Xinlai He

Effects of binder compositions on characteristics of feedstocks of microsized SiC ceramic injection moulding

Abstract In the present study, ceramic injection moulding was used to fabricate SiC preform for pressureless infiltration. The influence of various binder compositions consisting of paraffin wax (PW), high density polyethylene (HDPE), polypropylene (PP) and stearic acid (SA) on the characteristics of injection moulded SiC feedstocks has been investigated over the 150–180°C temperature range and the 4–1259 s−1 shear range. The results show that all the feed stocks exhibit pseudoplastic flow behaviour. Compared with 65PW–15HDPE–5SA or 65PW–15PP–5SA, the wax based binder of multi polymer components (65PW–15HDPE–15PP–5SA) is a more suitable binder for SiC ceramic injection moulding due to its good comprehensive rheology and pyrolytic properties. Over a range of temperatures and shear rates, the feedstock made of 65PW–15PP–5SA binder has the lowest viscosity than the other two, which can be attributed to the low molecule weight of PP.

Characteristics of complex oxides in Co based ODS alloys

Abstract Co based oxide dispersion strengthening (ODS) alloys strengthened by oxide nanoparticles and γ′ precipitates are promising high temperature structural materials. Phase evolution, thermal stability and interfacial structure of the oxides were characterised, and the formation mechanism of the complex oxides was clarified. Co based ODS alloys exhibit inhomogeneous dispersion of the oxides due to the presence of relatively large Y–Al–O and fine Y–Hf–O complex oxides. Oxide particle size was controlled by the relative amount of Y–Al–O and Y–Hf–O oxides. The addition of Hf inhibits the formation of Y–Al–O oxides, resulting in the refinement of the oxides. The coherency of the matrix/oxide interface is size dependent. Large Y–Al–O complex oxides are incoherent with the matrix, while small Y–Hf–O nanoparticles tend to be partially coherent with the matrix. The extremely fine Y2Hf2O7 complex oxides demonstrate excellent thermal stability during heat treatment. Based on the observations of partially crystallised nanoparticles and the oxides with core/shell structure, a three-stage mechanism was used to elucidate the formation mechanism of the complex oxides.

Effects of particle characteristics on homogeneity of green bodies in powder injection moulding

Abstract A homogeneous powder and binder distribution in the green body in powder injection moulding (PIM) is important. In the present study, the mould filling model of PIM has been developed, based on the multiphase fluid theory, viscosity model of feedstock and powder-binder drag force model. The particle Reynolds number is influenced by the particle size and density, resulting in the different drag force between powder and binder. Furthermore, the varied velocity of binder and powder will be obtained with numerical calculation of the continuity equations, leading to the change of green body homogeneity. CFX was used to simulate the mould filling in PIM. The results showed that the homogeneity of green bodies was relative to the filling patterns, which varied with different powder densities. The powders were not suitable for PIM when the particle size was bigger than 20 μm, and the fine powders were beneficial to improve the homogeneity.

Microstructure characterisation and tensile properties of MIM418 superalloy

Metal injection moulding in conjunction with hot isostatic pressing was applied to prepare MIM418 superalloy. Densification behaviour, precipitated phases and tensile properties of the alloy were characterised. Relative density of 97.6% is obtained after vacuum sintering at 1240°C, and near full dense alloy is achieved after HIPing. High volume fraction (57%) of near cuboidal γ’ phase with average particle size of 0.52 μm precipitates homogeneously in γ matrix. Small amount of fine carbides with average particle size of 0.22 μm distribute mainly along grain boundaries and in grain interior. Average grain size of the superalloy is estimated to be ∼26 μm. Uniformly distributed precipitates and fine grain size lead to significant improvement of mechanical properties. The tensile strength, yield strength and ductility of MIM418 superalloy achieve 1425 MPa, 1004 MPa and 19.4% respectively, which are much higher than that of the cast K418 superalloy.

SiCp/Cu composites prepared by pressureless infiltration of copper into porous SiC preforms

Abstract SiCp/Cu composites with high reinforcement content (56–65 vol.-%) were fabricated by spontaneously infiltrating copper alloy into porous SiC preform obtained by powder injection moulding. The main influencing factors of the preparation of the preform and the infiltration behaviour of various preforms were studied. The results indicate that the viscosity increases with an increase in powder loading or decreases with an increase in particle size. The feedstocks with bimodal mixture exhibit low viscosity due to the improvement in packing efficiency. When small particles or bimodal mixture with large size ratio were used, the preforms were difficult to be fully infiltrated. The preforms debound in air exhibit lower infiltration rate than the ones debound in vacuum due to the reduction of porosity and the poor wettability between the oxidised layer and copper alloy. Kinetic analysis on the infiltration curves yielded activation energies of 253 kJ mol–1.

In Situ Observation of the Microstructure Evolution in HAZ and Analysis by EBSD

The microstructure evolution of pressure vessel steels by different smelting technology was observed in situ by the laser scanning confocal microscopy the second phase particles of the steels were observed by TEM, The results showed that there are many TiN particles in the Al killed steel smelted by traditional method and the TiN particles could pin austenite grain boundary and refine the austenite grain size. While there were many IAF formed in the HAZ of steel smelted by Oxide Metallurgy technology and the IAF divided the prior austenite grains into smaller and separated regions and refine the microstructure of the HAZ, and the analysis of the effective grain size of two steels carried out by electron back scattering diffraction(EBSD)show that Refining Effect of the steel smelted by Oxide Metallurgy technology is better.

Fractographic evaluation of high cycle fatigue strength of a high V alloyed powder metallurgy tool steel

Abstract The high cycle fatigue strength and the fracture surface morphology of a high V alloyed powder metallurgy tool steel were investigated and connected with microstructure. The inclusion with size above 30 μm was demonstrated to be responsible for the reduction in fatigue strength. The shape and size of inclusion and the stress amplitude affected the position of crack origin in fish eye. Based on fractographic evaluation and statistical analysis, the most harmful inclusions with the largest size proportion for steel A are ∼30 μm, and that of B1, B2 and B3 is roughly 50, 60 and 60 μm respectively. A distinctive granular-like zone with specific morphology was found to be confined in a very small area, suggesting that the fatigue crack propagation was mainly controlled by Paris Law regime. A linear fracture mechanics approach was implemented to evaluate the effect of grain size, yield stress and inclusion size on fatigue strength.

Thermal expansion behaviour of SiCp/Cu composites prepared by spark plasma sintering and pressureless infiltration

Abstract The coefficient of thermal expansion and thermal strain response behaviour of SiCp/Cu composites prepared by spark plasma sintering (SPS) and pressureless infiltration were studied. The CTEs decreased with increasing reinforcement content. For a fixed SiC content, pressurelessly infiltrated specimens exhibited lower CTE than the SPS specimens. The CTEs of SPS specimens agreed well with the ROM model while the CTE of pressurelessly infiltrated specimens was close to Kerners predictions. The strain hysteresis loops were developed during thermal cycling. The residual stress increased with increasing reinforcement content. The SPS specimens experienced large dimensional change due to the weak interfacial bond while the pressurelessly infiltrated specimens showed negligible strain hysteresis. The interfacial condition has a pronounced effect on the strain hysteresis behaviour. The improved interfacial bond was beneficial to the decreasing of the CTE and improving the dimensional stability of the composite.

Production of iron matrix composite reinforced by TiN particulate through nitriding by powder metallurgy

Abstract An iron matrix composite reinforced by TiN particulate is fabricated by nitriding and by powder metallurgy. The results show that chromium nitrides and titanium nitride can be in situ synthesised in the prealloyed powder when nitriding at 1100–1250°C in pure nitrogen atmosphere. After the denitrification at 1300°C in vacuum below 10−2 Pa, the chromium nitrides decompose, but titanium nitride still exists stably. The titanium nitride particulates have fine size and are homogeneously distributed in the matrix. The relative density of the sintered compact is ∼99% by spark plasma sintering at 1000°C and 40 MPa. However, it is very low (∼90%) when the denitrified powder is sintered at 1400°C in vacuum.