Quantitative Analysis of Alloy Structures Solidified Under Limited Diffusion Conditions
Solidification of Containerless Undercooled Melts
The need for an instrumented drop tube More than 90% of all metallic materials are manufactured starting from their liquid state. Designing the solid structure produced during solidification can have major savings in downstream processing. This is evident in the continued development of near-net shape processing. Examples such as shape castings, strip casting, powder processing and numerous welding and joining processes form part of that development. These processing routes offer great
... ffer great advantages to reduce cost of processing due to the refined structure of the solidified alloy, formation of metastable structures or reduced macrosegregation. In these processes, melt undercooling can play a major role in determining the final solidified structure or the type and morphology of precipitates. Under undercooled liquid conditions, there are numerous feasible paths which are also affected by interdependent and complex transport issues of alloy chemistry, heat, momentum and mass transfer. Each path may achieve a different solidified structure. Empirical approaches to optimize near-net shaping routes in materials processing from the liquid state are expensive and time consuming. The development of experimentally verified models of solidification under a wide range of conditions would provide a large leap forward in developing cost effective manufacturing strategies with products having superior properties. Such models must provide reliable information verified by experiments to describe any solidification pathway from the melt. In order to develop such models, a reliable, robust and reproducible experimental technique is required. Such a capability must meet the requirements of a range of experimental conditions such as temperature, melt reactivity and composition, gas atmosphere, total pressure of gas, predictable and reproducible transport conditions for the solidifying sample and controlled and reproducible size of the sample. Electromagnetic levitation (EML) of droplets can meet many of these requirements. EML allows for direct observation of undercooling and solidification and can achieve very high undercoolings of large droplets (~6mm) that solidify in a containerless environment. However, EML alone cannot provide many of the desired quantitative data required for validating mathematical models of the evolution of microstructure during rapid solidification. For example, most of the volume of the droplet experiences a relatively low cooling rate. Also, the solidification of a droplet in such a system is strongly disturbed by large stirring effects from the electromagnetic forces used for positioning and heating the sample. Working with smaller sized droplets would be ideal.