Study of single point incremental sheet forming with low-frequency vibration : 저주파 진동을 이용한 단점 점진 판재 성형에 관한 연구

다국어 초록 (Multilingual Abstract)

Single point incremental sheet metal forming is a new type of sheet metal plastic forming technology. A specific tool is used to locally load the sheet, resulting in overall cumulative deformation under a die-less and unconstrained condition, and finally, the target shape is obtained. It integrates the design and manufacture, has the advantages of flexible, green, fast, and low cost. However, due to the local loading characteristics of the tool, the sheet is prone to instability during the forming process, leading to large springback, and ultimately making the forming accuracy difficult to meet the requirements of use. In addition, due to the uneven deformation of the material and the complex evolution of the structure, as well as the interaction of the forming parameters on the forming accuracy, the control of the forming accuracy becomes more difficult. This article attempts to introduce low-frequency vibration into single point incremental sheet forming technology to improve the quality problems of formed products.
In this research, a low-frequency vibration-assisted single point incremental forming method is proposed. The unit-body in the contact area between the tool and the sheet is analyzed, and the mechanical equation is established. According to the assumption of the relationship between tensile stress and shear stress in sheet plastic forming, an approximate solution to the mechanical equation is obtained.
A finite element model of the low-frequency vibration single point incremental sheet forming is established in which the low-frequency vibration is applied to the forming tool. Through the explicit and implicit numerical simulation of the whole forming process, the effects of low-frequency vibration on equivalent stress, equivalent strain, thickness distribution, forming force, residual stress, and springback are studied. The results showed that the low-frequency vibration assisted single point incremental forming could effectively reduce the equivalent stress and forming force. At the same time, the reduction of residual stress significantly reduced springback and improved the accuracy of formed products. A set of low-frequency vibration single point incremental sheet forming systems was developed. After that, through a series of experiments, the influence of low-frequency vibration on formability, forming force, surface quality, and forming accuracy was analyzed. In addition, based on the RSM analysis, the main parameters in the forming process of low-frequency vibration were analyzed and optimized. The results showed that the low-frequency vibration significantly reduces the forming force and improves the formed product's surface quality and forming accuracy. At the same time, there is no coupling relationship between low-frequency vibration and other main parameters, which is suitable for popularization and use in actual production.

국문 초록 (Abstract)

단점 점진 판재 성형공정은 금형이 없이 간단한 공구를 사용하여 판재의 국부적인 소성변형을 가하여 점진적으로 성형하는 새로운 금속 소성성형 기술이다. 동 기술은 시제품이나 다품종 소량 제품의 빠른 생산을 위해 특화된 기술로 기존의 프레스 현장에서와 대용량의 대형 프레스와 상하 금형을 사용하지 않는다. 그 대신에 공구의 경로를 제어할 수 있는 기능을 갖는 CNC 머신이나 로봇 등을 통해 제품의 외부 형상의 궤적을 따라 이동하고 가압하면서 국부적인 소성을 유도하는 것에 의해 제품을 축차적으로 장출성형하는 혁신적인 기술이다. 그러나, 공구의 국부적인 하중 특성과 금형이 없기 때문에 성형 공정 중에 판재의 불안정 현상이 쉽게 발생하고, 이에 따라 상대적으로 큰 스프링 백을 발생하여 제품의 성형 정확도가 떨어진다. 또한, 가공 공구 경로로 인해 성형품의 표면 품질 저하, 큰 가공력이 필요하다는 문제들이 존재하여 실제 산업 분야에서 단점 점진 판재 성형 기술을 적용하는 것이 제한적이다. 본 연구에서 기존 단점 점진 판재 성형에 성형품의 품질 문제를 개선하기 위해 단점 점진 판재 성형 공정에 저주파 진동 보조 기술을 도입하고자 한다.
우선, 단점 점진 판재 성형공정의 메커니즘을 분석하여 단점 점진 판재 공정의 성형 특성을 기반으로 기계적 해석 모델을 구성하였다. 이에 따라 저주파 진동을 포함한 SPIF 과정의 역학 해석 모델을 도출하였다.
이어서 공구가 저주파 진동을 가할 수 있는 단점 점진 판재 성형 유한 요소 모델을 구성하였다. 명시적 및 암시적 수치 시뮬레이션을 통해 전체 성형 공정에 저주파 진동은 등가 응력, 등가 변형률, 두께 분포, 가공력, 잔류 응력 및 스프링 백에 미치는 영향을 해석하였다. 결과적으로 저주파 진동으로 인해 등가 응력과 가공력을 효과적으로 줄일 수 것을 확인되었고 특히 잔류 응력이 감소함으로 인해 스프링백의 발생이 억제되며 성형품의 성형 정밀도가 크게 향상되었다.
저주파 진동에 대한 실험적 연구를 수행하기 위해 저주파 진동 단점 점진 판재 성형 시스템을 개발하였고 실제 실험을 통해서 가공력 및 성형 정확도에 대한 저주파 진동의 시뮬레이션 결과를 검증하였다. 특히 저주파 진동으로 인해 판재와 공구 사이의 마찰 조건을 효과적으로 개선되며 윤활제의 사용을 크게 줄일 수 있다는 결과를 확인되었다. 또한, 응답표면법을 사용하여 저주파 진동 단점 점진 판재 성형 공정의 주요 공정변수를 분석하여 최적화하였다. 결과적으로 저주파 진동으로 인해 성형 공정에 필요한 가공력이 크게 감소하며 성형품의 표면 품질과 성형 정확도가 크게 향상된다는 것을 확인되었다.

목차 (Table of Contents)

• Chapter 1 Introduction 1
• 1.1 Research background and significance 1
• 1.2 Overview of single point incremental sheet forming technology 3
• 1.3 Status of research on the single point incremental sheet forming 8
• 1.3.1 Status of research on the forming accuracy 8
• 1.3.2 Status of research on the mechanics mechanism 11
• 1.3.3 Status of research on the numerical simulation 13
• 1.3.4 Status of research on the surface quality 15
• 1.3.5 Status of research on the optimization of processing parameters 16
• 1.3.6 Status of research on the incremental sheet forming with auxiliary technology 17
• 1.4 Status of research on the plastic forming process with vibration 20
• 1.4.1 Status of research on the incremental sheet forming with auxiliary technology 20
• 1.4.2 Application of low-frequency vibration in the plastic forming process 23
• 1.5 Problems in current research of single point incremental sheet forming 24
• 1.6 Main research content and chapter arrangement of this topic 25
• Chapter 2 Analysis of the forming process of low-frequency vibration single point incremental sheet forming 29
• 2.1 Forming principle of low-frequency vibration single point incremental forming 29
• 2.2 Definition and mechanism analysis of the geometric error 30
• 2.2.1 Definition of geometric error 30
• 2.2.2 Generation mechanism of axial error 31
• 2.2.3 Generation mechanism of normal error 33
• 2.3 Establishment and solution of the mechanical model 35
• 2.3.1 Single point incremental sheet forming process 35
• 2.3.2 Low-frequency vibration single point incremental sheet forming process 42
• Chapter 3 Finite element modeling and analysis of low-frequency vibration single point incremental sheet forming process 49
• 3.1 Research on numerical simulation of LFV-SPIF Process 49
• 3.1.1 Finite element modeling of the LFV-SPIF process 49
• 3.1.2 Characteristics of the LFV-SPIF process 49
• 3.1.3 Establishment of finite element model 50
• 3.2 Influence of low-frequency vibration on the forming process 61
• 3.2.1 Simulation analysis of equivalent stress 61
• 3.2.2 Simulation analysis of equivalent strain 66
• 3.3 Thickness distribution analysis 70
• 3.4 Effect of low-frequency vibration on residual stress 73
• 3.5 Influence of low frequency vibration on forming force 78
• 3.5.1 Forming force in single point incremental forming 78
• 3.5.2 Influence of low-frequency vibration on forming force F_z 79
• 3.6 Numerical simulation analysis of forming accuracy 84
• 3.7 Summary 86
• Chapter 4 Experimental study on single point incremental sheet forming process with low-frequency vibration 89
• 4.1 Experimental equipment and methods 89
• 4.1.1 Low-frequency vibration system 91
• 4.1.2 Modal analysis of low-frequency vibration system 94
• 4.1.3 Formability evaluation 100
• 4.1.4 Measurement system of parts accuracy 101
• 4.1.5 Measurement system of axial forming force and surface quality 101
• 4.2 Effect of low-frequency vibration on the formability 102
• 4.3 Effect of low-frequency vibration on the axial forming force F_z 107
• 4.3.1 Influence of frequency on axial forming force F_z 109
• 4.3.2 Influence of amplitude on axial forming force F_z 111
• 4.4 Influence of low-frequency vibration on surface quality 114
• 4.5 Influence of low-frequency vibration on geometric accuracy 118
• 4.6 Parameter optimization of low-frequency vibration single point incremental sheet forming process 121
• 4.6.1 Experimental Design 121
• 4.6.2 RSM modeling 123
• 4.6.3 Axial forming force F_z 125
• 4.6.4 Surface roughness 128
• 4.6.5 Geometric accuracy 131
• 4.7 Multi-objective optimization 134
• 4.8 Summary 136
• Chapter 5 Conclusion and future works 138
• 5.1 Conclusion 138
• 5.2 Future works 141
• References 143
• Publications 157