Fast aging kinetics of the AA6016 Al-Mg-Si alloy and the application in forming process
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DUCTION
WEIGHT and, consequently, fuel reduction is a primary concern in the automobile industry. As a result, more and more aluminum alloys are being used for inner and outer panels, where high formability and dent resistance are required. Unfortunately, the inferior formability of aluminum alloys to deep-drawing steels does limit their applications in areas where severe deformation is involved in the stamping process. There are ways of achieving better forming performance for aluminum alloys. Forming at elevated temperatures is an effective approach to improve the deep-drawing performance.[1] However, the additional equipment needed to create a thermal gradient in sheet forming is generally complex and expensive. Introducing a microstructural gradient is a way to create an inhomogeneous material and, correspondingly, inhomogeneous properties. A cold-rolled aluminum alloy differentially annealed, leaving the center area hard over the punch but soft in the flange, gave an improved deep-drawing ratio of 2.5 compared to 2.0 for a completely soft blank.[2] Recently, similar experimental work has been reported with a fully annealed AA2014 alloy. The center region is heated to dissolve precipitates, followed by quenching and artificial aging to achieve a higher strength than the flange. As a result, the deep-drawing ratio is increased from 1.8 to 2.6.[3] Most sheet materials of Al-Mg-Si alloys used for outer panels in the automotive industry are in the naturally aged state, with a microstructure containing fine Guinier-Preston (GP) zones. They show a weak bake-hardening response in a typical paint bake cycle, e.g., 180 ⬚C for 30 minutes. Y.G. AN, Senior Researcher, and H. VEGTER, Principal Scientist, Automotive Applications, and L. ZHUANG, Principal Scientist, and A. HURKMANS, Department Manager, Aluminium Metallurgy, are with Corus Research Development and Technology, 1970 CA IJmuiden, The Netherlands. Contact e-mail: [email protected] Manuscript submitted March 30, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
Presently, much effort is being devoted to developing materials with an improved bake-hardening response for the automotive industry.[4,5,6] The microstructure in the conventional naturally aged state is modified by a preaging treatment after solid-solution heat treatment to create a microstructure that has sufficiently large and stable clusters in the matrix, so that a continuous growth of the clusters to ordered precipitates could occur rapidly to provide a strong bake-hardening response in the paint bake cycle. It is well known that the aging temperature and time can influence the strength of heat-treatable aluminum alloys significantly. The aim of this research was to find a possible fast aging process to change the strength of materials within a very short time and to apply it to making tailored blanks with microstructural gradients or property gradients, so the forming performance of such treated blanks at room-temperature stamping can be improved.
II. EXPERIMENTAL PROCEDURE A. Materials A preage
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