EFFECT OF SHORT HEAT TREATMENT ROUTES ON THE TRIBOLOGICAL PROPERTIES OF TI-۶AL-۴V ALLOY

In this research, the effect of annealing and aging temperature as well as the effect of quenching media on the microstructure, hardness and dry sliding wear behavior of Ti-۶Al-۴V alloy has been studied. Cylindrical samples with the diameter of ۱۰ mm and the height of ۲۰mm were solutionized at ۹۳۰˚C and ۱۰۶۰˚C for ۶۰۰ seconds and then were quenched in the cold water and in the air. The samples were aged at different temperature of ۴۸۰˚C, ۵۵۰˚C and ۶۱۰˚C for ۳۶۰ s to increase the hardness. Heat treated samples were tested using standard pin-on-disc test machine at the applied loads of ۱۰۰, ۱۵۰ and ۲۰۰ N. Microstructural investigations using scanning electron microscope revealed that for the samples solutionized at ۹۳۰˚C and quenched in the water, the microstructure is composed of primary α and high volume fraction of martensitic α΄ phase with fine precipitated of β between martensitic lathes. In the case of air cooled samples, transformed β has also been  appeared in the vicinity of primary α. For the samples solutionized at ۱۰۶۰˚C and quenched in the water, fully martensitic micro structure with fine β precipitates was observed. For the sample solutionized at ۱۰۶۰˚C and air quenched, plate like α and lamellar grain boundary β were detected. The maximum hardness value relates to the sample solutionized at ۱۰۶۰˚C and quenched in water which is equal to ۴۳۳ HV. Different wear mechanisms, including oxidative wear, scratch, and delamination occurred at the worn surfaces at different applied loads. For the samples quenched in the water, the oxidative wear mechanism governing at low applied load and oxide debris was observed as separate or compacted particles which formed in the contact between pin and steel counter face detached from the sample. At higher applied loads, delamination and scratch mechanism was also observed and metallic plate like debris was detached from the sample. Mechanically mixed layer (MML) was formed on the surface of the pin at high applied loads and for the samples with low hardness value adhesion marks were also revealed on the steel disc. The minimum weight loss in the wear test is related to the sample quenched from the ۱۰۶۰ºC in the cold water and aged at ۵۵۰ ºC .

Keywords:

Ti-۶Al-۴V alloy , Tribology , Heat treatment , Microstructure , Hardness