Mg2Si Phase Research Articles

Microstructure and Mechanical properties of CMT welded AA6063-T6/B4C/ES metal matrix composite using ER5356 filler wire

Aluminium alloy 6063-T6 is a popular choice among different industrial sectors as it portrays various desirable properties like moderate strength, improved corrosion resistance, better weldability and formability etc. These advantages make AA6063-T6 a common choice for matrix in a metal matrix composite. Mostly conventional fusion welding methods like Metal Inert Gas (MIG) and Tungsten Inert Gas (TIG) seem preferable options for welding aluminium and its alloys but it remains a challenge to use them to weld aluminium metal matrix composite. A modified version of MIG i.e. CMT welding, can be explored to overcome the challenges. In this study, an aluminium metal matrix composite has been fabricated with AA6063-T6 as matrix and two reinforcements −one organic, i.e., Eggshell (ES) and one inorganic, i.e., Boron Carbide (B4C), using the conventional stir casting method. CMT welding has been performed on plates cut out from the composites. Microstructure, hardness, tensile strength and residual stress of weld joint of composite have been observed. From micrographs, it is observed that a finer grain has been formed at the Fusion Zone (FZ) in comparison to the Heat Affected Zone (HAZ) and Parent Material (PM). XRD analysis at the fusion line has identified several intermetallic stoichiometric compounds (Al12Mg17, Mg2Si, CaMgSi, CaO, B2O3) and reinforcement B4C and CaCO3 within the matrix volume. The FZ exhibits higher hardness than the HAZ and the PM. Further, the tensile strength of the FZ exceeds that of the unwelded composite due to the presence of metastable strengthening phases of Mg2Si in the FZ, along with reinforcement (B4C and ES) particles. Residual stress analysis reveals tensile stress in the FZ and more compressive stress in the HAZ than in the PM.

Effect of post-weld heat treatment methods on microstructure and fatigue behavior of Al-Mg-Si alloy oscillating laser welding

To address the softening issue in aluminum alloy weld joints, post-weld heat treatment(PWHT) techniques were employed to enhance the mechanical properties of Al-Mg-Si alloy weld joints joined by oscillating laser welding(OLW). A systematic investigation was conducted on the impact of two different PWHT methods, artificial aging(AA) and solution treatment followed by aging(STA), on the static mechanical properties, fatigue performance, and microstructure of the weld joints. The results indicated that both AA and STA could improve the tensile strength and fracture strain of the welded joints. Compared to AA, STA did not show a significant improvement in increasing tensile strength and elongation at break. Notably, AA significantly increases the fatigue life of the joints, while STA also contributes to enhanced fatigue life, albeit to a lesser degree. At the lowest stress level, the fatigue life of both STA and as-welded(AW) joints exceeded 106 cycles, with the STA joints exhibiting approximately 57.9% higher fatigue life than the AW joints. Microstructural analysis of the AW joints revealed the presence of dislocations and a few large-sized precipitate phases, primarily Mg2Si and AlFeSi phases. After AA, nano-scale second phases, mainly the AlFeSi phase, appeared in the weld joints, accompanied by a reduction in dislocation density. Following STA, there was a significant increase in the density and size of precipitate phases, with the coarse AlFeSi phase becoming more prominent and large-sized grain boundary precipitates appearing. The AA treatment, due to grain refinement and second phase precipitation, enhanced the hardness, tensile strength, and fatigue strength of the joints. As the STA joints exhibited more β-AlFeSi precipitates and grain boundary precipitates, the second phase strengthening effect was reduced. Although the mechanical properties of STA joints were better than those of AW joints, they were inferior to those of AA joints.

From crack-prone to crack-free: Eliminating cracks in additively manufacturing of high-strength Mg2Si-modified Al-Mg-Si alloys

Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification. In this work, we develop novel crack-free Al-Mg2Si alloys fabricated by laser powder-bed fusion (L-PBF). The results indicate that the eutectic Mg2Si phase possesses a strong ability to reduce crack susceptibility. It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidification. The crack-free L-PBFed Al-xMg2Si alloys (x = 6 wt.%, 9 wt.%, and 12 wt.%) exhibit the combination of low crack susceptibility index (CSI), superior ability for liquid filling, and grain refinement. Particularly, the L-PBFed Al-9Mg2Si alloy shows improved mechanical properties (e.g. yield strength of 397 MPa and elongation of 7.3 %). However, the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg2Si alloy with a large solidification range and low eutectic content for liquid filling. Correspondingly, the L-PBFed Al-3Mg2Si alloy shows poor bearing capacity of mechanical properties. The precise tuning of Mg2Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.

Morphology and properties of Mg2Si phase modified by Ge in Mg-Si alloys

The modification effects of Ge on Mg2Si phase in a Mg-2.5Si alloy were studied by computational simulation and experimental investigation. Based on first-principles structural prediction and electronic structure calculation, Mg2(SixGe1-x) phases were structurally stable and had sufficient hardness and inherent toughness as a reinforcement phase. With the increase of Ge content, due to the preferential adsorption of Ge atoms on the {100} planes of Mg2Si crystal, the morphology of primary Mg2Si crystal changed from equilateral-dendrite to polygonal, and finally to quadrilateral. In Mg-2.5Si-0.5Ge alloy, Ge was almost completely dissolved in the Mg2Si phase, but a small amount of dispersed solute segregation with a size of 10–20 nm can be observed. In Mg-2.5Si-1Ge alloy, many Mg2Ge precipitates with a small amount of Si dissolved having a size of 20–50 nm and a lattice constant a of 0.644 nm were uniformly distributed in primary Mg2Si, and only one orientation relationship (OR) in form of [001]Mg2Ge || [001]Mg2Si, (001) Mg2Ge || (001)Mg2Si with a tiny rotational angle of 0.39° was found. The stiffness and hardness of Mg2Si phase decreased with the increase of Ge content. Despite Mg2(SixGe1-x) crystal models only considering the solid solution of Ge and neglecting segregation and precipitation, the theoretical predictions of mechanical properties remained consistent with experimental results. This agreement was primarily due to the unique OR between Mg2Ge precipitates and Mg2Si phase.

Morphology and Properties of Mg2Si Phase Modified by Pb in As-Cast Mg-2.5Si-xPb Alloys.

Pb plays an important role in determining the morphologies and mechanical properties of the Mg2Si phase in Mg-2.5Si-xPb alloys. As the amount of Pb increases from 0.4 wt.% to 1 wt.%, the primary Mg2Si phase is refined during solidification. Its morphologies transform from equiaxed-dendrite to polygonal and finally to roughly circular. The key reason for morphology evolution is the preferential adsorption of Pb atoms on Mg2Si {100} surfaces to suppress the growth rate along the ⟨100⟩ directions, which is demonstrated by the adsorption model based on first principles. In addition, the hardness of the Mg2Si phase decreases with the increasing solution content of Pb according to the results of the nanoindentation. With the addition of Pb at 1 wt.%, Pb content in the primary Mg2Si phase reaches a maximum of 0.4 wt.%, and the hardness of the primary Mg2Si phase reaches a minimum of 3.64 GPa. This reduction in hardness is attributed to the augmented ionic bond ratio resulting from the solution of Pb, which concurrently enhances the toughness of the Mg2Si phase.

Localized corrosion characteristics of Al-Mg-Si aluminum alloy with aging time and pre-delayed aging condition

The effects of single aging, delayed aging and pre-delayed aging treatment on the corrosion resistance of Al-Mg-Si alloy were studied by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), three-dimensional X-ray tomography (3D-XRT), confocal laser scanning microscopy (CLSM) and electrochemical measurements. The surface potentials of Al(MnFe)Si, Mg2Si, and Si-rich remnant phases are determined, identifying Al(MnFe)Si phase as the crucial cathodic site for corrosion initiation, and the increase in surface potential implies an accelerated corrosion rate after immersion. The intergranular corrosion susceptibility of the alloy is predominantly influenced by the microstructure caused by aging conditions. The factors affecting corrosion resistance are summarized as (i) adjustment of electrochemical reaction activity by controlling the area fraction of Al(MnFe)Si phase, (ii) the spacing of grain boundary precipitates (GBPs), and (iii) the width of the precipitation-free zone (PFZ). The delayed aging (DA) sample exhibits inferior corrosion resistance due to the continuous GBPs and extended sub-grain boundaries corrosion propagation paths. The pre-delayed aging treatment successfully mitigates the corrosion degree of DA by widening the GBPs spacing and narrowing the PFZ width. This suggests that pre-aging treatment is a potent strategy for enhancing corrosion resistance without compromising the alloy's strength.

First-principles investigation on adsorption poisoning and heterogeneous nucleation mechanisms in Ca modified primary Mg2Si phase

We employed first-principles calculations to investigate the adsorption poisoning and heteroepitaxial nucleation mechanisms in the Ca modification of the primary Mg2Si phase. The extensive computations considering various factors such as lattice mismatch, interface energy, adsorption energy, charge density, work of adhesion, and interface mechanical properties, as well as comparisons of interface stacking modes reveal that Ca modification of the primary Mg2Si phase can occur through both adsorption poisoning and heteroepitaxial nucleation mechanisms simultaneously. The adsorption poisoning mechanism implies that Ca preferentially adsorbs on the Mg-terminated surfaces of Mg2Si(100) and Mg2Si(111). Meanwhile, the heteroepitaxial nucleation mechanism indicates the nucleation interface of Mg2Si(100)//CaSi2(001), where the Mg2Si(100) surface is terminated by Si, and the CaSi2(001) surface is terminated by Ca, with stacking modes of Si-Top and Si-Center. In addition, the inclusion of Ca in the primary Mg2Si phase through heteroepitaxial nucleation not only leads to refinement of the primary Mg2Si phase but also displays excellent interface mechanical properties, which suggests that Ca can be considered as an optimal modifier for the primary Mg2Si phase. This study provides a fundamental insight into the role of Ca in modifying the primary Mg2Si phase and has potential implications for the development of Si-containing magnesium alloys.

Investigating the Microstructural and Mechanical Characteristics of Laser Additive‐Manufactured AlSi10Mg Specimens Through Experimental and Phase‐Field Modeling Approaches

Metal‐based additive manufacturing can make complicated parts that are complex or expensive to cast and process. Rapid cooling rates increase laser powder bed fusion (LPBF's) mechanical properties during manufacturing. The objective of this study is to examine the impact of process parameters in the L‐PBF technique on the characteristics of microstructure and mechanical properties, specifically, on the nanohardness influenced by Si segregation. The microstructures of the produced specimens are examined using field‐emission scanning electron microscopy and the analysis identifies the existence of bimodal equiaxed α‐Al grains, accompanied by Si phases located within their grain boundaries. In addition, the solidified sample exhibits the segregation of secondary precipitates, particularly Mg2Si, which results in enhanced mechanical properties. Both cellular walls and Si precipitates impede the motion and generation of dislocations, thereby influencing the overall behavior of dislocations. The examination of segregation at the top layer is conducted in a comprehensive manner, subsequently using energy‐dispersive X‐ray spectroscopy for analysis. The presence of Mg2Si, Al2MgSi2, and other phases in all samples is confirmed through X‐ray diffraction. The as‐built samples’ residual stress under different process conditions is also investigated. Additionally, the obtained microstructure is compared to a phase‐field model to forecast the evolution of the microstructure.

Effects of Si Addition on Interfacial Microstructure and Corrosion Resistance of Hot-Dip Zn–Al–Mg–Si Alloy-Coated Steel

Alloy coatings protect steel from corrosion in various applications. We investigated the effects of Si addition on the microstructure, electrochemical behavior, and corrosion resistance of steel sheets coated with a hot-dip Zn–Mg–Al–Si alloy using a batch-type galvanization process. Microstructural analysis revealed that the Zn–Al–Mg alloy coating layer contained a significant amount of Fe that diffused from the substrate, leading to delamination due to the formation of brittle Fe–Zn intermetallic compounds. However, the introduction of Si resulted in the formation of a stable Fe2Al3Si inhibition layer at the substrate–coating interface; this layer prevented interdiffusion of Fe as well as enhanced the coating adhesion. Additionally, the formation of acicular Mg2Si phases on the coating surface improved the surface roughness. As the Si content increased, the corrosion resistance of the coating improved. Specifically, the Zn–Al–Mg coating layer with 0.5 wt.% Si exhibited excellent anti-corrosion performance, without red rust formation on its surface even after 2600 h, during a salt spray test.

Aragonite crystallization in a sulfate-rich hypersaline wetland under dry Mediterranean climate (Laguna Honda, eastern Guadalquivir basin, S Spain)

This research investigates the influence of water composition, the presence of seasonal algal mats, detrital inputs and the activity of microorganisms on the crystallization of aragonite in the sediments deposited in the hypersaline Laguna Honda wetland (S of Spain). The high alkaline and hypersaline waters (pH > 9.2 and C.E. > 70 mS/cm) of the wetland lake are rich in SO42− (>24,000 mg/l), Cl− (>21,000 mg/l), Na+ (>11,000 mg/l) Mg2+ (>8400 mg/l) and Ca2+ (>1000 mg/l), and are supersaturated for dolomite, calcite and aragonite. Sediments have lower pH values than column waters, oscillating from 8.54 in the low Eh (up to −80.9 mV) central deep sediments and 6.33 in the shallower higher Eh (around −13.6 mV) shore sediments. Erosion of the surrounding olive groves soils produced detrital silicates rich sediments with concretions of carbonate or sulfate. Aragonite (up to 19 %) and pyrite (up to 13 %) are mainly concentrated in the organic matter rich samples from the upper part of the sediment cores, whereas gypsum is preferably concentrated in low organic matter content samples. Mineral crusts containing a MgAl silicate phase, epsomite, halite and gypsum are precipitated on the floating algal mats covering the wetland waters. Floating algal mats deposit increased the organic matter content of the upper sediments which promoted the presence of fermentative microorganisms, sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) communities and variations of Eh that influence the authigenesis of carbonate and S-bearing minerals. Replacement of poorly crystalline MgSi phases infilling algal cells by aragonite was favored in the organic matter rich sediments with low Eh values and important SRB communities that promoted sulfate bioreduction processes to form pyrite. Aragonite precipitation was favored by the increase of carbonate and bicarbonate concentration produced by the SRB oxidation of organic matter, the CO2 degassing by high summer temperatures and the CO2 uptake by photosynthesis of the algal mats.

Characterization of an Active Soldering Zn-Mg Alloy and the Study of Ultrasonic Soldering of SiC Ceramics with Copper Substrate

The aim of this study was to characterize a Zn-Mg type soldering alloy and direct soldering of SiC ceramics with a copper substrate. The Zn1.5Mg solder exerts a wider melting interval. The temperature of the eutectic reaction was 365 °C, and the liquidus temperature was 405 °C. The microstructure of the soldering alloy is comprised of a zinc matrix. Segregation of binary eutectics in the form of (Zn) + Mg2Zn11 lamellas occurred on the boundaries of Zn grains. Additionally, the presence of a MgZn2 magnesium phase was observed in the solder matrix. The SiC/solder bond was formed due to magnesium distribution from solder to the boundary with SiC ceramics, where magnesium reacts with silicon, which is infiltered in SiC ceramics. By a mutual interaction, a new Mg2Si phase is formed. An interaction between the molten zinc solder and Cu substrate occurred on the boundary of the Cu/substrate joint at the formation of a transition zone composed of two new phases, namely the γCu (Cu5Zn8) non-wettable phase type, approximately 30 µm wide, and a wettable (CuZn4) phase type ε, approximately 12 µm wide. The average shear strength of a combined SiC/Cu joint fabricated using Zn1.5Mg solder was 44 MPa.

Effect of load and sliding velocity on wear behaviour of magnesium AZ91D composites reinforced with Ca2SiO4 at ambient temperature

This article reports the dry sliding wear behaviour of AZ91D-Mg and AZ91D-Ca2SiO4 composites at different sliding velocities (1–3 m/s) and different normal loads (10 N, 30 N, and 50 N). The post-wear characterizations were carried out to determine the wear mechanisms under each tested condition. XRD analysis confirms the presence of CaO, SiO, MgO, Mg2 Si, and MgAl2O phases which play a vital role in improving the wear resistance of AZ91D-Ca2SiO4 composites. The oxidation mechanism was predominant at low sliding velocity and loading conditions. Further, plastic deformation, delamination and abrasive wear were dominant at high loading conditions. Furthermore, detailed study on the influence of Ca2SiO4 and wear parameters on the dry sliding wear behaviour of AZ91D-Mg and AZ91D-Ca2SiO4 composites are reported.

CALPHAD-aided design of high-strength Al-Si-Mg alloys for sufficient laser powder bed fusion processability

This study proposes a design concept for an Al-Si-Mg ternary alloy to achieve both high strength and laser-based powder bed fusion (PBF-LB) processability using calculation of phase diagrams (CALPHAD). The thermodynamic calculations using the CALPHAD method evaluated the Al-10Si-4.5Mg (wt%) alloy composition for strengthening by both the Si and Mg2Si phases and high PBF processability by reducing the risk of hot tearing behind the small freezing range of 27 °C and crack susceptibility index (|dT/d(Fs1/2)|) of 77 °C. The PBF-LB Al-10Si-4.5Mg alloy sample (processed by laser power of 128 W and laser scanning velocity of 1000 mm/s) exhibited a fine three-phase microstructure and exceptional hardness of 199 HV. No hot tearing occurred, whereas many cracks preferentially propagated along the melt pool boundaries during the PBF-LB process. The severe lateral cracking resulted in a maximum relative density of 96 % in the Al-10Si-4.5Mg alloy samples. Preferential cracking could be responsible for the localization of the relatively coarse particles of the brittle Mg2Si phase in the cellular microstructures of the melt pool boundaries. Understanding the lateral cracking mechanism can provide new insights into alloy modification using the controlled solidification path to avoid a specific two-phase eutectic reaction (L→α-Al+Mg2Si) to improve the PBF processability. A modified alloy composition of Al-10Si-3Mg (wt%) was assessed using the CALPHAD approach. PBF-LB processing using the modified alloy powder alleviated cracking and improved the maximum relative density to 98 % for the PBF-LB manufactured samples. This result accentuated the validity of the design strategy for Al-Si-Mg ternary alloy utilizing the CALPHAD method.

Effect of rare Y-microalloyed Al–Mg filler materials on the microstructure and tensile properties of laser welded Al–Mg–Si alloys

The Al–Mg filler metals containing various amounts of yttrium (Y) were obtained and used to study the effect of the Y element on the microstructure and tensile properties of laser welded Al–Mg–Si alloys. The results revealed that compared with the unmodified welded joints, 0.1 wt%Y addition cannot effectively refine the grains. However, the 0.1 wt%Y addition effectively decreased the rich-Fe phases size and transformed the bulk β-AlFeMnSi phases into the fine granular AlFeMnSiY phases. Due to the precipitation strengthening, the welded joints with 0.1 wt%Y exhibited excellent mechanical properties, the highest tensile strength and elongation increased by 53.42 MPa and 5.58 %, respectively. As the Y content further increased, a significant grain refinement was observed in the weld zone. However, due to the small growth restriction factor of Y in aluminum alloys, the degree of grain refinement was limited. When 0.3 wt% Y was added, the grain size of the weld was the smallest, which was only reduced by 7.7 μm compared to the unmodified weld and similar to the weld with 0.5 wt%Y addition. Furthermore, the increase in solidification temperature range led to the growth of the second phase, as well as an increase in the number of coarse rich-Fe and Mg2Si phases. These combined reasons prevent further improvement of tensile properties of the welded joints.

Development of a double-layered AA6061 build using additive friction stir processing

In this study, a double-layered AA6061 build was developed using solid-state additive friction stir processing (AFSP) at different linear and tool rotational speeds. The cross-section of the build was examined using optical macroscopy, and microhardness was measured using the Vickers hardness tester. Microstructural evolution was examined using advanced characterization techniques such as electron back scattered diffraction and transmission electron microscopy, while bulk texture evolution was investigated using the X-ray neutron diffraction method. Defect-free builds were attained at rotational speeds of 1100 and 1300 rpm. Compared to the starting alloy, hardness values were significantly lower along the builds, demonstrating loss in the material's strength due to precipitate dissolution. A maximum average hardness of 63.73 HV was observed on the build developed at 1100 rpm, 60 mm/min. AFSP caused grain refinement and reduced grain size to 4.95 μm in the build zone, but dissoluted strengthening β Mg2Si and AlFeMnSi phases due to the intense heat. The microstructure of base material exhibited a texture index of 1.4432, while the microstructure of the build zone showed a reduced texture index of 1.1742. Intense deformation caused randomized grain orientation with reduced texture but retained the shear type S {123}<634>, copper {112}<111> and brass {011}<211> components in the build zone.

Corrosion Behavior of Al Alloy A6061 Jointed with High-Tensile Strength Steel By Friction Stir Welding in NaCl Solution

In recent years, weight reduction of automobile bodies has been effective in reducing carbon dioxide emissions. For this reason, the replacement of steel, which has a large specific gravity, with aluminum alloys and carbon fiber reinforced plastics is being promoted for the right materials in the right places. In this case, an appropriate dissimilar material joining method is needed in terms of ease of assembly, joining performance, and cost. Friction stir welding (FSW) has emerged as a promising joining method for a variety of engineering applications. FSW is a solid-phase joining technique that has been shown to have significant advantages over conventional welding in terms of the strength and quality of the joints produced. However, during joining of metallic materials by FSW, microstructural changes may occur that affect their corrosion degradation. In this study, the corrosion behavior of A6061 jointed with high-strength steel (SPCC980) by FSW in aqueous NaCl solution and the effect of Al alloy microstructural changes on it were investigated.In this study, FSW was performed on A6061 and SPCC980 steel plates, and galvanic corrosion behavior was investigated by immersion tests and surface observations, as well as electrochemical measurements of Al alloys. Immersion tests were performed in 1 M NaCl solution. Micro electrochemical polarization tests of A6061 were also performed to investigate the corrosion behavior.In the immersion test of the galvanic couple of A6061 and SPCC980 steel plates in NaCl solution, galvanic corrosion occurred with the Al alloy as the anode and the steel plate as the cathode, and hydrogen evolution was observed mainly from the BM site on A6061 in the initial stage of immersion. This is due to the hydrogen evolution reaction occurring inside the pitting corrosion of the A6061. The pits was formed near Fe-rich inclusions in the BM site. In the HAZ site, corrosion propagated from Fe-rich inclusions along grain boundaries, but no pitting corrosion was formed. In the NZ site, minor localized corrosion was developed. These results indicate that the susceptibility to localized corrosion and its morphology differ in each site on the A6061. The difference in localized corrosion susceptibility on the A6061 was considered to be caused by the difference in the distribution of Fe-rich inclusions. However, the fact that the size and number of Fe-rich inclusions in each site did not differ suggests that the difference in anodic dissolution activity near Fe-rich inclusions affects pitting corrosion susceptibility in each site. A6061 alloy is a precipitation-strengthened Al alloy reinforced by β'' phase, which is a metastable phase of Mg2Si, and dense precipitation of β'' phase was observed in the BM site. Therefore, the reason for the higher localized corrosion susceptibility of BM is related to the precipitation of the β'' phase.This paper is based on results obtained from a project, JPNP14014, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Microstructure and mechanical properties of longitudinal weld in 6005A aluminum alloy profile extruded by a porthole die

Porthole die extrusion experiments were carried out on 6005A aluminum alloy under the billet heating temperatures of 420–570 °C and ram speeds of 1–120 mm/min. The effects of extrusion parameters on the microstructure and mechanical properties in the longitudinal weld of profiles were investigated. It was found that the crystal orientation of longitudinal weld region showed the characteristics of gradient arrangement with different oriented grains wrapped in layers and symmetric along the welding line. The Copper{112}<111> grains are formed at the welding line under the combined effect of shear and compression, and the Goss{110}<100> grains are formed on their both sides due to dynamic recrystallization. A shear band is formed when part of metals flow through the 45° inclination angle of port bridge due to a sudden change in velocity, causing the Goss{110}<100> texture in local region to rotate to Brass{110}<112> texture. The 6005A aluminum alloy profiles mainly contain β (Mg2Si) phase, Q (Al3Mg9Si7Cu2) phase, AlCrMnSi phase and AlFeMnCrSi phase. Appropriately increasing billet heating temperature and ram speed can promote dynamic recrystallization and the dissolution of β-phase and Q-phase, thus improving the strength and hardness of the profile. However, when the billet heating temperature is too high, the strength and hardness are reduced instead due to grain coarsening. The profile extruded at billet heating temperature of 510 °C and ram speed of 120 mm/min has the best mechanical properties, with the tensile strength in 0° direction and hardness at the welding line of 179.93 MPa and 52.2 HV, respectively.

Achieving strength-ductility synergy in a novel Al-Mg-Zn-Cu-Si lightweight multi-component alloy via eutectic structure refinement

The development of lightweight multi-component alloys (MCAs) with good strength plasticity synergy faces a significant challenge, owing to the enormous number of potential component combinations, serious microstructure defects caused by poor castability, and the inefficiency of conventional empirical trial-and-error approaches. To surmount this challenge, we proposed an alloy design method, using the CALPHAD-based high-throughput calculation (HTC) to quickly identify compositions in Al(100−w-x-y-z)MgwZnxCuySiz MCAs over high-dimensional composition space that can make α-Al and Mg2Si nucleate at the same time. Then successfully low-cost prepared large-sized MCA ingots with less segregation and porosity, and good performance according to the HTC screening results, and the calculated nucleation temperatures were in high agreement with the experimental values, with an error ranging within 0.5%. The granular eutectic Mg2Si phase exhibited exceptional uniformity and dispersion within the microstructure of the Al77Mg8Zn5Cu6Si4 alloy, with an average size of 1.8 µm. Consequently, the as-cast Al77Mg8Zn5Cu6Si4 alloy displayed superior strength plasticity synergy at room temperature (RT), with the ultimate compressive strength reaching 706 MPa and the strain reaching 14.2%. This work demonstrates that the CALPHAD-based HTC approach provides an effective strategy for accelerating the design of lightweight MCAs with large-size and excellent mechanical properties.

Growth of Mg2Si thermoelectric eutectics by unidirectional solidification

Mg2Si/Si eutectics are grown from melt by the Vertical Bridgman method using a carbon crucible at three pulling rates. All the Mg2Si/Si eutectics are composed of Mg2Si and Si phases, and the eutectic structure becomes finer with an increase in the pulling rate. Increasing the pulling rate systematically decreases the thermal conductivities of the Mg2Si/Si eutectics, suggesting that a decrease in the thermal conductivity is attributed to an increase in phonon scattering at the boundaries between the two phases. The power factor and figure of merit of the Mg2Si/Si eutectics are larger than those of Mg2Si above 400 °C because of the increase in the Seebeck coefficient. The figure of merit, ZT, of the Mg2Si/Si eutectics solidified at a 103 mm/min pulling rate reached 0.076 at 600 °C.

Effect of strontium addition, cooling rate and T6 heat treatment on the corrosion behaviour of Al-20%Mg2Si composite

Mg2Si reinforced AMCs are considered a promising structural material as the in-situ formation of Mg2Si particle offers excellent matrix/reinforcement stability that allow effective load transfer. Addition of Strontium (Sr) could minimize sharp edges shape of Mg2Si particles in as-cast condition improving the mechanical properties. Incorporating Strontium (Sr) can reduce the sharp-edged morphology of Mg2Si particles in their initial cast state, leading to enhanced mechanical properties. However, existing research on Mg2Si reinforced AMCs does not show sufficient data on corrosion behavior. This research aims to investigate the microstructural changes and corrosion performance of as-cast Al-20 %Mg2Si under various influencing factors including varying solidification rate, addition of Sr and post- fabrication metalwork such as T6 heat treatment. The composite materials were prepared using in-situ casting procedure with high purity of the elements. Microstructure characterization was carried out by optical microscopy, in which the primary Mg2Si particle sizes were measured by image analyser. Corrosion behaviour of the fabricated composite was analysed using electrochemical test. Optical microscopic images reveals that the primary Mg2Si was refined and transform to polyhedron upon addition of 0.01w.t. % Sr into Al-20 %Mg2Si and resulted in improved corrosion resistance upon immersion testing and potentiodynamic polarization testing in 3.5 % NaCl with decreasing the Icorr from 0.80 to 0.41 μA/cm2.The Al-20 %Mg2Si which solidified under lower cooling rate have coarser dendritic primary Mg2Si which results in higher corrosion rate (Icorr = 4.98 μA/cm2). T6 heat treatment transformed the primary Mg2Si to fine dendrites with spherical secondary Mg2Si phase and consequently enhancing the corrosion resistance by decreasing the Icorr to 2.93 μA/cm2.