Study on microstructure and residual stress analysis of additive manufactured AlSi10Mg alloy using Powder Bed Fusion
- 주제(키워드) Additive manufacturing , Residual stress , XRD , Nanoindentation , ECCI
- 발행기관 고려대학교 대학원
- 지도교수 오승주
- 발행년도 2023
- 학위수여년월 2023. 2
- 학위명 석사
- 학과 및 전공 대학원 신소재공학과
- 세부분야 해당없음
- 원문페이지 100 p
- 실제URI http://www.dcollection.net/handler/korea/000000270086
- UCI I804:11009-000000270086
- DOI 10.23186/korea.000000270086.11009.0001428
- 본문언어 영어
초록/요약
AlSi10Mg alloy is a representative aluminum-based alloy composition used in additive manufacturing (AM) as a lightweight material with low coefficient of thermal expansion, high specific strength and wear resistance. In particular, through the laser beam powder bed fusion (PBF-LB) process, it is possible to produce complex shaped products with a small size and is applied to engineering parts manufacturing, automobiles and aerospace industries. However, aluminum alloy requires higher energy to produce high-density products than other materials due to the characteristics of high heat reflectivity. Repeated heating and cooling by rapid cooling rate induces thermal gradients and expansion and shrinkage, resulting in residual stress (RS) the products. RS should be limited to a minimum because it affects mechanical properties and causes product deformation and destruction of products. There are limitations in applying the destructive method commonly used for the RS evaluation of products manufactured through PBF due to the difficulty of repeated measurement and inaccuracy due to the small product size. Therefore, it is necessary to apply the non-destructive evaluation method and verify the effectiveness of the method. In this study, AlSi10Mg powder was used for AM by using PBF-LB, and the microstructure changed as well as surface RS generated throughout the AM process were observed. For RS observation, Non-destructive methods including X-ray diffraction (XRD), Nano-indentation, and electron backscattering diffraction (EBSD) was employed. After XRD analysis, Williamson-Hall plot, sin2ψ, and cos α method were used to analyze the data. In addition, heat treatment was performed to change RS, and the effectiveness of a non-destructive measurement method that does not require a stress-free sample was discussed by comparing the results between the methods used. Electron Channeling Contrast Imaging (ECCI) technique was used for observation dislocations inside precipitates generated and coarsened by heat treatment. The field emission scanning electron microscope (FE-SEM) was chosen to observe stacking defects on the internal scale of particles. It was discussed that ECCI could be an alternative to the transparent electron microscope (TEM).
more초록/요약
AlSi10Mg 합금은 낮은 열팽창 계수, 높은 비강도 및 내마모성을 가진 경량 소재로써 적층 제조(Additive manufacturing, AM)에 사용되는 대표적인 알루미늄계 합금 조성이다. 특히 powder bed fusion (PBF) 방식을 통해 작은 사이즈를 갖는 정교한 제품의 생산이 가능하며 엔지니어링 부품 제조, 자동차 및 항공우주 산업에 응용된다. 하지만 열반사율이 높은 특성 때문에 다른 소재에 비해 고밀도 제품을 생산하기 위해서는 적층 시 더 높은 에너지를 갖는 레이저 열원이 필요하다. PBF 공정 시 반복되는 급속 가열 및 냉각은 제조품에 열 구배와 팽창 및 수축을 일으키며 잔류 응력이 발생하게 된다. 기존의 연구들을 통해 적층 제조품 표면에는 인장 잔류 응력이 존재하는 것으로 알려져 있는데, 이는 기계적 특성에 영향을 미치고 제품 변형 및 제품 파손을 유발하므로 최소한으로 제한해야 한다. 잔류 응력 평가를 위해서 보편적으로 사용되는 파괴적 방법을 PBF 적층 제조품에 적용하기에는 반복 측정의 어려움, 제품 크기, 비용 문제 등으로 인해 한계가 존재한다. 따라서 비파괴적인 평가 방법의 적용과 그 방법의 유효성에 대한 검증이 필요하다. 이를 위해 본 연구에서는 AlSi10Mg 분말을 laser beam PBF 방식의 적층 제조에 사용하였으며, 공정 시 발생하는 표면 잔류 응력과 미세구조를 관찰하였다. 표면 잔류 응력 관찰을 위해 X선 회절 (X-ray diffraction, XRD), 나노 인덴터 측정 (Nanoindentation) 및 전자 후방 산란 회절 (Electron backscatterd diffraction, EBSD)을 포함하는 비파괴적 방법을 사용하였다. XRD 측정 후에는 Williamson-Hall plot, sin2ψ 및 cos α 방법을 사용하여 분석했다. 또 잔류응력 완화를 위해 열처리를 진행하고 그 변화를 관찰했으며 사용된 방법 간의 결과 비교를 통해 stress-free sample을 필요로 하지 않는 비파괴적 측정 방법의 효율성에 대해서도 논의하였다. 열처리에 따라 생성 후 조대화 된 침전물 내부의 전위 및 미세조직 변화를 관찰하기 위해 ECCI (Electron channeling contrast imaging) 기법을 사용하였다. 전계 방사형 주사 전자 현미경 (Field emission scanning electron microscope, FE-SEM)을 사용해 입자 내부 규모에서 적층 결함 관찰을 수행했으며, 이는 투과 전자 현미경 (Transparent electron microscope, TEM)의 대안이 될 수 있음을 논의했다.
more목차
ABSTRACT ................................................................................................................................... ⅰ
국문 초록 ........................................................................................................................................ ⅲ
PREFACE....................................................................................................................................... ⅴ
TABLE OF CONTENTS .............................................................................................................. ⅵ
LIST OF TABLES ......................................................................................................................... ⅷ
LIST OF FIGURES ........................................................................................................................ ⅸ
NOMENCLATURE ........................................................................................................................ ⅹⅰ
CHAPTER 1. INTRODUCTION .................................................................................................... 1
1.1 Metal additive manufacturing .......................................................................................... 1
1.2 Al-Si-Mg alloy ................................................................................................................. 2
1.3 Residual stress ................................................................................................................. 2
1.4 Microstructural analysis ................................................................................................... 4
CHAPTER 2. THEORETICAL BACKGROUND ......................................................................... 5
2.1 Non-destructive techniques.............................................................................................. 5
2.1.1 Williamson-Hall plot method .............................................................................. 5
2.1.2 sin2ψ method ....................................................................................................... 8
2.1.3 cos α method ..................................................................................................... 16
2.2 Destructive and semi-destructive techniques ................................................................. 22
2.2.1 Destructive techniques ...................................................................................... 22
2.2.2 Nanoindentation ................................................................................................ 23
2.3 Electron channelling constrast image (ECCI)................................................................ 33
CHAPTER 3. RESEARCH STRATEGIES .................................................................................. 35
CHAPTER 4.EXPERIMENTAL PROCEDURE .......................................................................... 37
4.1 Materials chaeracterization ............................................................................................ 37
4.2 Additive manufacturing and heat treatment ................................................................... 40
4.3 Residual stress measurement ......................................................................................... 42
4.4 Microstructural characterization .................................................................................... 44
CHAPTER 5. RESULTS ............................................................................................................... 45
5.1 Microstructure ................................................................................................................ 45
5.2 XRD ............................................................................................................................... 49
5.3 Residual stress ............................................................................................................... 51
5.3.1 Williamson-Hall plot method ............................................................................ 51
5.3.2 sin2ψ method ..................................................................................................... 54
5.3.3 cos α method ..................................................................................................... 57
5.3.4 Nanoindentation ................................................................................................ 61
5.4 Microstructural characterization .................................................................................... 64
5.4.1 ECCI ................................................................................................................. 64
5.4.2 EBSD ................................................................................................................ 66
CHAPTER 6. DISCUSSION ........................................................................................................ 69
6.1 Differences according to sample orientation ................................................................. 69
6.2 Differences according to measurement method ............................................................. 71
6.3 Nanoindentation ............................................................................................................. 72
6.4 Effectiveness of the measurement methods ................................................................... 72
CHAPTER 7. CONCLUSION ...................................................................................................... 76
REFERENCES .............................................................................................................................. 78
