Hot isostatic pressing of high temperature materials

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Sergi, Alessandro (2022). Hot isostatic pressing of high temperature materials. University of Birmingham. Ph.D.

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Abstract

Near Netshape Powder Metallurgy Hot Isostatic Pressing (NNS PM HIP) is an advanced manufacturing process capable of manufacturing a variety of materials with superior mechanical properties, improved buy-to-fly ratio and complex geometrical design. NNS PM HIP could represent an advantageous alternative to conventional manufacturing routes for the manufacture of high-value components across multiple industrial sectors. However, despite the evident advantages of NNS PM HIP technology and the recent research interest in the topic, there is a current lack of knowledge associated with the processability of some materials, which limits the potential benefits of this manufacturing technology. Thus, the
main scope of this thesis is to investigate some key challenges, report the main findings and demonstrate the possibility of manufacturing different classes of high temperature materials through NNS PM HIP. This will be achieved by assessing the HIP behaviour for a wide variety of high temperature materials including IN625, IN625-based metal matrix composites (MMCs), Nb and C-103 Nb alloy. The first study concerns the HIP behaviour of Ni-base superalloys using IN625 as case study. To this end, four different IN625 powders obtained using four different atomisation routes including argon, nitrogen, plasma and water atomisation (AGA, NGA, PA and WA) were characterised through scanning electron microscopy (SEM), energy dispersive X-ray (EDS) and X-ray photoelectron spectroscopy (XPS) techniques. The differences in as-HIPed microstructure and mechanical properties were assessed for the four powders via SEM, EDS, tensile and Charpy impact tests. The results indicate a direct correlation between powder quality, presence of PPBs and mechanical properties, highlighting the importance of powder characteristics on PPBs formation for Ni-base superalloys.
The knowledge acquired in the first study was exploited to develop a high temperature wear resistant material through NNS PM HIP as part of the second study. To enhance the wear properties of IN625, two different reinforcements respectively SiC and TiB2 with various reinforcement fractions ranging from 5vol% to 25vol% were used. The microstructural characterisation of the HIPed material performed using SEM and EDS techniques highlighted the presence of a fully dense microstructure with a continuous network of ceramic reinforcements at PPBs. The wear test performed on IN625-MMCs demonstrated a progressive improvement in tribological properties with the increase of the reinforcement volume fraction for both IN625-SiC and IN625-TiB2. Finally, IN625-10v%SiC was downselected for room temperature and high temperature tensile tests. The results indicate a drastic reduction in elongation, which is a direct consequence of the presence of ceramic reinforcement at PPBs.
As part of the third study, the HIP behaviour of refractory metals with pure Nb as case study was investigated. In this study, three Nb powders with varied particle size ranges (fine, midrange and coarse) and C-103 Nb alloy powder were investigated to understand the differences in particle size distribution (PSD), morphology and oxygen content present in the alloy chemistry. The as-HIPed microstructures of pure Nb and C-103 were near to fully dense with the absence of PPBs. Additionally, it was observed that oxygen content plays a crucial role in the microhardness and mechanical properties of pure Nb. On the other hand, as-HIPed C-103 powder showed superior tensile properties if compared to the minimum specifications for wrought C-103. Finally, in the fourth study, the potentials of HIP diffusion bonding (DB) were exploited to demonstrate the possibility of manufacturing high temperature oxidation resistant coatings for pure Nb using Pt and the feasibility of joining Nb-Ti6Al4V.
To appraise the industrial viability of the NNS PM HIP process route and materials studied in this thesis, three different prototypes were manufactured. In particular, the manufacturing steps involved in the production of IN625 Y-shaped pipe, IN625-10v%SiC mechanical seals and Nb combustion chamber were presented and discussed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):