【正文】 metal 基體金屬，母材 。 Molten weld metal 熔化的焊接金屬，熔池金屬 。 Solidified weld metal 凝固焊縫金屬，焊縫 Figure The gas tungstenarc welding process, formerly known as TIG (for tungsten inert gas) welding. Tungsten electrode: Thorium（釷） , cerium（鈰） or zirconium（鋯） may be used in the tungsten electrodes, to improve their electron emission characteristics. Contamination of the tungsten electrode Contamination （污染） of the tungsten electrode by the molten metal can be a significant problem, particularly in critical applications, because it can cause discontinuities in the weld. Contact of the electrode with the molten metal pool should be avoided. How to influence the weld？ Power supply Depending on the metals to be welded, the power supply is either DC (include pulsedDC) or AC. In general, AC is preferred for aluminum and magnesium, because the cleaning action of AC removes oxide and improves the weld quality. Figure Equipment for gas tungstenarc welding operations. Process Capabilities: ① The process has better arc stability, and is especially suitable for thin metals (normally ≤4mm) 。 ② It provides welds with high quality and surface finish 。 ③ It can be used in all welding position。 ④ It is used for a wide variety of metals and applications, particularly aluminum, magnesium, titanium and the refractory metals (難熔金屬） 。 ⑤ The cost of the inert gas makes this process more expensive than SMAW. ? PlasmaArc Welding (PAW) （等離子弧焊） In PAW, a concentrated plasma arc (壓縮等離子弧 ) is produced and is aimed at the weld area. The arc is stable and reaches temperatures as high as 33000℃ . A plasma is ionized hot gas (完全電離了的氣體 ), posed of nearly equal numbers of electrons and ions. When a filler metal is used, it is fed into the arc, as is done in GTAW. Arc and weldzone shielding is supplied by argon, helium, or mixtures. How to concentrate the arc? plasma gas (離子氣 )。 shielding gas (保護(hù)氣 )。 power supply (電源 ) Figure Two types of plasmaarc welding processes: (a) transferred, (b) nontransferred. Deep and narrow welds can be made by this process at high welding speeds. There are two methods of PAW: ① transferredarc method: (轉(zhuǎn)移電?。? The workpiece being welded is part of the electrical circuit, the arc transfers from the electrode to the workpiece. It is mainly used for welding and cutting of metals. ② nontransferred method: (非轉(zhuǎn)移電弧） The arc occurs between the electrode and the nozzle, workpiece is not linked into the circuit, and the heat is carried to the workpiece by the plasma gas. It is mainly used for welding and cutting of nonmetals. Keyhole technique (小孔型等離子弧焊 ) Process Capabilities: ① PAW has higher energy concentration ( and so permits deeper and narrow welds), better arc stability, less thermal distortion, and higher welding speeds. It has higher productivity 。 ② With keyhole technique, it can perate pletely through the joint of thicknesses as much as 20mm for some Ti and Al alloys ? Effective heat in welding (a) Carbon (electrode) arc (b) Gas metal arc (c) Submerged arc Thermit Welding （ 熱劑焊 /鋁熱焊 ） ElectronBeam Welding (EBW) 電子束焊 In EBW, heat is generated by highvelocity narrowbeam electrons. The kiic 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 perates and the greater the depthtowidth ratio. Generally, no shielding gas, flux, or filler metal is required. Electron gun: Electrons are accelerated and focused Process Capabilities: ① Almost any metal can be welded by EBW (the weld quality is high due to the vacuum). ② The process has the capability of making highquality welds that are almost parallelsided, deep and narrow, and have small heataffected zones（熱影響區(qū)） . Distortion（變形） and shrinkage（收縮） in the weld area is minimal. Depthtowidth ratios（深寬比） range between 10 to 30. ③ The intense energy is also capable of producing keyholes in the workpiece, and workpiece thicknesses can range from foil (箔，薄片 ） up to 150 mm. ④ Typical applications include the welding of aircraft, missile, nuclear, and electronic ponents and of gears and shafts for the automotive industry. (important parts) ⑤ EBW equipment generates xrays。 hence, proper monitoring and periodic maintenance are essential. LaserBeam Welding (LBW) 激光焊 LBW utilizes a highpower laser beam as the source of heat, to produce a fusion weld. The laser beam can be focused onto a very small area, it has high energy density and, therefore, deepperating capability. This process is particularly suitable for welding deep and narrow joints, with depthtowidth ratios range from 4 to 10. The laser beam may be pulsed for applications such as spot welding of thin materials. Continuous multikW laser systems are used for deep welds on thick sections. Figure Comparison of the size of weld beads in (a) electronbeam or laserbeam welding to that in (b) conventional (tungstenarc) welding. Source: American Welding Society, Welding Handbook (8th ed.), 1991. Comparison of LaserBeam and TungstenArc Welding Process Capabilities: ① LBW produces welds of good quality, with minimum shrinkage and distortion. Laser welds have good strength and are generally ductile and free of porosity。 ② It is particularly effective on thin workpiece 。 it can be automated to weld the materials with thicknesses of up to 25mm . Welding speeds range from metals。 ③ Typical metals and alloys welded include aluminum, titanium, ferrous metals