TiAl合金의 界面反應 및 流動性에 關한 硏究 = (A) study on metal/mold reaction and fluidity of TiAl alloys

다국어 초록 (Multilingual Abstract)

TiAl alloys have great potential for high temperature structural applications such as turbine blades, turbochargers and exhaust valves since they have an attractive combination of high melting point, low density, high specific yield strength, good oxidation resistance and excellent creep properties up to high temperatures. However, the use of TiAl alloys has been limited, because it is highly reactive in the molten state and its low ductility and poor workability at ambient temperature make it difficult to machine. In order to minimize their drawbacks, it is required to establish a novel melting and investment casting process by which complicated, thin-wall and small size TiAl castings could be obtained.
The investment casting process of TiAl alloys is generally influenced by three barriers. Firstly, TiAl alloys are extremely reactive with oxygen and nitrogen in the molten state. So TiAl alloys cannot be melted with conventional melting process, therefore these alloys must be melted, cast and cooled under a vacuum or inert atmosphere. Secondly, the reactivity of TiAl alloys in the molten condition requires special non-reactive molds making practices to reduce a very brittle reaction layer, commonly called α-case. This α-case leads to difficulty in machining and crack initiation and propagation due to the hardening of the surface layer. Finally, the fluidity of molten TiAl alloys is very poor. This leads to shrinkage and gas defects as well as misruns.
The aim of the present work is to optimize the investment casting of TiAl alloys. Therefore, the effect of mold material, binder and mold preheating temperature on the metal/mold reaction were investigated between TiAl alloys and investment molds using a vacuum induction melting furnace. And the effect of mold preheating temperature on the fluidity of TiAl alloys was evaluated by using a spiral shell mold.
In the investment casting process of TiAl alloys, colloidal silica bonded Al2O3 mold is regarded as a promising mold in terms of the formability, enough handling strength, cost and the thermal stability. The fluidity of TiAl alloys was able to be controlled by mold preheating temperature.

목차 (Table of Contents)

• 목차
• 목차 = ⅰ
• List of Tables = ⅲ
• List of Figures = ⅳ
• 1. 서론 = 1
• 2. 이론적 배경 = 3
• 2.1 TiAl 합금 = 3
• 2.2 TiAl 합금 성형기술 = 3
• 2.2.1 분말사출성형 = 3
• 2.2.2 항온단조 = 7
• 2.2.3 주조 = 7
• 2.3 TiAl 합금 용융기술 = 8
• 2.4 TiAl 합금 주조기술 = 10
• 2.5 연구현황 = 12
• 3. 실험방법 = 15
• 3.1 TiAl 합금 제조 = 15
• 3.2 Metal/Mold 계면반응 조사 = 15
• 3.2.1 정밀주조 주형제작 = 15
• 3.2.2 주형재료 및 점결제에 따른 계면반응 조사 = 18
• 3.2.3 주형예열 온도에 따른 계면반응 조사 = 21
• 3.3 TiAl 합금 유동성 측정 = 22
• 3.3.1 Spiral 주형제작 = 22
• 3.3.2 주형 예열온도에 따른 TiAl 합금 유동성 측정 = 25
• 4. 실험결과 및 고찰 = 26
• 4.1 Metal/Mold 계면반응 = 26
• 4.1.1 주형재료 및 점결제에 따른 영향 = 26
• 4.1.2 주형 예열온도에 따른 영향 = 38
• 4.2 TiAl 합금 유동성 평가 = 47
• 5. 결론 = 55
• 6. 참고문헌 = 57
• Abstract = 61