Microstructure and mechanical properties of spray-formed AZ91 magnesium alloy

The AZ91 magnesium alloy, preformed with complete shape, has been prepared using spray forming technology under a protective atmosphere. The microstructure and mechanical properties have been investigated. Initially, a homogeneous and equiaxed-grain structure with average grain size of 20 μm was obtained. The tendency for segregation of the divorced eutectic β(Mg₁₇Al₁₂) phase towards the grain boundary was greatly reduced. Further grain refinement was attributed to dynamic recrystallization during extrusion processing. When solution treated at 415 °C and aged at 175 °C, two kinds of β(Mg₁₇Al₁₂) precipitates are formed: the majority are lamellar discontinuous precipitates, in addition to a small amount of dispersed continuous precipitates. The average tensile ultimate and yield strength of the spray-formed and extruded AZ91 magnesium alloy samples were 435 MPa and 360 MPa with a room temperature elongation of 9.2%, indicating an enhanced combination of toughness and strength.     

Microstructural stability and high-temperature mechanical properties of AZ91 and AZ91 + 2RE magnesium alloys

The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg₁₇Al₁₂ intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg₁₇Al₁₂ and retention of the thermally stable Al₁₁RE₃ intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg₁₇Al₁₂ after exposure to high temperature.     

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of β-Mg₁₇Al₁₂ phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.     

Effects of T5 heat treatment on extrusion welds in AZ80 hollow profile

AZ80 hollow profile was produced by porthole-die extrusion. The mechanical behavior and microstructure characteristic of extrusion welds in AZ80 hollow profile during different T5 heat treatments were investigated. Five kinds of morphologies of β-Mg₁₇Al₁₂ were observed in turn in weld regions under various temperatures. Extrusion weld is not weakest region in AZ80 hollow profile in terms of hardness, the peak hardness of weld regions is roughly equal to the weld-free regions at different aging temperatures, which is attributed to similar volume fraction, morphology, and size of β-Mg₁₇Al₁₂ precipitate in weld regions and weld-free regions. The precipitate-free zones (PFZs) usually form near the welds interface in aging process, and the hardness of PFZs is usually lower than those of weld regions and weld-free regions. Aging at 200°C for 24?h is suitable heat treatment system for AZ80 hollow profile, which can achieve excellent mechanical property and simultaneously reduce the width of PFZs.     

Effects of samarium on microstructures and tensile properties of Mg-5Al-0.3Mn alloy

Microstructures and tensile properties of Mg-5Al-0.3Mn-xSm (x = 0, 1, 2 and 3 wt.%) alloys prepared by metal mould casting method were investigated. It was demonstrated that Mg-5Al-0.3Mn alloy was mainly composed of α-Mg and β-Mg₁₇Al₁₂ phases. However, the other two precipitates (Al₁₁Sm₃ and Al₂Sm) were observed along grain boundaries in the alloys containing Sm. The amount of Al₁₁Sm₃ and Al₂Sm precipitates was increased with the increment of Sm content. Meanwhile, volume fraction of β-Mg₁₇Al₁₂ phase was decreased. Moreover, the morphology of β-Mg₁₇Al₁₂ was altered from bulk bone-like shape to spherical one. Tensile results showed that Mg-5Al-0.3Mn-2Sm alloy exhibited the highest tensile properties both at room temperature and 150 °C. Compared with ultimate tensile strength (UTS), yield strength (YS) and elongation (?) of Mg-5Al-0.3Mn alloy, UTS, YS and ? of Mg-5Al-0.3Mn-2Sm alloy were enhanced by 30%, 45% and 35% at room temperature, and by 17%, 48% and 96% at 150 °C, respectively. The improvement of tensile properties was attributed to the decreased amount of β-Mg₁₇Al₁₂ and its refined morphology, and high thermal stable Al₁₁Sm₃ and Al₂Sm precipitates which effectively prohibited dislocation movement and grain boundary sliding during deformation process.     

Microstructure investigation and first-principle analysis of die-cast AZ91 alloy with calcium addition

In order to get improved mechanical properties of die-cast AZ91 alloy under elevated temperatures, Ca element was added as a cost-effective alloying constituent. It appeared that minor Ca addition less than 0.5 wt% would result in no apparent change in microstructure, but the tensile strength at elevated temperatures was improved considerably. When increasing Ca addition to more than 1.0 wt%, Al₂Ca phase will precipitate during solidification, no Mg₂Ca phase was discovered. Homogeneous microstructure and high temperature stability in tensile strength of die-cast AZ91 alloy with Ca addition was mainly attributed to the precipitation of Al₂Ca phase, which considerably refined the bulky β-Mg₁₇Al₁₂ phase distributed originally at the grain boundaries of die-cast AZ91 alloy with no Ca addition. The priority of Al₂Ca phase compared to Mg₂Ca phase in precipitation sequence was verified by first-principle calculation of their cohesive energy and formation enthalpy, and can also be associated with more bounding electrons between Al and Ca atoms.     

In situ observation of ageing process and new morphologies of continuous precipitates in AZ91 magnesium alloy

An in situ observation of the precipitating process of γ-Mg₁₇Al₁₂ phase in die-cast AZ91 magnesium alloy, was carried out with a transmission electron microscope equipped with a heating stage maintained at 473 K for 8 h. In addition to the thin plate-shaped continuous precipitates, continuous precipitates with rod-shaped and the Potter orientation relationship were observed and analyzed with transmission electron microscopy including high-resolution transmission electron microscopy techniques. It was also observed firstly that there exist plate-shaped continuous precipitates with the Pitsch-Schrader orientation relationship in the die-cast AZ91 magnesium alloy.     

Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy

Mg-9Al-6Sn-3Zn (wt%) alloy was extruded and heat treated in T5 and T6 conditions, and its mechanical properties and microstructures were investigated. The extruded product can be slightly strengthened by the T5 treatment as a result of sparse and heterogeneous precipitation. Significant increase in strength is achieved by the T6 treatment, and this is mostly attributed to the formation of lamellar discontinuous Mg₁₇Al₁₂ precipitates. The segregation of Al and Zn at grain boundaries is responsible for the discontinuous Mg₁₇Al₁₂ nucleation. The T6-treated alloy exhibits a tensile yield strength of 341 MPa and an ultimate tensile strength of 409 MPa, together with an elongation to fracture of 4%.     

Calorimetric analysis of AZ91D magnesium alloy

In order to study the activation energies and microstructure evolution of β-Mg₁₇Al₁₂ and eutectic structure in AZ91D magnesium alloy, calorimetric analyses were carried out using non-isothermal measurements by DSC and scanning electron microscopy (SEM) with EDX. During DSC tests, two peak temperatures of β-Mg₁₇Al₁₂ dissolution/precipitation and eutectic transformation were delayed in the heating/cooling process with the increase of scanning rates due to the occurrence of superheating/supercooling. Activation energies of β-Mg₁₇Al₁₂ dissolution and eutectic transformation were derived using multiple heating rates. Furthermore, the results of SEM and EDX revealed that the size and morphology of eutectic structures in the alloy had a close relationship with the time of β-Mg₁₇Al₁₂ phase dissolution during the heating process. More eutectic structures formed at low scanning rate due to sufficient time for the diffusion of Al in α-Mg regions.     

Microstructural characteristics and mechanical properties of non-combustive Mg–9Al–Zn–Ca magnesium alloy friction stir welded joints

Non-combustive Mg–9Al–Zn–Ca magnesium alloy was friction stir welded with rotation speeds ranging from 500 to 1250 rpm at a constant welding speed of 200 mm/min. Defect-free joints were successfully produced at rotation speeds of 750 and 1000 rpm. The as-received hot extruded material consisted of equiaxed α-Mg grains with β-Mg₁₇Al₁₂ and Al₂Ca compounds distributed along the grain boundaries. Friction stir welding produced much refined α-Mg grains accompanied by the dissolution of the eutectic β-Mg₁₇Al₁₂ phase, while Al₂Ca phase was dispersed homogeneously into the Mg matrix. An increase in rotation speed increased the α-Mg grain size but not significantly, while microstructure in the heat affected zone was almost not changed compared with the base material. The hardness tests showed uniform distributed and slightly increased harness in the stir zone. Results of transverse tensile tests indicated that the defect-free joints fractured at the base material, while longitudinal tensile tests showed that the strength of the defect-free welds was improved due to microstructural refinement and uniform distribution of intermetallic compounds.     

T4 热处理对石膏型 AZ91 铸造镁合金组织和性能的影响

用石膏型熔模铸造技术,成功制备了AZ91镁合金铸件.用金相显微镜(OM)、扫描电镜(SEM)、能谱(EDS)以及电子万能实验机等,研究了AZ91镁合金铸态及T4热处理态的显微组织演变和力学性能.结果表明,分布在铸态AZ91镁合金晶界的网状β-Mg17Al12相在T4热处理过程中逐渐溶解,铸态和T4热处理态中均存在大量的A18Mn5化合物,T4处理后,其力学性能显著提高.     

Interface characteristics and mechanical properties of the induction brazed joint of magnesium alloy AZ31B with an Al-based filler metal

In this paper, wrought magnesium alloy AZ31B sheets were brazed by means of high-frequency induction heating device using a novel Al-based filler metal in argon gas shield condition. The interfacial microstructure, phase constitution and fracture morphology of the brazed joint were studied. The experimental results show that α-Mg solid solution and β-Mg₁₇Al₁₂ phase were formed in brazing region. Moreover, the homogeneous Mg₃₂(Al, Zn)₄₉ phase in the original Al-based filler metal disappeared entirely after the brazing process due to the fierce alloying between the molten filler metal and the base metal during brazing. Test results show that the shear strength of the brazed joint is 45 MPa. The fracture morphology of the brazed joint exhibits intergranular fracture mode and the crack originates from the hard β-Mg₁₇Al₁₂ phase.     

Microstructure and tensile properties of as extruded and as aged Mg–Al–Zn–Mn–Sn alloy

The effect of 0–4 wt-% Sn addition on the microstructure and tensile properties of AZ80 alloys was investigated. The results indicated that Mg₂Sn particles were barely formed during the extrusion process until the content of Sn is >2 wt-%. The dislocation density in alloys after extrusion declined with the addition of Sn due to the promotion of dynamic recrystallisation after adding Sn. In aging treatment, Mg₁₇Al₁₂ precipitates were promoted by Sn and the phases distributed uniformly at low density level of dislocation. The AZ80-2 wt-% Sn alloy possessed the excellent tensile properties in as extruded and as aged state.     

Effect of heat input on the microstructure and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates

In this paper, the effects of heat input on the microstructures and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that with an increase of the heat input, the grains both in the fusion zone and the heat-affected zone coarsen and the width of the heat-affected zone increased. Moreover, an increase of the heat input resulted in a decrease of the continuous β-Mg₁₇Al₁₂ phase and an increase of the granular β-Mg₁₇Al₁₂ phase in both the fusion zone and the heat-affected zone. The ultimate tensile strength of the welded joint increased with an increase of the heat input, while, too high a heat input resulted in a decrease of the ultimate tensile strength of the welded joint. In addition, the average microhardness of the heat-affected zone and fusion zone decreased sharply with an increase of the heat input and then decreased slowly at a relatively high heat input.     

Evolution of the microstructure and hardness of a rapidly solidified/melt-spun AZ91 alloy upon aging at different temperatures

The effect of aging at different temperatures on a rapidly solidified/melt-spun AZ91 alloy has been investigated in depth. The microstructures of as-spun and aged ribbons with a thickness of approximately 60 µm were characterized using X-ray diffraction, transmission electron microscopy and laser optical microscopy; microhardness measurements were also conducted. It was found that the commercial AZ91 alloy undergoes a cellular/dendritic transition during melt-spinning at a speed of 34 m/s. A strengthening effect due to aging was observed: a maximum hardness of 110 HV/0.05 and an age-hardenability of 50% were obtained when the ribbon was aged at 200 °C for 20 min. The β-Mg₁₇Al₁₂ phase exhibits net and dispersion types of distribution during precipitation. The dispersion of precipitates in dendritic grains or cells is the main source of strengthening.     

Fracture behaviour of a magnesium–aluminium alloy treated by selective laser surface melting treatment

Fracture behaviour of AZ91D magnesium alloy is dominated by the brittle fracture of the β-Mg₁₇Al₁₂ phase so its modification is required to improve the toughness of this alloy. The novel laser treatment named as Selective Laser Surface Melting (SLSM) is characterized by the microstructural modification of the β-Mg₁₇Al₁₂ phase without altering the α-Mg matrix. We have studied the effect of the selected microstructural modification induced by the laser treatment in the fracture behaviour of the alloy has been studied using in situ Scanning Electron Microscopy bending test. This test configuration allows the in situ observation of the crack progression and the record of the load–displacement curve. It has been observed that the microstructural modification introduced by SLSM causes an increase of 40% of the fracture toughness of the treated specimen. This phenomenon can be related with the transition from brittle to ductile fracture behaviour of the laser modified β-phase.     

Superplastic behavior and microstructural stability of friction stir processed AZ91C alloy

Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300–375 °C and strain rates (SRs) in the range of 1 × 10^?4–3 × 10^?3 s^?1. Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33–0.39 and grain size stability till 350 °C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, β-Mg₁₇(Al,Zn)₁₂, Mg₂Si, and Al₈Mn₅ in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.     

Thermodynamic analysis of processability of Mg–Al–Zn–Mn alloys for rheocasting

A two-stage thermodynamic calculation procedure was developed to analyse alloy compositions for rheocasting. Based on Kazakov's criteria, the processability of AZ91, AZ61 and AM60B for rheocasting was investigated. The results show that these alloys do not satisfy selected criteria fully, and only AZ91 is possible to be used for rheocasting with semisolid slurry having lower fraction solid (0.2–0.4). In microstructure produced by rheocasting, the β-Mg₁₇Al₁₂ phase forms continuous network along primary α-Mg gain boundary and is concentrated in this limited contiguous space. The routes to modify and optimise alloy composition were proposed.     

Research on microstructures of sub-rapidly solidified AZ61 Magnesium Alloy

AZ61 magnesium alloy foils of 0.5–3.0 mm thick were successfully produced by using sub-rapid solidification technique. Microstructures of conventionally solidified (CS) and sub-rapidly solidified (sub-RS) alloys were examined by optical microscope (OM) and scanning electron microscope (SEM). The results showed that the cellular grain of 1.8–13.5 μm can be obtained during sub-rapid solidification process. Phase compositions and microdistribution of the alloying elements in the foils were analyzed by X-ray diffraction (XRD) and electron probe microanalyzer (EPMA), respectively. The eutectic transformation L → α-Mg + β-Mg₁₇Al₁₂ and microsegregation in conventionally solidified AZ61 alloy were remarkably suppressed in sub-rapid solidification process. As a consequence, the alloying elements Al, Zn, Mn showed much higher solid solubility and the sub-rapid solidification microstructures dominantly consisted of supersaturated α-Mg solid solution. Meanwhile, the β-Mg₁₇Al₁₂ phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate.     

Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

The microstructures and age hardening behaviours of a series of Mg–Al–rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, Al₄MM and Al₁₂Mg₁₇, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al₄MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al₁₂Mg₁₇ partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of Al₁₂Mg₁₇ precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al₁₂Mg₁₇ formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al₁₂Mg₁₇ also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.

MST/3382