I defended my dissertation in December (2010); click here to download my dissertation. A video of my defense, filmed and edited by Jared Cook, is available on YouTube in four parts (Part 1, Part 2, Part 3, and Part 4). I am currently working as a post-doc at BYU to publish journal articles summarizing a novel interlayer selection procedure for an advanced bonding process. The first article has been published by the Journal of Materials Science and is a current finalist for the Sapphire Prize. The second has been submitted, and work is well underway for the third article.
Abstract: Partial transient liquid phase (PTLP) bonding is a high-level bonding process mainly used to join ceramic materials using an interlayer composed of multiple layers. While PTLP bonding is currently an esoteric joining process with limited applications, it has desirable advantages compared to typical joining techniques and is the best joining technique for certain applications. Specifically, it can bond hard-to-join materials as well as dissimilar materials, and bonding is performed at relatively low temperatures.
Part of the difficulty in applying PTLP bonding, though, is finding suitable interlayer combinations. A novel interlayer selection procedure has been developed to facilitate the identification of successful PTLP bond interlayer combinations and is explained in this and a companion article. This article presents a filtering routine that identifies ideal interlayer combinations for many different applications. The routine integrates every important characteristic of PTLP bonding that can be generalized, and also includes a set of user-customizable parameters. These parameters include important design considerations, such as bonding temperature, target remelting temperature, bond solid type, and interlayer thicknesses. The output from this routine provides a detailed view of each candidate interlayer combination as well as a broad view of the entire set of candidates, greatly facilitating the selection of ideal interlayer combinations. It is expected that the use of this routine and the selection procedure detailed in the companion article will expand PTLP bonding as an industrial joining process.
Journal of Materials Science, Volume 46, Number 16, pp. 5305–5323, August 2011, current finalist for the Sapphire Prize
_bin_sys.jpg)
Abstract: Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented.
Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side.
This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.
This article briefly outlines the PTLP bond selection procedure that I developed and that will be explained in great detail in some upcoming journal articles. It was published in FEA Information, an online publication of LS-DYNA.
This article details the PTLP bond filtering procedure that I developed as part of my doctoral research and have recently enhanced. The procedure is general so that it can be used to identify ideal interlayer combinations for any PTLP bond. It is expected that the implementation of this procedure will greatly expand the number of PTLP bonding applications.
My research involved joining tungsten carbide (WC) and polycrystalline cubic boron nitride (PCBN) by a joining process known as partial transient liquid phase (PTLP) bonding. PTLP bonding is a joining process whereby two materials are joined by a thin interlayer. PTLP bonding can be used on many hard-to-join materials (e.g., ceramics, metal matrix composites). The interlayer is composed of a refractory core and suitable diffusants on each side. Upon heating, the diffusants melt and quickly diffuse into the refractory metal core. After a sufficient amount of diffusion has occurred, the entire interlayer solidifies isothermally due to changes in the interlayer's composition. The resulting ceramic joint then has a higher melting point than the temperature used for joining.
When all the elements alloys that exist are considered, there are essentially an infinite number of possible combinations for the bonding interlayer. To solve this problem, I developed a systematic method of filtering out material combinations that will not work. This method involves (1) assuming three layers in the bond (a refractory core surrounded by two diffusant layers); (2) analyzing all elements of the periodic table with respect to the bonding criteria; (3) using a database of existing binary phase diagrams which includes data such as maximum solubilities, room-temperature solubilities, melting points, and eutectic and peritectic points; (4) conducting sessile drop (or wetting) tests to determine surface energies for candidate elements; and (5) developing a numerical model to determine critical interlayer thicknesses for PTLP bonding.
The material combinations resulting from the filtering process combine to form a set of ideal PTLP bond setups. More importantly, this filtering method utilizes a simple, but robust method to quickly determine ideal setups which can be validated against data that has been obtained empirically. This method is being outlined in detail in two upcoming journal articles. The use of this filtering method will greatly extend the number of bonds possible between materials that are typically difficult to join, such as ceramics and metal matrix composites.
.jpg)
A finite-difference model was developed for two plates of metal being friction stir welded together. The steel backing plate is included in this analysis. The parameters of the model were chosen to produce an error less than or equal to 0.1%. Also, a grid refinement study was conducted and the nodal stability criterion was checked to verify the numerical model. Then, the output of the model was compared to thermocouple data gathered from friction stir welding Inconel 718 plates. The results demonstrate the overall usefulness of this type of model in qualitatively predicting temperature profiles during FSW. However, certain modifications must be made to this model to obtain more accurate results.
Friction stir welding (FSW), a relatively new solid-state welding process, uses frictional heat to stir metals together without melting them. The heat dissipation during FSW is formulated using the heat equation and appropriate boundary conditions. Analytical model results are compared to experimental data for FSW of steel X65. The analytical model produced higher temperatures than the experimental data, likely because of the insulated boundary condition on the bottom of the steel plate. Improvements to the analytical model can be made by incorporating a conducting boundary condition at this surface and by modifying the energy source term. Despite the differences in results, the analytical model provides a good qualitative estimate of heat dissipation during FSW.
Homogenization theory is a method of obtaining effective, or continuum-level, properties for a given material based on the properties of given phases within the material. Sampled sections from the material are analyzed by orientation imaging microscopy, producing volume fraction, phase location, and phase orientation data. These data are used in conjunction with correlation functions and the Green's function to predict the effect that the material's microstructure has on local properties. The results are used to predict the effective material properties.
Specifically, the two-point correlation function parameterizes the effect of all other material points in the microstructure on a given material point as a function of distance from that given point. For most materials, the two-point correlation function will eventually converge to a theoretical value after passing the coherence limit or radius. A notable exception to convergence is a microstructure with long-range periodicity.
This paper presents two-point correlation functions for a two-phase eigen microstructure. (An eigen microstructure has only one phase in each sub cell of the material domain, and therefore, the properties are constant over each sub cell.) In particular, this paper discusses convergence studies for the two-point correlation functions with respect to the sampled cube size and the ensemble size.
Samples of nickel were rolled to different reduction levels, namely: 0, 31, 64, and 95 percent (with and without a post anneal). These five samples were analyzed by orientation imaging microscopy. Detailed results from this analysis include grain distributions, pole figures, grain images, and inverse pole figures of the fundamental zone for each sample.
Tensile samples at the 0 percent and 95 percent (without anneal) reduction levels were tested and analyzed for the modulus of elasticity and other pertinent material properties. The 95 percent reduction level samples had greatly increased properties such as 0.2% offset yield and ultimate tensile strength, while some properties such as ductility decreased, thus indicating the expected brittle nature of the 95 percent reduced samples.
An upper-bound (Hill-Paul) and a lower-bound model were used to predict the modulus of elasticity, critical resolved shear stress, and yield strength for various nickel samples using data from the orientation imaging microscopy analysis. The data from these models was in relative agreement with the actual data obtained from the tensile testing analysis.
The microstructure of an as-received nickel sample was characterized using tools in an orientation imaging microscopy (OIM) software package. This process was repeated for a nickel sample rolled to 64 percent reduction. Results from the analysis for the as-received sample were used to predict the evolution of the nickel microstructure after a 64% rolling reduction. These predictions were compared to the actual results obtained from the OIM software. The discrepancies between the predictions and the actual data are likely due to cold rolling having been done transverse to the hot rolling direction.
The soundboard of a piano is an integral part of its construction. This paper describes a design method to create a composite soundboard which matches all 88 piano frequencies and could therefore replace the typical Sitka spruce soundboard. This method optimizes design variables such as soundboard size, composite lay-up, and composite material. The composite soundboard is optimized with respect to the number of natural frequencies required to match the piano's frequencies. The final result is a 4-ply graphite/epoxy (T300/5208) composite soundboard with a lay-up of [67.5/90]s and dimensions of 1.2 by 0.8 m. Suggestions for future work to improve on this design method are given.