AZ Series Research Articles

Biodegradability of β-Mg17Al12 phase in simulated body fluid

In vitro degradation behaviour of β phase (Mg17Al12), commonly present in AZ series magnesium alloys, was studied using electrochemical techniques in simulated body fluid. The experimental results suggested that the degradation rate of β phase was significantly lower than that of pure magnesium. Scanning electron microscopy (SEM) analysis of the polarized β phase revealed localized corrosion.

Effects of the types of overlap on the mechanical properties of FSSW welded AZ series magnesium alloy joints

The effects of the types of overlap on the mechanical properties of the friction stir spot welding (FSSW) welded AZ series magnesium alloy joints were investigated by microstructural observations, microhardness tests, and tensile tests. The results show that the microstructure of the stir zone adjacent to the periphery of the rotating pin is mainly composed of the upper sheet. The average distance D between the longitudinal segment of the curved interface and the keyhole periphery, the tensile shear force, and the microhardness of the stir zone of the FSSW welded AZ61 alloy joint are the highest in all samples. During FSSW of AZ31 and AZ61 dissimilar magnesium alloys, the irregular deformation of the longitudinal segment of the curved interface appears, while the microhardness of the stir zone is higher when AZ61 alloy is the upper sheet. Moreover, the microhardness of the stir zone increases initially and then decreases sharply in the longitudinal test position.

An exponential material model for prediction of the flow curves of several AZ series magnesium alloys in tension and compression

This paper is concerned with flow behaviors of several magnesium alloys, such as AZ31, AZ80 and AZ81, in tension and compression. The experiments were performed at elevated temperatures and for various strain rates. In order to eliminate the effect of inhomogeneous deformation in tensile and compression tests, the Bridgeman’s and numerical correction factors were respectively employed. A two-section exponential mathematical model was also utilized for prediction of flow stresses of different magnesium alloys in tension and compression. Moreover, based on the compressive flow model proposed, the peak stress and the relevant true strain could be estimated. The true stress and strain of the necking point can also be predicted using the corresponding relations. It was found that the flow behaviors estimated by the exponential flow model were encouragingly in very good agreement with experimental findings.

The Evaporation Effect of Front Keyhole Wall in Penetration Welding with an Electron Beam

The process of vacuum electron beam welding is characterized by deep-penetration with the action of keyhole effect. The assumption of simple cylindrical physical model of keyhole is reasonable according to the thermal transfer of the keyhole effect during welding. There is an intensive evaporation arises from the front keyhole wall owing to the high energy density of electron beam. Therefore, an analysis model of heat transfer at the interface of vapor phase and front keyhole wall was proposed to the temperature calculation on the basis of heat transfer theory. The evaporation of the primary elements, which are Mg, Al, Zn and Mn in AZ series magnesium alloy, can be analyzed by the model, as well as the influence of keyhole radius varying on the temperature at vapor-solid interface offront keyhole wall. And dimensionless parameters are introduced to analyze the influence of the process condition on the thermal effect. The calculation results show that Mg and Zn are vulnerable to vaporize loss during the vacuum electron beam welding on AZ series magnesium alloy, and the evaporation of Mg occurs earlier than Zn. A longer electron beam acting duration and smaller keyhole size will increase the temperature of the front keyhole wall significantly, which has a considerable influence on the evaporation effect of the elements.

Hot working and crystallographic texture analysis of magnesium AZ alloys

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10−3 and 5 s−1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol−1 and Z values range from 10 to 10 s−1. Textural examinations show basal texture as the predominant orientation.

Electrochemical investigation of the influence of laser surface melting on the microstructure and corrosion behaviour of ZE41 magnesium alloy – An EIS based study

Surface melting of a magnesium alloy, ZE41 (4%-Zn, 1%-RE) was performed to achieve electrochemical homogeneity at the surface by microstructure refinement. Large secondary precipitates are particularly known to cause severe pitting in magnesium alloys. The corrosion resistance of the laser treated and untreated alloy was investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy. Contrary to the reported behaviour of other magnesium alloys (such as AZ series alloys), laser surface melting did not significantly improve the corrosion resistance of ZE41. This observation is attributed to the absence of beneficial alloying elements such as Al in ZE41 alloy.

Surface Heat Treatment of Magnesium Alloys by Plasma Electrolysis from Phosphate Electrolytic Solution

This study examined the possibility of surface heat treatment by the plasma electrolysis from phosphate electrolytic solution on magnesium alloys. Effects of the anodic plasma electrolysis onto various AZ series magnesium alloys on the mechanical properties and microstructure were examined.The tensile test revealed that the anodic electrolytic treatment at final bias voltage from 250 V to 400 V influenced the tensile strength. The tensile strength of AZ61 and AZ91D substrate after anodic electrolytic treatment increased or decreased, and this change of tensile strength is attributable to the precipitation of intermetallic compound (β phase, Mg17Al12) as sparks occurred due to dielectric breakdown during anodic electrolysis. These results demonstrate the utility of this electrolytic treatment on AZ61 and AZ91D magnesium alloys.

Application of X-ray microtomography to analysis of cavitation in AZ61 magnesium alloy during hot deformation

The efficiency of particles in acting as cavity formation sites during hot deformation was investigated for a fine-grained wrought magnesium–aluminium–zinc (AZ series) alloy using X-ray micro tomography. Two methodologies were developed to determine the particle/cavity association from 3-dimensional data, each clearly demonstrating that particles act as a major formation site for cavitation. The particles forming cavities were identified and characterised. It is shown that progressively smaller particles nucleate cavities as strain increases. This is due to concurrent grain growth which reduces the critical particle diameter for cavity nucleation during testing, leading to a continuous cavity nucleation. Particle agglomerates are shown to be particularly potent sites for cavity formation, leading to large and complex shaped cavities even if the individual particles within the agglomerate are below the critical particle diameter for cavity nucleation.

Microstructure evolution of AZ series magnesium alloys during cyclic extrusion compression

Electron Backscatter Diffraction (EBSD), transmission electron microscopy (TEM) and optical microscopy (OM) are employed to characterize the microstructure evolution of AZ31, AZ61 and AZ91 Mg alloys during cyclic extrusion compression (CEC). The results show that CEC is an efficient grain refinement method for magnesium alloys. The most effective CEC pass is the first pass. There exists a critical CEC pass for three Mg alloys and beyond this critical pass the grain size and the homogeneity of microstructure retains almost constant. With increasing CEC passes, the fraction of low angle grain boundaries (LAGBs) tends to decrease and the average misorientation angle increases. Typical network shape structure of fine grains is obtained in the microstructure of AZ31 Mg alloy with fewer Mg 17Al 12 particles after CEC 7 passes. The microstructure of the different Mg alloys at the same number of passes becomes more homogeneous as Mg 17Al 12 particles increase from AZ31, AZ61 to AZ91. The Mg 17Al 12 particles promote increases of the fraction of high angle grain boundaries (HAGBs), the dislocation density, the refinement process of coarse grains at initial CEC pass, and the texture randomization and delay the formation of network shape structures.

The effect of substrate composition on the electrochemical and mechanical properties of PEO coatings on Mg alloys

Abstract Plasma electrolytic oxidation (PEO) is a unique surface treatment technology which is based on anodic oxidation forming ceramic oxide coatings on the surface of light alloys such as Mg, Al and Ti. In the present study, PEO coatings prepared on AZ91D, AZ31B, AM60B and AM50B Mg alloys have been investigated. Surface morphology and elemental composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS). SEM results showed that the coating exhibited a porous top surface layer and a subsequent dense layer with micro-pores and shrinkage cracks. Phase analysis of coatings was carried out by X-ray diffraction (XRD). XRD analyses indicated that PEO coatings on AZ alloys had higher amount of Periclase (MgO) followed by the presence of Spinel (MgAl 2 O 4 ) e.g. on the AZ91D alloy compared to that on AM series alloys. In order to examine the effect of substrate composition on adhesion strength of PEO coating scratch tests were carried out. Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization technique in 3.5% NaCl solution at room temperature (20 ± 2 °C). Corrosion test results indicated that the corrosion rates of coated Mg alloys decreased by nearly two orders of magnitude as compared to bare Mg alloys. PEO coatings on AZ series alloys showed better corrosion resistance and higher adhesion properties than AM series alloys. In addition to the PEO processing parameters, such are mainly attributes of the compositional variations of the substrate alloys which are responsible for the formation, phase contents and structural properties of the PEO coatings.

High corrosion resistance of magnesium coated with hydroxyapatite directly synthesized in an aqueous solution

Anticorrosion coatings are crucial for practical applications of magnesium alloys, which are used to reduce the weight of vehicles, aircraft, electronics enclosures etc. Hydroxyapatite (HAp) potentially offers high corrosion resistance and no environmental toxicity because its thermodynamic structural stability is high and it is a basic component of bone. However, direct synthesis of HAp on magnesium in aqueous solutions has been a scientific challenge because Mg ions prevent HAp crystallization. A new method of direct synthesis of HAp on magnesium was developed using a Ca chelate compound, which can maintain a sufficiently high concentration of Ca ions on the magnesium surface to overcome prevention of HAp crystallization with Mg ions. Highly crystallized HAp coatings were successfully formed on pure magnesium and AZ series alloys. Corrosion behavior of HAp-coated pure magnesium was examined by cyclic dry and wet tests with 1 g m −2 NaCl on the surface and polarization tests in a 3.5 wt% NaCl solution. A HAp-coated pure magnesium showed no noticeable corrosion pits after the dry and wet test. HAp-coated specimens showed 10 3–10 4 times lower anodic current density than as-polished specimen in the polarization test. The results demonstrate the remarkable anticorrosion performance of HAp coatings on magnesium for the first time.

Wrought magnesium alloys for structural applications

Wrought magnesium alloys are of special interest as lightweight structural components as a result of their more homogeneous microstructures and improved mechanical properties compared to cast components. Extrusion as a shaping technology offers the possibility to produce a wide variety of magnesium alloy profiles. In this contribution, the authors describe the role of extrusion parameters such as extrusion rate, ratio and temperature as well as the type of extrusion process on the microstructure of AZ series magnesium alloys and the resulting mechanical properties. The effect of microstructure on the tensile/compression deformation behaviour of the alloys has also been investigated. In a further step, extruded material has been used in die forging experiments. Aspects of alloy development and process optimisation designed to overcome technical and economic limitations and thus establish magnesium alloy profiles for industrial applications will also be discussed.

Effect of Al and Mn additions on rolling and deformation behavior of AZ series magnesium alloys

Hot rolling and tensile deformation characteristics of six AZ series magnesium alloys containing 3, 4, and 6 wt% aluminum were studied at two different levels of manganese. It was found that increasing the Al content to 6 wt% resulted in edge cracking during hot rolling. On the other hand, addition of Mn to some extent compensated for the detrimental effect of Al. Tensile deformation at various strain rates and temperatures revealed that the AZ series alloys undergo different combinations of deformation mechanisms, from dislocation creep dominating at high strain rates to grain boundary sliding dominating at low strain rates. The Al content of the alloy did not appear to affect these mechanisms significantly, but Mn showed a noticeable effect. The effect of Al content on flow stress was evident at high strain rates, but was not at low strain rates. Analysis of elongation to fracture at two levels of Mn revealed that Mn improves ductility by increasing the extent of uniform elongation.

Optimal parameters for low and high voltage electron beam welding of AZ series magnesium alloys and mechanism of weld shape and pore formation

The dependence of ultimate tensile strength (UTS) on the parameters of low voltage and high voltage electron beam welding (LV- and HVEBW) has been analyed by changing one factor at a time (COFAAT) and by Taguchi's method for AZ series magnesium alloys. COFAAT is a conventional method for trend analysis of each parameter. However, its cost is high and a long time is needed, and interactions between parameters are ignored. On the other hand, Taguchi's method cannot only reduce the number of experiments, but also provide precise optimum parameters and predicted UTS even when a few data points are lost. According to the results of analysis of variance, the strength of a weldment is affected by oscillation of the beam, the focal position, the welding speed, the accelerating voltage, the beam current and stress relief, in order of impact. The general trend is for the optimum parameters to include a non-oscillating beam, focus at the bottom and no stress relief. With regard to welding speed, accelerating voltage and beam current, there are no specific trends. Finally, the differences in welds between LV- and HVEBW for AZ series forged alloys have been examined to study the mechanism of pore formation, which results from the dissolution of precipitates to reform molecular oxygen. It is also emphasised that the area and shape of the weld cross-section are related to the input power and welding speed.

AZ 系 Mg 合金の自然浸漬環境下における腐食特性に及ぼす Al 含有量の影響

Though Mg alloys are the outstanding candidates for the use in automotive, aerospace and electronic industries because of their low density and excellent strength to weight ratio, they have a great disadvantage in corrosion resistance. In this study, their corrosion behavior has been investigated by the polarization test and electrochemical impedance spectroscopy test for their practical use. AZ series Mg alloy samples produced by conventional ingot metallurgy showed a little slight decrease Ecorr with an increase of Al content, while they indicated a remarkable decrement of Icorr with increasing amount of Al content.

Effects of Alloying Elements on Characteristics of Anodic Oxidized Coatings in Various Mg-Al-Zn Series Alloys

Effects of the alloying element on processing of the environmental-friendly anodizing were investigated in various AZ series magnesium alloys. The phosphate solution was used mainly in the current process without the need for the deleterious materials such as heavy metals or fluoride. Also the characteristics of the formed coatings, such as their structure, composition, and corrosion resistance, were investigated. Anodized coatings became dense with increasing aluminum content of the alloy substrate. X-ray diffraction analysis revealed that the anodized coatings changed amorphous structure in the cases of the magnesium substrate containing aluminum concentration from 1 to 9 mass%. The anticorrosion performance of these coatings with an average thickness of 10 mm varied depending on the aluminum content, and it is concluded that the anticorrosion performance improved with increasing aluminum content except for the anodized AZ61 specimen. [doi:10.2320/matertrans.MC200729]

Solidification Curves for Commercial Mg Alloys Determined from Differential Scanning Calorimetry with Improved Heat-Transfer Modeling

An improved method is presented for determination of solidification curves, i.e., fraction solid vs temperature, for commercial Mg alloys using heat-transfer model (HTM) differential scanning calorimetry (DSC) curves. A better evaluation of the measured DSC heat flow signal is attained through an independent measurement of the time constant as a function of temperature for the applied equipment. A further improvement is achieved through a more impartial interpretation of the measured DSC curves. Both improvements enable a better desmearing of the DSC signal and reduce the error induced by the operator. Challenging Mg alloys could be appropriately handled by optimizing the tantalum encapsulation. Highly reproducible DSC-HTM solidification curves are determined for seven Mg alloys of the AZ and AM series. The results indicate a potential of this method for simple, fast, and reliable determination of solidification curves even for such very reactive alloys with high vapor pressure.

Anodizing from phosphate electrolyte onto various AZ series magnesium alloys and corrosion resistance

We studied the effects of alloying element on environmental-friendly anodizing onto various AZ series magnesium alloys. This process uses mainly phosphate solution without the need for deleterious materials such as heavy metals or fluoride. We also investigated the characteristics of the coatings formed, such as their structure, composition, and corrosion resistance. Anodized coatings became dense, as the aluminum content increased. X-ray diffraction analysis revealed that the anodized coatings onto magnesium substrate containing aluminum concentration from 1 to 9 mass% had a glass-like structure. The formation of this structure is explained in terms of the film composition and freezing point. In salt spray tests, the anticorrosive performance of these coatings having an average thickness of 10 μm was changed, depending on the aluminum content, and the anticorrosive performance was improved with the increase in aluminum content. The coating onto AZ91D magnesium alloy retained the anticorrosive performance over 1000 hours.

Acoustic emission during stress relaxation of pure magnesium and AZ magnesium alloys

Acoustic emission was monitored and analysed during tensile tests of pure Mg and Mg alloys of the AZ series in order to study the influence of alloy composition on plastic deformation. The Kaiser effect was used to determine the stability of the microstructure. During the tests at room temperature, repeated stress relaxations were performed. The solute–dislocation interaction in the alloys leads to ageing phenomenons, which manifest themselves in a post-relaxation effect and subsequent serrated flow (Portevin–Le Chatelier effect). The post-relaxation effect was sensitive to alloy composition and the strain at which the stress relaxation was performed.

A study of weldability and fracture modes in electron beam weldments of AZ series magnesium alloys

Given the growing need for environmental protection and lightweight construction, electron beam welding (EBW) is becoming the most important welding technology because it can compensate for the poor formability of magnesium alloys. This paper examines interactions between the properties of three AZ series magnesium alloys and welding conditions. The EBW process can yield four kinds of defect in a weld: cavities, the heat-affected zone (HAZ), undercuts, and root concavities. These defects obviously induce stress concentrations in the weld, and may seriously damage its strength. Additionally, the distribution of precipitates (γ phase, Mg 17Al 12) in the fusion zone (FZ) changes from a relatively small number of scattered particles to a dense population of dendrites as the Al content of the magnesium alloy increases. Under excessive tensile stress, alloy weldments break in one of two fracture modes: an irregular FZ fracture, or a regular HAZ fracture. AZ31B usually exhibits the former mode and AZ91D the latter, while AZ61A exhibits each mode half the time. The overall weldability, which depends on the random distribution of these precipitates and defects, is found to be greatest for the AZ61A alloy. The best process window, on the other hand, is found for the AZ91D alloy. Finally, we obtain optimum parameters for the EBW process and empirical formulae for the weldment strength as a function of these parameters. These results are closely related to each other.