Constitutive behaviour and hot tearing during aluminium DC casting

The constitutive behaviour of two non-heat treatable industrial aluminium alloys (AA3104 and AA5182) is investigated. This is done by testing the as-cast material in tension at low strain rates and from room temperature to semi-solid temperatures, similar to the conditions during DC casting. The parameters of two constitutive equations, the extended Ludwik equation and a combination of the Sellars-Tegart equation with a hardening law, were determined. To evaluate the quantified constitutive equations, tensile tests were performed simulating the deformation and cooling history experienced by the material during casting. In the semi-solid state the behaviour is dominated by the solid network but the geometry of the liquid determines how much of the solid network contributes to the strength. A simple modification of a standard creep law, which takes into account the geometry of the liquid film, provides a continuous description of the constitutive behaviour of these alloys from the creep regime into the semi-solid state. It is concluded that the constitutive behaviour of the alloys in as-cast condition is well described by both the extended Ludwik equation and an adapted form of the Sellars-Tegart equation. Although the extended Ludwik equation describes the data better, the adapted form of the Sellars-Tegart equation is easier to implement in DC casting models, because the temperature appears explicitly in this equation. In order to study the hot tearing mechanism, tensile tests are carried out in semi-solid state and at low strain rates and crack propagation is studied in-situ by SEM. Microstructural investigations of these cracks indicates that they initiate at any weak spot such as a pore or partially liquid grain boundary and occur by a combination of fluid film separation and rupture of solid bridges. This leads to brittle behaviour on the large scale although local deformation can be very ductile. Similarities between hot tears in the industrial ingot and cracked specimens indicate that important aspects of hot tearing during casting can be simulated by tensile experiments at semi-solid temperatures.