Destructive Test Methods

Earlier in the book, within the process development section, destructive testing was mentioned as being used during parameter studies and during the fabrication of test samples. The samples are sectioned and subjected to a variety of metallographic tests to determine the micro-structural character, defect morphology, and mechanical properties of the deposit as they relate to processing conditions. Small lot production may rely on destructive testing of witness coupons fabricated layer by layer or within the same build cycle of the actual part to infer proper machine function. Figure 10.11 lists categories of destructive test methods being applied to AM parts.

Micro-structural analysis will often entail preparing small samples of representative regions of interest within a part and by mounting, polishing, and in some cases etching the samples to reveal the structure of grains, phases, inclusions, and defects such as pores, voids, cracks, and other discontinuities. As-polished samples may rely on camera-based microscopic inspection, coupled with image processing, to provide deposit density estimation, (% voids), or other semi-quantitative determination.

Hardness values and phase identification can be used to verify time-temperature transformations related to cooling rates and infer property determination confirmed by standard mechanical test procedures. Micro-structural analysis coupled w'ith nondestructive testing and proof testing is often all that is needed to qualify a noncritical component or process. A source for procedures used for micro-structural analysis can be found in the tech sheets of the Beuhler.

For common engineering materials in commercial shapes, mechanical property data contained within engineering data books such as those provided by the Society of Automotive Engineers (SAE), military specifications (MIL), and the American Society of Mechanical Engineers (ASME) is often sufficient to provide the confidence and documentation required by a quality trail. In cases where standard test data is unavailable, a full material and process qualification may need to be performed to assure the material properties and part performance. Vendors of AM

FIGURE 10.11 AM metal destructive tests.

machines often supply this information for their proprietary material, produced using their standard parameter sets and using their stated test conditions. In some cases, this may be sufficient to infer the integrity of a functional prototype, although the material test conditions and properties cited may not be representative of those of any AM produced part.

Service providers of metallurgical and micro-structural analysis are readily available via the Web and can provide rapid turnaround for a full range of services to characterize and formally perform analysis for process development and qualification needs. Design definition of standard test specimens may be enhanced to include standard AM deposition and testing conditions with which to compare material lot chemistry variations or machine-to-machine differences.

Chemical analysis can be applied to verify the integrity and purity of materials and processing conditions used in AM. Samples may be sent to service providers to identify levels of contaminants such as oxygen, hydrogen, nitrogen, or trace elements such as iron, vanadium, or aluminum. This analysis can be useful to determine the pickup of contaminants or loss of alloying constituents. Results may be compared with feed powder or wire chemistry specifications as required for process or part acceptance. Standardized mechanical test specimens and testing procedures have been used for technical generations to assist in alloy development and process material and process certification. ASTM International (referred to as ASTM) leads the development and delivery of voluntary consensus standards to improve product quality, enhance health and safety, strengthen market access and trade, and build consumer confidence. The American National Standards Institute (ANSI) serves to administer and coordinate the private-sector voluntary standardization system, for creation, dissemination, and use of standards and specifications for United States’ industry.

They work closely with similar international organizations such as ISO to harmonize and enhance the global conformity of products to these standards. Both of these organizations are active in the identification and to speed up the development of standards related to the growth of additive manufacturing as in the America Makes ANSI Additive Manufacturing Standardization Collaborative and the ASTM Committee F42 on Additive Manufacturing Technologies for the development of AM standards.

As applied to metals, common standard test specimens include tensile bars, Charpy V-notch tests, and creep specimens. Many material scientists and materials engineers have spent their technical lives studying the mechanical properties of metals. These test methods are actively being researched, modified, and applied to AM materials. These active areas of development are leading to identifying the need for new standards to speed the characterization and certification and adoption of AM processes and materials. An introductory description of ASTM standards can be found in student material provided on their website. Other destructive tests that may be applied to AM material could include something as simple as the standard guided bend tests as applied to 3D weld deposits. Guided bend testing is used in weld processing or metal to reveal internal flaws in weld deposits and HAZ regions. Flaws such as voids, cold laps, lack of fusion, poor fusion, porosity, or other micro-structural defects can be revealed in surface or near surface regions. Flat plate samples taken from bulk

AM coupons may be surface machined and bend tested representative of orientations related to the X, Y, Z orientation of the part within the AM build chamber.

Nondestructive testing of machined test coupons such as those described above may lend themselves more readily to inspection by UT, RT, and PT. While not testing an actual part, these samples can infer a represented characterization of bulk AM material, parameters, and procedures as qualified for an actual build procedure. As you can see from the discussion above, the inspection of AM parts remains a challenge as the procedures and technology are still being developed.