Page 10 - 3D Metal Printing magazine Fall 2022

3D
AM INSIGHTS By William Mohr
Choices for Mechanical Testing for Qualification of Additive Manufacturing Processes
When additive manufacturing (AM) processes are qualified, it is common to perform mechanical tests of deposited material, as we expect that the material will carry mechanical loading. The assumptions about how AM material might behave differently from material pre- pared by other means (forging or casting, or even welding) have driven the choices of the required tests. As more AM has been performed and more combinations of material and process qualified, it has become obvious that experience does not justify some of those assumptions.
Two leading categories of metal AM are powder-bed fusion (PBF) and directed energy deposition (DED), both fusion- based involving the generation of melt pools and repeated cycles of heating and cooling, and similar to welding. And, welding has some common qualification requirements that have been developed and standardized by organizations such as the American Welding Society (AWS) for tests such as cross- weld tensile tests, all weld-metal tensile tests, Charpy tests for toughness (primarily of heat-affected zones, HAZs) and bend tests that primarily check for weld soundness.
Processes
PBF uses a controlled-atmosphere chamber around the powder bed, so chamber size limits the total size of any one build. As items are built above the base plate they remain in contact with the powder, which helps keep the temperature more uniform as the build continues and the heat source impinges on layers farther and farther from the base. Initial worries about leaving gaps between the heated passes and about gradual variations of the process across the build vol- ume have been reduced, partly due to trying to achieve the desired flatness of the layer top surface to act as a base for the next layer, which is checked on less-complicated builds than those used for mechanical test specimens.
While some DED processes may employ a build chamber larger than those for PBF, some DED can occur without a chamber. DED usually prints larger parts than PBF, even though the moving source of added metal creates more varia-
William Mohr, principal engineer for structural integrity at EWI, is responsible for initiating, conducting and reporting research and contract work. He is an expert in the areas of fitness-for-service assessment, design and fatigue of welded structures; bmohr@ewi.org.
tion in temperature through the build process. Like PBF, DED processes have developed to avoid gaps between beads, and often will create relatively high heat input per filler-metal vol- ume. This leads to remelting more of the previous layers and helps to achieve the desired flatness.
Testing Assumptions
Mechanical test specimens built by PBF and DED have the frustrating requirement that the material for the grip sections of mechanical tests must be made with the AM process. This differs from welding, where weld processes can be qualified with many specimens, including cross-weld tensile tests and transverse bends, oriented to put the grip sections into adja- cent base metal. Since DED generally makes larger parts, bend specimens and other larger-size test pieces are easier to make than those for PBF.
It is difficult to test the material out to the build-volume cor- ners without adding material to make the grip section. This resembles the problem of not being able to test through the thickness of thin castings or the through-thickness properties of welds.
Initial assumptions about mechanical testing for PBF, embedded in the qualification requirements for machines and procedures in AWS D20.1, consider that material position in the build volume, as-built thickness and as-built orientation are parameters that can affect tensile properties. These should be tested exhaustively. Other qualification documents have used a less-extensive material position matrix. Because PBF commonly uses different path directions for the laser for fill passes with each layer, there is no obvious directionality of the heating within a layer. However, the inherent anisotropy between properties in and out of the layer means that proper- ties must be tested in at least two directions.
Initial assumptions about mechanical testing for DED focused on differences of orientation in a more limited way, as well as thickness of the deposit. Position in the build plays less of a role, partly because there is no obvious outer envelope to the build volume to act as a reference. DED more likely will have a single direction of heating within a layer than will PBF, similar to the travel direction for welding. More-elaborate DED qualifications can use configurations, where specimens come from thicker block areas and from thinner walls, both in differ- ent directions and with specimens targeted at the deposited
8 | 3D METAL PRINTING • FALL 2022
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