Laser Surface Processing

Lasers with continuous outputs of 0.5 to 10 kW can be used to modify the metallurgical structure of a surface and to tailor the surface properties without adversely affecting the bulk properties. Surface modification of steels via lasers can take the following four forms:

• Laser transformation hardening

• Laser surface melting

• Laser surface alloying

• Laser cladding (hardfacing)

Figure 24 shows typical ranges of conditions for various processes. The laser power, power density, and interaction time are the primary variables. Other variables, such as the composition of the atmosphere during treatment or the rate of material addition, are determined by the details of the processing--for example, the necessity of shielding against oxidation and the desired thickness, composition, and structure of the surface layer.

Figure 24 shows typical ranges of conditions for various processes. The laser power, power density, and interaction time are the primary variables. Other variables, such as the composition of the atmosphere during treatment or the rate of material addition, are determined by the details of the processing--for example, the necessity of shielding against oxidation and the desired thickness, composition, and structure of the surface layer.

Fig. 24 Interaction times and power densities necessary for various laser surface modification processes

The laser beam modified layer can range in thickness from 0.01 to 5 mm (0.4 to 200 mils), depending on the processing variables, although thicknesses of 0.05 to 1 mm (2 to 40 mils) are more common. The longer the interaction time of the laser beam with the material, the deeper the processed layer will be. Of the processes shown in Fig. 24, the area labeled

"Cladding and surface melting" delineates process parameters that typically affect the material to depths from 0.5 to 5 mm (20 to 200 mils) and result in metallurgical structures similar to welded structures. The parameters designated "Rapid surface alloying and melting" affect a surface layer only 0.02 to 0.6 mm (0.8 to 24 mils) thick, but result in quench rates to 107 K/s and therefore allow for the production of novel metallurgical structures and alloys. Because laser transformation hardening is the most commercially viable (with regard to steels) of the laser processing methods listed above, it will be described further below. Additional information on laser surface processing can be found in Ref 61. Laser cladding, or laser hardfacing, is also described in Ref 53.

Laser surface heat treatment is widely used to harden localized areas of steel machine components such as gears and bearings. The heat generated by the absorption of the laser light is controlled to prevent melting, and therefore is used to selectively austenitize local surface regions which transform to martensite as a result of rapid cooling by the conduction of heat into the bulk of the workpiece. This process is referred to as laser transformation hardening to differentiate it from laser surface melting and alloying phenomena (Fig. 24). There is no chemistry change produced by laser surface heat treating of steels.

Laser heat treatment produces thin surface zones which are heated and cooled very rapidly, resulting in very fine martensitic microstructures, even in steels with relatively low hardenability. High hardness and good wear resistance with less distortion result from this process.

Steels which have been successfully laser surface hardened include plain carbon steels (1040, 1045, 1050, 1070) and alloy steels (4140, 4340, 52100) (Ref 62, 63). Typical case depths for steels range from 0.75 to 1.3 mm (0.030 to 0.050 in.) depending on the laser power range (Fig. 25). Hardness values as high as 60 HRC are routinely achieved using laser surface hardening.

I

V

~ Surface melt

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^^ 2 kW

Processing spaect, cm/s

0.16

o.oe

0,04

Processing spaect, cm/s

Fig. 25 Effect of laser processing speed and power output on case depth thickness of laser hardened 1078 carbon steel

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