3.1

Basics of fiber lasers

Fiber lasers that are used in laser material processing typically operate at wavelength of 1070 nm. The lasing dopent is ytterbium (Yb). The fiber core is doped with Yb. The lasing dopent is pumped with diode lasers that are spliced into the fiber. Fiber grating are used to establish the resonator such that mirrors are not required. Fiber lasers have been built with powers over 30kW with wall-plug efficencies greater than 25%. This opperating efficency is greater than that of Nd:YAG or CO₂ lasers that are typically used for material processing. Since there are minimum consumerables in operating such lasers, the maintainence is also a minimum. This helps to maximize the fiber laser up time as compared to previously used material processing lasers. Figure 2 highlights differences of the fiber laser than the more traditional material processing lasers.

Figure 2.

The fiber laser incorporates all the good characteristics of both the Nd:YAG and the CO₂ lasers.

3.2

Process Map

The operational regimes for various laser processing techniques are a function of the laser interaction time and the power density of the delivered laser beam. Laser interaction times range for very short pulses to a continuous wave (CW) mode [1]. Power densities include peak powers from a laser that’s being operated in a pulse mode to average and CW. It can be seen from Figure 3 below that laser welding falls within an approximate power density range from 10⁵ to 10⁷ W/cm² and a laser interaction time from CW to a pulse width (length) that is as short as 10^-4 seconds (0.1ms). The power density for a given laser process is dependent on the laser output power and the focus spot size which is directly related to the beam quality of the laser. When the beam quality of a laser is low, smaller focused spot sizes can be produced.

Figure 3.

The operational regimes for various laser material processing techniques are shown. The regime shown for laser welding is also the regime for laser cladding

3.3

Initial Thick Section Welding with Fiber Lasers

To establish the entitlement of thick section welding with high power fiber lasers, test trials were performed with a typical stainless steel alloys. Bead on plate and butt weld trials were performed. Welding was conducted at power levels ranging between 10-20kW. The focus lens had a focal length of 310 mm. On the average, focus was ~6 mm into the surface of the 304L stainless steel substrate. The range of the welding speed was between 1-2 m/min. Argon was used as the welding cover gas and most welds were performed in the 1G position with a few in the 2G position. Figure 4 shows the cross section of butt welds for thickness between 0.75 and 1.0 inches. By welding for from opposite sides of a butt joint, successful welds achieved of thickness of 1.5 and 2.0 inches.

Figure 4a.

1.5 inch double sided butt weld (20kW, 2.0 m/min).

Figure 4b.

2.0 inch double sided butt weld (20kW, 0.6 m/min).

Figure 5 illustrates how weld penetration varies as a function of welding speed for different power levels of laser energy. These welding results were achieved with a 20kW IPG fiber laser. The welding results were achieved on 304 stainless steel. As the welding speed increases for a fixed power level, weld penetration decreases.

Figure 5.

The graph depicts how the weld penetration decreases monotonically with increasing laser welding speed.

4.1

GE Nuclear Fuels

GE has used laser technology to weld fuel rod assemblies for over 25 years where CO₂ lasers were initially used. There were engineers at the Research Center that developed a weld seam tracking system because there wasn’t anything that was available at that time with the desired resolution. The time frame when CO₂ lasers were being used started in 1986. This was the case because there weren’t Nd:YAG lasers at that time that had sufficient power when operated in a CW mode. The switch over to the 1.06μ wavelength lasers took place in the mid-1990s. These lasers were flash-lamp pumped. In the early 2000s, there another switch to diode pumped Nd:YAG lasers. They were more efficient with reduced maintenance because changing flash lamps were not need. It is quite expensive to replace the pumping diodes in a diode pumped laser, perhaps $50-70K. It was decided in 2010-2011 to go with fiber laser technology instead of replacing the pump diodes in the old Nd:YAG laser. The fiber laser was more efficient than the diode pumped laser and there was no free-space optics in the resonator head to care for. The fiber laser was very attractive with having a 25% operating efficiency and with no or very little maintenance required.

The component that requires laser welding is the tubes of the control rod blade assembly. These tubes are ~12 feet long and are made of 304L SS. There are two opposing laser autogenously welds that are the full length of the tubes. There are two 2kW fiber lasers that are used for this task. Figure 7 shows the end of the welded control blade assembly.

Figure 7.

The end of a complete welded control blade is shown for the 14-tube D-Lattice Design.

The laser power to affect these welds is ~750W with a focus spot size of 375μ. The laser beam is focused at the surface. Helium is used as a shielding gas. Welding speed may range between 4-5 m/min. The effective weld depth is ~1.0 mm for the Marathon and Ultra Designs. These designs and related weld cross sections are depicted in Figure 8 below.

Figure 8a.

Marathon Design and Weld Cross Section.

Figure 8b.

Ultra Design and Weld Cross Section.

Figure 8.

A completed welded diesel cylinder head-liner is shown.

4.2

GE Transportation

The cylinder head-liner for one of the business’s diesel engine products was once electron beam welded and then the process was switched over to laser welding. A high power CO₂ laser had been used for over three decades. The welding process required between 12-13kW of CO₂ laser power. To assure a robust welding process, the laser was on continuously to assure that the laser beam quality remained stable. If the beam quality was unstable, the laser beam focus could vary, leading to weld penetration variations. The operating efficiency of the laser was 7-8% which leads to a large consumption of electricity with the system on continuously. It was estimated that an annual savings between $200-250K could be realized if the CO₂ laser was replaced with a fiber laser due to a reduction in electricity usage alone. This reduction in electricity usage was due to the higher efficiency of the fiber laser and that the fiber laser only needed to on when the welding process is taking place. There was other reduced maintenance costs that were identified such as the reduction in water usage.

The weld to that is required is for a butt joint with a 0.55 inch wall thickness. The bore diameter is approximately 11 inches. The head is a cast steel (Mg, Mo) and the liner is ASTM A106 steel. A keyhole weld was performed in a 1G position with the CO₂ laser and was change to a 2G position with the new fiber laser workstation. A 14kW IPG fiber laser was selected. It is operated at 13kW to affect the desired weld. The head-liner assembly is first tact welded in four locations equally spaced around the circumference of the assembly. The continuous weld is then performed at a speed of ~1.0 m/min. A welded cylinder head liner is shown in Figure 8 and a weld cross section is seen Figure 9.

Figure 9.

A cross section of the keyhole butt weld is shown with evidence of no major defects. The ASTM A106 steel is in the top of the picture and the cast steel (Mg, Mo) is in the bottom of the photo. This weld was performed at 13kW and 1.5 m/min.

4.3

GE Healthcare

The welding of two components for an x-ray tube has been developed with the use fiber lasers. The first component involves the welding of a flat tungsten emitter and the second component is a shaft that has been previously welded by electron beam. The tungsten emitter requires a pulse laser operation and the shaft weld requires a 10kW CW keyhole weld. An 11kW fiber lasers was configured to be shared between at tungsten emitter welding workstation and a shaft welding workstation. The flexibility of the fiber laser enables a pulse mode operation at ~1kW average power at one workstation and a CW mode operation at 10kW at the second workstation.

Figure 10 shows a schematic of a flat tungsten emitter that be joined initially to three tungsten posts and then to three molly post. The weld involves the use of platinum material to affect the joining process of the refractory materials. Many such welds were made and successfully tested hours at high temperatures. One important attribute of the flat emitter was its flatness. The welding process did not compromise the emitter’s flatness. The tungsten emitter is only 0.2 mm thick.

Figure 10.

Shown is the schematic of the tungsten emitter mounted on the posts. It is fixture in place for the welding process. The cross section shows the flow of the platinum.

The rotor shaft weld is shown in Figure 11. In this case, a 10 mm deep weld required. The welding direction is parallel to axis of the rotor shaft assembly and is in a 1G position. The materials that are being welded are 909 Incoloy and 1018 Steel. The Incoloy cylindrical component is pressed fitted into the counter bore of the steel component thus forming an ideal interface for a keyhole weld. The weld does not require a vacuum chamber. It only requires a cover has such as nitrogen, argon or helium.

Figure 11.

The schematic depicts the weld process of the rotor with an E-Beam process. A high brightness fiber laser is being used in place of an E-beam system to affect the required 10 mm weld depth.