Affordable and Effective Lasers Find Their Way Into More Heat Treating Operations.
Heat treatment alters the microstructure of a metal by controlled heating and cooling. The process can harden the surface of a metal, temper it or soften it and make it more ductile. Laser heat treatment is a sophisticated variation of the commonly used flame heat treatment process.
A coating is applied to the workpiece surface for facilitating absorption of the laser's energy as selected areas of the part are exposed to that energy. As these areas heat up, their temperature and how fast they cool determines the alteration of the metal's microstructure.
Because today's lasers accurately control the area of the workpiece heated and the amount of heat applied, laser heat treating accomplishes what no other type of heat treatment process can. For example, the cams on an automotive camshaft need to be hardened by heat treatment to improve wear resistance. Using traditional flame heat treatment subjects the entire cam shaft to enough heat to cause some distortion in the shaft. But laser heat treatment can be applied only to the cam surface, thus eliminating any possible shaft distortion.
Another example is the use of laser heat treating to improve the formability of today's new ultrahigh-strength steel sheets that are so hard they have poor formability qualities. While a general heat treatment of the sheets reduces their desired hardness, a local treatment with a laser softens the material only in those places where high formability is needed. Hardness in those places is then restored by subsequent forming processes.
Industrial lasers concentrate the energy from beams of photons into small, well-defined areas. The resulting energy density is so high that it heats, melts or vaporizes materials only in those areas, thus minimizing surrounding heat-affected zones.
There are several types of lasers, and each generates a different wavelength of energy and beam quality. These differences influence how well a particular laser couples with (is absorbed by) a material.
Common types of industrial lasers are CO2 gas, Neodynium YAG (Nd:YAG) and semiconductor (diode) solid because they all work well with metal. CO2 lasers exhibit less surface absorption in most metals than Nd:YAG and diode lasers, and CO2 lasers require that material surfaces be coated to improve absorption.
Since the surface absorption of Nd:YAG lasers is significantly higher than CO2 lasers, Nd:YAG lasers generally require less power to do the same job. Laser diodes, on the other hand, are absorbed reasonably well by uncoated steel, are much more efficient than CO2 and Nd:YAG lasers, can easily generate several kilowatts of power and are much more affordable than they were in the past.
A laser system used for heat treatment has a laser source, a beam-delivery system and focusing optics. The system is normally incorporated into a multi-axis machine that allows for at least 5-axis numerically controlled work.
The beam exiting the laser is poorly formed for heat treating because it has hot spots and weak areas. It can, however, be shaped into almost any desired pattern using a number of different devices, such as multifaceted segmented mirrors, integrators and scanners. Mirrors always produce individual stable beam patterns that are built into the devices. Integrators and scanners are adjustable and generate a wide range of beam patterns. While these devices tended to be too fragile for production environments, recent developments in scanners have made them more robust.
CONSIDERATIONS FOR LASER HEAT TREATING
According to U.S. Laser Corp. (www.uslasercorp.com), there are several factors that must be considered before taking full advantage of laser surface heat treating.
- Microstructure of parts — the most desirable types of microstructures for the laser process are quenched and tempered or austenitized and tempered conditions. Fully annealed and spherodized structures are not recommended for this process.
- Microstructural homogeneity of parts — laser surface heat treating requires a homogeneous structure because there is little time in which to diffuse and redistribute the alloy elements throughout the material. Parts with heavy segregation will not respond uniformly to the laser process.
- Fine microstructure or small grain size — the smaller the grain size in the part, the faster the response to the laser process. Grain size is one of the major factors in determining the hardenability of parts.
- Hardness of core — core hardness is important for parts installed for use in high-pressure conditions after heat treating. If the background material is dead soft, the hardened layer will peel off quickly in high-pressure conditions.
- Parts cleaning — part surfaces to be laser heat treated should be thoroughly cleaned. Heavy dirt, rust and grease will cause uneven case depth.
- Surface coating — when using CO2 lasers, a thin layer of coating is commonly applied to the metal surface to enhance the absorptivity of the metal by the laser beam. Phosphate and black paint are the most common coatings used because of their low susceptibility to moisture, but oxide and graphite also work. The optimal thickness for the coating is around 0.02 mm to 0.05 mm. Coatings are generally not required when Nd: YAG lasers are used.
LASER HEAT TREATING PARAMETERS
Power density — generally speaking, the higher power density, the deeper the case depth. However, if all other variables are fixed, there is a maximum depth that can be achieved. When that limit is exceeded, surface melting will occur. If some finish machining is a requirement as a final step after hardening, then some surface melt of several thousandths, for instance, can be tolerated.
Travel speed — if, after maximizing all variables, travel speed increases, case depth decreases until there is no reaction with the material. Decreased travel speed causes significant surface melting and/or lower hardness.
Hardness requirement — the maximum hardness that can be achieved on a given material is governed by the carbon content in the material. When a maximum hardness is required for a certain carbon content, then the case depth is controlled by the cooling condition of the part. If the hardness requirement is lower, then the power density and travel speed can be reduced to allow more time to drive the heat down deeper and create a deeper case depth.
Cooling condition — as a general rule of thumb, at least six or seven times the case depth thickness of material is needed beneath the surface to ensure self-quenching and the required case depth and hardness. This requirement can sometimes be circumvented by using various methods to assist in quenching, such as air jets, water mist or, if the part geometry allows, oil. These processes aid in obtaining maximum surface hardness.
How Lasers Work
There are two basic excitation mechanisms, or "pumps" for pumping up the laser energy. The first is optical — used in most solid state lasers where wavelength-specific light is energized into a solid material, such as a man-made crystal, by "pumping" light into the crystal from a special flash lamp, laser diode or diode array.
The other mechanism types are electrical. These use DC or RF signals to excite atoms. These mechanisms cause the electrons in an atom to absorb energy and move to higher energy levels and then release them in the form of a photon (a short wave of light).