Background
Porosity and other defects resultant by additive manufacturing processes are well-known to affect mechanical properties. However, there remains limited understanding regarding how the internal defect structure influences the evolution of the local strain field, as experimental investigations have not presented direct measurements of the evolving internal strain field in the presence of defects.
Objective
Interrupted in-situ tensile tests in a lab-based X-ray computed tomography machine were used to investigate the evolution of the strain field around internal defects. The evolution of the internal strain field facilitated examination of the role of specific defects in the macroscopic deformation of additively manufactured tensile coupons.
Methods
Samples were produced in 316L stainless steel by laser powder bed fusion. An in situ loading device was utilized to subject the samples to tensile failure within a tomographic scanning environment. Digital volume correlation was utilized to directly determine local strain levels within the additively manufactured components in the vicinity of porosity defects.
Results
Effects of porosity on strain localization and eventual failure of the samples were evaluated. Characteristics of the porosity distribution, including presence of porosity at the surface or near-surface of components, as well as the proximity of pores to each other were found to influence the evolution of failure. Early onset of failure was found to be associated with the availability of neighboring porosity that allowed for rapid progression of the fracture path.
Conclusions
The direct measurements of strain field evolution in the present study established understanding regarding how internal defect structure characteristics influence the evolution of the local strain field for additively manufactured components. This high fidelity characterization and the associated phenomenological observations have bearing for supporting validation of numerical modeling frameworks for describing failure in these materials.
more »
« less
Deformation Behavior of Grains Near Defects in Direct Metal Laser Sintered Inconel 718 During Indentation
The present work utilizes Orientation Imaging Microscopy and Finite Element Modelling to analyse microstructure evolution in grains near defects during plane strain indentation of direct metal laser sintered Inconel 718. Defects are inevitably produced during printing of metals and they degrade the mechanical behaviour of parent components. Understanding microstructure evolution of grains present near defects can help create better predictive models of mechanical behaviour of components resulting from additive manufacturing. In this work, an ex-situ study of microstructure evolution during plane strain indentation of DMLS Inconel 718 specimens is performed. Regions that lie near volumetric porosity defects were studied. Grain Orientation Spread was utilized as a metric to quantify intra-granular deformation. It was seen that microstructure evolution of grains near defects is enhanced due to strain concentrations whereby they exhibit larger orientation spread after plastic deformation. Finite Element Analysis was used to simulate the plane strain indentation test on the specimen in which, porosity defects and roughness textures similar to those seen in the as-received specimen were programmed using the python scripting interface of Abaqus. Results from finite element analysis were compared with insights from microstructure analysis to describe evolution of microstructure during deformation near defects.
more » « less- Award ID(s):
- 1825686
- NSF-PAR ID:
- 10483142
- Publisher / Repository:
- American Society of Mechanical Engineers
- Date Published:
- Journal Name:
- Proceedings of the ASME 2020 15th International Manufacturing Science and Engineering Conference MSEC2020
- Format(s):
- Medium: X
- Location:
- Virtual, Online
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACTElectro-chemical polishing (ECP) was utilized to produce sub-micron surface finish on Inconel 718 parts manufactured by Laser Powder-Bed-Fusion (L-PBF) and extrusion methods. The L-PBF parts had very rough surfaces due to semi-welded powder particles, surface defects, and difference layer steps that were generally not found on surfaces of extruded and machined components. This study compared the results of electro-polishing of these differently manufactured parts under the same conditions. Titanium electrode was used with an acid-based electrolyte to polish both the specimens at different combinations of pulsed current density, duty cycle, and polishing time. Digital 3D optical profiler was used to assess the surface finish, while optical and scanning electron microscopy was utilized to observe the microstructure of polished specimens. At optimal condition, the ECP successfully reduced the surface of L-PBF part from 17 µm to 0.25 µm; further polishing did not improve the surface finish due to different removal rates of micro-leveled pores, cracks, nonconductive phases, and carbide particles in 3D-printed Inconel 718. The microstructure of extruded materials was uniform and free of processing defects, therefore can be polished consistently to 0.20 µm. Over-polishing of extruded material could improve its surface finish, but not for the L-PBF material due to defects and the surrounding micro-strain.more » « less
-
Microstructure evolution modeling using finite element crystal plasticity (FECP), Monte- Carlo (MC), and phase field (PF) methods are being used to simulate microstructure evolution in Ti-6Al-4V under thermomechanical loading conditions. FECP is used to simulate deformation induced evolution of the microstructure and compute heterogeneous stored energy providing additional source of energy to MC and PF models. The MC grain growth model, calibrated using literature and experimental data, is used to simulate α+𝛽 grain growth. A multi-phase field, augmented with crystallographic symmetry and orientation relationship between α-𝛽, is employed to model simultaneous evolution and growth of all twelve α-variants in 3D. The influence of transformation and coherency strain energy on α-variant selection is studied by coupling the model with the Khachaturyan-Shatalov formalism for elastic strain calculation. This FECP/MC/PF suite will be able to simulate evolution of grains in the microstructure and within individual 𝛽- grains during typical thermomechanical processing conditions.more » « less
-
Purpose AlSi10Mg alloy is commonly used in laser powder bed fusion due to its printability, relatively high thermal conductivity, low density and good mechanical properties. However, the thermal conductivity of as-built materials as a function of processing (energy density, laser power, laser scanning speed, support structure) and build orientation, are not well explored in the literature. This study aims to elucidate the relationship between processing, microstructure, and thermal conductivity. Design/methodology/approach The thermal conductivity of laser powder bed fusion (L-PBF) AlSi10Mg samples are investigated by the flash diffusivity and frequency domain thermoreflectance (FDTR) techniques. Thermal conductivities are linked to the microstructure of L-PBF AlSi10Mg, which changes with processing conditions. The through-plane exceeded the in-plane thermal conductivity for all energy densities. A co-located thermal conductivity map by frequency domain thermoreflectance (FDTR) and crystallographic grain orientation map by electron backscattered diffraction (EBSD) was used to investigate the effect of microstructure on thermal conductivity. Findings The highest through-plane thermal conductivity (136 ± 2 W/m-K) was achieved at 59 J/mm 3 and exceeded the values reported previously. The in-plane thermal conductivity peaked at 117 ± 2 W/m-K at 50 J/mm 3 . The trend of thermal conductivity reducing with energy density at similar porosity was primarily due to the reduced grain size producing more Al-Si interfaces that pose thermal resistance. At these interfaces, thermal energy must convert from electrons in the aluminum to phonons in the silicon. The co-located thermal conductivity and crystallographic grain orientation maps confirmed that larger colonies of columnar grains have higher thermal conductivity compared to smaller columnar grains. Practical implications The thermal properties of AlSi10Mg are crucial to heat transfer applications including additively manufactured heatsinks, cold plates, vapor chambers, heat pipes, enclosures and heat exchangers. Additionally, thermal-based nondestructive testing methods require these properties for applications such as defect detection and simulation of L-PBF processes. Industrial standards for L-PBF processes and components can use the data for thermal applications. Originality/value To the best of the authors’ knowledge, this paper is the first to make coupled thermal conductivity maps that were matched to microstructure for L-PBF AlSi10Mg aluminum alloy. This was achieved by a unique in-house thermal conductivity mapping setup and relating the data to local SEM EBSD maps. This provides the first conclusive proof that larger grain sizes can achieve higher thermal conductivity for this processing method and material system. This study also shows that control of the solidification can result in higher thermal conductivity. It was also the first to find that the build substrate (with or without support) has a large effect on thermal conductivity.more » « less
-
The objective of this study is to evaluate the radiation induced microstructural and mechanical differences influenced by alloying elements including phosphorus, chromium, and nitrogen and crystal orientation in iron-based binary alloys. Fe-4.5at%P, Fe-9.5at%Cr, and Fe-2.3at%N binary model alloys were irradiated with 4.4 MeV Fe++ ions at 370 °C to 8.5 displacements per atom (DPA). Transmission electron microscopy (TEM) characterization including brightfield scanning electron microscopy (BFSTEM), diffraction, and TEM in situ irradiation, energy dispersive spectroscopy (EDS) compositional analysis, and nanoindentation were used to evaluate the radiation induced microstructural evolution and mechanical responses in these model alloys. Microstructure is of particular interest in irradiated nuclear structural materials because it plays an integral role in the mechanical integrity of these materials. Radiation induced defects present obstacles to dislocation motion and thus lead to hardening and embrittlement. P is highly undersized and forms a strong covalent bond with Fe which progresses to an Fe3P beta phase in BCC iron when the solubility limit is reached. The covalent nature of the bonding as well as the smaller atomic volume of P leads to enhanced radiation induced defect nucleation, phosphorus segregation, and radiation induced precipitation. The high density of defects in the Fe-P alloy contributed to enhanced hardening of the irradiated Fe-P alloy in comparison to the Fe-Cr and Fe-N alloys. The density of these defects and depth of the ion irradiated damaged layer and thus the mechanical response is also heavily dependent on orientation and is made evident by nanoindentation and indentation cross section BFSTEM imaging.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1115/MSEC2020-8442
