WELDING (comprehensive)

Welding is a joining process that produces coalescence of materials (typically metals or thermoplastics) by heating them to welding temperature, with or without the application of pressure or by the application of pressure alone, and with or without the use of filler materials.

Most commonly, workpieces are welded by melting both of them and adding more molten metal or plastic to form a pool that cools to form a strong joint. The energy to form the joint between metal workpieces most often comes from a flame (e.g. oxy-acetylene) or an electric arc, but welding by laser beam, electron beam, ultrasound and friction processes is well established. Energy for fusion welding of thermoplastics typically comes from direct contact with a heated tool or a hot gas.

Welding differs from soldering and brazing in that enough heat is applied to melt the materials to be joined. Soldering uses solder, a lower-melting-point material.

RESISTANCE WELDING

Resistance welding is a group of welding processes in which coalescence is produced by the heat obtained from resistance of the work piece to electric current in a circuit of which the work piece is a part and by the application of pressure. There are at least seven important resistance-welding processes. These are flash welding, high-frequency resistance welding, percussion welding, projection welding, resistance seam welding, resistance spot welding, and upset welding. They are alike in many respects but are sufficiently different.

Resistance spot welding (RSW) is a resistance welding process which produces coalescence at the faying surfaces in one spot by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. The equipment for resistance spot welding can be relatively simple and inexpensive up through extremely large multiple spot welding machines. The stationary single spot welding machines are of two general types: the horn or rocker arm type and the press type.

The horn type machines have a pivoted or rocking upper electrode arm, which is actuated by pneumatic power or by the operator’s physical power. They can be used for a wide range of work but are restricted to 50 kVA and are used for thinner gauges. For larger machines normally over 50 kVA, the press type machine is used. In these machines, the upper electrode moves in a slide. The pressure and motion are provided on the upper electrode by hydraulic or pneumatic pressure, or are motor operated.

For high-volume production work, such as in the automotive industry, multiple spot welding machines are used. These are in the form of a press on which individual guns carrying electrode tips are mounted. Welds are made in a sequential order so that all electrodes are not carrying current at the same time.

Projection welding (RPW) is a resistance welding process which produces coalescence of metals with the heat obtained from resistance to electrical current through the work parts held together under pressure by electrodes.

The resulting welds are localized at predetermined points by projections, embossments, or intersections. Localization of heating is obtained by a projection or embossment on one or both of the parts being welded. There are several types of projections: (1) the button or dome type, usually round, (2) elongated projections, (3) ring projections, (4) shoulder projections, (5) cross wire welding, and (6) radius projection.

The major advantage of projection welding is that electrode life is increased because larger contact surfaces are used. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly.

Resistance seam welding (RSEW) is a resistance welding process which produces coalescence at the faying surfaces the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The resulting weld is a series of overlapping resistance spot welds made progressively along a joint rotating the electrodes. When the spots are not overlapped enough to produce gaslight welds it is a variation known as roll resistance spot welding. This process differs from spot welding since the electrodes are wheels. Both the upper and lower electrode wheels are powered. Pressure is applied in the same manner as a press type welder. The wheels can be either in line with the throat of the machine or transverse. If they are in line it is normally called a longitudinal seam welding machine. Welding current is transferred through the bearing of the roller electrode wheels. Water cooling is not provided internally and therefore the weld area is flooded with cooling water to keep the electrode wheels cool.

In seam welding a rather complex control system is required. This involves the travel speed as well as the sequence of current flow to provide for overlapping welds. The welding speed, the spots per inch, and the timing schedule are dependent on each other. Welding schedules provide the pressure, the current, the speed, and the size of the electrode wheels.

This process is quite common for making flange welds, for making watertight joints for tanks, etc. Another variation is the so-called mash seam welding where the lap is fairly narrow and the electrode wheel is at least twice as wide as used for standard seam welding. The pressure is increased to approximately 300 times normal pressure. The final weld mash seam thickness is only 25% greater than the original single sheet.

Flash Welding (FW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces, by the heat obtained from resistance to electric current between the two surfaces, and by the application of pressure after heating is substantially completed.

Flashing and upsetting are accompanied by expulsion of metal from the joint. During the welding operation there is an intense flashing arc and heating of the metal on the surface abutting each other. After a predetermined time the two pieces are forced together and coalescence occurs at the interface, current flow is possible because of the light contact between the two parts being flash welded.

The heat is generated by the flashing and is localized in the area between the two parts. The surfaces are brought to the melting point and expelled through the abutting area. As soon as this material is flashed away another small arc is formed which continues until the entire abutting surfaces are at the melting temperature. Pressure is then applied and the arcs are extinguished and upsetting occurs.

Upset welding (UW) is a resistance welding process which produces coalescence simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area where those surfaces are in contact.

Pressure is applied before heating is started and is maintained throughout the heating period. The equipment used for upset welding is very similar to that used for flash welding. It can be used only if the parts to be welded are equal in cross-sectional area. The abutting surfaces must be very carefully prepared to provide for proper heating.

The difference from flash welding is that the parts are clamped in the welding machine and force is applied bringing them tightly together. High-amperage current is then passed through the joint, which heats the abutting surfaces. When they have been heated to a suitable forging temperature an upsetting force is applied and the current is stopped. The high temperature of the work at the abutting surfaces plus the high pressure causes coalescence to take place. After cooling, the force is released and the weld is completed.

Percussion welding (PEW) is a resistance welding process which produces coalescence of the abutting members using heat from an arc produced by a rapid discharge of electrical energy.

Pressure is applied progressively during or immediately following the electrical discharge. This process is quite similar to flash welding and upset welding, but is limited to parts of the same geometry and cross section. It is more complex than the other two processes in that heat is obtained from an arc produced at the abutting surfaces by the very rapid discharge of stored electrical energy across a rapidly decreasing air gap. This is immediately followed by application of pressure to provide an impact bringing the two parts together in a progressive percussive manner. The advantage of the process is that there is an extremely shallow depth of heating and time cycle is very short. It is used only for parts with fairly small cross-sectional areas.

High frequency resistance welding (HFRW) is a resistance welding process which produces coalescence of metals with the heat generated from the resistance of the work pieces to a high-frequency alternating current in the 10,000 to 500,000 hertz range and the rapid application of an upsetting force after heating is substantially completed. The path of the current in the work piece is controlled by the proximity effect.

This process is ideally suited for making pipe, tubing, and structural shapes. It is used for other manufactured items made from continuous strips of material. In this process the high frequency welding current is introduced into the metal at the surfaces to be welded but prior to their contact with each other.

Current is introduced by means of sliding contacts at the edge of the joint. The high-frequency welding current flows along one edge of the seam to the welding point between the pressure rolls and back along the opposite edge to the other sliding contact.

The current is of such high frequency that it flows along the metal surface to a depth of several thousandths of an inch. Each edge of the joint is the conductor of the current and the heating is concentrated on the surface of these edges. At the area between the closing rolls the material is at the plastic temperature, and with the pressure applied, coalescence occurs.

RESISTANCE WELDING

Resistance welding is a process that takes advantage of a workpiece's inherent resistance to the flow of electrical current. As current is passed through the parts to be welded, the parts resist the passage of the current, thus generating the welding heat. A force is simultaneously applied, and the parts are joined together.

Unlike other forms of welding, resistance welding does not utilize additional materials such as fluxes and filler rods. The weld nugget is formed directly from the base materials.

Spot Welding

Spot welding is a process typically used in high-volume, rapid welding applications, such as those found in the automotive, appliance and aerospace industries, to join sheet metal up to 1/8 of an inch (3mm) in thickness. The advantages of this process are that it is not labor-intensive and can easily be automated.

In spot welding, the pieces to be joined are clamped between two electrodes under force, and an electrical current is sent through them. Resistance to the flow of current heats the material. Pressure is simultaneously applied to the joint, forming a solidified nugget that attaches the pieces.

Spot Welding Equipment

Equipment for spot welding is available in a wide range of power levels, welding forces, and hardware configurations.

The most commonly used power supply systems for resistance welders are AC transformers. However, single-phase DC, three-phase DC, three-phase frequency converter, and medium frequency DC power systems are also used.

Pneumatic cylinders are the most common type of forcing system. Alternate forcing systems can range from air-over-oil cylinders to hydraulic cylinders and spring-assisted systems.

As for hardware, spot welding systems are available in a range of configurations including pedestal, gun and hard tooling welders. Pedestal welders use standing frames to integrate the transformer and forcing systems. Gun welders are essentially portable systems for use with flexible automation. Hard tooling welders are built into other materials handling tooling.

Resistance spot welding is usually accomplished through voltage control, however, the current is conventionally controlled indirectly through the applied voltage. Control systems now allow direct control of the current and also can be used with monitoring systems to report process parameters.

The Benefits of Spot Welding

The advantages of spot welding are many and include the fact that it is:

• An economical process
• Adaptable to a wide variety of materials including low carbon steel, coated steels, stainless steel, aluminum, nickel, titanium, and copper alloys
• Applicable to a variety of thicknesses
• A process with short cycle times
• A robust process
• Tolerant to fit-up variations

Projection Welding

Projection welding is a variation of resistance welding in which current flow is concentrated at the contact surface of interest by an embossed or machined projection. Because the projection effectively localizes the current, resistance welding can be done in a wide range of applications not addressable by conventional resistance spot welding.

Projection welding is a variation of resistance welding that utilizes a projection to concentrate the current flow at the exact point where the joint is desired.

As mentioned, the function of the projection is to localize current flow at the contact surface where the joint is desired. While the projection usually collapses early in the weld cycle, the localized heating raises the material resistivity locally and promotes further heating and finally weld development at the initial contact point.

Because the formation of the weld is highly localized, the process is considerably more energy efficient than other resistance welding processes.

Projection Welding Equipment

Projection welding uses conventional resistance welding equipment, however, due to the uniqueness of projections, systems must be designed specifically for each application.

Virtually all types of resistance welding power supplies (AC, half-wave DC, full-wave DC, frequency inverter type, capacitive discharge, etc.) and configurations (straight-acting, series, pinch, etc.) are used with projection welding.

In selecting welding machines, care must be taken to accommodate both power demands and force requirements for the application. Because this type of welding is largely dependent upon the collapse of a projection, the force application provided by the welding head is vital to producing good quality welds.

Benefits of Projection Welding

• Ability to simultaneously perform multiple welds
• Reduced weld spaces
• Ability to join sheets of widely dissimilar thicknesses
• Can be used to minimize surface marking when joining sheets
• Increased energy efficiency

Seam Welding

Seam welding is similar to spot welding except that rotating wheel electrodes are used. The process is used when leak-tight welds or long strings of spot welds are required. Three forms of seam welding exist: standard seam, mash seam, and roll spot welding.

In standard seam welding, a series of overlapping weld nuggets are formed by rotating the wheel electrodes along the workpieces and firing a continuous series of current pulses. This action forms a continuous, leak-tight joint.

In mash seam welding, there is a small overlap of sheets, typically about one to two times the sheet thickness. Sheets are then mashed together, making a solid state joint. The resulting welded joint is generally 110-150% of the original sheet thickness. This final joint thickness can be reduced by postweld planishing.

In roll spot welding, the current pulses as wheels traverse the workpieces to form a line of separate spot welds (not a leak-tight scam).

Seam Welding Equipment

The equipment used in seam welding is similar to spot welding in both the power supplies and forcing systems. The difference lies in the rotating configurations of the wheel electrodes. Two basic wheel configurations are commonly used: stationary and traversing.

For stationary wheel machines, the workpiece moves through the wheels. The position of the wheels is stationary with respect to the frame of the machine, and the wheels can be oriented either parallel or perpendicular to the throat of the welding machine.

With traversing wheel machines, the workpiece is stationary with respect to the frame of the welder, and the wheels travel across the workpiece.

Cooling is critical for seam welding in order to control the geometry of the individual spots and prevent damage to the wheels. Both external and internal cooled wheels are used.

For external, or flood cooling, the weld is flooded with or immersed in water. For internal cooling, the wheels contain flow channels with cooling water circulating inside.

Benefits of Seam Welding

• Development of process requirements
• Welding of dissimilar steels
• Utilization of different electrode materials
• Investigation of new variants of the seam welding process
• Mash seam welding for tailored blanks applications