Thixoforming

Aluminium alloys have favourable physical and mechanical properties, and their recycling ability is good as well, so therefore they are increasingly used in the automotive industry. However, majority of these alloys do not have satisfactory tribological properties. Possible solutions for improvement of these properties are use of new composite materials and use of new technologies of aluminium alloys production. In this work, both solutions were considered together with presentation and analysis of results of previous investigations.

This paper describes the development of a financial model for evaluating the benefits of introducing a new technology. Thixoforming is a semi-solid metal processing technique that can offer potential users a number of technical and economic advantages. This process has been commercialized for a number of products, particularly in the United States and Italy, but there is significant scope for its further exploitation, especially within UK manufacturing. One of the major obstacles to the further adoption of this technique is the ability of potential users to evaluate and quantify the benefits offered. The model developed adopts a business process perspective to this and demonstrates how `intangible' benefits can be quantified.

The present work deals with the microstructure evolution of 7075 aluminum alloy which has been backward thixoextruded in the temperature range of 550-600°C using different ram diameters and ram displacement velocities. The recrystallization and partial melting (RAP) route has been used to obtain the semi-solid feedstocks for thixoforming. The results indicate that the back extruded microstructures mostly consist of semisolid grains which have been elongated along extrusion direction. The finest semisolid grain size has been obtained at lower deformation temperature and higher equivalent strain. This is attributed to the higher imposed shearing force on the liquid phase which could in turn fragment the grains. The current work also explores the room temperature mechanical properties of thixoformed work-pieces using shear punch testing method. It is found that the room temperature shear strength and ductility values have been substantially influenced by the deformation temperature, ram displacement velocity, and ram diameter.

Microstructural evolution and mechanical properties of thixoformed 7075 aluminum alloy prepared by conventional and new modified SIMA processes The microstructural evolution during semi-solid processing and thixoformability of a 7075 alloy prepared by conventional and new modified strain induced melt activation (SIMA) processes were comparatively investigated in this paper. The semi-solid slurries were thixoformed at 600 8C, at which temperature the solid fraction was estimated to be 0.8. The coarsening process of the semi-solid samples was described using Lifshitz-Slyozov-Wagner theory and the effect of pre-deformation on the coarsening kinetics of the solid particles was discussed. The coarsening rate constant of the new modified SIMA sample showed a remarkable decrease compared to that of the conventional SIMA sample. Microstructural and mechanical investigations indicated that the sample with a near-equiaxed microstructure deforms through the plastic deformation of solid grains mechanism. However, the sliding of solid grains and flow of liquid incorporating solid grains mechanisms were dominant in the sample with a globular microstructure. Also, it was observed that the yield and ultimate strengths and hardness of the sample prepared by the new modified SIMA process after thixoforming and T6 heat treatment increased by about 15 %, 10 % and 25 % respectively, compared to those of the conventional SIMA sample.