Additive Manufacturing of Metals: Establishing Location-Specific Processing-Microstructure-Property Relationships: Microstructure and Microstructural Evolution
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: High Temperature Alloys Committee, TMS: Shaping and Forming Committee, TMS: Additive Manufacturing Bridge Committee
Program Organizers: Eric Lass, NIST; Judy Schneider, University of Alabama-Huntsville; Mark Stoudt, National Institute of Standards and Technology; Lee Semiatin, AFRL; Kinga Unocic, Oak Ridge National Laboratory; Joseph Licavoli, Michigan Technological University; Behrang Poorganji, YTC America Inc.

Thursday 2:00 PM
March 2, 2017
Room: 7B
Location: San Diego Convention Ctr

Session Chair: Eric Lass, NIST; Phil Prangnell, The University of Manchester

2:00 PM  Invited
Microstructure and Mechanical Properties Evolution of Biomedical Co-Cr-Mo Alloys Produced by Electron Beam Additive Manufacturing: Akihiko Chiba1; 1Tohoku University
    Recently, electron beam melting (EBM) has become an established additive manufacturing technology to produce any three-dimensional (3D) complex structures from precursor powders of advanced metal alloys. The purpose of this work is to clarify the effect of EBM process on phase distribution of biomedical Co-Cr-Mo (CCM) alloy and to examine how the fatigue property of it can be improved by post –built heat treatments. In addition, the constituent phase variation along the build height will be investigated because the post-built heating process may change the constituent phase.The samples were fabricated on an Arcam A2 EBM system. The powder used in the experiment consisted of spherical particles and attached small satellite particles, with an average particle size of 64 μm,. The chemical composition of the Co–28Cr–6Mo–0.23C–0.2N alloy powder was within the range of ASTM F75 standards. Higher carbon and nitrogen contents are known to provide a large amount of precipitates and stabilize γ-phase. The cylindrical samples of Co–28Cr–6Mo–0.23C–0.2N alloy rods were fabricated along the building direction. The rods were 18 mm in diameter, and 160 mm in height. A series of isothermal heat treatment from 750 ˚C to 1000 ˚C were carried out on the as-built samples to refine the microstructure consisting of γ-fcc. The microstructures at the middle position (80 mm from the bottom) of the sample rod were investigated by scanning electron microscopy (SEM), EBSD, and X-ray diffraction (XRD) on the longitudinal cross section consisting of the cylinder and z axes. The tensile and fatigue tests were conducted using the specimens cut from the middle position of the rod samples. It was revealed that building horizontal position of the rod samples affects the phase distribution. During the EBM process of CCM alloy, build parts experiences different thermal histories and have different microstructures depending not only on their height but also their position in building space. Three types of heat treatments were conducted. ε or g phased heat treatments can homogenize and refine the microstructure of EBM built CCM alloy. Low cycle fatigue tests were conducted. It was found that these heat treatments can improve the fatigue life by suppressing the formation of sharp surface relief.

2:30 PM  
Additively Manufactured 17-4 PH Stainless Steel: Toward Conventional Wrought Behavior: Eric Lass1; Mark Stoudt1; Sudha Cheruvathur1; Lyle Levine1; Yaakov Idell1; 1National Institute of Standards and Technology
    17-4 precipitation hardenable (PH) stainless steel is a useful material when a combination of high strength and good corrosion resistance up to about 315 °C is required. In the wrought form this steel has a fully martensitic structure that can be strengthened by precipitation of fine Cu-rich FCC phase upon aging. When fabricated via additive manufacturing (AM), specifically laser powder-bed fusion, 17-4 PH steel exhibits a dendritic structure containing a substantial fraction of nearly 50 % of retained austenite along with BCC/martensite and fine niobium carbides preferentially aligned along interdendritic boundaries. It is found that a relatively simple post-build heat treatment effectively homogenizes the material and produces a near fully martensitic microstructure, similar to conventional wrought of cast 17-4 PH. The mechanical properties, precipitation hardening, and electrochemical behavior of the post-processed AM 17-4 PH are investigated and compared to conventionally produced material.

2:50 PM  
Grain Structure Engineering for Metal Additive Manufacturing: Fuyao Yan1; Wei Xiong1; Gregory Olson1; 1Northwestern University
    The engineering application of additively manufactured alloys is closely related to microstructure optimization in materials and process design. Due to the extremely high thermal gradient during laser melting, it is easy for grains to grow epitaxially along the build direction into columnar structures resulting in undesired anisotropic properties, which degrade materials performance in applications. In this study, the primary object is to control and optimize grain structure in several alloys prepared by laser melting, such as austenitic steels, martensitic steels and titanium alloys. The approach for impeding grain growth in as-fabricated alloys is to introduce Zener-pinning particles, which are designed to form during rapid solidification and thereafter pin grain boundaries during multiple heating and cooling cycles. For martensitic steels, it is also possible to perform cyclic post heat treatment to promote recrystallization.

3:10 PM  
On the Development of a α+α' Dual-Phase Microstructure for Electron Beam Melted Ti-6Al-4V: Tensile Behavior and Thermal Stability: Charlotte de Formanoir de la Cazerie1; Alice Brulard1; Guilhem Martin2; Frédéric Prima3; Sébastien Michotte4; Edouard Rivière5; Adrien Dolimont5; Stéphane Godet1; 1Université Libre de Bruxelles; 2Université Grenoble Alpes; 3PSL Research University, Chimie ParisTech – CNRS; 4Sirris; 5Université de Mons
    Electron beam melting offers the opportunity to build complex Ti-6Al-4V parts. However, the microstructure cannot be controlled during the process, and post-process deformation cannot be performed on near-net shape parts. To improve the mechanical properties of additively manufactured parts, specific heat treatments must be developed. Here, annealing of electron beam melted Ti-6Al-4V at sub-transus temperatures followed by water quenching was performed. This treatment generates a α+α' dual-phase microstructure. The heat-treated specimens exhibit a broad range of tensile properties, depending on the annealing temperature. An increase in both strength and ductility, related to a remarkable work-hardening behavior, can be achieved. This substantial improvement in hardening behavior was investigated through Baushinger tests. TEM and ACOM-TEM analyses were performed in order to characterize each phase. The thermal stability of this dual-phase microstructure was also investigated.

3:30 PM Break

3:50 PM  
In Situ Characterization of Defects Formation and Microstructure Evolution in Selective Laser Melting of Metals: Lianyi Chen1; 1Missouri University of Science and Technology
    The quality of the parts manufactured by selective laser melting (SLM) is determined by the defects and microstructure characteristics formed during processing. Understanding how defects form and how microstructure evolves during laser melting and solidification is critical for eliminating defects and obtaining desired microstructures in the as-built parts produced by SLM. Unfortunately, the highly localized and very short interaction of laser beam with metal powders during SLM prevents the direct observation of this process by conventional characterization tools. The details of the defects formation and microstructure evolution during SLM are still unknown. Here we report the characterization of the formation and evolution of defects and microstructures during selective laser melting of metals by high speed x-ray imaging and diffraction. The results obtained in this work are critical for establishing the processing-microstructure relationships in SLM of metals.

4:10 PM  Cancelled
Size Dependence of Deformation Response of 316 Steel Made by Additive Manufacturing: Minh-Son Pham1; 1Imperial College London
    A strong size dependence of deformation response of additively manufactured 316 steel will be presented. In order to understand this strong dependence, we use X-ray tomography and optical/electron microscopy to quantify the pore density/distribution in the steel. In addition, high resolution scanning/transmission electron microscopy and electron backscattered diffraction are used to investigate microstructures such as texture, dislocations and chemical element segregation. In-situ micro-mechanical tests are also carried out to obtain direct observations and get deeper insights into the relationships between microstructures/pores and deformation response. In this talk, we will present our latest findings on the size dependence of 316 steel as well as on the roles of microstructures (e.g., dislocations, metastable phases, textures) on deformation mechanisms and failures of AM materials.

4:30 PM  
Microstructure and Mechanical Behavior of Additively Manufactured Austenitic Stainless Steel: Thale Smith1; Kaka Ma2; Baolong Zheng3; Joshua Sugar4; Chris San Marchi4; Julie Schoenung3; 1University of California, Davis; 2Colorado State University; 3University of California, Irvine; 4Sandia National Laboratories
    Highly localized heating, rapid cooling, and extensive thermal cycling occur during directed energy deposition (DED) processes, giving rise to complex, spatially varying thermal histories that affect local microstructures and mechanical properties in as-deposited materials. In this study, characteristics of local microstructural constituents in a 304L grade austenitic stainless steel fabricated by time-invariant DED were evaluated through electron backscattered diffraction, scanning electron microscopy, transmission electron microscopy, and electron probe microanalysis. Tensile properties were determined for three distinct combinations of build geometry and specimen orientation. Mechanical properties of the as-deposited material were found to exceed those typical of wrought and annealed 304L, with yield strengths ranging from 437-554 MPa and elongations to failure between 50-69%. The relationship of spatial and orientation dependent tensile properties to characteristics of local microstructural constituents was assessed to identify fundamental microstructural strengthening mechanisms responsible for variability in mechanical behavior.

4:50 PM  
Massive Transformation in Ti-6Al-4V Additively Manufactured by Selective Electron Beam Melting: Ma Qian1; Shenglu Lu2; Huiping Tang2; David StJohn3; 1Royal Melbourne Institute of Technology University; 2State Key Laboratory of Porous Metal Materials, Northwest Institute for Nonferrous Metal Research; 3The University of Queensland
    A massive transformation of the type from β (bcc) to αm (hcp) has been identified in Ti-6Al-4V (wt.%) additively manufactured by selective electron beam melting (SEBM). A variety of different patch-shaped massive grains were observed and characterised, including grain boundary (GB)-dependent but non-GB-crossing and GB-crossing massive grains which are characteristic of massive transformations. The massive grains in the as-fabricated Ti-6Al-4V consisted of ultrafine lamellar α and β phases (100 nm wide β strips) not seen in conventional Ti-6Al-4V due to in situ decomposition of the massive grains during additive manufacturing (AM). The massive transformation start temperature, growth characteristics and microstructural evolution of the massive phase grains during AM were discussed. The identification of massive transformation assists in the understanding of grain formation and microstructural development in Ti-6Al-4V during AM and provides a different pathway to the creation of novel titanium alloy microstructures by AM.

5:10 PM Concluding Comments