Hot isostatic pressing of titanium metal matrix composites for tribological applications

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Al Lawati, Malallah Mohamed Malallah (2022). Hot isostatic pressing of titanium metal matrix composites for tribological applications. University of Birmingham. Ph.D.

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Abstract

An investigation into developing titanium metal matrix composites (TMCs) for tribological applications while still possessing good mechanical properties has been performed using powder metallurgy (P/M) and hot isostatic pressing (HIP) process. TMCs were successfully synthesized using a wide range of process parameters such as : investigating the influence of HIP temperature and dwell time. In addition to that, the influence of reinforcement volume fraction, size, type, blending route and in situ reactions were studied in terms of the microstructural homogeneity, mechanical and tribological properties. The as-HIPped samples were analysed using scanning electron microscopy (SEM), optical microscopy (OM), x-ray diffraction (XRD) and Raman spectroscopy for the samples containing graphene. A wide range of reinforcements were investigated, such as silicon carbide (SiC), titanium diboride (TiB2), boron (B), graphene nanoplatelets (GNP) and titanium carbide (TiC). In addition to that, the influence of reinforcement size from micron size (5 µm) to nano-size (20 nm) on the mechanical and tribological properties were investigated thoroughly. Furthermore, the influence of reinforcement volume fraction ranging from 5 volume percent (vol.%) to 10 vol.% on the microstructural homogeneity, mechanical and tribological properties was studied. Reinforcements such as B and GNP were used in situ formation work in order to synthesize phases such as TiC and TiB during the HIP process and process parameters such as the HIP temperature was investigated in terms of in situ phase formation and completion. Finally, functionally graded material (FGM) using SiC as a reinforcement and Ti-6Al-4V (Ti64) matrix was successfully synthesized using the HIP process and the tribological and mechanical properties were investigated. The work will aim to solve issues of high brittleness due to the high reinforcement volume fractions, microstructural homogeneity due to powder clustering , while trying to enhance interfacial bonding and solve issues concerning TMCs blending due to the high reactivity of Ti with oxygen. Moreover, issues such as incomplete in situ phase formation will be investigated by HIPping at different temperatures and to reduce the size of the brittle diffusion zone due to iron (Fe) diffusion at high temperatures. The main novelty of the work will look into improving microstructural homogeneity by controlling the blending route using mechanical alloying (MA) via wet ball milling, reducing the brittle intermetallic diffusion zone by studying various HIPping temperatures and using different elemental reinforcements with high specific strengths in order to reduce agglomeration and inhomogeneity as a result of high volume fractions. The rationale for conducting the research is that ti64 has a high specific strength to weight ratio and could provide weight savings in a wide range of engineering applications including the marine and oil and gas industry. Furthermore, Ti64 has good corrosion resistance but low tribological properties such as coefficient of friction and sliding wear properties which is an issue at harsh environmental conditions. Therefore, reinforcing the Ti64 matrix with various chemically stable reinforcements that are stiffer than the matrix such as SiC were suggested to increase the strength and wear resistance of the composites. Furthermore, in-situ phase formation was chosen using reinforcements such as boron (B) and graphene nanoplatelets (GNP) to obtain clean in situ strengthening stiffer phases to enhance the tribological properties of the composites. TMCs were developed for high wear resistance applications that also possess good mechanical properties using a Ti64 matrix reinforced with different additions and volume fractions of B (1 vol.%) and GNP (1-2 vol.%). The study looked into in situ formation of hard phases and how the HIP temperature can influence that including the consolidation behaviour. Different HIP conditions were investigated such as HIPping below the ß-transus temperature at (920℃), 1040℃ and super ß-transus temperature (1160℃). 1040℃ was selected as an optimal HIP temperature based on calculations that estimate the ß-transus temperature by taking into account the diffusion of oxygen (O) and carbon (C). The main findings of the work is that HIPping at 1160oC ensured full consolidation, but resulted in grain growth, while HIPping at 920oC there was lack of consolidation and no in situ reaction. At 1040oC there was retention of TiC phase with some unreacted graphene, which improved tribological performance. Furthermore, TMCs using SiC as a reinforcement and Ti64 as the metal matrix were developed for high wear resistant applications. The study looked into the influence of reinforcement volume fraction (5-10 vol.%) and reinforcement size (5 µm , 20 nm) on the mechanical and tribological properties. In addition to that, blending routes were investigated such as MA by ball milling and roll blending and how it affects powder homogeneity, hence the properties. The main findings of the work is that increasing the reinforcement vol.% to 5 vol.%, changes the wear mechanism from abrasive wear to mainly delamination wear, which in turn reduces the wear rate of the composite and improves the wear resistance. This could be mainly attributed to the hard in situ formed TiC and titanium silicide (Ti5Si3) phases. Ti64 based FGMs were prepared using P/M HIP using SiC as a reinforcement. Three layers of an FGM was produced, with the bottom layer being monolithic Ti64, the second layer reinforced with 5 vol.% SiC and the third layer with 10 vol.% SiC. A super transus HIP temperature of 1160℃ was selected for the work to ensure full consolidation, in situ reaction at the expense of grain growth. The different layers of the FGM showed good bonding and no cracking was observed along the gradient layers as seen by the micro-hardness indentations. The synthesized FGM showed promising compressive properties such as high compressive yield strength and very good ductility values even at the higher 6 vol.% regions. On the other hand, there was a very clear trend of a reduction in ductility and an increase in compressive yield strength as the reinforcement volume fraction was increased.Ti64 reinforced with varying volume fractions (5 vol.%, 10 vol.%) of TiB2 was prepared by P/M HIP. The influence of the HIP temperature (1160℃) on the microstructural evolution, mechanical and tribological properties were investigated. Furthermore, as-received MA titanium-silicon carbide (Ti�SiC) nanocomposite and titanium-titanium carbide (Ti TiC) nanocomposite were successfully prepared by HIP at various temperature such as (950℃,1040℃) and how that influenced the mechanical and tribological behaviour. Some of the key findings of the study were that the increase of micro-hardness observed with the with the 10 vol.% TiB2 is mainly be attributed to the load transfer mechanism and grain refinement. The large standard deviation could be attributed to the inhomogeneity, which is inherent to the blending technique used and the relatively large reinforcement size of 5 µm. Some of the promising applications of TMCs can be used in the oil and gas industry such as pipelines as titanium is highly corrosion resistant, and the reinforcement in situ formed hard phases will provide good wear resistance properties. other areas could include engine valves and connecting rods due to the weight savings of titanium and good tribological properties provided by using graphene for example as a reinforcement that would provide lubrication.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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