Franz-Josef Kahlen

Tensile Strengths for Laser-Fabricated Parts and Similarity Parameters for Rapid Manufacturing

This paper presents a set of process parameter selection rules to deposit a good metal part. A CO 2 laser is used to melt metal powder to achieve layer by layer deposition for fabricating three-dimensional parts. Dimensionless numbers characterizing this powder deposition process are identified using Buckinghams Π-Theorem. These dimensionless numbers are used to identify a range of values for the process parameters, such as the laser power, spot diameter, speed of the xyz stage and powder flow rate, to achieve good quality layers for different materials. The yield and ultimate strengths are examined for parts fabricated with stainless steel 304 (SS 304) powder under three different processing conditions. These stresses are correlated to the operating conditions and physical dimensions of the deposit through the dimensionless similarity parameters. Experimental data indicate that the yield strength of the part is close to the value of standard sample (250 MPa, same as wrought stainless steel SS304). It is also observed that the direction of maximum yield strength is oriented very close to the dominant direction of material solidification. The ultimate strength is found to be considerably less than the ultimate strength of wrought SS 304 (540 MPa) which may be due to the residual stresses generated in the part.

Effects of intrapulse structure on hole geometry in laser drilling

Lasers have been established as a commercial tool for drilling holes in a variety of materials. The beam quality and temporal shape of the laser pulse have been identified as critical parameters to create high quality (deeper holes, minimum taper angle, and minimum recast layer) holes. However, little attention has been directed toward an analytic investigation of the effect of different temporal pulse shapes on these product variables. In this study, a leading-edge, sharp-spiked, and a rectangular Nd:YAG laser pulse were used to drill holes in stainless steel 304 to examine the effects of these temporally different laser pulses on the drilling process. It is found that holes drilled with the sharp-spiked laser pulse are significantly deeper than for the case of rectangular pulses. Also an optimum processing regime is identified to drill holes with minimum taper angle and recast layer. A mathematical model describing the drilling process is presented in this article for different temporal profiles of the ...

Hardness, chemical, and microstructural studies for laser-fabricated metal parts of graded materials

Laser deposition of metal layers has been recognized, in recent years, as a one-step process to fabricate metal parts instead of the two-step process of producing a mold and then using the mold to cast a metal part. The authors have employed this one-step technique to create graded materials by varying the part’s composition from 100% stainless steel to a 100% nickel-based superalloy. Mechanical properties of these graded materials are measured and the effects of slow solidification rates are investigated. A multimode CO2 laser is operated at 270 W to produce wall-like structures of graded materials. The CO2 laser beam is focused to a spot size of 600 μm using a 127 mm focal length lens.Laser deposition of metal layers has been recognized, in recent years, as a one-step process to fabricate metal parts instead of the two-step process of producing a mold and then using the mold to cast a metal part. The authors have employed this one-step technique to create graded materials by varying the part’s composition from 100% stainless steel to a 100% nickel-based superalloy. Mechanical properties of these graded materials are measured and the effects of slow solidification rates are investigated. A multimode CO2 laser is operated at 270 W to produce wall-like structures of graded materials. The CO2 laser beam is focused to a spot size of 600 μm using a 127 mm focal length lens.

Effects of intra-pulse structure on hole geometry in laser drilling

Lasers have been established as a commercial tool for drilling holes in a variety of materials. The beam quality and temporal shape of the laser pulse have been identified as critical parameters to create high quality (deeper holes, minimum taper angle and minimum recast layer) holes. However, little attention has been directed towards an analytic investigation of the effect of different temporal pulse shapes on these product variables. In this study, a leading-edge sharp-spiked and a rectangular Nd:YAG laser pulse were used to drill holes in stainless steel 304 to examine the effects of these temporally different laser pulses on the drilling process. It is found that holes drilled with the sharp-spiked laser pulse are significantly deeper than for the case of rectangular pulses. Also an optimum processing regime is identified to drill holes with minimum taper angle and recast layer. A mathematical model describing the drilling process is presented in this paper for different temporal profiles of the pulsed laser beams. The model assumes one-dimensional heat conduction in the material and it incorporates the Stefan conditions at the solid-liquid and liquid-vapor interfaces. The predicted drill depth and recast layer thickness are larger than the experimental data but follow the same trend.Lasers have been established as a commercial tool for drilling holes in a variety of materials. The beam quality and temporal shape of the laser pulse have been identified as critical parameters to create high quality (deeper holes, minimum taper angle and minimum recast layer) holes. However, little attention has been directed towards an analytic investigation of the effect of different temporal pulse shapes on these product variables. In this study, a leading-edge sharp-spiked and a rectangular Nd:YAG laser pulse were used to drill holes in stainless steel 304 to examine the effects of these temporally different laser pulses on the drilling process. It is found that holes drilled with the sharp-spiked laser pulse are significantly deeper than for the case of rectangular pulses. Also an optimum processing regime is identified to drill holes with minimum taper angle and recast layer. A mathematical model describing the drilling process is presented in this paper for different temporal profiles of the puls...

Plasma Instability and Optimum Utilization of Laser Energy in Laser Materials Processing

A plume consisting of vapor and ionized particles of the workpiece is usually formed during various types of laser materials processing. The process parameters such as the laser power, spot diameter, scanning speed, material properties and shielding gas affect the properties of this plume. A one- dimensional model is presented to understand the effects of the vapor-plasma plume in continuous wave (cw) laser materials processing. A model for pulsed laser materials processing is also discussed. These models are used to analyze the transmission of the laser beam through the plume and the deposition of energy on the melt pool at the substrate surface. An experimental technique described as the pinhole experiment is devised for pulsed laser operations to measure the partitioning of laser energy between the plume and workpiece and to identify the process parameter regime for efficient energy transfer from the laser beam to the workpiece. The attenuation coefficient of the vapor-plasma plume was measured during cw CO2 laser-assisted metal deposition conditions by directing a CO2 probe laser beam horizontally through the plume and determining the ratio of irradiance of the beam after and before the plume. Assuming an isotropic attenuation coefficient through the plume, the energy partitioning between the plume and workpiece was determined.

Geometry and Heat Transfer Considerations in Rapid Manufacturing

The authors review a laser-assisted rapid manufacturing process which allows to manufacture components from the original powderized material, overcoming functionality test limitations of conventional rapid prototyping processes. In a comprehensive experimental and analytical investigation, a set of rules which predicts the heat transfer and the geometrical properties of the fabricated parts was derived and verified. Further, the resulting set of equations was connected to a second model which predicts the mechanical properties of the fabricated parts. The results agree very well with measured mechanical properties, showing that, depending on the operating parameters laser power, scanning speed and powder delivery rate, the mechanical strength is anisotropic.

Laser glass machining for crack initiation and edge seaming in flat panel display manufacturing

A laser-based edge seaming process to improve the edge quality of laser-cut glass panels for the display industry is introduced. Laser cutting of glass typically results in 90° comers at the edges. These sharp comers pose a major problem during assembly and shipment because (i) they can scratch or damage electrical leads in the frame, and (ii) micro-cracks can grow due to vibrations during shipment. These micro-cracks can develop into small chips of glass which can also damage the electrical leads and the frame itself. A laser beam is focused on the edge of the glass panel and material is removed due to melting and ablation. Experimental results show that a rounded edge is produced with a radius of curvature in the order of 50 μm. An optical microscope is used to examine the processed edges. No molten material or bubble entrapment was found in the machined area.A laser-based edge seaming process to improve the edge quality of laser-cut glass panels for the display industry is introduced. Laser cutting of glass typically results in 90° comers at the edges. These sharp comers pose a major problem during assembly and shipment because (i) they can scratch or damage electrical leads in the frame, and (ii) micro-cracks can grow due to vibrations during shipment. These micro-cracks can develop into small chips of glass which can also damage the electrical leads and the frame itself. A laser beam is focused on the edge of the glass panel and material is removed due to melting and ablation. Experimental results show that a rounded edge is produced with a radius of curvature in the order of 50 μm. An optical microscope is used to examine the processed edges. No molten material or bubble entrapment was found in the machined area.

Residual stresses in laser-deposited metal parts

Several laser-based techniques to fabricate parts by depositing metals or ceramic powders or a combination thereof have been developed in recent years. These fabrication techniques are incomplete and not fully useful to an operator without any predictive capability to calculate the geometries of the fabricated parts or equations to calculate their expected yield and ultimate strengths. Data concerning the energy transfer from the processing laser beam to the material powder, such as the metal vapor-plasma plume temperature and plume absorption coefficient, the efficiency of laser energy transfer and mathematical analysis for the thermal and dimensional process characteristics are unavailable. Also the characterization of the mechanical properties of such laser-fabricated parts has just begun. A one-dimensional model to calculate the thermal and dimensional process characteristics is developed. The model accounts for the transmission of the laser beam through the plume, energy transfer in the molten phase and the Stefan conditions at the solid-liquid and liquid-vapor interfaces. The yield and ultimate strengths of laser-fabricated stainless steel (SS 304) parts have been measured. A mathematical model is developed accounting for directionally preferred solidification to calculate the residual stresses generated in the part during solidification.Several laser-based techniques to fabricate parts by depositing metals or ceramic powders or a combination thereof have been developed in recent years. These fabrication techniques are incomplete and not fully useful to an operator without any predictive capability to calculate the geometries of the fabricated parts or equations to calculate their expected yield and ultimate strengths. Data concerning the energy transfer from the processing laser beam to the material powder, such as the metal vapor-plasma plume temperature and plume absorption coefficient, the efficiency of laser energy transfer and mathematical analysis for the thermal and dimensional process characteristics are unavailable. Also the characterization of the mechanical properties of such laser-fabricated parts has just begun. A one-dimensional model to calculate the thermal and dimensional process characteristics is developed. The model accounts for the transmission of the laser beam through the plume, energy transfer in the molten phase ...

Hardness, chemical and microstructural studies for laser-fabricated metal parts of graded materials

Laser deposition of metal layers has been recognized in recent years as a one-step process to fabricate metal parts instead of the two-step process of producing a mold and then using the mold to cast a metal part. The authors have employed this one-step technique to create graded materials by varying the part’s composition from 100 % stainless steel to a 100 % nickel-based superalloy. Mechanical properties of these graded materials are measured and the effects of slow solidification rates are investigated. A multimode CO2 laser is operated at 270 W to produce wall-like structures of graded materials. The CO2 laser beam is focused to a spot size of 600 μm using a 127 mm focal length lens.Laser deposition of metal layers has been recognized in recent years as a one-step process to fabricate metal parts instead of the two-step process of producing a mold and then using the mold to cast a metal part. The authors have employed this one-step technique to create graded materials by varying the part’s composition from 100 % stainless steel to a 100 % nickel-based superalloy. Mechanical properties of these graded materials are measured and the effects of slow solidification rates are investigated. A multimode CO2 laser is operated at 270 W to produce wall-like structures of graded materials. The CO2 laser beam is focused to a spot size of 600 μm using a 127 mm focal length lens.

One-step microlithography

Subject of this investigation is a one-step rapid machining process to create miniaturized 3D parts, using the original sample material. An experimental setup where metal powder is fed to the laser beam-material interaction region has been built. The powder is melted and forms planar, 2D geometries as the substrate is moved under the laser beam in XY- direction. After completing the geometry in the plane, the substrate is displaced in Z-direction, and a new layer of material is placed on top of the just completed deposit. By continuous repetition of this process, 3D parts wee created. In particular, the impact of the focal spot size of the high power laser beam on the smallest achievable structures was investigated. At a translation speed of 51 mm/s a minimum material thickness of 590 micrometers was achieved. Also, it was shown that a small Z-displacement has a negligible influence on the continuity of the material deposition over this power range. A high power CO2 laser was used as energy source, the material powder under investigation was stainless steel SS304L. Helium was used as shield gas at a flow rate of 15 1/min. The incident CO2 laser beam power was varied between 300 W and 400 W, with the laser beam intensity distribute in a donut mode. The laser beam was focused to a focal diameter of 600 (Mu) m.