Laser surface alloying and in-situ formation of aluminium metal composites reinforced with ceramics and intermetallics.
Date
2012-01-31
Authors
Mabhali, Luyolo Andrew Baxolise
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Abstract
This thesis describes the novel laser surface alloying of aluminium AA1200 with various
combinations of Ni, Ti and SiC powders, using a 4.4kW Rofin Sinar Nd:YAG laser in order
to improve its mechanical and tribological properties. The laser alloying parameters were
optimized on the breakdown systems of the complex Al-Ni-Ti-SiC system. Various analytical
techniques were used to study the microstructures produced. Wear testing was conducted
under sliding and abrasion conditions while the fracture mechanisms were investigated using
impact tests. Aluminium surfaces reinforced with metal matrix composites and intermetallic
phases were achieved. The phases present depended on the composition of the alloying
powder mixture. Al reacted with Ni to form Al3Ni and Al3Ni2 intermetallic phases while Ti
reacted with Al to form an Al3Ti intermetallic phase. Some of the SiC particles dissociated
and reacted with either Al or Ti to form Al4C3, Al4SiC4, TiC or Ti3SiC2 phases. Si reacted
with Ti to form a Ti5Si3 phase. An increase in surface hardness was achieved, up to a
maximum of 13 times that of aluminium when alloying with 80wt%Ni + 15wt%Ti +
5wt%SiC. The increase in hardness was attributed to the intermetallic phases especially the
Al3Ni2 phase. Alloying led to a 4-38% improvement in the wear resistance of the pure
aluminium under sliding wear conditions and a 19-82% improvement under three body
abrasion wear conditions. The predominant wear mechanisms for both wear types were
groove formation by ploughing and cutting action of the abrasive particles, smearing,
material pile-up, extensive cracking of the intermetallic phases and fracturing of the
embedded SiC particles in the MMCs. Alloying led to a 31-50% decrease in the impact
resistance of the pure aluminium. Brittle fracture of the SiC particles and transgranular
cracking of the intermetallic phases were observed for the laser alloyed surfaces while ductile
fracture was observed for the bulk aluminium.