Laser+plasma: search for new possibilities in surfacing

Article: Authors - Som А.I., Krivtsun I.V.
The Paton Welding Journal - 2000. - №12. - 36-41.

More and more publications appear in a scientific and technical literature about application of combined processes of welding based on a combined use of a laser radiation and electrical arc, including plasma arc [1-7]. Such combination leads to the improvement of a space stabilization of an arc spot at the surface of metal being welded and to the increase in stability of its burning at low currents and high rates of displacement relative to the work piece. Simultaneously, the coefficient of absorption of a laser radiation by the metal surface is increased that is especially important when the low-power lasers are used. All this provides, from the one hand, the significant increase in productivity and stability of arc (plasma) welding, and from the other hand, the increase in effectiveness and decrease in the cost of the laser welding.

Our work was aimed at the conductance of theoretical and experimental investigations of effect of a laser beam on the process of a plasma-powder cladding (PPC) and revealing of new technological capabilities of a combined method, i.e. laser-plasma powder cladding (LPPC). The most rational scheme of realization of this combined process is, in our opinion, a coaxial combination of a laser beam and plasma arc during a distributed feeding of a filler powder into a discharge plasma. As in this case the laser beam should pass a definite distance in the arc plasma, then, during development of the above-described method it is necessary to take into account not only the plasma and laser effect on the filler material and the surface treated, but also the direct interaction of the focused laser beam with a compressed arc plasma.

It is known that such interaction can lead to the occurrence of a special type of a gas discharge, i.e. a combined laser-arc discharge [5]. The necessary condition of its occurrence is the commensurability of energy, put into arc plasma by a laser radiation, with energy, generated in the plasma at the expense of the electric current passing. When this condition is fulfilled, a significant change in energy balance of the arc discharge takes place, thus resulting in the fact that both integral and local characteristics of the plasma of the combined discharge are differed greatly from the corresponding characteristics of the initial arc plasma. As to the characteristics of the laser beam, then they undergo the significant changes due to absorption of beam and its additional focusing in the plasma of the laser-beam discharge.

Unfortunately, it is almost impossible to use the traditional arc plasma torches for the creation of a combined discharge and its use for LPPC according to the above-described scheme. The coaxial combination of the laser beam with a plasma arc requires the creation of special devices, i.e. laser-arc plasma torches [8-10]. These devices are distinguished mainly by a design of the cathode unit (refractory tubular cathode or a system of pin cathodes located around the circumference). This peculiar feature makes it possible to introduce a focused laser beam to arc plasma along the axis of the plasma-shaping channel.

For a practical realization of the LPPC process and revealing the technological capabilities of a combined use of the plasma arc and laser radiation, a prototype of a specialized laser-arc plasma torch LPP-22 was designed and manufactured (Figure 1). During its design a large experience of Plasma-Master Co., Ltd. on the creation of plasma torches for PPC [11], and also results of theoretical studies of a combined laser-arc discharge and devices for its realization, made at the E.O. Paton Electric Welding Institute, were used [5, 9].

In this plasma torch (Figure 1, b) the DC arc is burning in an axial flow of the plasma gas (argon) between the refractory (tungsten) tubular cathode / and the workpiece being subjected to cladding (anode) 2. At initial region a discharge is stabilized by a wall of the plasma-shaping nozzle 3, which is coaxial with a cathode. The nozzle 4 serves for a distributed feeding of the filler powder to the discharge. Both nozzles are made from copper and cooled by water 5. The beam 6 of a continuous CO2-laser radiation, focused by an optical system and spreading along the plasma torch axis, is introduced to the discharge through the cathode orifice. The plasma gas 7, 8 is supplied to a plasma-shaping channel both through the cathode orifice and to a gap between the cathode and a wall of the nozzle 3. The gas consumption here may vary independently of one another. The filler powder is fed by a flow of the transporting gas 9 (argon) to the gap between nozzles 3 and 4. The cladding zone is protected by an argon flow. To protect the laser focusing system in plasma torch LPP-22 an additional gas gate is provided, and also appropriate correctors of the cathode positioning relative to the beam axis are provided to provide the coaxiality of a laser beam and a plasma arc.

The plasma torch was designed for operation at the arc current of 100 A < I < 300 A using a laser beam having a mode TEM 2.0, power Qo<= 5 kW and angle of beam focusing 0.053. To define the optimum conditions of the LPP-22 plasma torch operation, characteristics of the generating plasma and an interacting laser beam, a detailed computer modelling of a combined discharge generated by the mentioned plasma torch without taking into account the feeding of the filler powder was made. Here, the following sizes of the nozzle 3 were used (Figure 1, b): 4.5 mm length of a cylindrical channel (Ln1 = 5.5 mm), radius Rn1 = 5.0 mm. Respectively, for nozzle 4, the length of the outlet channel is 2.5 mm (Ln2 = 10.0.mm), radius Rn2 = 5.0 mm. Consumption of the plasma gas, supplied through the cathode orifice is G1 = 0.5 L/min, while between the cathode and the channel wall it is G2 = 2.5 L/min. Consumption of transporting gas was 5.0 L/min, and its initial temperature was taken equal to the temperature of water-cooled walls of the channels (300 K). Distance f from the cathode edge till the focus of the initial beam was varied within the range of 14 - 22 mm, and the length d of the open region of the discharge was 4-12 mm.

Calculations of characteristics of plasma and a laser beam, interacting with it, were carried out on the basis of a model of the combined discharge in a laser-arc plasma torch, described in detail in [9]. Results of numerical modelling of the discharge for different conditions of the plasma torch operation are presented partially in Figures 2-4. Thus, Figure 2 presents space distributions of plasma temperature T, while Figure 3 shows a distribution of gas-dynamic pressure of its flow pu*u/2 (p - density, kg/m3; u - rate, m/s). According to curves given in Figure 2, the absorption of the laser radiation by an arc plasma leads to a significant increase in temperature of its central regions. Moreover, the maximum achieved values T are increased with a growth in beam power Qo. The mentioned growth in the plasma temperature promotes the increase in its electroconductivity and, consequently, to the increase in current density in a near-axial zone of the discharge. Thus, the discharge, generated by a laser-arc plasma torch, is characterized by an increased concentration of heat and electrical energy in that region of the plasma, which is subjected to the action of the laser beam and rigidly connected with its axis and also by a high space stability of this zone.

The described change in heat condition of the plasma arc burning under the action of the laser radiation causes a significant redistribution of gas-dynamic characteristics of the plasma flow with increase in Qo. One of the main causes is the decrease in a viscosity of the argon plasma with the temperature increase. Another cause is the above-mentioned redistribution of current density in the discharge, thus increasing the role of electromagnetic forces in accelerating of the plasma flow [5].

As a result, the axial component of the plasma rate at the discharge axis is increased noticeably. In spite of increase in rate, the decrease in plasma density at the temperature increase leads to the fact that the gas-dynamic pressure of the plasma flow pu*u/2 in a near-axial zone of the combined discharge is somewhat decreased (Figure 3). It should be noted that this decrease, which causes the dynamic action on the surface of the molten metal, is important for the cladding process using a laser-arc plasma torch [12].

The interaction of a laser radiation with an arc plasma, proceeding in the above-described plasma torch leads to the redistribution of not only its characteristics but also the laser beam itself (due to absorption and refraction in the discharge plasma). Thus, for example, the absorption of the laser radiation leads to the fact that at the distance of 20 mm from the cathode edge the beam power for the mentioned conditions is only about 30 % of its initial power Qo, whereas the intensity of radiation, Po, at its axis increases by more than twice (Figure 4).

Thus, the interaction of the laser beam with plasma of a combined discharge causes its additional focusing, which is increased with a growth of I and Qo. Consequently, by varying these two parameters, it is possible to control effectively the beam focusing in plasma of the combined discharge created with the help of a laser-arc plasma torch that is important when similar devices for welding and cutting are used.

Experimental investigations of the plasma torch LPP-22 were performed in Fraunhofer Institute of Technology (Germany) using CO2-laser RS-5000 and plasma arc power source Messer Griesheim Uniting GW 30. The plasma torch was fastened to a focusing system by a special adapter which makes it possible to coincide the axis of the laser beam with the plasma torch axis. The movement of the workpiece to be treated with respect to the plasma torch was realized by a programming welding manipulator.

Experiments were performed in two stages. First, the peculiarities of burning the combined discharge without feeding the filler powder were studied, then the laser-plasma cladding of steel samples at different conditions was performed. During all the experiments the arc current was varied in the range of 100 - 280 A, the laser beam power - 0 - 4 kW. The distance f was changed within 14 - 22 mm, and d (from the plasma torch edge to the anode-workpiece surface) was set equal to 4, 8 and 12 mm. Consumptions of transporting and plasma gas were constant and corresponded to the above-mentioned values.

The experiments showed that the plasma torch LPP-22 operated with a high space-time stability of parameters of the generating plasma within the entire examining range of arc current and laser beam power. Volt-ampere characteristics of discharge using a copper water-cooled anode at different values Qo and I were measured (Figure 5). The experimental data confirm the theoretical results [5]: under the action of the laser beam the arc voltage is decreased, and, as follows from Figure 5, a (solid curves), its main drop occurs at the laser power Qo < 2.5 kW. In the same Figure the calculated dependence (dashed curve) of a full voltage of discharge on the beam power demonstrates quite satisfactory its coincidence with experimental values. As to the plasma arc itself, then under the action of the laser radiation it is somewhat compressed that can be observed visually or from the rising volt-ampere characteristics of the discharge (Figure 5, b).

Investigation of technological capabilities of the plasma torch LPP-22 was performed on flat samples made from 20 mm thick low-carbon steel using LPPC. Powdered alloys of Hastelloy C and Stellite 6 grades of 20 - 63 mkm fractions were used as filler materials. These investigations showed that the coaxial combination of the plasma arc with a laser beam in the laser-arc plasma torch gives an opportunity to increase the speed of cladding of single beads by 2 - 3 times as compared with conventional PPC due to improving the space stability of the arc burning. This is similar to the increase in productivity of cladding works for such parts as screw conveyors of extruding machines, flat and disc knives, milling cutters, etc. Simultaneously, the melting y of the parent metal during cladding of separate beads was at the level of 5 - 10 %. This small value became possible due to the above-mentioned dynamic action of the plasma flow to the melt surface.

In addition, the above-mentioned decrease in arc voltage in the combined process reduces the risk of a double arcing that is especially important in operation at high currents (more than 300 A). This is a good premise for increasing the cladding efficiency by increasing the arc current. Unfortunately, it was not possible to realize this opportunity in our series of experiments, as the available power source could not operate at currents of more than 300 A.

Figure 6 presents two examples of a wide-layer LPPC, made at Qo = 2 kW, 18 L/min, full gas (argon) consumption and 2.7 kg/h powder (Hastelloy C) consumption. In one case the cladding was performed with transverse oscillations of the workpiece relative to the plasma torch (Figure 6, b), and in the second case - with longitudinal overlap beads (Figure 6, c). In both cases a good formation of the deposited layer was provided at minimum melting of the parent metal (y < 5 %).

This method of surfacing opens up the feasibility of cladding of large surfaces of the products using wear- and corrosion-resistant alloys providing a high efficiency and quality, for example, in production of bimetals. For comparison, such efficiency of cladding (2.7 kg/h) can be attained in case of using only a laser radiation at the beam power of not less than 10 kW.

In the work, the effect of LPPC speed (Figure 7) and laser beam power (Figure 8) on the formation of single beads and also on the melting y of the parent metal was also investigated. In experiments, with increase of the cladding speed the section of the beads tried to be kept constant by the appropriate increase in the powder consumption.

As follows from the appearance of beads and macrosections of their transverse section, a good formation of beads is provided in the wide range of cladding speeds (from 10 to 50 m/h). In addition, the nature and level of melting of the parent metal depends both on a total power of the laser beam and plasma arc and also on their ratio. With increase of the laser beam power a peak of melting of the parent metal is shifted from the edges of the bead to its axis (Figure 8). The structure of metal deposited with LPPC method (Figure 9) is similar to that produced by using the conventional PPC.

Thus, the combined application of the laser beam and plasma arc opens up new possibilities in cladding, consisting of increase in speed, stability and reproducibility of the process results, reducing in its cost, as well as in improving the use of the laser energy. However, the authors understand that the results given in this article are preliminary.

For a practical realization of the combined process the further investigations are required both from the point of view of optimizing the process parameters and also the selection of rational fields of its application.