Mechanical Engineering / Magnesium (Mg) alloys provide material properties that make them attractive for structural components. In particular Mg alloys can be used to produce components with lighter weight than most alloy sheets currently used. However, the insufficient ductility of Mg alloy sheet materials at room temperature can require these to be formed at elevated temperatures to achieve suitable formability. In this research, wrought Mg alloy ZEK100 is studied at 300 °C and lower temperatures. Behavior at these lower temperatures is compared to behavior of 450 °C and 350 °C. A goal of this study is to determine the possibilities for future forming technologies at these lower temperatures. The deformation mechanisms at these temperatures are examined, including their relation to plastic anisotropy. Knowledge of the active deformation mechanisms is used to formulate descriptive models of plastic deformation. Material constitutive models are constructed and used in finite element method (FEM) simulations of gas pressure bulge tests. Finally, results of FEM simulations are compared with experimental results, and the accuracies of the material constitutive models are validated. / text

The current study investigates the formability of ZEK100, a rare-earth containing magnesium alloy, using an in-house developed technique of pneumatic bulge forming. The thesis pursued innovation of sample preparation, testing, and experimental data analysis in order to create several forming limit diagrams (FLDs) of critical importance for determining a methodology for Mg formability. Samples were bulged through elliptical and circular dies at room temperature, 150 C, and 250 C, in two orientations, rolling direction (RD) and transverse direction (TD), in order to determine temperature dependence and orientation characteristics. The current research concluded ZEK100 is not a suitable alloy for room temperature forming processes used in automotive industries. Little difference between safe and marginal, as well as marginal and failure strain ratios was seen for RD orientation testing, while greater resolution is evident for TD orientation testing. ZEK100 exhibits a temperature dependence in relation to limiting strain between RD and TD.

The current research addresses the formability of two magnesium sheet alloys, a conventional AZ31B and a rare earth alloyed ZEK100. Both alloys had a nominal thickness of 1.6 mm. Both Limiting Dome Height (LDH) and Cylindrical Cup Draw experiments were performed between room temperature and 350°C. To examine the effect of sheet directionality and anisotropy, LDH experiments were performed in both the sheet rolling and transverse directions. In addition, strain measurements were performed along both sheet orientations of the cylindrical cup and LDH specimens for which the geometry is symmetric. The LDH tests were used to study the formability of ZEK100 and AZ31B (O and H24 tempers) magnesium alloy sheet between room temperature and 350°C. At room temperature, AZ31B-O and AZ31B-H24 exhibit limited formability, with dome heights of only 11-12 mm prior to the onset of necking. In contrast, the dome heights of ZEK100 at room temperature reached 29 mm (a 140% improvement over AZ31B). Increasing the temperature above 200°C did not affect the relative ranking of the three sheet samples, however it did reduce the magnitude of the difference in dome heights. The rare earth alloyed ZEK100 had pronounced benefits at intermediate temperatures, achieving an LDH of 37 mm at 150°C; this dome height was only reached by AZ31B at a much higher temperature of 250°C. To further characterize the formability of ZEK100, forming limit curves (FLCs) were developed from the LDH tests in both the rolling and transverse directions. Comparisons to AZ31B were made at selected temperatures. Surface strain data was collected with an in situ digital image correlation (DIC) system incorporating two cameras for stereo observation. Results from these experiments further highlighted the enhanced formability relative to AZ31B over the entire temperature range between room temperature and 350°C, with the most dramatic improvements between room temperature and 150°C. The plane strain forming limit (FLC0) for ZEK100 at 150°C was 0.4 which equals that of AZ31B at 250°C. At higher temperatures (300°C), the two alloys exhibited similar performance with both achieving similar dome heights at necking of 37 mm (AZ31B) and 41 mm (ZEK100). To round out the investigation of ZEK100 for industrial applications, cylindrical cup deep drawing experiments were performed on ZEK100 sheet between 25°C and 250°C under isothermal and non-isothermal conditions. Draw ratios of 1.75, 2.00 and 2.25 were considered to examine the effects of draw ratio on draw depth. The effect of sheet anisotropy during deep drawing was investigated by measuring the earring profiles, sheet thickness and strain distribution along both the rolling and transverse directions. Isothermal test results showed enhanced warm temperature drawing performance of ZEK100 over AZ31B sheet; for example, a full draw of 203.2 mm (8”) blanks of ZEK100 was achieved with a tool temperature of 150°C, whereas a tool temperature of 225°C was needed to fully draw AZ31B-O blanks of this diameter. Non-isothermal deep draw experiments showed further improvement in drawability with significantly lower tooling temperatures required for a full cup draw using ZEK100. ZEK100 achieved a full draw of 228.6 mm (9") blanks with a die and blank holder temperature of 150°C and a cooled punch (25°C) while the same size blank of AZ31B required a die and blank holder temperature 225°C and a cooled punch (150°C). Temperature process windows were developed from the isothermal and non-isothermal results to show a direct comparison of drawing behaviour between ZEK100 and AZ31B. Overall, ZEK100 offers significantly improved forming performance compared to AZ31B, particularly at temperatures below 200°C. This lower temperature enhanced formability is attractive since it is less demanding in terms of lubricant requirements and reduces the need for higher temperature tooling.

The extent to which dissolved Li2CO3 can inhibit corrosion of lightweight Mg alloy sheet metal in contact with aqueous NaCl solutions was determined. Two Mg alloy sheet metal alloys were studied, which include: AZ31B (3% Al, 1% Zn, 0.5% Mn, balance Mg) and ZEK100 (1.3% Zn, 0.2% Nd, 0.25% Zr, balance Mg). Corrosion inhibition was first determined for each alloy separately when immersed in 0.1 M NaCl (aq), with and without dissolved Li2CO3 added. The addition of 100 mM Li2CO3 (aq) reduces the corrosion rate of AZ31B by a factor of ~10 and ZEK100 by a factor of ~12. Inhibition involves a reduction in both global anodic dissolution and cathode (H2 gas evolution) kinetics. It also involves suppression of localized filament-like corrosion and associated anode/cathode activation. Site specific cross-sectional analysis of the surface film formed during forced anode activation (polarization) revealed the formation of a Li-doped MgO film, akin to what forms, and provides protection to, Mg alloys with Li added as an alloying element. Such film formation was used to explain all corrosion inhibition aspects. Corrosion inhibition was then determined for ZEK100 when immersed in 0.1 M NaCl (aq) with and without a spray-deposited Li2CO3 surface coatings added. A commercial hexafluoro-titanate/zirconate-polymer conversion coating (Bonderite® MNT 5200) also served as the comparative basis. The Li2CO3-coated surface exhibits the lowest relative corrosion, whereas the conversion-coated surface exhibits the highest. Improved corrosion control is attributed to the formation of a compact coating (physical contribution) and the ability of dissolved Li2CO3 to inhibit both the anode and cathode kinetics (electrochemical) contribution. The findings are of interest to automotive industry as a possible means to effectively control corrosion of Mg alloy sheet metal using Li2CO3 as a surface pre-treatment or the inclusion of Li2CO3 to a polymer as an inhibitor additive. / Thesis / Doctor of Philosophy (PhD) / The objective of this research was to determine the extent to which dissolved lithium carbonate (Li2CO3) can inhibit corrosion of lightweight magnesium (Mg) alloy sheet metal in contact with aqueous NaCl solutions. Corrosion inhibition by dissolved Li2CO3 in 0.1 M NaCl (aq) was demonstrated for two Mg alloy sheet metal alloys: AZ31B (3% Al, 1% Zn, 0.5% Mn, balance Mg) and ZEK100 (1.3% Zn, 0.2% Nd, 0.25% Zr, balance Mg). As a next step towards the development of a protective coating scheme, corrosion inhibition of ZEK100 by Li2CO3, as a surface coating applied, is achieved through a reduction of both the anodic dissolution and the cathode (H2 gas evolution) kinetics in large part by the formation of a Li-doped MgO film at anodic dissolution sites.

Magnesium (Mg) alloy sheets possess several key properties that make them attractive as lightweight replacements for heavier ferrous and non-ferrous alloy sheets. However, Mg alloys need to be formed at elevated temperatures to overcome their limited room-temperature formabilities. For example, commercial forming is presently conducted at 450°C. Deformation behavior of the most commonly used wrought Mg alloy, AZ31B-H24, and two potentially competitive materials, AZ31B-HR and ZEK100 alloy sheets, with weaker crystallographic textures, are studied in uniaxial tension at 450°C and lower temperatures. The underlying physics of deformation including the operating deformation mechanisms, grain growth, normal and planar anisotropy, and strain hardening are used to construct material constitutive models capable of predicting forming for all three Mg alloy sheets at 450°C and 350°C. The material models constructed are implemented in finite-element-method (FEM) simulations and validated using biaxial bulge forming, an independent testing method. Forming limit diagrams are presented for the AZ31B-H24 and ZEK100 alloy sheets at temperatures from 450°C down to 250°C. The results suggest that forming processes at temperatures lower than 450°C are potentially viable for manufacturing complex Mg components. / text

Parsing the effect of slip-twin interactions on the strain rate and thermal sensitivities of Magnesium (Mg) alloys has been a challenging endeavor for scientists preoccupied with the mechanical behavior of hexagonal close-packed alloys, especially those with great latent economic potential such as Mg. One of the main barriers is the travail entailed in fitting the various stress-strain behaviors at different temperatures, strain rates, loading directions applied to different starting textures. Taking on this task for two different Mg alloys presenting different textures and as such various levels of slip-twin interactions were modeled using VPSC code. A recently developed routine that captures dislocation transmutation by twinning interfaces on strain hardening within the twin lamellae was employed. While the strong texture was exemplified by traditional rolled AZ31 Mg alloys, the weak texture was represented by ZEK100 Mg alloy sheets. The transmutation model casted within a dislocation density based hardening model showed tremendous flexibility in predicting the complex strain rate and thermal sensitive behavior of Mg textures’ response to various mechanical loadings schemes.