Sr Modification of Eutectic Al-Si Alloys
Inal Kaan Duygun Özen Gürsoy2 Eray Erzi Derya Dışpınar
AbstractStrontium 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.