Showing papers on "Powder metallurgy published in 2002"

Finite element analysis of single layer forming on metallic powder bed in rapid prototyping by selective laser processing

Abstract: A method for calculating the distribution of temperature and stress within a single metallic layer formed on the powder bed in rapid prototyping with the selective laser melting method is proposed. The solidified layer is assumed to be subjected to plane-stress deformation and the two-dimensional finite element methods for heat conduction and elastic deformation are combined. In the simulation, the finite element mesh is constructed on the surface of the powder bed. The heat caused by laser irradiation is given to the elements under the laser beam. Shrinkage due to solidification is assumed to result in only the change of the layer thickness. In the elastic finite element simulation, the Young's modulus of the solidified part is expressed as a function of temperature. To simplify the calculation, the whole area is treated to be continuous, and the powder bed and the molten part are assumed to have a very small Young's modulus. The heat conduction and the elastic finite element calculations are carried out alternately. The obtained results of deformation and tensile stress distribution show the possibility and places of cracking of the layer during forming.

Direct Selective Laser Sintering of Hard Metal Powders: Experimental Study and Simulation

Abstract: Direct selective laser sintering (SLS) technology can be used to produce 3D hard metal functional parts from commercial available powders. Unlike conventional sintering, it does not require dedicated tools, such as dies. Hence, total production time and cost can be reduced. The large shape freedom offered by such a process makes the use of, for example, sintered carbides components viable in domains where they were not applied before. Successful results have been obtained in the production of sintered carbide or hard metal parts through SLS. The investigation focuses on tungsten carbide-cobalt (WC-Co) powder mixture. This material is characterised by its high mechanical properties and its high wear resistance and is widely used in the field of cutting tools. This paper is devoted to the experimental study and the simulation of direct selective laser sintering of WC-Co hard metal powders.

A Review on the various synthesis routes of TiC reinforced ferrous based composites

Abstract: The major thrust underlying the processing of Fe-based composites has been directed towards improving the wear resistance of steel or castiron by incorporating some reinforcing phase, e.g., carbides, oxides, etc. The present article provides a review on the various synthesis routes of TiC reinforced Fe-based composites, i.e., powder metallurgy, conventional melting and casting, carbothermic reduction, combustion synthesis, aluminothermic reduction, electron beam radiation, laser surface melting, and plasma spray synthesis, highlighting the advantages and disadvantages associated with the different routes of synthesis.

Powder metallurgical fabrication processes for NiTi shape memory alloy parts

Abstract: NiTi components with reproducible and stable shape memory properties are attractive for various technical applications (e.g. couplings). With the aim of producing NiTi components on an industrial scale, near-net-shape fabrication routes are preferred considering the limited machinability of NiTi alloys. Powder metallurgy (PM) is known to provide the possibility of material-saving and automated fabrication of at least semi-finished products as well as net-shape components. As promising PM routes hot isostatic pressing (HIP) and metal injection moulding (MIM) were used for the fabrication of NiTi compacts. Microstructural investigations, chemical analysis, X-ray diffraction (XRD) measurements and also differential scanning calometry (DSC) measurements were performed in order to characterize the produced parts. Additionally, the mechanical properties of HIPed samples were measured by tensile tests at room temperature. The components from both fabrication routes show reversible austenite↔martensite transformations which are a prerequisite for shape memory effects.

Structure–property correlation in discontinuously reinforced aluminium matrix composites as a function of relative particle size ratio

Abstract: Metal matrix composites made by a powder metallurgy route often exhibit clustering of the reinforcements due to geometrical reasons. The clustering tendency was studied in 2124 Al/30 v/o SiCp composites as a function of relative particle size ratio between the matrix and reinforcement particles. Dry blended composite powders, with different RPS ratios, were vacuum hot-pressed and microstructures were examined to assess the uniformity of the microstructure by measuring the local area fraction of the constituents. It was found that a decrease in RPS ratio resulted in an increase in strength as well as ductility, as a result of improved distribution of the SiCp. The improved response to the homogenisation treatment, observed in the composite with lower RPS ratio, is attributed to the smaller diffusion path length for the alloying element.

High density net shape components by direct laser re-melting of single-phase powders

Abstract: Direct Metal Laser Re-Melting is a variant of the Selective Laser Sintering process, a Rapid Prototyping (RP) technology. This tool-less manufacturing technology has the potential of producing complex, high quality components from single-phase metal powders in short time scales. This is made possible by the production of consecutive two-dimensional layers. Unfortunately, finished components manufactured by this technique have their integrity and material properties dictated by the porosity within the laser re-melted structure. In order to maintain structural integrity comparable to conventionally produced components, metal components produced by the rapid prototyping method should exhibit a porosity of the order of maximum of ∼2% with corresponding bulk material properties. To achieve these objectives, process and laser parameters require optimisation for maximum densities to be attained. This paper reports on the development of a scanning strategy that produces stainless steel (316L) laser re-melted components which exhibit porosities of <1%, while maintaining the concept of rapid prototyping.

The influence of the atmosphere on the sintering of aluminum

Abstract: Alloys of Al, Al-0.15Mg, and Al-12Sn made using air atomized aluminum powder and pressed to green densities of 75 to 98 pct were sintered under argon or nitrogen. Sintering in argon is only effective at high green densities when magnesium is present. In contrast, highly porous aluminum can be sintered in nitrogen without the need for magnesium. The oxygen concentration in the gas is reduced by the aluminum through a self-gettering process. The outer layers of the porous powder compact serve as a getter for the inner layers such that the oxygen partial pressure is reduced deep within the pore network. Aluminum nitride then forms, either by direct reaction with the metal or by reduction of the oxide layer, and sintering follows.

Microstructures and Mechanical Properties of Porous Titanium Compacts Prepared by Powder Sintering.

Abstract: Using pure Ti powder with particle sizes from 300 to 500 μm prepared by the plasma rotating electrode process (PREP), porous pure Ti compacts for biomedical applications were synthesized by powder sintering, and microstructures and mechanical properties of the compacts were investigated in this study. Porous compacts having porosity of 19-35 vol% are successfully fabricated by controlling sintering condition. It is found that Young's modulus and compressive yield strength decrease linearly with increasing porosity, and porous Ti compacts having porosity of about 30-35 vol% exhibit identical Young's modulus of human bone.

Preparation of Ti-TiB-TiC & Ti-TiB composites by in-situ reaction hot pressing

Abstract: Discontinuously reinforced Titanium matrix composites, with TiB and TiC reinforcements, have been produced by the reaction hot pressing process. Powder metallurgy approach has been adopted with two different starting powder mixtures, namely Ti+TiB2 and Ti+B4C, the former leading to a composite with only TiB reinforcement, while the latter resulted in a combination of TiB+TiC reinforcement. The initial composition was adjusted in such a way as to have a reinforcement content of 70% by volume, the rest being matrix titanium. The X-ray diffraction analysis confirmed the completion of reaction and the scanning electron microscope studies revealed the morphology of the reinforcement. The effect of morphology and reinforcement type on mechanical properties was evaluated with hardness measurement and 3-point bend tests. It is concluded that the composite with the TiB+TiC reinforcement has a better strength and toughness, due to the presence of needle shaped TiB, while the composite with only TiB reinforcement in the form of TiB particulate has inferior strength and fracture resistance.

Hot isostatic pressing of ultrafine tungsten carbide-cobalt hardmetals

Abstract: Hot Isostatic Pressing (HIP) has been successfully used to consolidate tungsten carbide-10 weight% cobalt (WC-10 wt% Co) powders mixtures with WC powder particle sizes in the range of 100 nanometers. Fully dense specimens of this composition have been obtained by HIP at 1000°C, a temperature well below those usually required for reaching the closed porosity stage in the WC-Co system. Conventional processing by vacuum sintering has also been carried out to study the individual effects of high isostatic pressure and vanadium carbide additions on densification and WC grain growth control of these hardmetals. The finest WC mean grain size after sintering has been obtained for the combined action of applied isostatic pressure and vanadium carbide (VC) additions. These results show that VC additions are effective in controlling WC grain growth even at temperatures as low as 1000°C.

Processing and Characterization of NiTi Porous SMA by Elevated Pressure Sintering

Abstract: Currently, three methods are commonly used for producing porous NiTi shape memory alloys (SMAs) from elemental powders. These include conventional sintering, Self-propagating High temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press (HIP). Conventional sintering requires long heating times and samples are limited in shape and pore size. SHS is initiated by a thermal explosion ignited at one end of the specimen, which then propagates through the specimen in a self-sustaining manner. One of the difficulties with SHS is the inability to control intermetallic phases. This work will focus on the fabrication and characterization of porous NiTi SMA material produced from elemental powders via HlPping. Porous NiTi SMA was produced from elemental Ni and Ti powders at elevated temperature and pressure using a HIP. Small and large pore specimens containing average pore sizes ranging from 20 μm up to 1mm have been produced by slightly varying the HIPping sintering temperatures and times. Quasi-static and dynamic loading experiments are conducted on various samples produced using the presented methodology and their shape recovery and energy absorption characteristics are measured during the forward and reverse phase transformation and detwinning. Their phase transformation characteristics were found using calorimetric measurements and their composition has been studied using optical and electron microscopy and microprobe X-ray analysis.

Blended powder solid-supersolidus liquid phase sintering

Abstract: A green article comprising an A-B powder mixture and methods of manufacturing such green articles and corresponding sintered articles are disclosed. The A-B powder mixture consists of a minor volume fraction of a relatively fine powder A and a complementary major volume fraction of a relatively coarse prealloyed powder B wherein the A-B powder mean particle size ratio is at least about 1:5. Metal powder A consists of one or more elemental metals or alloys which has a melting or solidus temperature above the highest sintering temperature at which the A-B powder mixture may be sintered without slumping. Prealloyed metal powder B consists of one or more alloys which are amenable to supersolidus liquid phase sintering. Green articles made from the A-B powder have a wider sintering temperature window than do articles made from prealloyed metal powder B alone.

Synthesis and processing of nanostructured WC-Co materials

Abstract: In this study a novel approach, termed the integrated mechanical and thermal activation (IMTA) process, was used to synthesize nanostructured WC-Co powder. As a result of the integration of mechanical and thermal activation, nanostructured WC-Co powder was synthesized below 1000°C, starting from WO3, CoO and graphite powder mixtures. Furthermore, consolidation of the nanostructured WC-Co powder via high velocity oxy-fuel (HVOF) thermal spraying and solid state sintering was investigated. The results demonstrated the feasibility of converting the nanostructured WC-Co powder to coatings and bulk components, the properties of which are either comparable to or better than that of the conventional coarse-grained counterparts.

SYNTHESIS OF ALLOY Ti − 6Al − 4V WITH LOW RESIDUAL POROSITY BY A POWDER METALLURGY METHOD

Abstract: The possibility of producing titanium alloy Ti ― 6Al ― 4V with minimal residual porosity from mixtures of elemental powders by the method of pressing and sintering without hot deformation during or after sintering was investigated. Various powder mixtures based on titanium and titanium hydride with alloying additions of either elemental powders having different particle sizes, or master alloys, were studied. It was shown that the synthesis of Ti ― 6Al ― 4V from mixtures of titanium hydride and master alloys is optimal with respect to the attainment of high relative density. In this case the sintered material has density up to 99%, homogeneous microstructure with relatively small (100-120 μm) β-phase grains, and a low concentration of impurities, in particular oxygen, which provide a high level of mechanical properties σ(ten = 970 MPa, δ = 6%).

High performance P-type thermoelectric materials and methods of preparation

Abstract: The present invention is embodied in high performance p-type thermoelectric materials having enhanced thermoelectric properties and the methods of preparing such materials. In one aspect of the invention, p-type semiconductors of formula Zn 4−x A x Sb 3−y B y wherein 0≦x≦4, A is a transition metal, B is a pnicogen, and 0≦y≦3 are formed for use in manufacturing thermoelectric devices with substantially enhanced operating characteristics and improved efficiency. Two methods of preparing p-type Zn 4 Sb 3 and related alloys of the present invention include a crystal growth method and a powder metallurgy method.

Effect of nano-micro TiN addition on the microstructure and mechanical properties of TiC based cermets

Abstract: In this paper the titanium carbide based cermets with addition of nano-micro titanium nitride were fabricated by conventional powder metallurgy techniques. The microstructure of TiC based cermets with nano-micro TiN addition has been investigated by transmission electron microscope (TEM) and a scanning electron microscope (SEM). Bending strength, fracture toughness and hardness were also measured. Results reveal that mechanical properties do not change monotonously with increasing nano TiN addition. Results also show that nano titanium nitride addition can prevent the coalescence of TiC grains. It is also found that nano TiN is distributed at the interface of TiC/TiC grains and that the growth of TiC grains is impeded by the presence of nano TiN particles and that the result is an increase in mechanical properties.

Processing and mechanical properties of Ti–6Al–4V/TiC in situ composite fabricated by gas–solid reaction

Abstract: Titanium carbide (TiC) reinforced Ti–6Al–4V powder metal (P/M) composites were developed using a gas–solid in situ reaction process. The elemental blended powders were cold compacted in a die and reacted with methane (CH 4 ) to form titanium carbide. The reacted compacts were then sintered in vacuum and HIPed without canning to close residual pores. Titanium carbide, aluminum carbide and vanadium carbide were formed during the reaction but only titanium carbide remained after the sintering. Although sintering densified the composites to fairly high densities (over than 95% of the theoretical), full densification could not be obtained by sintering alone. The fully densified composites after HIPing showed higher hardness, higher elastic modulus and tensile strength, and better wear resistance than wrought Ti–6Al–4V alloys, but were much less ductile at room temperature.