Micromechanical analysis of fatigue crack initiation and growth behaviour of recycled cast aluminium silicon piston alloys

Abstract

There are significant economic and ecological benefits of aluminum recycling. However, recycled aluminum alloys have not yet found widespread application in premium components. This is mainly due to difficulties in controlling the chemical composition of the recycled alloys attributed to the varied compositions of cast aluminum scrap. For this reason, existing secondary alloys (e.g. 319 type alloys) tend to have relatively more tolerant impurity element specifications and broader limits for major alloying elements. As useful as this may be, it does not alleviate the problems associated with recycling as regards to chemical composition control. This research is part of a wider investigation whose overall aim is to develop new high performance recycled alloys for selected automotive component applications. Previous work within the group has mainly concentrated on the development of recycle-friendly alloys for automotive piston applications. A preliminary evaluation of mechanical performance has been carried out on a suggested model secondary piston alloy that can be obtained via direct reuse of piston scrap. The current work involved a more detailed analysis of the microstructure-mechanical property characteristics of this alloy as affected by selected minor elements. The emphasis here was to assess the effect of microstructure on the fatigue performance of the base alloy as well as the effect of Sr, Mn, Cr and grain refinement.

Fatigue crack initiation was investigated at room temperature using S-N and short fatigue crack growth experiments. A 4-point bend test configuration was adopted for laboratory tests. Post failure analysis indicated that pores and intermetallic particles were detrimental in causing fatigue crack initiation. Porosity was observed to act as fatigue crack initiation sites in the base alloy, the alloy containing Cr, and in the alloy with a combined addition of Sr and Al-Ti-B grain refiner. In the alloy containing a combined addition of Mn and Cr, intermetallic particles were observed to cause fatigue crack initiation. Using EDX, the intermetallic particles were identified to be Al9FeNi phases. Different fatigue crack growth behaviour was observed for all the alloys investigated. The highest crack growth rates were observed in the base alloy, and in the alloy with a combined addition of Sr and Al-Ti-B grain refiner as compared to the alloys containing Cr, or a combined addition of Mn and Cr.

From SEM images, all the alloys under investigation exhibited complex multiphase intermetallic phases. As result, the micromechanisms of fatigue are due to particle fracture and debonding for crack propagation continuity. The level of crack continuity depends largely on micro-damage behaviour experienced at the crack tip. It is the extent of this damage in the alloys that control the fatigue crack growth rates exhibited.