Quality and formability in hemming of automotive aluminum alloys.
Lin, Guosong
2006
Abstract
As a manufacturing process to fold a panel over itself or another sheet, hemming is frequently utilized in the final stages of exterior automotive closures production. The dimensional quality of hems, e.g., roll-in/roll-out, warp/recoil and springback, critically impacts the assembly quality of autobody panels. Furthermore, when aluminum alloys are used for autobody manufacturing, a formability problem emerges as they tend to crack at the hemlines (bent corners) before desired hems can be achieved. Therefore, quality and formability (or hemmability) are the key issues in hemming of automotive aluminum alloys. Due to its process complexity, analytical study is difficult, and experimental or numerical investigations have been very limited for aluminum hemming processes. The hemline surface cracking of aluminum hemming is not well understood and designers are short of a physics-based failure criterion for formability evaluations. As a result, design of aluminum hems is currently trial-and-error based, thus time-consuming and cost-ineffective. Addressing these deficiencies, a systematic study on quality and formability of aluminum hemming was carried out in this dissertation using finite element (FE) modeling, plasticity and ductile fracture theories, physical experiments, and statistical methods. The study mainly consists of: First, a combined explicit-implicit 2D FE model was developed and validated for flat surface-straight edge aluminum hemming modeling, and key process parameters for 2D hemming are identified based on a computational parametric study. Second, the explicit-implicit FE procedure was extended to 3D curved surface-curved edge aluminum hemming simulations. In balancing the numerical accuracy and computational efficiency, an innovative solid-to-shell mapping FE approach was developed. Using this mapping approach, the 3D hemming process was quantitatively explored via a response surface study. Third, maximum equivalent surface strain was proposed as a failure criterion based on the strain/stress nature on hemline surfaces in evaluating the hemmability of aluminum alloys. The criterion was experimentally verified and it is proven that fracture strain for hemming can be simply approximated by plane-strain tensile fracture strain. Fourth, an algorithm is developed for the detection and characterization of surface cracking in aluminum hemming by monitoring the statistical behaviors of micro-cracks on the hemline surface. A transition point when micro-cracks merge into macro-cracks was identified, and verified to be applicable for crack detections in engineering applications. Through this study, the major quality and formability issues in hemming of aluminum alloys become predictable. The findings provide the quantitative insights for the product/process design of aluminum hemming, and it is expected the knowledge obtained can be eventually implemented into industry practice for design and production improvements. Future research directions are suggested.Subjects
Aluminum Alloys Automotive Ductile Fracture Formability Hemming Quality
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