Laser Forming

The effect of passive water cooling in laser forming of thin sheets made of AISI 304 stainless steel is experimentally investigated. Indeed, since each laser scan can produce only small bending angles, multiple laser scans are required to produce a given deformation with a significant increase of production time due to cooling between consecutive scans. Therefore, passive water cooling is tested to verify its influence on minimum time between consecutive scans (cooling time), bending angle, and surface quality. A parametric approach is involved in the investigation and main process parameters are changed among the experiments by varying laser scanning speed, laser beam power, sheet thickness, and cooling media among several levels. It was discovered that the employment of passive water cooling in laser forming of thin sheets would be beneficial since the capability to dramatically reduce the cooling time and oxidation of both irradiated and cooled surfaces. In addition, the bending angle is only marginally affected by employment of water cooling. The effect of water cooling on stress and deformations are discussed by developing a numerical model based on finite element model.

An analytical solution for temperature and deformation fields produced by laser forming is derived. The effects of process parameters are investigated on samples of different materials and thicknesses. Experimental tests were carried out using a high-power diode laser on a poorly conductive material (AISI 304) and a highly conductive one (AA 6013) to evaluate the accuracy of the proposed model. A 2D FEM is developed to calibrate and validate the new model. The model predictions are in good agreement with experimental measurements even under low scanning speed due to the effect of conductive heat exchange of the irradiated zone with surrounding material. Based on the achieved results, a simple method for optimizing the process productivity is also discussed.

An evolutionary process was used to develop the toroidal field (TF) coil design for the ARIES-spherical torus (ST). Design considerations included fabricability, assembly, maintenance, energy efficiency, and structural robustness. Design options were identified early in the process. Trade studies were carried out to identify preferred choices. Design points were re-optimized based on the design choices in the TF and other systems. An attractive design for the ARIES-ST TF coil system evolved. This design addresses a number of the concerns (complexity) and criticisms (high cost, high recirculating power) of fusion. It does this by: applying advanced, but available laser forming and spray casting techniques for manufacturing the TF coil system; adopting a simple single turn TF coil system to make assembly and maintenance much easier. The single turn design avoids the necessity of using the insulation as a structural component of the TF coils, and hence, is much more robust than multi-turn designs; using a high conductivity copper alloy and modest current densities to keep the recirculating power modest. # (W. Reiersen).

In this paper a computational model for the numerical simulation of coupled thermoplastic problems within a fully nonlinear kinematic framework, involving large deformations, is presented. The final goal is to get an accurate, efficient and robust numerical model for the simulation of laser heat-forming processes. This innovative technique, of crucial interest nowadays to the aeronautical industry, allows the forming of a sheet metal part by means of the plastic deformations induced by a heat source provided by a laser beam. The performance of the model is shown in the numerical analysis of a laser heat-forming of an aluminium sheet. The data of the test have been provided by British Aerospace. INTRODUCTION: In spite of important progresses achieved lately in computational mechanics, the large scale numerical simulation of coupled thermomechanical problems at finite strains continues to be nowadays a very complex task due mainly to the highly nonlinear nature of the problem. Sheet and bulk metal forming have been one of the topics of main interest in the past years, as it can be shown in a number of International Conferences and Symposiums devoted to these topics. Within the sheet metal forming processes, a very innovative technique, of crucial interest to the aeronautical industry, has been the laser heat-forming process, where a sheet metal part is formed as a result of the plastic deformations induced by a heat source provided by a laser beam. In this paper an attempt to deal with the numerical simulation of such interesting forming procedure is presented. The performance of the model is shown in the numerical analysis of a test provided by British Aerospace, where a laser heat-forming of an aluminium sheet is performed. The formulation of the model has been thermodinamically consistently derived from a thermoelastoplastic free energy function, including temperature-dependent material properties. See Agelet de Saracibar (1998), Agelet de Saracibar et al. (1999), Cervera et al. (1999) and Chiumenti et al. (1999) for a description of the model within the infinitesimal regime. Time integration algorithm to update the stresses and internal variables is based on the exponential return mapping algorithm for multiplicative plasticity introduced by Simo (1991), including plastic hardening and thermal softening. Spatial discretization has been performed by means of Q1/P0 finite elements at finite strains. The resulting model has been incorporated to the in-house developed computer code COMET.

Laser assisted self-piercing riveting (LSPR) is a new solid state process that enables low ductility materials to be mechanically joined without cracking. A simple but effective thermal analysis of LSPR is presented that enabled both the absorption of the laser radiation and heat transfer between plies to be determined. The approach was applied to experimental data for LSPR joining of AZ31B-H24 magnesium alloy sheets. It is shown that by using this analytical approach, the temperature at the onset of joining could be estimated and related to observations of joint quality. It was found that crack-free joints were produced at strip temperatures above 200°C at the time of rivet insertion.

This paper deals with the development of a computational procedure for the inverse analysis of the laser forming process of three-dimensional metal sheet, characterized by double curvature. The procedure is based on the minimization of a vectorial fitness function, obtained by the comparison of the considered target surface with reference deformed surfaces, represented as sixteen point bicubic patches. An FEM-based computational approach has been adopted to evaluate the reference deformed surfaces for the instruction of the processor.