Browsing by Author "Piek, Jacques"

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    Slurry formulation for gel-cast titanium
    (Stellenbosch : Stellenbosch University, 2020-04) Piek, Jacques; Blaine, Deborah; Sigalas, Iakovos; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: In this study, commercially pure titanium (CPTi) parts were gel-cast. Gel-casting is a ceramic forming technology developed in the early 1990’s. Titanium is popular in the aerospace and biomedical industries for its excellent corrosion resistance, high mechanical strength, high strength-to-weight ratio and excellent biocompatibility. A novel process for titanium slurry gel-casting was developed, studying the sedimentation behaviour of a methacrylamide (MAM)/methylene bisacrylamide (MBAM) and an Isobam® polymer binder system, respectively. Factors influencing the sedimentation behaviour of titanium particles in a binder are the monomer content, monomer:cross-linker ratio, dispersant content, slurry mixing time and solid loading of the slurry. An optimum slurry was developed with 20 wt% monomers, at a 6:1 MAM:MBAM ratio, with dispersant content of 0.8 wt% ammonium hydroxide (NH4OH). CPTi powder with a particles size distribution of 15-45 μm was used at a solid loading of 55 vol%. Stokes Law was used tosuccessfully suspend the powder particles in the cast slurry to obtain an evenlydense microstructure. The slurry was cast into a resin 3D printed rectangular bar-shape mould, polymerized at 60 ˚C for 2 hours and dried in air at room temperature for 12 hours. Thermal gravimetric analysis (TGA) was conducted on the dried samples to determine the temperatures where the various binder constituents debind. Binder burnout was achieved by heating the dried parts to 400 ˚C at 1 ˚C/min and holding for 30 min, before presintering the parts at 650 ˚C for 30 min to obtain handling strength. The parts were vacuum sintered at 1200 ˚C for 2 hours at a heating rate of 10 ˚C/min. The shrinkage measured from cast to sinter, was 10.4 % and 9.03 % in the length and width of the rectangular bars, respectively. Optical microscopy was used to study the sintered microstructure of the gel-cast parts, finding an evenly dense microstructure. Scanning electron microscopy (SEM) was used to study the fracture surfaces of the tensile test specimens, confirming that only intermediate sintering has taken place. Energy dispersive spectroscopy (EDS) was used to determine the elemental composition of the sintered microstructure, observing that carbon and oxygen contamination has taken place. Finally, the mechanical properties were evaluated: a yield- and ultimate tensile strength of 323 MPa and 378 MPa, respectively, and a hardness value of 60 HRBW, which is 86 % of wrought.