Effects of reinforcement particle size in MMCs on extrusion die wear

The thermal conductivity of a 40 vol% silicon carbide‐particulate‐reinforced aluminum matrix composite was determined as a function of silicon carbide mean particle size ranging from 0.7 to 28 μm. A size dependence was found consisting of a decrease in thermal conductivity with decreasing SiC particle size. This effect is in accordance with theoretical expectations for composites with an interfacial thermal barrier at the dispersion–matrix interface. At the finest particle size of the silicon carbide, the composite thermal conductivity approached the value for the matrix with pores, as expected from theory. Only at the largest SiC particle size did the composite thermal conductivity exceed the value for the matrix. These results suggest that in maximizing the thermal conductivity of composites with an interfacial thermal barrier, the reinforcement particle size should be as large as practically possible.

The decrease of the bearing length in extrusion processes results in increasing of the material flow and offers, through this, the possibility for manipulation and optimization. This paper presents a simulation based optimization technique which uses this effect for optimizing the material flow in direct extrusion processes. Firstly, the method is used in a multi-extrusion process with equal pitch circle profiles, then in an extrusion process of an asymmetric profile. Furthermore, a composite extrusion process is analyzed where endless wires of high strength steel are embedded in a base material of aluminum. The insertion of reinforcement elements into the base material flow, especially within the small ratio between profile thickness and the reinforcement diameter, can lead to significant local disturbances inside the die, which result in undesirable profile defects. Hence, the simulation-based optimization method is especially used to optimize inhomogeneous wall thicknesses in composite profiles.

Flexural strength, elastic modulus, and hardness were used to characterize the mechanical properties of ZrB 2 –SiC composites that contained varying SiC particle sizes. Dense ZrB 2 –SiC composites consisting of 70 vol% ZrB 2 and 30 vol% α‐SiC were produced via hot pressing. This series of composites had maximum SiC particle sizes that ranged from 4.4 to 18 μm. The mechanical properties scaled with the maximum SiC particle size, not with ZrB 2 grain size. Flexural strength decreased as the maximum size of SiC particles increased from 1150 MPa at 4.4 μm to 245 MPa at 18 μm with an abrupt decrease in strength at ~11.5 μm. Elastic modulus remained constant at ~530 GPa for compositions containing SiC particles smaller than 11.5 μm, but exhibited a decrease with larger SiC particle sizes. Vickers and Knoop hardness were 21.4 and 17.2 GPa, respectively, for ceramics with SiC particle sizes <11.5 μm, but hardness decreased for larger SiC particle sizes. The decreases in strength, elastic modulus, and hardness with SiC particles larger than 11.5 μm were coincident with stress‐induced microcracking in the composites.

Effect of SiC particle size and severe deformation on mechanical properties and thermal conductivity of Cu/Al/Ni/SiC composite fabricated by ARB process

The available temperature range of the self-healing induced by high temperature oxidation of SiC can be controlled by the particle size of the contained SiC particles. In this study, three types of alumina–SiC composites were prepared. The SiC particle sizes of the composites were 270, ∼30 nm, and less than 10 nm. The self-healing abilities were estimated by the strength recovery behavior at several temperatures. The use of nanometer-sized dispersed SiC particles as healing agent decreases the activation energy of the SiC oxidation obtained from the differential thermal analysis with several heating rates. This implies that smaller SiC particles can give rise to the oxidation at lower temperature. Moreover, the lowest temperature at which the cracked strength was completely recovered for 10 h was strongly affected by the SiC particle size. As the SiC particle size varied from 270 to ∼30 nm, the lowest temperature varied from 1300 to 950 °C. However, alumina composite containing SiC particles whose particle size is less than 10 nm cannot recover completely the cracked strength under every condition, because the space between crack walls cannot be filled with the formed oxide due to the small volume of SiC on the crack walls. Therefore, it was found that there is an optimal SiC particle size for endowing self-healing ability.

The aluminum metal matrix composites (Al MMCs) reinforced by SiC particles with different sizes (25 nm, 150 nm and 3.5 m) were fabricated by powder metallurgy technique, and the microstructure and tensile properties of the Al MMCs were investigated. When the volume fraction of SiC particles is fixed to be 5 %, the Al MMCs reinforced by nanosized and submicron particles exhibit much higher ultimate tensile strength and yield strength, and much lower ductility compared with those of the non-reinforced aluminum. However, for the MMC reinforced by 3.5 m SiC particles, almost no strengthening effect is found. The strengthening effect of the Al MMCs is analyzed in terms of dislocation mechanism. Moreover, the tensile fracture surface shows that the damage mechanism of the Al MMCs can be changed as the size of SiC particles is changed.

The structure, hardness, tensile and wear behavior of Mg–5Sb–xSiC hybrid composites experimented where the amount of SiC particles were 1, 3, and 5 wt% SiCp before and after applying hot extrusion process. Micron size SiC particles were added to Mg–5Sb alloy via stir casting technique to form both Mg3Sb2 and SiC particles in the as-cast microstructures. Although SiC addition reduced the grain size of the Mg–5%Sb in-situ metal matrix composite (MMC), no improvement was observed on ultimate tensile strength (UTS) and the ductility (EL.%) in the as-cast state. The use of the extrusion process led to a remarkable fracturing of Mg3Sb2 intermetallic along the extrusion direction, grain refinement, and reduction in the size of SiC clusters. The optimal amount of SiC was found to be 3 wt%, which reduced the grain size from ~ 827 to ~ 4 µm and promoted the UTS amounts from 120 to 244 MPa for as-cast and hot extruded composites. Besides, the enhancement of yield strength of extruded MMCs was rationalized depending on the grain size compared to the as-cast condition. The fractography of the as-cast composites revealed more cleavage planes in contrast with hot extruded composites, which proved much more dimples. It was also found that the use of hard SiC particles in hybrid composite improved wear properties just in the extruded state. According to observations, in various sliding test conditions, the abrasive, oxidation and delamination are mostly operated in combination.

Charge accumulation phenomenon on gas–solid interface greatly restricts the development of HVDC energy transmission system. In this study, the surface charge transport behavior and flashover performance on alumina/epoxy spacer coated by SiC/epoxy composites are experimentally investigated under DC stress. SiC/epoxy composites with varied SiC particle size are fabricated and deposited on spacer surface. Nearly a charge free surface is achieved especially at smaller SiC particle size, even when metallic wires are adhered on spacer surface and connected to high voltage electrode. The DC flashover voltage increases with the decrease of SiC particle size. On the one hand, smaller SiC particle size exhibits more outstanding nonlinear conductivity characteristics, contributing to accelerating charge dissipation, whereas on the other hand, it would introduce plenty of shallow traps due to the increased interfacial regions between SiC particle and epoxy matrix, resulting in aggravating charge accumulation. A theoretical model is proposed to reveal the control mechanism of SiC/epoxy coating with different SiC particle size on flashover performance. The validity of this model can be confirmed based on the surface trap distribution, carrier mobility and our previous investigations on gas–solid interface flashover development process.

An overview of the effect of stirrer design on the mechanical properties of Aluminium Alloy Matrix Composites fabricated by stir casting

This paper deals with the investigation of mechanical properties by introducing micro size SiC particulates and Graphite particulates into Al-4.5%Cu alloy matrix. SiC and Graphite particle reinforced Al alloy metal matrix composites were prepared by stir casting method. Al-Cu alloy was taken as the base matrix to which SiC and Graphite particulates were used as reinforcements. 6 wt. % of SiC and Graphite particulates were added to the base matrix to fabricate Al-4.5Cu – 6 wt. % SiC and Al-4.5%Cu – 6 wt. % Graphite composites. For each composite, the reinforcement particles were pre-heated to a temperature of 600oC and then dispersed in steps of two into the vortex of molten Al-Cu alloy to improve wettability and distribution. The Micostructural study was done by using optical microscope, which revealed the uniform distribution of SiC and Graphite particles in matrix alloy. Mechanical properties like hardness, tensile and impact strength were evaluated as per ASTM standards. Hardness and tensile strength increased with addition of 6 wt. % of SiC particulates in the base matrix Al-Cu alloy compared to Al-Cu-6wt. % Graphite composites.

MICROSTRUCTURAL EFFECTS ON FRACTURE MICROMECHANISMS IN LIGHTWEIGHT METAL MATRIX COMPOSITES

In this study the effects of reinforcement particle size and cutting parameters on machining performance variables like cutting force, maximum tool-chip interface temperature and surface roughness of the machined surface have been investigated while machining Aluminum based metal matrix composites (MMCs). MMC bars with silicon carbide reinforcement having 10 % volume fraction and particle sizes of 5 μm, 10 μm and 15 μm are machined with polycrystalline diamond (PCD) inserts. Experiments are performed using central composite design (CCD) having four parameters with three levels. Response surfaces for each performance variables are generated using polynomial models. Single variable and interaction effects have been investigated using principal component analysis and 3D response charts. Multi-response optimization has been performed to minimize surface roughness and maximum tool-chip interface temperature using non-dominated sorting genetic algorithm II (NSGA-II). In addition, constraints have been applied to the optimization search to filter design points with high cutting forces and low material removal rate. Most of the optimal solutions are found to be with moderate cutting speeds, low feed rate and low depth of cuts.

The texture development was investigated in the extruded Al and Al metal matrix composites (MMCs) reinforced with SiC particles of different sizes and volume fractions. During extrusion, both the unreinforced Al and the MMCs develop a strong fiber texture with two components: <111> and <100>. When SiC is introduced into aluminum, the main component of texture is not modified, but the intensity of the component evolves with the volume fraction and average size of SiC particles. For the MMCs reinforced with 3.5μm SiC particles, the texture intensity of the Al matrix tends to decrease as the SiC volume fraction increases, and it is lower than that in the unreinforced Al. However, for the MMCs reinforced with 25 nm and 150 nm SiC particles, the texture intensity of the Al matrix is higher than that in the unreinforced matrix, and it increases with increasing the SiC volume fraction. It is found that superfine particles may introduce some new component into the deformation texture, and the texture intensity increases as the SiC particle size decreases.

In this study, 35 vol.% SiC particles with different sizes reinforced 6061 aluminium alloy matrix composites were prepared by a powder metallurgy method. The Scanning Electron Microscope (SEM) images of composites were observed, the Coefficient of Thermal Expansion (CTE) and tensile strength of composites were examined, and the influences of SiC particle size on microstructures and properties of the composites were analyzed. Furthermore, the SiCp/6061Al composites with SiC particle size of 7.5 µm were selected to investigate the SiCp/Al interface microstructure and precipitated phases by the means of SEM, TEM and HRTEM. The study indicated that, with the increase of SiC particle size, the SiC particles distributed more uniformly in the matrix, the CTE of composites increased, but the tensile strength of composites decreased. The SiCp/Al interface in this experiment is clean and smooth, and the combination mechanism of SiC and Al is the formation of a half coherent interface by closely matching of atoms. Some micron-sized coarse intermetallic particles existed in the hot-pressed composites, such as random-shaped Mg2Si, long stick shaped Al15(Mn, Fe, Cu)3Si2. When the composites were solution treated at 510 °C for 2 h and then aging treated at 190 °C for 9 h, except long stick shaped Al15(Mn, Fe, Cu)3Si2, numerous nano-sized precipitated phases (Mg2Si) with diameters of 50-200 nm dispersively distributed in the matrix. After heat treatment, the tensile strength of composite with SiC particle size of 7.5 µm enhance from 298 MPa to 341 MPa.

Silicon carbide particle–reinforced aluminum matrix (SiCp/Al) composites were milled at a high speed by adopting polycrystalline diamond (PCD) tools, which the diamond grain sizes were 5, 10, 25, and 32 μm. The machined materials were SiCp/Al composites which the volume fraction was 45% and grain size of SiC particles was 5 μm. The tool wear resistance and wear morphology of polycrystalline diamond cutters were investigated. And it centered on the effects of diamond grain sizes on the cutting forces as well as the machined surface roughness. The obtained results indicated the difference of corresponding tool wear resistance between larger SiC particles and smaller SiC particles during high-speed milling SiCp/Al composites with higher volume fraction. When the grain size and volume fraction of SiC granules are 5 μm and 45%, the tool wear resistances of four diamond grain sizes were all far higher than those of tools in machining composites with higher volume fraction (56%) and larger 60-μm SiC particles. And the PCD tools of larger diamond grain size acted on better wear resistance. Both the corresponding cutting forces and surface roughness were also smaller. As cutting distance increased, the variation law of cutting forces had a good correspondence with the tool wear of PCD cutters of different diamond grain sizes. The machined surface roughness had a decreasing trend in general, but the fluctuation range was tiny. The main wear morphology was flank wear and slight wear groove marks. And there was no chipping of cutting edge and coarse wear groove marks. A large amount of the machined materials was adhered on flake face in the cutting process, but built-up edge was not formed.