• International Journal of Korean Welding Society
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• v.2 no.2
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• pp.26-31
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• 2002

Considering the fuel consumption of car, a light structure of aluminum alloys is desired fer car body nowadays. However, fusion welding of aluminum alloys has some problems of reduction of joint efficiency, porosity formation and hot cracking. In the present work, investigation to improve the joint performance of laser welded joint has been carried out by addition of Cu, Ni, and Zr to A6NO 1 alloy welds. Aluminum alloy plate of 2.Omm in thickness with filler metal bar was welded by twin beam Nd: YAG laser facility (total power: 5kW). The filler metals were prepared by changing the chemical compositions for adding the elements into the weld metal. Thirteen filler metal bars were prepared and pre-placed into the base metal before welding. Ar gas shielding with a flow rate of 10 1/min was used. The defocusing distance is kept at 0 mm. At travel speeds off 3 to 9 and at laser power of 5kW (front beam 2kW rear beam 3kW), full penetration welds were obtained, whereas at travel speeds of 12 to 18 m/min and same power, partial penetration was observed. The joint efficiency of laser-welded joint was improved by the addition of Cu, Ni, and Zr due to the solid solution hardening, grain refining and precipitation hardening. The type of hardening has been further considered by metallurgical examination.

• Journal of Korean Society of Steel Construction
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• v.28 no.5
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• pp.345-354
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• 2016

In the present study, hybrid moment connections of welding and bar reinforcement for composite beam-column connections were proposed. Concrete-filled octagonal tube and U-section were used for the column and beam, respectively. In the beam-column connection, the top flange and web of the beam U-section were connected to the column plate by welding. However, to reduce stress concentration at the weld joints, the bottom flange of the beam was not welded to the column plate. Instead, to transfer the tension force of the beam flange, reinforcing bars passing through the column plate were used. Four exterior connections with conventional welded and hybrid moment connections were tested under cyclic loading and their cyclic behaviors were investigated. The test results showed that the hybrid moment connections successfully transferred the beam moment to the column. The strength and ductility of the hybrid moment connections were comparable to the conventional welded moment connection with exterior diaphragm; however, the connection performance was significantly affected by the details of the hybrid moment connection.

• Laser Solutions
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• v.7 no.2
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• pp.1-10
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• 2004

This work was attempted to join SCP sheet and STS304 sheet by using Nd:YAC laser beam. SCP sheet has good formability and low cost, while STS304 has excellent corrosion resistance and mechanical properties in high temp. In this experiment, butt joint type was used to develop the tailored blank welding for dissimilar steel. Sheets which have different thermal properties. Computer simulation was conducted to obtain the off-set position for efficient welding by considering laser power, scanning speed, focal length and basic properties. The result showed that the optimum thermal distribution was obtained when the laser beam was irradiated at $0.05{\sim}0.1$ mm off-set toward the SCP sheet side. The experiment was conducted based on the result of computer simulation to show the same optimum conditions. Optimum conditions were 3KW in laser beam power, 6m/min in scanning speed, -0.5mm in focal position, 0.1mm off-set toward SCP. Microhardness test, tensile test, bulge test, optical microscopy, EDS, and XRD were performed to observe the microstructure around fusion zone and to evaluate the mechanical properties of optimum conditions, The weld zone had high microhardness values by the formation of the martensitic structure. Tensile test measured the strength of welded region by vertical to strain direction and the elongation of welded region by parallel to strain direction. Bulge test showed $52\%$ formability of the original materials. Bead shape, grain size, and martensitic structure were observed by the optical microscopy in the weld zone. Detailed results of EDS, XRD confirmed that the welded region was connected of martensitic structure.

• Transactions of Materials Processing
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• v.17 no.7
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• pp.523-527
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• 2008

In this study, mechanical properties of Alloy 718 welded after forgings for jet propulsion component was investigated. Hot-forged and machined work-pieces($230mm\times70mm\times15mm$) which have different grain sizes are welded by electron beam welding technique. After welding, the components were solution heat-treated and aged. Samples were sectioned to analyze the microstructural evolution and formation of micro-crack. It was found that HAZ grain boundary liquation crack generally initiates in the coarse grains rather than the fine grains. Needle-like phases with high Nb contents were found at the outer part near the base metal. Vickers hardness and tensile tests were carried out at room temperature and at $649^{\circ}C$. The tensile properties of electron beam welding specimens exhibited around 100MPa and 10% decrease in strength and elongation, respectively.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.20 no.3
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• pp.342-349
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• 2011

The purpose of this paper is to describe experimental results about the butt welding and bead on plate welding of the high power Continuous Wave (CW) Fiber laser for Ultra-low carbon Interstitial Free(IF) steel plate for gear part of car. After being welded of the gear parts by the fiber laser and electron beam Microstructures of melting zone had been mixed acicular, granular bainitic, quasi-polygonal and widmanstatten ferrite because of a radical thermal diffusion after welding, difference of critical volume and grain size. As a result of experiment, when gear parts were welded by the fiber laser and electron beam, the fiber laser welding has been stable properties without internal defects more than the electron beam welding. Therefore it has the very advantages of welding high quality and productivity more than conventional melting method. The optimal welding processing parameters for gear parts were as follows : the laser power and welding speed were 3kWatt, 30mm/sec respectively. At this time heat input was $21.2{\times}10^3J/cm^2$.

• Journal of Korean Society of Steel Construction
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• v.28 no.2
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• pp.75-83
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• 2016

Existing tubes for concrete filled tubular structure are made through welding of four plates irrespective, but the production performance is poor and special welding technique is needed to weld the internal or through diaphragm. Accordingly, We developed a welded built-up square steel tube having a welding lines and a stiffeners at location out of stress concentration. The welded built-up square steel tube occurred a interference with stiffeners at the internal or through diaphragm, therefore researches of a external diaphragm for welded built-up square CFT column connections are needed for the purpose of avoidance of a interfere with stiffeners. In this study we suggest a design formulation for external diaphragm of the welded built-up square CFT external diaphragm connections. Four specimens were manufactured for a experimental test, then we analyzed the behaviors of the specimens.

• Proceedings of the KSR Conference
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• 2001.10a
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• pp.235-239
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• 2001

The bogie of high speed freight car running on conventional line is welded bogie modeled on Y25 bogie developed in Europe. Y25 bogie has speed limit of 110km/h. But it has limiting factors to speed-up as increasing maintenance cost and friction parts pedestal. And also it was reported that cracks are found in the parts where center beam meet bolster and endbeam near bracket for braking part. This study includes stress analysis improved structure of welded bogie for strength evaluation by numerical method and experimental method. According to the study, new configuration bogie shows improvement in strength.

• Laser Solutions
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• v.1 no.1
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• pp.4-10
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• 1998

A basic research for tailored blank welding between mild steel and Zn-coated steel was carried out with $CO_2$ laser beam. The materials used in this work were low carbon steel sheet with a thickness of 1.2mm and Zn-coated steel sheet with the same thickness and 6.3$\mu$m Zn coating. Experiments were carried out by applying the Taguchi method in order to obtain optimized conditions for the application of tailored blank laser welding method in practical manufacturing process. Optical microscopy, XRD, SEM and TEM analysis were performed to observe microstructures and to determine the solidification mode of welded zone. Also mechanical properties were measured by microhardness test tensile test and Erichsen test in order to evaluate the formability of welded specimen. There was no trapped Zn in the fusion zone, and the phases in this region consisted of polygonal ferrite, quasi-polygonal ferrite, banitic ferrite and martensite. The elongation value of welded specimen was more than 80% of that value in the substrate and LDH value was more than 90% of that value in the substrate metal.

• Journal of the Society of Naval Architects of Korea
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• v.45 no.6
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• pp.669-678
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• 2008

The fatigue life of ship structure under cyclic loading condition is made up of initiation and propagation stages. In this study, crack growth test is carried out on large scale structure test specimens and fracture mechanical analysis is performed. The fatigue lives measured from fatigue tests are compared with DNV, Matsuoka and BS 5400 S-N curve. And to predict the crack initiation life, S-N curve, corresponding to crack length 20mm at welded joint, is developed based on hot spot stress range. Also crack propagation life is calculated using crack growth equation. Consequently, computed crack propagation life is compared with experiment results.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1996.04a
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• pp.176-180
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• 1996

Continuous wave CO$_{2}$laser beam welding and formability of zinc coated steel shets were investigated. First, optimal welding condition could be obtained in but welding by using the data for heat input and welding velocity. The highest value of Erichsen test is 79.3% compare to that of base matel. Secondly, Formability of laser welds was investigated by using ball punch tester. Finally, the forming results of butt-welded sheets showed that the joing design was important to apply the laser welded blank in the automotive production.