Tube bending locally heated by a laser beam is analysed. This heating reduces the yield stress and prevents stress hardening thus making easier the process of plastic deformation. In the present work simple model of elasto-plastic bending of circular tubes is developed and some experimental results of laser assisted bending are shown.
Laser-assisted forming is a method based on contactless shaping of profiles with the use of a laser beam impact. This method has been developed from the early 1980s by the Centre for Laser Technology of Metals at the Kielce University of Technology (PŚk) and the Polish Academy of Sciences (PAN), among others [1-7]. In general, the mechanism allowing for changing the shape of profiles is the material’s thermal expansion. Suitable profile heating (on programmed paths with an adequately selected temperature, etc.) allows for obtaining the planned shapes.
Despite certain areas of application (electrical mechanics, precision mechanics, micro-positioning and others), this method is ineffective [4-5]. When changing the shape of large-diameter profiles, this method is power and time consuming, thereby eliminating it from industrial applications. Thus, the concept of a hybrid method was created, i.e. laser forming with mechanical assistance. I
n 2015, PŚk established co-operation with the Metal Forming Institute (INOP) in Poznań, PAN’s Institute of Fundamental Technological Research, and the Rzeszów University of Technology in terms of research in the aforementioned topic. The above consortium commenced the execution of the project titled “Laser forming of thinwalled profiles with mechanical assistance, financed by the National Centre for Research and Development as part of subsidy no. PBS3/A5/47/2015.
The paper’s authors will present, among other things, the main concept of hybrid laser and mechanical forming, one of the concepts selected for the execution, design and construction of a station for bending thin-walled tubes and cone diffusers used in the construction of aircraft engines. The target materials of the research are Inconel 618 and Inconel 625 refractory nickel superalloys, as well as AISI 410 and AISI 325 heat-resistant Martensitic steels. These materials, due to their good mechanical properties when working at higher temperatures, are used for building turboprop engines. For economic reasons, the testing was conducted on X5CrNi18-10 acid-resistant austenitic steel. The hybrid method (assumptions, concept, design) presented in the paper was subjected to validation in laboratory conditions. The testing featured measurements of the forces required to obtain plastic deformations in the profile, bending angle, and determination of the process temperature. Furthermore, the paper will feature a presentation of future plans concerning the work executed as part of the said project.
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.
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.
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.
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.
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.
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.