Characterisation of semi-solid deformation behaviour of aluminium-copper alloys via combined X-ray microtomography and finite element modelling

• Main Author: Fuloria, Devashish
• Other Authors: Lee, Peter
• Published: Imperial College London 2009
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.513391

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.