Intermetallic Diminution During Friction Stir Welding of Dissimilar Al/Mg Alloys in Lap Configuration Via Ultrasonic Ass
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TRODUCTION
ALUMINUM (AA6061-T6) and magnesium (AZ31B) alloys are essential materials and have been employed in several structural components for aerospace and automotive applications.[1] A successful joining of these dissimilar metals is vital to ensure their full-fledged integration and to gain a higher strengthto-weight ratio with economic viability. Various fusion welding techniques have been attempted in the past to join the two metals.[2–4] Unfortunately, due to the formation of several weld defects and the enormous number of IMCs, most of them failed to produce high-strength dissimilar joints.[5] Recently, the friction stir welding (FSW) technique has been found effective for fabricating high-strength joints of Al and Mg alloy.[6–10] As observed, the joints made with the FSW process have fewer weldment defects and a thin IMC layer, thereby possessing superior joint strength compared to that of fusion welded joints.[11,12] In the FSW process, the maximum temperature encountered in the stirred zone (SZ) is
SACHIN KUMAR, CHUANSONG WU, and LEI SHI are with the MOE Key Lab for Liquid-Solid Structure Evolution and Materials Processing, Institute of Materials Joining, Shandong University Jinan, 250061 China. Contact E-mail: [email protected] Manuscript submitted on May 26, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
significantly less than in the fusion welding processes,[13,14] which is vital for IMC suppression. Additionally, a high strain rate and intensive plastic deformation are the primary characteristics of the FSW technique and have been proven quite effective at curbing various weld defects and enhancing the joint quality.[15] The quality of FSWed AA6061-T6-AZ31B Mg alloy can fulfill industrial norms; hence, it finds applicability in numerous structural components such as automotive chassis, internal gear components, and engine parts of military helicopters, etc.[1] As evident from the binary phase diagram of Al/Mg alloys, the Al and Mg-rich phases react to each other in the solid state to form the intermetallic nature of Al3Mg2 or Al12Mg17 during the FSW process.[16] The individual nature of formation primarily depends on the local composition of the respective material as well as the temperature gradient.[16] At a temperature of 437 C the eutectic reaction (Mg) + Al12Mg17fi L (liquid) and at 450 C another eutectic reaction (Al) + Al3Mg2 fi L take place.[16,17] The values of these eutectic temperatures are notably below the melting point temperatures of respective alloys. As per Sato et al.,[16] depending on the welding parameters used during the FSW of Al/Mg alloys, a small amount of liquid film is formed across their dissimilar interfaces. This liquation process takes place far below the melting point temperature of the respective materials and is termed as constitutional liquation.[16] During cooling of the weld, the IMC phases of Al3Mg2 or Al12Mg17 nature are precipitated
depending on the localized material concentration. The nucleation, as well as the formation of IMCs, is a critical concern during th
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