Novel aluminium alloys for thermal spray applications to steel substrates.

?Extensive research on alloying aluminium (Al) with zinc (Zn), indium (In) and titanium (Ti) to improve corrosion and wear resistance has been conducted since 1950's to meet the increased demand of Al in construction, automotive, aerospace and food industries in the form of coatings deposited using thermal spraying, hot dipping and cladding. This research has investigated the performance of arc sprayed Al-5wt%Zn-(0.02-0.05wt%)ln and Al-3wt%Ti-0.15wt%C coatings to protect steel structures from corrosion and wear.The accelerated and electrochemical tests conducted on Al-5wt% Zn-(0.02-0.05wt%) In showed that its superior corrosion performance was attributed to the synergetic interaction of Zn and In. Cyclic formation and rupturing of passive Al oxide layer was found to be the underlying mechanism of activation produced by combination of both Zn and In. The presence of In in the corrosion product or on the surface of the coating In was not detected, hence validation of surface enrichment theory was improbable.The micro-scale abrasive wear test showed that the wear coefficient of an arc sprayed Al-3wt%Ti-0.15wt% C coating was found to be very close to that of an arc sprayed 13wt%Cr steel coating with a much higher hardness. This was attributed to the formation of Al[3]Ti and (Ti,Al)C particles due to rapid cooling that takes place in arc spraying . This resulted in precipitation strengthening, explAlning the increase in the hardness of the sprayed Al-3wt%Ti-0.15wt%C coating compared to its feedstock alloy. To establish a mechanism by which the identified phases in the microstructure of arc sprayed Al-3wt%Ti-0.15wt%C alloy coating showed improved hardness and wear resistance than the bulk alloy, growth of identified phases in the coating was promoted by two different heat treatment cycles. The coated samples were held at three chosen temperatures of 150°C, 350°C and 550°C separately for 1 hour and 3 hours, followed by two different cooling regimens; the first involved rapid quenching of samples and the other was room temperature exposure for 5 hours. After both heat treatment cycles; Precipitations of Ti rich high contrast and near spherical brittle phases were observed in the microstructure of Al-3wt%Ti coatings. A reduction in the spread of micro-hardness value with increase in exposure temperature from 150°C to 550°C was also observed.An overall increase in the wear coefficient value of rapidly quenched coatings in comparison to as-sprayed Al-3wt%Ti-0.15wt%C coating was seen. A similar increase in the wear coefficient value was also observed after room temperature cooled coatings. However, an exceptional 15 % reduction in wear coefficient value was seen in the room temperature cooled coating after 3 hours of exposure at 550°C. The role of precipitated phases and possible mechanism of their effect on micro-hardness and wear resistance of Al-3wt%Ti-0.15wt%C alloy coating has also been discussed.