Lasers in Surface Engineering

Increasing engineering applications demand the materials to possess high strength, wear and corrosion resistance with less cost. In particular, the surface of the materials plays a vital role in the engineering applications. Many surface related failure mechanisms involve wear, corrosion, erosion or high temperature oxidation which could be minimized by modifying the surfaces. To improve the surface properties of the engineering materials, in general there are several conventional methods available such as thermal spraying, oxy-fuel spraying, flame spraying, etc.,. However, they exhibit high porosity, non uniform thickness, poor bonding between the coatings and substrate.

Laser surface modification techniques such as laser alloying and laser cladding involves rapid heating and cooling rates which produce meta-stable and non-equilibrium phases and offer the possibility of new materials with novel properties. Coating and substrate are metallurgically bonded. Surface properties could be tailored according to the application without sacrificing the bulk characteristics of the structure. The additional advantage of this method is to conserve the strategic materials which are expensive or scarce. Very good coatings with uniform thickness could easily be achieved.

Also, laser offers many benefits which include localized treatment, low thermal distortion, non contact, ability to address complex shapes. Importantly, free to choose metals, carbides and nitrites or combination of all as a coating material for the surface modification.

Laser transformation hardening

Laser transformation hardening is the simplest process method since there is no materials are added and no melting is involved. It is commonly known that the heat treating makes use of the rapid heating and cooling rates produced by scanning laser beams on metal surface. Laser heat treating involves solid state transformation, so the surface of the metal will not be melted. The fraction of the beam power absorbed by the material is controlled by the absorptivity of the material surface. Steels and cast irons are particularly good candidates for laser transformation hardening. The process has unique advantages, particularly when used to enhance surface properties in local areas without affecting other areas of the component surface.

Laser Melting

The laser beam with high irradiance is rapidly scanned over a metal surface and hence it produces a thin layer of molten material near the surface. Rapid laser irradiation at localized surface leads to very rapid quenching of the molten material by conduction into the workpiece. It gives refined and homogeneous microstructure which results in enhanced surface hardness in the laser scanned regions.

Laser Alloying

Laser alloying of a surface involves melting the surface of a material along with some material added to the surface. The added material is mixed with the surface during the melting and the surface resolidifies rapidly. The surface chemical composition of the material could be modified by this technique. The result can be a thin hardened layer at the surface. The motivation for surface alloying is to provide a hardened layer on a relatively inexpensive substrate. A wide range of alloying elements can be added to enhance the surface properties including metals, carbides and nitrides. Figure shows the schematic diagram of laser alloying of preplaced method.

Laser Cladding

Laser cladding is a process whereby a thick/thin protective layer would be formed on the materials or components with external materials (fig). Laser cladding usually involves coating a relatively inexpensive substrate material with a more expensive alloy that will increase its wear or corrosion resistance. However, unlike laser alloying the overlay material does not intermix with the surface to a high degree and it is only limited mixing on the surface, which is required for effective bonding at the interface.

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