AZ91D Magnesium Alloy Research Articles

Effect of Y2O3 Addition on Microstructure and Properties of Laser Cladded Al-Si Coatings on AZ91D Magnesium Alloy

The effect of Y2O3 addition on the microstructure and properties of the laser cladded Al-Si alloy coating on the surface of AZ91D magnesium alloy was investigated in this study. The experimental results showed that the Al-Si + Y2O3 cladding layers contained α-Mg, Mg2Si, Al4MgY and a small amount of Al12Mg17 phases. The coarse dendrites, reticulated eutectic structures and massive phases in the coatings tended to be refined and gradually uniformly distributed with the increased amount of Y2O3. The introduction of Y2O3 into the cladding layer favored the improvement of microhardness and wear resistance due to the grain refinement strengthening and dispersion strengthening. The addition of Y2O3 also promoted the reduction of localized corrosion sites and made the corrosion surface smoother, implying that the corrosion resistance of the Y2O3-modified coatings was better than that of the unmodified cladding layer.

Microstructure and mechanical properties of GTA-based wire arc additive manufactured AZ91D magnesium alloy

Wire arc additive manufacturing offers advantages in producing large metal structures. The current research on GTA-based wire arc additive manufacturing (GTA-WAAM) of magnesium alloys is focused on deformed magnesium alloys, mainly on the Mg-Al alloy system. However, there is little research on GTA-WAAM for casting magnesium alloy. This study investigates the microstructural characteristics and mechanical properties of AZ91D magnesium alloy (AZ91D-Mg) deposited by GTA-WAAM. Single-pass multilayer thin-walled components were successfully fabricated. The results show that equiaxed grains dominate the microstructure of the deposited samples. During the remelting process, the precipitated phases dissolve into the matrix, and they precipitate and grow from the matrix under the thermal effect of the subsequent thermal cycle. The mechanical properties in the vertical and horizontal directions are similar, showing higher overall mechanical properties than the casting parts. The average yield strength is 110.5 MPa, the ultimate tensile strength is 243.6 MPa, and the elongation is 11.7%. The overall hardness distribution in the deposited sample is relatively uniform, and the average microhardness is 59.6 HV0.2.

Grain Refinement, Texture Evolution, and Tensile Performance Improvement of AZ91D Magnesium Alloy Processed by Multi‐Cycle Accumulative Back Extrusion

Accumulative back extrusion (ABE) process is performed on an AZ91D Mg alloy at 310 °C. Microstructure, texture, and mechanical properties affected by the ABE cycles are elucidated in detail. Results reveal that the initial as‐cast grain structure (≈190 μm) is significantly refined (≈2.98 μm) after being imposed by six ABE cycles. The β‐Mg17Al12 phases undergo fragmentation and spheroidization. The dynamic recrystallization (DRX) process is accelerated through the particle stimulated nucleation (PSN) mechanism during initial deformation. Meanwhile, twins are inclined to occur in barren areas of the second phases and play a role of twinning‐induced dynamic recrystallization (TDRX). Grain refinement is governed by the combined function of PSN, TDRX, continuous dynamic recrystallization (CDRX), and discontinuous dynamic recrystallization (DDRX). Basal textures with low intensities are developed due to the fully developed recrystallized grains. Schmid factor analysis indicates that the {0001} <11–20> basal slip and the {11–22} <11–23> pyramidal slip contribute to ABE deformation. Finally, excellent ultimate tensile strength and fracture elongation are obtained after a three‐cycle ABE process, which are estimated to be 296 MPa and 21.43%, respectively. The resulting tensile performance is attributed to the refined grains, fragmented β‐Mg17Al12 particles, and the weakened basal texture.

Effect of Phosphate Anodizing on Adhesive Property of AZ91D Magnesium Alloy under Constant Temperature and Humidity

Effects of constant temperature and humidity testing on the adhesive properties of phosphate-anodized AZ91D magnesium alloy were evaluated to produce multiple materials for lightening the fabrication of these materials. Phosphate-anodized AZ91D magnesium alloy, after immersion in dilute nitric acid solution, maintained strong adhesive strength even after constant temperature and humidity testing. These findings suggest that the anodization film is bonded chemically to the adhesive by the dilute nitric acid solution treatment.

SmCl3’s effect on plasma electrolytic oxidation coating’s corrosion characteristics on AZ91D

Rare earth salt SmCl3 was added to the electrolyte to improve the corrosion performance of the AZ91D magnesium alloy plasma electrolytic oxidation (PEO) coating. The influence of SmCl3 on coating formation, morphology and performance was investigated. The results showed that adding SmCl3 can not only refine the discharge holes but also improve the coating’s formation efficiency and its’ combination with the substrate. Potentiodynamic polarisation (PDS) and electrochemical impedance spectroscopy (EIS) tests showed that the corrosion characteristics of the PEO coating were greatly improved with SmCl3’s addition. The corrosion current density decreased from 8.208 × 10−4 to 3.733 × 10−8 A cm−2 when the presence of optimum level of SmCl3 in solution (0.25 g L−1). Cyclic polarisation test indicated that the pitting corrosion of AZ92D was improved.

Properties of AZ91D magnesium alloy Anodized in Oxalic and Sulfuric Acids and its corrosion resistance in 3.5% NaCl solution

Properties of AZ91D magnesium alloy Anodized in Oxalic and Sulfuric Acids and its corrosion resistance in 3.5% NaCl solution

Effect of Polyaniline and Graphene Oxide Composite Powders on the Protective Performance of Epoxy Coatings on Magnesium Alloy Surfaces

Composite fillers are often used to improve the protective properties of coatings. To obtain a high protective performance of epoxy coatings for magnesium alloys, polyaniline (PANI) and graphene oxide (GO) composite powders were selected because of their corrosion inhibition and barrier performance, respectively. The paper mainly focuses on the effect of the preparation methods of the composite powders on the protective performance. PANI and GO composite powders were prepared by in situ polymerization and blending, respectively. First, the composite powder was characterized by X-ray diffractometer, Fourier transform infrared spectroscopy, and scanning electron microscopy. Then, the different composite powders and pure PANI powder were dispersed uniformly in epoxy resin, and the coating was prepared on the surface of the AZ91D magnesium alloy and studied by an electrochemical impedance test, adhesion strength test and physical properties test. The results show that the impedance value of the coating with the added PANI and GO composite powders by in situ polymerization was 4 × 109 Ω·cm2 and higher than that with the added pure PANI (4 × 109 Ω·cm2) and PANI and GO mixed powders (1 × 109 Ω·cm2) after 2400 h immersion in a 3.5% NaCl solution; the former also had better flexibility, ss impact resistance, and adhesion strength. Compared with the direct blending method, the PANI and GO polymerization powders can exert the shielding effect of GO and PANI corrosion inhibition better and achieve a better protective effect on the magnesium alloy.

Effect of Nd2O3 on microstructure, corrosion and wear properties of laser cladding Zr-based amorphous composite coatings on AZ91D magnesium alloy

The preparation of amorphous matrix composite coatings by laser cladding is a applicable approach to improve the surface properties of Mg and Mg alloys. Rare earth elements significantly affect the corrosion and wear resistance of the coatings by influencing the quantity, morphology and distribution of crystalline phases. In the presented work, Zr55Cu30Al10Ni5 + x wt% Nd2O3 (x = 0, 0.5, 1, 1.5, 2, 3) composite coatings were prepared by laser cladding on the AZ91D Mg alloy. The coatings were composed of dendrites, cellular grains, Nd enriched spherical structure and amorphous matrix. Appropriate Nd2O3 addition can refine the size of crystalline microstructure, purify oxygen impurities and increase amorphous content of the coatings, but excessive addition of Nd2O3 (2–3 wt%) causes aggregation and abnormal growth of some crystalline phases and deteriorates the glass forming ability of the coatings. Corrosion resistance of the coating with 1.5 wt% Nd2O3 is the most excellent relying on the highest amorphous and the lowest crystalline phases contents in this coating. Wear resistance of the coatings is all greatly improved compared with the Mg substrate, however that are no linearly related to the hardness and do not follow completely the Archard's wear equation.

Analysis of Multilevel Otsu Thresholding for Corrosion Rate of Anodised AZ91D Magnesium Alloy ​

Magnesium and its alloy are renowned for their high strength-to-weight ratio, excellent machinability and good castability, making it a favourable material for manufacturing various automotive parts. But when exposed to the environment, magnesium and its alloy have a weak ability to resist corrosion. AZ91D magnesium alloy was anodised and immersed in a salt solution to accelerate deterioration. The image of the corroded surface resulting from the immersion test was analysed by quantising the corroded regions. This paper studies the feasibility of an image processing approach using the multilevel Otsu thresholding method by quantising the corroded region on the AZ91D magnesium alloy. This method converts the original colour image to a greyscale image and then converts it to a binary image and RGB image for the multilevel thresholding. The pixel distributions from the multilevel Otsu thresholding display a trend that, with a higher corrosion rate, the pixel’s frequency of occurrence shifted more to the darker regions. This conforms with the Otsu thresholding, where the darker region is identified as the corroded area of the AZ91D magnesium alloy. By implementing the negative image, the binary image is segmented into non-corroded and corroded areas and labelled as 0 (black) and 1 (white,) respectively. As for multilevel Otsu thresholding, the RGB image is assigned in three colours representing different corrosion severity regions. The regions of the corrosion severity are categorised into severe corrosion, moderate corrosion and low corrosion, coloured in dark blue, light blue, and yellow, respectively. Thus, the trend of the corrosion activity can be seen by implementing the multilevel Otsu thresholding and correlating it with the corrosion rate value. Furthermore, obtaining data on corrosion attack severity can provide helpful information in predicting a component’s failure and future maintenance planning.

Effect of Zinc Oxide Concentrations on Anodic Coated AZ91D Mg Alloy for Surface Modification Applications

The AZ91D Magnesium (Mg) Alloy is broadly known as an attractive material with many applications ranging from military goods and aerospace to automobile applications due to its excellent properties in lightweight, machinability, and castability. Unfortunately, Mg Alloys are also prone to corrosion resulting in degradation of the components, which can lead to severe component failure and injuries. Thus, corrosion prevention is required for the AZ91D Mg alloy to increase its performance in the environment. This paper uses the anodising method to study the influence of Zinc Nitrate’s electrolyte concentrations on anodic coated AZ91D Mg Alloys. This method formed a thin film layer on the AZ91D Mg Alloy’s samples known as an anodic film, which can potentially reduce the corrosion activity on the surface of the substrate. To investigate the corrosion activity of the anodised samples, the corrosion rate value was calculated by immersion of coated samples in a salt solution. The coated and corroded surfaces' morphology, structure, and thickness are observed using a Scanning Electron Microscope (SEM). At the same time, the element content of the material will be determined using X-ray diffraction (XRD). Thus, the resulting influence of Zinc Nitrate electrolyte concentration by anodising method shows which concentration value successfully formed the best Zinc Oxide (ZnO) coating layer on AZ91D Mg Alloy samples. Furthermore, an anodisation method using suitable concentrations of Zinc Nitrate electrolyte can be further developed to form an appropriate surface treatment for automotive applications in resisting corrosion in its environment.

The Effect of Sodium Tetrafluoroborate on the Properties of Conversion Coatings Formed on the AZ91D Magnesium Alloy by Plasma Electrolytic Oxidation

Magnesium and its alloys are widely used in many areas because of their light weight, excellent dimensional stability, and high strength-to-weight ratio. However, the material exhibits poor wear and corrosion resistance, which limits its use. Plasma electrolytic oxidation (PEO) is an effective surface modification method for producing ceramic oxide layers on Mg and their alloys. The influence of the additions of sodium tetrafluoroborate (NaBF4) and sodium fluoride (NaF) into alkaline-silicate electrolyte on the properties of the conversion layers formed in the magnesium AZ91D alloy has been investigated. Surface morphology and chemical composition were determined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The anticorrosive properties of the layers were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods in simulated body fluid (SBF). The presence of NaBF4 or NaF in the electrolyte increases the corrosion resistance of the protective layer. However, the best anticorrosive properties show the layers obtained in the presence of NaBF4. This is probably caused by the incorporation of boron and fluorine in the form of Mg (BF4)2 mainly in the barrier layer.

Effect of Oxidation Time on the Structure and Corrosion Resistance of Micro-Arc Oxidation Coating of AZ91D Magnesium Alloy in (NH4)2ZrF6 Electrolyte System

Micro-arc oxidation (MAO) coatings were obtained from an AZ91D magnesium alloy at different oxidation times (5, 10, 15, and 20 min), using a zirconium salt electrolyte system, with (NH4)2ZrF6 as the main salt. The morphology of the coatings was studied using a scanning electron microscope (SEM) and confocal laser scanning microscopy (CLSM). Energy dispersive spectrometry (EDS), X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS) were employed to determine the type of element and the composition of its phase. The potentiodynamic polarization curve (PDP) was applied to illustrate the corrosion resistance of the coatings. We found the coatings had minor porosity and the best compactness when the MAO treatment time was 10 min. The coatings mainly comprised MgO, ZrO2, MgF2, and Zr3O2F8 phases and amorphous magnesium phosphate. Among the MAO coatings prepared in this experiment, the 10 min coating had the lowest corrosion current density (Icorr), and the Icorr was 4.864 × 10−8 A/cm2, which was three orders of magnitude lower than the uncoated AZ91D magnesium alloy.

Optimization of process parameters for electromagnetic induction heating assisted extrusion of magnesium alloy AZ91D internal thread

The forming of magnesium alloy AZ91D internal thread by electromagnetic induction heating assisted extrusion is a new processing method. In this work, on the basis of this process, the torque in the forming process is selected as the evaluation index, and the response surface method is used to analyze the influence of the process parameters on the internal thread forming process. The range of process parameters (auxiliary heating temperature, hole diameter, machine tool speed) is determined by slip line and empirical method, the test data are simulated, modeled and compared with the response surface analysis method, and the best mathematical model is selected to establish the regression model. the three-dimensional response surface curve of torque is obtained. Through simulation and prediction, it is found that the hole diameter and auxiliary heating temperature have significant influence on the torque in the forming process, and the order is that the hole diameter is larger than the auxiliary heating temperature than the machine tool speed. At the same time, it is found that the interaction between the auxiliary heating temperature and the machine tool speed has a significant effect on the torque. The research results show that the measured value of torque is close to the predicted value, and the predicted value is 3.1% lower than the actual value, and the model fits well. The better forming process parameters are as follows: auxiliary heating temperature to be 240°C, hole diameter to be 11.45 mm, machine tool speed to be 100 r/min. under this parameter, the torque in the forming process to be 24.751 N·m.

Analysis of microstructure, treatment pattern, and corrosion behaviour of AZ91D magnesium alloy processed by selective laser surface remelting

In this work, we show that selective laser surface remelting, apart from modifying the microstructure, can also act as a design method to treat the surface of AZ91D magnesium alloy with varying treatment patterns, thus providing a feasible approach to tailor the electrochemical behaviour and to improve its corrosion resistance. Two treatment patterns, spots and stripes, are designed, in which cases, the boundary ratio and geometrical complexity are analyzed and compared. Besides microstructure, Hydrogen evolution, and potentiodynamic polarization curves are studied. It is found that in the central treated area, corrosion is hindered by formation of the greatly refined β-Mg17Al12 continuous network, together with the uniform distribution of surface potential. As a comparison, in the boundary region, extensive micro/galvanic couples are detrimental to the corrosion resistance, due to the oriented dendritic microstructure. As a result, good corrosion resistance, 3–5 times higher than the as-received material, is obtained despite of the variation of treatment patterns and presence of extensive micro/galvanic couples in the boundary region, indicating that selective laser surface remelting could act as a feasible way to tailor the corrosion behaviour of AZ91D. It is suggested that this technique provides not only a metallurgical approach, and also a design one to improve the corrosion resistance of AZ91D, which may lead to numerous applications of both AZ91D and laser surface treatment.

Construction of a sandwich-like Gr/Ni composite coating on AZ91D magnesium alloy to achieve excellent corrosion and wear resistances in the seawater

Preventing AZ91D magnesium alloy from severely wear and corrosion should be of significance so as to extend its applications. In this work, we systemically investigated graphene (Gr) doping concentration to influence the structure and performance of Gr/Ni composite coatings on AZ91D Mg alloy in the seawater. The results showed that the optimal Gr doping Gr/Ni composite coating on Mg alloy could possess much higher hardness and better adhesive strength as well as much lower wear rate in the seawater. Moreover, the novel composite sandwich structure was utilized to construct multi-interfacial Gr/Ni composite coating on AZ91D Mg alloy, which successfully achieved much more excellent long-term corrosion and wear resistances in the marine. Density functional theory calculations clarified the mechanism of sequential adsorption of nickel to graphene, indicating that it easily gave rise to graphene agglomeration in electroplating process. However, the multi-interfaces could provide more stretching space for graphene sheets and reduce the folding tendency of graphene. These results indicated that the sandwich-like structure Gr/Ni composite coating brought excellent properties for AZ91D magnesium alloy to enhance its services in the marine and aviation environments.

Coupled effect of ultrasonic vibration and post-weld remelting on the porosity and microstructure of die-cast magnesium alloy AZ91D

Ultrasonic vibration (UV), post-weld remelting (PWR), and UV coupled with PWR (UV + PWR) were used to reduce the fusion weld porosity of die-cast magnesium alloys. In addition, the microstructures of the resulting alloys were evaluated. Unlike conventional methods, this work adopted a novel approach to introducing UV. All three treatments reduced the porosity of the alloys. In particular, single and double PWR treatments reduced the porosity by 74.45 % and 69.92 %, respectively, whereas UV treatment reduced the porosity by 79.6 %. Furthermore, single and double UV + PWR treatments reduced the porosity by 92.23 % and 94.23 %, respectively. Further analysis suggests that the cavitation effect and the movement of the tungsten needle facilitated the uplifting of bubbles in the melt pool. The increase in the number of PWR applications increased the β-Mg17Al12 phase growth, aggregation, and grain size. However, multiple UV + PWR applications achieved finer and more diffused β-Mg17Al12 with smaller grain sizes, thereby demonstrating the effectivity of the novel ultrasonic introduction method in reducing porosity and refining microstructure. This work is expected to contribute in improving the welding performance of magnesium alloys.

Investigation on microstructure and tribological performance of zirconium boride reinforced AZ91D magnesium alloy: Effect of processing routes

The present research explored the effects of spatially dispersed ZrB2 particles on mechanical characteristics, surface-modified tribo layer and the dry sliding wear of the AZ91D alloy composite. The objective of this study was to reduce the porosity and improve the tribo performance of the AZ91D/ZrB2 composite by a squeeze casting, and the results were compared with those of a stir-cast composite. The influence of ZrB2 on mechanical properties such as hardness, tensile strength, and wear analysis of composites by the pin-on-disk technique was studied. The squeeze cast composite showed a higher hardness of 115 HV and ultimate tensile strength of 275 MPa than the stir cast composite. Stir-cast composite reduces particle rearrangement due to cluster formation, resulting in higher porosity and lower mechanical strength, reducing tribo performance. The refinement of grain structure and porosity reduction of up to 31.61% improves strength, leading to better tribological properties for the squeeze cast composites. The squeeze cast composite containing 12 wt% of ZrB2 showed a minimum wear of 0.024 mm3/m, which is 25.14% less than the stir cast composite. The improved wear resistance resulted from a lubricating boron oxide layer (B2O3), confirmed through Raman spectroscopy. Tribo layer, delamination, and plastic deformation were found to be the primary wear mechanisms for the AZ91D/ZrB2 composite that were active throughout the sliding period.

Electrochemical studies on ammonium magnesium carbonate tetrahydrate/calcium carbonate composite coating on AZ91D magnesium alloy

At present, it is still a challenge to efficiently and environmentally prepare corrosion-resistant coating with good adhesion on magnesium alloy. In this paper, the composite coating of ammonium magnesium carbonate tetrahydrate (AMCT) and calcium carbonate (CC) was prepared on AZ91D magnesium alloy by a simple one-step in-situ growth process. The surface morphology and composition of the samples were characterized. The corrosion resistance of the sample in 3.5 wt%NaCl solution was evaluated, and the electrochemical test results showed that the Icorr of the coated samples decreased by 3 orders of magnitude compared with that of the Mg substrate, and the corrosion resistance was significantly improved. In addition, the forming mechanism and adhesion properties of the coating are also discussed. The results show that the ratio of AC/CG in the reaction solution and the reaction time are important factors to determine the coating formation. When the molar ratio of AC/CG is 5:1 and the reaction time is 89 h, the prepared coating has good adhesion to the substrate and the best corrosion resistance. The method does not use expensive drugs and instruments, and does not need complicated pretreatment, so it has great advantages in reducing cost and improving efficiency.

Effect of cathode pulse square-wave on the microstructure and corrosion resistance of PEO coatings of AZ91D magnesium alloy prepared in the Na2C4H4O6–KF–Na2SiO3 system

ABSTRACT Plasma electrolytic oxidation coatings were formed in Na2C4H4O6–KF–Na2SiO3 system on AZ91D magnesium alloy by controlling the single-pulse square-wave and bipolar pulse square-wave, respectively. Potentiodynamic polarisation (PD), electrochemical impedance spectroscopy (EIS) and electrochemical noise measurement (EN) were employed to investigate their corrosion behaviour in 3.5% NaCl solution. The results show that with bipolar pulse mode for PEO process, the growth rate of PEO coating on magnesium alloy first decreased and some micropores were present inside the PEO coating. With the increasing oxidation time, the PEO coating became much homogeneous and the porosity of the coating decreased largely, and the corrosion resistance of PEO coating was enhanced. The use of the bipolar pulse mode is beneficial to the improvement of the compactness and corrision performance of magnesium alloy with PEO treatment for the higher voltage stage and longer oxidation time.

Preparation of corrosion-resistant hydrophobic composite films on magnesium alloy

ABSTRACT Magnesium alloy has attracted wide attention for excellent properties but poor corrosion resistance hinders its application. The method of preparing high-performance corrosion-resistant films with low cost and easy operation is significantly crucial to the corrosion protection of magnesium alloy. By synthesizing zeolite film directly on AZ91D magnesium alloy, the work presented a simple, efficient and environmentally-friendly preparation method. In this study, a one-step synthesis method of multi-structure composite film on AZ91D magnesium alloy was explored. The prepared film surface had dense Mg(OH)2 bottom layer and K2Al2Si4O12·xH2O upper layer with porous support microstructure. Compared with the substrate, the corrosion current on the sample surface modified by stearic acid was reduced by about three orders of magnitude, and the corrosion inhibition rate reached 99.96% with a stable contact angle of about 125°. The results showed the excellent corrosion resistance and stable hydrophobicity of the prepared sample.