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Automotive - Heat Treating and Surface Modification

Laser heat treatment and surface modification are the key technologies available today to enhance the effective use of materials and achieve the desired properties of the components used in the automotive industry to improve the performance of the car.

The high power heat source produced by a laser beam is ideal for surface modification. Laser heating produces local changes at the surface of the material whilst, leaving the properties of the bulk of a given component unaffected. The principle laser surface engineering applications can be divided into three broad areas. The following processes can also be divided into those relying on metallurgical changes in the surface of the bulk material i.e. transformation hardening, annealing, grain refining, glazing and shock hardening, and those involving a chemical modification to the surface by addition of new material i.e. alloying and cladding.

Heating without melting, commonly know as heat-treating. This involves solid-state transformation, so that surface of the metal is not melted. The fraction of the beam power absorbed by the material is controlled by the absortivity of the material surface. Both mechanical (hardness, abrasion, resistance etc.) and chemical properties, (corrosion resistance etc.) can often be greatly enhanced through the metallurgical reactions produced during these heating and cooling cycles.

Heating with melting, i.e. laser glazing, surface homogenization, remelting. This method produces very rapid heating, melting and cooling to modify the surface properties.

Melting with addition of material, i.e. cladding, alloying impregnation, which involves melting of the surface plus material added to the surface to form a modified surface layer.

The principles of laser heating are similar to those of conventional through heating. The time scales involved in the former are, however, typically an order of magnitude shorter. Whereas heating is conventionally induced by a furnace, flame, arc or induction coil, the laser beam is focused or shaped into a suitable pattern and scanned over the component. The high energy density laser beam heats the surface much more rapidly, reducing the time for conduction into the bulk of the component. Laser heat treatment and surfacing techniques must compete directly with a wide range of comparatively low cost conventional processes and must therefore offer significant advantages. The common advantages of laser surfacing compared to alternative processes are:

• Chemical cleanliness and cosmetic appearance
• Minimal heat input, since the source temperature is so high, transformation occurs so quickly and the heat input to the part is very low. This reduces the distortion and the heat-affected zone is very small.
• No post machining required
• Non-contact process
• Ease of integration

The range of alloys that can be transformation hardened by laser techniques covers all those than can be hardened by conventional methods. The response of steel to hardening increases with increasing carbon content, and hardness values have exceeded 700HV for steels containing 0.75% carbon content. In addition because of the high cooling rates plain carbon steel (0.2%C) will harden.

The hardenability of cast irons is controlled by the amount of pearlite present, and only martensitic stainless steels will respond to heat-treating.

Transverse section of laser hardened track in low carbon steel

Automotive industry has been responsible for much of the laser heat- treatment process development and some of the applications are listed in below table.

Typical lasers that have been used for this application

JK2003SM
Lamp-Pumped Nd:YAG, 2000W, 4000W with SuperModulation

JK401M & JK501M
Lamp-Pumped Nd:YAG, 400 & 500W

JK401SM & JK501SM
Lamp-Pumped Nd:YAG, 400 & 500W, 800 & 1000 with SuperModulation

JK802 & JK1002
Lamp-Pumped Nd:YAG, 800 & 1000W, 1600 & 2000 with SuperModulation

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