Cavitation erosion of laser-melted ductile iron

The influence of laser processing on the cavitation erosion of ductile iron, a material employed extensively for components in marine applications, including diesel-engine cylinder liners, valves and pumps, is addressed here. Pearlitic ductile iron has only modest resistance to cavitation erosion; the mean depth of penetration rate (MDPR) was $10-12\mu mh^{-1}$ in distilled water. Ferritic ductile iron had consistantly better resistance to erosion than pearlitic iron, particularly in corrosive media. The cavitation erosion rates in an aqueous slurry of 35 wt%. 220 grit SiC, Synthetic sea water (3.4 wt%) and centinormal dilute $H_2SO_4$ were 24, 40 and $86\mu mh^{-1}$ respectively, whilst the corresponding rates were reduced to 3, 6 and $12\mu mh^{-1}$ for laser-melted samples. The synergistic effect of corrosion and cavitation erosion was more pronounced in the dilute acid than in the salt water. The cavitation erosion rates decreased linearly with increase in the pH value of the cavitation bath. Laser treatment was very effective in bringing about a nearly seven-fold enhancement of the erosion resistance of ductile iron in mild corrosive media. This has enabled the delineation of the mechanism of cavitation erosion of ductile iron before and after laser processing, particularly in corrosive baths. A comprehensive review of published technical literature concerning the influence of laser surface treatment on the cavitation erosion of selected steels and cast irons in different baths is included

Dry sliding wear and friction: laser-treated ductile iron

Pin-on-disc wear and friction of hypereutectic ductile iron, the type employed for automotive components, was investigated at sliding speeds of 5 and 7.5 m $s^{-1}$, before and after laser surface treatment by $CO_2$ continuous- wave and Nd-YAG pulsed lasers. A significant increase in transition load and wear resistance upon laser treatment has been attributed to the ultrafine microstructure and high hardness; laser-melted ledeburite was superior to martensite by transformation hardening. Wear rate at a specific contact pressure and sliding speed bears a log-linear relationship with the harmonic mean of tensile and fatigue stress of ductile irons. The role of lubrication by graphite during mild wear and plastic deformation in severe wear of pearlitic ductile iron, and its enhanced resistance to plastic flow on laser melting, have been confirmed. The coefficient of friction of a ductile iron pin sliding on a steel disc before and after laser melting has been determined and compared with that of white iron of identical composition and structure obtained by conventional chilling

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: from power (0.5-2.5 kW) and scan rate (7.5-25 mm s(-1)) leading to solid state transformation, all the way through to melting followed by rapid quenching. Superfine dendritic (at 10(5) degrees C s(-1)) or feathery (at 10(4) degrees C s(-1)) ledeburite of 0.2-0.25 mu m lamellar space, 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 transformed or melt zone structures, parameters such as dendrile arm spacing, volume fraction of carbide and surface hardness bear a direct relationship with the energy intensity P/UDb2, (10-100 J mm(-3)). There is a minimum energy intensity threshold for solid state transformation hardening (0.2 J mm(-3)) and similarly for the initiation of superficial melting (9 J mm(-3)) and full melting (15 J mm(-3)) in the case of ductile iron. Simulation, modeling and thermal analysis of laser processing as a three-dimensional quasi-steady moving heat source problem by a finite difference method, considering temperature dependent energy absorptivity of the material to laser radiation, thermal and physical properties (kappa, rho, c(p)) and freezing under non-equilibrium conditions employing Scheil's equation to compute the proportion of the solid enabled determination of the thermal history of the laser treated zone. This includes assessment of the peak temperature attained at the surface, temperature gradients, the freezing time and rates as well as the geometric profile of the melted, transformed or heat-affected zone. Computed geometric profiles or depth are in close agreement with the experimental data, validating the numerical scheme