Powder Metallurgy

For very accurate positioning of satellites μN-thrusters are required. These thrusters consist of ion emitters, which are typically made of W or Re. This study focuses on the investigation of needles (diameter 200–400 μm) and emitter-crowns manufactured by MIM. Realizing extremely high aspect ratios (up to 1:200) in combination with controlling of porosity and density gradient in the material is essential for this application and needs to address the following topics: advanced binder system, suitable starting powders, carefully adjusted injection molding parameters. After the development of the injection step, the sintering process and its influence on the densification behavior was studied. The above described application of a space propulsion system needs precise control of porosity and resulting capillary effects within the manufactured W and Re structures. Distinctive open porosity and density gradients were realised including denser cores and porous outer layers. The produced needles and crowns are currently tested for the use in ultra-precise formation flying space systems and terrestrial applications.

Selective electron beam melting (EBM) is a powder-bed based additive manufacturing technique for the fabrication of dense metallic parts. As a free-form deposition process EBM offers a high degree of freedom in design and a highly efficient utilization of material. For a widespread industrial application the achievable build rate is a critical factor. In this paper the process window for Ti-6Al-4V is explored by varying beam current and scan speed in a wide range. The EBM specimens were characterized by their density, porosity size and distribution, chemical composition, microstructure, hardness and tensile strength. Key findings include: (1) the minimum line energy necessary to achieve full density, (2) parameter sets for increased build rates at full density, and (3) formation of α'-martensite at high scan rates.

The demand for clean and green energy has raised the consumption of hydrogen continuously during the last years. Hydrogen is most economically produced in large scale systems by methane steam reforming followed by pressure swing adsorption (PSA). However, with a rising demand for small-scale production of hydrogen, and as down-scaling to smaller PSA-systems ( < 500 Nm³/h H₂) is not economic, a substantial demand for hydrogen generation using palladium membranes has emerged.

Porous tubes made of an oxide dispersion strengthened powder metallurgy Fe-Cr alloy (trade name ITM) constitute the backbone for the thin solid Pd films. The tubes provide mechanical and chemical long-term stability in atmospheres with hydrogen- and carbon-species at operation temperatures up to 600°C. A porous ceramic diffusion barrier layer (DBL) is deposited between the ITM-backbone and the Pd thin-film to avoid Pd diffusion into the Fe-Cr substrate and thereby ensure long-term integrity of the system. The Pd thin-film with a thickness < 10 μm is applied onto the DBL by a proprietary coating technology.

This paper describes the production route of a tube/diffusion-barrier-layer/Pd-membrane system, its structure and permeation properties.

Pressureless sintering and the two pressure assisted sintering technologies – sintering + hot isostatic pressing (sinter-HIP) and field assisted sintering technology (FAST) were used to produce a fine grained WC-6 Co hardmetal.

In order to compare the mechanical properties of the hardmetals, which result from different sintering technologies, the material strength was determined. In the present study the uniaxial three point bending test and the biaxial ball on three balls test were used as strength measuring methods. The results were described by the Weibull distribution function.

Additionally, the paper will analytically compare the two tests by describing the relation of the strength of the samples to their effective volumes. Based on this procedure components having arbitrary shapes and volumes can be designed.

In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have a major impact on the microstructure as well as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings.

The effect of milling parameters, such as ball/powder ratio (BPR), milling speed and time, on the microstructural and phase development of powder metallurgy processed Cu–Al–Ni–Ti shape memory alloys has been studied. It has been found that milling for 40 h at a BPR of 40:1, 200 rev min⁻¹ is the optimised condition for mechanical alloying in an attritor mill in order to get a uniform distribution of the alloying elements. A lower BPR caused insufficient milling, whereas a higher BPR led to oxidation of the powder. The complete formation of martensite occurred in the sample prepared under the optimised milling condition followed by cold compaction, sintering at 900°C for 1 h in vacuum and finally quenching. The austenite transformation temperature of the prepared shape memory alloy was found to be ∼208°C. Thus, the developed process enables to curtail the long milling time as well as decrease the prolonged homogenisation treatments.

Porous TiC–TiB₂–NiAl composites were successfully synthesised by self-propagating high temperature synthesis (SHS) with Ti–B₄C–Ni–Al mixture as reactant. The effects of Ni+Al contents and particle size of B₄C powders on pore structure and microstructure of composites were investigated. The results show that the porous composites are composed of open and closed pores with desired TiC, TiB₂ and NiAl phases. The closed pores are mainly from the decomposition of B₄C particles, while TiB₂ and TiC particles develop at the surface of B₄C particles, which can provide a shell surrounding the remnant B₄C particles and finally form a shell core structure. With the increase of Ni+Al contents, enough melted NiAl is formed during the SHS process. The NiAl liquid can cause the TiB₂–TiC shell to collapse, then lead to the distribution of TiB₂ and TiC particles in the NiAl matrix, and even fill in the space between the shells and cores, which result in the reducing or even disappearing of closed pores, and the increasing of open porosity.

Hot quasi-static pressing and dynamic compaction of Al7075–B₄C composite powder are studied in this work. Micron sized Al7075 and B₄C particles were milled to obtain uniform dispersion of B₄C reinforcing phase. Samples with relative densities up to 97.5, 99 and 100% were fabricated using gas gun, drop hammer and hot quasi-statically compactions respectively. The results showed that the increase of B₄C content improved the hardness of the compacts but did not improve the compressive strength in the samples compacted by the gas gun. However, the ductility of the samples was reduced significantly. The samples compacted using the drop hammer and under hot quasi-static loading experienced ∼25 and 11% enhancement in the compressive strength respectively. The investigation indicated that the dynamic compaction using the gas gun was more successful if it is performed after quasi-static pre-compaction and under warm compaction process. It was concluded that the quasi-static pressing is the best fabrication technique to achieve the highest density, microhardness and strength.

In the present study, porous titanium based composite has been developed by powder metallurgy route using cenosphere and sodium chloride (NaCl) as space holders. The effect of cenosphere particle size and sintering parameters on density, microstructure, wear and corrosion resistance has been studied. The microstructure of the composite consists of uniform distribution of cenosphere in α-Ti matrix with the presence of few TiO₂ and SiO₂ phases. The density of the composites varies from 1331 to 1812 kg m^− 3. The kinetics and mechanism of fretting wear against hardened steel ball (at an oscillation frequency of 10 Hz) as a function of applied load (5 and 10 N) have been studied. There is a significant reduction in coefficient of friction in porous composite as compared to that of pure titanium. There is improvement in corrosion resistance in porous titanium composites as compared to as received titanium in 3.56 wt-% NaCl solution.

Coarsening of the grains and pores during sintering has a pronouncedly negative effect on the densification of prealloyed brass powder compacts. This investigation examines the role of sintering variables in realising the complicated effects on densification and microstructure. Experiments were designed to model and evaluate the effect of sintering parameters such as temperature, time and furnace atmosphere on densification, grain and pore intercept as well as pore number. The study of microstructures suggests that there is a good correlation between grain and pore intercepts. It is concluded that pore coarsening is a result of supersolidus liquid phase sintering of Cu28Zn powder, and it can retard densification, which is in acceptable agreement with the experimental data.

The effects of annealing temperature and annealing time on microstructure evolution and mechanical properties of Ti–6Al–4V titanium alloy prepared using powder hot isostatic pressing were investigated. The results showed that when annealing temperature was < 1173 K, the globularisation of primary α was observed, and the morphology of secondary α changed from acicular shapes to acicular and continuous lathy grains. Scanning electron microscopy fracture surface showed that the fracture mechanism changed from transcrystalline dimple type to a mixture of transcrystalline and intercrystalline dimple types with increasing annealing temperature. The tensile strength and elongation varied with the temperature in an opposite trend. However, they varied similarly with increasing annealing time. Microhardness was sensitive to the annealing time, resulting into a maximum microhardness of 396.95 HV at 1173 K for 3 h.