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Principles of Arc Welding

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Principles of arc welding Arc welding is the welding process, in which heat is generated by an electric arc struck between an electrode and the work piece. Electric arc is luminous electrical discharge between two electrodes through ionized gas. Any arc welding method is based on an electric circuit consisting of the following parts: Power supply (AC or DC); Welding electrode; Work piece; Welding leads (electric cables) connecting the electrode and work piece to the power supply. 1 2 Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them. When a long join is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint. 3 Metal Inert Gas Welding (MIG, GMAW) Metal Inert Gas Welding (Gas Metal Arc Welding) is the arc welding process, in which the weld is shielded by an external gas (argon, helium, CO2, argon + oxygen or other gas mixtures). Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint. Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed. 4 5 Advantages of Metal Inert Gas Welding (MIG, GMAW): Continuous weld may be produced (no interruptions); High level of operators skill is not required; Slag removal is not required (no slag); Disadvantages of Metal Inert Gas Welding (MIG, GMAW): Expensive and non-portable equipment is required; Outdoor application are limited because of effect of wind, dispersing the shielding gas. 6 Tungsten Inert Gas Arc Welding (TIG, GTAW) Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is the welding process, in which heat is generated by an electric arc struck between a tungsten non-consumable electrode and the work piece. The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the molten metal from atmospheric contamination. The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, if required. Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used in the process. 7 8 Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): Weld composition is close to that of the parent metal; High quality weld structure Slag removal is not required (no slag); Thermal distortions of work pieces are minimal due to concentration of heat in small zone. Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): Low welding rate; Relatively expensive; Requires high level of operators skill. 9 Shielded Metal Arc Welding (SMAW) Shielded metal arc welding (Stick welding, Manual metal arc welding) uses a metallic consumable electrode of a proper composition for generating arc between itself and the parent work piece. The molten electrode metal fills the weld gap and joins the work pieces. This is the most popular welding process capable to produce a great variety of welds. The electrodes are coated with a shielding flux of a suitable composition. The flux melts together with the electrode metallic core, forming a gas and a slag, shielding the arc and the weld pool. The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation and stabilizes the arc. The slag is removed after Solidification. 10 11 Advantages of Shielded Metal Arc Welding (SMAW): Simple, portable and inexpensive equipment; Wide variety of metals, welding positions and electrodes are applicable; Suitable for outdoor applications. Disadvantages of Shielded Metal Arc Welding (SMAW): The process is discontinuous due to limited length of the electrodes; Weld may contain slag inclusions; Fumes make difficult the process control. 12 Submerged Arc Welding (SAW) Submerged Arc Welding utilizes a bare consumable metallic electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld. The arc heats and melts both the work pieces edges and the electrode wire. The molten electrode material is supplied to the surfaces of the welded pieces, fills the weld pool and joins the work pieces. Since the electrode is submerged into the flux, the arc is invisible. The flux is partially melts and forms a slag protecting the weld pool from oxidation and other atmospheric contaminations. 13 14 Advantages of Submerged Arc Welding (SAW): Very high welding rate; The process is suitable for automation; High quality weld structure. Disadvantages of Submerged Arc Welding (SAW): Weld may contain slag inclusions; Limited applications of the process - mostly for welding horizontally located plates. 15 Electro slag Welding (ESW) Electro slag Welding is the welding process in which the heat is generated by an electric current passing between the consumable electrode (filler metal) and the work piece through a molten slag, which covers the weld surface. Prior to welding the gap between the two work pieces is filled with a welding flux. Electro slag Welding is initiated by an arc between the electrode and the work piece (or starting plate). Heat, generated by the arc, melts the fluxing powder and forms molten slag. The slag, having low electric conductivity, is maintained in liquid state due to heat produced by the electric current. The slag reaches a temperature of about 3500°F (1930°C). This temperature is sufficient for melting the consumable electrode and work piece edges. Metal droplets fall to the weld pool and join the work pieces. Electro slag Welding is used mainly for steels. 16 17 Advantages of Electro slag Welding: High deposition rate - up to 45 lbs/h (20 kg/h); Low slag consumption (about 5% of the deposited metal weight); Low distortion; Unlimited thickness of work piece. Disadvantages of Electro slag welding: Coarse grain structure of the weld; Low toughness of the weld; Only vertical position is possible. 18 Resistance Welding (RW) Resistance Welding is a welding process, in which work pieces are welded due to a combination of a pressure applied to them and a localized heat generated by a high electric current flowing through the contact area of the weld. Heat produced by the current is sufficient for local melting of the work piece at the contact point and formation of small weld pool (”nugget”). The molten metal is then solidifies under a pressure and joins the pieces. Time of the process and values of the pressure and flowing current, required for formation of reliable joint, are determined by dimensions of the electrodes and the work piece metal type. AC electric current (up to 100 000 A) is supplied through copper electrodes connected to the secondary coil of a welding transformer. The following metals may be welded by Resistance Welding: Low carbon steels - the widest application of Resistance Welding Aluminum alloys Medium carbon steels, high carbon steels and Alloy steels (may be welded, but the weld is brittle) 19 Advantages of Resistance Welding: High welding rates; Low fumes; Cost effectiveness; Easy automation; No filler materials are required; Low distortions. Disadvantages of Resistance Welding: High equipment cost; Low strength of discontinuous welds; Thickness of welded sheets is limited - up to 1/4” (6 mm); Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks, domestic radiators, pipes of gas oil and water pipelines, wire ends, turbine blades, railway tracks. 20 The most popular methods of Resistance Welding are: Spot Welding (RSW); Flash Welding (FW); Resistance Butt Welding (UW) ; Seam Welding (RSEW). 21 Spot Welding (RSW) Spot Welding is a Resistance Welding (RW) process, in which two or more overlapped metal sheets are joined by spot welds. The method uses pointed copper electrodes providing passage of electric current. The electrodes also transmit pressure required for formation of strong weld. Diameter of the weld spot is in the range 1/8” - 1/2” (3 - 12 mm). Spot welding is widely used in automotive industry for joining vehicle body parts. 22 23 Flash Welding (FW) Flash Welding is a Resistance Welding (RW) process, in which ends of rods (tubes, sheets) are heated and fused by an arc struck between them and then forged (brought into a contact under a pressure) producing a weld. The welded parts are held in electrode clamps, one of which is stationary and the second is movable. Flash Welding method permits fast (about 1 min.) joining of large and complex parts. Welded part are often annealed for improvement of Toughness of the weld. Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper alloys and Nickel alloys may be welded by Flash Welding. Thick pipes, ends of band saws, frames, aircraft landing gears are produced by Flash Welding. 24 25 Resistance Butt Welding (UW) Resistance Butt Welding is a Resistance Welding (RW) process, in which ends of wires or rods are held under a pressure and heated by an electric current passing through the contact area and producing a weld. The process is similar to Flash Welding, however in Butt Welding pressure and electric current are applied simultaneously in contrast to Flash Welding where electric current is followed by forging pressure application. Butt welding is used for welding small parts. The process is highly productive and clean. In contrast to Flash Welding, Butt Welding provides joining with no loss of the welded materials.
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