Powder Metallurgy Approach Research Articles

Fabrication of Mg-Al alloy foam with close-cell structure by powder metallurgy approach and its mechanical properties

A powder metallurgy (PM) approach was developed to fabricate Mg-(15–50)at.% Al foams with closed-cells, where CaCO3 is selected as the blowing agent. The key point of successful foaming is Al addition and the proper sintering treatment, which lead to the formation of intermetallic compounds and can make the gas release reaction between Mg melt and CaCO3 happen during the foaming process. Besides, the high precursor compact degree, proper foaming temperature (∼620°C) and foaming time (∼150s) are required to fabricate Mg-Al alloy foam with good cellular structure. The compressive stress–strain curves of Mg-Al alloy foams are serrated during the plateau region due to the intermetallics in the cell walls, and the less Al addition, the less intermetallics formed in the cell walls, the higher yield strength of the Mg-Al alloy foams.

Porous TiAl alloys fabricated by sintering of TiH2 and Al powder mixtures

Porous TiAl alloys with different nominal powder compositions were fabricated through a cost-effective powder metallurgy (PM) approach by sintering TiH2 and Al mixed powders. It is found that the pore properties vary with the TiH2 and Al contents, i.e., the nature of the pores can be improved by using different compositions. Furthermore, by detailed structural characterizations, four crystalline phases, i.e., α2-Ti3Al, γ-TiAl, TiAl3, and Ti2Al5, were observed in the fabricated porous TiAl alloys with different compositions. An optimal composition of TiH2-50wt% Al is preferred since its maximum open porosity is about 70% and its viscous permeability coefficient for nitrogen is 1.11 μm2.

Microwave-Assisted Preparation of High Entropy Alloys

Microwaves at the ISM (Industrial, Scientific and Medical, reserved internationally) frequency of 2450 or 5800 MHz have been used to prepare FeCoNiCuAl, FeCrNiTiAl and FeCoCrNiAl2.5 high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave-assisted near-net-shape technology, using a powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). High entropy alloy compositions have been selected so as to comprise at least one ferromagnetic element and one highly reactive couple, like Ni-Al, Ti-Al, Co-Al or Fe-Al. Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the load. Microwaves have been applied both for the ignition and sustaining of such reactions, showing that by the proposed technique, it is possible to control the cooling rate of the newly-synthesized high entropy alloys. Results showed also that microwave heating in predominant magnetic field regions of the microwave applicator is more effective at controlling the cooling rate. The herein proposed microwave-assisted powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the prepared high entropy alloys in all cases was good, without the dendritic segregation typical of arc melting, even if some partially-unreacted powders were detected in the samples.

Growth kinetics of intermetallic layer in lead-free Sn–5Sb solder reinforced with multi-walled carbon nanotubes

In this study, the effects of multi-walled carbon nanotubes reinforcement on the thickness and growth kinetics of interfacial intermetallic compound (IMC) layer in Sn–5Sb solder system are investigated. In the typical study, plain Sn–5Sb solder system and carbon nanotubes reinforced solder systems (Sn–5Sb−xCNT; x = 0.01, 0.05 and 0.1 wt%) were developed via the powder metallurgy approach. The solder/Cu joints subjected to the as-reflow and isothermal aging (170 °C) conditions were systematically characterised to evaluate the growth of the IMC layer. Results suggest that the composite solders showed better results for all subjected conditions compared with the plain solder. In particular, 0.05 wt % CNTs reinforced solder system exhibited the most appreciable IMC layer suppression and diffusion coefficient among all the samples analysed in this study.

A Comparative Study on the Sintering and Casting of a Blended Elemental Ti-6Al-4V Alloy

Over the past years, the blended elemental powder metallurgy (PM) approach has been identified as one of the most promising strategies to reduce the cost of titanium-based components. However, oxygen pick-up, inhomogeneity of the microstructure and chemical composition are sometimes reported for PM parts. This work compares properties of a blended elemental Ti-6Al-4V alloy obtained by sintering under argon gas atmosphere with those of a vacuum cast alloy. Argon was purified by passing it through a series of oxygen and moisture traps prior to being introduced into the sintering furnace. Casting was performed under vacuum (1 x 10-3mbar). The starting material in both processes was the cold isostaticaly pressed blended elemental (BE) Ti-6Al-4V powder compact. The BE powder was prepared by mixing 60Al-40V master alloy powder with commercial Grade 4 titanium powder (0.377 wt.% O2). The sintered and cast alloys were compared on the basis of oxygen pick-up, density, microstructure, chemical composition and hardness to determine which method is better. Although the BE approach could not eliminate the common challenges associated with powder metallurgy processing of Ti alloys, oxygen pick-up and additional contamination was lower compared vacuum casting. Sintering at 1350°C for 1 h could not achieve full density compared to casting, but the microstructure appeared more homogeneous. Both sintered and cast Ti6Al4V alloys were harder than wrought Ti6Al4V due to a high concentration of interstitial oxygen. The sinterered and sintered plus HIPed Ti6Al4V alloys were softer than as-cast Ti6Al4V due to lower oxygen pick-up and incomplete densification. From the contamination and homogeneity perspective, the BE approach is an attractive technique for processing of Ti6Al4V alloy.

Investigating the effect of isothermal aging on the morphology and shear strength of Sn-5Sb solder reinforced with carbon nanotubes

An analysis of the role played by the addition of carbon nanotubes (CNTs) to the solder matrix of conventional Sn-5Sb lead-free solder was performed. In a bid to determine the potential of this new solder system, the powder metallurgy approach was used to synthesise a plain Sn-5Sb solder system and CNTs reinforced composite solder formulations of Sn-5Sb-xCNT; x = 0.01wt.%, 0.05wt.% and 0.1wt.%. Isothermal aging study was conducted on the solder joints, to examine the evolution of the interfacial intermetallic compound (IMC) layer between solder and the adjoining copper (Cu) substrate. Similarly, shear strength analysis was performed on as-reflow and aged solder joints. A considerable improvement in the wetting properties, the microstructural evolution, and the interfacial intermetallic compound (IMC) layer growth was observed in the composite solder joints. Owing to the excellent mechanical properties of CNTs, the shear strength assessment revealed that the composite solder joints gave a superior shear strength property, especially the Sn-5Sb-0.01CNT solder joint sample.

The Development of High Performance Ti-6Al-4V Alloy via a Unique Microstructural Design with Bimodal Grain Size Distribution

The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique microstructure with bimodal grain size distribution, termed as “harmonic structure.” The Ti-6Al-4V compacts with harmonic structure design were successfully prepared via a powder metallurgy approach consisting of controlled mechanical milling and spark plasma sintering of the pre-alloyed Ti-6Al-4V powders. The microstructural evolution at each stage of processing has been investigated to establish a correlation between the processing conditions and the microstructural evolution. The Ti-6Al-4V compacts with heterogeneous harmonic structure exhibited better mechanical properties as compared to their homogeneous fine/coarse-grained counterparts. An attempt has also been made to explain the deformation mechanism of the harmonic-structured Ti-6Al-4V specimens with the help of the experimental evidences. The superior mechanical properties of the harmonic structure Ti-6Al-4V were found to be related to the peculiar topological distribution of strong fine-grained and ductile coarse-grained regions, which promotes uniform distribution of strain during plastic deformation and results in improved mechanical properties by avoiding the localized plastic deformation in the early stages of deformation.

Microstructure and mechanical properties of Ti–22Al–25Nb alloy fabricated by vacuum hot pressing sintering

Abstract A study has been undertaken to verify the feasibility of using a powder metallurgy (P/M) approach to fabricate Ti–22Al–25Nb alloys. Pre-alloyed powders with a nominal composition of Ti–22Al–25Nb (at%) obtained by argon atomization were sieved to the spherical size less than 180 μm and used for the fabrication of P/M Ti–22Al–25Nb alloys via hot pressing in vacuum. Vacuum hot pressing sintering was carried out in a temperature range of 950–1200 °C with a pressure of 35 MPa for 1 h followed by furnace cooling. Microstructure and phase composition examinations of the as-atomized powders and hot pressed (HP׳ed) samples were conducted by applying optical microscopy, back-scatter electron imaging and X-ray diffraction analysis. Tensile tests were studied at room temperature and 650 °C, respectively. The results showed that all HP׳ed samples were composed of coarse equiaxed B2 grains, fine lamellar O phase inside the B2 grains, and some α2 along B2 grain boundaries. The elongations of HP׳ed samples were less than 3.95%, indicating the bad ductility at room temperature. However, the elongations were improved as the tensile temperature increased to 650 °C. The sample sintered at 1050 °C exhibited a better ductility with the elongation of 7.97% at 650 °C than that of other samples.

Investigating aluminum alloy reinforced by graphene nanoflakes

As one of the most important engineering materials, aluminum alloys have been widely applied in many fields. However, the requirement of enhancing their mechanical properties without sacrificing the ductility is always a challenge in the development of aluminum alloys. Thanks to the excellent physical and mechanical properties, graphene nanoflakes (GNFs) have been applied as promising reinforcing elements in various engineering materials, including polymers and ceramics. However, the investigation of GNFs as reinforcement phase in metals or alloys, especially in aluminum alloys, is still very limited. In this study, the aluminum alloy reinforced by GNFs was successfully prepared via powder metallurgy approach. The GNFs were mixed with aluminum alloy powders through ball milling and followed by hot isostatic pressing. The green body was then hot extruded to obtain the final GNFs reinforced aluminum alloy nanocomposite. The scanning electron microscopy and transmission electron microscope analysis show that GNFs were well dispersed in the aluminum alloy matrix and no chemical reactions were observed at the interfaces between the GNFs and aluminum alloy matrix. The mechanical properties׳ testing results show that with increasing filling content of GNFs, both tensile and yield strengths were remarkably increased without losing the ductility performance. These results not only provided a pathway to achieve the goal of preparing high strength aluminum alloys with excellent ductilitybut they also shed light on the development of other metal alloys reinforced by GNFs.

Flake thickness effect of Al2O3/Al biomimetic nanolaminated composites fabricated by flake powder metallurgy

Al2O3/Al biomimetic nanolaminated composites were fabricated by means of a flake powder metallurgy approach, in which pure Al flake powders with native Al2O3 nano-skins were assembled into a biomimetic nanolaminated structure. The tensile properties of the composites were investigated experimentally and numerically, and a flake thickness effect was revealed. It was found that the tensile strength increased monotonously with decreasing flake Al powder thickness (FAPT) and reached a maximum of ~435MPa at a FAPT of ~180nm. At a FAPT of ~500nm, a well-balanced strength and ductility was achieved, resulting in a maximum work-of-fracture. Both the decrease in grain size of Al matrix and the increase in volume fraction of Al2O3 contribute to the increase in tensile strength caused by decreasing FAPT. At FAPTs greater than 500nm, the former is dominant; at FAPTs less than 500nm, the latter is dominant. For Al2O3/Al nanolaminated composites, a FAPT of ~500nm is suggested for maximum toughness and smaller for higher tensile strength.

High-gravity combustion synthesis and in situ melt infiltration: A new method for preparing cemented carbides

A new method of high-gravity combustion synthesis and in situ melt infiltration is reported for preparing cemented carbides, where hot nickel melt is in situ synthesized from a highly exothermic combustion reaction and then infiltrated into tungsten carbide powder compacts. The as-prepared sample showed a homogeneous microstructure, and its relative density, hardness and flexural strength were 94.4%, 84 HRA and 1.49 GPa, respectively. Compared with conventional powder metallurgy approaches, high-gravity combustion synthesis offers a fast and furnace-free way to produce cemented carbides.

Flexural properties, thermal conductivity and electrical resistivity of prealloyed and master alloy addition powder metallurgy Ti–6Al–4V

A comparison between the properties achievable by processing the Ti–6Al–4V alloys by means of two powder metallurgy approaches, precisely prealloyed and master alloy addition, was carried out. Prealloyed and master alloy addition hydride–dehydride powders characterised by an irregular morphology were shaped by means of cold uniaxial pressing and high vacuum sintered considering the effect of the variation of the sintering temperature and of the dwell time. Generally, the higher the temperature and the longer the dwell time, the higher the relative density and, consequently, the better the mechanical performances. Nevertheless, a higher processing temperature or a longer time leads also to some interstitials pick-up, especially oxygen, which affects the mechanical behaviour and, in particular, lowers the ductility. Although some residual porosity is left by the pressing and sintering route, mechanical properties, thermal conductivity and electrical resistivity values comparable to those of the wrought alloy are obtained.

In situ formation of Ti alloy/TiC porous composites by rapid microwave sintering of Ti6Al4V/MWCNTs powder

The rapid in situ formation of a Ti alloy/TiC porous composite was achieved using the powder metallurgy approach. Ti6Al4V powders were mixed with 14.5vol.% of multiwalled carbon nanotubes (MWCNTs) and the particle size of Ti6Al4V was reduced to ∼5μm by ball milling. The MWCNTs acted as microwave susceptors as well as reactants for the rapid sintering of Ti alloys. The green compact of the Ti6Al4V/MWCNTs powder was sintered in a 1.4kW, 2.45GHz bi-directional microwave emission furnace without using any protective gas environment. The microwave sintering was completed within 2min. The surface temperature of the sample reached 1620°C during sintering, as measured by a pyrometer. Titanium carbide was formed after the sintering as confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The composite has a porosity of approximately 25%, compressive strength of 270.41±24.97MPa, compressive yield stress of 145.48±27.28MPa, compressive Young’s modulus of 10.87±2.46GPa and compressive strain of 3.75±1.11%. The Vickers hardness value in the TiC region (545.4±13.9HV) is significant higher than that in the Ti alloy region (435.9±12.9HV). An overall Rockwell hardness of 47HRA was measured.

Alloy development for highly conductive thermal management materials using copper-diamond composites fabricated by field assisted sintering technology

Improving thermal management materials is critical to many industries including the microelectronics where smaller components lead to larger heat densities. Performance of thermal management materials are largely governed by the thermal conductivity of the material. Commercial diamond particles have a very high thermal conductivity, but their inclusion into conductive metals such as copper is challenging due to a lack of compatibility which creates a poor interface between materials. This study uses a powder metallurgy approach employing an alloyed powder blend to act as an agent to improve the interface leading to improved thermal properties. Results show that minor additions of zirconium to the copper matrix is successful in improving the interface and allows for improvement of thermal conductivity up to a maximum of 533 W/m K with 40 vol.% diamond. Energy-dispersive X-ray spectroscopy mapping analysis shows that additions of zirconium are vastly better than chromium for improvement of interface. It is shown that caution must be taken when adding alloying elements because small additions result in a rapid decrease in conductivity of the alloy before diamonds are added.

Processing and electrochemical characterization of Ni2Si intermetallic compound produced by vacuum sintering

In this study, a powder metallurgy approach for fabrication of Ni2Si intermetallic compound was utilized. In this regard, mechanically activated Ni-33 at.% Si powders were used as feedstock. The powders were investigated by differential scanning calorimetry to study phase transformations that occurred during heating in the calorimeter. It was shown that the milled powders reacted after heating up to 850 °C. In order to fabricate bulk Ni-33 at.% Si, the as-milled powders were cold-pressed and subsequently sintered at 900 °C for 1 h in a vacuum furnace. X-ray diffraction results indicated that the phase composition of sintered materials consisted of only δ-Ni2Si intermetallic. Consolidated samples exhibit 12% porosity and microhardness up to 773 HV0.05. Further investigations showed the corrosion rate of sintered compound is higher than that of Ni–Si alloys reported in the literature. Here, anodic dissolution near the surface pores seems to be corrosion mechanism which resulted in increase in surface porosity up to 27% after corrosion.

Nanostructured bainitic steel obtained by powder metallurgy approach: structure, transformation kinetics and mechanical properties

A PM approach is investigated in the context of extremely fine low temperature bainitic steels with a reasonable combination of ultimate tensile strength and uniform elongation. The strategy consists of three steps. First, ball milling is used for mechanical alloying and to create a large density of interfaces prior to sintering. In the second step, spark plasma sintering is used as a rapid consolidation process to retain a very fine and homogeneous structure after sintering. The isothermal heat treatment which leads to bainite can then be done at a low austenitisation temperature (860°C) for a few minutes, leading to a small initial austenite grain size. A fine grain size promotes the rapid transformation of the austenite into bainite. The characteristics of a nanostructured Fe–Ni–C bainitic steel obtained after mechanical alloying and nanostructuring of the powder, spark plasma sintering and final heat treatment are reported.

Role of Surface Contamination in Titanium PM

The powder metallurgy (PM) approach is widely used for cost-effective production of titanium alloys and articles. In the PM approach the large specific surface of starting powders heightens the risk of excessive impurity presence and, hence, degradation of final alloy properties. The present study analyzes the opportunity to produce sintered commercially pure titanium (CP-Ti) with acceptable impurity content from powder materials. Starting titanium and titanium hydride powders were comparatively examined. The impurity elements (oxygen, chlorine, carbon) and their conditions on the powder particle surface, as well as the surface processes and gases emitted from powders upon heating, have been analyzed by means of surface science techniques. The role of hydrogen emitted from titanium hydride in material purification has been discussed. The opportunity to produce titanium materials with final admissible content of interstitials (O, C, Cl, and H) using starting titanium hydride powder has been demonstrated.

Fabrication of aluminium foam through pressure assisted high frequency induction heated sintering dissolution process: an experimental observation

A process based on powder metallurgy approach was developed to produce open celled aluminium foam. In the preparation of foam specimens, the Al powder and the NaCl (leaching agent) were dry mixed together in order to prepare a homogeneous mixture. The blended mixture was then subjected to pressure assisted sintering in which a pressure beyond atmospheric level is externally applied to the specimen during high frequency induction heated sintering. The embedded leaching agent was then dissolved in order to leave behind an open celled Al with the same chemical composition as that of the original Al powder. The final material is highly porous and has an interconnected porosity network. The structure of the resulting material has three levels of porosity (i.e. main cells, windows and microporosity). The X-ray diffraction analysis shows that, as the content of NaCl is increased to the volume fraction of 60%, no traces of NaCl presents in the foam.

A flake powder metallurgy approach to Al 2O 3/Al biomimetic nanolaminated composites with enhanced ductility

A simple and scalable methodology called “flake powder metallurgy” (flake PM) has been developed to fabricate biomimetic Al2O3/Al composites. Nanoflake Al powders with native Al2O3 skins were used as building blocks to rapidly assemble into biomimetic nanolaminated structures via compacting and extrusion, giving rise to strong and ductile composites with tensile strength of 262 MPa and plasticity of 22.9%. Thus, it is evidenced that well-balanced strength and ductility can be achieved in biomimetic metal–matrix composites by flake PM.

The reactive stabilization of Al–Zn foams using a powder metallurgy approach

Abstract The Al–Zn binary system was chosen in order to study the possibility of generating a reactive foam system within the semi-solid region. The idea is to create foam at lower temperatures than the melting point of pure aluminum using a transient liquid phase that softens the matrix prior to bulk expansion. This minimizes crack formation, collapse, drainage and deformation generated during processing. The Al–Zn foams were fabricated via the powder metallurgy route by hot compaction and subsequently foamed using TiH 2 as a blowing agent. The investigated systems consist of low, medium and high concentrations of Zn (10 wt%, 33 wt% and 50 wt%) in an Al based matrix containing 0.8 wt% TiH 2 . High temperature in situ confocal microscopy was used to study the formation of the transient liquid phase of the compacted elemental powders. As the percentage of Zn was increased, the liquidus temperature of the melt was lowered along with an increase in the volume of transient liquid phase. This reduces the mismatch between the hydrogen release temperature of the blowing agent and the liquidus temperature of the melt, thus increasing foaming stability. Reasonable foam structures near 300 vol% expansion and fair pore distributions were achieved at low concentrations of Zn (10 wt%) only above the alloy liquidus point. The mechanical compressive strength properties of the alloyed foam systems were also assessed.