The following points highlight the four main types of resistance welding. The types are: 1. Spot Welding 2. Projection Welding 3. Seam Welding 4. Percussion Welding.
Type # 1. Spot Welding:
By definition, spot welding is a form of resistance welding in which the parts or pieces are joined in spots, accompanied by heated relatively small sections of the parts or pieces between suitable electrodes under pressure.
Spot welding is perhaps the most widely used form of resistance welding. It is characterized by low cost, speed, and dependability making it a common electrical resistance welding process.
Spot welding is usually used for joining or fabricating sheet metal structure. This type of welding only provides mechanical strength and is neither airtight nor watertight. For obtaining good welds having strength, and to avoid the excessive heating effects due to foreign films on sheet surfaces, it is desirable to clean the sheets thoroughly before welding. In this type of welding the workpieces to be welded are pressed together by mechanical pressure exerted through electrodes (made of pure copper or alloy of copper) and then supply is switched on for a particular period.
The mechanical pressure may be developed by a foot lever or by air pressure or by hydraulic cylinders. The metal in the zone of pressure gets up heated to fusion and the joint thus made gets cooled under pressure. The pressure remains applied till the weld cools and regains sufficient strength. Current required is above 5,000 A and the voltage between the electrodes is usually less than 2 volts.
The open- circuit voltage is less than 12 volts. The time period of flow of current varies widely depending upon the thickness of sheets, kinds of metal etc. For thin sheets this period is about 1/50 second for each 0.25 mm of total thickness of the two sheets to be joined.
In order to avoid electrode sticking it is necessary to avoid heating of junction faces between electrodes and work.
This is achieved:
(i) By using water-cooled electrodes and
(ii) By employing electrodes made of material having high electrical as well as thermal conductivity.
In order to minimise the wear and tear at the tip of the electrodes it is necessary to employ electrodes made of material having high mechanical strength.
Efficient spot welds are possible only by a correct combination of current, pressure, and welding time. The heat developed being proportional to the product of the welding time and the square of the current, it is possible to obtain good welds by employing either low currents for a longer duration or higher currents for a shorter duration. Longer welding time usually produces stronger welds but this involves lot of distortion of workpieces, electrode maintenance and high energy expenditure.
The welding pressure required is determined on the basis of a specific electrode pressure of 7 kg per sq. mm over the electrode tip area. In view of the fact, however, that plates will not always be perfectly flat, a certain additional pressure is usually recommended, which will overcome slight distortion of the plates without the resultant necessary welding pressure.
For obtaining good joints spot welding machine, which makes the welding operation independent of the operator by automatically controlling welding current, pressure, and timing, once the same have been set correctly, may be employed. By toggle mechanism electrodes are pressed on the work and according to the requirement the current can be turned on and off.
In certain machines the electrodes of spot welders are equipped with water cooling arrangement. In such cases the electrodes are made hollow and the water is circulated through them. Spot welding machines are of capacities ranging from 10 to 150 kVA.
Spot welding is applicable for joining components made from plate material, the plate thickness being limited (usually to 10 or 12 mm) by the pressure and current capacity of the available spot welding machine. It has an advantage over projection welding in cases where the number of welds required is relatively small, where the size of the job is too large for the capacity of projection welders, and where the design allows the necessary spacing and arrangement of the welds to be obtained. The lower cost of the equipment makes spot welding economical for small quantities.
Type # 2. Projection Welding:
Projection welding is a resistance welding process that produces coalescence by the heat obtained from the resistance to the flow of the welding current. The resulting welds are localized at predetermined points by projections, embossments or intersections.
Projection welding uses the same equipment as spot welding. The only difference is that the electrodes used are flat on the ends and larger in diameter than the flat electrodes used in spot welding. Successful projection welding depends greatly on the surface preparation of the pieces to be welded.
Projections, small deformations that will touch the surface of the material to be welded, are made on the weld areas. One of the main advantages of these projected points is that welding area can be located easily, which makes projection welding a high-production welding technique.
As the current flows through the two parts to be welded, the projected points are the main contact area. Whenever a point or projection touches the metal, a weld nugget will start. These weld points soon reach the plastic state, and the force applied by the electrodes finishes the weld nugget. The cycle time is the same as the spot-weld time. As the points reach their plastic state, the metal is compressed so that the finished weld is similar to the spot weld with the exception of the small indentations created by projections.
Projection welding, as spot welding, needs no protective atmosphere in order to produce successful results. It is also useful in securing small circular component, to larger pieces of metal. For projection welding, a large flat-surfaced electrode is used, which covers the areas of the projections.
The areas of projection, one or more, and the spots where they touch on the other piece of metal will be the only places where fusion takes place, even with a large electrode. Projection welding has been used by auto manufacturers for many years. Auto bodies have many areas that are projection welded. Another area that uses projection welding is stud welding, which has also been utilized for car bodies.
Projection welding reduces the amount of current and pressure required in order to form a good bond between two surfaces. With this reduction there is less chance of shrinkage and distortion in the areas surrounding the weld zone, one reason why it has been incorporated into many manufacturing processes.
Projection welding has the following advantages over spot welding:
(i) Projection makes the welding process simple.
(ii) It is possible to join several welding points in projection welding.
(iii) More output is obtained since more than one weld are done at a time.
(iv) Electrode life is increased due to use of low current density and low pressure.
(v) Good finished appearance is obtained as surface remains undented by the electrodes.
(vi) Welds are automatically located by the position of projections.
(vii) As the electrodes are flat, therefore the contact area over the projection is sufficient. It facilitates the easier maintenance of electrodes, alignments of parts and construction of various jigs and fixtures.
(viii) With the projection welding, it is easy to weld certain parts which cannot be welded by spot welding.
When the two plates to be welded are of different cross- sectional area then in order to obtain desirable strength it becomes necessary to have the projections on the thicker plate.
When the two plates to be welded are of different material then the projections should be made on the high conductivity materials.
The thickness of plates to be jointed is limited to 2 x 6 mm in case of steel, 2 x 4 mm in case of aluminium, and 2 x 2 mm in case of copper. This reduction in maximum thickness of different metals is due to increase in conductivity.
The initial cost of the equipment required for projection welding is high as press type machines are required for this type of welding.
This type of welding is usually employed on punched, formed or stamped parts, where the projections automatically exist.
Small fasteners, nuts etc. can be welded to larger components.
Projection welding is particularly applicable to mass production work, e.g., the welding of refrigerator condensers, crossed-wire welding, refrigerator racks, gratings, grills etc.
Type # 3. Seam Welding:
Seam welding can be defined as series of continuous spot welds. This process is employed for making a continuous joint between two overlapping pieces of sheet metal. In this process the work to be welded is placed between the two wheels which apply sufficient mechanical pressure and also carry sufficient current for producing continuous welds. The mechanical pressure applied is kept constant and the current is regulated by a timer (the current is kept on for a definite time and then off for another fixed interval).
As the pressure is applied, the drive is started, the welding current switched on and simultaneously the overlapping surfaces of the metal are forced together as fast as they are heated. The heat is generated due to flow of current through the resistance in the welding circuit. The heat generated can be controlled by either varying the current or pressure between the sheets which varies the contact resistance. If the rate of heat generation is high then the speed of the rollers is increased thereby reducing the welding time and vice versa.
The electrodes are made of copper alloys and are water circulated in order to dissipate heat from them. In high speed seam welding using continuous current the frequency of the current acts as an interrupter. In order to localise the current and pressure to the welding point, the contact area of the electrodes is kept small.
Usually numbers of spots obtained per metre of the joint are between 200 and 400 depending upon the nature of the joint required. Welding speed ranges from 0.25 metre to 10 metres per minute. This type of welding is very important as it provides pressure tight or leak proof joint.
It is very essential in case of seam welding that the surfaces to be welded are cleaned and made dust and dirt free. This is not necessary in other forms of resistance welding, e.g., in spot welding the detrimental effect of scale and surface dirt can be compensated for by adjustment of time, pressure, current or by other means but this is not true of seam welding.
Materials which can be seam welded after they are cleaned include high-carbon, stainless and coated steels and alloys of aluminium, nickel and magnesium. Seam welding of copper and high copper alloys is not recommended. Seam welding is employed for welding pipes, conduits, tanks, transformers, refrigerators, gasoline tanks, aircrafts and various types of containers. This process is best adopted for metal thickness ranging from 0.25 mm to 3 mm.
Type # 4. Percussion Welding:
It is a recent development in welding. It is a resistance welding process where in coalescence is produced simultaneously over the entire area of abutting surfaces by heat obtained from an arc produced by a rapid discharge of stored electrical energy. This is a self-timing spot welding method.
In this process, the pieces to be welded are held apart, one in a stationary holder and the other in a clamp mounted in a slide and backed up against heavy spring pressure. When the movable clamp is released, it moves rapidly carrying with it the piece to be welded. When the two pieces are about 1.5 mm apart, there is a sudden discharge of electrical energy, causing intense arcing between the two surfaces and heating them to high temperature. As pieces come in contact with each other under heavy pressure the arc is extinguished due to the percussion blow of the two parts and the force between them effects the weld.
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. The advantage of the process is that there is an extremely shallow depth of heating and the time cycle is very short. This process is so fast (takes about 0.1 s) that there is little heating effect in the material adjacent to the weld. It may be noted that this process is limited to only small areas (up to 3.2 cm2) of nearly regular sections.
Thin sheets of equivalent area cannot be joined by this process. It can be used for welding a large number of dissimilar metals. It is used for very specialized applications and the process is entirely automatic. Metals which can be percussion welded include, copper alloys, aluminium alloys, nickel alloys, low-carbon steel, medium-carbon steel and stainless steels.
This process is used for welding satellite tips to tools, copper to aluminium or stainless steel, silver contact tips to copper, cast iron to steel, lead-in-wires on electric lamps and zinc to steel. Gold, silver, copper-tungsten, silver-tungsten and silver-cadmium oxide percussion welded to copper alloys for commonly used assemblies for electric contacts.