The effect of different addition ratio of rare earth element erbium and europium on microstructure and mechanical properties of A356 (Al-7Si-0.3Mg) alloy

Abstract

A356 (Al-7Si-0.3Mg) alloy is widely used in automotive, aerospace and defense industries due to its good specific strength, castability, corrosion resistance and weldability. The main microstructural features of Al-Si alloys are primary α-Al dendrites, secondary denrite arm space (SDAS), eutectic Si and intermetallic phases. The size and morphology of primary α-Al, the acicular and coarse plate-like structure of eutectic Si cause poor mechanical properties. α -Al grain refinement and eutectic Si modification techniques are used to improve the mechanical properties of the alloy. Alloying is widely used in industry for grain refinement and modification because of its simple handling and low cost advantages. Ti, B, and their combinations are commonly used as grain refiners, while Na and Sr are used to modify the eutectic Si. It is seen that there are more effective grain refinement and/or modification effects with the addition of rare earth elements in trace amounts. The present work investigates the optimum addition amount of Er and Eu to achieve the best mechanical properties on sand mold casting. Sand casting is one of the most traditional casting methods. It has advantages such as slower solidification and ability to mold complex parts. Defect free casting begins with starting melt cleanliness, casting speed and well-designed runner systems. Failure of one of these three main components has a significant negative impact on the quality of part. The ASTM B108/B 108M mold is the most common mold used to obtain a tensile specimen. New mold designs were proposed in this work that would provide statistically reproducible results. Filling analyzes of the designs were made in the Anycast software. In order to evaluate the tensile test results of the samples cast with A356 alloy. T6 heat treatment was carried out, and the quality index values were calculated. The reliability of the results was evaluated using survivability plots. Considering these results, V3 mold was casted by tilt casting method in subsequent alloying castings. Alloying additions were applied in the best mold design and casting method. The effect of adding different amounts of Er and Eu to the A356 alloy was investigated: •0.0 •0.1 Er wt% •0.1 Er + 0.1 Eu wt% •0.3 Er wt% •0.3 Er + 0.1 Eu wt% T6 heat treatment was applied to examine the heat treatment effect of the same addition rates. To the heat-treated and non-heat-treated samples; OES, XRF, RPT, microstructure, hardness, tensile test, microstructure and SEM tests were employed to the fractured surfaces. Grain refinement was observed with the addition of Er, while grain coarsening was observed with the addition of Eu. However, while there is no change in the eutectic Si length with the addition of Er, a shortening is observed as the amount of Er increases. Eutectic Si is modified by the addition of Eu. The best eutectic Si aspect ratio was obtained with the addition of Eu with the heat treatment. The mechanical properties were improved with the addition of alloying elements. The best mechanical properties were obtained by heat treatment and the addition of alloying elements. The hardness value decreased with the addition of alloying elements, although the heat treatments increased. In SEM examinations, acicular Fe intermetallic phases in unalloyed structure were observed. With the addition of the alloying element, there wer acicular intermetallic phases containing Er or Er, Eu elements together in accordance with the alloying element. The frequency of intermetallic phases in the structure increased with the addition of alloying element. Although these intermetallic phases shortened after heat treatment, they could not be fully spheroidized.