Browsing by Author "Govender, Preyin"

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    Influence of powder characteristics on the spreadability of pre-alloyed tungsten-carbide cobalt
    (Southern African Institute for Industrial Engineering, 2021) Govender, Preyin; Blaine, Deborah Clare; Sacks, Natasha
    With rising interest in additive manufacturing (AM) techniques, there is an increased focus on research that evaluates critical parameters that guide the selection of powders that are suitable for AM. One such parameter is a powder’s spreadability, described by metrics such as powder bed density and percentage coverage. This study focused on three spray-dried WC-Co powders (two 12 wt% and one 17 wt% Co) and evaluated the influence of typical powder characteristics, such as particle size and shape, apparent density, and flow rate, on their spreadability. It was found that particle size distribution influenced the powder spreadability. Larger particles hindered the even spreading of powder over the base plate, resulting in low powder bed density and percentage coverage. This also correlated with the powders’ apparent densities. The flow rate and angle of repose gave an indication of how cohesive the powders are. The more cohesive a powder, the poorer the spreadability, resulting in a lower powder bed density and percentage coverage.
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    Sinter infiltration of TI-6A1-4V
    (Stellenbosch : Stellenbosch University, 2020-03) Govender, Preyin; Blaine, Deborah; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Ti-6Al-4V is a widely used titanium alloy within the aerospace and medical industry with powder metallurgy (PM) becoming a fast growing industry within. The press-and-sinter technique is one such PM process that can be used to manufacture parts. In order to improve mechanical properties, dimensional tolerances and machinability, ferrous PM press-and-sintered parts are often infiltrated with molten Cu during the sintering heat treatment process. This project investigates the feasibility of infiltrating sintered Ti-6Al-4V compacts with molten Al, in order to improve its properties. Two Ti-6Al-4V powder blends were mixed, namely a blended elemental (BE) blend consisting of elemental Ti, Al and V powder in a 90:6:4 wt% ratios and a master alloy (CPTi+MA) blend consisting of commercially pure Ti (CPTi) powder with a master alloy (MA) powder of Al and V mixed in a 90:10 wt% ratio. These powder blends were compacted in 10 mm right cylinders with a relative green density (ρg) of approximately 75%. The compacts were sintered under high vacuum at temperatures of 1100 ºC and 1200 ºC, respectively, to achieve relative sintered densities (ρs) of < 92%. At ρs > 92 %, open pore channels close off preventing infiltration. Infiltration disks were compacted from pure Al spherical powder. The mass of the disks was calculated by taking the residual porosity of the sintered Ti-6Al-4V compacts, and providing enough molten Al to infiltrate the pores exactly. Infiltration took place under a nitrogen atmosphere at various temperatures between 700 °C - 900 °C, above Al melting point (660.6 ºC) to ensure melting, and for various dwell times. Slices of wrought Al bar stock were also evaluated for infiltration. Neither the Al powder compacts nor the wrought Al slices melted, with the result that all attempts to infiltrate the Ti-6Al-4V with Al failed. Characterization of the sintered and infiltrated samples was performed. Optical microscopy as well as energy dispersive X-ray spectroscopy (EDS) analysis was used to view the microstructures and elemental distribution in the microstructures, respectively. From these analyses, it was confirmed that infiltration with Al did not occur for any of the samples. CPTi+MA samples showed, on average a change in relative density from 74.5% to 90.2%, while BE samples showed negligible change from 73.6% to 74.9% relative density. The microstructure of both blends were observed with the CPTi+MA having a ɑ+β microstructure; while the BE having a predominantly ɑ-microstructure. EDS imaging for the CPTi+MA samples showed a fairly homogenous elemental distribution for both sintering temperatures. The BE blends showed an inhomogenous distribution at both sintering temperatures. Large pores were visible as Al particles melted and diffused into Ti and V, leaving high Al concentrations surrounding the pores. The results indicate that Ti and V diffuse into the Al disks during the infiltration heat treatment process, raising the melting temperature of the Al disks. By observing the concentration of Ti in Al disks, we see on the Ti-Al phase diagram that an intermetallic may have formed, thus increasing the melting temperature above the temperatures used for infiltration.