27 2001 February • JOM Overview Wrought and Sheet Sheet-metal forming is a multi-billion- dollar industry in the United States, with an overwhelming portion of its use in automo- tive markets as formed-sheet components. With roughly one-third of the weight of an auto in this form, the Big Three automakers (Ford, General Motors, and Daimler- Chrysler) are looking for lighter, yet high- strength alternative sheet materials. Such materials would reduce the overall weight of vehicles, helping the automakers achieve gas mileage goals set down in the Partnership for a New Generation of Vehicles. Progress has been slow, however, because of a lack of knowledge and experience in forming these new materials. The National Institute of Standards and Technology, in keeping with its mission to assist industry by developing technology and standard test methods, has initiated a program to help the auto indus- try, as well as other industries that produce and utilize sheet metal, make the transition to these new materials. INTRODUCTION While the U.S. automotive industry spends approximately $700 million per year producing new sheet-metal form- ing die sets, 1 roughly half of that amount covers the cost of tryout, a process in which inaccurate die designs are cor- rected on the production floor. Many of the die errors are believed to result from limited understanding of material be- havior, 1 especially surface roughening, friction, biaxial strain limits, and springback. By understanding these fac- tors on a fundamental level, the number and/or severity of the problems with the dies can be reduced before produc- tion through use of improved finite ele- ment analysis (FEA) of the forming pro- cess during the design phase. Of the above-mentioned material be- haviors, most, if not all, of the springback compensations in die sets are introduced on the production floor on a trial-and- error basis. This approach has proven to be expensive and time consuming. An alternative could be FEA simulations, which, given the appropriate mechani- cal properties, may be able to prevent problems caused by surface roughening of the sheet. Changes in the surface fin- ish caused by the forming process result Sheet Metal Formability Studies at the National Institute of Standards and Technology T. Foecke, S.W. Banovic, and R.J. Fields Figure 1. A schematic of the NIST version of the Marciniak biaxial stretching test. in unusable or failed components and, therefore, increased expenditures by automotive manufacturers. From an aes- thetic perspective, orange peeling leads To improve material-property treat- ment in the finite element codes used to design sheet-metal forming dies, the National Institute of Standards and Tech- nology (NIST) program in sheet-metal formability has been developed with a number of parallel components. One project involves linking dislocation sub- structure morphologies to flow behav- ior in metals, with the goal of developing more accurate, physically-based consti- tutive laws. 2 Another NIST project deals with the development of surface rough- ening and changes in texture during mul- tiaxial forming operations, and the re- sulting friction and strain limits on a number of material systems. A third project involves determining mechani- cal properties and developing standard test methods and new metrologies for multiaxial deformation of sheet metal. This paper concentrates on the develop- ment of standard mechanical test meth- ods and how these techniques are being used to study the interplay of roughen- ing and crystallographic texture under biaxial-strain response. STANDARD TEST DEVELOPMENT When being formed in a die, sheet metal can undergo a complex strain path before taking its final shape. (A useful review of sheet-metal forming processes was published in the November 1999 issue of JOM and on the journal’s web site. 3 ) Quite often, the stretching and drawing processes involve placing cer- tain regions of the sheet into plane strain tension or various states of balanced and unbalanced bi-axial strain. In addition, the sheet experiences friction as it is drawn over the edges and corners in the die, or is in moving contact with one face of the die tool set. This coefficient of friction is not constant, but can vary with the changing surface topography, the smoothing of this topography by con- tact with the die, and the presence, ab- sence, or thickness of any applied lubri- cating layer. Thus, the variables needed to simulate forming behavior are both numerous and complex, and must be determined in strain states as close as possible to real behavior. However, this This paper concentrates on the development of standard mechanical test methods and how these techniques are being used to study the interplay of roughening and crystallographic texture under biaxial-strain response. to poor surface quality of the finished component and the need for costly and labor-intensive post-fabrication steps (e.g., wet sanding) prior to painting. Be- cause of the in-situ change in surface topography, FEA has been unsuccessful at predicting or calculating the behavior of the material in terms of the complex stress states occurring within the sheet and the frictional forces occurring along the die-work piece interfaces.