Machining of powder metal titanium

[摘要] ENGLISH ABSTRACT: The purpose of this study is to investigate the machinability of commercially pure (CP)titanium, manufactured using the press-and-sinter PM process.To this end, CP titanium powder (-200 mesh) was compacted and sintered in vacuum(10-4 torr) for two hours at 1200°C. Small cylindrical samples were compacted atpressures varying from 350 to 600 MPa in order to determine the compressibility of thepowder. Following these tests, four larger stepped-cylinder samples were compacted atpressures close to 400 MPa and sintered under similar conditions. These samples hadsintered densities varying between 3.82 and 4.41 g/cm3. They were used to evaluate themachinability of the sintered titanium using face turning machining tests.The samples were machined dry, using uncoated carbide (WC-Co) cutting tool. Cuttingspeeds between 60-150 m/min were evaluated while keeping the feed rate and depth ofcut constant at 0.15 mm/rev and 0.5 mm, respectively. The final machined surface finishand the tool wear experienced during the face turning machining tests were monitored inorder to evaluate PM titanium's machining performance.This study showed that it is possible to use the press-and-sinter PM process with CPtitanium powder, with a particle size of less than 75 μm (-200 mesh), to manufacturesintered titanium. However, particle shape influences the compressibility of the powderand pressing parts of larger volume, such as the machining test sample shape, ischallenging when using such small particle size powder. Processing conditions, such ascompaction pressure, sintering temperature and sintering time, influence the sintereddensity.Results from the machinability tests show that tool wear increases with a decrease in theporosity of the sintered titanium. A more porous sintered material has both lower strengthand thermal conductivity. As these factors have opposing effects on the machinability ofmaterials, it is concluded that the strength of the sintered titanium has a stronger influenceon its machinability than the thermal conductivity.The cutting tool wear was uniform but showed indications of crater wear. The machinedsurface of the denser parts had minimal defects compared to less dense parts. Chip shapeis long for the dense parts, and spiral for the less dense parts. The chips formed were allsegmented, which is typical for titanium.The machinability of the sintered CP titanium was compared to that of wrought titaniumalloys. As expected, it was found that the machinability of the sintered titanium was poorin comparison.

[发布机构] Stellenbosch University