EFFECT OF LASER PROCESSING PARAMETERS ON THE STRUCTURE OF DUCTILE IRON

Laser processing of structure sensitive hypereutectic ductile iron, a cast alloy employed for dynamically loaded automative components, was experimentally investigated over a wide range of process parameters: f rom power (0.5-2.5 kW) and scan rate (7.5-25 mm s(-1)) leading to soli d state transformation, all the way through to melting followed by rap id quenching. Superfine dendritic (at 10(5) degrees C s(-1)) or feathe ry (at 10(4) degrees C s(-1)) ledeburite of 0.2-0.25 mu m lamellar spa ce, gamma-austenite and carbide in the laser melted and martensite in the transformed zone or heat-affected zone were observed, depending on the process parameters. Depth of geometric profiles of laser transfor med or melt zone structures, parameters such as dendrile arm spacing, volume fraction of carbide and surface hardness bear a direct relation ship with the energy intensity P/UDb2, (10-100 J mm(-3)). There is a m inimum energy intensity threshold for solid state transformation harde ning (0.2 J mm(-3)) and similarly for the initiation of superficial me lting (9 J mm(-3)) and full melting (15 J mm(-3)) in the case of ducti le iron. Simulation, modeling and thermal analysis of laser processing as a three-dimensional quasi-steady moving heat source problem by a f inite difference method, considering temperature dependent energy abso rptivity of the material to laser radiation, thermal and physical prop erties (kappa, rho, c(p)) and freezing under non-equilibrium condition s employing Scheil's equation to compute the proportion of the solid e nabled determination of the thermal history of the laser treated zone. This includes assessment of the peak temperature attained at the surf ace, temperature gradients, the freezing time and rates as well as the geometric profile of the melted, transformed or heat-affected zone. C omputed geometric profiles or depth are in close agreement with the ex perimental data, validating the numerical scheme.