Elsevier

Additive Manufacturing

Volume 34, August 2020, 100918
Additive Manufacturing

Full length article
Combined effect of surface anomalies and volumetric defects on fatigue assessment of AlSi7Mg fabricated via laser powder bed fusion

https://doi.org/10.1016/j.addma.2019.100918Get rights and content

Abstract

In recent years, the fabrication of aluminum alloy parts via laser powder bed fusion has been extensively considered in the biomedical, aerospace, and other industrial sectors, as it provides advantages such as the ability to manufacture complex shapes with high performance associated with lightweight design. However, surface irregularities and sub-surface defects limit the full exploitation of such parts in fatigue-critical applications. Moreover, most of the commonly used metrological methods for surface characterization have proven to be unsuitable for determining important features such as undercuts and sub-surfaces pores. Hence, a comprehensive coupled investigation of metrological methods and cross-sectional analysis were performed in this study to evaluate the effects of surface features and volumetric defects typical of additively manufactured materials. Fatigue tests and fractographic analyses were conducted to support the finite element simulations and proposed fracture mechanics model. The results demonstrate that the standard metrological methods cannot provide all of the data needed to model the fatigue behaviors of additively manufactured materials robustly. Moreover, a statistical model describing the competition between volumetric defects and surface irregularities was developed and validated.

Introduction

Additive manufacturing (AM) technologies enable the production of structural and functional parts via layer-by-layer addition of material, starting from 3D digital computer-aided designs (CADs). Among the various techniques available, laser powder bed fusion (L-PBF) is the most common technique utilized for printing nearly net-shaped metallic parts from the base metallic powder. L-PBF has received extensive attention in the past few years for the production of complex geometries, customized parts, and open cell structures (3D lattices) with little material waste for the aerospace, biomedical, and automotive industries [[1], [2], [3], [4]].

Many previous studies have been focused on optimization of the processing or post-processing techniques such as heat treatment and surface finishing of the as-built parts due to their poor surface quality compared to conventionally manufactured parts [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. Surfaces produced by L-PBF suffer from high roughness and irregularities due to the layer-by-layer nature of the building process and the presence of particular contaminants, such as spatter, balling, and partially melted metal powder [[17], [18], [19], [20], [21], [22], [23], [24]]. Furthermore, the presence of the so-called altered material zone (AMZ), corresponding to the sub-surface layers of parts whose properties differ from those of the bulk material due to the use of different contour process parameters also requires thorough assessment [5,25]. The aforementioned surface and sub-surface anomalies could cause additional notch effects and trigger fatigue crack initiation, thereby affecting the performance of the parts [26,27]. Therefore, a compatible approach to surface metrology is urgently necessary to understand the quality of manufactured parts. AM surfaces have freeform geometry and are combinations of structured surface textures with random features, which makes them very difficult to analyze using conventional metrological methods [28,29].

Numerous reports have been published on the surface metrology of additive manufactured parts based on roughness evaluation using stylus profilometers [6,18,24,[30], [31], [32], [33], [34], [35]] and other non-contact methods such as confocal microscopy [[36], [37], [38]], focus variation microscopy [31], coherence scanning interferometry [39,40] and X-ray micro-computed tomography (μCT) [[41], [42], [43], [44], [45], [46], [47]]. However, a combined study of the surfaces of AM materials using metrological methods and cross-sectional analysis aimed at coupling the surface roughness parameters with the features of the AMZ has not yet been performed. Among the various aspects affected by the surface quality, one of the most complex and widely debated is the fatigue strength. In fact, the fatigue properties of net-shaped AM parts are largely affected by the previously described surface features (SFs) and flaws [48]. Considering parts with machined or high-quality surfaces, fatigue strength is mostly driven by volumetric defects (VDs) falling in the surface or sub-surface regions [49,50]. In these cases, several authors have shown that defects can be treated in the form of short cracks by describing their size using the area parameter proposed by Murakami [51]. The effects of such defects on the fatigue strength can be assessed by using the Kitagawa-Takahashi diagram [45,48,[52], [53], [54]] or performing fatigue crack growth simulations [50,55,56]. In contrast, the effects of the typical SFs caused by AM are currently being debated and no standardized methods of robustly determining the effects of the surface on fatigue performance are available.

Two primary approaches have been adopted in previous work to consider the effects of net-shape surface conditions: (i) evaluation of the stress concentration induced by SFs [46,53,[57], [58], [59]] and (ii) estimation of the empirical [53,[60], [61], [62]] or fictitious (i.e., equivalent initial flaw size approach) [[63], [64], [65]] size of the SFs and application of fracture mechanics concepts [65].

As fatigue phenomena are driven by the largest and most detrimental features present in the most loaded volume, the ability to measure or estimate the size of the most detrimental feature is essential to perform fatigue assessment of parts fabricated via AM correctly [66]. Thus, fatigue strength must be evaluated as a “competing” risk between the VDs and SFs. In this paper, a new approach is proposed whereby both VDs and SFs are treated as short cracks and their criticality is evaluated via fracture mechanics approaches, by describing their sizes using the area parameter.

The present report describes the experimental investigation and modeling performed to evaluate the effects of the surface quality of AM materials on the fatigue strength. For this purpose, various metrology methods and fracture mechanics modeling were considered to select the most significant parameter determining the fatigue properties of the investigated material. Based on this approach, a probabilistic model usable for estimating the critical failure mechanism by competing-risk assessment of the VDs and SFs of AM parts was developed. The model was finally validated by performing fatigue tests on AlSi7Mg samples produced by L-PBF, according to various process conditions.

Section snippets

Material and sample manufacturing

All of the samples in this study were manufactured via L-PBF by utilizing a Renishaw AM250 system (Wotton-under-Edge, UK) with a single-mode pulsed fiber laser with a maximum power of 200 W, focused to a spot size of 75 μm. The powder adopted was a commercial gas atomized AlSi7Mg (A357) alloy supplied by LPW Technology Ltd. (Runcorn, UK), whose chemical composition is reported in Table 1. The alloy belongs to the Al-Si-Mg alloy system, which is widely used for L-PBF processing.

Cubic specimens

Morphology of surfaces

Representative SEM images for a selection of three of the investigated surfaces (S01, S05, and S07) are presented in Fig. 2. The S01 specimen, with highest line energy (EL = 350 J/m), shows a fairly smooth surface, occasionally covered with typical L-PBF SFs such as spatter, partially unmelted metal powder, and balling (Fig. 2a). The formation mechanisms of these features are explained elsewhere [17]. In comparison, the S07 specimen (Fig. 2c), featuring the lowest line energy (EL = 83 J/m),

Fracture mechanics analysis of surface and sub-surface irregularities

To achieve better understanding of the effects of the various surface irregularities detected in the investigated samples, the stress intensity associated with the surface and sub-surface defects was calculated by finite element (FE) analyses. The steps performed in these simulations are listed below and summarized in Fig. 11:

  • 1

    Extract a relevant section of the surface (with a length of at least 1 mm);

  • 2

    Perform thresholding to distinguish the material from the background;

  • 3

    Mesh the image with a

Discussion

Based on the topological measurements of the SFs, one could expect that a surface having large roughness peaks, such as S01, would be more detrimental than one featuring lower roughness, such as S07. Conversely, fractography analyses of the fatigue-tested specimens showed that all of the failures detected in S01 were caused by VDs, while only a small percentage of internal defects (specifically LoF) occurred at the fracture origin of the S07 samples.

The reason for this finding is that the large

Conclusions

The effects of the surface conditions on the fatigue properties of L-PBF AlSi7Mg were investigated considering samples fabricated with different process parameters for the contour layers, resulting in different surface characteristics. The quality of the surfaces was then investigated using contact and non-contact metrological methods, followed by analysis of the polished cross-sections. The fracture surfaces were carefully investigated after performing bending fatigue tests on three sets of

Acknowledgements

The support by the Italian Ministry for Education, University and Research (MIUR) through the project “Department of Excellence LIS4.0″ (Integrated Laboratory for Lightweight and Smart Structures) is acknowledged.

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