Heat-resistant Magnesium Alloys Research Articles

Microstructure and Mechanical Properties of Mg-Al-La-Mn Composites Reinforced by AlN Particles.

The Mg-Al-RE series heat-resistant magnesium alloys are applied in automotive engine and transmission system components due to their high-temperature performance. However, after serving at a high temperature for a long time, the Al11RE3 phase coarsened and even decomposed, while the Mg17Al12 phase grew and dissolved, which limits the service temperature of Mg-Al-RE series heat-resistant magnesium alloys to a maximum of 175 °C. In this study, a new preparation method for in situ AlN particles was presented. The AlN/Mg-4Al-4La-0.3Mn composites were prepared by a master alloy and casting method. The effects of various contents of AlN (0.5-3.0 wt.%) on the microstructure and mechanical properties of the Mg-4Al-4La-0.3Mn (AE44) alloy at room (25 °C) and high temperatures (150-250 °C) were investigated. Microstructure analysis revealed that the inclusion of AlN led to a reduction in both the grain size and second phase size in the AE44 alloy, while also improving the distribution of the second phase. The average grain size, Al11La3 phase, Al2La phase, and Al3La phase of the 2.0 wt.% AlN/AE44 composite were 135.7, 9.6, 1.9, and 12.6 μm, respectively, which were significantly lower than those of the AE44 matrix alloy (179.8, 12.6, 3.3, 17.8 μm). The refinement was attributed to the ability of AlN particles to serve as heterogeneous nucleation cores for α-Mg and, at the same time, impede the growth of the solid-liquid interface, eventually leading to grain refinement. With the increase in the AlN content, the mechanical properties of composites initially exhibited an increase at both room and high temperatures, followed by a subsequent decrease. When the AlN content was 2.0 wt.%, the composite exhibited optimal strength and plasticity matching. At room temperature, the TYS, UTS, and EL values of the 2.0 wt.% Mg-4Al-4La-0.3Mn composite were 96 MPa, 175 MPa, and 7.0%, respectively, which were increased by 26 MPa, 27 MPa, and 0.7% when compared with the base alloy. The TYS of the 2.0 wt.% Mg-4Al-4La-0.3Mn composite at 150 °C, 200 °C, and 250 °C were 17 MPa, 14 MPa, and 22 MPa higher than those of the matrix alloy, respectively. The main strengthening mechanisms were second phase strengthening, load transfer strengthening, and thermal mismatch strengthening. At elevated temperatures, AlN particles effectively pinned the grain boundaries, inhibiting their migration, and hindered dislocation climbing, resulting in excellent mechanical properties of the composites at high temperatures. This study contributes to the advancement of in situ AlN particle preparation methods and the exploration of effects of AlN on the properties and microstructure of Mg-Al-RE alloys at high temperatures (150-250 °C).

Research Progress of Heat Resistant Magnesium Alloys

Magnesium alloy has extremely excellent properties and is known as “21st Century Green Engineering Material”. This article mainly introduces the influence of the heat resistance and comprehensive performance of the three series of Mg-Al, Mg-Zn and Mg-RE heat-resistant magnesium alloys after adding rare earth elements, alkali metal elements and other elements. Three development directions of improving the heat resistance of magnesium alloys are prospected. These are: 1. Using cheap alloy elements (such as Ca, Si, etc.) to replace rare earth elements of the heat-resistant magnesium alloy, 2. Titanium element is added to improve heat-resistant magnesium alloy’s mechanical properties and its strength, 3. The new casting process and processing technology are used to improve the heat-resistant magnesium alloy’s properties. This article aims to provide technical reference for the development of my country’s magnesium alloy industry.

Abnormal creep stress exponents in a high-pressure die casting Mg–Al−RE alloy

Abnormal creep stress exponents (n) are frequently reported for heat-resistant magnesium alloys, but hitherto without in-depth elucidation or rationalization from the underlying creep mechanisms. In this work, tensile creep behavior of a high-pressure die casting Mg−3.76Al−3.52RE−0.23Mn (wt.%) alloy at 200 °C were studied, adopting small applied stress intervals of 1–2.5 MPa. The result indicates a roller-coaster n-value evolution of 5 → 23 → 38 → 8 → 45 → 76, as the applied stress increases from 80 MPa to 100 MPa. Microstructural analysis using transmission electron microscopy suggests that basal slip of <a> dislocations is dominant at lower stresses while dislocation climb at higher stresses and, simultaneously, twinning becomes more prominent and other sub-microstructures such as dislocation bands gradually occur as the applied stress increases such as to 87 MPa. By simultaneously introducing a threshold stress and an internal stress, the abnormal n-values are rationalized to 3.0 at relatively lower stresses and gradually to 7.7 at higher stresses, corresponding basal slip of dislocations to dislocation climb. Therefore, we suggest that alloy design aimed to improve creep resistance of magnesium alloys should pay more attention to impeding dynamic twinning during creep.

Microstructure and Hot Deformation Behavior of the Mg-8 wt.% Sn-1.5 wt.% Al Alloy.

Mg–Sn–Al alloy is a new type of heat-resistant magnesium alloy with great potential and the hot deformation process of this alloy is of great significance for its application. The microstructure, hot deformation behavior, textural evolution, and processing map of a Mg–8 wt.% Sn–1.5 wt.% Al alloy were studied. A Gleeble 1500 D thermo-mechanical simulator was used. The temperature of deformation was 653 to 773 K, the strain rate was 0.001–1 s−1, and the maximum deformation degree was 60%. The obtained results show that the rheological stress of the alloy decreases with an increase in deformation temperature and increases with an increase in the strain rate. The alloy is completely dynamically recrystallized at 653 K, and the entire structure is formed of homogeneous crystals/grains, with small secondary phase particles distributed at the crystal boundary. The mean apparent activation energy of hot compression deformation is 153.5 kJ/mol. The Mg–8 wt.% Sn–1.5 wt.% Al alloy exhibits excellent plastic deformation properties, an expansive thermal processing interval, and a narrow instability zone under the test temperature and deformation rate. The optimal process parameters of the alloy comprise deformation temperatures between 603 and 633 K and strain rates of 0.03 to 0.005 s−1.

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.

Study on the Constitutive Behavior and Hot Deformation Characteristic of Mg–4Sm–2Zn–0.5Zr Alloy

Mg–4Sm–2Zn–0.5Zr alloy is a new type of heat-resistant magnesium alloy, it has the mechanical properties similar to EZ33 alloy but is much cheaper than EZ33 in cost since Samarium (Sm) rather than Nd was employed in the alloy. In this paper, based on isothermal compression tests in the temperature range of 350–450 °C and strain rate range of 0.001–1.0 s−1, the Arrhenius-type constitutive equations for this alloy were established, and the hot working flow behavior and the microstructures of the alloy were examined. To evaluate the hot deformation mechanisms and determine the optimal processing parameters, 3D processing maps under different deformation conditions were developed according to the dynamic material model and Muthy instability criterion. The hot extrusion experiments were carried out, and the results show the predictions of processing maps are in agreement with the experimental results.

Electrodynamic treatment by electric current pulses as effective method of control of stress-strain states and improvement of life of welded structures

The present work was aimed at the development of the new technological process: a postweld electrodynamic treatment of welded joints by pulses of a high-density current. Basing on the numerical solution of Maxwell’s equations for the case of a pulsed electromagnetic field the calculation model of distribution of field components was designed on the basis of integral equations, which allow determining the electromagnetic forces P and current density j in aluminium alloy AMg6 during the process of capacitive energy storage discharge. The calculation data allow accounting for influence of dynamic stresses and electric plasticity effect on change in a stressed state of a metal during the electrodynamic treatment. Comparison of calculated and experimental results showed a good correlation of the obtained data. Using the model, the parameters of the electrodynamic treatment, providing the reduction of residual stresses in welded joints of alloy AMg6, were determined. Generators of the pulse current and electrode devices, realizing the preset parameters of electrodynamic effects at single- and two-channel schemes were designed and manufactured. Results of experimental investigations, carried out on specimens of base metal and welded joints showed that the electrodynamic treatment has a positive effect on structure of the treated metal and allows decreasing greatly the residual stresses in welded joints, increasing their resistance to fatigue and brittle fracture, as well as eliminating the distortion deformations in thin-walled elements of structures. The developed technologies and equipment gave an opportunity to perform the electrodynamic treatment of welded joints of hull structures of ships of aluminium alloy AMg6 and repair welds of an intermediate casing of the aircraft engine of heat-resistant magnesium alloy ML10, thus promoting the increase in their service reliability and life.

Effect of Sn Content on Heat Resistance of Mg-3%Al-1%Si Alloy for Casting

Magnesium alloys have the characteristic with high specific strength and lightweight property, it is widely used for auto mobile industry. Heat-resistant magnesium alloy is focused as a suitable material for weight reduction of the engine and power train parts in automotive field. In this study, microstructure and heat-resistant property in Mg-3mass%Al-1mass%Si (Mg-3%Al-1%Si) alloy with containing large amount of Sn (tin) were investigated. The alloys produced by permanent mold casting were investigated by optical microscope (OM), scanning electron microscopy (SEM) and measuring of bolt load retention at 423K. The heat-resistant property of Mg-3mass % Al-1mass % Si alloy with containing 6-13masss%Sn was higher compared with Sn free alloy and conventional Magnesium alloys (e.g. AZ91 and AM60 alloys).

Development of Post-weld Electrodynamic Treatment Using Electric Current Pulses for Control of Stress-Strain States and Improvement of Life of Welded Structures

Abstract The present work was aimed at the development of a new technological process: a post-weld electrodynamic treatment of welded joints by pulses of a high-density current. Generators of the pulse current and electrode devices, realizing the preset parameters of electrodynamic effects at single- and two-channel schemes, were designed and manufactured. The results of experimental investigations, carried out on specimens of base metal and welded joints, showed that the electrodynamic treatment has a positive effect on the structure of the treated metal and allows the residual stresses in welded joints to decrease gently, increasing their resistance to fatigue and brittle fracture as well as eliminating the distortion deformations in thin-walled elements of structures. The developed technologies and equipment gave an opportunity to perform the electrodynamic treatment of welded joints of hull structures of ships of aluminum alloy AMg6 and repair welds of an intermediate casing of the aircraft engine of heat-resistant magnesium alloy ML10, thus promoting an increase in their service reliability and life.

Effects of Sn addition on the microstructure and tensile properties of AX55 alloys

The microstructures and tensile properties at both room and elevated temperatures for both the as-cast and as-aged Mg-5Al-5Ca (AX55) alloy with 0–2 wt% Sn addition were studied. The results indicate that the α-Mg dendrite is gradually refined and the interdendritic Al2Ca and Mg2Ca intermetallics become more connected with Sn addition. The as-cast AX55-1Sn alloy shows optimal ultimate tensile strength (UTS) at testing temperature from 25 to 225 °C. After T61 and T62 heat treatment, the eutectic-lamellar microstructure of the as-cast alloys tends to be spheroidized and distributed uniformly along the grain boundaries. While the alloys with higher Sn content show higher density of granulated and needle-shaped Al2Ca phases precipitated into α-Mg matrix, which results in the increase of UTS, yield strength (YS), elongation and microhardness with Sn addition. The morphology of CaMgSn phase can be improved by T62 treatment, which makes as-aged AX55-2.0Sn alloy exhibit a smaller decrease rate of the UTS at temperature up to 225 °C. The heat resistance of different heat-resistant magnesium alloys were compared and discussed by using the decrease rate of the UTS.

Strength Stability of Aging Hardened Mg-10Y-1.5Sm Alloy

The tensile strength stability of aging hardened Mg-10Y-1.5Sm (mass fraction, %) alloy was investigated by microstructure analysis and tensile tests. The results show that the microstructure of Mg-10Y-1.5Sm alloy consists of α-Mg matrix and Mg24Y5 phase and the formation of Sm-containing phase is not observed. With the increase of temperature from 20 to 300 oC, the tensile strength exceeds 200 MPa and has no obvious change. It is insensitive to temperature and the variation range is less than 10 MPa. The strength stability is superior to that of heat resistant magnesium alloy WE54 developed most successfully. It is mainly attributed to the hardness stability of strengthening phase Mg24Y5 enhanced by dissolved Sm.

Present Development Status of Anti-creep Magnesium Rare-Earth Alloys

The research status of creep resistance of rare earth magnesium alloy at home and abroad is reviewed, and the mechanism of high temperature creep resistance and the way of improving the creep resistance of magnesium alloy were also discussed. The problems of high temperature resistance and creep resistance of cast magnesium alloy are pointed out, and its future development direction is forecasted. The purpose of this paper is to provide the idea and basis for the development of creep resistant and heat resistant magnesium alloy.

Hot Compression Deformation Behavior of Mg-Gd-Nd-Zr Heat-Resistant Magnesium Alloy

The hot compression deformation of Mg-12Gd-2.5Nd-0.5Zr heat-resistant magnesium alloy was carried out on Gleeble-1500D thermo-mechanical simulator with the strain rate range of 0.002-1 s-1, the deformation temperature range of 623-773 K and maximum strain of 50 %. The results show that true stress decreases with the increase of deformation temperature at a constant strain rate, and the true stress increases with the increase of strain rate at a same temperature. The deformation constitutive equation was established, and the hot deformation activation energy of 292.679 kJ/mol is determined.

Thermodynamic stability of Al11RE3 intermetallic compounds from first-principles calculations

Phase stability of Al-RE intermetallic compounds is crucial particularly for high temperature mechanical properties of Mg–Al–RE based alloys as well as the possibility of their commercial applications. For a long time, a controversial issue exists concerning whether Al11RE3 phase in heat-resistant Mg–Al–RE based alloys decomposes into Al2RE and Al when treated at high temperatures. Herein, we performed a systematically study to the relative phase stability of all Al11RE3 with single RE element compared with Al2RE at 0–1000K using the state-of-the-art first-principles density functional calculation method. The results indicate that Al11RE3 phases with RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb are highly stable while those with RE=Ho, Er, Tm, Lu, Y, and Sc are always instable compared with the Al+Al2RE two-phase equilibrium at temperatures lower than 1000K. These calculations suggest that Al11RE3 phase in the Mg–Al–RE based alloys with Ce-rich misch metal or mixed RE elements would decompose into Al2RE and Al due to the instable Al11RE3 components, then decreased the alloy’s high temperature performances. The present work also provides important guidance for future optimization design of heat-resistant magnesium alloys.

Investigation of residual stresses in welded joints of heat-resistant magnesium alloy ML10 after electrodynamic treatment

Abstract In repair of aircraft structures of magnesium alloy ML10, the argon arc non-consumable electrode welding is used. In this case, the residual welding stresses occur in repair welds, being one of the causes for reducing the service characteristics of the restored products. Residual stresses arise as a result of welding. Post-weld heat treatment is used to reduce the residual stresses. The heat treatment, which occurs after welding, increases the cost of repair. This leads to the search for alternative methods to control the stressed state of welded joints, one of which is electrodynamic treatment, which reduces the level of residual stresses in repair welds, and as a consequence, the cost of the welding repair in restoring aircraft structures. It was found from the results of experiments carried out, that the electrodynamic treatment allows reduces the initial level of stresses in welded joints, reaching 120 MPa, to 30 MPa, and at definite geometric characteristics of the specimens forming the field of compressive stresses, the values of which are equal to –50 MPa. It is shown that the optimum distance between the zones of treatment, being 5 mm, provides the guaranteed covering the zones of electrodynamic effect and, as a consequence, the maximum efficiency of the electric dynamic treatment.

Development of heat resistant Mg-Zn-Al-based magnesium alloys by addition of La and Ca: Microstructure and tensile properties

In order to clarify the potential of developing Mg-Zn-Al-RE-Ca-based heat resistant magnesium alloy, the influences of different La and Ca combinations on the microstructure and mechanical properties of Mg-4Zn-4Al-xLa-yCa (x + y = 4) alloys have been investigated. The results show that the high content of La and Ca additions promotes Al2LaZn2, Al4Ca and Ca2Mg6Zn3 isomorphous phases forming in the as-cast Mg-Zn-Al-La-Ca alloys, instead of the formation of low melting point Mg-Al and Mg-Zn-Al eutectics. Among all the secondary phases, the Al2LaZn2 phase is prior to form by consuming up La atoms, and then the Al reacts with Ca to form Al4Ca, and finally the surplus Zn reacts with Ca and Mg to form Ca2Mg6Zn3 phase. The Ca2Mg6Zn3 isomorphous phase contains a certain amount of Al, and its volume fraction is increased with increasing Ca content. The tensile strengths at both room temperature and 175 °C are increased with increasing Ca addition, while the ductility is decreased due to the increased formation of networked Ca2Mg6Zn3 phase. The Mg-4Zn-4Al-2La-2Ca alloy shows the optimal tensile properties at both room temperature and high temperature.

Study on microstructure evolution of deformed Mg-Gd-Y-Nd-Zr heat-resistant magnesium alloys after solid solution and ageing

The microstructure evolution of Mg-Gd-Y-Nd-Zr heat-resistant magnesium alloy after deformation and T5 or T6 treatment were studied. In thermoplastic deformation, dynamic recrystallization and dynamic precipitation has been taken place at the same time. The dynamic precipitation reduces the recrystallization nucleation driving force in the grain; it will prevent to occur dynamic recrystallization partially. Solid solution temperature was 530oC and hold 4h. Age hardening treatments were performed at 225oC and hold 16h. The alloy showed the comprehensive properties are obviously improved from T6 to T5 heat treatment. After T5 heat treatment the tensile strength of alloy increased to 359.3 MPa, increased by around 48.5%; Elongation is increasing from 5.17% to 6.5%. After peak ageing treatment, the main precipitation is ?' phase, the precipitation phase have obvious pinning effect to grain boundary of the alloy, it will prevent the grain growth ageing for a long-time. At the same time, strengthening role of precipitate phase make its strength increased significantly.

難燃性マグネシウム合金や耐熱マグネシウム合金の開発と応用

難燃性マグネシウム合金や耐熱マグネシウム合金の開発と応用

408 耐熱マグネシウム合金とグラファイトの濡れ性と改善(生産加工・工作機械II)

408 耐熱マグネシウム合金とグラファイトの濡れ性と改善(生産加工・工作機械II)

Microstructures and mechanical properties of Mg–Al–Sm series heat-resistant magnesium alloys

The microstructures and phase compositions of the as-cast and die-cast Mg–6.02Al–1.03Sm, Mg–6.05Al–0.98Sm–0.56Bi and Mg–5.95Al–1.01Sm–0.57Zn alloys were investigated. Meanwhile, the tensile mechanical and flow properties were tested. The results show that the as-cast microstructure of Mg–6.02Al–1.03Sm alloy is composed of α-Mg matrix, discontinuous β-Mg17Al12 phase and small block Al2Sm phase with high thermal stability. Rod Mg3Bi2 phase precipitates when Bi is added, while the added metal Zn dissolves into α-Mg matrix and β-Mg17Al12 phase. The as-cast alloys exhibit the excellent tensile mechanical property. The tensile strength (σb) and elongation (Δ) can reach 205–235 MPa and 8.5%–16.0% at ambient temperature, respectively. Meanwhile, they can also exceed 160 MPa and 14.0% at 423 K, respectively. The die-cast microstructures are refined obviously, and meanwhile the broken second phases distribute dispersedly. The die-cast alloys exhibit better tensile mechanical properties with the values of σb and Δ of 240–285 MPa and 8.5%–16.5% at ambient temperature, respectively, and excellent flow property with the flow length of 1870–2420 mm. The die-cast tensile fractures at ambient temperature exhibit a typical character of ductile fracture.