تجزیه و تحلیل فرآیند تشکیل جامد مخصوص لیزر از ساختار دیواره نازک

Laser solid forming (LSF) is a promising manufacturing technology. Thermal behavior is very significant for the research of microstructure, performance and geometric dimension of the fabricated part. In this research, a two-dimensional transient analytical model was developed on a moving square heat source with a uniform heat intensity distribution, and applied to estimate the temperature distribution and deposition thickness of the LSF thin-wall structures. The effects of two ends of the thin-wall structure and the temperature decline after closing the laser beam were investigated. The deposition thickness with different process parameters was also calculated and agreed well with the data measured by a CCD camera system under the practical process parameters despite some differences. Finally, a unique strategy (adjusting the dwell time of laser beam at both ends) was proposed to improve the dimensional accuracy at two ends of the thin-wall sample, and the experimental results demonstrated the validity of the strategy proposed.

Laser solid forming (LSF) is a promising manufacturing technology that can build three-dimensional metallic parts directly from CAD model. During LSF, a laser beam scans on a substrate (or a previous clad layer) to create a moving molten pool into which metallic powders are injected through the powder nozzle synchronously. As powders are caught by the moving molten pool and then experience melting and re-solidification process, a clad layer is formed. Further, a three-dimensional part can be built in layer-by-layer fashion. This technique can significantly reduce the length of time between initial concept and finished part. It also allows the production of graded materials and the repair of damaged parts. Several similar methods have been developed to produce metallic parts, such as laser en- gineered net shaping (LENS) [1] , direct laser fabrication (DLF) [2,3] , directed metal deposition (DMD) [4,5] , and laser-based additive manufacturing (LBAM) [6] . Understanding thermal behavior in the LSF process is sig- nificantly helpful to research the microstructure, performance and geometric dimension of a deposited part. Therefore, many studies have been focused on this field. Griffith et al. [7] used the thermocouple and imaging techniques to monitor the thermal signature during LENS process and discussed the solidification behavior, residual stress, and microstructure evolution with respect to thermal behavior. Doubenskaia et al. [8] applied the pyrometer for surface temperature monitoring to control melting/solidification and avoid thermal decomposition in Nd:YAG laser cladding. Pinkerton and Li [9] developed a simple thermal model to analyze the temperature distribution and estimate the molten pool size in laser cladding. Liu and Li [10] established a model to investigate the effects of process variables on laser direct formation of thin wall. Jendrzejewski et al. [11] developed a two-dimensional thermal model to understand the temperature distribution in laser multi-layer cladding. However, the previous models mostly focused on analyzing the steady-state process, and hardly consider the influences of the transient stage and the edges of the samples. In this research, a two-dimensional transient model was developed to estimate the LSF thin-wall process by using a moving square heat source with uniform heat intensity. The effects of two ends of the thin-wall structure and the temperature decline after closing the laser beam on the thermal behavior were investigated. The deposition thickness with different process parameters were predicted and compared with the experimental data. Further, the process improvement was also discussed

(1) An analytical model for LSF thin-wall sample was developed using a square heat source with a uniform heat intensity distribution. The model consider the effects of the two ends of the thin-wall structure and the temperature decline process after closing the laser beam on the LSF thin wall, and can be used for the transient thermal analysis of the thin wall and the deposition thickness prediction. It can provide a better insight into not only the physical process of LSF thin wall but also the thermal behavior of other laser processing technologies. (2) The analysis was applied to the LSF 316 L stainless steel thin- wall process. It can be found that the effects of two ends on the thermal behavior and deposition thickness are noticeable near two ends; the temperature decline after closing the laser beam have some influences on the deposition thickness at the tail end, and the transient stage at the beginning and the closing of the laser beam are the very primary reasons that thermal behavior and deposition thickness at two ends are different from those at other zones. (3) The analytical solution can provide an estimation of the deposition thickness, and successfully predicted the serious collapse at two ends of the thin wall. The deposition thickness with different process parameters were calculated and agreed well with the experimental results under the practical process parameters despite some differences. (4) A unique strategy with adjusting the dwell time of laser beam at both ends was proposed, and the results show that the LSF thin-wall process can be improved effectively by using a suitable dwell time of laser at the beginning and at the end of the thin wall