Effect of powder mixing process on the microstructure and thermal conductivity of Al/diamond composites fabricated by spark plasma sintering

Two powder mixing processes, mechanical mixing (MM) and mechanical alloying (MA), were used to prepare mixed Al/diamond powders, which were subsequently consolidated using spark plasma sintering (SPS) to produce bulk Al/diamond composites. The effects of the powder mixing process on the morphologies of the mixed powders, the microstructure and the thermal conductivity of the composites were investigated. The results show that the powder mixing process can significantly affect the microstructure and the thermal conductivity of the composites. Agglomerations of the particles occurred in mixed powders using MM for 30 min, which led to high pore content and weak interfacial bonding in the composites and resulted in low relative density and low thermal conductivity for the composites. Mixed powders of homogeneous distribution of diamond particles could be obtained using MA for 10 min and MM for 2 h. The composite prepared through MA indicated a high relative density but low thermal conductivity due to its defects, such as damaged particles, Fe impurity, and local interfacial debonding, which were mainly introduced in the MA process. In contrast, the composite made by MM for 2 h demonstrated high relative density and an excellent thermal conductivity of 325 W·m-1·K-1, owing to its having few defects and strong interfacial bonding.     

Effect of H_2 Reduction Temperature on the Properties of Reduced Graphene Oxide and Copper Matrix Composites

Reduced graphene oxide(RGO) and copper composites(RGO/Cu) were successfully fabricated based on a molecular-level mixing method(MLM). The composite powders were reduced in H2 at 350, 450, and 550 °C and then consolidated by spark plasma sintering(SPS) in order to evaluate the effect of H2 reduction temperature on the properties of the composites. The results indicate that the strengths of the composite decrease with the increase of H2 reduction temperature, while the electrical conductivity reaches its maximum at 450 °C and minimum at 550 °C. Hot rolling could benefit the electrical conductivity. The yield strength of the RGO/Cu composite reduced to 337 MPa at 350 °C. The electrical conductivity of the RGO/Cu composite reduced at 450 °C after hot rolling reaches 60.26% IACS. The properties of the RGO/Cu composites can be designed by adjusting the reduction degree of RGO and by hot rolling.     

Electrochemical performances of LiFePO4/C composites prepared by molten salt method

LiFePO4/C composites were synthesized by a molten salt (MS) method using the mixture of LiCl,LiOH and NaCl.The prepared LiFePO4/C composites are characterized by X-ray diffractometry (XRD),field emission scanning electron microscopy (FESEM) and charge-discharge test.XRD patterns indicate that LiFePO4 prepared in the temperature range of 550-700 ℃ crystallizes well in an olivine-type structure.Through FESEM images,the sphere-like and homogeneous particles of 0.2 μm can be observed.The charge-discharge test shows that the materials prepared at 600 ℃ for 12 h have good electrochemical performance.At the rates of 0.2C (34 mA/g) and 0.5C,the discharge capacities are 144.6 and 122.3 mA·h/g,respectively,together with good cycle performances.     

Microstructural Characteristics and Mechanical Behavior of Spark Plasma-Sintered Cu–Cr–rGO Copper Matrix Composites

Copper matrix composites were prepared through spark plasma sintering(SPS) process, mixing fixed amount of reduced graphene oxide(rGO) with the different amounts of Cr. In the sintered bulk composites, the layered rGO network and uniform Cr particles distributed in the Cu matrix. Both of mechanical blending and freeze-drying stages of the wet-mixing process obtained the Cu/Cr/rGO mixture powders, and then SPS solid-phase sintering realized the faster densification of these mixture powders. The hardness and compressive yield strength of the Cu–Cr–rGO composites depicted the higher values than those of pure Cu and single rGO-added composite, and they were gradually increased with increasing Cr. The rGO/Cr hybrid second-phases are believed to be beneficial to strengthening Cu matrix. The relevant formation and strengthening mechanisms involved in Cu–Cr–rGO composites were discussed.     

Distribution of SiC particles in semisolid electromagnetic-mechanical stir-casting Al-SiC composite

The distribution of SiC particles in Al-SiC composite can greatly influence the mechanical performances of Al-SiC composite. To realize the homogeneous distribution of SiC particles in stir-casting Al-SiC composite, semisolid stir casting of Al-4.25 vol.%SiC composite was conducted using a special electromagneticmechanical stirring equipment made by our team, in which there are three uniformly-distributed blades with a horizontal tilt angle of 25 ° to mechanically raise the SiC particles by creating an upward movement of slurry under electromagnetic stirring. The microstructure of the as-cast Al-SiC composites was observed by Scanning Electron Mcroscopy(SEM). The volume fraction of SiC particles was measured by image analysis using the Quantimet 520 Image Processing and Analysis System. The tensile strength of the Al-4.25 vol.%SiC composites was measured by tensile testing. Results show that the Al-4.25 vol.%SiC composites with the homogeneous distrbutin of SiC particles can be obtained by the electromagnetic-mechanical stirring casting with the speed of 300 and 600 r·min-1 at 620 °C. The differences between the volume fraction of Si C particles at the top of ingot and that at the bottom are both ~0.04 vol.% with the stirring speed of 300 and 600 r·min-1, which are so small that the distribution of SiC particles can be seen as the homogeneous. The tensile strength of the Al matrix is enhanced by 51.2% due to the uniformly distributed SiC particles. The porosity of the composite mainly results from the solidification shrinkage of slurry and it is less than 0.04 vol.%.     

Microstructure of porous Cu fabricated by freeze-drying process of CuO/camphene slurry

Porous Cu with macroscopically aligned channels was synthesized using a freeze-drying process.Camphene-based CuO slurry was prepared by milling at 60 °C with a small amount of dispersant.Freezing of a slurry was done at 25 °C while unidirectionally controlling the growth direction of the camphene.Pores were generated subsequently by sublimation of the camphene during drying.The green body was hydrogen-reduced at 300 °C for 30 min,and sintered in the furnace at 700 °C for 1 h under a hydrogen atmosphere.Microstructural observation reveals that all of the sintered samples are composed of only Cu phase and show macroscopic open pores with an average size of 100 μm which are aligned along its macroscopic growth direction.The internal wall of the macroscopic aligned pore shows relatively small pores due to the traces of the camphene left between the concentrated Cu particles on the internal wall.Increase in the porosity and pore size with increasing camphene content was explained by the change of the growth behavior of the camphene crystals.     

Thermal Conductivity and Tensile Properties of Carbon Nanofiber-Reinforced Aluminum-Matrix Composites Fabricated via Powder Metallurgy: Effects of Ball Milling and Extrusion Conditions on Microstructures and Resultant Composite Properties

Carbon nanofiber(CNF)-reinforced aluminum-matrix composites were fabricated via ball milling and spark plasma sintering(SPS), SPS followed by hot extrusion and powder extrusion. Two mixing conditions of CNF and aluminum powder were adopted: milling at 90 rpm and milling at 200 rpm. After milling at 90 rpm, the mixed powder was sintered using SPS at 560 °C. The composite was then extruded at 500 °C at an extrusion ratio of 9. Composites were also fabricated via powder extrusion of powder milled at 200 rpm and 550 °C with an extrusion ratio of 9(R9) or 16(R16). The thermal conductivity and tensile properties of the resultant composites were evaluated. Anisotropic thermal conductivity was observed even in the sintered products. The anisotropy could be controlled via hot extrusion. The thermal conductivity of composites fabricated via powder extrusion was higher than those fabricated using other methods. However, in the case of specimens with a CNF volume fraction of 4.0%, the thermal conductivity of the composite fabricated via SPS and hot extrusion was the highest. The highest thermal conductivity of 4.0% CNF-reinforced composite is attributable to networking and percolation of CNFs. The effect of the fabrication route on the tensile strength and ductility was also investigated. Tensile strengths of the R9 composites were the highest. By contrast, the R16 composites prepared under long heating duration exhibited high ductility at CNF volume fractions of 2.0% and 5.0%. The microstructures of composites and fracture surfaces were observed in detail, and fracture process was elucidated. The results revealed that controlling the heating and plastic deformation during extrusion will yield strong and ductile composites.     

Fabrication and mechanical properties of MWCNTs-reinforced aluminum composites by hot extrusion

Over the past decade,the interest in aluminum composites reinforced with carbon nanotubes has grown significantly.Studies have been carried out to overcome problems with uniform dispersion,interfacial bonding,void formation and carbide formation of the composites.In the present work,multi-wall carbon nanotubes(MWCNTs)aluminum composites were produced.High-energy ball milling with the aim at developing well-dispersed MWCNTs Al composites was followed by cold compaction,sintering,and hot extrusion at 500 ℃.Different amounts of stearic acid as processing control agent(PCA)is used in order to minimize cold welding of the Al particles,and to produce finer particles.Differential scanning calorimetry(DSC),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and X-ray diffraction(XRD)were employed to analyze the MWCNTs,the aluminum powder,and the composites' microstructural behavior.The hardness and tensile properties of the composites are also evaluated.The results showed 500％ increase in yield stress after the addition of 1 wt％ MWCNTs in Al-MWCNTs based composite.The ball-milling time of 4 h is found to be sufficient as excessive milling time destroys a vast number of MWCNTs.     

Preparation of multi-walled carbon nanotube-reinforced TiNi matrix composites from elemental powders by spark plasma sintering

Carbon nanotube (CNT)-reinforced TiNi matrix composites were synthesized by spark plasma sintering (SPS) employing elemental powders.The phase structure,morphology and transformation behaviors were studied.It was found that thermoelastic martensitic transformation behaviors could be observed from the samples sintered above 800 ℃ even with a short sintering time (5 min),and the transformation temperatures gradually increased with increasing sintering temperature because of more Ti-rich TiNi phase formation.Although decreasing the sintering temperature and time to 700 ℃ and 5 min could not protect defective MWCNTs from reacting with Ti,still-perfect MWCNTs remained in the specimens sintered at 900 ℃.This method is expected to supply a basis for preparing CNT-reinforced TiNi composites.     

Manufacturing of cast A356 matrix composite reinforced with nano- to micrometer-sized SiC particles

In this study, large micron-sized Si C particles were fragmented via ball-milling process in the presence of iron and nickel powders, separately, to fabricate composite powders of Fe–Si C and Ni–Si C. Continuous fracturing of brittle Si C powders leads to the formation of multi-modalsized Si C powders with size of from 50 nm to slightly higher than 10 lm after 36-h ball milling. The milled powders were then incorporated into the semisolid melt of A356 aluminum alloy to ease the incorporation of fine Si C particles by using iron and nickel as their carrier agents.The final as-cast composites were then extruded at 500 °C with a reduction ratio of 9:1. Lower-sized composite powders with slight agglomeration are obtained for the36-h milled Ni–Si C mixture compared to that of Fe–Si C powders, leading to incorporation of Si C particles into the melt with a lower size and suitable distribution for the Ni–Si C mixture. It is found that lower-sized composite particles could release the fine Si C particles into the melt more easily, while large agglomerated composite particles almost remain in its initial form, resulting in sites of stress concentration and low-strength aluminum matrix composites. Ultimate tensile strength(UTS) and yield strength(YS) values of 243 and 135 MPa, respectively, are obtained for the aluminum matrix composite in which nickel acts as the carrier of fine ceramic particles.     

放电等离子烧结TiB/Ti-1.5Fe-2.25Mo复合材料的微观组织与力学性能

采用放电等离子烧结技术原位合成了TiB增强Ti?1.5Fe?2.25Mo复合材料,研究了烧结温度对复合材料微观组织和力学性能的影响规律。结果表明,随着烧结温度的升高,钛合金中 TiB 晶须的长细比迅速减小;然而,复合材料的相对密度及TiB的体积含量随着烧结温度的升高而不断增大。由于TiB晶须长细比的减小会导致复合材料强度的降低,而复合材料的相对密度及TiB体积含量的增大又会带来复合材料强度的增加,因此,在这两种因素的共同作用下,最终导致 TiB/Ti?1.5Fe?2.25Mo复合材料的弯曲强度随着烧结温度的升高而缓慢增大。在烧结温度为1150°C 时,TiB/Ti?1.5Fe?2.25Mo复合材料具有最大的弯曲强度1596 MPa。     

Enhanced thermal diffusivity of copperbased composites using copper-RGO sheets

The synthesis of copper-reduced graphene oxide (RGO) sheets was investigated in order to control the agglutination of interfaces and develop a manufacturing process for copper-based composite materials based on spark plasma sintering. To this end, copper-GO (graphene oxide) composites were synthesized using a hydrothermal method, while the copper-reduced graphene oxide composites were made by hydrogen reduction. Graphene oxide-copper oxide was hydrothermally synthesized at 80 °C for 5 h, and then annealed at 800 °C for 5 h in argon and hydrazine rate 9:1 to obtain copper-RGO flakes. The morphology and structure of these copper-RGO sheets were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. After vibratory mixing of the synthesized copper-RGO composites (0-2 wt%) with copper powder, they were sintered at 600 °C for 5 min under100 MPa of pressure by spark plasma sintering process. The thermal diffusivity of the resulting sintered composite was characterized by the laser flash method at 150 °C.     

升温速率对烧结ITO靶材密度和组织的影响

采用化学共沉淀法制备ITO粉体前驱物,在600℃煅烧粉体前驱物4h,得到粒径为20~30nm的ITO粉体。添加1%的聚乙烯醇(PVA)造粒,模压成型制备ITO靶材素坯,设置不同的升温速率,在1550℃氧气氛下烧结素坯,得到ITO靶材。研究了烧结过程升温速率对ITO靶材密度和微观组织的影响。结果表明,在低温阶段(0~500℃)升温速率为3℃/min,高温阶段(500~1550℃)升温速率为8℃/min时,ITO靶材相对密度为99.58%,孔洞极少,近乎完全致密,且靶材宏观上无裂纹。     

Interfacial structures and mechanical properties of a high entropy alloy-diamond composite

A FeCoCrNiMo high-entropy alloy (HEA)/diamond composite prepared by spark plasma sintering (SPS) was investigated. Sintering the HEA/diamond composites at different temperatures leads to different interfacial structures, which have an impact on the mechanical properties. The multiple microstructures at HEA/diamond interface have different effects on the retention of the HEA matrix on diamond particles. It was found that the interstitial strengthening effect, amorphous carbon and nano-scale ordered carbon complex were beneficial to the mechanical properties. Due to the good interfacial bonding strength between the HEA matrix and un-failed diamond particles, the composite sintered at 950 °C exhibited an optimized combination of mechanical properties, with a hardness of 630 HV, transverse rupture strength of 1310 MPa, and optimal wear resistance. The failure of the diamond particles and the formation of brittle chromium carbides at sintering temperature at 1000 °C can deteriorate the properties of HEA/diamond composites.     

Powder processing and properties of zircon-reinforced Al-13.5Si-2.5Mg alloy composites

Zircon, ZrSiO₄, is a thermally stable mineral requiring expensive and energy-intensive process to reduce. Owing to its abundance, high hardness, excellent abrasion/wear resistance, and low coefficient of thermal expansion, a low-cost alternative use of the mineral for medium-strength tribology was investigated. The present study has developed a conventional low-cost, double-compaction powder metallurgy route in the synthesis of Al-13.5Si-2.5Mg alloy reinforced with zircon. The mechanical and physical properties were determined following the development of optimum conditions of cold pressing and reactionsintering. Reinforcing the hypereutectic Al-Si alloy with 15 vol% zircon particles (size < 200 μm) and cold pressing at 350 MPa to near-net shape, followed by liquid-phase reaction sintering at 615 °C in vacuum for 20 min, improved the ultimate tensile strength, 0.2 % yield strength, and hardness of the alloy by 4,12.8, and 88%, respectively. At values of more than 9 vol% zircon, percent elongation and the dimensional changes of the sintered composites remained virtually unchanged. At a critical volume fraction of zircon, between 0.03 and 0.05, a sharp rise in hardness was observed. Microstructural and mechanical property analysis showed that the improvement in the mechanical properties is attributable largely to the load-bearing ability and intrinsic hardness of zircon, rather than to particulate dispersion effects. A good distribution of the dispersed zircon particulates in the matrix alloy was achieved.     

Effects of porous carbon on sintered Al-Si-Mg matrix composites

The influence of microporous particulate carbon char on the mechanical, thermal, and tribological properties of wear-resistant Al-13.5Si-2.5Mg alloy composites was studied. Large increases in surface area due to the formation of micropores in coconut shell chars were achieved by high-temperature activation under CO₂ gas flow. Activated char particles at 0.02 V _f were used to reinforce the alloy. The composites were fabricated via a double-compaction reaction sintering technique under vacuum at a compaction pressure of 250 MPa and sintering temperature of 600 °C. At more than 35% burn-off of the carbon chars at the temperature of activation, 915 °C, the total surface area remained virtually unaffected. The ultimate tensile strength and hardness decreased by 23% and 6 %, respectively; with increasing surface area of the reinforcement from 123 to 821 m²g⁻¹. The yield strength and the percentage of elongation decreased by a factor of 2 and 5, respectively. No significant change in sliding wear rate was observed but the coefficient of friction increased by 13 % (0.61 to 0.69). The coefficient of linear thermal expansion was reduced by 16 % (11.7 × 10⁻⁶ to 9.8 × 10⁻⁶ °C⁻¹), and remained unaffected at more than 35 % burn-off. Energy-dispersive spectrometry of the particles of the activated chars showed that oxides of potassium and copper coated the open surfaces. Failure at the matrix-char interface was observed, and this was attributed to localized presence of oxides at the interfaces as identified by electron probe microanalysis. Poor wetting of the oxides by magnesium at the sintering conditions resulted in formation of weak matrix-char interface bonds. J.U. Ejiofor, formerly of the Department of Metallurgical and Materials Engineering, The University of Alabama     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiC Composite Prepared by Spark Plasma Sintering Process: Evaluation of Mechanical and Tribological Properties

Al₂O₃-10TiC composites were synthesized by spark plasma sintering (SPS) process. Microstructural and mechanical properties of the composite reveal homogeneous distribution of the fine TiC particles in the matrix. The samples were produced with different sintering temperature, and it shows that the hardness and density gradually increases with increasing sintering temperature. Abrasion wear test result reveals that the composite sintered at 1500 °C shows high abrasion resistance (wt. loss ~ 0.016 g) and the lowest abrasion resistance was observed for the composite sample sintered at 1100 °C (wt. loss ~ 1.459 g). The profilometry surface roughness study shows that sample sintered at 1100 °C shows maximum roughness (R_a = 6.53 µm) compared to the sample sintered at 1500 °C (R_a = 0.66 µm) corroborating the abrasion wear test results.     

Effect of Particle Size on Wear of Particulate Reinforced Aluminum Alloy Composites at Elevated Temperatures

The present paper describes the effect of particle size on operative wear mechanism in particle reinforced aluminum alloy composites at elevated temperatures. Two composites containing zircon sand particles of 20-32 μm and 106-125 μm were fabricated by stir casting process. The dry sliding wear tests of the developed composites were performed at low and high loads with variation in temperatures from 50 to 300 °C. The transition in wear mode from mild-to-severe was observed with variation in temperature and load. The wear at 200 °C presented entirely different wear behavior from the one at 250 °C. The wear rate of fine size reinforced composite at 200 °C at higher load was substantially lower than that of coarse size reinforced composite. Examination of wear tracks and debris revealed that delamination occurs after run in wear mode followed by formation of smaller size wear debris, transfer of materials from the counter surfaces and mixing of these materials on the contact surfaces. The volume loss was observed to increase with increase in load and temperature. Composite containing bigger size particles exhibit higher loss under similar conditions.