Investigation of grain growth and microstructural evolution in electron beam melting additive manufacturing process

• Main Author: Chandra, Shubham
• Other Authors: Tor Shu Beng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Mechanical engineering::Prototyping; Engineering::Materials::Mechanical strength of materials
• Online Access: https://hdl.handle.net/10356/143917
• Institution: Nanyang Technological University

Selective Electron Beam Melting (SEBM) is a representative powder bed fusion metal Additive Manufacturing (AM) technique that employs a high-energy electron beam for sintering and melting metal powder bed in a layer-wise fashion. High build temperatures (500 – 1100 oC) and vacuum fabrication environment are inherent to SEBM process, leading to residual stress- and contamination-free as-built parts. However, there is a lack of understanding in grain growth behaviour and control during microstructural evolution which eventually governs the mechanical properties of as-built parts. Additionally, the prevailing knowledge on hot cracking during AM of non-weldable Ni-based superalloys lacks a framework to relate the solidification parameters and microstructural features with crack susceptibility. This study addresses the limitations in state-of-the-art research and utilizes multi-scale experimental and numerical simulation methods to investigate the grain growth behaviour and microstructural evolution with varying build geometries and metal powder sizes for three different engineering alloys, i.e. Ti-6Al-4V, Stainless Steel 316L (SS316L), and Ni-based Single Crystal (SX) superalloy. Furthermore, it also provides an analytical framework to correlate solidification parameters and microstructural features with the contingency of hot cracking in AM of Ni-based superalloys. In general, the key results of this thesis work can be clarified in the following sections: First, a multi-scale numerical model is created based on SEBM processing of Ti-6Al-4V to investigate the grain growth behaviour and microstructural evolution with varying build geometries, e.g. V-, I-, and A-shaped parts. Model validation was carried out with the aid of a series of quantitative experimental observations on the as-built Ti-6Al-4V microstructure. It is found that the difference in Heat Affected Zone (HAZ) depths caused by varied beam scanning lengths resulted in the periodic microstructural banding in SEBM-built Ti-6Al-4V samples. Moreover, build geometry is determined to impact a dominant effect on solidification parameters, hence on the crystallographic texture of the as-built microstructure. The V-shaped part is identified to favour a strong <001> crystallographic texture with increasing build height in comparison to the A- and I-shaped counterparts. Further insight into grain growth control is advanced by SEBM processing of austenitic stainless steel – SS316L. Effects of build geometry and powder size on crystallographic texture in the as-built parts are demonstrated by employing various and multi-scale characterization techniques. Thermal conductivities of the sintered powder bed and as-built samples were determined using the Laser Flash Apparatus (LFA). Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM), and Back-Scattered Electron (BSE) imaging were utilized to investigate melt pool topology and single-phase dendritic microstructure. X-ray Diffraction (XRD) and Electron Back-Scattered Diffraction (EBSD) were utilized for bulk phase analysis and crystallographic texture measurements, respectively. Transmission Electron Microscopy (TEM) was employed to determine the deformation mechanisms in the as-built samples subjected to uniaxial tensile testing. It was observed that a fine columnar-dendritic microstructure with primarily <001> crystallographic texture could be achieved by varying the build geometry. Furthermore, the powder size distribution is found to have an important effect on the as-built crystallographic texture. Changing the powder size from fine to coarse led to a uniform equiaxed-grained microstructure with significant improvement in tensile strength and ductility by ~17% and ~62%, respectively. Despite its advantages, SEBM processing of Ni-based SX superalloys is still challenging due to their hot cracking susceptibility during rapid solidification. Additionally, elevated build temperature facilitates the precipitation of unfavourable Topologically Close-Packed (TCP) phases. Examination of the as-built microstructure using OM, FESEM, and EBSD revealed severe hot cracking which was confined only to divergent Grain Boundaries (GBs). Investigation of TCP phases using Energy-Dispersive Spectroscopy (EDS) and Atom Probe Tomography (APT) confirmed them as Ta-rich C14 Laves phase. Finally, a generalized hot cracking criterion is presented in this study that can account for the mutual inclination of grains at a GB. This generalized criterion is a major advancement over state-of-the-art and can be used to quantitatively predict cracking susceptibility of a GB under any solidification conditions associated with AM processes universally.