In an effort to improve the fuel efficiency of automobiles, car designers are investigating new materials to reduce the overall vehicle weight. Magnesium alloys are good candidates to achieve that weight reduction due in part to their low density and high specific strength. To support their introduction into vehicle body structures, the dynamic behavior of magnesium alloys must be determined to assess their performance during a crash event. In this work, the tensile high strain rate behavior of AZ31B magnesium alloy sheets was characterized. Two different temper conditions were considered: AZ31B-O (fully annealed) and AZ31B-H24 (partially hardened). Three different sheet thicknesses were considered for the O temper condition, 1.0, 1.6 and 2.5 mm, while the H24 temper was 1.6 mm in thickness. The sheet condition of the magnesium alloys implies an in-plane anisotropy induced by the rolling process. Therefore, both the rolling and transverse directions were investigated in the current research. In order to characterize the constitutive behaviour of AZ31B-O and AZ31B-H24 magnesium alloy sheets, tensile tests were performed over a large range of strain rates. Quasi-static experiments were performed at nominal strain rates of 0.003s-1, 0.1s-1 and 1s-1 using a servohydraulic tensile machine. Intermediate strain rate experiments were performed at 30s-1 and 100s-1 using an instrumented falling weight impact (IFWI) apparatus, and high strain rate experimental data at 500s-1, 1000s-1 and 1500s-1 was collected using a tensile split Hopkinson bar (TSHB) apparatus. Elevated temperature experiments (up to 300°C) were also performed at high strain rates using a radiative furnace mounted on the TSHB apparatus. The tensile experiments show a significant strain rate sensitivity of the constitutive behavior of both the O and H24 temper conditions. The two tempers exhibit an average increase of stress level of 60-65 MPa over the range of strain rates considered. As the strain rate increases, the strain rate sensitivity of both tempers also increases. The strain rate has a different effect on the ductility of the two material conditions. The ductility of AZ31B-O is significantly improved under high strain rate deformations, whereas the AZ31B-H24 exhibits similar ductility at low and high strain rates. Both material conditions presented a strong in-plane anisotropy, with an average stress level in the transverse direction higher than in the rolling direction by 15 MPa and 35 MPa for the O and H24 tempers, respectively. The thermal sensitivity for both tempers at high strain rates was obtained. The two material conditions exhibit a clear thermal softening. From room temperature to 250°C, the loss in strength at 5% plastic strain was found to be 55 MPa and 125 MPa for the AZ31B-O and AZ31B-H24 materials, respectively. The thickness of the AZ31B-O sheets has a mild effect on the measured constitutive behavior. The flow stress increases with increasing thickness. An average difference of 10-15 MPa was seen between the flow stress of the 1.0mm and 2.5mm sheets. However, similar strain rate sensitivity was seen for the three thicknesses. The experimental data was fit to three constitutive models: the Johnson-Cook model, its modified version with a Cowper-Symonds strain rate sensitivity formulation, and the Zerilli-Armstrong model. The three models were evaluated by numerical simulation of the TSHB experiment under various testing conditions. It was found that the Zerilli-Armstrong model was the most accurate in predicting the flow stress of the different material conditions. However, finite element models incorporating the three constitutive fits failed to predict necking in the specimen.

In this study, a new consistent formulation coupling kinematics, thermodynamics, and kinetics with damage using an extended multiplicative decomposition of the deformation gradient that accounts for corrosion effects is presented. The technical approach used for modeling the corrosion behavior of magnesium alloys was divided into three primary steps. First, a predictive corrosion model was developed based on experimental corrosion observations. The experimentally-observed corrosion mechanisms of pitting, intergranular, and general corrosion on the AZ31 magnesium alloy were quantified in 3.5 wt.% NaCl immersion and salt spray environments using optical microscopy and laser profilometry to document the changes in the pit characteristics. Although both environments showed similar trends, the immersion environment was more deleterious with respect to intergranular and general corrosion. On the other hand, the salt-spray environment allowed deeper pits to form throughout the entirety of the experiments, which led to a substantial thickness drop (general corrosion) compared with the immersion environment. Next, the complete corrosion model based upon the internal state variable theory was formulated to capture the effects of pit nucleation, pit growth, pit coalescence, and general corrosion. Different rate equations were given for each mechanism. Following the formulation of the model, the aforementioned experimental work and experimental work on four other magnesium alloys (AZ61, AM30, AM60, and AE44), was used to validate the model.

In this work, a crystal elasto-viscoplastic model was modified to account for the anisotropic mechanical response of magnesium aluminum alloys. Crystal plasticity may offer new understanding of these alloys by explicitly modeling the texture development that profoundly affects the properties of magnesium. The model is able to account for the individual slip systems of both the cubic and hexagonal phases. The constants of the model were determined from published experimental AZ31 data, and the plastic hardening response is shown to match these results well using a modification to the hardening rule to approximate the kinetics of twinning. Model aggregates were created with aluminum compositions representative of common magnesium structural alloys. This approach allows the effect of varying percentage of cubic phase on the hexagonal magnesium alloy aggregate to be studied both in terms of macroscopic response and the crystallographic changes occurring within the system.

<p>A study has been conducted in mildly aggressive saline solutions to indentify subtle yet important difference in the anodic and cathodic process of three Mg-Al alloys, AZ31B, AM30 and AM60B in a partially passive state and a localized corrosion state. The influence of metallurgical factors and environment variables on the corrosion resistance and surface film breakdown process has been investigated using potentiodynamic and potentiostatic tests combined with optical microscopy.</p> <p>All three Mg-Al alloys corroded in a partially protected state under open circuit conditions in the test solution and the surface film formed on each exhibited a similar breakdown potential. This indicates that metallurgical factors such as alloying additions and the presence of the β-phase (Mg₁₇Al₁₂) did not significantly influence the surface film breakdown process. AM60B exhibited improved corrosion resistance at potentials below the breakdown potential due to the formation of a more protective surface film. The β- phase, however, did not strongly influence either the anodic process at potentials above the breakdown potential or the cathodic process. It was determined that increasing the alloy Al content increases the corrosion potential of Mg-Al alloys, but also increases the risk of localized corrosion. The similar anodic and cathodic polarization behaviour exhibited by AZ31B and AM30 indicates that a 1 wt % Zn alloying addition does not strongly influence the corrosion resistance of these alloys. The die-cast skin of AM60B exhibited better corrosion resistance than the interior at potentials below the breakdown potential due to the higher fraction of Al-rich β-phase, which improved the protective ability of the surface film. The semi-continuous β-phase network did not provide a strong micro-galvanic activity to drive anodic dissolution of the α-phase, therefore, did not significantly affect the corrosion resistance above the breakdown potential. The crystallographic texture exhibited by the extruded AM30 did not strongly affect the short-term and long-term corrosion resistance. AM30 showed a relatively steady passive state during long time exposures in a room temperature 0.01 M NaCl solution.</p> / Master of Science (MSc)

Pure magnesium (Mg) and its alloys with calcium (Ca) and both Ca and zinc (Zn) have potential as bioresorbable bone implant materials provided the corrosion rate can be controlled. Thus, corrosion behaviour was investigated for pure Mg, Mg-2Ca, and Mg-2Ca-1Zn cast alloys subjected to either no heat treatment or to solutionizing and aging heat treatment. In addition, corrosion behaviour was investigated for surface modifications involving the deposition of silver (Ag) nanoparticles. These materials and constructs were all nominally biocompatible in that they would not elicit a strong and immediate adverse tissue reaction when implanted in bone. Static immersion tests in Hanks’ balanced salt solution were performed to evaluate the corrosion behaviour. The Mg-2Ca alloy exhibited the highest corrosion rate when compared with pure Mg and Mg-2Ca-1Zn for any length of immersion time. For short immersion times (48 hours), solutionizing followed by natural aging reduced the corrosion rate of Mg-2Ca alloy, but this heat treatment did not seem to have an effect on the corrosion rate of Mg-2Ca-1Zn alloy. As well, for short immersion times (48 hours), solutionizing and artificial aging also did not seem to have a large effect on corrosion rates for either Mg-2Ca or Mg-2Ca-1Zn, when compared to solutionizing and natural aging. Corrosion behaviour of surface-modified samples was sensitive to certain features of the Ag depositions. It was found that when the deposited Ag tracks were thick and wide, the corrosion rate of Ag-deposited samples increased significantly when compared to samples without any Ag deposition. However, when the Ag tracks were thinner and somewhat narrower, the corrosion rate did not appear to be much higher than that of samples without Ag deposition. Therefore, controlled Ag deposition may not be too detrimental to the corrosion behaviour of Mg and Mg alloys. The corrosion product morphology appeared to be similar for both the samples deposited with Ag and samples without any Ag. Needle-like formations were observed in small areas on the corroded surfaces. X-ray diffraction revealed Mg(OH)₂ as the main corrosion product. Because energy dispersive X-ray analysis consistently revealed multiple elements in the corrosion products (such as Mg, O, Ca, P, small amounts of C, and sometimes Cl), it was concluded that other compounds (possibly hydroxyapatite, magnesium chloride, and/or magnesium- and calcium-containing phosphates) may have formed in addition to the Mg(OH)₂.

This study investigated effects of the process parameters on the forging load and metal flow pattern during forging and thread-rolling these two process of LZ91 magnesium alloy small size screw by the finite element analysis. At first, Compression tests were carried out under various forming temperatures to study the flow stress. Then, FEM software DEFORM-2D is adopted to simulate forging and thread-rolling processes of small screw to analyze the formability and parameters. In one of this study, there are two stages in forging process, and found out that up-die velocity, temperatures and friction factors will affect the product quality and appearance; on the other part, it investigated the effect of friction factor and temperature under thread-rolling process, and found out that effective stress, effective strain, metal flow and height of thread will be affected. In addition, conduct forging and thread-rolling experiments by using universal testing machine with the mold self-designed, and MoS2 of lubricant, and comparing the analytical results to verify the suitability and accuracy of FEM for forging process. Finally, according to the analysis result of this study, engineers can take it as reference.

This study investigates analytically and experimentally extrusion processes of magnesium hollow tubes by a single-cylinder extrusion machine and double-cylinder extrusion machine. The first part of this study is to conduct analysis and experiment of hollow helical tube extrusion by single-cylinder extrusion machine. Firstly, a design criterion is proposed to determine the forming parameters and discuss the effects of product size, extrusion ratio, billet length, etc. on the mandrel surface stress. The effects of the die bearing part length, angle of rotation, extrusion speed, initial temperature, petal number, etc. on the radial filling ratio are also investigated. Better parameters are chosen from analytical results to conduct hot extrusion experiments for obtaining sound products. Microstructure observation and hardness test are conducted at the cross-section of the product. The experimental values of extrusion load and product¡¦s dimensions are compared with the analytical values to verify the validity of the analytical models. The second part of this study is to conduct analysis and experiment of hollow tubes extrusion by a double-cylinder extrusion machine. The effects of extrusion ratio, billet length, mandrel diameter, etc. on the drawing force on the mandrel and critical conditions without mandrel fracture are discussed.

This book has the introduction of the friction stir welding and friction stir processing, and introduces the newest development in FSW.Finding out the appropriate paraments of the grain size refinement in AZ31 Mg. The relationship between the resulting grain size and the applied working strain rate and temperature for the friction stir processing in AZ31 Mg is systemically examined. The Zener-Holloman parameter is utilized in rationalizing the relationship. The grain orientation distribution is also studied using the X-ray diffraction.

The friction stir processing is applied in mixing elemental thin sheets of Mg, Al, and Zn in various portions to result in hard intermetallic alloys with Vicker¡¦s hardness in excess of 350. The Mg3Al2Zn3

Weight reduction is a hot topic in automotive industry. Both the applications of tube hydroforming technique and magnesium alloys offer a large potential for reducing the weight of automotive components. In this research, the relationship between forming pressure and bulge height, the maximum forming pressure and the forming limit during the tube hydraulic bulging process are first analysed. A self-designed warm hydraulic bulge forming equipment and the seamlessly extruted magnesium alloy AZ61 tubes, are used for carrying out a series of warm hydraulic bulge tests, and discussing the formalibility of the magnesium tubes at various temperatures. Furthermore, the flow stress curves are determined by the mathematical model in this paper with the bulge forming test results. Then the validity of the analytical model is verified by comparing the forming pressure and bulge height between analytical and experimental values.