The Influence of Iron Content on the Porosity of AlSi9 Alloy Intended for Alfining Piston Ring Inserts

Due to its tendency to increase the power of engines, improving their reliability and operational efficiency, the compression ring in combustion engine pistons is embedded in a cast iron insert, which is subjected to the process of “alfining”. This involves covering the insert with an Al–Si alloy, which increases the iron content. Research has shown that the β-Al5FeSi phases crystallizing in the area of the insert–piston connection are the main cause of an unstable connection between the silumin casting of the piston and the ring insert. Their unfavourable lamellar morphology and large dimensions are the main causes of weakening in the connection between the insert and the piston, resulting in an unacceptable number of defective products. It has also been found that up to approx. 0.59 wt.% Fe, the pore volume fraction is very small (up to 3%), and there is no correlation. However, after exceeding this value, both the volume fraction of the β-Al5FeSi phase and the number of pores increase monotonically to values of approximately 18% and 14%, respectively, and the correlation between the examined features is statistically significant. These results were compared with known theories of the influence of iron on the porosity of Al–Si alloys, showing that the precipitates of the β-Al5FeSi phase are more important in the porosity fraction than the two-layer oxide films called “bifilms”. This research was carried out and verified under industrial conditions in one of the largest piston foundries (Federal-Mogul Gorzyce sp. z o.o., F-MG) on a separate line intended for alfining ring inserts intended for combustion pistons.