Slurry formulation for gel-cast titanium

Piek, Jacques (2020-04)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

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.

AFRIKAANSE OPSOMMING: In hierdie studie is kommersiële suiwer titaan parte geproduseer deur jelgieting. Jelgieting is ‘n produseringstegniek vir keramiek wat in die vroeë 1990’s ontwikkel was. Titaan is gewild in die lugvaart- en biomedese bedryf vir die sy uitstekende weerstand teen korrosie, hoë meganiese sterkte, hoë sterkte-tot-gewig verhouding en uitmuntende verenigbaarheid met menslike been as biomateriaal. ‘n Nuwe proses vir titaanmengsel jelgieting was ontwikke deur die sedimentasie gedrag van ‘n meta-akrielamied (MAM)/metileen-bisakrielamied (MBAM) en ‘n Isobam® polimeer bindmiddel te bestudeer. Faktore wat die sedimentasie gedrag van die titaanpartikels in the bindmiddel beïnvloed is die monomeer inhoud, monomeer:kruis-verbinder verhouding, dispergeermiddel inhoud, die titaanmengsel mengtyd en die soliede belading van die mengsel. ‘n Optimum mengsel was ontwikkel met 20 wt% monomere, teen ‘n 6:1 MAM:MBAM verhouding, met dispergeermiddel inhoud van 0.8 wt% ammoniumhidroksied (NH4OH). Titaan poeier met ‘n deeltjiegrootteverspreiding van 15-45 μm was gebruik teen ‘n soliede belading van 55 vol%. Stoke se Wet was gebruik om die poeier partikels suksesvol in suspensie te hou in die gegiete mengsel om ‘n mikrostruktuur met eweredige digtheid te verkry. Die mengsel was gegiet in ‘n reghoekige staafvormige gietvorm wat drie-dimensioneel gedruk is van harsmateriaal, gepolimeriseerd teen 60 ˚C vir 2 ure en gedroog in lug teen kamertemperatuur vir 12 ure. Termiese gravimetriese analise (TGA) was uitgevoer op die gedroogde jelgegiete monsters om die temperature waarteen die verskillende komponente van die bindmiddel uitbrand, te bepaal. Die uitbranding van die bindingsmiddel was bereik deur die gedroogte stafies te verhit tot 400 ˚C teen 1 ˚C/min en dit daar te hou vir 30 min. Die stafies was dan verder verhit tot 650 ˚C vir 30 min om hanteringskrag te verkry deur oorspronklike sintering. Die stafies was gesinter in ‘n vakuum teen 1200 ˚C vir 2 ure teen ‘n verhittingstempo van 10 ˚C/min. Van giet tot sinter was ‘n inkrimping van 10.4 % en 9.03 % in die lengte en breedte van die stafies, onderskeidelik, gemeet. Optiese ligmikroskopie was gebruik om die gesinterde mikrostruktuur van die jelgegiete stafies te bestudeer. ‘n Eweredige digtheid in die mikrostruktuur was waargeneem. Skanderings-elektron-mikroskopie (SEM) was gebruik om die fraktuur oppervlaktes van die trektoets spesimens te bestudeer, bevestigend dat slegs intermediêre sintering plaasgevind het. Energie-verstrooings-spektroskopie (EVS) was gebruik om die elementele komposisie van die gesintreerde mikrostruktuur te bepaal. Hierdeur was waargeneem dat koolstof en suurstof kontaminasie plaasgevind het. Ten slotte was die meganiese eienskappe geevalueer: ‘n swig- en trekkrag van 323 MPa en 378 MPa, onderskeidelik, en ‘n hardheidswaarde van 60 HRBW, wat 86 % van gesmede eienskappe is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108304
This item appears in the following collections: