Powder Metallurgical Process Research Articles

Densification and micro-structural characteristics of spark plasma sintered Ti-Zr-Ta powders

The fabrication of Ti-based alloys with enhanced properties for extreme applications is an insatiable quest in the materials engineering field. Optimization of powder metallurgical processing routes is an area of interest for more efficient and cost effective manufacturing techniques. Ti-Zr-Ta alloys of different characteristics have been synthesized from their elemental powders, using Spark Plasma Sintering (SPS): Temperature (1100°C), Pressure (50MPa) and holding times (5min and 10min) respectively. Microstructures, chemical and phase analyses were conducted using FE-SEM, EDS, Oxygen analyser and XRD. Results revealed significant amount of martensite phase evolution irrespective of the holding time. The evolution of β-Ti phase and decrease in oxygen contents was also noticed with the addition of Ta. Relative density measurement conducted using Archimedes principle revealed the bulk samples had densifications close to theoretical values and micro-hardness of significantly high magnitudes (above 500Hv). This suggests the suitability of the fabricated products for applications requiring high hardness parameters such in dental technologies.

Influence of α-Phase Field Heat Treatment on the Tensile and Primary Creep Resistance of a Powder Metallurgical Processed Ti-45Al-5Nb-0.2B-0.2C Titanium Aluminide Alloy

In powder metallurgical processing the sintering process, as well as heat treatments, can drastically influence microstructure formation. In the case of γ-titanium aluminides, it is critical to achieve certain microstructure parameters, such as colony size, porosity and grain boundary morphology in order to obtain appropriate mechanical properties. In this study, the effect of a heat treatment implemented after sintering with the objective of varying the colony size was investigated. Specimens of Ti-45Al-5Nb-0.2B-0.2C prepared by metal injection moulding and uniaxial pressing of feedstock were used to evaluate the tensile and creep properties. Heat treatments conducted at 1350 and 1400 °C for 3 h led to colony sizes of approximately 100 and 200 μm, respectively. Classically, there is an inverse relationship between grain size and creep resistance, nonetheless, for γ-titanium aluminides, the morphology of the colony boundaries was also found to play a role. The larger colony sizes achieved with the heat treatments did not improve the primary creep resistance, which was explained by the change in the morphology of the colony boundaries as they became larger.

The Development of Lead-Free Sliding Contacts Based on Bronze-Graphite Composites through Powder Injection Moulding

Sliding electrical contacts are traditionally produced by conventional compacting technologies. Employing the powder injection moulding process (PIM) as a new manufacturing method can offer several advantages such as the fabrication of complex net-shaped parts, cost-effectiveness and high volume productions. The PIM process route consists of the following steps: powder processing, compounding, injection molding, debinding and sintering. A two-stage process consisting of solvent debinding and thermal debinding is often used to remove the moulding binder. In the present paper, the suitability of the powder metallurgical processes: mechanical alloying and powder mixing for the preparation of bronze-graphite powder mixtures for the compounding and injection moulding of sliding contacts is discussed. The use of a suitable binder is of central importance for the preparation of injection-moldable feedstocks. For this purpose, two commercial ready-to-use binder systems were utilized and evaluated. The essential challenge of the process route is to optimize all parameters of the subprocesses to achieve a damage-free debinding and sintering of the injection-moulded parts. First results on the influence of the graphite content, the binder fraction, the debinding and sintering parameters are presented and discussed.

TFP Reinforced Metal - One Way to Increase the Specific Strength

The integration of fibers, especially tailor fiber placement (TFP), in metal matrices offers one way to generate composite materials with increased specific strength compared to the unreinforced metal matrix. The TFP can be adapted according to the final load paths through the component and can be covered partially or fully with the metal. Following this approach load transfer elements can be built, transferring much load and having low mass. First fields of application are identified in building and automotive industry. This work includes the powder metallurgical manufacturing process using Spark Plasma Sintering (SPS) technique, the characterization of the microstructure and the tensile test of different specimens (sintered copper, TFP (as received) and TFP (Cu covered) reinforced copper). Experimental result on 19.5 vol.% TFP (Cu covered) reinforced copper shows an increase of specific strength around a factor of 2.2 compared to pure copper.

The effect of Cu-based core-sheath configurations on the processing of Nd-Fe-B-based permanent magnets via equal-channel angular pressing

Equal channel angular pressing (ECAP) has been used as an alternative manufacturing route for preparation of Nd2Fe14B-based anisotropic magnets, facilitating processing temperatures much lower than conventional die upsetting. While this method can produce a suitable texture and microstructure in permanent magnetic materials, it still remains novel; involving extremely high pressures which present a high risk of both process failure and die and tooling damage. Powder metallurgical processes frequently incorporate an external layer of secondary material (commonly an outer foil layer or can) for separation between the primary material and die as well as the control of surface effects such as friction through appropriate choice of secondary material. This work implements such modifications to this manufacturing route by incorporation of an outer layer of Cu foil, the addition of which negatively affected both the powder compaction and strength of texture produced via ECAP. Also investigated was the incorporation of solid Cu bar as part of the sample cross section. This modification facilitated processing without any compromise on observed magnetic properties, whilst also reducing damage to both the die and tooling. This type of methodology may aid in improving the reliability of producing bulk anisotropic permanent magnets via ECAP.

Process-Oriented Microstructure Evolution of Vss-V3Si-V5SiB2 Materials

Abstract Vanadium silicide alloys are potentially interesting high temperature materials, since they combine high mechanical strength at temperatures of up to 1 000 °C with a low density. In this study, the microstructures of innovative V-Si-B high temperature materials are examined using different analytical methods. The selected V-9Si-13B model alloy was manufactured using a powder metallurgical process route as well as an ingot metallurgical process. The alloys show a vanadium solid solution phase as well as the high-strength silicide phases V3Si and V5SiB2. Especially for the powder metallurgically fabricated alloy, showing finely dispersed phases, the quantification of microstructural constituents is difficult. The phases, however, can be separated from one another via computer tomography.

Bronze Alloy Development for Zinc Vapor Capture

After gamma-emitting 65Zinc was detected in a vacuum pumping system contained in a tritium glovebox, a series of experiments were undertaken to develop a method and material to trap zinc vapors in an area that is more suitable for preventing dose to workers. In this study, bronze alloys with 0–30% tin were prepared using a powder metallurgical process and exposed to three levels of zinc vapors. All of the alloys demonstrated acceptable zinc gettering capacity; however, low tin content bronzes are considered for further testing.

Microstructural and magnetic properties of Mn-Fe-P-Si (Fe2 P-type) magnetocaloric compounds

Fe2 P-based magnetocaloric compounds are an important and industrially relevant material class for magnetic refrigeration, yet their microstructure and its influence on the magnetic properties is hardly discussed in literature. We prepared Mn-Fe-P-Si-based samples using a powder metallurgical process and analyzed their microstructural and thermomagnetic properties. XRD, SEM, EDX and EBSD analysis reveal a phosphorous depleted cubic secondary phase in many samples with distinct microstructural properties giving an insight into the phase formation process. A porous morphology was found, hindering the direct application of the materials a magnetocaloric heat exchanger in bulk-like structures. The “virgin” effect could be in-situ observed for the first time on a macroscopic scale using temperature-dependent optical microscopy. Thermomagnetic measurements reveal a difference in transition temperature Tt in comparison to literature values which is attributed to a processing induced deviation from the nominal composition. The isothermal entropy change ΔST and adiabatic temperature change ΔTad were studied as well as their cyclic behavior. The effect of secondary phases is discussed and the importance of the metal/non-metal (M/NM)-ratio is shown. The article presents a road map for the preparation of Mn-Fe-Si-P-based alloys with highest quality.

Robot based deposition of WC-Co HVOF coatings on HSS cutting tools as a substitution for solid cemented carbide cutting tools

Cemented carbide (hard metal) cutting tools are the first choice to machine hard materials or to conduct high performance cutting processes. Main advantages of cemented carbide cutting tools are their high wear resistance (hardness) and good high temperature strength. In contrast, cemented carbide cutting tools are characterized by a low toughness and generate higher production costs, especially due to limited resources. Usually, cemented carbide cutting tools are produced by means of powder metallurgical processes. Compared to conventional manufacturing routes, these processes are more expensive and only a limited number of geometries can be realized. Furthermore, post-processing and preparing the cutting edges in order to achieve high performance tools is often required.In the present paper, an alternative method to substitute solid cemented carbide cutting tools is presented. Cutting tools made of conventional high speed steels (HSS) were coated with thick WC-Co (88/12) layers by means of thermal spraying (HVOF). The challenge is to obtain a dense, homogenous, and near-net-shape coating on the flanks and the cutting edge. For this purpose, different coating strategies were realized using an industrial robot. The coating properties were subsequently investigated. After this initial step, the surfaces of the cutting tools were ground and selected cutting edges were prepared by means of wet abrasive jet machining to achieve a smooth and round micro shape. Machining tests were conducted with these coated, ground and prepared cutting tools. The occurring wear phenomena were analyzed and compared to conventional HSS cutting tools. Overall, the results of the experiments proved that the coating withstands mechanical stresses during machining. In the conducted experiments, the coated cutting tools showed less wear than conventional HSS cutting tools. With respect to the initial wear resistance, additional benefits can be obtained by preparing the cutting edge by means of wet abrasive jet machining.

Controlled release of chlorhexidine from a mesoporous silica-containing macroporous titanium dental implant prevents microbial biofilm formation.

Roughened surfaces are increasingly being used for dental implant applications as the enlarged contact area improves bone cell anchorage, thereby facilitating osseointegration. However, the additional surface area also entails a higher risk for the development of biofilm associated infections, an etiologic factor for many dental ailments, including peri-implantitis. To overcome this problem, we designed a dental implant composed of a porous titanium-silica (Ti/SiO2) composite material and containing an internal reservoir that can be loaded with antimicrobial compounds. The composite material consists of a sol-gel derived mesoporous SiO2 diffusion barrier integrated in a macroporous Ti load-bearing structure obtained by powder metallurgical processing. The antimicrobial compounds can diffuse through the porous implant walls, thereby reducing microbial biofilm formation on the implant surface. A continuous release of µM concentrations of chlorhexidine through the Ti/SiO2 composite material was measured, without initial burst effect, over at least 10 days and using a 5 mM chlorhexidine solution in the implant reservoir. Metabolic staining, CFU counting and visualisation by scanning electron microscopy confirmed that Streptococcus mutans biofilm formation on the implant surface was almost completely prevented due to chlorhexidine release (preventive setup). Moreover, we demonstrated efficacy of released chlorhexidine against mature Streptococcus mutans biofilms (curative setup). In conclusion, we provide a proof of concept of the sustained release of chlorhexidine, one of the most widely used oral antiseptics, through the Ti/SiO2 material thereby preventing and eradicating biofilm formation on the surface of the dental implant. In principle, our flexible design allows for the use of any bioactive compound, as discussed.

Powder metallurgically synthesized Cu12Sb4S13tetrahedrites: phase transition and high thermoelectricity

A powder metallurgical process was used to synthesise Cu12Sb4S13−xcompounds as natural powders for use as high performance thermoelectric materials.

Physical, mechanical and magnetic properties of cobalt-chromium alloys prepared by conventional processing

In this work, the cobalt-chromium alloys (Co-xCr) with various compositions, x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 wt.%, were prepared by conventional powder metallurgical processing cooperated with cold isostatic press technique (CIP) followed by traditional sintering in argon media. The effects of chemical compositions on phase formation, microstructure and properties of cobalt-chromium alloys were studied. Phase evolution, densification, microstructure, mechanical and magnetic properties of this system were investigated. X-ray diffraction patterns show the existence of cobalt peak along with chromium peak and the intensity of Cr peaks increase with increasing of x. SEM micrographs reveal high dense of very large irregular-shaped-grains and the average grain size of this alloy decreases with increase of x. Physical, mechanical and magnetic properties strongly depend on chemical composition.

Viewpoint on fast creation of dense MgB2 phase in wires made by internal Mg diffusion process

Md Shahriar Al Hossain, University of Wollongong, shares his views on a letter by P KovaAcA○ and colleagues on fast creation of dense MgB phase in wires made by internal Mg diffusion process. According to him, In situ and ex situ powder metallurgical processes are currently used for the industrial fabrication of mono- and multi-filamentary MgB wires. The mass density in the MgB filaments of the wires after the final heat treatment is quite low with respect to the theoretical density. The typical effective superconductor area in the wire is only about 10%. Cold high pressure densification (CHPD) has been suggested as an alternative to enhance filament mass density. The authors showed that 8 min sintering time is sufficient for the formation of a uniform MgB2 phase 90 μm in thickness and without cracks, which is nearly identical to samples produced under the same sintering temperature conditions but longer sintering time of 3?5 h that were previously reported by the NIMS group. This finding is important for economical industrial production, but a question still remains as to how the round wires with a hollow filament will perform in magnet design and application, where enhancement of the filling factor, reduction of filament size, and mechanical and electrical stability and reliability are key factors.

Experimental Analysis of Hardness and Densification of Microwave Sintered AL/SIC/AL2O3/Flyash Composites

Objectives: Powder metallurgy is one of the best methods to achieve uniform distribution of reinforcement in matrix. In the present study, silicon carbide, alumina and fly ash are reinforced in Aluminium matrix at various proportions through powder metallurgical process using both conventional and microwave sintering and the mechanical properties were evaluated. Methods/Statistical Analysis: The compaction load has varied from 4000 kg to 8000 kg, and it was observed that the maximum densification of 84.04% at 8000 kg was obtained. All the samples were prepared at 8000 kg load and sintering was done using microwave furnace. For comparison purpose, all the samples were sintered conventionally using tubular furnace. Findings: Hardness and densification tests were conducted on both conventional and microwave sintered specimens. Composites, which are, processed through microwave sintering shows a better result than conventionally sintered specimens with saving the process timing and power consumption. It is being observed that the role of SiC in microwave sintering is very vital as it is an absorber of microwaves. Application/Improvements: The working area of a microwave furnace is very less and it restricts the size of the working sample. The role of other absorbers (semiconductors) of microwaves can be studied.

Compressive and tensile deformation behaviour of TRIP steel-matrix composite materials with reinforcing additions of zirconia and/or aluminium titanate

The mechanical properties and the related microstructure of metal-matrix composites (MMC) based on a high-alloyed CrMnNi steel with varying particle reinforcements (5% or 10 vol%) of magnesia partially stabilized zirconia and/or aluminium titanate were investigated. The powder metallurgical processing comprised the cold extrusion and conventional solid-state sintering at 1350 °C. The mechanical properties were examined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics contributing to significant changes in strength and ductility and affecting the failure mechanisms during deformation were characterised by scanning electron microscopy including energy-dispersive X-ray spectroscopy and electron backscatter diffraction, and by X-ray diffraction. For all compositions the stress-strain and the deformation behaviour were mainly controlled by dislocation hardening and α’-martensite formation in the steel matrix, but it was further improved by the particle strengthening of the ceramic reinforcement. Thus, the composite materials showed higher strength and work hardening than unreinforced steel specimens over a wide strain range. The pressureless sintering triggers several reactions at the metal/ceramic interface, which leads to pronounced destabilisation of the initial metastable zirconia particles with a lack of transformation toughening capability, and the formation of invalid olivine. These MMC suffer from early particle/matrix displacement and particle fracture under loading. A more positive effect of interfacial reactions was observed in the composites with addition of aluminium titanate. Here, the formation of a dense spinel structure and the reliable matrix/particle interface bonding during firing provides a significant particle strengthening effect. The combination of zirconia and aluminium titanate provides mechanical and microstructural benefits as well. Thus, aluminium titanate facilitates the consolidation of powder metallurgical processed steel-matrix MMCs via the conventional sintering for advanced load applications. The results of the study help to understand essential hardening mechanisms in composite materials and provide potential for future cellular MMC structures.

Powder metallurgical processing of equiatomic AlCoCrFeNi high entropy alloy: Microstructure and mechanical properties

Phase formation, microstructural evolution and the mechanical properties of novel multi-component equiatomic AlCoCrFeNi high entropy alloy synthesized by high energy ball milling followed by spark plasma sintering have been reported here. The microstructure of the mechanically alloyed (MA) powder and sintered samples were studied using X-ray diffraction, scanning electron and transmission electron microscopy, whereas the detailed investigation of the mechanical properties of the sintered samples were measured using micro and nano hardness techniques. The fracture toughness measurements were performed by applying single edge V notch beam (SEVNB) technique. The MA powder shows the presence of FCC (τ) and BCC (κ) solid solution phases. Extended ball milling (up to 60h) does not change the phases present in MA powder. The sintered pellets show phase-separated microstructure consisting of Al-Ni rich L12 phase, α′ and tetragonal Cr-Fe-Co based σ phase along with Al-Ni-Co-Fe FCC solid solution phase (ε) for sample sintered from 973 to 1273K. The experimental evidences indicate that BCC (κ) solid solution undergoes eutectoid transformation during sintering leading to the formation of L12 ordered α′ and σ phases, whereas FCC (τ) phase remains unaltered with a slight change in the lattice parameter. The hardness of the sample increases with sintering temperature and a sudden rise in hardness is observed 1173K. The sample sintered at 1273K shows the highest hardness of ~8GPa. The elastic modulus mapping clearly indicates the presence of three phases having elastic moduli of about 300, 220 and 160GPa. The fracture toughness obtained using SEVNB test shows a maximum value of 3.9MPam1/2, which is attributed to the presence of brittle nanosized σ phase precipitates. It is proposed that significant increase in the fraction of σ phase precipitates and eutectoid transformation of the τ phase contribute to increase in hardness along with better densification at higher sintering temperatures.

Microstructure and thermoelectric properties of WSi2-added CrSi2 composite

CrSi2-based composite materials with submicrometer-sized grains were fabricated by the powder metallurgical process using ball-milling and pulse-current sintering. By the solid-state reaction of submicrometer-sized fine powders of CrSi2 and WSi2 during the sintering, the composites consisting of Cr-rich hexagonal (Cr,W)Si2 phase and W-rich tetragonal (W,Cr)Si2 phase were obtained. The grain size of CrSi2 phase was effectively reduced by the WSi2 addition because of the suppression of grain growth during sintering. Thermal conductivity was then decreased with the WSi2 content by the enhancement of phonon scattering resulting from the reduction of grain size, the introduction of the CrSi2/WSi2 interface and the crystal lattice distortion induced by the W substitution for CrSi2 phase. Seebeck coefficient and electrical resistivity were also decreased due to the addition of metallic WSi2 phase and the deformation of CrSi2 electronic structure resulting from the W substitution. The WSi2 addition had then little effect on the thermoelectric power factor. Consequently, the thermoelectric figure-of-merit, ZT, was enhanced by the reduction of thermal conductivity and reached 0.3 around 700 K.

Synthesis of Niti Based Nanocomposites Reinforced by Ha Addition

Abstract NiTi alloy is well known for its unique properties, such as good ductility at room temperature, good corrosion resistance and also thermal shape memory effects. On the other hand hydroxyapatite has a combination of desirable properties, such as low density and excellent compatibility with the bone which used as ceramic reinforced phases can change the properties and thermal stability of the NiTi alloy. In this study, the NiTi alloy matrix shape memory composite reinforced by hydroxyapatite particles was successfully fabricated using mechanical alloying and powder metallurgical process. The structural evaluation of milled and heat treated powders was studied by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The differential scanning calorimetry was used to measure the phase transformation temperatures. The porosity, Vickers’ hardness and corrosion resistance of the TiNi-HA composites were investigated. The results show that the increase of the weight ratio of hydroxyapatite causes increase the porosity and decrease the corrosion resistance. The fabricated NiTi alloy matrix composite possesses lower density and higher Vickers’ hardness as the pure NiTi shape memory alloy, yet still exhibiting the shape memory effect.

Sintering and properties of MIM M2 high speed steel produced by prealloy and master alloy routes

The advantages of powder metallurgical processing of tool steels have been recognized for many years with water atomized [1] powder being used in press & sinter valve seat parts for many years and gas atomized tool steel powders consolidated by HIP used in high performance applications [2] . While coarse HIP powder grades dominate high volume tool steel markets, the advent of fine powder gas atomization has enabled the use of MIM for the production of an increasing number of industrial components where hardness and wear resistance is required. Examples include hand- and power-tool components, pump impellers and textile machinery. Tool steels typically have carbon content in the range 0.5–1.5% and heat treatment cycles are carefully controlled to achieve the optimum distribution of matrix carbides to deliver required wear performance.

Apatite Wollastonite-Reinforced Hydroxyapatite Biocomposites

The paper is focused on the reinforcement of hydroxyapatite (HA) by apatite wollastonite (AW) because of its aptitude for combining biocompability with mechanical properties superior to those of the bone. HA powders were produced by thermal extraction technique involving calcination from natural source. HA-AWP biocomposites containing from 0 to 20 wt.% of AW in particulate form were prepared starting with wet ball milling during 24 h, followed by powder metallurgical processes using HA powders calcined at 800 ◦C, followed by compaction by cold pressing and by cold isostatical pressing at 250 MPa and subsequent sintering at 1100–1300 ◦C. The phases and compositions in the resulting products were identified by XRD and XRF. Reinforcement particle distribution were investigated on polished surfaces of the sintered samples by SEM analysis and by optical microscopy. Mechanical properties of the sintered samples was evaluated via Micro Vickers hardness testing.