| dc.description.abstract |
The aim of this study is using finite element method (FEM) to develop a computer-aided-engineering (CAE) technique for application in simulation of various laser processes, such as fixed-point irradiation, moving irradiation, and multilayer powder deposition. Thermal and mechanical analyses are conducted through FEM modeling to calculate the distributions of temperature and stress in various laser processes. Experiments, including fixed-point laser irradiation and line segment of laser irradiation are conducted to validate the FEM models developed in this study. Furthermore, effects of laser processing parameters, such laser scanning speed and laser scanning strategies, are analyzed to find a proper process window for laser additive manufacturing (LAM) in making a build with low residual stress.
In validating experiments, CO2 laser of 30 W and fiber laser of 50 W and 80 W are employed for fixed-point irradiation, while CO2 laser of 30 W and fiber laser of 80 W are used in line segment irradiation. The temperature and strain variations at selected points are measured by thermocouples and strain gages in experiment and are compared with the FEM simulations. For fixed-point laser irradiation with a 30-W CO2 laser or a 50-W fiber laser, temperature histories at measurement points concur with the simulation results. Scattering of temperature data is observed in the fixed-point irradiation tests using an 80-W fiber laser, but the temperature changes at selected positions in simulation still have a fair agreement with experimental measurements. For a line segment irradiation, the temperature variations at selected points in experiment using a 30-W CO2 laser or an 80-W fiber laser agree well with the simulations. For strain measurement in a line segment irradiation by 80-W fiber laser, the strain variation at each measurement point in simulation shows a fair agreement with the experimental results.
Simulation of thermal-to-structural changes in a multilayer powder deposition is also performed. A concentric temperature distribution is observed at the location irradiated by laser on the top surface of the build. For moving laser irradiation, a tail-shape temperature distribution is found and the temperature gradient along the laser scanning direction is extremely large. Compressive stress is induced at the region right after irradiating by the laser beam, and this local stress becomes tensile one when the irradiated area cools down. As the temperature gradient along the direction of moving laser is quiet large, the residual normal stress in this direction is much larger than that in the transverse direction. Distribution of von-Mises equivalent stress indicates that residual stress increases with increasing number of deposited layers. In addition, highly residual stresses are present in the build and at the base nearby the interface between the build and base. Such residual stress may have a detrimental effect on the structural robustness and integrity of the build made by LAM. The maximum temperature during LAM process and induced residual stress are affected by several laser processing parameters. The residual stress in the scanning path of an “S” pattern is smaller than that in a unidirectional path. The residual stress in the build and base increases with a decrease in the laser scanning speed. | en_US |