Jeong Il Youn

Environmentally Conscious Gas Mixtures for Magnesium Melt Protection

The candidate gases for the replacement of SF6 in magnesium melt protection were reviewed from the viewpoint of environmental effects, melt protection properties, and chemical properties. The surface films formed under fluorine bearing gases were analyzed by using XPS. The XPS results revealed that the surface films consist of MgO and MgF2. The fluorine content of the surface film increases with increasing gas concentration and exposure time. The Pilling- Bedworth ratio (PBR) of MgO was 0.73, and the porous magnesium oxide could not cover the whole melt surface. PBR of MgF2 was 1.29, and the surface film which contains MgF2 could cover a lager area of the melt surface. This higher PBR of composite surface film prevented further oxidation and evaporation of magnesium.

Nucleation Enhancement of Al Alloys by High Intensity Ultrasound

The utilization of ultrasound is expected to be applied to many casting process because of the high efficiency of ultrasound energy. The present study attempts to evaluate the effect of ultrasound energy on microstructure of castings, and high intensity ultrasound was injected into A356 and A390 alloy melt for refinement of primary phase without adding a chemical refiner. The microstructure refinement, especially primary alpha phase, could not be achieved in A356 alloy with simply ultrasound injection into alloy melt, although ultrasound was injected for 20 min. However, the influence of ultrasound on the microstructure in A390 alloy is very clear. Primary silicon of A390 alloy with ultrasound injection into melt was very small and dispersed uniformly in alloy matrix. In addition, the primary Si size was decreased with an increase the ultrasound injection time from 82 to 8 µm. As compared with A356 alloy, however, A390 alloy shows the nucleation enhancement by ultrasound vibration of melt only. The reason that there is a difference of the refinement of primary phase between A356 and A390 alloy could be explained by solute cluster theory.

Ultrasonic Effect on Refinement of Mg2Si and Mechanical Properties of In Situ Al-Mg-Si Composites

Coarse primary Mg2Si was usually found in Al-Mg-Si alloys. In this experiment, to refine the coarse primary Mg2Si size, ultrasound was used as a new process. Ultrasound was injected to the molten Al-Mg-Si melts directly and its effect was significant. Average size of primary Mg2Si was reduced from 70㎛ to 15㎛, morphology was also changed to polygonal from irregular shape. Moreover tensile strength was improved by 30%, from 112 to 148 after ultrasonic injection.

In Situ Synthesis and Wear Resistance of Titanium Matrix Composites

The main purpose of this research is to evaluate the wear resistance of titanium matrix composites (TMCs). Reinforcements, TiB and TiC, were formed by in-situ reaction between boron carbide and commercial pure titanium. The confirmation of the sound synthesis of TMCs was done by phase identification. And then, sliding wear test were carried out to verify the wear resistance of TMCs by means of the coefficient of friction, wear loss and morphology wear track. The results of wear test indicate that TMCs have superior resistance than AISI H13 tool steel at the condition of severe loads.

Effect of sonotrode material on grain refining of Mg–3Al and Mg–9Al alloys by ultrasonic melt treatment

The new process, nucleation enhanced ultrasonic melt treatment (NEUMT), was proposed to increase the refining efficiency through heterogeneous nucleation by using the sonotrode which has been only concerned with the medium to transfer the ultrasonic energy. In the processing, the metal atoms and/or clusters eroded from the sonotrode were supplied and were simultaneously mixed uniformly into the melt by the ultrasound. These particles act as potential nuclei and refine the structure. The process was applied to assess grain refinement of Mg alloys, especially Mg–3Al and Mg–9Al. The refining efficiency was affected by the sonotrode material, and Ti was very effective in this process by the formation of proper intermetallic compound in the Mg alloy melt. The intermetallic compound was searched by the calculation of plane disregistry of the crystallographic orientation, and Al3Ti was suggested to be the heterogeneous nuclei.

Soldering Properties of Sn-Zn-X Micro Solder Powders Fabricated by Melts Dispersion Technique

Melts dispersion technique is a new process to produce solder powders in oil surroundings by the formation of molten metal’s droplets. The entire viscosity of the fluid after uniform mixture of melts and oil mainly affected the shape and size of the solder powder. The size of solder powder was increased and the irregular shape was altered into the spherical shape with increasing the fluid viscosity. The effect of alloying elements, such as Mg, Al and Ti, on Sn-Zn solder for the oxidation resistance was investigated using the solder paste mixed solder powder fabricated by melts dispersion technique at optimum conditions and RMA type flux. The oxygen concentration was reduced by the addition of alloying elements, especially, by Ti adding.

Study on Phase Stability of New Ni Based Superalloy Less 1 Designed to Low the ETA and Sigma Phase Fraction at 700°C

The alloys required for fossil power plants are altered from stainless steel that has been used below 600 °C to Ni-based alloy that can operate over 700 °C for advanced ultra super critical (A-USC) steam turbine. The IN 740 alloy is proposed for improved rupture strength and corrosion resistance at high temperature. However, previous studies with experiments and simulations on stable phases at over 700 °C have indicated the formation of the eta phase with the wasting of the gamma prime phase, which is the most important reinforced phase in precipitation hardened Ni alloys. This results in the formation of precipitation free zones to decrease the strength. LESS 1 alloy designed through some modifications of IN 740 was suggested in this study. LESS 1 showed the phase stability more than IN 740 due to the optimum composition of Cr, Mo, Ti and Al. The experimental results established that a needle-shaped eta phase was formed in the grain boundary and it grew to intra-grain, and a precipitation free zone was also observed in IN 740, but these defects were entirely controlled in LESS 1.

Effect of Sonotrode Design on Simultaneous Grain Refinement and Degassing of Al Alloys by Ultrasound

Grain refinement and degassing of Al alloys are the fatal processes to keep the high quality castings. The most widely used process for the grain refinement and the degassing is the addition of inoculants and the rotary degassing, respectively. However, these processes affect each other and make low efficiency of each process, in addition, these increase the cost. The application of the ultrasound for grain refinement and degassing has been studied to modify the processes, but the ultrasound could be only applied in special casting condition, such as solidification or in the semi-liquid state, and the effect of ultrasound in this state seriously decreases because the fluidity of melt extremely dropped. In this study, new process using ultrasound, named as NUMT (Nucleation Ultrasonic Melt Treatment), is suggested as the melt treatment for grain refinement and degassing simultaneously in melt. NUMT process could be established by the special design of the sonotrode made by Ti, and it makes supply the nuclei in Al alloys melt continuously and efficiently. The grain size and the content of hydrogen drop extremely by NUMT.

The Effect of Ultrasonic Injection on the Microstructure and Mechanical Properties of A356 Alloy

A356 alloy is one of the wide used casting aluminum alloys and intense research activities of worldwide on casting process or refinement agent and procedure. In this study, we attempted refinement on Al primary phase and eutectic phase of A356 alloy by ultrasonic injection into the melts. The effect of ultrasonic injection into the melts could be summarized as follows: the morphology of the Al primary phase was refined and changed shape from dendritic to nondendritic and the eutectic phase of A356 was also refined and changed morphology from dendritic to plate shape.

Hydrogenation and Degradation Behaviors of Mg-Ni Alloys

In this study, melting and casting of Mg-Ni alloys in ambient atmosphere for mass production, nickel content adjustment for enhancing hydrogen capacity and reducing alloy weight, and cyclic hydriding/dehydriding test for evaluation of degradation properties were investigated. Mg-Ni alloys were fabricated by Rotation-Cylinder Method (RCM) in ambient atmosphere. Hydriding/dehydriding reactions were repeated 350 times at 350°C to evaluate degradation property. SEM-EDS analysis revealed that the lamellar eutectic structure of Mg-Ni alloy consists of Mg-rich α-phase and β-Mg2Ni. It is supposed that the lamellar eutectic structure enhances hydrogenation properties of Mg-Ni alloys.