In this work, T-shaped mould design was used to generate hot spot and the effect of Sr and B on the hot tearing susceptibility of A356 was investigated. The die temperature was kept at 250o C and the pouring was carried out at 740o C. The amonut of Sr and B additions were 30 and 10 ppm, respectively. One of the most important defects that may exist in cast aluminium is the presence of bifilms. Bifilms can form by the surface turbulence of liquid metal. During such an action, two unbonded surfaces of oxides fold over each other which act as a crack. Therefore, this defect cause many problems in the cast part. In this work, it was found that bifilms have significant effect over the hot tearing of A356 alloy. When the alloy solidifies directionally, the structure consists of elongated dendritic structure. In the absence of equiaxed dendrites, the growing tips of the dendrites pushed the bifilms to open up and unravel. Thus, leading to enlarged surface of oxide to become more harmful. In this case, it was found that these bifilms initiate hot tearing.
A356 is one of the widely used aluminium casting alloy that has been used in both sand and die casting processes. Large amounts of scrap
metal can be generated from the runner systems and feeders. In addition, chips are generated in the machined parts. The surface area with
regard to weight of chips is so high that it makes these scraps difficult to melt. Although there are several techniques evolved to remedy
this problem, yet the problem lies in the quality of the recycled raw material. Since recycling of these scrap is quite important due to the
advantages like energy saving and cost reduction in the final product, in this work, the recycling efficiency and casting quality were
investigated. Three types of charges were prepared for casting: %100 primary ingot, %100 scrap aluminium and fifty-fifty scrap
aluminium and primary ingot mixture were used. Melt quality was determined by calculating bifilm index by using reduced pressure test.
Tensile test samples were produced by casting both from sand and die moulds. Relationship between bifilm index and tensile strength were
determined as an indication of correlation of melt quality. It was found that untreated chips decrease the casting quality significantly.
Therefore, prior to charging the chips into the furnace for melting, a series of cleaning processes has to be used in order to achieve good
quality products.
Recyclability is one of the great features of aluminium and its alloys. However, it has been typically considered that the secondary aluminium quality is low and bad. This is only because aluminium is so sensitive to turbulence. Uncontrolled transfer and handling of the liquid aluminium results in formation of double oxide defects known as bifilms. Bifilms are detrimental defects. They form porosity and deteriorate the properties. The detection and quantification of bifilms in liquid aluminium can be carried out by bifilm index measured in millimetres as an indication of melt cleanliness using Reduced Pressure Test (RPT). In this work, recycling efficiency and quality change of A356 alloy with various Ti additions have been investigated. The charge was recycled three times and change in bifilm index and bifilm number was evaluated. It was found that when high amount of Ti grain refiner was added, the melt quality was increased due to sedimentation of bifilms with Ti. When low amount of Ti is added, the melt quality was degraded.
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