Asymmetric Diffusional Solidification during Transient Liquid Phase Bonding of Dissimilar Materials
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THE application of fusion welding for the fabrication and repair of parts made of precipitation-hardened nickel-based superalloys that are used in aerospace and power generation industries is severely limited because of the high susceptibility of these materials to fusion zone and heat-affected cracking.[1,2] Conventional brazing has evolved consequently as an alternative method for joining these difficult-to-weld structural alloys. Nevertheless, brazed joint properties are generally inferior to those of the base superalloy because of the formation of hard and brittle eutectic microconstituents along the brazed joint from residual interlayer liquid during cooling from joining temperature.[3] The prevention of the deleterious formation of nonequilibrium solidification reaction products can be achieved through the transient liquid phase (TLP) bonding process.[4] The process involves sandwiching a filler material containing a melting point depressant (MPD) element, such as boron, silicon, and phosphorus in nickel, between the substrate layers and heating the whole assembly to a high temperature, usually between the liquidus temperature of the filler and the solidus temperature of the base alloy. At the bonding temperature, the interlayer alloy melts and attains equilibrium rapidly with the solid base metal through the process of melt-back dissolution of the substrate. Subsequent to this, interdiffusion of alloying elements between the base metal and the liquid A. GHONEIM, PhD Candidate, and O.A. OJO, Associate Professor, are with the University of Manitoba, Winnipeg, MB R3T 5V6, Canada. Contact e-mail: [email protected] Manuscript submitted January 15, 2011. Article published online December 14, 2011 900—VOLUME 43A, MARCH 2012
commences and causes the melting point of the interlayer liquid at the liquid–solid interface to increase, which results in isothermal solidification of the liquid. As the MPD solute diffuses continuously from the liquid into the base metal, the volume of liquid that can be maintained at equilibrium decreases, which causes solidification to proceed towards the center of the joint from the mating solid surfaces. If sufficient time for complete isothermal solidification is not allowed at the bonding temperature, then the formation of eutectic microconstituents could occur along the braze centerline. Therefore, an important process parameter in the consideration of TLP bonding for commercial applications is the holding time (tf) required to achieve complete isothermal solidification that is necessary for preventing the formation of eutectic that degrades properties of materials joined by conventional brazing techniques. A major attractive potential of TLP bonding process is its capability of producing high-performance joints in dissimilar materials, if properly optimized.[3] Despite the potential for joining dissimilar alloys, the optimization of the process for joining of dissimilar materials is limited. This is largely because of the lack of appropriate theoretical models for simulating and understanding
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