열처리를 통한 Ti-6Al-4V 주조재 합금의 기계적 성질 개선 = Improved Mechanical Properties of Cast Ti-6Al-4V by Heat Treatment

다국어 초록 (Multilingual Abstract)

Titanium alloys have been used in a number of parts as not only aerospace components but also marine and biomedical applications due to lightweight, high specific strength, and corrosion resistance. Particularly, Ti-6wt%Al-4wt%V alloy which is one of α+β titanium alloys has been utilized in the various applications due to its excellent combination of strength and ductility. In addition, Ti-6Al-4V alloy is heat-treatable to develop a variety of microstructure and mechanical property combinations. Thus, in wrought Ti-6Al-4V alloy, the improvement of their mechanical properties by a heat treatment has been studied for a long time. However, in cast Ti-6Al-4V alloy, it is still required to study on heat treatment process and strengthening mechanism to derive the increased strength and ductility of cast Ti-6Al-4V alloy.
From the above viewpoint, in this study, the microstructure refinement which is one of the strengthening mechanisms is applied to improve the mechanical properties by the heat treatment on cast Ti-6Al-4V alloy. Additionally, the effect of the heat-treatment condition and stabilizer partitioning (Al,V) on the microstructure control and phase transformation of the β phase was investigated.
Moreover, the further strengthening mechanism was investigated to obtain the improvement of the strength and ductility. The precipitation strengthening by α2 (Ti3Al) precipitates is well known in Ti-6Al-4V alloy to improve the further mechanical properties. However, it needs a long time to precipitate the α2 phase. To overcome this disadvantage, the short-time aging treatment was performed after the solution treatment in present study. With the relatively short aging time, the further strengthening mechanism of the retained β phase on microstructure and mechanical properties was investigated.
From the results in present study, the microstructure of cast Ti-6Al-4V alloy was greatly refined through the solution and subsequent aging treatments due to α and β phases decomposed from α' martensite. As well as, the microstructure control was determined by the prior solution treatment. The solution heat-treated microstructure and phase transformation of the β phase was strongly dependent on the concentration of vanadium in the β phase in cast Ti-6Al-4V alloy. Furthermore, although the short-aging treatment for 5 minutes was conducted, its combination of tensile strength and elongation was further improved, relatively compared to that of aged specimen for 1440 minutes. Its effects and reasons were discussed in present study.

목차 (Table of Contents)

• 1 Introduction 1
• 1.1 Necessity of the study 1
• 1.2 Objective of the study 6
• References 7
• 2 Theoretical background 9
• 2.1 Classification of titanium alloys 9
• 2.2 Phase transformations of titanium alloys 13
• 2.3 Microstructure and mechanical properties of Ti-6Al-4V alloy 17
• 2.3.1 Microstructure control 17
• 2.3.2 Microstructure and mechanical properties 22
• 2.4 Strengthening mechanisms of α+β titanium alloys 23
• 2.5 Casting and Vacuum Arc Remelting (VAR) Skull method 26
• Reference 29
• 3 Experimental procedure 30
• 3.1 Fabrication of specimens 30
• 3.2 Heat treatment 31
• 3.3 Microstructure and phase analysis 31
• 3.4 Mechanical properties 33
• 4 Results and discussion 34
• 4.1 Cast Ti-6Al-4V alloy 34
• 4.1.1 Microstructure and phase 34
• 4.1.2 Mechanical properties 38
• 4.1.3 Summary 38
• 4.2 Effect of cooling rates on microstructure and mechanical properties of cast Ti-6Al-4V alloy 39
• 4.2.1 Heat treatment procedure 40
• 4.2.2 Microstructure and mechanical properties 41
• 4.2.3 Summary 46
• 4.3 Effect of stabilizers (Al,V) on microstructure and phase transformation of β phase during heat treatments of cast Ti-6Al-4V alloy 47
• 4.3.1 Heat treatment procedure 48
• 4.3.2 Microstructure and phase of solution-treated specimens 50
• 4.3.3 Microstructure of aged specimens 54
• 4.3.4 Hardness 56
• 4.3.5 Discussion 58
• 4.3.6 Summary 62
• 4.4 Strengthening mechanism by short-time aging on mechanical properties in as-quenched Ti-6Al-4V alloy 63
• 4.4.1 Heat treatment procedure 64
• 4.4.2 Microstructure of solution treated and aged specimens 65
• 4.4.3 Hardness 68
• 4.4.4 Tensile properties 70
• 4.4.5 Discussion of strengthening by retained β phase 72
• 4.4.6 Summary 77
• Reference 78
• 5. Conclusions 80
• 국문 초록 81
• 감사의 글 83