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Completed TRL 1 (started at 1, targeting 3)
Accurate high-temperature thermophysical property data for liquid metals and alloys are important for simulation of laser-based 3D printing processes. To understand and better control such additive manufacturing (AM) processes, knowledge of properties such as melting temperature, density, viscosity, and surface tension of liquid metals and alloys is needed.
Likewise, thermochemical property data information regarding alloys, including partial pressures, specific heat, latent heat of fusion, and chemical activities relative to composition and temperature aid in the understanding and development of phase data important in the material design process. Vacuum electrostatic levitation (ESL) is an important technique through which both thermophysical and thermochemical property measurements can be conducted without physical contact with the liquid.
ESL provides a container-less, noncontact measurement technique for the accurate measurement of many important properties. ESL facilitates the generation of high-temperature data and includes the capability to produce significant under-cooling, since there are no nucleation sites in contact with the sample. ESL has been utilized to generate an abundance of test data available for the properties of elemental liquid metals, however, there is little in the form of multiple component alloys, especially ternary and quaternary alloys. The ESL technique provides the stability to maintain constant sample temperatures while in a levitated state. This allows the conversion of equilibrium vapor pressure data, generated through mass loss experiments, into thermochemical properties as a function of composition.
Thermophysical and thermochemical properties are necessary for computational modeling of the AM process and parts, and these are not readily available for the alloys of interest at temperatures close to and above the melt point. Often, models extrapolate or use educated assumptions for the required properties. Solid state properties of specific heat, enthalpy of fusion, melting range and density are all excellent inputs for a CALPHAD model, while liquid state density and vapor pressure are useful inputs as well. These collective property measurements (thermophysical and thermochemical) are the primary objective of this proposal in addition to gaining a physical understanding of the measured properties through comparison with simulations. The proposed project will study several important existing and proposed titanium and nickel based engineering alloys through a container-less oscillating drop technique utilizing laboratories at the NASA Marshall Space Flight Center, the University of Louisiana at Lafayette, and the Louisiana based, NSF funded, Consortium for Innovation in Manufacturing and Materials (CIMM). Funded by other sources, the CIMM will provide additional resources to extend the reach of the funds provided by NASA. Thermodynamic data will be generated and compared to CALPHAD (Calculation of Phase Diagram) simulation results while thermophysical data of liquid alloys will be compared to molecular dynamics results. The test and simulation procedure, consistent with ICME (Integrated Computational Materials Engineering) principles and with the federal MGI (Materials Genome Initiative), will provide support for the development of improved AM simulation, and processing and control strategies.
The proposal aims to support the NASA initiatives involving the development of selective laser melting additive manufacturing, optimizing and refining the process and building materials properties databases for alloys of aluminum, titanium, and Inconel. The ultimate goal of in-space advanced manufacturing and repair is supported by these initiatives. A selection date and/or award and project start date beyond the target periods/dates listed in the CAN is acceptable. This proposal is valid through FY2019.
The Goal of this research is to support the measurement, and subsequent prediction of material properties relevant to the 3D printing of metals, including new and higher-order alloys, in order to support reliable printing of 3D metal/alloys parts with improved physical characteristics and functionality. The proposal aims to support the NASA initiatives involving the development of selective laser melting additive manufacturing, optimizing and refining the process, and building materials properties databases for alloys of aluminum, titanium, and Inconel. The ultimate goal of in-space advanced manufacturing and repair is supported by these initiatives. Objectives of this proposal include tackling major technical barriers facing SLM additive manufacturing including the lack of certification for 3D printing due to the lack of knowledge at powder-bed level and the dearth of material thermophysical and mechanical property data for process design and optimization. The first objective is to generate data relative to thermophysical and thermochemical properties of liquid titanium and nickel alloys which are otherwise difficult to measure and to model. Data relating to the higher order alloys in these systems will be generated for the first time. The generation of this data will facilitate the process modeling of direct selective laser melting (SLM) as well as other powder metallurgy manufacturing processes. The experimental extraction of thermochemical properties as a function of temperature and composition, as an integrated part of the thermophysical test procedure can lead to an improved materials characterization and design process relative to high- temperature and liquid metals. The chemical data can then be used to improve the CALPHAD database, which aids in new material design as the CALPHAD technique is often used as a first pass materials screening method. Similarly, the thermophysical data can be utilized to improve and validate molecular dynamics (MD) modeling which can be used to more rapidly generate data across temperature and composition ranges as well as generate elastic property predictions. The design of materials specifically for use in additive manufacturing (AM) processes as an outcome helps fill a critical void in AM commercialization. Improvement of test procedures to increase throughput and reduce uncertainty will also serve to support this objective. The second objective is to support and validate MD and CALPHAD simulation efforts that can then be expanded into larger ranges of temperature and composition measurements for material property prediction. A third objective is to characterize the microstructural evolution due to solidification processes relevant to SLM in order to optimize the SLM process for of the metal/alloys under consideration.
The process of SLM is one of the most popular methods of 3D printing of metals and involves the firing of a laser into a metallic power bed. As an emerging manufacturing method, SLM promises increased economy and speed when material volumes and batch sizes are small, key attributes for NASA and aerospace industry applications. Using fine metal/alloy powders (10 to 45 µm in diameter), SLM can produce complex shaped components while integrating flexibility in material composition and with near full density, attributes that cannot be produced by conventional methods.
During the SLM process, upon laser irradiation, the powder material is heated to melt, and forms a liquid pool. Thereafter, the molten pool interacts with the substrate, solidifies, and starts to form the consolidated product with the desired shape, layer-by-layer. This transient process, including non-uniform heating, molten pool/solid interaction, and uneven solidification, introduces complexities to the analysis of microstructural evolution and results in properties not typically found in conventional processes. Due to the lack of process optimization, SLM parts
usually have many defects, from atomic level point defects to large scale pores and flaws. These flaws lead to a poor mechanical strength and a shortened service life for SLM parts. “Some 47% of manufactures surveyed indicated that uncertain quality of the final product was a barrier to adoption of additive manufacturing (AM).”
The optimization and control of the process for generation of high quality outcomes requires high fidelity modeling of the multi-physics process which involves the melting- solidification of the metallic powder. The dynamics of the melt pool and the path of crystallization will ultimately determine structural properties of the part. In order to fully describe the process, thermophysical properties of the liquid metal must be quantified including density, surface tension, and viscosity as a function of temperature. In addition to SLM, surface tension and viscosity are key parameters in other liquid phase processes such as casting and joining while density, in combination with thermal conductivity and specific heat, is important for heat and mass transfer processes such as solidification. Indeed, the actual production of metal powders for use in additive processes are heavily influenced by the thermophysical properties of liquid metal droplets that cool into spherical powder particles.
When not experimentally available, property data is obtained from semi-empirical extrapolations derived from free energies and may significantly differ from experiment. Titanium and nickel based alloys are widely used in aerospace, biomedical and other areas with a growing global demand because of their high strength and stiffness, low density, and resistance to corrosion at high temperatures. As such, they have become a material of choice in the implementation of SLM processes. One common titanium alloy is Ti-6Al-4V, found in many industrial applications, and a strong candidate for use in 3D printing applications. A suggested improvement to the Ti-6Al-4V ternary alloy for the SLM process is the quaternary Ti-6Al-4V- 10Mo, which was found to have improved tensile properties. It would therefore be beneficial to have the appropriate material properties of this proposed alloy as a function of temperature to model the SLM process. The quaternary will be investigated, along with traditional alloys Inconel 718, Ni-Al, and time permitting, Haynes 230 for the needed thermodynamic properties. Thermochemical evaporation studies of Ti-xAl, Ni-xAl, and Ni-xCr will serve as inputs, along with the thermophysical data, to CALPHAD databases for model improvement.
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