RESISTANCE SEAM WELDING
Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material. A transformer supplies energy to the weld joint in the form of low voltage, high current AC power.
The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed together by the welding pressure that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.
Seam welding is restricted to the welding of thin materials with the metal ranging from 0.10 to 0.187 inch thick. Seam welding is further restricted to metals that have a low hardenbility rating , such as hot-roll grades of low alloy steel. When seam welding is done for heavier or thicker pieces of metal, higher current and greater welding pressure is used. Greater pressure and higher current densities settings results into deeper surface indentation of the seam weld.
Basically two types of welds can be formed in seam welding with the individual nuggets
The stitch weld and
The roll weld.
The stitch weld is made by turning the current on the rolls off and on quickly enough so that a continuous fusion zone is maintained. The fusion zone will not be parallel but will be in the shape of each overlapped bead. The stitch weld is also called as Lap seam weld. The roll weld occurs when the current to the copper rolled electrodes is turned off and on intermittently which causes formation of individual nuggets.
Stitch weld is used more for joints for use with liquid or gas , while roll welding is used for simple joining of two pieces of metal . the roll weld will not be liquid tight , water tight or gas tight.
Testing of Strength of seam weld :
The pillow test is the most common test for determining the strength of seam welds. It involves seam welding two pieces of metal to enclose a cavity . An appropriate pipe fitting is either put on with a nipple or welded on to the two pieces that were seam welded together. Hydraulic or air pressure is pumped through the fitting, expanding the cavity into a pillow shape. The pressure at which the pillow bursts is recorded and compared to the fracture strength of the base metal. Failure should always occur in the base metal and not in the welded seam. If the weld seam fractures, then the weld will not support that particular metal.
ADVANTAGES
GAS-TIGHT OR LIQUID-TIGHT JOINTS CAN BE PRODUCED (NOT POSSIBLE WITH SPOT WELDING OR PROJECTION WELDING). SEAM WIDTH MAY BE LESS THAN THE DIAMETER OF SPOT WELDS, BECAUSE THE ELECTRODE CONTOUR CAN BE CONTINUOUSLY DRESSED AND IS THEREFORE OF A STABLE SHAPE. HIGH-SPEED WELDING (ESPECIALLY ON THIN STOCK) IS POSSIBLE. COATED STEELS ARE GENERALLY MORE WELDABLE USING SEAM WELDING THAN SPOT WELDING, BECAUSE COATING RESIDUE CAN BE CONTINUOUSLY REMOVED FROM THE ELECTRODE WHEELS IF SPECIAL PROVISIONS ARE MADE.
RESISTANCE SEAM WELDING IS NOT PARTICULARLY FITUP-SENSITIVE COMPARED TO LASER WELDING. THE HARDNESS OF RESISTANCE SEAM WELDS MADE WITH AIR COOLING IS LESS THAN THAT OF LASER WELDS . LIMITATIONS
EXTERNAL WATER COOLING OF THE ELECTRODES AND THE WELD ZONE MAY BE REQUIRED FOR HIGH-SPEED WELDING. (UNDER HIGH-SPEED CONDITIONS THE WELD NUGGETS ARE STILL MOLTEN AS THEY LEAVE THE PRESSURE FIELD OF THE WHEELS). EXTERNAL COOLING MAY ADD TOOLING COST FOR WATER CONTAINMENT AND WATER REMOVAL FROM THE PARTS AFTER WELDING. CHROMATES AND INSULATING COATINGS ARE NOT RESISTANCE SEAM WELDABLE. COMPONENTS USING MULTIPLE CROSSING SEAM WELDS CAN BE QUALITYSENSITIVE AT THE WELD INTERSECTIONS.
APPLICATIONS
A common use of seam welding is during the manufacture of round or rectangular steel tubing.
Lap seams are popular in automotive applications, such as automotive fuel tanks, roof joints, as well as in nonautomotive applications, such as furnace heat exchangers, water tanks, and certain types of can making Lap seam welding of multiple stackups also possible.
and dissimilar thicknesses is
Most low-carbon, high-carbon, low-alloy, and stainless steels, can be seam welded. Aluminum and aluminum alloys can be lap seam welded . Nickel and nickel alloys can also be seam welded, but seam welding is not recommended for copper and high-copper alloys. Compatible combinations of dissimilar metals and alloys also can be seam welded.
PROBLEM
Calculate the work speed and electrode RPM for a seam welding job. Data: Supply frequency 50Hz Welding cycle = 3 cycles on and 2 cycles off Electrode diameter = 200mm No. of welds required for air tightness in the tube = 3 welds per cm
Work speed (mm/min) = welds per min/welds required per mm Welds per min = supply frequency x 60/ No. of cycles for weld + interval cycles
R.P.M. of the electrode
= work Speed(mm/min)/ x electrode diameter(mm)
