Rapidly Solidified Microstructure in Surface Layers of Laser-Alloyed Mo on Fe-C Substrates
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RAPIDLY SOLIDIFIED MICROSTRUCTURE IN SURFACE LAYERS OF LASERALLOYED Mo ON Fe-C SUBSTRATES T. R. Tucker, A. H. Clauer, S. L. Ream, and C. T. Walters Battelle Columbus Laboratories, Columbus, Ohio 43201 ABSTRACT Shaped continuous-wave laser beams have been used to alloy molybdenum coatings into Fe-C substrates. The Mo overlay was first deposited by the plasma spray process in an inert gas environment. Slowly scanned laser radiation then melted a layer 2 to 5 times the thickness of the original Coating causing the two metallic species to mix. Melt proceeded from the coating/substrate interface, the Mo melt temperatures being significantly higher than that of the base material. Fusion zone microstructures for individual melt passes appeared to be homogeneous, and molybdenum composition was found to be relatively uniform. With overlapping melt passes, the microstructures became more complex with heat effects apparent in the overlap regions. INTRODUCTION The use of the continuous wave, high power laser as a rapid heating tool for the surface alloying of metals has been explored in a number of processes. Variations include the fusion of vapor deposited layers of Cr (1) or noble metals (2), the surface cladding of a preplaced composite powder (3), and the introduction of metallic carbides into laser surface melted layers (4). As part of a program to improve the wear properties of iron-based materials, we have studied the laser alloying of molybdenum into steels and cast irons. The objective in using Mo was to utilize its carbide stabilizing and secondary hardening properties in the same way that Mo is used in conventional tool steels. The specific process chosen to achieve surface alloying was the deep consolidation of a pure Mo plasma-spray coating on an Fe part. That is, the laser beam was used to melt a coated metal surface to a depth greater than the original coating thickness. Plasma spray deposition was employed because it is an efficient practical method for producing the desired coating thicknesses with minimal substrate effects. A second advantage of plasma spray coating was the irregular surface finish which provided improved absorption for the far infrared laser radiation. EXPERIMENT Test coupons were machined from iron-carbon binary castings of 0.8, 1.4, and 2.8 w/o carbon. The 15 x 15 x 3 mm coupons were coated with molybdenum by plasma spray deposition in an argon environment. The coating thickness varied from 40 to 60 pm. The coated test pieces were laser surface melted to fuse the Mo overlay into the substrate melt layer. The experimental configuration employed in the laser alloying procedure (Fig. 1) contained a cylindrical cup which was mounted to a linear travel stage. The top of the vessel was open to admit the converging laser beam. This research was sponsored by the U.S. Army Research Office, Research Triangle Park, North Carolina.
542 Argon gas was introduced at the bottom of the test volume at flow rates of 203 50 ft /hr. A graphite block supported the sample, absorbed any laser radiation nor
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