Cooling Rate Determination in Additively Manufactured Aluminum Alloy 2219
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ADDITIVE manufacturing (AM) comprises a family of processes that utilize a focused heat source to melt a feedstock material to create a three-dimensional shape directly from a computer model.[1] The processes can be broadly characterized as being either high fidelity with low volumetric deposition rate, low fidelity with high volumetric deposition rate, or some variation in between. Within this envelope, the thermal input from the heat source can vary from as little as a few hundred watts to tens of kilowatts. The process pressure can vary from high vacuum for electron beam processes (10 mPa) to ambient pressure for most other processes. The geometry of the deposited structure also has a significant impact on the heat flow during the AM process. Thin-wall geometries can limit heat conduction away from the molten pool in two dimensions, while bulk deposits have a much larger heat sink with a more three-dimensional heat flux. All of these factors create a processing environment with highly variable heat flux throughout the deposition process. Variability in heat flux can induce variability in the cooling rate of the deposited material, which in turn can affect the mechanical and physical properties, especially as the change in heat flux becomes extreme. CRAIG A. BRICE, Materials Research Engineer, is with the NASA Langley Research Center, Hampton, VA 23681. Contact email: [email protected] NOAH DENNIS, Graduate Associate, is with the Georgia Institute of Technology, Atlanta, GA 30332 . Manuscript submitted September 30, 2014. Article published online February 10, 2015 2304—VOLUME 46A, MAY 2015
This paper examines the variability in additively manufactured aluminum deposits made using the electron beam freeform fabrication process (EBF3) currently being developed at NASA Langley Research Center in Hampton, Virginia, USA.[2] The EBF3 process uses an electron beam to melt feedstock wire and create freeform objects in a layered manner. The process takes place inside a vacuum chamber with an operating pressure of about 10 kPa, which greatly limits thermal convection. The dominant heat transfer modes are radiation to the atmosphere and conduction through the substrate plate into the worktable. In this environment, the first few layers are deposited at ambient temperature and experience vastly different thermal boundary conditions than the last few layers, when the environment has accumulated excess heat. It is important to understand and quantify this environmental variability to determine what effect it might have on microstructure and properties of AM structures. The objective of the experiment was to estimate the cooling rate in an EBF3-deposited aluminum alloy. First, the secondary dendrite arm spacing (SDAS) was measured in aluminum alloy 2219 material that was remelted using a beam-only pass under variety of preheat conditions. The SDAS was used to estimate a cooling rate during solidification using established empirical relationships. The resulting data were then compared to a twenty-layer additive deposit made using
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