Study into surface properties of plasma nitrided and laser melted workpieces
Mohammed, Mahmoud A (1998) Study into surface properties of plasma nitrided and laser melted workpieces. PhD thesis, Dublin City University.
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The present work was carried out to study the surface properties of plasma nitrided, T iN PVD coated and laser-melted Ti-6A1-4V workpieces. The work consists of two phases. The first phase of the study was conducted to investigate the tribological and mechanical properties of plasma nitriding and T iN PVD coating of Ti-6A1-4V alloy. Specimens were nitrided in a N 2/H2 (8/2 ratio) plasma. Workpiece temperature was varied from 450-520°C during the nitriding process. Pin-on-disc wear tests were carried out to evaluate the wear properties of the resultant samples and ball-on-disc experiments were conducted to measure the friction coefficient. Micro-hardness tests, SEM, EDS and XRD were carried out to investigate the phases developed in the nitrided zone. It was found that the wear resistance improved considerably after the nitriding process. Three distinguished layers were identified. These include an inner layer, where 5 -T iN + e -T ^N phases formed, an intermediate layer, where a -T iN with or without 8-phase developed and an outer layer, where precipitation was dominant.
In the second phase, the surface properties of the Ti-6A1-4V alloy due to laser melting after plasma nitriding process was investigated. A CO2 laser with nominal output power of 1.6 kW was employed to melt the nitride layers. In order to achieve low and high melting regions, the laser output power intensity was varied. It was found that the laser melting altered the friction coefficient considerably as compared to untreated samples. The microstructures were analyzed before and after the laser melting process using SEM microphotography. It was found that the scratch developed at the untreated surface was deeper than those compared to plasma nitrided and nitrided/laser-melted surfaces. A mathematical model governing the laser melting process was developed using a Fourier theory, which permitted the heating and cooling rates to be predicted.
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