Laser Speckle Photometry For Monitoring Of Welding Process

The laser welding will cause lack of fusion defect,hole defect,cracks and other defects.Those defects which are hard to be detected,will cause very big impact when there are ignored.Therefore,we need to monitor laser welding,but the traditional detection methods are not suitable for this situation,so a new way should be proposed to detect the laser welding process.Laser Speckle Photometry(LSP)is a newly developed contactless,fast and completely optical non-destructive testing method based on the detection and analysis of thermally or mechanically activated characteristic speckle dynamics.When the heat energy propagation or tension process,the surface of the object will change and have little displacements which are caused by the locally different degrees of thermal/mechanical expansion.This displacement will make the laser speckle change at time domain,also it can be said the change of laser speckle is affected by the thermal/mechanical expansion.Normally the displacement at the surface of the sample is regular,but if the sample's nearfield has any defect,it will be different,and it is caused by the defect of locally different degrees of thermal/mechanical expansion.The diffidence is the fundamental to inspect the defect.After analysing the video which recorded the movement of the laser speckle,the defect can be detected through use of a special correlation function to combine speckle intensity's change and local spatial gradient at time domain.It is the principle that provides the possibility of defect detection with LSP technique.In the current work,the objective is to examine the suitability of LSP technique for the detection of the defect in the laser welding process.In this thesis,Chapter 1 states the motivation for this work.Chapter 2 presents the state-of-the-art of some NDT techniques,which are highly related to LSP techniques,and especially presents the principle of the laser speckle and LSP.Chapter 3 clarifies the objective and scope of current work.Chapter 4,5,and 6 show the experimental parts by which the results were obtained.This thesis proposes a new method for laser welding process monitoring,this thesis combined the LSP method and laser-multi-pass-welding(laser-MPNG)technology.The principle of LSP detection for monitoring welding process has been proved.After experiments,the feasibility of this method is proved,LSP method is developed the new application.After improving algorithm of LSP,LSP method is applied from static measurement to dynamic measurement.The experiments prove the feasibility of the new algorithm when the old algorithm cannot detect the defect.For the current research stage,lots of experiments have been finished,experiment system has been adjusted and results have been analyzed.Thus,the LSP method can be used in welding process monitoring and it can be used to detect the lack of fusion defect and whole defect.