Fusion Welding Processes Sumber: Manufacturing Engineering & Technology Oleh: Serope Kalpakjian

Manufacturing Processes: Joining Merupakan proses untuk menyambung atau menyatu- kan bagian logam yg terpisah. Proses penyambungann bisa dilakukan dgn cara: Pengelasan (welding) Brazing Soldering Adhesive bonding Mechanical fastening Figure I.7f Schematic illustration of various joining processes

Fusion Welding Merupakan proses pengelasan (penyambungan logam) di mana kedua logam yg akan disambung dilelehkan dengan pemanasan (chemical or electrical ), kemudian logam yg meleleh disatukan (bisa dengan logam pengisi /filler metal atau tanpa logam pengisi)

Fusion Welding Processes

Oxyfuel Gas Welding (OFW) Proses pengelasan yg menggunakan fuel-gas yg dicampur dengan oksigen untuk menghasilkan flame (nyala api sbg sumber panas). Fuel-gas yg umum digunakan adalah acetylene fuel (karbit), sehingga dikenal sbg oxy-acetylene welding (las karbit). Harga peralatan relatif murah, mudah dipindahkan. Dapat digunakan u/ pabrikasi maupun repair, kapasitas pekerjaan rendah/tidak banyak.

Oxyacetylene Flame Types Figure 30.1 Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.

Oxyacetylene Flame Types Neutral flame: rasio acetylene dgn oksigen 1:1. Oxidizing flame: suplai oksigen lbh besar drpd acetylene. Tidak baik u/ pengelasan baja karena menyebabkan karat. Reducing or carburizing flame: suplai acetylene lbh besar drpd oksigen. Digunakan u/ pengelasan dgn panas yg tidak terlalu tinggi (brazing, soldering, flame-hardening)

Oxyacetylene Torch Figure 30.2 (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed. Oxygen regulators are usually painted green, and acetylene regulators red.

Pressure-Gas Welding Process Figure 30.3 Schematic illustration of the pressure-gas welding process; (a) before, and (b) after. Note the formation of a flash at the joint, which can later be trimmed off.

Gas-Tungsten Arc Welding Tungsten electrode tidak ikut meleleh, sehingga busur listrik lebih konstan. Menggunakan gas pelindung argon atau Helium untuk mencegah oksidasi dari udara sekitar. Filler metal umumnya disuplai dari filler Wire. Kualitas pengelasan sangat baik, surface Finish juga sangat baik. Daya yg diperlukan berkisar 8 – 20 kW. Bisa menggunakan arus DC (200 A) atau Arus AC (500 A) Figure 30.4 (a) The gas tungsten-arc welding process, formerly known as TIG (for tungsten inert gas) welding. (b) Equipment for gas tungsten-arc welding operations.

Plasma-Arc Welding Process (PAW) Plasma adalah gas panas yg di ionisasi (mempunyai electron dan ion sama banyak). Nyala busur plasma dimulai antara ujung elektroda tungsten dan ujung bukaan nozzle (orifice) dgn bantuan low current pilot arc. Bukaan/celah nosel sempit, sehingga busur nyala terkonsentrasi. Temperatur nyala busur plasma mencapai 33.000 oC

Plasma-Arc Welding Process (PAW) Figure 30.6 Two types of plasma-arc welding processes: (a) transferred, (b) nontransferred. Deep and narrow welds can be made by this process at high welding speeds.

Dua metode pada PAW Transferred-arc method, logam yg dilas dihubungkan dengan power supply (+), sehingga busur plasma bergerak (transferred) dari elektroda tungsten ke benda kerja. Nontransferred method, logam yg dilas tidak terhubung ke power supply. Busur plasma terbentuk antara elektroda dan nozzle. Panas busur plasma dialirkan ke logam yg dilas melalui gas plasma (mekanisme transfer panas ini = pada las acetylene).

Shielded-Metal Arc Welding (SMAW) SMAW merupakan las busur listrik tertua dan paling sederhana. Hampir 50% pengelasan diindustri menggunakan SMAW. Elektrode ikut meleleh dalam las SMAW. Temperatur busur listrik yg dicapai sekitar 30.000 oC. Karena elektrode berbentuk batang kecil (stick), SMAW juga disebut dgn stick welding. Elektrode dilengkapi dgn coating yg saat dipanaskan menghasilkan gas pelindung u/ mencegah oksidasi dari udara sekitar.

Shielded-Metal Arc Welding Figure 30.7 Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process. Figure 30.8 A deep weld showing the buildup sequence of eight individual weld beads.

Efek Polaritas pada SMAW Polaritas dlm arus DC menunjukkan arah aliran arus listrik. Straight polarity (DCEN), Reverse polarity (DCEP), Bolak-balik (AC). Figure 30.5 The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

Efek Polaritas pada SMAW Straight polarity (DCEN): B kerja (+), elektroda (-) Penetrasi las dalam, cocok untuk pengelasan thick metal. Weld bead sempit. Figure 30.5 The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

Efek Polaritas pada SMAW Reverse polarity (DCEP): B kerja (-), elektroda (+) Penetrasi las dangkal, cocok untuk pengelasan sheet metal. Weld bead lebar. Figure 30.5 The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

Efek Polaritas pada SMAW Bolak-balik (AC): Penetrasi las dalam, cocok untuk pengelasan plat tebal. Bisa menggunakan elektrode dengan diameter besar. Weld bead sedang. Figure 30.5 The effect of polarity and current type on weld beads: (a) dc current straight polarity; (b) dc current reverse polarity; (c) ac current.

Submerged-Arc Welding Figure 30.9 Schematic illustration of the submerged arc welding process and equipment. The unfused flux is recovered and reused.

Gas Metal-Arc Welding Figure 30.10 (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. (b) Basic equipment used in gas metal-arc welding operations.

Fluxed-Cored Arc-Welding Figure 30.11 Schematic illustration of the flux-cored arc welding process. This operation is similar to gas metal-arc welding, shown in Fig. 30.10.

Electrogas-Welding Figure 30.12 Schematic illustration of the electrogas welding process.

Electrogas-Welding Daya yg diperlukan 20 kW Ampere 400 – 750 A Pelindung inert gas (CO2, argon, helium) Untuk butt-joint Ketebalan las 12 – 75 mm One pass welding system (dari bawah ke atas).

Electroslag-Welding Figure 30.13 Equipment used for electroslag welding operations.

Electroslag-Welding Daya sampai 30 kW Ampare sampai 600 A Ketebalan pengelasan mulai 50 – 900 mm One pass welding system (mulai dari bawah ke atas)

Electrode Designations

Elektrode untuk Las Panjang coated elektroda 150 – 460 mm Diameter elektroda 1,5 – 8 mm Coating pada elektroda terdiri dari silicate binder dan powder material seperti oksida, carbon, fluoride, metal alloy dan selulose (cotton cellulose) Coating berfungsi menstabilkan busur nyala, menghasilkan gas pelindung, menghasilkan slag (lapisan) u/ melindungi lasan dari oksidasi.

Laser Beam Welding Menggunakan sinar laser sebagai sumber panas u/ melelehkan logam, daya bisa sampai 100 kW Sinar laser bisa difokuskan pada area yg sempit, shg memiliki high energy density, Mampu melakukan penetrasi panas pada celah sempit dan dalam, shg cocok u/ pengelasan dgn celah sempit dgn kedalaman besar. Depth to width ratio: 4 – 10. Hasil pengelasan bagus, ketebalan bisa s/d 25 mm Sinar laser bisa kontinyu atau intermittent (u/ spot welding). Harga peralatan mahal (USD 40.000 – 1.000.000)

Weld Bead Comparison (a) (b) Figure 30.14 Comparison of the size of weld beads: (a) laser-beam or electron-beam welding, and (b) tungsten-arc welding. Source: American Welding Society, Welding Handbook (8th ed.), 1991.

Example: Laser Welding of Razor Blades Figure 30.15 Detail of Gillette Sensor razor cartridge, showing laser spot welds.

Flame Cutting Figure 30.16 (a) Flame cutting of steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate, showing drag lines.

Weld Joint Structure Figure 30.18 Grain structure in (a) deep weld and (b) shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal (see also Fig. 10.3). (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead. Figure 30.17 Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding.

Discontinuities and Defects in Fusion Welds Figure 30.19 Examples of various discontinuities in fusion welds. Figure 30.19 Examples of various defects in fusion welds.

Cracks in Welded Joints Figure 30.21 Types of cracks developed in welded joints. The cracks are caused by thermal stresses, similar to the development of hot tears in castings (see also Fig. 10.12).

Crack in Weld Bead Figure 30.22 Crack in a weld bead. The two welded components were not allowed to contract freely after the weld was completed. Source: Courtesy of Packer Engineering.

Distortion of Parts After Welding Figure 30.23 Distortion of parts after welding. (a) Butt joints and (b) fillet welds. Distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly.

Residual Stresses and Distortion Figure 30.24 Residual stresses developed in a straight butt joint. Note that the residual stresses in (b) must be internally balanced. (See also Fig. 2.29.) Figure 30.25 Distortion of a welded structure.

Weld Testing Figure 30.26 (a) Specimen for longitudinal tension-shear testing; (b) specimen for transfer tension-shear testing; (c) wraparound bend test method; (d) three-point bending of welded specimens (see also Fig. 2.11).

Welded Joints Figure 30.27 Examples of welded joints and their terminology.

Weld Symbols Figure 30.28 Standard identification and symbols for welds.

Weld Design Figure 30.29 Some design guidelines for welds. Source: After J.G. Bralla.

Example 30.2: Weld Designs Figure 30.30 Examples of weld designs used in Example 30.2.