Abstract
Abstract Semisolid state processing is a methodology that transforms dendritic as-cast microstructure into globular non-dendritic microstructure with optimized mechanical properties. Rheocasting process involves mechanically stirring metals in a semisolid state and its benefits are associated with processing parameters. In order to evaluate stirring time effects, magnesium alloy MRI 230D (Mg–Al6.45–Ca2.25–Mn0.27–Sr0.25–Sn0.84) was melted and cooled down to semisolid processing temperature (595oC). Moreover, isothermal stirring was executed for 1, 2, 4 and 8 minutes. In each experiment, cylindrical samples with different diameters (6 and 12 mm) were collected to evaluate cooling rates effects. The microstructure was analyzed through optical and electron microscopes. The results showed that in the longest stirring times, the primary α-Mg phase got coarser and the interglobular region became more refined. Although shape factor initially increased, subsequently there was a tendency to stabilize. Regarding cooling conditions, it was found that the interglobular region became more refined at higher cooling rates. Mechanical tests revealed an initial decrease in ultimate tensile strength (UTS) and ductility, but afterwards increased due to a more regular microstructure and superior shape factor. The 8-minutes stirring sample achieved the best results: an increase of 12% in UTS and 16% in ductility.
Highlights
Magnesium alloys have an excellent weight/resistance ratio, high recycling capacity and low density
Based on the characteristics of the MRI 230D alloy, this work aims to analyze the influence of cooling rates and stirring time on the microstructure and mechanical properties of the alloy when submitted to the rheocasting process
The blue dashed lines indicate the temperatures of each inflection point in the slow cooling curve
Summary
Magnesium alloys have an excellent weight/resistance ratio, high recycling capacity and low density. Based on these characteristics, in recent years the research and development of magnesium alloys for industrial application have increased significantly, replacing materials such as steel, aluminum and structural plastics. The application of magnesium alloys is limited due to their low creep resistance[1,2,3]. The SSM (semisolid metal) processing can cause significant changes in the microstructure of materials such as shape factor, refining, primary particle distribution, among others. The purpose of SSM processing is to form a uniform and refined microstructure, with a high shape factor and improved mechanical properties, aiming at an increase in toughness and ductility. The optimization of processing parameters becomes a possible solution to the problems found in Mg alloys[4,5,6]
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