Effect of Beam Scan Length on Microstructure Characteristics of EBM Manufactured Alloy 718

Sammanfattning: Additive Manufacturing (AM) as a method is on the rise and allow for a high freedom to create unique shapes without being limited by conventional machining methods. The Electron Beam Melting method, developed by Arcam AB in Mölndal, Sweden, use Powder Bed Fusion together with an electron beam and at an elevated temperature (+1000ºC) to lower stress due to thermal gradients. The purpose of this paper is to study the influence of Scan Length during Electron Beam Melting of Alloy 718 in regards to the appearance of shrinkage, porosity, primary carbide precipitation (mainly NbC), primary dendrite width and hardness. Samples built had the dimensions of 10x15xVar mm3 (Height x Depth x Width) with widths ranging from 10 mm in steps of 5 mm up to a maximum of 90 mm. The parameters were set as a single entry within the build project and as such each layer was melted as a single unit. A Light-Optical Microscope (LOM) and a Scanning Electron Microscope (SEM) was used to obtain images for manual counting to calculate the fraction of porosity and NbC-precipitates as well as the columnar grain width. The space between lines of interdendritical precipitation of NbC was used to determine the dendrite arm widths and a series of Hardness Vickers (500g for 15s) indents was performed. An Energy-Dispersive X-Ray Spectroscope (EDS) was used to help identify precipitates and phases. Columnar grain width and the spacing between vertical bands of interdendritical NbC was measured according to ASTM112-13 while porosity and hardness was measured according to ASTM562-11. Both of these only looked at the XZ-plane instead of all three planes. The columnar grain width was measured in the 10 mm, 40 mm and 90 mm samples at a distance of 4 mm from the top and with a slight spread over the sample width according to ASTM112-13 but using only one plane for counting. No significant change to columnar width was found. Primary dendrite arm width was measured on the 10 mm, 40 mm and 90 mm samples at about 5 mm from the top. An average for all samples was found to be 7.82 μm ± 2.89. No significant trend could be found with increased sample width. A total average porosity of 0.33% ± 0.16 was found. Variations between samples were less than the standard deviation. Even though the variations were not high enough to be significant, no obvious trend could be seen in regards to sample width, position on the base plate or heat transfer through the build. The presence of NbC was investigated in all samples with a total average of 0.36% ± 0.23 with variations between sample lengths being within the standard deviation. An insignificant trend could be seen between the smaller samples together with the wider samples having a higher degree of NbC compared to the middle samples. No significant trend could be seen in NbC based on row. Across all samples, the mean hardness was found to be 406.75 HV0.5 ± 16.53. No significant trend could be seen with increased sample width. Based on sample rows no significant trend could be seen.