Formability of Textured Sheets from Magnesium Alloys
Improvement of Formability of Textured Sheets from Magnesium Alloys
Tech Area / Field
- MAN-MPS/Manufacturing, Planning, Processing and Control/Manufacturing Technology
- MAT-ALL/High Performance Metals and Alloys/Materials
3 Approved without Funding
All-Russian Institute of Light Alloys, Russia, Moscow
- VNIITF, Russia, Chelyabinsk reg., Snezhinsk\nMISIS (Steel and Alloys), Russia, Moscow
- Thyssen Krupp AG / MgF Magnesium Flachprodukte, Germany, Freiberg
Project summaryIt is well known, that sheet materials intended for making details and car nodes (hoods, wings, doors and others) must posses sufficiently high formability parameters R and n, that can be defined from mechanical tensile tests for flat samples. In this case, normal anisotropy parameter R can be calculated from the following equation:
where fb and fs are logarithms of strain relation on the width to the thickness for the flat tensile sample, b0 and s0 are the width and the thickness of the sample before deformation, b1 and s1 are the width and the thickness of the sample after deformation. Hardening factor n can be found from the flow curve that can be approximated using a function:
where s is tensile stress, k is constant, e is degree of deformation. The value n is possible to determine after taking a logarithm of the flow curve. The value that is equal to:
is called a formability parameter and characterises the tendency of the sheet material to the deep drawing. The higher is the value of K, the greater is the formability parameter of the investigated sheet material. If a technique of the magnesium alloy sheet making is known, determing of R and n values after tension tests of the samples at the axial loading gives a tendency estimation of the sheet material to the deep drawing. R and n, in terms, depend on a chemical composition of the alloy and the structure and texture parameters, that are formed as a result of the rolling and heat treatment processes.
In fact, the value of the normal anisotropy parameter R depends mostly on a crystallographic texture, that is formed during the rolling and annealing in the sheets of the investigated material. The value of R parameter can be quantitatively found from pole figures, by modelling the axial tension process taking into account with known mechanisms of the sliding and twinning and values of critical-resolved shear stress (CRSS) on the active strain systems [1-4].In VILS's laboratory of mathematical simulation of plastic deformation processes it was worked out several computer programmes for calculating R on the pole figures and on the CRSS values at the active strain systems, afforded an axial tension of the investigated alloy sheets. These programmes were used for prediction of dependence R(a) (a is an angle between the rolling direction and the chosen one in the plane of sheet) in the low-carbon steel and titanium alloy sheets [5,6].The calculated results agree with the experimental ones within the limits of experimental accuracy. Using such programmes with X-ray experimental procedure for pole figure definition, that our laboratory also can organise, gives a good opportunity to investigate influence of the special types of texture on the R for determination of the optimal textures ensured the maximal values of R. Knowing the defining termomechanical parameters, that lead to formation of necessary textures, it is possible to optimize the rolling and heat treatment conditons to obtain the maximal value R. So it is offered a scientific work on the investigation of possibilities of increasing of the sheet formability parameters from the magnesium alloy AZ31, which consists of the following parts:
1. To develop plastic deformation models of magnesium alloys in view of the chemical composition, structure and texture parameters at the axial loading.
2. To define the formability parameters (R and n) from mechanical tests for some magnesium alloy sheets, obtained on the known technology. To investigate the sheet textures with the help pole figure measurement. On the basis of the elaborated plastic deformation model to make the computer calculation of coefficient R on the pole figures (evaluation of and dependence R(a) in sheet plane). To compare with the calculated and experimental dependences R(a). To correct the calculated results.
3. To investigate behaviour of the texture forming during the rolling and heat treatment of the given magnesium alloy sheets using the computer simulation. To establish the texture behaviour from the termomechanical parameters of the technological process and chemical composition of the under study alloy. To predict the normal anisotropy coefficient on the under investigation textures. To choose the optimal textures, affording the maximal values and minimal spread in values R in the different directions of the sheet. To work out recommendations to the parameters of rolling and annealing, affording the optimal texture securing or the most closed ones to them and causing to the increasing of formability parameter of the investigated magnesium sheet.
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