Effect of Ti addition holding time on casting quality and mechanical properties of A356 alloy
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Effect of Ti addition holding time on casting quality and mechanical properties of A356 alloy Mikdat Gurtaran1 · Muhammet Uludağ1 Received: 13 June 2020 / Accepted: 8 October 2020 © Springer Nature Switzerland AG 2020
Abstract Bifilms are casting defects that occur due to the incorporation of surface oxide film into the melt. Thus, they act as a crack in the liquid. Once they remain in the structure, they decrease mechanical properties owing to their negative effect on porosity formation. One of the most fundamental problems to be avoided in this frame is the reduction of the bifilms or the cleaning of the liquid metal to prevent the porosity formation. In this study, A356 (Al–7Si–0.3 Mg) alloy was used. AlTi5B1 master alloy was added as a grain refiner. The effect of Ti on casting quality and mechanical properties changes by holding time was evaluated. The results were examined statistically by Weibull analysis. It was found that the presence of bifilms can be reduced as holding time increases after adding Ti. The tensile properties were increased as the holding time increased. Additionally, the optimum holding time may be 40 min after Ti addition for high casting quality and good mechanical properties. Keywords A356 alloy · Grain refinement · Holding time · Mechanical properties · Casting quality
1 Introduction
melt [12]. Campbell [13] describes the effect of oxide films on porosity formation by three mechanisms:
Bifilms are casting defects that occur due to the incorporation of surface oxide film into the melt. Thus, they act as a crack in the liquid. Once they remain in the structure, they decrease mechanical properties [1–7]. On the other hand, hydrogen has always been blamed for the reduction of mechanical properties. It has been emphasized that the removal of hydrogen from the melt by degassing would reduce the porosity and thus improve the mechanical properties [8, 9]. However, Campbell and Dispinar [10, 11] carried out several works on the mechanism of porosity formation and it has been shown that dissolved hydrogen is not the main factor but only a trigger. As is known, Al alloys can easily form oxides during casting. Bifilms are formed by entrainment of oxide films such as the use of poor feed systems and turbulence during casting into the
1. Gas pressure within the folded oxide films 2. The negative pressure which occurs during solidification (shrinkage) 3. Dendrite arms that grow during solidification The type of oxide that may form on the surface of liquid aluminum can be in three structures: amorphous, young and old [3]. Amorphous is in nanoscale which is the one instantly forms in milliseconds. Young oxide is the crystalline form in micrometer thickness which is also known as γ-Al2O3. Old oxide is the α-Al2O3 which is corundum. The type of oxide that may be present in the final microstructure significantly determines the casting quality. As can be expected, the older and the thicker the oxide predominantly decreases the mechanical properties. Tiryakioglu
* Muhammet Uludağ, [email protected]
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