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.     

Preparation and some properties of SiC particle reinforced aluminium alloy composites

Aluminium alloy composites containing various particle sizes of 10 and 20 wt.% SiC particles were prepared by molten metal mixing and squeeze casting method under argon gas. The stirring was carried out with graphite impeller during addition of particle. The molten mixture was poured into a die when the stirring was completed and metal matrix composites were produced by applying the pressure. Optical microscopic examination, hardness, density and porosity measurement were carried out. Moreover, metal matrix composites were machined at various cutting speeds under a fixed depth of cut and feed rate using different cutting tools. It is observed that there was a reasonably uniform dispersion of particles in the matrix alloy. The density decreased with decreasing particle sizes, but porosity decreased considerably with increasing particle size. In addition, the tool life decreased considerably with increasing cutting speeds for all tests. Among cutting tools, the wear resistance of Al₂O₃ coated tools showed better performance than those of the other tools without chip breaker geometries in the machining of SiCp-reinforced composites.     

Sliding wear behaviour of SiC particle reinforced 2024 aluminium alloy composites

Abstract

Sliding wear tests on SiC particle reinforced 2024 aluminium alloy composites fabricated by a powder metallurgy technique were carried out, and the effects of SiC particle content, size, and the wear load on the wear properties of the composites were systematically investigated. It was found that the wear resistance of the composites was about two orders of magnitude superior to that of the unreinforced matrix alloy, and increased with increasing SiC particle content and size. Under the conditions of sliding wear used, the effect of SiC particle size on the wear resistance was more significant than that of particle content.

MST/3161     

High pwder Nd-YAG laser welding of SiC particle reinforced aluminium alloy 2124

Abstract

The effects of different Nd-YAG laser output waveforms on the weldability of a SiC particle reinforced aluminium alloy 2124 have been studied. The results show that although the square waveform can produce the greatest depth-of-penetration among the three different waveforms studied, i.e. continuous wave, sine wave and square wave, a high level of porosity was observed in the weld. Alternatively, porosity free welds with a reasonable depth-of-penetration can be obtained by using a sine waveform operated at high peak powers. However, the results also show that it would be difficult to stop theformation of aluminium carbides entirely in the fusion zone simply by varying the laser output parameters and the waveform. In order to stop the, formation of carbides, a new laser joining technique has been developed. This involved brush plating of nickel on the joining surfaces before laser welding. The results of the corrosion tests of the weld zone show that welding with a nickel coating would result in a much lower corrosion current than that without nickel coating.     

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Microstructure and properties of SiC whisker reinforced ceramic composites

Microstructures of SiC whisker reinforced alumina and tetragonal zirconia polycrystals (TZP) were investigated using analytical electron microscopy. In the Al₂O₃-SiC system, amorphous phases between the whisker and matrix were observed; these amorphous phases were virtually eliminated when the whiskers were leached with HF acid before being incorporated into the matrix. In the TZP/SiC system, reaction between the whisker and matrix had taken place during fabrication and resulted in the formation of a glassy phase. This reaction appeared to be associated with the presence of SiO₂ impurity present in the TZP matrix.Mechanical properties of the composites were measured both at ambient and elevated temperatures and fracture surfaces were examined. The results indicated that fracture of the composites was sensitively influenced by the whisker-matrix interface. The presence of amorphous interfacial phases was detrimental to the properties of the composites and caused a reduction in fracture energy. High temperature tests showed that the TZP composite had a structural transition with extensive cracking occurring at 1000 °C, whilst the alumina composites retained their properties up to 1200 °C, whereupon they deteriorated rapidly.     

Workhardening effects in SiC particle reinforced aluminium alloys

Abstract

Previous research by the current authors has demonstrated that thermomechanical treatments may significantly influence the monotonic properties, especially fracture toughness, of Al-SiC_p materials through modification of grain size. The present paper gives afurther assessment of these effects. A metal matrix composite (MMC) of aluminium alloy 2124 containing 20 wt-% of 3 μm SiC particles was produced via a powder route at Aerospace Metal composites and rolled to 1.8 mm sheet at DERA, Farnborough. Tensile testing showed the finest grained MMC to exhibit the lowest workhardening rate in the naturally aged condition, which is in disagreement with conventional polycrystalline behaviour. Artificial aging and testing in liquid nitrogen were found to increase the workhardening rate of this material. It was argued, therefore, that the low workhardening rate was attributed to grain boundary sliding in the highly stressed particle/matrix interface region.     

Extrusion modelling of 6061 aluminium alloy and particle reinforced MMCs

Abstract

Extrusion modelling was performed for 6061 aluminium alloy and three particle reinforced MMCs (10%Al₂O₃/6061, 15%SiC/6061, 20%Al₂O₃/6061) using constitutive equations previously obtained from torsion test data. In applying the finite element software DEFORM, suitable heat transfer, friction, and velocity boundary conditions were chosen based on a direct extrusion press. Simulations were run for various extrusion conditions and the outputs for the four materials were compared. The simulation results were validated by comparison with real life extrusions and modelling of other researchers. The results showed that an increase in billet temperature, a reduction in ram speed, or a reduction in extrusion ratio had the effect of reducing the ram load. In consequence, extrusion conditions could be selected so that extrusion of the composite was carried out with the same peak ram load as the alloy.     

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al-Cu alloy composites

This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al-Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.     

Dry sliding wear of TiB2 particle reinforced aluminium alloy composites

Abstract

Al–4 wt-%Cu alloy and composites reinforced with 10 and 20 vol.-% of TiB₂ particles were prepared by powder metallurgy followed by hot isostatic pressing. The dry sliding wear behaviour of specimens of these materials was investigated. Pin-on-disc measurements showed that the wear resistance of Al–4Cu alloy can be improved dramatically by the addition of 20 vol.-%TiB₂ particles. This was due to the high hardness of the TiB₂ particles, and to strong particle–matrix bonding. The wear data were found to correlate with SEM observations.     

Tensile and fatigue properties of a thermomechanically treated 7475 aluminium alloy

High-purity Al-Zn-Mg alloy was thermomechanically treated. The process included solution treatment, pre-ageing, cold-working by rolling and final ageing. Pre-ageing was carried out at 100°C (TAHA1) and room temperature (TAHA2). Experimental results indicated that the TAHA1 process improved the tensile strength significantly while the TAHA2 process improved the fatigue life more substantially. Fatigue crack initiation sites were examined carefully by scanning electron microscopy. A correlation between fatigue crack initiation, fatigue striation, tearing ridge, dimple distribution and fatigue life was observed. The experimental results are discussed in terms of substructure and are also compared with the tensile and fatigue properties of a thermomechanically treated 7075 Al-Zn-Mg alloy which were previously reported by one of the authors.     

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

Abstract

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.     

Effect of ZnO coating of whiskers on thermal expansion properties of aluminium borate whisker reinforced aluminium composites

Abstract

The pure aluminium composites reinforced by ZnO coated aluminium borate whiskers were fabricated by squeeze casting. The microstructure of the composite was observed using an optical microscope and the thermal expansion behaviours of the composites were investigated in the range from 50 to 400°C. In addition, the effects of heat treatment and thermal cycling on the thermal expansion behaviours of the composite were also investigated. The results show that the coefficient of thermal expansion of as cast composites decreases with the ZnO coating content increasing. However, heat treatment time and thermal cycling lead to an increase in the CTE of the composite.     

Effect of particle size on fatigue behaviour in SiC particulate-reinforced aluminium alloy composites

The effect of particle size on rotary bending fatigue behaviour was studied for powder metallurgy 2024 aluminium alloy composites reinforced with 10 wt% silicon carbide particles (SiC_p ). Average particle sizes of 5, 20 and 60 μm were evaluated. Particle size had a significant influence on fatigue strength, indicating an increased fatigue strength with decreasing particle size. The composite with 5 μm SiC particles showed higher fatigue strength than the unreinforced alloy. The incorporation of 20 μm SiC particles led to an increase in fatigue strength at a high stress level, but the improvement diminished with decreasing stress level, and a slightly decreased fatigue strength was observed at low stress level, as compared with the unreinforced alloy. The composite with 60 μm SiC particles exhibited a considerable decrease in fatigue strength. Fatigue cracks initiated at several different microstructural features, e.g. surface defects, inclusions and particle–matrix interfaces, and crack initiation was considerably affected by particle size. Fatigue strength was found to depend strongly on the resistance to crack initiation, because there was no discernible difference in small crack growth between the unreinforced alloy and the composites, particularly at a low maximum stress intensity factor.     

Tensile properties and thermal expansion of discontinuously reinforced aluminium composites at subambient temperatures

The effects of temperature on the mechanical properties and thermal expansion of two discontinuously reinforced aluminium composites have been determined over the range 300–100 K. Silicon carbide particulate-reinforced 2009 and 6092 aluminium alloys were studied by tensile testing, in which both longitudinal and transverse strains were recorded, and by thermal expansion measurements. The test results clearly show that cooling to 100 K induces plastic flow in the aluminium alloy matrices due to the thermal expansion difference between aluminium and silicon carbide. At very low temperatures, the linear region of the stress-strain curve is greatly reduced or eliminated and the Poisson’s ratio, ν, increases. For the higher yield strength 2009 matrix composite, ν increases from a room-temperature value of 0.28 to 0.35 at 100 K. For the lower-strength 6029 matrix composite, ν increases from a room-temperature value of 0.33 to a value of 0.5 at 100 K. A Poisson’s ratio of 0.5 is the value characteristic of plastic flow in an incompressable material. Changes in yield strength, Young’s modulus and thermal expansion with decreasing temperature are also consistent with thermally induced plastic flow in the composite matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of impact behaviour of aluminium based SiC particle reinforced metal–matrix composites

In this paper, the impact behaviour of aluminium and silicon carbide (SiC) particle reinforced aluminium matrix composites under different temperature conditions was determined. Charpy impact tests were performed on as extruded and heat treated specimen at temperatures varying from −176 to 300 °C. Composite specimens based on aluminium alloys of 2124, 5083 and 6063 and reinforced by SiC particles were manufactured. Two different SiC sizes of 157 μm and 511 μm and two different extrusion ratios of 13.63:1 and 19.63:1 were used. The results of instrumented impact tests were compared with the microstructural and fractographic observations. The failure mechanisms and deformation behaviour of unreinforced alloys and composites were assessed. The impact behaviour of composites was affected by clustering of particles, particle cracking and weak matrix-reinforcement bonding. Agglomeration of particles reduced the impact strength of Al 2124 and 6063 based composites. Alumınum 6063 alloys and composites showed a better impact strength. The impact strength of 6063 composites increased with particle size and extrusion ratio. The effects of the test temperature on the impact behaviour of all materials were not very significant.