Resistance welding equipment is utilized by a group of welding processes in which the joining of metals is produced by the heat obtained from resistance of the work to the electric current, in a circuit of which the work is a part, and by the application of pressure. The three factors involved in making a resistance weld are the amount of current that passes through the work, the pressure that the electrodes transfer to the work, and the time the current flows through the work. Heat is generated by the passage of electrical current through a resistance current, with the maximum heat being generated at the surfaces being joined. Pressure is required throughout the welding cycle to assure a continuous electrical circuit through the work. The amount of current employed and the time period are related to the heat input required to overcome heat losses and raise the temperature of the metal to the welding temperature. The selection of resistance welding equipment is usually determined by the joint design, construction materials, quality requirements, production schedules, and economic considerations. Standard resistance welding machines are capable of welding a variety of alloys and component sizes. There are seven major resistance welding processes: resistance projection welding, resistance spot welding, resistance flash welding, resistance upset welding, resistance seam welding, resistance percussion welding, and resistance high frequency welding.
Principal Elements of Resistance Welding Machines
A resistance welding machine has three principal elements:
- An electrical circuit with a welding transformer and a current regulator, and a secondary circuit, including the electrodes which conduct the welding current to the work.
- A mechanical system consisting of a machine frame and associated mechanisms to hold the work and apply the welding force.
- The control equipment (timing devices) to initiate the time and duration of the current flow. This equipment may also control the current magnitude, as well as the sequence and the time of other parts of the welding cycle.
Electrical Operation. Resistance welds are made with either semiautomatic or mechanized machines. With the semiautomatic machine, the welding operator positions the work between the electrodes and pushes a switch to initiate the weld; the weld programmer completes the sequence. In a mechanized setup, parts are automatically fed into a machine, then welded and ejected without welding operator assistance. Resistance welding machines are classified according to their electrical operation into two basic groups: direct energy and stored energy. Machines in both groups may be designed to operate on either single-phase or three-phase power.
There are several types of spot welding machines, including rocker arm, press, portable, and multiple type. A typical spot welding machine, with its essential operating elements for manual operation, is shown in figure 5-39. In these machines, the electrode jaws are extended in such a manner as to permit a weld to be made at a considerable distance from the edge of the base metal sheet. The electrodes are composed of a copper alloy and are assembled in a manner by which considerable force or squeeze may be applied to the metal during the welding process.
Rocker arm type. These machines consist essentially of a cylindrical arm or extension of an arm which transmits the electrode force and in most cases, the welding current. They are readily adaptable for spot welding of most weldable metals. The travel path of the upper electrode is in an arc about the fulcrum of the upper arm. The electrodes must be positioned so that both are in the plane of the horn axes. Because of the radial motion of the upper electrode, these machines are not recommended for projection welding.
Press type. In this type of machine, the moveable welding head travels in a straight line in guide bearings or ways. Press type machines are classified according to their use and method of force application. They may be designed for spot welding, projection welding, or both. Force may be applied by air or hydraulic cylinders, or manually with small bench units.
Portable type. A typical portable spot welding machine consists of four basic units: a portable welding gun or tool; a welding transformer and, in some cases, a rectifier; an electrical contactor and sequence timer; and a cable and hose unit to carry power and cooling water between the transformer and welding gun. A typical portable welding gun consists of a frame, an air or hydraulic actuating cylinder, hand grips, and an initiating switch. The design of the gun is tailored to the needs of the assembly to be welded.
- Multiple spot welding type. These are special-purpose machines designed to weld a specific assembly. They utilize a number of transformers. Force is applied directly to the electrode through a holder by an air or hydraulic cylinder. For most applications, the lower electrode is made of a piece of solid copper alloy with one or more electrode alloy inserts that contact the part to be welded. Equalizing guns are often used where standard electrodes are needed on both sides of the weld to obtain good heat balance, or where variations in parts will not permit consistent contact with a large, solid, lower electrode. The same basic welding gun is used for the designs, but it is mounted on a special “C” frame similar to that for a portable spot welding gun. The entire assembly can move as electrode force is applied to the weld location.
- When spot welding aluminum, conventional spot welding machines used to weld sheet metal may be used. However, the best results are obtained only if certain refinements are incorporated into these machines. These features include the following:
- Ability to handle high current for short welding times.
- Precise electronic control of current and length of time it is applied.
- Rapid follow up of the electrode force by employing anti-friction bearings and lightweight, low-inertia heads.
- High structural rigidity of the welding machine arms, holders, and platens in order to minimize deflection under the high electrode forces used for aluminum, and to reduce magnetic deflections, a variable or dual force cycle to permit forging the weld nugget.
- Slope control to permit a gradual buildup and tapering off of the welding current.
- Postheat current to allow slower cooling of the weld.
- Good cooling of the Class I electrodes to prevent tip pickup or sticking. Refrigerated cooling is often helpful.
Projection Welding. The projection welding dies or electrodes have flat surfaces with larger contacting areas than spot welding electrodes. The effectiveness of this type of welding depends on the uniformity of the projections or embossments on the base metal with which the electrodes are in contact (fig. 5-40). The press type resistance welding machine is normally used for projection welding. Flat nose or special electrodes are used.
Seam Welding. A seam welding machine is similar in principle to a spot welding machine, except that wheel-shaped electrodes are used rather than the electrode tips used in spot welding. Several types of machines are used for seam welding, the type used depending on the service requirements. In some machines, the work is held in a fixed position and a wheel type electrode is passed over it. Portable seam welding machines use this principle. In the traveling fixture type seam welding machine, the electrode is stationary and the work is moved. Seam welding machine controls must provide an on-off sequencing of weld current and a control of wheel rotation. The components of a standard seam welding machine include a main frame that houses the welding transformer and tap switch; a welding head consisting of an air cylinder, a ram, and an upper electrode mounting and drive mechanism; the lower electrode mounting and drive mechanism, if used; the secondary circuit connections; electronic controls and contactor; and wheel electrodes.
Upset and Flash Welding. Flash and upset welding machines are similar in construct ion. The major difference is the motion of the movable platen during welding and the mechanisms used to impart the motion. Flash weld-fig is generally preferred for joining components of equal cross section end-to-end. Upset welding is normally used to weld wire, rod, or bar of small cross section and to join the seam continuously in pipe or tubing. Flash welding machines are generally of much larger capacity than upset welding machines. However, both of these processes can be performed on the same type of machine. The metals that are to be joined serve as electrodes.
A standard flash welding machine consists of a main frame, stationary platen, movable platen, clamping mechanisms and fixtures, a transformer, a tap switch, electrical controls, and a flashing and upsetting mechanism. Electrodes that hold the parts and conduct the welding current to them are mounted on the platens.
Upset welding machines consist of a main frame that houses a transform and tap switch, electrodes to hold the parts and conduct the welding current, and means to upset the joint. A primary contactor is used to control the welding current.
Percussion Welding. This process uses heat from an arc produced by a rapid discharge of electrical energy to join metals. Pressure is applied progressively during or immediately following the electrical discharge. The process is similar to flash and upset welding. Two types of welding machines are used in percussion welding: magnetic and capacitor discharge. A unit generally consists of a modified press-type resistance welding machine with specially designed transform, controls, and tooling.
High Frequency Welding. This process joins 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 completed. The process is entirely automatic and utilizes equipment designed specifically for this process.