Extrusion textures in Al, 6061 alloy and 6061/SiCp nanocomposites

The 6061 alloy matrix composites reinforced with 10 wt.% and 15 wt.% of SiC nanoparticles with an average diameter of ~ 500 nm were hot extruded in strip shape from ball milled powders. The microstructures and textures of the hot extruded nanocomposites have been investigated by means of three dimensional orientation distribution functions and electron backscatter diffraction (EBSD) techniques. Pure Al and 6061 alloy extruded strips from atomised powders have been produced for comparison purposes. The results show that the non-deformable SiC particulates have a strong influence on the formation of extrusion textures in the matrix. Pure Al and 6061 alloy develop a typical β fibre texture after extrusion in strip shape. For 6061/SiC_p nanocomposites, the intensities of major texture components decrease with increasing amount of SiC particles. The total intensities of Brass, Dillamore and S components have decreased by 19% for 6061/10 wt.% SiC_p and 40% for 6061/15 wt.% SiC_p composites when compared with the 6061 alloy. EBSD analysis on local grain orientations shows limited Al grain rotations in SiC rich zones and decreased texture intensities.     

Effect of heat treatment on strength and abrasive wear behaviour of Al6061-SiC<Subscript>p</Subscript> composites

In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several aerospace and automobile applications. Aluminum 6061 has been used as matrix material owing to its excellent mechanical properties coupled with good formability and its wide applications in industrial sector. Addition of SiC_p as reinforcement in Al6061 alloy system improves its hardness, tensile strength and wear resistance. In the present investigation Al6061-SiC_p composites was fabricated by liquid metallurgy route with percentages of SiC_p varying from 4 wt% to 10 wt% in steps of 2 wt%. The cast matrix alloy and its composites have been subjected to solutionizing treatment at a temperature of 530°C for 1 h followed by quenching in different media such as air, water and ice. The quenched samples are then subjected to both natural and artificial ageing. Microstructural studies have been carried out to understand the nature of structure. Mechanical properties such as microhardness, tensile strength, and abrasive wear tests have been conducted both on matrix Al6061 and Al6061-SiC_p composites before and after heat treatment. However, under identical heat treatment conditions, adopted Al6061-SiC_p composites exhibited better microhardness and tensile strength reduced wear loss when compared with Al matrix alloy.     

A study of the strength of P/M 6061Al and composites during high strain rate superplastic deformation

Metal Matrix Composites (MMCs) are potential candidate materials in the aerospace and automobile industries because of its attractive properties, in particular, their high specific properties, and Superplastic forming (SPF) is a good solution to the problems in the forming process of MMCs due to their low ductility resulting from the incorporation of reinforcement. High strain rate superplasticity (HSRS) is attractive for industrial applications because superplastic forming at high strain rates can reduce forming time greatly. The strength of P/M 6061 Al and 6061 Al/SiC_p (3 m) composites during superplastic deformation at temperatures of 853 K–871 K and a high strain rate of 0.1 s^–1 has been studied in this paper. Experimental results presented a softening effect by the SiC_p reinforcement. Mechanical and microstructural analyses show that the decrease in the strength during high strain rate superlastic (HSRS) deformation is associated with the decreased grain size of the Al matrix with increase of the SiC_p volume fraction or the extrusion ratio, and the occurrence of liquid phase. The formation of the liquid phase was related to segregation of the solute atom during HSRS deformation.     

High strength Al2O3p/6061 Al composites: effect of particles,subgrains and precipitates

Abstract

Composites of 6061 Al reinforced by Al₂O₃ particles have been produced by squeeze casting followed by hot extrusion and a T4 precipitation hardening treatment. The tensile properties at room temperature have been determined and analysed based on microstructural parameters. The strength contributions from the matrix, particles, subgrains and precipitates have been estimated individually, and then based on an assumption of linear additivity, the yield stress values of the composites under the extruded and heat treated conditions have been calculated. Good agreement between the calculated and experimental values has been found, illustrating the suitability of the process for the manufacture of strong composites, with a maximum yield stress of 524 MPa obtained for a composite containing 60 vol.-%Al₂O₃.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Compressive stress-strain response of directionally aligned SiC<Subscript>w</Subscript>/Al composite

A SiC_w/6061Al composite was fabricated through a squeeze-casting route and hot extruded to obtain a composite with directionally aligned whiskers. Based on observed changes in whisker orientation and length before and after deformation, compressive deformation behaviour of the directionally aligned SiC_w/Al composite was investigated. It is found that when the compressive temperature is much lower than the solidus of the matrix alloy, the compressive flow stress of the directionally aligned composite is increased with compressive strain first and then decreased. When the compressive temperature equals the solidus of the matrix, however, the compressive flow stress of the directionally aligned composite is increased monotonously with compression strain. During compression, whisker rotation and breakage occurred, and the higher the compressive temperature, the easier the whisker rotation and hence the smaller the degree of whisker breakage. When the compressive strain was quite high, the degree of whisker breakage was serious even at the temperature as high as the solidus of the matrix. Analyzing changes in whisker orientation and breakage before and after compression indicates that the decreased compressive flow stress with compressive strain is the result of the decreased load carrying ability of whiskers caused by whisker rotation and breakage. Compared with whisker rotation, whisker breakage has a bigger contribution to the decreased compressive flow stress. No strain softening in the composite compressed at 580°C can be thought to be a result of the very low strengthening effect of whiskers at such a high temperature. From the point of view of whisker breakage, to get higher properties of SiC_w/Al composite parts made by means of plastic forming, too high plastic strain should not be suffered by SiC_w/Al composites during the plastic forming.     

Fatigue Crack Growth and Fracture of 30 wt% B4C/6061Al Composites

This paper focuses on studying the fatigue crack growth (FCG) characteristics and fracture behaviours of 30 wt% B₄C/6061Al composites fabricated by using powder metallurgy and hot extrusion method. Compact tension (CT) specimens having incisions parallel to the extrusion direction (T‐D) and perpendicular to the extrusion direction (E‐D) were investigated through FCG tests. Results show that, at low/medium stress‐intensity factor range levels (ΔK ≤ 9), crack propagation rate in E‐D specimens is lower than that in T‐D specimens because the elongated B₄C particles parallel to the extrusion direction in E‐D specimens can deflect the crack. The scanning electron microscope micrographs of the fractured surface illustrate that crack mainly propagates in the matrix alloy at the initial stage of its propagation and propagates more remarkably near the particle‐matrix interface with the increase of ΔK value. B₄C particles are also found to be easy to fracture during the rapid crack propagation. Based on fracture analyses, considering the impacts of factors like crack deviation, plastic zone size at the crack tip, and crack driving force, a 2‐D crack propagation model was developed to study the fatigue crack propagation mechanism in the 30 wt% B₄C/6061Al composite.     

Incorporation of new technique for processing of Al/SiCp composites based on evaporative pattern casting (EPC) method

Abstract

A336 Al matrix composites containing different volume fraction and mean mass particle size of SiC particles as the reinforcing phase were synthesised by evaporative pattern casting (EPC) route. The process consisted of fabricating of EPS/SiC_p composite pattern followed by EPC of A336 Al alloy. The EPS/SiC_p pattern was made by blending SiC particles with expandable polystyrene (EPS) beads and placing them in expanding mould heating with steam until EPS beads expand completely. Uniform distributed SiC particles around the EPS beads and locally movement of them during pouring and degradation leads to homogenous distribution of particles in final Al/SiC_p composite. Higher modulus, strength and hardness were observed in the composites than the unreinforced Al alloy part. The fracture surfaces of the composite samples exhibited dimple surfaces and fracture in SiC particles.     

ECAPT 工艺对 SiCp/ Al 基复合材料组织和性能的影响

探究了用等径角挤扭(ECAPT)法制备的SiC_p/Al基复合材料组织演化过程和力学性能。比较了2种体积分数的SiC_p(8.75%和35%)复合材料在ECAPT中组织演化规律,测试了其硬度值及致密度。结果表明,ECAPT形变量大,细化晶粒能力强,SiC_p分布均匀性好,其致密效果SiC_p体积分数大的(35%)比小的(8.75%)差;并且体积分数为35%的SiCp复合材料存在"拱桥效应",但颗粒细化效果优于体积分数为8.75%的SiC_p。     

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.     

Refining SiCp in reinforced Al–SiC composites using equal-channel angular pressing

Aluminum–silicon carbide composite (Al–SiC_p) is one of the most promising metal matrix composites for their enhanced mechanical properties and wear resistance. In the present study, Al–SiC (average size 55 μm) composites with 5% and 10% by volume were fabricated by stir casting technique. The equal-channel angular pressing (ECAP) was then applied on the cast composites at room temperature in order to study the effect of ECAP passes on the SiC_p size and distribution. The ECAP process was successfully carried out up to 12(8) passes for Al–5%(10%)SiC samples. Microstructure study revealed that the highest refinement by breakage of SiC_p was achieved after the first ECAP pass and that further refinement took place in the next passes. More breakage of the SiC_p was found in the composite richer in reinforcing particles so that the SiC_p reached approximately 1 μm in the Al–10%SiC after 8 passes and 4 μm in Al–5%SiC after 12 ECAP passes. The distribution of SiC reinforcement particles also improved after applying ECAP. The factors including decrease in reinforcing particle size, improvement in their distribution, decrease in porosity in addition to strain hardening and grain refining of the matrix resulted in enhancement of tensile and compressive strengths as well as hardness by more than threefold for the Al–5%SiC after 12 passes and for Al–10%SiC after 8 passes compared to the cast composites. Additionally, the composite remained ductile after the ECAP process. The fracture surface indicated good bond between the matrix and the reinforcement.     

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.     

Mg_2B_2O_(5W),SiC和Gr颗粒增强6061Al基复合材料的摩擦磨损行为

采用粉末热挤压法制备2%Mg2B2O5w/6061Al,2%Gr/6061Al,2%SiCp/6061Al,2%Mg2B2O5w+2%Gr/6061A,2%Mg2B2O5w+2%SiCp/6061Al,2%Mg2B2O5w+2%Gr+2%SiCp/6061Al单一及混杂增强的铝基复合材料,并对其耐磨性和摩擦行为进行研究。结果表明:随着载荷的增大,各种复合材料的磨损率均增大,石墨的添加增大了铝基复合材料的磨损率;复合材料的摩擦因数随载荷的增大而降低并趋于稳定,摩擦因数均介于0.22~0.32之间。未加入石墨的复合材料的磨损机制以磨料磨损和轻微的黏着磨损为主,加入石墨后复合材料的磨损机制转变为剧烈的黏着磨损。     

SiCw/6061Al复合材料冲击破坏行为

本文用夏比冲击试验(Instrumented Charpy Testing)与SEM断口分析研究了SiCw/6061Al复合材料的冲击破坏行为.与SiCw是混乱分布的铸态SiCw/6061Al复合材料相比,挤压变形后材料的冲击韧性明显提高.热挤压变形改善了材料的性能.研究观察发现了分层开裂的现象.本文详细讨论了SiCw/6061Al复合材料的冲击断裂方式、比较了变形前后冲击破坏方式的差异,并分析了热挤压变形后韧性提高的原因.     

Corrosion behavior of SiC p /6061 Al metal matrix composites in chloride solutions

The corrosion behavior of silicon carbide particulates-aluminum metal matrix composites was studied in chloride solution by means of electrochemical techniques, scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope. The materials under investigation were compocasting processed 6061 Al reinforced with increasing amounts of SiC particulates. Electrochemical tests such as potentiostatic polarization were done in 0.1 kmol·m^–3 NaCl solutions that were aerated and deaerated to observe overall corrosion behavior. In addition, pit morphology was observed after immersion tests. It was seen that the pitting potentials did not vary greatly or show definite trends in relation to the amounts of SiC _p reinforcement. However, the degree of corrosion increased with increasing SiC _p content; probably mainly due to galvanic couple. No intermetallics layer was found at the SiC _p /Al interface. Based on pitting potentials of Al-Si alloys, a pitting process around SiC particulate was proposed.Abbreviations SiC _p (silicon carbide particulates) - SiC _f (silicon carbide fibers) - SiC _w (silicon carbide whiskers) - E_pit (pitting potential) - E_prot (protection potential) - E _corr (corrosion potential) - i _galv (galvanic current density) - E _galv (galvanic potential)     

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.     

Constitutive modelling concerning the effects of particulate shapes for the cyclic deformation of SiCP/6061Al composites at high temperature

Abstract

Based on the Eshelby equivalent inclusion theory and the extended Mori–Tanaka method, a new meso-mechanical cyclic visco-plastic constitutive model was obtained to describe the cyclic deformation of SiC_P/6061Al composites at high temperature. In the proposed model, a thermal recovery term was introduced into the newly developed non-linear kinematic hardening rule to represent the thermal recovery effect of the matrix alloy produced at high temperature. Meanwhile, the Eshelby tensor for ellipsoidal inclusions with different aspect ratios was employed into the new model to reflect the effect of particle shapes on the cyclic deformation of the composites. Finally, the simulated results of the proposed model to the time-dependent monotonic tension and uniaxial cyclic deformation of SiC_P/6061Al composites with different particulate shapes at high temperature was verified to be reasonable by comparing with the corresponding experiment ones and finite element simulations.     

Studies on the fracture and flexural strength of Al/Sip composite

Pure aluminum matrix composite reinforced with a high volume fraction of silicon particles (Al/Si_p) was fabricated by gas-pressured infiltration. The results of four point flexural strength tests show that Al/Si_p has low flexural strength. The analysis of the fractograph reveals the fracture mechanism of Al/Si_p. The fracture of Al/Si_p is primarily dominated by the fracture of brittle silicon particles and the subsequent link up of damage through the matrix. The pre-existent microcracks in silicon particles that were made during the process of compacting will also lower the flexural strength of Al/Si_p composite. The hybrid particle reinforced pure aluminum matrix composite (Al/Si_p+SiC_p) was fabricated in the same way. Results show the flexural strength can be improved by 11.3% compared with Al/Si_p when 6 vol.% silicon particles are replaced by silicon carbide particles with the same volume fraction and size. The reason is that SiC_p with higher fracture stress and higher elastic modulus can prevent the rapid expansion of cracks through the composite and lower the stress in silicon particles.     

Low temperature extrusion of 6061 aluminum matrix composite reinforced with SnO2-coated Al18B4O33 whisker

The 6061 aluminum matrix composite reinforced with SnO₂-coated Al₁₈B₄O₃₃ whisker was fabricated by squeeze casting and following by extrusion extruded at elevated temperatures from 300 °C to 400 °C. Optimization of the extruding process, microstructure, texture and mechanical properties of the extruded composites were investigated. The lowest extrusion temperature at which a composite rod with high surface quality was successfully produced was 300 °C. The yield strength of composites is much improved after extrusion, and especially their elongation is increased by 300%. Such big improvements depend on a fact that SnO₂ coating can introduce low-melting-point Sn phase into the interface through an interfacial reaction. The melting of interphase and their surrounding areas is the main reason for the excellent extrusion ability of the composite. Besides, detailed X-ray diffraction analysis of the extruded composite textures reveals the significant effects of extrusion temperatures on their features.     

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.