The high temperature properties of Al-Si alloys and fatigue resistance can be insubstantially improved as well as reducing of shrinkage. Whenever Mn is available in alloys, it has the compensating task regarding the detrimental effects of Ferrous by reshaping the compound Al15(Fe, Mn)3Si2 into Chinese script. Needle shaped Fe phase (FeSiAl5) with embrittling effect can be also minimized .
Nickel presence in Al- Si alloys influence the strength of alloys and has the increasing effect on ductility, as long as it appears as Fe corrector in Al- alloys. The opposite will occur, if Ni does not act as a Fe corrector and may reduce the ductility . Chromium (Cr)
As mentioned before, Chromium has almost the same characteristic as Nickel and acts also as Fe corrector. The increasing content of Chromium in Al-Si alloys might reduce the ductility of alloys .
When Zinc is available in Al-Si alloys, the high temperature strength will decrease and prove an increasing hot tearing tendency. The machinability and corrosion resistance of alloys will improve due to compensate task of Zn for Cu and Ni . Effect of impurity elements
Certainly there are many common alloying elements which can be intentionally added into Al casting alloys to improve the all-round properties and performance of alloys, but also undesirable impurities can occur in alloys. Due to low concentration of impurity elements, which are available in most commercial Al casting alloys, the impurities dissolve in the Al and their influence on Al- alloys property is negligible. Only under some circumstances, intermetallic compounds may occur, which may appreciably affect the properties and performance of Al- alloys.
The shape of Al- Si eutectic is determinant for mechanical properties of the Al-Si alloys. Proper eutectic modification of Al-Si alloys becomes more crucial than the grain refinement regarding the casting and mechanical properties of alloys. Changes in the morphology of Al- Si eutectic can be interpreted as a kind of modification. Since the formed Al- Si eutectic in an unmodified Al-Si alloy appears as a coarse flake-like structure, which ensure brittleness and degradation in mechanical properties of Al-Si alloys, therefore it is desirable to reshape the Al- Si eutectic structure shape from coarse to more fibrous fine flake structure in order to improve the mechanical properties of Al-Si alloys. (Figure 2.5 and 2.6)
The modification is possible by the introduction of a low concentration of specific elements such as Strontium (Sr), Sodium (Na) or Antimony (Sb). The modification fulfills refinement and redistribution of Si- phase, which leads to improvement of the mechanical properties in Al-Si alloys.
There are two types of modification, which are applicable in most foundries; quench modification by rapid freezing or chemically induced modification, which includes addition of specific alloying elements. In quench modification, the rapid freezing induces twinning in the growing Si crystal lattice and the growth direction of the crystal lattice might be forced to change several times and results in a fibrous or branched structure. By chemically induced modification, the modifier inhibits the growth of the eutectic silicon phase which nucleates on the primary aluminium dendrites during the solidification of Al-Si alloys. Also the refinement and consistent distribution of Si crystals improves the ductile behavior of Al-Si cast components, since the mechanical properties of Al-Si alloys are strongly dependent of the morphology of Si particles. [11, 12, 13]
In this case, the chemically induced modification of AlSi7Mg0.3 alloy with strontium results in transforming the morphology of Al- Si eutectic phase from plate like in to fibrous one. It is as well mentionable, that adding of strontium in higher level than needed (over modification), leads to reduction of the Al-Si alloy properties as well as coarse Si structure and the strontium will interact with intermetallic phases. [4, 12, 13]
2 THEORETICAL BACKGROUND
3 EXPERIMENTAL METHODS
4 RESULTS AND DISCUSSION
6 FUTURE WORK .