Effects of Cr/Ni equivalent ratio and cooling rates on phase transformations and residual stresses in 17-4 PH alloy fabricated by LPBF

국문 초록 (Abstract)

본 연구는 17-4 PH 합금의 LPBF 공정 및 후열처리 동안의 상변태에 대해 연구하여 LPBF로 출력된 17-4 PH 합금의 결함을 개선하고자 하였다. LPBF 출력 17-4 PH 합금의 미세구조를 관찰하고, 응고 모드가 합금 원소 편석과 출력 중 석출물 형성에 미치는 영향을 파악하였다. 또한 마르텐사이트 변태를 통해 인장 잔류 응력장을 제어하는 방법을 제안하였으며, 고전적 핵생성 및 성장 이론에 기반하여 17-4 PH 합금의 LPBF 출력 중 상변태를 예측하는 모델링을 수행하여 최종 미세조직을 예측하는 모델을 제시하였다.
기존의 17-4 PH 스테인리스강과 달리, LPBF로 출력된 17-4 PH 합금에서는 급속 냉각으로 인한 Cu 편석과 반복적인 가열로 인한 ε-Cu 석출물이 관찰된다. Cu 편석과 ε-Cu 석출물이 시효 처리 중 석출 경화에 미치는 부정적인 영향을 개선하기 위한 용체화 처리 최적화를 수행하였다. ε-Cu 석출물은 고온, 장시간의 용체화 처리 후에도 기지 내로 용해되지 않았으나, 용체화 처리 온도와 시간이 증가할수록 석출물의 평균 직경이 감소하는 경향이 나타났다. 이에따라 1200 ℃-8시간 용체화 처리가 기존의 1050 ℃ 용체화 처리보다 시편의 경도를 향상시켰으며, 적층 제조 시편에서는 적어도 1100 ℃ 이상의 용체화 처리가 효과적임을 확인하였다.
17-4 PH 합금은 Creq/Nieq 및 냉각속도에 따라 다양한 미세조직으로 출력될 수 있다. 본 연구는 Creq/Nieq 비율과 VED를 변화시켜 미세조직을 제어하고 이를 통해 인장 잔류 응력을 제어하는 방법을 제안하였으며, 17-4 PH 합금의 LPBF 출력 미세 조직을 예측하는 다이어그램을 제시하였다. 마르텐사이트 변태 시 발생하는 인장잔류 응력 상쇄의 메커니즘을 규명하였으며, 마르텐사이트 변태는 Creq/Nieq 비율이 낮고 냉각속도가 느릴수록 더욱 용이하게 발생하여 LPBF 공정 중 인장 잔류 응력을 효과적으로 상쇄함을 확인하였다.
빠른 냉각속도로 인해 LPBF 출력 17-4 PH 합금의 최종 미세조직은 델타 페라이트, 마르텐사이트, 잔류 오스테나이트로 구성된다. 최종 미세조직을 결정하는 것은 델타 페라이트로 부터 오스테나이트가 형성되는 과정으로, 최종 미세조직을 예측하기 위해서는 이 상변태를 예측하는 것이 중요하다. 따라서 고전 핵생성 및 성장 이론을 기반으로 한 상변태 모델을 시뮬레이션된 온도프로파일과 결합하여 최종 미세조직 예 측을 수행하였다. 시뮬레이션 결과를 실제 상분율과 비교하여 모델이 높은 정합성 (R2 =0.9405)을 달성한 것을 확인하였다. 본 연구는 LPBF에서 최종 미세조직을 예측하기 위해 위치별 온도 프로파일을 고려하는 것이 중요하다는 것을 보여준다.

다국어 초록 (Multilingual Abstract)

This study investigated the phase transformations and changes in residual stress fields during the laser powder bed fusion (LPBF) process and post heat treatment of 17-4 PH alloys. Aimed at advancing both theoretical understanding and practical applications, this research bridges academic insights with industrial relevance. By providing a comprehensive analysis of microstructure control, phase transformation modeling, and residual stress management, the study establishes a scientific foundation for obtaining reliability of LPBF-fabricated 17-4 PH alloy. The microstructures of LPBF-fabricated 17-4 PH alloys were observed, and the influence of solidification modes on solute segregation and early precipitation was investigated. Various conditions of solution treatments were conducted to assess their effectiveness in homogenizing the matrix. Additionally, effects of chemical compositions and volumetric energy densities (VED) on the printed microstructures were studied and a microstructure prediction diagram was proposed. In addition, a method for controlling residual stress fields through martensitic transformation was also proposed based on the established relationship between microstructures and residual stress fields. Recognizing the critical importance of microstructure prediction, numerical modeling for phase transformation of the 17-4 PH alloy was conducted based on the classical nucleation and growth theory.
1. Homogenization through solution treatment and its effects on the precipitation-hardening
17-4 PH stainless steel (SS) exhibits high strength and good corrosion resistance via Cu-precipitation hardening. Unlike conventional wrought 17-4PH SS, Cu segregation and ε-Cu precipitates are observed in additively manufactured (AM) 17-4PH SS owing to the repeated rapid cooling after heating, which characterizes the AM process. In this study, solution treatment was conducted under various temperatures (1,000, 1,050, 1,100, and 1,200 °C) and durations (1, 2, 4, and 8 h) to minimize the negative effects of Cu segregation and ε-Cu precipitates on precipitation hardening. Because the Cu segregation and precipitates formed during the LPBF process are challenging to fully homogenize even with the most intensive solution treatment, it is important to minimize Cu segregation during the printing. The mechanical properties and microstructures of each condition for the Cu precipitation behavior were examined. Although the ε-Cu precipitates did not disappear after solution treatment, the average diameter of the ε-Cu precipitates tended to decrease with increasing solution treatment temperature and duration. Therefore, solution treatment at a temperature of 1,200 °C for 8 h was the best, resulting in increase of hardness about 128 HV xi compared to the conventional solution treatment at 1,050 °C. Solution treatment on at least 1,100 °C is effective in AM.
2. Understanding the relationship between martensitic transformation and residual stress fields
The 17–4 PH alloy can be printed into diverse microstructures. Existing reports on the alloy have focused on the effect of Creq/Nieq ratios and the processing parameters of LPBF on the final microstructures of the δ-ferrite and α’-martensite phases in the alloy. However, the effect of microstructural variations on the residual stress fields of LPBF-fabricated 17–4 PH SS has yet to be fully understood. This study bridges the gap between what is known about the microstructure and residual stresses by proposing an alloy-equivalent phase prediction diagram for the AM of the 17–4 PH alloy by varying the Creq/Nieq ratio and VED to control residual stresses via microstructure manipulation. Dilatometry and Satoh tests were performed to elucidate the mechanism of the offset effect in martensite transformation. To control the microstructures, samples were fabricated with various Creq/Nieq ratios (2.05, 2.15, 2.30, and 2.49) and laser powers (130, 150, 170, and 190 W). The 17–4 PH alloy exhibited a low Ms temperature and approximately net zero residual stresses in the Satoh test. The study confirmed that martensitic transformation, which occurs more readily at lower Creq/Nieq ratios and higher laser powers, effectively offsets tensile residual stresses during the LPBF process, resulting in almost zero tensile residual stresses on fully martensitic specimens.
3. Kinetic modeling of the δ-ferrite to γ-austenite phase transformation
17-4 PH SS fabricated by LPBF retains δ-ferrite due to rapid cooling rates of up to 107 K/s. γ-austenite, transformed from δ-ferrite, further transforms into α’-martensite under sufficient cooling rates, resulting in a final microstructure composed of a mixture of retained δ-ferrite, α’-martensite, and retained austenite. The cooling rate, controlled by the VED, and the alloy composition (Creq/Nieq ratio) can influence the microstructure in LPBF-fabricated 17-4 PH SS. These microstructures influence not only mechanical properties but also tensile residual stress fields, highlighting the importance of predicting the final microstructure. This study focuses on the kinetic modeling of δ-to-γ phase transformation under various thermal profiles, induced by different VEDs (Laser powers: 130 W, 150 W, 170 W, and 190 W), in alloys with distinct Creq/Nieq ratios (2.05, 2.15, 2.30, and 2.49). Numerical modeling, based on nucleation and growth theory, was combined with simulated thermal profiles. The model’s predictions were validated by comparing the simulated microstructures with experimentally observed ones, achieving a high accuracy (R² = 0.9405). This study emphasizes the importance of considering site-specific thermal profiles to predict final microstructures in LPBF, offering new insights for microstructure prediction in additive manufacturing.

목차 (Table of Contents)

• TABLE OF CONTENTS LIST OF FIGURES ············································································· iv
• LIST OF TABLES··············································································· ix
• ABSTRACTS ······················································································ x
• 1. Introduction····················································································· 1
• 1.1 Additive manufacturing processes ····················································· 1
• 1.2 Characteristics and current research areas of the AM process··················· 4
• 1.3 Applications of the AM process in industry ·········································· 6
• 1.4 References··················································································· 9
• 2. Literature Review·············································································11
• 2.1 Additive Manufacturing (AM)·························································11
• 2.1.1 Various Types of Metal Additive Manufacturing ·····························11
• 2.1.2 Powder Bed Fusion and Direct Energy Deposition System··················13
• 2.1.3 Process Parameters for Laser Powder Bed Fusion System··················17
• 2.2 Phase Transformation ···································································20
• 2.2.1 Solidification mode··································································20
• 2.2.2 Nucleation and Growth·····························································25
• 2.2.3 TTT diagram and CCT diagram ·················································29
• 2.2.4 Microstructure of 17-4 PH alloy fabricated by LPBF ························34
• 2.3 Residual Stress ············································································45
• 2.3.1 Mechanism of Residual Stress ····················································45
• 2.3.2 Offset of Residual Stress from Martensitic Transformation ················49
• 2.3.3 Post Heat Treatment································································52
• 2.4 References··················································································56
• 3. Research Objectives ··········································································61
• 4. Homogenization through solution treatment and its effects on the precipitationhardening··························································································64
• 4.1 Introduction ···············································································64
• 4.2 Experimental ··············································································66
• 4.3 Results and Discussions··································································71
• 4.3.1 Microstructure of the as-built specimen ········································71
• 4.3.2 Microsegregation in the as-built specimen······································76
• 4.3.3 Effects of the solution treatment on the microstructure and precipitation ·································································································82
• 4.3.4 Effects of the solution treatment on mechanical properties·················92
• 4.4 Summary ···················································································97
• 4.5 References··················································································98
• 5. Understanding relationship between martensitic transformation and residual stress fields ······················································································ 101
• 5.1 Introduction ············································································· 101
• 5.2 Experimental ············································································ 104
• 5.2.1 Materials and Processing Parameters········································· 104
• 5.2.2 Phase fractions····································································· 109
• 5.2.3 Characteristic physical testing and analysis·································· 112
• 5.3 Results and discussions································································ 113
• 5.3.1 Microstructures of as-fabricated bulks using unblended powders······· 113
• 5.3.2 Dilatometry and Satoh test ······················································ 115
• 5.3.3 Effect of Creq/Nieq ratio and laser power on the microstructures········· 120
• 5.3.4 Residual stress ····································································· 128
• 5.3.5 Alloy-equivalent Phase Predicting (APP) Diagram ························· 133
• 5.4 Summary ················································································· 136
• 5.5 References················································································ 137
• 6. Kinetic modeling of the δ-ferrite to γ-austenite phase transformation············ 141
• 6.1 Introduction ············································································· 141
• 6.2 Experimental ············································································ 144
• 6.2.1 Materials and microstructural observation ·································· 144
• 6.2.2 Thermal history profile simulation ············································ 147
• 6.3 Model description······································································· 150
• 6.3.1 Nucleation and Growth··························································· 150
• 6.3.2 Calculation of TTT, CCT curve and phase fraction ························ 154
• 6.4 Results and discussion ································································· 158
• 6.4.1 Nucleation rates, TTT diagrams and CCT diagrams······················· 158
• 6.4.2 Phase transformation during LPBF process ································· 165
• 6.4.3 Comparison between calculated and observed microstructures·········· 170
• 6.5 Summary ················································································· 175
• 6.6 References················································································ 176
• 7. Conclusions··················································································· 179
• Remarks on Copyright········································································ 181
• Abstract in Korean ············································································ 182