Intermediate Structures & Properties of Materials

Fatigue properties gained from Additive Manufacturing of AlSi10Mg through defect replication testing and estimation of fatigue strength.

ABSTRACT

This review serves as the basis of an article aimed at locating and replicating the products via as samples from post-manufactured defects associated with Additive Manufacturing (AM). AM stands to be looked upon as a standing solution for mass production savings as well as the production of complex orientations within components. Of the replicated manufactured samples, they went through three separate processes with which were utilized throughout the experimental tests. The microstructures were analyzed for each sample to see any defects post-AM. The samples were of cylindrical geometery and during manufacturing all five samples consisted of both vertical orientation and horizontal orientation during their respective production. Testing was done with two starting factors post-AM, production orientation (both vertical and horizontal) and processing method. Typically two primary defects are recognized through AM, lack of fusion within the material and gas porosity (Seifi, 2016). This literature aims to tackle other defects that could or can be recognized for fatigue failure other than the more common but not limited to, surface voids and subsurface voids. Emphasizing homogeneity in regards to spatial defect distribution is an important focus of surface effects of post-AM products. One of the tests imposed was artificial micronotches produced by femtosecond laser ablation. Post discussion of the test results yields that the utilization of SIF distribution came to be a clear indicator for determining the criticality of defects but along with other similar concepts to be able to predict the sizes of critical defects as well. The primary focus was the post-testing results which collected the results of fatigue testing produced by SLM on AlSi10Mg. A model was built for estimating the fatigue life assesment and strength associated with the defects of post-AM production.

RELEVANCE

In regards to Processing-Structure-Properties-Performance relationship, this study listed the relationship as such. During sample preparation in the beginning of the article, a glaring feature that explains clearly the significant reduction in fatigue strength for as-built AM parts is the noted presence of diffused porosity within the subsurface region. Which is an issue that cannot be quickly rectified as even if the pores' diameter were small, clusters will form which creates a dangerous region that simple polishing cannot remove but the application of chemical milling or machining is required (Yadollahi, 2017).

As well as during analysis of the stress intensity factor, (SIF), at the fracture origin revealed that the internal crack rate of crack propagation is generally lower than it is for surface cracks (Itziar Serrano-Munoz, 2017). Therefore structure is impeded during vacuum testing as the internal cracks become more elongated versus the surface cracks which makes it more difficult to detect but are dependent upon manufacturing. The processing of the samples through AM already provides its own embedded defects that of which hinders the structure portion of the relationship through AM side effects alone. Platform pre-heating alone aided in the absence of residual stresses throughout the specimens manufactured, per the authors' findings.

One of the aims of the authors was to asses any possible anisotropy developed during the manufacturing process and crack propagation threshold (Yadollahi, 2017). As well as to consider the resistance of fracture toughness and fatigue crack growth. Through the authors' aims the properties were measured of which gave the estimated performance through Kitagawa diagrams, S-N curves and a model to show experimental scatter of high cycle fatigue, (HCF), regimes.

In regards to the performance relation to this article, improvement for fatigue resistance was the end-goal for the Processing-Structure-Properties-Performance relationship. Due to the improvement of the SLM in terms of output performance has seen a significant improvement in quality, which stands to a better method compared to basic casting. Electron beam melting, (EBM), has some benefits that SLM does lack as in the capability to control the build environment using a controlled vacuum and elevating the processing temperature within the building enclosure to reduce residual stresses (Arun Ramanathan Balachandramurthi J. M., 2018).

EXPERIMENTAL PROCEDURE

Some of the static tests utilized to test the relationship was building direction of the specimens, a series of CT scans and an axial strain-controlled test. Purpose of utilizing the varying orientations, horizontal and vertical, was to asses if any anisotropy was present during the AM process (Yadollahi, 2017), as mentioned earlier. The series of computed tomography, (CT), scans served to determine any population of effects. Reconstruction was also done by performing an optimization for beam hardening, post-alignment correction and ring artifacts. Through these scans and reconstruction, the authors found the presence of a large amount of manufacturing defects as well as within the varying processes of the specimens. After plotting their findings, their projected √area was perpendicular towards the printing direction. Which made the vertical printing orientation provide a property of lower fatigue resistance.

Investigating the microstructure thru chemical etching revealed that the build orientation of the samples clearly showed some voids and melting pools. The voids and pools themselves displayed, dimensionally anyways, were of an inhomogeneous pattern. Through closer examination of the melting pools, the authors' noted that the fine microstructures aggregated nanocells. The nanocells' outer edges themselves were bedeck with a phase of Eutectic (AlSi), with which are also found around the boundaries of the melting pools (B.Chena, 2017).

An axial strain test was performed to measure the cyclic response with a servo-hydraulic machine at frequency of 0.5Hz attached to a gauge extensometer. The findings of this test showed a progressive trend of cyclic yield stresses that increased as the processed samples were of finer manufacturing in a detail orientated sense by AM. Where the vertical oriented samples had higher yield stresses compared to that of the horizontal samples (S. Romano, 2017).

Another test performed by the authors which is basic and yet essential was a tensile test at room temperature. Variables being the machine was equipped with a 100kN load cell, probably suggest doing another load cell test above this measure of 100kN and another load cell test below as an added measure if I had to perform a similar test myself. Following material testing ISO standards, (6892-1, 2017), the data showed an increase in both UTS and YS. Which the only distinguishing varying factor was the sample orientation of horizontal versus vertical. The results concluded from this test was that their values for maximum strain at failure was positive, horizontal samples outperforming the vertical samples only slightly.

A few other varying tests that may have also been considered for this article as an alternative can be suggested from the following. One of the options being subjecting the specimens to solution treatment and ageing, (STA), with which as a pre-treatment hot isostatic pressing, (HIP), can also be done (Arun Ramanathan Balachandramurthi J. M., 2018). Utilizing the two test either in conjunction, versus STA alone, has shown to improve fatigue strength and hardness. Which could've proven to be an additional method of examination. Possibly machining the surface post production and performing some of the authors' tests may also result in some other fatigue resilience findings.

Believe performing some of the authors' tests as well as some of the other sugeestions made expanding upon them on samples not only manufacutured by SLM but as well as EBM. Interested in seeing if the typical microstructure associated with EBM processed samples, columnar microstructure, would have features that would alternate some of the findings based of texture (Raghavan, et al., 2016). Alloys manufactured by the use of EBM tend to display aboveaverage mechanical properties due to a lower brittle phase formation and higher ductility present (Murr, et al., 2009).

DISCUSSION

Some lacking details within this journal are some of the representations of the samples processes of manufacturing that differed between all three samples. No mention of how the original orientation was altered to fit within the cylindircal desired measurements to perform the fatigue tests necessary. Also adding on to additional potential testing, performing a fractographic analysis of the specimens based on a tensile test has the probability of displaying closure of shrinkage porosity. As well as the healing of the lack of fusion, (LoF), defects that of which are responsible for improving properties (Arun Ramanathan Balachandramurthi J. M., 2018).

Testing the potential difference in fatigue resistance and/or performance by removal of the outer surface could provide some varying results since there will be no lack of fusion defects present. Now the authors primarily focused on a 'fine microstructure' versus referencing their findings against a coarse-grained microstructure as well. Would be interesting to see how the authors' testing methods, alongside the additional tests suggestions made, would have yielded different results if the samples were of either microstructure. For if the sample were to be a more coarse-grained microstructure, the fatigue crack growth would be the result of cyclic cleavage mode due to the mechanism of single shear. The cyclic hardening behavior varies differently for a polycrystalline material, fine-grained, versus a more textured one, coarse-grained (Suresh, 1998). Hence there stands to be a difference in behavior for both deformation and crack propagation. Primarily due to the difference in both the texture of the microstructure and overall size of the grains at or near the region of maximum stress experienced (Arun Ramanathan Balachandramurthi J. M., 2018).

The authors did note the fact that some of their graphs and resulting data can also be slightly hindered by but not limited to initial crack size, example being intrinsic material scatter, fatigue crack growth curve, etc. that resulted in such a scatter of data points. Also noted was the fact that the polygon appearance of the fatigue fracture surfaces themselves have been affected by the attributes of the texture of the microstructure as well as the size of the grain. Would be interesting to find out if they were chemically treated prior to see how the texture alone would vary, if at all.

CONCLUSION

The fatigue tests performed on the material AlSi10Mg fabricated by SLM were of all three seperately processed samples ina cylindrical geometric output. The authors set out to determine the strength as well as the fatigue life based off the influence of AM. The advancement of AM has been ever so rapid in its' development that their samples investigated contained properties that exceeded dated casting methods. There samples were properly categorized by manufacturing defects in order of evaluation methods, i.e. CT, HCF, LCF, etc. I believe a few other tests and/or an additional fabrication method not limited to SLM, such as EBF, may have benefited in seeing the expanded results. Which this experimental journal article displays prominately the Process-Structure-Properties-Performance in such that the process of AM by SLM contains a certain order leading to performance.

The microstructure alone created by SLM varies in properties had compared to other methods with AM being a seperate contributer. Based off the microstructure being more fine in grain structure, with its issues in porosity and lack of fusion, it still tends to have positive fracture mechanic properties. Therefore its' performance still operates as such as a viable upgrade in terms of fabrication of AM by SLM compared to lesser quality of casting. Overall the authors analyzed an adequate amount of data based off the fabrication end results of AM by SLM. Providing sufficient investigation into the positives and negatives of AlSi10Mg and its' performance, minus the effect of plasticity, gives AM a considerable upgrade as a means of fabrication.

REFERENCES

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