Thermodynamically, at infinite dilution, the free energy of formation of any solution from its pure components decreases at an infinite rate with increase in mole fraction of solute. This is tantamount to stating that there is always a thermodynamic driving force toward some mutual dissolution of pure substances to form a solution. Accordingly, unalloyed aluminum has a strong thermodynamic tendency to take into solution the iron in the steel dies commonly used in the die casting process. This explains why approximately 1% iron is added to die cast Al-Si alloys, as this addition drastically decreases the aluminum's tendency to want to take into solution more iron from the die. The problem with this solution is that the iron used to avoid die soldering drastically decreases mechanical properties. This is because the iron, which has a low solubility in aluminum, appears in the microstructure as a "needle-like" phase that hinders feeding (i.e. contributes to porosity) and acts as a stress riser. The needle-like morphology may be modified to a "Chinese script" morphology with the addition of manganese, but these Mn-phases are still stress risers that negatively effect mechanical properties. Thus, up until now. there has not been an element that could be added to aluminum that would provide solder-resistance and would not form compounds with aluminum that would act as stress risers. The new solution which is expected to change the future chemistries of die casting alloys relies on the effect strontium has on increasing the surface tension of aluminum and promoting "non-wetting" conditions.
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