Example 1 Laser Surface Hardening of Cast Iron Camshaft Lobes

(Ref 18). The surface of the lobes of an automotive camshaft made from ductile cast iron (see Fig. 13) was to be surface hardened to increase wear resistance. The desired case depth, defined as the depth where the hardness was 50 HRC, was 0.5 to 1.0 mm (0.02 to 0.04 in.).

Cam Lobe Surface Uneven
Fig. 13 Ductile cast iron cam from automotive camshaft. Source: Ref 18

A 15 kW CO2 laser was used for the processing. The optical system delivered a focused spot, with a diameter of 10 mm (0.4 in.) to the workpiece. This spot was scanned over a distance of 22 mm (0.9 in.) normal to the direction of processing. The frequency of scanning was 125 Hz in the normal direction, forming a rectangular spot 22 mm by 25 mm (0.9 by 1.0 in.) on the camlobe surface.

To obtain an even hardened case around the periphery of the camlobe, it was necessary to vary the angular speed of rotation of the lobe under the laser beam. The reason is that the angle of incidence of the laser beam to the workpiece changed during rotation, from nearly normal incidence at the cylindrical portion of the lobe to a grazing incidence of only 20 to 30° at the flat portion. Furthermore, at constant rotational speed, the linear speed of processing would vary as the lobe rotated. This was obtained by mounting the workpiece on a rotary table. The speed of rotation was varied by means of an electromechanical controller in a predetermined manner.

The camlobe was laser hardened using a manganese phosphate coating to increase energy absorption. Because of the design of the workpiece, it was difficult to predict the optimum processing parameters by calculations and the parameters were, therefore, evaluated by trial and error. The results were:

Power input

9 kW

Power density

1600 W/cm2 (10,300 W/in.2)

Linear speed of processing

at the cylindrical portion

760 mm/min (30 in./min)

at the flat portion

180 mm/min (7 in./min)

Depth of case

0.55 mm (0.022 in.)

The hardness profile of the surface layer of the camlobe is shown in Fig. 14.

Fig. 14 Hardness profile of laser surface hardened cast iron camlobe. Source: Ref 18

Laser melting requires higher power densities than the levels used for laser transformation hardening. The workpiece is often made absorptive either by using coatings similar to those used for laser heating or by increasing surface roughness, for example, by sand blasting. Laser melting can harden alloys that cannot be hardened by laser transformation hardening. In ferritic malleable gray iron, melting enhances the diffusion of carbon, and the ensuing rapid quench produces a hardened region.

Processing conditions for laser melting are typically a power density from 10 to 3000 MW/m2 (6.5 to 1935 kW/in.2) and an interaction time from 0.01 to 1 s. Inert gas shielding is used to prevent oxidation of the surface.

Microstructural changes with laser melting are in the forms of grain refinement, solid solutions, and fine dispersions of precipitates. All of these can contribute to the hardening and strengthening of the surface. Laser-melted surfaces of cast irons appear dendritic, as shown in Fig. 15(a) for gray iron and in Fig. 15(b) for ductile iron. Below the melt zone is the heat-affected zone, which appears in lighter contrast in Fig. 15.

Fig. 15 Cross section of laser-melted cast iron surfaces. (a) Gray iron. (b) Ductile iron. Source: Ref 13

In the solidified melt in cast irons, a ledeburite (mixture of austenite and cementite) structure generally forms. Hardening is caused by graphite dissolution to form cementite and austenite transformation to martensite. Molian and Baldwin (Ref 14) described the formation of predominantly dendritic ledeburite with small amounts of plate-like, high-carbon martensite and retained austenite in the melt zone in gray and ductile irons.

Chen et al. (Ref 19) determined that microstructures in laser-melted ductile iron depended on the solidification rate. Dendritic retained austenite with a continuous interdendritic carbide, with a microhardness from 400 to 650 HV, formed at high solidification rates. A lamellar mixture of ferrite and cementite plates, with a microhardness from 1000 to 1250 HV, formed at low solidification rates.

Bamberger et al. (Ref 20) found that in laser-melted gray cast iron, full dissolution of the graphite occurred, leading to the formation of hot tears on the surface. On the other hand, in laser-melted nodular cast iron, partial dissolution of the graphite occurred, resulting in increased ductility and no hot tearing.

Bergmann (Ref 21) has summarized the wear properties of laser-melted cast irons. For laser-melted gray iron containing flake graphite in a pearlitic matrix, the dry pin-on-disk test demonstrated that the wear behavior improved by an order of magnitude and was better than a fully martensitic structure. In a test involving rolls that ran against one another with a fixed relative slip, it was found that the wear resistance of laser-melted ductile iron was superior to that of 0.6% C steel, case-hardened 16MnCr5 steel, nitrided or carburized 16MnCr5 steel, and a gas-tungsten arc welded melted surface.

Ju et al. (Ref 22) reported significant improvement in erosive wear behavior in laser-melted ductile and gray irons, as shown in Fig. 16. This improvement was due to the presence of mechanically metastable austenite that transforms to martensite when plastic deformation of the near-surface region occurs during wear.

Fig. 16 Erosive wear behavior of as-received and laser-melted gray and ductile irons. Source: Ref 22

Tomlinson et al. (Ref 23) reported that laser melting reduced the amount of cavitation erosion of gray cast iron in distilled water by a factor of 0.3, and, in 3% saltwater, by a factor of 0.57.

Laser Alloying. A technique of localized alloy formation is laser surface melting with the simultaneous, controlled addition of alloying elements. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface; the degree of microstructural refinement will depend on the solidification rate. The surface of a low-cost alloy, such as cast iron, can be selectively alloyed to enhance properties, such as resistance to wear, in such a way that only the locally modified surface possesses properties typical of tribological alloys. This results in substantial cost savings, and reduces the dependence on strategic materials. Typical processing parameters for laser alloying are a power density from 10 to 3000 MW/m2 (6.5 to 1935 kW/in.2) and an interaction time from 0.01 to 1 s. An inert shielding gas is normally used.

One method of alloying is to apply appropriate mixtures of powders on the sample surface, either by spraying the powder mixture suspended in alcohol to form a loosely packed coating, or by coating a slurry suspended in organic binders. The use of metal powders in laser alloying is the least expensive, but, with appropriate process modifications, alloys in the form of rods, wires, ribbons, and sheets can also be added.

Cast irons, primarily gray irons, have been laser alloyed with chromium, silicon, carbon, nickel, nickel-aluminum, cobalt, and cobalt-chromium powders. Figure 17 shows the improved cavitation erosion resistance of a gray cast iron containing 2.9 to 3.2% C, 1.7 to 2.1% Si, 0.5 to 0.8% Mn, < 0.1% P, and 0.06 to 0.11% S that was laser alloyed with chromium. The resulting coating, which was approximately 0.5 mm (0.02 in.) thick, contained 22% Cr and had a hardness value of 700 HV.

Fig. 17 Effect of laser alloying with chromium on the cavitation erosion resistance of gray cast iron in distilled water (a) and 3% sodium chloride solution (b). Source: Ref 24

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