MEMO

To: Dr. Hugh Jack

From: Matt Remelts

Subject: Laser Cladding Investigation

Date: 1-31-97

Based on the subsequent research analysis, I believe we have a viable solution to our specific manufacturing problem. The major economic impact of scrapping and rework will be significantly reduced if we implement a laser cladding process. Although the acquisition price of this devise is fairly high, it will make up for itself in waste reduction and rework time. This is the major benefit of laser cladding. Enhanced surface properties such as wear resistance, environmental protection, and dimensional restoration are the major positive offsetting factors in this process. This means we can make repairs that were previously unmanageable. The bond created in this process is more diffuse than conventional surfacing techniques, requires lower overall heat input, and uses a minimally diluted filler.

The rapid heating and cooling effects produced with lasers is the foundation for forming solid meta-stable materials without the constraint of a phase diagram. Laser cladding is the process by which the surface chemistry of a substrate is altered by heating, mixing a filler, and cooling at a rate of about 10^6 K/s. There are two major components of all laser cladding operations: an energy transportation device and a momentum and mass transfer device. Lasers provide the energy transportation which controls the rate of heating and cooling. Most often, a powder injector is used for the mass transport which controls the mixing and final composition of the clad. The basic set-up of a laser cladder is shown in figure 2 (The Fabricator, p.59). The way it actually works is that the cladding powder is deposited at a constant velocity and laser melted in a parallel process onto the substrate. There has been some theoretical work done in the prediction of the formation of an extended solid solution under rapid cooling. In this work, the pool of melted cladding material is considered to have three distinct surfaces. The first is adjacent to the substrate. The second is adjacent to the solidified clad and the third is open to an inert atmosphere (an inert atmosphere is necessary to avoid oxidation). Since the solidified clad is at a very high temperature and the heat loss due to radiation from the open surface is small, heat loss due to conduction is only considered between the pool and the solid substrate. From this thermodynamic/mathematical model and the use of space-time variables, a relatively accurate prediction can be made as to the final composition of the clad. The quality of a clad is divided into three physical zones: the heat affected zone (HAZ), the unmixed zone, and the partially melted zone. The most prominent and critical of these zones is the HAZ. Figure 1 (The Fabricator, p.59) shows a typical cross-section of this effect. It is the region of pure substrate that is affected by the cladding process. For a high quality clad, all of these regions should be minimized.

The speed of cladding processes is relatively versatile. They range from speeds of .05 to 10 lb./hr of cladding filler. For applications such as surfacing the tip of a turbine blade, a speed on the lower end of the spectrum is used. For situations in which large surfaces are hardfaced, a speed near 10 lb./hr is applied.

There are basically two types of lasers used for laser cladding. The first and most common is the CO2 laser. With this type, the beam is delivered using hard optics, mirrors, and lenses. The second type is the continuous wave Nd: YAG laser. The advantage of this laser is that fiber-optics are a viable method of beam delivery as well. This makes it especially conducive to robotic beam motion systems. Another difference between the two lasers is the operating wavelength. The CO2 laser uses a wavelength of 10.6 micrometers whereas the Nd:YAG laser has a much smaller wavelength of 1.06 micrometers. This difference affects the rate at which the beam is absorbed. The shorter wavelength is absorbed quicker and thus makes the Nd:YAG a theoretically more efficient machine. It is more effective on aluminum, copper, and highly polished surfaces. However, beam absorption depends mainly on surface condition and virtually nullifies any advantage gained based on operating wavelength.

Beam quality is also an important factor. A perfect beam is defined as having a true Gaussian energy distribution and beam divergence, i.e. angle of expansion. For this theoretical beam, the hottest point is right in the center. This is a very desirable condition for cutting and welding purposes as well as in laser cladding. However, in laser cladding, the beam is generally a little out of focus in order to increase the pool size. A perfect beam could thus be manipulated with a scanner to better enable one to control the size of this pool. Beam integrators are sometimes used, though, to homogenize the beam density and control the pool size in that manner.

There are two methods of filler metal addition. The first is the wire method. For good reason, it is not a popular option. The wire can jam into the molten pool and cause serious defects in the final clad. Another good reason is that if the laser is scanned over a continuous half-inch length (or more) this method is simply not possible. The second method is through powder injection. The advantages are that there are more compositions available in this form and it eliminates all of the problems of the wire method. One problem with powder injection is that providing a constant mass flow is difficult to accomplish. The rate can be measured before and after but not during the procedure.

The following case study illustrates the benefits of laser cladding. GE Aircraft Engines (GEAE) implemented a new cladding process in 1993 to surface and repair engine turbine blades. They used a CO2 laser with a high nickel alloy powdered filler. Other enhancements such as vision guidance, sophisticated motion mechanics, and software control were used as well. Compared with conventional welding techniques that deposited filler at a rate of 6-8 ipm, the laser cladding process did it at 22 ipm. This is an invaluable benefit. Also, the cladding process produced a far better surface finish that reduced the necessary hand grinding by 50%. After some testing, the engineers improved on it even further. They added a new controller and modified the powder-and-beam nozzle to meet production demands. Another improvement which impressed me very much was an added capability of three or five axis motion with a precision linear, rotational, and tilt mechanism. This allows the machine to adjust its tool path in real time and produce a like-new blade.

As I mentioned at the beginning, there are several ways that laser cladding will benefit our manufacturing facility. It is useful for repairing grain boundary cracking in the HAZ. Since laser cladding involves heating the substrate, the HAZ is consequently a more brittle region and susceptible to cracking. Heavy castings and large grained wrought materials are the most likely materials to see this type of problem. Another benefit occurs if the base and filler metals are substantially different. Our current methods require a highly diluted filler with probably two layers of application. Since laser cladding is a low heat input process, this dilution is not necessary and would require only a single application. The last major benefit of laser cladding is the novelty of dimensional restoration. Expensive parts that are warped or worn can be restored to their original shape instead of thrown away. Again, the low heat input is necessary to avoid a reduction in base metal properties in the HAZ. Shafts for power transmissions are highly stressed and fatigued making them a likely candidate for this treatment.

For the reasons previously cited, I believe we have a viable solution to our specific manufacturing problem. We can substantially reduce the costs of scrapping and rework if we implement a laser cladding process. We will be able to increase wear resistance and environmental protection and utilize the process' dimensional restoration characteristics. The start-up costs for this are fairly high but will be offset by more efficient and versatile rework capabilities. Most importantly, we will be able to serve our customers better with shorter lead time and higher quality work.

Bibliography

1. Laser Cladding Takes Off, Manufacturing Engineering, Jan. 1993, pp. 84-85.

2. Laser Surfacing Through Laser Cladding, Meinert, Kenneth Jr. and Whitney, Eric, The Fabricator, Jan. 1997 vol. 27, no. 1, pp. 58-60.

3. One-Dimensional Diffusion model for Extended Solid Solution in Laser Cladding, Kar, A. and Mazumder, J., Journal of Applied Physics, April 1 1987 vol. 61 (7), pp. 2645-2653.