Bong Jae Choi

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 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.

Titanium Alloys Casting for Structural Application

The aim of this study is to clarify the metal-mold reaction for the economic net-shape forming of titanium and titanium alloys. The metal-mold reactions between pure titanium, TiAl alloys and Al2O3 mold were examined in a plasma arc melting furnace. The alpha-case generation between pure titanium and Al2O3 mold could not be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, based on the interstitial and substitutional α-case formation mechanism, the α-case formation between pure titanium and Al2O3 mold, and α-case free casting of TiAl alloys against Al2O3 mold can be explained. On the basis of the interstitial and substitutional α-case formation mechanism, α-case controlled net-shape forming of titanium and TiAl alloys can be possible for the structural materials.

In Situ Synthesis of TiAl Matrix Composites by Melting Route

TiAl alloys Al composition range between 45 and 49 at%, includes γ-TiAl and α2-Ti3Al, are an emerging high temperature materials which has higher specific strength, oxidation ratio and specific modulus than Ni base superalloy. In this study, TiAl alloys were manufactured by plasma arc melting (PAM) and then TiAl and granular boron carbide were in-situ synthesized in PAM method again. The in-situ synthesized TiAl matrix composites were investigated by using X-ray diffractometer, optical microscope, and electron probe micro-analyzer.

Evaluation of Interfacial Reaction between Titanium Matrix Composites and Aluminium Alloy

The main purpose of this study is to evaluate the interfacial reaction between titanium matrix composites (TMCs) and A380 alloy in aluminum die-casting. In-situ synthesized titanium matrix composites and H13 tool steel were immersed in molten A380 alloy in a mold at 993 K for times varying from 0 to 1200 s. In-situ synthesis TMCs and interfacial reaction between TMCs and A380 alloy were examined by X-ray diffraction, optical microscope, scanning electron microscope and electron probe micro-analyzer. The reaction behavior shows that TMCs can a substitution for H13 tool steel.

Casting and Modeling of Titanium Matrix Composites

The aim of this study is to establish the net-shape forming of titanium matrix composites (TMCs) shot sleeve for Al alloys die-casting using a casting route. In-situ synthesis and casting of TMCs were carried out in a vacuum induction melting furnace. The synthesized (TiC+TiB) TMCs were examined using an scanning electron microscopy and electron probe micro-analyzer. The thermo-physical variables estimated by casting process were applied to the modeling of TMCs shot-sleeve casting using the Magmasoft®. The results of the investment casting and modeling of TMCs confirm that the casting route can be an effective approach for the economic net-shape forming of TMCs shot sleeve.

Thermodynamic Calculation of Alpha-Case Formation in Titanium Alloys

The alpha-case formation mechanism was elucidated for the economic titanium casting. The α-case formation reaction between Ti and Al2O3 mold was examined in a plasma arc melting furnace. The reaction products were characterized by electron probe micro-analyzer and transmission electron microscopy. The α-case generation between Ti and Al2O3 mold was not able to be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, from our experimental results and thermodynamic calculations and Calphad modeling (Thermo-calc®), it was confirmed that the α-case is formed not only by an interstitial element but also by substitutional metallic elements dissolved from mold materials. Our newly established α-case formation mechanism will surely lead to a variety of significant applications of the α-case controlled Ti casting.

Interfacial Reaction between Molten Al Alloys and Titanium Matrix Composites

The aim of this study is to investigate the applicability of titanium matrix composites (TMCs) sleeve to Al alloys die-casting. Ti and 1.88 mass% B4C were prepared for the synthesis of 10 vol% (TiC+TiB) hybrid TMCs. In-situ synthesis and net-shape forming of TMCs were carried out in a vacuum induction melting furnace. The synthesized (TiC+TiB) TMCs were examined using scanning electron microscopy, an electron probe micro-analyzer, X-ray diffraction and transmission electron microscopy. The resistance-ability of (TiC+TiB) TMCs to molten Al alloys attack was also examined. Their reactions were carried out in a furnace at 993 K for times varying from 0 to 1200 s. In the case of conventional sleeve material, H13 steel, there were severe interfacial reactions and erosion after 60 s. On the other hand, the resistance of (TiC+TiB) TMCs to interfacial reactions and erosion by molten A380 alloy was significantly increased.

(TiB+TiC) Hybrid Titanium Matrix Composites Shot Sleeve for Aluminum Alloys Diecasting

The aim of this study is to fabricate an α-case free (TiB+TiC) hybrid titanium matrix composites (TMCs) shot sleeve for aluminum alloy diecasting by in-situ synthesis and investment casting. Granular 1.88 wt% B4C was added to a titanium matrix in a vacuum induction melting furnace. The synthesized (TiB+TiC) TMCs were examined using electron probe micro-analysis and transmission electron microscopy. The results of the in-situ synthesis and investment casting of the TMCs show that our casting route constitutes an effective approach to the economic net-shape forming of TMC sleeves.