해양구조물 체결을 위한 Al6061 합금 볼트의 통합 제조공정 설계

최근 수송 분야에서 경량화에 대한 관심이 증가하면서, 각종 수송 기계 부품에 비철금속을 적용하는 사례가 늘고 있다. 특히, 6xxx 계열의 알루미늄 합금은 낮은 비중량, 우수한 기계적 특성 및 높은 내식성으로 인해 소형 선박에서부터 해양 구조물에 이르기까지 다양한 해양 분야에 적용되고 있다. 패스너를 사용한 기계식 체결은 사용이 쉽고 적용이 용이하여 알루미늄 합금과 강재의 접합에 널리 사용되어왔다. 현재까지도 산업 현장에서는 알루미늄 합금과 강재의 체결을 위해 스테인리스강 볼트가 사용되고 있다. 그러나 알루미늄 합금의 모재에 기존의 스테인리스강 볼트를 사용하게 되면 중량의 증가뿐만 아니라 알루미늄 모재에서 갈바닉 부식이 발생하게 되고 이것은 특히 해양 환경에서 더욱 가속화된다. 따라서 스테인리스강 볼트를 알루미늄 합금 볼트로 대체하는 것은 추가적인 경량화를 도모할 수 있을 뿐만 아니라 해양 환경에서 이러한 갈바닉 부식의 생성을 미연에 방지할 수 있다. 현재까지 알루미늄 합금 볼트 성형에 대해 일부 연구들이 수행되어 왔다. 그러나 대부분의 연구들은 재료의 성형성 평가에 국한되어 있으므로 성형 공정에 대한 명확하고 체계적인 설계 기준이 없다. 또한, 볼트를 제조하기 위해 필수적인 트리밍 및 전조 공정의 설계가 누락되어있다. 따라서 본 연구에서는 해양구조물 체결을 위한 Al6061 합금 볼트의 통합 제조공정 설계를 수행하였다. 이를 위해 이론 설계 및 수치 해석을 진행하였다. 이론 설계에서는, 스크류 규격과 기하학적 관계를 고려하여 전조 공정의 초기 소재경과 압입 깊이를 계산하였다. 헤딩 공정을 위한 초기 소재 치수는 체적일정법칙으로 구하였다. 결함 예측에 사용되는 공정 한계를 고려하여, 공정 단수를 설정하고, 헤딩 공정의 설계 규칙에 의해 예비 형상을 결정하였다. 또한 설계된 헤딩 공정에 대해 슬래브법을 사용하여 성형 하중을 예측하였다. 이론 설계를 바탕으로 헤딩, 트리밍 및 전조 공정에 대한 유한요소해석을 수행하였다. 헤딩 및 트리밍 공정 중에 발생하는 결함을 예측하기 위해 연성파괴기준을 적용하였다. 또한 전조 공정에서 금형의 압입량, 이송 속도 및 분당 회전수, 트리밍 공정에서는 펀치의 곡률 반경, 하부 금형의 랜드 폭 및 펀치와 하부 금형간의 정지 거리를 각각 설계 변수로 선정하고 다구찌법을 통해 공정을 최적화하였다. 제안 된 설계 방법을 검증하기 위해 알루미늄 합금 볼트 성형 실험을 수행하였고 높은 치수 정확도를 갖는 건전한 형상의 Al6061 합금 볼트를 얻을 수 있었다.|In recent years, with the increasing interest in weight reduction in the transportation sector, the application of non-ferrous metals to various transportation machinery parts has been increased. In particular, the 6xxx series of aluminum alloys are commonly used in marine applications from small vessels to offshore structures due to its low specific weight, high quality mechanical properties, and high resistance to corrosion. In order to join aluminum alloy and steel, mechanical joint by fasteners has been widely used because it is easy-and-practical for the application. Until now, stainless steels bolts have been used for fastening aluminum alloys and steels to industrial sites. However, when the stainless steel bolt is used for the base material of aluminum alloy, weight increases and galvanic corrosion occurs in aluminum, which is accelerated in marine environment particularly. Therefore, substituting aluminum alloy bolts for stainless steel ones can prevent such corrosion promotion in marine environment as well as extra weight increase. To date, several studies have been conducted on aluminum alloy bolt forming. However, most studies are limited to the evaluation of formability of materials, so there is no clear and systematic design criterion for the forming process. Furthermore, the design of the trimming and thread-rolling process, essential for manufacturing bolts, was not considered. Therefore, the integrated manufacturing process design of Al6061 alloy bolts for fastening offshore platforms was performed in this study. To do this, we carried out theoretical design and numerical analysis. At theoretical design, the initial rod diameter and the penetration depth in thread-rolling process were calculated according to the screw standards and geometric relation. Thereafter, the dimensions of the initial workpiece for the heading process were obtained by volume constancy law. Considering process limitations to predict the defects, the number of the stage was set, and preform was determined by the design rule in the heading process. In addition, the forming load for the designed heading process was predicted by using the slab method. Based on theoretical design, FE-analysis for the heading, trimming and thread-rolling process was conducted. The ductile fracture criterion was applied to predict the defects during the heading and trimming process. In addition, the Taguchi method is used to optimize the trimming and thread-rolling process with the set of design parameters, such as the penetration depth, transfer velocity, and revolutions per minute of rolling dies in the thread-rolling process, and the blade radius of the punch, land width of the bottom die, and stop distance between the punch and bottom die in the trimming process, respectively. To verify the proposed design, the aluminum alloy bolt forming experiment was carried out and sound Al6061 alloy bolt with high dimensional accuracy was obtained.1. Introduction 1 1.1 Research background 1 1.2 Precedent research 4 1.3 Objective and scope of study 8 2. Theoretical design of Al6061 alloy bolts manufacturing process 9 2.1 Procedures for theoretical design 9 2.2 Process design for thread-rolling 12 2.2.1 Calculation of initial diameter in thread-rolling process 12 2.2.2 Determination of penetration depth 13 2.3 Process design for heading 16 2.3.1 Determination of initial height of head section 16 2.3.2 Theoretical prediction of defects and setting of the number of process stages 17 2.3.3 Preform design for heading process 20 2.3.4 Forming load prediction by slab method 23 3. FE-analysis of cold-former forging process 29 3.1 Conditions of FE-analysis in heading process 29 3.2 Results of FE-analysis in heading process 31 3.3 Design of experiment for trimming process 34 3.3.1 Taguchi method 34 3.3.2 Parameter design for the Taguchi method 36 3.4 Conditions of FE-analysis in trimming process 43 3.5 Results of FE-analysis in trimming process 45 4. FE-analysis of thread-rolling process 55 4.1 Design of experiment for thread-rolling process 55 4.2 Conditions of FE-analysis in thread-rolling process 58 4.3 Results of FE-analysis in thread-rolling process 60 5. Experiment for manufacturing Al6061 alloy bolts 71 5.1 Experimental procedure 71 5.2 Experimental results 72 6. Conclusions 75 References 77 감사의 글 82Maste

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