Rapid graphitization of a pulsed laser remelted ductile cast iron during multipass overlap melting
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INTRODUCTION
LASER surface melting of materials can produce novel high-performance materials having ultrafine solidified structures; some metastable, quasicrystalline, or even amorphous phases; supersaturated solid solutions; ultrahigh density of crystal defects, etc. ~ In the last decade, increasing attention from both the academic and industrial communities has been attracted in the area of laser materials processing, and it is anticipated that this trend will grow significantly within the next decade, t3] Graphite cast irons (gray and ductile) are one of the most widely used engineering materials because of their excellent combination of low price, good castability, machinability, tribological, damping, heat conducting, and mechanical properties. When remelting surface of cast irons with high-power laser beams, the surface melt-pool solidifies metastablly, i . e . , following the Fe-Fe3C phase diagram, producing a rapidly solidified metastable, ultrafine Fe/Fe3C structure or ledeburite having excellent wear resistances, t4-71In the last decade, considerable works have been done on aspects such as processing, physical metallurgy, evaluation of properties, etc. for various ductile and gray cast irons. Some general reviews on the topic have also published, t4] Concerning the processing aspect, the effect of materials surface conditions and coatings, tS~ shielding gas, t4~ beam shaping, IBm preheating, tl~ polarized irradiation, tS,~) twin beam irradiation, [~z] and coirradiation of TEA and CO2 laser beams I131 on the processing efficiency and remelting quality has been investigated. As far as the metallurgical aspect was concerned, the rapid solidification H.M. WANG, formerly Research Fellow, Alexander von Humboldt Foundation of Germany, Institut for Werkstoffwissenschaften 2 (Metalle), Universit~it Edangen-Niirnberg, is Associate Professor, Division 102, Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China. H.W. BERGMANN, Professor, is with the Institut for Werkstoffwissenschaften 2 (Metalle), Universit/it Erlangen-Niirnberg, D-91058 Erlangen, Germany. Manuscript submitted September 8, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
process and especially the rapidly solidified microstructure were intensively studied. Depending on the processing conditions, two types of microstructure were observed: a feathery ferrite-cementite eutectic with a hardness ranging from 1000 to 1250 HV and a austenite dendritic structure with some interdendritic cementite whose hardness is relatively low (400 to 650 H V ) . 04'151 To process components with large surfaces, multipass overlap remelting is necessary. Much literature has reported the tempering phenomenon of the underlying melted track caused by the heating of succeeding overlap r u n . [13'16"171 However, no literature studied the details of the microstructure in the tempered zone. The mechanical properties of the laser glazed surface, especially wear resistance in various wear systems, have been
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