Elsevier

Optics & Laser Technology

Volume 34, Issue 8, November 2002, Pages 639-648
Optics & Laser Technology

Transient deformation of thin metal sheets during pulsed laser forming

https://doi.org/10.1016/S0030-3992(02)00088-9Get rights and content

Abstract

The transient deformation of thin grade 304 stainless steel metal sheets heated by a single pulse of a CO2 laser beam is simulated in this paper. The laser beam is assumed to be line-shaped and the problem is treated as three-dimensional thermo-elastoplastic. The temperature field, deformation pattern, stress–strain states and the residual stress distribution of the specimens have been calculated numerically and the transient response of the bending angle has been validated by experiments. Good agreement has been obtained between the numerical simulation and the experiments under various operating conditions. The numerical study reveals that a high temperature gradient exists for a positive bending angle and a low one for a negative angle. It transpires that the mechanisms of pulsed laser forming are dependent mainly upon the laser power, the heating time, the clamping arrangement, as well as the geometry, the thermal properties and the original stress states of the specimen.

Introduction

Laser forming has been thoroughly studied by various investigators in the metal forming industries [1], [2], [3], [4], [5] since its development in the recent years. It is a new form of laser technology which allows a metal sheet to be stressed thermally under a controllable laser beam heat source without the use of a mold or forming press. The forming mechanism of the complex sheet geometry has been analyzed using various computational softwares based on the finite element method (FEM). The basic focus is on the influence of material properties, the beam mode and the heating paths of the laser. In 1998, Zhong and Wu [1] solved the temperature distribution of laser forming for a moving laser beam problem using the FEM. Kyrsanidi et al. [2], on the other hand, simulated the bending angle of metal sheets under laser forming using ANSYS. More recently, Hennige [3] used the FEM code, SYSWELD to simulate the laser forming of a flat metal sheet into a bowl-shape deformation with straight and curve laser beam paths. The above studies, however, involve only moving laser sources. Open literatures on stationary pulsed laser forming are scarce due to the comparatively smaller heat affected zone, especially in the transient phase of the deformation process during laser radiation. As far as the authors are aware, a slight mention of some related experimental work was made by Widlaszewski [4] in 1997. Recently, however, Chen [5] calculated the bending angle of laser forming using ABAQUS by assuming 2-D plane strain and a line shape heating source and the final deformation predictions were compared with experimental results. Nevertheless, the transient deformation mechanism of a thin metal sheet under pulsed laser beam is to date still unclear. The only certainty is that the unpredictable deformation and dynamic characteristics of thin metal sheet in many similar pulsed laser applications such as laser trimming and laser micro-welding for miniature devices are increasingly important in modern manufacturing technologies.

In this paper, the transient deformation response of a thin metal plate heated by a stationary pulsed CO2 laser beam has been simulated and compared with experiments. The problem has been assumed to be 3-D thermo-elasoplastic and is solved using ABAQUS [6] in order to investigate the influence of temperature, the stress state and the residual stress distribution of the specimen on the deformation mechanism. As a means of validation, the bending angle of the specimen was measured in the experiments.

Section snippets

Numerical computation

Laser forming, as with other laser material processing applications such as laser heat treatment, involves many non-linear physical phenomena that include temperature distribution, stress field and microstructure variation, all of which are significantly inter-related. In comparison with the amount of laser beam energy, the heat generation due to the strain energy in the bending process is negligible. Therefore to simplify the analysis, the laser forming problem herein will be decoupled by two

Experimental arrangement

The experimental arrangement for the pulsed laser forming of the stainless steel sheets is shown in Fig. 16. A CO2 laser beam was directed into a cylindrical focusing lens with a focal length of 250mm which collimated a circular beam with a raw diameter of 16mm into a line beam. The pulse duration of the laser was adjusted by a CNC controller and a mechanical chopper of the laser system. The specimen was clamped at one end and irradiated by the laser beam at its center. A laser displacement

Conclusions

Due to the non-linear coupling effects among the stress, strain and temperature of the material properties of the stainless steel specimens in pulsed laser forming, a number of phenomena, namely, heat conduction, plasticity and the microstructure variations of the specimen occur during the laser–material interactions. This study has been investigated numerically, using a three-dimensional meshed domain, and experimentally the thermal deformation of thin stainless steel sheets under laser

References (8)

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