On Asymmetric Diffusional Solidification During Transient Liquid Phase Bonding
- PDF / 2,325,238 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 94 Downloads / 223 Views
ades, developments in alloy design have resulted in advanced materials with remarkably improved properties for structural engineering applications. Nevertheless, these developments have not been matched with adequate understanding of the advanced techniques required for joining these materials during component fabrication. Transient liquid phase (TLP) bonding has evolved as an attractive technique for joining advanced materials for aerospace, biomedical, and electronic applications.[1–5] It involves sandwiching a filler material containing a melting point depressant (MPD) element between the substrates 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 rapidly attains equilibrium with the solid base material through the process of meltback dissolution of the substrate. As the MPD solute diffuses from the liquid into the substrate, the volume of liquid that can be maintained at equilibrium decreases, which causes solidification to proceed toward the center of the joint from the adjoining substrates. If sufficient time for complete isothermal solidification is not allowed at the bonding temperature, formation of deleterious non-equilibrium solidification microconstituents would occur within the joint region. Therefore, the most crucial stage of TLP bonding, which influences the microstructure and properties
of the joint, is the diffusional solidification of the interlayer liquid during holding at the bonding temperature. Asymmetric diffusional solidification that can alter joint microstructure and properties has been reported during TLP bonding of dissimilar materials due to mismatch between diffusional properties of the dissimilar base materials.[6] Apart from dissimilar substrates, asymmetric distribution of microconstituents can also occur in TLP joints between substrates with similar chemical composition and microstructure. However, the underlying mechanism of such occurrence in similar substrate materials, which is imperative to controlling its formation, is not fully understood, and it is the primary focus of this study. Practically, some degree of unintentionally imposed temperature gradient, although often of low magnitudes, exists in vacuum furnaces that are used to perform TLP bonding. Its presence is generally ignored during theoretical modeling of TLP bonding. Shirzadi and Wallach[7] have, however, reported that even a small degree of unintentionally imposed temperature gradients may have significant effect on TLP bonding process. Due to asymmetric boundary conditions at the two propagating liquid–solid interfaces, a non-symmetrical numerical model that also effectively conserves solutes, as opposed to symmetrical models that are often used for conventional TLP bonding process, is required for a proper theoretical simulation of TLP bonding under temperature gradient (TLP-UTG). In the present work, the challenges of developing an asymmetric numerical model which
Data Loading...
