By ALEXEI YELISTRATOV edited by BRUCE VERNYI, editor-in-chief

Fig. 1 Experimental setup, processing area: 1 - Beam's heat spot on top of the molten pool. 2 - Strip guide. 3 - Strip. 4 - Shielding gas nozzle.

Fig. 2 Main components of laser-strip system, orientation of the strip to laser beam: a - Orientation of the strip to beam's heat spot (front strip feeding, (=90=const). b - Orientation of the laser beam to travel direction.

Fig. 3 Main components of laser-strip system, orientation of the strip (dotted lines) to the surface of the molten pool at front and rear strip feeding: A - Front feeding, above the pool. B - Front feeding, inside the pool. C - Rear feeding, inside the molten pool. D - Rear feeding, above the molten pool. E - Rear feeding, strip slides on the surface of the pool.

Fig. 4 Feeding the strip into the laser beam, above the molten pool.

Fig. 5 Bead with fine ripples.

Fig. 6 Feeding the strip inside the molten pool, with targeting of the strip: a - To the middle of the pool's height. b - To the bottom of the pool.

Fig. 7 Bead with even surface.

Fig. 8 Deposition/penetration areas vs. travel speed, process parameters: P = 4kW, Strip feeding speed: 140

Fig. 9 Cross section of the beads at the strip feeding speed of 130 cm/min, for different travel speeds: a - 10 cm/min; b - 25 cm/min.

Fig. 10 Cross section of the beads at the strip feeding speed of 150 cm/min for different travel speeds: a - 10 cm/min; b - 15 cm/min.

Fig. 11 Microstructure of the deposit.

Fig. 12 Diffusion of elements across the interface: EDS data (right) and measurement points across the interface (left).

Fig. 13 Micro hardness survey across the interface.

In hardfacing, strip filler metal is used for high productive deposition and to obtain low dilution with the base metal. Typically, strip filler is applied in electric arc or electroslag hardfacing. Most research in the field of laser deposition is concentrated on powder feeding.

In hardfacing, strip filler metal is used for high productive deposition and to obtain low dilution with the base metal. Typically, strip filler is applied in electric arc or electroslag hardfacing. Most research in the field of laser deposition is concentrated on powder feeding.

An average feed rate for a 4-5 kW CO2 laser beam is 18-20 g/min of Ni-alloy powders (Ref. 1) and as much as 100 g/min of Ni-Cr powder for a 6 kW diode laser (Ref. 2). There was an attempt to deposit a layer of a Ni-base super alloy on a low-alloyed steel surface by melting a preplaced sheet with dimensions of 8 mm by 50 mm by 0.9 mm using a 10 kW, CO2 laser with a special beam integrator to obtain a heat spot 10 mm by 10 mm (Ref. 3, 4). Deposits, made with a travel speed of 0.25-0.5 m/min had a depth of penetration up to 3 mm.

The diode laser has a much better electrical efficiency versus traditional lasers. Furthermore, a NUVONYX laser produces a heat spot characterized by a linear shape, with dimensions of 12 mm by 0.5 mm at a normal focal distance. The lower power density and larger heat spot area made by a diode laser deposition results in minimal penetration of the base metal and a smooth appearance of the deposit. In that case, the less concentrated heat source — the beam's heat spot — retains a stable volume of the molten pool while penetration of the base metal is provided mainly by the molten pool's enthalpy. The diode laser beam's energy is high enough to melt thin wire — 0.9mm (0.035 inch) — and a metal strip with a cross section four times larger. Experiments (Ref. 5) have verified that high deposition rates can be achieved with strip feeding using diode laser processing.

Process description
A High Power Direct Diode Laser, such as the NUVONYX - ISL-4000L, provided the output beam power of 4 kW to the work piece that was installed at focal distance of 94 mm (3.74 inches) below the laser head. The processing area is presented in Fig 1.

Process parameters: Strip feeding rate: 80-150cm/min. Travel speed: 10-20 cm/min. Shielding gas: Argon, 30 CFH. Strip: Nickel base super alloy 625, size: 6 mm by 0.413 mm (.236 by .016 inch).

Base metal: ASTM A36, 12 by 12 by 3/4 inch flat plates.

The direction of strip feeding was only across to the laser beam's heat spot, as shown in Figure 2-a ( =90). Straight single and multi-layer beads were deposited for studying the feasibility of the new process.

Results and discussion
With strip feeding across the long axis of the beam, the molten metal was distributed evenly along the beam's heat spot. For a strip cross-section of 6 mm by 0.413 mm, the following dimensions of the molten pool were obtained:

Width: 12 mm to 14 mm.

Height: 9 mm to 11 mm.

To achieve an even cross section of the deposit, a proper ratio of strip feed rate to travel speed is required. If the strip feed speed is insufficient for a given travel speed, the deposit has a lower height and an uneven shape.

To get better formation of the deposit at a wider range of travel and strip feeding speeds, a change in orientation of the beam's heat spot to the travel direction, as shown in Figure 2-b, angle "a", is beneficial. In that case, the direction of the strip feeding to the beam heat spot (B = 90), but the width of the melted zone decreases and an even bead can be formed at a lower strip feed rate or at a higher travel speed.

The flat surface of the strip absorbs the beam energy more effectively compared with the cylindrical surface of a wire, and so the strip can be effectively heated and melted with wide clearance for feeding angles. In that case, the strip guide can be compact and, with front and rear feeding, it increases the technological possibilities of the laser deposition.

Front feeding of the strip
When the strip is fed ahead of the molten pool, the direction of feeding is opposite to the travel direction. In that case, two different targeting points are available, as shown in Figure 3.

Orientation A
The strip enters inside the front part of the pool. Because the thickness of the molten pool in the direction of strip feeding does not exceed 2 mm to 3mm, strip melting/feeding has limited feeding speed range. For a strip size of 6 mm by 0.413 mm, which was used in the experiments, a uniform clad deposit was achieved at strip feed speed of 80 cm to 140 cm/min. The angle of the strip entering the molten pool and the location of occurrence of the strip melting are important parameters.

Orientation B
The strip melts inside the laser beam, above the molten pool. In this case, the molten metal is transferred to the molten pool by drops. Usually one or two drops are simultaneously formed on the strip's tip and flow to the pool, as seen in Figure 4. Those drops generate an oscillation of the pool that results in a solidified bead with fine ripples (Figure 5).

To get an even bead, the strip should be fed inside the molten pool (Figure 6). Because of absence of the transfer of drops and the reduced oscillation of the pool, the bead has smooth appearance (Figure 7).

With a higher strip feed speed, bead cross section becomes larger, reaching 20 mm ² to 22 mm ² at a feeding speed of 140 cm/min to 150 cm/min (Figure 8, 10). Penetration depth is very shallow and very uniform. The average penetration area for maximal beam power and a high feeding speed is 0.2 mm ² to 0.4 mm ² , deviation: ±0.02 mm ² . Bead cross section changes with increasing travel speed. The bead becomes narrower and lower for strip feed speeds of 130 cm/min (Figure 9) and 150 cm/min (Figure 10). At the same travel speed, a higher strip feed speed corresponds to a wider bead (Figure 9-a, Fig. 10-a). Higher strip feeding provides a larger bead cross section (Figure 10-a).

Rear feeding of the strip
It is possible to feed the strip from behind the laser beam (Figure 3). In that case different targeting points for strip feeding are:

C - The strip can be directed into the upper rear part of the molten pool at an angle to horizon (y=10º to 15º), to provide a completed melting by the enthalpy of the pool.

D - The strip can be directed to the laser beam above the molten pool (clearance: .035 inches to .08 inches (1 mm to 2mm) with drop transfer from the strip to the pool.

E - The strip feeding can be oriented parallel to the pool's upper surface, touching the pool on its way to the laser beam spot (no clearance between the strip and the pool).

Orientation C
When the strip immerges into the pool, it changes the temperature conditions in the pool, and causes a crystallized front "pool-bead" to approach the strip because of heat sinking to cold bulk and the shadow effect. In that case, the strip can be welded (frozen) to the bead through an interruption of deposition process or it can push the liquid metal along the direction of travel, depending on the process parameters and the location of the targeting point in relation to the laser beam. In experiments, we did not achieve an even formation of the bead with C-orientation.

Orientation D
With the strip's melt separated from the molten pool and drops transferred to the molten pool, the strip is melted by the laser beam and does not cause any problems for the formation of the pool. This orientation provides free conditions for the molten pool to solidify under the influence of the forces of surface tension. Therefore, when the amount of filler metal is too small for a given travel speed, the pool shrinks and forms a bead with a width that is only a portion of the fused zone. A similar effect was noticed with front strip feeding. When the strip melting rate is close to normal, the bead is characterized by an even shape with fine ripples that is similar to front feeding (Figure 5).

Orientation E
When the strip is touching the upper surface of the molten pool, the strip acts like sliding crystallizator. It supports the liquid metal of the pool in the middle of the fused zone and provides smooth forming of the bead. For this orientation of the strip, beads with a smooth surface were formed at a wider range of process parameters and at lower strip feed speeds. When there was not enough molten metal to get an even deposit, a sliding strip provided the even formation of the bead in the middle of the fused zone. The deposit cross section reached 20 mm2, a result similar to front feeding (Fig 8), with maximal strip melting nearing a rate of 27 g/min.

The strip deposition process is very robust, and requires minimal preliminary adjustment of the strip guide and minimal "on-the-fly" controlling. For deposition over a wide area, each bead should be overlapped by 20 percent to 30 percent to obtain a flat profile with maximum width. Because of rapid cooling, it is possible to use this process to deposit on tilted surfaces and edges.

Samples cut from multilayer beads were used for the next set of metallographic research.

Metallographic investigation
Metallographic investigations of strip deposits reveal sound metal and the absence of defects (Figure 11). Multi-layer (8 layers) deposit has a dendrite microstructure with a direction corresponding to the direction of heat flow during cooling. There is no demarcation indication at the points at which the borders between the deposited layers are located. Grains are disoriented at the bottom of the area because of multidirectional heat sinking, and the grains are more directional at the upper area of the deposit. The heat affected zone of the base metal has relatively large grains directly adjacent to the interface, and they become finer as one moves further into the base metal.

Due to the high cooling rate of the clad, diffusion of Fe from the base metal to the deposit through the interface is within a 200 m layer (Figure 12). Concentration of other alloying elements - Cr, Ni and Mo - in the deposit decreases inside the layer, which is less than 300 m above the interface. There is no noticeable mutual diffusion across the interface.

A micro hardness survey was conducted along the vertical line in the middle of the deposit on distances ±4000 m in both directions from the interface with 250 HK to 350 HK, at 100g for 10 seconds (Figure 13). Some increase in hardness of the base metal was observed near the interface. This was probably connected to the phenomena in the heat affected zone.

Conclusion

1. Strip filler can be used with a direct diode laser to enhance the application and productivity of resurfacing. With 4 kW laser, melting rates up to 3.72 cm3/min for nickel-base super alloy 625 can be achieved.
2. With strip feeding across the heat spot's long axis, the laser deposition process became very stable. Targeting the strip to the molten pool can be easily accomplished and controlled. Process provides minimal penetration of the base metal.
3. There are several possible orientations of the strip feeding direction to the beam's heat spot and to the travel direction. This enlarges the application possibilities for the diode laser strip deposition.

References Kathuria, Y.P. 2000. Some aspects of laser cladding in the turbine industry. Surface & Coatings Technology, No. 132, pp: 262-269. Vuoristo, P. and Vihinen, J. 2002. High-Performance Laser Coatings for Manufacturing and Maintenance of Industrial Components and Equipment. Maintenance Research in Finland. Kunnossapito No 5, 2002 Tosto, S. and Piedominici, F. 1994. Laser cladding of Ni-based super-alloy. Journal of Materials Science, vol.29, No 2, pp: 504-509. Nacey, T. 2001. Diode Lasers Offer Welding Advantages. Welding Journal 80 (6): 28-30. Malin, V, Johnson, R and Sciammarella, F. Laser Cladding Helps Refurbish US Navy Ship Components. The AMPTIAC Quarterly, Vol.8, #3, 2004, p 3 to 9.

A. Yelistratov, PhD., is a welding engineer, e-mail: alexeiyel@yahoo.com