Improving the Effectiveness of Laser Heat Treatment with Laser Beam Profiling

In the past various methods have been employed for heat treating metals, which is usually carried out using large furnaces where an entire object undergoes the heating process. These processes have higher energy requirements and may not be feasible for the heat treatment of a particular portion or area of the equipment.

Therefore, utilizing lasers for the heat treatment process is a widely accepted method. The efficiency of the laser heat treatment method can be enhanced by recognizing the uniformity of intensity of the laser beam, beam shape, and the cleanliness of the delivery optics.

Ophir-Spiricon BeamGage®

When the laser heat treatment process is applied during the manufacture of locomotive engine cylinders, it is vital to make sure that the specifications are met by the delivered beam at the beginning of the process.

In this case, the size, shape, and uniformity of the beam were set up, quantified, and graphically displayed as a baseline by using the Ophir-Spiricon BeamGage® beam profiling software.

The main advantage of laser heat treatment is the localized and uniform heating approach. However, if imaging and quantitative analysis are not performed, then the heat treatment process will be non-uniform. The cylinder can be rejected when it is assembled, increasing the overall manufacturing cost.

Moreover, if a cylinder manufacturer wants to obtain the values of power density/cm2 for the laser beam, the beam diagnostics can be calibrated to power output via the BeamGage software, and the power density of the beam at points on the beam is provided by the resulting image.

The power density value is a highly significant parameter, because if this value is not equal to the optimum level, then the heat from the laser beam may penetrate the cylinder material, which is a special composite made of high-strength steel, to a depth that affects the grain structure of the material, causing a premature failure in the final cylinder.

This article presents an example where the BeamGage software was calibrated to the measured laser output power of 6 kW (Figure 1). The power density/cm2 for the entire beam is shown on the left hand side of the graph. This figure shows the beam is delivered at its point of contact on the material 1330 W/cm2, and is highly uniform from side to side, which is absolutely necessary for uniform heating.

Another parameter to be closely monitored during laser heat treatment is the delivery optics of the laser beam. This can be carried out in foundries or clean room environments. Here, the engine components have been treated in a foundry, so the delivery optics of the laser must be cleaned regularly to make sure that the delivered beam has accurate shape, size, and intensity.

Figure 1. A beam profile during cylinder heat treatment with a dirty laser cover glass; the top of the beam is not uniform in intensity.

The beam can be affected by dirty optics such as delivery optics, turning mirrors, or laser cover glass which usually expand the beam size and reduce the power delivered to the target point. In both cases, there are undesirable results on the cylinder during the heat treatment process.

The difference prior to and after a simple cleaning of the final cover glass over the laser output optics can be demonstrated by using the beam measurement and graphic analysis.

The most frequently affected surface is cover glass, because debris from the laser process and ambient dust can accumulate on the cover glass.

Therefore, the cover glass should be cleaned regularly. The cleaning frequency is decided by the periodical performance comparison of the beam, e.g. once a day, and by determining the time period for which cleaning can be tolerated before the specification is failed by deteriorating beam quality.

Figure 1 depicts the beam profiled with a laser cover glass in place that had not been cleaned for many days. As displayed, the intensity of top of the beam is not uniform, and the beam has a different profile in the X and Y axes. In the case where a moving optic is treated, the results produced while moving in the X versus the Y direction of the heat treatment movement may highly vary, which is undesirable.

The final cover glass over the laser optics delivery head was cleaned and reinstalled to demonstrate the results of thorough cleaning. As a result, the same 6 kW power was delivered. Figure 2 illustrates the beam shape, size, and intensity, which validate that this method has a highly desirable effect, i.e. a highly uniform beam pattern.

Figure 2. A beam profile during cylinder heat treatment after cleaning the cover glass shows a much more uniform beam pattern.

The subsequent step in this process is cleaning the other optics in the beam delivery path to make sure that the lingering negative effects are eliminated. As described above, the same procedure can be performed on the optics: determining the frequency at which the process has to be carried to provide continuous and constant performance of the laser to specification.

Delivering a Uniform, Consistent Beam

The locomotive engine cylinder manufacturer utilized the laser beam profiling and beam diagnostics software for the laser-based heat treatment process to deliver a uniform beam with a consistent shape and the necessary power density to ensure the desired grain structure of the steel composite that was processed.

Usually, the steel composites utilized in high-temperature applications should be heat-treated to decrease the grain structure size by a factor of 100. This enables the production of physically stronger steel that isn't distorted, even in Kelvin environments. This process also eliminates the problem of brittleness of the untreated steel.

Conclusion

Laser heat treatment significantly reduces the diagnostic costs by eliminating the need for destructive testing, unlike the destructive testing method using a thorough metallurgical analysis, which is a common method for verifying the production of correct gain structure within the metal and the penetration of the heat treatment to a proper depth of the material.

This information has been sourced, reviewed and adapted from materials provided by Ophir Optronics Group.

For more information on this source, please visit Ophir Photonics Group.

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