Page 11 - 3D Metal Printing magazine Fall 2022

material and the interface. The isotropy in the X-Y plane for DED does not hold the same way as it does for PBF.
Another set of initial assumptions relates to the mechanical testing methods themselves. Standard test methods, as those in AWS 4.0 or the nearly equivalent ASTM standards, are assumed. We can obtain additional information beyond the standard test methods using digital image correlation on ten- sile tests, allowing checks for variation of material properties across gradients, including surface-to-core, deposits on inte- grated build plates and functionally graded deposits.
Tests such as fatigue, creep, FGCR and corrosion are “appli- cation-specific” tests activated and included in the qualifica- tion schema as you start qualifying a process for a particular service condition and for a family of parts in those service conditions.
Recommendations
Both PBF and DED, by building layer by layer, create an effective thermal gradient in the build direction (often desig- nated the Z direction). This often creates a texture, with prefer- ential grain growth in this direction. It only takes a few layers above the base plate or the bottom of an outstand to set up this texture, which can be modified by solid-state transformations
during cooling. Thus, it is crucial to test for strength if the resulting items will need strength in that direction. Many pre- liminary demonstrations have avoided this direction because it takes longer to build in this orientation than it does parallel to the base plate. Other directions of testing can be considered interchangeable for blocky deposits—those not affected by the surface-to-core gradient.
Unlike testing of manual welding procedures, where lack of sidewall fusion is particularly important, the connection to the build plate may not even remain in the final part. Thus, the equivalent of cross-weld tensile tests and transverse bends will not be needed for every case, but can be used specifically to qualify cases with integrated build plates. Toughness testing around the interface also would be desirable when the deposit and the HAZ material can crack around the first layer interface, such as when the toughness is reduced by the more rapid cool- ing of the material next to a heavy base plate.
Overall, looking for effects of texture on strength and tough- ness seem to be more important for PBF and DED than looking for soundness. Since the heating gradient will remain an inte- gral part of metal AM, we can focus more attention on this with mechanical tests and less on expecting soundness variation
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