Surface remelting of ductile iron with a high power pulsed Nd:YAG laser

Ductile iron in as-machined surface was remelted by a high power pulsed Nd:YAG laser. Processing efficiency, microstructure and hardness of the laser remelted layers were investigated as a function of processing parameters. Microstructural changes after successive multi-pass overlap remelting were studied. It is demonstrated that the processing results were sensitive to pulse frequency, pulse power density and spot diameter, while surface conditions have only minor influence. Depending on processing conditions, two types of microstructures in the laser remelted zone were observed. An extremely fine dendrite-free lamellar eutectic structure occurred in the surface layer of the melted track. In the lower part of the remelted track, a plate-like ledeburite interspersed by small amount of primary austenitic dendrite was present. In overlapping tracks, the previous rapidly solidified ledeburite was ultra-rapidly graphitized by the heat input from the successive remelting track. The thickness of the graphitization zone and the graphitized microstructure were closely related to the processing conditions. The phenomenon of the rapid graphitization has not been reported so far and is discussed as a consequence of the cycling temperature fluctuations during pulsed laser heating. The results were finally compared with those obtained with a CO₂ laser using cw-radiation.