Bab 27B Pengelasan Fusi
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Pengelasan Fusi (Fusion Welding) Welding) Ir. Tri Prakosa, M. Eng.
Proses Manufaktur II, Februari 2011
Fusion Welding 1. Oxyfuel Gas Welding 2. Arc-Welding Processes: Consumable Electrode 3. Arc-Welding Processes: Nonconsumable Electrode
– Thermit Welding – Electrom Beam Welding – Laser Beam Welding
OXYFUEL GAS WELDING
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Oxyfuel gas welding /OFW Developed in the early 1900s, this process utilizes the heat generated by the combustion of acetylene gas (C2H2) in a mixture with oxygen. The heat is generated in accordance with the following chemical reactions. The primary combustion process, which occurs in the inner core of the flame, is C2H2 + O2 2CO + H2 + Heat
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Oxyfuel gas welding /OFW This reaction dissociates the acetylene into carbon monoxide and hydrogen and produces about 1/3 of the total heat generated in the flame. The second reaction is: 2CO + H2 + 1.502 2C02 + H2O + heat
which results in burning of the hydrogen and combustion of the carbon monoxide, producing about 2/3 of the total heat. 5
Types of flames The proportions of acetylene and oxygen in the gas mixture are an important factor in oxyfuel gas welding. At a ratio of 1 : 1, that is, when there is no excess oxygen, it is considered to be a neutral flame.
a) Neutral Flame 6
Types of flames With a greater oxygen supply, it becomes an oxidizing flame. This flame is harmful, especially for steels, because it oxidizes the steel. Only in copper and copper-base alloys is an oxidizing flame desirable because a thin protective layer of slag forms over the molten metal.
b) Oxidizing Flame 7
Types of flames If the supply of oxygen is lowered, it becomes a reducing or carburizing flame. The temperature of a reducing, or excess-acetylene, flame is lower. Hence it is suitable for applications requiring low heat, such as brazing, soldering, and flame hardening.
c) Carburizing Flame 8
Filler metals Filler metals are used to supply additional material to the weld zone during welding. They are available as rods or wire, and are made of metals compatible with those to be welded. These consumable filler rods may be bare, or they may be coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded, by generating a gaseous shield around the weld zone. 9
Filler metals The flux also helps dissolve and remove oxides and other substances from the workpiece, resulting in a stronger joint. The slag developed protects the molten puddle of metal against oxidation as it cools.
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Welding Torch yang digunakan pada Pengelasan Oxyacetylene (a) Gambar umum dan (b) Penampang torch yang digunakan pada pengelasan oxyacetylene.
Pertama kali katup acetylene dibuka; gas dinyalakan dengan korek api atau penyala; kemudian katup oksigen dibuka dan nyala diatur.
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High Pressure Gas Cylinders yang Digunakan pada Pengelasan Oxyacetylene (c) Peralatan dasar yang digunakan pada oxyfuelgas welding.
Untuk memastikan hubungan adalah benar, semua ulir pada sambungan acetylene adalah ulir kiri, Tabung oksigen sedangkan pada oksigen biasanya dicat merah, adalah ulir kanan. sedangkan tabung acetylene dicat hijau. 12
ARC-WELDING PROCESSES: ARCCONSUMABLE ELECTRODE 13
Shielded metalmetal-arc welding (SMAW) (Las Busur Listrik Listrik)) Shielded metal-arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes. Currently, about 50% of all industrial and maintenance welding is performed by this process. The electric arc is generated by touching the tip of a coated electrode against the workpiece and then withdrawing it quickly to a distance sufficient to maintain the arc.
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Las Busur Listrik (Shielded Metal-- Arc Welding/SMAW) Metal Ilustrasi skematik proses pengelasan dengan elektroda terselubung (shielded metalarc welding). Sekitar 50% operasi di industri ukuran besar menggunaan proses ini. Ilustrasi skematik operasi shielded metal-arc welding (dikenal juga sebagai stick welding, karena bentuk elektroda berupa batang). 15
Las Busur Rendam Submerged arc welding/SAW) In submerged arc welding (SAW), the weld arc is shielded by granular flux, consisting of lime, silica, manganese oxide, calcium fluoride, and other elements. The flux is fed into the weld zone by gravity flow through a nozzle. See the following figure.
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Las Busur Rendam (Submerged Submerged-Arc Welding/SAW)
Ilustrasi Skematik Las Busur Rendam (submerged-arc welding) dan peralatannya. Flux yang tidak terproses, akan digunakan kembali. Sumber: American Welding Society. 17
Gas metalmetal-arc welding /GMAW In gas metal-arc welding (GMAW), the weld area is shielded by an external source, such as argon, helium, carbon dioxide, or various other gas mixtures Ilustrasi Skematik proses pengelasan busur logam-gas (gas metal-arc welding), dahulu sering disebut dengan pengelasan MIG (metal inert gas). 18
Gas metalmetal-arc welding /GMAW Peralatan dasar yang digunakan pada operasi gas metal- arc welding. Sumber: American Welding Society.
The consumable bare wire is fed automatically through a nozzle into the weld arc. 19
Gas metalmetal-arc welding /GMAW In spray transfer, small droplets of molten metal from the electrode are transferred to the weld area at rates of several hundred droplets per second. The transfer is spatter-free and very stable. High dc current and voltages and large diameter electrodes are used, with argon or argon-rich gas mixtures used as the shielding gas.
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Gas metalmetal-arc welding /GMAW The average current required in this process can be reduced by pulsed arcs, which are high-amplitude pulses superimposed over a low, steady current, and the process can be used in all welding positions. In globular transfer, CO2 rich gases are utilized, and globules propelled by the forces of the electric arc transfer the metal, resulting in considerable spatter.
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Gas metalmetal-arc welding /GMAW High welding currents are used, with greater weld penetration and welding speed than in spray transfer. Heavier sections are commonly joined by this method. In short circuiting, the metal is transferred in individual droplets, at rates of more than 50 per second, as the electrode tip touches the molten weld metal and short circuits.
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Flux--cored arc welding /FCAW Flux The flux-cored arc welding (FCAW) process is similar to gas metal-arc welding, with the exception that the electrode is tubular in shape and is filled with flux (hence the term flux cored). Iustrasi skematik proses pengelasan busur - dengan inti flux (flux-cored arcwelding). Operasi ini mirip dengan gas metal-arc welding.
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Flux--cored arc welding /FCAW Flux Cored electrodes produce a more stable arc, improve weld contour, and improve the mechanical properties of the weld metal. The flux in these electrodes is much more flexible than the brittle coating used on SMAW electrodes. Thus the tubular electrode can be provided in long coiled lengths. The electrodes are usually 0.5-4 mm (0.020-0.15 in.) in diameter. 24
Flux--cored arc welding /FCAW Flux The power required is about 20 kW. Self-shielded cored electrodes are also available. These electrodes do not require external gas shielding because they contain emissive fluxes that shield the weld area against the surrounding atmosphere.
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Flux--cored arc welding /FCAW Flux Small-diameter electrodes have made welding of thinner materials not only possible but often desirable. Also, small-diameter electrodes make it relatively easy to weld parts out of position, and the flux chemistry enables welding of many base metals.
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Electrogas welding /EGW Electrogas welding (EGW) is used primarily for welding the edges of sections vertically in one pass with the pieces placed edge to edge (butt).
It is classified as a machine welding process because it requires special equipment 27
Ilustrasi skematik proses pengelasan electrogas. Sumber:American Welding Society.
Electrogas welding /EGW The weld metal is deposited into a weld cavity between the two pieces to be joined. The space is enclosed by two water-cooled copper dams (shoes) to prevent the molten slag from running off. Mechanical drives move the shoes upward. Circumferential welds such as on pipes are also possible, with the workpiece rotating.
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Electrogas welding /EGW Single or multiple electrodes are fed through a conduit and a continuous arc is maintained, using flux-cored electrodes at up to 750 A, or solid electrodes at 400 A. Power requirements are about 20 kW. Shielding is by inert gas, such as CO2, Ar, or He, depending on the type of material being welded. The gas may be provided from an external source, or it may be produced from a flux-cored electrode, or both. 29
Electroslag welding /ESW Developed in the 1950s, electroslag welding (ESW) and its applications are similar to electrogas welding. Peralatan yang digunakan untuk operasi pengelasan electroslag. Sumber: American Welding Society.
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Electroslag welding /ESW The main difference is that the arc is started between the electrode tip and the bottom of the part to be welded. Flux is added and melted by the heat of the arc. After the molten slag reaches the tip of the electrode, the arc is extinguished. Energy is supplied continuously through the electrical resistance of the molten slag.
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Electroslag welding /ESW Thus because the arc is extinguished, ESW is not strictly an arc welding process. Single or multiple solid as well as flux-cored electrodes may be used. The guide may be non-consumable (conventional method) or consumable.
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ARC-WELDING PROCESSES: ARCNONCONSUMABLE ELECTRODE 33
Gas tungstentungsten-arc welding /GTAW In gas tungsten-arc welding (GTAW), the filler metal is supplied from a filler wire.
Proses pengelasan busur - gas tungsten, dahulu dikenal sebagai pengelasan TIG (tungsten inert gas).
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Gas tungstentungsten-arc welding /GTAW Because the tungsten electrode is not consumed in this operation, a constant and stable arc gap is maintained at a constant current level. The filler metals are similar to the metals to be welded, and flux is not used. The shielding gas is usually Ar or He, or a mixture of the two. Welding with GTAW may be done without filler metals, as in welding close-fit joints. 35
Gas tungstentungsten-arc welding /GTAW The power supply is either dc at 200 A, or ac at 500 A, depending on the metals to be welded.
Perlengkapan untuk operasi pengelasan busur - gas tungsten. Sumber: American Welding Society. 36
Gas tungstentungsten-arc welding /GTAW In general, ac is preferred for aluminum and magnesium because the cleaning action of ac removes oxides and improves weld quality. Thorium or zirconium may be used in the tungsten electrodes to improve their electron emission characteristics. Power requirements range from 8 kW to 20 kW. Contamination of the tungsten electrode by the molten metal can be a significant problem, particularly in critical applications, as it can cause discontinuities in the weld. 37
Gas tungstentungsten-arc welding /GTAW Thus contact of the electrode with the molten metal pool should be avoided.
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Atomic hydrogen welding /AHW Atomic hydrogen welding (AHW) uses an arc in a shielding atmosphere of hydrogen. The arc is between two tungsten or carbon electrodes. Thus the workpiece is not part of the electrical circuit, as it is in GTAW. The hydrogen gas also cools the electrodes.
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Atomic Hydrogen Welding Torch
Sumber: http://www.gbwelding.com/data/welding_processes/ahw.htm
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Atomic hydrogen welding
Sumber: http://www.gbwelding.com/data/welding_processes/ahw.htm
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Plasma--arc welding /PAW Plasma In plasma-arc welding (PAW), developed in the 1960s, a concentrated plasma arc is produced and aimed at the weld area. The arc is stable and reaches temperatures as high as 33,000 °C (60,000 °F). A plasma is ionized hot gas, composed of nearly equal numbers of electrons and ions. The plasma is initiated between the tungsten electrode and the orifice, using a low-current pilot arc. 42
Plasma--arc welding /PAW Plasma Unlike other processes, the plasma arc is concentrated because it is forced through a relatively small orifice. Operating currents are usually below 100 A, but they can be higher for special applications. When a filler metal is used, it is fed into the arc, as in GTAW. Arc and weld-zone shielding is supplied through an outer shielding ring by gases such as Ar, He, or mixtures. 43
Plasma--arc welding /PAW Plasma There are two methods of plasma-arc welding. { Transferred-arc method { Nontransferred method
In the transferred-arc method (Figure a), the workpiece being welded is part of the electrical circuit. The arc thus transfers from the electrode to the workpiece-hence the term transferred.
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Plasma--Arc Welding /PAW Plasma
Dua jenis proses pengelasan busur plasma: (a) transferred, (b) nontransferred. Lasan yang dalam dan sempit dapat dilakukan dengan proses ini, dengan kecepatan pengelasan yang tinggi. 45
Plasma--arc welding /PAW Plasma In the nontransferred method (Figure b), the arc is between the electrode and the nozzle, and the heat is carried to the workpiece by the plasma gas. The thermal transfer mechanism is similar to that for oxyfuel flame.
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THERMIT WELDING
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Thermit welding Thermit welding (TW) gets its name from thermite, which is based on the word therm meaning heat; the word Thermit is a registered trademark. The process involves exothermic (heat producing) reactions between metal oxides and metallic reducing agents. The heat of the reaction is then utilized in welding. This process dates back to the early 1900s.
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Thermit welding The most common mixture of materials used in welding steel and cast iron is finely divided particles of iron oxide (Fe3O4), aluminum oxide (Al203), iron, and aluminum. The basic reactions are: ¾Fe 3O4 + 2Al 9⁄4Fe + Al2O3 + heat 3 FeO + 2Al 3 Fe + Al2O3 + heat Fe3O4 + 2Al 2 Fe + Al2O3 + heat
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Thermit welding This nonexplosive mixture produces a maximum theoretical temperature of 3200 °C (5800 °F) within less than a minute. In practice, however, this temperature is only about 2200-2400 °C (4000-4350 °F). The mixture may also contain other materials to impart special properties to the weld.
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Thermit welding The reaction is started by applying a magnesium fuse to special compounds of peroxides, chlorates, or chromates, known as oxidizing agents, with an ignition temperature of about 1200 °C (2200 °F). Welding copper, brasses, and bronzes, and copper alloys to steels, involves the following reactions: 3CuO + 2Al 3Cu + Al2O3 + heat 3Cu2O + 2Al 6Cu + Al2O3 + heat 51
Thermit welding Oxides of copper, nickel, chromium, and manganese are also used in Thermit welding, resulting in temperatures ranging up to 5000 °C (9000 °F).
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ELECTRON--BEAM WELDING ELECTRON
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Electron--Beam Welding /EBW Electron In electron-beam welding (EBW), heat is generated by high-velocity narrow-beam electrons. The kinetic energy of the electrons is converted into heat as they strike the workpiece. This process requires special equipment to focus the beam on the workpiece in a vacuum. The higher the vacuum, the more the beam penetrates and the greater the depth-to-width ratio is. 54
Electron--Beam Welding /EBW Electron Almost any metal can be welded by EBW, with workpiece thicknesses ranging from foil to plate. The intense energy is also capable of producing holes in the workpiece. Generally, no shielding gas, flux, or filler metal is required. Capacities of electron beam guns range to 100 kW. Developed in the 1960s, EBW has the capability to make high-quality welds that are almost parallel sided, are deep and narrow, and have small HAZ (heat-affected zones). 55
LASER--BEAM WELDING LASER
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Laser--Beam Welding /LBW Laser Laser-beam welding (LBW) utilizes a high-power laser beam as the source of heat to produce a fusion weld. Because the beam can be focused to a very small area, it has high-energy density and, therefore, has deep penetrating capability. The beam can be directed, shaped, and focused precisely on the workpiece.
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Laser--Beam Welding /LBW Laser Consequently, this process is particularly suitable for welding deep and narrow joints, with depth-towidth ratios typically ranging from 4 to 10. The laser beam may be pulsed (milliseconds) for applications such as spot welding of thin materials, with power levels up to 100 kW. Continuous multi-kW laser systems are used for deep welds on thick sections.
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Contoh Pengelasan dengan Laser
Pengelasan Laser pada pisau cukur. 59
TERIMA KASIH
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