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Industrial Machinery Repair Episode 2 Part 11

Tham khảo tài liệu 'industrial machinery repair episode 2 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 484 Maintenance Welding Argon is used more extensively than helium for GTAW because 1 It produces a smoother quieter arc action 2 It operates at a lower arc voltage for any given current and arc length 3 There is greater cleaning action in the welding of materials such as aluminum and magnesium in the AC mode 4 Argon is more available and lower in cost than helium 5 Good shielding can be obtained with lower flow rates 6 Argon is more resistant to arc zone contamination by cross drafts 7 The arc is easier to start in argon. The density of argon is approximately 1.3 times that of air and 10 times that of helium. For this reason argon will blanket a weld area and be more resistant than helium to cross drafts. Helium being much lighter than air tends to rise rapidly and cause turbulence which will bring air into the arc atmosphere. Since helium costs about three times as much as argon and its required flow rate is two to three times that for argon the cost of helium used as a shielding gas can be as much as nine times that of argon. Although either helium or argon can be used successfully for most GTAW applications argon is selected most frequently because of the smoother arc operation and lower overall cost. Argon is preferred for welding thin sheet to prevent melt-through. Helium is preferred for welding thick materials and materials of high thermal conductivity such as copper and aluminum. Electrode Material for GTAW In selecting electrodes for GTAW five factors must be considered material size tip shape electrode holder and nozzle. Electrodes for GTAW are classified as pure tungsten tungsten containing 1 or 2 thoria tungsten containing 0.15 to 0.4 zirconia and tungsten that contains an internal lateral segment of thoriated tungsten. The internal segment runs the full length of the electrode and contains 1 or 2 thoria. Overall these electrodes contain 0.35 to 0.55 thoria. All tungsten electrodes are normally available in diameters from 0.010 to 0.250 and lengths .