1997 TMS Annual Meeting: Wednesday Session

AUTOMOTIVE ALLOYS: Session I: Fundamental Studies

Session Chairperson: Dr. Subodh K. Das, ARCO Aluminum, Inc., P.O. Box 32860, Louisville, KY 40232

8:30 am

PREDICTING THE FORMABILITY OF ALUMINUM AUTOBODY SHEET ALLOYS: J. Daniel Bryant, Tatsuhito Koya, Armand J. Beaudoin, Reynolds, Metals Corporate Research and Development, Fourth and Canal Streets, P.O. Box, 27003, Richmond, VA 23261

While aluminum alloys based on the Al-Si-Mg-Cu system are currently being used in the production of a number of automotive body panels, the continuing demands for improved formability in these alloys have led alloy developers to the study of novel compositions and processing practices. Predicting stamping performance for a wide range of alloy variants, however, remains a difficult problem. Uniaxial tension tests, while providing reproducible assessments of mechanical behavior, are performed in a strain state that is quite different from that encountered in most stamping operations. As such, more elaborate, and often and less reproducible, methods of assessment are often employed, such as limiting dome height measurements and the construction of forming limit curves. In the present work, we have analyzed the tensile behavior of a series candidate alloys and used these data to predict the forming limit curve over a range of strain states. Using a modified form of the Voce work-hardening model, a Marciniak-Kuczinski simulation has been constructed and used to predict the forming limit curve minimum (FLC0). These predictions are then related to experimentally determined forming limit curve data for the candidate alloys. The comparison of predicted and measured forming limit curve data indicates that the model can be successfully used to predict mechanical behavior under a state of plane strain using uniaxial tensile data. The results of the model indicate that the slope of the work-hardening curve, particularly at high strain values, is intimately related to the forming limit curve minimum. These correlations provide alloy developers with an inexpensive method of comparing the formability new alloy variants by more thoroughly exploiting the data concealed within the full uniaxial tensile curve.

9:00 am

TOOL WEAR DURING MACHINING OF AA356 ALUMINUM ALLOYS: Zhongnan Dai, J.G. Morris, Light Metals Research Labs, Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506

As a casting material, AA356 Aluminum alloy is widely used in the automotive industry. During machining, the tool wear mechanism for this alloy is significantly different from that for steel. For different as-cast metallurgical microstructures, materials with harder dispersed particles and stronger matrices show higher resistance to seizure because of an increase in the pressure requirement above which seizure occurs. This results in better machinability. On the other hand, hard particles act as small cutting edges on the tool materials, thereby causing tool wear due to the particle's abrasive characteristics. The wear characteristics of cutting tools were investigated in this study. Scanning electron microscopy, optical microscopy, EDX analysis were used to identify wear mechanisms. The results obtained show that there are a number of different wear mechanisms that contribute to tool damage, and hence, to tool-life. The role of the as cast microstructure on tool wear was also studied in order to determine under what structural states tool wear would be reduced.

9:30 am

THE EFFECTS OF PREAGING TREATMENTS ON FORMABILITY AND PAINT BAKE RESPONSE IN ALUMINUM AUTOBODY SHEET ALLOYS: J. Daniel Bryany, Reynolds Metals Company, Corporate Research and Development, Fourth and Canal Streets, P. O. Box 27003, Richmond, VA 23261

The use of heat treatable aluminum alloys for automotive body panels is presently increasing, due to aluminum's attractive combination of low density and compatibility with current production methods. In these applications, the automotive paint bake cycle is used to impart a modest artificial aging response, referred to as the paint bake response (PBR), in the alloys following stamping. Unfortunately, the short duration of the paint bake cycle (as dictated by production demands) is generally insufficient to exploit more than a small fraction of the age hardening potential of the alloys. Reynolds Metals Company has developed a thermal treatment which has been shown to be effective in increasing the paint bake response by up to a factor of four, while at the same time improving the formability of alloys and reducing the natural aging rate. Through the use of atomic resolution microscopy and differential thermal calorimetry, the mechanism of pre-aging can be shown to be the result of changes in the precipitation sequence of metastable variants of Mg2Si, resulting in an increase in the precipitation kinetics during the paint bake cycle and a finer distribution of strengthening precipitates in the painted component. Through the use of pre-aging, our research has shown that 6XXX autobody sheet alloys may be produced which have both superior stamping performance as well as higher strengths in the stamped and painted components, resulting in automotive body panels with up to 50% higher dent resistance.

10:00 am

THE EFFECT OF THE DISPLACEMENT CONTROL ROUTINE ON THE ELONGATION TO FAILURE AND FAILURE MORPHOLOGIES IN SUPER PLASTIC AA 5083: A.L. Lund, S.G. Pitman, M.A. Khaleel, M.T. Smith, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352

It has been noted in the literature that superplastically formed Al alloys fail by cavitation. The morphology of the final cavitated fracture region may be important in determining optimum forming conditions, by determining whether failure occurs parallel or perpendicular to the principal stresses. In addition, the displacement control routine may have a large effect on the total elongation that can be expected during forming operations. Tests were performed to evaluate failure in a AA5083 base alloy, by the following methods: 1) uniaxial tension tests at a constant true strain rate were interrupted at 80%, 60,G, 40%, and 5% of the maximum load, and microstructurally evaluated to identify the failure path and 2) tests were performed with various displacement control routines, and the elongation to failure was measured. The following displacement routines were used: smooth test, two-step strain test, multi-bump strain rate tests with various sizes of bumps scheduled at various intervals, and variable load oscillation tests.

10:30 am BREAK

11:00 am

EVOLUTION OF ALUMINUM MICROSTRUCTURES IN THE ALUMINUM EXTRUSION PROCESS: Woiciech Z. Misiolek, Aluminum Processing Program, Rensselaer Polytechnic Institute Troy, New York 121803590

The final microstructure and its uniformity is responsible for the physical properties and surface quality of the extruded profiles. Extrusion is time dependent process and its deformation variables such as, deformation zone and dead zone geometry as well as extrusion speed and temperature, change in time. The final microstructure depends on the above mentioned extrusion parameters as well as billet microstructure and post processing treatment. Metal flow conditions are influenced by the process parameters. Both physical and numerical process modeling techniques have been applied to predict metal flow behavior during extrusion through dies with different geometries. Additional information, allowing understanding of microstructure evolution, can be obtained from the crystallographic characterization of the typical deformation zone regions such as dead metal zone, main deformation zone and recrystallized zone on the billet-container interface using the electron backscattering diffraction (EBSD) technique. This technique has been utilized to follow in detail the orientation aspects of the deformed grains in extruded aluminum. This analysis provides information which can be used in the die and process design to improve metal flow uniformity and therefore the microstructure of the final product. It also allows prevention of the typical extrusion defects like surface tearing.

11:30 am

METALLURGICAL SAMPLE PREPARATION AND IMAGE ANALYSIS TECHNIQUES USED FOR THE EVALUATION OF AUTOMOTIVE MATERIALS: Matthias Hoffman, George A. Blann, Buehler Ltd., 41 Waukegan Rd., P.O. Box One, Lake Bluff, IL 60044; William R. Creech, BMW Manufacturing Corp., Greer, SC

This paper focuses on the commercial application of aluminum alloys and composite materials used in the automotive market. Today's automotive materials require a high degree of reliability. Therefore, efficient and accurate material testing and characterization methods are essential. Automotive alloys and composite materials often times are evaluated for microstructural properties. This work focuses on the developments in the metallurgical Q & A and research lab by highlighting the latest advancements using automated and semi-automated sample preparation techniques. The interpretation and quantitative analysis of these microstructures Ts accomplished by utilizing automated image analysis techniques. Image Analysis allows for an efficient as well as accurate analysis of dimension measurements, constituent analysis, porosity measurements, etc.