열처리를 통한 Ti-6Al-4V 주조재 합금의 기계적 성질 개선

오성탁 전북대학교 일반대학원 2016 국내석사

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.

선택적 레이저 용융 방식으로 제조한 Ti-6Al-4V 합금의 후열처리 설계

조윤환 창원대학교 일반대학원 2024 국내석사

The demand for lightweight materials to reduce carbon emissions has been steadily increasing. Ti-6Al-4V alloy is regarded as a key lightweight material due to its superior combination of specific strength and toughness. Furthermore, additive manufacturing (AM) process such as selective laser melting (SLM) or direct energy deposition (DED) has been applied to Ti-6Al-4V alloy to reduce high production costs and achieve much more complex shapes. However, due to the rapid cooling rate of AM process, AMed Ti-6Al-4V alloys inevitably have α′ martensite structure, providing elevated strength but exhibiting significantly reduced ductility. Therefore, post-heat treatment with optimized condition must be conducted to ensure sufficient strength and ductility of AMed Ti-6Al-4V alloy. In this study, the effects of post-heat treatment conditions on the microstructural evolution and its correlation with the room-temperature tensile properties have been investigated in Ti-6Al-4V alloy fabricated by SLM process. As-built Ti-6Al-4V alloy consisted of a fine α’ martensite structure within prior columnar β grains due to a high cooling rate during the SLM process. The β-stabilizer elements (V and Fe) were enriched along α’ lath boundaries in a solid-solution state. The stress-relief heat treatment (593 ℃ for 2 h) promoted the formation of fine particle shaped β phases at the α’ lath boundaries with the retention of α’ martensitic lath structure, leading to a slight increase of microhardness (about 20 Hv). High-temperature heat treatment in the range of 800-950 ℃ with different holding times led to a substantial martensite decomposition (α’→α+β lamellar). The stress-relief heat treatment had a negligible effects on the microstructure and room-temperature tensile properties of the SLMed Ti-6Al-4V alloy as long as the high-temperature heat treatment was applied. With increasing temperature and holding time, microhardness decrease and lamellar width became larger in the wake of static α phase coarsening, by which ductility was enhanced. Excellent strength-ductility balance (yield strength of 920 MPa and fracture elongation of 15%) can be achieved when the as-built Ti-6Al-4V alloy was heat-treated at 950 ℃ for 1 hour. The heat-treated microstructure was composed of completely decomposed α+β lamellar with a width of 2.56 μm, showing 367 Hv microhardness.

지르코니움 코팅된 Ti-6Al-4V 합금에 대한 골모세포의 행동양식

김해진 전남대학교 대학원 2012 국내박사

The use of zirconium (Zr) as a biomaterial has increase, due to its good chemical and dimensional stability. Zr coating on Ti-based metals is considered to increase the life expectancy of surgical implants and prosthesis due to its greater wear and corrosion resistance than titanium and its alloys. However, surface characteristics and cell responses to Zr coating on Ti-based metals have poorly been understood. Thus, the purpose of this study was to assess the surface characteristics and the biocompatibility of Zr coating on Ti-6Al-4V alloy surface by radio frequency (RF) magnetron sputtering method. Ti-6Al-4V disks were formed into discs 15 mm diameter and 1 mm thickness. The discs were divided into 2 groups. Group I was a non-coated machined Ti-6Al-4V surface (Ti-6Al-4V group). Group II was a zirconium coated surface by RF magnetron sputtering on Ti-6Al-4V alloy (Zr-coating group). SEM and EDX analysis showed that zirconium deposition on Ti-6Al-4V alloy substrate was performed successfully. Profile roughness measurements showed 0.27 ㎛ in Ti-6Al-4V group, 0.34 ㎛ in Zr-coating group. The surface roughness appeared similar in two groups. Under SEM, no difference in cell morphology was observed between Ti-6Al-4V group and Zr-coating group. In MTT assay, there was no significant difference between Ti-6Al-4V group and Zr-coating group at 3 and 5 days. Zr-coating group showed higher ALP activity level than Ti-6Al-4V group (p<0.05). In RT-PCR analysis, bone sialoprotein and osteocalcin mRNA expression on Zr-coating group increased approximately 1.2-fold and 2.1-fold respectively, compared to that of Ti-6Al-4V group. These results suggest that zirconium coating on Ti-6Al-4V alloy could enhance the early osteoblast responses, such as ALP activity and bone matrix-related gene expression, which are important for the subsequent cell interactions and bone healing in vivo. This property could make non-toxic metal coatings on Ti-6Al-4V alloy suitable for orthopedic and dental implants.

Ti-6Al-4V 합금의 절삭가공 시 절삭성에 관한 실험 및 FE 해석

이철용 인천대학교 대학원 2023 국내박사

Titanium, which has small size, light weight, excellent corrosion resistance, and heat resistance, can be used to the maximum in aerospace, field industries, and strategic materials. Among titanium alloys, Ti-6Al-4V alloy, which has the most common defect in the industrial field, has poor productivity at the contact point, causing great difficulties in occurrence. Therefore, in this paper, the normal condition of the proof machining is first confirmed casting in the machining center and turning center in order to identify the main composition and their phenomenon tendency, such as normal temperature or normalization when opening and normalizing the normalization speed, standard depth and feed rate. Support force and measured temperature were observed using a flow sensor, infrared thermometer, and thermal imaging camera. In addition, bearing analysis was performed using the finite element analysis tools AdvantEdge and Deform, and the following conclusions were reached. 1) When changing the number of rotations of the spindle in cutting using a machining center, the processing load (current value) all showed high STD61, but the actual tool wear (7,000 rpm) and damage (11,000 rpm) were higher in Ti-6Al-4V occurred. Therefore, stable cutting conditions are judged to be 4,000 to 5,000 rpm of spindle rotation and 500 to 600 mm/min of feed rate.2) In a cutting experiment using a turning center, the Al7075 and STD61 increased the overall processing load (current value by about 5A) as the rotation increased, but the Ti-6Al-4V was unable to perform cutting at a spindle speed of 2,000 rpm or more, It is judged that cutting is possible under the cutting conditions for the production of tensile test pieces: rotation speed of 1,200 to 1,300 rpm, feed rate of 0.08 to 0.12 mm/rev, and cutting depth of about 0.3 mm. 3) As a result of measuring the cutting temperature, Al7075 started at 19℃ lower than room temperature and gradually decreased to 17.5℃ during cutting. STD61 decreased from an average of around 30℃ to about 26℃, but it also rose to a maximum of 65℃ due to frictional heat caused by chip curling. However, Ti-6Al-4V was 27~28℃ at the start of cutting, but in a short time, heat was generated around the tool and tool, and the temperature rose up to 190℃.4) In the cutting force test and cutting temperature test of Ti-6Al-4V, the cause of the rapid wear and tear of the tool could be proved through finite element analysis. In the AdvantEdge analysis, STD61's main component and required power were much higher than Ti-6Al-4V, but the tool temperature and boundary shear stress were 1.5 and 2 times higher in Ti-6Al-4V, respectively. Although there are differences by condition, the overall tool temperature and boundary pressure were high. 5) Due to the characteristics of Ti-6Al-4V, the high pressure at the interface generates frictional heat at the contact surface between the tool and the chip, causing the coating of the tool to peel off and high-temperature microparticles to adhere, causing rapid abrasion to the tool edge and spreading to the cutting edge. It is judged to be It is thought that the change in adhesion behavior that occurs at this time will require many studies through flow analysis in the future. 경량 고강도와 우수한 내식성, 내열성을 갖고 있는 타이타늄(Titanium)은 항공우주, 산업분야, 전략물자에 이르기까지 광범위하게 활용되고 있다. 타이타늄 합금 중 산업분야에서 가장 널리 사용되고 있는 Ti-6Al-4V 합금은 그 기계적 성질로 인하여 절삭성이 좋지 않아 절삭에 큰 애로를 겪고 있다. 따라서, 본 논문에서는 절삭가공의 일반적인 조건인 절삭속도, 절삭깊이 및 이송속도를 변화시키며 절삭할 때 절삭온도와 절삭력 등의 주요 절삭성 인자들의 변화 경향을 규명하기 위하여 우선 머시닝센터와 터닝센터에서 절삭 시 전류센서, 적외선 온도계 및 열화상 카메라 등을 이용하여 절삭력과 절삭온도를 관찰하였다. 또한, 유한요소해석 툴인 AdvantEdge와 DEFORM을 이용하여 절삭해석을 수행하였으며 다음과 같은 결론을 얻었다. 1) 머시닝센터를 이용한 절삭가공에서 주축의 회전수를 변화시킬 때 가공부하(전류값)는 모두 STD61이 높게 나왔지만 실제 공구의 마멸과(7,000rpm) 파손(11,000rpm)은 Ti-6Al-4V에서 발생하였다. 따라서, 안정적인 절삭조건은 주축의 회전수 4,000~5,000rpm, 이송속도 500~ 600mm/min로 판단된다. 2) 터닝센터를 이용한 절삭실험에서 AL7075와 STD61은 회전이 증가할수록 전체적인 가공부하는 상승(전류값 약 5A씩) 하였으나, Ti-6Al-4V는 주축 회전수 2,000rpm 이상에서 절삭가공이 불가능하였으며, 인장 시험편 제작 절삭조건인 회전수 1,200~1,300rpm, 이송속도 0.08~0.12mm/rev, 절삭깊이 0.3mm 정도 조건으로 절삭이 가능할 것으로 판단된다. 3) 절삭온도 측정 결과 Al7075는 상온보다 낮은 19℃에서 시작하여 절삭가공 동안 점차적으로 17.5℃까지 감소하였다. STD61은 평균 30℃ 내외에서 약26℃까지 감소하였으나 부분적으로 칩 말림에 의한 마찰열로 최고 65℃까지 상승하기도 하였다. 그러나 Ti-6Al-4V는 절삭 시작점에서 27~28℃ 였으나 짧은 시간에 공구와 공구 주변에 발열이 생성되면서 최고 190℃까지 온도가 상승하는 부분도 나타났다. 4) Ti-6Al-4V의 절삭력 실험과 절삭온도 실험에서 공구의 급속한 마멸과 파손의 원인을 유한요소해석을 통하여 이론적 근거를 증명할 수 있었다. AdvantEdge 해석에서 STD61의 주분력과 소요동력이 Ti-6Al-4V보다 훨씬 높았지만, 공구온도와 경계전단응력은 각각 1.5배 및 2배 정도 Ti-6Al-4V가 높다는 점과 DEFORM 해석에서 수치 차이와 조건별 편차는 있지만, 전체적으로 공구온도와 경계 압력이 높게 나타났다. 5) Ti-6Al-4V의 특성상 경계면의 높은 압력이 공구와 칩의 접촉면에 마찰열을 발생시켜 공구의 코팅이 박리되고 고온의 미소입자들이 응착되면서 공구인선에 급속한 마멸이 일어나면서 절삭날까지 확산되는 것으로 판단된다. 이때 발생하는 응착의 거동 변화는 앞으로도 유동해석을 통하여 많은 연구가 필요할 것으로 사료된다.

TiH_(2) 분말을 이용한 Ti-6Al-4V의 온도에 따른 소결 특성

허경호 연세대학교 대학원 2000 국내석사

티타늄 및 티타늄 합금 제품은 경량-고강도-고내식성의 우수한 특성에도 불구하고, 고융점-고반응성-난가공성 때문에 다른 구조용 재료에 비하여 고가인 단점을 가지고 있다. 따라서 가공시 소모되는 부분을 줄이며, 복잡한 형상의 제품 제조에 유리한 분말야금법이 티타늄·티타늄 합금 제조에 많이 이용되고 있다. 소결재는 다른 제품에 비해 상대적으로 기계적 성질이 취약하지만, 뛰어난 성능을 요구하지 않는 환경에서 이들의 경제성은 최대의 장점이다. 근래에는 소결재의 기계적 성능을 향상시키기 위해 많은 연구와 노력이 모아지고 있다. 한편, 금속 티타늄 분말 제조는 티타늄 고유의 고반응성으로 인한 오염문제와 고순도 티타늄의 강한 연성 때문에 파·분쇄에 어려움이 존재한다. 이러한 문제점을 해결하기 위해 취성이 강한 수화물을 이용해 분말을 제조하는 공정이 개발되었으며, 이 공정을 수소화-탈수소화법(Hydride-DeHydride process)이라고 한다. 현재 티타늄 분말을 제조하는 대표적인 공정 중에 하나이다. 이 공정은 티타늄 스폰지를 고온 수소 분위기에서 수소화티타늄으로 합성한 후, 분쇄·분급하고, 다시 고온 진공 분위기에서 탈수소 처리하여 고순도 금속티타늄을 만든다. 이를 다시 분쇄·분급으로 최종 티타늄 미분말을 생산한다. 본 실험에서는 앞서 설명한 HDH법의 중간 산물인 수소화 티타늄을 소결에 직접 이용하여, 탈수소 공정을 줄임으로서 공정 단순화로 제조경비를 줄이고, 열처리 단계를 줄여 산소와 같은 가스 불순물의 오염을 줄이고자 했다. 또, 수소화 티타늄의 소결 밀도가 순수 금속 티타늄보다 우수하다는 기존의 연구의 결과와 마찬가지로, 순수 티타늄 대신 수소화 티타늄을 사용하여 소결한 Ti-6Al-4V 합금의 소결 밀도도 향상될 것인지 확인하고자 했다. 먼저, 초기 진공은 10^(-5)torr로 하고, 소결온도는 700, 800, 900, 1000, 1100, 1200 그리고 1300℃로 정하였으며, 4시간동안 소결한 후 노냉하였다. α+β상 영역과 β상 영역의 사이 온도(β transus)인 1000℃에서 용존 산소량의 급격한 변화와 모합금인 알루미늄-바나듐 합금의 X-ray 피크가 사라지는 점 그리고 β상이 형성되기 시작했다는 증거인 widman-statten 조직이 나타나는 것으로 보인다. 1000℃ 아래 온도에서 소결했을 경우, 수소화 티타늄 분말을 사용한 Ti-6Al-4V 소결 밀도가 높게 나와서 기존의 수소화 티타늄의 소결 밀도 향상효과를 확인할 수 있었으나, 그 위 온도에서는 진공도의 변화로 봤을 때, 급격한 탈가스는 승온 시 끝나고, X-ray 피크에가 소멸되는 것으로 보아 알루미늄과 바나듐의 본격적인 확산으로 합금화를 동반한 소결 과정이 지배적인 것으로 사료된다. 소결 온도가 증가함에 따라 소결 밀도와 더불어 경도값도 증가했다. 같은 소결 밀도일 경우 수소화 티타늄을 사용한 Ti-6Al-4V가 경도값이 더 작게 나왔다. 이는 용존 산소량이 더 적어서 낮은 경도값이 나오는 것으로 보이며, 잔류 수소량은 과거 실험에서처럼 대부분 환원되어 경도에 영향이 미미한 것으로 사료된다. 앞서 말한 바와 같이 1000℃이하의 소결온도에서 수소화 티타늄을 이용한 Ti-6Al-4V의 더 큰 밀도값이 1000℃이하에서 경도 차를 좁히는 요인인 작용하고 있는 것으로 보인다. 산소농도는 1000℃이상에서 알루미늄의 확산이 충분히 일어나 시편의 표면 근처에 산소와 반응해서 알루미나(Al_(2)O_(3))가 생성되며, 이것이 산소의 내부확산을 억제하여, 순수 티타늄 소결 실험에서 보인 소결온도에 따른 산소량 증가가 나타나지 않은 것으로 사료된다. 그 이하의 온도에서는 소결이 미미해서, 개기공이 많아 소결재의 비표면적이 넓을 것이며, 따라서 산소 오염 가능 면적도 넓어 결과적으로 많은 산소량이 많이 검출된 것으로 보인다. 또, 낮은 소결온도에서 탈가스 반응이 충분하지 않아 분해능이 떨어지는 수소 잔류량에 의한 오차도 포함되었을 것으로 사료된다. 1000℃이하 소결온도에서 산소량은 소결온도에 반비례하게 나타났다. 탈수소는 500℃ 부근에서 발생하여, 소결온도에 상관없이 70분 가량 지속되는 것으로 나타났다. Titanium is a good material which is high strength-to-weight and high corrosion resistance. On the other hand, It is more costly than other structural materials for high melting temperature, high reactive behavior especially on oxygen and poor workability. For the sake of overcoming these handicap, powder metallurgy (PM) has been long applied to titanium products. Recently many efforts and researches have improved mechanical properties of PM's products which is worse than other structural materials. In producing titanium powder are there usually two problems; the contamination and the difficulty in milling due to ductility of pure metals and therefore Hydride-DeHydride(HDH) process is developed and this process utilizes brittleness of hydride and a nearly reversible reaction of hydrogenation and dehydrogenation. At first sponge titanium is hydrogenated at high-temperature and then titanium hydride is milled and sieved and then dehydrogenated again at high-temperature and -vacuum. In this study, titanium hydride, a mid-product in HDH process, was used to prepare the blended elemental(BE) Ti-6Al-4V powder instead of metal titanium, a final-product in the same process. This experiments have advantage of reducing the stage of dehydrogenation and verified whether the titanium hydride improves the sinterability of Ti-6Al-4V alloy as the case of pure titanium hydride. Two kind of powders were prepared. One is a Ti-6Al-4V powder using TiH_(2). The other is a Ti-6Al-4V powder using pure Ti. As the condition of experiments, initial pressure was 10^(-5) torr and sintering temperatures were 700, 800, 900, 1000, 1100, 1200 and 1300℃ and the duration of sintering was 4 hours. After sintering, the specimens were cooled in the furnace. There is β-transus between α+β phase and β phase and the temperature is 999℃(□ 1000℃). This sintering temperature seemed to be a specific point according to measuring data. That is to say, X-ray peaks of master alloy disappeared at that point and the content of oxygen was dramatically changed at that temperature and Widmann-statten structure began to appear. Hardness and density increased with the sintering temperature. In the case of same density, Ti-6Al-4V using TiH_(2) had lower hardness. This may be because its oxygen content was smaller and hydrogen in it was also removed to the limit of tens ppm as the former experiments. Ti-6Al-4V using TiH_(2) showed better density at 1000℃ below and this seems to make the difference of Hardness smaller. The content of oxygen was not changed with sintering temperature above 1000℃. The reason is that aluminum is transformed into alumina close to the surface and then alumina prevents oxygen from diffusing inside. But at 1000℃ below, the content of oxygen decreased against sintering temperature and this seemed to be caused by the large surface-to-volume which had many possibilities attacked by oxygen and poor resolution of a measuring equipment. The content of hydrogen was expected to reduce against sintering temperature. This is consistent with the measuring content of oxygen. Dehydrogenation started about 500℃ and continued during 70 minutes without regard to sintering temperature.

(A) study on tribological properties of laser textured Ti-6Al-4V alloy

Leilei Wang 영남대학교 대학원 2018 국내석사

마찰 손실이 세계 공업 에너지 손실의 주요한 원인들 중에 하나이다. 에너지 손실을 줄이기 위하여 연구자들은 소재 표면에서 여러 가지의 처리를 연구하였다. 레이저 텍스쳐는 바로 그 중 하나이다. 저자는 이 논문에서 새로운 조합형 텍스쳐 패턴의 Ti-6Al-4V 합금에 대한 마찰 성능에 관한 연구를 하였다. 이 조합형 패턴에서 원과 타원이 정육각형으로 분포한다. 저자는 단일 요소 실험과 L9(34) Taguchi 방법 ( 3가지 실험 요인: Dimple distance, Sliding speed, Applied load ) 실험을 하였다. 단일 요인 실험은 패턴 시편이MoS2 고체 윤활제가 없는 경우와 Ti-6Al-4V 합금의 표면에서 이러한 조합형 패턴을 가공하고 MoS2를 채우면 마찰계수가 감소되는 결과를 밝혔다. Taguchi 방법 실험은 Dimple distance가 마찰계수에 대한 영향을 가장 많이 끼치고 Applied load의 영향이 가장 작은 결과를 밝혔다. 3가지 요인이 평균 마찰 계수에 미치는 영향력의 순위는 Dimple distance (D) > Sliding speed (S) > Applied load (L)이다. Friction loss has become an important cause of energy consumption in the world's industry. In order to reduce the energy loss, various kinds of processing are performed on the surface of the material. Surface laser processing micro-structure is one of them. In this paper, we studied the tribological properties of a novel composite surface microstructure for Ti-6Al-4V titanium alloys, the novel composite surface microstructure are mixed with circles and ellipses. Single factor experiments and three factors and three levels of Taguchi method (three factors: dimple distance, sliding speed, applied load) experiments were carried out. Single factor experimental results show that the dimple-textured Ti-6Al-4V surfaces filled with MoS2 solid lubricants can effectively reduce the friction coefficient as well as its fluctuation compared with the untextured samples and the textured samples without lubricants. And the Taguchi method experimental results show that Dimple-distance had the greatest influence on the coefficient of friction and Load had the least effect. The influence degree of factors for average friction coefficient was: Dimple distance (D) > Sliding speed (S) > Applied load (L).

Hybrid Process and Improvement of Grain Refinement Efficiency in Wire Arc Additive Manufactured Ti-6Al-4V Alloy

마우리아 아눕 쿠마르 경상국립대학교 대학원 2024 국내박사

와이어 아크 적층 제조법(WAAM)은 Ti-6Al-4V을 사용한 다양한 부품 생성에 새로운 길을 열었다. 그러나, 예기치 않게 적층 방향에 따른 열의 영향으로 큰 주상정 B 결정립을 생산함으로써, 결과적으로 거친 질감과 이방성 기계적 특성을 가진다. WAAM 기술은 Ti-6Al-4V 와이어를 공급 재료로 사용한다. 따라서 와이어의 비용은 초기에 Ti-6Al-4V 빌렛 비용에 따라 달라진다. 본 연구에서는 가공 단계를 최소화하여 와이어 비용을 줄이기 위해 주조 및 β 단조 Ti-6Al-4V 빌렛바를 사용하여 필러 와이어를 제조하는 가공 경로를 개발하고있다. 열역학 시뮬레이터 gleeble-3800을 사용하여 700°C – 1200°C의 온도 범위와 0.01 – 10s-1의 변형속도 범위, 50%까지의 신장 수축률에서 주조 및 β 단조 Ti-6Al-4V의 열변형 특성을 조사했다.단열 가열로 인한 비등온 유동 응력을 보정하기 위한 인공 신경망(ANN) 모델이 개발되었다.보정된 유동 응력 값을 이용하여 0.6 % 실제 변형률에서 공정 지도를 구성했다. 미세구조 특성화는 공정 지도의 불안정하고 안전한 도메인의 저효율 영역과 고효율 영역을 기반으로 수행되었다. 공정 지도의 불안정성 영역은 유동 국소화, α 라멜라에 꼬임 및 미세 공공 형성과 관련된 낮은 온도(<800°C)와 높은 변형률에서 획득되었습니다. 동적 재결정(DRX) 및 동적 복구(DRV)와 관련된 변형률 1s-1 미만의 온도 범위 850°C - 950°C에서 고효율 및 안전 영역이 획득되었습니다. 공정 지도의 최적 변형 조건은 필러 와이어 개발을 위한 주조 및 β 단조 Ti-6Al-4V 빌렛바의 열간압연에 추가로 사용됩니다. 최근 개발된 와이어 아크 적층 가공 하이브리드법이 단일 단계의 열간 단조 공정으로 완제품과 거의 유사한 중간재를 생산한다. 하이브리드법은 가공된 중간재의 결함과 이방성을 극복하고 더 나은 기계적 특성으로 부품의 정형을 생산한다. 또한 하이브리드법에 의해 생산된 widmanstatten 조직(0.34 - 0.48µm)을 포함하는 Ti-64 합금의 WAAM의 열간성형거동과 기계적특성을 연구한다. 열간성형 시험은 열역학 시뮬레이터 gleeble-3800을 사용하여 700°C – 1000°C의 온도 범위와 0.01s-1 - 10s-1 변형속도 범위, 60%까지의 신장 수축률로 수행되었다. 인공 신경망(ANN) 모델은 실험 조건의 미세한 간격에서 비등온 흐름 곡선을 수정하기 위해 개발되었다. 미세 구조 연구는 개발된 공정 지도를 사용하여 다양한 영역에서 수행되었습니다. 미세 구조는 국한된 흐름 및 라멜레 꼬임과 관련된 높은 변형속도와 낮은 온도에서 불안정한 영역을 보여줍니다. 동시에 높은 효율과 안정된 영역은 1s-1 이하의 변형률에서 900 - 950 °C 온도 영역에서 동적 재결정화와 관련이 있습니다. α+β와 β상 영역에서 자기변형 활성화 에너지는 각각 308.7kJ/mol과 493.2kJ/mol이었습니다. 920 °C와 변형률 0.6, 0.8, 0.9에서 단조된 시료는 받은 시료와 응력 완화된 시료에 비해 높은 강도와 연신율, 약한 조직감을 보여줍니다. WAAM 기법에서 큰 주상정 β 결정립은 해머 피닝을 통해 퇴적된 각 트랙의 소성 변형을 사용하여 미세화 할 수 있습니다. 이 방법에서는 C형 필러를 사용하여 WAAM Ti-6Al-4V의 피닝 및 증착 속도 동안 β 결정립 미세화의 효율을 향상시키기 위한 추가 접근법을 조사했습니다. 결과는 피닝 WAAM Ti-6Al-4V에서 기존의 상용 모양 필러 와이어에 비해 C형 필러를 사용할 때 결정립 미세화 효율이 크게 향상됨을 보여줍니다. C형 필러 와이어를 사용하여 얻은 WAAM Ti-6Al-4V의 용융 풀 침투 깊이는 상용화된 둥근 모양(R-Type) 필러 와이어(4.48mm)보다 낮습니다. 소성 변형된 영역은 변형 깊이(844±32.65μm)까지 미세하고 무작위로 배향된 β 결정립 성장합니다. C형 필러를 사용한 피닝 WAAM Ti-6Al-4V는 상용 필러 와이어(원형)와 비교하여 작은 등축 β 결정립과 얇은 α lath(0.56±0.18μm)의 존재로 인해 양방향(수평 및 수직)으로 등방성 기계적 특성과 높은 강도를 보입니다. The wire and arc additive manufacturing (WAAM) technique provided a new route to produce metrically complex parts of Ti-6Al-4V. However, it produces undesirable large columnar prior β grains created by the directional heat flow along the building direction, resulting in a strong texture and anisotropic mechanical properties. The WAAM technique uses Ti-6Al-4V wire as feeding material. So, the cost of wire initially depends on the Ti-6Al-4V billet cost. In this study, we are developing the processing route to manufacture filler wire using as cast and β forged Ti-6Al-4V billet bars to reduce the wire cost by minimizing processing steps. The hot deformation characteristics of as cast and β forged Ti-6Al-4V have been investigated using thermal-mechanical simulator gleeble-3800 in the temperature range of 700°C – 1200°C and strain rate range of 0.01 – 10s-1 up to height reduction of 50%. An artificial neural network (ANN) model has been developed to correct the non-isothermal flow stress caused by adiabatic heating. The model-corrected flow stress values were used to construct the processing map at the 0.6 true strain. The microstructural characterization was carried out based on the low and high-efficiency region of the instable and safe domain of the processing map. The instability domains in the processing map were obtained at lower temperatures (<800°C) and high strain rates, associated with flow localization, α lamellae kinking, and micro-voids formation. High efficiency and safe domain were obtained in the temperature range 850°C- 950°C below strain rate 1s-1 associated with dynamic recrystallization (DRX) and dynamic recovery (DRV). The optimum deformation condition of the processing map is further used for hot rolling of as cast and β forged Ti-6Al-4V billet bar for filler wire development. The study revel that the filler wire can be manufactured directly using as cast Ti-6Al-4V ingot. The developed wire were oxidation and crack free. The hybrid method has recently been developed in which wire and arc additive manufacturing (WAAM) produces the near net shape preform for the single-step hot forging process. The hybrid method overcomes the defects and anisotropic properties of WAAM processed preform and produces the component's net shape with better mechanical properties. We also investigate the hot deformation behavior and mechanical properties of WAAM Ti-6Al-4V alloy containing widmanstatten microstructure (0.34-0.48 µm) produced by the hybrid method. Hot deformation tests were conducted in the temperature range 700°C – 1000°C and strain rate range 0.01s-1 – 10s-1 up to the height reduction of 60%, using the thermal-mechanical simulator gleeble-3800. The artificial neural network model (ANN) has been developed to correct the non-isothermal flow curve at finer intervals of experimental conditions. The microstructural studies were carried out at various regions using a developed processing map. The microstructures show an instability region at a high strain rate and lower temperature, associated with flow localization and lamellae kinking. At the same time, the high efficiency and stable areas are related to dynamic recrystallization in the temperature range 900 - 950 °C at a strain rate below 1s-1. The self-deformation activation energy in the α+β and β phase regions was 308.7kJ/mol and 493.2kJ/mol, respectively. The forged sample at 920°C and strain 0.6, 0.8, and 0.9 show high strength, elongation, and weak texture compared to the received and stress-relieved samples. The large columnar prior β grains in the WAAM technique can be refined using plastic deformation of each deposited track through hammer peening. In this method, we have investigated an additional approach to enhance the efficiency of β grain refinement during the peening and deposition rate of WAAM Ti-6Al-4V using C-type filler. The results show that the grain refinement efficiency is significantly improved when using C-type filler compared to the existing commercial-shape filler wire in peening WAAM Ti-6Al-4V. The obtained melt pool penetration depth of WAAM Ti-6Al-4V using a C-type filler wire is lower (3.3 mm) than the commercially available round shape (R-Type) filler wire (4.48 mm). The plastically deformed region develops fine and randomly orientated β grains up to the deformation depth (844± 32.65 μm). The peening WAAM Ti-6Al-4V using C-type filler shows isotropic mechanical properties in both directions (horizontal and vertical) and high strength due to the presence of small equiaxed β grains and thin α lath (0.56 ± 0.18 μm) as compared to the available commercial filler wire (round shape).

진공 원심 주조된 Ti-6Al-4V 합금의 주조 및 열처리에 따른 미세조직과 기계적 특성

이현우 창원대학교 2021 국내석사

This study confirmed the casting properties according to the thickness and cooling rate of the vacuum centrifugal cast Ti-6Al-4V alloy and studied the microstructure and mechanical properties according to the heat treatment after casting. In order to check the casting characteristics according to the thickness and cooling rate, the thickness of the cast product was manufactured with steps of 2x2mm, 4x4mm, 6x6mm, and 8x8mm, respectively. The microstructure of the cast product was observed in the α+β phase of the Widmanstätten and lamellar structure, and it was confirmed that the larger the thickness, the larger the grain size and the α-lath width. In addition, the thicker the cast product, the thicker the α-case of the surface reaction layer. This means that the larger the thickness of the cast, the relatively slower the cooling rate, and the longer the crystal grains, α-lath and α-case grow It is believed to be due to. Through the XRD test, it was confirmed that the fraction of the β phase was low regardless of the thickness, and it was confirmed that the α’ martensite structure appeared at 2mm thickness. It is judged that this is formed due to the very fast cooling rate due to the thin thickness of the cast product. And it was confirmed that the thicker the thickness of the cast product, the lower the known hardness value, but the higher the surface hardness value. It is judged that the reason why the hardness value of the matrix is low is that the microstructure becomes coarser as the thickness of the cast product is thicker, and the reason why the surface hardness value is higher is the surface reaction that forms the surface hardened layer. It is judged that this is due to the increase in the thickness of the layer α-case. After casting, to check the microstructure and mechanical properties due to heat treatment after casting, round bar specimens were prepared 750℃×1hr, F.C. condition of Stress Relieving(SR), 960℃×1hr, W.Q. and 540℃×4hr, A.C. Condition of Solution Treatment & Aging(STA), 900℃×2hr×100MPa, F.C. Conditions of Hot Isostatic Pressing(HIP) was performed, respectively. It was observed that the width of α-lath increased compared to As-cast in SR and HIP, which were heat-treated below the β-phase transformation temperature. On the other hand, it was confirmed that the α’ martensite structure appeared in the STA. It is believed that this is caused by rapid cooling in the β-phase transformation temperature range. And it was confirmed that the casting defect observed in As-cast was removed after HIP. As a result of observing the mechanical properties by heat treatment after casting, it was observed that the hardness, YS, and UTS of SR and STA, which were heat treated in the air, were higher than that of As-cast, while the elongation decreased significantly. In particular, the hardness, YS and UTS of STA were higher than that of SR, and the elongation was lower than that of the SR. HIP, which was heat-treated in vacuum, had similar hardness, YS, and UTS to As-cast, whereas elongation was observed to be higher than that of As-cast. The cause of the increase in the strength of the STA can be considered the contribution of the α’ martensite structure, but this is difficult to see as the main reason for the collapse of the elongation rate. This is because the α’ martensite structure was not observed in SR, and the elongation was observed similar to that of STA, although a microstructure similar to HIP was observed. Titanium alloys increase the reactivity with hydrogen, nitrogen, and oxygen gases in the atmosphere at high temperatures, and when reactions with these gases occur, mechanical properties may be affected. Particularly, when the solid solubility of hydrogen increases at high temperatures and the solid solution of hydrogen increases in the titanium alloy, brittleness increases, and the elongation decreases significantly. Therefore, after STA heat treatment was performed in air and vacuum, respectively, after checking the mechanical properties, hydrogen, oxygen, and nitrogen were analyzed. STA performed in the atmosphere had relatively high YS and UTS, while relatively low elongation. and it was confirmed that the STA performed in the vacuum had lower YS and UTS than the STA performed in the atmosphere but had a much higher elongation than the STA performed in the atmosphere. It was confirmed that the oxygen and nitrogen content of STA performed in the atmosphere and in vacuum both had similar contents, but the content of hydrogen was found to be much higher than that of the STA performed in the atmosphere. Therefore, it is judged that the vacuum centrifugal cast Ti-6Al-4V alloy reacts with hydrogen in the atmosphere when heat treatment is performed in the atmosphere, resulting in increased brittleness.

석회화 순환처리와 flute 형성이 Ti-6Al-4V 합금제 교정용 미니스크류의 생체활성도에 미치는 영향

강재연 전북대학교 일반대학원 2021 국내박사

Objectives: The purpose of this study is to evaluate the effect of structural change and implementation of bioactivity to the surface of Ti-6Al-4V alloy orthodontic miniscrews through anodization and pre-calcification treatment. Materials & Methods: Ti-6Al-4V ELI alloy rods were CNC-machined to fabricate orthodontic miniscrews with diameter of 1.4㎜ × 3.3㎜ in length. A spiral flute was designed to change the structure of the miniscrews. To create a large surface area for the macroscopic and microscopic attachment of bioactive materials, the surface of the orthodontic miniscrews were immersed in an aqueous electrolyte solution by adding 20wt% H2O and 1.5wt% NH4F to glycerol. Voltage of 20V with current density of 20 ㎃/㎠ was applied for 1 hour to form a nanotube TiO2 layer. Afterwards, to improve the bioactivity, miniscrews were immersed in 0.5 vol% silica aqueous solution at 37ºC for 5 minutes, and then cyclic pre-calcification treatment with 0.03M NH4H2PO4 and 0.01M Ca(OH)2 solution at 90 ºC was repeated for 20 times. And to evaluate the effect of the change in concentration of NH4H2PO4 solution on bioactivity, these processes were also repeated with 0.06M NH4H2PO4 solution. X-ray diffraction analysis was performed to investigate the crystalized precipitated material, and cytotoxicity test was performed to evaluate the biocompatibility of treated surface. In order to investigate the bioactivity, the presence or absence of hydroxyapatite precipitation was investigated in vitro. Also, to evaluate the effect of this bioactive surface treatment on retention of miniscrews in vivo, miniscrews with and without treatment or with and without spiral flute were implanted into rabbit’s tibia. Removal torque was measured after 4-week installation. The surface of removed miniscrews were examined with optical microscope and confocal laser scanning microscope. Results: There were TiO2 nanotubes completely self-aligned and formed in a dense structure on the surface of the anodized miniscrew. A fine granular cluster layer of hydroxyapatite and octacalcium phosphate were formed on the surface of miniscrew subjected to pre-calcification treatment, and it was confirmed that the bioactivity was improved. The bioactivity was enhanced with higher NH4H2PO4 concentration. Removal torque and bone density in surface treated groups and groups with spiral flute increased when compared with the untreated group and groups without flute, resulting in improved initial stability. However, there was no significant change depending on the concentration of NH4H2PO4. According to the results of confocal laser scanning microscopic observation, new bone formed from the basal bone towards the surface of the miniscrew in untreated groups. However, in surface treated group, new bones formed from the surface of the miniscrew towards the basal bone, showing a directionality. Conclusions: With spiral flute and cyclic pre-calcification treatment, bone adhesion to Ti-6Al-4V alloy orthodontic miniscrew surface and removal torque increased which will be advantageous in enhancing the stability of orthodontic miniscrews in orthodontic treatment.

이반드로네이트 탑재가 Ti-6Al-4V 합금제 교정용 미니스크류의 골유착에 미치는 영향

전진 전북대학교 일반대학원 2021 국내박사

Objective This study was conducted to evaluate the effect of the loading of ibandronate, a drug that inhibits osteoclast function, on osseointegration for orthodontic miniscrews that obtain stability mainly from mechanical retention. Methods To investigate the loading and release characteristics of ibandronate, a 0.1 mm thick Ti-6Al-4V ELI alloy plate was cut into 10 mm × 10 mm and used for the test. Miniscrews 1.4 mm in diameter × 3.3 mm in length were produced using Ti-6Al-4V ELI alloy rods 1.4 mm in diameter. In order to remove the oxide layer formed on the surface during the manufacturing process, all specimens were etched by immersing them in a mixture of HNO3 : HF : H2O at a ratio of 12 : 7 : 81 for 10 seconds. To provide a large surface for loading of ibandronate via the formation of nanotube TiO2 layer, 20 V was applied for 60 minutes in a glycerol solution containing 20 wt% H2O and 1.5 wt% NH4F. To evaluate the effect of the loading conditions of ibandronate on its release characteristics, in vitro test of the ibandronate release was carried out using the group that was lyophilized after continuous immersion for 60 minutes, as well as the group that repeated 10 minutes immersion and lyophilization for 6 times. For in vivo experiments, the miniscrews were divided into three group: (1) untreated (UN), (2) anodized and heat-treated (AN), and anodized, heated, and ibandronate-treated (IB) groups. The miniscrews were implanted into both tibias of rats and at 4 weeks post-operation, removal torque was analyzed to evaluate the effect of ibandronate loading on in vivo osseointegration. Results The nanotubes were fabricated by electrochemically anodizing Ti-6Al-4V alloy miniscrews, in which a completely self-organized and dense structure was formed. Each of the nanotubes has a hollow, independent tube structures with each tube being bound to each other in the outer wall. Miniscrews were anodized and heat-treated at 500° C for 2 hours and then immersed in SBP solution. From the fifth day, protuberances that are detectable in the initial stage of HA precipitation were formed thinly over the entire surface, thereby indicating an improved bioactivity of miniscrew by anodization. It was found that the group immersed continuously for 60 minutes shows an acute decrease in the release of ibandronate, whereas the group immersed repeatedly 6 times at intervals of 10 minutes exhibits its stable release over 7 days. When the removal torque of implanted miniscrews was measured at 4 weeks post-operation, the values of removal torque were statistically high in the order of IB group>AN group>UN group. While the untreated UN group also showed interfacial fracture without adhesion of bone material after 4 weeks of implantation, both AN and IB groups revealed interfacial fracture and cohesive fracture of the adhered bone material. Conclusions This study demonstrates that the formation of a nanotube TiO2 layer along with the loading of ibandronate to miniscrews may improve effectively osseointegration of orthodontic miniscrews.