Effect of thermal cycling on the expansion behavior of Al/SiCp composite

The coefficient of thermal expansion (CTE) and accumulated plastic strain of the pure aluminum matrix composite containing 50% SiC particles (Al/SiC_p) during thermal cycling (within temperature range 298–573 K) were investigated. The composite was produced by infiltrating liquid aluminum into a preform made by SiC particles with an average diameter of 14 μm. Experiment results showed that the relationship between the CTE of Al/SiC_p and temperature is nonlinear; CTE could reach a maximum value at about 530 K. The theoretical accumulated plastic strain of Al/SiC_p composites during thermal cycling has also been calculated and compared with the experimental results.     

真空压力浸渗制备SiCp/AZ91复合材料的显微组织、力学性能和磨损行为

采用真空压力浸透法制备SiC_p/AZ91复合材料,研究其显微组织、力学性能和耐磨性。结果表明,SiC颗粒均匀分布于金属基体中,并与基体界面结合良好。Mg₁₇Al₁₂相在SiC颗粒附近优先析出,SiC与AZ91基体的热膨胀系数失配导致高密度位错的产生,加速基体的时效析出。与AZ91合金相比,SiC颗粒的加入提高了复合材料的硬度和抗压强度,这主要是由于载荷传递强化和晶粒细化强化机制。此外,由于SiC具有优异的耐磨性,在磨损过程中形成稳定的支撑面保护基体。     

Near-net shaped Al/SiCP composites via vacuum-pressure infiltration combined with gelcasting

To solve the problem of difficult machining, the near-net shaped Al/SiC_P composites with high volume fraction of SiC particles were fabricated by vacuum-pressure infiltration. The SiC_P preform with a complex shape was prepared by gelcasting. Pure Al, Al4Mg, and Al4Mg2Si were used as the matrices, respectively. The results indicate that the optimal parameters of SiC_P suspension in gelcasting process are pH value of 10, TMAH content of 0.5 wt.%, and solid loading of 52 vol.%. The Al matrix alloyed with Mg contributes to improving the interfacial wettability of the matrix and SiC particles, which increases the relative density of the composite. The Al matrix alloyed with Si is beneficial to inhibiting the formation of the detrimental Al₄C₃ phases. The Al4Mg2Si/SiC_P composite exhibits high relative density of 99.2%, good thermal conductivity of 150 W·m⁻¹·K⁻¹, low coefficient of thermal expansion of 10.1×10⁻⁶ K⁻¹, and excellent bending strength of 489 MPa.     

Microstructures and mechanical properties of Al 2519 matrix composites reinforced with Ti-coated SiC particles

Ti-coated SiC_p particles were developed by vacuum evaporation with Ti to improve the interfacial bonding of SiC_p/Al composites. Ti-coated SiC particles and uncoated SiC particles reinforced Al 2519 matrix composites were prepared by hot pressing, hot extrusion and heat treatment. The influence of Ti coating on microstructure and mechanical properties of the composites was analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the densely deposited Ti coating reacts with SiC particles to form TiC and Ti₅Si₃ phases at the interface. Ti-coated SiC particle reinforced composite exhibits uniformity and compactness compared to the composite reinforced with uncoated SiC particles. The microstructure, relative density and mechanical properties of the composite are significantly improved. When the volume fraction is 15%, the hardness, fracture strain and tensile strength of the SiC_p reinforced Al 2519 composite after Ti plating are optimized, which are HB 138.5, 4.02% and 455 MPa, respectively.     

Using ARB process as a solution for dilemma of Si and SiCp distribution in cast Al-Si/SiCp composites

A major challenge in achieving the best potential of SiC_p-reinforced aluminum composites is to homogeneously disperse SiC particles within the aluminum alloys. The presence of coarse Si fibers with non-uniform distribution in cast Al-Si alloys, which may lead to poor mechanical properties, is another important problem that limits the application of these alloys. In order to eliminate these problems, accumulative roll bonding (ARB) process was used in this study as a very effective method for improving the microstructure and mechanical properties of the Al356/SiC_p composite. It was found that when the number of ARB cycles was increased, the uniformity of the Si and SiC_p in the aluminum matrix improved, the Si particles became finer and more spheroidal, the free zones of Si and SiC particles disappeared, the porosity of composite decreased, the bonding quality between SiC_p and matrix improved, and therefore mechanical properties of the composites were improved. The microstructure of the manufactured Al356/SiC_p composite after six ARB cycles indicated a completely modified structure so that its tensile strength and elongation values reached 318 MPa and 5.9%, which were 3.1 and 3.7 times greater than those of the as-cast composite, respectively.     

SiC_p/2024与2219铝合金电子束焊接

分别采用电子束对中焊、偏束焊技术,研究了Si C颗粒增强铝基复合材料Si Cp/2024与2219铝合金的接头组织及力学性能.结果表明,对中焊时接头易出现Si C增强相的偏聚,同时发生严重的界面反应,生成大量脆性相Al4C3,接头抗拉强度最高为104 MPa.采用偏束焊工艺可以很好地抑制界面反应,通常只在焊缝上部与Si Cp/Al热影响区上部生成少量脆性相Al4C3,接头抗拉强度最高可达131 MPa.试件均断裂在母材界面反应层上,且为明显的脆性断裂.不同工艺下接头横截面硬度分布存在突变区,该区域在Si Cp/2024熔合区附近,该处脆性相Al4C3的生成导致硬度升高.     

Preparation and characterization of SiCp/2024Al composite foams by powder metallurgy

SiC_p/2024Al composite foams were manufactured by powder metallurgical methods using foaming agent CaCO₃ in order to enrich the foam fabrication process and promote its development and extensive application. The effects of CaCO₃ and SiC volume fractions on the foaming behaviours were investigated by means of SEM and Magiscan-2A image analysis technique. The influence of SiC content on the compressive behaviour was analyzed using Gleeble 1500 thermal simulation testing machine. The experimental results show that with increasing the foaming agent, the porosity and pore dimension increase first and decrease later. With increasing the reinforcement content, the porosity and pore dimension decrease. The compressive curves reveal that the introduction of SiC particles can improve compressive yield strength and energy absorption capacity. Meanwhile, it is found that SiC_p/2024Al composite foams are the brittle foam materials.     

Electron beam welding of SiCp/2024 and 2219 aluminum alloy

SiC_p/2024 matrix composites reinforced with SiC particles and 2219 aluminum alloy were joined via centered electron beam welding and deflection beam welding, respectively, and the microstructures and mechanical properties of these joints were investigated. The results revealed that SiC particle segregation was more likely during centered electron beam welding (than during deflection beam welding), and strong interface reactions led to the formation of many Al₄C₃ brittle intermetallic compounds. Moreover, the tensile strength of the joints was 104 MPa. The interface reaction was restrained via deflection electron beam welding, and only a few Al₄C₃ intermetallic compounds formed at the top of the joint and heat affected zone of SiC_p/Al. Quasi-cleavage fracture occurred at the interface reaction layer of the base metal. Both methods yielded a hardness transition zone near the SiC_p/2024 fusion zone,and the brittle intermetallic Al₄C₃compounds formed in this zone resulted in high hardness.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.     

Microstrucmral characteristics and properties in centrifugal casting of SiCp/Zl104 composite

The microstructural characteristics and Brinell hardness of a cylinder produced by centrifugal casting were investigated using 20% (volume fraction) SiC_p/Zl104 composites. Macrostructure and XRD analysis show that most of SiC particles segregate to the external circumference of the cylinder, the other SiC particles maintain in the inner circumference of the cylinder, and a free particle zone is left in the middle circumference of the cylinder. Microstructural characteristics and quantitative assessment of SiC particles show that most of congregated SiC particles in 20%SiC_p/Zl104 composites are dispersed by centrifugal force, and the other congregated SiC particles and most of alumina oxide are segregated to the inner circumference of the cylinder. The SiC particles in aluminum melt can promote the refinement of primary α(Al) during solidification, and fine primary α(Al) grains can also promote the uniform distribution of SiC particles. Brinell hardness of SiC_p/Zl104 composites is connected with not only the volume fraction of SiC particles, but also the distribution of SiC particles in matrix alloy.     

Influence of addition of Grp/Al2O3p with SiCp on wear properties of aluminum alloy 6061-T6 hybrid composites via friction stir processing

Aluminum alloy base surface hybrid composites were fabricated by incorporating with mixture of (SiC+Gr) and (SiC+Al₂O₃) particles of 20 μm in average size on an aluminum alloy 6061-T6 plate using friction stir processing (FSP). Microstructures of both the surface hybrid composites revealed that SiC, Gr and Al₂O₃are uniformly dispersed in the nugget zone (NZ). It was observed that the addition of Gr particles rather than Al₂O₃ particles with SiC particles, decreases the microhardness but immensely increases the dry sliding wear resistance of aluminum alloy 6061-T6 surface hybrid composite. The observed microhardness and wear properties are correlated with microstructures and worn micrographs.     

Thermal residual stresses in co-continuous composites

The thermal residual stresses in two types of co-continuous composites copper/aluminum oxide (Cu/Al₂O₃) and aluminum/aluminum oxide (Al/Al₂O₃) were measured by neutron diffraction experiments. These stresses were generated during the cooling after high processing temperature. The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases led to significant amount of thermal stresses. In both the composites, the metallic phase was found to be under tension and aluminum-oxide phase under compression. Even though the magnitude of compressive stress in both the composites was similar; the two metal-phases had very different magnitude of tensile stresses. The difference in volume fraction, CTE, elastic stiffness and plastic flow properties led to this difference. The hydrostatic stresses were found to be predominant in both the phases. Finite element simulations were used to predict the stress distributions inside each phase and at the interfaces. A representative unit cell approach was considered to represent the composite. Concept of effective ΔT was utilized to simulate the thermal stress distribution inside the two phases in the unit cell. This model utilized the neutron diffraction measurements to predict the stress distribution inside each phase and at the interface. The simulations showed that significant amount of tensile stresses develop at the metal–ceramic interfaces.     

Microstructure and Mechanical Properties of Al356/SiCp Cast Composites Fabricated by a Novel Technique

In this study, SiC_p containing composite powders were used as the reinforcement carrier media for manufacturing cast Al356/5 vol.% SiC_p composites. Untreated SiC_p, milled particulate Al-SiC_p composite powder, and milled particulate Al-SiC_p-Mg composite powder were injected into Al356 melt. The resultant composite slurries were then cast from either a fully liquid state (stir casting) or semisolid state (compocasting). The results revealed that by injection of composite powders, the uniformity of the SiC_p in the Al356 matrix was greatly improved, the particle-free zones in the matrix were disappeared, the SiC particles became smaller, the porosity was decreased, and the matrix microstructure became finer. Compocasting changed the matrix dendritic microstructure to a finer non-dendritic one and also slightly improved the distribution of the SiC_p. Simultaneous utilization of Al-SiC_p-Mg composite powder and compocasting method increased the macro- and micro-hardness, impact energy, bending strength, and bending strain of Al356/SiC_p composite by 35, 63, 20, 20, and 40%, respectively, as compared with those of the composite fabricated by injection of untreated SiC_p and stir casting process.     

Cold spray of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with soft metal incorporation: A technical contribution

Al + SiC, Al + Al₂O₃ composites as well as pure Al, SiC, and Al₂O₃ coatings were prepared on Si substrates by the cold gas dynamic spray process (CGDS or cold spray). The powder composition of metal (Al) and ceramic (SiC, Al₂O₃) was varied into 1:1 and 10:1 wt.%, respectively. The propellant gas was air heated up to 330 °C and the gas pressure was fixed at 0.7 MPa. SiC and Al₂O₃ have been successfully sprayed producing coatings with more than 50 μm in thickness with the incorporation of Al as a binder. Also, hard ceramic particles showed peening effects on the coating surfaces. In the case of pure Al metal coating, there was no crater formation on hard Si substrates. However, when Al mixed with SiC and Al₂O₃, craters were observed and their quantities and sizes depended on the composition, aggregation and size of raw materials.     

SiCp/Al复合材料电子束焊接接头组织及性能

对SiCp/Al复合材料自身进行电子束焊接,研究了其接头成形、焊缝组织、热影响区组织及接头力学性能.结果表明,SiCp/Al复合材料自身直接电子束焊接时,接头的主要缺陷是焊缝成形差、易形成两侧堆积颗粒物的凹槽;焊缝组织中存在界面反应产生的灰白色初生硅、深灰色针状相Al4C3以及Al-Si共晶中的浅灰色针状共晶硅,形成脆性区,拉伸断裂位置便在此处,断裂为脆性断裂.熔合区附近硬度较高,与焊缝区组织及硬度差异较大.接头的最高强度为73 MPa,仅占母材平均抗拉强度的41％.     

Dry Sliding Wear Behavior of a Novel 6351 Al-(Al4SiC4 + SiC) Hybrid Composite

In this research study, the dry sliding wear behaviors of 6351 Al alloy and its composites with single and hybrid reinforcements (ex situ SiC and in situ Al₄SiC₄) were investigated at low sliding speed (1 ms^?1) against a hardened EN 31 disk at different loads. In general, the wear mechanism involved adhesion (coupled with subsurface cracking) and microcutting-abrasion at lower loads. With higher loads, abrasive wear involving microcutting and microplowing along with adherent oxide formation was observed. At higher loads, the abrasive wear mechanism caused rapid wear loss initially up to a certain sliding distance beyond which, by virtue of frictional heat generation and associated temperature rise, an adherent oxide layer was developed at the pin surface, which drastically reduced the wear loss. Moreover, the overall wear rates of all the composites (either single or hybrid reinforcement) were found to be lower than that of the 6351 Al alloy at all applied loads. The ex situ SiC particles were found to resist abrasive wear; while, in situ Al₄SiC₄ particles offered resistance to adhesive wear. Accordingly, the 6351 Al-(SiC + Al₄SiC₄) hybrid composite exhibited the best wear resistance among all composites.     

Effect of α-Al2O3 coatings on the interface of Ni/SiC composites prepared by electrodeposition

This paper describes an investigation on the effect of α-Al₂O₃ coating on the interface between nickel and SiC particle. Uniform, dense and well-adhered α-Al₂O₃ coatings were obtained on the surface of SiC particles by sol–gel technology. The nickel-based composites reinforced with α-Al₂O₃-coated SiC particles (CS_p) or uncoated SiC particles (UCS_p) prepared by composite electrodeposition were heated at 600 °C to study the reactivity and the resulting interfaces. The results showed that the Ni/CS_p composites presented excellent thermal stability without interfacial reaction, while nickel silicide formed in the Ni/UCS_p composites. It indicated that high-temperature interfacial reaction between SiC particles and nickel matrix was efficiently inhibited by the α-Al₂O₃ coatings. Moreover, great differences of the local mechanical properties of interfaces were observed by the nanoindentation characterization.     

Production of High-Strength Al/Al2O3/WC Composite by Accumulative Roll Bonding

In this study, Al/Al₂O₃/WC composites were fabricated via the accumulative roll bonding (ARB) process. Furthermore, the microstructure evolution, mechanical properties, and deformation texture of the composite samples were reported. The results illustrated that when the number of cycles was increased, the distribution of particles in the aluminum matrix improved, and the particles became finer. The microstructure of the fabricated composites after eight cycles of the ARB process showed an excellent distribution of reinforcement particles in the aluminum matrix. Elongated ultrafine grains were formed in the ARB-processed specimens of the Al/Al₂O₃/WC composite. It was observed that as the strain increased with the number of cycles, the tensile strength, microhardness, and elongation of produced composites increased as well. The results indicated that after ARB process, the overall texture intensity increases and a different-strong texture develops. The main textural component is the Rotated Cube component.     

Fabrication and characterization of cast magnesium matrix composites by vacuum stir casting process

A vacuum stir casting process is developed to produce SiC_p reinforced cast magnesium matrix composites. This process can eliminate the entrapment of external gas onto melt and oxidation of magnesium during stirring synthesis. Two composites with Mg-Al9Zn and Mg-Zn5Zr alloys as matrices and 15 vol.% SiC particles as reinforcement are obtained. The microstructure and mechanical properties of the composites and the unreinforced alloys in as-cast and heat treatment conditions are analyzed and evaluated. In 15 vol.% SiC_p reinforced Mg-Al9Zn alloy-based composite (Mg-Al9Zn/15SiC_p), SiC particles distribute homogenously in the matrix and are well bonded with magnesium. In 15 vol.% SiC_p reinforced Mg-Zn5Zr alloy-based composite (Mg-Zn5Zr/15SiC_p), some agglomerations of SiC particles can be seen in the microstructure. In the same stirring process conditions, SiC reinforcement is more easily wetted by magnesium in the Mg-Al9Zn melt than in the Mg-Zn5Zr melt. The significant improvement in yield strength and elastic modulus for two composites has been achieved, especially for the Mg-Al9Zn/15SiC_p composite in which yield strength and elastic modulus increase 112 and 33%, respectively, over the unreinforced alloy, and increase 24 and 21%, respectively, for the Mg-Zn5Zr/15SiC_p composite. The strain-hardening behaviors of the two composites and their matrix alloys were analyzed based on the microstructure characteristics of the materials.     

Fabrication and characterization of Al356/SiCp semisolid composites by injecting SiCp containing composite powders

Al356/5 vol.% SiC_p cast composites were fabricated by the injection of reinforcement particles into the melt in three different forms, i.e. as untreated SiC_p, milled particulate Al-SiC_p composite powder, and milled Al-SiC_p-Mg composite powder. The resultant composite slurries were then cast in the semisolid temperature range of the alloy, upon which the effects of the type of injected powder on the distribution and incorporation of the reinforcement particles, along with the hardness of the cast composites, were investigated. Injection of milled composite powders resulted in considerable improvement in SiC_p wetting as well as the incorporation and distribution of SiC_p in the Al356 matrix alloy. Al356/5 vol.% SiC_p composite with well dispersed reinforcement particles of less than 3 μm average diameter was successfully produced by injecting Al-SiC_p-Mg composite powder into the melt. The best microstructural characteristics in terms of the reinforcement incorporation and distribution, and the highest hardness value of the cast composites, were achieved when magnesium was added through the injected composite powder and not directly into the melt.