Mg2Si Particles Research Articles

Effects of Bi-Sb Addition and Solution Treatment on Microstructures and Mechanical Properties of Al-20 wt.% Mg2Si Composites

In this study, the effects of compound modification with Bi and Sb and solution treatment on the microstructures evolution and the mechanical properties of Al-20 wt.% Mg2Si composites were investigated. The specimens were observed by optical microscope, scanning electron microscope equipped with an energy-dispersive spectrometer and x-ray diffraction. The characterization results showed that 2.0 wt.% Sb and 0.2 wt.% Bi addition refined the primary Mg2Si from 95 to 10 μm, accompanied by a change in morphology from coarse dendrites to dispersive fine polygons. After solution treatment at 540 °C for 15 h, the sharp polygonal primary Mg2Si particles were blunted and the flake-like eutectic Mg2Si phases were changed to small spherical particles, resulting in the increase in the Brinell hardness of the composites from HB 94.53 to 134.67. Mechanical tests revealed that both tensile strength and hardness of the composite were improved by compound modification with Bi and Sb, and the ultimate tensile strength (UTS) was increased from 216 MPa unmodified to 257 MPa with optimum modification of 2 wt.% Sb and 0.2 wt.% Bi. A propounded change in hardness from HB 94.53 to 130.67 was also investigated.

Effect of Si and Ni on microstructure and mechanical properties of in-situ magnesium-based composites in the as-cast and extruded conditions

The effects of silicon and nickel addition and hot extrusion process on the microstructure and mechanical properties of in-situ magnesium-matrix composites were studied. By increasing the amount of Si, the volume fraction of primary Mg2Si particles increased toward more pronounced long dendritic forms, which resulted in the deterioration of as-cast tensile properties. A new primary intermetallic phase containing Ni, Si, and Mg appeared in the microstructure upon the addition of Ni to Mg–Si system, where its effect on the as-cast tensile properties was insignificant. However, after extrusion process, the tensile strength of composites enhanced with increasing the amount of primary phases. Moreover, major enhancements in mechanical properties, tensile toughness, and fracture surface appearance were observed for extruded composites compared with the as-cast counterparts. These results can shed light on the capability of these alloys for potential applications.

Influence of extrusion parameters on microstructure, texture, and second-phase particles in an Al-Mg-Si alloy

The extrusion parameters greatly influence the microstructures of aluminum alloys, and further determine the mechanical properties of the final products. In this work, using a self-designed experimental device, a series of extrusion experiments was conducted to investigate the influence of the extrusion parameters on the microstructure, crystallographic texture, and second-phase particles in the Al-Mg-Si aluminum alloy (AA6N01), which is widely applied in the bodies of high-speed trains. Firstly, the billet discard in the press container was taken out for analyzing the material flow and microstructural evolution during the extrusion process. The results showed that continuous dynamic recrystallization occurred, and the initially coarse equiaxed grains in the as-homogenized billet evolved gradually into fine equiaxed grains at the die exit due to their dynamic recovery and dynamic recrystallization. More importantly, coarse grain layers were observed on the surface of the extruded profiles under specific extrusion conditions (460 °C/48 mm/min and 520 °C/48 mm/min). Then, the textures on the surface and at the center of the profiles were analyzed under different extrusion conditions by electron backscatter diffraction. The results showed that the textures of all the extruded profiles were mainly along <100> and <111 > , but the texture intensity was higher at the profile's center than that on the surface under the same extrusion condition. Finally, the second-phase particles in the as-homogenized billet and as-extruded profiles were compared and analyzed. The Fe-rich particles were broken under deformation conditions and their sizes decreased during the extrusion process. However, they were all still distributed on the grain boundaries after deformation, and the composition and content remained unchanged. The Mg2Si particles served as the main strengthening phase in AA6N01, and they were greatly aggregated as the extrusion temperature increased and the extrusion speed decreased.

Effect of post-weld heat treatment on the mechanical behavior and dislocation density of friction stir welded Al6061

Al6061 is a heat-treatable aluminum alloy and an extremely versatile material used in settings where medium to high strength is required. To produce good joints using this alloy, a particular solid-state welding process called friction stir welding (FSW) is preferable instead of the conventional welding methods. In this study, the effects of T6 post-weld heat treatment (PWHT) on the mechanical properties, dislocation density, and microstructure of a friction stir welded (FSWed) Al6061 aluminum alloy were investigated. Results indicated that FSW had degraded the mechanical properties of Al6061. The welding efficiency decreased to 65% compared with the tensile strength of the base metal. After FSW, the samples demonstrated a serrated flow behavior known as the Portevin–Le Chatelier (PLC) effect. This effect was attributed to the nonuniform distribution of hard Mg2Si particles, coarsening and/or solutionizing of the strengthening elements in the stir and thermomechanically affected zones, and an over-aging effect in the heat-affected zone of the FSWed sample. However, the T6-PWHT performed on the FSWed sample diminished the PLC effect and concurrently improved the mechanical properties back to its original state. The PWHT also promoted precipitation hardening through a better distribution of Mg2Si particles that prevented grain growth and increased the dislocation density due to the applied strain, which subsequently improved the mechanical properties of the FSWed Al6061 alloy.

Preparation of millimeter scale second phase particles in aluminum alloys and determination of their mechanical properties

The mechanical parameters such as elastic moduli E and shear moduli G of second phases are critical inputs for modeling and optimizing mechanical properties of aluminum alloys. However, some of these mechanical parameters are lacking because these micron or even nano scale phases are hard to be tested. The aim of this paper is to present an efficient approach to test the mechanical properties of second phases in aluminum alloys accurately. The α-AlFeSi, β-AlFeSi and Mg2Si particles at the millimeter scale were prepared by directional solidification with the aid of calculation of phase diagrams (CALPHAD). Such a large size can effectively reduce the impact of particle penetration into matrix in nanoindentation and Vickers hardness experiments. The elastic moduli of α-AlFeSi, β-AlFeSi and Mg2Si were determined as 183.81 ± 8.64 GPa, 174.13 ± 6.81 GPa and 116.38 ± 4.06 GPa, respectively, and their Vickers hardness values were 883 ± 64, 765 ± 21 and 475 ± 22 HV, respectively. Besides, the continuous uniform plastic deformation (without pop-in phenomenon in load-displacement curve) of Mg2Si was observed and is attributed to the anti fluorite structure of Mg2Si. Further, by Vickers hardness and electron backscatter diffraction (EBSD) it was found that the hexagonal α-AlFeSi had greater hardness anisotropy than the near-tetragonal β-AlFeSi. Overall, the hardness values of α-AlFeSi particles in <u 0u¯w> direction were higher than others, the hardness values of β-AlFeSi particles near <001> were lower than others.

Aluminum Matrix In-Situ Composites Reinforced with Mg2Si and Al3Ti

Cast aluminum matrix composites are promising materials for the production of responsible and particularly critical parts used in the high-technology fields of industry such as aerospace, automotive, electronics, etc. In this work, hybrid aluminum matrix composite reinforced with in-situ formed Mg2Si and Al3Ti particles were synthesized successfully in gravity casting conditions, and the influence of thermal conditions of solidification on the structural parameters of endogenous reinforcing phases was investigated. The results show that an increase in the cooling rate during solidification due to the application of a copper mold instead steel mold leads to reduction in the average size of the Mg2Si primary crystals, improvement of the distribution uniformity and significant increasing of their total quantity; at the same time, the average size of Al3Ti particles decreases substantially but their quantity is approximately the same for both types of molds.

Fracture and determination of a lifetime of in-service mechanical components with porosity cluster in an aluminium alloy

This paper focuses on the fatigue lifetime estimation of metallic components containing group of porosities as manufacturing defects. In order to represent these defects, six test specimens with different Side Drilled Holes (SDH) clusters, have been made of 7075-T6 aluminium alloy. Main characteristics of such defect (elementary size of porosities, spatial density or gradient of elementary sizes) have been studied by taking it into account in a FEM modelling in fatigue to determine their influence on the lifetime. Experimental tensile fatigue tests have been performed on all SDH samples with load ratio R=0.1. Semi-elliptical surfaces with a size of a typical Mg2Si particle of the studied alloy have been chosen as initial cracks for multiple crack propagation. Indeed, such a particle is often responsible for the initiation of a crack in this type of material. For crack growth, Paris’ type propagation law has been used. Local stress redistribution due to presence of the SDH implies mix-mode behaviour which has been considered by using G-theta method. Simulations results allowed realistic lifetime estimation according to the experimental ones. Moreover, a strong dependency of the crack paths and fatigue lifetime on the defect characteristics has been proved and quantified.

Experimental Investigations on the Failure of Diesel Engine Piston

Piston failure is not a common incidence usually associated with micro-structure aspect. Pin hole or skirt of the piston is affected that most of the failure was initiated by cracks. In this study, investigations of piston failure using experiments have been presented. The average value of the hardness is 61 HRB. Hardness of the surface is clearly differs from the crown to skirt of the piston. The high percentage of silicon, increased the piston life, but could not retain high temperatures during the operating time and finally failed. Copper percentage decreases the mechanical properties of the alloys. The micro-structure of the piston particles like small, angular and primary silicon and Mg2Si particles distributed in a matrix of aluminum solid solution throughout structure.

Characterization of the Density and Spatial Distribution of Dispersoids in Al-Mg-Si Alloys

Al-Mg-Si alloys often contain small additions of, for example, Mn or Cr, to form dispersoids, which may act as nucleation sites for Mg2Si particles after homogenization. The purpose of adding Mn and Cr is to ensure a high density of uniformly distributed small β’-particles, which can be dissolved during further processing prior to the final age hardening step. However, their density and spatial distribution are critically dependent on the homogenization procedure. It is therefore important to have a robust and reliable method for assessing their spatial distribution. In the present work, an existing methodology for assessing spatial uniformity, the Global Shannon Entropy (GSE), was implemented and evaluated for different dispersoid structures characterized by scanning electron microscopy. This metric is highly dependent on the parameters used, but by careful selection of adequate parameters, it can be effective in detecting non-uniformity. An important weakness with the GSE was identified, and a modification to improve on the ability to differentiate degrees of non-uniformity is suggested. To evaluate the proposed methodology, the effect of heating rate on dispersoid precipitation behaviour during homogenisation of four Al-Mg-Si alloys with different Mn/Cr-content was investigated. In general, the dispersoid density increased with decreasing heating rates as longer times in the furnace resulted in particle coarsening. Slower heating rates were found to promote a more uniform dispersoid structure for most alloys investigated in this study. The metric with the new term demonstrated promising results, and improved the ability to differentiate degrees of spatial uniformity.

Centrifugal casting of in-situ Al-Mg2Si(1-x)Snx composites: a study of radial segregation

ABSTRACTDuring centrifugal casting of Al-Mg2Si hypereutectic cylinders, the light Mg2Si particles are commonly segregated in centripetal direction towards the inner periphery. In order to change this inevitable centripetal segregation of Mg2Si particles, some amounts of tin up to 8 wt pct were added to Al-15wt.% Mg2Si alloy to enhance the formation of Mg2Si(1−x)Snx particles with variant stoichiometry thus variant density. XRD, EDS, optical metallography and hardness measurements were used to evaluate the stoichiometry and segregation patterns of in situ formed Mg2Si(1−x)Snx particles. According to the results, the cylinder containing 6 wt pct Sn illustrates a rather uniform distribution of particles thus hardness in all radial sections. However, the cylinders with higher amounts of Sn exhibit a radial segregation of Mg2Si(1−x)Snx particles in centrifugal direction towards the outer periphery. The results are interpreted in view of the Stokes’ law and the difference in density of the particles and the melt.

Effect of SIMA Process on Microstructure and Wear Behavior of Al-Mg2Si-3% Ni Composite

The microstructural evolution and wear behavior of Al-Mg2Si-3% Ni in situ composite during strain-induced melt-activated (SIMA) process were investigated in this study. In order to induce strain in samples, they were cold deformed by the compression ratio of 10% reduction. Then, they were isothermally re-heated at 540 °C for 20, 40 and 60 min. It was observed some cracks in primary Mg2Si particles which lead to reduce the size of these particles and distribute more uniformly in matrix. Also, they became more globular after SIMA process. Furthermore, the brittle network of eutectic phase has altered remarkably during this process from flake-like to rod- or dot-like. The wear resistance of specimens improved remarkably after SIMA process. It was found that this improvement can attribute to microstructural changes, especially eutectic Mg2Si phase, because this microstructure can curb the crack nucleation in delamination mechanism and consequently reduces the wear rate.

Understanding the effect of the reinforcement addition on corrosion behavior of Fe/Mg2Si composites for biodegradable implant applications

Abstract Iron-based materials showed a high potential for degradable biomaterial applications. However, their slow corrosion rate limits their use as a biodegradable implant material. One approach to control and modify their mechanical properties is the use of reinforcement particles, to create metal matrix composites in which the second phase is aimed at tuning not only the mechanical properties but also the corrosion mode and rate in a physiological environment. Here, Fe/Mg2Si composites were produced via powder metallurgy from pure Fe and Mg2Si powders. The different conditions were fabricated by various combinations of milling or mixing processes followed by hot rolling consolidation. The effect of the Mg2Si addition on corrosion behavior of Fe/Mg2Si composites was studied by performing cyclic polarization and static corrosion tests for an immersion time of 24 and 240 h. The presence of the reinforcement particles played a crucial role in the susceptibility of Fe-based composites to localized corrosion attack. The corrosion initiation and its development were systematically monitored. Scanning electron microscopy, X-ray diffraction, and atomic emission spectroscopy were employed to investigate the corrosion mechanism. The importance of Mg2Si particles in the triggering of corrosion processes was explained. Electrochemical measurements and static immersion tests implied that the introduction of Mg2Si particles could accelerate the corrosion rate of Fe. It was confirmed that the size and distribution of the reinforcement influenced considerably the uniformity of the corrosion attack.

In-situ study of phase transformations during homogenization of 6060 and 6063 Al alloys

In as-cast 6xxx aluminium billets β-Al5FeSi intermetallic phase and coarse Mg2Si particles have negative effects on subsequent processes such as extrusion. To achieve extruded products with a high surface quality the billets are heat-treated by a so-called homogenization before extrusion. During the heat treatment undesired intermetallic particles, i.e. β-AlFeSi platelets, are transformed to rounded α-Al(FeMn)Si intermetallic particles. The transformations occurring during homogenization have up until now only been studied by interrupted experiments giving some insight into the process, but a complete picture of how these phase transformations are coupled to the geometrical changes has not evolved. In the present study transformations in 6060 and 6063 alloys have been observed in-situ by TEM, and several nucleation and growth modes have been identified. Boundary layer diffusion was found to be important for the understanding of how the transformations proceed, and also diffusion on the free surface contributed to higher transformation rates compared to industrial processes due to thin characteristics of TEM samples. Together with an earlier study of 6005 and 6082 alloys the homogenization process for all important industrial 6xxx alloys is now characterised in detail by in-situ monitoring.

In situ synthesis of carbon doped porous silicon nanocomposites as high-performance anodes for lithium-ion batteries

We demonstrate the in situ synthesis of carbon doped porous silicon (Si/C) nanocomposites by a simple thermal displacement process between Mg2Si and inorganic gas CO2 in one-step. Via the decomposition of Mg2Si, the reduction process occurred between Mg and CO2, leading the uniform doping of many distributed tiny carbon nanoparticles into Si. Meanwhile, the porous structure was formed after an acid treatment. When worked as anodes for lithium-ion batteries, the as-prepared s-porous Si/C nanocomposites exhibited good cycling stability and high-rate capability, which were superior to the porous Si and porous Si/C nanocomposites. It was revealed that the enhanced electrochemical properties could be ascribed to the novel porous structure and doped carbon nanoparticles that can buffer the volume expansion, as well as enhance the electronic conductivity of Si. The reaction mechanism was well investigated by studying the influence of reaction temperature and raw Mg2Si particle size on the morphology and component of the porous Si/C nanocomposites.

Modelling and experimental validation of microstructure evolution during the cooling stage of homogenization heat treatment of Al–Mg–Si alloys

A CALPHAD-coupled multi-component multi-phase Kampmann–Wagner Numerical modelling framework has been adapted and coupled with a homogenization model to predict the competitive nucleation and growth of multi-sized metastable/stable phase particles during the cooling stage of a homogenization heat treatment. The reported model is the continuation of our previous work on the homogenization soaking modelling (Du et al., 2013). It takes the experimentally verified soaking modelling prediction results as the initial microstructural status and predicts the microstructural evolution during the very last step of the homogenization treatment, i.e. the final cooling process. The model has been applied to two Al–Mg–Si alloys: AA6061 and AA6082. The simulation shows a multi-modal MgSi particle size distribution forms due to multiple nucleation events. For the AA6082 alloy the multiple nucleation events occur mainly due to the local micro-chemistry variations along a dendrite while for AA6061 it is due to the two opposite contributions to supersaturation: the positive contribution by the cooling and the negative contribution by the particles' growth. The interaction of intergranular and intragranular MgSi containing particles have been captured, and the partitioning of Mg and Si solutes among the two different sized MgSi particles has been predicted. The model predictions are in reasonable agreement with the microstructure characterization results obtained with Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Electrical Resistivity Measurement on the quenched samples from some dedicated laboratory-scale homogenization heat treatment experiments. It is concluded that the extended KWN modelling framework is applicable to predict the microstructure evolution during the non-isothermal heat treatment of multicomponent aluminium alloys.

Effect of Copper and Magnesium on the Microstructure of Centrifugally Cast Al-19%Si Alloys

Hypereutectic Al-Si alloys can be used in applications that require high wear resistance. Such wear resistance is achieved by the presence of hard primary silicon particles, allied to the formation of Mg2Si intermetallic phase when magnesium is added in this alloy. Centrifugal casting generates a gradient in the microstructure of hypereutectic Al-Si alloys that can favor such applications. Cylindrical components of Al-19%Si alloy containing added copper and magnesium contents were processed by centrifugal casting. The purpose of this study is to investigate the formation and segregation of particles of primary silicon (β) and Mg2Si in Al-19%Si alloy containing additions of copper and magnesium. Because the density of silicon (2.33 g/cm3) and Mg2Si (1.88 g/cm3) is lower than that of aluminum (2.67 g/cm3), centrifugal casting causes primary silicon (β) and Mg2Si particles to concentrate more at the outer wall of the centrifuged pipe. In this study, primary silicon (β) and Mg2Si particles were found to be retained at the outer wall of the pipe. It is believed that the rapid cooling of the molten metal in the region of contact with the mold, whose temperature is lower than that of the molten metal, allied to the centrifugal force, prevented the particles from migrating to the inner wall of the pipe. The microstructure shows a gradient in the distribution of these phases, enabling the production of a functionally graded material. The addition of copper and magnesium leads to the formation of Mg2Si and Al5Cu2Mg8Si6 phases, reducing the amount of primary β phase (Si) particles. In all the evaluated conditions, a tendency is also observed for a gradual increase in the segregation of silicon towards the inner wall along the entire length of the centrifuged pipe.

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg2Si in situ composites

The effect of high-speed direct-chill (DC) casting on the microstructure and mechanical properties of Al–Mg2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg2Si particles (from 28 to 12 μm), the spacing of eutectic Mg2Si (from 3 to 0.5 μm), and the grains of AA6061 alloy (from 102 to 22 μm). The morphology of the eutectic Mg2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg2Si phase and the grains in the Al–Mg2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg2Si composites and AA6061 alloy billets.

Effect of Ce Inoculation on Microstructure and Mechanical Properties of in situ Al–20%Mg2Si Composite

In this study, an in situ Al–20%Mg2Si composite fabricated with varying levels of cerium (Ce) as an inoculation was investigated. Microstructural examinations were carried out via scanning electron microscope and X-ray diffraction. The results showed that Ce inoculation changed the morphology of primary Mg2Si from coarse lotus-type particles to fine polyhedral shape. Furthermore, the average size of primary Mg2Si particles decreased obviously. When the Ce addition reached to 0.6%, the refinement of Al–20%Mg2Si composite was the best. Similarly, the superior ultimate tensile stress, elongation at fracture (%El), and Vicker’s hardness (Hv) of Al–20%Mg2Si composite were obtained. However, excess Ce inoculation addition reduced refinement and mechanical properties because of Al11Ce3 phase aggregating.

Experimental investigation on microstructure, mechanical properties and dust emission when milling Al-20Mg2Si-2Cu metal matrix composite with modifier elements

Sustainable manufacturing regulations are pushing manufacturing towards decreasing of manufacturing hazards including microparticles and ultrafine particles. Machining process such as milling produces dust that can be harmful for operators’ health. The emission of this dust depends on workpiece materials (microstructure, mechanical properties) and machining conditions. The aim of this paper is to determine the effect of the microstructure and machining conditions on dust emission during dry milling of Al-20Mg2Si-2Cu metal matrix composite with addition of bismuth (Bi) and barium (Ba). Experiments were carried out using dry CNC milling by uncoated carbide tools. An aerodynamic particle sizer (APS) and a scanning mobility particle sizer (SMPS) were used to measure microparticles and ultrafine particles emission, respectively. It was found that the addition of 0.4 wt% Bi and 0.2 wt% Ba changed Mg2Si particle size and improved the hardness of composite. In addition, ultrafine particle number concentration, specific area concentration and mass concentration decreased with the addition of modifiers. It is also confirmed that cutting conditions and microstructure of workpieces have a direct effect on dust emission during the milling process.

Pengaruh Solution Heat Treatment terhadap Sifat Fisis dan Mekanik Proses Pengelasan Fssw AA6063-T5

Friction stir spot welding (FSSW) of aluminum alloy AA6063-T5 will decrease the strength and hardness. Physical and mechanical properties of aluminum alloy can be improved by heat treatment. Heat treatment is affected by temperature and holding time. The heat treatment process used for this research was solution heat treatment with temperature variation 470, 500, 530 oC and holding time 1 and 2 hours. The tests included microstructure test, tensile shear test and vickers hardness test. Result of the research, it was found that Mg2Si particles were precipitated from the grain boundaries part into the aluminum matrix with the coarser size and the distance was increasing as the temperature and the holding time increases. The highest shear tensile strength of 3735,2 N was obtained from the temperature variation of 470 oC and the holding time of 1 hour. The lowest tensile shear strength of 3172,6 N was obtained from temperature variation of 530 oC and 2 hours of holding time. The highest hardness value was obtained of 470 oC variation and 1 hour was 43,7 HVN and the lowest hardness value at 530 oC variation and 2 hours of holding time was 30,1 HVN.