1,720,999 research outputs found
An assessment of the corrosion resistance of powder sprayed titanium coatings
No full textAluminium and stainless steels are susceptible to pitting corrosion in sea water. Mild steel is susceptible to corrosion in general. Titanium however, has good corrosion resistance in chloride solutions and thus is not susceptible to pitting in sea water. The following is an assessment of the corrosion resistance of Titanium powder coatings. Potentiostatic electrochemical experiments were carried out on three substrates (stainless steel, mild steel and Aluminium) coated using two spray methods (plasma and HVOF). A discussion of the results was given with reference to the polarisation curves that were created, SEM images, XRD results, estimates of porosity, and Pourbaix diagrams. It was concluded that the plasma coating provides the best corrosion resistance due to the fact that it had less porosity than HVOF and that it is made up of Titanium oxides. Also, the corrosion mechanism for the coatings is pitting of the substrates at the end of pores. The extent of this is far greater for the HVOF than the plasma coating. It was found that features found in the polarisation curves for the substrates are present in the curves for coatings. This is more evident in the HVOF polarisation curves. Mild steel benefited the most from the Titanium coatings compared to the substrate (more so for the plasma coating). Finally, the plasma coating improves the substrate based on corrosion rate and thermodynamics, except for Aluminium which remains the same thermodynamically
Direct Metal Laser Sintering of Titanium implant with Tailored structure and Mechanical Properties
No full textDirect Metal Laser Sintering has attracted much attention over the last decade for producing complex parts additively based on digital models. The capability and reliability of this process has stimulated new design concepts and has widened the manufacturing perspective of product customisation. This research work is designed to gain a deep understanding of laser sintering processing parameters, the corresponding microstructures and the mechanical properties. The main purpose is to have a body of fundamental knowledge about the laser and titanium powder material interactions, thus establishing the factors that influence the process-structure-properties relationships of the Direct Metal Laser Sintering process. Finally, a route for manufacturing customised craniofacial implants was described. This is to evaluate the DMLS processing capabilities in medical areas, particularly those parts having porous and lattice design structures.
The interaction between a laser beam and the powder bed creates a distinctive structure; a ball shaped (blob) consists of solid and porous regions. All the blobs have the same shape and morphology which may well suggest that there is a tendency for the powder particles to form a spherical droplet prior to a movingless laser beam. Surrounding the melted core is a sintered region of partially melted powder particles where the powder particles were fused together to form inter-particle necks. There is a linear relation between size, weight and density of a blob and the laser power. The surface temperature obtained exceeds the melting and vaporization temperature of the titanium and this creates a hole on the top part of a blob as a result of a massive temperature rise. Laser power of 140W gives a consistent structure and hardness in a blob. Metallographic analyses of a blob’s cross-section show an α+β structure with prior-beta grains. The morphology of the lamellar structure consisted of acicular needles with a basket-weave pattern. The pores were characterised as having flat and spherical features with the size ranging from 2µm to 6µm. The micro-porosity observed may be associated with shrinkage which occurs during solidification or with the presence of entrapped gases from the atmosphere or argon gas from the shrouding environment
Laser power and scan speed are the two most crucial factor controlling the laser-powder interactions. Result shows that laser power is capable of widening the processing parameters particularly the scan speed. Increased laser power causes more powder to melt thus creating a bigger melt pool. Contrary to this, increasing the scan speed reduces the interaction time thus a smaller amount of powder melts. The right combination of these two parameters results in inducing an appropriate exposure time where continuously scanned tracks can be formed. Most of the parts were successfully built using a specific volume energy density of 50Jmm⁻³, which was considered to be the optimum processing parameter for this research work. The ideal laser-material interaction time was calculated at 0.0008secs.
The microstructural analysis revealed a fully lamellar structure with acicular morphology. XRD analysis confirmed the presence of α’ martensite, which explains the thermal history of a high isothermal condition and rapid cooling. The cross section of a solid part exhibited an acicular, needle-like structure with a herring bone pattern, parallel to building direction, due to directional solidification. The microstructure had a high tensile strength but with low ductility. It is also worth mentioning that a slight change in scan speed, with the intention of providing more energy density to the powder, may cause instability in the melt pool and cause deterioration in the mechanical properties. It is therefore confirmed that there is an upper limit and allowable processing window where a good balance of tensile strength and hardness in a DMLS part is achievable.
A framework prior to an implant’s fabrication was established and the associated design and manufacturing software are reported. The processing route required software like MIMICS and MAGICS to manipulate the medical images and design data and equitable skills must be acquired to handle the machine in order to successfully fabricate the desired parts. Employing MAGICS new lattice function proved to be more efficient, saving time compared to a manual procedure, especially when dealing with large medical data manipulation. In conclusion, the proposed method from this study is capable of producing a customised part with the highest degree of design complexity compared with other conventional manufacturing methods. This has proved to be very suitable for manufacturing titanium medical implants, particularly craniofacial implants which require a customised and lightweight structure and at the same time still provide good mechanical propertie
Effects of powder conditioning on the quality, microstructure and mechanical properties of sintered titanium alloys
No full textA small amount of internal lubricant (Stearic acid (SA) or Magnesium stearate (MgSt)) was added to Ti powder for the purpose of improving the compressibility and the green density homogeneity. Lubrication improved the compressibility of Ti powder compacts, especially in the low pressure region (<400MPa), yet at higher pressures, lubricants hindered further improvement. An addition of up to 0.3wt.% SA improved the green density. MgSt showed a better lubrication effect and an addition of up to 0.6wt.% improved the density. The difference in the lubrication effect between SA and MgSt was due to their mixing or blending characteristics. A homogenous mixing of lubricant led to an increased lubricated area and the effectiveness of the lubrication. Lubricants had a strong influence on shape retention, green strength and ejection behaviour of Ti powder compacts.
The green density distribution was improved by adding a lubricant. The density profiles were measured experimentally using a coloured layer method. SA gave better results compared with MgSt, with 0.6wt.% of SA considered to be the optimum addition. The mixing or blending characteristics of SA additions accounted for the better improvement in performance.
By adding a lubricant, the sintered density distribution in Ti compacts was improved by controlling the pore morphology with respect to their size, aspect ratio and orientation. But 1wt.% SA created many pores ranging in size from 50-100μm, both in the top and bottom regions, and this led to very bad ductility. The consistency in mechanical properties of a sintered Ti Φ40 mm compact was significantly improved by adding 0.6wt.% SA. Such improvement was achieved because of a lower sintering mismatch initiated from a more homogenously distributed green density, both in the horizontal and transverse directions. MgSt was not recommended because of higher oxygen pick-up.
Rare earth elements (RE) were added to Ti metal and alloy for the purpose of scavenging oxygen and to promote better microstructural control. Three additive forms were studied. Er and LaB6 additions were added to Ti and Ti6Al4V alloys directly. The Ti(Ti6Al4V)-Y alloy was made by mechanical milling (MM). But direct Er additions caused a segregated microstructure and processing involving the MM of Ti(Ti6Al4V)-Y leaded to oxygen pick-up. These methods are therefore not recommended. Direct LaB6 additions achieved excellent results. The reinforcement was uniformly distributed with various orientations and the microstructure was refined. The TiB reinforcement gave excellent strength and good ductility. The acicular TiB phase seemed to be the only problem, because while it significantly improved the strength it decreased the ductility.
Theoretical research was carried out on the compaction process for internally lubricated Ti powder. A modified Cooper-Eaton formula was employed to analyse the compaction behaviour of Ti powder. There was a good fit between the simulation results and the experimental data. The theoretical research indicated that cold compaction of titanium powder could be separated into two stages: a particle rearrangement (PR) stage, which occurred in a compacting pressure range of 0-200MPa, followed by a plastic deformation (PD) initiated (PR) stage from 200-1000MPa. The existence of stage II was due to the low plastic deformability of titanium and the low density achieved at the end of stage I. Ti particles needed to be plastically deformed or even cracked into fragments to fill the gaps in-between Ti particles. At pressures between 200-600MPa, the use of an internal lubricant improved consolidation, leading to better densification because a lubricant facilitates particle motion. On the basis of this discussion, the approaches for modifying the cold compaction behaviour of Ti powder were studied
Design of a Stable Traffic Cone
Full text linkTraffic cones are a common sight on roads in most countries but due to their commonplace nature people tend not to think too much about their design and the importance of them in maintaining safe traffic flow is often overlooked. Traffic cones have become a product that is taken for granted in daily life for most people. Traffic cones are so easy to see on the road and yet are readily neglected by people. Never the less traffic cones are still important for safety and guiding traffic flow even though they are so simple.
A traffic cone consists of a relatively thin walled cone above a square hollow base. In order to increase the cones stability, the base is usually weighted therefore lowering the centre of gravity of the entire traffic cone. Traffic cones are typically produced in plastic or composite materials for reasons of safety and cost.
A literature search of a range of books and professional magazines has turned up little related research and few reports on the topic of traffic or road cones. It appears that road- cones receive little attention by people, due to their simple profile and inexpensive production cost.
Most current products are designed with a considerable amount of weight in the base in order to increase the stability of the traffic cone, which consumes a relatively large amount of material. Stability cannot be significantly increased using this design strategy without increasing the amount of material and the cost of production. Therefore optimization of the design to improve flexibility and stability would be investigated without compromising the appearance or manufacturing costs
Synthesis, Microstructure and Mechanical Properties of Titanium with Controlled Levels of Porosity
No full textPorous titanium (Ti) products have many applications due to their excellent corrosion resistance, high strength-to-weight ratio, high specific surface area, etc. However, the major limitation is the expensive cost of porous Ti products because of the high raw materials costs and the difficulty of processing. The novelty of this research work is concerned with a study of a ceramic casting technique, slip casting, to create porous Ti products and the effect of porosity on the mechanical properties and gas permeability of Ti compacts.
An optimised Ti slurry was developed from 43 vol.% of Ti powder, 0.3 dw.% of dispersant, 0.8 dw.% of plasticiser and 0.8 dw.% of binder, mixed with a balance of distilled water, which produced a viscosity of 290 cP. It was then poured into a plaster mould to form compacts with a green density of 45%. Thermal debinding was carried out at 320°C with an argon flow for 2 hours, followed by vacuum sintering different samples at 1000°C and 1200°C for 0.5 hours, respectively. The porous sintered compacts had satisfactory tensile strength with some plastic deformation. The increase in oxygen and carbon content during processing was minor. The results from this investigation suggested that slip casting is a potentially low-cost, simple manufacturing route for porous Ti products.
Porous Ti compacts with porosity in the range of 12.3 vol.% to 35.3 vol.% were fabricated by slip casting. The mechanical properties, fracture morphology, gas permeability, pore size analysis and pore shape factors for the porous Ti compacts were determined for different porosity levels. A decreasing porosity level resulted in less open porosity and gas permeability, reduced pore size, and an increased tensile stress and elongation. The mechanical properties of porous Ti compacts produced by slip casting were comparable with more conventional press and sintered materials at the same porosity levels. Theoretical models for tensile stress and ductility as a function of porosity were examined and incorporated into the results and differences between the theoretical models and experimental results are discussed. The pore shape factor analysis showed that tensile loading would stretch the pores in the compacts to produce more irregular pores, which were acting as linkage sites to allow the propagation of cracks. Additionally, a novel interconnected pore characterisation method using ammonium meta-tungstate solution is presented. By using backscatter scanning electron microscopy, the interconnected pores can be directly observed.
The packing behaviour and sintering behaviour of Ti compacts fabricated with different particles size distribution were characterised and studied, in terms of porosity, pore size analysis, tensile properties and gas permeability. Two different particles size (avg. 14 μm & avg. 56 μm) of pure Ti hydride-dehydride powder were used in five volume proportions (20:80, 40:60, 60:40, 80:20 & 100:0). A theoretical model predicted that the green density reaches a maximum when the volume fraction of fine particles is 0.35. It was found that although experimental results showed similar behaviour there was no well-defined maximum green density. The sintered compacts showed that an increase in the volume fraction of fine powder particles reduced the porosity, permeability level and pore size, and increased the tensile properties. The relationship between permeability and porosity level was non-linear and this was caused by the differences in pore diameters in the compacts. The capillary tube model was used to discuss the relationship between the permeability, pore diameter and porosity. Using this information, a graded porosity compact was designed and fabricated by slip casting. A future study on this graded porosity compacts is recommended to undertake in more depth
Investigation of Flow Parameters for Titanium Cold Spraying using CFD Simulation
No full textA comprehensive study of cold gas dynamic spray technology is required for optimising performance and gun design for spraying various materials. Cold spraying technology is a new technique in industry and very limited data is available. This thesis focuses on the investigation of cold spray parameters for spraying ductile titanium alloys through a de-Laval convergent-divergent nozzle and optimisation of the nozzle dimensions. This work describes a detailed study of the various parameters, namely applied gas pressure, gas temperature, size of titanium particles and dimensions of the nozzle on the outlet velocity of the titanium particles. A model of a two-dimensional axisymmetric nozzle was used to generate the flow field of titanium particles with the help of a gas stream flowing at supersonic speed. ANSYS FLUENT software was used for the simulation of a cold spray nozzle. A standard k-ɛ model has been used to account for the turbulence produced due to the very high velocity flow. Differences in the velocity of titanium particles were modelled over the range of applied gas pressure, gas temperature and size of titanium particles. From the CFD simulation results optimum values of gas pressure and temperature were found for making a successful coating of titanium particles. The optimum nozzle dimensions were also found as the diverging length and exit diameter of the nozzle were found to affect the outlet velocity of titanium particles. The simulation results show good agreement with previous cold spray work using different spraying materials. Validation of the CFD model was done by referring to the experimental work and CFD work done for a similar kind of flow field. The grid quality of the model was investigated to get the results to converge and be independent of the grid size to give good agreement between the accuracy of results and the computational time
Recovery and Purification of Titanium Dioxide and Aluminium Compounds from Corundum By-product of the TiPro Process
No full textThe TiPro process developed at the University of Waikato utilises an aluminothermic reaction to create titanium rich intermetallic alloys or alloy powders by solid-liquid separation. The product is then reduced with calcium vapour or calcium hydride to lower the oxygen content. This process involves intimately mixing titanium oxide powder with stoichiometric quantities of aluminium powder and heating the mixture to initiate a thermite reaction that transfers oxygen from the titanium oxide to the aluminium to produce titanium aluminide alloy and aluminium oxide. The liquid alloy is drained off from the by-product, called corundum, before going on to further processing. The corundum is left behind and contains significant quantities of valuable titanium alloy and aluminium and titanium oxides which must be recovered to increase the efficiency and lower the cost of the process.
This thesis describes and explains proof-of-concept wet chemistry methods of processing the corundum by-product to recover and separate the valuable metal oxides. Using acid-base equilibria, over 90% of the titanium was recovered from the corundum phase as impure titanium oxide. Aluminium was recovered as aluminium hydroxide and aluminium oxide, both of which can be processed into feedstock for fresh aluminium smelting and hence represent a potential revenue stream. The products were characterised using SEM-EDX and XRD to determine their composition and structure. The proposed treatment produces no waste other than environmentally benign neutral salt water
Study of Titanium based Composite Coatings for Resistance against Molten Aluminium Soldering on H13 Tool Steel
No full textThe service life of industrial components is limited predominantly by chemical corrosion, mechanical failure or mechanical wear. In the aluminium high pressure die casting industry, liquid aluminium is extremely reactive with the constituents of H13 die steel and has a tendency to form intermetallic layers. This chemical interaction results in sticking of molten metal to the die surface which produces defective castings and also damages the die surface. The use of thermal spray coatings provides protection to the surfaces operating in severe environments. An HVOF thermally sprayed coating has the advantage of having excellent bond strength and very low porosity levels (< 1%). This research work is concerned with producing and evaluating the performance of titanium/alumina based composite coatings to improve the service life of tool steel (H13) used for dies in aluminium high pressure die casting and dummy blocks used in Al extrusion.
In this research work, the powder feedstocks for making the composite coatings were produced by high energy mechanical milling of a mixture of Al and TiO₂ powders in two different molar ratios followed by a thermal reaction process. The feedstock powder was then thermally sprayed using a high velocity oxygen fuel (HVOF) technique on H13 steel substrates to produce Ti(Al,O)/Al₂O₃ and TiAl/Al₂O₃ composite coatings. The performance of the coatings was assessed in terms of Al soldering, liquid metal corrosion resistance, thermal shock resistance and wear resistance.
In an immersion test, the coated specimens were dipped into molten Al at a temperature of 700 ± 10 °C for different intervals of time. The performance of the coatings was tested in terms of liquid metal corrosion resistance and propensity to Al soldering. The dissolution behaviour of the coatings was evaluated by measuring weight loss after dipping the samples in to molten aluminium. The immersion test results showed that the coated samples have relatively few locations where aluminium soldering (reactive/chemical) occurred, however, an H13 steel surface showed more tendency for aluminium soldering. It was found that composite coatings changed the molten Al attack on H13 tool steel from a generalized to a localized one. No reaction between molten aluminium and a Ti(Al,O)/Al₂O₃ composite coating was identified. The TiAl/Al2O₃ composite coating was found to be attacked by molten aluminium as a result of a reaction between the coating and molten aluminium. The metallic phase TiAl in the composite coating is believed to be attacked by the molten Al. A Ti(Al,O)/Al₂O₃ composite coating was found to be a better protective coating than the TiAl/Al₂O₃ composite coating due its stability against molten aluminium attack.
The thermal shock behaviour of the composite coatings was investigated by subjecting the coated coupons to a number of cycles, each cycle consisting of a holding time of 30 seconds in molten aluminium at 700 ± 10 °C followed by quenching into water. The surfaces of the coupons were examined for Al soldering and an evaluation of surface spallation. Any cracks found in the coatings were studied to explain their thermal shock behaviour. A Ti(Al,O)/Al₂O₃ composite coating on H13 tool steel produced from a fine feedstock has better thermal shock resistance than the Ti(Al,O)/Al₂O₃ TiAl/Al₂O₃ composite coatings produced from the agglomerated feedstocks.
The study also describes and compares the tribological properties such as friction and sliding wear rate of the composite coatings both at room and high temperature (700.°C) under dry and lubricating conditions. The wear resistance of the coatings was investigated by a tribometer using a spherical ended alumina, flat ended high speed steel and spherical ended hardened steel pins as counter bodies. The experimental results show that the composite coatings look promising for high temperature applications due to their low wear rate at high temperature. However room temperature applications of the composite coatings can be improved under lubricated conditions.
Successful trials of a Ti(Al,O)/Al₂O₃ composite coated dummy block revealed that the coating has potential as an industrial coating
Simulation of the powder forging process for titanium components using a porous metal plasticity model
No full textPowder forging is a technique that has been used to produce fully dense near-net parts from metal powders. Due to a “low cost titanium” product manufacturing initiative, targeting a reduction in the cost of titanium components, a titanium powder forging technique has gained significant interest. In titanium powder forging, powder consolidation is a key factor that influences successful component manufacture. Consolidation during titanium powder forging is dependent on the densification and deformation mechanisms involved.
In this study, a finite element method is used to model the densification and deformation behaviour of titanium powder compacts during powder forging. The research focuses on developing a simulation capability and identifying a suitable constitutive model to simulate the powder forging process that can predict the relative density distribution. The simulation is carried out in Abaqus software and the results are compared with experimental results. A gamma particle radiography technique is used to compare the experimental density results with the simulated results.
The Gurson and Gurson-Tvergaard models are used to predict the relative density of porous titanium powder compacts during upset-powder forging and are used to include the effect of hydrostatic stresses and the extent of densification. Three different modes of densification, related to powder forging were studied i.e. upset forging, hot-repressing and closed die forging.
The simulation results indicate that both models can be used to determine the relative density during powder forging. By comparing the simulated results with the experimental results, it is found that the density prediction given by the Gurson-Tvergaard model showed closer agreement with the calculated parameters
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