As-cast Magnesium Alloy Research Articles

Microstructure and mechanical properties of ultrafine grained Mg15Al alloy processed by equal-channel angular pressing

An as-cast magnesium alloy with high Al content Mg15Al was subjected to equal-channel angular pressing (ECAP) through a die with an angle of ϕ=90° at 553 K following route Bc. It is found that the network β-Mg17Al12 phases in the as-cast Mg15Al alloy are broken into small blocks and dispersed uniformly with increasing numbers of pressing passes. Moreover, many nano-sized Mg17Al12 particles precipitate in the ultra-fine α-Mg matrix. The grains are obviously refined. However, the grain structure is inhomogeneous in different areas of the alloy. The average size of the primary phase α-Mg is reduced to about 1 μm while grains of around 0.1–0.2 μm are obtained in some two-phase areas. With additional ECAP passes (up to 8), coarsening of the grains occurs by dynamic recovery. Room temperature tensile tests show that the mechanical properties of Mg15Al alloys are markedly improved after 4 ECAP passes. The ultimate tensile strength and elongation to failure increase from 150 MPa to 269.3 MPa and from 0.05% to 7.4%, respectively. Compared with that after 4 passes, the elongation to failure of the alloy increases but the strength of the alloy slightly decreases after 8 ECAP passes. Fracture morphology of the ECAP-processed alloy exhibits dimple-like fracture characteristics while the as-cast alloy shows quasi-cleavage fractures.

Effects of Al and Zn on the Microstructure and Mechanical Properties of Magnesium Alloys

Two groups of magnesium alloys with various Al and Zn components are studied in this paper. One group of alloys are constant Al content of about 6% and various Zn content from 0 to 3%, another group are constant Zn content of about 0.4% and various Al content from 0 to 6%. The microstructures and mechanical properties of these alloys are investigated in as-cast and homogenized at 380°C for 15h. The results show that the tensile strength increases but yield strength decreases after homogenizing treatment. It can also be found that the morphology of second phrase and the size of grain exert the more effect on the mechanical properties than Zn content does. The alloys with uniform, fine and non-dendrite microstructure exhibit both high strength and elongation regardless of Zn content. On the other hand, the tensile strength and yield strength elevate significantly as Al content increases, and the elongation has a peak value in Al content of about 1.90%. The results show that the as-cast magnesium alloys with Al content of 5.6~6.0% and Zn content of 0.6~1.0% exhibit the best comprehensive mechanical properties.

The Development of Superplastic Magnesium Alloy Sheet

While magnesium alloys are routinely used in engineering applications in the form of net shape castings, applications for sheet product have been limited due to the poor cold formability of magnesium combined with the perceived expense of sheet. The issues associated with poor cold formability could largely be overcome if magnesium alloys were to be superplastically formed. Superplasticity in magnesium is well established with research papers on the subject dating back to the late 1960s. In recent years, interest in this area has grown to the point where a number of companies have successfully superplastically formed prototype automotive panels from magnesium alloy sheet. Concurrent to this the scientific community have demonstrated superplasticity in a wide range of magnesium alloys using processing techniques ranging from the exotic (severe plastic deformation) to the mundane (traditional warm rolling). Work by the current authors has shown, rather surprisingly, that superplasticity can be achieved in magnesium alloys in the as-cast condition. This has led to some initial exploratory work involving twin roll casting. The concept being that affordable superplastic magnesium sheet could be produced via twin roll casting with only limited rolling reduction to final gauge. This paper describes the superplastic behaviour (in uniaxial tension) and microstructure of sheet processed from strip cast AZ31 and AZ91. The experimental material has included strip cast AZ91 subjected to large shear strains immediately prior to casting. The material was tested in the as-cast condition and after warm rolling to a number of gauges. Industrially useful superplastic capability was demonstrated in the strip cast alloys. Furthermore, good superplastic capability was also demonstrated in sheet subsequently rolled from the cast metal and rolling strain did not significantly influence the ductilities obtained. The mechanism for achieving superplasticity in as-cast magnesium alloys will be considered and the contrasting deformation characteristics of AZ31 and AZ91 will be discussed in terms of m value analysis and microstructural characterisation.

Thermal Cycle Characteristics of AM 50 Magnesium Alloy Welded by FSW

By means of Friction stir welding (FSW), as-cast AM50 magnesium alloys were welded with travel speed of 50 mm/min and rotating rate of 1200 r/min. The thermal cycle was investigated by thermocouples and a paperless recorder. The results show that phenomenon of dual peaks and hysteresis exist in temperature curves of featured points at the starting part while only hysteresis in the middle part. The finishing part has the highest peak temperature while the starting part corresponds to the lowest one. Featured points for different depths in the same place share the same shape of temperature curves but with different peak temperature. The retaining time over recrystallization temperature at middle part is double that at finishing part.

Research on the Microstructures and Mechanical Properties of Annealed Cold Upsetting AZ31 Magnesium Alloy

The microstructures and mechanical properties of cold upsetting magnesium alloys were investigated upon anneal under different conditions. The results show that a large amount of twins were observed in the original grains of cold upsetting AZ31 magnesium alloys. The twins disappeared gradually and recrystal grains formed after anneal. The volume fraction of the recrystal grains increases as the strain of samples rises. Recrystal grain size grows large with the elevated annealing temperature. Recrystal grain size reduces at first and then grows as the annealing time is prolonged. In addition, compared with as-cast magnesium alloys, the yield strength of cold upsetting samples increase apparently due to grain refinement after anneals.

Constitutive behavior of as-cast magnesium alloy Mg–Al3–Zn1 in the semi-solid state

The first reported tensile semi-solid stress/strain data for as-cast magnesium alloy Mg Al3 Zn1 are provided. The results show that the maximum tensile stress at the solidus temperature (460 degrees C) was 13 MPa. Furthermore, this alloy has no ductility above 540 degrees C. and cannot sustain tensile stresses above 560 degrees C. Based on these tests, an equation relating the maximum tensile stress with temperature was derived. The microstructure of the tested specimens was also examined to link the tensile properties to fraction Solid and microstructure. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Dry sliding wear behavior of magnesium alloys

Dry sliding tests were performed on as-cast magnesium alloys Mg 97Zn 1Y 2 and AZ91 using a pin-on-disc configuration. Coefficients of friction and wear rates were measured within a load range of 20–380 and 20–240 N at a sliding velocity of 0.785 m/s. X-ray differactometer, scanning electron microscopy, tensile testing machine were used to characterize the microstructures and mechanical properties of Mg 97Zn 1Y 2 alloy and AZ91 alloy. Worn surface morphologies of Mg 97Zn 1Y 2 and AZ91 were examined using scanning electron microscopy. Five wear mechanisms, namely abrasion, oxidation, delamination and thermal softening and melting, have been observed. Mg 97Zn 1Y 2 exhibited good wear resistance compared with AZ91 for applied loads in excess of 80 N, which have been explained in terms of thermal stability of intermetallic phase and elevated temperature mechanical properties of the two materials tested, using surface temperature analysis.

The Influence of Zinc and Zirconium on the Microstructure of the As-Cast Magnesium Alloy ZK60

In the present paper, nine kinds of alloys with different contents of zinc and zirconium were cast and the influence of zinc and zirconium individually and jointly on the microstructure of the ZK alloys were investigated. The results indicated that the shape and amount of the eutectic were closely related to the content of Zn. With the increase of Zn content, the eutectic was prone to have fish-bone-like shape. When the eutectic grew up to contact with each other, then network eutectic formed; Zr could intensively refine not only the grain size but also the eutectic, but when Zr>0.6%, compound containing Zr or pure Zr granules would appear. That the increase of the Zn must go with the increase of the Zr proved the combined influence of the two alloy elements.

The Effect of Ca and Mn Addition on Ignition Temperature, Castability, and Electrochemical Properties of Mg-Alloy by Casting

The effect of Ca and Mn additions on cast ability, ignition temperature, and electrochemical properties has been investigated in the as-cast magnesium alloys. Not only the remarkable improvement of electrochemical properties but also the ignition prevention of magnesium alloys during melting and casting is achieved by small addition of Ca and Mn into Mg. Flow length of the specimens decrease with an increase of Ca content. Serious decrease of flow ability in Mg melt is observed in Mg-1.31wt%Ca alloy. However, flow length of Mg-Ca alloys is negligibly influenced by small amounts of Mn addition. The improved performance of Mg-Ca alloys is attributed to the homogeneous and refined microstructures with extremely electronegative potential of Ca. It is considered from that serious reduction in flow length of Mg-1.31wt%Ca alloy is due to cast from large solidification interval.

Fracture Analysis and Mechanical Property of AZ91D Magnesium Alloy Chips Prepared by Solid State Recycling

AZ91D magnesium alloy is prepared by hot extrusion of recycled machined chips and its fractures and mechanical properties are investigated at various extrusion conditions. Cold-press is employed to prepare extrusion billets of AZ91D magnesium alloy chips, and then the billets are hot extruded at 573K-723K with an extrusion ratio of 11:1. The results show that tensile strength and elongation of the extrusion magnesium alloy with the extrusion temperature of 673K and the extrusion rate of 0.08mm/s can reach 380MPa and 6%, respectively. Fracture surface presents a mix mechanism of dimple-like fracture and gliding fracture. Due to grain refinement by cold-press and hot extrusion, mechanical properties of extruded rods are much higher than those of as-cast AZ91D magnesium alloy. Also, much lower energy consumption is necessary for this recycling compared to the conventional casting process. Solid state recycling is an efficient method for magnesium alloy chips recycling.

Corrosion behavior of AZ91D magnesium alloy in sodium sulfate solution

The corrosion behavior of as-cast AZ91D magnesium alloy in 0.1M sodium sulfate solution at the corrosion potential (E-corr) was investigated by using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that the corrosion of AZ91D started at both the primary alpha-Mg and the eutectic alpha-Mg. The surface first was covered by a film (MgO, Mg(OH)2) which became thicker with time. Due to the dissolution of the eutectic alpha-Mg, the concentration of aluminum increased, MgAl2(SO4)(4). 2H(2)O precipitated at the primary alpha-Mg and progressively spread to the eutectic alpha-Mg areas. The surface film changed from two-layer to three-layer structure with the increase of immersion time.

Influence of initial microstructure on the hot working flow stress of Mg–3Al–1Zn

The hot working flow stress is examined for wrought and as-cast magnesium alloy AZ31, deformed in both compression and torsion. It is found that the hot deformation behaviour is sensitive to the deformation conditions, initial microstructure and the deformation mode. The peak stresses in compression are higher for as-cast material at low strain rates and high temperatures, whereas they are higher for fine-grained wrought material at high strain rates and low temperatures. Also, the stress–strain curves in torsion exhibit considerably higher strains to the peak flow stress than in compression, particularly at lower temperatures. Most noticeably in the compression of the wrought material, the shape of the stress–strain curves change considerably as the temperature is reduced and the strain rate is increased. These key features of the deformation behaviour are explained in terms of initial grain size, texture, twinning and dynamic recrystallization.

Effect of Yttrium on Morphologies of Precipitation Phases along Grain Boundaries of As-Cast ZK60 Magnesium Alloy

Morphologies of precipitation phases along grain boundaries of the as-cast Mg-6.0Zn-0.45Zr, Mg-6.0Zn-1.2Y and Mg-6.0Zn-0.6Zr-1.0Y alloys were investigated. The experimental results indicated that the main precipitation phase along grain boundary of the as-cast Mg-6.0Zn-0.45Zr was a Mg-Zn binary compound. There were two kinds of precipitation phases along grain boundaries for Mg-6.0Zn-1.2Y and Mg-6.0Zn-0.6Zr-1.0Y alloys. One grew at triangular grain boundary, which was fish-bone -like in shape and a Mg-Zn binary compound. The other distributed mainly around the grain, which was net-like in shape and a Mg-Zn-Y ternary compound. In addition, there were lots of granular phases, extending to the interior of the grain, at the rim of grain boundary phases. Y was found to affect obviously morphology of precipitation phases along grain boundary of the as-cast Mg-Zn-Zr (ZK60) alloys.

Effect of Deformation on the Damping Capacity of Magnesium Alloys

Equal channel angular extrusion (ECAE) was applied to an as-cast AZ91D magnesium alloy. The strain amplitude dependence and temperature dependence of damping capacities of the as-cast and ECAE processed AZ91D alloys were investigated by dynamic mechanical analyzer (DMA). Microstructures of AZ91D alloys after ECAE were observed by optical microscopy (OM). In higher strain region, the damping value of 4-pass ECAE deformed AZ91D alloy was the highest among all the AZ91D alloys under different conditions. The damping peaks of ECAE deformed AZ91D alloys detected during heating from room temperature to 400°C were considered to be related to the migration of grain boundaries and the movement of dislocations during recrystallization.

Research on Technology and Microstructure Performance in Profile Extrusion of AZ31 Mg-Alloy

Aimed at characteristics of the profile such as complex shape, high dimensional accuracy and mechanical properties, the reduction zone length and devided flow taper of female die were designed by FEM and extrusion experiments were carried out for as-cast AZ31 magnesium alloy. Technological parameters were determined and microstructure and properties of the extruded profile were examined. The results show that magnesium alloy profile can be extruded if technological parameters such as deformation temperature, tool temperature and ratio of extrusion are controlled strictly. Each of the above mentioned technological parameters has its different influence on microstructure and properties of extruded profile. In comparison with as-cast alloys, microstructure is refined from 120～140μm to about 10μm and tensile strength is enhanced from 171～200MPa to above 260Mpa. This suggests that fine grain and high strength are attained for AZ31 magnesium alloy by extrusion deformation. The extruded profile can meet high properties demands for carrying parts. It provides a possibility for broader usage of magnesium alloy profiles.

Particle-Stimulated Nucleation of Dynamic Recrystallization in AZ31 Alloy at Elevated Temperatures

Particle-stimulated nucleation (PSN) was investigated in magnesium alloy AZ31 to study the effect of the evolution of second-phases during extrusion and other metal forming processes. Compression tests were carried out on samples taken from coarse-grained as-cast magnesium alloy billets containing a lamellar Mg17All2 eutectic phase and (Al, Mn) particles. These revealed that particle-stimulated DRX nucleation (PSN) was taking place during hot deformation and that this is facilitated by the fragmentation of the Mg17All2. When Mg17All2 dissolves into the matrix at about 350°C, the (Al, Mn) particles remain effective in producing PSN at temperatures up to at least 400°C. This suggests that alloy design leading to a suitable distribution of second-phase particles can improve the properties and formability of wrought magnesium alloys.

Modeling of hardening and softening processes in Mg alloys

The tensile deformation behavior of a range of supersaturated Mg–Al solid solutions and an as-cast magnesium alloy AM60 has been studied. The Mg–Al alloys were tested at room temperature while the alloy AM60 was tested in the temperature range 293–573 K. The differences in the deformation behavior of the alloys is discussed in terms of hardening and softening processes. In order to identify which processes were active, the stress dependence of the strain-hardening coefficient was assessed using Lukáč and Balı́k’s model of hardening and softening. The analysis indicates that hardening involves solid solution hardening and interaction with forest dislocations and non-dislocation obstacles such as second phase particles. Cross slip is not a significant recovery process in the temperature range 293–423 K. At temperatures between 473 and 523 K the analysis suggests that softening is controlled by cross slip and climb of dislocations. At temperatures above 523 K softening seems to be controlled by dynamic recrystallisation.

The effects of the distribution aspect of precipitate on the corrosion behavior of as-cast magnesium alloys

In the present study, the corrosion behavior of AZ91D as-cast alloy was investigated form the viewpoint of the distribution aspect of precipitate (Mg17Al12) and the variation of Al concentration in the Mg-rich matrix. The dendrite arm spacing (DAS) of an as-cast specimen was measured as a function of degree which describes the distribution aspect of the precipitate, and the salt spray test was conducted for various grain-sized specimens for 20 days. The dendrite arm spacing increased as the grain size increased to about 150 μm, but a constant value is indicated when the grain size exceeds that range. Although the relationship between the corrosion rate and grain size is of a nonlinear type, the linear trend between the corrosion rate and the dendrite arm spacing is maintained for the overall range of dendrite arm spacing. Since the precipitate in the as-cast alloy is discontinuously distributed, this linear relationship means that the variation of Al-solute concentration in the Mg-rich matrix has a more potent effect than the protective action of the precipitate on the corrosion behavior of an as-cast alloy.

AC Impedance Spectroscopy in Characterizing Time-Dependent Corrosion of AZ91 and AM50 Magnesium Alloys Characterization with Respect to Their Microstructures

The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the α-eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of in the magnesium oxide (or hydroxide) layer, a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance. This was attributed to an increase of the number of particles, accelerating the dissolution of eutectic areas. © 2001 The Electrochemical Society. All rights reserved.

Effects of chunk breakage and surface protective film on negative difference effect of magnesium alloys

In the present study, the factors having an affect on the difference effect of Mg-alloy, such as the role of monovalent ion, chunk breakage and surface protective film are discussed in detail. The degree of difference effect of as-cast magnesium alloy increases with Al content and applied external potential. The chunk breakage of precipitate was observed under open circuit potential, as well as applied external potential. Also, the precipitate which is disrupted as chunk type maintained its original morphology as that of as-cast alloy among the corrosion products after a corrosion test. It is thought that chunk breakage is a secondary factor bearing on the difference effect, which varies only the degree of the difference effect, Δ. The main cause for the difference effect of Mg-alloy which reveals a negative sign is due to the corrosion behavior of the surface protective film which is influenced on the Al content of the Mg-rich matrix, e.g., the repassivation tendency.