Abstract
In situ measurement of residual stress is a challenge, and it is a source of many defects during additive manufacturing (AM). Usually, postmortem measurement is too late to save the product once a defect appears. Most of the existing technologies are predictive simulations and postmortem analysis. However, these technologies cannot directly reflect the stress evolution during the fabrication process. This paper introduces a computer vision-based stress monitoring system combined with finite element method (FEM) technology to estimate the stress development inside of the deposition layer. The system uses a CCD camera and a line laser beam to measure the height of the melt pool and solidified layer, forms a real-time FEM model, and uses the surface displacement between the two states to calculate the stress development during the solidification process. The results show that there is no obvious shape change after solidification. The shape of the melt pool and its solid state is similar. The stress distribution obtained through online monitoring is similar to that from the traditional thermal-stress simulation.
Highlights
Due to the complicated thermal history of laser-based additive manufacturing (AM) processes, uneven thermal contraction and expansion occur, which generate a massive amount of residual stress inside the solidified material[2]
Moat[12] and Pratt, P13 use neutron diffraction to measure the stress in direct metal deposition (DMD) manufactured blocks
Based on computer vision and finite element method (FEM) analysis, this method can make a quick prediction of the stress generated during the laser deposition process
Summary
Due to the complicated thermal history of laser-based AM processes, uneven thermal contraction and expansion occur, which generate a massive amount of residual stress inside the solidified material[2]. Qi10 and Xinran Zhao[11] added solidification phase changes to the model to make the result more closely resemble the experimental data Another method to measure residual stress is to do postmortem analysis after manufacturing and analyse it with the laser parameters. There is limited room to integrate X-ray or neutron diffraction equipment into the DMD system and insufficient time to do the online simulation based on the traditional method Another way to monitor the process is to evaluate the strain evolution on the substrate. Andreas[15] uses a LVDT (linear variable differential transformer) gauge to measure the out-of-plane deformation at the centre of the bottom surface of the substrate This method cannot directly reflect the real condition inside the deposition layer. Based on computer vision and FEM analysis, this method can make a quick prediction of the stress generated during the laser deposition process
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