Hee-Seon Bang

Measurement and numerical simulation of particle velocity in cold spraying

The velocity of cold spray particles was measured by a diagnostic system designed for thermal spray particles that is based on thermal radiation. A laser beam was used to illuminate the cold spray particles in cold spraying to obtain a sufficient radiant energy intensity for detection. The measurement was carried out for copper particles of different mean particle sizes. The particle velocity was also estimated using a two-dimensional axisymmetric model developed previously. The simulated velocity agreed well with the measured result. This fact indicates that particle velocity in cold spraying can be predicted reasonably by simulation. Therefore, it is possible to optimize the cold spray process with the aid of the simulation results.

A Study on the Weldability and Mechanical Characteristics of Dissimilar Materials Butt Joints by Laser Assisted Friction Stir Welding

* 조선대학교 공과대학 선박해양공학과 ** 조선대학교 대학원 선박해양공학과 *** 한국생산기술연구원 동력부품센터 **** 조선이공대학 선박해양·기계과 Abstract This study intends to investigate the weldability and mechanical characteristics of butt weld joints by LAFSW for dissimilar materials (Al6061-T6 and SS400). At optimum welding conditions, the tensile strength of dissimilar materials joints made by FSW is found to be lower than that of LAFSW. Due to the increase in plastic flow and formation of finer recrystallized grains at the TMAZ and SZ by laser preheating in LAFSW, the hardness in LAFSW appeared to be higher than that of FSW. Compared with FSW, finer grain size is observed and elongated grains in parent metal are deformed in the same direction around the nugget zone in TMAZ of Al6061-T6 by LAFSW. Whereas, at weld nugget zone, coarse grain size is appeared in LAFSW compared to FSW, which is owing to more plastic flow due to laser preheating effect. In dissimilar materials joints by LAFSW, ductile mode of fracture is found to occur at Al6061 side with fewer brittle particles. Mixed mode of cleavage area and ductile fracture is observed at SS400 side.

Mechanical characteristics of resistance multispot welded joints

Abstract In the present study, three-dimensional thermal elastoplastic analysis has been carried out in order to clarify the mechanical phenomena of the thermal elastoplastic behaviour of multispot welded joints. As the shape of multispot welded joints is not axisymmetrical, unlike single spot welded joints, the solution domain for simulation should be three-dimensional. Therefore, the present study first developed three-dimensional heat conduction and thermal elastoplastic programs using the isoparametric finite element method. Second, from the results analysed by the developed programs, thermal and mechanical characteristics and their production mechanisms on single and multispot welded joints were clarified. Moreover, effects of pitch length on the temperature distribution, welding residual stresses and plastic strain of multispot welded joints were evaluated, indicating that a pitch of 30 mm was more advantageous compared to a pitch of 15 mm.

Application of Taguchi Approach to Optimize Laser-Arc Hybrid Welding Parameters of Galvanized Steel

This study aims to establish the weld quality of the Nd:YAG laser-MIG hybrid welding on galvanized steel (SGACC60). In order to do so, the Taguchi analysis optimized the laser-arc hybrid welding process for SGACC60 by varying four parameters: welding speed, welding current, laser-arc distance and shielding gas. The experiment produced 18 combinations of these four essential welding parameters, which were estimated in terms of ultimate tensile strength, welding depth and width ratio, and hardness. The optimum result exhibited that the tensile strength of welds was approximately 101% of that of the base metal, and that their hardness was within the acceptable range (the maximum value 350 Hv).

Mechanical and microstructural characteristics of the dissimilar materials butt joints by hybrid CO2 laser-gas metal arc welding

This paper intends to present the applicability of hybrid [CO2 laser+gas metal arc (GMA)] welding process to dissimilar materials AH32 and STS304L by clarifying the mechanical and microstructural characteristics of hybrid welded joints. In order to observe the mechanical phenomena on dissimilar material hybrid welded butt joints, two-dimensional thermal elastoplastic numerical analysis has been carried out. For this, the quasistate heat source model for simulation has been determined considering the hybrid welding characteristics. Based on the thermal history obtained by using this heat source model, the residual stress distribution in welded joints is predicted and found to be in reasonable agreement with the experimentally measured values. The result shows that maximum welding residual stress in longitudinal direction (σzz) in dissimilar material (AH32-STS304L) welded joints has occurred in the STS304L side by about 5% than in the AH32 side of welded joints. However, the equivalent stress of dissimilar ...

The Comparison of the Thermal and Mechanical Characteristic in Butt Joint for Ship Structure Thick Plate AH32 Steel by SAW & Hybrid(CO 2 Laser+MIG)Welding

In this paper concentrate on the comparison of the thermal and mechanical characteristics in Butt joint of ship structure AH32 steel by using hybrid welding and conventional SAW. For this purpose, fundamental welding phenomena of hybrid process using Laser and MIG is investigated by the experiments and characteristics of thermal and welding residual stress distribution of welded joint in SAW and hybrid welding are understood from the result of FE numerical simulation and experimental values. From the result of this study, it is understood that Laser-MIG hybrid welding have high potential, make substantial saving of time and manufacturing cost and may proves its self robust in the butt joining of thick AH32 steel ship structural plate in the near future. 缀Ѐ㘰〻ሀ䝥湥牡氠瑥捨湯汯杹

Effect of Tungsten-Inert-Gas Preheating on Mechanical and Microstructural Properties of Friction Stir Welded Dissimilar Al Alloy and Mild Steel

The purpose of this study is to clarify the effect of tungsten-inert-gas (TIG) welding preheating on the mechanical properties of Al6061-T6 and SS400 welded joints by friction stir welding (FSW). FSW joints with and without TIG welding preheating are characterized and compared in terms of their mechanical and microstructural properties. The results show that the TIG assisted hybrid FSW welded joints (TIG-HFSW) provide an enhanced joint strength. The transversal tensile strength of the TIG-HFSW joints exhibited approximately 104% of the Al6061-T6 base metal tensile strength and was higher than that of the FSW joints. Microstructural investigations also reveal that in the HAZ and TMAZ of TIG-HFSW joints, the grains of Al6061-T6 are smaller than those of the FSW welds.

Narrow gap laser welding by multilayer hot wire addition

Welding of thick section butt joints experiences limitations for different techniques. One option is to fill a narrow gap layer by layer with laser melted wire, a laser metal deposition technique where the complexity of a keyhole is avoided. The presented results show that wire addition can enable relatively thick layers. In particular, when electrically preheating the wire the process becomes more energy-efficient and favorable wetting conditions might be achieved. Since the wire was preheated by an electric current conducted through the wire to the workpiece, high speed imaging has shown that the wire tip can occasionally ignite small electric arcs. The wire deposited in the narrow gap also shows a fluctuating but self-stabilizing movement of the tip. Imperfections that have to be avoided are hot cracks, cavities, lack of fusion, and an irregular final weld surface topology. The technique shows high potential.

Redistributions of Welding Residual Stress for CTOD Specimen by Local Compression

주성민 * ·윤병현 * ·장웅성 * ·방한서 ** ·방희선 **,† ·노찬승 *** * 포항산업과학연구원 용접센터 ** 조선대학교 선박해양공학과 *** 조선이공대학 선박해양·기계공학과 Abstract When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

Effects of processing parameters on the friction stir spot joining of Al5083-O aluminum alloy to DP590 steel

Two dissimilar materials, aluminum alloy Al5083-O and advanced high strength steel DP590, were successfully joined by using friction stir spot joining (FSSJ). Satisfactory joint strengths were obtained at a rotational speed of 300 rpm and a plunge depth of 0.7 mm. Resulting joints were welded without a non-welded zone. This may be attributed to the enhanced smooth material flow owing to sufficient stirring effect and tool down force between the upper Al5083-O side and the lower DP590 side. The maximum tensile shear strength was 6.5 kN, which was higher than the joint strength required by the conventional method of resistance spot welding. The main fracture mode was plug fracture in the tensile shear test of joints. An intermetallic compound (IMC) layer with <6 μm thickness was formed at the joint interface, which meets the allowance value of <10 μm for the dissimilar material Al-Fe joints. Thus, the use of FSSJ to weld the dissimilar materials Al5083-O and DP590 resulted in mechanically and metallurgically sound joints.