• Journal of the Korean institute of surface engineering
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• v.45 no.5
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• pp.181-187
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• 2012

The Friction Stir Welding (FSW) has mainly been used for making butt joints in Al alloys. The development of Friction Stir Lap Welding (FSLW) would expand the number of applications. In this study, microstructures and mechanical properties of FSLW in A5052 alloy were investigated under varying rotating speed and probe length. Investigating the characteristics as FSLWed conditions were as below ; Failure Maximum load by shear fracture was increased proportional to the width of joint area, which was increased by input heat, stirring intensity in the case of 2.3 mm probe length. Tensile fracture occurred, and maximum load was determined due to side worm hole of joint area and softening of microstructure in the case of 3.0 mm probe length. In the case of 3.7 mm probe length, material hook and bottom worm hole were appeared at the end interface of joint area. The most sound FSLW condition with no defects was 3.0 mm probe length and 1500 rpm-100 mm/min. No defects were showed in 1500 rpm-100 mm/min and 1800 rpm-100 mm/min, but Vickers microhardness distribution in TMAZ/HAZ which was fracture zone was lower in 1800 rpm-100 mm/min than in 1500 rpm-100 mm/min. In this condition highest tensile strength, 215 MPa (allowable rate 78% of joint efficient) was obtained.

• Journal of Convergence for Information Technology
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• v.9 no.7
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• pp.94-99
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• 2019

Friction stir welding was first reported by TWI(The Welding Institute) in 1991, and this welding method has been rapidly used in various industrial areas such railway, automobile, aerospace and shipbuilding industry. Here, we study core characteristics of friction stir welding (FSW) applied to Al 6005-T6 extruded sheets, which is the typical alloy used for railway car bodies. With the fixed welding speed of 500 mm/min, the rotating tool speed was varied from 600 to 1800 RPM. The results of hardness measurement revealed that the hardness of nugget area is ~70% with respect to the parent material, and for the selected range of rotation speed, no clear dependence was observed and the hardness values close to the parent materials were achieved for the area located 5 mm away from the welding interface. The tension test shows that yield strength and tensile strength were slightly decreased with increasing RPM, with no observed difference for the elongation.

• Korean Journal of Metals and Materials
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• v.49 no.4
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• pp.321-328
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• 2011

The 30 nm-thick Ni layers was deposited on a flexible polyimide substrate with an e-beam evaporation. Subsequently, we deposited a Si layer using a catalytic CVD (Cat-CVD) in a hydride amorphous silicon (${\alpha}$-Si:H) process of $T_{s}=180^{\circ}C$ with varying thicknesses of 55, 75, 145, and 220 nm. The sheet resistance, phase, degree of the crystallization, microstructure, composition, and surface roughness were measured by a four-point probe, HRXRD, micro-Raman spectroscopy, FE-SEM, TEM, AES, and SPM. We confirmed that our newly proposed Cat-CVD process simultaneously formed both NiSi and crystallized Si without additional annealing. The NiSi showed low sheet resistance of < $13{\Omega}$□, while carbon (C) diffused from the substrate led the resistance fluctuation with silicon deposition thickness. HRXRD and micro-Raman analysis also supported the existence of NiSi and crystallized (>66%) Si layers. TEM analysis showed uniform NiSi and silicon layers, and the thickness of the NiSi increased as Si deposition time increased. Based on the AES depth profiling, we confirmed that the carbon from the polyimide substrate diffused into the NiSi and Si layers during the Cat-CVD, which caused a pile-up of C at the interface. This carbon diffusion might lessen NiSi formation and increase the resistance of the NiSi.

• Journal of the Korean institute of surface engineering
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• v.54 no.3
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• pp.102-111
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• 2021

The corrosion behaviors of laser-welded super duplex stainless steel tubes with post-weld heat treatment(PWHT) conditions(950, 1000, 1050, 1100 ℃ for 5 and 30 min) were evaluated by electrochemical potentiodynamic polarization and critical pitting temperature measurements. This study showed that the critical metallurgical factors affecting the degradation of corrosion resistance of a steel tube in as-welded condition were the unbalanced phase fraction(ferrite:austenite = 94:4), Cr₂N precipitation, and phase transformation from the austenite phase to ɛ-martensite(via stress-induced phase transformation). The improvement in the corrosion resistance of the welded specimen depends greatly on the PWHT conditions. The specimens after PWHT conducted below 1000 ℃ showed inferior corrosion resistance, caused by precipitation of the sigma phase enriched with Cr and Mo. At 1100 ℃ for a longer duration in PWHT, the ferrite phase grows, and its fraction increases, leading to an unbalanced phase fraction in the microstructure. As a result, pitting can be initiated primarily at the interface between the ferrite/austenite phase, particularly in base metal.

• Corrosion Science and Technology
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• v.20 no.6
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• pp.426-434
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• 2021

The effect of two different annealing temperatures on the level of the second phase precipitated in the microstructure and the corrosion behaviors of super austenitic stainless steel were examined. The sample annealed at a higher temperature had a significantly lower fraction of the sigma phase enriched with Cr and Mo elements, showing more stable passivity behavior during the potentiodynamic polarization measurement. However, after the welding process with Inconel-type welding material, severe corrosion damage along the interface between the base metal and the weld metal was observed regardless of the annealing temperature. This was closely associated with the precipitation of the fine sigma phase with a high Mo concentration in the unmixed zone (UMZ) during the welding process, leading to the local depletion of Mo concentrations around the sigma phase. On the other hand, the fraction of the newly precipitated fine sigma phase in the UMZ was greatly reduced by post-weld heat treatment (PWHT), and the corrosion resistance was greatly improved. Based on the results, it is proposed that the alloy composition of welding materials and PWHT conditions should be further optimized to ensure the superior corrosion resistance of welded super austenitic stainless steel.

• Progress in Superconductivity and Cryogenics
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• v.8 no.1
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• pp.9-14
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• 2006

Sample preparation plays a critical role in microstructure analysis using TEM. Although TEM specimen has been usually prepared by jet-polishing or Ar-ion beam milling technique. these methods could not be applied to YBCO CC which is composed of IBAD or RABiTS substrates, several buffet layers, and YBCO superconducting layer because of big difference in mechanical strengths between the metallic phase and oxide phases. To obtain useful cross-sectional information such as interface between the phases or second phases in YBCO CC, it is prerequisite to secure the large area of thin section in the cross-sectional direction. The superconducting layer or the buffer layers are relatively weak and fragile compared to the metallic substrate such as Ni-5wt%W RABiTS of Hastelloy-based IBAD, and preferential removal of weak ceramic phases during polishing steps makes specimen preparation almost impossible. Tripod polisher and small jig were home-made and employed to sample preparation. The polishing angle was maintained <$1^{\circ}$ throughout the polishing steps using 2 micrometers attached to the tripod plate. TEM specimens with large and thin area could be secured and used for RABiTS/IBAD substrate analyses. In some cases, additional Ar-beam ion milling with low beam current and impinging angle was used for less than 30 sec. to remove debris or polishing media attacked to the specimens.

• International Journal of Korean Welding Society
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• v.2 no.1
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• pp.21-24
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• 2002

The main purpose of this study was to investigate the formation mechanisms of imperfections such as irregular humps, outer cavity and inner cavity in the laser fusion zone of diamond saw blade. Laser beam welding was conducted to join two parts of blade; mild steel shank and Fe-Co-Ni sintered tip. The variables were beam power and travel speed. The microstructure and elements distributions of specimens were analyzed with SEM, AES, EPMA and so on. It was found that these imperfections were responded to heat input. Irregular humps were reduced in 10.4∼l7.6kJ/m heat input range. However there were no clear evidences, which could explain the relations between humps formation and heat input. The number of outer cavity and inner cavity decreased as heat input was increased. Considering both possible defects formations mechanisms, it could be thought that outer cavity was caused by insufficient refill of keyhole, which was from rapid solidification of molten metal and fast molten metal flow to the rear keyhole wall at low heat input. More inner cavities were found near the interface of the fusion zone and sintered segment and in the bottom of the fusion zone. Inner cavity was mainly formed in the upper fusion zone at high heat input whereas was in the bottom at low heat input. Inner cavity was from trapping of coarsened preexist pores in the sintered tip and metal vapor due to rapid solidification of molten metal before the bubbles escaped.

• Composites Research
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• v.19 no.1
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• pp.36-42
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• 2006

MWNT(multi-walled carbon nanotube)-filled plain-weave glass/epoxy composites were fabricated and electromagnetic characteristics of the composites were investigated. The observation of the microstructures of the composites revealed that MWNTs are mostly distributed in matrix rich region and the interface between glass fiber yarns in warp and fill direction. The permittivity of the composites, measured in X-band (8.2-12.4 GHz) frequency range, increased with weight fraction of MWNTs and remained almost constant with frequency. The measured permittivity was used to investigate the reflection loss of radar absorbing structures (RAS) composed of MWNT-filled composites according to thickness and a RAS were constructed with 10 dB absorbing bandwidth 4.2 GHz and 3.3 mm in thickness.

• Korean Journal of Materials Research
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• v.12 no.10
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• pp.814-819
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• 2002

TiN and TaN films as electrode materials of reactive sputtered $Ta_2$$O_{5}$ were prepared by sputtering to compare their thermal stabilities with $Ta_2$$O_{5}$ The microstructural change of $Ta_2$$O_{5}$ films with annealing was also investigated. As- deposited $Ta_2$$O_{5}$ film on $SiO_2$ was amorphous and annealing of 80$0^{\circ}C$ for 30 min made it transform to $\beta$-Ta$_2$O$_{5}$ crystalline which contains amorphous particles with the size of a few nm. Crystallization temperature of Ta$_2$Ta_2$$O_{5}$ on TaN is higher than that on TiN electrode. The interface between TaN and Ta$_2$O$_{5}$ maintained stably even after vacuum annealing up to $800^{\circ}C$ for 1 hr, but TiN interacted with $Ta_2$$_O{5}$ and so interdiffusion between TiN and $Ta_2$$O_{5}$ occurred by vacuum annealing of 80$0^{\circ}C$ for 1 hr. It indicates that TaN is thermally more stable with $Ta_2$$O_{5}$ than TiN.N.

• Journal of the Korean Ceramic Society
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• v.39 no.10
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• pp.955-962
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• 2002

The microstructure change of brazed $Si_3N_4$/Stainless steel 316 joint with Cu buffer layer were examined to clarify the effects of brazing process conditions such as brazing time and temperature on the mechanical properties and reliability of brazed joints. For the brazed joint above 900${\circ}C$, the Cu buffer layer was completely dissolved into brazing alloy and the thickness of reaction product formed at $Si_3N_4$/brazing alloy joint interface was abruptly increased, which could increase the amounts of residual stress developed in the joint. The fracture strength of brazed $Si_3N_4$/Stainless steel 316 joint with Cu buffer layer at 950${\circ}C$ was much reduced comparing to those of joints brazed at the lower temperature. But, it was found that the effects of brazing time was not critical on the mechanical properties as well as the reliability of $Si_3N_4$/Stainless steel 316 joint with Cu buffer layer brazed at the temperature below 900${\circ}C$.