Commercial AZ31 Research Articles

In vitro biodegradation behavior of magnesium and magnesium alloy

Magnesium has the potential to be used as degradable metallic biomaterial. For magnesium and its alloys to be used as biodegradable implant materials, their degradation rates should be consistent with the rate of healing of the affected tissue, and the release of the degradation products should be within the body's acceptable absorption levels. Conventional magnesium degrades rapidly, which is undesirable. In this study, biodegradation behaviors of high purity magnesium and commercial purity magnesium alloy AZ31 in both static and dynamic Hank's solution are systematically investigated. The in vitro test results show that magnesium purification and selective alloying are effective approaches to reduce the degradation rate of magnesium. In the static condition, the corrosion products accumulate on the materials surface as a protective layer, which results in a lower degradation rate than the dynamic condition. Anodized coating can significantly further reduce the degradation rate of magnesium. This study strongly indicates that magnesium can be used as degradable implant material as long as the degradation is controlled at a low rate. Magnesium purification, selective alloying, and anodized coating are three effective approaches to reduce the rate of degradation.

Effects of Mixed RE on Die-Casting Microstructures and Properties of Commercial Mg Alloy

In this paper, the commercial AZ91 Mg alloy with mixed rare earth (RE) Y、Gd and Nd has been investigated and effects of RE on die-casting microstructure and properties of die-casting Mg alloy has been also explored. The results indicate that RE can effectively refine the grain size of the alloy and improve inner microstructures transforming the reticular structure of Mg17Al12phrase into granular or dotlike structure. At the same time, the amount of the compound at crystal boundary is less than that in the alloy without RE. In addition, the mechanical properties of AZ91 Mg alloy with mixed RE has been improved efficiently, and its tensile strength sbcan reach over 260MPa; its elongation δ can also reach over 4.5％. When the amount of mixed RE exceeds 5%, it will form coarse Al-RE compounds, which can low the mechanical properties of Mg alloy.

Improving the sheared edge in the blanking of commercial AZ31 sheet through texture modification

Commercial rolled magnesium sheets of alloys AZ31 (Mg–3 mass%Al–1 mass%Zn) and ZE10 (Mg–1 mass% Zn–<1 mass% Rare Earths) in O-temper condition were used for blanking experiments near room temperature. A serrated fracture surface can be observed in case of AZ31 but not in case of ZE10. During the shearing process of the AZ31 sheet, many micro cracks parallel to the sheet plane are generated in the shearing zone. These micro cracks lead to the formation of loose particles during the shearing operation, which interfere with further processing of the part and incur additional costs by increasing the scrap rate. It is found that the strong basal texture of this alloy is an important reason for the generation of such serrated cracks. In this paper a new method of selective texture modification is described to locally change the mechanical properties of the AZ31 sheet. Subsequent shearing experiments show a significant change in the material behavior, especially regarding the direction of crack propagation, which leads to a better shearing performance. The commonly observed serrated crack does not occur any more after this local treatment and the sheared edge is clearly improved.

Discharge behavior of Mg–4 wt%Ga–2 wt%Hg alloy as anode for seawater activated battery

Magnesium–gallium–mercury alloy is one of the new developed anode materials for seawater activated batteries. The potentiodynamic polarization, galvanostatic discharge and electrochemical impedance spectroscopy of Mg–4%Ga–2%Hg alloy in seawater are studied and compared with commercial AZ31 and AP65 alloys in this study. The results show that Mg–4%Ga–2%Hg alloy exhibits different discharge behaviors in as-cast, homogenizing, rolling and annealing conditions. The annealing Mg–4%Ga–2%Hg sheet obtains the most negative corrosion potentials in different current densities. And the Mg–4%Ga–2%Hg alloy provides more negative corrosion potentials than AZ31 and AP65 alloys. EIS studies reveal that the Mg–Ga–Hg alloy/seawater interfacial process is determined by an activation-controlled reaction. The Mg 3Hg and Mg 21Ga 5Hg 3 phases improve the electrochemical properties of Mg–4%Ga–2%Hg alloy. The assembled prototype battery with Mg–4%Ga–2%Hg alloy as anode and CuCl as cathodes exhibits a satisfactory discharge performance because of the advantages in discharge characterizations and microstructure of the Mg–4%Ga–2%Hg alloy.

Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes

A severe plastic deformation (SPD) technique based on tubular channel angular pressing (TCAP) is proposed suitable for deforming cylindrical tubes to extremely large strains without changing their dimensions. The tube constrained by inner and outer dies is pressed by a hollow cylindrical punch into a tubular angular channel with three shear zones. This technique was applied to a commercial AZ91 magnesium alloy and a significant grain refinement was achieved even after single cycle TCAP. Microhardness of the tube increased to 78 Hv from an initial value of 51 Hv. This new SPD process is promising for future industrial applications.

Solidification Microstructure and Mechanical Properties of Hot Rolled and Annealed Mg Sheet Produced through Twin Roll Casting Route

The use of wrought magnesium for automobile structural components is an important component of the mass reduction strategy for automobiles to improve their fuel efficiency. Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31 magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions. Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and mechanical properties were comparable with commercial AZ31 sheet.

Effect of the chemistry and structure of the native oxide surface film on the corrosion properties of commercial AZ31 and AZ61 alloys

The purpose of this study has been to advance in knowledge of the chemical composition, structure and thickness of the thin native oxide film formed spontaneously in contact with the laboratory atmosphere on the surface of freshly polished commercial AZ31 and AZ61 alloys with a view to furthering the understanding of protection mechanisms. For comparative purposes, and to more fully describe the behaviour of the native oxide film, the external oxide films formed as a result of the manufacturing process (as-received condition) have been characterised. The technique applied in this research to study the thin oxide films (thickness of just a few nanometres) present on the surface of the alloys has basically been XPS (X-ray photoelectron spectroscopy) in combination with ion sputtering. Corrosion properties of the alloys were studied in 0.6 M NaCl by measuring charge transfer resistance values, which are deduced from EIS (electrochemical impedance spectroscopy) measurements after 1 h of exposure. Alloy AZ61 generally showed better corrosion resistance than AZ31, and the freshly polished alloys showed better corrosion resistance than the alloys in as-received condition. This is attributed to a combination of (1) higher thickness of the native oxide film on the AZ61 alloy and (2) greater uniformity of the oxide film in the polished condition. The formation of an additional oxide layer composed by a mixture of spinel (MgAl 2O 4) and MgO seems to diminish the protective properties of the passive layer on the surface of the alloys in as-received condition.

Negative lateral strain ratio induced by deformation twinning in magnesium alloy AZ31

We show that any highly textured metal that deforms predominantly by deformation twinning can exhibit a negative lateral strain ratio under uni-axial loading. Theoretical calculations of lateral strains caused by tension twinning on the { 1 0 1 ¯ 2 } plane in a magnesium single crystal predicts this behavior, and we verify this by direct measurements of lateral strains in highly textured commercial magnesium alloy AZ31 deformed in uniaxial compression. The R values of AZ31 plate compressed in the rolling direction at −100 °C and 25 °C show negative values at compressive strain up to ∼0.03, and increase with strain and temperature. These evolution trends of R value are found be closely related to activity of { 1 0 1 ¯ 2 } twinning. This approach is extended to compression twinning in zinc, for which theoretical calculations show the same effect on lateral strain ratio.

Influence of strain rate on the characteristics of a magnesium alloy processed by high-pressure torsion

A commercial magnesium AZ31 alloy was processed by high-pressure torsion (HPT) through 5 turns at 453 K using three different rotation rates. The results show that HPT introduces significant grain refinement to an average grain size of ∼0.5 μm. The processed structures were heterogeneous in the centers of the disks and homogeneous at distances at and above ∼1 mm from the disk centers. The measured values of the microhardness were also reasonably homogeneous after HPT except only near the disk centers. The hardness values and the average grain sizes were correlated with the local strain rates by using the imposed rotation rates and estimates of the strains within the disks. The results demonstrate the hardness and the average grain size both exhibit a small dependence on the local strain rate.

Influence of extrusion parameters on grain size and texture distributions of AZ31 alloy

Grain size and texture distributions have great influences on the mechanical properties of extruded rods. In order to study grain size and texture evolution during the hot extrusion process, direct extrusion tests were carried out with a variety of extrusion parameters (extrusion ratio, temperature and velocity) for commercial as-cast AZ31 magnesium alloys. Extruded specimens were investigated by optical microscopy (OM) and electron backscattered diffraction (EBSD). Experimental results show that extrusion ratio is the most important parameter for grain size refinement. Basal fiber textures with various (0 0 0 2) pole intensities are observed in extruded rods. Maximum intensities increase with the decreasing extrusion ratio and the increasing velocity, while the influence of temperature depends on the value of extrusion ratio and velocity.

Crystallographic Orientation Relationship between Discontinuous Precipitates and Matrix in Commercial AZ91 Mg Alloy

Crystallographic Orientation Relationship between Discontinuous Precipitates and Matrix in Commercial AZ91 Mg Alloy

Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming

Cavitation behaviors of fine grained AZ31 alloy sheet during high temperature blow forming have been investigated in terms of deformation and microstructure evolution. Especially, effect of hydrostatic stress by applying backward pressure on blow formability has been investigated by examining cavitation behavior. A commercial grade AZ31 magnesium alloy sheet with the grain size of 15μm could have an elongation over 300% over 400°C. However, elongation did not show a strong strain rate and temperature dependencies which could be easily found in most superplastic aluminum alloys. It might result from that AZ31 would be vulnerable to cavitation even at the early stage of deformation. It was also found that hydrostatic stress was not effective to prevent evolution of cavities during blow forming, which was much different with the case in most superplastic alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature where non basal slip systems were activated.

Native Air-Formed Oxide Film and its Effect on Magnesium Alloys Corrosion

The present work uses X-ray photoelectron spectroscopy (XPS) analysis to compare the chemical composition of native oxide films formed spontaneously on commercial pure magnesium and AZ31, AZ80 and AZ91D magnesium alloys. The study considers both the outer surface and inner regions of the films with the assistance of argon ion bombardment. Possible relationships are established between the alloy Al content and the native oxide film characteristics. The Al content is very similar in the oxide films on all three studied alloys. XPS identifies a much greater film thickness on AZ80 and AZ91D specimens than on AZ31 and pure Mg specimens, which seems to be related with the presence ofphase (Mg17Al12) on the AZ91D alloy surface and of the eutectic � -Mg/� on the AZ80 alloy surface. Considerable Ca segregation is observed (directly related with the calcium impurities content in the bulk material) towards the outer surface of the metal, where it appears in the form of calcium oxide. Direct correspondence is found between the thickness of the native oxide film formed spontaneously on the surface of magnesium and its alloys and their subsequent corrosion resistance in exposure to a 3.5 wt% NaCl solution.

Stress-corrosion cracking susceptibility of AZ31 alloy after varied heat-treatment in 3.5 wt.% NaCl solution

Abstract The stress corrosion cracking behavior of a commercial AZ31 magnesium alloy after T5 and T6 heat treatments under a vacuum of 10−3 torr has been evaluated via slow strain rate tensile tests in an aqueous 3.5 wt.% NaCl solution. The experimental results show that this material displays a high sensitivity to stress corrosion cracking. The fractured surface was consistent with a significant transgranular component. Double-line cracking that resulted from twinning could be readily spotted on the cleavage plane. The presence of such deformation twins accompanied by the formation of voids is the main reason for the stress corrosion cracking susceptibility of this alloy. The disappearance of twins after artificial aging at 260 °C for 4 h significantly improves the corrosion resistance, increases the breakdown potential difference ΔΦ (thus lowering the pitting tendency), and decreases the corrosion current density in a 3.5 wt.% NaCl solution.

Comparison of the corrosion behaviour of bulk and thin film magnesium alloys

The corrosion behaviour of bulk and thin film specimens consisting of commercial magnesium alloys AZ31 and AZ80 was investigated by electrochemical tests in 3.5 wt.% NaCl solution and in phosphate buffered saline solution (PBS) at room temperature. Corrosion tests indicated that the corrosion rates are nearly equal, but localized attack was more pronounced for the bulk specimens than for the thin film samples. XRD analyses showed that this behaviour can be attributed to the fact that the as-deposited thin film specimens do not contain any precipitates, which are present in the bulk samples and are considered to enhance localized corrosion.

Corrosion protection of different environmentally friendly coatings on powder metallurgy magnesium

Pure magnesium was processed by a powder metallurgy (PM) route to generate microstructural features that provide higher mechanical properties than those of cast pure magnesium and commercial AZ31 alloy. Nevertheless, corrosion resistance of PM Mg needs to be improved if this material is to be used for structural applications in a corrosive medium. In the present work, the corrosion protection effectiveness of three simple, economical and environmentally friendly coatings has been evaluated over immersion time in a chloride-containing solution. A silane coating, an anticorrosive paint formulated with ion-exchangeable pigments (IEPs) and a chemical conversion treatment to form a MgF 2 layer have been studied. Silane film and anticorrosive paint enhance the corrosion behaviour of PM Mg during the first hours of immersion, but their protection effectiveness completely disappears after 2 days. For longer immersion times, the fluoride conversion coating may be considered the only viable and effective barrier to protect PM magnesium from degradation.

Orientation dependent slip and twinning during compression and tension of strongly textured magnesium AZ31 alloy

Over recent years there have been a remarkable number of studies dealing with compression of magnesium. A literature search, however, shows a noticeably less number of papers concerned with tension and a very few papers comparing both modes, systematically, in one study. The current investigation reports the anisotropic deformation behavior and concomitant texture and microstructure evolution investigated in uniaxial tension and compression tests in two sample directions performed on an extruded commercial magnesium alloy AZ31 at different Z conditions. For specimens with the loading direction parallel to the extrusion axis, the tension–compression strength anisotropy was pronounced at high Z conditions. Loading at 45° from the extrusion axis yielded a tension–compression strength behavior that was close to isotropic. During tensile loading along the extrusion direction the extrusion texture resists twinning and favors prismatic slip (contrary to compression). This renders the shape change maximum in the basal plane and equal to zero along the c-axis, which resulted in the orientation of individual grains remaining virtually intact during all tension tests at different Z conditions. For the other investigated sample direction, straining was accommodated along the c-axis, which was associated with a lattice rotation, and thus, a change of crystal orientation. Uniaxial compression at a low Z condition (400 °C/10 −4 s −1) yielded a desired texture degeneration, which was explained on the basis of a more homogeneous partitioning of slip systems that reduces anisotropy and enhanced dynamic recrystallization (DRX), which counteracts the strong deformation texture. The critical strains for the nucleation of DRX in tensiled specimens at the highest investigated Z condition (200 °C/10 −2 s −1) were found to range between 4% and 5.6%.

Improvement of Magnesium Sheet Formability by Alloying Addition of Rare Earth Elements

The influences of rare earth elements addition on the crystallographic texture and microstructural evolutions are examined during rolling and annealing of Mg-sheets. In case of Nd or Y additions, dynamic recrystallisation is suppressed such that the deformed microstructure is observed after hot rolling with relatively large strain per pass. Cold rolled binary Mg-Nd alloy sheet shows strong texture with splitting of the basal poles in the rolling direction, however, the texture intensity decreases significantly during the recrystallisation annealing. From the comparison of deep drawing behaviours between commercial ZE10 and AZ31 sheets, it is observed that the addition of the rare earth elements and accompanying texture changes result in the improved formability.

Surface Protection of Magnesium Alloys via Pack Cementation Coatings with Aluminum Powder and Chlorides

Magnesium alloys have been received an attention for structural applications due to their low density compared to other alloys, and intensive studies have been focused for enhancing mechanical strength and surface protection as well. Especially, for environmental reasonings, coating processes in a dry condition have been recently received a great attention. In this study, diffusion coatings via Al powders with an aluminum chloride activator have been investigated in order to examine surface protection effect on magnesium alloys. The commercial AZ31 magnesium alloy has been subjected to diffusion coatings in an Al alloy powder for various time frames. An intermediate layer of Mg17Ag12 was successfully synthesized via diffusion annealing. The underlying mechanisms for surface layer formation are discussed together with growth kinetics and microstructural observations

Mg alloy for hydrogen storage processed by SPD

AbstractMg-based nanocomposites are promising candidates for hydrogen storage applications exhibiting fast H-sorption kinetics at reasonably low temperatures when processed by high-energy ball milling techniques. However, since compaction of the highly reactive nanometric powder is desirable before application, the search for other effective processing routes for the preparation of Mg-based nanocomposites is relevant. In this work, we have used a combination of equal channel angular pressing, cold rolling and high-energy ball milling in the processing of the commercial AZ31 extruded alloy to evaluate its use as a hydrogen storage material. Severe plastic deformation carried out at different temperatures, combined with further mechanical processing resulted in a controlled texture and signifiant grain refinement, which are desirable microstructural characteristics for hydrogen storage applications.