Development of Al3Ti and Al3Zr intermetallic particulate reinforced aluminum alloy AA6061 in situ composites using friction stir processing

Aluminum rich intermetallic particles are potential reinforcements for discontinuously reinforced aluminum matrix composites (DRAMCs). The objective of the present work is to produce AA6061/Al₃Ti and AA6061/Al₃Zr composites using in situ casting technique and applying friction stir processing (FSP) to enhance the distribution and morphology of Al₃Ti and Al₃Zr particles. AA6061/Al₃Ti and AA6061/Al₃Zr DRAMCs were produced by the in situ reaction of inorganic salts K₂TiF₆ and K₂ZrF₆ with molten aluminum. The microstructure was observed using optical and scanning electron microscopy. AA6061/Al₃Ti DRAMC exhibited clusters of Al₃Ti particles while the segregation of needle shape Al₃Zr particles was observed in AA6061/Al₃Zr DRAMC. The prepared composites were subjected to FSP. Significant changes in the distribution and morphology of Al₃Ti and Al₃Zr particles were observed after FSP. The changes in microhardness and sliding wear behavior of AA6061/Al₃Ti and AA6061/Al₃Zr DRAMCs before and after FSP is detailed in this paper.     

6061 Al reinforced with zirconium diboride particles processed by conventional powder metallurgy and mechanical alloying

The homogenous distribution of the reinforcement phase is an essential condition for a composite material to achieve its superior performance. Powder metallurgy (PM) can produce metal matrix composites in a wide range of matrix reinforcement compositions without the segregation phenomena typical of casting processes. Particularly, mechanical alloying can be used to mix the matrix and reinforcement particles, enhancing the homogeneity of the reinforcement distribution. This work investigates the production of aluminium 6061 reinforced with zirconium diboride by mechanical alloying followed by cold pressing and hot extrusion, and compares the results with the same composite produced by conventional PM and hot extrusion. The incorporation of the ZrB₂ particles produces only a small increase in the material hardness, but a small decrease in the UTS when conventional PM is employed. Mechanical alloying breaks the reinforcement particle clusters, eliminates most of the cracks present in the surface of the reinforcement particles, decreases its size and improves its distribution. This enhancement of the composite structure, in addition to the metallurgical aspects promoted by mechanical alloying in the matrix, brings approximately 100% improvements in the composite UTS and hardness, compared with the composites obtained by PM.     

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.     

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.     

Particle reinforced AA 6061 and A359 aluminium alloys under cyclic thermal loading conditions

Abstract

The thermal cycling creep characteristics of aluminium matrix composites AA 6061+22 vol.-%Al₂O₃particles and A359+20 vol.-%SiC particles were analysed with respect to the dependence of the behaviour on the applied external stress and on different heat treatment conditions. The specimens were thermally cycled using a triangular waveform between 50 and 300°C with dwell times of 3 s at the minimum and maximum temperatures. An enhanced lifetime was found for reinforced aluminium alloy 6061 compared with A359+20 vol.-%SiC and it was found that overaging of the matrix prior to the thermal cycling experiments increases the creep rates significantly in both composites. The determination of the true, thermally compensated strain revealed a much higher in cycle strain evolution in A359+20 vol.-%SiC. Reverse creep during the cooling sequence was observed in both composites where it depends on the applied stress.     

A nano-indentation study on the mechanical behaviour of the matrix material in an AA6061 - Al2O3 MMC

The nano-indentation technique is a suitable technique to measure hardness and elastic moduli profiles of AA6061 reinforced with Al₂O₃ particles, since it allows measurements of mechanical properties on a micrometer range. To investigate possible local variations in mechanical behaviour of the matrix material due to precipitation reactions being affected by the presence of ceramic reinforcements, nano-indentation tests were done on both metal matrix composite (MMC) as well as unreinforced reference material, in three different heat treatment conditions. Matrix response depends on heat treatment condition, but is approximately equal for the MMC and the base reference alloy. Due to the various imposed heat treatments, magnesium enrichment around the ceramic particles was observed, but hardness and elastic modulus of this interfacial layer could not be measured. To confirm the preferential segregation of Mg near the particle/matrix interface, linescans were made with a Scanning Electron Microscope (SEM) equipped with EDS (Energy Dispersive Spectrum) facilities. The limited width of the Mg rich zone explains the absence of typical 'interphase' indentations in this investigation. Hardly any differences in calculated elastic moduli and hardness values were found for the three heat treatment conditions investigated, when comparing results of AA6061 reference material with results of an AA6061 matrix in an MMC. This result is of great importance when modelling the mechanical behaviour of MMCs using the finite element method, since it permits the assumption that the MMC matrix material behaves similar to the same aluminium alloy without ceramic reinforcements.     

A study on microstructure and mechanical properties of Al 6061-TiB2 in-situ composites

In situ reinforced aluminium based metal matrix composites (AMMCs) are emerging as one of the most promising alternatives for eliminating the inherent defects associated with ex situ reinforced AMMCs. Researchers in recent past have attempted various processing techniques for the development of in-situ composites, of which liquid metallurgy is the most widely adopted technique. Development of in-situ composites via liquid metallurgy route using master alloys is a relatively new processing technique. Very little information is available providing the usage value of these reinforcing materials.The present study is an attempt to explore the processing and characterization of in situ AMMCs using master alloys as reinforcement materials. Al 6061-TiB₂ in-situ composites were fabricated by liquid metallurgy route using Al 6061 as the matrix material and Al-10%Ti and Al-3%B as reactive reinforcements. Tests carried out on the fabricated composites include XRD, metallographic studies, EDAX analysis, microhardness, grain size analysis and tensile strength tests. The developed composites exhibited superior structural properties when compared with base alloy.     

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.     

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.     

Effect of friction stir welding on microstructure,tensile and fatigue properties of the AA7005/10 vol.%Al2O3p composite

Several studies have been recently focused on friction stir welding of aluminium alloys and some data are also reported on FSW of aluminium-based composites. The application of this solid state welding technique to particles reinforced composites seems very attractive, since it should eliminate some typical defects induced by the traditional fusion welding techniques, such as: gas occlusion, undesidered interfacial chemical reactions between the reinforcement and the molten matrix alloy, inhomogeneous reinforcement distribution after welding. The present work describes the effect of the FSW process on the microstructure and, consequently, on the tensile and low-cycle fatigue behaviour, of an aluminium matrix (AA7005) composite reinforced with 10 vol.% of Al₂O₃ particles (W7A10A). The microstructural characterization evidenced, in the FSW zone, a substantial grain refinement of the aluminium alloy matrix (due to dynamic recrystallization induced by the plastic deformation and frictional heating during welding) and a significant reduction of the particles size (due to the abrasive action of the tool). Tensile tests showed a high efficiency of the FSW joints (about 80% of the ultimate tensile strength). The low-cycle fatigue tests evidenced a fatigue life reduction for the FSW material respect to the base composite, particularly for high values of total strain range. The fracture mechanisms for the FSW specimens were those typical of metal matrix composites: interfacial decohesion, void nucleation and growth, as well as fracture of reinforcing particles, as shown by SEM analyses of the fracture surfaces.     

Optimization of friction stir welding process parameters to maximize tensile strength of stir cast AA6061-T6/AlNp composite

Aluminium Matrix Composites (AMCs) reinforced with particulate form of reinforcement has replaced monolithic alloys in many engineering industries due to its superior mechanical properties and tailorable thermal and electrical properties. As aluminium nitride (AlN) has high specific strength, high thermal conductivity, high electrical resistivity, low dielectric constant, low coefficient of thermal expansion and good compatibility with aluminium alloy, Al/AlN composite is extensively used in electronic packaging industries. Joining of AMCs is unavoidable in many engineering applications. Friction Stir Welding (FSW) is one of the most suitable welding process to weld the AMCs reinforced with particulate form of ceramics without deteriorating its superior mechanical properties. An attempt has been made to develop regression models to predict the Ultimate Tensile Strength (UTS) and Percent Elongation (PE) of the friction stir welded AA6061 matrix composite reinforced with aluminium nitride particles (AlN_p) by correlating the significant parameters such as tool rotational speed, welding speed, axial force and percentage of AlN_p reinforcement in the AA6061 matrix. Statistical software SYSTAT 12 and statistical tools such as analysis of variance (ANOVA) and student’s t test, have been used to validate the developed models. It was observed from the investigation that these factors independently influenced the UTS and PE of the friction stir welded composite joints. The developed regression models were optimized to maximize UTS of friction stir welded AA6061/AlN_p composite joints.     

Microstructure and mechanical properties of cast (Al–Si)/SiCp composites produced by liquid and semisolid double stirring process

Abstract

A stirring process containing two steps, i.e. liquid and then semisolid stirring, was used to produce SiC particle reinforced aluminium matrix composites. The major advantages of this process are that full wetting of SiC particles by molten aluminium can be readily achieved at relatively low stirring rates, and undesirable Al₄ C₃ is not formed at the Al/SiC interface due to lower processing temperatures. Cast Al–Si matrix composites reinforced with 15 and 20 vol.-%SiC particles were produced in the present work. The mechanical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained without employing semisolid stirring, the aggregation of SiC particles observed in the microstructure of composites resulted in quite poor mechanical properties. Observations and analyses indicated that some Al/SiC interfaces were very clean, and a reaction product of spinel MgAl₂O₄ was also found at some Al/SiC interfaces. Silicon dioxide (SiO₂ ) was found to exist on the surface of as purchased and 250°C dried SiC powders. This SiO₂ is involved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.     

Slurry erosive wear behavior of Ni–P coated Si3N4 reinforced Al6061 composites

Ni–P coated Si₃N₄ reinforced Al6061 composites were fabricated by liquid metallurgy route. Percentage of reinforcement was varied from 4 wt% to 10 wt% in steps of 2. The developed composites were subjected to microstructure and sand slurry erosive wear studies. The influence of experimental parameters such as slurry concentration, rotational speed of slurry, size of impinging particles and the test duration on slurry erosive wear behavior of developed composites have been studied. Results reveals that, Al6061–Si₃N₄ composites exhibited improved wear resistance when compared with the matrix alloy under identical test conditions. With increase in slurry concentration, rotational speed of slurry, test duration, size of impinging particles, the slurry erosive wear rates of both matrix alloy and developed composites increases. However, under all the tests conditions studied, the developed composites possess higher wear resistance when compared with that of matrix alloy. Energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to identify the oxides/passive layer formed on the worn surfaces. Scanning Electron Microscopy (SEM) examinations were also carried out on worn surfaces to observe the possible mechanisms of material removal in the matrix alloy and developed composites.     

Microstructural evolution in ultrasonically processed in situ AZ91 matrix composites and their mechanical and wear behavior

AZ91 alloy matrix composites reinforced with phases formed in situ from the addition of Si particles were fabricated by solidification under ultrasonic vibrations. Application of high-intensity ultrasonic field to the melt resulted in optimized size, morphology and distribution of in situ formed Mg₂Si particles. The amount of Mg₂Si particles increased, its size was refined and the distribution became uniform. Heterogeneous nucleation from the addition of silicon particles and enhanced nucleation from rapid cooling refined the grain size of the matrix in the composites. Hardness and ultimate compressive strength of the composites increased as compared to that of the cast AZ91 alloy. Composites exhibited improved sliding wear behavior of under varying normal loads. Identified dominant wear mechanism at lower sliding velocities is abrasion. Improvement in mechanical and sliding wear properties of the composites is attributed to the refinement of both matrix and reinforcement phases and improved dispersion of the reinforcement under ultrasonic vibrations.     

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.     

Potentiality of artificially aged aluminium 6061 hybrid composites reinforced with granite and graphite micro–fillers for structural applications

The current study explores the feasibility of artificial aging treatment to aluminium alloy 6061 hybrid composites reinforced with graphite and fine granite particulates to suit structural applications. Aging at 100 °C retrieved a better response of the hybrid composites as compared to that at 150 °C in terms of peak hardness and strength. Among the five different stir‐cast compositions, the composition of aluminium 6061 with 2 wt. % graphite and 4 wt. % granite showed enhanced mechanical properties than that of the base alloy as well as mono reinforcement type composites which was attribued to the formation of rod‐shaped β‐Mg₂Si precipitates.     

Development of light composite materials with low coefficients of thermal expansion

Abstract

Composite materials based on aluminium are used in different fields where weight, thermal expansion, and thermal stability are key requirements. The aim of the present study was to develop a universal method and scientific approach for evaluating the design of lightweight, Al matrix composites with low coefficients of thermal expansion (CTE) and high dimensional stability, and to produce such composites using the vacuum plasma spray (VPS)process. The methodology is general and could be applied to other composite systems. The VPS-produced Al and Al alloy 6061 based composites were reinforced with a variety of ceramic particles including Si₃N₄, B₄C, TiB₂, and 3Al₂O₃.2SiO₂. These composites have low CTE values ((12–13)×10^-6 K^-1), similar to that of steel, and high dimensional stability (capable of keeping dimensions stable with changes in temperature). They have low porosity (98–99%dense) and a uniform distribution of the strengthening particles. Hot rolling of the VPS-formed composites, followed by heat treatment, resulted in a significant improvement in the mechanical properties. Deformed and heat treated 6061 based composites, containing 20 wt-%TiB₂ and 40 wt-%3Al₂O₃?2SiO₂, showed excellent mechanical properties (ultimate tensile strength 210–250 MPa, elongation >4%).     

Microstructure investigations and mechanical properties of an Al‐Al2O3 MMC produced by semi‐solid solidification

The influence of the semi‐solid solidification production parameters (shear rate and agitation time) and the concentration of reinforcing particles on the microstructure formation and mechanical properties of a 520 aluminum alloy reinforced with Al₂O₃ particles was investigated. Depending on the content of reinforcing particles and the stirring conditions different rosette structures were formed. The type of wear mechanism (delamination or adhesion) depends on the size of the rosettes and the distribution of Al₂O₃ reinforcements. Best mechanical properties were obtained for metal matrix composites reinforced with 12 wt% of Al₂O₃ stirred at a shear rate of 2100 s^–1 for 1800 s. These samples showed tensile strength and yield stress similar to the commercial A520 alloy. The hardness and wear resistance were improved by the addition of Al₂O₃ particles, meanwhile the elongation to fracture was reduced.     

Fatigue properties of friction stir welded particulate reinforced aluminium matrix composites

Few papers have discussed the friction stir welding (FSW) of particulate reinforced aluminium matrix composites and most of them focused on the set-up of the welding process parameters and their effect on microstructure, hardness and tensile behaviour. The aim of this study was to investigate the fatigue resistance of FSW joints on an as-cast particulate reinforced aluminium based composite (AA6061/22 vol.%/Al₂O_3p). The welding process was performed using different process parameters, also investigating their effect on joint microstructure. The mechanical properties of the FSW composites were compared with those of the base material and the results were correlated to the microstructural modifications induced by the FSW process on the aluminium alloy matrix and the ceramic reinforcement. FSW reduced the size of both particle reinforcement and aluminium grains, and also led to a significant increase in interparticle matrix microhardness, for all process parameters. The FSW specimens belonging to a different set of parameters, tested without any post-weld heat treatment, exhibited a very high joint efficiency (ranging from 90% to 99%) with respect to the ultimate tensile strength of the base material. The stress controlled fatigue test showed a high spread both for the base and FSW composites. Statistical analysis disclosed that all FSW specimens belonging to different process parameters showed apparently slightly worse fatigue behaviour than that of the base composite. Statistical processing applied to the different welding parameters revealed that all the welded specimens belonged to the same population. Therefore it can be concluded that the parameters used produced joints with similar microstructure and comparable fatigue behaviour. The slight difference in the fatigue behaviour of the FSW specimens whose process parameters differed form those of the unwelded composite was explained by the different microstructural homogeneity in the transition from the base to the FSW zone.     

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.