Automotive magnesium sheets typically exhibit poor room temperature ductility which makes them unsuitable for room temperature sheet stamping applications. This research involved aspects of re-processing and forming of AZ31 automotive magnesium sheet to improve its room temperature ductility and bendability (and, more generally, formability). The sheet re-processing studies for formability improvement were carried out by two different methods, (i) cyclic bending-unbending and annealing (or CBUA) and (ii) wire brushing and annealing (or WBA). These two processing methods led to a complex stress and strain distribution through the thickness and a multi-layered microstructure after annealing. The grain structure, micro-texture, and micro-hardness of each of the layers were studied by optical microscopy, electron back-scattered diffraction (EBSD) and indentation measurements, respectively. The through-thickness grain structure study indicated grain refinement and texture randomization in the surface layers for both CBUA and WBA processed materials. In addition, the as-received (and fully annealed) sheet as well as processed materials were subsequently deformed in uniaxial tension and bending by a process referred to in the literature as pre-strain annealing (or PSA). The PSA process was studied as a single step as well as multi-step process to assess its effect on formability improvement, underlying changes in microstructural and mechanical behavior, and to explore practical limitations and advantages of the process. The results from single-step PSA process were also used to develop a microstructure-based constitutive material model to capture and predict the observed mechanical and microstructural response of AZ31 sheet to PSA variables. This model explicitly considered the effect of recovery on recrystallization kinetics, and non-constant nucleation and growth rate. The model was extended to predict the grain size at the end of recrystallization and within the grain growth stage as well as post-PSA yield and work hardening characteristics. The mechanical property prediction was based on considering the microstructure as a composite of un-recrystallized, recrystallized and coarsened grain structure and by employing a rule of mixture. The processing and forming methods led to significantly increased cumulative uniaxial tensile ductility and plane strain cumulative bendability of AZ31 sheet at room temperature depending upon PSA process parameters. The experimental and modeling studies collectively helped correlate mechanical properties from various processing conditions and forming methods with microstructural parameters, and to explain the improvement in room temperature formability based on microstructural and textural considerations. / Dissertation / Doctor of Philosophy (PhD)

The automotive industry has become a major user of magnesium components. However, use of magnesium sheet products is quite limited, due to difficulties in producing cost effective components. Any sheet currently produced is formed at elevated temperatures, making magnesium parts relatively expensive. Knowledge of the microstructural effects on magnesium formability will help reduce the cost of these products. In this thesis, the microstructural factors that affect the formability of rolled and extruded magnesium sheet were compared. It was found that the degree of dynamic recrystallization was the factor that controlled elongation. Dynamic recrystallization produced a finer grain size, which resulted in a transition in deformation mechanism from dislocation slip to grain boundary sliding. Digital image correlation was used to study local stresses during tensile deformation, and to determine if magnesium satisfies Considere's criterion before failure. The results indicated that local stresses developed during deformation satisfied Considere's criterion, although the global strains were lower than the theoretical predictions. / Thesis / Master of Applied Science (MASc)

The concept of the Forming Limit Diagram (FLD) has proved to be useful for representing conditions for the onset of sheet necking, and is now a standard tool for characterizing materials in terms of their overall forming behavior. In this study, the M-K approach, in conjunction with Gurson model, is used to calculate FLDs. The influences of mechanical properties, including strain hardening, strain rate sensitivity, as well as the void nucleation, growth and coalescence, on the FLDs are examined. Most sheet metals undergo multiple deformation modes (strain paths) when being formed into complex manufacturing parts. When the strain path is changed in the deformation processing of metal, it's work-hardening and flow strength differs from the monotonic deformation characteristics. As a consequence, sheet metal formability is very sensitive to strain path changes. In this study, the hardening behavior and damage evolution under non-proportional loading paths are investigated. The effect of strain path change on FLDs is studied in detail. FLDs are conventionally constructed in strain space and are very sensitive to strain path changes. Alternatively, many researchers represented formability based on the state of stress rather than the state of strain. They constructed a Forming Limit Stress Diagram (FLSD) by plotting the calculated principal stresses at necking. It was concluded that FLSDs were almost path-independent. In this work, the FLSD has been constructed under non-proportional loading conditions to assess its path dependency when damage effect is included. / Thesis / Master of Applied Science (MASc)

The feasibility of utilizing TiB₂-reinforced near-gamma TiAl intermetallic matrix composites (IMCs) as a reinforcing entity within a commercially pure Ti matrix was investigated. IMCs are "ceramic-like" at ambient to moderate temperatures, and “metallic-like" in their deformation behavior above their brittle-to-ductile transition temperature, thus IMCs create opportunities to create unique in-situ composite microstructures otherwise unattainable using conventional ceramic reinforcements. CP titanium composites reinforced with 20 vol% of near-gamma TiAl IMC were produced by powder blending and densifying via high temperature extrusion deformation processing. The microstructures of the in-situ processed composites were characterized in terms of size, aspect ratio and average spacing of the IMC reinforcement. The microstructural features were correlated to observed mechanical behavior of the composites relative to the unreinforced matrix. The results indicate that the strengthening is derived from microstructural changes within the matrix due to the presence of the IMC particles, and solid solution strengthening due to the diffusion of Al from the reinforcing IMC phase into the Ti matrix. The increase in flow strength due to the former contribution correlates with the inverse square root of the IMC interparticle spacing. / Master of Science

This thesis deals with metal foam, a relatively new material whose tremendous potential has been identified early on. The material is an excellent shock absorber and also has a very high strength-to-weight ratio, properties that are highly desirable particularly within the aerospace and automotive industries. Despite the material’s immense potential, hardly any metal foam products have made it past the prototype stage. The reason is that the material is difficult to manufacture in the shapes required in industrial applications. Oftentimes, applications require sheets to be bent into specific shapes, yet bending is not possible with conventional methods. Laser forming is currently the only method that shows promise to bend metal foam panels to a range of shapes. In this thesis, the analysis of laser forming of metal foam was taken far beyond the experimental work that has been delivered thus far. A thorough analysis was performed of the thermo-mechanical bending mechanism that governs the deformation of metal foam during laser forming. This knowledge was then used to explain the effect of the process condition on the bending efficiency and the bending limit. Additionally, the impact of laser forming on the metal foam properties was explored. Experimental results were complemented by numerical results that were validated both thermally (using infrared imaging) as well as mechanically (using digital image correlation). Numerical models with different levels of geometrical complexities were used, and the effect of the model geometry on the predictive accuracy was explored. In the second half of the thesis, the aforementioned effort was extended to metal foam sandwich panels, in which metal foam is sandwiched between two sheets of solid metal. The material again has a high strength-to-weight ratio and excellent shock absorption capacity, while also being stiff and core-protective. Just like metal foam alone, metal foam sandwich panels are typically manufactured in flat sheets, and failure-free bending can only be achieved using lasers. The analysis was again initiated with the bending mechanism. It was revisited whether the foam core still follows the same bending mechanism, and how its deformation is affected by the interaction with the solid facesheets. This insight was then used to elucidate the bending efficiency and limit at different process conditions, as well as the impact of the process on the material performance. Additionally, the effect of the sandwich panel manufacturing method on the process outcome was investigated. This was achieved by contrasting two sandwich panel types with a different foam core structure, foam core composition, facesheet composition and facesheet attachment method. Lastly, three-dimensional deformation of metal foam sandwich panels into typical non-Euclidean shapes such as bowl and saddle shapes was explored. It was shown that a significant amount of 3D deformation can be induced. At the same time, it was discussed that the achievable deformation is limited to moderate curvatures, since only a limited amount of in-plane strains may be induced using laser forming. The aforementioned experimental efforts were again accompanied by numerical efforts. Sandwich panel models with different levels of geometrical complexity were used to study all aspects pertaining to the process, and the properties to the facesheet/foam core interface were discussed. Overall, the work in this thesis demonstrated that laser forming is capable of bending metal foam panels and metal foam sandwich panels up to large bending angles without causing failures, while maintaining the favorable properties of the material. Conceptual, experimental and numerical groundwork was laid towards a successful implementation of the material in industrial applications.

This thesis presents the results of experimental and numerical work carried out to determine if electromagnetic forming (EMF) increases the formability of aluminum alloy sheet and, if so, to determine the mechanisms that play a role in the increased formability. To this end, free form (open cavity) and conical in-die samples were produced to isolate high strain rate constitutive and inertial effects from the effects of the interaction between the die and the sheet. Aluminum alloys AA5754 and AA6111 in the form of 1mm sheet were chosen since they are currently used in automotive production and are candidates for lightweight body panels. The experiments showed significant increases in formability in the conical die samples in areas where significant contact with the tool occurred, with no significant increase recorded for the free-formed samples. This indicates that the tool/sheet interaction is playing the dominant role in the increase in formability observed. Metallographic and fractographic analysis performed on the samples showed evidence of microvoid damage suppression, which may be a contributing factor to the increase in formability. Numerical modeling was undertaken to analyse the details of the forming operation and to determine the mechanisms behind the increased formability. The numerical calculations were performed with an explicit dynamic finite element structural code, using an analytical electromagnetic pressure distribution. Microvoid damage evolution was predicted using a microvoid damage subroutine based on the Gurson-Tvergaard-Needleman constitutive model. From the models it has been determined that the free forming process is essentially a plane-stress process. In contrast, the tool/sheet interaction produced in cone forming makes the process unique. When the sheet makes contact with the tool, it is subject to forces generated due to the impact, and very rapid bending and straightening. These combine to produce complex non-linear stress and strain histories, which render the process non-plane stress and thus make it significantly different from conventional sheet forming processes. Another characteristic of the process is that the majority of the plastic deformation occurs at impact, leading to strain rates on the order of 10,000 s^-1. It is concluded that the rapid impact, bending and straightening that results from the tool/sheet interaction is the main cause of the increased formability observed in EM forming.

This thesis presents the results of experimental and numerical work carried out to determine if electromagnetic forming (EMF) increases the formability of aluminum alloy sheet and, if so, to determine the mechanisms that play a role in the increased formability. To this end, free form (open cavity) and conical in-die samples were produced to isolate high strain rate constitutive and inertial effects from the effects of the interaction between the die and the sheet. Aluminum alloys AA5754 and AA6111 in the form of 1mm sheet were chosen since they are currently used in automotive production and are candidates for lightweight body panels. The experiments showed significant increases in formability in the conical die samples in areas where significant contact with the tool occurred, with no significant increase recorded for the free-formed samples. This indicates that the tool/sheet interaction is playing the dominant role in the increase in formability observed. Metallographic and fractographic analysis performed on the samples showed evidence of microvoid damage suppression, which may be a contributing factor to the increase in formability. Numerical modeling was undertaken to analyse the details of the forming operation and to determine the mechanisms behind the increased formability. The numerical calculations were performed with an explicit dynamic finite element structural code, using an analytical electromagnetic pressure distribution. Microvoid damage evolution was predicted using a microvoid damage subroutine based on the Gurson-Tvergaard-Needleman constitutive model. From the models it has been determined that the free forming process is essentially a plane-stress process. In contrast, the tool/sheet interaction produced in cone forming makes the process unique. When the sheet makes contact with the tool, it is subject to forces generated due to the impact, and very rapid bending and straightening. These combine to produce complex non-linear stress and strain histories, which render the process non-plane stress and thus make it significantly different from conventional sheet forming processes. Another characteristic of the process is that the majority of the plastic deformation occurs at impact, leading to strain rates on the order of 10,000 s^-1. It is concluded that the rapid impact, bending and straightening that results from the tool/sheet interaction is the main cause of the increased formability observed in EM forming.

In order to effectively and efficiently hydroform new automotive components, the formability of new tubular steels must be evaluated. Standard forming limit diagrams have been used for decades to evaluate and predict the formability of sheet steel formed along linear strain paths. However, tube hydroforming can present a problem since the pre-bending stage used in many hydroforming operations causes multiple non-linear strain paths. This thesis has modified a formability test method that deforms small-scale sheet steel samples in a single plane. The sample geometries were designed such that the strain paths achieved at the center of the samples were very near the plane strain condition. The four steels chosen for this study were: a deep drawing quality (DDQ), a high strength low alloy (HSLA) and two dual phase steels (DP600 and DP780). The plane strain formability for each of the four steels was tested in both the rolling and transverse directions. Three objective criteria were employed to evaluate and directly compare the formability of the four steels tested: difference in strain, difference in strain rate and local necking. The DDQ steel showed the highest formability followed in order by the HSLA, DP600 and DP780 steels. The repeatability in determining the forming limit strains using the difference in strain, the difference in strain rate and the local necking criteria for a 95% confidence interval was ± 1.5%, ± 1.2% and ± 3.2% engineering strain, respectively. The forming limit data collected for this thesis has been compared to results from full-scale tube hydroforming operations and free expansion tube burst tests carried out by researchers at the University of Waterloo on the same four materials. It was found that local necking results could be used to predict failure of hydroformed HSLA steel tubes with low levels of end-feed. However, this same method could only predict the failure of hydroformed DP600 steel tubes at higher levels of end-feed. The three objective criteria were not found to be suitable for predicting failure of free expansion tube burst tests. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2007-09-27 15:00:35.873

Transformation induced plasticity (TRIP) steels are a class of steels with exceptional formability properties, due mainly to the presence of meta-stable retained austenite which transforms to martensite under loading, locally hardening the steel. The volume fraction and mechanical stability of the retained austenite play an important role in producing the high formabilities of TRIP steels. In this thesis, two separate morphologies of retained austenite, equiaxed versus lamellar, have been produced through thermo-mechanical processing of a single common TRIP steel chemistry. The sheet formability characteristics of these two microstructures were examined, with varying volume fractions of retained austenite, through uniaxial tensile and in-plane plane-strain (IPPS) testing. It was found that higher levels of retained austenite produced better formability properties for both microstructures and strain paths. In uniaxial tension it was seen that the the lamellar microstructure attained higher strains at maximum load, and exhibited more sustained instantaneous n values than the equiaxed structure, despite having a lower volume fraction of retained austenite. IPPS testing was performed using an optical measurement of local strain and a comparative forming limit based on differences in strain rate between a developing neck and the surrounding material. It was found that the lamellar microstructure performed better than the equiaxed microstructure for this strain path, achieving higher strains before reaching the comparative forming limit. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-01-25 16:36:07.598

Les procédés de mise en forme des tôles minces sont largement utilisés dans l'industrie. L’utilisation optimale des alliages légers ou des aciers à haute résistance, propices à des économies d’énergie dans le domaine des transports, nécessite une connaissance approfondie de leurs limites de formabilité. Classiquement, la formabilité d’une tôle est caractérisée par l’apparition d’une striction localisée. Cependant, pour des chargements spécifiques (chemins de déformation complexes ...), la rupture caractérise la formabilité du matériau, la courbe limite de formage à rupture (CLFR) plutôt que celle à striction (CLFS) doit alors être considérée. Pour identifier la CLFS et la CLFR pour des chemins de déformation linéaires et non-linéaires, les méthodes conventionnelles requièrent différents dispositifs expérimentaux et différentes formes d'éprouvette pour atteindre une large gamme de chemins de déformation. L'essai de traction biaxiale, associé à une éprouvette cruciforme, est possible pour la réaliser. De plus, le changement de chemin est activé au cours de l’essai, sans déchargement. Le premier objectif de cette étude est de montrer que l'essai de traction biaxiale, associé à une forme unique d'éprouvette cruciforme, permet de tracer la CLFS et la CLFR pour plusieurs chemins de déformation, qu’ils soient linéaires ou non-linéaires. En premier lieu, des essais ont été réalisés sur des tôles d’alliage d’aluminium 5086 (épaisseur initiale de 4 mm) à partir d’une forme d’éprouvette déjà proposée au laboratoire. Une nouvelle forme d'éprouvette cruciforme a été proposée pour des tôles moins épaisses (2 mm), plus répandues. Cet éprouvette a été validée pour étudier la formabilité d’un acier dual phase DP600 pour plusieurs chemins de déformation. Le deuxième objectif est de discuter la validité de critères classiques de rupture ductile. Pour les deux matériaux, un critère a finalement été identifié pour prédire assez précisément les résultats expérimentaux. / Sheet metal forming is very common in industry for producing various components. The optimal use of light alloys or high strength steels in transportation for energy economy, requires in-depth analysis of their formability. Usually, the formability of sheet metal is controlled by the onset of localized necking. However, under specific loadings (complex strain paths...), fracture characterizes the formability and the forming limit curve at fracture (FLCF) instead of the forming limit curve at necking (FLCN) should be considered. For identifying FLCN and FLCF under linear and non-linear strain paths, conventional methods require different experimental devices and geometrical specifications of specimen to cover a wide range of strain paths. However, using the in-plane biaxial tensile test with just one shape of cruciform is sufficient for that, even changes of strain path without unloading can be made during the test. The first objective of this study is to show that the in-plane biaxial tensile test with a single type of cruciform specimen permits to investigate the FLCN and FLCF of sheet metals under different linear and non-linear strain paths. Firstly, the forming limit strains at fracture of AA5086 sheet (t=4 mm) under linear and non-linear strain paths have been characterized, by testing an existed dedicated cruciform specimen. Thinner sheet metals are often used in industry, so a new shape of cruciform specimen with an original thickness of 2 mm was proposed. This specimen is successfully used to investigate the formability of DP600 sheet under linear and two types of non-linear strain paths. The second objective is to discuss the validity of commonly used ductile fracture criteria to predict the onset of fracture. Some ductile fracture criteria were used to produce numerical FLCFs for AA5086 and DP600 sheet. Finally, for the two tested materials, it is possible to find a criterion to predict the experimental FLCFs for either linear or non-linear strain paths.