Papers by Keyword: AZ31

Abstract: This work briefly discusses the applications of Layered Double Hydroxides (LDHs) to medicine and presents a study regarding the growth of LDHs on the biodegradable AZ31 alloy foreseen to manufacture a rib-fixator. Mg is one of the most investigated metallic materials for biomedical applications owing to its high biocompatibility and osteointegration, as well as a value of the elastic modulus close to that of human bone. Since Mg is essential for metabolism, when it degrades forming Mg²⁺ ions, it promotes healing and growth of bone tissue. Experiments have been carried out to grow LDHs on the alloy surface in view to retard corrosion in human body and intercalate drugs to be released in-situ, with anti-inflammatory, analgesic, and antimicrobial action.

Abstract: Sometimes, in surgical procedure following an accident or illness it is necessary to use metal prostheses or implants to ensure the functionality of bones and joint systems. From time to time, at the end of the patient's healing process, it is necessary to remove the medical device used. In these cases, it would be useful to use resorbable devices to avoid further surgery. A possible solution to the problem could be to use metal alloy devices that degrade over time, while ensuring the functionality of the system. Unfortunately, the chemical compounds generated by the corrosion processes of metal alloys used in the medical field are almost always harmful to human health. The products generated by the degradation processes of some magnesium alloys, on the other hand, are not considered toxic or harmful to human health, so that a device showing controllable degradation rate can be used, guaranteeing the functionality of the implanted device. To achieve this goal, a possible solution could include the use of one or more coatings, capable of controlling the metal degradation process. To do this, we used a first coating obtained by subjecting the samples to a direct current (DC) plasma electrolytic oxidation (PEO) treatment, carried out in an alkaline solution based on silicates and sodium. Subsequently, the samples were coated with a polydopamine (PDOPA) film by dip-coating, and, at the end, a l-polylactic acid (PLLA) coating was applied on the sample by hot-pressing. The coupons were subjected to morphological characterization by Scanning Electron Spectroscopy (SEM) and to electrochemical characterization in Hank's solution at 37°C by means of electrochemical impedance spectroscopy (EIS). The experimental results obtained demonstrate that the coupling of the PEO oxide with the polydopamine and the polymeric film show properties such as to allow the creation of devices which permit the control of the metal degradation process.

Abstract: Magnesium alloys are highly desirable for weight critical applications owing to their high weight to strangth ratio. However, their poor formability at room temperature limits their widespread use in industrial applications. In this study, we invstigate the hot deformation behaviour of AZ31 and AZ31-0.7% Ca magnesium alloys and explore their microstructural and thermal properties. Our findings reveal that dynamic recrystallization during hot deformation leads to successful grain refinement in the AZ31 alloy, resulting in a normal grain size distribution. In contrast, the AZ31-0.7% Ca alloy shows bimodal grain size distribution due to the addition of calcium. Additionally, the number and size of β-Mg₁₇Al₁₂ particles were found to increase with the addition of a small amount of calcium. These particles are responsible for the discontinuous precipitation phenomenon, which strongly influences microstructural changes during hot rolling. Our study provides valuable insights into the dynamic recrystallization and discontinuous precipitation phenomena of magnesium alloys, which can aid in the development of novel alloys with improved formability and mechanical properties.

Abstract: It is known that metallic materials are characterized by anisotropy of their mechanical properties, with this being attributed to the conditions during the manufacturing process. For sheet metals, this anisotropy occurs symmetrically to the three orthogonal axes of the rolling, transverse and normal direction. This characteristic is referred to as orthotropic behaviour and manifests itself, for example, in earing during cupping tests. Therefore, orthotropic yield criteria are highly relevant for the numerical simulation of sheet metal forming processes. The Lankford coefficient, also known as the r-value, is a good experimental measure for characterizing orthotropic ductile behaviour of sheets, and can easily aid in parameter identification for yield criteria such as the Hill approaches. In the present investigations, Lankford coefficients were determined as a function of local strain in uniaxial tensile tests through high-resolution digital image correlation. The sample direction was varied between 0°, 45° and 90° to the rolling direction and the test temperature varied from RT to 350 °C at three different strain rates (0.01-1 s^-1). By means of a novel backward analysis, the measuring range for the Lankford coefficients was positioned exactly in the necking area. An increase in temperatures showed a decrease in the initial Lankford coefficient. The results showed non-constant Lankford coefficients and commence the course of a natural exponential function depending on the local strain. Regardless of strain rate, the results revealed that the Lankford coefficients (r-values) at 150 °C, 250 °C and 350 °C approaches a steady-state of r = 1.14 with strains greater than 50 %.

Abstract: Magnesium alloys are important engineering materials due to their good combination of strength and very low densities. However, the low ductility imposed by the hcp-lattice has thus far limited the application of magnesium alloys as sheet material. The use of the electroplastic effect offers a route to increase formability of magnesium alloys while being more energy efficient than conventional hot forming. The underlying mechanism (s) of this effect have not yet been fully understood. This study investigates the impact of high current density electrical pulses on magnesium alloys. Special consideration was given to the effect of the orientation of the applied electric current relative to the mechanical loading of the specimens. The results show that the mechanical properties of coarse-grained materials are more strongly affected by the current pulses than finer grained material. Applying the current parallel to the compressive load shows a more pronounced softening of the material than pulses applied perpendicular to the mechanical stress. Microstructure investigations revealed the formation of twinning solely in the interior of grains even at stresses below the yield point for both configurations.

Abstract: Since 2018, the institute of metal forming has been studying the novel twin-roll casting (TRC) of magnesium wire at the pilot research plant set up specifically for this purpose. Light microscopic and scanning electronic investigations were carried out within this work and show the unique microstructure of twin-roll cast AZ31 magnesium alloy with grain sizes of about 10 μm ± 4 μm in centre and 39 μm ± 26 μm near the surface of the sample. By means of a short heat treatment (460 °C/15 min), segregations can be dissolved and grain size changes in centre to 19 μm ± 12 μm (increase) and near the surface to 12 μm ± 7 μm (decrease). Further, the mechanical properties of the twin-roll cast and heat-treated wire were analysed by tensile testing at room temperature. By heat treatment, the total elongation could be increased by a third whereas the strength decreases slightly. In heat-treated state, no preferred orientation is evident. In addition to the twin-roll cast and the heat-treated condition, the rolled state was analysed. For this purpose, the twin-roll cast wire was hot rolled using an oval-square calibration. After hot rolling, a dynamic recrystallization and grain refinement of the twin-roll cast wire could be achieved. It can be seen, that an increase in strength as well as in total elongation occur after wire rolling. Beside this, a rolling texture is evident.

Abstract: The paper investigates deformations and plastic properties received from different material volumes and tests of magnesium samples. Small volume characteristics gained on single Mg crystals are compared to polycrystalline AZ31 alloy. Results of tests employing nanoindentation, focused ion beam milling and electron backscatter diffraction techniques are presented. Large differences were found between micro-beam testing and spherical indentation tests having the volume one order of magnitude apart. The plastic strength scaling factor was found 1.7 for the studied grain configurations and volumes.

Abstract: In order to study the plastic deformation mechanism of AZ31 magnesium alloy, in situ texture measurement during uniaxial tensile deformation is conducted by using neutron diffraction. The specimen is prepared from a rolled sheet so that the deformation axis is parallel to the rolling direction. By increasing strain, the alignment of <10-10> along the tensile axis is strengthened, which is due to the activation of the prism slip system. The basal pole concentration at the prior sheet normal direction is slightly decreased by the deformation and the new texture component is formed at the transvers direction. This can be understood by activation of the {10-12} tension twinning. These results indicate that the tension twinning plays an important role even when the tensile deformation is applied parallel to the basal plane.

Abstract: Due to their advantageous strength-weight ratio, good recyclability, excellent noise and vibration damping properties as well as excellent dent resistance, magnesium materials offer a variety of uses in modern lightweight structures. As a result, a comeback of magnesium alloys is to be seen in multiple industrial applications, especially in the automotive industry, as the potential for lightweight, cost-effective lightweight industrial construction associated with the lightest metallic engineering material helps to meet even stricter energy and environmental efficiency guidelines. Traditionally, casting processes have been the dominant magnesium manufacturing processes, but in the last decade, an increasing number of lightweight structural applications have been implemented with magnesium wrought alloys. In this paper, the current state of developments focusing the twin-roll casting (TRC) of semi-finished products of magnesium alloys will be presented. By reducing the number of process steps in combination with a unique microstructure, the near-net-shape TRC process offers new options to provide magnesium materials meeting current and future requirements for semi-finished products, constructions and moldings in terms of type, quantity and quality.

Abstract: Low Al single-phase magnesium alloy surfaces with dense magnesium oxide films were uniformly formed. The films were deposited with a radio frequency magnetron sputtering process with a planar magnetron sputtering system. The thickness of deposited magnesium oxide thin films was around 240 nm. According to the XRD results, a magnesium oxide phase film was formed on the substrate. The surface was uniform, and no cracks or exfoliation were observed. The deposited magnesium oxide film did not have any cracks or pores, and the surface of the sample was covered by magnesium oxide. The hardness of the magnesium oxide-coated magnesium alloy reached around Hv200, while that of the uncoated Mg-alloy was around Hv80. Moreover, the Vickers indenter under a 10-mN load indented the magnesium alloy substrate coated with the magnesium oxide film to a depth of around 640 nm, while that for the uncoated magnesium alloy substrate was around 620 nm. Meanwhile, the elasticity value for the magnesium alloy substrate coated with magnesium oxide film was around 5.3×10¹⁰ Pa, while that of the uncoated magnesium alloy substrate was around 4.2×10¹⁰ Pa.