Effects of Post-Processing Routines

Several of the trials listed in Table 3.2, T4 through T9, are designed to use samples sourced from the tensile tests conducted in the recrystallization study outlined in Table 3.1, and are meant to mimic those tests. These tests are designed specifically to evaluate the ability of the small punch test as a means to track gradual changes in microstructure due to variations in the

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recrystallization implemented in the post-processing of IN939V to reduce the anisotropic characteristics of the as-manufactured material. The responses shown in Figure 5.1 show the P-δ curves of these samples, while the equivalent tensile responses can be seen in Figure 3.3. These curves show a variation in the response depending on the post-processing recrystallization cycle implemented on the samples, and are labeled by their respective recrystallization study designation, with RX7 and RX8 having forgone the recrystallization cycle and instead received only the hot isostatic pressing and successive heat treatments and serve as a basis for comparison of the effectiveness of the recrystallization cycle treatment. Samples for RX1 and RX2, trials T6 and T7, respectively, were treated for a base recrystallization time at the recrystallization temperature, TRX, while samples for RX3 and RX4, trials T4 and T5, respectively, were treated for a time of 3X base in otherwise identical conditions.

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Figure 5.1 - Small punch tech test response of samples sourced from tensile tests conducted during the recrystallization study outlined in section 3.2.1.

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The most prominent feature of note when simultaneously comparing all six trials is an evidentiary reduction in strength, which corresponds to the occurrence of Pmax, with the implementation of a recrystallization cycle. This seems to continue as the recrystallization time increases, as Pmax generally occurs at lower values and also earlier on, at a lower value of δm. The samples which were not recrystallized, RX7 and RX8, diverge in the first and second regions, overlap for most of the third region, then diverge and switch position during the latter portion of the third region, of which RX8 has a longer duration, as the sample for RX7 has a longer plastic instability region, the fourth zone of the P-δ curve. Sample RX8, however, peaks earlier and at a higher level than RX7, and the two seem to converge again later in the failure region, suggesting similarities in the failure region and latter stages of ductility. To aid in inspection of the fractures of each specimen and ascertain their differences, however, samples were not tested until total failure, but stopped after a significant load drop indicating the appearance of a fracture and a limited amount of growth of the crack. Closer inspection of the elastic regions of these curves reveals a notable difference in their slopes, much larger than any of the other equivalently-treated pairs of differently-oriented samples. Given the collective differences in elasticity, strength, and ductility, a significant amount of anisotropy can be deduced to exist between the two directions.

Samples from RX1 and RX2, treated with a Base recrystallization cycle, conserve some of the behaviors of those displayed by the non-recrystallized samples. In the primary section of the responses of RX1 and RX2, the elastic portion, the gap is considerably reduced from that between RX7 and RX8, the non-recrystallized tests. The slopes of these portions of the curves appear almost parallel, which would make the elastic moduli equal, though they diverge somewhat in the latter portion of the elastic zone, prompting an assumption that they are not. The

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two curves begin to merge at the end of the elastic zone, but then begin to diverge soon after, as they cross and diverge early in the third region, then cross again as RX1 resists instability and fracture for much longer than RX2, as δm for RX1 occurs at 0.64mm, as opposed to 0.49mm for RX2, accompanied by a higher strength at 1.30kN for RX1 versus 1.18kN for RX2, though the rate of increase in the third region, and thus the plastic behavior up to the instability zone just before fracture seems to be equal for both orientations. This is akin to the behavior displayed by samples from RX7 and RX8, as both RX1 and RX7, which were manufactured in the transverse direction, have a higher δm than their longitudinally-manufactured counterparts, suggesting higher ductility and resistance to loading. A comparison of these two sets of trails also indicates that the recrystallization heat treatment initially impacts the strength of the longitudinally-manufactured samples more so than the transverse ones, as the elastic portion and the peak load of RX2 change more dramatically when compared to RX8 than RX1 does when compared to RX7.

This is continued, logically, when inspecting the curves of RX3 and RX4, which were recrystallized at a 3X Base time, relative to RX7 and RX8, as they are now nearly overlapping and remain parallel in the elastic region, suggesting equal moduli. The elastic regions persist for a longer range of displacement than those of either RX1 and RX2 or RX7 and RX8, and the modulus also appears higher for the longer treatment, reinforcing the direct correlation between the increasing modulus with treatment time, and a reduction in the anisotropy of such, as the initial slope of the P-δ curve has been linked as a direct correlation with the Young's modulus. In a macroscopic view, the two curves match up very well, with only a small gap separating them for the majority of their length, with the peak loads being only 3.5% different, from 1.214kN for

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RX3 to 1.171kN for RX4, and a difference of only 4.3% when normalized with respect to thickness and displacement. This indicates a significant reduction in anisotropy between the two directions, when compared to the un-recrystallized samples of RX7 and RX8, and even in comparison to the shorter Base treatment times undergone by the samples for RX1 and RX2.

There is also, however, an overall considerable reduction in strength when compared to RX7 and RX8, and another smaller further reduction of the transversely-built samples between the Base and 3X Base trials (RX1 vs. RX3). The successive reductions in strength, however, are crucial to the reduction in anisotropy, as the recrystallization treatment affects the transversely-built samples more dramatically than the longitudinally-built samples, closing the gap between the two directions over time. This suggests the possibility of a limit to the effectiveness of the recrystallization cycle for reducing anisotropy, and it’s unclear whether a further extension of the cycle time would lead to any further homogenization of the two directions, or just continue to reduce strength while maintaining a difference between them.

It must be considered, however, that the correlation of Pmax with the ultimate strength of a material has been shown to be dependent on the displacement, δm, at which Pmax occurs as well as the thickness, t, of the sample. In effect, this will work to reduce the differences in strengths when peak loads between two samples occur at very different values of δm. As such, Table 5.1 more directly quantifies the differences between the samples as they occur in the P-δ curves shown in Figure 5.1. These values show similar trends to those ascertained by visual comparison of the curves but show significant differences dependent on sample thickness and the value of the displacement at maximum load.

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Table 5.1 – Relevant inflection points and normalized values corresponding to material properties for small punch tests of recrystallization study test samples.

RX1 RX2 RX3 RX4 RX7 RX8

Inspecting the values in Table 5.1 and the percent differences between the directions for each set of treatment reveals similarities and differences to the visual trends. A notable reduction of Pmax is seen as recrystallization time is implemented and with the increase in time. The implementation of the cycle also increases the anisotropy, but the extended duration reduces the difference between orientations. This trend continues when the values are normalized, though normalization with respect to thickness and displacement shows a much higher difference than those normalized with the square of the thickness, which are included here solely for the sake of showing this disparity. Considering the P/δmt values reveals some inconsistencies, as these values actually denote an increase in strength with increasing recrystallization time, as well as a dominance in strength favoring the vertical orientation. Normalization with respect to thickness only shows a general trend of decreasing strength with respect to increasing treatment time, indicating either errors in the displacement values or an incompatibility of AM materials tested in SPT with the normalization techniques which utilize this metric. Inspecting the normalized

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values corresponding to the modulus and yield strength shows a general reduction in anisotropy with implementation and extension of a recrystallization cycle. Notable is that both of these traits seem to switch in terms of the dominant orientation when considering the extended recrystallization cycle as opposed to the base cycle or the non-recrystallized samples. This is denoted by a change in sign from negative, which indicates higher values for vertical samples, to positive, which indicates higher values for horizontal samples.

The effects of post-processing can be better assessed by comparing samples produced from uniform donor geometries, such as those acquired from the large blocks. Using these, samples for each condition can be sourced from the same donor for both orientation, so that geometry and its effects on initial and post-processed microstructures are not a factor, as they are in the recrystallization study, where the transverse samples were produced with the long direction parallel to the build plate and the longitudinal samples were produced with the long side perpendicular to the build plate. As seen in section 5.2.2, donor geometry differences such as these induce differences in cooling rate between layers, which will inherently affect microstructure and resultant material properties. The responses of monotonic room temperature tests for samples sourced from large blocks in both the as-manufactured and fully post-processed states are shown in Figure 5.2. Like in the recrystallization study trials, there is a severe indication of anisotropy in the as-manufactured samples, which is significantly reduced after heat treatment. As established previously in the tensile RX study, samples labeled as fully heat treated herein will have been treated with the longer recrystallization cycle, in an effort to reduce anisotropy as much as possible.

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Figure 5.2 - Monotonic SPT responses of SLM IN939V samples tested at room temperature in as-manufactured and heat-treated conditions for transverse and longitudinal orientations.

Upon primary inspection of the curves, further notable traits include similar moduli for all trials, distinguished by varied responses in the plastic region and significantly higher strength for the as-manufactured samples. The as-manufactured samples, T10 and T11, show higher strength throughout the entirety of the curves, with the horizontal sample displaying increased resistance to the onset of plastic deformation up to peak loading, absorbing a larger amount of strain energy in the early stages of displacement than the vertical sample. After the horizontal sample peaks, however, the vertical sample continues to steadily increase in load and deflection, highlighting a large disparity in directional strengths. Considering normalization factors

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emphasizes the anisotropy present in the as-manufactured samples, and the effectiveness of the post-processing routine for reducing it, as shown by the values in Table 5.2. There is a marked difference when considering normalized and percentage difference values for strength and moduli. Normalization shows an increase in ultimate strength for the transverse orientation, with a reduction for the longitudinal orientation, and a decrease for both in yield when normalized using the t/10 offset method, and moduli are increased for both orientations. Percent difference between orientations is reduced for all properties for the post-processed samples to the range of 2.7-3.2%, as compared to the as-manufactured absolute difference ranging from 13-17%. This shows the effectiveness of the post-processing routine in homogenizing the microstructure and the minimization of scatter due to the reduction in anisotropy. Of note, as differences are calculated using an error formula with longitudinal values as the base, some are calculated as negative, indicating higher values in the horizontal orientation. For the sake of comparison, discussions are typically done in absolute values of these. It can also be deduced, whether or not considering absolutes, that percent difference between directions, and thus anisotropy, is greatly reduced when donor geometry is kept constant between orientations, as the percentages for the samples sourced from the large blocks, regardless of manufacturing condition, are consistently lower than those sourced from the bars produced for the recrystallization study.

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Table 5.2 – SPT material property value results of SLM IN939V samples tested at room temperature in as-manufactured and heat-treated conditions for both orientations.

T2 long. HT T3 trans. HT T10 long. as-man T11 trans. as-man

The effects of testing temperature were examined by comparing several otherwise identical test trials. This included both conventionally and additively manufactured materials. As is seen in conventional tests, testing temperature affects material properties such as strength and ductility, and the extent of which can be shown to interact with other sample variations. The variations in materials properties between room temperature (RT) and elevated temperature test trials due to SPT response will be shown here, and comparisons to equivalent tensile testing data will be drawn in Chapter 6, using percentage differences for context.

Tensile equivalent monotonic tests which were conducted with stainless steel 304 at room and elevated temperature served as a basis for testing of the thermal capabilities of the SPT fixture developed for this study. Elevated temperature monotonic testing of 304 stainless was conducted at 300°C, with test parameters otherwise matching those of the room temperature test.

The responses of these trials, T1 and T12, are shown in Figure 5.3. Additionally, the response of the initial loading cycle of test F5, which was cycled at 200°C with R=0 to 1kN, is given. This is