Conventional Powder Metallurgy Techniques Research Articles

Designing, processing and characterisation of titanium cylinders with graded porosity: An alternative to stress-shielding solutions

Bone resorption events and consequent failure of titanium implants are frequently related to stress-shielding problems, due to stiffness mismatch with respect to bone. This is a mechanical incompatibility problem, which is difficult to resolve because of the challenge of replacing highly anisotropic biomechanical systems, as is the case of dental implants. This work describes the designing, processing and characterisation of cylindrical titanium samples with a longitudinally graded porosity obtained by conventional powder-metallurgy techniques. The design concept used was biomimetic, based on the stiffness properties of the tissues to be in contact with titanium dental implants. Processing conditions were optimised in terms of different parameters: structural integrity, porosity and mechanical properties. The influence of sintering temperature was evaluated in search of optimum results under the above criteria. The behaviour of longitudinal porosity and Young’s modulus were consistent with the preliminary design concept from the original biomechanical system. Mechanical strength results were reasonably suitable for dental applications and they were favourably sensitive to increasing sintering temperature, due to a stronger adhesion between initial green layers of cylindrical samples. Results showed that it is possible to obtain a desired longitudinal gradient in Young’s modulus, as well as suitable yield strength values. The optimised processing described suggests that it is a plausible candidate for manufacturing dental implants with a good balance between reduced stress shielding and suitable mechanical strength, which encourages us to undertake further work along the same lines.

Innovative Metal Injection Molding (MIM) Method for Producing CoCrMo Alloy Metallic Prosthesis for Orthopedic Applications

Powder injection molding (PIM) is a powder metallurgy process currently used for the production of complicated and near net shape parts of high performance materials [. This technique basically combines the advantages of plastic injection molding and the versatility of the conventional powder metallurgy technique. The process overcomes the shape limitation of powder compaction, the cost of machining, the productivity limits of isostatic pressing and slip casting, and the defect and tolerance limitations of conventional casting [1, 2, . According to German and Bose [, the technology of metal injection molding (MIM) is more complicated than that of the plastic injection molding, which arises from the need to remove the binder and to densify and strengthen the part. The process composed of four sequential steps: mixing of the powder and organic binder, injection molding, debinding where all binders are removed and sintering [1, 2, 3, 4]. If it necessary, secondary operations such as heat treatments after sintering can be performed [1, 2, 3, 4, .

Colloidal processing of Fe-based metalceramic composites with high content of ceramic reinforcement

Major difficulties of processing metal-matrix composites by means of conventional powder metallurgy techniques are the lack of dispersion of the phases within the final microstructure. In this work, processing through colloidal techniques of the Fe-based metal-matrix composites, with a high content of a ceramic reinforcement (Ti(C,N) ), is presented for the first time in the literature. The colloidal approach allows a higher control of the powders packing and a better homogenization of phases since powders are mixed in a liquid medium. The chemical stability of Fe in aqueous medium determines the dispersion conditions of the mixture. The Fe slurries were formulated by optimising their zeta potential and their rheology, in order to shape bulk pieces by slip-casting. Preliminary results demonstrate the viability of this procedure, also opening new paths to the microstructural design of fully sintered Fe-based hard metal, with 50 vol. % of Ti(C,N) in its composition.

Thermal Expansion Behavior of the Electroless Copper Coated Cu-SiC<sub>p</sub> Composites Fabricated via the Conventional Powder Metallurgical Technique

The introduction of the metal matrix composites as the advanced electronic packaging materials is highly anticipated because their thermal properties can be engineered to match those of semiconductors, ceramics substrates and optical fibers. Among these advanced packaging materials, silicon carbide particles reinforced copper matrix (Cu-SiCp) composites are highly rated due to the high thermal conductivity of copper and low coefficient of thermal expansion (CTE) of silicon carbide. However, the Cu-SiCp composites fabricated via the conventional powder metallurgy (PM) technique usually have immature thermophysical properties due to the weak bonding between the copper matrix and the SiCp reinforcement. In order to improve the bonding between the two constituents, the SiCp were coated with copper via electroless coating process prior to PM fabrication processes. Based on the experimental results, The CTE and porosity of the Cu-SiCp composites were significantly affected by the volume fraction of SiCp. Furthermore, the CTE and porosity of the Cu-Coated Cu-SiCp composites were significantly lower than the non-Coated Cu-SiCp composites. These differences were mainly contributed by the nature of the bonding between the copper matrix and SiCp reinforcement.

Effects of alignment on the magnetic and mechanical properties of sintered Nd–Fe–B magnets

Sintered Nd–Fe–B magnets with various degrees of grain alignment are prepared by the conventional powder metallurgy technique. The magnetic properties, bending strength and fracture morphologies of sintered magnets with different alignment degrees are investigated. The results show that the remanence Br and maximum energy product (BH)max increase, while intrinsic coercivity Hcj decreases gradually with increasing the alignment degree. Moreover, the bending strengths measured by applying force parallel (σbb∥) and perpendicular (σbb⊥) to the alignment magnetic field Happl present opposite variation tendency, and σbb∥ is larger than σbb⊥, suggesting the more significant anisotropy of mechanical properties with increasing the degree of grain alignment. Furthermore, the related mechanisms are systematically analyzed.

High temperature transformations in a steel-TiCN cermet

The influence of the carbon content on the microstructure, phase transformation and hardness of an iron-based cermet is studied. The cermet is constituted by a high-alloyed steel as matrix, and TiCN particles (50vol.%) as reinforcement. The material is produced by conventional powder metallurgy techniques, that is, uniaxial pressing and sintering, and the carbon content is varied from 0wt.% to 1wt.%. The aim of the research is the understanding of the transformations undergone by the material with increasing C amounts when temperature is increased. For this purpose, the cermet is studied by mechanical spectroscopy (MS) and differential thermal analysis (DTA) and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and hardness measurements. The equilibrium phase diagram calculated by ThermoCalc software contributes to explain the differences found on phase transformations with respect to the C content of the cermet.

Effect of WC on the microstructure and mechanical properties of nano Ti(C,N)-based cermets

In this paper, nano Ti(C,N)-based cermets with different WC additions were prepared by conventional powder metallurgy techniques. The microstructure of nano cermets was investigated by scanning electron microscope (SEM). The results showed that there existed three kinds of “core–rim” structure in nano cermets, i.e., “black core–white rim”, “white core–gray rim” and “gray core–white rim” structures. With the increase of WC addition, there were more “white core–gray rim” and “gray core–white rim” grains. The amount of the abnormal growth grains decreased and the grains became more homogeneous with the WC addition. The lattice parameter of Ti(C,N) would increase with the WC addition. The mechanical properties of nano cermets were also improved with the increase of WC addition.

Effect of h-BN Content on the Sintering of SS316L/h-BN Composites

In this work, the three compositions of hexagonal boron nitride (10, 15 and 20 vol. %)-embedded 316L stainless steel (SS316L/h-BN) composites were prepared by a conventional powder metallurgy technique and then sintered at varying temperatures of 1100 to 1250°C for 60 min in H2 atmosphere. The h-BN content and sintering temperature were found to affect the microstructure and hardness of the composites. The hardness decreased with increasing h-BN content and was improved by increasing the sintering temperature. Microstructure results revealed that the boride phase was formed at the grain boundary at the sintering temperature higher than 1150°C and the boride phase formation was observed to transform the h-BN in the composites.

Effect of Ceramic Powder Size on Microstructure and Mechanical Properties of Ti(C, N)-Based Cermets

Four Ti(C, N)-based cermets was fabricated from different size of ceramic powder by the conventional powder metallurgy technique. Microstructure, fracture surfaces and crack propagation path of samples were observed by SEM, and mechanical properties such as transverse rupture strength, hardness and fracture toughness were measured. Results show that with the refinement of the raw TiC/TiN powders, the black cores and ceramic grains tend to be finer, the gains with white core-grey rim structure obviously increase, and the microstructure of cermet becomes inhomogeneous. The cermets fabricated from fine powders possess higher porosities. Compared with the cermets made from micron ceramic powders, the cermets fabricated from ceramic powders containing nano powder possess higher transverse rupture strength and lower fracture toughness. When the cermet is fabricated from micron TiC/nano TiN powders, its combination property is the best.

Effect of Nano-Carbides on the Microstructure and Mechanical Properties of Ti(C,N)-Based Cermets

In this paper, Ti(C,N)-Based cermets with addition of nano-carbides were fabricated by conventional powder metallurgy techniques. Then the mechanical properties, microstructure and corresponding formation mechanism have been investigated. According to the results, the microstructures of sintered cermets are composed of three kinds of hard grains: one with black core/grey shell, one with white core , and the other with no core. In addition, with more nano-carbides adding, the number of white core grains increases and that of black core grains decreases. It is also found that sintered cermets with nano-carbide additions exhibit higher transverse rupture strength and higher hardness than the others originated from much finer and more homogeneous microstructure. However, cermets with nano-micro carbide additions show the most poor mechanical properties among all sintered cermets, including conventional cermets, due to the obviously heterogeneous microstructure. And this can be explained by Ostwald ripening theory.

Improved mechanical performance of solution-processed MWCNT/Ag nanoparticle composite films with oxygen-pressure-controlled annealing

A method for the synthesis of solution process-based MWCNT/Ag nanoparticle composite thin films as electrode or interconnect materials for flexible electronic devices is presented. The method produces homogeneously-dispersed CNT networks and increases the density of the Ag matrix, which are major factors in determining the mechanical performance of CNT/Ag films. By introducing nanometer-sized Ag particles as a matrix material, the agglomeration of CNTs is suppressed. In addition, the generation of pores during the synthesis procedure is effectively restrained by oxygen-pressure-controlled annealing. The elastic modulus of the pristine Ag films was observed to increase by 34% by adding 5 wt% CNTs. An improvement in the fatigue resistance of the CNTs under cyclic tensile deformation was confirmed. The normalized resistance change (( R − R o)/ R o) of the Ag films containing 5 wt% CNTs after fatigue testing was reduced by about 27% compared to that of the pristine Ag films. For industrial application the process has the advantage of relatively low-temperature processing without any high pressure compaction compared to the conventional powder metallurgy techniques normally used.

Conventional Powder Metallurgy Process and Characterization of Porous Titanium for Biomedical Applications

Commercially pure titanium (c.p. Ti) is one of the best metallic biomaterials for bone tissue replacement. However, one of its main drawbacks, which compromises the service reliability of the implants, is the stress-shielding phenomenon (Young’s modulus mismatch with respect to that one of the bone). Several previous works attempted to solve this problem. One alternative to solve that problem has been the development of biocomposites implants and, more recently, the fabrication of titanium porous implants. In this work, porous samples of c.p. Ti grade 4 were obtained using conventional powder metallurgy technique. The influence of the processing parameters (compacting pressure and sintering temperature) on the microstructure features (size, type, morphology, and percentage of porosity), as well as on the mechanical properties (compressive yield strength, and conventional and dynamic Young’s modulus) were investigated. The results indicated that there is an increment in density, roundness of pores, and mean free path between them as compacting pressure and/or sintering temperature is increased. The Young’s modulus (conventional and dynamic) and yield strength showed the same behavior. Better stiffness results, in the central part of cylindrical samples, were obtained for a uniaxial compression of 38.5 MPa using a sintering temperature of 1273 K and 1373 K (1000 °C and 1100 °C). An evaluation of porosity and Young’s modulus along a cylindrical sample divided in three parts showed that is possible to obtain a titanium sample with graded porosity that could be used to design implants. This approach opens a new alternative to solve the bone resorption problems associated with the stress-shielding phenomenon.

Research on Process and Microstructure Formation of W-Ni-Fe Alloy Fabricated by Selective Laser Melting

W-Ni-Fe alloys are important materials for many practical applications; however, at present, they are usually fabricated by conventional powder metallurgy techniques, which is difficult in fabrication with complex shapes. In this work, a selective laser melting (SLM) processing method was developed for fabricating W-Ni-Fe alloys parts. A process map was obtained for selection of proper laser parameters by optimizing processing conditions. Microstructures of laser fabricated samples in different laser energy inputs were investigated. There are two coexisting forming mechanisms in the SLM process: (i) liquid phase sintering (LPS) with full melting of Ni and Fe powders but non-melting of W powders and (ii) melting/solidification with full melting of W powders. Moreover, with increasing laser energy, a transition trend of forming mechanism from LPS to full melting/solidification can be observed.

Influence of Yb 2O 3 addition on microstructure and corrosion resistance of 10Cu/(10NiO-NiFe 2O 4) cermets

10Cu/(10NiO-NiFe 2O 4) cermets doped with Yb 2O 3 were prepared by conventional powder metallurgy technique. The effects of Yb 2O 3 content and sintering temperature on the relative density, phase composition, microstructure of the sintered cermets and the corrosion resistance to Na 3AlF 6-Al 2O 3 melts were investigated by sintered density test, XRD analysis and SEM. YbFeO 3 phase, which distributes in the ceramics grain boundary as particles or film, is produced by the reaction between Yb 2O 3 and ceramics. The addition of Yb 2O 3 accelerates the sintering process of ceramics matrix, eliminates pores in the boundary and results in coarsened crystalline grain. The relative density of the cermets with about 1% (mass fraction) Yb 2O 3 sintered at 1275 °C increases to above 95%. Addition of about 1.0% Yb 2O 3 can inhibit obviously the corrosion of NiFe 2O 4 grain boundary and Cu phase in Na 3AlF 6-Al 2O 3 melts.

Selective laser melting W–10 wt.% Cu composite powders

Tungsten–Copper (W–Cu) alloys are promising materials for electrical and thermal applications. However, its forming method still remains limited in conventional powder metallurgy technique which is not suitable for manufacturing parts with intricate shapes. In this work, selective laser melting technology was introduced for fabricating W–10 wt.% Cu alloys parts. The morphological feature of a single molten track was analyzed. The results show that liquid phase sintering with complete melting of the binder (Cu), but nonmelting of the structural metal (W), acts as the main mechanism in this process. The melting conditions of single layers in different processing parameters were investigated. The results show that a moderate melting zone can be acquired from an established process map. Moreover, investigations on multilayers forming show that the final density increases with the decrease of scan speed until it reaches a plateau due to the insufficient rearrangement in liquid phase sintering and the balling effect.

Cold compaction of metal–ceramic powders in the preparation of copper base hybrid materials

The compressibility behavior of Cu–Al2O3 powders in the preparation of composite compacts was studied. The effect of ceramic content on densification during compaction and resulting microstructure was evaluated through two methods, a conventional admixture powder metallurgy technique, and a modified chemical deposition and powder metallurgy route. Experiments were performed using plain and electroless copper coated alumina particles with a medium particle size of 58.3μm; two sizes of copper powders, 17.6 and 60μm, were used to adjust the ceramic content to 30, 40, 50 and 60vol.%. The pressure–density response showed a decrease of porosity with increasing metal content but no effect with copper particle size. Densification was slightly higher when using coated fillers; the surface treatment also eliminated interparticle contacts and reduced the amount of voids providing clean interfaces.

Effect of molybdenum content on the microstructure and mechanical properties of ultra-fine Ti(C, N) based cermets

Effect of molybdenum as a sintering aid on the microstructure and mechanical properties of Ti(C, N)–20WC–15Co–Mo–2.5C system cermets was studied in this work. Ti(C, N)-based cermets with variable Mo content were fabricated by conventional powder metallurgy techniques. The microstructure was investigated by means of X-ray diffractometer, scanning electron microscopy in combination with an energy dispersive spectrometer. Mechanical properties such as transverse rupture strength, fracture toughness and hardness were also measured, respectively. Results revealed that finer grains, as well as larger amount of bright core/grey shell structures could be obtained by increasing the molybdenum content. Transverse rupture strength and hardness were also improved. Fracture toughness, however, was decreased with the increase of Mo content. The optimized content of Mo was chosen as 15 wt.% for Ti(C, N)–20WC–15Co–Mo–2.5C system cermets.

Fabrication of nickel aluminide intermetallic-alumina nanocomposite via reaction synthesis

Nickel aluminide–alumina nanocomposite was prepared by a conventional powder metallurgy technique. Nanosize alumina as dispersed phase was mixed with Ni and Al powder in a planetary ball mill for 4 h at 175 rpm. The mixture was then compacted using a hydraulic press at 400 MPa for 15 min. Sintering was done under inert condition in a tube furnace at 660 °C with 5 h soaking time. The occurrence of reaction synthesis during sintering was detected by the presence of a ‘large’ exothermic peak of the differential thermal analysis curve, which occurred below 600 °C. The hardness value for the composite containing 5% alumina was about two times higher than Ni 3 Al intermetallic. Its saturation magnetization ( M s ) was very low indicating the presence of a small amount of elemental Ni. X-ray diffraction (XRD) measurements showed peaks corresponding to Ni–Al and Ni 3 Al indicating the formation of the desired intermetallics.

Carbon nanotube-reinforced aluminium strips

Carbon nanotubes (CNTs) have recently emerged as materials with outstanding properties. Researchers have investigated their use as reinforcements in – mainly – polymer, and ceramic matrices. Due to the anticipated fabrication difficulties, a few research groups have explored their use to reinforce metal matrices. Recently, conventional powder metallurgy techniques (compaction and sintering) were used with some success. In this paper, a powder can rolling technique is used to fabricate carbon nanotube-reinforced aluminium strips. The Al–CNT mixtures are blended in either a mixer-shaker at a rotary speed of 46 rpm, or under argon in a planetary mill at a rotary speed of 300 rpm, prior to rolling. The dispersion of the nanotubes is shown to be better under the higher energy planetary action. The strength of the rolled strips is evaluated for various wt% CNT samples. The Al–0.5 wt% composite strips exhibited enhanced mechanical properties. The CNT-reinforced aluminium strips can have numerous attractive applications in the aerospace, automotive and electronics industries.

Effects of TiC/TiN addition on the microstructure and mechanical properties of ultra-fine grade Ti (C, N)–Ni cermets

Two series of Ti (C, N)-based cermets, one with TiC addition and the other with TiN addition, were fabricated by conventional powder metallurgy technique. The initial powder particle size of the main hard phase components (Ti (C, N), TiC and TiN) was nano/submicron-sized, in order to achieve an ultra-fine grade final microstructure. The TiC and TiN addition can improve the mechanical properties of Ti (C, N)-based cermets to some degree. Ultra-fine grade Ti (C, N)-based cermets present a typical core/rim (black core and grayish rim) as well as a new kind of bright core and grayish rim structure. The average metallic constituent of this bright core is determined to be 62 at% Ti, 25 at% Mo, and 13 at% W by SEM–EDX. The bright core structure is believed to be formed during the solid state sintering stage, as extremely small Ti (C, N)/TiC/TiN particles are completely consumed by surrounding large WC and Mo 2C particles. Low carbon activity in the binder phase will result in the formation (Ni 2Mo 2W)C x intermetallic phase, and the presence of this phase plays a very important role in determining the mechanical properties of TiN addition cermets.