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Index >> Welding of Magnesium and its Alloys >> Adhesive Bonding for Magnesium Alloys

Gas Tungsten Arc Welding (TIG or GTAW) for Magnesium Alloys

Gas Tungsten Arc Welding (TIG or GTAW) for Magnesium Alloys - It is perhaps the most popular process for welding magnesium alloys. Direct current straight or reverse polarity and alternating current with superimposed high frequency for arc stabilization are commonly used. Both manual and automatic methods are suitable. GTAW is used principally on materials of thickness from 0.020 inch (0.5 mm) to 0.375 inch (9.375 mm).On materials over 4.5 mm thick, A.C. is preferred, because it provides deeper penetration.
Welding Equipment A.C. Power sources should be equipped with a primary contactor operated by a control switch on the torch or by a foot switch for starting and stopping the arc. Otherwise, the arcing that occurs while the electrode approaches or draws away from the job may result in burned spots on the job. D.C. Power sources should be equipped with a continuous amperage control to obtain fine current adjustments. DCRP is preferred since DCSP is difficult to handle manually and its arc lacks cleaning action. Welding torches in a variety of sizes and tip angles are available for use with AC and DC power sources. Helium is often preferred for TIG welding using DCRP. However, as much as two to three times more helium (by volume) than argon is required for a given amount of welding.
	Argon as shielding gas is most commonly used with A.C. Helium is preferred for automatic welding because it produces a more stable arc than argon and permits the use of slightly longer arc lengths. Pure tungsten, zirconiated and thoriated tungsten electrodes, from 0.25 to 6.25 mm in diameter, are used for TIG welding of magnesium alloys. Joint Design The following points may be taken into consideration when designing joints for TIG Welding. (i) Less joint preparation is usually required for DCRP because of its better weld penetration. (ii) Use sheared edges on thin sheet up to 2 mm and double sheared edges on thicker sheet. (iii) Double bevelled joints result in less weld distortion than single bevel joints.
(iv) Parts should fit tightly without any gaps at abutting edges or with a gap of less than 1.5 mm. (v) Use 1.5 mm tack welds spaced 25 to 50 mm on centres in 1.5 mm sheet to about 6 mm tacks 100 to 125 mm on centres in 6 mm plate to help maintain fit up and prevent weld distortion. (vi)A backing plate also is useful for holding molten metal in place, minimizing distortion and preventing excessive metal drop through. The following joints are used for TIG (and MIG) welding of magnesium alloys. (i) Square groove butt joint on smaller thicknesses (t) (t up to 6 mm) (ii) Single Vee butt joint is used on thick material (t 6 to 9 mm). (iii) Double Vee butt joint is used on still thicker materials. It minimizes distortion by equalizing shrinkage stresses on both sides of joint (t ≥ 9 mm). Butt and fillet joints are the easiest to make and provide more consistent results than other types of joints. Lap joints are used sometimes, but generally are not as satisfactory as butt joints for stressed applications.
Welding Technique (i) Arc length maintained should be about 1132 inch (0.8 mm). (ii) Forehand welding is preferred. (iii) Weaving should be used only for fillet welds or large corner joints. (iv) Minimize the number of stops during welding. After a stop, the weld should be restarted on weld metal about 12 mm from the end of the previous weld. (v) To prevent weld cracking: (a) Make use of starting and run off plates (or tabs) to start and end the weld. (b) Weld from middle of the job towards the ends. The base metal and the fixture if used, also, should be preheated at least 94 to 150°C. (vi) Welding conditions for manual Tungsten Argon Arc welding of butt joints in magnesium alloys are given below:

Plate thickness (mm)	Type of joint	No.of passes	Electrode Diameter (mm)	Current AC (Amp.)	Filler rod diameter (mm)
1.5	X	1	2.3	50	2.3
3	X	1	2.3	125	3
6	Y	2	4	175	3
12.5	Z	2	4.5	250	3

X - Square groove butt joint, with zero root opening.
Y - 60°-bevel, single V-groove butt joint, 1.5 mm root face and zero root opening.
Z- 60°-bevel, double V-groove butt joint, 2.3 mm root face and zero root opening.

Thoriated tungsten electrodes will require currents about 20% higher. After welding, magnesium parts can be readily straightened by heating to temperature between 150 and 205°C.

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