Advanced Materials Research Vols. 26-28

Abstract: Grain refinement of magnesium alloy by Friction Stir Processing (FSP) was investigated. It is assumed that dynamic recrystallization (DRX) is occurred by frictional heat and plastic flow during FSP. This process is the effective method of the grain refinement for light metals. In this study, FSP was conducted to cast Mg alloys for and the difference of the grain refinement by DRX in these alloys was examined. As a result, in comparison with commercial Mg-Al-Zn alloy and Mg-Y-Zn alloy have finer microstructure. The grain size of FSP-ed Mg-Y-Zn alloy was ~1.7 [/m], however, that of AZ31 alloy was 20~30 [/m].

Abstract: In this study, we investigate the effects of high energy milling under Ar atmosphere on the morphology, size and microstructure of Cu-Cr alloy powders prepared by gas atomization. The attrition milling using stainless steel balls is performed up to 60 hrs. The observation by SEM and TEM shows apparent sequential changes of morphology and size of powders similar to those that are typical for mechanically alloyed elemental powders. Prolonged milling in the steady state regime produces a nano-crystalline structure, consisting of extremely small grains of 20 to 50 nm in size and finely dispersed Cr particles. It is also shown that the uptake of Fe from the stainless steel balls and vessel is appreciable. The microhardness of milled powder increases with the milling time, reaches its peak and then slightly decreases.

Abstract: Laser welding has not easily been adopted in shipbuilding industry because of its poor gap bridging ability. Recently, laser-GMA hybrid welding process showed possibility to overcome the tight gap tolerance with improved productivity. The laser-arc hybrid welding process is inherently complex because it has three kinds of process parameters: arc welding, laser welding and hybrid welding parameters. In this study, welding phenomena were investigated to optimize the hybrid process parameter; interspacing distance between laser and arc. The bead surface and cross-sectional shapes were evaluated, and weld pool shape and droplet transfer were monitored by high speed camera to clarify welding phenomena.

Abstract: This work sought to examine the suitability of twin roll strip casting for Ag-27%Cu- 25%Zn-3%Sn brazing alloy (BAg-7A) and to investigate the mechanical properties and microstructure of the strip. The effect of aging heat treatment on the properties was also studied. This new manufacturing process has applications in the production of the brazing alloy. XRD and microstructural analysis of the Ag-27%Cu-25%Zn-3%Sn strip represented eutectic microstructure of a Cu-rich phase and a Ag-rich matrix regardless of heat treatment. The results of mechanical tests showed tensile strength of 470MPa, a significant enhancement; and an 18% elongation of the twin roll casted strip, due mainly to the solid solution strengthening of Zn atoms (~20%) in the Cu-rich phases. Tensile results showed gradually decreasing strengths and increasing elongation with aging heat treatment. Microstructural evolution and fractography were also investigated and related to the mechanical properties.

Abstract: Isothermal grain coarsening of AlCu5MnCdVA aluminum alloy prepared by permanent mould casting during semi-solid treatment process was studied. It was found that the relationship between the average diameter of α (Al) grains and the isothermal holding time fitted the formula r − r = K ⋅ t − − ' 3 0 3 , where the coarsening rate K’ varied with the holding temperature, which equaled to 4.288×10-10m3/s and 5.962×10-10m3/s at the holding temperature. A modified model of liquid film migration has been proposed for explaining of 622°C and 631°C respectively the coarsening rate variation with annealing temperature. The diffusion of the solutes as well as vacancies, and the grain boundary tension are responsible for the microstructure evolution of AlCu5MnCdVA cast alloy.

Abstract: We have proposed a novel bonding process using composite Ag nanoparticles composed of Ag metallo-organic nanoparticles and Ag2CO3 for an application to the assembly of electronic devices. In this research, the sintering mechanisms of the composite Ag nanoparticles are discussed based on the results of the observation of the sintering behaviors and the investigation of the thermal characteristics. Moreover, Cu specimens were bonded using the composite Ag nanoparticles for measuring the bonding strengths. Based on the results, the effects of the Ag2CO3 contents in the composite Ag nanoparticles and the bonding conditions on the bondability were evaluated. As a result, it was found that the composite Ag nanoparticles were sintered rapidly because of the interaction between the Ag metallo-organic nanoparticles and Ag2CO3. Thereby, the bondability was improved by optimizing the contents of Ag2CO3 in the composite Ag nanoparticles.

Abstract: Due to the environmental problem, automotive companies are trying to reduce the weight of the car body. Therefore, TRIP (TRansformation Induced Plasticity) steels, which have high strength and ductility, have been developed. However, there are only a few publication reported on the characteristics of GMA weld on TRIP steel. In this study, we will evaluate which of the shielding gases, i.e. Ar 100% CO2 100% and Ar 85% + CO2 15%, and welding speed make a better quality GMA welded TRIP steel. Bead-on plate welding and butt welding were performed with various welding conditions. In order to identify mechanical properties of welded TRIP steel experimental methods including fractography, tensile test, and hardness test were performed.

Abstract: The membrane, which forms the primary barrier of the cargo tank onboard LNG carrier, is made of corrugated sheets, angle pieces and sealing caps and so on. These components are manufactured from SUS 304L sheets of 1.2mm thickness and assembled by the plasma welding. There are several welding defects such as overlap, excessive convexity, excessive concavity of weld and incomplete inclusion in the plasma welding. These unacceptable portions of the weld should be removed and repaired in order to prevent propagation of the defects. So a study is undertaken to determine the optimum process parameters such as the welding current, traveling speed, gas flow rates and frequency in the plasma lap welding process for LNG ships in order to prevent these defects. The polished and etched surface of the welded membrane sheet is examined if the cross section shows a properly made weld. And the tensile tests are conducted to consider if the tensile strength is less than the minimum required for the parent metal of the membrane sheet.