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The ends to be welded touch each other before the current is switched on. A heavy current is then passed from one piece to another and the contacting faces are heated up due to the contact resistance. The two pieces are pressed together firmly after the desired welding temperature of 870 to 925 oC is reached. The pressing action which results in the increase in lateral dimension of the workpieces is called upsetting. Upsetting takes place both during and after the current flow. The upsetting action results in welding of end faces with squeezing of a part of the softened metal to form a fin, which can be removed later, if required, by machining.
Resistance butt welding is used for end joining of rods, tubes, bars and similar other sections. However, an important application of this process is the large scale production of tubes and pipes at a high rate of production and the process is then referred to as resistance butt-seam welding or simply as electric resistance welding (ERW). In ERW process, the strip for tube making is continuously edge shared and rolled into tube for forming a longitudinal seam.
Current of upto 40,000 amperes at 5 volt is introduced across the joint by split electrode rollers and the force is applied by the pressure rolls. In this process, both the work motion and current and supply are continuous. The flow of current through the shunt path is avoided or reduced by the use of ferrite or wrought iron ‘impeder’ placed inside the tube.
The maximum speed of production is controlled by the frequency of the current as that decides the number of zero-current periods per unit time. Frequencies upto 350 Hz are commonly used which results in the production rate of about 36 m/min. The fin formed due to extruded metal is continuously removed by cutters and the desired length of the tube or pipe is cut on the production table without any interruption in the process of welding.
This is a process of welding stud (a headless threaded bolt) or stud-like pieces (e.g. bolts, screws, rivets, rods, etc.) to flat workpieces like plates. Its a unique process which combines arc and forge welding processes and results in tremendous cost saving when compared to the conventional methods like drilling and tapping.
Stud welding was first used by British Navy in 1918 but its regular and extensive use started from 1938. There are four variations of the process viz., capacitor discharge stud welding, the drawn arc capacitor discharge stud welding, the consumable ferrule stud welding and the drawn arc stud welding. The last variation of the process is the most popular and the following description pertains to that only.
The main equipment for stud welding consists of a stud welding gun, a time control unit, a dc power source of 300 to 600 amperes current capacity, studs, and ceramic ferrules.
A stud is held in the welding gun and a ferrule is slipped on it. The stud is then made to touch the cleaned spot (shot blasted, ground, or wire brushed) where it is to be welded and the switch in the form of gun trigger is pressed and the process is completed in a couple of seconds. This necessitates the use of ultra-high speed power source to supply the desired welding current. A stud about 40 mm diameter requires about 5000 ampere current at 65 to 70 volts for 2 seconds. Therefore, motor-generator sets with their higher overload capacities are preferred over the rectifier welding sets.
For efficient results the plate on which the stud is to be welded must have the minimum thickness at least 20% that of the diameter of the stud, however for developing full strength it should not be less than 50% of the diameter of the stud base.
Current and Power Source Rating for Different Stud Sizes
|Stud Diameter (mm)||Current Required (amp)||Power Rating (amp)||Source Number|
Stud welding is used mainly for mild steel, low alloy steels, and austenitic stainless steels. Drawn arc stud welding is not used for non-ferrous metals but other variants of the process can be utilized for welding lead free brass, bronze, chrome plated metals and aluminum. However heat-treatable aluminum alloys are not recommended for stud welding.
Typical applications of stud welding include steel decks of ships, for attaching brackets, hangers, cover plates, conduits, piping, etc. to metal workpieces. The process also finds wide use in automotive rail road machinery manufacturing and construction industries.
Principle of Operation
Welding current, inert gas supply, water is turned on. Electric arc is struck between the non-consumable electrode and the work piece by touching the electrode with work piece or using a high frequency until. In 1st method arc is initially struck on a scrap metal piece and then broken by increasing arc length.
In 2nd method a high frequency current is supper-imposed on the welding current. Welding torch is brought near to the job when electrode tip reaches within a distance of 3 to 2 mm from job. A spark jumps across the air gap between the electrode and job. Air path gets ionized and arc is established.
TIG welding is also known as Gas Tungsten Arc Welding Machine (GTAW) both the AC and DC power source can be used for GTAW electrode employed varies in dia from 0.5 to 6.5 mm carrying current from 5A to 650A; welding torch used for carrying current higher than 1000A is normally water cooled. GTAW is all position (1G, 2G, 3G, 4G.6G) welding and gives the highest quantity weld amongst commonly used arc welding process.
Equipment used in TIG Welding Machine
a) Welding torch, tungsten electrode and filler rod.
b) Welding power source, high frequency unit, DC suppressor unit and cables.
c) Inert gas cylinder for shielding purpose, pressure regulator, flow meter.
d) Cooling water supply.
e) Gas and water solenoid valve.
Advantage of TIG Welding Machine
a) TIG welding is very suitable for high quality welding of thin material.
b) Deeper penetration can be achieved through this process.
c) No flux is used so there is no danger of flux entrapment.
d) As this process can be seen with the help of goggle therefore operator can exercise a better control on the welding process.
e) TIG is very suitable for welding of non-ferrous metals and stainless steel.
f) TIG welding is suitable for welding in all positions (1G, 2G, 3G, 4G, 6G).
a) MIG welding is much faster process as compared to TIG welding under similar application.
b) Tungsten if transfer to molten weld pool can contaminate the same.
c) Tungsten inclusion is hard and brittle.
d) TIG welding equipment is more costly as compared to other welding equipment.
e) Skill or trained operator is required for welding.
Application of TIG Welding Machines
a) Basically welding of aluminium, magnesium, copper, nickel and their alloys, carbon, alloys or stainless steel, high temperature and hard surfacing alloys such as zirconium, titanium etc.
b) Welding sheet metal and thinner section.
c) Precision welding in automatic energy aircraft chemical and instrument industries.
The process involves fusing two pieces of sheet metal together by penetrating entirely through one piece into the other. No joint preparation is required except proper cleaning of the overlap areas. The main operation in arc spot welding is to strike and hold an arc without travel at a point where the two parts to be joined are held tightly together.
A vented metal nozzle of a shape to suit the application is fitted to the MIG gun and is pressed against the workpiece at the desired area. The operation is carried out for a period of 1-5 seconds and a slug is melted between the parts to be joined. Timing is usually controlled automatically with the help of a timer. Thus, the process time can be varied to achieve welds of different sizes depending upon the thickness of the sheets. Arc initiation is a critical part of the process and therefore must be reliable and consistent. This is easy to achieve by a flat characteristics power source and clean surface of the work.
GMAW spot welding is a highly adaptable process which requires very little manipulative skill; does not require the use of a welding helmet. It is an extremely fast process and can be fully automated. Due to addition of extra metal the weld slug is free from piping defects. A wire composition different from the base metal may be used to control cracking, porosity, or strength. Argon and CO2 are shielding gases commonly used for GMAW spot welding.
GMAW arc spot process can be used more efficiently for downhand welding position. It can be successfully employed for horizontal position but fails for overhead welding position.
This process does not require a hole to be made in either member, thus in differs from plug welding in that respect. As the upper member is required to be melted through and through, its thickness is normally restricted to 3 mm. The thickness of the second member is not important. Through lap joints are the most often used type of joint for arc spot welding but fillet joints can also be successfully made by this process.
In this process the metal from the electrode wire scours deeply into the weld crater. This breaks up the oxide films at the faying surfaces so that the process can be used as successfully on aluminum as on mild, low alloys, and stainless steels.