Modeling of the heating process during the laser transmission welding of thermoplastics and calculation of the resulting stress distribution

Abstract

At present, various welding techniques are used in many different process chains and steps. Moreover, when joining metallic materials, welding processes are used to create integrated material bonds of organic as well as inorganic materials. One of the methods used to join thermoplastics is laser transmission welding. In addition to the low heat introduced into the welded parts, high welding speed is one of the reasons why this process is used in industry. Highly accurate simulation of the heating process would improve understanding of the process, facilitate and reduce the time required for process installation in the long run, and provide a significant contribution for computer-aided component design. The quality of the weld line in laser transmission welded plastic parts is affected by material properties, machine parameters, and their interactions with each other. For these reasons, a thermal simulation model was developed as part of a research project due to support the operator of the laser transmission welding process in setting up the process faster and more reliably. First, the presented model considers the various interferences of the welding process by the material, the thermal, and optical properties which are often not known exactly enough. Second, interaction between the laser beam and the material was determined and implemented. The developed calculation model allows for highly reliable determination of the size of the heat affection zone. The exact process window can be identified and predicted. Furthermore, the heating and cooling process can be reproduced using the spatial temperature distribution, which allows cycle time to be optimized on the basis of these results. In addition to the impact on weld quality, residual stresses can be found in welded thermoplastic parts as a result of local thermal expansion and shrinkage. Especially, tensile stresses are undesired effects of the welding process because they can have an unfavorable impact on the local mechanical properties of the welded components. Mainly, in laser transmission welded parts, these tensile residual stresses occur in the weld seam. These tensile stresses can overlap with permitted operating stresses, and this may result in early failure of the component. This can take place, in particular, when the structural part is exposed to oscillating and/or highly corrosive stresses. These are the reasons why it is important to know the residual stresses introduced into the component, in order to minimize the distribution of these stresses by means of suitable process management. To calculate the mechanical behavior in the structural simulation of a welded part, precisely mapping the temperature field is of fundamental importance. Residual stresses and warping are directly based on the temperature field. Based on thermal calculations, the paper presents thermo-mechanical simulations for the determination of residual stress distribution in the weld seam, depending on the welding parameters.