The effects of magnesium additions on the structure and properties of Al-7 Si-10 SiCp composites

The additions of magnesium to an aluminium alloy matrix, which contains insufficient magnesium, was found to be essential during the synthesis of composites by the stir-casting technique. Magnesium promotes interfacial wetting between the dispersoid surface and the matrix. Dispersion of SiC_p in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeration and rejection was not possible. Hence, the addition of up to 3 wt% Mg was made to the alloy matrix during the dispersion of 10 wt% SiC_p (34 m), and the microstructure and mechanical properties of the composites were investigated with a view to optimize the magnesium content. With a magnesium content less than 1 wt% in the matrix, the SiC_p particles were essentially in agglomerated form. The highest UTS of 280–300 MPa was obtained with 1 wt% Mg content and SiC_p was uniformly distributed in the matrix. A higher magnesium content (>1.0 wt%) did not further improve the uniformity in the dispersion of SiC_p but the ultimate tensile strength properties deteriorated. This decrease in strength was attributed to the observed coarseness of the Mg₂Si phase, the precipitation of Mg₅Al₈ phase and the presence of a higher amount of porosity in the composites in the heat-treated condition. The aspect ratio (length/width) of precipitates changed from 1–3 for 1% Mg to 3–9 for 3.2% Mg in the matrix. Corresponding values for per cent porosity were 2% and 6%, respectively.     

Processing,microstructure and properties of ultrasonically processed in situ MgO–Al2O3–MgAl2O4 dispersed magnesium alloy composites

AZ91 alloy matrix composites are synthesized by in situ reactive formation of hard MgO and Al₂O₃ particles from the addition of magnesium nitrate to the molten alloy. The evolved oxygen from decomposition of magnesium nitrate reacts with molten magnesium to form magnesium oxide and with aluminium to form aluminium oxide. Additionally, these newly formed oxides react with each other to form MgAl₂O₄ spinel. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, and decreased porosity in the castings. Ultrasound induced physical phenomena such as cavitation and melt streaming promoted the in situ chemical reactions. Well dispersed, reactively formed hard oxides increased the hardness, ultimate strength, and strain-hardening exponent of the composites. Presence of well-dispersed hard oxide particles and stronger interface resulting from cavitation-enhanced wetting of reactively formed particles in the AZ91 alloy matrix improved the sliding wear resistance of the composites.     

Influence of B4C on the tribological and mechanical properties of Al 7075–B4C composites

In the present investigation, the influence of B₄C on the mechanical and Tribological behavior of Al 7075 composites is identified. Al 7075 particle reinforced composites were produced through casting, K₂TiF₆ added as the flux, to overcome the wetting problem between B₄C and liquid aluminium metal. The aluminium B₄C composites thus produced were subsequently subjected to T6 heat treatment. The samples of Al 7075 composites were tested for hardness, tensile, compression, flexural strengths and wear behavior. The test results showed increasing hardness of composites compared with the base alloy because of the presence of the increased ceramic phase. The wear resistance of the composites increased with increasing content of B₄C particles, and the wear rate was significantly less for the composite material compared to the matrix alloy. A mechanically mixed layer containing oxygen and iron was observed on the surface, and this acted as an effective insulation layer preventing metal to metal contact. The coefficient of friction decreased with increased B₄C content and reached its minimum at 10 vol% B₄C.     

Effect of Al2O3 particulates as reinforcement in age hardenable aluminium alloy composites

Abstract

This paper investigates the behaviour of aluminium based composites, reinforced with aluminium oxide particulates, manufactured using a powder processing route and various sintering times. It is found that a softening effect, compared with the monolithic material, coincides with migration of magnesium from the matrix to the matrix/reinforcement interface and to the specimen surface. The addition of 5 wt-%Si to the matrix alloy results in a retention of hardness in the specimens given short sinter times (0.5 and 1 h). This coincides with reduced migration of magnesium from the matrix. It is postulated that the excess silicon in the matrix results in sufficient magnesium being retained in the matrix during short sinter times to take part in the formation of Mg₂Si precipitates necessary for the hardening of the Al alloy 6061 matrix.     

Flexural damping of a P55 graphite/magnesium composite

Recent studies have shown that when magnesium is alloyed with particular solute species having very low solid solubilities (< 1%), such as aluminium, copper, tin, zirconium, manganese or silicon, the characteristically high damping is preserved while the mechanical properties are enhanced. Moreover, both damping, and the amplitude dependence of damping, increase with decreasing solute atom concentration. Accordingly, these materials are considered candidates in the fabrication of metal matrix composites (MMCs) for use in large space structures. This paper presents damping data on two magnesium alloys, Mg-0.6%Zr and Mg-1.0%Mn, and a recently developed magnesium MMC, a [0₈] P55Gr/Mg-0.6%Zr. The alloy data demonstrate the increase in damping and amplitude dependence which accompanies a decrease in alloy concentration. A comparison between the damping of the Mg-0.6%Zr alloy and the [0₈] P55Gr/Mg-0.6 %Zr composite shows that the addition of the strength-enhancing fibres reduces the high damping properties of the matrix.     

Flux-assisted infiltration of liquid Al-6063 into TiC beds in air

This study details trials to produce aluminium metal matrix composites reinforced with TiC particles by means of a flux-assisted infiltration technique. Whilst no infiltration of TiC beds occurred, by using a K-Al-F flux infiltration was successful at temperatures as low as 680°C. Some reaction of TiC with the Al matrix, forming TiAl_2.3Si_0.1 and Al₄C₃, was observed in the microstructure along with flux trapped within the Al-6063 matrix. DSC showed exothermic oxidation of TiC to occur, until the flux melts at 545°C arresting and preventing further oxidation by spreading over, coating and cleaning the particle surfaces. As soon as the flux melts, it also starts dissolving the oxide layer on the Al alloy and prevents any re-oxidation by isolating the surface from the surrounding atmosphere. Sessile drop experiments suggest that when the alloy melts and the oxide layer has been dissolved by the flux, intimate contact occurs between the liquid and the particles. The low tensions for the solid/flux and liquid metal/flux interfaces facilitates spreading and wetting of liquid Al on the TiC particles, followed by infiltration of the bed and the displacement of the flux to the outer surfaces of the sample.     

Microstructure,mechanical properties and corrosion behaviour of Al–Si–Mg alloy matrix/zircon and alumina hybrid composite

In the present study, the effect of reinforcement on microstructure, mechanical properties and corrosion behaviour of aluminium–silicon–magnesium (Al–Si–Mg) alloy matrix hybrid composites reinforced with varying amounts of zircon and alumina has been investigated. Hardness and room temperature compressive tests were performed on Al–Si–Mg alloy as well as composites. Hardness and compressive strength was found to be higher for composites containing 3.75?% ZrSiO₄?+?11.25?% Al₂O₃. Similarly, Al–Si–Mg alloy and its composites were studied for corrosion behaviour in 1 N HCl corrosive media. The weight loss of all the composites was found to decrease with time due to the formation of passive oxide layer on the sample surface. The results obtained indicate that composites exhibit superior mechanical properties and corrosion resistance compared to unreinforced alloy.     

Welding of rapidly solidified Alloy 8009 (Al–8·5Fe–1·7Si–1·3V): preliminary study

Abstract

Alloy 8009 is a rapidly solidified, dispersion strengthened Al–8·5Fe (wt-%) alloy designed for high temperature (up to 400°C) aerospace applications. Both fusion and solid state joining techniques were shown to produce bonds. Fusion techniques destroyed the base metal microstructure with primary Fe₃Al, loss of solute, formation of larger aluminium grains, and the formation of grain boundary FeAl₃ and intermetallics enriched with silicon and vanadium. Solid state friction stir welding did not cause a significant modification to the dispersoid population but there was a loss of solute to dispersoid/matrix interfaces.

MST/3500     

Experimental investigation on mechanical properties and wear characterization of Al-Si12CulMg1 aluminium alloy reinforced with titanium diboride and boron carbide particulate hybrid composites using stir casting process

LM13 aluminium alloy (Al−Si12CulMg1) with titanium diboride (TiB₂) and boron carbide (B₄C) particulate hybrid composites have been prepared using stir casting process. Wt% of titanium diboride is varied from 0–10 and constant 5 wt% boron carbide particles have been used to reinforce LM13 aluminium alloy. Microstructure of the composites has been investigated and mechanical properties viz., hardness, the tensile strength of composites have been analyzed. Wear behavior of samples has been tested using a pin on disc apparatus under varying load (20 N–50 N) for a sliding distance of 2000 m. Fracture and wear on the surface of samples have been investigated. Microstructures of composites show uniform dispersion of particles in LM13 aluminium alloy. Hardness and tensile strength of composites increased with increasing wt % of reinforcements. Dry sliding wear test results reveal that weight loss of composites increased with increasing load and sliding distance. Fracture on the surface of composites reveals that the initiation of crack is at the interface of the matrix and reinforcement whereas dimples are observed for LM13 aluminium alloy. Worn surface of composites shows fine grooves and delamination is observed for the matrix.     

Interfacial reactions and wetting in Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique

The present paper considers the microstructures of Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique. The composites were produced using an Al–10 wt.% Mg alloy with two oxide ceramic foams, spinel (MgAl₂O₄) and mullite (Al₆Si₂O₁₃), at 915 °C in a flowing N₂ atmosphere. Full infiltration of the aluminium alloy into the ceramic preform has been achieved with good bonding between the metal and ceramic phases. The composites were characterised by a range of techniques and compared with those for alumina from the literature. It has been found that the metal–ceramic interface of the composite consisted of an oxide layer near the ceramic phase and a nitride layer from Mg₃N₂ to AlN near the metal phase. The improvement of Al wetting and adhesion on the oxide ceramics by the addition of Mg and in the presence of N₂ was studied by a sessile drop technique to clarify which compound that formed at the interface contributed to the spontaneous infiltration.     

Enhanced mechanical properties of Al–Si–Cu–Mn–Fe alloys at elevated temperatures through grain refinement and dispersoid strengthening

The effect of Ti content on the microstructure and mechanical properties of heat-treated Al–Si–Cu–Mn–Fe alloys was investigated. It was found that the mechanical properties increased with the increase of Ti content. This was attributed to the refinement of grain size, the increased amount of T (Al₂₀Cu₂Mn₃), the α-Fe (Al₁₅(FeMn)₃(CuSi)₂) precipitated particles, and the decrease in Al₂Cu. At an elevated temperature of 300°C, the heat-treated Al–Si–Cu–Mn–Fe alloy with 0.5% Ti demonstrated the best mechanical properties, which are superior to those of commercial aluminium alloys. The yield strength contribution at 300°C was quantitatively evaluated based on the dispersoid, solid solution, and matrix contributions. It was confirmed that the main strengthening mechanism in the experimental alloys was the dispersoid strengthening.     

Microstructure and mechanical properties of cast (Al–Si)/SiCp composites produced by liquid and semisolid double stirring process

Abstract

A stirring process containing two steps, i.e. liquid and then semisolid stirring, was used to produce SiC particle reinforced aluminium matrix composites. The major advantages of this process are that full wetting of SiC particles by molten aluminium can be readily achieved at relatively low stirring rates, and undesirable Al₄ C₃ is not formed at the Al/SiC interface due to lower processing temperatures. Cast Al–Si matrix composites reinforced with 15 and 20 vol.-%SiC particles were produced in the present work. The mechanical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained without employing semisolid stirring, the aggregation of SiC particles observed in the microstructure of composites resulted in quite poor mechanical properties. Observations and analyses indicated that some Al/SiC interfaces were very clean, and a reaction product of spinel MgAl₂O₄ was also found at some Al/SiC interfaces. Silicon dioxide (SiO₂ ) was found to exist on the surface of as purchased and 250°C dried SiC powders. This SiO₂ is involved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.     

Influence of in situ formed ZrB2 particles on microstructure and mechanical properties of AA6061 metal matrix composites

Particulate reinforced metal matrix composites (PMMCs) have gained considerable amount of research emphasis and attention in the present era. Research is being carried out across the globe to produce new combination of PMMCs. PMMCs are prepared by adding a variety of ceramic particles with monolithic alloys using several techniques. An attempt has been made to produce aluminium metal matrix composites reinforced with zirconium boride (ZrB₂) particles by the in situ reaction of K₂ZrF₆ and KBF₄ salts with molten aluminium. The influence of in situ formed ZrB₂ particles on the microstructure and mechanical properties of AA6061 alloy was studied in this work. The in situ formed ZrB₂ particles significantly refined the microstructure and enhanced the mechanical properties of AA6061 alloy. The weight percentage of ZrB₂ was varied from 0 to 10 in steps of 2.5. Improvement of hardness, ultimate tensile strength and wear resistance of AA6061 alloy was observed with the increase in ZrB₂ content.     

Influence of particle size of the dispersoid on compressibility and sinterability of TiO2 dispersed Al 7075 alloy composites prepared by mechanical milling

Particulate TiO₂ (with varying particle size produced by mechanical milling) dispersed AA7075 composites are synthesised by short duration milling (10 min) followed by room temperature unidirectional compaction (with varying pressure) and sintering. Apparent and relative density of the alloy powder and composites are measured. The effect of reinforcement particle size on the compressibility behaviour of the composites is demonstrated. Mechanically milled (for 25 h) alloy powder shows lower relative density than coarse alloy powder. In addition, compressibility of the alloy composites decreases with decreasing particle size of the reinforcement. In contrast, the sinterability of the composites increases with decreasing dispersoid’s size due to easy filling up of finer pores and particle induced precipitation.     

Interfacial reaction and its effect on strength of Al18 B4O33 /Al composites

Abstract

Aluminium alloy 6061, AC8A, Al–1Mg, Al–9Cu and pure aluminium composites reinforced with aluminium borate whiskers were fabricated by a squeeze casting process. The interfacial reaction in the composites and its effect on the bending strength are discussed, together with the results from SEM, TEM, and X-ray diffraction. A slight interfacial reaction is favourable for composite strength as it has the effect of anchoring the whiskers. A T6 treatment can enhance the strength of an Al–9Cu matrix composite, but is not efficient for magnesium containing 6061 and AC8A matrix composites. Furthermore, if heated at temperatures higher than 793 K for a long time, the composite strength drops rapidly owing to whisker damage and shortening during the interfacial reaction. It is suggested that the interface in an Al₁₈ B₄O₃₃ /Al alloy composite is stable below 623 K which is the temperature requirement for automobile engine components.     

Filamentary preforms of Al-13wt % Si alloy reinforced with TiC-coated and pre-treated carbon fibres

Filamentary preforms of aluminium-13 wt % Si alloy reinforced with TiC-coated carbon fibres were obtained by liquid metal infiltration using K₂ZrF₆ as wetting agent. The interfacial structure was investigated using analytical electron microscopy (TEM, SEM) and the fibre strength measured following each step of processing. The reaction between the K₂ZrF₆ deposit and the molten alloy is discussed on the basis of the reaction products formed at the fibre-matrix interface, which mainly consisted of fluoride compounds (K₃AlF₆), different zirconium-rich suicides (Si₂Zr, SiZr (Al), (Al, Si)₂Zr) and crystalline -alumina. Large amounts and/or inhomogeneous distributions of the K₂ZrF₆ deposit should be avoided, because the massive precipitation of colonies of intermetallics around fibres is conducive for the formation of harmful aluminium carbides and favours brittle fracture of the composite. The utilization of TiC-coatings formed by the reactive chemical vapour deposition process gave satisfactory results, both in terms of composite elaboration and fibre protection during liquid infiltration.     

Fabrication and corrosion behavior of HA/Mg-Zn biocomposites

The thermal-treated hydroxyapatite (HA) particles, Mg and Zn powders were used to prepare the HA/Mg-Zn composites with different HA contents by means of powder metallurgy technology. The microstructures, formation phases, and corrosion behaviors in simulated body fluid (SBF) were studied in comparison with pure magnesium and HA/Mg composites fabricated by the same preparation technology. As a result, no evident reaction happened between HA particles and Mg matrix during sintering process, and Zn atoms diffused into Mg matrix to form a single phase Mg-Zn alloy matrix. The addition of HA particles changed the corrosion mechanism of Mg matrix. During the corrosion process, HA particles would adsorb PO₄³⁻ and Ca²⁺ ions efficiently and induce the deposition of Ca-P compounds on the surface of composites. HA could improve the corrosion resistance of magnesium matrix composites in SBF and restrain the increase of pH of SBF. Furthermore, the addition of Zn was favorable to improve the corrosion resistance of HA/Mg composites due to the densification of composites and the formation of Mg-Zn alloy matrix.     

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.     

A study on microstructure and mechanical properties of Al 6061-TiB2 in-situ composites

In situ reinforced aluminium based metal matrix composites (AMMCs) are emerging as one of the most promising alternatives for eliminating the inherent defects associated with ex situ reinforced AMMCs. Researchers in recent past have attempted various processing techniques for the development of in-situ composites, of which liquid metallurgy is the most widely adopted technique. Development of in-situ composites via liquid metallurgy route using master alloys is a relatively new processing technique. Very little information is available providing the usage value of these reinforcing materials.The present study is an attempt to explore the processing and characterization of in situ AMMCs using master alloys as reinforcement materials. Al 6061-TiB₂ in-situ composites were fabricated by liquid metallurgy route using Al 6061 as the matrix material and Al-10%Ti and Al-3%B as reactive reinforcements. Tests carried out on the fabricated composites include XRD, metallographic studies, EDAX analysis, microhardness, grain size analysis and tensile strength tests. The developed composites exhibited superior structural properties when compared with base alloy.     

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.