Chang Seog Kang

Microstructure and Formability of Different-Speeds Rolled AZ31 Mg Sheet

The aims of this study ares to investigate the microstructure evolution of AZ31 Mg alloys with normal rolling and different speeds rolling during hot rolling affects microstructure, texture and mechanical properties of AZ31 Mg alloy. In the microstructures of as-rolled both samples, twins are clearly apparent, small and recrystallized grains are visible along some grain boundary and twinned regions. The tensile strength and yield strength of DSR sample were slightly higher than that of NR sample. Also, in the case of the NR sample, tensile strength indicated different values to the rolling directions. From this result, NR sample compared to DSR sample strongly indicated to the plastic anisotropy tendency. Therefore, it is noted that DSR sample could be presented to the good formability, comparing to the NR sample. DSR samples deformed at 473K and 523K could be perfectly formed, indicating the potential application of the DSR process to improve formability of the Mg alloys at warm temperatures.

Joining of Aluminum Foam/Aluminum Metal by Spark Plasma Sintering Process

Aluminum foam is lightweight structures, energy absorption and thermal management. In this reason, there is a lot of attention on the aluminum foam as a structural material. The present papers showed various conventional joining techniques can be applied for foam-sheet structures, i.e., riveting, screwing, welding, gluing and soldering. This research presents new joining technique of Aluminum foam/Aluminum metal using spark plasma sintering (SPS) process. The Aluminum foam/Aluminum metals were fabricated by changing of various SPS holding temperature and holding time conditions. With increasing holding temperature and holding time, the tensile stress increased. The specimen sintered at 550°C for 20 min shows σts = 1.47 MPa. Also, it was found that the SPS holding time is dominant factor than the holding temperature for sound joining of two joint materials.

Mechanical Properties and Biocompatibility of Porous Titanium Prepared by Powder Sintering

Porous Ti compacts were fabricated by spark plasma sintering (SPS) method and their Youngs modulus and biocompatibility were investigated in this study. Ti powders were made from commercially available pure Ti (grade 2) using the plasma rotating electrode process (PREP) in an Ar atmosphere. Porous Ti compacts for biomedical applications were successfully fabricated in the porosity range from 5.1 to 39.2 vol%. Youngs modulus of porous Ti compacts having porosity from 31 to 35 vol % is almost the same as that of human cortical bone. To investigate the biocompatibility of pure Ti, fibroblast-like L-929 and osteoblast-like MG-63 cells were cultured with the presence of the extract solution from pure Ti for 72 hours. The viabilities of cells cultured with the diluted extract solutions were almost the same compared to that of the control. Also, porous Ti compacts did not induce any morphologic damage of cells.

Microsegregation Effect of Copper in Solidification of A356 Alloy

In this study, the microsegregation effect of copper during solidification of A356.2 aluminum casting alloy was evaluated by using solidification simulation and a series of experimental works. Electron probe micro analysis (EPMA) and the computational simulation revealed clearly that copper as a solute element segregates during solidification and it becomes more significant as the solidification approaches its completion. This microsegregation effect of copper is due to accumulated copper solute ahead of the eutectic interface during solidification. Consequently, the presence of a higher content of copper in A356.2 aluminum casting alloy causes influence on the solidification characteristics of the alloy and forms copper enriched phases at the last region to solidify in the microstructures.

Structural and Mechanical Behaviors of Partially Devitrified Ti-Based Bulk Metallic Glass

The mechanical properties of the nano-size primary α-Ti phase in Ti55Zr18Ni6Cu7Be14 bulk metallic glass (BMG) have been examined through various annealing temperatures and times. The yield strength and ductility were changed upon variation of α-Ti volume percent originate from heating conditions. The strength mechanism and associated heating conditions (heating rate, time and temperature) are discussed in terms of the microstructural observations and mechanical properties.

Tensile Deformation and Fracture Behavior of a Commercial Al Alloy with Nano-Sized Grains

The nano-structure in the commercial 5083 Al alloy was introduced by the equal channel angular pressing technique. The nano-sized grains of ~300 nm were obtained after 8 ECAPs at 373 K and 473 K. It was also obvious that much improvement in strength was obtained at lower pressing temperature, 373 K, than 473 K. In particular, there were mainly two different tensile characteristics; one was that the strength in the alloy ECA pressed at 373 K was much higher than that of as-annealed alloy but the elongation to failure was significantly smaller, while, in case of pressing at 473 K, the improvement in strength was found without sacrificing much of the elongation. The other was that the work hardening with increasing the amount of deformation was found in the alloy ECA pressed at 473 K. These tensile deformation characteristics were analyzed based on the observations of microstructure by TEM and fracture surface by FE-SEM.

Influence of the STD61/S45C Substrate and added Si content on the Die Soldering Behavior both Al and Iron

Die soldering is one of common defect generating during the die casting process of aluminum alloys. Until now, the various researchers have been usually studied about die soldering phenomenon in relation to the aluminum components. However, the influence of the mold materials was rarely studied. So, in this study, we have carried out the die-soldering tests using both STD61 and S45C steel to confirm the effect of die materials on die-soldering. Each Aluminum alloys were melted and held at 680 °C and then STD61 and S45C steel were dipped for 2 hr in the aluminum melt. After dipping , the substrate was cooled in the air and analyzed using a SEM and EPMA. The result of soldering test, reaction layer of STD61 substrate has increased according to Si contents of aluminum, however, reaction layer thickness was deceased in the S45C substrate. †(Received June 25, 2013)

Effects of Mn and Ca Additions on Microstructure and Mechanical Properties of Al-Mg Alloy

The effects of Mn and Ca contents on the microstructure and mechanical properties in Al-Mg alloy were investigated. The results showed that mechanical properties of Al-4.5wt.%Mg were increased as Mn content was raised from 0.1 to 0.5wt.%. Thermodynamic analysis and FE-SEM observation showed that Al15(Fe,Mn)3Si2 phase began to form as Mn content exceeded 0.3wt.%. The case of Ca addition, the tensile strength and elongation of Al-4.5wt.%Mg were decreased as Ca content was increased from 0 to 3wt.%. The maximum solubility of Ca for Al is very lower to 0.074wt.%, the most of Ca precipitated in the form of Al2Ca phase which is very brittle and the increase of Ca content was reduced the mechanical properties.

Influence of Process Parameters on the Fluidity of High Pressure Die-Casting Al-Si Alloys

It is generally known that silicon influences the fluidity of aluminum alloys. There are several techniques to evaluate the fluidity of aluminum for gravity casting such as using spiral or serpentine type mold and vacuum suction test. However, fluidity of aluminum in high pressure die-casting has not been sufficiently studied. Therefore, in this study, the relationship between the fluidity and superheat of pouring aluminum alloy as well as injection speed was studied. A serpentine and step type die for evaluating the fluidity of aluminum alloys was designed and actual experiments were conducted for aluminum by varying many parameters such as pouring and injection speed and the content of silicon. The results showed that fluidity of aluminum in die-casting was quite similar to the gravity casting. Under high pressure die casting conditions, increased fluidity was measured as the silicon content, superheat, G.B.F treatment time and injection speed were increased.

Corrosion and Soldering Behaviour of STD61 Coated by Arc Ion Plating Coatings

In order to investigate corrosion and soldering behaviour of STD61, TiN and TiAlN were coated on the surface of STD61 by using Arc Ion Plating. The structure of the coatings was examined as a function of deposition conditions by X-ray diffraction, and the crystallographic orientation was determined by use of a texture coefficient. TiN coating was grown with a strong (111) preferred orientation of a typical NaCl-type crystal structure. This strong (111) preferred orientation had been commonly observed from the TiN coatings deposited by physical vapour deposition techniques. TiAlN coatings, however, showed relatively multiple orientations mainly of (111) and (200). Furthermore, TiAlN film demonstrated superior corrosion resistance in a molten aluminum alloy at 680°C. This paper described in detail the corrosion and mass loss phenomena related to this steel-cast metal interaction.