It is well-known that the better the control of the liquid aluminium allows obtaining of better properties. One of the most important defects

that is held responsible for lower properties has been the presence of porosity. Porosity has always been associated with the amount of

dissolved hydrogen in the liquid. However, it was shown that hydrogen was not the major source but only a contributor the porosity. The

most important defect that causes porosity is the presence of bifilms. These defects are surface entrained mainly due to turbulence and

uncontrolled melt transfer. In this work, a cylindrical mould was designed (Ø30 x 300 mm) both from sand and die. Moulds were produced

both from sand and die. Water cooled copper chill was placed at the bottom of the mould in order to generate a directional solidification.

After the melt was prepared, prior to casting of the DC cast samples, reduced pressure test sample was taken to measure the melt quality

(i.e. bifilm index). The cast parts were then sectioned into regions and longitudinal and transverse areas were investigated

metallographically. Pore size, shape and distribution was measured by image analysis. The formation of porosity was evaluated by means

of bifilm content, size and distribution in A356 alloy.

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.

In the present study, the corrosion behaviour of A356 (Al-7Si-0.3Mg) alloy in 3.5% NaCl solution has been evaluated using

cyclic/potentiodynamic polarization tests. The alloy was provided in the unmodified form and it was then modified with AlTi5B1 for grain

refinement and with AlSr15 for Si modifications. These modifications yield to better mechanical properties. Tensile tests were performed.

In addition, bifilm index and SDAS values were calculated and microstructure of the samples was investigated. As a result of the corrosion

test, the Ecorr values for all conditions were determined approximately equal, and the samples were pitted rapidly. The degassing of the

melt decreased the bifilm index (i.e. higher melt quality) and thereby the corrosion resistance was increased. The lowest corrosion rate was

founded at degassing and as-received condition (3.9x10-3 mm/year). However, additive elements do not show the effect which degassing

process shows.

The formation of oxide film on the surface of aluminium melts, i.e. bifilms, are known to be detrimental when they are incorporated into

the cast part. These defects causes premature fractures under stress, or aid porosity formation. In this work, Al-12 Si alloy was used to cast

a step mould under two conditions: as-received and degassed. In addition, 10 ppi filters were used in the mould in order to prevent bifilm

intrusion into the cast part. Reduced pressure test samples were collected for bifilm index measurements. Samples were machined into

standard bars for tensile testing. It was found that there was a good agreement with the bifilm index and mechanical properties.

The microstructure of Al-Si alloy has coarse silicon and this structure is known dangerous for mechanical properties due to its crack effect. Sr addition is preferred to modify the coarse silica during solidification. Additionally, bifilms (oxide structure) are known as a more dangerous defect which is frequently seen in light alloys. It is aimed at that negative effect of bifilms on the properties of the alloys tried to be removed by the degassing process and to regulate the microstructure of the alloy. In this study, the effect of degassing and Sr modification on the mechanical properties of AlSi12Fe alloy was investigated, extensively. Four different parameters (as-received, as-received + degassing, Sr addition, Sr addition + degassing) were studied under the same conditions environmentally. The microstructural analyses and mechanical tests were done on cast parts. All data obtained from the experimental study were analyzed statistically by using statistical analysis software. It was concluded from the results that Sr addition is very dangerous for AlSi12Fe alloy. It can be suggested that to reach high mechanical properties and low casting defects, the degassing process must be applied to all castings whereas Sr addition should not be preferred.