Loose Sintering Research Articles

Limits of powder metallurgy to fabricate porous Ti35Nb7Zr5Ta samples for cortical bone replacements

The use of β-Titanium alloys to fabricate metal implants with Young's modulus that resembles bone tissues is presented as an alternative to commercially pure titanium or α-Titanium alloys, although it is still necessary to introduce proper implant porosity to reach the Young's modulus of cortical bones. In this work, porous samples were fabricated by loose sintering (0 MPa) and compared to samples manufactured at 1000 MPa, both sintered under the same conditions. Raw powders and sintered samples of the β-Titanium alloy, Ti35Nb7Zr5Ta, were characterized in detail in terms of both physicochemical and microstructural properties. Moreover, the tribo-mechanical behavior of sintered samples was evaluated by performing ultrasound technique, instrumented micro-indentation (P-h curves), and scratch tests. The bio-functional behavior was studied by impedance spectroscopy and contact angle measurements. The results allowed the evaluation of the limits of conventional powder metallurgy (percentage of porosity, size, and morphology of pores), as well as the influence of the porosity and chemical composition to achieve a better biomechanical and bio-functional behavior that would guarantee bone requirements. The Ti35Nb7Zr5Ta alloy showed relatively high electrical impedance values compared to commercially pure titanium, indicating an improved bio-corrosion behavior. Furthermore, wettability measurements indicated that porous disks fabricated by loose sintering exhibit higher hydrophilicity, often associated with a better antibacterial response.

Performance improvement and microstructure evolution of powder metallurgy high silicon steel with phosphorus addition

ABSTRACT In this study, phosphorus-containing Fe-5 wt.% Si silicon steel sheet was prepared by powder loose sintering followed by hot rolling. Defect-free sheet with high P content was achieved. The study found that P effects on sintering promotion and processing deterioration were significantly suppressed by the presence of Si in steel. Appropriate P content would significantly improve the mechanical and magnetic properties of the sample. Under a comprehensive consideration, P content of 0.3-0.5 wt.% was suitable for silicon steel. Compared with P-free silicon steel, with 0.3 wt.% P addition, the tensile strength of silicon steel increased from 521 MPa to 680 MPa. As to the magnetic performance, the sample with 0.5 wt.% P had B8 of 1370.5 mT, and W10/50 0f 1.16 W·kg−1, while those for P-free sample were 1332.3 mT, and 1.46 W·kg−1, respectively. FeP phase with special morphology would be precipitated in samples with P addition.

Pressure drop in sintered wicks with spherical or irregular particles

The pressure drop in the wick significantly affects the heat pipe heat transfer rates. This study experimentally investigated the pressure drop in wicks with spherical and irregularly shaped particles with comparisons with existing correlations to evaluate their applicability for predicting the pressure drops in wicks. Ten types of copper particles with various shapes and sizes were used to fabricate wicks using loose sintering. The particles included four spherical and six irregular particles with particle sizes from 25 μm to 150 μm. The results showed that the 75 μm spherical particles had the lowest pressure drop and that the pressure drop of the irregular particle wicks decreased with increasing porosity. The Morcom correlation most accurately predicted the pressure drops in the spherical particle wicks with a mean average error (MAE) of 20.11%. For the irregular particle wicks, the Meyer and Smith correlation gave the best predictions for small particle sizes while The Morcom correlation gave the best predictions with larger particles.

Electric discharge machining using rapid manufactured complex shape copper electrode with cryogenic cooling channel

In this study, a rapid manufacturing process based on the combination of polymer three-dimensional printing and pressureless loose sintering was explored for the fabrication of complex shape electric discharge machining pure copper electrodes with the cryogenic cooling channel. The fabricated electrodes were used to perform electric discharge machining on D2 steel workpiece. The comparative study was performed on material removal and electrode wear rates between the solid copper electrode, rapid manufactured electrode without cryogenic cooling and with cryogenic cooling. Also, the surface characteristics of the worn electrode and the machined workpiece were studied with and without cryogenic cooling. The significant effect of the cryogenic cooling on the electrode wear rate and the surface roughness was observed. Better surface finish, small cracks and less debris were notified on the workpiece surface machined with rapid manufactured electrode with cryogenic cooling due to rapid dissipation of the heat from the surface of the electrode after machining. Similarly, few cracks and low carbon deposition was observed on the rapid manufactured electrode with cryogenic cooling surface after machining as compared to rapid manufactured electrode without cryogenic cooling. The sharp corner edges of the complex shape tool in rapid manufactured electrode with cryogenic cooling were retained after machining due to low melting and vaporization of the electrode material. The dimensional deviation of the machined surface with respect to computer-aided design model was compared. The rapid manufactured electrode with cryogenic cooling was found to machine the more accurate complex shape features in terms of dimensions on the workpiece as compared to rapid manufactured electrode without cryogenic cooling.

In Vitro Bone Cell Behavior on Porous Titanium Samples: Influence of Porosity by Loose Sintering and Space Holder Techniques

A great variety of powder metallurgy techniques can produce biomimetic porous titanium structures with similar mechanical properties to host bone tissue. In this work, loose sintering and space holder techniques, two frequently used metallurgical techniques, are compared to evaluate the influences of porosity (content, size, morphology and wall roughness), mechanical properties (stiffness and yield strength) and in-vitro cellular responses (adhesion and proliferation of myoblasts and osteoblasts). These comparisons are made to achieve the best balance between biomechanical and bifunctional behavior of a partial porous implant for cortical bone replacement. Cell adhesion (filopodia presence) and spreading were promoted on both porous surfaces and fully dense substrates (non-porous control surfaces). Porous scaffold samples designed using 50 vol.% NaCl space holder technique had an improved bioactive response over those obtained with the loose sintering technique due to higher roughness and scaffold pore diameter. However, the presence of large and heterogeneous pores compromises the mechanical reliability of the implant. Considering both scenarios, the substrates obtained with 40 vol.% NH4HCO3 and pore size ranges between 100 and 200 μm provide a balanced optimization of size and strength to promote in-vitro osseointegration.

Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications

In the present work, the use of porous titanium is proposed as a solution to the difference in stiffness between the implant and bone tissue, avoiding the bone resorption. Conventional powder metallurgical technique is an industrially established route for fabrication of this type of material. The results are discussed in terms of the influence of compaction pressure and sintering temperature on the porosity (volumetric fraction, size, and morphology) and the quality of the sintering necks. A very good agreement between the predicted values obtained using a simple 2D finite element model, the experimental uniaxial compression behavior, and the analytical model proposed by Nielsen, has been found for both the Young’s modulus and the yield strength. The porous samples obtained by the loose sintering technique and using temperatures between 1000 °C −1100 °C (about 40% of total porosity) are recommended for achieving a suitable biomechanical behavior for cortical bone partial replacement.

Process optimization for rapid manufacturing of complex geometry electrical discharge machining electrode

Rapid manufacturing techniques permit tools and dies to be fabricated in short duration of time with complex geometry. The major contribution of the present research was to fabricate copper complex geometry electric discharge machining electrode by using amalgamation of three-dimensional printing along with pressureless loose sintering. Response surface methodology was employed to study the sintering parameters’ (sintering temperature, heating rate and soaking time) effect on electric discharge machining electrode important characteristics such as density, shrinkage and electrical conductivity. Analysis of variance was used to investigate the significant contribution of the parameters on the responses. Density and electrical conductivity of fabricated electric discharge machining electrode were revealed to increase with respect to rise in soaking time and sintering temperature. The interaction between the heating rate and sintering temperature for density and electrical conductivity responses signified the less effect of heating rate at high temperatures. Further, multi-objective optimization was used to maximize density and electrical conductivity and to minimize volumetric shrinkage. Different shapes of electric discharge machining electrodes were fabricated at optimized parameters. In addition, the fabricated electrodes were tested on electric discharge machining of D2 steel for 5 mm depth. The dimensional analysis was carried out between the computer aided design (CAD) model, fabricated electric discharge machining electrode and the obtained cavity by electric discharge machining process. The results depicted high efficacy of the process to fabricate complex geometry electric discharge machining electrodes.

Cyclic voltammetry measurements of electroactive surface area of porous nickel: Peak current and peak charge methods and diffusion layer effect

Surface area is a key parameter for porous metals for electrode applications. Here we measured the electroactive surface area of porous nickel samples using the cyclic voltammetry peak current and peak charge methods. The peak current method measures the contributions from primary pores, while the peak charge method measures the contributions from both primary and secondary pores. The electroactive surface area measured by both methods decreases with normalised diffusion layer thickness. It follows the semi-infinite model at low normalised diffusion layer thicknesses (<0.35) and the thin-layer model at high normalised diffusion layer thickness (>0.35). The correcting factors obtained from the semi-infinite model provide quantitative information on the pore surface roughness and the secondary porosity contribution. The surface roughness of the samples produced by Lost Carbonate Sintering is 2.15. The relative contribution of secondary porosity depends on the type of porous nickel and increases with scan rate, due to reduced diffusion layer thickness. It is in the range of 0.14–0.3 for the samples produced by Lost Carbonate Sintering and loose sintering, and 0.02–0.05 for the sample produced by electrodeposition, for scan rates in the range of 0.005–0.05 V/s.

A comparative study of oxygen pick-up of TiHDH powder during press and sinter and loose sintering processing

Press and sinter is considered to be the most cost effective powder metallurgy process for producing parts. However, loose powder sintering shows to be a more promising cost effective powder metallurgy process as it entails pouring powder into a mould followed by sintering. The differences in their behaviour with respect to densification, maintaining dimensional stability and their oxidation behaviours determine the choice of their industrial applicability. Titanium has a high affinity for oxygen which in turn makes it difficult to process components from powder. It also affects the mechanical properties of products significantly; particularly in applications where ductility is imperative. The focus of this study was therefore to evaluate the oxygen pick-up of the two cost effective powder metallurgy processes (press and sinter and loose sintering). A 100 Mesh TiHDH powder was used for sintering. Sintering was performed at 1500 °C for 4 hours. The oxygen contents of the green and sintered compacts were then compared. High oxygen contents were observed in tap density powder and pressed samples. The loosely sintered components showed high oxygen pick-up after sintering while oxygen pick-up decreased with increasing pressure in pressed samples. These results show that press and sinter is advantageous over loose sintering where oxygen control is critical.

Interface Structure and Wear Behavior of Cr26 Ferrous Matrix Surface Composites Reinforced with CTCP

Using cast tungsten carbide particles (CTCP) and reduced iron powders as raw materials, the porous ceramic preforms with honeycomb, strip, and layer structure, respectively, were prepared by loose sintering process; then, the CTCP/Cr26 ferrous matrix composites were fabricated by casting infiltration process. The microstructure of the composites was analyzed by SEM, XRD, and EDS. The results show that a sintered shell forms as a result of the reaction of Fe and W2C in the CTCP during loose sintering process; the inner part of the shell around the CTCp consists of WC and Fe3W3C phases, while the outer part between the particles is dominated by Fe3W3C. Therefore, the strength of preforms is enhanced because the particles are connected with each other by sintered shell. During casting infiltration process, a transition layer constituted by WC and Fe3W3C formed at the interface of CTCp and the matrix due to the dissolution and precipitation of the sintered shell in the high-temperature liquid iron. The three-body abrasive wear behavior of the composites was investigated. The result shows the wear resistance of honeycomb structure composite is comparable to that of whole layer (WL) structure composite, which is three times of heat-treated Cr26. However, the honeycomb structure composite has higher performance/cost ratio owing to the lower CTCp volume fraction and higher strength and toughness compared with the WL structure composite.

Development of porous titanium for biomedical applications: A comparison between loose sintering and space-holder techniques

One of the most important concerns in long-term prostheses is bone resorption as a result of the stress shielding due to stiffness mismatch between bone and implant. The aim of this study was to obtain porous titanium with stiffness values similar to that exhibited by cortical bone. Porous samples of commercial pure titanium grade-4 were obtained by following both loose-sintering processing and space-holder technique with NaCl between 40 and 70% in volume fraction. Both mechanical properties and porosity morphology were assessed. Young's modulus was measured using uniaxial compression testing, as well as ultrasound methodology. Complete characterization and mechanical testing results allowed us to determine some important findings: (i) optimal parameters for both processing routes; (ii) better mechanical response was obtained by using space-holder technique; (iii) pore geometry of loose sintering samples becomes more regular with increasing sintering temperature; in the case of the space-holder technique that trend was observed for decreasing volume fraction; (iv) most reliable Young's modulus measurements were achieved by ultrasound technique.

Study on molding process of UHMWPE microporous filter materials

AbstractA method called loose sintering was first introduced to prepare ultrahigh‐molecular weight polyethylene (UHMWPE) microporous materials. The pore size was predicted by the face‐centered cubic structure model while considering the particles' arrangement and melt. The results showed that the experimental pore diameter was close to that calculated by the present model. The effects of UHMWPE molecular weight, particle diameter, packing density, sintering temperature, and sintering time on pore size, compressive strength, pore diameter distribution, and density were presented. The morphology of micropore and the uniformity of pore distribution were analyzed by scanning electron microscopy and fractal geometry. The results showed that average pore diameter and porosity both increased with the UHMWPE particle diameter while decreased with compressive strength and bulk density. Sintering temperature and sintering time determined whether the heat was redundant to melt the particles. They also determined the pore size and the uniformity. UHMWPE microporous materials could be successfully prepared with suitable processing conditions. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Fabrication and characterisation of powder metallurgy parts having porosity gradient

Powder metallurgical preforms, which were fabricated by compaction and sintering as well as by loose sintering, were subjected to surface densification as a secondary operation by means of a specially designed indenter. The resulting structure of each specimen consisted of three zones, namely a fully dense layer, an intermediate zone with gradually increasing porosity, and the initial porous core that did not undergo any densification. Such a unique structure was created to mimic natural bones and the potential application for such parts would therefore be bone implants. The morphology of various specimens was characterised using image analysis with optical microscopy, physical measurements of density distribution, as well as scanning electron microscopy. Results indicated that the thickness of the fully dense layer increased with increasing depth of indentation and can be controlled as required.

The Preparation of Aluminum-Bronze/Steel Duplex Metal by Loose Sintering

Aluminum-bronze/steel duplex metal has been prepared by aluminum-bronze powder whose compact oxide film was removed by dilute HCl solution before sintering. Scanning electronic microscope (SEM), energy spectrum analysis (EDS), density measurement and compression shear strength test were used to investigate the structure, composition, relative density and bond strength of aluminum-bronze/steel duplex metal. The results show: liquid phase could wet the aluminum-bronze powder washed by acid, improve sintering activity of the powder and increase bond strength of aluminum-bronze/steel interface. The relative density of sintered aluminum-bronze is 99.6%, and shear strength of aluminum-bronze/steel interface is 132.6MPa.

Porous Metal Bond Matrix with High Porosity and Large Pore Size Fabricated Using Pore-Forming Agent

In order to improve the machining performance of the new type porous metal bond wheel, a new attempt were carried out for fabricating porous metal bond matrix with adjustable and high porosity as well as large pore size by use of pore-forming agent ammonium bicarbonate (NH4HCO3) and a vacuum loose powder sintering process. The porosity, bending strength and microstructure of the as-sintered porous metal bond matrix have been studied. The results show that the pore-forming agent NH4HCO3 with a certain amount can effectively increase porosity and pore size. The porosity of the fabricated porous metal bond matrix increases linearly with addition of ammonium bicarbonate and reaches as high as 67.7%, while the size of pores is larger than 300 μm. Meanwhile, the use of the raw materials were given and the sintering mechanism was discussed.

Development of sintered Ni-Cu wicks for loop heat pipes

The present study fabricated a series of capillary wicks for loop heat pipes (LHPs), using two different methods, the cold-pressing sintering and direct loose sintering, and experimentally investigated the effect of different methods, compositions and sintering parameters on their properties in terms of porosity, permeability and pore radius. Porosity and pore radius were measured by the Archimedes method and Scanning Electron Microscope (SEM), respectively. Permeability of the wicks was compared by calculation using empirical equation. Results show that capillary wicks were successfully fabricated by using two different methods; the optimal capillary wick was found to be sintered at 650°C for 30 min, using direct loose sintering technique, with 90% nickel and 10% copper. The wicks could reach the porosity of 70.07% and the permeability of 10−13 m2 order, with mean pore radius of 0.54 μm.

Microstructure and Performance of Porous Ni-Cr Alloy Bonded Diamond Grinding Wheel

Although the porous metal bonded diamond grinding wheel, which has recently been developed, had an excellent grinding performance for hard-brittle materials, its applications were only in precision grinding in past study. A new method for fabricating the new porous metal bonded diamond grinding wheel by Ni-Cr alloy as bond and vacuum loose powder sintering was proposed in this paper. The morphology of cross section of the segments and microstructure of interface between diamond grits and bond were analyzed. The wetting mechanism between Ni-Cr alloy bond and diamond is reacting wetting. Machining performance experiments about grinding ratio and surface roughness have been carried out. The testing results show that the porous Ni-Cr alloy bonded diamond grinding wheel has certainly fine machining performance in high grinding force occasion, and the maximal grinding ratio and best surface roughness were 6660 and 1.08 m, respectively.

The Resorbable Bioglassceramics Based on Calcium Biophosphate

ABSTRACTNovel resorbable bioglassceramic artificial tooth roots based on calcium biphosphate were synthesized, implanted into rabbits, and assessed. They were prepared by melting a glass composition of CaO/(CaO+P205) = 0.33, casting, crushing, loose powder sintering and crystallization annealing. The resultant porosity is 3 to 36 μm. The three point bending strength is 33 to 150 MPa depending on annealing treatment. The crystallized phases are β-Ca2P2O7 and CaP206. These porous implants show remarkable biocompatibility with the old bone and induce the growth of new bone within 30 days. The implants are partly resorbed after 90 days and replaced by new bone. The ingrowth of blood vessels into the implant is abundantly seen after 90 days despite a small original porosity, due probably to enlarged pores by preferential resorption of the implant material. The new ceramics are good candidates for resorbable bone bioimplant.

Metric and Topological Characterization of the Advanced Stages of Loose Stack Sintering

A quantitative stereological study, including serial sectioning analysis, has been carried out to determine the topological and metric properties of the pore and grain boundary networks in a series of samples prepared by loose stack sintering a 5-μm nickel powder at 1250 °C. The evolution of the structure is found to be determined by an interplay between the rates of three proceses: channel closure in the pore network, annihilation of isolated pores, and grain boundary migration. For the conditions of this study, this interplay produced a path characterized by isolation of pores from the starting network; these pores effectively pinned moving grain boundaries, arresting grain growth. The isolated pores, thus pinned in grain boundaries, then disappeared, so that at the end of the sequence, only network porosity remained. The traditional view that holds that isolated porosity is stable is thus found to be naive; a valid description of the geometry of the porosity cannot be obtained unambiguously without serial sectioning analysis.

Kinetics of neck growth during loose stack sintering

The kinetics of loose stack sintering of spherical iron powder were monitored by measuring some global microstructural attributes associated with the interparticle contact necks. The neck size and the number of interparticle contacts per unit volume were calculated from these data. A direct comparison of the experimental and theoretically predicted neck sizes has been carried out. The neck sizes predicted by the model for surface diffusion controlled sintering are muchhigher than the corre-sponding experimental values. The number of interparticle contacts per particle does not change with isothermal sintering time or temperature, and it is predominantly determined by the initial stacking of particles in the powder mass. The change of atmosphere from dry hydrogen to argon does not affect the neck growth kinetics significantly.