Pretreatment process of SiC particles and fabrication technology of SiC particulate reinforced Zn–Al alloy matrix composite

Abstract

In the present paper, a novel pretreatment process for SiC particulate and a new mechanical–electromagnetic combination stirring process for fabricating Zn–Al(ZA27)/SiC_p composites are described. The optimal pretreatment route and the most appropriate SiC particle parameters were experimentally determined. The pretreated SiC particles were easily incorporated and dispersed in the ZA27 alloy melt and were not agglomerated before addition to the melt. The surface status of the SiC particles before and after pretreatment was observed and analysed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. It was found that gas existing on the SiC particle surfaces by physical and chemical absorption was a significant hindrance to the incorporation and dispersion of SiC particles in the alloy melt. The gas absorption was induced by ultrafine SiC powders, fracture steps, and ions existing on the SiC particle surfaces. The carbon, silicon, and oxygen contents on the SiC surface were varied with different pretreatment techniques. Moreover, a dense layer of amorphous SiO₂, which improves wetting of SiC particles in the ZA27 melt, was formed owing to calcination of SiC particles in air. The new combined stirring process exploits the advantages of both mechanical and electromagnetic stirring of the melt at the different processing stages during fabrication. The microstructural characteristics of the resulting composites are: homogeneously distributed SiC particles, fewer macro gas blows and inclusions, and little shrinkage porosity in comparison to composites fabricated by a mechanical stirring process. Finally, the mechanisms of degassing and reducing the porosity and the number of oxide inclusions are discussed.     

Microstructural characterisation of sand cast aluminium alloy A356–SiC particle metal matrix composite

Abstract

The microstructure of a cast aluminium alloy A356 reinforced with 15 vol.-%SiC particles has been investigated using analytical microscopy. It is shown that the morphology of the silicon phase, as well as that of the Al–Si eutectic structure, which are the features of this alloy system, are dramatically changed by the presence of the SiC particles. Significant effects of grain refinement werefound to occur not only on the primary aluminium grains, but also on the primary and eutectic silicon phases. Twinning and dislocations were often observed within that silicon phase which was situated adjacent to the SiC particles. Microcracks were also observed at the SiC/silicon interface. The presence of such microcracks suggests that a stress concentration had developed at the silicon/SiC particle interfaces, probably as a result of the thermal expansion mismatch occurring between the silicon and SiC particles. The microcracks which consequently develop are formed as a result of the poor silicon–SiC particle bonding. Other intermetallics, notably Mg₂Si and FeSiAl₅, which exhibit cubic and tetragonal symmetries respectively, were also identified as being present in the microstructure.

MST/1460     

Preparation and properties of cast aluminium alloy–sillimanite particle composite

An attempt has been made to explore the possibility of using natural mineral namely sillimanite for synthesizing aluminium alloy composite through a solidification technique. The sillimanite particles were characterized in terms of X-ray, differential thermal analysis in order to examine their suitability for preparing the composite. An aluminium alloy (BS:LM6) was used as the matrix alloy. The sillimanite particles of mean size 140 μm (major axis) were used as reinforcement. The sillimanite particles were added into the matrix melt by creating a vortex with the help of a mechanical stirrer and the melt temperature was maintained between 750 and 800°C. The cast composite was characterized in terms of microstructural, mechanical and abrasive wear properties. It was noted that the sillimanite particles were reasonably uniformly distributed within the matrix and exhibited good mechanical bonding with the matrix. The strength of the composite was noted to be marginally lower than that of the base alloy but the hardness and the wear resistance of the composite were found to be significantly higher than those of the base alloy.     

Insight of the interface of electroless Ni–P/SiC composite coating on aluminium alloy,LM24

Electroless nickel composite coatings with silicon carbide, SiC, as reinforcing particles deposited with Ni–P onto aluminium alloy, LM24, having zincating as under layer were subjected to heat treatment using air furnace. The changes at the interface were investigated using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to probe the chemistry changes upon heat treatment. Microhardness tester with various loads using both Knoop and Vickers indenters was used to study the load effect clubbed with the influence of second phase particles on the coating at the vicinity of the interface. It was observed that zinc was absent at the interface after elevated temperature heat treatment at 400–500 °C. Precipitation of copper and nickel with a distinct demarcation (copper rich belt) along the coating interface was seen with irregular thickness of the order of 1 μm. Migration of copper from the bulk aluminium alloy could have been the factor. Brittleness of the coating was confirmed on heat treatment when indented with Vickers. However, in composite coating the propagation of the microcrack was stopped by the embedded particles but the microcracks continue in the matrix when not interrupted by second phase particles (SiC).     

Effect of casting techniques on tensile properties of cast aluminium alloy (Al–Si–Mg) and TiB2 containing metal matrix composite

Abstract

An analysis of the statistical distribution of the tensile strength of a TiB₂ containing aluminium matrix composite and its matrix alloy (Al-7Si-0.35Mg) was carried out using different casting techniques. The scatter of the tensile strength data was assessed by Weibull statistics. Results for the metal matrix composite (MMC) and the matrix alloy in as cast and heat treated conditions were compared. It was found that a low turbulence casting technique resulted in less scatter of tensile values, confirming the greater reliability of the cast material. Fractographic examination of the fractured faces of lower strength specimens showed that entrained oxide films play an important role in failure of the specimens. Heat treatment causes increased scatter in strength values, reflected in the lower Weibull modulus.     

Fabrication and characterisation of aluminium clad aluminium–copper alloy cored rod

Abstract

The Ohno continuous casting process was successfully applied to cast cored rod with Al–Cu alloy as a core material and Al as a clad material. Good metallurgical bonding was observed at the core/clad interface. Ultrasonic characterisation and pulse–echo measurements were performed to assess the suitability of the material as an acoustic buffer rod. Acoustic velocity profiles across the rod diameter and acoustic anisotropy were measured using a 225 MHz line focus beam scanning acoustic microscope. Longitudinal wave measurements with excellent signal/noise ratios were obtained.

MST/3154     

Cracking of Al–4.5Zn–1.5Mg aluminium alloy propellant tank – A metallurgical investigation

The medium strength aluminum alloy AFNOR 7020 (Al–4.5Zn–1.5Mg) is extensively used in the fabrication of common bulk head propellant tank of liquid propulsion engine. The rings used in fabrication of the tank were of different sizes and were processed almost 4 years back in T651 condition. The propellant tank developed a leak at 2.7 bar(a) during proof pressure charging to 5 bar. The cracked portion of the ring was removed from the failed tank and subjected to detailed metallurgical investigations to understand the cause of failure. This paper brings out the details of investigations carried out and thereafter conclusion arrived on.     

Wear behaviour of a novel aluminium–nanozirconia composite

A novel aluminium–zirconia composite, with improved wear resistance, has been produced. The method consists of compacting aluminium powder mixed with cellulose fibres, which are then eliminated by a heat treatment. The compacted piece was immersed several times in a colloidal (i.e. nanosized) zirconia solution until a constant weight was reached. The solution penetrated and infiltrated the preform. Then, the aluminium matrix presented zirconia tetragonal and monoclinic phases. Wear test was carried out in a pin-on-disc machine. By measuring the weight loss it was shown that the composite had significant improvement in wear resistance, as compared to pure aluminium.     

Effect of inclusions on the tensile properties of Al–7% Si–0.35% Mg (A356.2) aluminium casting alloy

The present study was performed on an A356.2 alloy. Two types of initial materials were used, i.e. fresh and recycled. A total of 13 operations representing those normally applied in aluminium foundries were simulated under dry atmospheric conditions (humidity 15%–20%). The molten metal was cast into test bars which were T6 tempered prior to tensile testing. The results show that holding the liquid metal for a long time, i.e. 72 h at 735°C leads to sedimentation of most inclusions towards the bottom of the melting crucible. However, a change in the surrounding humidity may cause absorption of hydrogen and, hence, a large amount of porosity. Degassing using dry argon injected into the liquid metal through a rotary impeller (speed 160 r.p.m) appears to be the best technique for inclusion removal. The efficiency of this process is significantly improved when it is coupled with filtration using ceramic foam filters (10 and 20 p.p.i). A linear relationship between alloy ductility and logarithm of percentage inclusions has been established. Owing to decohesion between the inclusions/oxide films and the surrounding matrix, cracks are easily initiated at their interfaces, leading to unpredicted failure. © 1998 Chapman & Hall     

Effect of microstructure on fatigue behaviour of cast Al–7Si–Mg alloy

Abstract

The fatigue behaviour of a cast Al–7Si–Mg alloy, conforming to A356, has been studied. Specimens of this material were tested in both the as cast condition and a solution treated and aged condition. It was observed that the size, number, and position of casting defects influenced the fatigue life very strongly. This marked effect nearly hides that of the heat treatment. Nevertheless, if the analysis is carried out considering only results obtained from sound specimens it is revealed that the heat treatment causes an improvement in the fatigue resistance of the alloy.     

Influence of SiC particle distribution and prestraining on fatigue crack growth rates in aluminium AA 6061–SiC composite material

Abstract

The fatigue crack growth properties of an aluminium AA 6061 alloy containing 15 vol.-% particulate SiC and of the corresponding matrix alloy with the same grain size were investigated. The composite was tested in the undeformed condition and after undergoing a plastic prestrain of 0·33% which changes the residual microstress state of the matrix from hydrostatic tension to longitudinal compression. The composite showed superior resistance to crack growth compared with the matrix material irrespective of whether nominal or effective values of stress intensity range ?K were considered. A 50% increase in the nominal threshold value of ?K was observed for the undeformed composites. Prestraining did not affect the intrinsic fatigue crack growth properties but resulted in a twofold increase of crack closure stress intensity. Quantitative fractography showed that SiC particles deflect the propagating crack, thus enhancing roughness induced crack closure. Local relief of ?K by crack deflection and diminished crack tip opening are the main causes of the improved intrinsic fatigue crack growth properties of the composite.

MST/1356     

Experimental correlations between wear rate and wear parameter of Al–Cu–Mg/bagasse ash particulate composite

The experimental correlations between wear rate and wear parameter of Al–Cu–Mg alloy composite reinforced with 10 wt.% bagasse ash particles produced by double stir casting method was developed in terms of applied load and sliding speed using the empirical linear regression and analysis of variance method. The wear behaviour of the specimen was investigated using pin-on-disc method. An empirical linear regression equation was used in predicting wear rate within a selected experimental domain. The predicted wear rate of the alloy and composite samples were found to lie close to that of the experimentally observed ones. The confirmation of experiments was conducted using analysis of variance (ANOVA) to verify the optimal testing parameters. The interactions of applied load and sliding speed of the composite had no significant effect, while the alloy had a significant effect on the wear rate.     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Precipitation hardening of Zr-modified Mg–Ca–Zn alloy

The microstructure and mechanical properties of Mg–Ca–Zn alloys with 1 wt.% Zr were investigated in as-cast and heat-treated conditions. A substantial decrease in grain size (from 65 μm for the Mg–Ca–Zn base alloy to 22 μm) was observed. The alloy was solution treated at 410 °C for up to 96 h followed by aging at 175 °C for up to 24 h. Conventional techniques, X-ray diffraction, EM + EDS, and TEM were used to characterize the microstructure of the alloy. The microstructure obtained after heat treatment had equiaxed grains with evenly distributed binary phase Zn₂Zr. The binary Mg₂Ca and ternary Mg₂Ca₆Zn₃ phases were identified in the matrix and at grain boundaries surrounded by precipitate-depleted zones (PDZs). The thermal stability of the Zr-modified alloys was examined by microhardness measurements conducted after prolonged exposures of the alloys to elevated temperatures. It was found that Zr is a structure-stabilizing factor. Its influence was associated with the formation of Zn₂Zr phase that does not undergo coarsening at the elevated temperatures used (due to the low diffusivity of Zr). The nanoscale mechanical properties of grain boundary PDZs were analyzed using combined nanoindentation and atomic force microscopy. These mechanical properties were then correlated to the composition and precipitate distribution in PDZs. An increase in the solution treatment duration from 10 to 96 h at 410 °C resulted in expansion of PDZs from ~0.75 to ~3 μm, while the following aging at 175 °C for up to 24 h did not lead to a detectable change in PDZs. The analysis indicates that the lowest hardness was found in the region where Zn₂Zr precipitates density was low, regardless of the solute concentration.     

Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

Underwater shock consolidation of Mg–SiC composites

The shock consolidation of magnesium (Mg)/silicon carbide (SiC) composites using axisymmetric explosive fabrication setup is reported. Pure Mg and SiC powders are consolidated in a three-layered cylindrical assembly with the energy being derived from a high-detonation velocity explosive. The pressure of underwater shock wave is experimentally measured and simulated using AUTODYN 2D. Microstructural characterization of the samples revealed a well-flown Mg matrix enveloping near homogeneous SiC particles. Occasional clustering of SiC particles and interparticle melting is evidenced. Results of microhardness revealed that the presence of SiC particulates led to a substantial increase in the hardness of the composite. Fractography results indicate the lack of formation of ductile dimples, which is attributed to the presence of discontinuous SiC particles. The strengthening mechanism, the absence of reaction products, the structure–property correlation of the shock consolidated composite are discussed.     

Grain refinement of Mg–Al alloys with Al4C3–SiC/Al master alloy

A novel Al₄C₃–SiC/Al master alloy for grain refinement of Mg–Al–Zn alloys has been developed in the present work. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) results show the existence of Al₄C₃ and SiC particles in this master alloy. The master alloy presents good grain refining efficiency on both AZ31 and AZ63 alloys, but little effect on AZ91 alloy. After addition of 0.5 wt.% Al₄C₃–SiC/Al master alloy, the average grain size of AZ31 and AZ63 decreased dramatically from 1300 to 225 μm, and from 300 to 200 μm, respectively. However, no further refinement of grain size was achieved with additional amount of Al₄C₃–SiC/Al master alloy exceeding 0.5 wt.% for both AZ31 and AZ63 alloys in the present investigation. Duplex phase of Al₄C₃ and SiC was found to be located at the grain center of α-Mg and is proposed to be the nucleating agent during solidification of α-Mg.     

Fabrication and characterization of bioactive composite coatings on Mg–Zn–Ca alloy by MAO/sol–gel

High corrosion rate and accumulation of hydrogen gas upon degradation impede magnesium alloys’ clinical application as implants. In this work, micro-arc oxidation (MAO) was used to fabricate a porous coating on magnesium alloys as an intermediate layer to enhance the bonding strength of propolis layer. Then the composite coatings were fabricated using sol–gel method by dipping sample into the solution containing propolis and polylactic acid at 40°C. The corrosion resistance of the samples was determined based on potentiodynamic polarization experiments and immersion tests. Biocompatibility was designed by observing the attachment and growth of wharton’s jelly-derived mesenchymal stem cells (WJCs) on substrates with MAO coating and substrates with composite coatings. The results showed that, compared with that of Mg–Zn–Ca alloy, the corrosion current density of the samples with composite coatings decreased from 5.37 × 10⁻⁵ to 1.10 × 10⁻⁶ A/cm² and the corrosion potential increased by 240 mV. Composite coatings exhibit homogeneous corrosion behavior and can promote WJCs cell adhesion and proliferation. In the meantime, pH value was relatively stable during the immersion tests, which may be significant for cellular survival. In conclusion, our results indicate that composite coatings on Mg–Zn–Ca alloy fabricated by MAO/sol–gel method provide a new type bioactive material.     

Evaluation of mechanical properties of aluminium alloy–alumina–boron carbide metal matrix composites

This paper deals with the fabrication and mechanical investigation of aluminium alloy, alumina (Al₂O₃) and boron carbide metal matrix composites. Aluminium is the matrix metal having properties like light weight, high strength and ease of machinability. Alumina which has better wear resistance, high strength, hardness and boron carbide which has excellent hardness and fracture toughness are added as reinforcements. Here, the fabrication is done by stir casting which involves mixing the required quantities of additives into stirred molten aluminium. After solidification, the samples are prepared and tested to find the various mechanical properties like tensile, flexural, impact and hardness. The internal structure of the composite is observed using Scanning Electron Microscope (SEM).