Alloy Based Casting equipment Design

AA7xxx are widely used in high-performance structural aerospace and transportation applications. There is an increasing demand of A7xxx billets of larger dimensions for these applications. However, these alloys are hard to cast as their chemistry make them prone to both hot and cold cracking. The difficulty to cast sound billets increases with billet diameter due to increasing thermal gradients and associated stress build-up in the center. The Low Pressure Casting (LPC) technology developed by Hycast for aluminium extrusion billet casting is able to produce billets with improved surface appearance and sub-surface microstructure. In this project, the applicability range of the LPC equipment has been extended towards casting of 7xxx alloys with larger dimensions (up to 520 mm in diameter) by increasing the flexibility of the technology. Equipment design changes as well as process parameter variations and asssociated effects on the cracking tendency have been assessed using process modelling before testing in pilot casting trials. The effect of varying alloy composition on the hot-tearing sensitivity was also assessed through a combination of modelling and industrial casting trials. Finally, the equipment was updated with a wiper to eject cooling water from the ingot surface to decrease residual stresses in the solid. The large diameter of the billets combined with large solidification intervals result in high hot-tearing tendencies in the billet centre in the start-up phase. The bottom block properties and design was identified as essential to improve heat-extraction from the centre to avoid centre cracks. The material choice and design of the bottom block has been improved and the new solution has been tested in casting of 520 mm billets showing a clear reduction of the hot-tearing sensitivity. Guidelines for a better casting practise has been established based on the findings in the project.

In this project Hycast aims at upgrading LPC technology towards automated casting of large dimension billets of 7000 alloys. The alloy-composition based model will be developed to link with the automation system for calculation of casting parameters enabling alloy-based casting parameter design. The updated LPC technology will be designed with a wiper to reduce the cracking tendency for large dimension billets. In addition to the substantial commitment of Hycast and Hydro, this innovation will be implemented via the research activities carried out in internationally renowned research institutes SINTEF and IFE. The main research methodology of this project is to combing advanced numerical simulation tools, Alstruc and Alsim, with dedicated laboratory and industrial scale experiments and trials. The modeling tool Alstruc will be further developed to predict the correlation of the 7000 alloys composition and solidification conditions with as-cast microstructure. Alsim will be employed to address the relation between casting parameters and castability. The successful execution of this project will enable the reduction in the thickness of the machined-off layer of an extrusion billet giving a potential of less scrapped material, or even eliminating the need for machining of the billets completely. The project represents an innovation at an international level and gives a unique business opportunity to Hycast. The concept of alloy based casting design can be used to other alloy groups and can also be applied to the other casting technologies in the Hycast portfolio and at Hydro.