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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.
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.