1. Field of the Invention
The present invention relates to a welding portion inspection device and an inspection method therefor, particularly to an inspection device which inspects a welding condition of a welding portion formed when a plurality of works are welded together by means of laser beam and an inspection method therefor, for example.
2. Description of Related Art
When two steel sheets are welded together by means of laser beam such that they are stacked one upon another, the quality evaluation of a welding portion formed by laser welding has been performed. As an example of the quality evaluation on the welding portion formed by laser welding, for example, Japanese Patent Application Publication No. 2008-87056 (JP 2008-87056 A) has disclosed a technology concerning the quality evaluation on the laser welding using reflected light of the laser beam.
A laser welding quality determination system disclosed in JP 2008-87056 A radiates. YAG laser, for example, from a laser torch. A laser reflected light is received by a first reflected light receiving/outputting device from a forward-diagonally upward side in the advance direction of welding operation. A welding light including evaporative light (plume) and reflected laser light is received by a second reflected light receiving/outputting device coaxially with the radiation direction of laser beam. The laser reflected light and welding light received from two predetermined directions at the same time are converted to an electric signal corresponding to their intensities. This system determines the welding quality based on the signal intensity of this electric signal and changes in the intensity thereof.
According to the laser welding quality determination system disclosed in JP 2008-87056 A, the reflected laser light and the welding light are received from two different directions at the same time. By comparing the signal intensity of each of received lights with a threshold set appropriately, it is possible to determine that any one of various welding defects such as underfill in which welding bead sinks to fill a gap between steel sheets, joint failure in which upper and lower steel sheets fail to joint due to an excessive gap between the steel sheets, bead depression in which bead is depressed due to an excessive gap between steel sheets, fusing in which bead vanishes abruptly due to a change in heat balance, boring has occurred.
However, according to the laser welding quality determination system disclosed in JP 2008-87056 A, for example, if a laser torch and a work (steel sheet) are set apart from each other, the electric signal obtained from the received reflected laser beam or welding light may be very weak. Thus, the accuracy for determination of welding failure may drop. Particularly, in the case of bead depression in which bead is depressed upon laser welding, the change in electric signal due to welding failure decreases. Thus, sometimes, welding failure in a work cannot be detected precisely. Further it has been known that evaporation luminescence generated due to melting/evaporating of a work or heat radiation light radiated from a molten pool in the work changes depending on a work temperature and that a threshold for determining the electric signal obtained from the received reflected laser beam or welding light and the laser welding quality changes depending on the work temperature. If the change in the work temperature upon laser welding is large, the determination accuracy for the work welding defect sometimes may further drop.