Showing papers on "Powder metallurgy published in 2017"

Balanced strength and ductility in CNT/Al composites achieved by flake powder metallurgy via shift-speed ball milling

Abstract: Flake powder metallurgy via shift-speed ball milling (SSBM) was proposed to combine the mechanisms of low-speed and high-speed ball milling (LSBM and HSBM) for carbon nanotube (CNT)-reinforced, strong and ductile aluminum composites. LSBM featured mild ball-to-powder collision and slow flattening of spherical Al powders into flakes, leaving enough time for CNTs to be uniformly dispersed on the surface of Al flakes and causing little damage to CNTs; HSBM featured much stronger collision and cold welding of Al flakes, leading to clustered CNTs within the welded Al particles with serious damage but better inter-flake bonding. Therefore, the coordination of CNT dispersion, integrity and interfacial bonding could be achieved by a smart powder processing of SSBM, namely a longer starting LSBM and a shorter following HSBM. As demonstrated, such SSBM resulted in comparable strength in 1.5 wt.% CNT/Al composites but doubled and tripled ductility as those fabricated via LSBM and HSBM.

Review of the Methods for Production of Spherical Ti and Ti Alloy Powder

Abstract: Spherical titanium alloy powder is an important raw material for near-net-shape fabrication via a powder metallurgy (PM) manufacturing route, as well as feedstock for powder injection molding, and additive manufacturing (AM). Nevertheless, the cost of Ti powder including spherical Ti alloy has been a major hurdle that prevented PM Ti from being adopted for a wide range of applications. Especially with the increasing importance of powder-bed based AM technologies, the demand for spherical Ti powder has brought renewed attention on properties and cost, as well as on powder-producing processes. The performance of Ti components manufactured from powder has a strong dependence on the quality of powder, and it is therefore crucial to understand the properties and production methods of powder. This article aims to provide a cursory review of the basic techniques of commercial and emerging methods for making spherical Ti powder. The advantages as well as limitations of different methods are discussed.

Microstructure evolution and superior tensile properties of low content graphene nanoplatelets reinforced pure Ti matrix composites

Abstract: Titanium matrix composites with the discontinuous reinforcement of graphene nanoplatelets (GNPs) were produced by powder metallurgy and subsequent hot-rolling. In the process of spark plasma sintering (SPS), the GNPs were well preserved at low temperature and high compressive pressure. Hot-rolling process was applied to improve the microstructure and properties of the GNPs-Ti matrix composites. The GNPs were uniformly distributed and arranged along with the rolling direction (RD). Also, the GNPs blocked slipping so that the matrix generated {10 1 1} 1 2 > compressive twining to be compatible with deformation in the rolling process with the increase of GNPs content. Tensile strength test demonstrated an excellent ultimate tensile strength that was 54.2% higher than pure titanium with merely 0.1 wt% GNPs addition. The strengthening mechanism of composites was discussed by three main strengthening factors combined with a modified load transfer model and it was thought that the composites were strengthen by grain refinement, load transfer from Ti matrix to GNPs and texture strengthening.

Properties of aluminum matrix Nano composites prepared by powder metallurgy processing

Abstract: Pure aluminum Nano composite reinforced with Nano titanium dioxide was produced by powder metallurgy route. Measurements of tensile strength, hardness, and density showed that the porosity and the tensile strength of composites increased with an increase in volume fraction of nanoparticles; however ductility of aluminum was decreased. Wear test revealed that composites offer superior wear resistance compared to alloy.

Powder Metallurgy Processing of a W x TaTiVCr High-Entropy Alloy and Its Derivative Alloys for Fusion Material Applications

Abstract: The WxTaTiVCr high-entropy alloy with 32at.% of tungsten (W) and its derivative alloys with 42 to 90at.% of W with in-situ TiC were prepared via the mixing of elemental W, Ta, Ti, V and Cr powders followed by spark plasma sintering for the development of reduced-activation alloys for fusion plasma-facing materials. Characterization of the sintered samples revealed a BCC lattice and a multi-phase structure. The selected-area diffraction patterns confirmed the formation of TiC in the high-entropy alloy and its derivative alloys. It revealed the development of C15 (cubic) Laves phases as well in alloys with 71 to 90at.% W. A mechanical examination of the samples revealed a more than twofold improvement in the hardness and strength due to solid-solution strengthening and dispersion strengthening. This study explored the potential of powder metallurgy processing for the fabrication of a high-entropy alloy and other derived compositions with enhanced hardness and strength.

Mechanical and microstructural characterization of powder metallurgy CoCrNi medium entropy alloy

Abstract: The present study is focused on synthesis and mechanical properties characterization of equiatomic CoCrNi medium entropy alloy (MEA). Powder metallurgy processes of mechanical alloying (MA) with subsequent spark plasma sintering (SPS) for bulk alloy densification have been utilized. As opposed to the single-phase alloys of identical composition fabricated via casting routes, the alloy after SPS compaction consisted of a major FCC solid solution phase (94.4%), minor fraction of secondary BCC phase (5.6%, precipitated at the FCC grains boundaries), and negligible amount of oxide inclusions. The alloy exhibited high ultimate tensile strength of 1024 MPa and a elongation to fracture of 26%. Elastic modulus of the alloy reached 222 GPa and the thermal expansion coefficient (CTE) was measured as 17.4 × 10−6 K−1 The plastic deformation in the alloy is carried out by a combination of dislocation glide and mechanical nano-twinning at room temperature.

Effect of Milling Time on the Microstructure, Physical and Mechanical Properties of Al-Al₂O₃ Nanocomposite Synthesized by Ball Milling and Powder Metallurgy.

Abstract: The effect of milling time on the morphology, microstructure, physical and mechanical properties of pure Al-5 wt % Al2O3 (Al-5Al2O3) has been investigated. Al-5Al2O3 nanocomposites were fabricated using ball milling in a powder metallurgy route. The increase in the milling time resulted in the homogenous dispersion of 5 wt % Al2O3 nanoparticles, the reduction of particle clustering, and the reduction of distances between the composite particles. The significant grain refining during milling was revealed which showed as a reduction of particle size resulting from longer milling time. X-Ray diffraction (XRD) analysis of the nanocomposite powders also showed that designated ball milling contributes to the crystalline refining and accumulation of internal stress due to induced severe plastic deformation of the particles. It can be argued that these morphological and microstructural variations of nanocomposite powders induced by designated ball milling time was found to contribute to an improvement in the density, densification, micro-hardness (HV), nano-hardness (HN), and Young’s modulus (E) of Al-5Al2O3 nanocomposites. HV, HN, and E values of nanocomposites were increased by ~48%, 46%, and 40%, after 12 h of milling, respectively.

Modelling of carbon nanotube dispersion and strengthening mechanisms in Al matrix composites prepared by high energy ball milling-powder metallurgy method

Abstract: Carbon nanotube (CNT) reinforced Al-5Mg composites were prepared by combining ball milling, hot-pressing and subsequent hot extrusion. CNT distribution during milling and strengthening mechanism of the composites were investigated. A model based on the ratio of minimum necessary time for uniformly dispersing CNT to flattening time of composite powders was proposed to analyze the effect of milling rotation rate on CNT distribution, and it indicated that both low and high milling rotation rates are not beneficial to CNT distribution, due to small deformation ratio and severe cold-welding, respectively. Under a milling rotation rate of 400 rpm, CNTs could be uniformly dispersed after 8 h of milling and aligned along the extruding direction after extrusion. Elastic moduli and strengths of the composites were significantly increased. Load transfer, grain refinement, and mismatch dislocation mechanisms were determined to contribute to the strength increase of CNT/Al-5Mg composites.

High-strength aluminum-based composites reinforced with BN, AlB2 and AlN particles fabricated via reactive spark plasma sintering of Al-BN powder mixtures

Abstract: Light (density 350 MPa) metal matrix composites (MMCs) are highly anticipated for aerospace and automotive industries. The MMCs application fields can be significantly expanded if they possess enhanced strength at elevated temperatures also. In the present study, Al-based composites loaded with either micro- or BN nanoparticles (BNMPs and BNNPs) with up to 10 wt% of BN phase were produced via spark plasma sintering (SPS) of ball-milled Al-BN powder mixtures. A dramatic increase in the composite tensile strength compared to pure Al samples (up to 415%) was demonstrated during tensile tests both at 20 °C and 500 °C. BNMPs were found to be a more preferred additive compared with BNNPs due to the formation of more homogeneous and uniform morphologies within the ball-milled powder mixtures and resultant SPS products. The most impressive tensile strength of 170 MPa at 500 °C was achieved for an Al-7 wt% BNMPs SPS composite, as compared to a value of only 33 MPa for a pure Al SPS-produced sample. The reinforcement mechanism was uncovered based on detailed X-ray diffraction analysis, differential scanning calorimetry, Raman spectroscopy, scanning and high-resolution transmission electron microscopy and energy-dispersion X-ray analysis. Microscale BN, AlB2 and AlN inclusions acting within Al-matrices in the frame of Orowan strengthening mechanism, and pre-formed during ball-milling-induced pre-activation of Al-BN powder mixtures, finally crystallized during SPS processing and ensured the dramatically improved tensile strength and hardness of the resultant composites.

Review of solid state recycling of aluminum chips

Abstract: In contrast with the conventional remelting recycling of aluminum and its alloy chips, the solid state recycling techniques, which can convert the chips directly into dense bulk materials, have attracted significant attention primarily because it possesses many advantages including lower energy consumption, lower metal loss as well as almost no emissions of harmful gases and solid wasters. In this keynote paper, with a view to the current researches of the solid state recycling techniques based on the severe plastic deformation (SPD) and powder metallurgy (P/M), the characteristics and applications of several typical methods, such as hot extrusion, equal channel angular pressing (ECAP), cyclic extrusion compression (CEC), friction stir extrusion (FSE), high pressure torsion (HPT), screw extrusion and spark plasma sintering (SPS), are introduced. A growing number of researches and literatures suggest that the mechanical properties of solid state recycled specimens are primarily dependent on the chip bonding quality and microstructure of the corresponding bulk materials. Then, the mechanism analysis of consolidation of chips is carried out, and three relevant theoretical modes, characterizing the bonding quality, are also mentioned. Moreover, the factors influencing the density and microstructure of chip-consolidated product are discussed comprehensively. Eventually, recommendations in the improvement of solid state recycling techniques and the future prospects are put forward.

Corrosion Properties of Powder Bed Fusion Additively Manufactured 17-4 PH Stainless Steel

Abstract: The corrosion susceptibility of a laser powder bed fusion (LPBF) additively manufactured alloy, UNS S17400 (17-4 PH), was explored compared to conventional wrought material. Microstructural charact...

Microstructure and Thermal Conductivity of Al-Graphene Composites Fabricated by Powder Metallurgy and Hot Rolling Techniques

Abstract: The objective of this research is to improve the thermal conductivity and mechanical properties of Al/GNPs (graphene nanoplatelets) nanocomposites produced by classical powder metallurgy and hot rolling techniques. The microstructural evaluation confirmed the uniform dispersion of GNPs at low content and agglomeration at higher contents of GNPs. The structure of graphene was studied before and after the mixing and the Raman spectrum proofs that the wet mixing has a great potential to be used as a dispersion method. There was no significant peak corresponding to the Al4C3 formation in both the differential scanning calorimetry curves and X-ray diffraction patterns. The microstructural observation in both fabrication techniques showed grain refinement as a function of the GNPs content. Moreover, the introduction of the GNPs not only improved the Vickers hardness of the composites but also decreased their density. The thermal conductivity investigations showed that in both the press-sintered and hot-rolled samples, although the thermal conductivity of composites was improved at low GNPs contents, it was negatively affected at high GNPs contents.

Flow behavior and microstructures of powder metallurgical CrFeCoNiMo0.2 high entropy alloy during high temperature deformation

Abstract: Dynamic recrystallization (DRX) refine grains of high entropy alloys (HEAs) and significant improve the mechanical property of HEAs, but the effect of high melting point element molybdenum (Mo) on high temperature deformation behavior has not been fully understood. In the present study, flow behavior and microstructures of powder metallurgical CrFeCoNiMo0.2 HEA were investigated by hot compression tests performed at temperatures ranging from 700 to 1100 °C with strain rates from 10−3 to 1 s−1. The Arrhenius constitutive equation with strain-dependent material constants was used for modeling and prediction of flow stress. It was found that at 700 °C, the dynamic recovery is the dominant softening mechanism, whilst with the increase in compression testing temperature, the DRX becomes the dominant mechanism of softening. In the present HEA, the addition of Mo results in the high activation energy (463 kJ mol−1) and the phase separation during hot deformation. The formation of Mo-rich σ phase particles pins grain boundary migration during DRX, and therefore refines the size of recrystallized grains.

High-entropy alloy CoCrFeMnNi produced by powder metallurgy

Abstract: Lately high-entropy alloys (HEAs) have been the topic of extensive research, as these materials are promising candidates for many challenging applications, as for example tools, moulds and functional coatings In contrast to conventional alloys, HEAs consist of five or more principal elements, each having a concentration between 5 and 35 at-% Against expectations, HEAs show a rather simple microstructure consisting preferentially of cubic phases Due to this microstructure, HEAs show promising properties, eg in terms of high-temperature stability, high strength and ductility Within this research, a single-phase CoCrFeMnNi HEA was produced by powder metallurgy (PM) In contrast to conventional metallurgy, PM offers a lot of advantages, eg good material efficiency and high shape complexity Gas atomised powder was used and selected PM methods are presented (eg pressureless sintering, spark plasma sintering, additive manufacturing (EBM)) The process methods were evaluated by characterising

Quantifying the properties of low-cost powder metallurgy titanium alloys

Abstract: The extensive industrial employment of titanium is hindered by its high production costs where reduction of these costs can be achieved using cheap alloying elements and appropriate alternative processing techniques. In this work the feasibility of the production of low-cost titanium alloys is addressed by adding steel to pure titanium and processing the alloys by powder metallurgy. In particular, a spherical 4140 LCH steel powder commonly used in metal injection moulding is blended with irregular hydride-dehydride Ti. The new low-cost alloys are cold uniaxially pressed and sintered under high vacuum and show comparable properties to other wrought-equivalent and powder metallurgy titanium alloys. Differential thermal analysis and X-ray diffraction analyses confirm that Ti can tolerate the employment of iron as primary alloying element without forming detrimental TiFe-based intermetallic phases. Thus, the newly designed α+β alloys could be used for cheaper non-critical components.