Sintering Cycle Research Articles

Design of an instrumented microwave multimode cavity for sintering of nuclear ceramics

This paper describes the development of an innovative instrumented and automated 2.45GHz multimode microwave cavity for sintering of uranium oxide based nuclear fuels (UO2, (U,Pu)O2, (U,Am)O2 …). The cavity and the sintering cell were designed using Finite Element simulation and the expertise acquired over years on microwave heating. This original setup allows for hybrid and homogeneous heating of nuclear fuels. Reliable in situ monitoring of the samples' shrinkage and temperature is performed with an optical dilatometer and infrared pyrometers calibrated with a specific protocol. A dedicated software including a Proportional-Integral-Derivative (PID) controller was developed to precisely control the sintering cycles like under conventional heating. The setup is also designed for operating in a glovebox under reducing sintering atmosphere.A stable and homogeneous heating with controlled sintering cycles is reached to successfully sinter UO2 crack free pellets to high density. This is a key point of innovation compared to previous studies in the literature on microwave sintering of nuclear ceramics. The pellets' final densities are equivalent to those obtained under conventional sintering, but with a significant decrease in the total duration of the sintering cycle.

Preparation and Microstructural Characterization of Five-layered Aluminium-Aluminium Oxide Functionally Graded Material

In this research, the metallic phase Aluminium (Al) and the ceramic phase Aluminium Oxide (Al2O3) were combined in different proportions for the preparation of five-layered Al-Al2O3 functionally graded material. Delamination or crack is the common problem at the metal-ceramic interface because of the large difference in material properties. Therefore, the main objective of this research was to investigate the defects within the layer during processing of the graded structure. The compositions of different weight percentages of five-layered structure are 100%Al+0%Al2O3, 95%Al+5%Al2O3, 90%Al+10%Al2O3, 85%Al+15%Al2O3 and 80%Al+20%Al2O3. During preparation of the graded composite structure, conventional two-step sintering cycle was followed. Heating rate 2°C/min, sintering temperature 630°C and sintering time 3 hours were maintained. It was observed that there was uniform distribution of Al2O3 ceramic particle in the Al matrix with the minimum of agglomeration, all interfaces were almost parallel with layers boundary and gradual transition occurred from first layer to fifth layer which confirmed the proper preparation processes of the five-layered graded composite structure. In addition, it was evident that there was no existence of crack within any layer or interface of the prepared composite structure.

Prospects for Electrical Performance Tuning in Ca3Co4O9 Materials by Metallic Fe and Ni Particles Additions.

This work further explores the possibilities for designing the high-temperature electrical performance of the thermoelectric Ca3Co4O9 phase, by a composite approach involving separate metallic iron and nickel particles additions, and by employing two different sintering schemes, capable to promote the controlled interactions between the components, encouraged by our recent promising results obtained for similar cobalt additions. Iron and nickel were chosen because of their similarities with cobalt. The maximum power factor value of around 200 μWm−1K−2 at 925 K was achieved for the composite with the nominal nickel content of 3% vol., processed via the two-step sintering cycle, which provides the highest densification from this work. The effectiveness of the proposed approach was shown to be strongly dependent on the processing conditions and added amounts of metallic particles. Although the conventional one-step approach results in Fe- and Ni-containing composites with the major content of the thermoelectric Ca3Co4O9 phase, their electrical performance was found to be significantly lower than for the Co-containing analogue, due to the presence of less-conducting phases and excessive porosity. In contrast, the relatively high performance of the composite with a nominal nickel content of 3% vol. processed via a two-step approach is related to the specific microstructural features from this sample, including minimal porosity and the presence of the Ca2Co2O5 phase, which partially compensate the complete decomposition of the Ca3Co4O9 matrix. The obtained results demonstrate different pathways to tailor the phase composition of Ca3Co4O9-based materials, with a corresponding impact on the thermoelectric performance, and highlight the necessity of more controllable approaches for the phase composition tuning, including lower amounts and different morphologies of the dispersed metallic phases.

Hybrid Metal/Polymer Filaments for Fused Filament Fabrication (FFF) to Print Metal Parts

The exploitation of mechanical properties and customization possibilities of 3D printed metal parts usually come at the cost of complex and expensive equipment. To address this issue, hybrid metal/polymer composite filaments have been studied allowing the printing of metal parts by using the standard Fused Filament Fabrication (FFF) approach. The resulting hybrid metal/polymer part, the so called “green”, can then be transformed into a dense metal part using debinding and sintering cycles. In this work, we investigated the manufacturing and characterization of green and sintered parts obtained by FFF of two commercial hybrid metal/polymer filaments, i.e., the Ultrafuse 316L by BASF and the 17-4 PH by Markforged. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS) analyses of the mesostructure highlighted incomplete raster bonding and voids like those observed in conventional FFF-printed polymeric structures despite the sintering cycle. A significant role in the tensile properties was played by the building orientation, with samples printed flatwise featuring the highest mechanical properties, though lower than those achievable with standard metal additive manufacturing techniques.

Binder jetting of “Hard-to-Weld” high gamma prime nickel-based superalloy RENÉ 108

Abstract The ultra-high strength of high γ’ nickel (Ni)-based superalloys with low ductility comes with the price of high susceptibility to cracking and a high residual stress state when processed with conventional powder bed fusion technologies. Binder jet additive manufacturing (BJAM) is an interesting alternative to other additive manufacturing (AM) modalities since it does not require melting to fuse powder material but selectively adheres powder with a binder. While there are several studies on the use of melting and solidification-based AM technologies, limited literature is available on the use of BJAM for high γ’ Ni-based superalloys. This study presents the advantages and disadvantages associated with the BJAM of RENE 108 superalloy. The BJAM microstructure is characterized in the green, brown and sintered states. The formation of parallel primitive lines perpendicular to the build direction is observed after printing. This leads to persistent lines of porosity and a relative density above 96% after a sintering cycle. A fully dense microstructure is obtained after hot isostatic pressing (HIP) of the material. The formation of HfO2 and Ta-rich carbides along prior particle boundaries (PPBs) is also observed during the sintering stage. Precipitation along PPBs is intrinsic to the powder metallurgy of Ni-based superalloys and influenced by the debinding conditions of the parts. They have a strong impact on the mechanical properties of the material. At low temperatures ( 760 °C), the deformation becomes intergranular, and fracture occurs at the PPBs. The ductility of BJAM RENE 108 becomes slightly lower than the DS material while the strength of the materials remains comparable.

Effect of Thermal Debinding Conditions on the Sintered Density of Low-Pressure Powder Injection Molded Iron Parts

Low-pressure powder injection molding (LPIM) is a cost-effective technology for producing intricate small metal parts at high, medium, and low production volumes in applications which, to date, have involved ceramics or spherical metal powders. Since the use of irregular metal powders represents a promising way to reduce overall production costs, this study aims to investigate the potential of manufacturing powder injection molded parts from irregular commercial iron powders using the LPIM approach. To this end, a low viscosity feedstock was injected into a rectangular mold cavity, thermally wick-debound using three different pre-sintering temperatures, and finally sintered using an identical sintering cycle. During debinding, an increase in pre-sintering temperature from 600 to 850 °C decreased the number of fine particles. This decreased the sintered density from 6.2 to 5.1 g/cm3, increased the average pore size from 9 to 14 μm, and decreased pore circularity from 67 to 59%.

Improved process for synthesizing n-type and p-type β-FeSi2 thermoelectric material from attritor milled powder

Abstract Semiconducting iron disilicide, β-FeSi2, has its application in thermoelectric generators using p-type and n-type semiconducting β-FeSi2. It is useful for energy conversion (waste heat energy to electrical energy) in the temperature range between 200 and 900 °C. The peritectoid and eutectoid reaction responsible in the formation of β-FeSi2 phase are very sluggish. This research work focuses on shortening of sintering period and thus conserving energy required for complete transformation of various phases to β-FeSi2. Total time required for the synthesis along with the sintering cycle is relatively less (about 20 h) than the other processes (other process durations varying from 37 h to 504 h) studied earlier. An innovative sintering cycle was developed to achieve nearly complete β-FeSi2 phase formation. p-type and n-type semiconducting β-FeSi2 were characterized for thermoelectric properties.

Sintering shrinkage of cold compacted iron: effect of thermodynamic driving force, structural and geometrical activity

ABSTRACT The sintering shrinkage of tapped powders and prior cold compacted specimens of sponge and water atomised iron powders was investigated. The behaviour of the tapped powders is only determined by the driving force and that of the green compacts is strongly affected by modifications caused by cold compaction, which introduce a structural and a geometrical activity that enhance the kinetics of shrinkage. While the tapped powders display only an isothermal shrinkage, the green specimens display an additional large and anisotropic anisothermal shrinkage which is independent on the surface area of the powders. The effect of prior cold compaction tends to run out during the heating step of the sintering cycle.

Effect of High Pressure Spark Plasma Sintering on the Densification of a Nb-Doped TiO2 Nanopowder

Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.

Reactive sintering of WC-Ni-Co-Cr-Ti-Al cemented carbides and precipitation of gamma prime in their metallic binder phases

Cemented carbides processed from WC-Ni-Co-Cr3C2-TiAl3 powder mixtures by HIP after sintering present a homogeneous precipitation of gamma prime in their metallic binder phases. Other reaction products include alumina precipitates and (Tix,W 1-x)C mixed carbides. The formation of these phases is consistent with the highly exothermic reactions detected by DSC during the heating ramp of the sintering cycle. The amount of alumina and gamma prime phases present in the sintered materials depends on the oxygen content of the powder mixtures. Refinement of gamma prime precipitates depends critically on the cooling rate after HIPing. Nanoindentation experiments have confirmed the presence of age hardening effects of samples which were previously solution treated. As expected, gamma prime precipitation is induced at lower temperatures in compositions with higher Al contents in the binder phase.

Influence of Different Sintering Protocols on Translucency and Fracture Resistance of Monolithic Zirconia Crowns

Purpose : The current study investigated the effect of different sintering protocols (long, speed and high- speed cycles) on translucency and fracture resistance of monolithic zirconia crowns. Materials and Methods: 30 monolithic zirconia crowns were CAD/CAM fabricated from translucent BruxZir zirconia blanks. The crowns were divided into three groups (n=10 each) according to the sintering protocols(temperature & speed); Group (I): samples sintered by long sintering cycle (1510°C , 120 min holding time and 8 hours total cycle time), Group (II): speed sintering ( 1540°C , 25 min holding time and 2 hours total cycle time) and Group (III): high-speed sintering (1580°C, 10 min holding time representing the total firing cycle).Translucency parameter (TP) and Contrast Ratio (CR) were measured using a spectrophotometer, then compressive load was applied till fracture in a universal testing machine. One representative disc sample (10mm diameter x 1 mm thickness) was fabricated and sintered according to the parameters of each group and analyzed by scanning electron microscope. Collected data were statistically analyzed. Results: Sintering speed had a statistically significant effect on (TP),(CR) and fracture resistance means. Long cycle showed the statistically significantly highest (TP) and lowest (CR) mean values. The high-speed cycle recorded the statistically significantly highest fracture resistance mean values. Conclusion: Shortening the sintering cycle significantly decreased translucency and increased fracture resistance of monolithic zirconia crowns. Regarding Translucency Perception Threshold ,speed cycle can be recommended for sintering of BruxZir monolithic zirconia crowns.

One step co-sintering of silicon carbide ceramic membrane with the aid of boron carbide

To reduce the sintering cycles and shorten the preparation periods of silicon carbide membrane, we report a fabrication route for silicon carbide membrane by co-sintering process with the aid of boron carbide. In this process, the SiC selective layer was coated on the SiC- B4C green substrate with spray method followed by sintering at 1900 ℃. With the increase of B4C content from 7 to 13 wt.%, the grain size and pore size of the substrate decreased, ascribing to the interface reaction between boron carbide and silica film on the silicon carbide surface. A straight and homogeneous selective layer formed on the substrate, presenting average thickness of 30 μm and average pore size of 0.28 μm. Thus, this technique is considered as a cost-effective and short-period method for the preparation of silicon carbide membrane.

Solidification Structures in Low-Alloyed Powder Metallurgy Steels Obtained through Permanent Liquid Phase Sintering Using a Master Alloy Containing Boron

Abstract Powder metallurgy (PM) steels containing boron form an attractive group of alloys because of the important densification that can be achieved through permanent liquid phase sintering (LPS). However, upon solidification, such liquid phase is known to form borides or borocarbides. Recent works have shown that some alloying elements have significant interactions with the LPS of PM steels containing boron. More specifically, it is suspected that the concentration of prealloyed molybdenum influences the formation of boride/borocarbide upon cooling at the end of the sintering cycle. Therefore, the main objective of this work is to describe the relationship that exists between the concentration of prealloyed molybdenum and the crystal structure of the boride/borocarbide eutectic component that typically forms in PM steels containing boron. A master alloy made of iron-manganese-nickel-boron-carbon was utilized to introduce boron, thus providing enhanced sintering through LPS. Characterization in optical and scanning electron microscopy combined with electron-backscattered diffraction and energy-dispersive X-ray spectrometry (EDS) revealed that increasing the prealloyed molybdenum content not only increased the volume fraction of liquid phase but also modified the morphology and the nature of the boron-rich eutectic. Changing the prealloyed molybdenum content from 0.5 to 0.85 wt.% transformed the discontinuous M2B boride to a continuous M23(C,B)6 borocarbide phase, causing a drastic decrease in strength despite the higher densification observed at 0.85 wt.% molybdenum. The effect of molybdenum on the LPS process of boron PM steels is undeniable and was found to occur after the initial formation of the liquid phase. Indeed, differential scanning calorimetry revealed no difference in the endothermic melting peaks temperature for both concentration of molybdenum.

The number of specimens in a furnace affects the biaxial flexural strength of veneered zirconia specimens after sintering

This study evaluated the effect of the number of specimens in a furnace during the firing of porcelain-zirconia (Y-TZP) discs on their biaxial flexural strength. Eighty bilayer disc-shaped specimens (∅: 15mm; thickness: 1.2 mm, ISO 6872) of zirconia (Vita In-Ceram YZ Cubes/Vita) and porcelain (Vita VM9) ceramics, were prepared and divided into four groups (n = 20) according to the number of specimens in a furnace for each porcelain sintering: G1- one disc/20 sintering cycles.; G2 – two discs/10 sintering cycles.; G5-five discs/4 sintering cycles; and G10 – ten discs/2 sintering cycles. Then, the specimens were submitted to biaxial flexure testing (BFS) (1 mm/min, water). The data (MPa) were analyzed with Kruskal-Wallis test followed by the Dunn's post hoc analysis at a 95% level of confidence (α = 0.05). Kruskal-Wallis showed that the number of samples in the furnace affected flexural strength (p < 0.0001). The highest BFS was observed for the G5 (132.3 MPa)A, which was significantly higher than G1 (53.6 MPa)B. No significant difference was observed for the Weibull moduli and characteristic strength among the groups. In terms of flexural strength, approximately five specimens at a time can be placed in a furnace for sintering.

On the reactive sintering of Heusler Fe2VAl-based thermoelectric compounds

Heusler Fe2VAl-based compounds are promising candidates for thermoelectric conversion of waste heat. In comparison to more conventional thermoelectric materials, the constitutive elements of these alloys are indeed abundant, cheap, and non-toxic. Furthermore, the thermoelectric properties of these compounds can be easily enhanced by either doping or controlled deviation from exact stoichiometry while large power factors compatible with energy harvesting applications can be reached.In the present study, Fe2VAl-based compounds were synthesised by powder metallurgy. Almost fully dense bulk materials were obtained either by field-assisted sintering, also referred to as spark plasma sintering (SPS), of pre-alloyed powders or by spark plasma reactive sintering of mixtures of elemental powders. The influence of the processing route and parameters on the resulting microstructure was assessed. In the specific case of the one-step synthesis and rapid densification of the Fe2VAl compound starting from the elemental powders, a detailed insight in the reaction path was obtained by interrupted sintering cycles followed by careful phase analysis of the partially sintered compacts by X-ray diffraction and elemental analysis. Finally, it is worth emphasising that the thermoelectric properties of the compounds processed by reactive sintering are as good as the ones of samples sintered from pre-alloyed powder or processed by melting and casting.

Evolution of surface chemistry during sintering of water‐atomized iron and low‐alloyed steel powder

Water‐atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X‐ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide‐forming elements.

Manufacturing Process Design of a Micro-Scale Liquid-Liquid Extractor and Multi-phase Separator

Abstract Advances in process innovation within the chemical process industry are impeded by the cost of producing new innovative components. In this paper, various manufacturing process routes were evaluated for scaling up a micro-scale liquid-liquid extractor and multi-phase separator capable of separating immiscible phases that are used for the extraction of a chemical product into a lower boiling point solvent to make downstream distillation easier. A manufacturing process design methodology was used to evaluate both photochemical machining and binder jetting as a means for producing the micropost arrays within the separator. Cost curves show that for a 0.3 mm deep micropost array, photochemical machining was found to be much cheaper than binder jetting. However, because binder jetting processes enable microposts with over 13 times higher depth-to-width aspect ratios, binder jetting was found to decrease the overall costs of the extractor/separator by more than half for a 500 barrel per day plant, with breakeven as low as 15 barrels per day. These economics were confirmed by building extractor/separator plates and characterizing them dimensionally and functionally. The binder jet plates were found to perform better than photochemical plates leading to the complete separation of the organic phase from water. The use of slower sintering cycles combined with surface grinding provided acceptable dimensional tolerances for the binder jet plates. Using pressurized air during depowdering improved the economics for the binder jetting process.

Experimental study on the effect of sintering cycle and reinforcement of 1% niobium carbide (NbC) in aluminium metal matrix composite (Al-MMC)

In Today’s production scenario, Aluminium Metal Matrix Composites (AMMC) play a key role in Automotive and aircraft sectors for their improved mechanical properties. In the current situation, a great deal of work has been done on Aluminium (Al) base material, which is reinforced with various carbides such as Silicon Carbide, Tungsten Carbide and Boron Carbide in different proportions through powder metallurgy route for numerous applications. In spite of this, no work has been done on the combination of pure Aluminium (Al) and Niobium carbide (NbC) in powder metallurgy route through different sintering cycles. The main objective of the present work was to study and compare the physical and mechanical properties of the processed specimen through two different sintering cycles and to compare the results obtained. In this experimental study, 1% NbC powder was mixed with pure Al powder in the High Energy Planetary Ball Mill (HEPBM) and processed through the sintering cycles namely Conventional Sintering and Chu’s Two-Step Sintering process (TSS-C) followed by compaction. Four samples were obtained, such as two pure Al and two Al-1% NbC. In addition, studies have been conducted to examine density and micro-hardness. Finally, the results show that the mechanical properties of the TSS-C specimen with the addition of 1% NbC have a higher density and micro-hardness than the rest of the samples.

Experimental study on fabrication and impact characteristics of PTFE/Al/W reactive materials with different molding pressures

Here we focus on the impact characteristics of polytetrafluoroethylene/aluminum/tu-ngsten (PTFE/Al/W) reactive materials under various molding pressures. Reactive materials of the density of 3.0 g/cm3 were fabricated via pressing and sintering processes. Drop weight tests were carried out to study impact characteristics of the reactive materials. Experiment shows that, critical forming pressure and compaction pressure are approximately 5MPa and 100MPa, separately, and meanwhile, the sintering cycle could enhance the strength of the material pressed under the critical forming pressure. Furthermore, reactive materials show strain rate effect on impact characteristics, and the increase of mounding pressure attributes to the transition from inertness to sensitivity of the material. Within the molding pressure range of 15MPa, impact sensitivity of the material increases exceedingly, whereas, the impact sensitivity remains relatively stable when the molding pressure ranging from 15 MPa to 100 MPa. Phenomenologically, with the increase of the molding pressure, the drop impact induced initiation flame transits from weak and unsustainable to intense and continuous. From the point of initiation mechanism, low molding pressure introduced high porosity plays negative role against the formation and growth of the hot spots in the microstructure of the material, which was demonstrated by the experiments. The conclusion drawn from this research reveals that a well pressed condition would be beneficial for increasing the impact sensitivities of low density reactive materials.

Synthesis and Characterization of Sintered Magnetic Abrasive Particles having Alumina and Carbonyl Iron Powder

The present workis focused on compacting, sintering, and characterization of sintered magnetic abrasive particles, which is composed of equal volume fraction of alumina and carbonyl iron powder. Powder metallurgy method is a well-developed technique for manufacturing of ferrous and nonferrous parts. AhO3–CIP composites are prepared through powder metallurgy method. Ball milling is used for mixing powders, and hydraulic Jack with die is used for compacting purpose. Solid and liquid phase sintering is performed at a high temperature tubular furnace under an inert gas atmosphere of argon. Solid and liquid phase sintering is done at 1000°C and 1545°C, respectively in proper consecutive sintering cycle. After sintering, the sintered pallets are crushed using ball miller to obtain the required size of the sintered powder. Energy Dispersive X-ray spectroscopy is used for elemental composition of all sintered powders. Vibrating sample magnetometer is used to see the magnetization of the particles. The saturation magnetization of the sintered abrasive obtained at 9–ton compaction pressure is found to be highest. Different phases of all prepared samples are studied using the X-ray diffraction technique. The morphology, as well as particle size, are studied using a scanning electron microscope. Also, the microstructure of sintered powders is studied using an optical microscope. Compression strength test of all sintered pallets is carried out using Universal Testing Machine. Bulk density of the pallets is measured using standard Archimedean principle. It is observed that the bulk density value increases with the compaction load. Micro hardness of the sintered pallets is measured using a Vickers micro hardness measuring instrument. The sintered pallet, fabricated at a compaction pressure of 9 ton shows the highest hardness.