Showing papers on "Powder metallurgy published in 2000"

Fundamental aspects of hot isostatic pressing: An overview

Abstract: Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Sintering activation by external electrical field

Abstract: Field assisted sintering technique (FAST) is a non-conventional powder consolidation method in which densification is enhanced by the application of an electrical discharge combined with resistance heating and pressure. Interest in FAST is motivated by its ability to consolidate a large variety of powder materials to high densities in short times. Full densification of metal and ceramic powders has been achieved within minutes, with a reduced number of processing steps, no need for sintering aids and more flexibility in powder handling. Although the electrical discharge effects have not been completely elucidated, distinct surface effects created by micro-discharges have been noticed in FAST consolidated specimens such as atomically clean grain boundaries and new resistivity peaks in superconductors. On-going experimental and theoretical studies to provide more quantitative insight into the relevant FAST mechanisms are presented.

Nickel, cobalt, and their alloys

Abstract: This book is a comprehensive guide to the compositions, properties, processing, performance and applications of nickel, cobalt, and their alloys. It includes all of the essential information contained in the 20-volume ASM Handbook series. Includes new or updated coverage in the following areas: Expanded corrosion coverage including guidelines for selecting the best alloy for specific environments or applications Data sheets covering the compositions, specifications, applications and properties for dozens of the most commercially important heat, corrosion and wear-resistant nickel and cobalt alloys Recent advances in superalloy development, including coatings to extend high-temperature service life Unique characteristics of nickel and cobalt which allow them to be used in special-purpose applications, e.g., magnets, controlled-expansion devices, electronics, and implants for the human body Engineering applications for nickel and cobalt coatings produced by electroplating, electroforming, electroless coating, thermal spraying, and weld surfacing. Contents include: Nickel and Its Alloys: The Nickel Industry, occurrence, recovery and consumption Uses of nickel Wrought and Cast Corrosion-Resistant Alloys Cast Heat-Resistant Ni-Cr and Ni-Cr-Fe alloys Superalloys Special-Purpose Alloys Nickel Coatings Corrosion Behavior, including performance in specific environments, Stress-Corrosion Cracking and Hydrogen Embrittlement, and High-Temperature Corrosion Fabrication and Finishing, including forming, forging, powder metallurgy, heat treating, machining, welding and brazing, cleaning and finishing, and high-temperature coatings for superalloys Metallography, Microstructures, and Phase Diagrams Cobalt and Its Alloys: The Cobalt Industry, occurrence, recovery and consumption Uses of cobalt Cobalt-base alloys Wear behavior Corrosion Behavior Fabrication Characteristics Metallography, Microstructures and Phase Diagrams.

Novel Metallic Hollow Sphere Structures

Abstract: The obvious advantages of metallic hollow sphere materials have lead to several attempts to produce such structures. None of these approaches has gained practical significance owing to the very high cost of the methods. A new powder metallurgical process using styrofoam spheres allows for the production of hollow spheres from arbitrary metals and alloys. The IFAM Department of Powder Metallurgy and Composite Materials, Dresden, has developed this process and added a method that produces random hollow sphere structures (RHS) directly from the green spheres.

Densification of Al2O3 Powder Using Spark Plasma Sintering

Abstract: Al2O3 powders with four different particle sizes were densified using a spark plasma sintering (SPS) apparatus under three different sintering conditions: holding time, heating rate, and mechanical pressure. The Al2O3 powder compact sintered at a higher heating rate produced a sample with a higher density and a fine-grained microstructure, while abnormal grain growth and a lower density resulted when a lower heating rate was applied, though the sintering temperature and holding time were the same in both cases. This revealed that rapid sintering by SPS was effective for promoting the densification of the powder. However, the powder with a coarse particle size was hard to sinter at a higher heating rate. Microstructural observation revealed that the edge part was denser than the inside of the sample when the holding time was short. Increasing the holding time made it possible for the inside to be sintered almost as dense as the edge part. Mechanical pressure was found to enhance densification of the Al2O3 powder. On the basis of these results, the SPS process is discussed.

Laser fabrication of discontinuously reinforced metal matrix composites

Abstract: Disclosed are reinforced metal matrix composites and methods of shaping powder materials to form such composites. Articles of manufacture are formed in layers by a laser fabrication process. In the process, powder is melted and cooled to form successive layers of a discontinuously reinforced metal matrix. The matrix exhibits fine grain structure with enhanced properties over the unreinforced metal, including higher tensile modulus, higher strength, and greater hardness. In some preferred embodiments, an in-situ alloy powder, a powder metallurgy blend, or independently provided powders are reinforced with boron and/or carbon to form the composite.

Microstructure and tensile properties of mechanically alloyed Ti–6A1–4V with boron additions

Abstract: Particulate titanium metal matrix composites (MMCs) offer advantages for a wide range of structural applications with increases in stiffness and possibly wear resistance. A powder metallurgy/mechanical alloy route was used successfully to give a fine, uniform distribution of boron in titanium alloy powder. This powder was hot isostatically pressed and during the process TiB reinforcement was formed by an in-situ reaction. Significant tensile ductility in MA titanium alloy (without boron), equivalent to wrought material was achieved, along with good tensile strength. In materials with high TiB volume fractions it is thought that the fine boride size and high strength matrix caused embrittlement. This suggests that the use of a lower strength matrix, such as commercial purity titanium, and shorter milling times would provide a better balance of strength, stiffness and ductility for this type of composite.

PM synthesis and properties of steel foams

Abstract: Steel foam steel was synthesized by a powder metallurgical route, resulting in densities less than half that of steel. Process parameters for foam synthesis were investigated, and two standard powder formulations were selected consisting of Fe–2.5%C and 0.2 wt.% foaming agent (either MgCO3 or SrCO3). Using the PM approach, foams with relative density ranging between 0.38 and 0.64 were obtained. Compression tests were performed on annealed and pre-annealed foam samples of different density to determine mechanical response and energy absorption behavior. The stress–strain response was strongly affected by annealing, which reduced the carbon content and converted much of the pearlitic structure to ferrite. These microstructural changes led to a more ductile response during compressive loading, in which a long stress plateau typically occurred after initial yielding. In fact, annealed steel foams behaved much like aluminum foams under compressive loading, despite pore structures that were considerably more coarse than those reported for aluminum foams.

Low oxygen refractory metal powder for powder metallurgy

Abstract: Process for producing formed Ta/Nb powder metallurgy products using Ta and/or Nb hydride powders with an oxygen content greater than a target level, e.g., 300 ppm, heating the metal hydride in the presence of another metal having a higher affinity for oxygen, removing the other metal and any reaction byproducts, to form a metal powder with an oxygen content less than the target level and forming a metallurgical product from said oxygen reduced Ta/Nb powder with an oxygen content less than the target level.

Titanium Particulate Metal Matrix Composites – Reinforcement, Production Methods, and Mechanical Properties

Abstract: Titanium metal matrix composites (MMCs) offer potential advantages for structural applications, where they combine the high strength, high temperature capability, and oxidation resistance of titanium with an increase in stiffness provided by the ceramic reinforcement. They have the advantage of being isotropic in behaviour, cheaper to manufacture and more amenable to subsequent processing and component forming operations. Of potential reinforcing phases for titanium, mcluding TiB, TiB 2 , SiC, Al 2 O 3 , and TiC, TiB offers the best balance of stiffness, stability, and similarity of thermal expansion coefficients. The methods used to proauce these Ti-TiB MMCs, such as arc melting, gas atomization, mpid solidification, and powder blending have been assessed and the benefits these composites offer over conventional titanium alloys including increased stiffness, good creep performance, fatigue resistance, and wear resistance are highlighted.

Selective laser sintering of gas atomized M2 high speed steel powder

Abstract: Selective laser sintering of the gas atomized M2 high speed steel powder was performed using laser powers of 2.5–100 W, scan rates of 1–30 mm/s and scan line spacings of 0.15–0.75 mm. With increasing laser power, the sintered surface varied from open/closed pores to a fully dense structure. Large lateral pores were found in the sintered surface of samples using high scan rates. For fully dense samples, smooth surfaces could be achieved using large scan line spacing. The as-supplied and sieved M2 powder particles with size ranging from 0.04 to 400 μm and 53 to 150 μm, respectively, were found to give better laser sinterability as compared with those powder particles with finer ( 150 μm) sizes.

Sputtering target, method of making same, and high-melting metal powder material

Abstract: There is provided a method of making a high-melting metal powder which has high purity and excellent formability and, particularly, of a metal powder of spherical particles made of Ta, Ru, etc. having a higher melting point than iron. There is also provided a target of high-melting metal or its alloy, which is made by the sintering under pressure of these powders and which has high purity and a low oxygen concentration and shows high density and a fine and uniform structure. A powder metal material mainly composed of a high-melting metal material is introduced into a thermal plasma into which hydrogen gas has been introduced, thereby to accomplish refining and spheroidizing. Further, an obtained powder is pressed under pressure by hot isostatic pressing, etc.

Application of superplasticity in commercial magnesium alloy for fabrication of structural components

Abstract: An investigation of the superplastic characteristics of magnesium alloys with several grain sizes revealed that grain boundary sliding took place more easily with grain refinement. The required grain size for high strain rate superplastic forming was estimated to be ∼2 μm. The required grain structure could be obtained by several procedures, hot extrusion with a high extrusion ratio, severe plastic deformation via equal channel angular extrusion, consolidation of machined chip, and/or powder metallurgy processing of rapidly solidified powders, on a laboratory scale. The processing route of hot extrusion was selected in this study. An experimental study of superplastic press forming was conducted for a commercially extruded ZK60 alloy. The fabricated product did not essentially contain macroscopic defects, i.e. cracks or cavities. From an examination of tensile characteristics, it was found that the post-formed alloy exhibited higher strength and higher ductility compared with some conventional cast magnesium alloys, aluminium alloys, and steels. The experimental results support the possibility of using superplastically formed magnesium to produce structural components.

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Abstract: Titanium diboride TiB2 and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB2 was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB2 compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB2 peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB2. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB2 after annealing at 800 °C.

Phase formation in mechanically alloyed Nb-Al powders

Abstract: Nb3Al intermetallic compound is of interest as material for use at elevated temperatures due to its high melting point, high elastic modulus and excellent oxidation resistance. Furthermore, the A15 phase superconductor Nb3Al has been considered as an alternative to Nb3Sn for high-magnetic field applications [1]. The Nb-Al phase diagram shows that the bcc Nb(Al) solid solution phase (A2) is extensive and the other two intermetallic phases Nb2Al and NbAl3 are stable up to high temperatures above 1000 ◦C [2]. Mechanical alloying has proved to be a useful technique for obtaining Nb-Al alloys [3–6]. The Nb3Al could, however, be synthesized only after heat treating the mechanically alloyed powder [4, 6]. In this work we have studied the mechanical alloying route to synthesize Nb3Al intermetallic compound starting from blended elemental powders. Of particular interest is to investigate the effects of a process control agent on the structural change during mechanical alloying. Pure elemental aluminum (purity 99.8%, −325 mesh) and niobium (purity 99.9%, −325 mesh) were mixed to yield an average starting composition of Nb-25at.%Al(Nb3Al). Mechanical alloying was carried out in a Spex 8000 mixer/mill with stainless steel balls of 6.30 mm in diameter. The ball-to-powder weight ratio was 8 : 1. Hexane (C6H14) was used as a process control agent (PCA). Addition levels of 0, 1 and 3 wt% were employed. The charging and milling were performed in an argon-filled glove box. A fan was used during the milling to cool the vial. The structural evolution in the mechanically alloyed powder was examined by X-ray diffraction using Cu Kα radiation. The milled powders were heat treated at different temperatures, after sealing them in Cu pan, under argon atmosphere to investigate the thermal stability and decomposition of the phases produced in the as-milled state. Fig. 1 shows the X-ray diffraction patterns of the Nb-25at.% Al powder mixture milled with no PCA for different times. The X-ray patterns relevant to pure elements are also shown, indicating that the Al (111) and Nb (110) reflections are overlapping completely. After 1 h of milling, the peaks of aluminum were barely detectable and only the Nb-based solid solution lines were present. With increasing milling time, the peaks became considerably broader and their intensities decreased. The powder became amorphous after milling for 16 h. In the case of the milling using hexane as the PCA, the progress of mechanical alloying was similar to that of the milling with no PCA, as shown in Fig. 2 for the milling with 3 wt% hexane, but there were several essential differences. First, the time to amorphization decreased with the addition of hexane. Second, as calculated from the integral breadths of the diffraction lines [7], in the beginning of the milling, the decrease in the crystallite size and the increase in internal strain appeared to be more drastic in the milling with hexane than in the milling with no PCA (data not shown). Fig. 3 shows the effect of PCA level on powder hardness as a function of milling time, indicating that hardness increased with milling time and as the PCA level

Preparation of high porosity metal foams

Abstract: Metal foams with porosities greater than 90 pct were prepared by a novel powder metallurgy route using a polymeric vehicle. Coarse titanium powder and fine carbonyl iron powder were tested. The powders were blended with each component of a two-part polyol-isocyanate foaming system, and the resulting suspensions were mixed and allowed to expand. Although the resulting polymer-metal foam was closed cell, particles were not retained in the windows. Upon pyrolysis to remove the resin, the windows opened and the final sintered metal foam was reticulated. Such foams present very low sintered density and are correspondingly weak after sintering but offer a fine reticulated structure with cell diameters in the region of 100 to 200 µm. They may have applications in the areas of catalysis, biomaterials, and composites.

Free-form laser consolidation for producing metallurgically sound and functional components

Abstract: Functional metallic components can be built layer by layer from a computer-aided design model, by using an optical fiber coupled Nd:YAG laser beam along with the simultaneous delivery of desired metal/alloy powders through a nozzle into the molten pool. Building of shapes using various alloys, including 316 L stainless steel, Ni-base IN-625 superalloy, and M4 tool steel, have been investigated. The components built using the free-form laser consolidation are metallurgically sound, free of cracks and porosity. Surface finish on the order of 1–2 μm (Ra) can be obtained on the consolidated samples. The microstructure of the laser-consolidated samples is similar to the rapidly solidified materials. Under certain conditions, directionally solidified microstructure can also be obtained. X-ray diffraction analysis reveals that laser-consolidated material maintains the same phase structure as the original powder. The tensile properties of the laser-consolidated IN-625 alloy and 316 L stainless steel are comparabl...

Materials and Process Design through Mechanochemical Routes

Abstract: Mechanochemical Processing (MCP) is the term applied to the powder metallurgy process in which chemical reactions and phase transformations take place due to the application of mechanical energy. Mechanical alloying (MA) is a powder-processing technique involving deformation, cold welding, fracturing, and rewelding of powder particles in a ball mill. The technique of MCP has had a long history and the materials produced this way have already found a number of potential technological applications, e.g., in areas such as hydrogen storage materials, heaters, gas absorbers, fertilizers, catalysts, cosmetics, and waste management. Mechanical alloying has become an established commercial technique to produce oxide dispersion strengthened nickel- and iron-based superalloys. The present paper briefly discusses the basic mechanisms of formation of phases by the techniques of MA and MCP; the variety of possible technological applications of mechanically alloyed or mechanochemically processed products are highlighted. The main focus of this paper is on exploring the feasibility of these processing techniques to produce novel advanced materials and their comparison with competing technologies.

Laser attenuation of the focused powder streams in coaxial laser cladding

Abstract: The optical attenuation of a laser beam passing through a powder stream during coaxial laser cladding has an important effect on the powder catchment efficiency for this new process. This attenuation for variations in the powder stream focus was studied both experimentally and theoretically in this article. Measurement of the laser attenuation caused by the streams was made to compare with the theoretical results. It shows that more than 50% of the laser energy could be lost in the powder stream and the focused stream gives less powder catchment than the nonfocused stream in coaxial laser cladding for 304 L stainless steel powder on mild steel with 1 kW CO2 laser radiation.