The production of aluminium sheet is expensive and energy intensive
despite the reduced environmental impact during use. Twin roll
casting is a method of directly producing aluminium alloys in near net
shape directly to sheet at a fraction of the energy costs of conventional
DC casting / hot rolling. It also requires a fraction of the capital cost.
Although sheet can be produced, defects (segregates, surface bleeds,
buckling, etc.) can arise which limit the range of alloys which can be
cast. This project aims to elucidate the complex mechanisms causing
these defects through a combined experimental and computational
study of semi-solid deformation in aluminium alloys.
Columnar dendritic structures were generated for Al-Cu alloys through
directional solidi cation experiments and quanti ed in three dimensions
(3D) using x-ray microtomography (XMT). The -Al and the
Cu-rich interdendritic liquid were segmented using image analysis.
These 3D datasets were exported as meshes to be used in control
volume and nite element codes. Firstly, the
ow between the dendrites
was simulated by solving the Stokes equation and permeability
tensor was calculated as a function of the fraction solid. The size of
representative volume element was estimated to be 4-6 times the characteristic
length scale in the microstructure. Secondly, nite element
simulations were performed on 3D columnar dendritic structures to estimate
their mechanical properties and derive constitutive behaviour
as a function of temperature, strain-rate and fraction solid. Temperature
and strain-rate dependent compression tests were performed
in the Gleeble on alloys with dendritic composition to determine the mechanical properties of the monolithic Al-dendrites. The fraction
solid dependency term in the constitutive equation was determined
as a purely geometric factor which could be easily replicated in other
alloys systems. Lastly, hot tearing was directly observed in an Al-
12 wt.%Cu alloy by combining x-ray/synchrotron radiography with a
new tensile/compression apparatus capable of measuring strain, load
and quantifying the microstructure during controlled solidi cation of
Al alloy specimen. Using this new apparatus, the deformation of primary
dendrites and the concomitant
ow of Cu-rich interdendritic
uid was observed during isothermal and constant cooling rate conditions.
Initially, strain was observed to be accommodated by liquid
ow, but as the load is increased, void formation combined with liquid
necking between grains was prevalent
