2 results listed
Improved mechanical properties in Al-Si alloys can be achieved by Sr modification. In the present study, the
addition of Sr is carried out by Al-15wt%Sr master alloy where the total quantitiy of Sr in the alloy is preferred to
be around 300 ppm. Since melt quality is significantly affected by bifilms which are folded oxide layers, the
influence of Sr modification in A360 alloy over holding time of the melt has been evaluated. Reduced pressure
test was used to assess melt quality. Microstructural analysis was carried out. Sample collection was made at 4 th
and 17th hours of holding time and it was found that both Sr modification and bifilm index were decreased over
time. It was also seen that Sr content decreases significantly with increasing holding time. Tensile test specimens
were also prepared in order to observe the effects of modification and different casting conditions on the
mechanical properties of A360 alloy.
Aluminium-silicon based alloys are widely used especially due to their good mechanical properties, corrosion
resistance and castability, for light weight components in automotive and aerospace industries where it is used for
cylinder blocks and heads and plain bearings [1-3]. The mechanical properties of Al-Si alloys depend significantly
on the morphology of eutectic silicon. Modification is one of the most important melt treatments for aluminium-
silicon alloys castings. Desired properties of Al-Si alloys can be achieved by modification in two diferrent ways,
either by rapid cooling rate or by addition of certain modifier elements, such as Sr or Na [2,3]. Both elements cause
multiplication of twins and coraline-like microstructure. However, Na has a high tendency to fade because of high
volatility. Therefore, Sr is a good alternative to Na [4]. The eutectic silicon phase crystallizes into a coarse, plate-
like morphology during eutectic formation. This coarse morphology of eutectic silicon is disadvantageous for the
casting. With modification the morphology of eutectic is changed from coarse, birttle flakes which leads to poor
mechanical properties to finer and fibrous structure [4-5].
International Symposium on Light Alloys and Composite Materials
UHAKS
Inal Kaan Duygun
Özen Gürsoy2
Eray Erzi
Derya Dışpınar
Strontium modification of Al-Si alloys has many advantages for the casting parts. Particularly, mechanical
properties are increased and castability of the alloy is improved. However, Sr has high affinity to oxygen and it
may form SrO. Al2O3 spinel oxides on the surface of the melt depending on the holding time of the liquid. Thus,
Sr quantity in the melt can decrease by time. In this work, this phenomenon was investigated in A413 alloy. The
melt was held at 700°C and 750°C for 4 and 17 hours. Metallographic examination was carried out for the Sr
fading effect measurement. In addition, melt quality change was recorded by means of bifilm index change.
Even low additions of Sr results in favorable changes in the quality of casting by affecting the nucleation and
crystallization mechanisms. It was proved that Sr levels in tha range between 100 ppm and 400 ppm is enough for
coarse, flaky eutectic phase to be transformed finer fiberous morphology [1] However it is proposed that even 50
ppm of Sr can change the morphology efficiently [2] Besides that the fading effect which is because of the
increasing holding time at high temperatures and varying treatment times may also affect the efficiency of Sr on
the modification of Al-Si alloys [3]. Another important factor which changes the melt quality is the oxide-flms
which can remain suspended in melts for a long periods. During casting, these oxide-films may fold double and
cause cracks after solidification [4]. In this work the effect of holding time on modification was investigated by
comparing modified and unmodified alloys at 700°C and 750°C. Reduced pressure test was also used to assess
changing metal quality by measuring the Bifilm indices of samples.
International Symposium on Light Alloys and Composite Materials
UHAKS
Inal Kaan Duygun
Özen Gürsoy2
Eray Erzi
Derya Dışpınar