• PDF / 1,535,357 Bytes
• 10 Pages / 593.972 x 792 pts Page_size
• 64 Downloads / 216 Views

DOWNLOAD

REPORT

---

TION

MAGNESIUM alloys have exceptional specific strength, stiffness, damping capacity, machinability, castability, and weldability, making it attractive for use in the automobile, aerospace, and electronic industries.[1] However, magnesium alloys have poor formability at room temperature because of the limited slip systems in the hexagonal close-packed lattice. Most magnesium alloys are readily weldable using gas tungsten arc (GTA), gas metal arc (GMA), plasma arc, electron beam, laser beam, friction, explosion, stud, ultrasonic, and spot welding processes.[2] GTA welding was selected for use in this study because it produces very high quality welds as opposed to other methods (e.g., GMA or laser), thus minimizing complicating effects arising from weld defects such as undercuts, porosity, and weld spatter.[3] The main problems associated with welding of magnesium alloys are caused by the high oxidizing properties of magnesium during heating to high temperatures and formation of oxide film. A significant difference in the melting temperatures of magnesium oxide and magnesium itself about 2273 K (2000 C) results in the surface of molten pool being covered by an oxide film during welding.[4] The density of magnesium oxide film is approximately 3.2 g/cm3, whereas that of magnesium itself in liquid state is 1.6 g/cm3. As a result, the film can immerse into the molten metal and form an incomplete N. KISHORE BABU, Scientist, is with the Joining Technology Group, Singapore Institute of Manufacturing Technology (SIMTech), Singapore 638075 Singapore. Contact e-mails: kishorebn@simtech. a-star.edu.sg; [email protected] C.E. CROSS, Staff Scientist, is with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted October 13, 2011. Article published online June 6, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

fusion of welded edges. The loose magnesium oxide film does not protect the metal from subsequent oxidation, especially within the range of welding temperatures. Thus, during the welding process, the oxygen and nitrogen present in the air can actively dissolve into the molten metal and decreases the mechanical properties. Hence, alternating current (AC) or variable polarity (VP) are preferred GTA welding techniques for magnesium alloys because they destroy the oxide layer present on the surface of base metal during the electrodepositive cycle. VP and AC GTA welding have been successfully applied to various aluminum and magnesium alloys.[5,6] VP is similar to AC in that it alternates between positive and negative electrode polarities. It is different from AC in that the balance of two polarities and the current at each polarity can be varied independently (e.g., compare Figures 1 (a) and (c)). The VPM mode is a variation of VP welding that allows programming a segment of VP welding followed by a segment of straight direct current (DC) welding (Figure 1(b)). This ability increases penetration while obtaining the required cleaning action of VP. AC pulsing (ACPC) is a variation of AC welding that involves vary