Jinglian Fan

The rheological and sintering behavior of W–Ni–Fe nano-structured crystalline powder

Abstract Nano-crystalline powder of W–Ni–Fe alloy used for metal injection molding (MIM) was studied. This nano-powder, prepared by mechanical alloying (MA), was mixed with a wax-based binder to form a feedstock. The effects of MA milling time, nano-powder volume fraction and temperature on the rheological behavior of the feedstock are discussed. With increase of the milling time, the viscosity of the feedstock and the sensitivity of viscosity to shear strain rate decrease. Therefore the fluidity and formalization of this powder feedstock under conditions of longer time milling become better. With increase of the powder volume fraction, the viscosity of the feedstock has a non-linear increase which follows the equation η=η0A[1−(Φ/Φm)]−n. Here n is equal to 0.68. The change of viscosity with temperature and shear strain rate is small for the MA powder, so the quality of these MIM products is not affected strongly by the changes of temperature and injection speed. The experiments show that the milling of W–Ni–Fe powder leads to high density below the liquid sintering temperature. The sintering characteristics of this nano-powder by MA are also discussed. Because the lattice distortion, grain refining and supersaturated solid solution of MA powder promote the consolidation process, higher density and strength of W–Ni–Fe nano-powder products can be obtained.

Thermal stability, grain growth and structure changes of mechanically alloyed W-Ni-Fe composite during annealing

Abstract Mechanical alloying of elemental tungsten, nickel and iron powders was carried out in argon for 20 h to produce a nano-crystalline composite. By using a combination of DTA, XRD, OM and EDX analyses, systematic investigations were carried out on thermal stability, phase and microstructure changes during annealing. The results have shown, during annealing, the as-milled powder goes through stress relaxation, recovering and grain growth, precipitation of Ni, Fe phase from W supersaturated solution, amorphous phase crystallization, and a series of phase and microstructure changes. The phase and microstructure changes go through six steps. A significant grain growth occurs above 1100°C. Milling increased solubility of W in γ phase. The solubility decreases with an increase of temperature.

Phase transition and mechanical properties of tungsten nanomaterials from molecular dynamic simulation

Molecular dynamic simulation is used to systematically find out the effects of the size and shape of nanoparticles on phase transition and mechanical properties of W nanomaterials. It is revealed that the body-centered cubic (BCC) to face-centered cubic (FCC) phase transition could only happen in cubic nanoparticles of W, instead of the shapes of sphere, octahedron, and rhombic dodecahedron, and that the critical number to trigger the phase transition is 5374 atoms. Simulation also shows that the FCC nanocrystalline W should be prevented due to its much lower tensile strength than its BCC counterpart and that the octahedral and rhombic dodecahedral nanoparticles of W, rather than the cubic nanoparticles, should be preferred in terms of phase transition and mechanical properties. The derived results are discussed extensively through comparing with available observations in the literature to provide a deep understanding of W nanomaterials.

Influence of surfactant addition on rheological behaviors of injection-molded ultrafine 98W−1Ni−1Fe suspension

Abstract Ultrafine 98W-1Ni-1Fe powder injection molding was studied. The influence of ball milling and stearic acid (SA) addition on ultrafine powder characteristics and feedstock rheological behavior was investigated systematically. Results show that ball milling and SA addition can effectively increase powder loading of feedstock. SA addition can also avoid surface interactions between powder and binder during feedstock preparation, thus shortening the mixing time. SA addition is effective in reducing feedstock viscosity, but this effect decreases with increasing temperature and becomes disadvantageous when temperature is higher than 125 °C. SA addition can also reduce the temperature required for injection molding. The feedstock of powder with SA addition gets its best shear stability and lowest viscosity at 115 °C, while the feedstock of powder without SA addition obtains its best shear stability and lowest viscosity at 135 °C. Meanwhile, the former has lower temperature sensitivity and better comprehensive rheology than the latter, so the former is more suitable for ultrafine 98W-1Ni-1Fe injection molding.

Influence of ball milling on powder characteristics and feedstock rheological behaviours of injection moulded ultrafine 98W–1Ni–1Fe

Abstract In the present study, powder injection moulding (PIM) was used to fabricate ultrafine 98W–1Ni–1Fe preform. The influence of various ball milling times (3, 5 and 10 h) on the characteristics of the ultrafine powder and the rheological behaviour of the injection moulded feedstocks has been investigated over the 105–145°C temperature range and the 1·79–596·1 s−1 shear range. The results show that all the feedstocks exhibit pseudoplastic flow behaviour. The increase in ball milling time could reduce the viscosity of the feedstock and enhance the shear stability by greatly diminishing the agglomeration of ultrafine particles. However, the powder ball milled for 5 h can produce a feedstock with better thermostability and comprehensive rheology. Therefore, compared with the powder ball milled for 3 and 10 h, the powder milled for 5 h is more suitable for ultrafine 98W–1Ni–1Fe PIM.