Investigation Of Laser-induced Keyhole Behavior And Its Effects On Weld Formation In Hybrid High-power Laser Welding Process

With the development of the mass-produced industry such as automobile industry,household product,and shipbuilding industry,the manufacturing technology are facing great challenges on its applications.The high-power laser welding technology comes as the preference because of its high manufacturing rate,deep penetration,small heat affected zone,and high ratio of depth to width.During the regular Gaussian laser welding,the welding status become unstable due to the intense interaction of laser-material.This unstable status can directly cause the weld defects in the welding.The development and application of the laser welding technology raise an urgent issue to engineer and researcher,which is how to maintain a stable keyhole and prevent the weld defects.Aiming to deal with the disadvantages of the current high-power Gaussian laser welding technology,this thesis focus on the effects of hybrid laser technology on welding performance and its application in industrial manufacturing.Based on the interaction mechanism of laser and MAG welding in the hybrid laser-MAG welding process,this thesis proposed a novel hybrid laser-based welding technology entitled Adjustable-ring Mode laser(ARM laser)and investigated its performance in welding process.Additionally,this work aims at exploring a novel method for stabilizing status in high-power laser-based welding process.The novel hybrid laser technology consists of two coaxial laser beams.A Gaussian laser beam located at the center part is surround by a ring shape laser beam.Therefore,the novel adjustable-ring mode laser is of deep penetration from the Gaussian laser and is also with the stable molten pool from the ring laser.These characteristics enable the advantages of the novel laser in the application of deep penetration with good welding quality.This thesis summarizes the current hybrid laser-based welding technologies and investigates the traditional hybrid laser-MAG welding technology.Next,this thesis mainly focusses on the novel hybrid laser-based welding technology entitled adjustable-ring mode laser technology.Based on the interaction theory of laser-MAG in the hybrid laser-MAG welding process,Chapter Two first discusses the design a double-camera sensing system for monitoring the hybrid laser-MAG welding.The effects of MAG heat source on the status of laser-induced keyhole is then investigated,and a novel evaluation method for evaluating dynamic status in hybrid laser-MAG welding process is proposed.Aiming to deal with the effects of auxiliary heat source on dynamic status in hybrid laser-based welding process,Chapter Three proposes a novel adjustable-ring laser and investigates the welding performance under different optical characteristics.Chapter Four investigates the cross-section characteristics,keyhole status,plume intensity and their physical relationship.Chapter Five investigates the mechanism effects of the novel adjustable-ring mode laser on the dynamic status in welding process,and analyzes the mechanism of the novel laser on stabilizing welding status and preventing the weld defects.Chapter Six investigates the stability effects of the auxiliary heat source on Gaussian laser-induced keyhole in the novel adjustable-ring mode laser welding of aluminum alloy,and analyzes the weld quality and energy-absorbed efficiency.(1)Firstly,the top and bottom status of traditional hybrid laser-MAG is obtained based on the double-camera sensing system.Next,the dynamic characteristics is synchronously analyzed based on the top and bottom visual status.The arc light of droplet transition,the area of metallic vapor,and the status of keyhole bottom are quantified with the image processing method.Experimental results showed that the droplet transition of the auxiliary MAG welding directly affected the laser-induced keyhole.The more times of droplet transition,the more times of keyhole collapse.The longer the arc is,the more time the keyhole recover costs.The droplet transition of hybrid laser-MAG welding is investigated by combining the spatial-frequency analysis technology.(2)A novel hybrid laser multi heat source technology entitled Adjustable-ring mode laser is proposed for the application in welding.The welding performances under different laser power,laser focal position,and welding speed are investigated by applying visual sensing and spectrometer sensing technologies.In order to analyze the effects of different laser source on keyhole and molten pool in the novel adjustable-ring laser welding,the keyhole area and the width of molten pool are quantified with image processing method.The optical characteristics of the adjustable ring mode laser under different focal positions are investigated,so are its effects on penetration and width and the intensity of metallic vapor.For preliminarily analyzing the stability of laser-induced keyhole and its relationship with intensity of metallic vapor under different flow rate of molten metal in adjustable-ring mode laser welding,the geometric features of keyhole and the intensity of metallic vapor are quantified by visual sensor and spectrometer sensor,respectively.This work can be an effective proof for quantifying the dynamic characteristics of the novel adjustable-ring mode laser welding.(3)Based on the visual sensing technology and the preliminary theory of the ARM laser welding,the dynamic characteristics of the ARM laser welding can be quantified.The mechanism effects of the laser heat sources on laser-induced keyhole and molten pool are investigated,and the effect of dynamic characteristic on weld quality is analyzed.Due to the instability of regular Gaussian laser welding,the laser-induced keyhole presents a periodical opening and collapsed status.This instability further contributes to the irregular flow of molten metal in the molten pool,and causes the generation of weld defects such as spatter and porosity.Therefore,the keyhole collapse becomes the indicator for evaluating the dynamic status of the ARM laser welding.Based on the dynamic characteristic and process mechanism of the ARM laser welding,the time percentages of keyhole collapsed and opening are quantified with image processing method and statistical theory.Next,the process stability is obtained by evaluating the time percentages in the ARM laser welding process.In order to evaluate the stabilized mechanism of the ARM laser welding of aluminum alloy,the times of keyhole collapsed is calculated for evaluating the keyhole status.The relationship between the keyhole status and weld quality is analyzed in the ARM laser welding.(4)The experimental results showed that the novel hybrid laser-based welding entitled Adjustable-ring mode laser welding technology has stabilized the keyhole and molten pool by maintaining an opening keyhole.Additionaly,this technology has significantly reduced spatter and highly improved the weld quality.This thesis provided a reliable proof for improving the welding performance and weld quality by using the novel Adjustable-ring mode laser.