Zygmunt Mucha

Mechanically assisted laser forming of thin beams

Laser-assisted forming techniques have been developed in recent years to aid plastic working of materials, which are difficult in processing at normal temperatures due to a high brittleness, effects of high work-hardening or a high spring-back phenomenon. This paper reports initial experimental investigations and numerical simulations of a mechanically-assisted laser forming process. The research is aimed at facilitating plastic shaping of thin-walled parts made of high temperature resistant alloys. Stainless steel plate, 1 mm thick, 20 mm wide, was mounted in the cantilever arrangement and a gravitational load was applied to its free end. A CO2 laser beam with rectangular cross-section traversed along the plate, towards the fixed edge. Laser spot covered the whole width of the plate. Experiments and simulations using the finite element method were performed for different values of mechanical load and with constant laser processing parameters. Experimentally validated numerical model allowed analysis of plastic deformation mechanism under the hybrid thermo-mechanical processing. The revealed mechanism of deformation consists in intense material plastic flow near the laser heated surface. This behavior results mainly from the tension state close to the heated surface and the decrease of material yield stress at elevated temperature. Stress state near the side edges of the processed plate favored more intense plastic deformation and the involved residual stress in this region.

Deformations and stresses induced in materials by moving beam of CO2 laser

Analytical modelling of thermal bending of plates by use of a laser beam with circular cross-section is presented. The most important parameters in this process have been found: the pick surface temperature and the Fourier number, which are the similarity numbers. Experimental verification was performed. Temperature distribution in the plastified layer of the material (where the material losses its elastic properties) was taken into considerations. Developed model proves that effective bending occurs when the material surface temperature is close to the melting temperature. Restrain rigidity coefficient of heated material in Presented model can be applied in various laser processing technologies, for instance in welding, cutting, surface treatment, including hardening and others. Theoretical results of residual stresses distribution in plates after laser beam pass is presented. Analysis of the solution proves possibility control of surface stress on both sides of the bent plates by change Fourier number and surface temperature.

Laser forming of materials without external forces

Investigations on bending plates by use of laser beam with rectangular cross-section are presented. Permanent deformation of plate is obtained without external forces, solely under influence of thermal stress induced by local heating of the material. Laser beam of narrow rectangular cross-section was modeled as a linear heat source moving over the plate surface. Temperature distribution in the plastified layer of the material was taken into considerations. The bend angle was investigated as a function of laser beam parameters: power, cross-section geometric parameters, velocity with respect to the material and material parameters of bend plates. It has been proven that effective bending occurs when the material surface temperature is close to the melting temperature. Presented advanced analytical model can be applied in design and control of laser forming of developable surfaces, as well as in other laser processing technologies, hardening, for instance. Derived dependence for the bend angle and curvature can be reduced in special case to well-known expressions describing welding distortions.

The influence of surface preparation on the absorption coefficient of laser radiation

The absorption coefficient of the surface of a workpiece is of importance in laser treatment, particularly in the treatment where the temperature of an element must be strictly controlled. Laser surface treatment (such as hardening, metallic glazing) and laser forming can be primarily included in this type of technology. In another case, surface temperature must be precisely controlled, especially if structural changes are to be avoided. There are a number of ways to increase the absorption coefficient of the surface of an element. Since the laser forming is the research subject of the authors of the presented paper, it was necessary to determine the absorption coefficient for the different surfaces preparation of workpieces. Raw surface, oxidized surface, sandblasted surface, black enamel covered surface and waterglass covered surface were examined, respectively. The experiment was performed using a CO2 laser with a head for a surface treatment which generates a rectangular beam of dimensions 2x20 mm, and the samples were made of X5CrNi18-10 stainless steel.

Modeling and experimental investigations of keyhole laser welding

In the paper are presented new analytical modeling of deep penetration laser welding and its experimental verification. The model is an extension of analytical model of 1973. The model allows the derivation of penetration and width of melting zone caused by moving laser beam. As a result there was derived dependences of penetration length, width of the melting zone and aspect ratio of the zone as a function of welding speed and laser power. The theoretical results was compared with experimental data. The results allows the determination of optimal conditions for keyhole effect. Results of the modelling are expressed in non dimensional parameters therefore can be applied to any metals and alloys for design of laser welding parameters.

Permanent and non-permanent deformations in samples caused by CO2 laser pulse

In the paper the authors present results about research dealing with deformations of low-carbon samples influenced by a stagnant or not moving CO2 laser impulse. Permanent and non-permanent deformations were analysed for two cases: for constant power and for constant maximal surface temperature. Laser-based forming methods known till now, use moving laser beams. As a results of the paper it is found, that in case of constant power (for each experimental points) non-permanent (temporary) deformation decreases with pulse duration and permanent (residual) deformation is maximal for some pulse duration. But in case when surface temperature is kept constant, non-permanent deformation is constant with pulse duration and curvature of permanent deformation can be positive negative or kept zero. Aim of the research is to show possibility of laser forming by a not moving laser beam with suitable power and pulse duration with control of sample residual curvature.