Paste development and co-sintering test of zirconium carbide and tungsten in Freeze-form Extrusion Fabrication

Ultra-high temperature ceramics are being investigated for future use in aerospace applications due to their superior thermo-mechanical properties, as well as oxidation resistance, at temperatures above 2000°C. However, their brittle properties make them susceptible to thermal shock failure. Components fabricated as functionally graded materials (FGMs) can combine the superior properties of ceramics with the toughness of an underlying refractory metal by fabricating graded composites. This paper discusses the grading of two materials through the use of a Freeze-form Extrusion Fabrication (FEF) system to build FGMs parts consisting of zirconium carbide (ZrC) and tungsten (W). Aqueous-based colloidal suspensions of ZrC and W were developed and utilized in the FEF process to fabricate test bars graded from 100%ZrC to 50%W-50%ZrC (volume percent). Following FEF processing the test bars were co-sintered at 2300°C and characterized to determine their resulting density and micro-structure. Four-point bending tests were performed to assess the strength of test bars made using the FEF process, compared to test bars prepared using conventional powder processing and isostatic pressing techniques, for five distinct ZrC-W compositions. Scanning electron microscopy (SEM) was used to verify the inner structure of composite parts built using the FEF process --Abstract, page iii

Development of Particle Reinforced Metal Matrix Composite Materials for Selective Laser Melting Applications

Maraging steels are often used for high temperature applications, like forging dies, casting dies, or parts which are used in harsh environments. They are based on Fe with a relatively high amount of Ni (i.e. 17 - 19 %) and were developed for the aero, space and tooling industry. Maraging steels feature a high strength and toughness, but lack of resistance against wear, which limits their applications in some areas. Due to the weak tribological behaviour, parts made out of maraging steel sometimes do not last very long, if they are involved in an abrasive movement. In this thesis work, the metal matrix of maraging steel MS1 was reinforced with different volume contents of either vanadium carbide or titanium carbide. The goal was to improve the hardness values and the tribological behaviour of the created metal matrix composite (MMC). As traditional manufacturing methods like casting are often not applicable, a powder processing route had to be chosen. For this, mechanical mixing of the MS1 and the composite material was chosen for the first step of the process route. After mechanical mixing, the powders were processed with a selective laser melting (SLM) machine to additively manufacture solid parts. Computer aided design (CAD) files were used to laser weld contours of the metal powder and layer by layer, parts were built three dimensional. The first investigation was about the influence of the initial particle size of the composite material. As vanadium carbide has a significantly higher melting temperature, the ability to melt these particles in a SLM machine had to be analyzed. Furthermore, the tendency of agglomerations due to Van der Waals forces had to be considered. After performing a Design of Experiments to find the optimal parameters for SLM processing, specimens with improved hardness were manufactured, but had some design flaws. To further improve the results and to better understand the effects of vanadium carbide on the matrix material, different volume contents were analyzed. However, after these first investigations it was found that solely mixing is insufficient for a homogeneous particle distribution, which is why mechanical alloying (MA) was performed as a post-powder processing technique. MA allowed to crack the agglomerations of the carbides and to embed the particles into the metal matrix material. This processing step was important for the stability of the SLM process and significantly improved the results. The metal matrix composite system of MS1 and TiC was the most promising and achieved the best results. A higher energy density "eta" improved the part density and lead to the complete melting of VC and TiC particles, which then solidified as primary carbides. While designing advanced materials with metal matrix composites, it is important to understand the phase formations and the mechanical properties of the new material. Several metallurgical, mechanical and tribological tests were performed after SLM processing the mechanically alloyed powders. To find the optimal mechanical alloying parameters, various milling times were applied. Afterwards, the morphology and the particle size distribution was investigated. It was found that the addition of carbides through mechanical alloying can significantly influence not just the mechanical properties, but also the phase formation. Some chemical transformations were observed in the VC and TiC particles, which influenced the solidification of the melt and the phase transformation of the gamma-Fe into alpha-Fe. Future scientific work is suggested in the optimization of SLM parameters for higher part densities and as a result, higher mechanical properties. Electron backscatter diffraction and transmission electron microscopy could furthermore, help to understand the microstructures of metal matrix composites.Englis

Materials, Design and Process Development for Additive Manufacturing

Additive manufacturing is already actively used in various high-tech industries today. At the same time, there is a certain limitation and imperfection of known and widely used conventional materials when they are used in additive manufacturing. In this regard, extensive research and development are aimed at the advancements of new materials by adjusting the chemical compositions of conventional alloys, new equipment with expanded functionality and the ability to work with a wide range of materials that were previously not available for additive manufacturing. This Special Issue covers a wide scope of additive manufacturing processes, comprising investigation, characterization of materials and their properties, development and application of new materials, structures designed for additive manufacturing, as well as processes and techniques that will expand the potential applications of layer-by-layer synthesis

Reactive Laser Synthesis of Ultra-high-temperature Ceramics HfC, ZrC, TiC, HfN, ZrN, and TiN for Additive Manufacturing

Ultra-high-temperature ceramics (UHTCs) are optimal structural materials for applications that require extreme temperature resilience, resistance to chemically aggressive environments, wear, and mechanical stress. Processing UHTCs with laser-based additive manufacturing (AM) has not been fully realized due to a variety of obstacles. In this work, selective laser reaction sintering (SLRS) techniques were investigated for the production of near net-shape UHTC ceramics such as HfC, ZrC, TiC, HfN, ZrN, and TiN. Group IV transition metal and metal oxide precursor materials were chemically converted and reaction-bonded into layers of UHTCs using single-step selective laser processing in CH4 or NH3 gas that might be compatible with prevailing powder bed fusion techniques. Conversion of either metals (Hf, Zr and Ti) or metal oxides (HfO2, ZrO2, and TiO2) particles was first investigated to examine reaction mechanisms and volume changes associated with SLRS of single-component precursor systems. SLRS processing of metal or metal oxide alone produced near stoichiometric UHTC phases with yields up to 100 wt% total for carbides and nitrides. However, for single component precursors, gas-solid reactivity induced volumetric changes resulted in residual stresses and cracking in the product layer. To mitigate conversion-induced stresses, composite metal/metal oxide precursors were employed to compensate for the volume changes of either the metal (which expands during conversion) or the metal oxide precursor (which contracts).Comment: 58 pages, 17 figure

Advanced Powder Metallurgy Technologies

Powder metallurgy is a group of advanced processes used for the synthesis, processing, and shaping of various kinds of materials. Initially inspired by ceramics processing, the methodology comprising the production of a powder and its transformation to a compact solid product has attracted attention since the end of World War II. At present, many technologies are availabe for powder production (e.g., gas atomization of the melt, chemical reduction, milling, and mechanical alloying) and its consolidation (e.g., pressing and sintering, hot isostatic pressing, and spark plasma sintering). The most promising methods can achieve an ultra-fine or nano-grained powder structure, and preserve it during consolidation. Among these methods, mechanical alloying and spark plasma sintering play a key role. This book places special focus on advances in mechanical alloying, spark plasma sintering, and self-propagating high-temperature synthesis methods, as well as on the role of these processes in the development of new materials

The critical raw materials in cutting tools for machining applications: a review

A variety of cutting tool materials are used for the contact mode mechanical machining of components under extreme conditions of stress, temperature and/or corrosion, including operations such as drilling, milling turning and so on. These demanding conditions impose a seriously high strain rate (an order of magnitude higher than forming), and this limits the useful life of cutting tools, especially single-point cutting tools. Tungsten carbide is the most popularly used cutting tool material, and unfortunately its main ingredients of W and Co are at high risk in terms of material supply and are listed among critical raw materials (CRMs) for EU, for which sustainable use should be addressed. This paper highlights the evolution and the trend of use of CRMs) in cutting tools for mechanical machining through a timely review. The focus of this review and its motivation was driven by the four following themes: (i) the discussion of newly emerging hybrid machining processes offering performance enhancements and longevity in terms of tool life (laser and cryogenic incorporation); (ii) the development and synthesis of new CRM substitutes to minimise the use of tungsten; (iii) the improvement of the recycling of worn tools; and (iv) the accelerated use of modelling and simulation to design long-lasting tools in the Industry-4.0 framework, circular economy and cyber secure manufacturing. It may be noted that the scope of this paper is not to represent a completely exhaustive document concerning cutting tools for mechanical processing, but to raise awareness and pave the way for innovative thinking on the use of critical materials in mechanical processing tools with the aim of developing smart, timely control strategies and mitigation measures to suppress the use of CRMs

Additive Manufacturing of Novel Cemented Carbides with Self-Lubricating Properties

In this research, WC-17Co and WC-Co-hBN cemented carbides were processed using Selective Laser Sintering (SLS) and heat treated at 400 C, 600 C, 800 C and 1000 C for 3 hours to understand the effect of processing and post-processing heat treatment on the structure and properties of the cemented carbide. Electron microscopy and X-ray diffraction (XRD) analysis revealed that the microstructure of the as-printed WC-17Co specimen was characterized by relatively large poly-angular WC/W2C chips, WC-Co dendritic structures, W-C-Co phase and Co-rich regions. WC-Co-hBN also revealed from the microstructure polyangular WC chips which were smaller in size with no W2C phases present in the sample. During heat treatment between 0 C to 600 C, the large poly-angular chips in both WC-17Co and WC-Co-hBN disintegrated to smaller poly-angular chips as a result of the conversion of the unstable W2C phase to the more stable WC phase and the generation of W-Co-N and Co-W-B phases respectively. Heat treatment above 600 C resulted in the coalescence and growth of relatively large WC phase chips. There was significant increase in hardness of the WC-17Co samples during heat treatment when compared with the as-printed WC-17Co sample, with the sample heat-treated at 600 C being 36% harder than the as-printed sample due to the breakdown of poly-angular WC chips and the increase in volume fraction and spatial distribution of the observed W-C-Co phase regions. The increase in hardness at 600 C was coupled with the highest fracture toughness, representing a 34% increase in fracture toughness, when compared with the as-printed sample. The high fracture toughness is attributed to the evolution of the ductile W6Co6C phase in the sample after heat treatment. Nevertheless, the as-printed sample had approximately 15% higher wear resistance than the sample heat-treated at 600 C. In the WC-Co-hBN, the heat-treated samples had lower hardness values compared to the as-printed WC-Co-hBN sample. However, the hardness values were 3 times higher than the hardness value of the WC-17Co sample. This was attributed to the lower grain sizes in the WC-Co-hBN as compared to the WC-17Co samples. The wear resistance of the WC-Co-hBN samples were much higher than the WC-17Co samples with the highest being on the WC-Co-hBN sample heat treated at 1000 C. It is concluded that post-processing heat treatment of SLS printed WC-17Co alloy at 600 C can be used to improve the structure and mechanical properties of the alloy. And a further improvement of the wear properties and hardness of the material can be done by adding a volume of hBN to the alloy

Near-net-shape Fabrication of Ni(x)Al(y) – TiC Cermets by Binder Jet Additive Manufacturing and Pressure-less Melt Infiltration

Binder jet additive manufacturing effectively replaces preform preparation by traditional powder metallurgy methods and allows more complex geometries to be potentially fabricated. The use of a pressure-less melt infiltration technique provides a method of achieving a fully dense composite in a near-net-shape fashion, cost effective and scalable. TiC preforms (15 x 15 x 10/7.5 mm) were fabricated via Binder Jet Additive Manufacturing and then infiltrated by NiAl3 and Ni3Al using a pressure-less melt infiltration technique. Two compositions, Ni3Al and NiAl3, were used as infiltrant materials in order to compare wetting behavior and infiltration kinetics.A stark difference in shape retention between Ni3Al and NiAl3 infiltrated preforms was observed after infiltration. It was found that the TiC particles in the as printed preform were arranged in an interconnected network structure as a result of the binder jet process and/or sintering step and is responsible for maintaining structural integrity of the printed preform. TiC dissolution by liquid Ni3Al was significant enough to disband this network structure and force particle rearrangement and lead to poor shape retention. Conversely, TiC particle rearrangement did not occur during infiltration of the Al-rich NiAl3 alloy and thus the network structure remained intact leading to excellent shape retention of the infiltrated preform.It was found NiAl3 exhibits a complex melting and solidification behavior where an Al-rich phase segregates heavily from the melted material. The presence of an Alrich inter-particle matrix phase indicates the possibility of a “metered” infiltration process where multiple liquid phases of variant compositions infiltrate the “macro” and “micro” capillaries of the TiC preform in a staggered fashion. The composition and time of infiltration remains unknown and a topic of future work. Additionally, Thermodynamic simulation predicts a reaction at the TiC interface by liquid NiAl3 to form Al4C3, with an interfacial reaction product potentially explaining the lack of TiC dissolution and volumetric shrinkage during and after infiltration by NiAl3

A study on ultrasonic energy assisted metal processing : its correeltion with microstructure and properties, and its application to additive manufacturing.

Additive manufacturing or 3d printing is the process of constructing a 3-dimensional object layer-by-layer. This additive approach to manufacturing has enabled fabrication of complex components directly from a computer model (or a CAD model). The process has now matured from its earlier version of being a rapid prototyping tool to a technology that can fabricate service-ready components. Development of low-cost polymer additive manufacturing printers enabled by open source Fused Deposition Modeling (FDM) printers and printers of other technologies like SLA and binder jetting has made polymer additive manufacturing accessible and affordable. But the metal additive manufacturing technologies are still expensive in terms of initial system cost and operating costs. With this motivation, this dissertation aims to develop and study a novel metal additive manufacturing approach called Acoustoplastic Metal Direct-Write (AMD) that promises to make metal additive manufacturing accessible and affordable. The process is a voxel based additive manufacturing approach which uses ultrasonic energy to manipulate and deposit material. This dissertation demonstrates that the process can fabricate near-net shape metal components in ambient conditions. This dissertation investigates two key phenomenon that govern the process. The first phenomenon investigated is ultrasonic/acoustic softening. It is the reduction in yield stress of the metals when being deformed under simultaneous application of ultrasonic energy. A detailed analysis of the stress and microstructure evolution during ultrasonic assisted deformation has been presented in this dissertation. Crystal plasticity model modified on the basis of microstructure analysis has been developed to predict the stress evolution. The 2nd phenomenon investigated is ultrasonic energy assisted diffusion that enables the bonding of voxels during the AMD process. High resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) analysis has been used to quantify this phenomenon and also distinguish the process mechanics from other foil or sheet based ultrasonic joining processes

Influence of Nano-Sized SiC on the Laser Powder Bed Fusion of Molybdenum

Consolidation of pure molybdenum through laser powder bed fusion and other additive manufacturing techniques is complicated by a high melting temperature, thermal conductivity and ductile-to-brittle transition temperature. Nano-sized SiC particles (0.1 wt%) were homogeneously mixed with molybdenum powder and the printing characteristics, chemical composition, microstructure, mechanical properties were compared to pure molybdenum for scan speeds of 100, 200, 400, and 800 mm/s. The addition of SiC improved the optically determined density and flexural strength at 400 mm/s by 92% and 80%, respectively. The oxygen content was reduced by an average of 52% over the four scan speeds analyzed. Two mechanisms of oxygen reduction were identified as responsible for the improvements: oxidation of free carbon and the creation of secondary phase nanoparticles. This study illustrates the promising influence of nanoparticle additions to refractory metals in laser powder bed fusion