Henrikki Pantsar

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...

The absorption of a diode laser beam in laser surface hardening of a low alloy steel

Laser surface hardening is a process in which a shaped laser beam is scanned across the surface of a hardened component. The beam heats the surface rapidly to the austenite phase, while the rest of the component remains close to room temperature. The heat conducts to the surrounding material and the surface cools down very quickly forming a martensitic layer to the surface.Defining and controlling the absorption of the 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 watts.In this study the absorption has been measured by a liquid calorimeter and the surface temperature has been measured with a dual wavelength pyrometer. The processing parameters used were the intensity of the beam, the interaction time and the angle between the beam and the surface. Surface temperatures during hardening varied from the Ac1 temperature to the melting point. Tests were done with a 3kW diode laser with a 10x5 mm hardening optic.Laser surface hardening is a process in which a shaped laser beam is scanned across the surface of a hardened component. The beam heats the surface rapidly to the austenite phase, while the rest of the component remains close to room temperature. The heat conducts to the surrounding material and the surface cools down very quickly forming a martensitic layer to the surface.Defining and controlling the absorption of the 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 watts.In this study the absorption has been measured by a liquid calorimeter and the surface temperature has been measured with a dual wavelength pyrometer. The processing parameters used were the intensity of the beam, the interaction time and the angle between the beam and the surface. Surface temperatures during hardening varied from the Ac1 temperature to the melting point. Tests were done with a 3kW diode l...

Advances in 3D laser processing in mold technology

Texturing of molds is a normal technique to give plastic products a surface shape which replicates the appearance of leather, wood or similar. Often the decorative features have been done by EDM processing or photochemical etching; both processes which present problems in production times and flexibility. EDM processing requires a numerous set of machined erosion electrodes. Changes to the geometries require production of new electrodes and the process is not flexible. The down side to photochemical etching is the use of chemicals, it is not repeatable and usually it’s not done in-house therefore the delivery cycle is usually too long.Laser engraving has found many applications in production of injection molds. To replace e.g. photochemical etching in decorative texturing applications, the laser as a tool needs to be able to process true 3D surfaces accurately and with high quality. In order to meet these requirements, developments in software and CAM-interfaces are needed. In this paper a method for creating decorative textures on 3D surfaces, starting from image mapping and texture synthesis, to real processed samples, is presented. It has been shown that new innovative techniques enable 3D-processing which by quality and flexibility challenge the conventional processes.Texturing of molds is a normal technique to give plastic products a surface shape which replicates the appearance of leather, wood or similar. Often the decorative features have been done by EDM processing or photochemical etching; both processes which present problems in production times and flexibility. EDM processing requires a numerous set of machined erosion electrodes. Changes to the geometries require production of new electrodes and the process is not flexible. The down side to photochemical etching is the use of chemicals, it is not repeatable and usually it’s not done in-house therefore the delivery cycle is usually too long.Laser engraving has found many applications in production of injection molds. To replace e.g. photochemical etching in decorative texturing applications, the laser as a tool needs to be able to process true 3D surfaces accurately and with high quality. In order to meet these requirements, developments in software and CAM-interfaces are needed. In this paper a method for crea...

Quality and costs analysis of laser welded all steel sandwich panels

Laser welding is a fast and flexible way to manufacture all steel sandwich panels. Economically the most feasible solution is often to weld as large panels as possible. In corrugated core all steel sandwich panel applications the effect of an air gap in welding is difficult to eliminate when welding large panels. A system for welding large panels with a laser is often considered to be complicated and expensive. The processibility of large panels with specified core types was estimated by welding large all steel sandwich panels with two different core types. Materials used were low-carbon steel DC01 and stainless steel ASTM 304 with sheet thickness from 0.5 to 1.5 mm. The panels were welded with a 6 kW CO2 laser. The manufacturing of large steel panels was possible with acceptable weld quality (EN-ISO 13919-1). The total flatness of the panels was better than 11.3 mm, depending on the manufacturing procedure, core type, and material. Total manufacturing costs of high power CO2, Nd:yttrium–aluminum–garnet a...

Advanced beam steering in helical drilling

Helical laser drilling is a method for producing high quality holes with defined geometries in different materials among industries such as aerospace, medical device manufacturing and electronics. If the aspect ratio of the hole is small, drilling can be done using a fast scanner. However, a special drill head is needed for higher aspect ratio holes and improved precision. The drill head typically comprises wedges or a Dove prism to rotate the laser beam at high velocities. Using a pulsed laser, each pulse removes a portion of the material. Thermal effects and the thickness of the recast layer are significantly smaller than associated with single pulse or percussion drilling.This paper presents a new helical drill head design. The developed optical device can be used for precise drilling, as well as for creating shaped entrance holes and non-circular patterns in a one-step process. The apparatus is mounted to a galvanometric scanner and the beam rotation is arranged using a rotating Dove prism. Steering the beam with the galvanometric scanner enables automated coincident beam movement with the rotation, enabling the machining of special geometries such as spirals, ellipses and rhodonea curves. Such geometries can be used either for improving material removal or shaping drilled holes or hole entrances. Operating principle and sample geometries are presented, and future applications are discussed.Helical laser drilling is a method for producing high quality holes with defined geometries in different materials among industries such as aerospace, medical device manufacturing and electronics. If the aspect ratio of the hole is small, drilling can be done using a fast scanner. However, a special drill head is needed for higher aspect ratio holes and improved precision. The drill head typically comprises wedges or a Dove prism to rotate the laser beam at high velocities. Using a pulsed laser, each pulse removes a portion of the material. Thermal effects and the thickness of the recast layer are significantly smaller than associated with single pulse or percussion drilling.This paper presents a new helical drill head design. The developed optical device can be used for precise drilling, as well as for creating shaped entrance holes and non-circular patterns in a one-step process. The apparatus is mounted to a galvanometric scanner and the beam rotation is arranged using a rotating Dove prism. Steering t...

Effect of processing parameters on the microstructure and hardness of laser transformation hardened tool steel

Laser transformation hardening of tool steel containing 0.64 wt% carbon and 4.48 wt% chromium was studied. The delivery condition of the steel was annealed, with a microstructure of M7C3 carbides in a matrix of ferrite. Hardening was performed with a direct diode laser. Surface temperature during hardening was measured with a dual-wavelength pyrometer and the traverse speed was varied between 1.1 and 33.3 m s−1. A set of samples was quenched in liquid nitrogen to reveal the effect of retained austenite on the resulting hardness. Ac1 isotherm depths were measured from all samples. Volume fraction of retained austenite and carbide dissolution were estimated from thermodynamic considerations.The surface hardness was greatly influenced by the dissolution of carbides during austenitization. The dissolution of chromium carbides requires a high temperature and is limited by chromium diffusion. Highest hardness values were achieved with slow traverse speeds and high surface temperatures for both normal and sub-zero hardened samples. Highest measured hardness in samples, in which the surface was cooled by self-quenching, was 822 HV and in samples quenched in liquid nitrogen 926 HV. A good agreement was found between the carbide dissolution and characteristic diffusion distance of chromium.Laser transformation hardening of tool steel containing 0.64 wt% carbon and 4.48 wt% chromium was studied. The delivery condition of the steel was annealed, with a microstructure of M7C3 carbides in a matrix of ferrite. Hardening was performed with a direct diode laser. Surface temperature during hardening was measured with a dual-wavelength pyrometer and the traverse speed was varied between 1.1 and 33.3 m s−1. A set of samples was quenched in liquid nitrogen to reveal the effect of retained austenite on the resulting hardness. Ac1 isotherm depths were measured from all samples. Volume fraction of retained austenite and carbide dissolution were estimated from thermodynamic considerations.The surface hardness was greatly influenced by the dissolution of carbides during austenitization. The dissolution of chromium carbides requires a high temperature and is limited by chromium diffusion. Highest hardness values were achieved with slow traverse speeds and high surface temperatures for both normal and sub-ze...

Comparison of diode laser transformation hardening of heat treatable steels and martensitic stainless steels

Laser transformation hardening is applicable for a number of different steel types ranging from structural steels to high alloy tool steels and martensitic stainless steels. The physical properties of these steels vary considerably and therefore it is important to recognize how the variation of steel properties affects the hardening process.In this study, two very different steel types, AISI 420 L martensitic stainless steel and 42CrMo4 heat treatable steel, were examined. Samples were hardened with a 3 kW direct diode laser with a 10x5 mm spot size. Surface temperatures during hardening were measured with a dual-wavelength off-axis pyrometer and the absorptivities with a liquid calorimeter. The reflectivity of the laser beam was found to have the greatest effect on the optimization of processing parameters. In these experiments the absorptivity of the laser beam varied from 35 to 70 percent depending on surface oxidation, material and surface temperature. Case depths measured from cross sections of the samples varied from thin surface layers up to ca. 1 mm.Laser transformation hardening is applicable for a number of different steel types ranging from structural steels to high alloy tool steels and martensitic stainless steels. The physical properties of these steels vary considerably and therefore it is important to recognize how the variation of steel properties affects the hardening process.In this study, two very different steel types, AISI 420 L martensitic stainless steel and 42CrMo4 heat treatable steel, were examined. Samples were hardened with a 3 kW direct diode laser with a 10x5 mm spot size. Surface temperatures during hardening were measured with a dual-wavelength off-axis pyrometer and the absorptivities with a liquid calorimeter. The reflectivity of the laser beam was found to have the greatest effect on the optimization of processing parameters. In these experiments the absorptivity of the laser beam varied from 35 to 70 percent depending on surface oxidation, material and surface temperature. Case depths measured from cross sections of the s...

Manufacturing procedure and costs analysis of laser welded all steel sandwich panels

Laser welding is a fast and flexible way to manufacture all steel sandwich panels. Economically the most feasible solution is often to weld as large panels as possible. In corrugated core all steel sandwich panel applications the effect of an air gap in welding is difficult to eliminate when welding large panels.The manufacturability of large panels with specified core types was estimated by performing welding experiments of small joint details and manufacturing of large panels with equivalent joint configurations. Materials used were mild steel and stainless steels with thicknesses from 0.5 to 1.5 mm. The panels were welded with a 6 kW CO2–laser. It was found that acceptable air gap and misalignment depend on the joint configuration. The manufacturing of large panels is possible with very low defect levels. The total flatness of the panels was better than 11.3 mm depending on the manufacturing procedure.Total manufacturing costs of high power lasers was compared in welding of large weight optimized panel...

Process optimization for improving drilling efficiency in EWT solar cell manufacturing

Pulse parameter optimization is a key element in reaching higher drilling rates in the production of Emitter Wrap Through (EWT) solar cells. Manufacturing of these cells relies on fast laser drilling of silicon wafers. In order to create a cost case for EWT manufacturing, not only the drilling rate, but also the investment costs and cost of ownership have to be taken into account. In this aspect using fewer lower power lasers with a capability to maximize energy efficiency in drilling is an important factor.Master Oscillator Power Amplifier fiber laser concept allows for independent adjustment of pulse width, energy and temporal shape. This capability can be exploited for improving the drilling process efficiency. Combined with a FPGA controller, one can drill holes at unprecedented speeds. It was shown that a single hole can be drilled through a 210 µm silicon wafer using less than 3 mJ of energy; consequently 6,250 holes per second could be drilled using 18.2 W laser power. The results were compared to q-switched lasers by mimicking the shape of a high peak power q-switched pulse.Pulse parameter optimization is a key element in reaching higher drilling rates in the production of Emitter Wrap Through (EWT) solar cells. Manufacturing of these cells relies on fast laser drilling of silicon wafers. In order to create a cost case for EWT manufacturing, not only the drilling rate, but also the investment costs and cost of ownership have to be taken into account. In this aspect using fewer lower power lasers with a capability to maximize energy efficiency in drilling is an important factor.Master Oscillator Power Amplifier fiber laser concept allows for independent adjustment of pulse width, energy and temporal shape. This capability can be exploited for improving the drilling process efficiency. Combined with a FPGA controller, one can drill holes at unprecedented speeds. It was shown that a single hole can be drilled through a 210 µm silicon wafer using less than 3 mJ of energy; consequently 6,250 holes per second could be drilled using 18.2 W laser power. The results were compared to ...

Examination of laser hardened steel surfaces using interference microscopy

A diode laser beam in the near-infrared region is absorbed more efficiently by a metal surface than a CO2 or a Nd:yttrium-aluminum-garnet laser beam. Diode laser transformation hardening without an absorptive coating is feasible even in an inert gas atmosphere. Absence of the absorptive coating, and the fact that the formation of an oxide layer is inhibited, makes it possible to study phase transformations occurring during hardening from the surface relief formed to the processed surface. In the present study diamond polished plain carbon and low alloy steel samples have been hardened in an inert gas atmosphere of argon using a high power diode laser. Surface reliefs caused by phase transformations during quenching have been examined using differential interference contrast (DIC) microscopy. Hardness of each sample was measured and the effect of the processing parameters on the surface hardness and microstructure has been established. DIC imaging was found to be a rapid and illustrative tool for examining...