Fatigue characteristics of plasma treated aluminium repaired by graphite/epoxy composite patch

Abstract

In the present study, the fatigue behaviour of plasma treated aluminium patched by a graphite/epoxy composite (carbon fibre reinforced plastic, CFRP) has been investigated. The aluminium was surface treated using a dc plasma containing acetylene gas and nitrogen gas at a volume ratio of 5 : 5 for 30 s. The effect of plasma treatment on the fatigue behaviour of the aluminium/CFRP specimen was determined from fatigue testing using two different single edge notched (SEN) specimens of cracked aluminium repaired with a CFRP patch and plasma treated aluminium also repaired with a CFRP patch. The load ratio and the frequency applied in the fatigue tests were 0.1 and 10 Hz, respectively. The surfaces of the aluminium specimens were examined using atomic force microscopy (AFM) to investigate the effect of plasma treatment on the surface morphology. The results showed that plasma treated specimens exhibited almost 12% more fatigue life than untreated specimens. The surface roughness of aluminium was increased ~1.5 times by plasma treatment. The increased surface roughness improved the bonding strength between aluminium and the CFRP patch, increasing the fatigue life of aluminium patched by CFRP.     

Fracture of alumina particulate reinforced aluminium alloy matrix composites

Fracture toughness tests were performed on two aluminium alloy matrices, 2014-0 and 2024-0 reinforced with alumina particulates of different volume fractions and particulate sizes so as to investigate the fracture mechanisms operative in such composites and to determine how microstructural parameters such as volume fraction, particulate size and interparticle spacing affect the fracture toughness. The results indicate that fracture occurred by a locally ductile mechanism. The fracture toughness increased with increasing particle spacing provided that the particle size was less than a limiting value, above which unstable crack growth occurred and the toughness lowered.     

Effect of silicon carbide particulate on cyclic plastic strain response characteristics and fracture of aluminium alloy composites

The cyclic stress-response characteristics of powder-metallurgy-processed high-purity aluminium alloy 2124 discontinuously reinforced with varying volume fractions of silicon carbide particulates were studied over a range of plastic strains. The specimens were cycled using tension/compression loading under total strain control. The composite material, in the heat-treated condition, displayed cyclic hardening at all cyclic strain amplitudes and for different volume fractions of the ceramic reinforcement in the aluminium alloy matrix. The degree of hardening was observed to be greater at the higher cyclic strain amplitudes than at corresponding lower strain amplitudes. Micromechanisms controlling the hardening response during cyclic straining are highlighted and rationale for the observed hardening behaviour is attributed to concurrent and competing influences of an increase in dislocation-dislocation interaction, dislocation multiplication and dislocation-particle interactions, and is a mechanical effect. The kinetics of the cyclic fracture process of the composite alloy is discussed in light of composite microstructural effects, plastic strain amplitude and concomitant response stress.     

Friction characteristics of aluminium silicon carbide graphite hybrid composites

Hybrid aluminum metal matrix composites reinforced with silicon carbide (SiC) and graphite (Gr) are extensively used due to high strength and wear resistance. Friction behavior of such hybrid composites is quite vital in deciding the optimal combination of SiC and Gr. The sliding friction response of stir cast hybrid aluminum composites reinforced with equal weight fraction of SiC and Gr particulates of 2.5%, 5%, 7.5% and 10% reinforcement is investigated. The influence of % reinforcement, load, sliding speed and sliding distance on friction coefficient is studied using pin-on-disk equipment with tests based on design of experiments. Hardness of the composites decreases with increase in % reinforcement. Friction coefficient is influenced by sliding speed as well as load and its average value is around 0.269. But, % reinforcement and sliding distance do not affect the friction coefficient.     

Analysis of residual stresses in particulate reinforced aluminium matrix composite after EDM

This paper reports the effect of properties of three types of particulate reinforced metal matrix composite materials (65?vol.-%SiC/A356·2, 10?vol.-%-SiC–5?vol.-% quartz/Al, 30?vol.-%SiC/A359) and machining parameters on residual stresses induced in the machined surface during powder mixed electric discharge machining. Three electrode materials (Cu, Gr and Cu–Gr) and three machining parameters, namely, peak current and pulse (on/off) duration, are varied to determine the magnitude of induced residual stresses. The result shows that the workpiece, electrode material properties, and pulse off time significantly contribute in the formation of residual stresses. Concentration of reinforced particulates and matrix conductivity also play a vital role in the development of residual stresses. The deposition of disintegrated particles of composite electrode (Cu–Gr) results in high magnitude of residual stresses.     

Response of aluminium/graphite composite to deformation in the semi-solid state

Semi-solid deformation of Al/graphite (Gr) composites was studied. The effects of deformation temperatures and deformation rates on the macrostructures, morphologies, the deformation force, and the formability of the composites were studied. For Al-10 wt% Gr composites, the semi-solid deformation stress increases as strain increases at lower strain rate. At higher strain rate, the stress first increases to a local maximum, then decreases before it starts to increase again as strain increases. For Al-30 wt% Gr composites, the stress decreases as strain increases. For both the Al-10 wt% Gr composites and Al-30 wt% Gr composites, the higher the deformation strain rate is, the more severe is the degree of cracking. The higher the deformation strain rate, the larger is the deformation stress, not counting the effects of fracturing. The deformation stresses required to semi-solid-deform Al-30 wt% Gr composites are larger than those for Al-10 wt% Gr composites, which is contrary to the normal solid-state deformation behaviours of Al-Gr composites. Composites deformed at 620 °C tend to require a higher deformation stress than at 630 °C for the same deformation strain, due to smaller liquid fraction and lower ductility at 620 °C. The effects of deformation temperatures on the ductility of the compacts are not significant at lower deformation strain rate. However, at the highest deformation strain rate, the ductility is better for higher deformation temperature.     

Formabilities of steel/aluminium alloy laminated composite sheets

The tensile properties and press formabilities of laminates experimentally produced from mild steel and various aluminium alloy sheets are examined. The tensile properties of the laminates are approximately predictable by the mixture rule of the properties of the individual sheets. The forming limits in deep drawing, as well as stretch forming due to various types of fractures of the laminated composite sheets, cannot be predicted without considering the stress and strain histories of the individual sheets during forming. Furthermore, it is found that the drawability, as well as the stretch formability, is improved by setting the mild steel sheet on the punch side for the case of aluminium alloy sheet with comparatively high ductility, and by sandwiching the aluminium alloy sheet with the mild steel sheets for the case of low ductility.     

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

Abstract

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500°C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength, however, was not only affected by the interfacial state but also by many other factors.     

Fabrication and mechanical properties of in situ formed carbide particulate reinforced aluminium composite

Stable carbide particles of TiC, ZrC and TaC were in situ synthesized in liquid aluminium by the reaction between Al-Ti, Al-Zr or Al-Ta systems liquid alloy and SiC or Al₄C₃ particles. It was possible to generate TiC particles of nearly 1 m diameter, even utilizing SiC of 14 m. However, the dispersion behaviour of TiC particles in the matrix depended on the size of the raw carbide. Finer SiC made the dispersion of TiC particles more uniform, resulting in a greater improvement of the mechanical properties. Furthermore, although Al-Ti-Si system intermetallic compound was detected in a TiC_p/Al-Si composite fabricated by the melt-stirring method, those compounds considerably decreased in the composite fabricated by the in situ method. The mechanical properties of in situ formed TiC_p/Al-5 wt% Mg and TiC_p/Al-5 wt% Cu composites were better than those fabricated by the melt-stirring method and by T6 heat treatment, the properties of in situ formed TiC_p/Al-5 wt% Cu composite were further improved. The experimental results were analysed by the reaction model based on the assumption that the overall reaction rate was controlled by both the reaction and the diffusion.     

Laser processing of an aluminium AA6061 alloy involving injection of SiC particulate

In the present laser processing work, the powder injection technique was investigated as a method for producing a surface metal matrix composite (MMC) containing large SiC particulates (SiC_p) (105–150 m). This size is known to enhance the wear resistance of bulk aluminium-based composites. The effects of the laser-processing conditions, the powder feeding rate and the surface situations necessary to produce a well incorporated MMC on the surface were studied, and the microstructure examined. In previous work, laser processing involving the preplacement of SiC_p was developed to create an AI-SiC_p (45 m) MMC layer on aluminium alloy surfaces. Some of these ideas were used in conjunction with the injection process in the present work to enhance the surface-wear resistance. The wear resistance of an MMC obtained by a single laser track with the injection technique was determined and compared with the base alloy and the MMC layer produced by the preplacement technique.     

Liquid phase bonding of aluminium and aluminium/Nicalon composite using interlayers of Cu-Ag alloy

Abstract

The liquid phase bonding in air of unreinforced and fibre reinforced aluminium using interlayers of Cu-Ag alloy has been investigated. Bond strengths were measured using a simple shear jig and the associated microstructures characterised by electron microscopy and electron probe microanalysis. A shear strength of 65 MN m^-2 was achieved when bonding unreinforced aluminium at 510°C using a 50 μm thick alloy interlayer, and a pressure of 10 MPa for 30 min; the bonded region covered ~85% of the area of the joint. With reinforced aluminium, a bonding pressure of 20 MPa was required to achieve sufficient contact and a similar area of bond; after application of pressure for 30 min at 510°C, a shear strength of 54 MN m^-2 was developed at the joint. The diffusional behaviour and interphase reactions which occurred during the process are discussed and a mechanism for bonding proposed.     

Extrusion characteristics of aluminium alloy/SiCpmetal matrix composites

Abstract

Systematic extrusion studies have been carried out on aluminium alloy 2124/SiC_p metal matrix composites. Effects of various extrusion process parameters, such as die design, ram speed, extrusion ratio, reheat temperature, and lubrication, on the pressure requirement and surface quality of the as extruded circular rods have been investigated. Different volume fractions of SiC particles (10, 15, and 20 vol.-%) were used for the synthesis of metal matrix composite billets. These composites were synthesised using two different techniques, namely, stir casting and powder metallurgy. These billets were then hot extruded on a laboratory scale 500 ton vertical hydraulic press. The significance of specially designed dish shaped dies, avoiding the dead metal zone, has also been highlighted. The results indicated that the best extrusion was possible when powder metallurgical processed billets were extruded. Volume fraction analysis of ceramic reinforcement in the extruded rod (typically 2 m long) and in the extruded discard showed no appreciable backward migration of these particles during extrusion.