Showing papers on "Powder metallurgy published in 2010"

Investigation of particle size and amount of alumina on microstructure and mechanical properties of al matrix composite made by powder metallurgy

Abstract: Al matrix composite is well known, in which Al2O3 is the most widely used reinforcement. The aim of this study is to investigate the effect of alumina particle size and its amount on the relative density, hardness, microstructure, wear resistance, yield and compressive strength and elongation in Al–Al2O3 composites. To this end, the amount of 0–20 wt.% alumina with average particle sizes 48, 12 and 3 μm was used along with pure aluminum of average particle size of 30 μm. Powder metallurgy is a method used in the fabrication of this composite in which the powders were mixed using a planetary ball mill. Consolidation was conducted by axial pressing at 440 MPa. Sintering procedure was done at 550 °C for 45 min. The results indicated that as the alumina particle size is reduced, density raises at first, then, declines. Moreover, as the alumina particle size decreases, hardness, yield strength, compressive strength and elongation increase and factors such as wear resistance, microstructure grain size and distribution homogeneity in matrix decreases. For instance, as the alumina particle size gets smaller from 48 to 3 μm at 10 wt.% alumina, hardness rises from 50 to 70 BHN, compressive strength improves from 168 to 307 MPa and wear rate rises from 0.0289 to 0.0341 mm3/m. On the other hand, as the amount of alumina increases, hardness and wear resistance increase and relative density and elongation is decreased. However, compressive and yield strength rise at first, then drop. For example, if the amount of alumina with 12 μm particle size increases from 5 to 10 wt.%, hardness increases from 47 to 62 BHN and compressive strength rises from 190 to 273 MPa. Nevertheless, erosion rate after 300 m decreases from 0.0447 to 0.0311 mm3/m.

Cytotoxicity of Titanium and Titanium Alloying Elements

Abstract: It is commonly accepted that titanium and the titanium alloying elements of tantalum, niobium, zirconium, molybdenum, tin, and silicon are biocompatible. However, our research in the development of new titanium alloys for biomedical applications indicated that some titanium alloys containing molybdenum, niobium, and silicon produced by powder metallurgy show a certain degree of cytotoxicity. We hypothesized that the cytotoxicity is linked to the ion release from the metals. To prove this hypothesis, we assessed the cytotoxicity of titanium and titanium alloying elements in both forms of powder and bulk, using osteoblast-like SaOS(2) cells. Results indicated that the metal powders of titanium, niobium, molybdenum, and silicon are cytotoxic, and the bulk metals of silicon and molybdenum also showed cytotoxicity. Meanwhile, we established that the safe ion concentrations (below which the ion concentration is non-toxic) are 8.5, 15.5, 172.0, and 37,000.0 microg/L for molybdenum, titanium, niobium, and silicon, respectively.

Cold compaction and sintering of titanium and its alloys for near-net-shape or preform fabrication

Abstract: The conventional cold-compaction-and-sinter powder metallurgy (PM) approach offers an efficient solution to the near-net shape or preform fabrication of titanium and its alloys for cost reduction and improved chemical homogeneity and refined microstructures. The methods for compacting titanium powder are similar to those used for other ductile powders. The high-purity titanium in the most ductile state is similar to annealed copper in terms of elastic modulus, hardness, elongation, and ultimate tensile strength. The properties of titanium are sensitive to the impurity level, in particular to nitrogen, oxygen, carbon, and iron. Hardness is a convenient measure of the quality of a titanium sponge product. Dilatometric studies of the sintering of titanium-nickel alloys show similar observations and confirm that oxide films on titanium powder surfaces do not need to be reduced by the atmosphere or disrupted by a chemical additive.

Microstructural and mechanical analysis of carbon nanotube reinforced magnesium alloy powder composites

Abstract: Carbon nanotube (CNT) is an effective reinforcement used to improve the mechanical and thermal responses of metal matrix composites. It is, however, obvious that segregation of CNTs due to their strong van der Waals forces will produce material defects, decreasing the material properties. An advanced powder metallurgy process that disperses un-bundled nanotubes has been developed, and it is applied to fabricate a Mg matrix composite reinforced with CNTs in the present study. When approximately 1 vol.% CNTs were added, the extruded pure Mg and AZ31B alloy composites displayed an extremely large increase of the tensile yield stress of 25–40%, compared to Mg materials containing no CNT. HR-TEM verified the presence of MgO thin layers of 2–4 nm thickness, originating in the oxide surface films of the raw Mg and its alloy powders. These layers exist at the interface between α-Mg and the un-bundled nanotubes. The oxide layer showed a homogeneous mixture containing both α-Mg and CNT. This mixture resulted in an effective tensile loading transfer at the interface, which significantly improved the tensile strength (TS) and yield stress (YS) of the Mg composites. However, the elongation was less than 5%, and the composites exhibited a very poor ductility. This was because the MgO layers at the interface between α-Mg and CNT were very ductile.

A critical review of mechanical properties of powder metallurgy titanium

Abstract: Mechanical properties are the primary concern in the development and application of powder metallurgy (PM) titanium and its alloys. Their mechanical properties are reviewed by comparing PM with ingot metallurgy (IM) and by examining the dependence of the mechanical properties on the microstructures that are unique to PM titanium. The effects of the most critical factors (porosity, oxygen content, and microstructure) on mechanical properties are discussed. Throughout this review, PM Ti refers generically to PM titanium and titanium alloys. IM Ti embraces ingot metallurgy titanium and titanium alloys. Static and dynamic properties are examined to illustrate the challenges as well as the opportunities for PM Ti. Selected recent PM Ti technologies and attendant mechanical properties are also assessed.

Titanium foams produced by solid-state replication of NaCl powders

Abstract: Open-celled titanium foams were fabricated by vacuum hot pressing of a blend of Ti and NaCl powders followed by NaCl removal in water. Densification kinetics of the Ti/NaCl blends are measured at 780 °C at various pressures (30–50 MPa), NaCl volume fractions (30–70%) and NaCl powder sizes (50–500 μm). As compared to pure Ti powders, densification kinetics of the blends is faster for relative densities below 92% due to rapid deformation of the NaCl powders. After dissolution, the flattened shape of the NaCl powders is replicated in the pores, resulting in an anisotropic porous structure. The foams exhibit good compressive strengths (e.g., 102 MPa for 50% porosity and 28 MPa for 67% porosity), low Young moduli (e.g., 29 GPa for 51% porosity) and ductile behavior up to compressive strain >60%.

Technological Advancement in Electrical Discharge Machining with Powder Metallurgy Processed Electrodes: A Review

Abstract: Electrical discharge machining (EDM) is a well-established machining option for processing hard materials with complex geometrical shapes which are extremely difficult-to-machine by conventional machining processes. These hard materials find applications where lower surface cracks, wear resistance, corrosion resistance, etc. are desirable surface properties. In recent years, research has been carried out to determine the possibility of employing electrode as feed stock material in an effort to produce significant surface alloying. These electrodes are generally produced through powder metallurgy (PM) technique in order to achieve necessary combination of operating characteristics. This paper reports state of art related to the usefulness of PM-processed electrodes in imparting desirable surface properties and modification of the machined surface. The final part of the paper outlines the trends for future EDM research using PM-processed electrodes.

Effect of Hot-Isostatic-Pressing Parameters on the Microstructure and Properties of Powder Ti-6Al-4V Hot-Isostatically-Pressed Samples

Abstract: Ti-6Al-4V powders have been hot-isostatically-pressed (“HIPped”) using a range of hot-isostatic-pressing (“HIPping”) conditions, and the effects on microstructure and mechanical properties have been assessed. The properties were measured on test samples machined from HIPped powder billets and on samples that contained the as-HIPped surface. The fatigue limit of samples that contained the as-HIPped surface was improved by using a new HIPping procedure. The machined samples that had been HIPped at 1203 K (930 °C) exhibited a better balance of properties than those HIPped at 1153 K (880 °C) or 1293 K (1020 °C). The fine microstructure, formed from the martensitic structure of the atomized powder, coarsens with the increase of temperature or time during HIPping. These changes have been correlated with the corresponding changes in properties and with the fracture surfaces. The significance of these observations, especially the fatigue properties of samples that contain the as-HIPped surface, is discussed in terms of the properties of net-shape HIPped components.

Artificial neural network model in surface modification by EDM using tungsten–copper powder metallurgy sintered electrodes

Abstract: Electro-discharge machining (EDM) is a widely accepted nontraditional machining process used mostly for machining materials difficult to machine by conventional shearing process. Surface modification by powder metallurgy sintered tools is an uncommon aspect of EDM. Of late, it is being explored by many researchers. In the present paper, attempts have been made to model the surface modification phenomenon by EDM with artificial neural networks. Two output measures, material transfer rate and average layer thickness, have been correlated with different process parameters and presented in the form of plots. The predicted results are matching well with the experimental results.

Comparison of sintering of titanium and titanium hydride powders

Abstract: The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green str...

Review of densification of titanium based powder systems in press and sinter processing

Abstract: The development of novel extractive metallurgy techniques for titanium offers the prospect of lower cost Ti powder and therefore wider application of Ti. This review is largely confined to coverage of the low cost press and sinter methods of powder metallurgy, consisting of cold pressing of mixed elemental powders followed by sintering without the application of external pressure. Cold die compaction, sintering behaviour and densification are reviewed in detail. Some information on powders and cold isostatic pressing is included. Microstructure, mechanical properties and applications are considered in less detail. The review deals mostly with the sintering of alloys, but there is some reference to synthesis of intermetallic compounds, such as the shape memory alloy NiTi and titanium aluminides for high temperature applications. Densification is discussed in terms of the four fundamental processing variables: compaction pressure, particle size, sintering temperature and sintering time. Other factors such as alloy composition, the form of alloying addition, type and impurity content of powders and heating rate are also considered. © 2010 Institute of Materials, Minerals and Mining.

Production of Ti–13Nb–13Zr alloy for surgical implants by powder metallurgy

Abstract: The Ti–13Nb–13Zr near-β alloy was developed aiming the replacement of the traditional Ti–6Al–4V alloy in surgical implants owing to its larger biocompatibility. Samples of this alloy were obtained using the blended elemental technique from hydrided powders. The isochronal sintering of the compacts for 2 h was carried out in the range 900–1,400 °C with a heating rate of 20 °C min−1. In this work, the behavior of the elementary powders during sintering and the corresponding microstructural evolution were investigated. The alloy was characterized by means of scanning electron microscopy (SEM) in the backscattered mode, X-ray diffraction, and density measurements. The results indicate that the homogenization of the alloy is diffusion-controlled. With increasing temperature, homogenization of the alloy takes place and a fine plate-like α + β structure is found throughout the microstructure in temperatures above 1,300 °C. The process variables were defined aiming to minimize interstitial pick-up (C, O, and N) and avoiding intensive grain growth.

Sinter-ability of nanocrystalline tungsten powder

Abstract: In this investigation, nanocrystalline tungsten powders prepared by either chemical method or high energy mechanical milling were sintered to study the size-dependence of the densification of tungsten powders. Results show that nanocrystalline tungsten powders produced via an ultra-high energy milling process have significantly enhanced sinterability compared to conventional submicron- or micron-sized powders. This study for the first time shows nanocrystalline tungsten powder can be sintered via a pressureless process to near-full densification at a temperature as low as 1100 °C.

Microstructure and mechanical properties of WC Ni–Si based cemented carbides developed by powder metallurgy

Abstract: In this paper the influence of the Ni binder metal and silicon as an additional alloying element on the microstructure and mechanical properties of WC-based cemented carbides processed by conventional powder metallurgy was studied. Microstructural examinations of specimens indicated the presence of a very low and even distributed porosity and the presence of islands of metal binder in the microstructure of the cemented carbides. Furthermore, despite the addition of silicon and carbon in the cemented carbides, it was not observed the presence of small fractions of undissolved SiC and free graphite nodules in their microstructure. Vickers hardness and Flexural strength tests indicated that the cemented carbide WC–Ni–Si with 10 wt.% of binder presented bulk hardness similar to the conventional WC–Co cemented carbides and superior flexure strength and fracture toughness.

Constitutive flow behavior and hot workability of powder metallurgy processed 20 vol.%SiCP/2024Al composite

Abstract: Constitutive flow behavior and hot workability of the powder metallurgy processed 20 vol.%SiC(P)/2024Al composite were investigated using hot compression tests. The modified Arrhenius-type constitutive equations were presented with the values of material constants in consideration as a function of strain. Dynamic material model (DMM) and modified DMM were used to construct the power dissipation efficiency maps, and Ziegler's instability criterion and Gegel's stability criterion were used to build instability maps. The presence of finer SiC(P) and more boundaries resulting from smaller 2024Al powders shifted the dynamic recrystallization domain of the 2024Al matrix to higher strain rate and lower temperature ranges and decreased the peak value of power dissipation efficiency. Large instable regions were found in the form of flow localization and cavitations located at the matrix/SiC(P) interfaces and within the SiC(P) clusters. By comparison, the Gegel's stability criterion was more sensitive to the instability zones than the Ziegler's instability criterion for this material. (c) 2010 Elsevier B.V. All rights reserved.

High temperature sliding wear behavior of press-extruded AA6061/fly ash composite

Abstract: The effect of fly ash particulate on high temperature dry sliding wear resistance of AA6061, developed by powder metallurgy and hot extrusion, was studied. The dry sliding wear behavior of the prepared composite was investigated by using pin-on-disc method at an applied load of 14.6 N for various temperatures (100, 200 and 300 °C). The study at room temperature was also carried out for comparison purpose. The results had shown transition from mild-to-severe wear for unreinforced alloy in the temperature range of 200–300 °C. With the addition of fly ash to AA6061, the mild-to-severe wear transition was not noticed even up to 300 °C. This behavior was attributable to the formation of protective transfer layers of comminuted reinforcing particulates and transferred steel debris from slider counterpaces. The absence of severe wear phenomena in this composite was due to the “particulate hardening” of subsurface layer in the temperature range investigated.