Additive Manufacturing: Building the Pathway towards Process and Material Qualification: Defects and Mechanical Properties
Sponsored by: TMS Extraction and Processing Division, TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Powder Materials Committee, TMS: Process Technology and Modeling Committee, TMS: Additive Manufacturing Bridge Committee
Program Organizers: John Carpenter, Los Alamos National Laboratory; David Bourell, University of Texas - Austin; Allison Beese, Pennsylvania State University; James Sears, GE Global Research Center; Reginald Hamilton, Pennsylvania State University; Rajiv Mishra, University of North Texas; Edward Herderick, GE Corporate

Thursday 2:00 PM
March 2, 2017
Room: 8
Location: San Diego Convention Ctr

Session Chair: Brad Boyce, Sandia National Laboratory; Robert Warren, Worcester Polytechnic Institute

2:00 PM  Invited
High-throughput Testing Reveals Rare, Catastrophic Defects: Brad Boyce1; Brad Salzbrenner1; Bradley Jared1; Jeffrey Rodelas1; Jonathan Madison1; Jay Carroll1; 1Sandia National Laboratories
    Agile qualification of additively manufactured (AM) structural components requires rapid, high-throughput post-process measurements of material properties. To this end, we have developed a tensile testing methodology that permits >200 tensile tests per day with no compromise in data quality. This method has been used to explore stochastic mechanical property distributions of an AM 17-4PH stainless steel alloy produced by laser powder bed fusion (selective laser melting). Substantial differences were revealed between vendors, and between builds for a single vendor. Detailed microstructural and fractographic analysis on weak statistical outliers reveal rare processing defects that limit reliability. For example, in one study on 945 tensile bars from 8 builds on one machine, a small subset suffered from exceedingly low ductility associated with large internal, interconnected pore networks oxidized during heat treatment. The high-throughput testing provides a pathway to rapidly qualify process improvements that eliminate such rare defects.

2:30 PM  
Characterization of the Elastic Properties and Microstructure of SLM Al-10Si-Mg: David Witkin1; Scott Sitzman1; Yong Kim1; Paul Adams1; Robert Castaneda1; 1The Aerospace Corporation
    The microstructure and elastic properties of SLM Al-10Si-Mg in as-built and heat treated conditions were characterized for commercially produced samples. Coefficient of Thermal Expansion (CTE) did not vary between the in-plane and vertical build directions. Elastic moduli did consistently vary in these directions, but at a similar magnitude to variation found between transverse and longitudinal directions in extruded Al 6061-T6 bar. Electron back-scatter diffraction (EBSD) revealed a bimodal microstructure consisting of a coarser grained interior of the melt pool transect that exhibited preferred crystallographic orientation surrounded by a fine-grained region that was more randomly oriented but had signatures of directional solidification. X-ray computed tomography (CT) was used to characterize porosity, which are not randomly distributed but more prevalent where contour intersected interior fill passes. Ultrasonic measurements of elastic properties also yielded two-dimensional information about the distribution and size of pores.

2:50 PM  
Normal Track Size Related and Abnormal Lack of Fusion Defects Formed during Selective Laser Melting of CoCrMo Alloy: Kourosh Darvish1; Z. Chen1; T. Pasang1; 1Auckland University of Technology
    Severe lack of fusion (LOF) is a severe quality problem in selective laser melting (SLM). Track profile (size and shape) is the geometrical factor determining the size and amount (fraction, fLOF) of the normal inter-track LOF and is a function of laser power (P) when other parameters are kept the same. However, the sensitivity of track profile and thus fLOF affected by P through the geometrical factor has not been well studied. In this study, through examining the SLM tracks made using CoCrMo alloy, a rapid decay of fLOF in a narrow range of P has been observed and the geometrical effect causing this is explained. It has also been observed that fLOF has remained above zero even at high P values due to the presence of LOFs in an abnormal form. Therefore, the current research presents and discusses how higher laser beam penetration affects the reduction of abnormal LOFs.

3:10 PM  
Stress State, Strain Rate and Temperature Dependence of an Electron Beam Additive Manufactured Ti6Al4V: Omar Rodriguez1; Paul Allison1; Wilburn Whittington2; David Francis2; Oscar Rivera1; Y. Kevin Chao1; Bo Cheng1; 1The University of Alabama; 2Mississippi State University
    The current investigation presents a comprehensive examination of the stress state, strain rate and temperature effects on plastic flow and failure modes of EBM-manufactured Ti6Al4V. The plastic flow of the EBAM Ti6Al4V has been evaluated by means of traditional electromechanical load frame acting within the quasi-static strain rate regime (ε ̇ =1 x10-3 s-1) while intermediate and dynamic experiments ( ε ̇= 500 s-1 and ε ̇ = 1500 s-1) were performed on modified Kolsky bar apparatuses. Recognizing the importance of accurate material behavior prediction a dislocation-based Internal State Variable (ISV) plasticity model was preferred over a phenomenological-based model to capture the material’s constitutive behavior. The capabilities of the determined material constants were examined by comparing the predicted stress flow against different testing conditions.

3:30 PM Break

3:50 PM  Invited
Positional Dependence of Pore Morphology, Size and Orientation in SEBM Ti-6Al-4V and Influence on Mechanical Properties: Ma Qian1; Joe Elambasseril1; Huiping Tang2; Shenglu Lu2; Wei Xu3; Milan Brandt1; 1RMIT University (Royal Melbourne Institute of Technology); 2State Key Laboratory of Porous Metal Materials, Xi'an, China; 3Macquarie University
    Understanding the formation of defects and their influence on mechanical properties is important to the further development of metal additive manufacturing (AM) processes. This study focuses on the positional dependence of pore morphology, size and orientation and their influence on the mechanical properties of Ti-6Al-4V long rods built vertically to the limit (300 mm) of a commercial selective electron beam melting (SEBM) system. A total of 18,337 consecutive slices were scanned from an as-built rod using X-ray micro-computed tomography (ÁCT) and reconstructed into a three-dimensional (3D) digital rod for quantitative analyses. Corresponding mechanical properties were produced from different sections of the long rods with and without hot isostatic pressing (HIP). In addition, the effect of pore size on plastic strain developments under tensile loading was analysed by converting quantitative ÁCT data into finite element (FE) meshes. The implications of the experimental findings are discussed from different perspectives.

4:20 PM  
Effect of the Isotropic and Anisotropic Work Hardening on the Micromechanics Behavior in Textured Inconel 718 by Electron Beam Additive Manufacturing: Qingge Xie1; Alexandru Dan Stoica1; Sarma B. Gorti1; Radhakrishnan Balasubramaniam1; Gian Song1; Hassina Z. Bilheux1; Michael M. Kirka1; Ryan R. Dehoff1; Jean-Christophe Bilheux1; Ke An1; 1Oak Ridge National Laboratory
    Inconel 718 alloys with a strong cube texture component and elongated grain shape were prepared via electron beam additive manufacturing. The time-of-flight neutron diffraction technique was employed to in-situ measure the lattice strain evolution of different families of grains with their specific crystallographic-plane normal either parallel or perpendicular to the loading direction. Elastic-plastic finite element modeling (EPFEM) framework was used to simulate the lattice strain and grain orientation evolution. Both the isotropic type of work hardening law and the anisotropic type of work hardening law were implemented respectively in the model to understand their effects on the plastic anisotropic behavior and their effects on the grain orientation evolution. Results demonstrated that the anisotropic work hardening law is necessary to accurately capture the plastic anisotropic behavior of each grain in the polycrystal. The correlation between the identified coefficients of the anisotropic work hardening law and the micromechanical behavior will be illustrated.

4:40 PM  
Multiscale Mechanical Property Measurement and Microstructural Characterization of Additively Manufactured Ti-6Al-4V Components: Tugce Ozturk1; Xinyi Gong2; Soumya Mohan2; Surya Kalidindi2; Anthony Rollett1; 1Carnegie Mellon University; 2Georgia Institute of Technology
    Ti-6Al-4V, one of the most popular alloys used in metal additive manufacturing, has been shown to hold highly heterogeneous characteristics when produced by electron beam melting (EBM), an emerging AM technique. The thermal gradient accompanying this manufacturing process directly affects the resulting microstructure and the mechanical properties of the components. This study shows a set of microstructural characterization techniques and multiscale mechanical property measurements, bridging the local heterogeneous fields with macroscopic properties. Optical microscopy, EDS and EBSD techniques have been combined in different length scales. Characterization results are used to determine regions of interest, and nanoindentation is performed on these regions. Small-scale properties are translated into large-scale by a homogenization scheme, and the results are compared with macroscopic tensile tests performed in various directions and build heights. It is shown that the local heterogeneous properties can be utilized to determine the overall hardening parameters for constitutive modeling.

5:00 PM  
Microstructure Evolution, Fatigue Crack Growth Mechanisms, and Effects of Heat Treatment and HIP in Ti-6Al-4V Alloys Fabricated by Electron Beam Melting: Robert Warren1; Haize Galarraga1; Diana Lados1; Ryan Dehoff2; Michael Kirka2; 1Worcester Polytechnic Institute; 2Oak Ridge National Laboratory
    Electron Beam Melting (EBM) is an Additive Manufacturing process that selectively fuses powder-bed particles using an electron beam as the power source. EBM can produce unique microstructures and mechanical properties which are related to the processes specific thermal history. In this study, Ti-6Al-4V alloys fabricated by EBM have been systematically investigated. The microstructures, tensile, and long and small fatigue crack growth (FCG) properties have been evaluated and compared in different orientations relative to the deposition direction. The effects of various heat treatments, Hot Isostatic Pressing (HIP), initial crack size, and stress ratio on the FCG mechanisms at the microstructural scale of the materials has been determined, and will be discussed from structural design perspective. Original analytical methods for FCG threshold correlations and computational modeling work for predicting temperature evolution during processing and microstructure development will also be presented and discussed for properties optimization and rapid qualification.