Additive Manufacturing of Metals: Microstructure, Properties and Alloy Development: Additive Manufacturing of Fe-based Alloys
Program Organizers: Prashanth Konda Gokuldoss, Tallinn University Of Technology; Ulf Ackelid, Freemelt AB; Andrzej Wojcieszynski, ATI Specialty Materials; Sudarsanam Babu, University of Tennessee, Knoxville; Ola Harrysson, North Carolina State University

Monday 8:00 AM
September 30, 2019
Room: B115
Location: Oregon Convention Center

Session Chair: Prashanth Konda Gokuldoss, Tallinn University of Technology

8:00 AM  
Characterization of the Balling Defect in Stainless Steels during Laser Powder Bed Additive Manufacturing: Debomita Basu1; Bryan Webler1; Jack Beuth1; 1Carnegie Mellon University
    A major challenge in laser powder bed additive manufacturing is the navigation on processing regions of higher power and velocity. This region of processing space is desirable because it allows a reduction in the time necessary to build components without compromising part resolution; however, high power and high velocity induce melt pool instabilities, known as balling, along the length of the laser track. This creates uneven surfaces for subsequent layers, which can result in embedded pore-type flaws. In this work we investigate how balling develops with increasing laser power and velocity in 300-series stainless steels. This is studied with surface profilometry and as-built track width measurements. Melt pool and as-built part cross-sections are also characterized to examine the impact of balling on microstructure and porosity. Possible processing strategies to change melt pool shape at high powers and high velocities and mitigate balling are also discussed.

8:20 AM  
Concurrent Chemical and Structural Microanalysis of 316L Stainless Steel Parts Built with Closed Lifecycle Additive-subtractive Manufacturing: Justin Morrow1; Marcus Jackson2; John Konopka1; Dan Thoma3; Frank Pfefferkorn2; 1Thermo Fisher Scientific; 2University of Wisconsin-Madison; 3Grainger Institute for Engineering
    The objective of this work is to compare the microstructure of a 316L stainless steel part produced with closed lifecycle additive-subtractive manufacturing to a part made with traditional melt-atomized powder feedstock. Closed lifecycle additive-subtractive manufacturing is a vision for sustainable material manufacturing wherein the machining chips produced by subtractive manufacturing are mechanically processed, without melting, into feedstock for additive manufacturing and thus recycled. In this work, 316L stainless steel test coupons are built using Laser Engineered Net Shaping (LENS) from two different feedstock materials, traditional melt-atomized powder and reclaimed powder from processed machining chips. To understand the impact of this new feedstock material on the final part, concurrent energy dispersive x-ray spectroscopy and electron backscatter diffraction (EDS-EBSD) is used to compare the as-built part microstructures. The overall feasibility of the closed lifecycle additive-subtractive manufacturing method is presented.

8:40 AM  
On the Contribution of Defects on Small-scale Mechanical Properties of Additive Manufactured Stainless Steel 316: Mahmoud Baniasadi1; Nicholas Sutton1; Meysam Haghshenas2; 1Georgia Southern University; 2University of North Dakota
    To be able to scale metal additive manufacturing (MAM) up toward real-size critical components, the performance of the MAM needs to be comparable to the part made by conventional processes (if not better). To this end, local mechanical properties in the MAM is considered an important factor to assess service performance and reliability. Intrinsically, additive manufacturing-induced defects, which could form in nano- and micro-scales, may directly, and adversely, affect the critical performance. In the present study, the variation of the mechanical properties (i.e. nano-hardness, modulus, etc.) as well as the microstructural evolution of 3D-printed stainless steel 316 materials, printed with different laser power settings, are assessed using depth-sensing nanoindentation technique, a non-destructive, reliable and convenient approach. The contribution of the defects is characterized through the addition of various shapes and sizes of the defects which added to the STL file of the models at various locations of the build plate.

9:00 AM  
On the Thermal Stability of the Cellular Microstructure of Additively Manufactured Stainless Steel 316L: Janith Wanni1; Andrew Birnbaum2; John Michopoulos2; Amit Bagchi2; Ajit Achuthan1; 1Clarkson University; 2U.S. Naval Research Laboratory
    Unlike the classical grain structure in conventional 316L stainless steel, the microstructure of additively manufactured (AM) 316L exhibits a number of process-specific microstructural/sub-structural features. The cellular sub-grain structure, one of the prominent features, are expected to influence the local deformation mechanisms significantly. In this study, the thermal stability of the cellular substructure is investigated. Samples manufactured using laser engineered net shaping (LENS) were subjected to high temperature, post-process annealing conditions. Different temperature levels and exposure times were considered, and the corresponding changes in cellular microstructure characterized. Preliminary results show that the cellular microstructure is stable up to a critical temperature. Above the critical temperature, characteristics of the microstructure change substantially. The development of a model to capture the relationship between cellular microstructural characteristics and the exposure conditions are currently ongoing. The determination of the variation in mechanical properties as a result of the changes of microstructural characteristics is also presented

9:20 AM  
Modeling and Characterization of as-built Microstructure of SLM-processed 17-4PH Stainless Steel: Abhinav Saboo1; Swathi Vunnam2; Dana Frankel1; Thomas Starr2; 1QuesTek Innovations LLC; 2University of Louisville
    Selective laser melting (SLM) processed 17-4 PH stainless steel usually exhibits an inhomogeneous microstructure in as-built condition. QuesTek Innovations, partnering with the University of Louisville’s Rapid Prototyping Center have been designing and developing a new powder specification for high-strength martensitic precipitation-hardenable stainless steel optimized for the unique processing conditions and challenges of SLM processing. In this study, the effect of powder composition on microstructural evolution of SLM processed 17-4 PH was studied in as-built condition. The variation in Cr/Ni ratio of the powder result in widely different microstructure with varying fraction δ-ferrite, Austenite and Martensite phases. This variation in the relative phase fraction results varying mechanical property in as-built condition. Development of models for different Processing-Microstructure-Property relationships and its application for alloy optimization will be demonstrated.

9:40 AM  
The Effect of Microstructural Features on the Local Stress Field in Additively Manufactured 17-4 Steel: Ajit Achuthan1; Robert Saunders2; John Michopoulos2; Amit Bagchi2; 1Clarkson University; 2US Naval Research Laboratory
    Microstructural features have an effect on the local deformation mechanism, which can then influence the macroscopic behavior of the material, especially its strength and toughness. Computational models that can capture the effect of these microstructural features can be a very useful design tool to determine stress localization around the microstructural features and predict failure. Crystal plasticity based finite element (CPFE) models are a quite promising framework to develop such models. In this paper, we will use CPFE to determine the local stress field in AM printed 17-4 samples using a feature size and shape dependent crystal plasticity constitutive model. The effect of grain size, shape, and texture on the local stress field will be investigated. In addition, the significance of capturing grain boundary geometries accurately on the stress field prediction will be examined by considering tetragonal and hexagonal elements. The model will be validated with experimental results.

10:00 AM Break

10:20 AM  
Microstructures and Properties of Tool Steels Produced by Laser Powder Bed Fusion: Yining He1; Jack Beuth1; Bryan Webler1; 1Carnegie Mellon University
    This work examined microstructure and properties of tool steel parts produced by laser powder bed fusion (LPBF) additive manufacturing (AM). These were examined with single re-melt tracks, re-melt pads, and small cubes of printed material. A wide variety of beam power (P) and travel speed (V) combinations were studied for single tracks, and then certain combinations were downselected for pads and cubes. The materials of interest were M2, D2, and H13 tool steels. Wide variations in microstructure and properties were observed due to the rapid solidification, compositional complexity, and multiple transformations possible in steels on cooling. D2 steels exhibited predominately columnar grains. M2 steels exhibited a columnar-to-equiaxed transition under certain P-V combinations, while H13 exhibited large regions of retained austenite. Some of these microstructural features were associated with hardness variations and the occurrence of cracks in the as-built state.

10:40 AM  
Correlation between Processing Parameters and Mechanical Properties of Additive Manufactured H13 Steel: Piter Gargarella1; Adriel de Oliveira2; Luiz Henrique de Lima1; Bianca Caroline Felipe1; Claudemiro Bolfarini1; Nelson de Alcântara1; 1Materials Engineering Department, Federal University of São Carlos; 2Posgraduate Program in Materials Science and Engineering, University of São Carlos
    This work aimed to evaluate commercial H13 powder to be used as raw material for the Powder Bed Fusion (PBF) process, to optimize the PBF parameters to produce high-density H13 parts and correlate the parameters used with the mechanical properties. The powder was separated in different size ranges and was evaluated by XRD, OM, SEM, TEM, EDS and DSC. The powder was used to build PBF 10x10x5 mm parts with different laser scanning speed, power and hatching and these parameters were correlated with the mechanical properties. The samples density was measured by the Archimedes method and evaluated considering the power density applied. Samples with high density and free of defects were evaluate by XRD, OM, SEM, TEM, EDS, bending and hardness tests. The results obtained will be discussed considering the correlation of process parameters, microstructure, phase formation and mechanical properties and understood by the effect of cooling rate and solidification.

11:00 AM  
Additive Manufacturing of Duplex Stainless Steel via Selective Laser Melting and Subsequent Heat Treatment: Greg Nigon1; Somayeh Pasebani1; Burkan Isgor1; 1Oregon State University
    Additively manufactured duplex stainless steel 2205 components were built via selective laser melting followed by heat treatment. Gas atomized duplex stainless steel 2205 powder (D90 <45m) was procured from Carpenter. Powder characterization was done to determine phases, morphology, particle size, and particle size distribution. Subsequently, an experimental matrix was executed which varied the laser parameters (power and scan speed) on an OR Creator SLM machine with 250W Laser and nitrogen environment. Density and microhardness of SLMed parts were measured. Optical and scanning electron microscopy were used to investigate the SLMed microstructure. The results were compared to those in a wrought duplex sample. The optimized SLM parameters were used to duplicate samples for heat treatment to achieve a duplex structure. SLMed and heat treated parts were characterized for microhardness and microstructural evolutions. 99% density parts were built with dual (austenite-ferrite) phase with microhardness comparable or superior to the wrought duplex.

11:20 AM  
Impacts of Microstructure on Mechanical Behavior of Additively Manufactured Metals: Allison Beese1; 1Pennsylvania State University
    The heterogeneous microstructures, which may include elongated grains, textured grains, and internal pores/defects, in additively manufactured metallic materials directly impact their mechanical behavior, namely large deformation and fracture behavior. In order to adopt additive manufacturing (AM) for components that will bear load during their lifetime, the deformation and fracture behavior must be understood and predictable, described by computational models. This presentation will detail our work on studying and modeling the impact of microstructure on multiaxial plastic deformation and defects on fracture behavior of additively manufactured steel and Ti-alloys.