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METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic - 1 - EFFECT OF THERMOMECHANICAL PROCESSING ON PROPERTIES OF Al-Mg AUTO BODY SHEETS Kemal Delijic* Mitar Misovic* *University of Montenegro, Faculty of Metallurgy, Cetinjski put, 81000 Podgorica, Montenegro, YU [email protected] Abstract The results of determination of forming properties of Al-Mg4.5 alloys that satisfy the criteria 300-30 (300Mpa of tensile strength and 30% of elongation) and its dependence on some parameters of thermomechanical processing are presented. The effect of alloying elements content on the sheet properties is considered. The level of earring <3% and plastic anisotropy of the sheet can be successfully controlled by appropriate parameters of thermomechanical processing. Sheet forming ability increases by increasing the content of alloying elements. INTRODUCTION One of the objects of automobile manufacturers is the developing of the lightweight but strength material to build large and fuel saving automobile without compromise from safety and comfort. This can be achieved by reducing the weight of the automotive structure and exterior panels – the body in white /1-4/. The replacement of steel with aluminum sheet provides a direct body-in–white weight reduction. The basic requirement for automobile sheet is to have a high formability, suitability for adhesive bonding, weldability and good corrosion resistance /5, 6/. A variety of alloys have been used for different automobile panels. The 5000 series alloys have very good formability, and generally used for interior structural application. In recent years Al-Mg alloys have been developed for a car body sheets for external application and introduced to the market especially by Japanese producers /3/. In this paper the influence of some thermomechanical parameters on mechanical properties of Al-Mg alloys sheets will be considered. EXPERIMENTAL DETAILS The tests were performed on three different alloys. The chemical composition of the basic alloy AlMg4.5Mn is standardized, while the chemical composition of the other two alloys should improve the strength and plasticity of the basic alloy (table 1). Table 1. Chemical compositions of the investigated alloys Chemical composition, % (wt) Type of alloy Mg Zn Cu Mn Fe Si AlMg4.5Mn 4.23 0.02 0.015 0.42 0.26 0.13 AlMg4.5MnZnCu 4.03 0.34 0.16 0.43 0.24 0.22 AlMg4.5Zn1.5MnCu 4.00 1.31 0.18 0.21 0.22 0.21

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METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

- 1 -

EFFECT OF THERMOMECHANICAL PROCESSINGON PROPERTIES OF Al-Mg AUTO BODY SHEETS

Kemal Delijic*Mitar Misovic*

*University of Montenegro, Faculty of Metallurgy,Cetinjski put, 81000 Podgorica, Montenegro, [email protected]

AbstractThe results of determination of forming properties of Al-Mg4.5 alloys that satisfy the criteria300-30 (300Mpa of tensile strength and 30% of elongation) and its dependence on someparameters of thermomechanical processing are presented. The effect of alloying elementscontent on the sheet properties is considered. The level of earring <3% and plastic anisotropyof the sheet can be successfully controlled by appropriate parameters of thermomechanicalprocessing. Sheet forming ability increases by increasing the content of alloying elements.

INTRODUCTIONOne of the objects of automobile manufacturers is the developing of the lightweight butstrength material to build large and fuel saving automobile without compromise from safetyand comfort. This can be achieved by reducing the weight of the automotive structure andexterior panels – the body in white /1-4/. The replacement of steel with aluminum sheetprovides a direct body-in–white weight reduction. The basic requirement for automobile sheetis to have a high formability, suitability for adhesive bonding, weldability and good corrosionresistance /5, 6/. A variety of alloys have been used for different automobile panels. The 5000series alloys have very good formability, and generally used for interior structural application.In recent years Al-Mg alloys have been developed for a car body sheets for externalapplication and introduced to the market especially by Japanese producers /3/.In this paper the influence of some thermomechanical parameters on mechanical properties ofAl-Mg alloys sheets will be considered.

EXPERIMENTAL DETAILSThe tests were performed on three different alloys. The chemical composition of the basicalloy AlMg4.5Mn is standardized, while the chemical composition of the other two alloysshould improve the strength and plasticity of the basic alloy (table 1).

Table 1. Chemical compositions of the investigated alloysChemical composition, % (wt)Type of alloy

Mg Zn Cu Mn Fe SiAlMg4.5Mn 4.23 0.02 0.015 0.42 0.26 0.13AlMg4.5MnZnCu 4.03 0.34 0.16 0.43 0.24 0.22AlMg4.5Zn1.5MnCu 4.00 1.31 0.18 0.21 0.22 0.21

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

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The sheets are manufactured entirely in the laboratory (casting, cold rolling and heattreatment). The testing consisted of: hardness test, tensile test with determining the S value/7/,cylindrical-cup drawing test (β=2) and optical microscopy. The technological scheme of sheetproduction in laboratory conditions is presented in figure 1.

Figure 1. The technological scheme of sheet of 1.25mm thickness production in laboratoryconditions

RESULTS AND DISCUSSION

The microstructures of AlMg4.5MnZnCu and AlMg4.5Zn1.5MnCu alloys in as-cast,homogenized and the final annealed states are shown in figure 2. The micrographs of alloys inthe as-cast state show the inhomogeneous solid solution and eutectic as a result of crystalsegregation. The effect of phases dissolving during the homogenization can be seen inmicrographs of homogenized state of alloys.

TYPE OFALLOY

AS-CAST HOMOGENIZEDFULLY

ANNEALEDsheet of 1.25mm

hickness

AlMg4.5MnZnCu

x100 x100 x500

AlMg4.5Zn1.5MnCu

x100 x100 x500

Figure 2. Micrographs of AlMg4.5MnZnCu and AlMg4.5Zn1.5MnCu alloys in as-cast,homogenized and final annealed states

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

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The mechanical properties of sheets in final annealed state vs. alloying elements content areshown in figure 3. The addition of Zn and Cu causes the increasing the hardness for 25%. Thetensile strength and yield stress also increase with increasing the alloying elements content.Growth of these properties is about 20% compared to the basic AlMg4.5Mn alloy. Theinfluence of alloying elements content on formability index S is shown in figure 3.c. Thevalue of S increases linearly with increasing the content of alloying elements.

(a) (b) (c)

Figure 3. The influence of content of alloying elements on (a) tensile strength and yield stress,(b) hardness and elongation, (c) formability index S of final annealed sheets of 1.25mmthickness.

The results of investigation of tensile properties anisotropy in the plane of the sheet arepresented in figure 4. The annealed sheets show the lowest yield stress and tensile strength butthe highest elongation in 45o direction related to rolling direction. The addition of alloyingelements increases the level of anisotropy of these properties.

(a) (b) (c)

Figure 4. The anisotropy of tensile properties in the plane of the annealed sheets: (a) yieldstrength, (b) tensile strength, (c) elongation.

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

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The influence of the cold rolling reduction on mechanical properties of alloys is shown in thefigure 5. Changes of the mechanical properties during cold rolling represent the usualbehavior of metal sheets: sudden increasing of strength and decreasing of elongation at thebeginning 20% of reduction. The origin differences of properties, that are the result ofalloying, remain in the whole cold rolling reduction range.

(a) (b) (c)

Figure 5. The influence of cold rolling reduction on tensile properties: (a) tensile strength, (b)yield strength, (c) elongation

The influence of cold rolling reduction and the initial earring of annealed sheets on the earringof the rolled sheet is presented in the figure 6. Cold rolling of sheets up to 20% of reductioncauses small changes of ears. The earring significantly increases by increasing of rollingreduction more than 20%.

Figure 6. The influence of cold rolling Figure 7. The influence of coldreduction and the initial earring on the rolling reduction before the finalearring of the rolled sheet annealing and the initial earring on

earring of annealed sheet.

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

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The results of earring investigation of sheets in annealed state are presented in the figure 7.The influence of cold rolling reduction before the final annealing and the initial earring of asheetwere analyzed. The cold rolling reduction more than 40% increases the ears in case of sheetwith low initial earring. The cold rolling of sheets with high initial ears, and subsequentannealing decreases the ears on deep drawn cups.

CONCLUSION

The sheet forming ability increases by increasing the content of alloying elements ininvestigated ranges. Changes of the mechanical properties during cold rolling represent theusual behavior of metal sheets. The origin differences of properties, that are the result ofalloying, remain in the whole cold rolling reduction range. The level of earring <3% andplastic anisotropy of the sheet can be successfully controlled by appropriate parameters ofthermomechanical processing.

LITERATURE

1. KITORA K. Recent technical development in the Japanese aluminum industry, Aluminum66, 1990 7/8.

2. BLECK W. Demands made on car body materials, Stahl und Eisen 115, 1995.3. BLOECK M., TIMM J. Aluminium-Karoseriebleche der Legierungsfamilie AlMg(Cu),

Aluminium 71, 1995 ¾4. BURGER G.B. et all. Microstructural Control of Aluminum Sheet Used in Automotive

Application. Materials Characterization 35:23-39, 1995.5. SCHULTZ R.A. Trends in Aluminum Use for Passenger Cars and Light Trucks in North

America, Ligt Metal Age, pg 108-113, 1999.6. ZEYTIN H.K. Utilisation of aluminum in automotive industry, 10th Int. Metall. and Mater.

Congress, Istanbul, 2000 Proc. ISBN 975-395-382-87. OSTERMANN F. Entwicklungen beim Umformen und Bearbeiten von Blechteilen aus

Aluminum, Werstatt und Betrieb 121 (1988) 7 545-550