Yeong-Do Park

Pure SF6 and SF6-N2 mixture gas hydrates equilibrium and kinetic characteristics.

Sulfur hexafluoride (SF6), whether pure or mixed with inexpensive inert gas, has been widely used in a variety of industrial processes, but it is one of the most potent greenhouse gases. For this reason, it is necessary to separate and/or collect it from waste gas streams. In this study, we investigated the pure SF6 and SF6-N2 mixture gas hydrates formation equilibrium aswell asthe gas separation efficiency in the hydrate process. The equilibrium pressure of SF6-N2 mixture gas was higher than that of pure SF6 gas. Phase equilibrium data of SF6-N2 mixture gas was similar to SF6 rather than N2. The kinetics of SF6-N2 mixture gas was controlled by the amount of SF6 at the initial gas composition as well as N2 gas incorporation into the S-cage of structure-II hydrate preformed by the SF6 gas. Raman analysis confirmed the N2 gas incorporation into the S-cage of structure-II hydrate. The compositions in the hydrate phase were found to be 71, 79, 80, and 81% of SF6 when the feed gas compositions were 40, 65, 70, and 73% of SF6, respectively. The present study provides basic information for the separation and purification of SF6 from mixed SF6 gas containing inert gases.

Metallographic and fracture characteristics of resistance spot welded TWIP steels

Abstract In this study, the microstructural characterisation, mechanical testing and fractography investigation were performed on twinning induced plasticity (TWIP) steels, fabricated with resistance spot welding. Failure mode during the cross-tensile test was found to follow the sequences of strain localisation of both sheets, crack initiation at notch tip, crack following along the fusion boundary and, finally, ductile shear fracture along the sheet thickness direction. On the other hand, failure in the tensile shear test was always directed along the sheet/sheet (s/s) interface; the interfacial failure and shear deformation were observed at the weld centreline. Solidification occurred as a primary austenitic solidification mode, and no martensitic transformations were detected through electron backscatter diffraction analysis. The fusion zone was mainly composed of austenite with directional solidification towards the centreline; the columnar dendritic and equiaxed structures were identified. Interdendritic C and Mn cosegregation were confirmed by electron probe microanalysis.

A Hybrid Joining Technology for Aluminum/Zinc Coated Steels in Vehicles

Currently, in the automotive industry, joining of the aluminum alloys with the steel is a crucial problem to be solved. Conventional joining techniques including resistance spot and gas metal arc welding are not acceptable for those applications due to a number of metallurgical problems. The investigation was carried out to develop the hybrid joining process combining the resistance spot welding and brazing. In this study, an attempt was made to apply hybrid process to the joining of dissimilar sheet metals, Al-Mg-Si (6000 series) alloy and low carbon steel sheet. Hybrid process (resistance spot weld/brazing) using filler metal was found to be effective to overcome the incompatibility between aluminum alloy and steel. Although hybrid joining process of Al alloy sheet and steel sheet did not produce acceptable bond strength, it was proved to have reasonable interfacial bond layer if the optimal process condition was applied.

Porosity formation mechanisms in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steels

The porosity formation in cold metal transfer (CMT) gas metal arc welding (GMAW) of zinc coated steel is studied over a wide range of the heat inputs (160–250 J mm− 1), which shows low porosity in weld bead ( < 2% of bead area) in low ( < 250 J mm− 1) and high (>350 J mm− 1) heat inputs and maximum at medium (250–350 J mm− 1) heat inputs. The high speed imaging of weld pool shows that the highest frequency of zinc vapour escapes at high heat inputs compared to other conditions. Numerous experiments show that size and location of pores along with escaping of zinc vapour are the results of competition of viscosity of weld pool against buoyancy and vapour pressure within the time required to reach solidification temperature. Based on this concept, mechanisms involved in porosity formation, growth and escape phenomena are disclosed, which can help select the optimised welding conditions to obtain porosity free welds in CMT-GMAW of zinc coated steels.

Coating behaviour and nugget formation during resistance welding of hot forming steels

Abstract In this work, the effects of the coating composition and the welding parameters on the heat generation and nugget formation during resistance spot welding of the hot press forming steels have been evaluated. Two types of coated steels were used, termed as Al–Si and Zn coating. Al–Si coated steel showed rapid nugget growth toward the electrode direction, which is the probable reason of the higher heat conduction from the electrode–sheet surface toward the bulk material. In Zn coated steel, heat was generated and localised at the faying interface and uniformly propagated to the steel substrate. High speed camera images showed that the presence of oxide at faying interface provides inhomogeneous current flow and violent heat generation in the Zn coated steel.

Evaluation of Resistance Spot Weld Interfacial Fractures in Tensile-Shear Tests of TRIP 1180 Steels

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

Study on Coating Melting Behavior on Weld Growth Mechanism for Al-Si coated Hot-Stamped Boron Steels in Resistance Spot Welding

The effect of coating melting behavior on nugget growth during resistance spot welding of hotstamped boron steels was evaluated. A comparative analysis of the dynamic resistance for bare hot stamped and Al-Si coated hot stamped boron steels was carried out. The dynamic resistance curve of Al-Si coated hot stamped boron steel was higher than bare hot stamped boron steel during the whole stages of weld time. Higher α-peak and β-peak for Al-Si coated hot stamped boron steel resulted in larger nugget size at higher than 4.0 kA weld current owing to non-linear layer structure of alloyed coating, which appears to restrict the current passage through the faying interface. In the case of Al-Si coated hot-stamped boron steel, most of the Al-Fe intermetallic coatings are expected to be pushed away while in the liquid state at the faying interface. The pressure build up during the early stage of weld pushes the liquid Al-Fe intermetallic coating towards the edge of the faying interface. The continuous liquid state of Al-Fe intermetallic through the faying interface ensures the large contact area for the current passage to enlarge the nugget diameter. †(Received June 1, 2014)

Effect of Chemical Composition of Nut Material on the Fracture Behavior in Nut Projection Welding of Hot-Stamped Steel Sheet

Abstract The use of materials for modern lightweight auto-bodies is becoming more complex than hitherto assemblies.The high strength materials nowadays frequently used for more specific fields such as the front and rear sub frames, seat belts and seats are mounted to the assembled body structure using bolt joints. It is desirable to use nuts attached to the assembled sheets by projection welding to decrease the number of loose parts which improves the quality. In this study, nut projection welding was carried out between a nut of both boron steel and carbon steel and ultra-high strength hot-stamped steel sheets. Then, the joints were characterized by optical and scanning electron microscope. The mechanical properties of the joints were evaluated by microhardness measurements and pullout tests. An indigenously designed sample fixture set-up was used for the pull-out tests to induce a tensile load in the weld. The fractography analysis revealed the dominant interfacial fracture between boron steel nut weld which is related to the shrinkage cavity and small size fusion zone. A non-interfacial fracture was observed in carbon steel nut weld, the lower hardnessof HAZ caused the initiation of failure and allowed the pull-out failure which have higher in tensile strengthsand superior weldability. Hence, the fracture load and failure mode characteristics can be considered as an indication of the weldability of materials in nut projection welding.Key Words : Nut projection welding, Hot stamped steel, Carbon steel nut, Boron steel nut, Failure mode, Shrinkage cavityISSN 2466-2232Online ISSN 2466-2100

A study on tensile shear characteristics for weld-bonded 1.2GPa grade TRIP steels with changes in nugget diameter for automotive body application

High strength steels have been continually being developed to improve in fuel economy in automotive and ensure safety of passengers. New bonding and welding methods have been required for improving weldability on high strength steels. In this study, resistance spot welding and Weld-bond with nugget diameters of 4.0mm, 5.0mm, 6.0mm and 7.0mm were produced and tested, respectively. In order to confirm the effect of nugget diameters on tensile shear characteristic of the Weld-bond, tensile shear characteristics of Weld-bond were compared with those of resistance spot welding and adhesive bonding. Peak load of Weld-bond were increased as the nugget diameter increases. After appearing maximum peak load continuous fracture followed with second peak owing to load being carried by resistance spot weldment. Fracture modes of the adhesive layer in Weld-bond fractures were represented by mixed fracture mode, which are cohesive failure on adhesive part and button failure at resistance spot welds. The results showed that the tensile shear properties can be improved by applying Weld-bond on TRIP steel, and more apparent with nugget diameter higher than 5t.

Welding representation for training under VR environments

In this paper, we present a virtual training system which realistically represents the situation of real welding. First of all, we built a database about welding outputs such as the shape of bead which is the deposit outcome resulting from inputs of real welding conditions and operations. Second, we performed an analysis of relations between input variables and output variables, and the major and minor factors influencing on the welding shape were classified. Finally, we designed an estimation process of outputs by various inputs such as users movement, and constructed the method of graphical representation for real-time visualization from heuristic sources. Additionally, we also installed a welding simulator for a teacher and trainees, which can support not only a variety of welding situations but prompt evaluation of results and educational guide by optimal welding conditions.