Fabrication and characteristics of AA6061/SiCp composites by pressureless infiltration technique

The tensile properties and microstructures of AA6061/SiC_p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten AA6061 into the powder bed containing SiC_p occurred at 800 °C for 1 hour under a nitrogen atmosphere, it was possible to fabricate composites reinforced with SiC_p. Reaction product (Al₄C₃) was formed at the interface between SiC_p and Al alloy matrix. In addition, the amount and size of the Al₄C₃ is increased significantly by increasing the infiltration temperature. The reaction product (AlN) was formed as a result of the in situ reaction in both the control alloy and the composite. A significant strengthening even in the control alloy occurred due to the formation of in situ AlN particle even without an addition of SiC_p. While a further strengthening of the composite was produced by the reinforced SiC_p, strain to failure of the composite fabricated at 800 °C showed the lowest value (1.3%) in the T6 condition due to the formation of the severe reaction product (Al₄C₃). The grain size of the control alloy significantly decreased to about 20 m compared to 50 m for the commercial alloy. In addition, the grain size in the composite reinforced with SiC_p further decreased to about 8.0 m. This grain refinement contributed to strengthening of the control alloy and composite.     

Fabrication and characterisation of AZ91/SiC composite by pressureless infiltration method

Abstract

Since the spontaneous infiltration of molten AZ91 Mg alloy into the powder bed containing SiC particles occurred at 700°C for 1 h under a nitrogen atmosphere, it was possible to fabricate Mg alloy composites reinforced with SiC particles. Since the fabrication conditions (e.g. temperature, time and atmosphere) of the composite are different from those of the other fabrication route, reaction products formed during the composite fabrication were investigated in detail using field emission scanning electron microscopy and high resolution transmission electron microscopy. From the analysis of reaction products, the authors can identify the formation of MgO, MgAl₂O₄, Al₁₂Mg₁₇ and an AlN phase containing magnesium.     

Interfacial reactions and wetting in Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique

The present paper considers the microstructures of Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique. The composites were produced using an Al–10 wt.% Mg alloy with two oxide ceramic foams, spinel (MgAl₂O₄) and mullite (Al₆Si₂O₁₃), at 915 °C in a flowing N₂ atmosphere. Full infiltration of the aluminium alloy into the ceramic preform has been achieved with good bonding between the metal and ceramic phases. The composites were characterised by a range of techniques and compared with those for alumina from the literature. It has been found that the metal–ceramic interface of the composite consisted of an oxide layer near the ceramic phase and a nitride layer from Mg₃N₂ to AlN near the metal phase. The improvement of Al wetting and adhesion on the oxide ceramics by the addition of Mg and in the presence of N₂ was studied by a sessile drop technique to clarify which compound that formed at the interface contributed to the spontaneous infiltration.     

The interface in Al2O3 particulate-reinforced aluminium alloy composite and its role on the tensile properties

The interface characterization of the aluminium alloy reinforced with Al₂O₃ particulates ((Al₂O₃)_p/AI composite) was performed using X-ray diffractometry and energy dispersive X-ray spectroscopy. A layer of MgAl₂O₄ single crystals was observed at the (Al₂O₃)_p/Al interface in the as-received extruded composites. Such MgAl₂O₄ crystals formed at the surface of (Al₂O₃)_p are believed to grow by consuming a certain amount of (Al₂O₃)_p. Upon loading, interfacial debonding was observed to occur at the boundary between MgAl₂O₄ and the aluminium alloy, or along the MgAl₂O₄ layer itself. These experimental observations are correlated with the tensile properties of such composites.     

Microstructural development in Al/MgAl2O4 in situ metal matrix composite using value-added silica sources

Abstract

Al/MgAl₂O₄ in situ metal matrix composites have been synthesized using value-added silica sources (microsilica and rice husk ash) containing ～97% SiO₂ in Al-5 wt.% Mg alloy. The thermodynamics and kinetics of MgAl₂O₄ formation are discussed in detail. The MgO and MgAl₂O₄ phases were found to dominate in microsilica (MS) and rice husk ash (RHA) value-added composites, respectively, during the initial stage of holding the composites at 750 °C. A transition phase between MgO and MgAl₂O₄ was detected by the scanning electron microscopy and energy-dispersive spectroscopy (SEM–EDS) analysis of the particles extracted from the composite using 25% NaOH solution. This confirms that MgO is gradually transformed to MgAl₂O₄ by the reaction 3SiO_2(s)+2MgO_(s)+4Al_(l)→2MgAl₂O_4(s)+3Si_(l). The stoichiometry of MgAl₂O₄, n, computed by a new methodology is between 0.79 and 1.18. The reaction between the silica sources and the molten metal stopped after 55% of the silica source was consumed. A gradual increase in mean MgAl₂O₄ crystallite size, D, from 24 to 36 nm was observed in the samples held for 10 h.     

Fabrication of nano-sized Al2O3 reinforced casting aluminum composite focusing on preparation process of reinforcement powders and evaluation of its properties

In this study, a novel approach was used to fabricate Al₂O₃ nanoparticle reinforced aluminum composites to avoid agglomeration of nanoparticles in matrix. Al₂O₃ nanoparticles were separately milled with aluminum and copper powders at different milling durations and incorporated into A356 alloy via stir casting method. The effects of milling process and milling time on mechanical properties of the composites were evaluated by hardness, tensile, and compression tests. Based on the results, some of the composites, reinforced with Al₂O₃-metallic mixed powders, showed higher mechanical performance compared with that of the pure Al₂O₃ nanoparticle reinforced composite. This enhancement is related to uniform distribution of individual nanoparticles and grain refinement of A356 matrix, shown in microstructural studies. Moreover, the results showed that an increase in milling time, led to a gradual decrease in mechanical performance of the samples. It can be related to further oxidation of metallic powders that can act as inclusions and also further probable contamination of nanoparticles with increase in milling time. Studies on the fracture surfaces revealed that the failure of matrix was the basic mechanism of fracture in the composites. Agglomerated nanoparticles were observed on dendrites in the fracture surface of the Al₂O₃–Al reinforcement samples.     

Formation of spinel (MgAl2O4), MgO and pure Cu particles in Al-2Mg alloy-CuO particle composites

CuO particles were introduced into liquid Al-2Mg alloy by the vortex method to prepare an Al alloy-MgAl₂O₄ in situ particle composite, by reaction between CuO particles and the Al-2Mg alloy melt. Pure Cu, MgAl₂O₄ and MgO particles were detected in the particles extracted from the composites. DTA study showed partial dissolution of Cu in the matrix. Microhardness and hardness of the composites are higher than those of the base alloy. Both microhardness and hardness are higher for the Al-2Mg-2CuO composite than those of the Al-2Mg-5CuO composite. The hardness of the Al-2Mg-2CuO composite is remarkably high. The increase in microhardness has been attributed to the solid solution hardening effect with Cu as well as to the difference in CTE between the Al matrix and the particles. On the other hand, the improvement in hardness resulted from both solid solution hardening as well as the presence of hard particles such as MgAl₂O₄ and MgO.Retired.     

Deformation behaviour and microstructure of a 20% Al2O3 reinforced 6061 Al composite

Deformation and microstructural behaviours of a 20% (volume percent) particle reinforced 6061 Al matrix composite have been studied by torsion from 25 to 540°C with strain rates of 0.1, 1 and 5 s⁻¹. The logarithmic stress versus reciprocal temperature relationship exhibits two slopes indicating different deformation mechanisms. The 20% Al₂O₃/6061 Al composite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal much smaller subgrain size and higher volume of non-cellular substructures, as well as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the 10% Al₂O₃/6061 Al composite and matrix alloy the same test condition. The torsion fracture surface was studied and compared to the three point bending failure specimens.     

Property-microstructure correlation in in situ formed Al2O3, TiB2 and Al3Ti mixture-reinforced aluminium composites

The in situ formed Al₂O₃, TiB₂ and Al₃Ti mixture-reinforced aluminium composites were successfully fabricated by the reaction sintering of the TiO₂-B-Al system in a vacuum. With increasing boron content in the TiO₂-B-Al system, the amount of generated TiB₂ in the composites increased and Al₃Ti content decreased. At the same time the distribution uniformity of the in situ formed Al₂O₃ and TiB₂ particulates was obviously improved, and the size of the Al₃Ti particles was reduced. The in situ Al₂O₃ and TiB₂ particulates had sizes from 0.096–1.88 m. The interface between the in situ formed particulates and the aluminium matrix was clean, and no consistent crystallographic orientation relationship was found. The strength and elastic modulus of the composites was significantly improved by lowering the Al₃Ti content. When the boron content in the TiO₂-B-Al system rose, the morphology of the tensile fracture surface of the composites was changed from large fractured Al₃Ti blocks and fine dimples, to fine dimples and pulled-out particulates. The strengthening and fracture of the composites have been modelled.     

Utilization of Chemically Synthesized Fine Powders of SiC/Al2O3 Composites for Sintering

Fine powders of (Al₂O₃)_100–x(SiC)_x (0 ≤ x ≤ 50) composites were prepared by chemical route (named as pyrophoric technique) to achieve a uniform mixture of SiC in an alumina matrix. The chemically synthesized fine SiC/Al₂O₃ composite powders were sintered to form composites at 1450°C which is well below the sintering temperature of SiC. Sintering was performed in an argon atmosphere. Highly dense SiC/Al₂O₃ microstructures were achieved. An improvement in bulk density and hardness has been achieved for SiC/Al₂O₃ composites with 20 wt% of SiC. Hexagonal-shaped grains have been obtained in (Al₂O₃)₅₀(SiC)₅₀ composite with well-connected grain boundaries. The peak position of alumina in SiC/Al₂O₃ composites shifts toward lower wavenumbers in Fourier transform infrared spectroscopy and higher wavenumbers in Raman spectroscopy due to the incorporation of SiC in the composites. The optical band gap decreases with the addition of SiC and the composite behaves more like a semiconductor rather than an insulator. These properties make SiC/Al₂O₃ composites attractive for various industrial applications.     

Deformation Behaviour and Microstructure of a 20% Al2O3 Reinforced 6061 Al Composite

Deformation and microstructural behaviours of a 20% (volumepercent) particle reinforced 6061 Al matrix composite have been studied bytorsion from 25 to 540°C with strain rates of 0.1, 1 and5 s^-1. The logarithmic stress versus reciprocal temperaturerelationship exhibits two slopes indicating different deformationmechanisms. The 20% Al₂O₃/6061 Alcomposite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal muchsmaller subgrain size and higher volume of non-cellular substructures, aswell as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the10% Al₂O₃/6061 Al composite and matrixalloy the same test condition. The torsion fracture surface was studied andcompared to the three point bending failure specimens.     

Fabrication of Al/AlN composites by in situ reaction

The tensile properties and microstructures of various Al alloys fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. The spontaneous infiltration of molten metal into the powder bed occurred at 800 °C for 1 h under a nitrogen atmosphere. As a result, it was possible to fabricate Al alloys reinforced with AlN particles formed by in situ reaction. A significant strengthening even in the control alloy occurred due to the formation of in situ AlN particle even without an addition of artificial reinforcement. Strength values of the control alloy were increased with decreasing Al powders in bottom powders bed. In addition, tensile strength in Al–Mg alloys was increased with Mg content.     

Crack growth resistance (R-curve) behaviour and thermo-physical properties of Al2O3 particle-reinforced AlN/Al matrix composites

Crack growth resistance behaviour and thermo-physical properties of Al₂O₃ particle-reinforced AlN/Al matrix composites have been studied as a function of AlN volume fraction as well as Al₂O₃ particle size. The fracture toughness of the composites decreased with increase in vol% AlN and decrease in Al₂O₃ particle size. All the composites exhibited R-curve behaviour which has been attributed to crack bridging by the intact metal ligaments behind the crack tip. The Young’s modulus of the composites increased with the vol% of AlN whereas the thermal diffusivity and coefficient of thermal expansion followed a reverse trend. The composites exhibited hysteresis in thermal expansion as a function of temperature and the hysteresis decreased with decrease in metal content of the composite.     

Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 aluminium matrix composite

An Al-Mg-Si alloy matrix composite reinforced with 10 vol.% of alumina whiskers (Al₂O₃w) has been processed by powder metallurgy and investigated. The Al₂O₃w were produced as single crystal c-axis alpha-alumina fibres at pre-pilot scale via vapour-liquid-solid (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this is the first report of the utilization of whiskers of Al₂O₃ as reinforcing elements for Al alloys. Tensile tests have been performed on the composite at room and high temperatures. Results show that the AA6061 alloy reinforced with the as-produced Al₂O₃ whiskers has remarkably high mechanical properties at room temperature. This is attributed to the high quality of the Al₂O₃ single crystals and to the strong bonding attained between them and the 6061 alloy matrix.     

Microstructural characteristics and mechanical behavior of B4Cp/6061Al composites synthesized at different hot-pressing temperatures

B₄Cp/6061Al composites have become important structural and functional materials and can be fabricated by powder metallurgy and subsequent hot rolling. In this work, the effects of the hot-pressing temperature on microstructures and mechanical behaviors of the B₄Cp/6061Al composites were investigated. The results showed that compared with the T4 heat treated B₄Cp/6061Al composite hot pressed at 560 °C, the yield strength and failure strain of the composites hot pressed at 580 °C were increased to 235 MPa and 18.4%, respectively. This was associated with the interface bonding strength between the B₄C particles and the matrix. However, the reaction products, identified to be MgAl₂O₄ phases, were detected in the composites hot pressed at 600 °C. The formation of the MgAl₂O₄ phases resulted in the Mg depletion, thus reducing the yield strength to 203.5 MPa after the T4 heat treatment due to the effect of the solid solution strengthening being weakened. In addition, the variation of hardness and electrical conductivity was mainly related to the Mg content in the matrix. Based on the as-rolled microstructures observed by SEM, SR-μCT and fracture surfaces, the deformation schematic diagram was depicted to reflect the tensile deformation process of the composites.     

Fabrication of particulate aluminium-matrix composites by liquid metal infiltration

Aluminium-matrix composites were fabricated by liquid metal infiltration of porous particulate reinforcement preforms, using AlN, SiC and Al₂O₃ as the particles. The quality of the composites depended on the preform fabrication technology. In this work, this technology was developed for high-volume fraction (up to 75%) particulate preforms, which are more sensitive to the preform fabrication process than lower volume fraction whisker/fibre preforms as their porosity and pore size are much lower. The technology developed used an acid phosphate binder (with P/Al molar ratio=23) in the amount of 0.1 wt% of the preform, in contrast to the much larger binder amount used for whisker preforms. The preforms were made by filtration of a slurry consisting of the reinforcement particles, the binder and carrier (preferably acetone), and subsequent baking (preferably at 200 °C) for the purpose of drying. Baking in air at 500 °C instead of 200 °C caused the AlN preforms to oxidize, thereby decreasing the thermal conductivity of the resulting Al/AlN composites. The reinforcement-binder reactivity was larger for AlN than SiC, but this reactivity did not affect the composite properties due to the small binder amount used. The Al/AlN composites were superior to the Al/SiC composites in the thermal conductivity and tensile ductility. The Al/Al₂O₃ composites were the poorest due to Al₂O₃ particle clustering.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

New Al–AlN composites fabricated by squeeze casting: interfacial phenomena

Three Al–AlN composites (2024, 6060, 5754 with ∼45 vol% AlN) fabricated by squeeze casting were studied by TEM. Chemical reactions occurring at the matrix–AlN interfaces have been investigated. MgAl₂O₄ spinel crystals were found in 6060 and 5754 composites. The magnesium element of the matrix reacts with a very thin alumina layer which is deposited on the AlN surfaces during the liquid infiltration step. The 5754 composites exhibited a stronger reaction leading to the formation of MgO phases with the spinel. Degradation of the mechanical properties was clearly shown in that case. A faceting of the AlN surfaces was observed in all composites.     

Microstructure,tensile and fatigue properties of AA6061/20 vol.%Al2O3p friction stir welded joints

Metal matrix composites reinforced with Al₂O₃ particles combine the matrix properties with those of the ceramic reinforcement, leading to higher stiffness and superior thermal stability with respect to the corresponding unreinforced alloys. However, their wide application as structural materials needs proper development of a suitable joining processes. The present work describes the results obtained from microstructural (optical and scanning electron microscopy) and mechanical evaluation (hardness, tensile and low-cycle fatigue tests) of an aluminium alloy (AA6061) matrix composite reinforced with 20 vol.% fraction of Al₂O₃ particles (W6A20A), welded using the friction stir welding process. The mechanical response of the FSW composite was compared with that of the base material and the results were discussed in the light of microstructural modifications induced by the FSW process on the aluminium alloy matrix and on the ceramic reinforcement. The FSW reduced the size of both particles reinforcement and aluminium grains and also led to overaging of the matrix alloys due to the frictional heating during welding. The FSW specimens, tested without any post-weld heat treatment or surface modification showed lower tensile strength and higher elongation to failure respect to the base material. The low-cycle fatigue life of the FSW composite was always lower than that of the base material, mainly at the lower strain-amplitude value. The cyclic stress response curves of the FSW composite showed evidence of progressive hardening to failure, at all cyclic strain-amplitudes, while the base material showed a progressive softening.