Papers by Keyword: CO₂ Laser Welding

Abstract: Characteristics of CO₂ laser welded 1000 MPa grade Transformation Induced Plasticity steel plate were investigated under different welding power, welding speed and shield gas. Decreasing welding power, increasing welding speed or using mixed shield gas (Ar+He) all reduced the porosity in the weld metal (WM). The weld metal and HAZ near the WM had maximum hardness. In tensile strength test of load perpendicular to the weld axis, the specimens had same tensile properties as that of the raw material. For the load parallel with the weld axis, the specimens prepared with Ar had equal yield strength (YS) and tensile strength (TS) to that of the raw material. But the elongation was lower than that of the raw material. The specimens prepared with shield gas He had higher YS, TS and lower elongation compared with that of the raw material. The welded joint had lower formability than that of the raw material. The formability of specimens prepared with low welding power or mixed shield gas Ar+He was improved compared with that of the specimens prepared using high power or single shield gas Ar.

Abstract: The traditional method for post-weld quality inspection is visual testing by people. Major limitations of this method are the subjectivity of judgments and time required to perform the inspection. With the increasing use of laser welding in the automotive industry, the manufacturers are struggling with the challenge to automate the quality inspection process. This paper discussed the automatic post-weld quality inspection technology by computer vision sensing for mono-thickness tailored blanks of CO2 laser welding. A visual sensor was developed for acquiring the original image of the weld seam. The sensor consists of a PC based vision camera and a stripe-type laser diode. An image processing algorithm was presented to detect the geometrical defects of the weld seam. Some experiments are carried out and some applications are given.

Abstract: Reducing car’s weight has become an important goal for automobile industry. Laser welding of automotive high-strength steel plays a significant role in producing light weight cars. Experiments of CO2 high power laser welding of 1000MPa grade complex phase steels with a thickness of 3mm for automobile were performed using a 15 KW CO2 laser. The macrostructure and microstructure of the welded joint were examined by optical microscope. Mechanical properties of the welded joint, fusion zone and base metal were assessed by microhardness distribution across the welded joint, uniaxial tensile test and charpy V-notch impact test. Fractographs of the impact specimens were studied by scanning electron microscopy (SEM). It is found that the fusion zone has a higher toughness than that of the base metal and fusion line. The test results show good mechanical properties of laser welds that can meet the technical requirements for automobile Industry.

Abstract: This study examines for keyhole behavior by observing the laser-induced plasma and investigates the relation between keyhole behavior and formation of weld defect. Laser-induced plasma has been accompanied with the vaporizing pressure of zinc ejecting from keyhole to surface of primer coated plate. This dynamic behavior of plasma was very unstable and it was closely related to the unstable motion of keyhole during laser welding. As a result of observing the composition of porosity, much of Zn element was found from inner surface of it. But Zn was not found from the dimple structure fractured at the weld metal. therefore we can prove that the major cause of porosity is the vaporization of primer in lap position. Mechanism of porosity-formation is that the primer vaporized from the lap position accelerates dynamic behavior of the key hole and the bubble separated from the key hole is trapped in the solidification boundary and remains as porosity.

Abstract: Welding dissimilar materials become inevitable in engineering industries. There are many issues/problems associated with the welding of dissimilar materials, related to the welding process and its parameters. The current work investigates the effect of laser welding conditions on the toughness of dissimilar welded components. In this study, CO2 laser welding has been successfully applied for joining 316 stainless steel with low carbon steel (F/A). Design of experiment techniques has been used for different effective welding parameters (laser power, welding speed, and focus position) to optimize the dissimilar F/A joints in terms of its mechanical properties. Taguchi approach was applied to optimize the welding parameters. Three factors with five levels each (L-25) were employed in these models. Impact strength was measured at room temperature by using the universal pendulum impact tester. The results were compared with the impact strength of the base material. The results were analysed using ANOVA and S/N ratio for optimal parameters combination. It is evident that Taguchi approach has decreased the number of experiments without negative effects on the result.

Abstract: The application of laser welding technology has been considered to shipbuilding structure. However, when this technology is applied to primer-coated steel, good quality weld beads are not easily obtained. Because the primer-coated layer caused the spatter, humping bead and porosity which are main part of the welding defect attributed to the powerful vaporizing pressure of zinc. So we performed experiment with objectives of understanding spatter and porosity formation mechanism and producing sound weld beads in 6
t primer coated steels by a CO2 CW laser. The effects of welding parameters; defocused distance, welding speed, coated thickness and coated position; were investigated in the bead shape and penetration depth on bead and lap welding. Alternative idea was suggested to suspend the welding defect by giving a reasonable gap clearance for primer coated thickness. The zinc of primer has a boiling point that is lower than melting point of steel. Zinc vapor builds up at the interface between the two sheets and this tends to deteriorate the quality of the weld by ejecting weld material from lap position or leaving porosity. Significant effects of primer coated position were lap side rather than surface. Therefore introducing a small gap clearance in the lap position, the zinc vapor could escape through it and sound weld beads can be acquired. In conclusion, formation and suspension mechanism of the welding defects was suggested by controlling the factors.

Abstract: The effect of welding conditions on defects, microstructure, mechanical properties and formability of CO₂welded 800MPa grade TRIP steel was investigated. Bead-on-plate welding was carried out under the various welding speed in 6kW with a shield gas of Helium. Extensive porosity was observed in the joints produced at a 4m/min of welding speed but was not observed in the joints when a welding speed greater than 6m/min was used. The maximum hardness was obtained at the weld metal and the HAZ near the weld metal and was the same regardless of the welding speed. The HAZ hardness near the base metal was reduced due to retained ferrite but was higher than that of the base metal. In a parallel tensile test to weld axis, the strength of the joints was higher than that of the base metal. Elongation was found to be lower than that of the raw material because a crack was initiated in the bead at the strength levels corresponding to the tensile strength of the base metal and was propagated perpendicular to tensile direction. The formability of joints was found to be 63% that of the raw material.