Planned Experimental Matrices

As stated previously, two sets of test matrices were designed for the completion of this study. The first set of tests composed of full-sized specimens, which were subjected to conventional testing techniques under varying conditions for the characterization of the material for subsequent use in life prediction models following standards noted previously. In this study, this matrix primarily served as a model and standard of comparison for the second matrix, which is the focus of this study using the SPT. These tests included conventional tensile, creep, fatigue, and fracture tests at varying testing conditions. These will evaluate mechanical properties of IN939V to establish a basis by which the efficacy of the following SP tests may be evaluated.

Material mechanical properties such as Young’s modulus, yield and ultimate tensile strength, creep rates, and more will be determined at varying temperatures on samples manufactured in both horizontal and vertical orientations.

The second matrix, designed for small punch tests, has been subdivided by the equivalent conventional test types for each SP test, as outlined in the following tables, and will use similar conditions to those of the traditional test matrix, but adapted to the limitations of the small punch test. Desktop test controls were provided through the use of MTS software package Testworks

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Elite, implementing controls for force or displacement as required by the test matrix. A displacement rate of 0.5mm/min was used, which is in the range of what is commonly seen in literature and dictated in the CWA 15627, 0.2-2.0mm/min. The matrices are divided by the tests to which they are equivalent; tension, creep, and fatigue, with a variation of conditions for each of these. Heading each of these matrices are trials listed as using conventionally produced 304 stainless steel. Tests with this material serve as verification runs of the capabilities of the SPT rig with both the monotonic and cyclic functions. This is necessary especially given the novelty of the design for incorporating reversible loading, as significantly greater amounts of data are available for traditionally produced 304 stainless, facilitating comparison of test results. The feedstock material from which these samples are taken was certified for composition and material properties by the manufacturer. Selective tests which are representative of the traditional tests conducted were used for constructing the SPT matrix utilizing the SLM samples in order to show the suitability of SPT to evaluate SLM material evolution and SLM materials in general. In order to do so, some tests will be repeated with materials in various stages of post processing as well as different material types from both the machine at Power System Manufacturing (PSM) and from other sites. This includes several trials using samples taken from the grip sections of the tensile specimens produced by PSM and used for the recrystallization study presented earlier in this chapter, in which materials were tested at various stages and variations of heat treatment.

Additional studies listed in the test matrices previously given include other popular SLM materials. These include GP1 stainless steel samples, for which the feedstock material is manufactured by EOS specially for their DMLS machines with the equivalent composition to 17-4PH, manufactured at Central State University, and IN625 samples manufactured by Solid

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Concepts. These add variability in terms of both materials and SLM machines used, to ensure those do not present additional factors that affect test results when gauging the suitability of SPT for evaluating SLM materials. Finally, samples produced from IN718 on an EOS 270 machine with the manufacturer optimized settings will provide further additional test pieces and variability. This will provide a factor of variability within the test for different materials, to show that SPT is viable for more than one type of SLM material. These additional samples were mainly tested with monotonic SPT, comparing results to traditional tensile test results available in literature, and source data from traditional testing conducted at PSM, where available and applicable, as well as with conventional tests completed for this purpose, with samples manufactured from the same stock as the SPT samples. Test conditions for additional tests replicated those already existing within the test matrices shown in Table 3.2 to provide comparisons between materials.

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Table 3.2 - Tensile test equivalent SPT matrix.

Test No. Temp. (°C) Orientation Material Condition

T1 21 - SS 304 As-received

T2 21 Long IN939V RX, HIP, HT

T3 21 Trans IN939V RX, HIP, HT

T4 21 Long IN939V RX 3X Base, HIP, HT

T5 21 Trans IN939V RX 3X Base, HIP, HT

T6 21 Long IN939V RX Base, HIP, HT

T7 21 Trans IN939V RX Base, HIP, HT

T8 21 Long IN939V No RX, HIP, HT

T9 21 Trans IN939V No RX, HIP, HT

T10 21 Long IN939V As-manufactured

T11 21 Trans IN939V As-manufactured

T12 300 - SS 304 As-received

T13 427 Long IN939V RX, HIP, HT

T14 427 Trans IN939V RX, HIP, HT

T15 21 Long IN625 As-manufactured

T16 21 Trans IN718 As-manufactured

T17 21 Trans [100] EOS SS GP1 (17-4) As-manufactured

T18 21 Trans [010] EOS SS GP1 (17-4) As-manufactured

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Table 3.3 - Fatigue test equivalent SPT matrix.

Test No. Temp

*Initial cycles of samples F14 and F15 were conducted as tensile equivalent trials T17 and T18.

The variability of tests in terms of materials and manufacturing ensures that the test is robust and sensitive enough to detect any differences these variables may present, rather than gauging the suitability with a single material, IN939V, in the same state of processing. Samples were subjected to small punch testing and the accompanying analyses as detailed in the literature

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review to determine how well the properties and constants determined correlate with those of the traditional test matrix for tensile, creep, and fatigue test. Previously published correlation values from the various literature reviewed were tested to expand the existing literature and support or oppose the use of the published correlation factors. Updated correlation factors will be proposed as necessary. Although, tensile tests were directly adapted from traditional tests using SPT with monotonic loading, fatigue tests were modified to accommodate the test limitations. This includes cyclic loading in only one direction as previously seen in literature, and a novel modified cyclic testing mechanism able to test reversed cyclic loading. The use of this novel mechanism design allows for reversed loading via application of twin punches on either side of the sample, producing the loading history seen in Figure 3.5. The tests in Table 3.3 with an R value of -1 utilize this configuration, and the range denotes whether control for each trial is imparted using peak load or deflection.

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Figure 3.5 - Reversed cyclic loading as imparted by dual-punch SPT configuration.

Additionally, the fatigue test matrix also includes cyclic loading with the inclusion of a hold time at loading, which is representative of creep-fatigue loading as defined by ASTM E2714, which, as of this writing, has not been studied with SPT. The creep-fatigue and reversible-loading fatigue tests were conducted using load-control, while the single-punch fatigue tests were conducted using displacement control to achieve testing at specified strain ranges. The test matrices designed for this study test the viability of SPT as a test method for evaluating the material properties of components produced via SLM with its inherent directional property dependence, as well as its suitability for evaluating Inconel alloys. These tests will

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evaluate how well SPT can track changes in material properties due to operating temperature, build orientation, and post processing variations. As such, an additional assessment for the use of SPT as a method for optimizing processing parameters can be also indirectly conducted. If so, this determination alone could reduce process parameter and post-processing optimization time and costs considerably from current practices.