Testing and Modelling of SLM manufactured Ti-6Al-4V alloy under low cycle fatigue and creep conditions
Creators: name: Xi, Jiangjing
Publisher: name: Imperial College London
Year: 2023

description: Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high strength or high manufacturing costs metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, the disadvantage or challenge of AM is the unpredictable fatigue and creep properties of AM components. To realize the wide application of structural components manufactured by AM technology, their creep/fatigue failure and damage mechanism need to be understood so that the life of AM components can be predicted reliably. In this thesis, tensile and fracture mechanics type specimens were machined in accordance with the relevant ASTM standards from fabricated SLM with different heat treatment methods. All the specimens were post-treated with the same machined smooth surface and heated to relieve the residual stress. Various mechanical tests of the standard samples were performed to obtain the properties aiming to identify the critical factors influencing these properties. The room temperature tests carried out in this work include the tensile test, low cycle fatigue (LCF) test, crack growth rate investigation, and fracture toughness test. The LCF test was conducted using the standard samples in the low cycle fatigue (LCF) regime. The failure mechanism is observed by identifying the influence of post-processing methods. Scanning electron microscopy (SEM) techniques and optical microscopy (OM) are used to investigate the microstructural and fracture surface features of different types of specimens. In addition, a microstructure-based multistage fatigue model was applied to predict the LCF lives, which shows good agreement with the experimental results. Crack growth rate and fracture toughness tests were carried out and analyzed on the CT samples. Different build orientations and heat treatment methods effects on the fracture toughness and FCG rate were observed. A novel approach is presented to show a relationship between the FCG rate and microstructure characteristic using the database for the FCG tests and their microstructure and fractography analysis established from the experiments carried out. It was found that HIP treatment can help to reduce the effects of building directions due to the elimination of anisotropy. Furthermore, when the crack growth direction is perpendicular to the manufacturing direction, the crack growth rate is sensitive to the heat treatment method. The high-temperature properties test was largely focused on constant load uniaxial and notched bar creep tests under a continuous temperature environment. The uniaxial creep test of four types of SLM manufactured samples was carried out under 600℃ conditions at two stress levels. The notch acuity sensitivity investigation was conducted using the double sharp and blunt notched bar at 90 MPa, 600℃ and 100 MPa, 500℃, respectively. It is observed that no obvious defects were found in the fracture surface and crack profile. By comparing and analyzing the creep test results, the HIP-treated sample have a prior creep failure duration time to all the other manufacturing parameters. This demonstrates that the slightly elongated microstructure samples can help to improve the creep resistance by the characterization investigation. From the notched bar experimental results, the sharp notched bars have longer creep time and less strain deformation in the whole creep test process. The creep cracking behavior at the specific proper temperature was investigated and verified. A continuum damage mechanics-based model was proposed using a custom user subroutine in FE analysis software Abaqus to simulate the creep cracking damage evolution behaviors. The model applied a grain and grain boundary scale in meshing the creep cracking region using a CT geometry the same as that for the creep cracking test. The high temperature and mechanical properties were obtained from the test in this project. Considering that the defects were not detected in the fracture surface or crack profile, the FE model was operated without any distribution of voids. The microstructural features like average grain size and shape were described and generated in terms of meshing elements. The simulation results were found to be conservative and are validated by comparing them with the fracture behavior and crack propagation of the CT samples. The present work shows that in future work on creep testing and modeling of the additively manufactured material, looking at the effects of voids and interstitials in the microstructure, the damage simulation can be developed to more accurately describe creep under both uniaxial and multiaxial conditions.