Influence of the processing route on the microstructure of aluminum alloy A356 for thixoforming

Abstract

Raw materials for thixoforming processes can be obtained by various routes. An analysis was made of the influence of five different processing routes on the production of thixotropic A356 alloy: 1) direct casting in a water-cooled mold; 2) direct casting as in (1), plus enhancement by electromagnetic stirring and grain refining; 3) as in (1), but enhanced by mechanical vibration; 4) as in (1), followed by one ECAP pass (Equal Channel Angular Pressing); and 5) using a commercial product as comparison. The alloy was characterized by both conventional and color microstructure for a comprehensive understanding of its structure prior to and after heating to 580°C, which is the ideal processing temperature for this specific raw material in the semi-solid state. B&W (black and white) and polarized light color metallography of the microstructure were employed to better characterize these structures. Among the five tested routes, the samples prepared by the ECAP route presented the most suitable characteristics for thixoforming, showing smaller grain sizes (measured by polarized light color metallography), smaller globule size and greater sphericity (both measured by B&W metallography) in the semi-solid state. Interconnections in the residual solid fraction in the semi-solid state are usually visible only by polarized light color metallography. However, the use of both color and B&W metallography enabled the best morphological structures for thixoforming to be clearly identified, confirming the promising potential of the ECAP technique in processing raw materials for thixoforming.