Sintering Of Powders Research Articles

Corrosion of a spark plasma sintered Fe-Cr-Mo-B-C alloy in 3.5% NaCl solution

In this study, the corrosion behavior of a Fe-Cr-Mo-B-C alloy, fabricated by spark plasma sintering of an amorphous alloy powder, in 3.5% NaCl solution was analyzed. Electrochemical impedance spectroscopy and potentiodynamic polarization are techniques which were used for electrochemical performance estimation of samples and the results were further compared with conventional alloys: 1080 carbon steel and 304 stainless steel. Corrosion surface products were characterized through Scanning electron microscopy, Energy dispersive x-ray spectroscopy and X-ray photoelectron spectroscopy. Specimens sintered at 800℃ (S1-800) had achieved 94% densification approximately while the sample sintered at 900℃ (S2-900), had densified more which was 98% approximately. S2-900 had better corrosion resistance than S1-800 while in comparison to conventional alloys; it was inferior to 304 stainless steel. It was concluded that the increase in density of sintered samples favoured the formation of more uniform surface products and enhanced the formation of the passive chromium oxide (Cr2O3) layer.

The relationship between structural evolution and high energy ball milling duration in tin reinforced Mg alloys

In this study, 9 wt% Sn reinforced Mg alloys are produced by high energy ball milling and conventional sintering method. Effect of ball milling time (30 min, 2 h, 4 h, 8 h, and 12 h) on particle and grain morphology is detected by SEM examination. Phase evolution and solid solution mechanism is monitored by XRD analysis. Sintering process is carried out at 400 °C with 2 h sintering duration under Ar atmosphere. Platelet structure in powder morphology is observed up to 4 h ball milling, which is linked with the texture formation on < 002 > direction in hexagonal structure of Mg. This platelet formation is broken into smaller and monodispersed particle morphology with further milling process due to excessive plastic deformation which eventually provide homogenous dispersion of reinforcement element. Furthermore, ball milling process after 4 h triggers solid solution along with formation of intermetallic phase (Mg2Sn) according to crystallographic approach. The highest relative density values is measured in 8 h milling duration. Besides, the highest hardness value is achieved along with proper distribution of reinforcement element in 8 h condition. The hardness values is increased almost 85% comparing with pure ball milled Mg and 74% increment comparing with as cast Mg. The highest elastic modulus values is measured at 8 h milling condition by ultrasound method and this value is found in reasonable range. By introducing 12 h ball milling process, density, hardness and elastic modulus properties of 9 wt% Sn-Mg alloy is diminished because excessive amount of intermetallic phase formation on grain and grain boundaries. Therefore, 8 h high energy ball milling process is chosen the most suitable condition for this materials system to obtain proper powder packing and sinterability along with superior physical and mechanical properties.

Flowability of polymer powders at elevated temperatures for additive manufacturing

The flowability and spreadability of three different polymer powders for selective laser sintering were investigated by analysing the powder at different temperatures. These measurements were performed using both the Revolution Powder Analyzer and the Schulze Ring Shear Tester. The latter device was modified with a self-developed heating system to allow testing at temperatures up to 170 °C. Overall, the results of both tests showed that the powders' flowability decreases as the temperature increases, with the largest decrease observed near the materials' melting point. Afterwards, recoating tests were conducted on an EOS P395 machine, which utilized a self-built AI monitoring system to detect and evaluate the severity of defects in the powder bed. The tests revealed that the number and severity of defects also increases with temperature. When approaching the sintering window, the number and severity of defects greatly decreases, and the powder bed becomes stronger and more resistant to defects.

Influence of a prolonged shelf time on PA12 laser sintering powder and resulting part properties

In the laser sintering technology, the semi-crystalline polymer material is exposed to elevated temperatures during processing, which leads to serious material ageing for most materials. This has already been investigated intensively by various authors. However, the ageing of the material at ambient temperatures during shelf life has not been the focus so far. The need to analyse the shelf life can be derived from an ecological and economic point of view. This work is focusing on the shelf life of PA2200 (PA12). To reduce the potential influences of powder production fluctuations, two different powder batches stored for 5.5 years and 6.5 years are investigated and compared to a reference powder produced 0.5 years before these investigations. Multiple powder analyses and part characterisations have been performed. A significant yellowing and molecular chain length reduction can be derived from the measurement results. Whereas the influence on mechanical part performance was minor, the parts built with the stored powders are more yellowish. As it is most likely that this is due to the consumption of polyamide stabilisers, it can be assumed that these parts will be subject to significantly faster ageing. Therefore, it is still not recommended to use the stored powders for critical parts or light intense and humid environments.

Computational Efficient Modeling of Supersolidus Liquid Phase Sintering in Multi-component Alloys for ICME Applications

One of the challenges in computational design of pre-alloyed powders for sintering is the absence of predictive, efficient, and fast acting models that enable the design space of alloys to be tractable. This study presents an efficient and predictive model to simulate the densification as well as shape distortion of pre-alloyed powder compacts during supersolidus liquid phase sintering (SLPS). The model combines the generalized viscous theory of sintering with microstructural models for diffusional creep accommodated by viscous grain boundary sliding. Critical model parameters are obtained from thermodynamic modeling based on the calculation of phase diagrams (CalPhaD) and simulations of diffusional transformations in metals. The model is validated by comparing simulation results with experimental data from the literature for various types of engineering alloys. In addition, a processing window for defect free sintering of samples is presented by defining a microstructural softening parameter for a sintering body. The model can be used in the design of pre-alloyed powders for SLPS within the context of an integrated computational materials engineering (ICME) frameworks.

Accelerated phase growth kinetics during interdiffusion of ultrafine-grained Ni and Sn

The phase growth is investigated during the interdiffusion of an ultra-fine grained Ni (UFG) (grain size, d ∼ 320 nm) and coarse-grained Sn diffusion couple in the temperature range of 175–215 °C. The UFG Ni was produced through spark plasma sintering (SPS) of ball-milled Ni powder. Formation of only Ni3Sn4 intermetallic phase is observed, as confirmed by electron probe microanalysis (EPMA), micro-x-ray diffraction (XRD), and Transmission Kikuchi Diffraction (TKD) measurements. Using the CALPHAD approach, the driving forces for the formation of possible intermetallic phases in the Ni-Sn system were estimated and the highest value was found for Ni3Sn4, which matches the experimental observations. Time-dependent measurements at 200 °C revealed an acceleration in the growth (∼2 orders of magnitude) of Ni3Sn4 compared to the scenario prevalent in the coarse-grained (CG) Ni/Sn diffusion couple. This is reflected in the calculated integrated diffusivities for the UFG-Ni/Sn diffusion couple. The accelerated phase growth kinetics are attributed to the increased fraction of grain boundaries (GBs) in the UFG-Ni, which leads to an increased contribution of GB diffusion to the overall diffusion flux. The formation of Ni3Sn4 at GB was further ascertained using correlative microscopy strategy including transmission electron microscopy, transmission Kikuchi diffraction and scanning transmission electron microscopy.

Obtaining and properties of rolled products from clad powders NPG-80 and Nibon-20

Antifriction tape sealing materials (TSM) are used in the manufacture of turbines. This work studied the mechanism of the increase in thickness of rolled products. The study showed that internal oxides of powder particles, which are reduced during sintering and annealing in hydrogen, cause a change in the size of tapes and compacts from NPG-80 and Nibon-20 clad powders. The distinctive feature of powders used in the work is the presence of a nickel shell around the particle of the solid lubricant (graphite or boron nitride). It was shown that an increase in sintering temperature and heating time to isothermal holding intensifies the growth of the tapes and samples The studies carried out point to a relationship between reduction processes occurring in hydrogen during heating, sintering, and annealing of NPG-80 and Nibon-20 powders with an increase in the thickness of the tape containing the solid lubricant in its composition. An increase in the duration of heating to isothermal holding at 1150 °C is accompanied by an increase in the thickness of rolled stock. Sintering of TSM Nibon-20 in the first mode (4 h to 1150 °C) gives an increase in thickness by 5-7 %, whereas sintering in the second mode (9 h to 1150 °C) gives an increase of 12-13 %. For NPG-80, the increase in thickness is 3-7 % and 8-11 %, respectively. This leads to some decrease in the physical and mechanical properties of TSM. Lower temperatures and higher heating rates are recommended for the sintering of sheet materials from NPG-80 and Nibon-20 in the hydrogen atmosphere. The increase in thickness of TSM from clad powders NPG-80 and Nibon-20 can be avoided if the sintering (heating) is carried out under pressure. By the method of electric rolling and subsequent hot rolling TSM is obtained from Nibon-20.

Optical properties of Er3+ doped NaLuF4 transparent glass-ceramics produced by spark plasma sintering

Transparent oxyfluoride glass-ceramics (GCs) containing NaLuF4 nanocrystals doped with Er3+ have been prepared by combination of glass melt-quenching and spark plasma sintering (SPS) of glass powders. The parameters, such as temperature, pressure, particle size and holding times were optimized to obtain the densification of the powdered samples together with the desired transparency. The major advantage of this technique is the shorter times (few minutes) compared with those used for the preparation of these GCs by conventional heat treatments (more than 10 h). SPS presents an important inconvenient, which is the carbon contamination coming from graphite punch, this has been improved using Pt foils.The structural study indicates the presence of nanocrystals corresponding to the crystalline phase α-NaLuF4, with a crystal size between 26 and 31 nm. The optical characterization, which includes near infrared emission and infrared-to-visible up-conversion, confirms the incorporation of Er3+ ions in the nanocrystals. The longer lifetimes of the 4I11/2 and 4I13/2 levels together with the enhancement of the upconverted green and red emissions (as well as the weak blue emission) in the SPS-GCs samples, if compared with the precursor glass, indicates the presence of Er3+ ions in a crystalline environment. Possible energy conversion mechanisms for the green and red emissions based on the temporal behavior of the upconverted luminescence are discussed.

4D printing of polyamide 1212 based shape memory thermoplastic polyamide elastomers by selective laser sintering

4D printing technology, also known as additive manufacturing technology for smart devices, offers a wide range of promising applications in aerospace, automobile, biomedical and soft robotics. However, there are only a limited number of commercialized elastomer materials for 3D printing which needs further development, especially smart elastomers. In this work, polyamide 1212 (PA1212) based thermoplastic polyamide elastomer (TPAE) was synthesized by one-pot melt polycondensation of 1,12-dodecanedioic acid (DA), 1,12-dodecanedioic amine (DN) and polyetheramine in a stainless steel reactor and then the TPAE powder for selective laser sintering (SLS) was obtained after freeze-crushing and sieving. Subsequently, the effects of sintering temperature, laser power, layer thickness, and scan spacing during SLS processing on the properties of the sintered parts were fully investigated. The tensile strength, elastic modulus and elongation at break of the sintered parts could reach 13.7 MPa, 124 MPa and 200 %, respectively, when the sintering temperature, laser power, layer thickness and scan spacing were set as 156 °C, 21 W, 0.10 mm and 0.10 mm, respectively. Moreover, PA1212-based TPAEs fabricated by SLS showed excellent and stable shape memory behavior. This work not only provides a designing strategy of 4D printable materials and referential SLS-processing parameters, but also broadens the application of polyamide-based elastomer materials into 4D printing fields.

In-situ synthesized TiC/Ti-6Al-4V composites by elemental powder mixing and spark plasma sintering: Microstructural evolution and mechanical properties

Titanium matrix composites synthesized using powder metallurgy methods have attracted significant attention due to their extraordinary mechanical properties. In this study, cost-effective and high-performance TiC/Ti-6Al-4 V matrix composites were synthesized and characterized using a combined elemental powder mixing and spark plasma sintering method. Studies on the effect of graphene nanoplate (GNP) content on microstructures of Ti-6Al-4 V matrix composites showed typical Widmanstatten structures without apparent pores and microcracks. The grain size of composites was decreased significantly with the increase of GNP content, mainly due to the pinning effect of in-situ generated TiC particles at grain boundaries, which limited the rapid grain growth of the matrix. Mechanical test results showed that their yield strength and ultimate tensile strength were 889.0 MPa and 988.3 MPa, respectively, and their total fracture elongation was maintained at ∼14.2 %. GNPs/Ti-6Al-4 V exhibited superior strength (e.g., yield and ultimate tensile strength of 1028.4 and 1121.6 MPa, which are 14.4 % and 13.5 % higher than those of the matrix) and maintained a good ductility of ∼ 9.8 % with only 0.1 wt% GNP addition. Carbon nanomaterial (such as graphene nanoplates) induced the precipitation of needle-like nano-secondary phases in trigeminal grain boundary β phases, which strengthened the β-Ti soft phase. The reinforced strength of the composite is mainly attributed to the grain refinement, secondary α phases precipitation strength and dislocations strengthening. This work provides a new methodology for fabrication of high-performance titanium matrix composites (TMCs) combination blended elemental powder metallurgy (BEPM) and sintering technology.

Influence of flow agent in laser sintering powders on powder behavior and critical process steps

Good powder flowability is a key prerequisite for laser sintering powders. In this paper, the influence of rheological properties on powder behavior during dosing and coating steps was investigated and related to the build behavior and part properties. Five mixtures of Polyamide 11 powder with 0–0.068 wt% flow agent were tested. An insufficient amount of flow agent resulted in less powder being dosed, which further decreased with subsequent dosing steps. Flow behavior of the powder in the recoater and the amount of powder deposited was evaluated both at room temperature and at 181 °C. At operating temperature, the filling level and amount of flow agent significantly influenced powder flow in the recoater and amount of powder deposited. Test jobs indicated that good flowability impacts the stability of the building process rather than part properties. In addition, the importance of temperature dependent powder flow measurements for powder development could be shown.

Sintering inhibition and oil-absorption properties of calcined coal-series kaolin

AbstractIn this study, aluminium fluoride trihydrate (AlF3⋅3H2O) was used to inhibit the sintering of calcined coal-series kaolin (CCSK). In addition, the oil absorption properties of CCSK were studied. The particle-size distribution, specific surface area and porosity of the samples were investigated as a function of calcination temperature and the addition of AlF3⋅3H2O. Moreover, the ability of AlF3⋅3H2O to improve the oil absorption of CCSK was explored. The morphology, structure and phase composition of the specimens were investigated using scanning electron microscopy and X-ray diffraction. The phase transition during heating of the samples was studied using thermogravimetric analysis. The preparation with 10 wt.% AlF3⋅3H2O had the optimal sintering inhibition effect on CCSK at 1000°C. The release of SiF4 gas during heating and the formation of mullite whiskers on the particle surface caused by AlF3⋅3H2O moderated the formation of molten-phase liquid bridges between particles and inhibited sintering of the raw material powder. Furthermore, the formation of submicron mullite whiskers on the surface of the CCSK particles enhanced the oil-absorption properties of the sample significantly.

Study on catalytic gasification of pine wood with cerium loaded dolomite based clay ceramics

With dolomite as the main aggregate, a clay ceramic carrier was prepared by one-step powder sintering method. After being impregnated with Ce and calcined, a Ce loaded dolomite based (Ce-Dol) clay ceramic catalyst was prepared, which was used for the catalytic gasification of pine wood. The influence of the addition amount of Ce, steam flow rate, gasification temperature and other parameters on the catalytic gasification of pine wood was investigated using a self-developed two-stage gasifier, and the optimal operating conditions were determined. The results show that the dolomite based clay ceramics loaded with cerium can effectively improve the catalytic activity, reduce the stretching vibration peak absorbance of some functional groups in the gasification products, effectively promote the secondary cracking of tar, and improve the quality of gaseous product. When catalyzed by the Ce-Dol clay ceramic containing 6% cerium, the volume fraction of H2 peaks with 32.43%. With the rise in gasification temperature, aliphatic carboxylic acids and ketones in biomass tar are gradually decomposed into small molecular compounds such as CO, CO2. The content of H2 shows an ascending trend, and a maximum value is reached at 900 °C. Moreover, adding an appropriate amount of steam promotes the forward water-gas reaction, and a maximum volume fraction of H2 (37.37%) is achieved at a steam flow rate of 4 mL/min.

Selective Laser Sintering of Co-Cr Alloy Powders and Sintered Products Properties

Selective Laser Sintering of Co-Cr Alloy Powders and Sintered Products Properties

Low-Temperature Fabrication of Porous Mg-Zn Foams by a Reactive Sintering Powder Metallurgy Approach

Low-Temperature Fabrication of Porous Mg-Zn Foams by a Reactive Sintering Powder Metallurgy Approach

Cold sintering of bioglass and bioglass/polymer composites

Cold sintering of bioglass and bioglass/polymer composites

Corrosion Resistance of Nickel-Aluminum Sinters Produced by High-Pressure HPHT/SPS Method

As part of extensive research on the properties of nickel-aluminum alloys, corrosion tests of sintered materials produced by the innovative HPHT/SPS (high pressure, high temperature/spark plasma sintering) method were performed in 0.1 molar H2SO4 acid. The hybrid, unique device used for this purpose (one of only two such devices operating in the world) is equipped with a Bridgman chamber, which allows heating with high-frequency pulsed current and sintering of powders under high pressure in the range of 4-8 GPa and at temperatures up to 2400 °C. Using this device for the production of materials contributes to the generation of new phases not obtainable by classical methods. In this article, the first test results obtained for the nickel-aluminum alloys never before produced by this method are discussed. Alloys containing 25 at.% Al, 37 at.% Al and 50 at.% Al were produced. The alloys were obtained by the combined effect of the pressure of 7 GPa and the temperature of 1200 °C generated by the pulsed current. The time of the sintering process was 60 s. The electrochemical tests, such as OCP (open circuit potential), polarization tests and EIS (electrochemical impedance spectroscopy), were carried out for the newly produced sinters and the results were compared with the reference materials, i.e., nickel and aluminum. The corrosion tests showed good corrosion resistance of the produced sinters, with corrosion rates of 0.091, 0.073 and 0.127 mm per year, respectively. It leaves no doubt that the good resistance of materials synthesized by powder metallurgy is due to the proper selection of the manufacturing process parameters, ensuring a high degree of material consolidation. This was further confirmed by the examinations of microstructure (optical microscopy and scanning electron microscopy) and the results of density tests (hydrostatic method). It has been shown that the obtained sinters were characterized by a compact, homogeneous and pore-free structure, though at the same time differentiated and multi-phase, while the densities of individual alloys reached a level close to the theoretical values. The Vickers hardness of the alloys was 334, 399 and 486 HV10, respectively.

In situ processing of Fe-based bulk metallic glass nanocomposites in supercooled liquid region by spark plasma sintering

Current study reports fabrication of in-situ Fe-based bulk metallic glass (BMG) nanocomposites by carrying out spark plasma sintering of Fe57Cr9Mo5B16P7C6 amorphous alloy powder. In situ BMG composites were synthesized with varying amount of crystalline phases by controlled partial devitrification in supercooled liquid region. Application of high consolidation pressure (400 MPa) during sintering contributed to enhanced viscous flow in the amorphous powder resulted in high densification (> 97 %) in the sintered compacts. Microstructural and phase evolution in the consolidated BMGs and BMG nanocomposites were systematically investigated with respect to sintering conditions. Microscale heterogeneity with respect to phase evolution in the sintered samples was evaluated by nanoindentation studies. Hardness of the sintered BMG nanocomposites was found to be significantly higher than the sintered BMGs ascribed to the evolution of hard intermetallic phases during devitrification.

Effect of sintering aids on mechanical properties and microstructure of alumina ceramic via stereolithography

In this study, the effects of sintering aids (CaO, TiO2, La2O3 and ZrO2) on the physical and mechanical properties and microstructure of alumina ceramics made by stereolithography were systematically investigated. The addition of 3% CaO promoted internal porous structure at high temperatures, increasing the possibility of interlayer microcracking along the inner pores. 0.5% TiO2 had the best bending strength enhancement of alumina, increasing from 110.4 MPa to 201.1 MPa at a sintering temperature of 1650 °C. La2O3 took effect after 1550 °C and basically had no effect on refining grains. 1% ZrO2 increased 15% apparent porosity and 88% interlayer shear strength and reduced sintering shrinkage by about 30%, while maintaining bending strength. Optimised alumina powder ingredients and sintering conditions that could meet the performance index requirements of the ceramic core were obtained: particle size distribution 10 μm/2 μm (the mass fraction ratio of coarse to fine particles was 6:4), solid content 55%, sintering aid 1% ZrO2, sintering temperature 1450 °C.

Powder Metallurgy Processing and Characterization of the χ Phase Containing Multicomponent Al-Cr-Fe-Mn-Mo Alloy

High entropy alloys present many promising properties, such as high hardness or thermal stability, and can be candidates for many applications. Powder metallurgy techniques enable the production of bulk alloys with fine microstructures. This study aimed to investigate powder metallurgy preparation, i.e., mechanical alloying and sintering, non-equiatomic high entropy alloy from the Al-Cr-Fe-Mn-Mo system. The structural and microstructural investigations were performed on powders and the bulk sample. The indentation was carried out on the bulk sample. The mechanically alloyed powder consists of two bcc phases, one of which is significantly predominant. The annealed powder and the sample sintered at 950 °C for 1 h consist of a predominantly bcc phase (71 ± 2 vol.%), an intermetallic χ phase (26 ± 2 vol.%), and a small volume fraction of multielement carbides—M6C and M23C6. The presence of carbides results from carbon contamination from the balls and vial during mechanical alloying and the graphite die during sintering. The density of the sintered sample is 6.71 g/cm3 (98.4% relative density). The alloy presents a very high hardness of 948 ± 34 HV1N and Young’s modulus of 245 ± 8 GPa. This study showed the possibility of preparing ultra-hard multicomponent material reinforced by the intermetallic χ phase. The research on this system presented new knowledge on phase formation in multicomponent systems. Moreover, strengthening the solid solution matrix via hard intermetallic phases could be interesting for many industrial applications.