Papers by Keyword: ND: YAG Laser Welding

Abstract: 21% Cr with Ti-Nb dual stabilized ferritic stainless was welded using Nd: YAG laser. The relationship between microstructure and parameters of laser welding was examined. The microstructure was investigated by using the optical microscope and scanning electron microscopes. The average grain size of the HAZ was increased with increasing heat input due to the slow cooling rate. Large precipitates as TiN, TiC and Nb(C,N) were dissolved in the HAZ. Fine precipitates which supposed to be TiC was formed uniformly distributed in the case of the fusion zones.

Abstract: In the present study, the characteristics of Nd:YAG laser welded joints of 600 MPa DP steel (HDT580X), 2.4 mm in thickness, in respect of hardness, microstructures and mechanical properties were investigated. The test joints have been welded under a shielding gas on the stand for robotic Nd:YAG laser welding at the beam power of 1.5 kW and 2.0 kW. Three combinations of welding parameters were used: 2.0 kW - 2.1 m/min, 2.0 kW - 1.5 m/min and 1.5 kW - 1.2 m/min. Detailed examinations were performed on the joint welded at the highest speed. The microstructure was examined by the optical-, scanning- and transmission electron microscope. The heat affected zone (HAZ) was composed of ferrite, bainite and lath martensite, the weld contained lath martensite. The maximum hardness in the HAZ did not exceed 343 HV. The tensile strength of the welded joint was at the same level as that of the base material. The results of fatigue tests and residual stress measurement of laser welded DP steel joints are also presented. The fatigue strength of the welded joint is lower than that of the base material. The fatigue class FAT was determined, which is equal to 284 MPa - for the base material and 150 MPa – for the welded joint. By means of the modified hole drilling method the following residual stresses were measured: σmax = 573 MPa and σmin= -126 MPa.

Abstract: Cracks induced by metal fatigue and structural aging effect can’t be fixed by traditional Gas Tungsten Arc Welding (GTAW), consequently cause the increasing of defect ratio. Although some cracks might be acceptable and qualified to field service standard subject to the military regulation, however, in certain areas, e.g. brazing and coating areas, the results are not ideal especially the HAZ and residual stress. In this study, Nd-YAG Laser welding and traditional GTAW processes were performed on Hastelloy X superalloy for comparison. Post-welding residual stress distribution was measured by X-ray diffraction method. Macro- and microstructure were observed by metallurgical OM and SEM in comparison to the hardness testing. Tensile test results show that traditional welding technology has better ultimate tensile strength and ductility. For the Nd:YAG laser welding, residual stress is limited to 3mm of the both sides of weld and drops drastically, while higher amplitude and widely spread in the GTAW welding. It is proposed that combining both methods, the repairing process can be optimized to reduce the defect ratio and save repairing time.

Abstract: The optimized condition has been researched, for welding super alloys (Inconel 600, Inconel 625 and Haynes 230), which are used for liquid thruster of an artificial satellite. In the experiment of “bead-on-plate”, these materials are welded using continuous Nd:YAG laser and electron beam. The penetration depth and bead width are measured by optical microscope. The hardness of 1mm depth from welding surface is measured by micro Vickers hardness tester. The tension is tested for measuring the strength of welding part. The tension strength of welding specimen using the Nd:YAG laser is compared with that of the welding specimen using the electron beam.

Abstract: Based on the bead on plate test and the effects of heat input on weld penetration, the equivalence between 2 kW CW (Continuous Wave) Nd:YAG laser power and pulsed GMA(Gas Metal Arc) power and the effect of welding speed on their equivalent ratios in welding carbon steel and Al alloy were studied. The studied results show that 9.8 kW pulsed GMA power is needed to reach the same weld penetration depth with 2 kW CW Nd:YAG laser power for welding of low carbon steel at v=1.2 m/min; for Al alloy, it is 3.92 kW pulsed GMA power. The equivalent ratios of 2 kW Nd:YAG laser power and pulsed GMA power are 4.9~6.4 and 1.85~2.1 times at different welding speed for low carbon steel and Al alloy, respectively. But their equivalent ratios have different variations with welding speed; at which the equivalent ratio of low carbon steel increases with welding speed and that of Al alloy decreases. The studied results offer the experimental boundary of heat input for analyzing the effect of Nd:YAG laser power and pulsed GMA power on laser+pulsed GMA hybrid welding process.

Abstract: The characteristics of Nd:YAG laser welded 600MPa grade TRIP(transformation induced plasticity) and DP(dual phase) steels with respect to hardness, microstructures, mechanical properties and formability was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. In the case of TRIP steel, the hardness was the highest at the fusion zone(FZ) and HAZ near the FZ and decreased as the base metal was approached. The maximum hardness increased with increasing welding speed to 3.6m/min and then remained. The microstructures of FZ and HAZ near the FZ consisted of ferrite and bainite for all welding speeds. In the case of DP steel, the maximum hardness was obtained at the HAZ near the FZ. It increased rapidly to 2.1m/min and then showed nearly the same value. The difference between the maximum hardness of HAZ and that of FZ increased with decreasing welding speed. The microstructure of FZ was composed of acicular ferrite but the HAZ near the FZ contained bainite and ferrite at a low welding speed. Both steels had similar tensile properties and formabilities. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the strengths were somewhat higher than those of raw metals. In a parallel tensile test, the strengths of the joints were higher than those of raw materials but the elongations were lower. Formability was determined to be approximately 80% as compared with raw material at the optimum conditions. Differences in hardness near the welded zone was dependant on the relative contents of ferrite.

Abstract: Laser-welded parts experience high local temperatures and severe heating-cooling cycles which lead to large local residual stresses. These stresses introduce unacceptable degradation of the mechanical properties of a weldment. Thermo-elasto plastic analyses with 3-D FE models, as well as experimental investigations were performed in order to predict temperature distribution and residual stresses of ND-YAG laser-welded joints with various gap widths between the dissimilar steel types of austenitic and precipitation-hardening stainless steel. The specimens have the shape of a pocket to optimize the weight of the structure, which consists of a thin skin (AISI304) and a thick skeleton (AISI630). The residual stresses at the surface of the weldments were measured using the instrumented indentation method. The residual stresses and melt-pool zone (MPZ) profiles show good agreement between the theoretical and experimental results. Considering the residual stresses, the allowable gap width range of the laser-welded joints for the pocket-shaped specimen was calculated. For a welding joint with gap widths, the longitudinal residual stress values at the yield stress level were observed. Melt-pool zone profiles described by the underfill and penetration depth also depend upon the gap size.