Multi-Length Scale Characterization of Microstructure Evolution and Its Consequence on Mechanical Properties in Dissimil
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KAR and SATISH V. KAILAS are with the Department of Mechanical Engineering, Indian Institute of Science, Bangalore, Bengaluru, India. Contact e-mail: [email protected] SATYAM SUWAS is with the Department of Materials Engineering, Indian Institute of Science, Bangalore, Bengaluru, India. Manuscript submitted December 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
INTRODUCTION
THE demand for hybrid structures in transportation industries including the aerospace and automobile sector has increased recently, requiring an adequate joining process for dissimilar materials. The welding of titanium (Ti) with aluminum (Al) can have a major application in aerospace structures, where a higher strength-to-weight ratio is the core requirement for higher fuel efficiency and reduction of body weight. Welding of Al to Ti, especially by a fusion-based
process, is a challenge due to the difference in their physical and thermal properties as well as the problem associated with solidification, which causes a reduction in the mechanical properties of the weld. Attempts have been made to employ solid-state joining methods, such as diffusion bonding,[1] friction welding,[2,3] roll bonding and ultrasonic welding,[4] with the purpose of improving the mechanical properties of the weld and reducing the problem associated with solidification in fusion welding. Each of these methods is associated with some advantages and limitations, which are listed in Table I. Some of the issues faced during solid-state welding and fusion welding are due to the differences in thermal properties, especially the melting temperature and differences in physical properties such as flow stress, hardness and tensile properties, which vary with temperature. Friction stir welding (FSW) is a ‘‘green welding’’ technology developed at The Welding Institute in the UK in 1991. It is regarded as the most significant invention in metal joining technology considering its efficiency, versatility in use and environment friendliness. The key benefits of FSW are listed in Table II. In the case of welding of aluminum to titanium, FSW was recently introduced because of its unique ability to mechanically stir and mix in the weld nugget.[2,29–33] The FSW process is associated with several other microstructural attributes, which eventually affect the properties of the weld.[34–38] Microstructural features such as mechanical mixing, distribution of second-phase particles and refined grain structure play an important role in the final mechanical properties of the weld.[20,22,39] A number of research articles on the dissimilar FSW of aluminum to titanium are available; each of these articles is discussed and reviewed in detail as listed in Table III. In the literature,[44,50,51] it is well established that the extent of mechanical mixing is higher when the location of the weld line is on the advancing side (AS) of the tool during FSW. However, such a situation also leads to an alternating band structure comprising the intermetallic layer; Ti and mechanically mixed layers were noticed near t
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