Showing papers on "Powder metallurgy published in 2004"

Processing of titanium foams

Abstract: Because of their excellent mechanical properties, low density and biocompatibility, titanium foams are attractive for structural and biomedical applications. This paper reviews current techniques for titanium foam processing, which are all based on powder-metallurgy because of the extreme reactivity of liquid titanium. A first group of processes is based on powder sintering with or without place-holder or scaffolds. A second group relies on expansion of pressurized pores created during prior powder densification.

Hydrogen as a temporary alloying element in titanium alloys: thermohydrogen processing

Abstract: Thermohydrogen processing is a technique in which hydrogen is used as a temporary alloying element in titanium alloys to control the microstructure and improve the final mechanical properties. Thermohydrogen processing can also be used to enhance the processability/fabricability of titanium products including sintering, compaction, machining, and hot working (forging, rolling, superplastic forming, etc.). In the case of near net shapes, such as castings and powder metallurgy products, thermohydrogen processing is the only method available for significant microstructural modifications and consequent enhancement in mechanical properties. This paper reviews the status of the methods and applications of thermohydrogen processing to titanium alloys. Principles of thermohydrogen processing, based on the hydrogen induced alterations of the phase compositions and the kinetics of phase reactions in hydrogenated titanium alloys, are overviewed. Stable and metastable phase diagrams of several titanium alloys...

Direct electrolytic preparation of chromium powder

Abstract: Chromium oxide powder (Cr2O3) was slip cast or pressed into small cylindrical pellets which were then sintered in air. The sintered pellets were attached to a current collector to form an assembled cathode. Constant-voltage (2.7 to 2.8 V) electrolysis, with a graphite anode, was performed in molten CaCl2 (950 °C). After electrolysis, the pellets were removed from the molten salt and washed in water. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and fusion elemental analysis all confirmed that, when electrolyzed for a period longer than 4 hours, the Cr2O3 pellets were fully reduced to Cr metal. The oxygen content in the product depended on electrolysis time. Typically, for a 6-hour electrolysis, less than 0.2 wt pct oxygen was found in the product, with the current efficiency and energy consumption being 75 pct and 5 kWh/kg, respectively. The fully reduced pellet had a friable strength and could be manually crushed into a powder composed of cubic crystallites, very uniform in size, that grew with the electrolysis time, up to 50 μm (15 hours). The unique product morphology (cubic crystallites) differs drastically from the nodular morphology observed in other metals prepared by similar methods and is rarely seen among various commercial metal powders. A reduction mechanism is proposed, emphasizing the surface metallization at the early stage of electrolysis through the propagation of the metal-oxide-electrolyte three-phase interline (3PI).

Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering

Abstract: We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100 and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average graun size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissions of 40% and 60% were measured compared to MgO single crystal, for the yellow and red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric graun size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.

Powder Metallurgical Processing of Intermetallic Gamma Titanium Aluminides

Abstract: Intermetallic γ-TiAl-based alloys represent a new class of light-weight structural materials for use at high temperatures. Because of their unique properties these alloys are considered for applications in aerospace and automotive industries. During the last decade both, alloy development and materials processing progressed significantly. New materials and powder metallurgy (PM) have formed a symbiosis since many decades. In fact, PM is an ancient technology which has been used for the processing for almost every metal or ceramic material. Therefore, it is hardly surprising that PM plays an important role in research and development of γ-TiAl-based alloys.

Modeling of Laser Cladding with Powder Injection

Abstract: Laser cladding is one of the material additive manufacturing processes used to produce a metallurgically bonded deposition layer. To obtain a high-quality resulting part, a deep understanding of the underlying mechanisms is required. In this article, a mathematical model is developed to simulate the coaxial laser-cladding process with powder injection, which includes laser- substrate, laser-powder, and powder-substrate interactions. The model considers most of the associated phenomena, such as melting, solidification, evaporation, evolution of the free surface, and powder injection. The fluid flow in the melt pool, which is mainly driven by Marangoni shear stress as well as particle impinging, together with the energy balances at the liquid-vapor and the solid-liquid interfaces, are investigated. Powder heating and laser power attenuation due to the powder cloud are incorporated into the model in the calculation of the temperature distribution. The influences of the powder injection on the melt pool shape, penetration, and flow pattern are predicted through the comparison for the cases with powder injection and without powder injection. Dynamic behavior of the melt pool and the formation of the clad are simulated. The effects of the process parameters on the melt pool dimension and peak temperature are further investigated based on the validated model.

Consolidation and properties of binderless sub-micron tungsten carbide by field-activated sintering

Abstract: The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m1/2 and 2480 kg/mm2, respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.

Pure Al matrix composites produced by vacuum hot pressing: tensile properties and strengthening mechanisms

Abstract: Al matrix composites reinforced by spherical intermetallic particles, were consolidated from gas atomized elemental Al and Al–Cu–Fe alloy fine powders (

The Role of Particle Size on the Laser Sintering of Iron Powder

Abstract: The effects of powder particle size on the densification and microstructure of iron powder in the direct laser sintering process were investigated. Iron powders with particle sizes ranging from 10 to 200 µm were used. It was found that the sintered density increases as the laser energy input is increased. There is, however, a saturation level at which higher density cannot be obtained even at very intensive energy input. This saturation density increases as the size of the iron particles decreases. Meanwhile fine powders with narrow particle size distributions have a tendency toward agglomeration, and coarse powders with broad particle size distributions have a tendency toward segregation, both of them resulting in lower attainable density. In order to investigate the role of particle size, a “densification coefficient (K)” was defined and used. This coefficient depends on the particle size and the oxygen content of iron powder. The results of this investigation demonstrate that the presence of oxygen significantly influences the densification and pore morphology of laser-sintered iron. At higher oxygen concentrations, the iron melt pool is solidified to agglomerates, and formation of pores with orientation toward the building direction is more likely to occur. When the oxygen concentration is kept constant, the densification coefficient decreases with decreasing the particle size, meaning the densification kinetics enhances. This article presents the role of powder characteristics and the processing parameters in the laser sintering of iron powder as a model material. The mechanism of particle bonding and microstructural features of laser-sintered parts are addressed.

Direct laser sintering of iron–graphite powder mixture

Abstract: In the present work, the role of graphite addition on the laser sintering of iron powder was studied. Powder mixtures containing iron and 0.4, 0.8, 1.2, and 1.6 wt.% graphite were prepared by blending elemental powders. These powders were sintered layer-by-layer under nitrogen atmosphere using a continuous wave CO 2 laser beam. A laser power of 70–225 W, scan rate of 50–600 mm s −1 , scan line spacing of 0.1–0.3 mm, and layer thickness of 0.1 mm was used. It was found that the processing parameters play a key role on the densification of the iron–graphite powder mixtures. The addition of graphite enhances the densification of the iron powder and improves the surface quality of the laser sintered parts when optimized manufacturing conditions are applied. The graphite content has a significant influence on the internal pore structure of the sintered parts. They are gradually changed from interconnected networks to closed and spherical shaped pores with increasing graphite content. The metal matrix structure consists of different phases such as ferrite, austenite, and tempered martensite, which highlights the heterogeneous distribution of dissolved carbon in the iron matrix. This article presents the experimental details of the microstructural evolution in laser sintered iron–graphite powder mixtures. The role and key importance of graphite addition to iron powder in the laser sintering process is addressed.

Sintering of nanophase WC–15vol.%Co hard metals by rapid sintering process

Abstract: A sintering method for the rapid sintering of a nanostructured hard metals in short time is introduced with a focus on the manufacturing potential of this novel high-frequency induction heated sintering (HFIHS) process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. In this work, we developed a new process of sintering for nanostructured WC–15vol.%Co hard metals. A highly dense WC–15vol.%Co with a relative density of up to 99.4% was produced with simultaneous application of 60 MPa pressure and induced current within 1 min. The average grain size was about 258 nm and mean free path was about 11.6 nm. The fracture toughness and hardness values obtained were 11.9 MPa m1/2 and 1992 HV30, respectively. Also, we produced dense WC–Co hard metals with a relative density of 97% without applying pressure.

Microstructure and martensitic transformation behavior of porous NiTi shape memory alloy prepared by hot isostatic pressing processing

Abstract: Porous NiTi shape memory alloy (SMA) has been successfully prepared by hot isostatic pressing (HIP) of elemental Ni and Ti powder. The alloy has isotropic and uniform pore distribution of spherical pores ranged from 50 to 200 μm. The microstructure, martensitic transformation behavior and mechanical properties of this porous NiTi SMA aged at different conditions were studied. It has been found that the phase transformation behavior of porous NiTi SMA is similar to that of the dense Ni-rich NiTi alloys. The porous NiTi SMAs can exhibit almost complete superelasticity at human body temperature.

Sintering and creep processes in plasma-sprayed thermal barrier coatings

Abstract: During operation at elevated temperatures, sintering processes can significantly influence the mechanical properties of thermal barrier coatings (TBCs) by increasing Young’s modulus and reducing strain tolerance. These changes of the mechanical response of TBCs were investigated using free-standing plasma-sprayed TBCs in a thermomechanical analysis (TMA) facility. The time-dependent change of Young’s modulus was determined in situ in a flexure mode at different annealing temperatures. In addition, relaxation processes during loading and unloading were monitored. The time-dependent deformation behavior of the TBC sample can be described by a simple viscoelastic approach (Burgers model). Viscosity data are determined as a function of annealing temperature and time.

High strain rate response of aluminum 6092/B4C composites

Abstract: Aluminum 6092/B 4 C p (boron carbide) metal-matrix composites (MMC) fabricated by two different powder consolidation routes, extrusion and sintering/hot isostatic-pressing (HIPing), were made and tested over a wide range of strain rates (10 −4 to 10 4 s −1 ). The strength of these MMCs increases with increasing volume fraction of particulate reinforcement. Strain hardening is observed to increase with increasing volume fraction of reinforcement at lower strains ( 5 μm) considered. Finally, the Li–Ramesh model captures the observed high-rate behavior exhibited by these powder-consolidated composites.

Titanium group powder metallurgy

Abstract: Methods and compositions relating to powder metallurgy in which an amorphous-titanium-based metal glass alloy is compressed above its glass transition temperature Tg with a titanium alloy powder which is a solid at the compression temperature, to produce a compact with a relative density of at least 98%.

Carbon nanotube/magnesium composites

Abstract: Novel magnesium matrix composites reinforced with carbon nanotubes have been processed by powder metallurgy. Blends of metal powders and multi-wall carbon nanotubes were compacted by uniaxial hot pressing followed by hot isostatic pressing. A uniform dispersion of nanotubes in the metal matrix was obtained. A coating method of nanotubes is described, which is promising to improve the matrix-reinforcement bonding strength. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Clot-forming: the use of proteins as binders for producing ceramic foams

Abstract: This project uses the change of functional properties of proteins for ceramic and powder metallurgical shaping. Albumin (Bovine Serum Albumin, BSA) as a major constituent of blood was added as a model binder to an aqueous powder suspension, which then showed a sufficiently low viscosity for mould filling. The flow behaviour showed a structural viscosity being best described by a Herschel–Bulkley model. Temperatures higher than 66 °C lead to a significant increase of viscosity caused by irreversible changes in the spatial-structure of the protein molecule. Albumin, however, had a second effect in this process. Since albumin has amphiphilic properties its solutions are prone to foaming. A fine cellular foam structure of approximately 50–300 μm cell diameter was formed, probably due to a stable arrangement at the liquid gas interface. The combination of foaming and increase of stiffness lead to a stable protein-ceramic foam structure. After burn out and sintering final densities in the range from 8 to 20% t.d. were achieved. Fine cellular structures have more isolated pores while larger cells are typically interconnected. Typical applications would be high temperature insulation, catalyst carriers or scaffolds for cell technology.

Flow behaviour of powders during die filling

Abstract: In this paper, the powder behaviour during die filling is discussed. Initially, methods used to test powder flow properties are reviewed and a novel method based on the measurement of critical shoe velocity is emphasised. It is shown that this method not only can be used to characterise the flow of powders, but it can also be used to assist process design in powder metallurgy and similar operations. The powder behaviour during die filling is then discussed in two successive stages: the flow of powder from a shoe and the packing of powder inside a die. Previous studies are reviewed and typical behaviour of powder during die filling is summarised. Finally, segregation during die filling is also discussed and further studies on die filling are suggested.

The electroless deposition of nickel on SiC particles for aluminum matrix composites

Abstract: Nickel coatings have been deposited by an electroless method on SiC powder particles. Three different SiC powder grades, in terms of average particle size, were chosen, i.e. 8, 14 and 70 μm. The coating process was performed in few steps consisting of the SiC powder cleaning by acetone, its sensitization by HCl aqueous solution containing Sn2+, followed by its activation by HCl aqueous solution containing Pd2+, and finally—hydrometallurgical nickel deposition using aqueous solution containing Ni2+, as a nickel carrier. The influence of the deposition conditions on the coating layer properties was studied. The thickness, morphology and microstructure of the layers were controlled by the growth conditions. SEM and digital image analyzing techniques were used for those purposes. Since the Ni-coated SiC powders were produced as reinforcement for Al matrix composites, their compatibility as compared with the uncoated SiC powders was also controlled by metallography.