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Effect of casting procedures in the structure and flow behaviour of semisolid A356 alloy

Proni C. T. W., Robert M. H., Zoqui E. J.

Archives of Materials Science and Engineering

2015

Vol. 73, nr 2

82--93

Purpose: Evaluation of different casting methods to produce raw material for thixoforming, aiming costs reduction in the production of thixoformable alloys. Reduction of costs may stimulate the commercial use of the semisolid processing technology (SSM). Design/methodology/approach: It is analysed the effect of different casting routes in the microstructure features in semisolid A356 alloy, and in its rheological behaviour. Different casting procedures were investigated: a) pouring in water cooled Cu mould; b) same as „a)” adding electromagnetic stirring; c) same as „a)” adding mechanical vibration; d) same as „b)” with addition of grain refiner; e) same as „c)” with addition of grain refiner. Cast materials were reheated to the semisolid sate and the effect of different holding times upon the globularization of the primary phase was analysed for each cast structure. The semisolid material in each condition was evaluated concerning rheological behaviour. Mechanical properties of thixoformed products were evaluated using flexion tests. Findings: Despite the several methods currently in use to produce raw material for thixoforming, this work shows that the best combination of quality of thixoformable material/ production cost /process operationality can be achieved using casting in permanent mould, under water cooling and mechanical vibration. Resulting cast material under this condition presents grain size smaller than 100 μm, ideal for SSM. Lower the grain size, lower the primary globule size and higher the roundness of the primary phase particle and lower the apparent viscosity of the semisolid. In the best condition achieved, apparent viscosity measured was circa 105 Pa.s (similar to the working range for glass), leading to a probable homogeneous die filling during thixoforming in high pressure die casting machines (HPDC). Research limitations/implications: Regardless the best microstructure for SSM resulting from casting under mechanical vibration, it is still necessary to reach the optimum casting condition in terms of vibration in order to improve, even more, refinement of the microstructure. Practical implications: The suggested process is a simple technique to reduce costs in the production of raw material for thixoforming. The technology is easily implementable in industries. Originality/value: The development of a simple, original, low cost method to produce raw material for SSM technology.

Producing thixotropic semi-solid A356 alloy: microstructure formation x forming behaviour

Robert M. H., Zoqui E. J., Tanabe F., Motegi T.

Journal of Achievements in Materials and Manufacturing Engineering

2007

Vol. 20, nr 1-2

19-26

Purpose: The work discuss the phenomena involved in the formation of the microstructure of semi-solid thixotropic alloy A356 produced by different techniques and the relation between microstructure and forming behaviour of the material. Design/methodology/approach: Thixotropic slurries of A356 alloy were produced from liquid and from solid conditions; in the first case either by stimulating nucleation rate or crystal multiplication during solid growth; in the second case by recrystallization and partial melting of deformed structures. Flow behaviour was analysed by viscosity measurements or by flow ability in forging operations. Findings: Results show that different production techniques activate different mechanisms, leading to distinct structure features in the semi-solid slurry, and, as consequence, in its forming characteristics. Techniques which promote formation of isolate globules of primary phase, like those involving nucleation stimulation and recrystallization, result in a semi-solid with better flowing behaviour. On the other hand, techniques based on crystal multiplication during growth lead to more interconnected globules in the slurry and poorer forming behaviour. Research limitations/implications: Semi-solid processing is suitable only for alloys with appropriate liquid fraction x temperature relation. Practical implications: the konowledge of the phenomena involved in the formation of thixotropic metallic slurries produced by different techniques, and their consequences in the material structure and flow behaviour, allow the decision of the adequate slurry production method for a specific application, in order to take the best advantage of the semi-solid technology. Originality/value: The analysis of the relation between production process x flow behaviour of semi-solid thixotropic A 356 is original.