Master of Industrial Sciences 2015- 2016Faculty of Engineering Technology, Campus Group T LeuvenThis paper is written by (a) student(s) in the framework of the course Advanced Manufacturing Processes and Systems

UNDERWATER LASER WELDING

Nils Broes1, Bert Korthoudt1, Nick Verlooy1

1Electromechanics, focus Intelligent Manufacturing Faculty of Engineering Technology, Campus Group T Leuven

Vesaliusstraat 13, 3000 Leuven, Belgium

Professor: Jef LoendersFaculty of Engineering Technology, Campus Group T, Leuven

Vesaliusstraat 13, 3000 Leuven, BelgiumJef.Loenders@kuleuven.be

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1 INTRODUCTIONLaser welding is a welding technique that's used more frequently since automation of production lines is introduced. It is a technique that is used especially in the automobile industry nowadays. Laser welding is especially suited for the use of welding robots or CNC machines, which is why it's suited for the use in automated processes. Underwater laser welding is a more recent technique, deviated from normal laser welding, where the process is done underwater. Several techniques have been introduced and researched, like laser welding in an underwater medium or laser welding in a local dry cavity. The process has some significant benefits for reparations done on nuclear power plants, which is why this technique was developed.

2 UNDERWATER LASER WELDINGThere are multiple techniques that can be used to perform underwater laser welding. These different techniques will be discussed here, with the pro's and con's of each of these techniques. Underwater laser welding can be performed in a direct water medium or in a local dry cavity, the last one is mostly used because of certain problems encountered when welding in a water medium. The technique is frequently used in the repair of nuclear power plants, where cracks in the surface of some components appear over time because of fatigue or stress corrosion. In the past, underwater welding was solely used with the arc welding process, for which a diver was needed. When welding in nuclear power plants, radiation causes a serious danger to the welders, performing the repairs. Most of the time, the nuclear reaction also had to be stopped to perform the repairs, since all cooling water had to be removed, reducing operation time. Underwater laser welding introduces the use of robots for these repairs, removing the need of human interaction in the repair process. Research shows that underwater laser welding can be done twice as fast as Gas Tungsten Arc Welding. This means that repair times could also be greatly reduced, reducing downtime of the installation.

Figure 1: principle of laser welding (Y. Sano & Obata,2008)

In a laser welding process, a continuous laser beam or short laser pulses are sent to a metal, which will melt two pieces together. The lasers usually used in underwater laser welding are solid state lasers such as Yd-YAG lasers. The delivery from the laser beam to the weld area is done by the use of fiber optics. Since these lasers have a high power density (typically on the order of 1MW/cm³) the heat affected zone will be significantly smaller in comparison with other welding techniques which is useful when the property of the surrounding material must not be influenced by the weld.Westinghouse, a company that offers maintenance services to nuclear power plants, has introduced this technique to make repairs for their clients. The system used by this company is shown in figure 1. A laser beam is pointed at a crack in the surface, which must be repaired. The laser beam is transported by of fiber optics, and then focused by specialized lenses or mirrors. Argon gas is blown around the weld area to create a local dry cavity. To keep the gas in place to sustain a stable dry cavity, sometimes a water curtain is blown around the local bubble. This is can be helpful for making the welding process stable. (Westinghouse Electric Company, 2014)

3 SYNTHESIS SCIENTIFIC PAPERS

3.1 Laser welding in an underwater medium

To begin the research on underwater laser welding, research has been done on the influence of water as a medium when welding with a laser beam. It's obvious that welding in a different medium can have several consequences. The new medium can have an influence on the metallurgical behavior of the welded materials during and after the weld. Another problem that might arise is the absorption of the power sent by the laser before it reaches the welding area, thus reducing efficiency.The centre for Laser Processing in Balapur, India, has done research by comparing the micro-hardness, coefficient of friction, fusion zone and weld pool to be able to correlate these to the micro structural properties of the welded material. (N. Kumar, 2010). A Multi-kilowatt CO2 laser was used to perform these experiments. In the first case, the material was kept in normal conditions with air acting as the medium, in the second case the material to be welded was placed 1.5 mm deep under water to perform the weld.

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Figure 2: typical weld pool of a laser welding process (N.Kumar, 2010)

To better understand the results of this experiment, one must understand the concept of keyhole welding (a form of deep penetration welding). By applying a high power to an area, the laser penetrates the materials so that a deep pool of molten material is formed. When moving the laser, the shape of a keyhole is formed, hence the name.A typical weld pool with keyhole welding is shown in figure 2. Vwel represents the velocity of the weld or in other words, the velocity at which the laser moves. A ring shaped structure is formed with the inner weld pool being fluid metal and more solid material is found at the outside of the circular weld pool.As is expected of course, the diameter of the weld pool region is declining when moving deeper into the material, since the power of the laser is being absorbed while traveling down the pool.In both cases the keyhole depth and width (at half the maximum dept of the keyhole) were measured in function of laser power, as can be seen in figure 3a and figure 3b respectively. The keyhole depth tends to rise with increasing laser power, but the depth is bigger in a water medium then in the normal case. The reason for this is the higher hydrostatic pressure and a higher vapor recoil pressure. The vapor recoil pressure is the pressure that is produced by the evaporated water in and near the weld pool. The surface tension on the weld pool region tends to close the keyhole. On the other hand, the keyhole is kept open by vaporization of material inside the keyhole. The buildup of vapor pressure is more intense in the case of a water medium overcoming the surface tension of the keyhole, which tends to a deeper penetration.

Figure 3: keyhole depth and width in relation to laser power (N. Kumar, 2010)

Because of the cooling properties of the water a narrower weld tends to be produced in the case where water is the medium. But no systematic difference was found between the keyhole width in both cases, in comparison to the keyhole depth.Another difference can be found when analyzing the welding bead of both cases after the welding has been done. The results are shown in figure 4. For higher laser power a bigger weld bead is formed, while in an air medum above 3kW laser power, the keyhole mould disappears, because of excess vapor pressure and humps are created at the top of the keyhole. In the case of a water medium, rapid melt solidification of the molten material prevents this from happening.

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Figure 4: weld bead in relation to laser power (N. Kumar,2010)

On both welds, microindentation was also tested with the use of a Vickers indenter. The indentation depths are higher outside of the keyhole region, because of a harder phase formation. Both results show a similar behavior, regardless of the used medium. In figure 5, these indentations are imaged and a different shape of indentation can be seen for both different mediums. A more circular deformation can be seen in the case of a weld in a water medium with a slightly bigger size.

Figure 5: closeup of the microindentations performed on a underwater welding process and an air welding process

The results of the measured hardness in both welds are shown in figure 6. Hardness is, as the indentation depths have already shown us, much higher in the keyhole region for the welds in both mediums. A difference can be found in the resulted hardness for different values of laser power. In the air medium, hardness increases for higher laser power, except for 1kW, while in the water medium the opposite occurs. Hardness increases for decreasing laser power except 1.5kW. The explanation of this is pore formation, which has different causes in both mediums. In the water medium, faster solidification of the molten material can cause bubble formation and entrapment causing pore formation. The bigger the laser power, the more bubbles are formed during solidification. In the air medium, insufficient laser beam energy or turbulent flow in the weld pool generated by higher laser power can lead to pore formation, causing the

same decline in hardness, but the opposite reaction to laser power.

Figure 6: hardness in relation to laser power in air medium and an underwater medium (N. Kumar, 2010)

When measuring the coefficient of friction with a scratch test in both cases, no significant differences could be found. The coefficient of friction is lower inside the keyhole region, while is gets bigger towards the outside. The results are shown in figure 7.With these test however, it is shown that direct underwater laser welding is possible. Even better: when water is used as a medium, a significant increase in depth and decrease in width of the weld occurs in comparison to air as a medium. The water, acting as a cooling liquid, also causes faster solidification of the molten material during welding, which induces a higher melt stability. The other test shows that using a water medium with laser beam welding causes no big problem and it has to be noted that during all tests, porosity or scratch formation was not observed.

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Figure 7: friction coefficient of the weld zone of an underwater weld and an air weld (N. Kumar, 2010)

However, welding conditions were precise, for example, the material was placed 1.5mm deep inside of the water medium. Problems might occur when changing these parameters, as will be shown next.

3.2 Underwater laser welding with local dry cavity

More research has been done by the Laser Processing Research Center at Tsinghua University, Beijing, China. (X. Zhang, 2006)The abosorvity of water a to 1.06 µm wavelength light (typical laser wavelength) is 0.014 mm-1. The transmission efficiency T(z), being the amount of power from the laser that reaches the area to be welded, as a function of water depth z can be found by the following formula:

T ( z )=1−e−αz .The result is shown in a graph in figure 8.In these results, direct underwater welding seems possible. Even at a depth of 30mm, around 75% of the laser power is left to perform the weld.

When testing this however, other problems seem to arise. Welds were performed at different heights in a water medium and the results were put in a graph in function of penetration and bead width. The results are shown in figure 9. They show a slight variation in penetration depth for a distance below 2mm. Once the distance becomes larger however, the penetration depth diminishes and nearly no welding is performed.

Figure 8: transmission efficiency of laser power in a water medium (X. Zhang, 2006)

Figure 9: pentration and bead width of the weld in relation to the water depth (X. Zhang, 2006)

Since the absorption of water can't be that high, there are other reasons for this phenomenon. Photos were taken during the welding process, as shown in figure 10, which show a blue plasma being formed when performing a weld in water deeper than 2mm. Below 2mm, the laser energy quickly evaporates the water and the laser beam retains enough energy to form a keyhole into the metal. Once a keyhole is formed, the front of the keyhole zone rapidly evaporates all the

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water so that the laser can directly interact with the metal to be welded. At depths deeper than 2mm however, the water evaporates and then immediately ionizes, which forms a plasma when interacting with the laser beam. This gives a shielding effect reducing the laser intensity at the surface of the metal. These results clearly show that direct underwater laser welding is impossible, since most welds in the nuclear power plants are done from a distance of several millimeters. A local dry cavity must be created at the surface to be welded.

Figure 10: photos of the welding process at various water depths (X. Zhang, 2006)

To form a local dry cavity a water curtain system is used. Argon gas is blown onto the metal surface at high pressure, this removes the water making welding possible. To improve this, a water curtain is formed arround the gas bubble to protect the gas bubble. The water curtain is formed by water that is sent through a nozzle at a certain velocity. The velocity and the angle of the water jets are important for the stability of the water curtain. A schematic of this is shown in figure 11. Combining a gas flow with a water curtain is more efficient than gas shielding since gas bubbles may grow to a bigger size, leading to oscillations and a less stable local dry cavity.

Figure 11: schematic of the local dry zone showing the water curtain and gas flow (X. Zhang, 2006)

Different experiments were done to optimize the parameters for the creation of a local dry cavity. The parameters to be changed are the gas flow rate, the water flow rate from the nozzle of the water curtain, the water flow angle, and the thickness of the water curtain. Three distinct results were observed, as shown in figure 12. The best condition for welding is of course the one with a very stable dry cavity, where

the least water droplets are formed, this is the condition in figure 12-A.

Figure 12: different possible local dry zones (X. Zhang,2006)

In figure 13, the influence of the gas flow rate and the water flow rate are depicted. The different zones, assigned with the letters A, B and C correspond to the different conditions in figure 12. The case where the water flow rate is zero, is the case of pure gas shielding without the use of a water curtain. From a certain water flow rate, a minimum gas flow rate must be obtained to exclude water out of the water curtain and to receive a stable local dry cavity. The minimum water flow rate to form a water curtain in shown by a dashed line. The area A is the area for which the parameters provide an optimal condition. Note that there is also a maximum for the water flow rate. If this rate is too high, the recoil of the water curtain becomes too big and water will bounce back inside the local dry cavity.

Figure 13: stability of the dry zone in relation to gas flow rate and water flow rate (X. Zhang, 2006)

Figure 14 shows the influence of the water flow angle to the condition of the local dry cavity. Different angles were tested and the optimal shielding zone moves to the right with rising angle, but the parameter range also widens. A bigger angle means there is a bigger area to be protected by the water curtain, for which a larger rate of water flow is needed. When the angle becomes smaller, water recoil will become more severe and the parameter range for an optimal condition becomes smaller. The optimal condition to be found in the experiments was found to be at an angle of 60°.

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The last parameter to be tested was the thickness of the water curtain, which was measured at exit of the nozzle. The influence is shown in figure 15. A bigger thickness leads to a bigger minimum water flow, which is normal, since a bigger water flow rate should be obtained to sustain the water flow speed.

Figure 14: stability of the local dry zone in relation to the water curtain angle (X. Zhang, 2006)

Figure 15: stability of the local dry zone in relation to the water curtain thickness (X. Zhang, 2006)

Now that the optimal conditions to form a local dry cavity are determined, the effect of the gas flow rate on the quality of the weld will be defined. Two types of laser welding will be considered. Deep penetration welding or and heat conduction welding. As shown in figure 16, for deep penetration welding, the gas flow rate has little influence on the penetration depth, though it tends to decrease. However, the weld bead width seems to rise with rising gas flow rate. For deep penetration welding, the keyhole depth mainly dominates the penetration depth, while the weld bead is defined by the cooling condition of the work piece surface. The variation in gas flow rate leads to a very little difference in pressure inside the local dry cavity. The bigger pressure and thus bigger absorption of the laser beam intensity, leads to a slight decrease in penetration depth. The bigger gas flow rate also leads to a bigger local dry cavity,

which decreases the influence of water in the cooling of the work piece surface and, as we already stated, to a wider weld bead.

Figure 16: influence of the gas flow rate on penetration depth and bead width (X. Zhang, 2006)

On a side note, for a gas flow rate of 60 l/min, severe undercut was observed and surface oxidation of the weld was seen. This is because the local dry cavity is not stable with these parameters. When water is inside the welding zone, the concentration of oxygen becomes higher and the surface tension of the weld pool changes.For heat conduction welding with a filler wire, the penetration depth and bead with for underwater laser welding is almost the same as that in air. This is shown in figure 17. This is not the case when the gas flow rate is 60 l/min at which the penetration is a lot higher and the bead with is smaller than the weld in air. This is due to the unstable local dry cavity and the presence of water in the welding zone. The discomposed oxygen from the water molecules will oxidize the weld pool, which leads to a bigger penetration and narrow weld bead. The introduction of a filler wire does not seem to have any influence on the weld quality, provided the local dry cavity is stable.

Figure 17: penetration depth and bead width in relation to gas flow rate (X. Zhang, 2006)

These experiments show that underwater laser welding with a dry cavity zone is possible with sound quality and no porosity or surface defects.

3.3 Underwater laser cladding

Stress corrosion cracking is a problem that affects some metals like stainless steel in nuclear power plants because of the corrosive environment. When

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neglected, these cracks can cause failure of important parts of the nuclear installation. A way of solving this is to weld a layer of corrosion resistant metal over these cracks, a process called ‘cladding’. The metal that is added is an alloy containing a high chromium content (up to 30%)Laser welding is an ideal technology to do this: because of the shielding effects of the water and the possibility of doing this in an automated way it is a lot safer for the maintenance teams, protecting them from harming radiation.Underwater laser welding is performed by a Nd:YAG laser in a local dry cavity as explained above. An advantage of this technique is that the welding speed can be up to twice as fast as welding techniques used in the past. (X. Zhang, 2006)Corrosion resistant layers are deposited on the damaged material in such a way that the cracks are covered which makes it impossible to grow bigger.

In most cases the inside of a pipe is the part that is most effected by stress corrosion cracking. But is the weld quality independent of the location in the pipe? To test this, researchers of the Power and Industrial Systems Research and Development Center introduced artificial cracks of 0.3mm on the inside of a pipe. Then they investigated the quality of a weld in the flat, vertical and overhead position as shown if figure 18. All tests were performed with the same welding parameters. The tests showed that there isn’t a difference in quality making it possible to weld in all directions.

Figure 18: weld results of the inside of a tube in various positions (Y. Sano & Obata, 2008)

A second experiment was set up to test if it was possible to perform laser cladding on a fillet weld in a corner between two plates. By simply adjusting the shape of the welding head and thus optimising the gas flow, a good weld was performed.

Previous experiments were all performed on simulated cracks induced by the researchers, but this experiment was performed on an actual crack. This crack was made by putting a stainless steel component in a MgCl2 solution. The tests showed that it isn’t a problem to weld on actual cracks. Pictures of the actual crack and the weld covering it can be seen in figure 19.

Figure 19: detail of a clad layer deposited on an actual crack (Y. Sano & Obata, 2008)

3.4 Underwater laser peening

A second possible application of the laser is to perform a process called Laser Peening. This process introduces residual compression forces in the surface of the metal by exposing them to laser pulses. This is comparable to Shot Peening, where small metal pellets are shot at a work piece to become compression forces in the surface layer.These compression forces prevent the formation of cracks by fatigue, Stress Corrosion Cracking and improve wear resistance.In figure 20 the process of underwater laser peening is shown. A laser beam focuses on the metal object that is submerged in water, absorbing the laser energy. This energy causes the metal to evaporate. Because the process is performed under water, the vapor is contained and heated even more to produce a high pressure plasma by a process called ‘Inverse bremsstrahlung’. When this plasma is heated even more by the laser energy, a shock wave forms which hits the surface of the material. This shock wave has a pressure of several GPa, and travels through the material losing energy this way, but leaving a permanent strain behind. This causes a compressive force in the surface layer of the material.A negative property of this process is that the surface of the material need to be coated because if it isn’t, it can melt or become damaged.

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Figure 20: schematic of the laser peening process (Y. Sano& Obata, 2008)

An experiment was designed to test the effects of underwater laser peening. Sample bars were tested by bending them while rotating, testing fatigue resistance.All test bars are made out of type 304 austenitic stainless steel and prepared in the same way by thermal treating, cold working and bending them. Next, half of them are laser peened under water with a laser intensity of 60 mJ, a pulse density of 70 pulses/mm² and a focus diameter of 1mm.Each sample was scratched with graphite wool. This scratch made the process of Stress Corrosion Cracking faster and more constant for each test object making the results comparable. Next, the bars were put in a bath containing corrosive water for 500 hours.In figure 21 the test results are shown. Two unpeened bars were compared to two underwater laser peened ones. The two unpeened bars are indicated in the graph by circles.In the unpeened test bars Stress Corrosion Cracking occurred due to the high stresses on the surface because of the bending stresses. In the peened bars no cracks appeared because of the compressive force on the surface, so underwater laser peening is a good way of protecting the metal from Stress Corrosion Cracking.

Figure 21: test results of a fatigue test on heat treated and/or laser peened test bars (Y. Sano & Obata, 2008)

Meanwhile, a second test was performed to see what the effects of heat treatment are. Two samples received a Full Heat treatment (shown in the graph by FH), it was put in a vacuum at 1373K for 1 hour. The other two samples were Stress Relieved (shown in the graph by SR) at 1173K for 1 hour.Next, both samples were peened underwater (pulse energy of 200mJ, a focal diameter of 0.8mm and 36 pulses/mm²).The rotating test was performed by rotating the samples at 47Hz or 2820 rpm. The samples were cooled by distilled water because otherwise the thermal treatment could be lost due to the heat generated by the friction forces in the material. The results show that the fatigue strength of materials that receive underwater laser peening have a fatigue strength that is 1.4 to 1.7 times greater than in the case where the samples weren’t peened.

4 INTERVIEW WITH DR. IAN RICHARDSON FROM THE UNIVERSITY OF DELFT

As stated above, underwater laser welding is used most of the time in highly automated environments. The Westinghouse company uses this technique for nuclear power plant reparations which are very task specific. Each reparation is in a different set-up with other environmental parameters involved. This contradiction of highly automated processes and task specific jobs raises some questions regarding the control and safety of the robots which are used to perform these tasks. Dr. Ian Richardson, Professor at the university of Delft states that “If the welding procedure is declared safe and the weld stays inside his defined perimeter than the weld is in fact safe. Nevertheless the persons which are in control of the robot will always stay responsible.” (I. Richardson, personal communication, December 9, 2015). Also stated is that the welding process varies a lot with changing depth where plasma will form a shield which inhibits the welding process in a significant way. Dr. Richardson states that the development of

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the welding techniques underwater are rather a new step than an evolution from welding in an air medium. There are a lot of variables which will change with changing depth. Each of these needs to be examined with trial and error studies to determine the effect of these variables on the welding process at a given depth. Dr Richardson developed a robot for arc-welding at deep depths and has knowledge about the underwater laser welding process. His focus lay on the creation of a robot which is stable for a wide range of pressure which is correlated to the depth. He thinks that if there is a possibility to make the underwater laser welding technique viable and economic, the petroleum industry may use this as standard attachment method rather than mechanical connection methods. For the petroleum industry the robots need to weld at depths deeper than 500m. Theoretically his robot could go to around 10 000m deep but unfortunately this method is still not commercialized enough to be put into practice for daily tasks. The highest challenge is to enlarge the welding quality which is highly affected by the fast cooling due to the water medium.

5 CONCLUSIONThis paper brings together information and research on the recently developed technique underwater laser welding. This technique is originally developed for the use of nuclear power plant reparations which can be done in a faster and safer way with underwater laser welding. Other research expanded the technique to other industries like for example the petroleum industry but are not commercialized today. Here they can use under water laser welding to connect and repair their pipeline systems. First of all the principle of the technique was discussed where the change from air to water as medium has obviously the most

consequences on the process. Laser-water interactions are complex, making it impossible to weld at large distances underwater. To improve performance and process stability, a local dry cavity is created to imitate the normal welding conditions as it is done with air as medium. For welding in an underwater medium a study was done to compare micro-hardness, coefficient of friction, fusion zone and weld pool to be able to correlate these to the micro structural properties of the welded material.When the welding position is moved further away from the laser origin, a plasma shield will be formed which inhibits the welding process. Therefore a local dry cavity must be created. This water curtain is stable at an optimal water flow angle of 60°, combined with a specific water and gas flow rate. Furthermore the technique of laser cladding can be used for reparation of small cracks due to stress corrosive cracking. To prevent the system from failure, a metal corrosive resistant layer containing high concentrations of chromium is put on top of the crack. Laser welding has some benefits for reparation tasks since it can be performed two times as fast as a human being and the process can be fully automated. Another possible application of crack prevention is to perform a process called Laser Peening where residual compression forces are introduced due to exposing the metal to laser pulses. Samples which are underwater laser peened and scratched afterwards showed no cracks and higher fatigue strength than unpeened samples.

6 REFERENCES

N. Kumar, S. K. (2010). Contact mechanical studies on continuous wave CO2 laser beam weld of mild steel with ambient and under water medium. Materials and design , 3610-3617.

T. Hino, M. T. (2008). Development of underwater laser cladding and underwater laser seal welding techniques for reactor components. journal of power and energy systems .

Westinghouse Electric Company. (2014, July). Underwater Laser Beam Welding. Opgeroepen op 11 10, 2015, van Westinghouse Nuclear: http://www.westinghousenuclear.com/Portals/0/operating%20plant%20services/plant%20modifications/modification/NS-IMS-0050%20ULBW.pdf

X. Zhang, E. A. (2006). Effect of shielding conditions of local dry cavity on weld quality in underwater Nd:YAG laser welding. Journal of materials processing technology , 34-41.

Y. Sano, N. M., & Obata, M. Y. (2008). Enhancement of surface properties of metal materials by underwater laser processing. The review of laser engineering .