Additive Manufacturing: Microstructure and Material Properties of Titanium-based Alloys: Electron Beam Powder Bed Fusion
Program Organizers: Ulf Ackelid, Freemelt AB; Andrzej Wojcieszynski, ATI Powder Metals; Ola Harrysson, North Carolina State University; Sudarsanam Babu, University of Tennessee, Knoxville
Wednesday 8:00 AM
October 2, 2019
Location: Oregon Convention Center
Session Chair: Ulf Ackelid, Freemelt AB
Characterization of Additively Manufactured Ti 6Al 4V in Hydrogen: Paul Korinko1; John Bobbitt1; Travis Hubbard1; 1Savannah River National Laboratory
The Savannah River National Laboratory (SRNL) is considering using additively manufactured titanium alloys, e.g., Ti 6Al-4V (Ti64) in low level hydrogen containing atmospheres. SRNL is using typical densification parameters for the Arcam A2X to prepare near fully dense tensile coupons. These coupons are being characterized for microstructure and tensile strength in the as fabricated condition. Subsequent to initial characterization they are being exposed to low partial pressures of hydrogen at ambient conditions for weeks to months. Samples are then characterized using microscopic techniques and tested. This presentation will describe how the samples were prepared, characterized, and the effects of the low-level hydrogen exposure on mechanical properties. Future efforts to determine the effects of other hydrogen isotopes on properties will also be described.
Crystallographic Texture Evolution in Additively Manufactured Ti-6Al-4V as a Function of Build Height and Scan Strategy: Alec Saville1; Matt Kenney2; Priyanka Agrawal2; Sabina Kumar3; Jonah Klemm-Toole1; Sven Vogel4; Sudarsanam Babu3; Pete Collins2; Amy Clarke1; 1Colorado School of Mines; 2Iowa State University; 3University of Tennesee - Knoxville; 4Los Alamos National Laboratory
Additively manufactured (AM) metallic alloys may exhibit intended or unintended anisotropic behavior as a result of the layer-by-layer build process intrinsic to AM. Thus, understanding the link between processing parameters and microstructural evolution is vital to controlling key microstructural characteristics and potential defects that dictate the properties needed for high-performance structural applications. One of the primary contributors to anisotropic microstructures and properties is the evolution of preferred crystallographic orientation (texture) throughout the build volume during AM. Here we analyze the texture evolution of Ti-6Al-4V built with different AM scan strategies as a function of build height with neutron diffraction, illustrating how local conditions and processing history impacts texture evolution. The insights gained from this work are discussed in the context of improved microstructure control during AM, which can lead to on-demand properties and broader application of AM technologies.
Deformation and Fracture Behavior of Electron Beam Melted Ti-6Al-4V under High Strain Rate Impacts: Reza Alaghmandfard1; Dharmendra Chalasani1; Akindele Odeshi2; Mohsen Mohammadi1; 1Marine Additive Manufacturing Centre of Excellence; 2University of Saskatchewan
The effect of building direction on the dynamic deformation behavior of electron beam melted Ti-6Al-4V alloy was investigated. Several dynamic compression tests were performed at strain rates varying from 150 s-1 to 2200 s-1 using a split-Hopkinson pressure bar apparatus. The maximum stress increased as the strain rate increased up to a peak value. It then dropped with further increase of the strain rate for horizontally and vertically printed samples. Under similar loading conditions, the strength of vertically printed specimens was consistently higher than that of the horizontal ones, which can be attributed to the finer microstructure of the vertically printed samples. The dynamic yield strength of the alloy in both directions was strain rate dependent, where the maximum of 2046 MPa was attained in the vertically printed sample at 1500 s-1. In horizontal samples, failure occurred beyond 2200 s-1, whereas fracture initiated at much lower strain rates of about 1300 s-1 in vertical samples.
Electron Beam Melted Ti-48Al-2Cr-2Nb Intermetallics: Microstructure, Room- and High-temperature Compression Properties, and High Temperature Creep Properties: Kee-Ahn Lee1; Young-Kyun Kim1; Seong-June Youn1; Seong-Woong Kim2; Jaekeun Hong2; 1Inha University; 2Korea Institute of Materials Science
A gamma Ti-48Al-2Cr-2Nb alloy was additively manufactured by using electron beam melting (EBM), and its microstructure, room- and high-temperature mechanical properties, and deformation behaviors were investigated. Additionally, improvement in the high temperature compression and creep properties were achieved by introducing the 2-step heat treatment. Initial microstructural observation confirmed that the EBM-built Ti-48Al-2Cr-2Nb alloy had a near-gamma structure, whereas 2-step heat-treated EBM-built Ti-48Al-2Cr-2Nb had a very fine lamellar structure with some equiaxed gamma grains. Room- and high-temperature compression tests confirmed that the 2-step heat-treated sample had higher strengths in all temperature ranges, and the yield-strength anomaly phenomenon occurred in both sample conditions. A 750 °C creep test confirmed that the 2-step heat-treated Ti-48Al-2Cr-2Nb alloy had higher creep resistance compared with as-built Ti-48Al-2Cr-2Nb alloy. The correlations between the microstructure evolution, strength, and high temperature creep resistance were discussed based on these findings.
Factors on Tensile Properties of EBM Fabricated Ti-6Al-4V Alloy: Yuta Tanaka1; Yutaro Ota1; Keiji Kubushiro2; 1IHI Corporation; 2IHI ASIA PACIFIC (Thailand) Co., Ltd.
Additively manufactured (AM) Ti-6Al-4V shows different microstructures and mechanical properties depending on various process conditions and AM-specific anisotropy. It is well known that the relation between yield stress and α lath thickness follows the Hall-petch type relationship. However, some specimens did not follow the relationship. Therefore, the influence of other factors on yield stress was investigated in this study. One of the contributing factors for the strength was solid solution strengthening, resulted in lattice constant dependence on oxygen content. In addition, tensile anisotropy due to the columnar grains morphology and the strong texture was evaluated using electron backscatter diffraction plane (EBSD) trace analysis and electron channeling contrast imaging (ECCI) technique. The different activated slip systems were observed depending on the process conditions. Therefore, it is suggested that critical resolved shear stress value is also contributing factors for the strength.
Identifying and Understanding the Influence of Columnar Prior-Beta Grain Boundaries on the Tensile and Fatigue Properties of Additively Manufactured Ti-6Al-4V Alloy: Ma Qian1; 1RMIT University (Royal Melbourne Institute of Technology)
Fine equiaxed grains are preferred to columnar grains for metallic materials that are not concerned with creep-resistant applications. Ti-6Al-4V (wt.%) is the workhorse alloy of the titanium industry and also the most extensively studied alloy for additive manufacturing (AM). Ti-6Al-4V manufactured by fusion-based AM processes typically solidifies as columnar beta-phase grains parallel to the build direction. These columnar grain boundaries are retained at room temperature and after post-AM heat treatments as well if the isothermal holding temperature is below the beta transus. Should such columnar grain boundaries be removed by either solidification control or post-AM heat treatments in the beta-phase region? This paper discusses this question through well-designed experiments and detailed assessments of the tensile and fatigue properties of the additively manufactured Ti-6Al-4V.
10:00 AM Break
Numerical Prediction of Fatigue Life of Additively Manufactured Ti-6Al-4V: Manisha Banker1; Mannur Sundaresan1; Carter Keough2; Cynthia Waters3; Harvey West2; Richard Wysk2; Ola Harrysson2; 1North Carolina A&T State University; 2North Carolina State University; 3NSWC Carderock Division
Conventionally fabricated Ti-6Al-4V are preferred widely for fatigue resistant aircraft structures. However, its usage is restricted to primary structures due to associated cost burden. Recently additive manufacturing (AM) is scrutinized for fabrication of aerospace grade Ti-6Al-4V parts, which would allow complex part fabrication at marginal material wastage. Extensive research is reported in past few years to appraise fatigue behavior of AM Ti-6Al-4V parts. While industry giants are envisaging virtual or hybrid simulation as a part of certification process, it is inevitable to address the numerical prediction aspects for AM parts. Few scholarly articles report on numerical prediction of fatigue life in presence of manufacturing uncertainties such as porosity, surface roughness etc. In this work, a fatigue life prediction methodology will be presented for EBM Ti-6Al-4V specimens subjected to four-point bending. The fatigue life sensitivity to the process inherent defects will be presented as predicted S-N curve.
The Role of Microstructural Heterogeneities and as-Built Defects in EBM-PBF Ti-6Al-4V: Mechanical Testing and Characterization at Appropriate Length Scales: Jake Benzing1; Nik Hrabe1; Li-Anne Liew1; Enrico Lucon1; Ryan White1; 1National Institute of Standards and Technology
Current work on Ti-6Al-4V, manufactured by electron beam melting powder-bed fusion, includes fracture toughness testing and a comparison of properties measured with mini-tensile and small-punch testing of specimens extracted from the same part. Processing parameters under investigation include the influence of scan length (a parameter specific to Arcam A- and S-series machines) and use of support structures. Further, meso-scale tensile specimens (gauge dimensions on the order of a few hundred microns) were also extracted from as-built parts. Coupling non-destructive imaging and tomographic techniques with meso-scale testing (before and after deformation) allows for tracking of specific sub-surface pores and lack-of-fusion zones, plus microstructural features that form along prior-β grain boundaries. The purpose of this work is to evaluate how mechanical properties are influenced by as-built defects, grain size, crystallographic texture and microstructural heterogeneities, when deformed at an appropriate length scale.
Local Microstructures in Electron Beam Melted Ti-6Al-4V Lattice Primitives with Slender Intersecting Struts: Sara Messina1; Connie Dong1; Toby Francis1; Andrew Polonsky1; Jean-Charles Stinville1; McLean Echlin1; Rachel Collino2; Matthew Begley1; Tresa Pollock1; 1University of California, Santa Barbara; 2Los Alamos National Laboratory
The microstructure of EBM additively manufactured Ti-6Al-4V is influenced by cooling rate, sample dimensions, and build orientation. The interaction of these parameters is not well-understood, and the resulting microstructural variations lead to a distribution of mechanical properties. In thin-walled Ti64 structures, sample geometry often dictates cooling rate and build orientation, producing location-dependent microtextures. To sample a suite of build orientations, complex cage geometries are studied using EBSD, EDS, and SEM micrographs, and an analytical framework is developed to account for build orientation and build location. It has been observed that, despite previous work indicating that prior β does not impact final α lath orientation, here bi-textured α laths exist in vertically-oriented struts which likely contained columnar beta grains spanning the build height, and such local microtextures can elicit premature plastic failure. Understanding the limitations of microstructural control in this scenario will inform performance bounds and guide design using EBM Ti-6Al-4V.
Toughness of EBM Additively Manufactured Titanium Alloy Octet Truss Lattice: Andrew Neils1; Liang Dong1; Abbas Moftakhar2; Haydn Wadley1; 1University of Virginia; 2General Electric Additive
Selective electron beam melting (EBM) additive manufacturing with Ti-6Al-4V alloy powder has been used to make an octet-truss lattice, which is a candidate isotropic load bearing metamaterial with potential application to aerospace structures. We compare the fracture toughness of as-built lattices with relative densities between 8 and 20% using a single edge notch bend (SENB) test. Since lattice fracture toughness scales with the square root of the failure strain, the effect of post-fabrication hot isostatic pressing (HIP) treatment that reduced internal porosity and enhanced elongation to break was investigated. Examination of the strut surfaces indicated the presence of significant surface roughness and weakly adhering powder particles which contributed to lattice density but inefficiently support applied loads. A post-HIP chemical etching technique has been investigated for removal of these features leading to better fracture toughness to weight ratios of the additively manufactured structures.