Journal
JOURNAL OF MATERIALS SCIENCE & TECHNOLOGY
Volume 32, Issue 1, Pages 76-88Publisher
JOURNAL MATER SCI TECHNOL
DOI: 10.1016/j.jmst.2015.08.012
Keywords
Friction stir spot welding; Magnesium alloys; Zn interlayer; Hook defects
Funding
- National R&D Program of China [2011BAE22B05]
- Liaoning Province Doctor Startup Fund Program [20131087]
- National Natural Science Foundation of China [51371179, 51331008]
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With the addition of a thin Zn interlayer, 2.4 mm thick Mg-3Al-1Zn alloy sheets were friction stir spot welded (FSSW) using a pinless tool with flat, convex and concave shoulder shapes. The results showed that an alloying reaction took place between the Mg substrate and Zn interlayer during FSSW, forming a discontinuous intermetallics layer composed of dispersive (alpha-Mg + MgZn) eutectic structure underneath the shoulder and a Mg. Zn intermetallics bonding zone at the outside of the joints. This alloying reaction increased the bonded area and eliminated the hook defects, thereby producing sound FSSW joints with a shallow keyhole without hook defects. The increase of plunge depth was beneficial to the Mg. Zn diffusion, thereby increasing the tensile-shear load of the joints. However, excessive plunge depths resulted in a decrease of the effective sheet thickness, reducing the strength of the joints. At a small plunge depth, the convex and concave shoulders were more beneficial to the interface reaction than the flat shoulder. The maximum joint load of 6.6 kN was achieved by using the concave shoulder at a plunge depth of 1.0 mm. A post-welding heat treatment promoted the dissolution of the discontinuous reaction layer in the joints; however, it led to the occurrence of void defects, influencing the bonding strength. Copyright (C) 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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