Cast Magnesium Alloy Research Articles

Development of Mg–Zn–Y–Zr casting magnesium alloy with high thermal conductivity

Magnesium alloys are used in aircraft because of their light weight. However, for these alloys to be applied in electronic devices, high thermal conductivities are required. Several high-potential compositions of Mg–Zn–Y–Zr alloys were selected by phase composition and their freezing ranges calculated using Thermo-Calc software. The alloys were prepared, and their fluidity, hot tearing susceptibility, mechanical properties, and thermal conductivity were obtained and compared. The alloy composed of Mg–4 wt% Zn–3 wt% Y–0.3 wt% Zr was selected for further investigation, because of its high thermal conductivity and satisfactory mechanical properties. The Mg–4 wt% Zn–3 wt% Y–0.3 wt% Zr alloy's fluidity and hot tearing susceptibility were similar to those of the widespread AZ91 commercial casting magnesium alloy. The influence of a heat treatment regime on the microstructure, thermal conductivity, and mechanical properties of the developed alloy was also investigated. It was established that the room temperature thermal conductivity of the Mg–4 wt% Zn–3 wt% Y–0.3 wt% Zr alloy after aging at 300 °C for 5 h was 105 W/mK. Additionally, the following tensile test results were obtained in aged condition: 120 MPa yield strength, 200 MPa ultimate tensile strength, and 4% elongation. The utilization of solid solution heat treatment at 520 °C for 8 h prior to aging can promote up to 9% increase in elongation. The Mg–4 wt% Zn–3 wt% Y–0.3 wt% Zr casting alloy can be used as a high thermal conductivity material with industrial applications.

Influence of applied pressures and casting section thicknesses on interfacial heat transfer in squeeze casting of magnesium alloy AZ91

The estimation for interfacial heat transfer coefficient (IHTC) between casting and mold is paramount to predict solidification phenomena. In this study, three hydraulic pressures of 30, 60, and 90 MPa were applied to indirectly squeeze cast magnesium alloy AZ91 in a P20 steel mold with five casting section thicknesses of 2, 4, 8, 12, and 20 mm, named steps 1, 2, 3, 4 and 5, by using a 75-ton hydraulic press machine. For comparison, the AZ91 alloy was also cast without the applied pressure (0 MPa). K-type thermocouples were utilized to measure temperatures simultaneously on the casting surface and at three different depths from the mold surface inward. Based on the temperature measurements, the IHTCs at the five steps were calculated by the inverse method. Despite of the differences in their peak values, the IHTC variation for the three applied pressures at the five steps during the squeeze casting process was similar to that of 0 MPa, in which the IHTC values abruptly rose to an apex first, and then declined slowly. The overall IHTC values of step 5 were higher than those of the other four steps, while the lowest peak IHTC value was present at step 1. When the applied pressure was increased, the IHTC peak value of each step became large accordingly. The empirical equations relating the IHTCs to the pressures, section thicknesses, and temperatures at the casting/mold interface were derived based on the multivariate linear and polynomial regression.

ТЕНДЕНЦИИ РАЗВИТИЯ ЛИТЕЙНЫХ МАГНИЕВЫХ СПЛАВОВ С ПОВЫШЕННОЙ ТЕМПЕРАТУРОЙ ВОСПЛАМЕНЕНИЯ (обзор)

Trends in the development of cast magnesium alloys with increased temperature, ignition and fire resistance are presented. The main world companies and institutions - developers of casting magnesium alloys are presented. The developed alloys are considered, indicating their chemical characteristics, mechanical characteristics at high and high temperatures and corrosion resistance, as well as the use of materials and design aspects of industry. The presented magnesium alloys with an increased ignition temperature allow expanding the scope of their use.

Influence of the Composition on the Environmental Impact of a Casting Magnesium Alloy

The influence of the composition of magnesium alloys on their environmental impact was analyzed. In order to perform a more accurate environmental impact calculation, life cycle assessment (LCA) with the ReCiPe 2016 Endpoint and IPCC 2013 GWP (100 y) methodology was used, taking the EcoInvent AZ91 magnesium alloy dataset as reference. This dataset has been updated with the material composition range of several alloys included in the European standard EN 1753:2019. The balanced, maximum, and minimum environmental impact values were obtained. In general, the overall impact of the studied magnesium alloys varied from 3.046 Pt/kg to 4.853 Pt/kg and from 43.439 kg CO2 eq./kg to 55.427 kg CO2 eq./kg, depending on the composition. In the analysis of maximum and minimum environmental impacts, the alloy that had the highest uncertainty was 3.5251, with a range of ±7.20%. The element that contributed the most to increase its impact was silver. The AZ91 alloy, provided by the EcoInvent dataset, had a lower environmental impact than all the magnesium alloys studied in this work. The content of critical raw materials (CRMs) was also assessed, showing a high content in CRMs, between 89.72% and 98.22%.

Progress in twin roll casting of magnesium alloys: A review

Twin roll casting was commercialized for a strip production from ferrous and non-ferrous alloys in the 1950s; however, its application to magnesium has proven difficult and still creates major challenges. This report describes global efforts in expanding manufacturing capabilities of magnesium sheet through twin roll casting path, offering many benefits, including a reduction in number of processing steps and energy savings. In addition to hardware design, alloy transformation during processing, product microstructure and properties, examples of successful solutions along with present technology and knowledge limitations are discussed. A particular attention is paid to developments at CanmetMATERIALS, having the only in North America pilot scale twin roll casting facility, devoted to magnesium. Efforts are described that aim at design of new magnesium alloys, which could take advantage of unique processing conditions during twin roll casting and contribute to the overall progress in magnesium sheet manufacturing.

Influence of nanoparticle additions to the electrolyte on the structure, composition and corrosion resistance of oxide layers formed by PEO on cast Mg alloy

The effect of various nanoparticles (NPs) added to the electrolyte on the composition, structure, and properties of oxide layers formed by plasma electrolytic oxidation (PEO) on a cast magnesium alloy AZ81A was studied in this work. The oxide layers were obtained by alternating adding silicon dioxide SiO2, silicon nitride Si3N4, yttrium oxide Y2O3, tungsten carbide WC, and titanium carbide TiC NPs to the electrolyte. The obtained oxide layers were studied by scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDXMA), X-ray diffraction analysis (XRD), instrumental indentation and potentiodynamic polarization (PDP). The thickness, roughness, microhardness, adhesion and corrosion resistance of the oxide layers were determined. The greatest increase in the oxide layer thickness, in its hardness and corrosion resistance was observed for TiC and WC NPs added to the electrolyte during PEO. The addition of Si3N4 NPs leads to a decrease in the oxide layer thickness and its hardness, while the corrosion resistance is comparable to the one of the uncoated magnesium alloy.

Investigation on Influence of the Cast Part Thickness on Interfacial Mold–Metal Reactions During the Investment Casting of AZ91 Magnesium Alloy

The interfacial mold–metal reactions are the main barrier for the development of the magnesium alloy components through the investment casting route, owing to the higher affinity of the magnesium melt with oxygen. The molds are developed with refractory ceramic oxides mainly in the investment casting process. In the present paper, an attempt was made to investigate the influence of the cast part thickness on interfacial mold–metal reactions during the investment casting of AZ91 magnesium alloy. The thermal profiles during the solidification were recorded using the thermocouples and infrared camera. The reactions on the cast surface were quantified and characterized by image analysis, XRD, SEM–EDX and microhardness profile. It was observed that with the increase in thickness, the interfacial mold–metal reaction increases. The surface hardness was also found to be increased with the increase in thickness of the cast part as a consequence of the oxide formation on the cast surface. The reactions on the casting having connected regions of the varying thickness were observed to be relatively more as compared to the casting with the uniform thickness.

Thermophysical properties of ML5 casting magnesium alloy

In the present work thermal conductivity, thermal diffusivity, heat capacity, melting enthalpy, and linear thermal expansion coefficient of ML5 casting magnesium alloy were investigated in the temperature range of 298–670…925 K. The measurements were performed using laser flash method, dilatometric method and differential scanning calorimetry on the samples made from the one initial material. Using the obtained data, the temperature dependence of the thermal conductivity was determined. For all studied properties the approximation equations were obtained which can be used for various scientific and practical applications.

Material testing of magnesium alloy AZ31B using a finite element polycrystal method based on a rate independent crystal plasticity model

Magnesium and its alloys are attractive structural light materials because they have a high spe-cific strength and high specific stiffness. That is why their usage in high performance automotive and aerospace applications has been increased. However, their high anisotropy and low ductility could bring to forming failure, to avoid these a better accuracy for metal forming simulation is re-quired. The identification of macroscopic parameters is expensive due to the experiments needed to find them, the use of multi-scale approach allows to find parameters for the model through easier experiments. A finite element polycrystal method, based on a rate independent polycrystal plasticity model, is implemented in order to perform the material testing of a cast magnesium alloy AZ31B. The parameters of the model are adjusted to better fit the experimental data through a trial and error process.

Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys

Direct chill (DC) casting is the widely used technology for the production of ingots or billets of nonferrous alloys. Non-uniform thermal gradients generated during the process play a predominant role in generation of cracks within the billet. In the present work, a temperature-based finite element technique is used to estimate thermal gradients developed in different regions of billet during DC casting of magnesium alloys. Shrinkage-dependent realistic heat transfer coefficient and temperature-dependent nonlinear boiling curves are used as boundary conditions in the mould and secondary cooling zones, respectively. The simulated temperature profiles during DC casting of AZ31 alloy are validated with real-time plant measurements. Secondary cooling strategies such as pulsed water flow and water removal with wiper are widely used in industry for reducing thermal gradients in DC casting. In this work, the influence of these two techniques on the reduction of thermal gradients is investigated for the DC casting of wrought AZ31 and cast AZ91 alloys. Usage of wiper reduces thermal gradients by 30% in case of AZ31 alloy casting. Surface reheating is observed with the use of wiper in AZ91 alloy. The use of pulsed water is found to reduce thermal gradients at the bottom of the sump in AZ31 casting, whereas no thermal gradient reduction is observed in AZ91 casting for the investigated conditions.

Evolution of the Microstructure and Microsegregation in Subrapidly Solidified Mg–6Al–4Zn–1.2Sn Magnesium Alloy

The evolution of the microstructure and microsegregation in a new type of Mg–6Al–4Zn–1.2Sn (wt%) cast magnesium alloy solidified at cooling rates from 4.5 × 101 °C s−1 to 2.3 × 103 °C s−1 is investigated. The results indicate that the secondary dendrite arm spacing and the average grain sizes decrease from 14.3 to 2.9 μm and from 187 to 78 μm, respectively, as the cooling rate increases from 4.5 × 101 °C s−1 to 2.3 × 103 °C s−1. The relationship between the secondary dendrite arm spacing (λ2) and cooling rate (R) is fitted as λ2 = 69.7 × R−0.42. When the alloy solidifies in the range of cooling rates, a new quasi‐crystalline I phase is formed due to faster solidification. The microsegregation ratios of Al, Zn, and Sn elements decrease significantly with the increase in cooling rate. Compared with the specimen solidified at 4.5 × 101 °C s−1, the microsegregation ratios of Al, Zn, and Sn of that solidified at a cooling rate of 2.3 × 103 °C s−1 decrease by 37.4%, 45.7%, and 31.6%, respectively.

Fabrication of AZ91D/Cu/Mg2Cu and AZ91D/Mg2Cu/MgCu2/MgO in-situ hybrid surface nanocomposites via friction stir processing

In this study, in situ surface nanocomposites based on Mg-CuO and Mg-Cu systems were developed via six passes of friction stir processing (FSP) on the surface of AZ91 magnesium casting alloy. In-situ phase evolution during FSP with the addition of Cu and CuO micro-powders was studied by x-ray diffraction (XRD) analysis and microstructural investigations. Here, AZ91/Cu nanocomposite was reinforced by the in situ formation of Mg2Cu intermetallic compound. In AZ91/CuO nanocomposite, CuO particles were reduced and MgO and MgCu2 reinforcing particles alongside Mg2Cu intermetallic compounds were formed during FSP. Grain refinement and in situ formation of reinforcement particles with different sizes ranging from nano- to micro-scale significantly improved mechanical performance of the specimens. Dynamic recrystallization was found to be the main mechanism of grain refinement. After six passes of FSP, the hardness values of AZ91/Cu and AZ91/CuO nanocomposites were increased by 69.1% and 91%, respectively. Besides, AZ91/CuO nanocomposite exhibited the best tensile strength and wear resistance among all the samples. The dominant wear mechanisms were abrasive and delamination wear in AZ91 magnesium alloy, while nanocomposite specimens were worn mainly by abrasive mechanism.

Synergetic influence of microconstituents on the damage accumulation and consequent effect on the flow behaviour in cast Mg–Ca–Sn alloys

Combined role of microconstituents (particles, eutectic fraction and its morphology) on the damage accumulation of three as-cast Mg–Ca–Sn alloys having distinctive microstructural features is studied. Flow stress normalized with that of a solid solution alloy brought out the sole contribution of microconstituents to the deformation. For identifying the influence of microconstituents on strain hardening rate, Kocks-Mecking analysis is utilized together with intermittent post-deformation microstructures. Comprehensive microstructural characterization as a function of strain facilitated to unravel the distinction in internal damage accumulation among the alloys. Further, the internal damage accumulation variation due to distinct variation in the microconstituents is attributed to the differences in hardening rate trends among the three alloys. The reduction in hardening rate is analyzed to occur due to damage accumulation through eutectic fracture or interphase cracking. Thus, the damage mechanism and its influence on strain hardening in cast magnesium alloys is proposed.

Effect of Rare-Earth Elements and Heat Treatment on the Structure and Properties of a Heat-Resistant Cast Mg–REM–Zr Magnesium Alloy

Experimental compositions of a new heat-resistant cast magnesium alloy are considered, the compositions are corrected, and the optimum compositions of the new cast magnesium alloy with rare-earth metals (REMs) are selected for durability tests at a high temperature. Heat treatment conditions are corrected. The structure and mechanical properties of the cast Mg–REM–Zr magnesium alloy are studied. Tests of the mechanical properties of individually cast samples and the samples cut from an experimental casting (aggregate cover) are performed at room and elevated temperatures.

An ICME Method for Predicting Phase Dissolution During Solution Treatment in Advanced Super Vacuum Die Cast Magnesium Alloys

An integrated computational materials engineering (ICME) methodology was applied in this study to systematically and quantitatively study the second phase dissolution kinetics during the solution treatment process of a high pressure die cast magnesium sample. The study was conducted on Mg–9 wt% Al, Mg–5 wt% Al, and Mg–11 wt% Al binary alloys after isothermal solution treatments ranging from 380 to 420 °C. The experimental measurements revealed an exponential decrease of the volume fraction of the second β-phase (Mg17Al12) during the solution treatment. A CALPHAD-based computational tool (Thermocalc DICTRA Diffusion Module) was used to simulate the dissolution process. In this study, measurements from as-cast samples were used as input parameters to improve the accuracy of the predicted results. An analytical, physics-based micro-model, based on the Johnson–Mehl–Avrami type equation, was also applied to study the dissolution kinetics. Both the simulation and micro-model quantitatively agree with the experimental results at all solution treatment temperatures. Calibration and verification of a few input parameters help to understand the assumptions made in the modeling, improve the accuracy of the prediction, and can be applicable in different Mg–Al binary alloys. Adopting such an ICME methodology for modeling development and verification in predicting the dissolution kinetics during solution treatment will allow for more rapid optimization of solution treatments procedures to be used in industrial applications.

Microstructure and Corrosion of Cast Magnesium Alloy ZK60 in NaCl Solution.

In this work, the effects of the microstructure and phase constitution of cast magnesium alloy ZK60 (Mg-5.8Zn-0.57Zr, element concentration in wt.%) on the corrosion behavior in aqueous NaCl (0.1 mol dm−3) were investigated by weight-loss measurements, hydrogen evolution tests, and electrochemical techniques. The alloy was found to be composed of α-Mg matrix, with large second-phase particles of MgZn2 deposited along grain boundaries and a Zr-rich region in the central area of the grains. The large second-phase particles and the Zr-rich regions were more stable than the Mg matrix, resulting in a strong micro-galvanic effect. A filiform corrosion was found. It originated from the second-phase particles in the grain boundary regions in the early corrosion period. The filaments gradually occupied most areas of the alloy surface, and the general corrosion rate decreased significantly. Corrosion pits were developed under filaments. The pit growth rate decreased over time; however, it was about eight times larger than the general corrosion rate. A schematic model is presented to illustrate the corrosion mechanism.

Simulation on DC casting of magnesium alloy under out-of-phase pulsed magnetic field with different coil connection strategies

Four annular coils arranged vertically that have been loaded with two pulsed currents with different initial phases were used to create out-of-phase pulsed magnetic field (PMF) during direct-chill (DC) casting of magnesium alloy. A 2D axisymmetric mathematical model that couples electromagnetic field with fluid field and thermal field was established based on the finite element method by using COMSOL Multiphysics software. A two-step approach was adopted to handle the coupling problems, which the varying Lorentz force and Joule heat were treated as the averaged values during the steady calculation of DC casting process. Effects of coil connection strategies on the resulting Lorentz force, melt flow and solidification characteristics during DC casting of magnesium alloy under out-of-phase PMF were investigated and discussed. Results show that the application of magnetic field greatly strengthens the melt convection when compared to no magnetic field. Using out-of-phase PMF could control the range and intensity of melt convection by changing coil connection strategies, achieving lower overheating in the central part and higher temperature uniformity from the center to the edge or reducing the sump depth than a single PMF in which four coils are connected in series and loaded with the same pulsed current.

Enhanced resistance to liquation cracking during fusion welding of cast magnesium alloys: Microstructure tailoring via friction stir processing pre-weld treatment

Magnesium alloys provide critical opportunities for energy and lightweight-materials challenges. The alloy design approach based on utilizing the second phase strengthening mechanism plays an essential role in producing low-density high strength Mg-based alloys. However, the second phase in the microstructure (e.g., Mg17Al12 phase in Mg–Al–Zn alloys) can deleteriously affect the weldability of the metallic materials by promoting the liquation and liquation cracking during fusion welding. The lack of sufficient weldability of Mg-based alloys is a crucial barrier to their potential use in safety-critical applications. In this paper, it is shown that the severe plastic deformation induced by friction stir processing, as an effective pathway to microstructure refinement, before fusion welding enables tailoring the initial microstructure of an Mg–Al–Zn cast alloy for enhancing its resistance to liquation cracking. The formation of ultra-fine sub-micron Mg17Al12 particles in friction stir treated base metal translates the liquation mechanism from sub-solvus constitutional liquation to super-solvus melting alloy, reducing the size of susceptible partial liquation zone to cracking. Moreover, the higher grain boundary density induced by dynamic recrystallization phenomena during friction stir processing can enhance the resistance to liquation cracking of the alloy. It is shown that this pre-welding strategy enables a significant enhancement in the weld mechanical properties compared to the conventionally produced welds. The joint efficiency, defined as the ratio of the joint tensile strength to the as-cast material tensile strength, was changed from 0.74 in case of welds made on cast base metal to 1.4 in case of welds made on friction stir processed base metal. This simple modification to the fusion welding process can be used as a practical pathway to enhance the weldability of liquation-sensitive second phase-strengthened alloys.

Enhancing mechanical properties of AZ61 magnesium alloy via friction stir processing: Effect of processing parameters

The purpose of this study was to modify the microstructure and mechanical properties of AZ61 cast magnesium alloy via solid-state friction stir processing (FSP). The effect of processing parameters on the microstructural evolution, texture and mechanical behavior was investigated. The results indicate that FSP was capable of producing fine-grained AZ61 magnesium alloy through dynamic recrystallization, and eliminating the unfavorable network-like large β-Mg17Al12 particles present along the grain boundaries in the cast magnesium alloy. With increasing ratio of rotation rate to traverse speed, the grain size and fraction of high-angle grain boundaries (HAGBs) increased. Compared with the original cast alloy, the microhardness and tensile properties of FSP specimens were significantly enhanced. The high strength and ductility of FSP specimens were achieved mainly due to the scattered distribution of crystallographic texture and the effective grain refinement along with a larger fraction of HAGBs.

Correlation of 3D defect-band morphologies and mechanical properties in high pressure die casting magnesium alloy

For understanding the processing-microstructure-properties correlation of high pressure die casting (HPDC) AZ91D alloy, 3D reconstruction was carried out to characterize the microstructural morphologies of castings produced by four designed HPDC processes. The influence of different parameters (vacuum, slow-shot speed, fast-shot speed) on porosity volume and morphology, defect band width and average externally solidified crystals (ESCs) size of castings were systematically studied. Microstructural characterization revealed that the defect band width was strongly related with the size and quantity of ESCs, rather than the porosity volume. Meanwhile, ESCs were mainly influenced by the slow-shot speed in shot sleeve (ESCs growth time) and fast-shot speed into the die cavity (shear stress of melt flow). Tensile fracture revealed that cracks easily propagated within the defect bands due to the accumulated ESCs and connected irregular pores, while the cracks were restrained in center zone. The mechanical properties of HPDC castings was inversely proportional to the defect band width, and is not dependent on the average volume fraction porosity.