Microstructure and Properties Analysis of Laser Welding and Laser Weld Bonding Mg to Al Joints
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DUCTION
THE driving force for joining Al and Mg arose from the need for weight savings, essentially from a need for energy efficiency in the automobile industry and the requirement for chemical plants and cryogenic applications.[1,2] To broaden the field of application of Mg alloys and Al alloys, joining processes such as tungsten inert gas welding, laser beam welding, friction stir welding (FSW), diffusion bonding, and electromagnetic impact welding have been applied to join Mg alloys to Al alloys.[3–9] In a Mg-Al dissimilar welding joint, the influences of Mg-Al intermetallics are discussed widely. When the formation of Mg-Al intermetallics was controlled, the property of the joint was controlled as well.[10] Laser weld bonding (LWB) is a new welding technique that was put forward as an alternative to pulse laser welding and adhesive bonding. It is used to join a Mg alloy to an Al alloy successfully.[11] In suitable parameters, the tensile shear strength of the LWB Mg to Al joint was approximately 80 pct of the Mg base metal.[12] A synergy develops in which the tensile and peel performance of the weld-bonded structure exceeds that of either technology alone. A laser beam is the thermal source for the LWB process, the same as that for laser welding. Thus, the main characters of the LWB joint are approximately the same as that of laser welding. The two kinds of welding modes in general laser-welding processing are the conductive mode and the keyhole mode.[13,14] In the process of laser welding the same metal, the effect of the laser welding mode on the microstructures is not LIMING LIU, Professor, and HONGYANG WANG, Doctor, are with the Department of Materials Science and Engineering, Key Laboratory of Liaoning Advanced Welding and Joining Technology, Dalian University of Technology, Dalian 116024, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 5, 2010. Article published online November 2, 2010 1044—VOLUME 42A, APRIL 2011
significant. However, in dissimilar welding joints, the microstructures are often decided by the laser welding mode, especially for the formation of intermetallics. The Mg-Al intermetallics were found in most kinds of melted welding joints. In the LWB Mg to Al process, the addition of the adhesive affects the microstructures of the joint, especially for the distribution of the Mg-Al intermetallics. In this article, the microstructures and properties of laser welding and LWB Mg to Al joints with different kinds of welding power density were investigated separately by using scanning electron microscopy (SEM) and an electronic probe microanalyzer (EPMA). The differences between the laser welding joints and LWB joints were analyzed comparatively. The effects of the adhesive on the formation of the microstructure were discussed in different welding modes.
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EXPERIMENTAL
Samples of extruded AZ31Mg alloy (80 9 25 9 1.2 mm) and 6061Al alloy (80 9 25 9 1.7 mm) were used during the LWB process. The experiments were carried out using an Nd: YAG laser with a maximum output power of 500 W. Th
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