Microstructure Evolution during Roller Hemming of AZ31B Magnesium Sheet
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TRODUCTION
WITH a lower density and higher specific strength than both aluminum and steel, magnesium has the potential to help the automotive industry realize the goal of vehicle weight reduction.[1] Thus far, cast magnesium alloys have found many automotive applications,[1] but their limited room-temperature formability has been a major hurdle for implementing applications with wrought materials. If magnesium is to be utilized successfully in automotive outer body panels, it must withstand numerous processing operations, including hemming. After forming to meet shape and design requirements, the inner and outer body panels are joined by a hemming operation. This process requires the edges of the outer body panel to be bent around the edge of the inner panel. Conventional hemming procedures are conducted at room temperature, but most commercially available magnesium sheet alloys cannot be successfully hemmed at room temperature without initiating defects. The visibility of the hemmed joint places additional design requirements that require the joint to be as sharp as possible (such that the gap between two neighboring joints is minimized) and exhibit a smooth surface finish. AMANDA LEVINSON, Graduate Student, and ROGER D. DOHERTY, Professor Emeritus, are with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104. RAJA K. MISHRA, Technical Fellow, is with the General Motors R&D Center, Warren, MI 48092. SURYA R. KALIDINDI, Professor, is with the Department of Materials Science and Engineering, Drexel University, and also with the Department of Mechanical Engineering and Mechanics, Drexel University. Contact e-mail: [email protected] Manuscript submitted August 19, 2011. Article published online May 22, 2012 3824—VOLUME 43A, OCTOBER 2012
Hemming is preceded by a flanging step (Figure 1(a)) used to bend the sheet approximately 90 deg. Both press-and-die (Figure 1(b)) and roller hemming (Figure 1(c)) have been performed with magnesium sheet. Press hemming is a plane strain bending operation where hem dies bend the flange approximately 90 deg in a two-step process. In roller hemming, a roller is driven around the edge of an outer panel at various angles to bend the flange gradually in multiple passes. Roller hemming differs substantially from press hemming as it is not a purely plane strain bending action, but it includes a shear component that distributes strain through a different path.[2] Previous research has demonstrated that magnesium sheets can be successfully hemmed at an elevated temperature. Press hemming of AZ31B sheet has been successfully performed at a temperature of ~543 K (270 C).[3] In contrast, roller hemming has been successfully performed in four steps using a laser source to heat the sample locally ahead of the roller to approximately ~443 K (170 C) (Figure 1(c)).[4] This process is referred to as laser-roller hemming. In this processing route, hemming could only be carried out successfully if the laser were employed. Conventional roller hemming (done at roo
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