Microstructural Characteristics of a Stainless Steel/Copper Dissimilar Joint Made by Laser Welding
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INTRODUCTION
HYBRID structures with dissimilar alloys combining their respective advantages have many industrial applications.[1–5] In heat exchangers, the corrosion resistance of stainless steel and heat conductivity of copper are required. The joining of stainless steel and copper is necessary to manufacture these structures. However, the dissimilar welding of stainless steel and copper presents a series of problems. First, there are large differences between their physical properties, including melting point, thermal conductivity, and thermal expansivity. These differences make defect-free dissimilar welding difficult using conventional welding methods such as tungsten inert gas (TIG) and metal inert gas welding. Second, although iron (Fe)copper alloys are completely miscible in the stable liquid state and do not form brittle intermetallic compounds, the system shows a wide metastable miscibility gap at an undercooling level.[6] Thus, the phenomenon seems to be unavoidable for fusion welding, which has a certain undercooling level. The segregations of high- or lowmelting-point phases inside the seam must be taken into account because they may have negative influence the mechanical properties of a joint. Generally, mixing liquid stainless steel and copper should be limited to control metastable liquid phase separation during welding. The use of high-power density heating sources such as laser beam and electron beam (EB) is possible to restrict metastable liquid phase separation by controlling mixing liquid stainless steel and copper. The EB welding is a fusion welding process that employs a narrow beam of highenergy electrons to melt a material and create a weld that SHUHAI CHEN, Assistant Professor, JIHUA HUANG, Professor, JUN XIA, Master Student, HUA ZHANG, and XINGKE ZHAO, Associate Professors, are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 23, 2012. Article published online March 16, 2013 3690—VOLUME 44A, AUGUST 2013
joins two pieces of metal. The EB should be slightly shifted to the steel or copper side to suppress their mixing. Although the EB welding of steel and copper has been realized, it is difficult to precisely control heating location because the electron beam is easily perturbed by electromagnetic environments, such as ferromagnetic of the steel. Thus, suppressing the mixing of molten steel and copper is not ideal for the EB welding.[7–9] In contrast, the heating position of the laser beam can be controlled accurately. Mai and Spowage[10] proposed a processing map in which butt joints are fabricated by focusing a 0.2-mm-long laser beam into a steel plate. Although the steel plate is completely melted at the top of the bead, fusion does not occur at the bottom. The main reason is the high thermal conductivity of copper. Preheating or a high-power density can improve the joint integrity. Consequently, Phanikumar et al.[11] studied the continuous CO2 laser welding of
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