Efficient high-power diode laser cladding

Award-winning research leads to the development of wire-fed laser cladding of aluminum-bronze on steel bearing surfaces in marine pumps

by Valdemar Malin and Stuart Woods

Moving parts on ships are subject to both long-term wear and the effects of saltwater corrosion. Cladding is a useful method of increasing the lifetime of such parts. Here we describe how Alion Science & Technology's Rockford Manufacturing Research Center has worked with the US Navy's Puget Sound Naval Shipyard Intermediate Maintenance Facility (PSNS/IMF) to develop laser technology for cladding steel pump components with a low-friction layer of aluminum bronze. Conducted under the Low Volume Productivity (LVP) program, this technique utilizes a Coherent high-power direct diode laser (HPDDL) as the key component of the automated laser cell (ALC), a highly flexible and efficient robotic system. The award-winning work described in this article also included extremely detailed characterization of test samples produced under a range of tightly controlled conditions.

What is laser cladding?

Surfacing is a process where the application of a layer of material to a substrate surface by welding, brazing, or thermal spraying produces desired properties or dimensions. When the deposited material provides corrosion or heat resistance the process is called cladding. When the deposited material provides wear resistance the process is called hardfacing.

Cladding is a widely used process for providing the desired surface properties of a part, or for rebuilding worn components to their original dimensions after machining. In the first case, cladding allows a layer of material that is different than the substrate (dissimilar filler metal) to be added to the surface. Ideally, the cladding process should result in minimal mixing between the deposited and substrate materials (called dilution), a high-strength (truly metallurgic) bond between the cladding and substrate, and a minimum thermal distortion of the part being worked. Various practical considerations are usually equally important, such as productivity, process compatibility, and cost.

FIGURE 1. The linear-beam profile from a HPDDL is well-matched to support both powder and wire-fed cladding.

Lasers have proven to be competitive or superior in some cladding applications where the laser is used to melt the surface of the substrate and the filler material, which can be in powder or wire form. Laser cladding has traditionally relied on CO₂ and various types of Nd:YAG, and more recently fiber lasers, but each of these presents some limitations. For example, in previous cladding applications at Puget Sound—for refurbishing worn shafts—the Navy employed two Nd:YAG laser cladding systems, but discontinued their use because of various problems.

However, ongoing advances in HPDDL technology have now made this laser type very attractive because of its small footprint and ease of integration into existing manufacturing lines. Plus, their near-infrared wavelength produces superior results in terms of reduced thermal effects such as alloying and physical distortion. One reason for this is that their shorter wavelength output is better absorbed by cladding materials than the beam from a Nd:YAG and especially a mid-infrared CO₂ laser. This means that a HPDDL can melt any material using substantially less output power than a CO₂ laser.

These lasers also offer a substantial cost advantage over other laser types, in part because their electrical efficiency (conversion of input electrical energy to useful light output) is four times higher than for CO₂ lasers, about three times higher than diode-pumped Nd:YAG lasers, and nearly twice that of fiber lasers. When combined with the higher absorption, this translates into increased deposition efficiency, lower operating costs, and a smaller carbon footprint.

FIGURE 2. The Alion automated laser cell (ALC) supports cladding and hardfacing applications.

Moreover, the rectangular (linear) beam shape of a HPDDL incident on a substrate surface is typically an advantage in cladding. For example, in powder and wire cladding, the linear beam is oriented perpendicular to the travel direction, enabling large areas to be processed rapidly (see FIGURE 1).

The Alion approach

In this Navy sponsored program, the initial goal was to develop an improved laser cladding technology specifically for new steel bearing plates, which are key components in shipboard pumps. Until now, these plates have been brazed using a traditional cladding approach, which was to use rosebud torches for heating the parts, and to melt flux and chunks of filler metal (copper alloy) in specially machined pockets on these plates. This is followed by a long cooling interval. The whole brazing procedure takes up to 50 minutes. After that, the blank was machined down to final tolerance.

There are several problems with this manual brazing approach, including:

• Extensive porosity in the weld deposit. To cope with porosity, the pockets in the blank are made 0.280 in. deep, which is much deeper than the required 0.093in. thickness of the bronze deposit after final machining.
• Unpredictable quality of the weld deposit. Poor control over the heat source and, thus, substrate temperature, along with the welder's subjectivity in determining when to end brazing, are the main causes for variability in quality.
• Safety issues. High temperature, noise, and emission of harmful gases potentially jeopardize the safety of the welder and pollute the environment.
• Low productivity (parts/hour) and high cost because this manual process is labor intensive.

To address these issues, Alion developed an ALC suitable for both cladding and hardfacing (see FIGURE 2). This is a robotic system where a 4 kilowatt Coherent HPDDL is mounted on the arm of the Panasonic robot together with the laser support system. The latter includes a vision system to provide real-time images of the weld pool zone on monitor screen, a wire positioning system for remote manipulation of the wire relative to the weld pool, an infrared temperature sensor measuring the temperature of the blank, a water-cooling station, and a specially developed fixture and other systems and devices.

Special robotic programs were developed to control the sequence of operations, laser head motion, and welding parameters. A more detailed description of the ALC and the described technology can be found in reference 1.

One important rule of welding automation (for welding, brazing, etc.) has always been to modify the part to be welded wherever necessary in order to make it suitable for automatic processing.2 The original blank of the bearing plate was designed for manual out-of-furnace brazing and was not suitable for automatic laser cladding using the new ALC for several reasons:

• The pocket configuration was not suitable for uninterrupted HPDDL cladding using the ALC.
• Excessive allowances for machining had been incorporated to make up for imperfections in the manual brazing process.
• The blank did not have any design details that could be used as references to allow accurate positioning and locating in a fixture.

For these reasons, the blank design was modified, but only in ways that left the final part's dimensions and shape unaffected. The original and new blank designs are shown in FIGURE 3.

FIGURE 3. Design of a steel blank of a bearing plate: (A) original; (B) new as modified for automated cladding.

Regarding cladding material, aluminum-bronzes are well suited to this marine application because they provide low friction, and show no serious deterioration in appearance or significant loss of mechanical properties on exposure to most atmospheric conditions, including acids and salt-laden water vapor. Therefore, copper alloy ER CuAl-A2 containing 8.5–11% aluminum was selected for this application. So, even accounting for dilution and evaporation losses, the final minimum surface layer concentration of >7% can be confidently assured.

Optimizing and evaluating performance

In operation, the blank is clamped in the fixture, and the water cooling is turned on. The robotic program is then initiated and the cladding cycle starts and continues automatically. FIGURE 4 shows a bronze layer of aluminum-bronze deposited on a blank. Each weld bead is deposited along the shorter edge of the pocket. The laser head then makes a U-turn. The cladding in this pattern continues until the pocket is completely filled.

Deposits of precise thickness (0.125 in. +0.020 in./–0.015 in.) and uniform weld width (5/8 in. ± 1/32 in.) were obtained with no surface defects (cracks, pores, under-filled spots, excessive ripples, or oxidation). Obtaining a flat deposit over the entire filled pocket was also critical, with no spots 0.015in. below the blank surface being allowed. To produce a flat surface of the specified thickness, it was found necessary to place 22 beads with 55% overlap to cover the pocket.

Samples cut from the welded blanks were analyzed by multiple techniques to systematically determine metallurgical and mechanical properties of the deposited metal, including chemical composition, aluminum and iron distribution, hardness, and microstructures.

Results from metallurgical tests confirm that the laser cladding technology produces aluminum-bronze deposits of a very high quality. Iron content on the surface of a machined bronze deposit was only slightly higher than that contained in the wire because of the very low dilution (0.3%) inherent in this technology. Excessive iron in the deposit manifests itself in elevated hardness on the surface of the clad and thus could cause premature wear in the steel counterpart.

FIGURE 4. A layer of aluminum-bronze deposited on the surface of steel bearing plate. For illustration purpose, the top of the deposit is machined off (on the left side), while the right side remains as-welded.

Iron distribution through the thickness of the bronze deposit is favorable for anti-friction/wear resistant service. It contains elevated iron content at the bronze-steel interface, which makes the metallurgical bond stronger. At the same time, iron content on the surface in contact with steel gears is only slightly higher than that in the wire, which precludes excessive wear of the gears.

The loss of aluminum (due to dilution and evaporation) is also minimal. Aluminum content on the machined surface is above 7%, which ensures a desired dual-phase microstructure. Hardness of the aluminum-bronze deposit is actually higher than that currently produced by the current brazing and, in general, by conventional arc welding processes (by 6% and 17%, respectively). And just as important, the local hardness is fairly uniform on the machined surface of the deposit with no local hard spots.

Conclusions

Alion's high-power direct diode laser cladding technology has proved successful in depositing 8.5–11% aluminum bronze on steel bearing plates produced by a shipboard pump manufacturer. The quality of the deposited metal exceeds considerably the quality of the current method. Plus, both production time and consumption of the copper alloy was cut in half, machining time was considerably reduced, and safety and pollution problems were eliminated.

Editor's note

Drs. V. Malin and F. Sciammarella published a detailed account of this study in the Welding Journal September, 2008, which was recognized by the American Welding Society with the award of the A.F. Davis Silver Medal Award as representing “the best contributions to the progress of welding” in the category of Maintenance and Surfacing. Dr. Malin and the Alion ALC of his design have also been recognized with two prestigious international R&D 100 Awards.

Valdemar Malin, Alion Science and Technology Corp., Mc Lean, VA, is director of Rockford Manufacturing Technology Research Center ([email protected]) and Stuart Woods is director of Marketing-Material Processing, Coherent Inc.

References

1. Valdemar Malin and Federico Sciammarella, “Cladding in marine applications using high-power direct diode laser,” Welding Journal, September 2008, pp 2-11.
2. Malin, V. 1985. Designer's guide to effective welding automation — Part I: analysis of welding operations as objects for automation. Welding Journal 64(11): 12–26.