. Material thickness is not a restriction, as the process can weld materials from 0.004-in.-thick foils up to 1/2-in.-thick, heavy-wall tube.
Two distinct types of high-frequency welding are:
- Induction welding, which uses an energized copper coil that is wrapped around the part without contacting it. As the material flows through the electromagnetic field created by the coil, current is induced into the material. The need to adjust, maintain, and replace contacts is eliminated.
- Contact welding, which sends current flowing into the material through copper alloy, tungsten, or silver-tungsten contacts that touch and ride on the material.
The high-frequency weld process allows very high line speeds, depending on the material and thickness, and is an efficient process for the production of relatively simple round, square, and rectangular tube and pipe commodities. For instance, a tube forming line producing 1-in.-diameter, 0.090-in.-thick-wall conduit can operate routinely at speeds up to 1,000 feet per minute (FPM).
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 roll form progression and is not recommended. at which to
The roll forming process requires that the material begin in flat coil stock form (no re-forming 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.
Rotary Spot Resistance Welding
In resistance spot welding, the material's resistance to electrical current flow creates the heat required to form a molten spot, or nugget, between the two pieces of material. After the current is turned off, forge pressure and hold time to contain the weld until it cools sufficiently, which helps produce a strong metallurgical grain structure within the weld.
Several types of material can be welded with the process, including low- or high-carbon steels and alloy steels. Nonferrous metals, such as aluminum and copper alloys, require some special considerations, as these materials tend to deposit more material on the electrodes; an effective electrode dressing system is a must. Aluminum is especially susceptible to distortion from the heat of the weld process and requires an efficient flood coolant and resizing system. Pure copper is such a good conductor that enough electrical resistance cannot be generated to produce an acceptable weld.
Rotary Spot Resistance Welding Integrated With Roll Forming
When resistance welding is integrated into a roll forming line, the weld nugget must be formed while the material is moving in a linear direction while trapped between two copper-alloy electrode wheels. As the material moves and the electrodes rotate, the current is pulsed, creating a series of individual spots where the material is fused together. The nugget of a roll spot weld is elongated or egg-shaped as compared to the conventional spot weld of a static part.
Seam welding is a variation on this theme, but the spots are overlapped, creating a continuous weld. This process often is used to create a gas- or liquid-sealing seam in containers and pressure vessels.
With a conventional AC power supply, weld speed varies with material thickness from about 5 to more than 20 FPM, with higher speeds possible when welding lighter gauges. The incoming line frequency becomes the effective speed limit, because a cycle of electricity takes a finite amount of time. Increasing power, voltage, or force does not allow the weld time to be decreased, because the decrease in weld time degrades the quality of the weld nugget to less than optimal.
Using a DC power rotary spot welder can increase the weld speed beyond this speed limit, because there are no interruptions to the power, as in AC. The cost of the DC welding equipment needs to be justified by the increased speed, because the equipment is significantly more expensive.
The electrodes in many rotary spot welding lines are mounted on a shaft and bearing assembly driven by a motor and gearbox. The speed of the driven wheels must be synchronized with the roll former to eliminate excessive material buckling. The shafts need to be internally cooled, and the bearings need to be electrically insulated and highly wear-resistant.
Some seam welders have nondriven, or idler, rotary electrodes. These electrodes simply turn as the material moves because of the friction created by the squeeze pressure of the upper and lower electrode wheels on the material, eliminating the need to synchronize the electrode speed with the drive of the roll former. In this case, the electrode's revolutions per minute increase automatically to keep a constant material speed, regardless of the electrode diameter.
Coolant must be liberally applied externally over the electrodes and the material being welded. Proper drainage of this area is important, and the welder often is mounted on a grate over baffles in the machine base that returns the coolant to its reservoir.
Electrode dressing is an important consideration in rotary spot welding. Some rotary spot welding/roll forming lines drive the periphery of one or both electrodes with an external knurl wheel. In addition to driving the electrode, the knurl continuously dresses the contact surface of the electrode, ensuring a good, conductive surface and current flow. However, the knurl leaves a visible mark on the part and can cut through thin material, limiting its usage to applications tolerant of these limitations.
Electrodes that are not knurled need to be removed for dressing in some offline area at regular intervals, or an abrasive dresser that leaves a smooth finish on the electrode contact surface can be mounted on the back side of the electrode, continuously dressing the electrode wheel as it rotates. The RPM of positively driven electrodes must be increased to compensate for the reduced diameter created after dressing to retain the same material speed between the electrodes and the material.
Roll spot welding requires a lap joint design. The process allows nontubular roll formed and welded parts to be produced. When designing a part to be roll formed and rotary spot welded, designers must consider proper access of the welding electrodes and equipment to the area to be welded.
If the surface to be roll-spot or seam welded is facing up and easily accessible, fairly standard roll-spot and seam welding units can be used. This is the most economical approach. However, the orientation of the weld in a roll-spot or seam weld is not as critical as in fusion welding processes, as gravity has less effect on weld formation. Thus, unusual weld orientations can be accommodated, if required, to balance the forming forces in the roll tooling.
The roll tooling should have material guides before and after the roll-spot or seam-weld station to position and control the material properly through the weld area. Thermal distortion is a concern for thin or thick materials that require a lot of heat, triple-thickness welds, tight spot spacing, and materials with good thermal conductivity, such as aluminum. For this reason, it is common to have a series of resizing stations after the weld area. This usually is a series of roll tool passes that are adjustable in all three spatial dimensions to eliminate up and down bow, twist, and camber induced by the heat of the weld process.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.