As-cast Composites Research Articles

Mechanical behaviour of As-Cast and Heat-Treated: AA2024 Alloy Silicon Carbide-Al2O3 Hybrid Composites

This paper reports on the mechanical properties of as-cast and heat-treated AA2024 alloy hybrid composites reinforced with SiC/ Al2O3 from 1 to 5 wt %. The hybrid composites were fabricated using the Compocasting method, in which short SiC/Al2O3 were introduced into the vortex created in the molten alloy through an impeller rotated at 500 rpm. The molten mass was thoroughly stirred and poured into permanent molds and squeezed under pressure. The specimens were heat treated at 575°C for 1, 2, 3, and 4 h. The tests on the as-cast hybrid composites revealed that as the glass content in the hybrid composites was increased, the ultimate tensile strength (UTS), compressive strength, and hardness of the composite increased, while the ductility and impact strength were decreased. Heat treatment was found to improve significantly the ductility, compressive strength, and impact strength, while the hardness and UTS were reduced. This paper discusses the behavior of these hybrid composites.

Development of Novel Stir Cast Aluminium Composite with modified Coconut Shell Ash Filler

Abstract Coconut shell is a bio-degradable agro-waste obtained from the coconut processing industry and is found abundantly in India. Due to its low density, suitable chemical composition, ease of availability and cost effectiveness, coconut shell ash (CSA) is a suitable candidate as reinforcement for metal-matrix composites for automotive and aerospace applications. In the present work, coconut shell powder was initially burnt at 700°C in air to form ash. The ash was further treated at 650°C in an inert atmosphere with magnesium and graphite. The composition and morphology of initial and modified CSA was studied using X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS). The density of the particles was also determined. The composite was prepared by compo-casting route by melting AA 1200 aluminium in a stir casting furnace and adding 2, 4 and 8 wt. % (weight percentage) of the modified CSA. The microstructure of the as-cast composite was studied using optical microscopy and SEM. The density of the composite was also determined and related to the amount of filler. Hardness and wear behaviour of the composite were determined using Brinell hardness test and dry sliding pin-on disc wear test at room temperature, respectively

Dry reciprocating wear and frictional behaviour of B4C reinforced functionally graded and homogenous aluminium matrix composites

A comparative study was drawn for evaluating mechanical and tribological properties between as-cast, heat treated (HT) Functionally Graded (FG) A359/10wt% B4C and homogenous A359/6wt% B4C composites (wt% equivalent to FGM outer zone, determined by chemical dissolution method) processed through horizontal centrifugal and gravity casting techniques respectively. Metallographic analysis confirmed decreasing gradation of reinforcement particles from outer to inner layers for as-cast FG composite, whereas even distribution of reinforcement was observed throughout for homogenous composite. Formation of fine alpha-spheroidized eutectic silicon was observed in HT composite. Comparative analysis carried out for mechanical properties of as-cast to HT composite confirmed an improvement of 37.9 % for outer layer of FG composite and 32.6 % for homogeneous composite in micro-hardness, while an improvement of 24.1 % for outer layer FG composite and 5.6 % for homogeneous composite was observed for tensile strength. Reciprocating wear analysis confirmed superior resistance to wear for outer zone of HT FG composite, with an increasing trend for wear rate and friction co-efficient under the effect of load and sliding distance. A decreasing trend was observed for wear rate and friction co-efficient as weight percentage of reinforcement increased. Worn morphological analysis confirmed predominant delamination wear mechanism for homogenous composites, whereas a combination of abrasive, adhesive and delamination wear was observed for FG composite.

Investigation of Mechanical and Tribological Behaviour of Heat-Treated Functionally Graded Al-7Si/B4C Composite

The current study focuses on improving the mechanical and reciprocating wear characteristics of centrifugally cast functionally graded A356(Al-7Si-0.3 Mg)/10 wt.% B4C composite through T6 heat-treatment. Comparative analysis was performed between as-cast and heat-treated composite. Metallographic analysis confirmed the presence of reinforcement particles alongside grain boundaries and spheroidised eutectic silicon particles in heat-treated composite. Outer layer of heat-treated composite displayed superior hardness (24.09%), ultimate tensile strength (44.8%) and tribological properties when compared to outer layer of as-cast composite. With increasing applied load (15, 25, 35 N), wear rate and co-efficient of friction increased whereas wear rate and co-efficient of friction decreased as sliding distance (500, 1000, 1500 m) increased. Worn morphologies confirmed extreme delamination wear and particle pull-out at high applied loads whereas traces of oxide formations were observed at high sliding distance. Heat-treated composites have superior characteristics due to the presence of spheroidised eutectic silicon particles and precipitate formation, which makes it an alternative material for reciprocating applications in aerospace and automotive industry.

Microstructure and Mechanical Properties of Millimeter WC Particle-Reinforced High-Chromium Cast Iron Composites

WC particle/HCCI composites were prepared using liquid-phase sintering technology. In this study, millimeter WC particles were used to ensure that minimal WC particle performance was compromised during the preparation process. Moreover, with the aim of controlling the evolution of microstructures and secondary carbide precipitation in the matrix, different heat treatment methods were adopted in the manufacturing process. The microstructures of the composites were investigated via SEM, XRD, EPMA and TEM. The results showed that perfect metallurgical bonding was formed between the LCS and the WC/HCCI composites, and the width of the diffusion layer was 120 to 129 µm after heat treatment. A large number of secondary carbides precipitated in the matrix during heat treatment, and martensite formation occurred in the matrix during the subsequent cooling process, which effectively increased the microhardness of the matrix. The impact toughness of the LCS-toughened composites after heat treatment was 1.6 times that of the WC/HCCI composites, and the shear strength was 5 times that of the as-cast composites. The wear resistance of the composites under quenching at 950 °C and tempering at 220 °C was 5 times that of the as-cast composites.

Mechanical and corrosion behaviour of hydroxyapatite reinforced Mg-Sn alloy composite by squeeze casting for biomedical applications

Abstract Magnesium alloys have gained increasing attention for biomedical applications due to their biocompatibility and the biodegradability. Hydroxyapatite (HA) is known to be a highly bioactive because of its similar chemical and crystallographic structures to bone. Therefore, HA is believed to be a potential ceramic material for the fabrication of Mg based composites, to combine the advantages of both Mg and HA. But, in general, the composites known to be more susceptible to corrosion attack than the matrix alloy. Hence, in the present work, Sn is used as an alloying element to evaluate its effect on mechanical as well as corrosion properties of Mg/HA composites. Mg with 5 wt% HA and Mg-1 wt% Sn-5 wt% HA composites were prepared separately by stir assisted squeeze casting route. The phase analysis and microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) respectively. Mechanical properties were evaluated by conducting the compression and micro hardness tests. Corrosion properties of as-cast composites were studied by linear polarization, Tafel and electrochemical impedance spectroscopy (EIS) techniques. The results of both XRD and SEM-EDS revealed that the main constitutional phases of as-cast Mg/HA composites were α-Mg and HA whereas, in Mg-Sn/HA composites, the phase Mg2Sn was observed along with fine distribution of HA particles. In both the cases, no interfacial reactions observed. The yield strength, ultimate compression strength and hardness were found to be increased with the addition of Sn in Mg/HA composites. Furthermore, the addition of Sn also played an important role in increasing the corrosion resistance of the Mg/HA composites which was attributed the refinement of grain size and the formation of Mg2Sn phase along the grain boundaries. Hence, it was concluded that the addition of Sn improves both mechanical and corrosion properties of Mg/HA composites.

Effect of Tool Pin Profile on the Microstructure and Tribological Properties of Friction Stir Processed Al-20 wt% Mg2Si Composite

Abstract This study was undertaken with the aim of modifying the microstructure and improving the tribological properties of in situ Al-20Mg2Si composite. For this purpose, friction stir processing (FSP) was applied on the composite at constant travel and rotation speeds of 110 mm/min and 1500 rpm, respectively, using three different tool pin profiles: threaded tapered cylindrical, triangular tapered, and triangular threaded tapered. The sliding wear tests were conducted using a pin-on-disc apparatus under the applied load of 30 N and sliding distance of 1000 m at room temperature. The results showed that FSP substantially improved tribological properties of the as-cast composite. The best result was observed in the sample processed by the threaded triangular tapered tool, where compared with the as-cast composite, its wear rate and average friction coefficient decreased by almost 40% and 18%, respectively. This improvement can be attributed to the significant refinement and uniform redistribution of Mg2Si intermetallics, especially the coarse irregular-shaped primary crystals, the formation of ultrafine grains, and elimination of casting defects from the substrate microstructure of the processed sample, which improves its hardness and increased its potential in supporting the oxide tribolayer on the composite surface.

Mg-1Sn/Al2O3 biodegradable composites: effect of Al2O3 addition on mechanical, in-vitro corrosion and bioactivity response

The present work investigates the mechanical, corrosion and bioactive behaviour of Mg-1Sn reinforced with 5, 10 and 15 wt% of Al2O3 fabricated by squeeze casting route. The phase analysis and microstructure of as-cast composites were characterized by x-ray diffraction (XRD) and scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) respectively. Mechanical properties were evaluated by conducting the compression test. In order to evaluate the in-vitro corrosion properties of as-cast composites, Tafel and electrochemical impedance spectroscopy (EIS) techniques were performed in simulated body fluid (SBF). In-vitro bioactivity test was also carried out in SBF for 7 days of immersion period. Compression test results have showed that the increase in Al2O3 content up to 10 wt% in Mg-1Sn matrix would increase the compression strength and deteriorate on further increase Al2O3 content. The similar trend was observed in case of corrosion behaviour indicating that beyond 10 wt% of Al2O3, both mechanical and corrosion properties diminish. However, bioactivity performance was almost similar in case of all the composites and was better compared to Mg-1Sn alloy without the addition of Al2O3. Hence, it was concluded that the addition of Al2O3 increases the bioactivity of the Mg-1Sn alloy and up to the 10 wt% addition would enhance the mechanical and corrosion properties and further increase in Al2O3 content, they seem to be declining.

Effect of CNT on microstructure, dry sliding wear and compressive mechanical properties of AZ61 magnesium alloy

Carbon nanotubes (CNTs) reinforced AZ61 magnesium alloy was successfully fabricated through stir casting method with the concentration of (0, 0.1, 0.5, 1) wt.% CNTs followed by age heat treatments. The influence of the CNTs concentration on compressive mechanical properties, dry sliding wear behavior and microstructure of synthesized composites have been investigated using compression test, hardness, block on wear apparatus, scanning electron microscope and X-ray diffraction (XRD) respectively. The comparative study of as-cast and aged CNTs/AZ61 composites have been analyzed. The experimental results reveal that the addition of CNTs content in AZ61 magnesium alloy causes the decrease in mass wear loss and coefficient of friction, improves grain refinement and compressive strength. Three dominant wear mechanism (i.e. abrasion, oxidation, and delamination) have been observed. The compression strength of composites increases with the increase in CNTs contents for both aged and as-cast composite. The addition of CNTs contents has very small effects on fracture strain of as-cast composites and aging has remarkable effects to increase fracture strain which increases up to 0.5wt%CNTs/AZ61 and then decrease for 1wt%CNTs/AZ61. Evaluation of microstructure revealed that CNTs have grain refining eligibility and cause the diffusion of secondary phases to improve the strength. The diffusion of phases in aged CNTs/AZ61 composites causes the formation of shallow cavities at grain boundaries.Results analysis indicated that aged-CNTs/AZ61 composites exhibit enhanced wear and mechanical properties with load-bearing capacity, grain refinement and modification at grain boundaries than as-cast composites.

Effect of Aging Time on Characteristics of ADC12/SiC Composite with Addition of Sr Modifier and Al-5TiB Grain Refiner

An aluminium composite is developed as a light material with high strength for transportation application. Aluminium ADC 12 matrix was reinforced with SiC and added with Al-5TiB grain refiner and Sr modifier which is fabricated by stir-casting. The composites then were T6 treated for the various aging time from 2, 6, 10, 15 and 24 hours. The composites were characterized including tensile strength, impact resistance, hardness testing, wear resistance, density and porosity as well. The microstructural analysis to obtain the microstructure and phases changed after T6 using SEM-EDS and confirmed by XRD. Results show that T6 treatment has affected the morphology of primary and eutectic Mg2Si, as well as precipitation of metastable β’-Mg2Si from the aging process. T6 heat treatment has improved mechanical properties for all composites compared to as-cast composites. The highest ultimate tensile strength value and hardness are shown to be 144.5 MPa and 64.467 HRB after T6 for 6 hours at 170°C.

The microstructures and properties of in-situ ZrB2 reinforced Cu matrix composites

In this work, ZrB2 particles have been in-situ synthesized in the Cu melts using Cu-B master alloy as the starting material. It is found that the non-uniformly distributed ZrB2 particles in the prepared as-cast composites tend to aggregate into agglomerations and this trend is more pronounced with the ZrB2 content increasing.In addition, the size of hexagonal plate-like ZrB2 whose diameter ranges from less than 1 μm to more than 10 μm accompanied by the thickness ranging from about 0.5 μm to 2 μm shows an uneven appearance. The increasing ZrB2 contents can give rise to the higher hardness values of 160–205% of the original pure Cu. However, the electrical conductivity which is about 52.9% IACS to 76.8% IACS will be severely affected. Besides, the decreased friction coefficient from 0.6 to 0.4 and the gradually declining lost mass of the samples indicates the better wear resistance property with increasing ZrB2 contents.

Effect of friction stir processing on corrosion of Al-TiB2 based composite in 3.5 wt.% sodium chloride solution

Abstract The microstructure and corrosion behavior of as-cast and friction stir processed in-situ Al-TiB2 based composite in 3.5 wt.% sodium chloride solution were investigated. The microstructure was characterized using X-ray diffractometry, scanning electron microscopy and electron backscattered diffraction technique while corrosion behavior was evaluated using linear/cyclic potentiodynamic, electrochemical impedance spectroscopy and ASTM-G67 tests. The composite contains sub-micron TiB2 particles in an aluminum matrix with both blocky and fine clusters of Al3Ti agglomerated around TiB2 and displays a low uniform corrosion rate. It is also resistant to pitting as substantiated by the absence of a positive loop in cyclic potentiodynamic tests. This is due to the non-conductive nature of TiB2 particles and a controlled amount of blocky Al3Ti phase. However, both friction stir processed and as-cast composites are susceptible to inter-granular corrosion where Al3Ti and TiB2 at grain boundaries provide initiation sites for corrosion. Electrochemical impedance study attributes this to the adverse effect of Al3Ti and TiB2 on the protective oxide surface film, which increases with immersion time.

Fabrication of 6061Al–5 wt%TiB2 In Situ Composite Strips by Mixed Salt Method and Twin-Roll Casting

6061Al–5 wt%TiB2 in situ composite strips were produced by a novel process. For comparison, the as-cast composites were prepared by the commonly used method and processed into strips. In the process, 6061Al–5 wt%TiB2 slurry was prepared by using the mixed salt method by making the molten Al react with stoichiometric mixed salts of KBF4 and K2TiF6. To fabricate the composite strips of as-cast state and cast-rolling state, the slurry of 50-min insulation was cast into a permanent mould and a twin-roll caster, respectively. Microstructures of both as-cast state and cast-rolling state were investigated by metallurgical microscope, X-ray diffraction, scanning electron microscope, and transmission electron microscope, and ultimate tensile tests were conducted to evaluate the mechanical properties thereof. The experimental results demonstrated that the twin-roll casting could significantly improve the microstructure and mechanical properties of the aluminium matrix composite strips. The microstructural improvement for the cast-rolling state was due to the quick cooling and deformation along the cast-rolling direction. The yield stress, the ultimate tensile strength, and the elongation of the cast-rolling state got improved by 67.9%, 31.9%, and 19.24%, respectively, compared with the as-cast state.

Microstructural Characterization and Phase Diagram Calculation of a Less Known Al–Fe–Mn–Si Phase in a SiCp/2014Al Composite

A systematic characterization of a less known Al, Fe, Mn, and Si phase in a SiC particulate-reinforced 2014Al composite (SiCp/2014Al) was performed. In addition to the expected CuAl2 phase, the Al, Fe, Mn, and Si phase was formed as either an adhesion (>1 µm) onto SiC in the as-cast composite, or as a precipitate (<100 nm) in the matrix after hot extrusion. The structure of the phase was identified as cubic by both X-ray diffraction and selected area electron diffraction (SAED). The SAED pattern also indicated that the structure belongs to the Pm space group instead of Im. The thermodynamic phase diagram was calculated, confirming the presence of an α-AlFeMn or α-AlFeSi phase in the Al-Fe-Mn and Al-Fe-Si ternary systems, respectively, within the Fe, Mn, and Si content range corresponding to 2014Al. Wavelength-dispersive spectroscopy analysis indicated that the composition of the phase is close to Al12(Fe, Mn)3Si2, in which the Mn/Fe ratio is in the range of 0.6-1.4. The determined Mn/Fe ratio corresponds to the nominal composition of Mn and Fe in the alloy.

Effect of Fe addition and moist environment on the high temperature oxidation behavior of Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) composites

The effect of Fe addition and the moist environment on the high temperature isothermal oxidation behaviors of multiphase Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) composites in the range of 1000–1300 °C have been investigated. The microstructure of all the composites with α–Mo solid solution (SS) and eutectic mixture of Mo3Si + Mo5SiB2 phases, has been refined upon Fe addition, which improves the oxidation resistance and reduces the mass loss up to 55% in dry air and 31% in moist air for 24 h exposure. Whereas, the oxidation resistance in moist air improves due to the formation of a protective Fe-rich glassy borosilicate scale, which reduces the vaporization of MoO3 by forming Fe2(MoO4)3. The cross-sectional area of the residual alloy for x = 1 were estimated to be 97% in dry air and 85% in moist air that of unoxidized coupons. The bulk hardness of the as-cast composite is 1031 Hv for x = 1, which reduced only by 6.7% (962 Hv) upon oxidation at 1300 °C in both dry and moist air, showing superior stability of the silicide matrix composites for high temperature application. X-ray diffraction, scanning electron microscopy and energy dispersive x-ray spectroscopy studies were performed to identify the various products of oxidation and to explore the mechanism of protection.

The in-situ formation of Al3Ti reinforcing particulates in an Al-7wt%Si alloy and their effects on mechanical properties

The formation of in-situ Al3Ti particulates (5 vol %) in an Al-7%Si alloy and their effects on the mechanical properties of the alloy with either the addition of Sr or not were studied under both as-cast and heat treated conditions. The in-situ Al3Ti particulates are having a blocky morphology with an average size of ∼3.5 μm. The major contribution of Sr addition is on modifying coarse plate-shaped eutectic Si-phase to a fine globular-fibrous morphology without a noticeable influence on the Al3Ti particulates. Energy dispersive X-Ray spectroscopic (EDS) analysis revealed the existence of Si in the blocky Al3Ti particulates as, most likely, (Al,Si)3Ti in the as-cast composites. Solutionization at 540 °C for 4 h resulted in the formation of a Si-rich phase inside the Al3Ti particulates, raising a question about the stability of (Al,Si)3Ti, τ1 and τ2 phases as reported by others. The introduction of in-situ Al3Ti particles improved the yield strength of Al-7Si alloy but showed adverse effect on the elongation without Sr addition. In comparison, the tensile strength and ductility of the composites were simultaneous improved with the modification of Sr under both as-cast and heat treated states due to the modification of eutectic Si plates.

Dry sliding tribological behavior of AZ31-WC nano-composites

Abstract The present study investigates the effect of WC nano-particles on tribological behavior of magnesium based nanocomposites for varying wt% of WC (0.5, 1, 1.5 and 2 wt%). Nanocomposites are fabricated using ultrasonic vibration assisted stir casting method. Optical microscopy, scanning electron microscopy and energy dispersive x-ray patterns are used to characterize the base alloy and fabricated composites. Micro-hardness values of as-cast composites are obtained through Vicker's micro-hardness tester. Friction and wear behavior of fabricated composites are studied using pin-on-disk type tribotester at room temperature under dry sliding condition. Operating load range is 10–40 N while sliding speed range is 0.1–0.4 m/s. Characterization of Mg-composites confirms particle inclusion as well as better microstructural integrity with increase in reinforcement. Hardness value of composites increases with increase in the amount of WC nano-particles. Wear and friction behavior of base matrix are also enhanced due to incorporation of WC nano-particles. Different wear mechanisms occurring at the contact surfaces are ascertained through SEM images and EDAX patterns. Oxidation in the tested surface is also identified through SEM and EDAX. This yields better tribological behavior in composites.

Mechanical properties and fracture of in-situ Al3Ti particulate reinforced A356 composites

The microstructure and mechanical properties of in-situ 5 vol. % Al3Ti/A356 composites were studied either taking account of the effects of T6 heat treatment and strontium (Sr) addition or not. For both as-cast and T6 treated composites, the in-situ Al3Ti particulates are in a blocky morphology with average size of 5–6 μm. Sr modified the Si particulates by reducing their size and aspect ratio, without a noticeable influence on the Al3Ti particulates. Energy dispersive X-Ray spectroscopy (EDS) revealed the existence of Si in the blocky Al3Ti particulates in the as-cast composites likely to form an (Al,Si)3Ti phase. Solutionization at 540 °C for 4 h resulted in the enrichment of Si inside the Al3Ti particulates, raising a question about the stability of (Al,Si)3Ti as reported by others. For both as-cast and T6 treated A356 alloy, the yield strengths were enhanced by in-situ Al3Ti particles, but the elongation was adversely affected. In comparison, the tensile strength and ductility of the Al3Ti/A356 were simultaneous improved with the modification of Sr due mainly to the modification of eutectic Si particles. Microstructure-based representative volume element (RVE) simulations revealed the microscale damage evolution of the studied materials and provided a good prediction of their fracture behavior, substantiated by fractography.

X-Ray Tomography Study on Porosity and Particle Size Distribution in In Situ Al-4.5Cu-5TiB2 Semisolid Rolled Composites

X-ray computed tomography (XCT) was used for three-dimensional (3D) visualization of the internal microstructure and quantification of the porosity and second-phase particles in Al-4.5Cu-5TiB2 composites prepared by an in situ liquid metallurgy casting route. The as-cast composites were subjected to hot rolling and mushy-state rolling for deagglomeration and to achieve a uniform distribution of CuAl2-TiB2 particle clusters. Qualitative results obtained by scanning electron microscopy (SEM) and quantitative results obtained by XCT both showed that mushy-state rolling as well as hot rolling resulted in fragmentation and a homogeneous distribution of the CuAl2-TiB2 particle clusters, with the mushy-state-rolled composite exhibiting the highest number of smaller-size particles. The porosity was increased in both rolling conditions through debonding of particles due to the compressive force during solid-state deformation along with the quick solidification of the solute-rich liquid during mushy-state rolling. These results show that application of secondary processes such as hot-rolling and mushy-state rolling can help to achieve a relatively more uniform particle distribution in Al-4.5Cu-5TiB2 in situ composites.

Microstructure, mechanical, and high-stress abrasive wear behaviour of as-cast and heat-treated Al–Si–SiCp composite

Abstract The microstructure, mechanical, and high-stress abrasive wear of as-cast and heat-treated LM25-SiC composites were compared with those of a matrix alloy and a low-cost hypereutectic alloy (LM30). The microstructure of the composite exhibits uniformly dispersed SiC particles and good interfacial bonding between the SiC particles and the matrix. Heat treatment caused the needle-shaped silicon to become spherical and improved the homogeneity of its dispersion in the matrix. The hardness, ultimate tensile strength, yield strength, and wear resistance of the materials were improved, but the elongation was reduced as a result of the heat treatment. The wear rate and friction coefficient of the materials decreased as the sliding distance increased for both the as-cast and heat-treated samples. The wear surface morphology and wear debris analyses were performed by using high-resolution field emission scanning electron microscopy.