Coupling Analysis Of Stress Fields On KDP Crystal Slicing By Fixed Abrasive Wire Saw

KDP crystal which has large electro-optical and nonlinear coefficient as well as high laser damage threshold is the preferred nonlinear optical crystal material used in the Inertial Confinement Fusion. However, KDP crystal with low hardness, high brittleness, easy deliquescence and temperature-sensitive is easy to crack during the crystal growth, taken out from crystallizer, and the process of slicing. As the first process of obtaining wafer, the slicing of KDP crystal is of vital importance to follow-up processes and even the qualified rate of the wafer. The fixed abrasive wire saw slicing technology has some advantages of tiny kerf loss, low sawing stress and the ability to slice large crystals, which is expected to solve the crack of KDP crystal during slicing. Based on the theoretical analysis and the finite element method, the distribution rule of KDP crystal internal stress field during the wire saw slicing process is researched in this paper, and the main work is as follows:According to the directional cutting principle of crystal, the wire saw slicing method on KDP crystal in different crystal faces is analyzed. By analyzing the property of (001) crystal face, doubler crystal face and tripler crystal face, sawing direction programs of these three crystal faces slicing by fixed abrasive wire saw have been established. The results show that the (001) crystal face just needs simply directional during sawing fixed clamping, and the tripler crystal face can be cut out the shape with one rotational direction, but the doubler crystal face needs two rotational directions.Based on the anisotropic elasticity theory and the FEM theory, and considering the key factors comprehensively in wire saw slicing simulation such as material model, initial stress field, dynamic loads, material removal, et al, models of KDP crystal’s (001) face and tripler face cut by wire saw have been set up. The distribution rule of stress field during the sawing process has been analyzed. The results show that stress changes smoothly in sawing process which belongs to low stress cutting way. When the KDP crystal is about to be cut off, the maximum tension stress significantly increased. With the material removed, the crystal initial internal stress releases, and the stress of crystal internal point which is closer to the new surface changes more significantly. Sawing stress and internal stress couple mutually around the sawing kerf, and appear stress concentration on it. The larger of crystal initial stress, the more obvious stress concentration around the kerf during slicing.The KDP crystal containing defect cut by wire saw has been numerical simulated. Combining the stress concentration theory, the stress distribution near the defect during the process of sawing has been analyzed, and the effect of defect size and position on the stress distribution has been further researched. The results show that crystal defect leads to local stress concentration, and the stress concentration factor which remained stable during wire sawing increases sharply when the wire saw is cutting to the defect position. There is also stress concentration on the sawing kerf. The coupling effect between these two kinds of stress concentration is enhanced when the sawing kerf is close to the defect, so the maximum tensile stress near the defect increases and to the extreme when the wire saw is cutting to the defect position. The change of the maximum tensile stress is more intense when defect position is closer to the material removal layer. The maximum tensile stress is larger when the defect is located on the terminal sawing removal layer.Based on the thermal analysis fundamental, finite element models of KDP crystal have been constructed to the analysis of temperature and thermal stress distribution during wire saw slicing, and the coupling analysis of thermal stress field and initial stress field. The results show that the highest temperature of the crystal located on the sawing kerf remains stable during slicing. The temperature inside the crystal rises slowly at the start of the slicing, and then tends steady with the continued increase of sawing depth. Because of the good cooling condition during the sawing process, the overall rise in temperature is small, and the maximum thermal stress on the sawing kerf is less than the tensile strength of KDP crystal. Although thermal stress has little effect on crystal cracking, but the crystal tensile stress increases obviously when it is coupled with the initial stress field.