Collaborative Research : Manipulating the Contact and Solidification of Molten Metal in Continuous Casting
A technology to enable continuous casting of molten metals, in a single step, to the specifications of the designer is being developed[7]. For example, casting aluminum foil in a single step could reduce CO 2 emissions to the atmosphere by 250,000 tons per year, in the US alone. Furthermore every kilogram of aluminum saved by reducing manufacturing waste translates into electricity saved at the energy-hungry smelter. The technology is based on controlling length scales previously uncontrolled. Tunable casting uses substrate modification to manipulate product quality. Fig. 1 Single roll casting schematic with ‘write’ and ‘erase’ head concept. 1. Overview: Single roll casting (Fig. 1) is an economical technique to produce foil or ribbon, a technique limited, however, by the ability to influence the contacting and solidification event within the contact zone. Molten metal is forced through a nozzle onto the substrate where it forms a puddle. Heat extracted to the rotating substrate causes a solidification front to grow from it as it translates and eventually a solid ribbon product is spun off. Casting rates of 10 m/s (tangential velocity) are achievable with our apparatus that uses a wheel of 3 m circumference. In the zone between nozzle and wheel, there is contact under pressure, phase-change, and large concentration (for alloys), velocity and thermal gradients. The complicated interplay between contacting, wetting and solidification physics is poorly understood. Features of the ribbon product range over 6 orders of magnitude in length scale, from the product macroscale (50 m) to the grain size (10 -5 m). Interest is in influencing the smaller scales (< centimeter) since these lead to surface irregularities, unacceptable product and waste. Controlling these scales would enable manufacturing strip product with properties of specified uniformity. The goal is to condition the substrate by imposing either thermal or compositional gradients before the contact zone[1]. To realize this goal, one first needs to understand how ‘natural’ substrate modifications influence the solidified ribbon product. For example, the cross-stream wave defect appears frequently in the cast product as a row of pits on the wheel side of the ribbon that correlates with trenches on the opposite side (air side). “How does pit formation on the wheel side relate to trenches on the air side?” is a central question. 2. Progress: Thickness can be predicted from gap variations that affect the resistance to fluid flow. That is, feed rate rather than heat-transfer rate limits the product thickness. Other progress includes: an improved understanding of the origin of ribbon defects; the relating of the substrate deformation (due to thermal loading) to the variations in the product thickness (which, coupled with the effect of gap on thickness, yields a deterministic model for through-cast thickness); and the development of a model for solidification rate which captures the important features in the dynamics throughout a cast. Visible surface defects in the ribbon product represent localized variations in thickness and may be indicative of other unintended variations in metallurgy, such as relative concentration of alloying agents. The presence NSF GRANT # 0726813 NSF PROGRAM NAME: MPM
In situ manipulation of cooling rates during planar-flow melt spinning processing
Abstract Planar-flow melt spinning (PFMS) is a single stage rapid manufacturing/solidification technique for producing thin metallic sheet or foil. A new technology, envisioned to allow real-time manipulation of the local cooling rates and properties in melt-spun ribbon, has been tested successfully when casting Al–7%Si. Pulsed laser heating, directed low on the upstream meniscus, or on the substrate, leaves patterns of ‘dimples’ in the ribbon. Typical cooling rates of 10 4  K/s have been measured using a control-volume approach. Secondary dendrite arm spacing (SDAS) has been measured through the thickness of ribbons showing areas both affected by the laser heating and unaffected by the laser. Through a correlation of cooling rates and SDAS, it is shown that the unmodified ribbon has an average cooling rate similar to that measured macroscopically. The cooling rate underneath a laser dimple is estimated to be six times slower near the contact surface. It is envisioned that the technology described may bring the concept of ‘casting-by-design’ one step closer to realization.
Composite monolayer fabrication by an arc-spray process
A single layer (monotape) technique for fabricating complex high-temperature tungsten-fiber-reinforced superalloy composites is proposed. The fabrication of sheets of arc-sprayed monotape 38 cm wide and 122 cm long has been demonstrated. Composites fabricated using the method are shown to have equal tensile strength and a cleaner matrix than composites fabricated from powder metal cloth monotapes, and the present technique is less expensive than the powder metal fabrication techniques.