Experimental and numerical analysis of the particle size effect on the densification behaviour of metal injection moulded tungsten parts during sintering
Graphical abstract
Introduction
Metal injection moulding (MIM) is a near net-shaping technology for manufacturing small and complex components in large quantities, which includes mixing of metallic powder with plastic binder to form a feedstock, shaping the green parts by injection moulding of the feedstock into the cavities with desired shapes, removing the binder using thermal, solvent or catalytic methods to obtain brown parts with a porous structure, and sintering to a nearly full dense material and final dimensions of the parts [1]. Sintering, as a final step of MIM determining the final density and grain size, has an important effect on the mechanical and physical properties of the MIM components [2]. In general, the relative density of the sintered MIM components in materials such as iron alloys, stainless steel, copper, and ceramics is higher than 95% [3]. However, this high dense level is difficult to achieve at sintering temperatures below 1650 °C for pure tungsten (W), which has the highest melting temperature (3420 °C) among the metals [2]. Because of the very high fusion point, the consolidation of conventional W powders is difficult and generally requires a temperature in excess of 2700 °C through solid state sintering in a self-resistance sintering furnace under hydrogen (H2) atmosphere [4]. The low as-sintered density is an obstacle to the production of pure W components by MIM for high temperature applications such as electrodes of high-intensity discharge lamps, plasma facing components for fusion reactors, and furnace parts.
Many efforts have been reported to improve the sinterability of W for the purpose of decreasing the sintering temperature. Hayden et al. [5] proposed to reduce the activation energy of sintering through the addition of small amounts (< 1 wt.%) of Group VIII transition metals. German et al. [6] reported that the sintering temperature of W can be decreased from 2800 °C to 1400 °C by using less than 1 wt.% addition of transition metals, such as palladium and nickel. Yu et al. [7] investigated the effect of 0.4 wt.% Ni additives on the densification of MIM W–1.5% Al2O3 alloy. Ni had an important effect on promoting the densification process, which reduced the sintering temperature greatly. The residual Ni element in sintered parts of approximately 0.008 wt.%, however, would not be acceptable for some applications. Another important approach to activate the sintering of W is through the selection of a nano-sized precursor W powder. However, this powder is expensive and prone to contamination [8]. Malewar et al. [9] reported that the sintering temperature of nano-sized W produced by high-energy mechanical milling can be decreased from the conventional temperature 2500 °C to 1700 °C. El-Atwani et al. [10] performed sintering tests using fine-grained, hard, and ductile pure W powders for future fusion reactor applications. The bottom-up approach via powder consolidation by spark plasma sintering (SPS) was used under different temperature (1300–1800 °C) and pressure (90–266 MPa) conditions. Pure W powder, with an average particle size of approximately 1 μm was sintered to high density, approximately 94% of the theoretical one, with almost no grain growth at a temperature below 1400 °C and an applied pressure up to 266 MPa. Chanthapan et al. [11] performed an experimental investigation using W powder (0.6–0.9 μm) sintered by field-assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen (H2) reduction pretreatment and pulsed electric current during heating contained the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which was for the sample sintered at 2000 °C, 85 MPa for 30-min holding time. Recently, Wang et al. [12] reported that nanocrystalline W powder can be sintered to near full density at a temperature as low as 1100 °C under a H2 atmosphere without external pressure. Prabhu et al. [13] led an experimental investigation with anther sintering heating mode; microwave sintering has been studied using as-received W and activated W powder. These authors concluded that the relative density of the sintered part using as-received powder was 85% and that using activated powder was 93%. Because MIM is a process for producing small parts in complex shapes, innovative sintering approaches such as SPS, FAST and microwave sintering have not been applied in MIM industries to date. Piotter et al. [14] performed a comprehensive study on the conventional sintering of W samples with a powder particle size of 2 μm in a dry H2 atmosphere at temperatures higher than 2000 °C. Thermal treatment resulted in a grain size of approximately 18 μm and a final density of 95% measured by a helium pycnometer. To reduce the grain size of the sintered W parts, one solution is to use fine powders (e.g., an average particle size of 0.7 μm), which can be sintered at lower temperature (e.g., 1650 °C) to obtain closed residual pores necessary for the subsequent hot isostatic pressing (HIP) treatment. With the HIP cycle at 1600 °C under 250 MPa for 3 h, the final MIM W parts can reach a near full density (relative density of 98.6–99%) and fine grain structures (grain size of approximately 5 μm) [15], [16]. In addition to the application for near fully dense parts, MIM has also been used to produce porous W skeletons [17]. In this case, the relatively coarse powders were crucial to the performance of the final parts.
In this study, conventional sintering experiments in a dilatometer is used to investigate the densification behaviours of different components made of fine (e.g., the Fisher Sub Sieve Sizer (FSSS) particle size of 0.4, 1.0 and 3.0 μm) and coarse (e.g., the FSSS particle size of 5.0, 6.0 and 7.0 μm) W powders, with a maximum sintering temperature 1700 °C. Three-point bending testing is employed to measure the viscosity of the components during the sintering process. The continuum sintering model with a viscoplastic constitutive law and the identified material parameters has been used to predict the shrinkage, distortion and density distribution of the sintered parts. The numerical simulation is implemented by using the finite element code ABAQUS and the user subroutine UMAT.
Section snippets
Materials and sample preparation
Fine and coarse W powders of various particle sizes (Xiamen Honglu, China) were used in this study. To obtain the MIM powders with good flowability, deagglomeration was performed first via rod milling. For each type of W powder, 5 kg of powder was placed into a 5-litre polyurethane bottle with 15 kg of W rods, which has the diameter of 8 mm and length of 10 mm. The powder was milled for 8 h at a rotation speed of approximately 28 rpm. Fig. 1a and b presents scanning electron microscope (SEM) images
Constitutive model for sintering process
To simulate the sintering process using a finite element method, several macroscopic models have been developed to predict the shrinkages and distortions of the components during the sintering process. In these works, the sintering body is regarded globally as a compressible continuum even though it is composed of a solid and pores. Green components are usually elastic at room temperature, and an elastic to viscous transition during sintering. The viscoplastic constitutive law in continuum
Densification of sintered W
The relative densities of W specimens using fine and coarse powders sintered at various sintering temperatures (1500 and 1700 °C) at a heating rate of 5 °C/min with a holding time for 5 min are shown in Fig. 11. The final density of the sintered specimens was tested using the Archimedes method. The sintered density generally increases with the sintering temperature for all components. However, the degree of densification is affected by the particle size of powders. There is a trend showing that
Conclusions
The W components obtained by MIM using fine and coarse powder were sintered under a pure H2 atmosphere at up to 1700 °C to simulate the sintering process. The effect of the powder particle size on the dimensional changes of the sintered components was simulated using a viscoplastic constitutive model, and the main concluding remarks are as follows.
The final relative density of sintered W components produced by MIM depends mostly on the particle size of the used powders. The fine W powder
Acknowledgements
This work was performed by FEMTO-ST Laboratory in France and Xiamen Honglu Tungsten Molybdenum Industry Co., Ltd in China. The authors gratefully acknowledge the different financial subsidies of FUI New PIM project (2009-2012) members for funding the research by the way of the FEMTO-ST Institute laboratory.
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