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.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Effect of copper addition on wear and corrosion behaviours of Mg2Si particle reinforced composites

The purpose of this study is to investigate the effect adding Cu has on the wear and corrosion properties of “in situ” Mg₂Si particle reinforced Al–12Si–20Mg matrix composites, produced with help of the nucleation and growth of the reinforcement from the source matrix, in order to overcome the disadvantages of composites produced by externally reinforcing ceramic particles. Composites known as Al–12Si–20Mg–XCu were produced by adding Cu, at the rate of 1%, 2%, and 4%, to the Al–12Si–20Mg alloy in order to achieve this purpose. The microstructural characterisation, hardness, wear and corrosion properties of composites, produced using the casting method, were analysed. Dry environment wear experiments for investigated alloys were conducted using a pin-on-disc type wear device under different loads and at different sliding distances. The change in weight loss of the solution containing 30 g/l NaCl + 10 ml/l HCl, and the tafel extrapolation method were used to analyse corrosion behaviour. Results of microstructural characterisation concluded that as the amount of Cu added to the Al–12Si–20Mg alloy increased, the size and volume of the Mg₂Si particle, formed within the matrix, decreased, and CuAl₂ intermetallics formed within the matrix. Results of wear experiments concluded that adding Cu developed wear resistance under small loads; however, reduced wear resistance under high loads. According to results of corrosion experiment, corrosion resistance increased with the addition of Cu.     

The sliding wear behavior of TiCp/AZ91 magnesium matrix composites

The AZ91 metal matrix composites (MMCs) reinforced with 5, 10 and 15 wt.% TiC particulates are fabricated by TiC_p–Al master alloy process combined with mechanical stirring. The effects of TiC particulate content, applied load and wearing time on the sliding wear behaviors of the composites were investigated using MM-200 wear testing apparatus. The results show that the wear resistance and friction coefficient of the composites increased and decreased with increase of the TiC particulate content, respectively. The wear volume loss and friction coefficient of the reinforced composites as well as the unreinforced AZ91 matrix alloy increased with increase of applied load or wearing time, but the increase rates of the reinforced composites in two performance is lower than those of the unreinforced AZ91 matrix alloy. Furthermore, the sliding wear behavior of the composites and the unreinforced AZ91 matrix alloy is characterized by ploughing, adhesion and oxidation abrasion.     

Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite

One kind of (submicron + micron) bimodal size SiCp/AZ91 composite was fabricated by the stir casting technology. After hot deformation process, the influence of bimodal size particles on microstructures and mechanical properties of AZ91 matrix was investigated by comparing with monolithic A91 alloy, submicron SiCp/AZ91 and micron SiCp/AZ91 composites. The results show that micron particles can stimulate dynamic recrystallized nucleation, while submicron particles may pin grain boundaries during the hot deformation process, which results in a significant grain refinement of AZ91 matrix. Compared to submicron particles, micron particles are more conducive to grain refinement through stimulating the dynamic recrystallized nucleation. Besides, the yield strength of bimodal size SiCp/AZ91 composite is higher than that of single-size particle reinforced composites. Among the strengthening mechanisms of bimodal size particle reinforced composite, it is found that grain refinement and dislocation strengthening mechanism play a larger role on improving the yield strength.     

Self-lubricate and anisotropic wear behavior of AZ91D magnesium alloy reinforced with ternary Ti2AlC MAX phases

The dry sliding wear behavior of Ti₂AlC reinforced AZ91 magnesium composites was investigated at sliding velocity of 0.5 m/s under loads of 10, 20, 40 and 80 N using pin-on-disk configuration against a Cr15 steel disc. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were examined by scanning electron microscopy, energy dispersive X-ray spectrometry and transmission electron microscopy in order to obtain the wear mechanisms of the studied materials. The main mechanisms were characterized as the magnesium matrix oxidation and self-lubrication of Ti₂AlC MAX phase. In all conditions, the composites exhibit superior wear resistance and self-lubricated ability than the AZ91 Mg alloy. In addition, the anisotropic mechanisms in tribological properties of textured Ti₂AlC-Mg composites were confirmed and discussed.     

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.     

Effect of secondary processing on the microstructure and wear behavior of spray formed Al–30Mg2Si–2Cu alloy

In the present work, Al–30Mg₂Si–2Cu alloy has been spray formed and subsequently hot pressed for densification. The alloy is then subjected to solutionizing and isothermal aging treatments. The microstructural features, hardness and wear behavior of spray formed and secondary processed alloys have been evaluated individually and compared with that of as-cast alloy. The microstructure of spray formed alloy showed refined and globular shaped primary Mg₂Si intermetallic particles and Al₂Cu precipitate particles uniformly distributed in Al matrix. The microstructure was refined further after hot consolidation. The microstructure after solution heat treatment appeared similar to that of the spray formed alloy but aging led to a further refinement in the microstructure compared to that of the hot pressed alloy. The evaluation of wear behavior of these alloys, under dry sliding condition, showed that the age hardened alloy exhibits maximum wear resistance and minimum coefficient of friction over the entire range of applied load (10–50 N) at a sliding speed of 2 ms⁻¹ followed by hot pressed, spray formed and solution heat treated alloys. The as-cast alloy showed the least wear resistance and highest coefficient of friction. Similar trend has been observed even in their hardness values too. The wear resistance of the alloys is discussed in light of their microstructural modifications induced during spray forming and subsequent secondary processing and also the topography of worn surfaces.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Abstract

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Microstructure and mechanical properties of SiC nanoparticles reinforced magnesium matrix composites fabricated by ultrasonic vibration

A particulate reinforced magnesium matrix nanocomposite was fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, grain size of matrix in the SiCp/AZ91 nanocomposite decreased and morphology of phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates. Although there were still some SiC microclusters in the nanocomposite, most of the SiC nanoparticles were dispersed well outside the microclusters. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite were simultaneously enhanced compared with that of the AZ91 alloy. The study of interface between the SiC nanoparticles and the matrix in the nanocomposite suggested that SiC nanoparticles bonded well with the matrix without interfacial activity.     

Microstructure and properties of Sip/Al–Si surface composites prepared by ultrasonic method

Primary Si particles reinforced Al–Si surface composites (Si_p/Al–Si surface composites) were prepared by means of ultrasonic equipment with a special horn crucible. The microstructure and properties of the surface composites were investigated using optical microscope, scanning electron microscopy (SEM), hardness meter and friction and wear tester. The results show that when Al–12%Si alloy was treated by ultrasonic, Si element was easy to move up because of the decrease of the viscosity of the melt, and the alloy composition at the top of the melt became hypereutectic. So, a mass of primary Si particles formed in this place. The thickness of the surface composite layer in the surface composites decreased with increasing the ultrasonic input power. The average size of the primary Si particles in the surface composite layer was larger than that of Al–Si alloy untreated by the ultrasonic and increased with increasing ultrasonic input power. The top layer hardness of Si_p/Al–Si surface composites is higher than that of Al–Si alloy without ultrasonic treatment and increased with increasing ultrasonic input power. The friction coefficients of the top layers of the surface composites are lower than that untreated by ultrasonic. The friction coefficient decreased with increasing ultrasonic input power. With the increase of the applied load, the friction coefficient of the top layer of the surface composites increased. The wear mass loss of Si_p/Al–Si surface composites is lower than that Al–Si alloy without ultrasonic treatment. The wear resistance of the surface composites was improved with increasing ultrasonic input power.     

Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders

To improve the wear resistance of AZ91D magnesium alloy, laser surface cladding with Al and Si powders was investigated using a Nd:YAG pulsed laser. With appropriate processing parameters and the suitable weight ratio of Al to Si in powders, a modified surface layer free of cracks and pores was formed by reaction synthesis of Mg with Al and Si. X-ray diffractometry (XRD) confirmed the main phases in the layer to be Mg₂Si and Mg₁₇Al₁₂. The surface hardness increased from 35 HV for as-received magnesium alloy to more than 170 HV for laser treated sample. Accompanying the increase in hardness, the wear resistance of the clad layer increased more than 4 times that of the substrate.     

Characterization of AZ91/alumina nanocomposite produced by FSP

The microstructures, wear property and micro-hardness of AZ91 Mg alloy/alumina particle reinforced nano-composite produced by friction stir processing (FSP) were investigated. The initial microstructures of the AZ91 were composed of irregularly distributed β-phases (Al₁₂Mg₁₇), while the FSPed specimens were characterized by the homogeneous distribution of alumina particles, the recrystallized grain structure and the dissolution of β-phase. The results showed an improvement in the hardness, wear property of the FSPed zone as results of more grain refinement and pinning effect of nano-alumina particles as compared to those of the base metal. The hardness of the FSPed zone was a higher and more homogeneously distributed and the wear resistance as evaluated by Dry sliding wear tests, was superior, as compared with the base metal.     

Mo对TiC/Al复合材料组织和性能的影响

为了细化TiC/Al基复合材料中的增强颗粒,进一步提高TiC颗粒对基体的强化效果,在锻铝6A02基体中加入适量Mo元素,用原位合成的方法制备TiC/Al基复合材料.对制备得到的铸态和轧制态材料进行了显微组织观察、拉伸和磨损实验.结果表明,TiC颗粒可以作为异质形核核心起到细化基体组织的作用.TiC颗粒的引入提高了材料在室温和高温的抗拉强度和屈服强度,同时改善了材料的耐磨损性能,且随着载荷的增加,耐磨性能的提高越明显.当加入质量分数1.0%的Mo时,可改善基体对TiC颗粒的润湿性,细化TiC颗粒的尺寸(0.5μm),使TiC颗粒分布更为均匀,材料的力学性能和磨损性能得到提高.然而,过高的Mo含量将导致在组织中出现粗大的脆性Al5Mo相,同时使材料的力学性能和磨损性能有所降低.     

Effects of Al2O3 particle reinforcement on the lubricated sliding wear behavior of ZA-27 alloy composites

The present investigation deals with the effect of Al₂O₃ particle reinforcement on the lubricated sliding behavior of ZA-27 alloy. The composites with 3, 5, and 10 wt% of Al₂O₃ particles were produced by the compocasting procedure. Tribological properties of alloy and composites were studied, using block-on-disk tribometer at different specific loads and sliding speeds. The test results revealed that composite specimens exhibited significantly lower wear rate, but higher coefficient of friction than the matrix alloy specimens in all the combinations of applied loads and sliding speeds. The improved antiwear characteristics of the composites were influenced by positive effects of higher frictional heating on compatibility of the composite phases and suppressing micro-cracking tendency. Due to that, effects of reinforcing hard particles were manifested through the reduced wear rate of composites, especially in conditions of higher load, lower sliding speeds and higher Al₂O₃ particle content. In present wear tests, the significant forming of mechanically mixed layers was not noticed, what is confirmed by the SEM microphotographs.     

Microstructure Control and Performance Evolution of Hypereutectic Al–20Mg2Si Alloy by Novel Al–Ca–Sb Masteralloy

Herein, the microstructure control and performance evolution of hypereutectic Al–20Mg₂Si alloy with the addition of novel Al–3.3Ca–10Sb master alloy are investigated. It is found that AlCa₁₁Sb₉ and CaSb₂ compounds are successfully synthesized through in situ melt reaction of masteralloy. With 0.45 wt% Al–3.3Ca–10Sb master alloy addition, primary Mg₂Si particles in hypereutectic Al–20Mg₂Si alloy are significantly refined from more than 150 μm to 10.7 μm, which are accompanied with the 3D morphologies changing from dendrites to octahedrons. After heat treatment, Brinell hardnesses of Al–20Mg₂Si alloys are remarkably improved to 112 HB. Furthermore, it is also found that the cooling rate of Al–3.3Ca–10Sb master alloys has certain influence on the refinement effect of Al–Mg₂Si alloys. The excellent complex modification of this master alloy on Al–20Mg₂Si alloy can be attributed to the existence of CaSb₂ particles as the heterogeneous nucleation sites of Mg₂Si particles and the inhibiting growth effect of residual Ca atoms adsorbed on the surface of Mg₂Si phase.     

Processing and Characterization of In Situ (TiC–TiB2)p/AZ91D Magnesium Matrix Composites

In this paper, a practical and cost‐effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB₂ particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B₄C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Ti_p + B₄C_p) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X‐ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four‐point‐bending test at room temperature. Microstructural characterization of the (TiB₂–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB₂ and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH₂ powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.     

Grain refinement of AZ91 alloy by introducing ultrasonic vibration during solidification

Ultrasonic vibration was introduced into the solidification of AZ91 alloy. Various microstructures were produced in this alloy using ultrasonic vibrations at different temperatures of the melt. The coarse dendrite microstructures were obtained with ultrasonic vibrations at temperatures below the liquidus temperature. The fine uniform grains were achieved under ultrasonic vibrations during the nucleation stage, which was mainly attributed to the cavitation and the acoustic flow induced by the ultrasonic vibration.