Fig.(2-10): Shielded arc (a) overall process; (b) welding area enlarged.

Functions of Covering

The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below:

A. Protection: It provides a gaseous shield to protect the molten metal from air. For a cellulose-type electrode, the covering contains cellulose, (C6 H10O5 )x. A large volume of gas mixture of H2, CO, H2O, and CO2 is produced when cellulose in the electrode covering is heated and decomposes. For a limestone- (CaCO3) type electrode, on the other hand, CO2 gas and CaO form when the limestone decomposes. The limestone-type electrode is a low-hydrogen- type electrode because it produces a gaseous shield low in hydrogen. It is often used for welding that are susceptible to hydrogen cracking, such as high-strength .

B. Deoxidation: It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation.

C. Arc Stabilization: It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly.

D. Metal Addition: It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate.

Advantages and Disadvantages

The welding equipment is relatively simple, portable, and inexpensive as com- pared to other processes. For this reason, SMAW is often used for maintenance, repair, and field construction. However, the gas shield in SMAW is not clean enough for reactive metals such as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering tends to over- heat and fall off when excessively high welding currents are used. The limited length of the electrode (about 35cm) requires electrode changing, and this further reduces the overall production rate.

2-1-5 Flux Cored Arc Welding (FCAW)

Flux cored arc welding (FCAW) shown in figure (2-11) is an electric arc welding process which fuses together the parts to be welding by heating them with an arc between a continuously fed flux filled electrode wire and the work. Shielding is obtained through decomposition of the flux within the tubular wire (self-shielded method). Additionally shielding may be obtained from an externally supplied gas or gas mixture (gas shielded method). Equipment is similar to that used for (GMAW). The flux cored arc welding process can be used to weld and alloy steels, cast and wrought and stainless steels. The process is also capable of producing hard surfacing deposits. The process is commonly used to weld medium to thick steels because of the high deposition rate (up to 4 times greater than SMAW) obtained with the larger electrode diameters. Welding is normally limited to the flat and horizontal positions with large diameter wires. Smaller diameter wires are used in all positions. A layer of slag is left on the weld bead that must be removed after welding.

Fig.(2-11): flux cored arc welding

2-1-6 Gas-Tungsten Arc Welding (GTAW) (TIG)

The Process

Gas–tungsten arc welding (GTAW) is a process that melts and joins metals by heating them with an arc established between a non-consumable tungsten electrode and the metals, as shown in Figure (2-12). The torch holding the tungsten electrode is connected to a cylinder as well as one terminal of the power source, as shown in Figure (2-12 a). The tungsten electrode is usually in contact with a water- cooled tube, called the contact tube, as shown in Figure (2-12 b), which is connected to the welding cable (cable 1) from the terminal. This allows both the welding current from the power source to enter the electrode and the electrode to be cooled to prevent overheating. The workpiece is connected to the other terminal of the power source through a different cable (cable 2). The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is much better in GTAW than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. For this reason, also called tungsten–inert gas (TIG) welding. However, in special occasions a non-inert gas (Chapter 3) can be added in a small quantity to the shielding gas. Therefore, GTAW seems a more appropriate name for this welding process. When a filler rod is needed, for instance, for joining thicker materials, it can be fed either manually or automatically into the arc.

Fig.(2-12): Gas–tungsten arc welding (a) overall process; (b) welding area enlarged.

Polarity

Figure (2-13) shows three different polarities in GTAW (5), which are described next:

Fig.(2-13): three different polarities in GTAW.

A. Direct-Current Electrode Negative (DCEN): This, also called the straight polarity , is the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. As shown in Figure (2-13 a), electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced.

B. Direct-Current Electrode Positive (DCEP): This is also called the reverse polarity . The electrode is connected to the positive terminal of the power source. As shown in Figure (2-13 b), the heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, a large- diameter, water-cooled must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, as shown in Figure (2-14), knocking off oxide films and producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong oxide- materials such as aluminum and , where deep penetration is not required.

Fig.(2-14): Surface cleaning action in GTAW with DC electrode positive.

C. Alternating Current (AC): Reasonably good penetration and oxide cleaning action can both be obtained, as illustrated in Figure (2-13 c).This is often used for welding aluminum alloys.

Electrodes

Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current-carrying capacity, and resistance to contamination than pure tungsten electrodes. As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature required to emit electrons and hence a greater risk of melting the tip.

Shielding Gases

Both argon and helium can be used. The ionization potentials for argon and helium are 15.7 and 24.5eV (electron volts), respectively. Since it is easier to ionize argon than helium, arc initiation is easier and the voltage drop across the arc is lower with argon. Also, since argon is heavier than helium, it offers more effective shielding and greater resistance to cross than helium. With DCEP or AC, argon also has a greater oxide cleaning action than helium. These advantages plus the lower cost of argon make it more attractive for GTAW than helium.

Because of the greater voltage drop across a helium arc than an argon arc, however, higher power inputs and greater sensitivity to variations in the arc length can be obtained with helium. The former allows the welding of thicker sections and the use of higher welding speeds. The latter, on the other hand, allows a better control of the arc length during automatic GTAW.

Advantages and Disadvantages

Gas–tungsten arc welding is suitable for joining thin sections because of its limited heat inputs. The feeding rate of the filler metal is somewhat independent of the welding current, thus allowing a variation in the relative amount of the fusion of the base metal and the fusion of the filler metal. Therefore, the control of dilution and energy input to the weld can be achieved without changing the size of the weld. It can also be used to weld butt joints of thin sheets by fusion alone, that is, without the addition of filler metals or autogenous welding. Since the GTAW process is a very clean welding process, it can be used to weld reactive metals, such as titanium and zirconium, aluminum, and magnesium.

However, the deposition rate in GTAW is low. Excessive welding currents can cause melting of the tungsten electrode and results in brittle tungsten inclusions in the weld metal. However, by using preheated filler metals, the deposition rate can be improved. In the hot-wire GTAW process, the wire is fed into and in contact with the weld pool so that resistance heating can be obtained by passing an electric current through the wire.

Lecture No.12 Week No.12 No. of hours: 2 theoretical and 1 tutorial 2-1-7 (PAW)

The Process

Plasma arc welding (PAW) is an arc welding process that melts and joins metals by heating them with a constricted arc established between a tungsten electrode and the metals, as shown in Figure (2-15). It is similar to GTAW, but an orifice gas as well as a shielding gas is used. As shown in Figure (2-16), the arc in PAW is constricted or collimated because of the converging action of the orifice gas nozzle, and the arc expands only slightly with increasing arc length. Direct-current electrode negative is normally used, but a special variable- polarity PAW machine has been developed for welding aluminum, where the presence of aluminum oxide films prevents a keyhole from being established.

Arc Initiation

The tungsten electrode sticks out of the shielding gas nozzle in GTAW (Figure 2-12 b) while it is recessed in the orifice gas nozzle in PAW (Figure 2-15 b). Consequently, arc initiation cannot be achieved by striking the electrode tip against the workpiece as in GTAW. The control console (Figure 2-15 a) allows a pilot arc to be initiated, with the help of a high-frequency generator, between the electrode tip and the water-cooled orifice gas nozzle. The arc is then gradually transferred from between the electrode tip and the orifice gas nozzle to between the electrode tip and the workpiece.

Fig.(2-15): Plasma arc welding: (a) overall process; (b) welding area enlarged and shown with keyholing.

Fig.(2-16): Comparison between a gas–tungsten arc and a plasma arc.

Keyholing

In addition to the melt-in mode adopted in conventional arc welding processes (such as GTAW), the keyholing mode can also be used in PAW in certain ranges of metal thickness (e.g., 2.5–6.4mm). With proper combinations of the orifice gas flow, the travel speed, and the welding current, keyholing is possible. Keyholing is a positive indication of full penetration and it allows the use of significantly higher welding speeds than GTAW. For example, it has been reported that PAW took one-fifth to one-tenth as long to complete a 2.5-m-long weld in 6.4-mm-thick 410 stainless as GTAW. Gas–tungsten arc welding requires multiple passes and is limited in welding speed. As shown in Figure (2-17), 304 up to 13mm ( 1/2 in.) thick can be welded in a single pass. The wine-cup-shaped weld is common in keyholing PAW.

Advantages and Disadvantages

Plasma arc welding has several advantages over GTAW. With a collimated arc, PAW is less sensitive to unintentional arc length variations during manual welding and thus requires less operator skill than GTAW. The short arc length in GTAW can cause a to unintentionally touch the weld pool with the electrode tip and contaminate the weld metal with tungsten. However, PAW does not have this problem since the electrode is recessed in the nozzle. As already mentioned, the keyhole is a positive indication of full penetration, and it allows higher welding speeds to be used in PAW.

However, the PAW torch is more complicated. It requires proper electrode tip configuration and positioning, selection of correct orifice size for the application, and setting of both orifice and shielding gas flow rates. Because of the need for a control console, the equipment cost is higher in PAW than in GTAW. The equipment for variable-polarity PAW is much more expensive than that for GTAW.

Fig.(2-17): A plasma arc weld made in 13-mm-thick 304 stainless steel with keyholing.

2-1-8 Gas Metal Arc Welding (GMAW) (MIG)

The Process

Gas–metal arc welding (GMAW) is a process that melts and joins metals by heating them with an arc established between a continuously fed filler wire electrode and the metals, as shown in Figure (2-18). Shielding of the arc and the molten weld pool is often obtained by using inert gases such as argon and helium, and this is why GMAW is also called the metal–inert gas (MIG) welding process. Since non-inert gases, particularly CO2, are also used, GMAW seems a more appropriate name. This is the most widely used arc welding process for aluminum alloys. Figure (2-19) shows gas–metal arc welds of 5083 aluminum, one made with Ar shielding and the other with 75% He–25% Ar shielding . Unlike in GTAW, DCEP is used in GMAW. A stable arc, smooth metal transfer with low spatter loss and good weld penetration can be obtained. With DCEN or AC, however, metal transfer is erratic.

Fig.(2-18): Gas–metal arc welding (a) overall process; (b) welding area enlarged.

Fig.(2-19): Gas–metal arc welds in 6.4-mm-thick 5083 aluminum made with argon (left) and 75% He– 25% Ar (right).

Shielding Gases

Argon, helium, and their mixtures are used for nonferrous metals as well as stainless and alloy steels. The arc energy is less uniformly dispersed in an Ar arc than in a He arc because of the lower thermal conductivity of Ar. Consequently, the Ar arc plasma has a very high energy core and an outer mantle of lesser thermal energy. This helps produce a stable, axial transfer of metal droplets through an Ar arc plasma. The resultant weld transverse cross section is often characterized by a papillary- (nipple-) type penetration pattern such as that shown in Figure (2-19 left). With pure He shielding, on the other hand, a broad, parabolic-type penetration is often observed.

With ferrous metals, however, He shielding may produce spatter and Ar shielding may cause undercutting at the fusion lines. Adding O2 (about 3%) or CO2 (about 9%) to Ar reduces the problems. Carbon and low-alloy steels are often welded with CO2 as the shielding gas, the advantages being higher welding speed, greater penetration, and lower cost. Since CO2 shielding produces a high level of spatter, a relatively low voltage is used to maintain a short buried arc to minimize spatter; that is, the electrode tip is actually below the workpiece surface.

Advantages and Disadvantages

Like GTAW, GMAW can be very clean when using an inert shielding gas. The main advantage of GMAW over GTAW is the much higher deposition rate, which allows thicker workpieces to be welded at higher welding speeds. The dual-torch and twin- wire processes further increase the deposition rate of GMAW. The skill to maintain a very short and yet stable arc in GTAW is not required. However, GMAW guns can be bulky and difficult-to-reach small areas or corners.

2-1-9 (SAW)

The Process

Submerged arc welding (SAW) is a process that melts and joins metals by heating them with an arc established between a consumable wire electrode and the metals, with the arc being shielded by a molten slag and granular flux, as shown in Figure (2-20). This process differs from the arc welding processes discussed so far in that the arc is submerged and thus invisible. The flux is supplied from a hopper (Figure 2-20 a), which travels with the torch. No shielding gas is needed because the molten metal is separated from the air by the molten slag and granular flux (Figure 2-20 b). Direct-current electrode positive is most often used. However, at very high welding currents (e.g., above 900A) AC is preferred in order to minimize arc blow. Arc blow is caused by the electro-magnetic (Lorentz) force as a result of the interaction between the electric current itself and the magnetic field it induces.

Fig.(2-20): Submerged arc welding: (a) overall process; (b) welding area enlarged.

Advantages and Disadvantages

The protecting and refining action of the slag helps produce clean welds in SAW. Since the arc is submerged, spatter and heat losses to the surrounding air are eliminated even at high welding currents. Both alloying elements and metal powders can be added to the granular flux to control the weld metal composition and increase the deposition rate, respectively. Using two or more electrodes in tandem further increases the deposition rate. Because of its high deposition rate, workpieces much thicker than that in GTAW and GMAW can be welded by SAW. However, the relatively large volumes of molten slag and metal pool often limit SAW to flat- position welding and circumferential welding (of pipes). The relatively high heat input can reduce the weld quality and increase distortions.

2-1-10 (ESW)

The Process

Electroslag welding (ESW) is a process that melts and joins metals by heating them with a pool of molten slag held between the metals and continuously feeding a filler wire electrode into it, as shown in Figure (2-21). The weld pool is covered with molten slag and moves upward as welding progresses. A pair of water-cooled copper shoes, one in the front of the workpiece and one behind it, keeps the weld pool and the molten slag from breaking out. Similar to SAW, the molten slag in ESW protects the weld metal from air and refines it. Strictly speaking, however, ESW is not an arc welding process, because the arc exists only during the initiation period of the process, that is, when the arc heats up the flux and melts it. The arc is then extinguished, and the resistance heating generated by the electric current passing through the slag keeps it molten. In order to make heating more uniform, the electrode is often oscillated, especially when welding thicker sections. Typical examples of the application of ESW include the welding of ship hulls, storage tanks, and bridges.

Fig.(2-21): Electroslag welding: (a) overall process; (b) welding area enlarged.

Advantages and Disadvantages

Electroslag welding can have extremely high deposition rates, but only one single pass is required no matter how thick the workpiece is. Unlike SAW or other arc welding processes, there is no angular distortion in ESW because the weld is symmetrical with respect to its axis. However, the heat input is very high and the weld quality can be rather poor, including low toughness caused by the coarse grains in the fusion zone and the heat-affected zone. Electroslag welding is restricted to vertical position welding because of the very large pools of the molten metal and slag.

Figure (2-22) summarizes the deposition rates of the arc welding processes discussed so far. As shown, the deposition rate increases in the order of GTAW, SMAW, GMAW and FCAW, SAW, and ESW. The deposition rate can be much increased by adding iron powder in SAW or using more than one wire in SAW, ESW, and GMAW (not shown).

Fig.(2-22): Deposition rate in arc welding processes.

2-1-11 Electron Beam welding (EBW)

The Process

Electron beam welding (EBW) is a process that melts and joins metals by heating them with an electron beam. As shown in Figure (2-23 a), the cathode of the electron beam gun is a negatively charged filament. When heated up to its thermionic emission temperature, this filament emits electrons. These electrons are accelerated by the electric field between a negatively charged bias electrode (located slightly below the cathode) and the anode. They pass through the hole in the anode and are focused by an electromagnetic coil to a point at the workpiece surface. The beam currents and the accelerating voltages employed for typical EBW vary over the ranges of 50–1000mA and 30–175kV, respectively. An electron beam of very high intensity can vaporize the metal and form a vapor hole during welding, that is, a keyhole, as depicted in Figure (2-23 b).

Figure (2-23) shows that the beam diameter decreases with decreasing ambient pressure. Electrons are scattered when they hit air molecules, and the lower the ambient pressure, the less they are scattered. This is the main reason for EBW in a vacuum chamber.

The electron beam can be focused to diameters in the range of 0.3–0.8mm and the resulting power density can be as high as 1010 W/m2. The very high power density makes it possible to vaporize the material and produce a deep- penetrating keyhole and hence weld. Figure (2-24) shows a single-pass electron beam weld and a dual-pass gas– tungsten arc weld in a 13-mm-thick (0.5-in.) 2219 aluminum, the former being much narrower. The energy required per unit length of the weld is much lower in the electron beam weld (1.5kJ/cm, or 3.8kJ/in.) than in the gas–tungsten arc weld (22.7kJ/cm, or 57.6kJ/in.).

Electron beam welding is not intended for incompletely degassed materials such as rimmed steels. Under high welding speeds gas bubbles that do not have enough time to leave deep weld pools result in weld porosity. Materials containing high-vapor- pressure constituents, such as Mg alloys and Pb- containing alloys, are not recommended for EBW because evaporation of these elements tends to foul the pumps or contaminate the vacuum system.

Fig.(2-23): Electron beam welding: (a) process; (b) keyhole.

Advantages and Disadvantages

With a very high power density in EBW, full-penetration keyholing is possible even in thick workpieces. Joints that require multiple-pass arc welding can be welded in a single pass at a high welding speed. Consequently, the total heat input per unit length of the weld is much lower than that in arc welding, resulting in a very narrow heat-affected zone and little distortion. Reactive and metals can be welded in vacuum where there is no air to cause contamination. Some dissimilar metals can also be welded because the very rapid cooling in EBW can prevent the formation of coarse, brittle intermetallic compounds. When welding parts varying greatly in mass and size, the ability of the electron beam to precisely locate the weld and form a favorably shaped fusion zone helps prevent excessive melting of the smaller part.

However, the equipment cost for EBW is very high. The requirement of high vacuum (10-3–10-6 torr) and x-ray shielding is inconvenient and time consuming. For this reason, medium-vacuum (10-3 –25torr) EBW and nonvacuum (1atm) EBW have also been developed. The fine beam size requires precise fit-up of the joint and alignment of the joint with the gun.

Fig.(2-24): welds in 13-mm-thick 2219 aluminum (a) electron beam weld; (b) gas–tungsten arc weld

Lecture No.13 Week No.13 No. of hours: 2 theoretical and 1 tutorial 2-1-12 (LBW)

The Process

Laser beam welding (LBW) is a process that melts and joins metals by heating them with a laser beam. The laser beam can be produced either by a solid-state laser or a gas laser. In either case, the laser beam can be focused and directed by optical means to achieve high power densities. In a solid-state laser, a single crystal is doped with small concentrations of transition elements or rare earth elements. For instance, in a YAG laser the crystal of yttrium– aluminum–garnet (YAG) is doped with neodymium. The electrons of the dopant element can be selectively excited to higher energy levels upon exposure to high-intensity flash lamps, as shown in figure (2-25 a). Lasing occurs when these excited electrons return to their normal energy state, as shown in Figure (2-25 b).The power level of solid-state lasers has improved significantly, and continuous YAG lasers of 3 or even 5kW have been developed.

In a CO2 laser, a gas mixture of CO2 , N2 , and He is continuously excited by electrodes connected to the power supply and lases continuously. Higher power can be achieved by a CO2 laser than a solid- state laser, for instance, 15kW. Figure (2-26 a) shows LBW in the keyholing mode. Figure (2-26 b) shows a weld in a 13-mm-thick A633 steel made with a 15-kW CO2 laser at 20mm/s .

Besides solid-state and gas lasers, semiconductor-based diode lasers have also been developed. Diode lasers of 2.5kW power and 1mm focus diameter have been demonstrated. While keyholing is not yet possible, conduction- mode (surface melting) welding has produced full-penetration welds with a depth–width ratio of 3:1 or better in 3-mm-thick sheets.

Fig.(2-25): laser beam welding with solid-state laser: (a) process; (b) energy absorption and emission during laser action.

Fig.(2-26): laser beam welding with CO2 laser: (a) process; (b) weld in 13-mm-thick A633 steel. (b) Courtesy of E.A. Metzbower.

Reflectivity

The very high reflectivity of a laser beam by the metal surface is a well-known problem in LBW. As much as about 95% of the CO2 beam power can be reflected by a polished metal surface. Reflectivity is slightly lower with a YAG laser beam. Surface modifications such as roughening, oxidizing, and coating can reduce reflectivity significantly. Once keyholing is established, absorption is high because the beam is trapped inside the hole by internal reflection.

Shielding Gas

A plasma (an ionic gas) is produced during LBW, especially at high power levels, due to ionization by the laser beam. The plasma can absorb and scatter the laser beam and reduce the depth of penetration significantly. It is therefore necessary to remove or suppress the plasma. The shielding gas for protecting the molten metal can be directed sideways to blow and deflect the plasma away from the beam path. Helium is often preferred to argon as the shielding gas for high-power LBW because of greater penetration depth.

Since the ionization energy of helium (24.5eV) is higher than that of argon (15.7eV), helium is less likely to be ionized and become part of the plasma than argon. However, helium is lighter than air and is thus less effective in displacing air from the beam path. Helium–10% Ar shielding has been found to improve penetration over pure He at high-speed welding where a light shielding gas may not have enough time to displace air from the beam path .

2-1-13 Lasers in Arc Welding

As shown in Figure (2-27), laser-assisted gas metal arc welding (LAGMAW) has demonstrated significantly greater penetration than conventional GMAW. In addition to direct heating, the laser beam acts to focus the arc by heating its path through the arc. This increases ionization and hence the conductivity of the arc along the beam path and helps focus the arc energy along the path. It has been suggested that combining the arc power with a 5-kW CO2 laser, LAGMAW has the potential to achieve weld penetration in mild steel equivalent to that of a 20–25-kW laser. Albright et al. have shown that a lower power CO (not CO2 ) laser of 7W and 1mm diameter can initiate, guide, and focus an Ar–1% CO gas–tungsten arc.

Advantages and Disadvantages

Like EBW, LBW can produce deep and narrow welds at high welding speeds, with a narrow heat-affected zone and little distortion of the workpiece. It can be used for welding dissimilar metals or parts varying greatly in mass and size. Unlike EBW, however, vacuum and x-ray shielding are not required in LBW, however, the very high reflectivity of a laser beam by the metal surface is a major drawback, as already mentioned. Like EBW, the equipment cost is very high, and precise joint fit-up and alignment are required.

Fig.(2-27): weld penetration in GMAW and laser-assisted GMAW using CO2 laser at 5.7kW

2-1-14 Exothermic Welding Exothermic welding, also known as exothermic bonding, welding (TW), and thermit welding, is a welding process for joining two electrical conductors, that employs molten metal to permanently join the conductors. The process employs an exothermic reaction of a thermite composition to heat the metal, and requires no external source of heat or current. The chemical reaction that produces the heat is an between powder and a metal oxide.

History. Exothermic welding was developed by Hans Goldschmidt around 1895. The first non-ferrous application for exothermic welding was developed in 1938 by Dr. Charles Cadwell, a professor at the Case School of Applied Science (now Case Western Reserve University), in Cleveland, Ohio. The original use of the process was to weld signal bonds to railroad tracks. The method was patented by John H. Deppeler in 1928 while working for the Metal and Thermit Corporation. It is United States patent number 1671412. In the United States, an investigation into exothermic rail welding was conducted by the Committee On Welded Rail Joints, with the goal of improving and standardizing rail welding. Composed of members from the American Bureau of Welding and the American Electric Railway Engineering Association, this committee conducted extensive physical testing of welded rail joints, as well as testing of various parameters of the welding process.

Overview In exothermic welding, aluminium dust reduces the oxide of another metal, most commonly iron oxide, because aluminium is highly reactive. Iron(III) oxide is commonly used:

Fe2O3 + 2 Al → 2 Fe + Al2O3 The products are , free elemental iron, and a large amount of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation. Commonly the reacting composition is five parts iron oxide red (rust) powder and three parts aluminium powder by weight, ignited at high temperatures. A strongly exothermic (heat-generating) reaction occurs that via reduction and oxidation produces a white hot mass of molten iron and a slag of refractory aluminium oxide. The molten iron is the actual welding material; the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction, so the set-up for welding must take into account that the actual molten metal is at the bottom of the and covered by floating slag. Other metal oxides can be used, such as chromium oxide, to generate the given metal in its elemental form. Copper thermite, using copper oxide, is used for creating electric joints in a process called cad welding:

3 CuO + 2 Al → 3 Cu + Al2O3 Thermite welding is widely used to weld railway rails. One of the first railroads to evaluate the use of thermite welding was the Delaware Hudson in 1935 The weld quality of chemically pure thermite is low due to the low heat penetration into the joining metals and the very low carbon and alloy content in the nearly pure molten iron. To obtain sound railroad welds, the ends of the rails being thermite welded are preheated with a torch to an orange heat, to ensure the molten steel is not chilled during the pour. Because the thermite reaction yields relatively pure iron, not the much stronger steel, some small pellets or rods of high-carbon alloying metal are included in the thermite mix; these alloying materials melt from the heat of the thermite reaction and mix into the weld metal. The alloying beads composition will vary, according to the rail alloy being welded. The reaction reaches very high temperatures, depending on the metal oxide used. The reactants are usually supplied in the form of powders, with the reaction triggered using a spark from a flint lighter. The activation energy for this reaction is very high however, and initiation requires either the use of a "booster" material such as powdered magnesium metal or a very hot flame source. The aluminium oxide slag that it produces is discarded. When welding copper conductors, the process employs a semi- permanent crucible mould, in which the molten copper, produced by the reaction, flows through the mould and over and around the conductors to be welded, forming an electrically conductive weld between them. When the copper cools, the mould is either broken off or left in place. Alternatively, hand-held graphite can be used. The advantages of these crucibles include portability, lower cost (because they can be reused), and flexibility, especially in field applications. Properties An exothermic weld has higher mechanical strength than other forms of weld, and excellent corrosion resistance. It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors).

Applications Exothermic welding is usually used for welding copper conductors but is suitable for welding a wide range of metals, including stainless steel, , common steel, , , and Monel. It is especially useful for joining dissimilar metals. The process is marketed under a variety of names: sucas Quikweld, Tectoweld, Ultraweld, Cadweld, Techweld,Thermoweldand Kumwell.

The Process Typically, the ends of the rails are cleaned, aligned flat and true, and spaced apart 25 millimetres (0.98 in). This gap between rail ends for welding is to ensure consistent results in the pouring of the molten steel into the weld mold. In the event of a welding failure, the rail ends can be cropped to a 75 millimetres (3.0 in) gap, removing the melted and damaged rail ends, and a new weld attempted with a special mould and larger thermite charge. A two or three piece hardened sand mould is clamped around the rail ends, and a torch of suitable heat capacity is used to preheat the ends of the rail and the interior of the mould. The proper amount of thermite with alloying metal is placed in a refractory crucible, and when the rails have reached a sufficient temperature, the thermite is ignited and allowed to react to completion (allowing time for any alloying metal to fully melt and mix, yielding the desired molten steel or alloy). The reaction crucible is then tapped at the bottom. Modern crucibles have a self- tapping thimble in the pouring nozzle. The molten steel flows into the mould, fusing with the rail ends and forming the weld. The slag, being lighter than the steel flows last from the crucible and overflows the mould into a steel catch basin, to be disposed of after cooling. The entire setup is allowed to cool. The mould is removed and the weld is cleaned by hot chiselling and grinding to produce a smooth joint. Typical time from start of the work until a train can run over the rail is approximately 45 minutes to more than an hour, depending on the rail size and ambient temperature. In any case, the rail steel must be cooled to less than 370 °C (698 °F) before it can sustain the weight of rail locomotives. When a thermite process is used for circuits – the bonding of wires to the rails with a copper alloy, a graphite mould is used. The graphite mould is reusable many times, because the copper alloy is not as hot as the steel alloys used in rail welding. In signal bonding, the volume of molten copper is quite small, approximately 2 cubic centimetres (0.12 cu in) and the mould is lightly clamped to the side of the rail, also holding a signal wire in place. In rail welding, the weld charge can weigh up to 13 kilograms (29 lb). The hardened sand mould is heavy and bulky, must be securely clamped in a very specific position and then subjected to intense heat for several minutes before firing the charge. When rail is welded into long strings, the longitudinal expansion and contraction of steel must be taken into account. British practice sometimes uses a sliding joint of some sort at the end of long runs of continuously welded rail, to allow some movement, although by using heavy concrete sleepers and an extra amount of ballast at the sleerer ends, the track, which will be prestressed according to the ambient temperature at the time of its installation, will develop compressive stress in hot ambient temperature, or tensile stress in cold ambient temperature, its strong attachment to the heavy sleepres preventing buckling or other deformation. Current practice is to use welded rails throughout on high speed lines, and expansion joints are kept to a minimum, often only to protect junctions and crossings from excessive stress. American practice appears to be very similar, a straightforward physical restraint of the rail. The rail is prestressed, or considered "stress neutral" at some particular ambient temperature. This "neutral" temperature will vary according to local climate conditions, taking into account lowest winter and warmest summer temperatures. The rail is physically secured to the ties or sleepers with rail anchors, or anti-creepers. If the track ballast is good and clean and the ties are in good condition, and the track geometry is good, then the welded rail will withstand ambient temperature swings normal to the region.

Detailed operating instructions are provided for each of our processes, but the welding methods all comprise of 6 main elements: 1-A carefully prepared gap must be produced between the two rails, which must then be accurately aligned by means of straightedges to ensure the finished joint is perfectly straight and flat.

2-Pre-formed refractory moulds which are manufactured to accurately fit around the specific are clamped around the rail gap, and then sealed in position. Equipment for locating the preheating burner, and the thermit container is then assembled

3-The weld cavity formed inside the mould is preheated using an oxy fuel gas burner with accurately set gas pressures for a prescribed time. The quality of the finished weld will depend upon the precision of this preheating process.

4- The thermit Portion is manufactured to produce a steel with a compatible with the specific type of rail to be welded. On completion of the preheating, the container is fitted to the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten thermit Steel. The container incorporates an automatic tapping system enabling the liquid steel - which is at a temperature in excess of 2,500°C - to discharge directly into the weld cavity.

5- The welded joint is allowed to cool for a predetermined time before the excess steel and the mould material is removed from around the top of the rail with the aid of a hydraulic trimming device.

6- When cold the joint is cleaned of all debris, and the rail running surfaces are precision ground the profile. The finished weld must then be inspected before it is passed as ready for service.

( 1 ) ( 2 ) ( 3 )

( 4 ) ( 5 ) ( 6 )

Fig.(2-27): six elements in thermit welding

Fig.(2-28): thermite reaction crucibles Fig.(2-29): thermite welding mould

Once correctly installed, the finished weld is expected to last the life of the rail, with no further maintenance.

It is estimated that over 100,000 Aluminothermic welds are manufactured in track each year, with a total population in excess of 1.8 million. The processes are used throughout the year on site with no heavy equipment other than that which may be carried to site by the two man welding team. All the equipment, ancillary tools such as lighting, gases and consumables can be transported to site by small commercial vehicles.

2-2 Electric Resistance Welding

Electric resistance welding (ERW) refers to a group of welding processes such as spot and seam welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electrical resistance of material vs. the time and the force used to hold the materials together during welding. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electrical current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance (the connecting or "faying" surfaces) as an electrical current (100–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high (although in production situations the cost per weld may be as low as $0.04 USD per weld depending on application and manufacturing rate).

Energy = I2 R t

The welding current is applied via copper electrodes under controlled force. The diameter of the electrode which make contact with the workpiece will determine the density of the electric current. The amount of applied electrode force will also affect the resistance across all interfacing layers including the weld nugget zone and the electrode to work piece interface areas. In practice, force is adjusted so that heat is immediately created at the interfacing areas. Whereas it is important to start heat build up at the faying surfaces of the work pieces, it is undesirable to create excessive heat marks at the electrode - work piece interface. It is therefore very important that the electrode cooling system be as efficient as possible to take away heat from the surface of the workpieces that make contact with the electrodes. An efficient cooling system will preserve the electrodes in order to control the current density.

Steel

Resistance welding of steel is relatively easier than welding of aluminum. The characteristics that makes steel easier to resistance weld than aluminum is its higher electrical resistivity and its lower thermal conductivity as compared to the copper electrodes. The cooling of the electrodes is very important since steel requires a build up of temperature in excess of 1300oC to melt which is well above the melting temperature of copper of 1115oC. The flow of water in the electrodes is necessary to take away heat that builds up at the electrode / workpiece contact area. This will also help in maintaining the surface contact area of the copper electrodes at a proper dimension which will result in maintaining the current density to melt the steel.

Aluminium

Aluminum has an electrical resistivity and thermal conductivity that is closer to that of copper. What makes it possible for resistance welding is that its melting temperature is much lower than that of copper. Due to aluminum's lower resistivity and higher thermal conductivity as compared to steel, resistance welding aluminum would require much higher levels of current but the weld must be accomplished in much less time.

Coatings on Steel

Characteristics of zinc are shown above to illustrate the approaches necessary to weld coated materials. As compared to bare steel, the coated steels would require a pulse of current prior to the weld to melt the coating. It only requires 435oC to melt the coating. The resistance to the pulse of current by the steel would create the heat that would boil off the zinc coating. Once melted however, the zinc would puddle around the weld zone and would provide lower resistivity as compared to bare steel onto bare steel. Because of this lowered resistivity, significant higher levels of current would be required to weld coated steel as compared to bare steel.

Lecture No.14 Week No.14 No. of hours: 2 theoretical and 1 tutorial 2-2-1

Spot welding is a resistance welding method used to join two or more overlapping metal sheets, studs, projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually power sources and welding equipment are sized to the specific thickness and material being welded together. The thickness is limited by the output of the welding power source and thus the equipment range due to the current required for each application. Care is taken to eliminate contaminants between the faying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes a heat buildup in the work pieces between the copper electrodes, the rising temperature causes a rising resistance, and results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the workpiece in less than a second (resistance welding time is generally programmed as a quantity of AC cycles or milliseconds) the molten or plastic state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the metal, since copper is an excellent conductor. Resistance spot welding typically employs electrical power in the form of direct current, alternating current, medium frequency half-wave direct current, or high-frequency half wave direct current. If excessive heat is applied or applied too quickly, or if the force between the base materials is too low, or the coating is too thick or too conductive, then the molten area may extend to the exterior of the work pieces, escaping the containment force of the electrodes (often up to 30,000 psi). This burst of molten metal is called expulsion, and when this occurs the metal will be thinner and have less strength than a weld with no expulsion. The common method of checking a weld's quality is a peel test. An alternative test is the restrained tensile test, which is much more difficult to perform, and requires calibrated equipment. Because both tests are destructive in nature (resulting in the loss of salable material), non-destructive methods such as ultrasound evaluation are in various states of early adoption by many OEMs. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods, limiting the usefulness of the process. It is used extensively in the automotive industry— cars can have several thousand spot welds. A specialized process, called shot welding, can be used to spot weld stainless steel. There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In a solid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grain structure, e.g. molybdenum to tungsten, are joined using a very short heating time, high weld energy, and high force. There is little melting and minimum grain growth, but a definite bond and grain interface. Thus the materials actually bond while still in the solid state. The bonded materials typically exhibit excellent shear and tensile strength, but poor peel strength. In a fusion bond, either similar or dissimilar materials with similar grain structures are heated to the melting point (liquid state) of both. The subsequent cooling and combination of the materials forms a “nugget” alloy of the two materials with larger grain growth. Typically, high weld energies at either short or long weld times, depending on physical characteristics, are used to produce fusion bonds. The bonded materials usually exhibit excellent tensile, peel and shear strengths. In a reflow braze bond, a resistance heating of a low temperature brazing material, such as gold or solder, is used to join either dissimilar materials or widely varied thick/thin material combinations. The brazing material must “wet” to each part and possess a lower melting point than the two workpieces. The resultant bond has definite interfaces with minimum grain growth. Typically the process requires a longer (2 to 100 ms) heating time at low weld energy. The resultant bond exhibits excellent tensile strength, but poor peel and shear strength.

Fig.(2-30): spot welding Applications Spot welding is typically used when welding particular types of sheet metal. Thicker stock is more difficult to spot weld because the heat flows into the surrounding metal more easily. Spot welding can be easily identified on many sheet metal goods, such as metal buckets. Aluminium alloys can be spot welded, but their much higher thermal conductivity and electrical conductivity requires higher welding currents. This requires larger, more powerful, and more expensive welding transformers. Perhaps the most common application of spot welding is in the automobile manufacturing industry, where it is used almost universally to weld the sheet metal to form a car. Spot can also be completely automated, and many of the industrial robots found on assembly lines are spot welders (the other major use for robots being painting). Spot welding is also used in the orthodontist's clinic, where small scale spot welding equipment is used when resizing metal "molar bands" used inorthodontics. Another application is spot welding straps to nickel-cadmium or nickel-metal- hydride cells to make batteries. The cells are joined by spot welding thin nickel straps to the battery terminals. Spot welding can keep the battery from getting too hot, as might happen if conventional soldering were done. Good design practice must always allow for adequate accessibility. Connecting surfaces should be free of contaminants such as scale, oil, and dirt, to ensure quality welds. Metal thickness is generally not a factor in determining good welds.

Processing and Equipment Spot welding involves three stages; the first of which involves the electrodes being brought to the surface of the metal and applying a slight amount of pressure. The current from the electrodes is then applied briefly after which the current is removed but the electrodes remain in place for the material to cool. Weld times range from 0.01 sec to 0.63 sec depending on the thickness of the metal, the electrode force and the diameter of the electrodes themselves. The equipment used in the spot welding process consists of tool holders and electrodes. The tool holders function as a mechanism to hold the electrodes firmly in place and also support optional water hoses which cool the electrodes during welding. Tool holding methods include a paddle-type, light duty, universal, and regular offset.