Jon T. Carter

Rate sensitivity and tension-compression asymmetry in AZ31B magnesium alloy sheet.

The constitutive response of a commercial magnesium alloy rolled sheet (AZ31B-O) is studied based on room temperature tensile and compressive tests at strain rates ranging from 10−3 to 103 s−1. Because of its strong basal texture, this alloy exhibits a significant tension–compression asymmetry (strength differential) that is manifest further in terms of rather different strain rate sensitivity under tensile versus compressive loading. Under tensile loading, this alloy exhibits conventional positive strain rate sensitivity. Under compressive loading, the flow stress is initially rate insensitive until twinning is exhausted after which slip processes are activated, and conventional rate sensitivity is recovered. The material exhibits rather mild in-plane anisotropy in terms of strength, but strong transverse anisotropy (r-value), and a high degree of variation in the measured r-values along the different sheet orientations which is indicative of a higher degree of anisotropy than that observed based solely upon the variation in stresses. This rather complex behaviour is attributed to the strong basal texture, and the different deformation mechanisms being activated as the orientation and sign of applied loading are varied. A new constitutive equation is proposed to model the measured compressive behaviour that captures the rate sensitivity of the sigmoidal stress–strain response. The measured tensile stress–strain response is fit to the Zerilli–Armstrong hcp material model.

Analytical Method for Forming Limit Diagram Prediction with Application to a Magnesium ZEK100-O Alloy

A significant barrier to broader implementation of magnesium alloys is their poor room temperature formability, a consequence of the anisotropic response of the Mg hexagonal closed-packed (hcp) crystal structure. Additions of rare earth (RE) elements, such as in the ZEK100 alloys, weaken the texture and improve formability. Room temperature forming limit analyses of RE-containing Mg alloys, particularly Mg ZEK100, have not been explored to any significant extent in the literature. In this paper, strain-based forming limit diagrams (FLDs) are derived for an Mg ZEK100-O alloy (Zn1.34Zr0.23Nd0.182, wt.%) using an analytical method that combines the vertex theory of Storen and Rice (J Mech Phys Solids, 23:421-441, 1979), the anisotropic yield criterion of Barlat and Lian (Int J Plast, 5:51-66, 1989), and a hardening law. The method does not rely on assumptions about pre-existing defects, is broadly applicable to sheet alloys exhibiting in-plane anisotropy requiring a higher-order yield criterion, and requires only minimal experimental inputs. Results from the analytical method are compared with experimentally derived FLDs based upon the well-known Nakajima test and tensile deformation, and with predictions from an existing analytical method for FLDs. Close agreement between the experimentally derived FLDs and the present theoretical method was obtained. Sheet materials where the theoretical method does not apply are also discussed.

High-Temperature Forming of a Vehicle Closure Component in Fine-Grained Aluminum Alloy AA5083: Finite Element Simulations and Experiments

Fine-grained AA5083 aluminum-magnesium alloy sheet can be formed into complex closure components with the Quick Plastic Forming process at high temperature (450oC). Material models that account for both the deformation mechanisms active during forming and the effect of stress state on material response are required to accurately predict final sheet thickness profiles, the locations of potential forming defects and forming cycle time. This study compares Finite Element (FE) predictions for forming of an automobile decklid inner panel in fine-grained AA5083 using two different material models. These are: the no-threshold, two-mechanism (NTTM) model and the Zhao. The effect of sheet/die friction is evaluated with five different sheet/die friction coefficients. Comparisons of predicted sheet thickness profiles with those obtained from a formed AA5083 panel shows that the NTTM model provides the most accurate predictions.

Comparison of Tensile Properties and Crystallographic Textures of Three Magnesium Alloy Sheets

The most common commercially available rolled magnesium sheet alloy is AZ31B (typ. 3% Al, 1% Zn, 0.4% Mn, balance Mg). One of the often-cited shortcomings of this sheet is its limited formability at room temperature, which is attributed in part to a strong crystallographic texture in which the basal planes of the hexagonal unit cell are parallel to the plane of the sheet. Attempts have been made to avoid this rolling-induced texture by changing either (a) the alloy composition or (b) the rolling process. Specifically, sheet has been made using the conventional rolling practice, but changing the alloy to ZEK100 (typ. 1% Zn, 0.2 % Nd, 0.2% Zr, balance Mg), or by keeping the AZ31B composition but rolling at a much higher temperature. In this report, both types of sheet are evaluated and compared with conventionally rolled AZ31B sheet. Both show reduced texture and attractive tensile properties, and therefore both are expected to show greater room-temperature formability than conventionally rolled AZ31B.

Hot rolling of AZ31 Magnesium alloy to sheet gauge

This study details preliminary results of hot rolling trials of AZ31 alloy sheet using a pilot-scale rolling mill. The aim is to design and optimize the hot rolling schedule for AZ31 in order to produce sheet with a fine and homogeneous microstructure. The study examined three different hot rolling temperatures, 350, 400 and 450°C and two rolling speeds, 20 and 50 RPM. A total thickness reduction of 67% was obtained using multiple passes with reductions of either 15% or 30% per pass. The entry temperature of each rolling schedule was kept constant, by reheating the strip between passes. It was found that the microstructure of the AZ31 alloy was sensitive to the rolling temperature, the reduction (i.e. strain) per pass and the rolling speed (i.e. strain rate). A combination of a rolling temperature of 400°C, reduction per pass of 15%, and rolling speed of 50 RPM produced the finest and most homogeneous microstructure. The finite element software ADINA was used to simulate the evolution of the thermal profile of AZ31 sheet during hot rolling. The predicted exit temperatures were in good agreement with the measured temperatures.

Static recrystallization and grain growth in AZ31B-H24 magnesium alloy sheet

The effects of static annealing on recovery, recrystallization and grain growth in a magnesium alloy sheet are investigated at 50°C to 450°C. Full recrystallization is observed after annealing at 250°C or higher temperatures. Recrystallized grain size increases with temperature through normal grain growth. Room-temperature hardness drops abruptly following recrystallization and then decreases with increasing grain size. Predictive relationships are proposed for recrystallized grain size as a function of temperature and time and for hardness as a function of recrystallized grain size.

Gas-Pressure Bulge Forming of Mg AZ31 Sheet at 450°C

Magnesium (Mg) sheet materials, such as wrought AZ31, possess low densities and high strength- and stiffness-to-weight ratios. These properties suggest that the use of Mg sheet is viable for reducing vehicle weight, an important goal of the automotive industry. Magnesium exhibits poor ductility at room temperature, but high-temperature forming processes may be used to manufacture complex vehicle closure panels. Tensile tests are the most common method of characterizing the plastic deformation of sheet materials. However, gas-pressure bulge tests may be more representative of the stress states that occur during the manufacture of sheet metal components. This study investigates the plastic deformation of AZ31 sheet during both biaxial and plane-strain gas-pressure bulge forming at 450°C. The heights and thicknesses of formed specimens are measured and compared. The deformation behaviors of the AZ31 sheet are related to observations of grain growth and cavitation that occur during forming.

A Time-Dependent Material Model for the Simulation of Hot Gas-Pressure Forming of Magnesium Alloy AZ31

A time-dependent material constitutive model is developed for the deformation of wrought Mg AZ31 sheet material at 450°C. This material model is used to simulate gas-pressure bulge forming of AZ31 sheet into hemispherical domes. Finite-element-method (FEM) simulations using this material model are compared against experimental data obtained for dome height as a function of forming time under forming conditions identical to those assumed in the simulations. The time-dependent material model predicts experimental dome heights during forming with a quite useful accuracy. The most significant advantage of the time-dependent material model is that it can address the effect of preheating time on forming. Preheating times shorter than ~120 s produce an increase in forming rate. This material model provides a quantitative means of accounting for that effect.

The Effects of Strain and Stress State in Hot Forming of Mg AZ31 Sheet

Wrought magnesium alloys, such as AZ31 sheet, are of considerable interest for light-weighting of vehicle structural components. The poor room-temperature ductility of AZ31 sheet has been a hindrance to forming the complex part shapes necessary for practical applications. However, the outstanding formability of AZ31 sheet at elevated temperature provides an opportunity to overcome that problem. Complex demonstration components have already been produced at 450°C using gas-pressure forming. Accurate simulations of such hot, gas-pressure forming will be required for the design and optimization exercises necessary if this technology is to be implemented commercially. We report on experiments and simulations used to construct the accurate material constitutive models necessary for finite-element-method simulations. In particular, the effects of strain and stress state on plastic deformation of AZ31 sheet at 450°C are considered in material constitutive model development. Material models are validated against data from simple forming experiments.

Tensile Properties of Three Preform‐Annealed Magnesium Alloy Sheets

Magnesium alloy sheet metal is potentially attractive for use in automotive structural applications due to its high strength-to-weight ratio. However, application has been hindered by the low room-temperature formability of typical sheet alloys. One approach to effectively increase formability is to change the forming process from one which involves a single stamping hit to one which utilizes two hits plus an intemediate anneal (i.e., “preform anneal process” ). The purpose of the intermediate anneal is to restore some of the softness and ductility which were reduced by deformation during the first hit.