Ravi Verma

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.

Local Mechanical Properties of a Magnesium Hood Inner Component Formed at Elevated Temperature

There is considerable worldwide interest in magnesium (Mg) sheet as a replacement for heavier steel and aluminum alloys in vehicle closure components. As Mg gains acceptance in the automotive industry, there will be an increasing demand for accurate material properties for finite element simulations of Mg structures. In this paper, we investigate the extent to which average grain size and postformed tensile properties vary across a Mg AZ31B hood inner component formed at 485°C for 20 min under a constant gas pressure. Tensile specimens were extracted from six regions of the hood inner, which underwent varying degrees of thinning. A state-of-the-art digital image correlation (DIC) algorithm and custom image acquisition software provided true stress-true strain data for each specimen. Tensile data acquired during room temperature testing was compared with that from baseline (undeformed) Mg AZ31B in a fully recrystallized condition (O-temper). Due to its importance in finite element simulations, particular emphasis was placed on the variation of postformed yield strength with specimen thickness and average grain size. Finally, we compute local strain fields during fracture in a tensile specimen with DIC grids positioned in the failure region.

Recrystallization in AZ31 Magnesium Alloy during Hot Deformation

In this study, isothermal torsion tests were carried out on magnesium AZ31B alloy under constant strain rate conditions, in the range of 250 to 400oC at 0.01, 0.1, and 1.0 s-1. Alloy flow stress dependence on strain rate and temperature can be described by a power law with activation energy of 130 kJ/mol. Microstructural examination of hot deformed samples shows very fine recrystallized grains decorating grain boundaries of larger gains in the form of a necklace. These fine grains are produced by dynamic recrystallization at the grain boundaries of original grains. Microstructure evolution, based on samples quenched at different strain levels, indicates that increasing deformation strain has little effect on recrystallized grain size but widens the recrystallized region, with full recrystallization achieved at a certain high strain level. Recrystallized grain size increases with increasing deformation temperature and strain rate. The latter suggests recrystallization in AZ31 to be essentially a time dependant phenomenon.

Local Thinning at a Die Entry Radius During Hot Gas-Pressure Forming of an AA5083 Sheet

Splitting at regions of local thinning below die entry radii is a critically important mechanism of failure in hot gas-pressure forming of sheet materials. Local thinning is controlled by sheet-die friction and die geometry, as well as sheet material properties. In this study, local thinning is investigated at a particularly severe die entry radius during hot forming of a fine-grained AA5083 sheet at 450°C. Particular emphasis is placed on the relationship between local thinning and sheet-die friction conditions. A simple analysis of the mechanics of this thinning phenomenon is presented. Finite element simulation results are presented for different sheet-die friction conditions. Sheet thickness profiles measured from parts produced in forming experiments using three different lubrication conditions are compared with predictions from simulations. Simulation predictions agree well with experimental data for the occurrence and location of thinning below a die entry radius. Additional insights into sheet-die friction for controlling local thinning and preventing premature necking failure are detailed.

The USAMP Magnesium Front End Research and Development Project — Results of the Magnesium “Demonstration” Structure

This paper summarizes the final results of the United States Automotive Materials Partnership’s (USAMP’s) Magnesium Front End Research and Development (MFERD) Phase I and Phase II project focusing on a Demonstration (“Demo”) structure consisting of a cast AM60B shock tower, extruded AM30 rail and a warm-formed AZ31 sheet component. The project team designed and fabricated piece parts, assembled, surface finished and tested over 200 “Demo” magnesium front end substructures employing friction-stir linear lap welding (FSLW) and laser-assisted self-pierce rivets (LSPR), both with and without adhesive bonding. The “Demo” structures also employed two different coating strategies to mitigate corrosion. The structures were tested in multi-axial fatigue and overload conditions and the results were compared to simulation. They were also tested using different corrosion testing protocols to determine the corrosion susceptibility of the joint regions and the base metal in a structure containing dissimilar Mg alloys.

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.

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.

Hot Formability Curves for Four AZ31B Magnesium Alloy Sheets Obtained by the Pneumatic Stretching Test

The formability of four Mg AZ31B sheets produced by either direct chill or twin roll continuous casting, and having different initial grain sizes, was investigated at 400 °C and 5x10−3 s−1 using the pneumatic stretching test. Blanks were pneumatically bulged through four elliptical die inserts, with aspect ratios ranging between 1.0 and 0.4, producing ellipsoidal domes with different biaxial strain combinations. Testing was carried out in two ways: with the major strains being aligned either along or across the rolling direction of the material. Strain combinations were measured in the deformed specimens, and the forming limit curves were constructed for each of the four sheets in two orientations. The results show great impact of sheet orientation on material formability limits. Additionally, the results reveal significant differences between the four sheets; those differences were correlated back to disparities in grain structure and material inhomogeneities.

Effect of initial microstructure under different solidification conditions on hot working behavior of AZ31 alloy

In order to investigate the effect of the as-cast microstructure on the hot working behavior of an AZ31 magnesium alloy, specimens were cast in copper moulds with and without water cooling. A series of compression tests were performed at a temperature of 350 °C, a strain rate of 0.01 s-1, and at strains up to 1.0. It was found that as-cast microstructure is very sensitive to the solidification conditions, which leads to a significant difference in flow behavior and dynamic recrystallization (DRX) characteristics. It appears that more uniform and refined as-cast grain size promotes dynamic recrystallization and reduces the flow stress. It is also possible that second phases (>1+m in size) contribute to DRX by acting as nuclei.