AZ91D Magnesium Alloy Research Articles

Improved Corrosion Resistance of AZ91D Mg Alloy by Cerium-Based Films. Formation of a Duplex Coating with Polypyrrole

In order to improve the corrosion resistance of die-cast AZ91D magnesium alloy in simulated physiological solution, two cerium-based coatings were synthesized. The coatings were electrodeposited from solutions containing Ce(NO3)3, H2O2 and C6H8O7 or C4H4O6Na2. The influence of the presence of sodium tartrate and citric acid in the preparation solution on the morphology, composition and anticorrosive performance of the generated coatings was evaluated. The corrosion properties were examined in Ringer solution by polarization studies, open circuit measurements and faradaic impedance spectroscopy. Results showed that the incorporation of additives improves the anticorrosive properties of the films formed without sodium tartrate or citric acid. The coating formed in the presence of sodium tartrate showed the best anticorrosive performance. Subsequently, the possibility of forming duplex coatings employing the cerium-based films as inner layers and polypyrrole as top coating was evaluated. At this point, it was found that polypyrrole can be electropolymerized on top of the formed cerium-based film modified with sodium tartrate.

Effect of friction stir processing on the wear rate of as-cast AZ91D Magnesium alloy

Magnesium (Mg) alloys known to have the lowest density amongst other engineering materials used for structural applications. They are emerging as a promising material, particularly in automotive and aerospace industry. But the poor wear characteristics of Mg alloys restrict their use in many applications. In the present investigation, friction stir processing (FSP) of as-cast AZ91D Mg alloy was performed with the objective to improve its wear properties. The FSP was carried out at a tool rotation speed (TRS) of 600, 900 and 1200 rpm. The tool travel speed was kept constant at 70 mm/min. A pin-on-disc tribometer was used to perform dry sliding wear experiments at an optimized set of parameters. FSPed specimens demonstrate a substantial grain refinement and decomposition of coarse β-Mg17Al12 particles into Mg matrix. At a TRS of 600 rpm, the average grain size was reduced to 25 μm and the microhardness was increased to 81 Hv. The refined microstructure and improved hardness are expected to have provided better wear resistance in FSPed samples.

Microstructure and mechanical properties of AZ91D magnesium alloy by expendable pattern shell casting with different mechanical vibration amplitudes and pouring temperatures

To refine the microstructure and improve the mechanical properties of AZ91D alloy by expendable pattern shell casting (EPSC), the mechanical vibration method was applied in the solidification process of the alloy. The effects of amplitude and pouring temperature on microstructure and mechanical properties of AZ91D magnesium alloy were studied. The results indicated that the mechanical vibration remarkably improved the sizes, morphologies and distributions of the primary α-Mg phase and β-Mg17Al12 phase, and the densification and tensile properties of the AZ91D alloy. With an increase in amplitude, the microstructures were gradually refined, resulting in a continuous increase in mechanical properties of the AZ91D alloy. While, with the increase of pouring temperature, the microstructures were continuously coarsened, leading to an obvious decrease of the mechanical properties. The tensile strength and yield strength of the AZ91D alloy with a vibration amplitude of 1.0 mm and a pouring temperature of 730 °C were 60% and 38% higher than those of the alloy without vibration, respectively.

Tension twin-induced premature fracture mechanism of aging-treated AZ91D magnesium alloy

Effects of discontinuous precipitate (DP) and continuous precipitate (CP) on the tensile elongation and fracture mechanism of aging-treated AZ91D Mg alloy were investigated via quantitative microscopy and fractography. Specimens with various distributions of DP and CP were prepared under a wide range of aging conditions. As quantitative microstructural factors of DP and CP, area fraction of DPs and CP/α-Mg interfaces per unit volume were measured by optical microscopy (OM) and scanning electron microscopy (SEM). To explain the tensile fracture mechanism, a deformation twinning behavior was analyzed using OM and electron back-scattered diffraction, and area fraction of stepped cleavage facet was quantitatively measured on a large number of fractographs. An increase in the area fraction of DPs caused a slight decrease in the tensile elongation and this reduction was attributed to cracking at the DP along grain boundary. For the specimens with high CP/α-Mg interfaces per unit volume, a large number of fine {10−12} tension twins were produced within a few percent of tensile elongation as well as premature fracture occurred. In this investigation, the premature fracture mechanism of the aging-treated AZ91D alloy with a large amount of fine CP distribution was explained in relation with the increased probability of {10–12} tension twin boundary cracking, although activation of {10–12} tension twinning can be a way to facilitate deformation of some Mg alloys in previous researches. Validity of the suggested premature fracture mechanism was supported by the quantitative measurement of area fraction of the stepped cleavage facet on the fractography as indirect evidence of the twin boundary cracking.

Prediction and validation of temperature distribution and material flow during refill friction stir spot welding of AZ91D magnesium alloy

Temperature distribution and material flow behaviours during refill friction stir spot welding (RFSSW) of AZ91D magnesium alloy were studied by a combination of numerical modelling and experimentation. The coupled Eulerian–Lagrangian (CEL) formulation was adopted to analyse processing thermal history and plastic deformation mechanisms of the RFSSW joint more accurately. Welding thermal cycles of varying positions in joint were measured by thermocouples so as to validate accuracy of this simulation process. The FEA (Finite Element Analysis) simulation to cover mechanical results was found to be consistent with the experimental values. Moreover, simulation analysis presented that material flow caused the clear hook defects and bonding ligament, and such material flow behaviours were experimentally verified via the macrostructure and microstructure of the RFSSW joint.

Optimization of the Corrosion Resistance of Electroless Ni–W–P Coatings on Magnesium Alloys by the Response Surface Methodology

Plating processes greatly affect the corrosion resistance of electroless Ni–W–P coatings on magnesium alloys. In the present research, the corrosion resistance of electroless Ni–W–P alloy-coated AZ91D magnesium alloy was optimized by the response surface methodology. The optimum technological parameters of the plating process were determined by establishing a quadratic regression model, and the influence of these variables and their interactions on the corrosion resistance of the coating was analyzed. The optimum technological parameters of the electroless plating process were nickel sulfate concentration = 20 g/L, sodium tungstate concentration = 15 g/L, sodium hypophosphite concentration = 30 g/L, bath temperature = 60 °C, and bath pH = 9.3. Under these conditions, the coating had the best corrosion resistance. Among the aforementioned five variables, sodium tungstate concentration had the most significant influence on the corrosion resistance of the coating. Different degrees of interactions among the variables greatly affected the corrosion resistance of the coating.

Biocorrosion evaluation and bonding strength of HA-CS/PLA hybrid coating on micro-arc oxidised AZ91D magnesium alloy

ABSTRACT In order to improve the corrosion resistance and bonding strength of the micro-arc oxidised (MAO) AZ91D magnesium (Mg) alloy, the hydroxyapatite (HA)-chitosan (CS) and polylactic acid hybrid (PLA) coatings on MAO Mg alloy were fabricated via electrochemical co-deposition and dip-coating method. The influences of different concentrations of PLA on the phase composition, surface morphology, hydrophobic, bonding strength, and corrosion resistance were investigated. The surface morphology and immersion test in simulated body fluid (SBF) showed that the PLA coating could further seal the surface and prevent the HA-CS coating from cracking, thus improving the corrosion resistance of the sample. The sample with 20% PLA had the lowest corrosion current density (1.13 × 10−8 A/cm2) and the highest strength (18.84 N). These results manifest that the MAO/HA-CS/PLA is a protective coating on magnesium alloys for implants application.

Effect of ageing temperature on the corrosion behaviour of UHTC particulates reinforced magnesium composites fabricated through ultrasonic assisted squeeze casting process

The corrosion behaviour of AZ91D magnesium alloy reinforced with 5-wt% of various Ultra High-Temperature Ceramics (UHTC) such as TaC, HfC, TiN, TiB2, and TiC has been analysed in this current research work. AZ91D magnesium alloy and proposed composites were fabricated through ultrasonic assisted squeeze casting process and followed by T6 heat treatment at different ageing temperatures (170, 200, 230, and 260 °C). As per ASTM standards, the fabricated composites were subjected to various analyses on density, porosity, microstructure, hardness, and corrosion behaviour. The distribution of reinforcements was revealed by the optical microscope and scanning electron microscope images of composites. The presence of primary phase (α-Mg), intermetallic phase (β-Mg17Al12) and reinforcements in the composites were confirmed in the XRD analysis. Because of high densification, lower grain size, and lesser porosity level, TaC and HfC particulate reinforced composites showed higher hardness (170.3 and 160.4 HV respectively) at 230 °C compared to other composites. The salt spray corrosion test (3.5 wt% NaCl) on composites with varying exposure times (24, 48, and 72 h) revealed that, due to the formation of a dense and stable anti-corrosion protective layer with reduced β-Mg17Al12 phase, TaC and HfC composites exhibited higher corrosion resistance up to 48 h and ageing temperature of 230 °C. The scanning electron microscope image of the corroded samples showed pits and crack formation at an exposure period of 72 h.

Electrodeposition and microstructure of Ni and B co-doped diamond-like carbon (Ni/B-DLC) films

Ni and B co-doped diamond-like carbon (Ni/B-DLC) films were synthesized on the surface of AZ91D magnesium alloy by electrodeposition under mild environment. The formation mechanism of Ni/B-DLC films was discussed. The microstructure of the films was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Results conveyed that the Ni-DLC and Ni/B-DLC films were hydrogenated amorphous carbon films. With the co-doping of Ni and B, the protruding mulberry-like structure of carbon-based films gave way to the compact and uniform granular structure. The intensity ratio (ID/IG) of D and G lines was decreased from 0.75 to 0.71. XRD patterns revealed that the doping of boron favored the sp3 hybridization of carbon atoms. Simultaneously, the infrared spectra demonstrated that hydrogen for Ni/B-DLC films was preferentially bonded with sp2 carbon atom. In addition, Ni/B-DLC films had excellent wear resistance, which was mainly attributed to the homogeneous and dense structure and considerable micro-indentation hardness.

Optimization of tribology parameters of AZ91D magnesium alloy in dry sliding condition using response surface methodology and genetic algorithm

Optimization of tribology parameters of AZ91D magnesium alloy in dry sliding condition using response surface methodology and genetic algorithm

Electrodeposition of B, Ni co‐doped diamond‐like carbon films on AZ91D magnesium alloy: Corrosion and wear resistance

AbstractDiamond‐like carbon (DLC) possesses brilliant and excellent properties, including excellent corrosion resistance as well as outstanding wear resistance. Ni and B co‐doped DLC films were deposited on AZ91D magnesium alloy by electrodeposition under mild conditions (300 V and 25°C). Uniform and dense morphology of co‐doped DLC films were observed, and Ni and B were uniformly incorporated into the carbon‐based films. Among all the electrodeposits, the appearance of D and G peaks near 1330 and 1570 cm−1 revealed that the as‐deposited films were typical DLC films. As the addition of Ni was increased to 0.05 g, the highest microindentation hardness, the lowest friction coefficient, and wear loss were achieved to be 164.5 HV, 0.3, and 0.6 × 10−5 kg/m, respectively. The amorphous carbon films fabricated at 0.05 g Ni had the lowest corrosion current density and the most positive corrosion potential, which was mainly due to the small and dense granular structure effectively hindering the penetration of corrosion media.

Experimental study on drilling force during drilling of AZ91D magnesium alloy

Abstract Magnesium Alloys used in medicine, sports, electronics, automobile, household equipment, aerospace etc due to their low density. The present study investigated the influence of speed, feed and point angle on drilling force in drilling of AZ91D Magnesium alloy. Experiments were devised and conducted as per Taguchi L16 orthogonal array. ANOVA was performed to determine the percentage of contribution of the input parameters. Results concluded that drilling force was significantly influenced by point angle followed by speed and feed. Lower drilling force was obtained at a speed of 400 rpm, feed 15 mm/min and point angle of 30°.

The inhibition behavior of novel ionic liquids for magnesium alloy in NaCl solution: Experimental and theoretical investigation

Three pyrazolium ionic liquids 1-n-butyl-2-hexylpyrazole bis(trifluoromethylsulfonyl)amide ([BHPz][NTf2]), 1-n-butyl-2-octylpyrazole bis(trifluoromethylsulfonyl)amide ([BOPz][NTf2]), and 1-n-butyl-2-decylpyrazole bis(trifluoromethylsulfonyl)amide ([BDePz][NTf2]) are firstly synthesized and their corrosion inhibition for AZ91D magnesium alloys in 0.05 wt% NaCl solution is investigated. The highest inhibition efficiency is 90.0% under optimal test conditions. The inhibition performance is measured by electrochemical tests, and follows the Langmuir adsorption model. The surface morphology is determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which testifies the formation of protective films on Mg alloy surface. The possible inhibited mechanism is proposed according to X-ray photoelectron spectroscopy (XPS) analysis and the density functional theory (DFT) calculation.

Microstructure and biological activity of silicon‐doped composite coatings fabricated by micro‐arc oxidation on magnesium alloy

Among biomaterials, the mechanical properties of magnesium and its alloys are closest to those of human bone tissue, making these ideal materials for bone tissue repair and replacement. In order to further improve the biocompatibility and biological activity of the magnesium alloy AZ91D, micro‐arc oxidation with a constant‐current power supply mode was employed in this study in a silicate electrolyte solution. The resultant coating surface had a porous structure, and silicon was evenly distributed in it. Further, the results of in vitro experimental studies showed that the silicon‐doped magnesium coating on the surface of the magnesium alloys has no obvious cytotoxicity, and it can induce the proliferation of osteoblasts and promote actin stretching. In addition, it has good biological activity and biocompatibility. Thus, the novel method developed in this study improves the potential for clinical applications of magnesium alloys.

Corrosion resistance of a nano-Mg modified silane conversion coating with cathodic protection on magnesium alloy AZ91D

In order to give the capability of cathodic protection to silane conversion coating (SCC), nano-Mg particles was used to modify SCC, and a nano-Mg modified SCC (nano-Mg@SCC) was prepared on magnesium alloy (MA) AZ91D. The microstructure and anti-corrosion of nano-Mg@SCC were characterized. Both Si-O-Mg and Si-O-Si bonds were mainly responsible for constructing the nano-Mg@SCC/AZ91D system: the former bond acted at the interfaces between nano-Mg particles and SCC as well as between nano-Mg@SCC and AZ91D substrate, and the latter bond acted in the interior of nano-Mg@SCC. The corrosion current density (ic) and the coating resistance (Rc) for the nano-Mg@SCC coated AZ91D were respectively lower and higher than those for the single-SCC coated AZ91D; at the same time, both stable open circuit potential (OCP) and corrosion potential (Ec) of nano-Mg@SCC coated AZ91D were more negative than those of bare AZ91D. The nano-Mg@SCC retained the original capability of physical barrier and possessed the new capability of cathodic protection, so nano-Mg@SCC provided better corrosion resistance to AZ91D substrate than single-SCC did.

Effects of the Power Supply Mode and Loading Parameters on the Characteristics of Micro-Arc Oxidation Coatings on Magnesium Alloy

Micro-arc Oxidation (MAO) is a technology for non-ferrous metal surface treatment through growth ceramic coating in situ. To determine the influence of the power supply mode and the loading parameters on the film forming of magnesium alloy micro-arc oxidation processing, the different power supply modes of pulsed direct current DC, pulsed bipolar current (BC) and the pulsed with a discharge loop current (DLC) was used with MAO technology on the AZ91D magnesium alloy. The power load parameters were optimized. The average energy consumption was calculated. Results showed that the role of the negative voltage in the bipolar pulse power supply is to restrain the large arc tendency. Under the pulse power supply with a discharge loop, the current and energy consumption decreases with the increase of the discharge resistance at the same pulse parameters. The big arc phenomenon can be effectively avoided and the impact of load capacitance could be effectively avoided by using the pulse power supply with a discharge loop. Moreover, the processing of the micro-arc oxidation is stable, the arc point is uniform, the surface of the film is smooth, the hole is uniform and the coating is dense, and the film efficiency is improved effectively.

Influences of Carbon Addition on Mechanical Properties and Hydrogen Evolution of Thixomolded AM60B and AZ91D Magnesium Alloys

In this study, the effects of carbon nanoparticles fixed to the surfaces of AM60B and AZ91D magnesium alloy chips on the mechanical properties were examined. The manufacture of magnesium–carbon alloy is not easy because carbon does not possess the property of wettability for magnesium. However, magnesium alloy chips fixed to carbon nanoparticles enable magnesium–carbon alloys to be produced by the thixomolding process. Mechanical properties such as the tensile and fatigue strengths were improved by only 0.1 mass% of the carbon addition because of the decrease in the void and the refinement of crystal grains. In addition, the AZ91D magnesium alloy was proven to be more effective than the AM60B magnesium alloy for the decrease in the void and the refinement of crystal grains by the carbon addition. The aluminum content of the AZ91D magnesium alloy is higher than that of the AM60B alloy. It seems that the void formation is based on the hydrogen by the reaction between aluminum in the magnesium alloy and water. Therefore, the effect of carbon addition on the mechanical properties was dependent on the aluminum content in the magnesium alloys.

Fretting-corrosion behavior of electroless Ni-P/Ni-P-TiO2 coatings obtained on AZ91D magnesium alloy by a chromium-free process

Ni-P electroless coatings are widely applied for protection of magnesium alloys and other materials due to the low energy consumption of the process and high resistance to corrosion and wear, properties that can be improved with the incorporation of particulate materials. Despite the attractive combination of properties of electroless coatings, studies in the tribocorrosion field, such as fretting-corrosion behavior of Ni-P electroless coatings, are very scarce. In this work, Ni–P-TiO2 composite electroless coatings with several variations on particles size and content applied on AZ91D magnesium alloys were analyzed under fretting-corrosion conditions in 3.5 wt.% NaCl. Coatings were obtained by direct electroless technique in multiple steps. Procedures were chromium-free and no activation pretreatment was needed. The tests were carried out using a fretting-corrosion tribometer from where open circuit potential and coefficient of friction were analyzed, as well as wear tracks on coatings surfaces were observed by SEM. The results obtained indicate that an improvement in the tribochemical behavior of Ni-P coatings can be achieved with the TiO2 codeposit, which modify the contact between body and counter body, the elastic accommodation and the dissipation of energy in the contact area.

Thermo-Fluid-Dynamic Modeling of the Melt Pool during Selective Laser Melting for AZ91D Magnesium Alloy.

A three dimensional finite element model (FEM) was established to simulate the temperature distribution, flow activity, and deformation of the melt pool of selective laser melting (SLM) AZ91D magnesium alloy powder. The latent heat in phase transition, Marangoni effect, and the movement of laser beam power with a Gaussian energy distribution were taken into account. The influence of the applied linear laser power on temperature distribution, flow field, and the melt-pool dimensions and shape, as well as resultant densification activity, was investigated and is discussed in this paper. Large temperature gradients and high cooling rates were observed during the process. A violent flow occurred in the melt pool, and the divergent flow makes the melt pool wider and longer but shallower. With the increase of laser power, the melt pool’s size increases, but the shape becomes longer and narrower. The width of the melt pool in single-scan experiment is acquired, which is in good agreement with the results predicted by the simulation (with error of 1.49%). This FE model provides an intuitive understanding of the complex physical phenomena that occur during SLM process of AZ91D magnesium alloy. It can help to select the optimal parameters to improve the quality of final parts and reduce the cost of experimental research.

A Facile Method to Prepare a Superhydrophobic Magnesium Alloy Surface.

The application of superhydrophobic materials has been handicapped by complex processes and poor environmental friendliness. Magnesium alloys are widely used in daily production due to their low density and good casting properties. A facile and environmentally friendly method was proposed to prepare a superhydrophobic layer with coral-like microstructure on the surface of AZ91D magnesium alloy by high temperature heating. The prepared superhydrophobic surface has a contact angle of 159.1° and a rolling angle of 4.8°. The corrosion current of superhydrophobic surface has been reduced by about two orders of magnitude relative to the magnesium alloy substrate and its inhibition efficiency is 96.94%, which demonstrates its great corrosion resistance. In addition, the superhydrophobic surface has great thermal stability. When the temperature rises to 190 °C, the contact is still above 150°. Excellent self-cleaning and advantages in preparation efficiency, environmental protection and cost-effectiveness will boost its good application prospects.