Resistance welding is a process in which the welding heat is generated in the parts to be: joined by resistance of the parts to the flow of an electric current. Spot welding, seam welding and flash welding are forms of resistance welding.
All the aluminum alloys can be resistance welded. Because the physical characteristics of aluminum are different than those of steel, somewhat different equipment may be required, although modified equipment is often adapted with excellent results. More electrical capacity is usually required for aluminum than for steel.
Advantages of resistance welding are low cost, high production speed, and automatic operation. The major disadvantage is the high initial cost of the equipment. Consequently, resistance welding is generally confined to mass production items where the low cost per weld will offset the high cost of the equipment.
Three types of resistance welding equipment are used for spot and seam welding aluminum alloys. These are classified on the basis of the electrical system supplying welding current as follows: standard alternating current (ac), energy storage, electromagnetic, and energy storage, electrostatic. Electrostatic welding may be either magnetic or condenser energy storage. The comparative current and pressure cycles for these systems are shown in Figure A-4.
Since aluminum and its alloys have comparatively high thermal and electrical conductivities, high welding currents and relatively short welding times are required in spot welding.
In the widely used alternating-current method for spot welding, the high welding current required is obtained out of the secondary coil of a welding transformer having a turns ratio in the range of 20:l to 100:l. The primary coil is usually connected to either 230 or 460– volt, 60 Hz power supply. An electronic control is used to time the application of welding current ranging of 1 to 30 cycles.
Current Regulation
The secondary current required varies with the thickness of the material to be welded, as shown in Table A-5. To obtain the correct current, an electronic control adjusts the current in steps of approximately 1000 amperes. Taps either on the primary of the welding transformer, or on a separate auto-transformer may be used. Where necessary, a series-parallel switch is provided on the welding transformer primary to permit adjustment of the current down to 25% of the maximum, which is usually sufficient to cover the normal range of material thickness.
Timing- The welding time is controlled by means of a switch in the supply line to the welding transformer primary. Both mechanically operated and magnetically operated welding contactors have been used for this purpose, but modern machines use solid state switches. Such devices should control the welding time to the values listed in Table A-5, with an accuracy of plus or minus one cycle. Improved welds result when the controls are adjusted to close the circuit at a uniform point in the voltage wave, and to open the circuit when the welding current passes through zero. However, some variations of this ideal condition is permissible for welding most of the aluminum alloys.
Electronic equipment for controlling the duration of welding current is widely used with alternating-current welding machines. When these machines are provided with means to start the flow of current in synchronism with the supply voltage, the consistency of weld strength and the appearance of the welds are improved over that obtained when less precise timing equipment is used. Electronic timing equipment for controlling the magnitude as well as the duration of welding current provides a smooth adjustment of the welding heat.
Current Demand- One of the chief objections to alternating-current spot-welding machines is that the high currents required for aluminum welding place a very high electrical demand on the system supplying the machines. This current demand is of intermittent nature, single-phase, and of very low power factor, and may cause disturbances in electric lights and other electric equipment. This condition can be alleviated to a large extent by installing static condensers in series with the primary of the welding transformer. The manufacturer of the welding equipment should be consulted to determine the size and number of condensers required.
Magnetic-Energy Storage Welding
The electrical current demand for spot welding aluminum can be reduced even further by using magnetic- energy storage equipment, which stores the welding energy in an inductor transformer by establishing a direct current of 100 to 400 amperes in the primary winding of this transformer. On interruption of the current by a contactor, a high value of current is established in the secondary circuit and through the work being welded. This current decays to a low value in 0.01 to 0.05 second.
Equipment for this process also has an electrode pressure system which permits the welding pressure to be varied during the welding operation. The combination of a short duration welding current impulse and a varying pressure results in welds of very sound structure and good appearance.
The maximum power demand for magnetic energy storage equipment is about one-tenth that required for alternating-current equipment, but this system can weld the same thickness of material because the energy is obtained by drawing a lower power for a longer time.
Condenser-Energy Storage Welding
The condenser-energy storage equipment utilizes static condensers to store the energy used for welding. Three-phase primary power is stepped up in voltage and rectified to charge the condensers to voltages of 1000 to 3000 volts. When this bank of charged condensers is connected to the primary of the welding transformer, an impulse of welding current rises rapidly to its maximum value and decays to zero at a somewhat slower rate. When welding with this equipment, a constant high value of welding pressure is generally used. In some cases a higher pressure is used at the end of the weld to provide a forging action on the solidified weld metal.
Welds produced on this type equipment are excellent in appearance and the structure is very sound. Another advantage is that the maximum demand on the power system is about one-tenth of that required for a-c welding equipment to join the same thickness of material.
Electrodes
The correct selection of electrode shape and the maintenance of this shape in production is essential to achieving consistent spot welds on aluminum. Welding electrodes serve three functions:
1) They conduct the welding current into the parts being welded.
2) They exert sufficient pressure on the material to hold it in place.
3) They conduct the heat out of the parts welded to aid in the prevention of outside materials being reached by the weld zone.
At least one of the electrodes must be shaped so that current will be highly concentrated in the weld. This electrode may be dome shaped with a 25 to 50 mm (1 to 2 in.) radius, or it may be conical with a 158″ to 166″ included cone angle. Another tip shape often used with the energy storage welding processes consists of a truncated cone with a 160″ to 130″ cone angle and a glat spot with a diameter equal to twice the thickness of the weld materials, plus 3 mm (1/8 in.).
The same sahpe electrode can be used on the other side of the work, or a flat electrode can be used on one side of the work to obtain a surface with the minimum of electrode marking. These flat electrodes may be in the range of 16 mm to 30 mm (5/8 in to 1-1/4 ibn.) in diameter. A further increase in diameter does not improve the appearance of the weld.
The electrodes must be of sufficient diameter to carry the required welding currents without undue heating. A 16 mm (5/8 in.) diameter electrode is suitable for currents up to 35,000 amp, and a welding time of 15 cycles when the rate of welding is not more than 40 welds per minute. When higher welding currents or greater welding speeds are used, electrodes of 22 to 30 mm (7/8 in to 1-1/4 in.) diameter should be used. For welding currents less than 20,000 amps and welding times less than 8 cycles, 12 mm (1/2in.) diameter electrodes are satisfactory.
A coating of aluminum alloy gradually forms over the face of the electrode. This alloy “pickup” is of low electrical conductivity, and eventually causes the electrodes to stick to the work and to melt the surface of the base material. The pickup can be removed off the electrodes with No.160 or No.240 abrasive cloth, but in removing pickup off the dome-shaped electrodes, it is important to maintain the original electrode shape.
On alternating-current welding machines, using dome or cone shaped electrodes, pickup must be removed off of the tips after 15-80 welds, depending on the material welded. On energy-storage equipment using the truncated cone electrodes, less pickup is formed, and 60-300 welds may be made before the electrodes require cleaning. The tip cleaning operation can require 2-3 seconds.
Equipment for seam welding aluminum is similar to a-c spot-welding equipment except that the electrodes are replaced by roller electrodes in the ranges of 10 to 16 mm (3/8 to 5/8 in.) thick and 15 to 22 cm (6 to 9 in.) in diameter. One or both of these wheels are trimmed to an included “V” angle of 158″ to 166″, or a 25 to 50 mm (1 to 2 in.) radius to concentrate the current in the weld. The wheels and the work are cooled by a water flow of 8 to 12 L/min (2 to 3 gal/min), directed against the periphery of the wheel near the weld. Usually one of the wheels is driven at an adjustable constant speed of 30 to 150 cm/min (12 to 60 in./min). It is essential in seam welding that the electronic timing control initiate and close off the weld current in synchronism with the supply voltage.
Aluminum alloys in the form of sheet, tubing, extrusions, and rolled bar can be butt- or miter-flash welded to form joints of equal or greater strength than those produced by fusion welding. In flash welding, the parts to be joined are securely clamped in dies on the welding machine, and an electric arc is established between the ends of the parts to be welded. This arc is maintained by placing the parts together as the aluminum material is consumed in the arc. When the ends of the parts are sufficiently heated by this arcing process, the weld is made by rapidly driving the heated ends together with sufficient pressure to hold the material in intimate contact until the weld metal has cooled.
Equipment- So that no arcing occurs when welding aluminum, the flash-welding machine must have sufficient transformer capacity to supply a current density of 15 500 amp/cm2 (100 000 amp/in2.) within the section welded, when the parts are in firm contact. The secondary voltage of the flash-welding transformer can be in the range of 2 to 20 volts. The machine must be equipped with appropriate dies and die-clamping devices to securely hold the parts being welded to prevent slipping during the upsetting action which takes place when the weld is formed. One of the clamping dies must be driven toward the other with an accelerated motion to establish and maintain the flashing, and to obtain a very rapid upset motion at the end of the flashing period. The mechanism for driving the movable die must be sufficiently rigid and strong to upset the largest area of section to be welded.
Clamping Dies- Dies are made with hard-drawn copper or copper alloys. Water cooling is not required except on very high production machines. The clamping dies should securely contact at least 80% of the outside circumference of the part to be joined. The length of the dies is usually in the range of 25 to 50 mm (1 to 2 in.) and is limited only by the possibility of crushing the material if too small a die length is used. In addition to holding the parts, the die blocks serve as a means of conducting electric current into the parts being welded and of conducting heat out of the parts during the welding process. A secure electrical connection between one of the dies contacting at least 40% of the circumference of the part must be made.
Flashing- The duration of the flashing motion must be sufficient to permit adequate coverage by the arc of the entire section welded. Considerable variation can be tolerated in both the amount of material flashed off and the time of flashing, providing a uniform, steady flash is maintained. Total material flashed off both pieces varies, starting at 6 mm (1/4 in.) for small diameter wires, and up to 18 mm (3/4 in.) for large diameter rod. Flashing times in the range of one-half to one second are used, although the flashing time can be reduced to as low as 1/20 second, if sufficient current is available to maintain flashing.
Welding Current- Welding current is adjusted by varying the secondary voltage applied to the dies. It is usually done with taps on the primary of the welding transformer. An adjustment which provides an upset current of about 15 500 amp/cm2 (100 000 amp. Per in.2) is used. The current obtained during flashing ranges between 1/5 to 1/3 of the current, which flows after the parts have come into good contact during the upset.
Welding Time- The transformer is energized before the parts to be welded have come into contact and is de-energized by opening a contactor (or by other means) in the primary supply to the welding transformer. The time relation between the beginning of the upset motion and the cutoff of power to the welding transformer is the most critical adjustment in the flash welding of aluminum. The current is removed after 1 to 5 cycles following the initiation of the upset cycle. The time delay of mechanical current interruptions is critical. If the current is shut off too early, oxide inclusions occur in the welds; if it is shut off too late, overheating of the weld and low weld strength are the result.
Costs- The economics of constructing special dies to hold the parts, and the time and material necessary to adjust the machine for production are such that 500 to 1000 joints can usually be required to justify the cost of setting up the flash-welding process. Production rates starting at 60 and ranging to 200 welds per hour can be obtained, depending on methods used in clamping the parts. The actual welding operation lasts only one second.
Finishing the Welds- Chipping or grinding methods are used to remove the excess upset material to finish the weld. Welds finished and treated by the anodizing process exhibit only a narrow line of slight discoloration at the weld.