Tensile behaviour of squeeze cast and forged short alumina fibre reinforced AA 6061

Abstract

The tensile properties of a composite consisting of 20 vol.-% short δ alumina fibres in an aluminium matrix (AA 6061) prepared by squeeze casting have been investigated, before and after 30% reduction by forging. By annealing the composite before forging, a 30% forging reduction could be achieved at room temperature, without crack formation. A reduction in mean fibre length from about 65 to 15 μm was observed but most fibre breaks were filled by matrix. By heat treating the composites after forging, their elongation to fracture was increased to about twice that of a similarly heat treated unforged composite of comparable strength. The improvement of ductility is attributed to break-up of the fibre skeleton structure inherited from the fibre preform. A model is presented that predicts that for these fibres an optimum effective reinforcement is achieved at fibre lengths of about 100 μm, which explains why the reduction in fibre length caused by forging does not result in significant strength loss.

MST/1720     

Joining of alumina short-fibre reinforced AA6061 alloy to AA6061 alloy and to itself

The weldability of aluminium short-fibre reinforced AA6061 alloy (FRM) to AA6061 alloy and to itself using aluminium brazing materials has been investigated. AA4045 and BA03 were selected as brazing materials. When FRM was brazed to AA6061 alloy with AA4045 sheet, a disorder of fibre orientation near the interface was recognized at a brazing temperature above 863 K. Furthermore, the interface became very irregular and porous. The tensile strength achieved was about 100 MPa on brazing below 863 K. On the other hand, BA03 sheet, which has thin AA4045 layers on an AA3003 alloy layer, made the joint strong. The strength was about 200 MPa. BA03 induced little disorder of fibre arrangement and better contact at the interfaces. The BA03/AA6061 alloy interface was more porous than the FRM/BA03 interface and, hence, weaker. FRM/FRM joints with BA03 sheet had good strengths above 200 MPa.     

Fabrication of gradient distribution alumina short fibre reinforced aluminium alloy

Gradient distribution alumina short fibre reinforced 6061 aluminium alloy have been fabricated by taking advantage of preform compressive deformation during squeeze casting. Pressure was applied mechanically by a punch. Velocity of the punch, pre-heat temperature of the preforms and pouring temperature were controlled during the infiltration of molten 6061 alloy into alumina short fibre preforms. The distribution of hardness along the infiltration direction in the composites was measured and the distribution of volume fraction along the infiltration direction was calculated by the hardness. Velocity of the inflow, pre-heat temperature of the preform, pouring temperature of the molten metal, binder content of the preform and volume fraction of fibres, all have a very great effect on the gradient distribution of alumina short fibres in the aluminium alloy composites.     

Fabrication of alumina short fibre reinforced aluminium alloy via centrifugal force infiltration

Abstract

A new casting process for fabricating short fibre reinforced metal matrix composites via the centrifugal force infiltration of fibre preforms with molten aluminium alloys is described. The effect of processing variables, such as pouring temperature, preheated mould temperature, and time of application of centrifugal force, on the infiltration kinetics and resultant microstructure is discussed. Composites having fibre volume fractions of 4·5, 8·0, 12, and 16% were obtained via this method. Comparisons with existing infiltration technology and the mechanical properties of the composite are presented.

MST/2000     

Thermal expansion properties of 6061 Al alloy reinforced with SiC particles or short fibers

Thermal expansion measurements are reported for 6061 Al alloy and drawn composite materials reinforced with SiC particles or aligned short fibers. Samples with volume fractions of 5 and 20% SiC were measured in the drawing direction. The measurements were obtained from repeated heating and cooling cycles between room temperature and 500°C. Cumulative plastic strains were measured for the repeated thermal cycles, in association with the observation that lower expansivities generally occurred on cooling as compared with heating. Model calculations for particulate and an aligned fiber are presented for the combined elastic-plastic deformation properties of the Al/SiC composite systems. Lower expansivities and greater plastic strains are accounted for in the fiber-reinforced material.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Creep behaviour of AA 6061 metal matrix composite alloy and AA 6061

Abstract

The tensile creep properties of a pure AA 6061 matrix and an AA 6061 matrix reinforced with 22% of irregularly shaped Al₂O₃ particles (metal matrix composite) are presented for a temperature of 573 K and initial stresses between 15 and 70 MPa (where 70 MPa is about one-half of the yield stress). The metal matrix composite (MMC) was fabricated by a stir casting process and both materials were extruded. All the specimens were overaged before testing. The MMC exhibits a higher secondary creep rate for the whole range of loads. A stress exponent of n ≈ 1 for stresses from 15 to 25 MPa for the unreinforced material indicates the dominating diffusional creep mechanism. A stress exponent of n ≈ 3 is found from 25 to 50 MPa concluding dominating dislocation creep for the unreinforced material. This mechanism is found to be dominating for the MMC from as low as 15 MPa to 50 MPa (n ≈ 3). Although the secondary creep rate of the reinforced samples is higher than that of the unreinforced, the exposure time is longer for the MMC at stress levels below 20 MPa. The transition between the secondary and the tertiary creep stage occurs earlier in the unreinforced material. Thus, the 1% creep limit of the unreinforced alloy is reached only in the tertiary creep stage, whereas it can be applied as a conservative design criterion for the composite in the whole stress range. Furthermore, the MMC promises at low stress levels higher creep lifetime than the unreinforced alloy. Creep damage in the tertiary stage of the MMC was found to be as a result of void nucleation resulting in particle decohesion from the matrix. Relatively high tertiary creep strains are produced by necking of the unreinforced samples.     

Influence of alkali treatment and fibre length on mechanical properties of short Agave fibre reinforced epoxy composites

Composites based on short Agave fibres (untreated and alkali treated) reinforced epoxy resin using three different fibre lengths (3 mm, 7 mm and 10 mm length) are prepared by using hand lay up and compression mould technique. The materials were characterized in terms of tensile, compressive, flexural, impact, water absorption properties and machinability behaviour. All mechanical tests showed that alkali treated fibre composites withstand more fracture strain than untreated fibre composites. As evidenced by the dynamic mechanical analysis (DMA) tests, the thermo-mechanical properties of the composite with alkali treated Agave fibre were considerably good as alkali treatment had facilitated more sites of fibre resin interface. The machinability and atomic force microscope (AFM) studies were carried out to analyze the fibre–matrix interaction in untreated and alkali treated Agave fibre–epoxy composites.     

The structure and tensile properties of Al-Si alloy hybrid reinforced with alumina-aluminosilicate short fibre

Alumina-aluminosilicate fibre hybrid reinforced aluminium-silicon alloy was fabricated by pressure infiltration route. Tensile test results at room temperature and at 300°C are reported. It is shown that the alumina-aluminosilicate fibre ratio does have a strong influence on the ultimate tensile strength (UTS) of these hybrid composites. At an alumina-aluminosilicate ratio of 32, the optimum UTS of hybrid metal matrix composites (MMC) was obtained. The UTS of only alumina fibre reinforced MMC was improved with increasing fibre volume fraction at 300°C. No fibre put-out was observed on the fracture surface and longitudinal section.     

Microstructure and some mechanical properties of fly ash particulate reinforced AA6061 aluminum alloy composites prepared by compocasting

Fly ash has gathered widespread attention as a potential reinforcement for aluminum matrix composites (AMCs) to enhance the properties and reduce the cost of production. Aluminum alloy AA6061 reinforced with various amounts (0, 4, 8 and 12 wt.%) of fly ash particles were prepared by compocasting method. Fly ash particles were incorporated into the semi solid aluminum melt. X-ray diffraction patterns of the prepared AMCs revealed the presence of fly ash particles without the formation of any other intermetallic compounds. The microstructures of the AMCs were analyzed using scanning electron microscopy. The AMCs were characterized with the homogeneous dispersion of fly ash particles having clear interface and good bonding to the aluminum matrix. The incorporation of fly ash particles improved the microhardness and ultimate tensile strength (UTS) of the AMCs.     

Thermomechanical fatigue of short fibre reinforced aluminium matrix piston alloy

Abstract

The technical potential of short fibre reinforced aluminium matrix composites lies in their higher stiffness and higher strength at elevated temperatures compared with unreinforced matrix alloys. In the present investigation, thermal cycling creep tests were conducted on the piston alloy AlSi12CuMgNi reinforced with 20% Saffil (Al₂O₃) short fibres, to simulate the cold start conditions of combustion engines. After processing of the metal matrix composite (MMC) by direct squeeze casting, four heat treatment conditions were produced. Specimens under constant load were thermally cycled between 50 and 300°C, whereby a heating and cooling speed of 12.5 K s1 was achieved. Series of up to 5000 cycles at tensile stresses between 20 and 80 MPa were executed, comparing reinforced specimens and unreinforced matrix material. The results of these experiments showed that the creep properties of the alloy, especially minimum creep rate and lifetime to fracture, were improved by the reinforcement. Furthermore, the creep rate of the MMC was essentially independent of the heat treatment condition, whereas the minimum creep rate was increased significantly for the matrix material by overaging. It can be concluded that precipitation strengthening influenced the creep properties of unreinforced specimens only, which is in good agreement with theoretical considerations. An analysis of fibre length revealed that the majority of the fibres broke at between 50 and 75% of the lifetime, just before the beginning of tertiary creep. Metallographic investigations using a scanning electron microscope did not show fibre pullout, but multiple fracture of fibres along the whole specimen. Micromechanical models for isothermal creep in short fibre reinforced aluminium alloys confirm the above results, since tertiary creep is assumed to be a consequence of fibre fracture.     

Properties of squeeze cast Mg-6Zn-3Cu alloy and its saffil alumina short fibre reinforced composites

In the present work, Mg-Zn-Cu alloy (ZC63) and its saffil alumina short fibre reinforced composites produced using the squeeze casting technique were evaluated for their properties. The unreinforced base alloys and their composites were characterized for their microstructure, hardness, yield strength, impact strength, wear resistance and corrosion resistance. The dependence of the properties of composites was studied as a function of fibre volume fraction. Results showed that the composites exhibited improved hardness, yield strength at elevated temperature and wear resistance in comparison to the monolithic alloy. However, ductility, impact strength and corrosion resistance of the composites were inferior to that of the base alloy. The nature of the base alloy matrix in determining the properties of the composites was discussed based on fractographic analysis.     

Age-hardening behaviour of liquid-processed 6061 alloy reinforced with particulates and short fibres

The age hardening of 6061-based metal matrix composites containing various types of reinforcement and produced by both rheocasting and preform infiltration has been investigated. It is shown that there is no real effect of the processing route on age hardening of these materials whatever the ageing temperature. Reinforcement size effects have also been investigated for particulate reinforcements and it is shown that, for particulates larger than 3 μm, mechanical effects dominate over those due to microstructure.     

Mechanochemical effect on the microstructure and mechanical properties in ultraprecision machining of AA6061 alloy

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the microgroove surface. There is a reduction in cutting and thrust forces with the application of ink during microcutting. Moreover, the microhardness of the ink-affected microgroove is lower than that of the ink-free microgroove. Numerous substructured grains exist in the ink-affected microgroove zone whilst deformed grains dominate in the ink-free microgroove zone produced by microcutting. Furthermore, the mechanochemical effect can facilitate the nucleation of precipitates in the microgroove zone and induce the formation of subgrains with multiple orientations. According to the analysis and calculation, the main texture components of the ink-affected sample are Goss {110}001 and R {124}211, and that of the ink-free sample are Brass {110}112, Copper {112}111 and S {123}634. Besides, a clear difference of slip systems is found between the ink-free and ink-affected microgrooves, and the results show that R texture is easier to form on the ink-affected microgroove.     

Dynamic mechanical properties of short sisal fibre reinforced polypropylene composites

The dynamic mechanical properties of short sisal fibre reinforced polypropylene composites containing both untreated and treated fibres have been studied with reference to fibre loading, fibre length, chemical treatments, frequency and temperature. By the incorporation of short sisal fibre into polypropylene, the storage moduli (E′)and loss moduli (E″) have been found to be increasing whereas the mechanical loss factor (tan δ) decreasing. The storage modulus decreases with increase in temperature. The treated fibre composites show better properties compared to untreated system. The Arrhenius relationship has been used to calculate the activation energy for the glass transition. The use and limitations of various theoretical equations to predict the tan δ and storage modulus of the fibre reinforced plastic composites have been discussed. Cole–Cole analysis has been carried out to understand the phase behaviour of the composite samples. A master curve for the modulus of the blend is drawn by applying the time–temperature super position principle.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.     

Formation of zinc phosphate coatings on AA6061 aluminum alloy

Conditions for forming zinc phosphate conversion coatings on AA6061 aluminum alloy have been investigated by characterizing coatings formed for different parameters of the coating bath. Morphological and compositional information on the coatings was assessed by SEM, EDX and XPS, and simple adhesion tests were undertaken to indicate the strengths of coating attachment. The emphasis was to identify conditions that give high coverage, uniform coatings of small, strongly adhered, zinc phosphate crystals. The use of low-zinc solutions (e.g. an atomic Zn/P ratio of 0.07) and normal-zinc solutions (Zn/P ratio 0.25) were compared; coatings formed by the two solution types appear comparable at pH 2, although at pH 4 the low-zinc solution is more effective. Fluoride in the concentration range 200–400 ppm is indicated to be a useful additive for the normal-zinc coating bath and in the 600–1000 ppm range for the low-zinc process. The use of acid etching in the pre-treatment appears to yield better coatings than when mechanical polishing alone is used.     

Influence of aging treatment on mechanical properties of 6061 aluminum alloy

Aluminum–magnesium–silicon (Al–Mg–Si) alloys show medium strength, excellent formability, good corrosion resistance and are widely used in extruded products and automotive body panels. The major advantage of these alloys is their age hardening response during the paint baking process as well as the fact that they exhibit no yield point phenomenon and Lüdering. In this study, the mechanical properties of a commercially available AA6061 alloy aged to various levels were studied. Peak-aged conditions were reached in this particular alloy after a 2 h heat treatment at 200 °C. The variation of the yield stress, ultimate tensile strength, ductility and strain hardening rate with aging time is measured and discussed in relation to the microstructural changes induced by the heat treatment.     

Influence of weathering on unreinforced and short glass fibre reinforced thermoplastic polyester

The influence of weathering on fracture toughness, Jc, yield strength, y, local ultrasonic velocity, VR, and microhardness, Hu, in unreinforced and a short glass-fibre-reinforced polyester-based thermoplastic Xenoy has been investigated. Unreinforced material weathered for 11 months outdoors in Perth, West Australia, exhibited a significant decrease in VR and Jc, whereas little change was shown in y, Hu and the fracture surface morphology. Irradiation for 1000 h by artificial ultraviolet rays upon the unreinforced material caused a considerable increase in Hu and only a slight deterioration in VR of the surface layer. Filling with the short fibres induced an improvement in y and a large reduction in Jc for the unreinforced material. The natural weathering of the reinforced material caused a small reduction in Jc but a large degradation in the slope of the R-curve. It was concluded that the measurement of Jc and the slope of R-curve in combination with VR and Hu was an effective way to study the effects of weathering on engineering plastics. © 1998 Chapman & Hall     

Short thermal fatigue crack propagation behavior of alumina short fibre reinforced aluminum matrix composites

Thermal fatigue resistance is one of the most important parameters to design engine materials. The thermal fatigue crack growth behavior of alumina short fibre (V _f = 18 vol.%) reinforced AlSi12CuMgNi aluminum alloy composite has been investigated under thermal cycling condition between room temperature and 280 °C. Initiation and propagation of thermal fatigue crack have also been discussed. The results show that in the range of short crack, the fibres play an important role in the path of thermal fatigue crack, and the crack propagation rate of composites is much larger than that of the matrix alloy.