Microstructure and Property Modifications of Cold Rolled IF Steel by Local Laser Annealing
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few exceptions, the present use of laser annealing is related to modification of the structure and properties of thin films. Examples include performance optimization of thin film cathodes in lithium-ion batteries,[1] improvement of the properties of thin films for photovoltaic applications,[2] fabrication of thin film diodes,[3] and control of grain growth in thin films.[4] Studies on laser annealing of larger specimens are scarcer. One example related to local modification of a thin Ni-Ti shape memory alloy strip with 0.5 mm thickness is given in Reference 5 and surface laser annealing of stainless steel is studied in References 6 and 7. Even less attention is given to the possibility of using laser annealing on larger samples of rolled sheet metal in order to manufacture functionally graded materials through local modifications of the microstructure, which is not achievable by standard annealing methods. This kind of applications are in focus of the present study. HA˚KAN HALLBERG is with the Division of Solid Mechanics, Lund University, P.O. Box 118, 221 00 Lund, Sweden, and also with the Lab Process and Engineering in Mechanics and Materials (PIMM-UMR 8006), ENSAM, CNRS, CNAM, Hesam, 151 Bd de l’Hoˆpital, 75013 Paris, France. Contact e-mail: [email protected] FRE´DE´RIC ADAMSKI, SARAH BAI¨Z, and OLIVIER CASTELNAU are with the Lab Process and Engineering in Mechanics and Materials (PIMM-UMR 8006), ENSAM, CNRS, CNAM, Hesam. Manuscript submitted January 11, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
The method will be possible to apply in all areas of sheet metal modifications, such as deep drawing and folding as well as stamping, blanking, and punching. It is envisaged that the method will permit reduced tool forces and tool wear, and less risk of fracture and that finer geometrical tolerances can be met. By local laser annealing of the material, a grain size gradient will develop in the microstructure and alter macroscopic material properties accordingly. As described by the classical Hall–Petch relation, the macroscopic yield stress of the material is related to the average grain size, and since the grain size can be altered by annealing—in the irradiated zone—laser annealing can be used to modify the yield stress of the material in selected regions. The laser annealing process also causes a reduction of the stored energy in the material by growth of new grains from (nearly) dislocation-free recrystallization nuclei. In addition, annealing also provides a characteristic recrystallization texture. In the case of the interstitial free (IF) steel, presently under consideration, cold rolling yields a texture dominated by two components comprising the α-fiber, having the h110i orientations parallel to the rolling direction (RD), and the γ-fiber having the h111i orientations parallel to the normal direction (ND). In Reference 8, the initial fractions of the α- and γ-fibers after 75 pct cold rolling are found to be 41 and 27 pct, respectively, clearly showing the texture to be dominated by these components. Similar
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