Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering

Selective laser sintering (SLS) technique is capable of rapidly fabricating customized implants with porous structure. A simple encapsulation process was developed to coat 316L stainless steel (316L SS) powder with ethylene-vinyl acetate copolymer (EVA). Subsequently, porous 316L SS was prepared by SLS preforming of EVA-coated metal powders, debinding and sintering in hydrogen atmosphere. The effects of processing parameters on pore characteristics and mechanical properties were analyzed. The results indicate that the porosity of green body mainly depends on laser energy density, while the pore features and mechanical properties of sintered specimens are largely dominated by sintering temperature. After sintering at 1100–1300 °C, the average pore size and porosity are 160–35 μm and 58–28%, respectively. In addition, the elastic modulus and compressive yield strength are 1.58–6.64 GPa and 15.5–52.8 MPa, respectively. It is revealed that the pore structural parameters and mechanical properties of the as-sintered porous 316L SS can be controlled readily to match with those of cancellous bone by modification of SLS processing parameters and subsequent sintering temperature.     

Self-compacting high density tungsten–bronze composites

Green compacts of W–bronze were encapsulated in shells of bronze powder, placed in a ceramic mold and sintered in alumina tube furnace at 1150 °C. Throughout the sintering cooling stage the differential coefficient of thermal expansion ΔCTE of W–bronze was employed to induce an external compressive densification action. The process included simultaneous sintering, hot isostatic pressing (HIP) and infiltration act to enhance densification. By this technique, pilot sintered compacts of different W50–80 wt.%–pre-mix bronze of 97–99% theoretical density were produced. This process resulted in compacts of higher hardness, higher sintered density and better structure homogeneity as opposed to similar compacts densified by the conventional sintering process. The results showed a gain in hardness by 10–20% and in density by 5–15%. The impact of different cooling rates of 3, 4, 8 and 30 °C min^?1 on sintered density, microstructure and densification mechanisms was examined and evaluated. Low cooling rates of 3 and 4 °C min^?1 gave the best results.     

Effects of processing parameters on density and electric properties of electric ceramic compacted by low-voltage electromagnetic compaction

In this research, low-voltage electromagnetic compaction (EMC) was applied to compact TiO₂ and PZT powders in the indirect way. After selecting the appropriate processing parameters, TiO₂ and PZT ceramics of higher density and better electrical properties were produced compared with traditional static compaction. The microstructures of two ceramics produced by two above-mentioned methods respectively show that, the average grain size of TiO₂ and PZT compacted by low-voltage EMC are about 8 μm and 4 μm which are smaller than that by static compaction respectively (15 μm and 7 μm) under the same sintered condition. Discharge voltage and charge capacitance are important factors to the green density and sintered part's density of each ceramics. Meanwhile, TiO₂ and PZT have their own discharge voltage range (700–1100 V for TiO₂ and 600–1000 V for PZT), during which each ceramic powder could be pressed effectively. With the same condition of charge capacitance, as the discharge voltage increases toward a peak value, the green density and sintered part's density increase, then tend to decrease after that peak value. The green density and sintered part's density of each ceramic increase and the above peak discharge voltage decrease slightly, as charge capacitance enlarges in the range investigated. In addition, effects of pancake coil turns and field shaper structure on the ceramic density were investigated. In most of cases investigated, the higher the ceramic part's density, the better the dielectric constants of TiO₂ parts and the piezoelectric constants of PZT parts.     

Fabrication of W–20 wt.%Cu alloys by powder injection molding

W–20 wt.% Cu balls were fabricated by powder injection molding using a binder system consisted of paraffin wax, high density polyethylene, ethylene vinyl acetate and stearic acid. By optimizing the injection molding parameters, defect-free green parts were obtained. A two-step debinding process was employed to extract the binders in the molded samples. All soluble ingredients of the binders in the green parts were extracted during solvent debinding, and the residual binders can be removed in thermal debinding. The debound W–Cu samples were sintered in H₂ atmosphere at temperatures ranging 1050–1150 °C for 2 h. It was shown that relative density of the sintered W–Cu samples increases from 87.37% of the theoretical to 95.58% as sintering temperature rises from 1050 °C to 1150 °C. Microstructures of the molded, the debound and the sintered W–Cu samples were observed by scanning electron microscope, and the sintered W–Cu balls have fine and homogeneous microstructures. Maximum compressive strength of W–Cu balls with 8.5 mm diameter reaches 58 kN.     

Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering

Al₂O₃–10TiC composite was synthesized by high energy ball milling followed by spark plasma sintering (SPS) process. Microstructure of the sintered composite samples reveals homogeneous distribution of the TiC particles in Al₂O₃ matrix. Effect of sintering temperature on the microstructure and mechanical properties was studied. The sample sintered at 1500 °C shows a measured density of 99.97% of their theoretical density and hardness of 1892 Hv with very high scratch resistance. These results demonstrate that powder metallurgy combined with spark plasma sintering is a suitable method for the production of Al₂O₃–10TiC composites.     

New Ferro Powder for Selective Laser Sintering of Dense Parts

The paper describes a new powder composition specially developed for selective laser sintering (SLS). The aim is to obtain a ferro powder that can be sintered without need for a (sacrificial) polymer binder and that results in quasi dense parts that do not need any post-processing like furnace sintering, infiltration or HIP. The powder is a mixture of different types of particles (Fe, Cu, Ni and Fe₃P). The composition and mixture ratio are justified by using phase diagrams. The powder has been tested using an own developed Nd: YAG SLS machine. The influence of process parameters (laser power, scan velocity, scan spacing and scan strategy) and the microstructural characteristics have been investigated. Attention is devoted to the binding mechanism (liquid phase sintering, through melting) and to the quality of resulting parts (density, balling effect,…).     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

Sintering of Al2O3–TiB2 nano-composite derived from milling assisted sol–gel method

Synthesis and sintering of an alumina /titanium diboride nano-composite have been studied as an alternative for pure titanium diboride for ceramic armor applications. Addition of TiB₂ particles to an Al₂O₃ matrix can improve its fracture toughness, hardness and flexural strength and offer advantages with respect to wear and fracture behavior. This contribution, for the first time, reports the sintering, microstructure, and properties of Al₂O₃–TiB₂ nano-composite densified with no sintering aids. Nano-composite powder was produced by combination of sol–gel and mechano-chemical methods. The densification experiments were carried out using both hot pressing and pressureless sintering routes. In the pressureless sintering route, a maximum of 92.3% of the theoretical density was achieved after sintering at 1850 °C for 2 h under vacuum. However, hot pressing at 1500 °C for 2 h under the same condition led to achieving a 99% of the theoretical density. The hot pressed Al₂O₃–TiB₂ nano-composites exhibit high Vickers hardness (16.1 GPa) and a modest indentation toughness (~ 4.2 MPa.m^1/2).     

Effect of ball-milling time on structural characteristics and densification behavior of W-Cu composite powder produced from WO3-CuO powder mixtures

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO₃-CuO powder mixtures was investigated. The mixture of WO₃ and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H₂ atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.     

Warm die compaction and sintering of titanium and titanium alloy powders

A study has been made of the effect of non-lubricated warm die (200 °C) compaction on the densification of hydride–dehydride (HDH) Ti powder, pre-alloyed (PA) Ti-6Al-4V and Ti-10V-2Fe-3Al powders, and HDH Ti and V-Fe-Al master alloy powder blends, compared to cold die compaction. Depending on the compaction pressure, which was varied from 200 to 1000 MPa, non-lubricated warm die (200 °C) compaction was very effective for −100 mesh HDH Ti powder, increasing the green density by 5.0–9.4% theoretical density (TD). Die wall lubrication with stearic acid showed no influence on the green density when compacted at 800 MPa. With warm die (200 °C) compaction, achieving a green density of greater than 90%TD was straightforward for HDH Ti powder when compacted at ≥750 MPa. Accordingly, near pore-free (≥99.5%TD) Ti microstructures were obtained after sintering at 1300 °C for 120 min in vacuum when compacted at 1000 MPa. The resulting increment in the sintered density was between 2.0%TD and 4.4%TD. Warm die (200 °C) compaction showed no effect on PA Ti-10V-2Fe-3Al powder and only a small effect on PA Ti-6Al-4V powder when compacted at 1000 MPa. However, it was still virtually effective for Ti-10V-2Fe-3Al powder blends made of HDH Ti powder and V-Fe-Al master alloy powder. The observations were compared with literature data and discussed in accordance with the yield strength of Ti, Ti-6Al-4V, Ti-10V-2Fe-3Al and Al₃V as a function of temperature.     

The effects of ball-milling treatment on the densification behavior of ultra-fine tungsten powder

Ultra-fine tungsten powder with a BET particle size of 210 nm was synthesized by sol spray drying, calcination and subsequent hydrogen reduction process. Then this powder was treated by ball-milling, the characteristic changes of this powder before and after milling were investigated. Then the sintering densification behavior of these powders with different ball-milling time (0 h, 5 h, 10 h) were also studied. The results show that ball-milling treatment greatly activates the sintering process of ultra-fine tungsten powder. The relative density of the powder ball-milled for 10 h could reach 97.3% of theoretical density (TD) when sintered at 1900 °C for 2 h, which is 600 °C lower than the required temperature of the traditional micro-scaled powder sintered for the same density. At the same time, ball-milling treatment could substantially reduce the onset temperature of sintering as well as recrystallization, and bulk tungsten materials with more uniform and finer microstructure and much better mechanical properties (hardness) could be obtained.     

Manufacturing of WC–Co moulds using SLS machine

Tungsten carbide–cobalt composite is widely used as a wear-resistant material for cutting tools, molds and other applications. Its production by Selective Laser Sintering (SLS) technique promises to combine the material properties of a composite with the flexibility of a production process. The present paper deals with SLS of a powder mixture of tungsten carbide and cobalt with an aim to make functional parts using a modified Rapid Prototyping (RP) machine (100 W DTM Sinterstation 2000). The associated manufacturing problems and their solutions are discussed. Various compositions of powders have been tried before being settled to a particular composition of WC–9 wt.% Co for making final parts. Bronze infiltration is done on laser sintered parts to enhance their mechanical properties. Fretting wear test is taken as a main characterizing test which is used for estimating the wear resistance of samples.     

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

TiB₂–TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB₂ and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB₂ grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB₂ grains. The good mechanical properties of TN1((TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B₄MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB₂–TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB₂ grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.     

Highly sinter-active (Mg–Cu)–Zn ferrite nanoparticles prepared by flame spray synthesis

Nanoscale ZnFe₂O₄, Mg_0.5Zn_0.5Fe₂O₄ and Mg_0.2Cu_0.2Zn_0.62Fe_1.98O_3.99 powders were prepared for the first time by flame spray synthesis (FSS). Solutions of metal β-diketonates in organic solvents were used as precursor. Crystalline particles of spinel structure with 6–13 nm primary particle size resulted from the flame process. Particle and crystallite size depended on the flow rate of the atomizing gas, the precursor and its molarity. Compacts prepared from Mg–Cu–Zn ferrite nanoparticles revealed an extremely high sinter-activity. A sintered density of 5.05 g cm⁻³ was achieved after firing for 2 h at 900 °C without any sintering additives, while a maximum density of 4.91 g cm⁻³ was obtained with particles from the conventional ceramic route. The permeability of the sintered Mg–Cu–Zn ferrite nanopowder compacts reached μ = 600 at 1 MHz and the saturation magnetisation was 80 emu g⁻¹. The outstanding sintering activity of the flame-made ferrite powders is attributed to their small primary particle size.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

Selective laser sintering/melting (SLS/SLM) of pure Al,Al–Mg,and Al–Si powders: Effect of processing conditions and powder properties

Selective laser sintering/melting (SLS/SLM) processing difficulties of aluminium powders had been attributed to issues associated with laser–materials interaction only while neglecting the role of powder properties. This study provides a wholistic understanding of factors that influence the development of SLS/SLM processing window, densification, and microstructure of pure Al, Al–Mg, and Al–Si powders, fabricated in single and multiple layer parts by exploring the roles of processing and material parameters. It was demonstrated that similarities existing in the SLS/SLM processing maps of the powders could be attributed to similarities in their packing densities with the alloying addition of magnesium and silicon having no predominant effect on their processing maps’ boundaries. Rather, alloying addition has significant effect on the nature of the evolved surface morphology of SLS/SLM processed aluminium powders in their processing windows. In addition, the flow and solidification behaviour of the melt pool of the powders during single layer scan was strongly influenced by the particle morphology and oxygen content of the powders as well as applied energy density. The energy density in the range of 12–16 J/mm² was found to be the threshold below which SLS was predominant and above which SLM occurred for the investigated powders. Moreover, successful oxide disruption phenomena which is necessary for inter-particulate coalescence in multi-layered SLS/SLM processed aluminium powders are found to be mainly controlled by the amount of oxide in the as-received powder, the degree of the uniformity of the distribution of the surface oxide film covering the aluminium particles, the nature of thermal mismatch existing between the oxide film and the parent aluminium particle which was dependent on the phase present in the oxide film. Al–12 wt% Si powder is hereby affirmed as a suitable candidate material for SLS/SLM process due to its low thermal expansion and uniform distribution of its surface oxide films as well as the mullite phase in its oxide film.     

Improving the uniformity in mechanical properties of a sintered Ti compact using a trace amount of internal lubricant

Large sintered powder compacts are likely to be associated with variability in mechanical properties; an improvement of the uniformity of the mechanical properties of sintered powder compacts is important for powder metallurgy. In this work 0.3–1 wt.% stearic acid (SA) or magnesium stearate (MgSt) was added to a 40 mm diameter Ti powder compacts with height to depth (H/D) ratio of unity to give a more uniform green density. Tensile test pieces were cut from selected positions in each sintered compact to obtain the distribution of mechanical properties. Results revealed that variations in mechanical properties are due to the pore morphology with respect to size, aspect ratio and preferred orientation. A trace amount of lubricant significantly improves the uniformity in mechanical properties by optimizing the porosity distribution and minimizing the pore size and aspect ratio of pores after sintering. Such an effect was achieved by reducing the initial green density inhomogeneity and the stress induced by the mismatch of sintering shrinkage. However a relatively high 1 wt.% SA addition with a large particle size created burnt-off pores in the top and bottom zones. MgSt is not recommended since it significantly increases the oxygen content. An addition of 0.6 wt.% SA is the best choice due to the even pore distribution, small pore size and acceptable level of oxygen pick up.     

Aqueous tape casting of PMN-PT powder prepared by the partial oxalate route

Aqueous tape casting of PMN-PT powder, prepared by the partial oxalate route, using PVA and glycerol as a binder and a plasticizer has been studied. The amount of PVA and glycerol required for forming green tape with good strength and flexibility has been optimized. The tape cast PMN-PT sintered to near theoretical density (>99% TD) at lower temperature (1200 °C) are compared to the samples prepared by uni-axial pressing (1270 °C). Flat PMN-PT tapes with good surface finish could be prepared by binder removal and sintering of the green tapes. The piezoelectric properties of the sintered tapes are comparable to that of PMN-PT ceramics reported in literature.     

Fabrication of W–Cu functionally graded material by spark plasma sintering method

Three-layered (W–25Cu/W–50Cu/W–75Cu, volume fraction) W/Cu functionally graded material (FGM) was synthesized by spark plasma sintering (SPS) at different temperatures for 5 min under a load of 40 MPa. The influences of different sintering processes on relative density, hardness, thermal conductivity and microstructure at various layers of sintered samples were investigated. The experimental results indicated that the graded structure of the composite could be well densified after the SPS process. The relative density increased with the increment of sintering temperature and it was up to 96.53% as sintered at 1050 °C. In addition, the thermal conductivity reached 140 W/m·K at room temperature and 151 W/m·K at 800 °C, which could be ascribed to the specific structure that W particles enwrapped by net-like Cu. And the Vickers hardness was converted from 4.11 to 4.68 GPa.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.