UBC Theses and Dissertations
Heat transfer and microstructure during the early stages of solidification of metals Muojekwu, Cornelius A.
The future of solidification processing clearly lies not only in elucidating the various aspects of the subject, but also in synthesizing them into unique qualitative and quantitative models. Ultimately, such models must predict and control the cast structure, quality and properties of the cast product for a given set of conditions Linking heat transfer to cast structure is an invaluable aspect of a fully predictive model, which is of particular importance for near-net-shape casting where the product reliability and application are so dependent on the solidification phenomena. This study focused on the characterization of transient heat transfer at the early stages of solidification and the consequent evolution of the secondary dendrite arm spacing. Water-cooled chills instrumented with thermocouples were dipped into melts of known superheats such that unidirectional solidification was achieved. An inverse heat transfer model based on the sequential regularization technique was used to predict the interfacial heat flux and surface temperature of the chill from the thermocouple measurements. These were then used as boundary conditions in a 1-D solidification model of the casting. The secondary dendrite arm spacing (SDAS) at various locations within the casting was computed with various semi-empirical SDAS models. The predictions were compared with experimental measurements of shell thickness and secondary dendrite arm spacing from this work as well as results reported in the literature. The effects of superheat, alloy composition, chill material, surface roughness and surface film (oil) were investigated. The results indicate that the transient nature of the interface heat transfer between the chill and casting exerts the greatest influence in the first few seconds of melt-mold contact. The interfacial heat flux and heat transfer coefficient exhibited the typical trend common to solidification where the initial contact between mold and melt is followed by a steadily growing gap. Both parameters increase steeply upon contact up to a peak value at a short duration (< 10s), decrease sharply for a few seconds and then gradually decline to a fairly steady value. Heat transfer at the interface increased with increasing mold diffusivity, increasing superheat, decreasing thermal resistance of the interfacial gap, increasing thermal expansion of the mold, decreasing shrinkage of the casting alloy, decreasing mold thickness and initial temperature, and decreasing mold surface roughness. The secondary dendrite arm spacing decreased with increasing heat flux for the same alloy system and depended on the cooling rate and local solidification time. The secondary dendrite arm spacing was also found to be a direct function of the heat transfer coefficient at distances very near the casting/mold interface. A three stage empirical heat flux model based on the thermo physical properties of the mold and casting was proposed for the simulation of the mold/casting boundary condition during solidification. The applicability of the various models relating secondary dendrite arm spacing to heat transfer parameters was evaluated and the extension of these models to continuous casting processes was pursued.
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