Abstract
Homogenisation is an essential step in the preparation of aluminium billets for extrusion. It is designed to produce an homogeneous solid solution and to transform the beta-Al5FeSi particles to the more equiaxed and thus more acceptable cubic alpha-Al-12(Fe-x,Mn1-x)(3)Si variety. The effect of the homogenisation conditions on high temperature surface damage and fracture is investigated through full scale industrial extrusion experiments on AlMgSi alloy. The transformation of AlFeSi intermetallic particles from the beta-Al5FeSi thin plate like shape to the more rounded alpha-Al-12(Fe,Mn)(3)Si form, quantified by EDS measurements, is shown to postpone the formation of surface defects. Transmission Electron Microscopy (TEM) analysis of extruded samples close to the location of extrusion defects, reveals that damage initiates by microcracks within the brittle beta-Al5FeSi particles or due to incipient melting caused by the eutectic transformation Al + beta-Al5FeSi + Si --> Liquid at 578degreesC. These microcracks open by plastic deformation around the cracked brittle phase and, after sufficient growth, coalesce into a large crack.
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