1.1        
INTRODUCTION

 

Hypereutectic Al–Si alloys are
attractive materials for numerous commercial applications such as liner-less
cylinder blocks, cylinder head, piston etc. owing to their low density, low
thermal expansion coefficient and high wear resistance. They
are perfect piston materials and are ideal for tribological applications 58–60. However, they
have low ductility and poor machining properties due to the formation of
faceted and blocky primary Si during conventional processing, which greatly
confine their application in potential fields. It is necessary to refine
primary silicon for improving the properties60.Several researches have been
carried out for studying the wear behaviour of different Al-Si alloys (e.g.
hypoeutectic, eutectic and hypereutectic).For example, it has been reported
that the wear resistance of Al-Si alloys increased up to eutectic composition61–64.The minimum wear at the eutectic
point was the result of balance between two opposing factors, the increased
hardness and decreased aluminium matrix 65. Unstable subsurface caused by the
formation of shear bands and cracks was considered to be responsible for
increased wear in hypereutectic alloys 66. In contrast, the wear resistance
of Al-Si alloys was found to increase constantly with the increase of the
silicon content under several experimental conditions66–68. Extra silicon content in alloys
amplified the load-bearing capacity of the material, decreased the matrix
deformation  and in turn improved the
wear resistance 67,68. Sand casted hypereutectic Al-18Si
(wt. %) is widely used as anti-wear component of engines ,in the automotive
industry, and its wear mechanism has been well analysed and developed 6970.Therefore, this composition was
chosen in this work to analyze the dry wear and friction characteristics of Ni
rich piston alloys.  This study aims to
investigate the effect of Ni on the mechanical and tribological characteristicsof
hypereutectic Al-18Si-5Cu-(2-2.6wt.%)Ni piston alloys and compare with the
already obtained wear result of eutectic Al-12.6Si-5Cu-2.3Ni piston alloy
discussed in section 6.3.6.

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