The following points highlight the five main types of resistance welding. The types are: 1. Spot Welding 2. Projection Welding 3. Seam Welding 4. Butt Welding 5. Percussion Welding.
Type # 1. Spot Welding:
This is probably the simplest form of resistance welding and best suited for ordinary sheet steel. 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 spot welding there are five zones of heat generation, viz., one at interface between the two sheets, two at the contact surfaces of the sheets with electrodes and other two in the metal of the sheets.
It is very much desirable for obtaining good weld that proper heat balance should be there. It is used for welding both ferrous and non-ferrous metals upto 8 mm thickness. The surfaces to be welded should be thoroughly cleaned by pickling or shot blasting to obtain strong welds.
The two pieces to be joined by spot welding are placed between the two electrodes and then electrodes are pressed against each other (by pressing a foot lever) so as to cause desired pressure (700—1000 kg/cm2) on the sheets. The cooling water is turned on, The cycle that follows further is as below.
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A current of low voltage and sufficient amperage (120—300 amperes per mm2) is passed between the electrodes causing the two parts to be brought to welding temperature and thus the metal under electrodes pressure is squeezed and welded. The duration of current is for about 0.3 to 2 seconds and this time is known as weld time. After this the current is switched off while the pressure is still acting.
The pressure is applied till the weld cools and regains sufficient strength. This period as known as hold time. The pressure is then released and work is removed from the machine and the next job is brought under the machine. This period is called off-time. The pressure is applied again, and this period till current is passed is called squeeze time. Efficient welds depend on the correct combination of pressure, current and cycle time.
Materials of higher thermal conductivity, like aluminium, require higher currents for shorter periods than materials of lower conductivity. Many ductile materials and dissimilar metals can be spot welded if there is not too great a difference in the conductivities and melting points of the two materials. Correct electrode diameter needs to be chosen depending on the material thickness to be joined.
The structure of the weld in welded zone (called weld nugget—Refer hatched area in Fig. 9.20) consists of hemispherical regions on both the sheets.
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For lap joints the nugget diameter D should be, D = 2t + 2.5 mm (t = thickness of sheet in mm)
Lap distance L = 2 D + 4 mm; Spacing between spot welds = 16t.
Penetration of weld nugget into the base metal varies between 0.4 to 0.7 t.
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For 0.3 mm thick mild steel sheet, the electrode tip diameter is 3 mm, electrode force 540 Newtons, welding current 6000 amperes, and welding time 4 cycles ((4/50) sec. with 50 Hz supply). The corresponding values for 2 mm and 6 mm thick mild steel sheets are 7 mm, 2780 N, 11000 A, 20 cycles; and 12 mm, 8900 N, 21000 A and 60 cycles respectively.
The electrodes for resistance welding are made of high strength, high conductivity precipitation hardened 1% chromium copper alloy or 2% beryllium copper alloy. The tip diameter of electrode is around √t.
The various spot welding machines may be classified as:
(1) Stationary single-spot machine:
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(a) Rocker arm type,
(b) Direct pressure type.
(2) Portable single-spot machine.
(3) Multiple spot machine.
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The one most commonly used spot welder is stationary simple spot rocker arm type and is shown schematically in Fig. 9.19. This is very simple and cheap type and is very suitable for most of the small size works requiring less heating. The pressure between the electrodes is applied by pressing the foot lever which rocks the upper arm down.
For works which cannot be brought to the machine, portable spot welders are used. The transformer is generally placed at one place and spot welder is moved to work using long and flexible cables. Pressure is generally applied manually. For production work it is desirable to use multiple spot welding machines which have two or three electrodes arranged at appropriate places.
The process of spot welding is very suitable for mass production and can be used for most of ferrous and non- ferrous alloys by changing the current required to complete the weld. This is limited to thin parts and cannot be used for those parts which are more than 10 mm thick as heavy force is required for welding them.
It is noted that expansion of electrodes occurs which directly affects the quality of weld. If current density is too low, then expansion rate is slow and weld is insufficiently fused. If current density is high, then weld splashes and the pressure on the electrodes causes the weld to collapse at an early stage, thus resulting in a poor weld.
Use of Computer in Spot Welding:
The objective of use of computer in any production process is to produce a more consistent product of higher quality at a lower cost. In spot welding, when the weld solidifies, strain energy develops and it has characteristics that change with discontinuities like porosity and residual strain. This energy can be sensed by a transducer as an acoustic event (acoustic emission).
Emission characteristics generated by a suitable spot weld are stored and profiled in a computer memory. The emissions characteristics of other welds are compared with those of ideal as a test of weld integrity.
This method can be used for inspection (acceptance/rejection of weld) and also for modifying weld parameters to produce welds closer to ideal welds. It is due to fast response of computer that comparison between profiles and acoustic emission of ideal and actual welds can be made and corrective timely action taken.
Type # 2. Projection Welding:
This is a slight modification of spot welding. In this process, current and pressure are localised at the weld section by the use of embossed or coined projections of diameter equal to thickness of sheet on one or both pieces of work. The depth of projection is about 60% of sheet thickness.
The work is held between two copper platens and pressure is applied by the movable arm, the fixed arm supporting the work. The current is then passed and good welds at all points of contact are made due to flattening out of the projections under heat and pressure.
The advantage of this process is that a large number of welds can be carried out simultaneously by having large number of projections; of course limitation being due to ability of press to furnish and equally distribute the pressure to the work. Another advantage is that electrode life is long as only flat surfaces are required and little less current is required and little maintenance is needed.
Spacing of welds can be reduced. If welds are made close together then a proportion of the welding current tends to shunt through the previous weld and causes a weak joint. The sequence of operation is same as for spot welding. The initial cost is high as press type machines are required for this type of welding. This process is also suited to nearly all types of metals.
It permits a greater variety of metals and sheet thickness to be joined than the spot welding. It order to enable equalise the heat losses, the projections are formed on the thicker components or on the component having the higher thermal conductivity.
Type # 3. Seam Welding:
Seam welding is used for making continuous welds between two overlapping pieces of sheet metals. In this process the current is not passed continuously but is regulated by a timer. The work to be welded is placed between the two copper alloy wheels which apply sufficient pressure between the sheets and also carry sufficient current for producing continuous welds.
The heat is generated due to passing of the 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 heat rate is high then the speed of rollers is increased thereby reducing the weld time and vice versa.
The electrodes are made of copper alloys and are refrigerant circulated in order to dissipate heat from them. Seam welding is generally used where a water or gas-tight joint is required. In high speed seam welding using continuous current, the frequency of the current acts as an interrupter.
In seam welding, a series of over-lapping spot welds are formed as shown in Fig. 9.22 and these have sufficient overlap to provide a pressure-tight joint. However the spacing of these spots can be regulated also by pulsing the current at regular intervals (stitch welding). The major application of seam welding is in manufacture of seam welded pipes (ERW pipes).
Problem:
Calculate the welds per minute, work speed, and RPM of circular electrode of 220 mm diameter for carrying out seam welding at 4 welds per cm on 1.6 mm thick mild steel tube. Welding cycle consists of 3 cycles ‘on’ and 2 cycles ‘off. Power supply is at 50 Hz. Also calculate energy requirement assuming effective resistance between electrodes as 100µ ohm.
Solution:
Type # 4. Butt Welding:
In butt-welding, the two pieces of metal of same cross-section are gripped together and pressed while heat is generated in the contact surface by electrical resistance when the current is passed. As pressure keeps on acting continuously, the joint is upset slightly which has to be rounded up by machining or grinding.
It is desirable that both parts are of same resistance in order to have uniform heating at the joint. In order to have good weld, first the pressure must be less and then increased steadily to a value sufficient of effect weld. In this process there is no flashing or arcing at the joint during the operation. This process is best suited to rods, pipes and many other parts of uniform cross-section.
Butt welding can be sub-divided into two groups, i.e.,
i. Upset butt welding and
ii. Flash butt welding.
In the first type, the parts to be welded are clamped edge to edge in clamps of the machine and brought together with light contact while current flows to heat the joint. The pressure applied upsets the joint. Upset butt welding is used principally for non- ferrous metals.
In the flash butt welding, the parts are brought together in a very light contact. A high voltage starts a flashing action. The parts keep on moving against each other till forging temperature is reached and then sufficient pressure is applied to effect the weld. The pressure squeezes out from the joint faces any unwanted slag, oxides and overheated metal.
Due to upsetting action, slight bulging occurs around the weld. In this process it is very important to have proper timing and current for the size and section of parts used. Usually large areas are welded by flash welding process.
This process requires less current and is quite rapid. Most of the non-ferrous metals except those containing high percentages of lead, zinc, tin and copper can be welded satisfactorily. It is used for welding bars, tubes, and extruded sections.
Type # 5. Percussion Welding:
This is a recent development in the field of welding which depends on the arc effect for heating and not on the resistance. One of the two pieces to be welded is held in a stationary holder and the other in a clamp mounted in a slide and backed up by a heavy spring pressure.
For welding, the movable clamp is released. When pieces are very close to each other a sudden discharge of electric energy takes place causing an intense arcing over the surfaces and heating them. As piece 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.
The action of the process is so rapid that there is little heating effect in the material adjacent to the weld. It is used for welding satellite tips to tools, copper to aluminium or stainless steel, silver contact tips to copper. The equipment used for this process is quite expensive as it must be extremely rugged and provided with accurate holding fixtures and sensitive timing devices etc.