The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over . Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels (, , , ) each with the pre-assigned strain rate levels of , and are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.

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Evaluation of Strain, Strain Rate and Temperature Dependent Flow Stress Model for Magnesium Alloy Sheets

This study aimed at providing an experimental database for the mechanical properties of AZ31B magnesium alloy sheet such as stress-strain curve, yield stress, R-value and forming limit diagram(FLD) at various strain-rates and temperatures. Tensile tests were carried out on specimens having the orientations of 0°, 45° and 90° to the rolling direction with different crosshead speeds in the range between 0.008 and 8 mm/s at temperature from 25(room temperature) to 300℃. The influence of the specimen gage length on the tensile properties was investigated. FLD tests were performed at punch speed of 0.1 and 1.0 mm/s in the same temperature range as that of the tensile tests. Swift cup tests were conducted to verify the usefulness of the material database and the reliability of the finite element analysis(FEA). The effects of strain-rate as well as temperature were taken into account in these simulations. It was shown that the FLD-based failure was reasonably well predicted by the thermal-deformation coupled analysis for this rate-sensitive material.

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Experimental and Analytical Evaluation of Forming Characteristics for AZ31B Magnesium Alloy Sheet

In this paper, the formability of E-FORM magnesium alloy sheet (as one of recent alloys for automotive magnesium sheets) is analyzed based on the comparison between the results of actual magnesium (Mg) roof forming and those of finite element (FE) simulations. The FE model considers anisotropic mechanical properties and forming limit diagram (FLD) of the investigated magnesium alloy sheet. Through the coupled experimental and numerical procedure, the dominating factors for improving the accuracy of the numerical simulations are further studied. A commercial finite element analysis program, AUTOFORM®, is used for the simulations, in which identified mechanical properties and forming limit criteria are implemented as functions of temperature and strain rate. The improvement in the prediction of formability during the warm forming of the E-FORM magnesium alloy sheet can be obtained by considering a modified hardening law at large strain region, sheet anisotropy, and the properly measured forming limit curve.

Effect of Anisotropic Plasticity on the Prediction of Formability of E-Form Magnesium Alloy Sheet

The roll die forming (RDF) process is a new manufacturing technique for producing gear parts such as clutch drum and clutch hub in automotive transmission. In the RDF process, the material is deformed by a roll installed on a die set. Excellent productivity, low forming load and improved dimensional accuracy have quantitatively been shown to be the benefits of the RDF. In this study, the RDF process is applied to manufacture a clutch hub with a gear shaped part. A finite element (FE) analysis was performed in order to investigate the material strain field and dimension of the final product. Based on the result of the FE analysis, a RDF experiment was performed and the dimensional accuracy of the final product was validated. This work demonstrates that RDF is a process capable of producing a sound clutch hub.

Roll Die Forming Process for Manufacturing Clutch Hub in Automotive Transmission

According to current development trends for automotive body parts, light weight is a key issue in improving fuel efficiency and CO2 reduction. Compared to steel and aluminum, magnesium has a relatively low specific gravity. However, it is challenging to use magnesium to produce a product at room temperature because magnesium has a Hexagonal Close-Packed (HCP) crystal structure. Therefore, the structure is not suitable for plastic deformation without using a heating system. As a result, a magnesium sheet and die need to be heated from 250 to 300°C. Unfortunately, this procedure has positive and negative effects. It increases not only the material’s elongation, but also the friction between the material and die during warm forming. Furthermore, lubricant cannot be used due to corrosion occurring on the surface of a part after manufacturing. To overcome these challenges, we developed a “Partition panel,” which is a warm-formed part with a non-lubricated die with a Diamond-Like Carbon (DLC) coating. A one-quarter sized prototype was successfully produced with the DLC-coated non-lubricated die, and its characteristics were verified through an evaluation of the friction coefficient and mechanical properties.

Development of non-lubricated die for magnesium sheet forming of car body part

In this study, a forming magnesium alloy circular cup was simulated accounting for heat transfer at elevated temperatures. In order to predict the failure of magnesium alloy sheet during simulation, the forming limit diagram, which is used in sheet metal forming analysis to determine the criterion for failure, was investigated. For the failure prediction in the simulation accounting for heat transfer, the forming limit diagram for a temperature the same as the temperature of the blank element was used. The result of the simulation showed that the drawn depth increases with the increase of the die-holder temperature, and is in accord with the experimental results above the die-holder temperature of . The forming limit diagram provided a good guide for the failure prediction of warm forming simulation accounting for heat transfer. In addition, the effect of the tool shoulder radius on the drawn depth at various tool temperatures is verified using the simulation conditions which agreed with the experimental results.

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Evaluation of the Formability of Warm Forming Simulation of Magnesium Alloy Sheet Using FLD

Due to the low densities and high specific strength and stiffness, magnesium alloy sheets are very attractive lightweight materials for automotive and electrical products. However, the magnesium alloy sheets should be usually formed at elevated temperature because of their poor formability at room temperature. For the use of the magnesium alloy sheets for an industrial, their mechanical properties at elevated temperature and appropriate forming process conditions have to be developed. In this study, non-isothermal simulation of a square cup drawing of magnesium alloy sheets have been conducted to evaluate a proper forming process conditions such as the tool temperature, the tool shoulder radius, friction between the blank and the tools. According to this study, appropriate forming process conditions of square cup drawing at elevated temperature from magnesium alloy sheets are suggested.

M.H. Lee

Papers

Evaluation of the Formability of Warm Forming Simulation of Magnesium Alloy Sheet Using FLD

Forming Analysis for Warm Deep Drawing Process of Magnesium Alloy Sheet