Browsing by Author "Bosman, Hendrik Ludolph"

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    Influence of powder particle size distribution on press-and-sinter titanium and Ti-6Al-4V preforms
    (Stellenbosch : Stellenbosch University, 2016-03) Bosman, Hendrik Ludolph; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: This research focusses on the press-and-sinter manufacturing process through which titanium powders are employed to produce dense titanium and the Ti-6Al-4V alloy; more specifically, the influence of particle size distribution (PSD) on the densification behaviour and material properties are investigated. Commercially pure (CP) titanium powders of -100 and -200 mesh sizes were blended in various proportions and used to conduct compressibility and sintering studies. To produce Ti-6Al-4V, a -200 mesh 60Al-40V master alloy (MA) powder was additionally blended with the CP titanium powders. Powders and powder blend were characterised using scanning electron microscopy and laser diffraction. A vast array of specimens was produced while varying the following production parameters: aspect ratio, compaction pressure, sintering time and sintering temperature. Aspect ratios of cylindrical specimens were varied to produce thin disks (1:3), as well as square (1:1) and long (3:2) cylinders. Compaction pressures were varied from 200 MPa to 600 MPa using double action compaction. Sintering was conducted under high vacuum (<10-4 mbar, or better) with sintering temperatures ranging from 1000°C to 1300°C; typical holding times were two hours, with certain specimens being re-sintered to four, and up to six hours. From the results of the compressibility and sintering studies, a baseline densification pathway was elected: compaction at 400 MPa followed by sintering at 1300°C for two hours. This allowed meaningful comparison of the behaviour of different powder blends. Several CP titanium and MA Ti-6Al-4V powder blends of known weight compositions were considered by creating a model using the precursor powder PSD data to predict the blended powder PSDs. A few promising CP and MA blends were prepared and specimens were produced according to the elected baseline process. The densification behaviour was studied at each process step. Densification trends similar to those indicated in literature for bimodal powder blends were found for the CP titanium blends; however, the effect of the MA powder alloying addition was dominant in the case of the MA Ti-6Al-4V blends’ densification behaviour. Mechanical properties were tested using three point bending and Vickers hardness (HV10), respectively. Transverse rupture bar specimens were pressed (400 MPa) and showed either brittle or ductile fracture after being sintered for two hours at either 1000°C or 1300°C, respectively. The thermal conductivity of specific specimens was measured and showed that the thermal conductivity of sintered titanium is lower than that of the equivalent wrought material. The sintered microstructure of various specimens was investigated to gain insight into differences in pore structures among the blend compositions. A vast range of densification results has been put forth from which to extract data for future research. Recommended future work would include: the procurement of tooling for tensile test specimens, a redesign of the thermal conductivity experimental setup, and the addition of fine -325 mesh CP titanium powders to widen the range of PSDs achievable.