Formation mechanism of in situ Al3Ti in Al matrix during hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al composites were fabricated by a combination of vacuum hot pressing (VHP) and friction stir processing (FSP). The formation mechanism of the Al₃Ti and the effect of VHP and FSP parameters on the resultant microstructure and mechanical properties were investigated. The Al₃Ti formed due to the reactive diffusion between Al and Ti during VHP, and the number of Al₃Ti particles increased with increasing the temperature and holding time of the VHP. FSP not only induced the Al–Ti reaction, but also resulted in significant refining of the Al₃Ti, thereby creating a homogeneous distribution of Al₃Ti particles in the Al matrix. These microstructural changes led to significant improvement in the tensile properties of the in situ Al₃Ti/Al composite. However, the change trends of the tensile properties of the FSP samples were dependent on the extent of the Al–Ti reaction during VHP.     

In situ Al3Ti and Al2O3 nanoparticles reinforced Al composites produced by friction stir processing in an Al-TiO2 system

Al and TiO₂ powders were selected to fabricate in situ Al composites via multiple pass friction stir processing (FSP) based on the thermodynamic analysis. The microstructural investigations indicated FSP would induce reaction between Al and TiO₂. Al₃Ti and Al₂O₃ particles were formed after 4 pass FSP with 100% overlapping. The in situ particles were about 80 nm in size at various FSP conditions, and ultrafine matrix grains 602 nm in size were obtained when water cooling was applied during FSP. Tensile tests indicated that the in situ nanocomposites exhibited pronounced work hardening behavior and a good combination of strength and ductility.     

Intermetallic-reinforced aluminum matrix composites produced in situ by friction stir processing

Friction stir processing (FSP) is applied to produce intermetallic-reinforced aluminum matrix composites from elemental powder mixtures of Al-Cu and Al-Ti. The intermetallic phases are identified as Al₂Cu and Al₃Ti, which are formed in situ during FSP. The volume fraction of the intermetallic phases in the in situ composites may reach as high as ∼ 0.5. The composites produced by FSP are fully dense with high strength, and the composite strength increases with the reinforcement content.     

Friction stir processing strategies to develop a surface composite layer on AA6061-T6

Friction stir processing (FSP) has been used to produce metal matrix composites by incorporating reinforcement particles in an AA6061-T6 matrix. Two types of particles (Al₂O₃ and SiC) were tested. Powder was placed into a mechanized square section groove on a plate surface and then sealed before FSP. This study investigates the effect of several strategies for reinforcement (number and direction of FSP passes) on the wear resistance behavior of friction stir-processed Al-SiC/Al₂O₃ composites. The distribution and size of the particles in the friction stir-processed zone were studied by optical and scanning electron microscopy. Ball-on-disk test was performed on both base material and surface metal matrix composites (SMMCs), and both friction coefficient and specific wear rate (SWR) were correlated with particle distribution and metallurgical effects on the metallic matrix. For all strategies and for both types of reinforcing particles used in this study, the friction coefficient decreases with respect to the base material. Moreover, the SWR is reduced for the conditions of one single FSP pass and two passes with opposite directions, when SiC are used. However, this positive effect has not been detected with Al₂O₃. Wear mechanisms in base metal and in SMMCs are compared and discussed in detail.     

Microstructural evolution of reinforcements in the remelting in situ TiC/Al-12Si composites treated by ultrasonic vibration

Clusters of reinforced particles and long rod-like Al₃Ti particles are usually present in the matrix of in situ TiC/Al alloy composites fabricated via SHS reaction of the Al-Ti-C system in the molten aluminum alloys. In order to improve the properties of the composites, the above issues should be solved effectively. In our research, high-intensity ultrasonic vibration was introduced into the remelting TiC/Al-12Si composites containing clusters of TiC particles and long rod-like Al₃Ti phase to optimize the microstructure of the composites. The results of SEM showed that long rod-like Al₃Ti particles were turned into small blocky ones and large clusters were broken up into small ones. In the meantime, individual TiC particles could be peeled off from the clusters and distributed uniformly in the matrix. An in situ TiC/Al-12Si composite with a homogeneous microstructure was attained successfully. The evolution of the morphology of Al₃Ti phase and the clusters in the ultrasonic field was also discussed.     

Uniting Strength and Toughness of Al Matrix Composites with Coordinated Al3Ni and Al3Ti Reinforcements

Hybrid aluminum composites are fabricated in a novel manner to characteristically induce a layer‐wise aligned distribution of micro‐scale Al₃Ni and Al₃Ti intermetallic particles that are formed in situ within a ductile Al matrix. The simple and unique Rolling of Randomly Orientated Layer‐wise Materials (RROLM) manufacturing methodology enables microstructural tailoring of the intermetallic reinforcing particles to prescribe enhanced crack tip deflection caused by the complex interaction of local veins of reinforcement particles, in an effort to overshadow the classical loss of toughness in large‐particle reinforced composites. The complimentary reinforcements and their interface with the Al matrix are revealed to have a gradual transition zone that functions to maintain critical cohesion with the particles and the matrix, empowering the superior load transfer capability of the particles, and reducing microvoid penetration into the matrix. In situ three‐point bending observations combined with a local strain field analysis, demonstrate the distinctive crack deflection mechanisms exhibit by the composite. Deviating from the norm, this specialized particle reinforced composite exhibited both strengthening and toughening mechanisms simultaneously, over control samples. The investigated design strategy and model material will assist materials development toward light‐weight, stronger, and tougher particle reinforced Al matrix composites.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

Fabrication of AA6061/Al2O3 nano ceramic particle reinforced composite coating by using friction stir processing

Nano ceramic particle reinforced composite coatings were created by incorporating Al₂O₃ ceramic particles into the surface of AA6061-T6 alloy plate with multiple pass friction stir processing (FSP). Optical microscopy and Micro-Vickers hardness tests were employed to investigate the influence of axial force and the number of FSP pass on the distribution of the ceramic particles and the hardness of the generated nano ceramic particle reinforced composite coating. Results show that the composite coating is as deep as the length of the pin probe. No distinct interface was developed between the coating and the base metal. The composite region becomes greater as the axial force and the number of FSP pass increased. At the same time, the distribution of the ceramic particles became more homogeneous. Nano particles in the coating have no significant effect on the macro-hardness of AA6061-T6 aluminum alloy even in the composite zone due to the softening of matrix material resulted from overaging. Spindle torque of the tool increased with increasing axial force, while it became less variable and smaller in subsequent pass compared to that in the first pass.     

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.     

Fabrication and properties of in situ synthesized particles reinforced aluminum matrix composites of Al–Zr–O–B system

A new in situ Al–Zr–O–B system is exploited. The Al–Zr(CO₃)₂–KBF₄ components are used to fabricate the particle reinforced aluminum matrix composites by the direct melt reaction method. The analytical results of XRD and SEM show that the in situ endogenetic particles are ZrAl₃, ZrB₂ and Al₂O₃, which are well distributed in the aluminum matrix. The sizes of reinforced particles are 0.5–2.5 μm. The results of mechanical properties of the composites show that the tensile strength and yield strength are improved with the increase of theoretical volume fraction of particles in matrix in the range of 0–12%, which are much superior to those of aluminum matrix. The best elongation of composites is 33% when the theoretical volume fraction is 3%. The fracture mechanism belongs to a ductile one. The wear resistance properties of the composites are much higher than that of aluminum matrix. The best abradability is got when the theoretical volume fraction of particles is 6%. The wear mechanism of the aluminum matrix is adhesive wear while the wear mechanism of (ZrAl₃ + ZrB₂ + Al₂O₃)_p/Al composites is abrasive wear.     

Fabrication of AA6061/Al2O3p composites from elemental and alloy powders

The tensile properties and microstructures of AA6061/Al₂O_3p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten metal into elemental powders bed as well as alloy powders bed occurred at 700°C for 1 hour under a nitrogen atmosphere, it was possible to fabricate 6061 Al matrix composite reinforced with Al₂O_3p irrespective of the type of metal powders. Both MgAl₂O₄ and MgO were formed at interfaces between Al₂O₃ and the matrix. In addition, MgAl₂O₄ was formed at within the matrix by in situ reaction during composite fabrication. Fine AlN was formed by in situ reaction in both composites. A significant strengthening in the composites occurred due to the formation ofin situ AlN particle and addition of Al₂O₃ particles, as compared to the commercial alloy, while tensile properties in the both elemental and alloy powders composites showed similar trend.     

Influence of in situ formed ZrB2 particles on microstructure and mechanical properties of AA6061 metal matrix composites

Particulate reinforced metal matrix composites (PMMCs) have gained considerable amount of research emphasis and attention in the present era. Research is being carried out across the globe to produce new combination of PMMCs. PMMCs are prepared by adding a variety of ceramic particles with monolithic alloys using several techniques. An attempt has been made to produce aluminium metal matrix composites reinforced with zirconium boride (ZrB₂) particles by the in situ reaction of K₂ZrF₆ and KBF₄ salts with molten aluminium. The influence of in situ formed ZrB₂ particles on the microstructure and mechanical properties of AA6061 alloy was studied in this work. The in situ formed ZrB₂ particles significantly refined the microstructure and enhanced the mechanical properties of AA6061 alloy. The weight percentage of ZrB₂ was varied from 0 to 10 in steps of 2.5. Improvement of hardness, ultimate tensile strength and wear resistance of AA6061 alloy was observed with the increase in ZrB₂ content.     

Fabrication of 5052Al/Al2O3 nanoceramic particle reinforced composite via friction stir processing route

In this research, microstructure and mechanical properties of 5052Al/Al₂O₃ surface composite fabricated by friction stir processing (FSP) and effect of different FSP pass on these properties were investigated. Two series of samples with and without powder were friction stir processed by one to four passes. Tensile test was used to evaluate mechanical properties of the composites and FSP zones. Also, microstructural observations were carried out using optical and scanning electron microscopes. Results showed that grain size of the stir zone decreased with increasing of FSP pass and the composite fabricated by four passes had submicron mean grain size. Also, increase in the FSP pass caused uniform distribution of Al₂O₃ particles in the matrix and fabrication of nano-composite after four passes with mean cluster size of 70 nm. Tensile test results indicated that tensile and yield strengths were higher and elongation was lower for composites fabricated by three and four passes in comparison to the friction stir processed materials produced without powder in the similar conditions and all FSP samples had higher elongation than base metal. In the best conditions, tensile strength and elongation of base material improved to 118% and 165% in composite fabricated by four passes respectively.     

Processing of heat-treated silicon carbide-reinforced aluminum alloy composites

The present study investigates the processing of heat-treated silicon carbide (SiC) particle-reinforced 6061 aluminum alloy (AA) composites. As-received SiC powders were heat treated at 1300ºC, 1400ºC, and 1500ºC in nitrogen atmosphere for 2 h, and the 6061 AA–SiC composites were developed by spark plasma sintering at 560ºC and 60 MPa for 5 min in argon atmosphere. The amorphous silicon nitride is found to form in SiC particles as a result of heating at 1400ºC. The microstructure of the composites exhibited uniform distribution of SiC or SiC/Si₃N₄ particles in 6061 AA matrix. Further, the heat-treated SiC-reinforced 6061AA composites exhibited improved mechanical properties. A typical combination of UTS of 240 MPa and elongation of 21% is obtained for the 6061 AA composites prepared using SiC powders heated at 1400ºC.     

Microstructural evolution and mechanical properties of ultrafine grained Al3Ti/Al–5.5Cu composites produced via hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al–5.5Cu composites fabricated by powder metallurgy and subsequent forging were subjected to multiple pass friction stir processing (FSP) with and without active cooling. The forged sample exhibited lower strength and ductility due to the presence of coarse Al₃Ti clusters with a size range of 50–100 μm and coarse matrix grains. Four-pass FSP in air resulted in the refinement and redistribution of the Al₃Ti clusters, and the generation of micron matrix grains, thereby increasing the strength and ductility of the composites. Furthermore, coarse Al₂Cu particles dissolved and re-precipitated due to a relatively long duration of thermal exposure. Additional two pass FSP with rapid water cooling (FSP-water) dissolved most of the Al₂Cu into the matrix and retained the solutes in solution due to the short duration of thermal exposure. Meanwhile, ultrafine matrix grains with a high density of dislocations were obtained. These microstructural changes led to significant increase in strength and a decrease in ductility in the FSP-water sample. After aging, the FSP-water sample exhibited further increased yield strength and ultimate tensile strength due to the precipitation of metastable Al₂Cu phases. However, the ductility did not decrease due to the decrease of dislocation density after aging.     

Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds

In this study, friction stir processing (FSP) was employed to modify cold-sprayed (CSed) AA2024/Al₂O₃ metal matrix composites (MMCs). Three different rotation speeds with a constant traverse speed were used for FSP. Microstructural analysis of the FSPed specimens reveals significant Al₂O₃ particle refinement and improved particle distribution over the as-sprayed deposits. After FSP, a microstructural and mechanical gradient MMC through the thickness direction was obtained. Therefore, a hybrid technique combining these two solid-state processes, i.e. CS and FSP, was proposed to produce functionally gradient deposits. The Guinier-Preston-Bagaryatskii zone was dissolved during FSP, while the amounts at different rotation speeds were approximately the same, which is possibly due to the excellent thermal conductivity of the used Cu substrate. Mechanical property tests confirm that FSP can effectively improve the tensile performance and Vickers hardness of CSed AA2024/Al₂O₃ MMCs. The properties can be further enhanced with a larger rotation speed with a maximum increase of 25.9% in ultimate tensile strength and 27.4% in elongation at 1500 rpm. Friction tests show that FSP decreases the wear resistance of CSed MMCs deposits due to the breakup of Al₂O₃ particles. The average values and fluctuations of friction coefficients at different rotation speeds vary significantly.     

Evaluation of microstructure and mechanical properties of Al/Al2O3/SiC hybrid composite fabricated by accumulative roll bonding process

In this investigation, a new kind of metal matrix composites with a matrix of pure aluminum and hybrid reinforcement of Al₂O₃ and SiC particles was fabricated for the first time by anodizing followed by eight cycles accumulative roll bonding (ARB). The resulting microstructures and the corresponding mechanical properties of composites within different stages of ARB process were studied. It was found that with increasing the ARB cycles, alumina layers were fractured, resulting in homogenous distribution of Al₂O₃ particles in the aluminum matrix. Also, the distribution of SiC particles was improved and the porosity between particles and the matrix was decreased. It was observed that the tensile strength of composites improved by increasing the ARB passes, i.e. the tensile strength of the Al/1.6 vol.% Al₂O₃/1 vol.% SiC composite was measured to be about 3.1 times higher than as-received material. In addition, tensile strength of composites decreased by increasing volume fraction of SiC particles to more than 1 vol.%. Scanning electron microscopy (SEM) observation of fractured surfaces showed that the failure mechanism of broken hybrid composite was shear ductile rupture.     

Microstructures and mechanical properties of Al/Al2O3 surface nano-composite layer produced by friction stir processing

In this study, a new processing technique, friction stir processing (FSP) was attempted to incorporate nano-sized Al₂O₃ into 6082 aluminum alloy to form particulate composite surface layer. Samples were subjected to various numbers of FSP passes from one to four, with and without Al₂O₃ powder. Microstructural observations were carried out by employing optical and scanning electron microscopy (SEM) of the cross sections both parallel and perpendicular to the tool traverse direction. Mechanical properties include microhardness and wear resistance, were evaluated in detail. The results show that the increasing in number of FSP passes causes a more uniform in distribution of nano-sized alumina particles. The microhardness of the surface improves by three times as compared to that of the as-received Al alloy. A significant improvement in wear resistance in the nano-composite surfaced Al is observed as compared to the as-received Al.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Material characterisation and mechanical properties of Al2O3-Al metal matrix composites

The mechanical properties of metal matrix composites (MMCs) are critical to their potential application as structural materials. A systematic examination of the effect of particulate volume fraction on the mechanical properties of an Al₂O₃-Al MMC has been undertaken. The material used was a powder metallurgy processed AA 6061 matrix alloy reinforced with MICRAL-20, a polycrystalline microsphere reinforcement consisting of a mixture of alumina and mullite. The volume fraction of the reinforcement was varied systematically from 5 to 30% in 5% intervals. The powder metallurgy composites were extruded then heat treated to the T6 condition. Extruded liquid metallurgy processed AA 6061 was used to establish the properties of the unreinforced material.