The melt cleaning is an important aspect in the production of high-quality aluminum castings. Specifically inclusions within the melt and an excessively high hydrogen content lead to defects and undesired porosity in the castings. Although it is possible to reduce the amount of hydrogen and oxidic inclusions by purge gas treatment and the use of melting salts, it is impossible to remove oxides (bifilms) created during filling of gating system. Paper deals with the effects of melt quality and the placement of a filter in the filling system on Al-7%Si-Mg alloy mechanical properties. Three different filters were used: (a) rectangular ceramic pressed filter with 3 mm thickness (b) cubical pressed ceramic filter with thickness 10 mm (c) cubical pressed ceramic filter with thickness 22 mm. The results showed that the highest tensile strength values were obtained from the filter with thickness of 22 mm. Numerical simulation analysis of the filling process showed that velocity reduction by filter is the major phenomenon affecting the mechanical properties. Another evaluated aspect during experiments was capability of filters to retain old bifilms. For this purpose multiply remelted alloy was prepared and analyzed. Results showed that filter efficiency increases with decreasing melt quality as a result of possibility to retain “old” bifilms better than small and thin “new” bifilms.
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 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.
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.
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.
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