고출력 CW Nd:YAG 레이저를 이용한 CSP 1N 냉연강판 절단특성 분석 및 3차원 유한요소 해석에 관한 연구

Abstract

Laser cutting process is one of flexible rapid manufacturing technologies with various advantages including a high cutting speed, manufacturing of parts with a complex shape, and others. In addition, laser cutting can obtain high precision parts due to a narrow heat affected zone, a small kerfwidth and the minimized post-deformation after cutting. Hence, an interest in laser cutting in the production line is the precision and the net shape cutting of a metallic thin sheet. CW Nd:YAG laser begin to use in cutting of parts due to its advantageous characteristics, such as a small spot size, a higher energy density and a higher energy absorption rate of the cut material, in recent years. The objective of this research works is to investigate into the influence of process parameters on the quality of the cut section and characteristics of the heat transfer for the net shaping in the laser cutting of CSP 1N sheet using a high power Nd:YAG laser with a continuous wave. In the experiments, power of laser, cutting speed of laser, material thickness and corner angles as well as size of loop are selected as process parameters. The thickness of specimen is less than 2 mm. The experiments are performed using a 5-axis controlled automatic robot laser cutting system. CW Nd:YAG laser with a 2.8 KWatt of the maximum power and 1.06 ㎛ of the wave length is employed as the laser source. In order to investigate the effects of process parameters, including the power of laser, the travel speed of laser and the thickness of the cut material, on the practical cutting region, the kerfwidth, the surface roughness, the formation of the striation, and the cut & brake ratio, several cutting experiments have been carried out. Using the results of the experiments, the practical cutting region for each thickness has been obtained. The dross area and the straightness in the lower surface of the cut section are chosen as the measure of the selection of the practical cutting region. The effective heat input is introduced to consider the power of laser and the travel speed of laser, together. From the results of the experiments, the relationship between the effective heat input and kerfwidth with respect to the thickness has been obtained. Through the investigation into the influence of process parameters on the surface roughness and the formation of striation using the results of the experiments, optimal cutting conditions have been obtained. In order to examine the influence of the corner angle and the size of loop on an amount of the corner melting and heat affected zone, the loop cutting experiments have been performed. Through the results of experiment, the relationship between the corner angle and the melted area in the back and forth of each optimal cutting condition has been obtained. In addition, it has been shown that the optimal overrun size, which is minimized the melted area in sharp corner, is approximately 3 mm. In order to investigate the characteristics of the heat transfer during the laser cutting, the three-dimensional quasi-steady heat transfer analysis has been performed by a commercial code SYSWELD. The acceptable finite element analysis model has been established through comparison the results of analysis with those of the experiment. Using the FE analysis model, the three-dimensional temperature distribution in the cut material for each cutting condition has been investigated. The results have been shown that a rapid change of temperature occurs within ± 0.5 mm from the center of the focus for laser. In addition, it has been known that it is possible to cut the CSP 1N sheet using CW Nd:YAG laser when the maximum temperature is greater than 2,000 oC. The above results of experiments and analyses will be applied to construction of knowledge base for knowledge-based path generation program with an algorithm of the offset and the path modification to obtain the optimal cutting path of laser cutting.