Al 7075合金의 高溫 크리프의 狀態依存性에 관한 硏究

조용이 檀國大學校 大學院 1993 국내박사

고온, 고압을 받고 있는 시설의 안전성 문제는 최근에 와서 더욱 그 중요성을 더해가고 있다. 특히 원자로 터어빈 및 고압용기의 대형사고는 크리프가 주원인이 되므로 근래에 와서 특수금속, 복합재료 및 기타 첨단재료의 크리프 거동에 새로운 이론은 물론 새로운 실험방법의 개발이 절실하게 되었다. 특히 작용응력과 온도는 크리프변형에 대단히 중요한 영향을 끼치므로, 본 크리프 연구에서는 Zener-Hollomon 매개 변수를 써서 고온 크리프 거동의 활성화 에너지 의존성을 설명하고, 실험을 통해서 이것의 응력과 온도 의존성을 연구 분석하였다. 사용한 재료는 Al 7075합금이고 발광분광 분석장치를 사용하여 화학성분을 분석하고 T_6 방법으로 열처리하여 시편을 가공하였다. Al 7075합금의 고온 정상 크리프 실험과 파단 실험을 통해서 고온 크리프의 활성화 에너지의 온도 의존성을 온도 보상 시간과 Zener-Hollomon 매개 변수로 분석한 결과, Al 7075의 정상 크리프 활성화 에너지는 온도, 변형륭 및 응력에 민감하지 않았고, 크리프 파단에 대한 활성화 에너지 역시 온도 및 변형률에 민감하지 않음이 증명되었으며 실험결과는 Miller에 의한 현상론적 크리프 모델과 잘 부합하였다. 또 Al 7075합금의 크리프는 □=□(□) 의 함수관계에 의해 수식으로 표현할 수 있었다. 고온 정상 크리프의 응력 및 온도 의존성을 실험한 결과, 온도조건 380~410℃, 응력조건 1.53~1.91 kgf/mm^2에서의 크리프 변형기구는 전위상승이라고 추측되었고, 470~500℃, 응력조건 0.64~1.02 kgf/mm^2에서는 확산이 변형을 주도 한다고 생각되며 부가응력 의존성 n의 계산치와 측정치도 잘 일치하였다. This paper is treated the dependences of activation energy for Al 7075 alloy creep and the temperature, atress dependences on the high temperatures steady state creep of Al 7075. The activation energy for high temperature creep is associated with stresses, temperatures, strains. And the creep strain is a function of a temperature, compensated time, namely t exp(-△H/RT) and the stress. In fact this functional relation appears to be isomorphic to material structure by x-ray analysis. Applying this functional relation, the dependance of activation of activation energy for Al 7075 creep was investigated. The apparent activation energy and the applied stress exponent have been determined in order to investigate the creep behavior. Constant load creep tests under constant temperature, step-wise temperature cycling, high temperature tension tests and stress trasient dip tests were carried out in the experiment. From the experimental results, the followings are concluded. The activation energy for creep is insensitive to stress, temperature, structure, and strin. And phenomenological model agrees with experimental creep data. At the temperature ranging from 380℃ to 410℃ under the stress level of 1.53~1.91 kgf/mm^2, the creep behavior seemed to be controlled by dislocation climb. And at the temperature ranging from 470℃ to 500℃ under the stree level of 0.62~1.02 kgf/mm^2, the creep behavior seemed to be controlled by diffusion process. Calculated value of applied stree dependence agrees well with it's measured value.

M. E. F. 複合組織鋼과 Al 7075-T651 合金의 X線 回折에 의한 疲勞破損解析에 관한 硏究

김득진 東亞大學校 大學院 1995 국내박사

This study verified the relationship between fracture mechanics parameters and X-ray parameters for normalized SS41 steel with homogeneous crystal structure, M.E.F. dual phase steel(martensite encapsulated islands of ferrite), and A1 7075-T651 alloy with high strength and corrosion resistance. The change in X-ray parameters (residual stress, half-value breadth and diffraction pattern) according to loading history, stress ratio and plastic strain were investigated through tensile and fatigue crack propagation test. The experimental results obtained were as follows. 1. The measurements of X-ray stress constant K was possible for normalized SS41 steel, but not for M.E.F. dual phase steel and A1 7075-T651 alloy because the former has crystal grain with residual stress and the latter has texture with preferred orientation. K=-326.20 MPa/deg was obtained for normalized SS41 steel. 2. In order to predict the plastic strain ε_(p), half-value breadth B was more effective than residual stress σ_(r) . The relationship between ε_(p) and B was as follows Normalized SS41 steel : B = 0.295 log ε_(p) + 2.175 M.E.F. dual phase steel : B = 0.209 log ε_(p) + 2.539 A1 7075-T651 alloy : B = 0.548 log ε_(p) + 2.326 3. In normalized SS41 steel and M.E.F. dual phase steel, the residual stress σ_(r) on fatigue fractured surface arranged by K_(max) were dependent upon stress ratio R, but σ_(r) arranged by △K were independent upon R. Therefore their relationship could be represented by individual curve corresponding to each experimental material. △K could be estimated by using following relationship. Normalized SS41 steel : σ_(r) = -138.60 log △K + 353.52 M.E.F. dual phase steel : σ_(r) = -217.71 log △K + 366.23 4. In normalized SS41 steel and M.E.F. dual phase steel, the half-value breadth B on fatigue fractured surface arranged by △K was dependent upon the stress ratio R, but B arranged by K_(max) was almost independent upon R and tended to increase slightly as K_(max) was increased. 5. The depth of maximum plastic zone w_(y) were determined on the basis of the distribution of the half-value breadth for normalized SS41 steel and that of the residual stress for M.E.F. dual phase steel. K_(max) could be estimated by the measurement of w_(y) using following relationship. Normalized SS41 steel : K_(max) = σ_(y)(w_(y)/0.136)^(1/2) M.E.F. dual phase steel : K_(max) = σ_(y)(w_(y)/0.143)^(1/2) 6. In A1 7075-T651 alloy, the half-value breadth B for fatigue fractured surface increased as stress intensity factor K and effective stress intensity factor range ratio U increased. The diffraction pattern at the position of △K=10 MPa√m in all stress ratios presented characteristic shape according to degradation degree and fatigue crack propagation rate. 7. A1 7075-T651 alloy had texture with preferred orientation, but the half-value breadth and diffraction pattern showed characteristic change according to loading history. Therefore, the failure analysis of texture with preferred orientation was possible by X-ray diffraction technique.

改良化處理하여 急冷凝固한 7075Al 合金의 組織에 관한 硏究

이홍규 漢陽大學校 1990 국내석사

항공기 구조용 A1 7075/CFRP 적층 복합재 제조에 관한 연구

이제헌 부산대학교 일반대학원 2001 국내박사

A study has been made to establish an optimum condition in the surface treatment and curing process. These will be an important parameters for the fabrication of multilayered hybrid composite material, Al 7075 / CFRP CARALL(CARALL: carbon fiber reinforced aluminum laminates). Considering of the above results, 2, 4, 6 and 8 ply CARALL were fabricated with 0.25mm thickness Al 7075-T6 alclad sheet (KAISER Co.), FM 300M adhesive film (CYTEC Co.) and 0.13mm thickness F584-4 carbon epoxy prepreg (HEXEL Co.) in autoclave. Effects of carbon fiber direction and thickness variation in tensile strength were investigated by experiment and theoretical approach. Damage behavior of carbon fiber laminates and CARALL laminates were investigated also. Moreover, in order to study the characteristic of strength decrease by drilling on manufacturing and assembling of aircraft parts, specimen were made with circular hole notch and tested on tensile strength experiment. Characteristic of notch tensile strength and modified failure prediction criterion were reviewed in this experiment. The results are as follows : 1. The PAA (phosphoric acid anodizing process) provided the best adhesive strength among the specimens. On the other hand, the poor adhesive strength was shown on the vapor degreased and CAA (chromic acid anodizing process) and FPL (sulfuric . sodium dichromate acid etching process) was nearly same to the PAA. 2. The adhesive strength of the co-cured Al and carbon fiber prepreg higher than that of secondary cured one. But with respect to the dimensional stability, the secondary curing method was better than co-curing method. In secondary curing method, the condition of 0.35 MPa pressure shown higher adhesive strength than 0.6 MPa pressure condition and it was shown that the increaser curing time, the increaser adhesive strength. The effectiveness of adhesive strength increasing according to curing time was effective in 90 min curing time rather than 120 min curing time. 3. DMA(Dynamic Mechanical Analysis) test revealed T_(g) in curing times over 60 min is nearly same, so it is estimated they will have similar degree of curing and joint durability in using FM300M adhesive film. Considering of the results of the above mention, the condition of 0.35 MPa, 90 min, secondary curing method with PAA treated Al is recommended to fabricate the precise, high adhesive and durable aircraft parts. 4. High strength CARALL hybrid composite was fabricated with thin Al 7075-T6 sheet and adhesive film instead of glass fiber. The effect of strength enhancement was shown in case of using more than 4 ply F584-4 carbon prepreg. 5. As the results of analyzing the state of stress distribution of section of CARALL, by the FEM analysis of CARALL of L(0°) direction, 4 ply, it was found that a partial stress increase in order of epoxy adhesive, Al 7075, CFRP. And the partial stress of CFRP carried out a great portion of applied stress. 6. The real failure strengths of L(0°) direction, 4 ply CARALL were a little lower than theoretical strength by the law of mixture, but in case of 6 and 8 ply CARALL , the strengths were nearly equal to theoretical strength. And in case of 45°and T(90°) direction, the strengths of 2, 4, 6 and 8 ply CARALL were nearly equal to theoretical strength also. So it is estimated CARALL nearly satisfy the law of mixture in spite of fiber direction and thickness. 7. The impact resistance of CARALL was higher than that of CFRP. This is because both side Al sheet of CARALL absorb a great of impact damage. 8. Notch sensitivity of CFRP was decreased by hybriding with the high strength and high notch sensitivity CFRP material and ductile Al 7075. 9. Characteristic length d_(0) was different in Al 7075, CFRP, CARALL and vary with hole diameter 2R and width W. Notch sensitivity coefficient k increase in order of Al 7075, CARALL, CFRP. In same material, coefficient k decrease with increasing width. 10. Kims' modified failure prediction criterion was satisfied in CARALL, CFRP, Al 7075 better than Whitney's criterion. Between characteristic length d_(0) and notch strength, there is in inverse proportion, d□ increase with width W in same notch strength.

急冷凝固한 7075알루미늄 合金의 粉末組織에 관한 硏究

조대형 漢陽大學校 大學院 1988 국내석사

다구치 실험계획법을 이용한 7075-T76 알루미늄 합금의 최적 열처리 조건선정에 관한 연구

이선곤 경상대학교 항공우주특성화대학원 2019 국내석사

Aluminum alloys have been consistently studied and widely applied in the field of aerospace industries due to their ideal strength to weight ratio, good mechanical properties, workability and formability. Especially, 7000 series aluminum alloys (aluminum is alloyed with zinc and magnesium) have been used for commercial and military aircraft and launch vehicle because it can be precipitation hardened to provide the highest strength. The purpose of this study is to improve the electrical conductivity and hardness of Al7075-T76. This paper is focused on determining the parameters and optimal levels of Al7075-T76 heat treatment process. In this study, Taguchi method was used to investigate the optimization pro¬cess parameters of heat treatment. Four controllable factors were selected, solution heat treated temperature, refrigerated storage time, 1st step artificial aging time and 2nd step artificial aging time. The experiment was performed using orthogonal ar¬rays of L9(34) and was conducted by using the S/N ratio, analysis of ANOVA and F-test. As a result, the solution heat treatment temperature is the most influential parameter for electrical conductivity and the 1st step artificial aging time is the most influential parameter for hardness. The optimal heat treatment conditions for electronical conductiv¬ity of the Al 7075-T76 alloys are a solution temperature of 466°C, 1st step artificial aging time of 7 hours and 2nd step artificial aging time of 18 hours. The optimal heat treatment condi¬tions for hardness of the Al 7075-T76 are a solution temperature of 493°C, refrigerated storage time of 3 days and 2nd step artificial aging time of 17 hours. Based on the results of this study, it has shown that Taguchi method is a great way to find the optimized condition of Al 7075-T76 heat treatment process. Further studies should be applied to other heat-treated materials such as steel and titanium alloys and characteristics that are changed by heat treatment such as ultimate tensile strength, yield strength and elongation.

열간 성형 후 급냉한 Al7075 판재 부품의 미세조직 및 기계적 특성에 관한 연구

김민석 경북대학교 대학원 2018 국내석사

자동차 산업에서는 연비향상을 위한 경량화 소재의 적용이 지속적으로 증가하고 있다. 하지만, 기존에 예상한 것과 달리 경량 비철금속의 적용이 큰 폭으로 증가하지 않고 있는데 이는 소재 물성에 대비하여 높은 비용이 원인으로 판단된다. 알루미늄 합금은 우수한 비강성과 내식성을 강점을 가지고 있지만 소재비용과 철강에 비하여 높은 공정비용으로 양산화에 걸림돌이 되고 있다. 본 연구는 알루미늄 7075 합금의 열처리 과정 중 고온변형에 의해 생기는 미세조직 변화를 분석하여 고온 변형기구를 규명하고, 성형 후 기계적 특성을 평가하고자 한다. 또한 높은 공정비용으로 인한 양산화 적용이 어렵다는 문제점을 극복하기 위해 알루미늄 7075 합금의 용체화 처리 온도구간에서 열처리와 동시에 고온 예비성형을 한 후 직접 냉각성형을 도입한 열처리 단축 공정울 제안하고, 연구된 결과를 통해 제안된 성형법의 가능성을 판단하고자 한다. In the automobile industry, the application of lightweight materials for fuel economy improvement is continuously increasing. However, unlike the previous forecast, the application of lightweight non-ferrous metals has not increased significantly, which is attributable to high costs in comparison with the material properties. Aluminum alloys have a strong nose and corrosion resistance, but they are obstacles to mass production due to material cost and high processing cost compared with steel. The purpose of this study is to investigate microstructural changes caused by high temperature deformation during heat treatment of aluminum 7075 alloy and to investigate mechanical properties after deformation. In order to overcome the problem that it is difficult to apply mass-production due to high process cost, we propose a heat treatment and shortening process that adopts direct cooling molding after high-temperature preforming with heat treatment at the solution treatment temperature range of aluminum 7075 alloy. To determine the feasibility of the proposed process.

强塑性 變形된 7075Al 合金의 熱處理에 따른 機械的 性質의 影響

박재순 서울産業大學校 産業大學院 2006 국내석사

금속 및 합금재료를 이용하여 초미세 결정립으로 제어하면 강도는 크게 증가하고, 인성의 저하는 크게 보이지 않는다. ECAP(Equal Channel Angular Pressing)법은 두 개의 교차각을 갖는 금형에 시험편을 통과 시켜 단면적수축 없이 가공하는 방법으로 재료를 초소성변형시키는 방법으로 보고되고 있다. 본 연구에서는 최근 항공기 재료 및 스포츠 용구에 쓰이는 고강도 합금으로 주목 받고 있는 상용 7075Al 합금을 ECAP법을 이용하여 강소성 변형 후 열처리 온도 변화에 따른 미세조직 및 기계적 성질의 변화에 대해 평가하였다. 7075Al 합금을 봉제로 가공하여 용체화처리 후 시효처리 하였다. 열처리 후 1~4pass까지 ECAP 가공을 실시하여 가공률에 따른 경도 및 미세조직을 확인하였다. 가공률의 증가에 따라 경도값은 2pass까지 큰 변화를 보였으나 2pass가공 이후에서는 큰 변화가 없었다. 이는 시료내의 석출물의 영향으로 사료되어 이를 규명하고자 가장 큰 경도값을 갖는 시험편을 선택하여 열처리를 실시한 후 OM과 SEM 관찰을 통해 미세조직 및 석출물의 형태 및 크기를 확인하였다. 또한 TEM 관찰을 통해 석출물의 조성을 확인하였다. In this study, experiments were conducted to evaluate the characteristics of the mechanical properties and microstructures of an 7075Al alloy with number of passes and stabilizing annealing after the equal channel angular pressing. 7075Al alloy was overaged after solution treatment. The ECAP was conducted from 1pass to 4pass after heat treatment. The hardness and microstructures were identified with number of pass. Hardness was significantly increased from 0pass(Hv 83) to 2pass(Hv 125). However, from 3pass(Hv 132) to 4pass(Hv 134) were slightly increased. It was considered effect of precipitation in specimen. Thus, experiments were conducted to find out in annealinged 2pass. Microstructures and the form and size of precipitation were identified through OM and SEM and dislocation and chemical composition of precipitation were identified through TEM.

7075-T651 알루미늄合金의 表面균열進展에 미치는 應力比의 影響

김성민 漢陽大學校 大學院 1985 국내석사

7075-T6 알루미늄 합금 판재에서 η´ 석출상의 재고용이 기계적 특성에 미치는 영향

김영위 부산대학교 대학원 2022 국내석사

본 연구에서는 항공기용 판재로 사용되는 고강도 7075-T6 합금 판재를 차체에 적용하기 위하여 성형성 향상을 목적으로 용체화처리를 수행하고 이에 따른 η´상의 재고용이 기계적 특성 및 후속 시효처리에 미치는 영향을 조사하였다. T6 합금 판재는 450 ℃ 이상에서 용체화처리 시 주요 강화상인 η´상의 재고용으로 인하여 연신율이 약 23%로 크게 증가하였다. 반면 400 ℃에서 용체화처리 시 η´상의 재고용이 적고 입계에 η상을 형성하여 비교적 낮은 연신율 및 강도가 나타났다. T6 합금 판재는 용체화처리 후 자연시효 시 연신율의 저하는 적었으며 기지와 정합한 GPⅠ이 형성됨에 따라 항복강도는 증가하였다. 자연시효 후 소부경화처리 시 GPⅠ의 재용해 및 η´상의 재석출이 나타났으며 추가적인 항복강도의 상승은 작았다. 특히 400 ℃에서 용체화처리한 경우 낮은 고용량으로 인하여 소부경화처리 시 η´상의 재석출이 적어 항복강도가 감소하였다. 따라서 소부경화특성을 크게 향상시킬 수 있는 예비시효를 도입하고 기계적 특성을 평가하였다. 용체화처리 직후 예비시효를 도입한 결과 성형 전 연신율은 약 20 % 이상을 유지하였으며 최종 소부경화처리 시 항복강도가 크게 증가하였다. 이 때 용체화처리의 온도가 증가할수록 미세한 η´상이 다량 재석출 되어 항복강도는 더욱 증가하였다. 즉, 7075-T6 합금 판재에서 성형 전 연신율 확보와 소부경화처리를 통한 항복강도 향상을 위해서는 적절한 용체화처리 온도의 설정이 중요한 것을 확인하였다. In this study, the 7075-T6 aluminum alloy sheet, which is used as a high-strength aircraft sheet, was subjected to solution treatment to improve the formability for application to the car body. And the effect of dissolution of the η' phase on the mechanical properties and subsequent aging treatment were investigated. The elongation of the T6 alloy sheet was significantly increased to about 23 % due to the dissolution of the η' phase, the main hardening phase, immediately after solution heat treatment at 450 °C or higher. In contrast, after solution treatment at 400 ℃, there was relatively low elongation and strength due to the precipitation of the η phase at grain boundaries and low dissolution of the η´ phase. After natural-aging, GPⅠ, which is coherent with the aluminum matrix, was formed and yield strength increased. When bake hardening after natural-aging was performed, partial dissolution of the GPⅠ and re-precipitation of the η´ phase were observed, and the additional increase in yield strength was low. In particular, in the case of after solution heat treatment at 400 °C, the yield strength decreased due to low solubility and re-precipitation of the η´ phase. Therefore, pre-aging, which can significantly improve the bake hardenability, was introduced and the mechanical properties were evaluated. As a result of introducing pre-aging immediately after solution heat treatment, the elongation was maintained at about 20% or more, and the yield strength was greatly increased after the final bake hardening treatment. As the solution treatment temperature increased, a large amount of fine η´ phase was re-precipitated, So the yield strength was more increased. Therefore, to secure the elongation before forming and improve the yield strength after bake hardening in 7075-T6 alloy sheet, it is important to set an appropriate solution treatment temperature.