Deformation and fracture behavior of two Al-Mg-Si alloys

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I.

INTRODUCTION

THE demand for more lightweight, fuel efficient, and enhanced performance automobiles stimulates the research and development of high-strength and high formability aluminum alloys. Recent research has focused on the 6xxx series aluminum alloys.[1–6] These alloys have the properties of medium to high strength, excellent formability, and good corrosion resistance. The combination of these properties makes them potentially more desirable for certain applications than the stronger 7xxx alloys. The basic requirement for automotive sheet is to have high formability so that the panels can be stamped while retaining, or preferably increasing, strength when the parts are painted and thermally cured.[7] Because of the short time during the paint baking process, high strength is usually difficult to achieve for commercial Al-Mg-Si alloys. Through composition design and special heat treatment techniques, the strength of new Al-Mg-Si alloys in the paint-baked conditions can be improved significantly.[8] Therefore, the prospects for practical use of these Al-Mg-Si alloys as automotive body sheet are good. In order to make full use of the potential of these alloys, detailed investigation of the deformation and fracture behavior in different aging conditions is of great importance.

out by the following process: 430 7C, 4 h → 530 7C, 20 h, then air cooled to room temperature. After hot rolling (rolling reduction is about 87 pct) and annealing (400 7C/1 h), the materials were cold rolled to 1 mm with the cold rolling reduction of 67 pct. After cold rolling, the specimens were solution treated at 550 7C for 30 minutes and then quenched into water at room temperature. Thereafter, specimens were divided into two groups. Specimens in the first group were held at room temperature for different times before tensile tests, and specimens in the second group were first naturally aged for 3 months and then artificially aged at 180 7C for different times. Tensile tests were performed on an Instron testing machine with a crosshead speed of 2.5 mm/min. Observations of dislocation distributions in tensile fractured specimens and microstructures in different aging conditions were carried out at 160 kV using a JEOL*-2000EX type trans*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.

mission electron microscope (TEM). The specimens for TEM observation were thinned by the standard chemical method. A JEOL JSM-35CF scanning electron microscope (SEM) was used to observe tensile fracture surfaces. III.

II.

EXPERIMENTAL

Compositions of the Al-Mg-Si alloys studied here are listed in Table I. The homogenizing treatment of the ingots was carried

L. ZHEN, Associate Professor, is with the Harbin Institute of Technology, Harbin 15001, People’s Republic of China. S.B. KANG, Director, is with the Korea Institute of Machinery and Materials, Changwon 641-010, Korea. Manuscript submitted November 4, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

RESULTS

A. Tensile Properties Figure 1 shows the change of tensile properties of alloy

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