Digital Holography used for measurement of deformations during gas quenching processes

摘要

The compensation of distortion in manufacturing processes is important for quality assurance in component production and product accuracy. Researches at the University of Bremen show that the mam distortion effects for metal components take place during the quenching process. To measure and finally control these deformations a measurement principle which has no influence to the quenching process is necessary. Any measurement principles contacting the object are therefore inappropriate. Capacitive measurement principles drop out because the sensors have to be placed close to the object, disturbing the volume flow of the quenching gas. However, in this presentation optical metrology is shown to be suitable. As a fast, contactless measurement technique Digital Holography was used to measure surface deformation during the quenching process with interferometric accuracy. The results of these measurements shall give input to control processes in the gas quenching stand. Observing an axle of about 25 cm height, having a diameter of 2 cm and a quenching process starting at temperatures of the axis of about 800℃, several problems had to be faced: The glow of the observed axis could be compensated by application of an interference filter. Vibrations of the axis and the setup due to the gas streaming and subsonic noise had been suspended by application of a Nd-YAG-Pulselaser with 8 ns pulse duration and 10 Hz repetition rate. Because of the high volume flow of the quenching gas nitrogen, smear effects can be neglected. Surface microstructure decorrelations are shown to be of minor interest. An interferometric correlation of two timely neighboured holograms in a series of holograms taken with a time gap of 0,5 s was possible. Difference phase maps of an area of 1 x 5 cm~2 showing out-of-plane deformation of a few micrometers show the Digital Holography to be a suitable measurement technique for the gas quenching process.