Magnesium Alloy Melt Research Articles

Magnesium alloy melt protection by high-efficiency phase transition of carbon dioxide

In this study, we developed and evaluated a highly efficient method for protecting magnesium melts by cooling and shielding the magnesium melt with carbon dioxide (CO2) snow using a newly modified technique to achieve denser CO2 snow than competing methods. Experiments, using a replicate CO2 snow nozzle in Bach's study, were conducted for protection of the AZ91D alloy and pure magnesium in a 3-kg furnace to identify the protectiveness. The issue of CO2 consumption was immediately apparent at the beginning of the phase-two tests using two Bach's nozzles. Hence, the modified denser CO2 snow nozzle was designed and optimized to generate more dry ice. The optimized trigger mode to improve the efficiency of the dry ice manipulation was also proposed. Finally, a specialized pilot run of the protective system with the proposed denser CO2 snow technique for a 200-kg melting furnace was developed and tested. This system had an excellent performance with a low CO2 consumption of 1.2 kg/h for the AM60 magnesium alloy and demonstrated a quite low running cost of 0.55 USD/h compared with a non-denser CO2 snow system. The proposed modified snow nozzle presents three major advantages: an improved cooling efficiency by almost 10 times, a longer lifetime for the solenoid valve, and a reduced need for furnace maintenance as a result of the sulfur-free operation.

액상가압공정을 이용한 CNF/Mg 복합재료의 제조 및 특성평가

ABSTRACT Carbon nano fibers (CNFs) reinforced magnesium alloy (AZ91) matrix composites have been fabricated by liquid pressing process. In order to improve the dispersibility of CNFs and the wettability with magnesium alloy melt, CNFs were mixed with submicron sized SiC particles (SiC p ). Also, the mixture of CNFs and SiC p were coated with Ni by electroless plating. In liquid pressing process, AZ91 melts have been pressed hydrostatically and infiltrated into three reinforcement preforms of only CNFs, the mixture of CNFs and SiC p (CNF+SiC p ), and Ni coated CNFs and SiC p ((CNF+SiC p )/Ni). Some CNFs agglomerates were observed in only CNFs reinforced composite. In cases of the composites reinforce with CNF+SiC p and (CNF+SiC p )/Ni, CNFs were dispersed homogeneously in the matrix, which resulted in the improvement of mechanical properties. The compressive strengths of CNF+SiC p and (CNF+SiC p )/Ni reinforced composites were 38% and 28% higher than that of only CNFs composite. 초 록 본 연구에서는 액상가압공정을 이용하여 탄소나노섬유(carbon nano fiber, CNF)를 강화재로 하는 AZ91 마그네슘 복합재를 제조하였다. CNF의 분산성 및 마그네슘 합금 용탕과의 젖음성을 향상시키고자 CNF를 마이크로 크기의 실리콘 카바이드 입자(silicon carbide particle, SiC

Effects of various parameters on ultrasonic separation of inclusions from magnesium alloy melt

AbstractThe acoustic radiation force generated by ultrasonic standing wave in the flow media can make solid particles suspending in the liquid agglomerate at the nodal planes of the waves and then realize their separation, which is also known as ultrasonic agglomeration in chemical industry. In this paper, ultrasonic waves were employed to promote and accelerate the separation of inclusions from magnesium alloy melt, and the effect of acoustic radiation forces on oxide inclusions removal from magnesium alloy melts were studied by numerical calculation. The agglomeration behavior of the inclusions was also obtained by solving the equations of motion for inclusions. Finally, parametric studies, usually very helpful for continued optimization and design efforts, were carried out to evaluate the effects of various parameters such as ultrasonic power, ultrasonic treating time, particle size and density difference between particle and melt on the inclusions distribution. The results indicate that when a moderate ultrasonic power was applied, most of inclusions could agglomerate at wave nodes in a short time which finally enhanced and accelerated the separation of inclusions from magnesium alloy melt.

Effect of minor elements on the creep resistance of Mg-Zn-Mn-(Ca) alloy

Along with alumium, titanium and composite alloys, magnesium alloys have been given much attention by industry for applications such as lightweight automobiles and electronics because of their high strength, low specific density and good damping characteristics. In this paper, creep tests were done with magnesium alloys (Mg-3% Zn-1% Mn, Mg-1.2% Zn-1% Mn, and Mg-3% Zn-1% Mn-0.3% Ca) containing different amounts of Zn to investigate the effect of Zn and Ca on the deformation behavior and the rupture time for Mg alloy creep under elevated temperatures. The alloys were obtained as follows: (1) pure magnesium (9.7 kg) was melted at 720°C in an SF6 atmosphere; (2) the temperature was increased up to 800–820°C after adding 0.3 kg of pure Mn to make the Mg-1% Mn master alloy; (3) the minor element (Zn, Ca) was added to the master alloy; and (4) the magnesium alloy melts were cast into a metallic mold preheated to 150°C. The creep tests were executed under a constant load and temperature to measure the steady-state rate and rupture time of creep. Based on the experimental results, the creep behavior of the alloys seemed to be controlled by dislocation climb at around 0.5∼0.55Tm (Tm; melting temperature). In addition, the results showed that the addition of Ca was effective for increasing the creep resistance of a Mg alloy: the more Zn present in the alloy, the stronger the creep strength of the alloy.

EET Research on Melt Structural Information of Magnesium Alloy

Abstract Through the empirical electron theory of solids and molecules (EET), the valence electron structure of Mg-Al alloy melt was analyzed. Based on the analysis, the basic structure model was established and the melt structural information was obtained. By analyzing the irreversible phenomenon of viscosity-temperature relationship curve V ( t ) of magnesium alloy melt in the temperature interval of ascent and descent, the irreversible change of the melt structural information would be judged indirectly. The results show that the number difference of covalent electron pair n α of each bond between atoms is the ultimate reason of the different melt structural information, which induces micro-inhomogeneity in alloy melt directly; at near 730 °C, Al-Mg ||| (C) bonds in the atomic clusters with relatively stable structure begin to break. At this time, the melt structure achieves a new equilibratory and relatively homogeneous state. Disappearance of micro-nonequilibrium structure inherited from solidified structure, breakage of Al-Mg ||| (·C) bonds in the atomic clusters and irreversible rearrangement of atoms are the essential reason of irreversible change of curve V ( t ). It is very important for further understanding the melt structural information of magnesium alloy and exploring solidification micro-mechanics.

Numerical simulation of standing waves for ultrasonic purification of magnesium alloy melt

It was attempted to enhance and accelerate the separation of oxidation inclusions from magnesium alloy melt by virtue of ultrasonic agglomeration technology. In order to investigate the feasibility and effectiveness of standing waves for ultrasonic purification of magnesium alloy melt, numerical simulation and relevant experiment were carried out. The numerical simulation was broken into two main aspects. On one hand, the ultrasonic field propagations within the cells with various shapes were characterized by numerical solutions of the wave equation and with a careful choice of geometry a nearly idealized standing wave field was finally obtained. On the other hand, within such a standing wave field the agglomeration behavior of oxidation inclusions in magnesium alloy melt was analyzed and discussed. The agglomeration time and agglomeration position of oxidation inclusions were predicted with numerical simulation method. The results show that the oxidation inclusions whose apparent densities are close to the density of the melt can agglomerate at wave nodes in a short time which to a great extent enhances and accelerates the separation of oxidation inclusions from magnesium alloy melt.

Rheo-diecasting of AZ91D magnesium alloy by taper barrel rheomoulding process

A self-developed taper barrel rheomoulding (TBR) machine was introduced, and the rheo-diecasting process was implemented by combining TBR machine with the high pressure die casting (HPDC) machine. Microstructural characteristics of the rheo-diecasting components were investigated at different rotation speeds. Flow characteristics and microstructural evolution of the semi-solid slurry during the rheo-diecasting process were analyzed and the mechanical properties of the rheo-diecasting components were studied. The experimental results show that the process is able to obtain such components in which the primary α-Mg particles are fine, nearly spherical and uniformly distributed in the matrix. When the rotation speed of internal taper barrel is 700 r/min, the primary α-Mg particles get a mean diameter of about 45 μm and a shape factor of about 0.81. The magnesium alloy melt has complex stirring-fixed flow characteristics when flowing in TBR machine. Compared with conventional die-casing process, the rheo-diecasting process can improve the mechanical properties of components; especially, the elongation is improved by 80%.

Effects of processing variables on melt cleanliness of AZ31B magnesium alloy investigated by quantitative evaluation of Fe and non-metallic inclusions

Impurities such as Fe, Ni and Cu and non-metallic inclusions such as oxides, nitrides, carbides, sulfides and fluorides are harmful to the quality and various properties of magnesium alloy sheets produced by twin-roll casting. In this study, the changes of the content of Fe and non-metallic inclusions in AZ31B magnesium alloys with melt temperature and isothermal holding time were quantitatively evaluated using EPMA and the metallographic method. The Fe content did not increase above the Fe content in the raw material, which implies that the dissolution of Fe from a steel crucible was suppressed effectively. The content of non-metallic inclusions, mainly consisting of oxide, fluoride and Fe-rich intermetallic compounds, did not change remarkably with the melt temperature but it increased with the isothermal holding time due to the continuous oxidation of the magnesium alloy melt on the melt surface.

Magnesium matrix nanocomposites fabricated by ultrasonic assisted casting

Magnesium matrix composites reinforced with nano-sized SiC particles (n-SiCp/AZ91D) were fabricated by high intensity ultrasonic assisted casting. The microstructure of the nanocomposites was investigated by optical microscopy, scanning electronic microscopy (SEM), high resolution transmission electronic microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) methods. The results showed that the dispersion and distribution of n-SiCp in magnesium alloy melts were significantly improved by ultrasonic processing. Compared to the unreinforced AZ91D matrix, mechanical properties of the nanocomposites including tensile and yield strengths were remarkably improved and the yield strength increased by 117% after gravity permanent mould casting.

Effect of Low-Levels of Strontium on the Oxidation Behavior of Selected Molten Aluminum–Magnesium Alloys

The effects of small additions of strontium on the oxidation behavior of aluminum–magnesium alloy melts were investigated by thermogravimetry at 750 °C for times up to 48 h. Oxidized samples were examined by FEGSEM, and phases formed within the oxide layer and in the alloy were identified by EDS, WDS and low-angle X-ray diffraction techniques. In the absence of Sr, the Al–Mg samples gained substantial amounts of weight by formation of spinel (MgAl2O4) at the oxide–metal interface. Samples containing Sr had significantly lower weight gains. A very significant decrease (98%) of total weight gain was observed for small Sr additions in the low Mg-bearing Al–Mg alloys. This change in oxidation behavior was linked to the presence of Sr enrichment of the liquid beneath the initial MgO layer suppressing the formation of spinel crystals.

Choice of Bubble Radius in Floatation Refining of Magnesium Alloy

The species and shapes of impurities in magnesium alloy melt were studied. Floatation Refining method of Magnesium Alloys melt and eduction dynamics analysis were used to judge the floatation condition of gas bubble with MgO impurity. Models of floatation eduction were established to study the dynamics relationship of weight, drag and buoyancy. The relation curve between refining gas bubble radius and floating up rate was established. The choice method of gas bubble radius was provided and a practical process scheme proposed for floatation refining of magnesium alloys melt.

Study on the Filtering Purification of AZ91 Magnesium Alloy

High quality magnesium casting depends significantly on the purification of its alloy melt. However, it is very difficult to obtain high-quality magnesium alloy melt due to its high reactivity and its tendency of combustion. The fluxing processing, a traditional purification method for magnesium melt, not only bears the risk of flux inclusions but also is facing more and more environmental pressure today. Therefore, the effective substitutes for fluxing processing, such as physical filtering method, are paid more attention recently. In this paper, the effects of technological conditions including the pore size of stainless steel mesh and the filtering temperature on the purification degree of AZ91 alloy melt were investigated in the mesh filtering processing, in which 2 wt.% calcium was added into the melt in order to obtaining ignition-proof property. The results indicated that mesh filtering processing could improve the purification degree markedly, and the purification degree increased with the increase of filtering temperature properly or the decreased of pore size of mesh. However, too high filtering temperature would lead to the increasing of the tendency of oxidation and combustion, and too small pore size would decrease the filtering ability of mesh which would lead to the interrupting of purification operation.

Numerical simulation of the twin-roll casting process of magnesium alloy strip

The magnesium twin-roll strip casting process is investigated with a test caster at the IME. The flow field in the melt cannot be investigated optically, therefore numerical simulations have been undertaken to show the flow field in the Mg melt, the solidification between the two rolls and the cooling of the strip. The three-dimensional, turbulent flow field for different steady state casting points, in the process gap, have been calculated, including the solidification of the magnesium alloy melt. Within the process gap of the casting process the flow field shows similar flow structures. A big vortex in the casting channel and a much smaller vortex between the two rolls occur. With the increase of the casting speed the mushy zone is moved towards the kissing point. This can be interpreted due to a smaller force on the rolls. Nevertheless, the solidification on the rolls is so strong, that rolling between the two rolls happens.

Formation of <I>In Situ</I> Composite Layer on Magnesium Alloy Surface by Casting Process

In order to develop a process to obtain a functional layer on the surface of a magnesium alloy, the composite layer including in situ Mg 2 Si was formed on the surface by a gravity die-casting. The optimum fabrication condition to form the composite layer and the formation mechanism of the layer were clarified by observing the microstructure, and then the wear properties of the composite layer were also investigated. By casting the magnesium alloy melt into the permanent mold on which the slurry mainly consisting of Si particles was coated, the magnesium alloy composite layer in which the in situ Mg 2 Si particles were dispersed was formed. The melt temperature and mold temperature required to form the composite layer, which perfectly covers the casting, were above 1073 K and above 673 K, respectively. The composite layer was a magnesium alloy in which the fine Mg 2 Si particles of approximately 40 vol% were dispersed. The thickness and hardness of the layer was about 600 μm and 180 HV, respectively. Under the dry sliding wear, the weight loss of the composite layer was lower than that of the magnesium alloy. These results lead to the conclusion that the wear resistant magnesium alloy composite layer in which the in situ Mg 2 Si particles were dispersed can be formed in the present process.

Mechanical properties and microstructure of SiC-reinforced Mg-(2,4)Al-1Si nanocomposites fabricated by ultrasonic cavitation based solidification processing

Nano-sized SiC enhanced magnesium matrix nanocomposites, Mg-2Al-1SiC with 2% SiC and Mg-4Al-1Si with 2% SiC, were successfully fabricated by ultrasonic cavitation based dispersion of SiC nanoparticles in Mg-(2,4)Al-1Si magnesium alloy melts. As compared to the magnesium alloy matrixes, the mechanical properties including tensile strength and yield strength of the Mg-2Al-1Si/2% SiC and Mg-4Al-1Si/2% SiC nanocomposites were improved significantly, while the ductility of magnesium alloy matrix castings was retained. While there were some SiC micro-clusters in the microstructure of nanocomposites, the SiC nanoparticles were dispersed well outside the areas of micro-clusters. Most micro-clusters were located along the grain boundaries while most separate SiC nanoparticles were embedded inside the grains. TEM study of the interface between SiC nanoparticles and Mg-(2,4)Al-1Si metal matrixes suggested that SiC bonds well with the metal matrixes without forming an intermediate phase.

Effect of Gas Bubbling on Tensile Elongation of Gravity Mold Castings of Magnesium Alloy

Recently application of magnesium alloys is expanded rapidly over the whole industry, especially in automotive and electronic industries. Most of magnesium alloy parts are produced by casting process and the quality control of castings is very important to reliability and durability of parts. Various properties of magnesium alloy castings are deteriorated by defects such as non-metallic inclusion and pore. 1–5) Especially control of non-metallic inclusion remained in magnesium alloy castings is very important because the possibility of entrance of non-metallic inclusion into melt is very high due to high reactivity of magnesium alloy melt with gas species above melt surface. There are various methods for removal of inclusions in melt including addition of flux, gas bubbling and filtration. 6–8) Formerly a method of addition of flux was mainly applied to remove inclusions from magnesium alloy melt. 9–11) But part of flux was entered into melt during pouring when flux could not be entirely removed as dross and/or sludge before pouring and the flux remained in casting degraded the quality of castings as harmful defect. So a method of bubbling of inert gas into melt has been paid attention as new method for removal of inclusions from melt, especially during steelmaking. 12–15) But studies on removal of inclusions from magnesium alloy melt by gas bubbling have not been carried out actively. In this study the effect of Ar gas bubbling on the removal of inclusions from AZ91D magnesium alloy melt is evaluated indirectly by measurement of tensile elongation of gravity mold castings of AZ91D magnesium alloy and analyzed qualitatively.

Ｃａ添加Ｍｇ合金の制御圧延技術の開発

In general, SF6 gas is widely used in order to protect the molten magnesium against oxidation. But SF6 gas is a typical green house gas being the source of global warming, so that the usage will be prohibited in near future. Ca is known to be effective for improving the oxidation resistance for magnesium alloy melts. However, it was reported that the addition of Ca would be resulted in the degradation of hot workability and mechanical properties. It is effective for the improvement of mechanical properties and formability to control the microstructure by controlled rolling process. The purpose of this study is the establishment of the technologies for controlled rolling process in Ca added magnesium alloys using isothermal rolling machine for suppression of edge cracks. In this study, the hot workability, recrystallization behavior and tensile properties after rolling of AZ61-xCa (x = 0∼0.45 mass%) was investigated. The results are as follows: 1.The addition of Ca was resulted in the significant degradation of hot workability because of the Al2Ca phase. The Al2Ca phase remained even after solution treatment at 698 K for 24 hr. 2.The recrystallization progression and grain growth rate of AZX610 alloys are obviously slower than that of steel. So multi-pass rolling with low reduction process can be effective to obtain fine microstructure without edge cracks. 3.Better in-plane isoropic ductility was obtained by texture control in controlled rolled AZX610 at 673 K compared with that of 573 K.

Study on mechanism of iron reduction in magnesium alloy melt

This article studies the mechanism of iron reduction in magnesium alloy melt by investigating the iron concentrations in different parts in the melt with different holding time. The iron concentrations at the top, the center and the bottom of the melt are measured with the holding time of 300, 1800 and 7200 s. The elements in the sludge settling down in the crucible are also determined by ICP. With the increasing holding time, the iron content in the upper part of the melt decreases and that in the lower part of the melt increases. And the iron content in the sludge increases rapidly. The iron concentrations in the melt and the sludge change little with longer holding time than 1800 s. Among the three iron reduction agents B2O3, MnCl2 and TiO2, B2O3 has the highest iron reduction efficiency (IRE). IRE of B2O3 is as two times and five times as IRE of MnCl2 and IRE of TiO2 respectively. Formation of FeB and settling of it into melting sludge are believed to be the primary mechanism of iron reduction by B2O3 processing. The mechanism of iron reduction in magnesium melt by MnCl2 or TiO2 processing is studied by thermodynamic analysis. Formation of substance containing MnFeAl or TiFe with high density and high melting point and settling of them into melting sludge are suggested to be the mechanism of iron reduction by MnCl2 or TiO2 processing.

Analysis of Pipe Flow during Transportation of Magnesium Alloy Melt

Magnesium alloys have been well known as active metals. Thus, magnesium alloys in molten state must be handled with extreme care during melting and casting. In this study, water model experiments and numerical analyses were carried out to optimize a pipe flow that can transport magnesium alloys in molten state safely from melting to casting furnace. Especially, during a transportation of molten magnesium alloys, a flow pattern in a pipe becomes important, because the interface between air and melt can be the source of the metal oxidation, and therefore, an air/melt interface area must be minimized. For these purposes, two vessels connected with a long pipe having two elbows with different diameters and radii of the curvature were simulated as melting and casting furnace for magnesium alloys. Optimized conditions with minimized air/melt interface area for the melt transportation were discussed in several pipe configurations.

Effect of the frequency of electromagnetic vibrations on microstructural refinement of AZ91D magnesium alloy

The static magnetic field and the alternating electric field were simultaneously imposed on AZ91D magnesium alloy melt, and α-dendrite particles were refined by the electromagnetic vibrations. The effect of the frequency of electromagnetic vibrations on microstructural refinement was quantitatively investigated. In the frequency range from 60 to 1000 Hz, the vibration frequency near 200 Hz was the most effective for the refinement of α-dendrite particles, and α-dendrite particles were refined up to approximately 100 μm from 1800 μm at this frequency. However, the effect of refinement by the electromagnetic vibrations became weak at frequencies above 400 Hz. Although the degree of refinement of the primary particles differed with the frequency of the electromagnetic vibrations, the dendrite arm spacing was almost constant, 30–40 μm, in our experiment. Therefore, the refinement of primary α-dendrite particles is likely to be caused by collapse of dendrite arms due to the cavitation phenomenon and the stirring of the melt.