응력제거 열처리 공정조건이 적층제조한 Ti-6Al-4V 합금의 잔류응력 및 경도에 미치는 영향
Effect of stress relief heat treatment on the residual stress and hardness of additively manufactured Ti-6Al-4V alloy
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본 연구에서는 Laser Powder bed fusion(L-PBF) 공정을 사용하여 제작된 Ti-6Al-4V 합금 적층성형품의 응력제거 열처리 온도와 시간의 변화에 따른 미세조직, 잔류응력 그리고 경도의 변화를 연구하였다 . 잔류응력 제거를 위한 열처리 시험 결과 823K에서는 240분, 873K에서는 60분 이상 열처리시 치수변화 및 기계적 특성 저하를 야기하는 결정립 성장 및 상변화 발생 없이 대부분의 잔류응력이 30MPa 이하로 감소되는 것을 확인하였다. 또한, 열처리 온도 및 시간의 증가와 함께 경도가 증가하는 경향을 보였다 . 이러한 결과는 XRD 및 SEM-EBDS의 phase map 분석을 통해 확인되지 않지만, 773~873 K 온도범위에서 등온 열처리시 국부적인 침상 Martensitic α' 상의 미세화가 원인으로 추정된다.
Abstract
The effect of stress relief heat treatment temperature and duration time on the microstructure, residual stress and Vickers hardness of additively manufactured Ti-6Al-4V alloy using laser powder bed fusion process was clarified. As a result of stress relief heat treatment for 240 minutes at 823 K and 60 minutes or more at 873 K, residual stress was decreased less than 30 MPa without grain growth and phase transformation which causes dimensional distortion and deterioration of mechanical properties. In addition, hardness was increased with increasing heat treatment temperature and duration time. It was deduced that the refinement of acicular martensitic α' phase due to the increasing duration time of isothermal heat treatment at 773~873 K, which was not detected by XRD and phase map analysis using SEM-EBSD, probably increases the hardness.
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