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Title:Rapid Solidification of Alloy Substrates by Lasers and Electron Beams: Heat Flow Modelling and Solidification Morphology
Author(s):Sekhar, Jainagesh Akkaraju
Department / Program:Metallurgy and Mining Engineering
Discipline:Metallurgical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Metallurgy
Abstract:A combined theoretical and experimental study is described for the rapid melting and solidification of pure aluminum and aluminum copper alloy substrates subjected to a high intensity heat flux. The problems addressed are for stationary or moving heat fluxes absorbed through a circular region on the bounding surface of the semi-infinite substrates. The most important contribution of this study is the development of a new heat flow model and solidification of an alloy substrate over a range of temperatures, and the verification of same with steady-state experiments on an Al-4.5 wt.% Cu alloy substrate.
For the case of a pure metal substrate, previously developed heat flow models are generalized and the effect of temperature-dependent absorptivity on surface melting is described using numerical examples.
In the new heat flow model for melting and solidification of an alloy surface, regimes where steady-state thermal conditions are possible are identified. Numerical solutions using the dimensionless temperature and enthalpy to formulate a single energy equation indicate that a wide range of average cooling rates during solidification may be possible by varying the process parameters.
Experiments were performed in a 10 KW electron beam machine. Comparison of theory and experiments of steady-state melt profiles is shown to be good considering the simplifying assumptions of the theory. Further changes in the melt profiles with changes in the beam-on times and incident power unit length (qa) are compared with theory.
Microstructural details reveal that the initial breakdown of the interface on resolidification yields cells which may not grow parallel to the heat flow direction. However, when the interface is moving at a lower velocity, the cells tend to align themselves in the heat flow direction.
A new technique to reveal segregation in aluminum copper alloys with low copper contents is developed using electrochemical means. This technique is used to demonstrate the morphological stability of rapidly growing interfaces in an Al-0.006 wt.% Cu alloy by comparing an electron beam melted sample which displayed morphological stability to a conventionally arc melted sample displaying interface breakdown. The calculated heat flow conditions (i.e., the interface velocity and temperature gradient in the liquid) to obtain morphological stability in aluminum copper alloys are also discussed.
Issue Date:1982
Description:248 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.
Other Identifier(s):(UMI)AAI8209628
Date Available in IDEALS:2014-12-16
Date Deposited:1982

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