Veli Kujanpää

Inert gas cutting of thick-section stainless steel and medium-section aluminum using a high power fiber laser

Inert gas assisted laser cutting of 10 mm stainless steel plate and 4 mm aluminum sheet was performed with a 5 kW fiber laser. The effects of laser power, cutting speed, focal point position, and assist gas pressure on the cutting performance and cut quality were investigated. Clean cut surfaces without or with minimal dross were achieved with some combinations of process parameters and attempts were made to define parameter windows in terms of cutting speed and laser power for good quality cutting. The maximum cutting speeds for acceptable cut quality were determined at different power levels. The range at which complete through cutting could be achieved (so-called parameter window) was limited upwards by insufficient power intensity to obtain through cutting at high cutting speeds and downwards by heat conduction at slow cutting speeds. The effects of focal point position and assist gas pressure on the striation pattern (cut surface roughness) were also examined. Low surface roughness was achieved with the focal point position inside the workpiece showing the need for a wider kerf for better melt ejection in thick-section metal cutting. There was also a reduction in surface roughness with increase in assist gas pressure, but there was no significant reduction in surface roughness above the gas pressure of 16 bar, which could be due to the gas flow dynamics inside the narrow cut kerf at high assist-gas pressures.

Characterization of the melt removal rate in laser cutting of thick-section stainless steel

The efficiency of the laser cutting process depends on both the rate of melting and rate of melt removal from the cut kerf. The depth of flow separation and the dross attachment on the lower cut edge relate to the efficiency of the melt removal process and can be used to characterize the rate of melt removal from the cut kerf. The melt flow velocity and melt film thickness are formulated in this study by consideration of the fundamentals of viscous incompressible fluid flow. The calculated melt flow velocity and melt film thickness are correlated with the depth of flow separation on the 10 mm stainless steel AISI 304 (EN 1.4301) laser cut edge. The effects of process parameters—including assist gas pressure, nozzle diameter, nozzle standoff, focal point position, and cutting speed—on the depth of flow separation and the dross attachment on the lower cut edge are investigated. The assist gas pressure, nozzle diameter, and focal point position are found to significantly affect the efficiency of melt removal...

Diode laser beam absorption in laser transformation hardening of low alloy steel

Defining and controlling the absorption of the laser beam is important since all of the heating energy is brought to the material through absorption. Even small variations in the absorption change the laser power needed by hundreds of W. In this study the absorption of a diode laser beam to low alloy steel has been measured by a liquid calorimeter and the surface temperature has been measured with a dual wavelength pyrometer. The varied processing parameters were the power intensity of the beam, the interaction time, and the angle between the surface and the optical axis of the laser beam. Surface temperatures during hardening varied from the Ac1 temperature to the melting point. Tests were done with a 3 kW diode laser with a 12×5 mm hardening optic. The absorptivity of a machined clean steel surface ranged from 46% to 72% depending on the processing parameters. Aluminum oxide blasting of the surface increased the relative amount of energy absorbed to the work piece. The coupling rates for blasted surfaces varied from 66% to 81%. Best absorptivity was achieved by applying graphite coating on the surface. Absorptivity values in excess of 85% were measured.Defining and controlling the absorption of the laser beam is important since all of the heating energy is brought to the material through absorption. Even small variations in the absorption change the laser power needed by hundreds of W. In this study the absorption of a diode laser beam to low alloy steel has been measured by a liquid calorimeter and the surface temperature has been measured with a dual wavelength pyrometer. The varied processing parameters were the power intensity of the beam, the interaction time, and the angle between the surface and the optical axis of the laser beam. Surface temperatures during hardening varied from the Ac1 temperature to the melting point. Tests were done with a 3 kW diode laser with a 12×5 mm hardening optic. The absorptivity of a machined clean steel surface ranged from 46% to 72% depending on the processing parameters. Aluminum oxide blasting of the surface increased the relative amount of energy absorbed to the work piece. The coupling rates for blasted surface...

High power Nd:YAG laser welding in manufacturing of vacuum vessel of fusion reactor

Abstract Laser welding has shown many advantages over traditional welding methods in numerous applications. The advantages are mainly based on very precise and powerful heat source of laser light, which change the phenomena of welding process when compared with traditional welding methods. According to the phenomena of the laser welding, penetration is deeper and thus welding speed is higher. Because of the precise power source and high-welding speed, the heat input to the workpiece is small and distortions are reduced. Also, the shape of laser weld is less critical for distortions than traditional welds. For welding thick sections, the usability of lasers is not so practical than with thin sheets, because with power levels of present Nd:YAG lasers depth of penetration is limited up to about 10 mm by single-pass welding. One way to overcome this limitation is to use multi-pass laser welding, in which narrow gap and filler wire is applied. By this process, thick sections can be welded with smaller heat input and then smaller distortions and the process seems to be very effective comparing “traditional” welding methods, not only according to the narrower gap. Another way to increase penetration and fill the groove is by using the so-called hybrid process, in which laser and GMAW (gas metal arc welding) are combined. In this paper, 20-mm thick austenitic stainless steel was welded using narrow gap configuration with a multi-pass technique. Two welding procedures were used: Nd:YAG laser welding with filler wire and with addition of GMAW, the hybrid process. In the welding experiments, it was noticed that both processes are feasible for welding thicker sections with good quality and with minimal distortions. Thus, these processes should be considered when the evaluation of the welding process is done for joining vacuum vessel sectors of ITER.

Welding of ship structural steel A36 using a Nd:YAG laser and gas-metal arc welding

Laser welding has the potential of offering both technical and economical advantages in many applications in the shipbuilding industry. A limiting factor is currently the power level available with commercial lasers, since steel plates of more than 5 mm in thickness are used in almost every shipbuilding application. In addition, the high hardness of the welds produced using laser welding is a disadvantage compared with the requirements of existing classification society standards. It has been reported that by using a hybrid welding method in which a laser beam and a gas–metal arc weld (GMAW) arc are combined it is possible to weld thicker sections, because the penetration is increased. Hardness values are correspondingly lower than those using the laser process because of the increased energy input. Results of a study of hybrid high power Nd:YAG laser and GMA welding are reported. All plates were welded in a butt joint configuration. When laser and GMAW were combined into a single process, I grooves were ...

The effect of shielding gas composition on welding performance and weld properties in hybrid CO2 laser–gas metal arc welding of carbon manganese steel

Metals industries producing large structures currently have a particular interest in hybrid laser welding processes, which possess advantages compared with conventional methods of welding. One major benefit is a reduction in deformation that enables the amount of postweld finishing work to be reduced. Assembly then also becomes simpler because of the greater accuracy that may be achieved. Larger joint tolerances may be accommodated compared with laser welding alone. By using appropriate filler metal, the weld metal composition may be controlled to meet metallurgical criteria. The hybrid CO2 laser–gas metal arc (GMA) welding process was investigated in this study; the aim being to clarify the effects of process gas composition on welding performance, weld cross section, quality, and mechanical properties, when welding carbon manganese steel. Helium, argon, and carbon dioxide were used in varying proportions as shielding gases for welding I-butt and T-butt joints. The composition of the shielding gas was fo...

Welding of thick austenitic stainless steel using Nd:yttrium-aluminum-garnet laser with filler wire and hybrid process

Autogenous laser welding has shown many advantages over traditional welding methods in numerous applications. However, there could be even more applications, but due to the power levels of present high power lasers, depth of penetration is limited. One way to overcome this limitation is to use multipass laser welding, in which a narrow gap and a filler wire are applied. By this process thick sections can be welded with a smaller heat input and therefore with smaller distortions, and the process seems to be very effective compared to “traditional” welding methods. Another way to increase penetration and fill the groove is by using the so-called hybrid process, in which laser and GMAW are combined. In this study thick section austenitic stainless steel is welded using a multipass technique with filler wire, and also by utilizing a hybrid process. For narrow gap conditions, groove angles of 8°, 10°, and 12°, are used with a partially grooved V joint. Parameters (e.g, filler wire feeding, placement of wire, a...

Review study on remote laser welding with fiber lasers

The appearance of the high-power fiber laser with brilliant beam quality enables a rapid development of remote laser welding (RLW). In this paper, a theoretical study of remote laser welding has been reviewed. As a promising technology, the RLW offers an increased flexibility, high operational speed, and reduced cycle time to process a wide range of workpieces. This study presents the feasibility and typical characteristics of RLW with high-power fiber lasers. Meanwhile, the influence of process parameters, such as laser power, welding speed, shielding gas supply, beam inclination, and focal position, on the weld seam quality has been investigated.

Cutting of Stainless Steel With Fiber and Disk Laser

Laser cutting is a major application of laser materials processing. The cutting is usually performed with CO2-laser due to its good beam quality and its relatively low costs of ownership. Ever since entering the market the high power solid state lasers have been expected to achieve a dominating role also in cutting applications. This has not happened mainly due to the fact that beam quality has not been sufficient. The introduction of new generation of solid state lasers has raised the interest of use of them in cutting application. This study was concentrated on use of fiber and disk lasers, the new laser types with a high beam quality, in cutting of austenitic stainless steel. The performance of these new lasers at power level of 4 kW was compared with CO2-laser in respect of cutting speed, kerf width, kerf edge roughness and perpendicularity (squarness) in order to validate the potential of both of the new lasers against traditional CO2-laser. The results showed that the new lasers offer a great potential in improving the productivity of cutting phase with an acceptable edge quality. This is emphasized in thin sheets of 1.3 and 2.3 mm thickness. In that case the width of the cut kerf is considerably narrow especially when using a fiber laser. In case of thicker sections (4.3 and 6.2 mm) the focal length was increased in order to reach an acceptable cut quality still providing a competitive cutting speed in comparison to a CO2-laser. The fiber laser was the fastest cutting laser in case of each thickness. The results were very promising and it can be stated that these new laser types have a great potential in cutting and will probably gain a considerable market share not only in 3D cutting applications but also in ordinary flat sheet cutting.Laser cutting is a major application of laser materials processing. The cutting is usually performed with CO2-laser due to its good beam quality and its relatively low costs of ownership. Ever since entering the market the high power solid state lasers have been expected to achieve a dominating role also in cutting applications. This has not happened mainly due to the fact that beam quality has not been sufficient. The introduction of new generation of solid state lasers has raised the interest of use of them in cutting application. This study was concentrated on use of fiber and disk lasers, the new laser types with a high beam quality, in cutting of austenitic stainless steel. The performance of these new lasers at power level of 4 kW was compared with CO2-laser in respect of cutting speed, kerf width, kerf edge roughness and perpendicularity (squarness) in order to validate the potential of both of the new lasers against traditional CO2-laser. The results showed that the new lasers offer a great potent...

The effect of parameters on laser transmission welding of polymers

A laser transmission welding technique is commonly used to join polymers. In this technique the laser beam penetrates through the first polymer, which is transparent to the wavelength of the laser used, typically in the near infrared region, and is absorbed by the surface layer of the second polymer. This polymer is heated, and when both parts are clamped together during welding, the heat is transferred to the first polymer and the surface layers of both parts are melted. After cooling and solidification of the molten material the weld is formed.In this study the welding parameters were defined for polymers widely used in consumer electronic devices, such as transparent polycarbonate (PC), which was laser transmission welded to colored acrylonitrile butadiene styrene/polycarbonate alloy (ABS+PC-blend). The welding techniques used were contour and quasi-simultaneous welding. The parameters varied were laser power, welding speed and size of the laser beam on the surface of the absorbing polymer. Also the tensile strengths per unit length of the laser welds made with constant line energy achieved by varying both welding speed and laser power were investigated.A laser transmission welding technique is commonly used to join polymers. In this technique the laser beam penetrates through the first polymer, which is transparent to the wavelength of the laser used, typically in the near infrared region, and is absorbed by the surface layer of the second polymer. This polymer is heated, and when both parts are clamped together during welding, the heat is transferred to the first polymer and the surface layers of both parts are melted. After cooling and solidification of the molten material the weld is formed.In this study the welding parameters were defined for polymers widely used in consumer electronic devices, such as transparent polycarbonate (PC), which was laser transmission welded to colored acrylonitrile butadiene styrene/polycarbonate alloy (ABS+PC-blend). The welding techniques used were contour and quasi-simultaneous welding. The parameters varied were laser power, welding speed and size of the laser beam on the surface of the absorbing polymer. Also the te...