Studies on carbon fibre reinforced aluminium composite processed using pre-treated carbon fibres

Carbon fibre reinforced Al-12% Si alloy composite has been fabricated by pre-treating the fibres with K₂ZrF₆ followed by molten alloy infiltration and subsequent hot pressing of the preforms. The infiltration conditions were arrived at based on the measurement of tensile strength of the fibres extracted from the preforms. The fibre volume per cent of 20 was found to result in composite tensile strength of about 240 MPa as compared to tensile strength of 100 MPa for the unreinforced matrix. Characterization of the interface revealed the formation of ZrSi₂ and diffusion of potassium and aluminium into the fibre. The interfacial bonding was strong as is evinced by the absence of fibre pull-out on to the fracture surface.     

A TEM study of the interfaces and matrices of SiC-coated carbon fibre/aluminium composites made by the K2ZrF6 process

Carbon fibre-reinforced aluminium composites were pressurelessly cast by using K₂ZrF₆ as the wetting promotion agent. Transmission electron microscopy (TEM) and energy dispersed analysis of X-rays, (EDAX) were used. The results showed that interfacial reactions were very active after K₂ZrF₆ treatment. This was caused by the diffusion and reaction of zirconium in the surface of carbon fibres or in the SiC coating. Silicon alloying of aluminium could suppress the interfacial reactions by decreasing the activity of zirconium and changing intermetallic Al₃Zr to Zr₃Al₄Si₅, and building up the phase equilibrium between SiC, aluminium and silicon. The requested silicon content was higher than the equilibrium content of Al-Si-SiC system to suppress the SiC/Al interfacial reaction. A perfect interface was achieved in SiC-coated carbon fibre Al-12 wt% Si composite.     

Corrosion protection of carbon fibre reinforced aluminium composite by diamondlike carbon coatings

Abstract

Carbon fibre reinforced aluminium exhibits poor resistance against electrochemical corrosion in 3·5 wt-%NaCl solution. Diamondlike carbon (DLC) coatings provide properties which make them interesting materials for external corrosion protection on metal matrix composites (MMCs). The electrochemical corrosion behaviour of uncoated and DLC coated carbon fibre reinforced aluminium was tested in 3·5 wt-%NaCl solution. It has been found that the pitting potential is shifted significantly in the anodic direction and the corrosion current density is much lower due to the presence of the sealing DLC coating. Additionally, scratch tests and SEM studies were carried out in order to characterise the adhesion of the DLC films on the heterogeneous MMCs. Reliable corrosion protection is connected with sufficient coating durability under loading. In order to ensure sufficient loading capacity of the DLC coating under tribological conditions, wear tests were undertaken which revealed a considerable improvement in wear resistance due to deposition of the DLC coatings.     

Wetting improvement of carbon or silicon carbide by aluminium alloys based on a K2ZrF6 surface treatment: application to composite material casting

A surface treatment with aqueous solutions of K₂ZrF₆ has been carried out to improve, in dramatic manner, the wetting of carbon (or SiC)-base ceramics by liquid light alloys at low temperatures (i.e. within the 700 to 900°C range). The mechanism which is thought to be responsible for the wetting improvement involves two steps: (i) K₂ZrF₆ reacts with aluminium with the formation of K₃AlF₆, other complex fluoride species and intermetallics, (ii) K₃AlF₆ dissolves the alumina thin layer, coating the liquid light alloy and enables the wetting of the ceramics. The mechanism has been worked out from sessile drop experiments, solid state chemistry experiments and composite casting. The K₂ZrF₆ surface treatment appears to be particularly suitable for processing composite materials made of carbon (or SiC) fibrous preforms and aluminium-base matrices according to techniques directly derived from the light alloy foundry.     

Interlayer structure of carbon fibre reinforced aluminium wires

As the extent of the interfacial reactions controls the properties of metal matrix composites, the microstructural features and the chemical composition of the interlayers in aluminium wires reinforced with unidirectional carbon fibres (volume fraction app. 55%) have been investigated. High voltage and high resolution transmission electron microscopy of fibres, matrix, and interlayers, combined with analytical methods (electron energy loss spectroscopy and energy filtered microscopy) revealed a nanometre-sized C/Al interdiffusion layer and aluminium carbide needles or platelets of 10–50 nm thickness and 50–500 nm length in the matrix material, starting from the interlayer, the extension of which strongly correlates with the duration of melt contact. The observed interlayer phenomena impose restrictions to the process parameters, as by massive interface reactions the fibre strength is degraded, and the formation of brittle reaction products such as Al₄C₃ provides sites for initiation of fibre cracking and can cause composite failure. With a newly developed continuous process, which is capable of infiltrating endless products, the fibre/melt contact duration could be reduced to less than one second resulting in carbide formation lower than 0.2 wt% as confirmed by chemical analyses. So it was possible to achieve strength values of the composite wires that are as high as the theoretical prediction.     

High-temperature strengths of aluminium composite reinforced with continuous SiC fibre

Aluminium alloys were reinforced unidirectionally with 30, 35, 40 and 50 vol %SiC fibres by a liquid-pressing method. The SiC fibres for reinforcement were produced from a polycarbosilane and were yarns consisting of 500 continuous filaments of length 300 m and diameter 13m, having a tensile strength of 2000 MPa. High-temperature tensile and bending strengths of the composites were examined in air in the temperature range from room temperature to 500° C. The strengths were not influenced by temperature up to 400° C, but were decreased at 500° C. The decrease is considered to be caused by the reduction in transfer efficiency of the stress accompanying the decrease in adhesion between fibres and matrix.     

Fabrication of a carbon fibre/aluminium alloy composite under microgravity

Fabrication of a composite material with ultra low density and high stiffness under microgravity is the objective of the present investigation. The composite structure to be obtained is a random three-dimensional array of high modulus, short carbon fibres bounded at contact points by an aluminium alloy coated on the fibres. The material is highly porous and thus has a very low density. The motivation toward the investigation, simulation experiments, choice of the component materials and in-flight experiment during ballistic trajectory of a National Space Development Agency rocket are described herein. Supporting experimental evidence shows that the cohesion between the carbon fibre and the aluminium alloy is excellent, by which the achievement of desired properties of such composites seems probable.     

Resistance welding of carbon fibre reinforced thermoplastic composite using alternative heating element

The focus of this work is the use of a metal mesh as an alternative heating element for the joining of carbon fibre fabric reinforced polyetherimide composite laminate. A more homogeneous temperature distribution was generated by the metal mesh at the bonding surface. Glass fibre fabric reinforced PEI (GF/PEI) was used as an electrical insulator between the heating element and adherend laminates. Experimental results show that the GF/PEI prepreg could effectively prevent current leakage and enlarge the welding area. Welding parameters, such as input power level, welding time and pressure, were optimized according to the results of mechanical and microstructure characterization. Mechanical performance of composite specimens joined using metal mesh, in terms of lap shear strength and Mode I interlaminar fracture toughness, was equivalent to that of compression moulded benchmarks. Fracture surfaces of welded specimens showed mostly cohesive failure or intralaminar failure, indicating that good bonding between the PEI matrix and metal mesh was achieved.     

Fabrication of aluminium composites reinforced with carbon fibres by a centrifugal infiltration process

Centrifugal-force infiltration was used for obtaining aluminium alloy composites reinforced with carbon fibre by the infiltration of preforms. The lost-wax-casting technique was used during the manufacturing process. Preforms fabricated with different percentages of reinforcement were heated to facilitate their filling with aluminium. Some samples were coated with nickel to favor the reinforcement wetting by the molten aluminium alloy. Composites with volume fraction of reinforcements above 7 vol.% and porosity values lower than 0.5 vol.% were obtained with this technique. The hardness of the composites increased with the volume fraction of reinforcement and the solution and the later precipitation of nickel coating caused an additional hardening effect.     

Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G_1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.     

Preparation of carbon fibre reinforced aluminium via ultrasonic liquid infiltration technique

Abstract

An ultrasonic liquid infiltration technique has been developed for the fabrication of carbon fibre reinforced aluminium (CF/Al) precursor wires. The principal effect of ultrasound on aluminium infiltration into carbon fibres is considered to be caused by cavitation. The acoustic power required to produce cavitation in the present experimental system has been approximately calculated to be about 150 W, which is much greater than the requirement, several tens of watts, for overcoming the capillary pressure among carbon fibres. The observations on the morphology of the CF/Al precursor wires show that there are generally four states of infiltration: totally non-infiltrated, non-infiltrated in the centre and in some local regions of the wires, and completely infiltrated. It is found that carbon fibres can be sufficiently impregnated by molten aluminium given the appropriate application of ultrasound. Furthermore, a single fibre tensile test shows that there is no strength degradation of carbon fibres after aluminium infiltration. The CF/Al precursor wires obtained have an average fibre volume fraction of 26%. The maximum longitudinal tensile strength of the CF/Al wires is 605 MN m^?2, which implies a fibre strength transfer efficiency of 0·76.

MST/1715     

Bending characterisation of a molten unidirectional carbon fibre reinforced thermoplastic composite using a Dynamic Mechanical Analysis system

The quality of forming simulations based on Finite Element methods is mainly determined by the accuracy of the material properties. Out-of-plane bending is one of the deformation mechanisms that govern the appearance of wrinkles while forming composite reinforcements. This paper proposes a new test method using a Dynamic Mechanical Analysis (DMA) system for the characterisation of longitudinal out-of-plane bending properties of molten unidirectional thermoplastics. Investigations are presented for a unidirectional carbon fibre reinforced polyamide 6 composite. Several standard bending test fixtures are assessed quasistatically at three temperatures and three test speeds with specimens of different geometries. Additional tests are conducted at forming temperature with the selected test arrangement. The evaluation of different approaches for the calculation of the bending modulus shows the interlaminar shear to be negligible. Results highlight an important material strain rate dependency. The evolution of the bending modulus satisfies a linear fitting within the range of data.     

Processing,structure and flexural strength of CNT and carbon fibre reinforced,epoxy-matrix hybrid composite

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in variety of properties, as compared to their bulk, monolithic counterparts. These properties include primarily the tensile stress, flexural stress and fracture parameters. However, till date, there are hardly any scientific studies reported on carbon fibre (C_f) and carbon nanotube (CNT) reinforced hybrid epoxy matrix composites (unidirectional). The present work is an attempt to bring out the flexural strength properties along with a detailed investigation in the synthesis of reinforced hybrid composite. In this present study, the importance of alignment of fibre is comprehensively evaluated and reported. The results obtained are discussed in terms of material characteristics, microstructure and mode of failure under flexural (3-point bend) loading. The study reveals the material exhibiting exceptionally high strength values and declaring itself as a material with high strength to weight ratio when compared to other competing polymer matrix composites (PMCs); as a novel structural material for aeronautical and aerospace applications.     

Design of composite structures reinforced curvilinear fibres using FEM

This paper describes a method of modelling a curvilinear reinforcement structure, for a composite plate with a hole that allows trajectories of fibres to be adapted to geometric discontinuities (holes, notches, bolts, etc.). For this method, it is assumed that the trajectories of fibres are curvilinear and continuous, as well as located along the trajectories of maximum principal stress. On the basis of these trajectories, the functionally graded material is simulated by means of the finite element method (FEM). Each element of this structure has its own mechanical properties, depending on the fibre direction and a change in the distance between the fibres. It is demonstrated that the maximum value of the stress concentration factor in the fibre direction for the plate with the curvilinear reinforcement structure reduces by 3.2 times in comparison with the same plate with a rectilinear reinforcement structure (orthotropic material).     

Development of the properties of a carbon fibre reinforced thermosetting composite through cure

In this paper, the development of physical and mechanical properties of a thermosetting composite which are relevant to the modelling of residual stresses and process induced deformations are discussed. Findings of previous work on cure kinetics and cure shrinkage of the composite are summarized. The development of resin modulus throughout the Manufacturer’s Recommended Cure Cycle (MRCC) is modelled by Group Interaction Modelling (GIM). The moduli of AS4/8552 composite are calculated by two micromechanics methods: by an analytical approach based on the Self Consistent Field Micromechanics (SCFM) and by Finite Element Based Micromechanics (FEBM). The predictions show good agreement with the available experimental data and provide a fundamental understanding of how the properties of a thermoset resin and its composite develop through cure.     

Fracture toughness of 2-D carbon fibre reinforced carbon composites

The fracture toughness of 2-D woven carbon fibre reinforced carbon laminate has been evaluated by linear elastic fracture mechanics (LEFM),R-curve andJ-integral analysis using the single edge-notched bending (SENB) specimen of edge and flatwise geometries. The edgewise specimens failed by a small extension of the self similar crack whereas the flatwise specimens failed by delamination. The surface damage developing from the tip of the initial crack was revealed by the brittle lacquer coating technique and the zone shape varied with the specimen geometry, i.e. the loading axis relative to the woven layers. Acoustic emission (AE) was also used to monitor crack growth, and the total ring down count of AE was observed to increase as the initial crack length was decreased. Both the damage zone size and total AE counts were found to increase in two linear stages as a function of the square of the stress intensity factor,K.     

The buckling response of carbon fibre composite panels with reinforced cut-outs

This paper investigates the influence of cut-out diameter and reinforcement type upon the buckling stability of square CFRP panels. The study undertaken at Cranfield University in GB uses NASTRAN Finite Element Analysis (FEA) extensively for this investigation. The FEA results have been compared with results from practical tests and good agreement was found. Diagrams showing the influence of cut-out diameter, reinforcement lay up and thickness on the buckling stability of clamped or simply supported CFRP panels are presented. The results are shown for 2 mm thick ((± 45/0)_s)_s square CFRP panels with 30 mm wide reinforcement rings bonded around the central circular cut-outs. The panels are loaded in pure shear or in compression. The effect of reinforcement rings bonded to one or both sides of the panel has been investigated and shown in diagrams. The results presented can be used to find the optimum reinforcement type and thickness for square CFRP panels with central circular cut-outs.     

On the self-piercing riveting of aluminium blanks and carbon fibre composite panels

In the present paper the possibility to join aluminium alloys blanks and carbon fibre composites panels by self-piercing riveting operation is considered. In particular a few case studies were carried out at the varying of the process parameters. The effectiveness of the obtained joints was tested through tensile tests and through fatigue ones; what is more the process mechanics was highlighted through proper macro and micro observations of the transverse sections of the joints. The failure mechanics of the obtained joints were also considered in order to highlight the mechanisms which occur and determine the lost of the load carrying capability of the joints. Finally a numerical model of the process was carried out and the residual stress state after piercing was highlighted. The developed experiments and simulations demonstrated that self-piercing riveting can be effectively used to join carbon fiber composite panels and aluminum blanks.