Fatigue strength assessment of “as built” AlSi10Mg manufactured by SLM with different build orientations

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Highlights

Abstract

This study investigated the effects of the ‘as-built’ condition on the fatigue properties of an AlSi10Mg aluminium alloy manufactured by Selective Laser Melting (SLM) in different orientations. The results showed a moderate correlation between defects/surface features at the fracture origin and the roughness measurements. On the other hand, a fatigue strength analysis based on the size of features at fracture origin and residual stress just below the surface proved to be successful. Although considering the synergistic effect of different variables increased the scatter in fatigue life data points, a fatigue strength model including short crack effect and residual stresses made it possible to estimate the EIFS close to the real critical features. Results for life prediction models based on EIFS showed that 87% of the estimated number of cycles to failure are within a factor of 0.5–2 with respect to the experimental number of cycles to failure.

Introduction

Additive Manufacturing (AM) is becoming a suitable technology in various industries such as aerospace, automotive, and biomedical, thanks to the many advantages it offers [1]. Before widening the range of applications for such promising technology, validation of SLM components involved in load-bearing applications is required [2] considering the different parameters that control scatter and variability of the material properties [3], [4]. As for traditional manufacturing processes, the main parameters controlling the fatigue properties are the presence of flaws/defects, the surface condition and the residual stresses (once again related to the process) [5].

The presence of micro-structural defects heavily affects the fatigue performance of any metallic material [6] and they are the responsible for the fatigue debit of AM materials [2], [7]. The complex thermal cycles during fabrication of AM parts, along with the variations in the part’s geometry, can influence the orientation and geometry of the defects [8], [9], [10]. In general, studies have shown that defects closer to the surface can be more detrimental to fatigue resistance [11], [12], [13] since these defects typically experience higher crack-driving force expressed by the Stress Intensity Factor (SIF) [6], [14], [15].

The build orientation of AM parts can generate an anisotropic structural response, especially under fatigue [1], [10], [16], [17]. This is due to the fact that layer orientation affects microstructure and defect directionality [10]. For fatigue properties, the effect of defect orientation is more significant for large defects with lower aspect ratios, such as Lack Of Fusion (LOF) defects, as opposed to spherical defects such as pores [8], [17], [18].

A literature review on additively manufactured AlSi10Mg and Ti-6Al-4V demonstrated that the presence of manufacturing defects (mostly pores and LOF) are the main causes of the characteristic scatter in fatigue data for AM materials [19]. This can be introduced by the dependence of fatigue strength on defect size (the so called Kitagawa diagram, which expresses the fatigue limit in the presence of short cracks [20]). Following Murakami’s concepts [21], [22], Romano et al. [14] assessed the influence of defect size on fatigue of AlSi10Mg manufactured by SLM by means of the Kitagawa diagram and by describing the defect dimension through the area parameter (i.e., the square root of the projected area of the defect [6]).

The same concepts, although adopting slightly different models, were then successfully adopted by several authors on different AM materials [18], [23], [24], [25], [26], [27].

Among the aforementioned significant fatigue parameters, a deeper understanding of the surface roughness effect is necessary, considering that AM components would be used even without machining or post processing [1], [28]. Surface irregularities of SLM parts are mainly due to the layer-by-layer effect of the build process and they can be described as: partially un-melted particles, spatter generation and balling effects [29], together with the staircase effect, that could be classified as a topographical defect typical of AM processes [28].

As it happens for traditional materials [5], a rougher surface significantly decreases the fatigue life of AM components, because micro-surface notches or any discontinuities on the surface act as crack initiation sites [24], [30], [31], [32]. Molaei et al. [33] showed that surface connected defect networks are oriented perpendicular to the build orientation and that these defect networks influence the fatigue properties. Many researchers also observed that the fracture surface of as-built AM materials is always characterized by multiple crack initiation sites at different layers [34], [35].

Furthermore, studies have indicated distinct surface roughness of as-built Laser Powder Bed Fusion (LPBF) specimens to be dependent on surface orientation [36], [37], especially for those that are fabricated in a diagonal direction with respect to the build plate. Downfacing surfaces have higher surface roughness than upfacing ones [38]. Pegues et al. [39] and Solberg et al. [12] showed that crack initiation occurs at the rougher downward-facing surface on diagonally built specimens.

For modelling the effect of ‘as-built’ rough surfaces, a line of approaches followed the definition of an equivalent stress concentration factor for the rough surface in different ways: i) a simple expression of Kt in [40] where the local notch radius and depth correspond to surface waviness; ii) a definition of a local fatigue stress concentration based on notch sensitivity and local radius at the tip surface valleys [41]; iii) complex finite element models of the rough surface (measured by CT scans) for calculating local fatigue concentrations by a fatigue criterion averaged over a critical distance/volume [42], [43], [44], [45].

Other approaches followed the concept of treating the roughness as an initial crack: i) Rt [16] or Rv for a life estimation based on Linear Elastic Fracture Mechanics (LEFM) [32], [46]; ii) estimating an Equivalent Initial Flaw Size (EIFS) able to provide a good description of the experimental data with simple LEFM models [31], [47] or better results in combination with the Kitagawa diagram [48]); iii) treating the real features at the crack origin as ‘short cracks’ [24], [49], [50] for life estimation or for correctly analysing the outcomes from different experimental tests [51], [52].

Residual stresses (RS) are caused by high spatial temperature gradients in AM parts. In fact, when an entirely new layer is melted simultaneously and deposited, the temperature of the new layer is higher than the underlying part which was cooling uniformly in building direction. The differences in temperature of newly solidified layers respect to the surrounding material, together with rapid cooling and the constraint of the material underneath, induces tensile residual stress [53]. Printing orientation and presence of supports will change the RS distribution because for each different orientation the boundary condition for heat dissipation will be different [53], [54], [55], [56]. While the RS adds complexity to the real state of stress in AM parts [57], it is widely known that tensile RS is detrimental to the fatigue strength of AM materials [58] because it alters the mean stress and load ratio during cyclical loading and has a significant influence on the crack propagation rate and leads to anisotropic crack growth behavior [40], [59]. However, since most of the studies in fatigue of AM deal with materials subject to heat treatment after printing (i.e. Ti6Al4V and Ni-based superalloys) there is a lack of evidence of the synergetic effect of residual stresses and rough surfaces.

It is therefore important to study the correlation between surface roughness (and features) and fatigue properties of the ‘as-built’ surfaces. Methods based on crack-size and EIFS concepts are appealing because they make it possible to adopt the usual concepts of component qualification [2], [60] and to apply knowledge developed so far for internal defects. However, there is no confirmation of these methods differentiating between different surface qualities and different surface orientations: on one hand the assumption of EIFS size is not strictly related to real surface features [31], [61], on the other hand roughness and surface features are only one of the factors involved in fatigue performance. In fact, the difference in thermal histories responsible for the different roughness can also lead to very different microstructures or residual stresses [8], [38], [62].

Based on this background we investigated the fatigue behaviour of as-built specimens in AlSi10Mg manufactured by SLM with different build orientations. The primary aims of this study were:

  • 1.

    To understand comprehensively the fatigue properties of additively manufactured AlSi10Mg parts, already extensively studied by the research group, by considering the effect of surface quality,

  • 2.

    To verify if the surface features are able to fully describe the fatigue properties of surfaces with a different building quality or if other significant parameters have to be included,

  • 3.

    To verify how far is the estimated value of the EIFS from the measured surface features when short-crack are taken into consideration.

The secondary objective of this work is associated with developing fatigue life prediction models for AM materials: if the effect of internal defects is well-understood and included in probabilistic fatigue assessment codes [63], [64], it is crucial to correctly model the surface features because of the competition with internal inhomogeneities [65], [66], [67], [68].

Section snippets

Experimental methods

The samples were printed by SLM and no stress reliefs or heat treatments were involved in the post processing. All the experiments and measurements were performed in air, at room temperature. The roughness and condition of a surface produced by SLM depends on the printing orientation. To evaluate the effect of this process feature on the fatigue properties, the three point bending specimens were oriented according to five build orientations and classified in series, as shown in Fig. 1(a). In

S-N curves

Table 4 and Fig. 5 summarize and compare the fatigue test results and S-N parameters of different series. Table 5 correlates the failures and their positions, it shows that most of the failures were within 15% of notch length respect to the mid-section.

The aim of the three-point bending tests for machined specimens (series AA, CC, and DD) was to determine a reference fatigue limit, which is in the range 160-180[MPa] in agreement with data in [83]. The lack of a significant orientation effect

Measurement of surface features at the fracture origin

In order to understand the variation of fatigue limit distributions for different series, a defect-based analysis was taken into consideration. The crack initiation site for each specimen was determined and the area of defect responsible for crack initiation was measured using a digital image analysis tool [95].

The size was determined based on the guidelines described by Murakami [6] in terms of the area parameter (square root of defect area), because it prospectively makes it possible to

Effective stress ratio at the fatigue limit

If for series A the residual stresses are negligible and so the effective stress ratio is Reff0.1, then for the other series the effective stress ratio at the fatigue limit is quite different from the load stress ratio RL=0.1 applied during the tests.

Considering the residual stress Sres,100μm, we can calculate the effective stress ratio at the surface of specimens, that is relevant for the threshold condition of surface defects. For series B and D the effective stresses of the fatigue cycles

Conclusions

This study summarized the results related to the fatigue behaviour of as-built AlSi10Mg alloys produced by the SLM process considering the effect of surface roughness as a result of various build orientations. Five series of the notched specimen with five different build orientations horizontal, vertical, diagonal (upward notch), diagonal (downward notch), and on the side with respect to the build plane were taken into account for three-point bending fatigue tests. Crack growth behaviour of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the support from the Italian Ministry for Education, University and Research (MIUR) through the Department of Excellence LIS4.0 project (Lightweight and Smart Structures for Industry 4.0).

The authors acknowledge support provided by BEAMIT (Fornovo, Italy) in terms of supplying SLM printed specimens for this research activity.

We are indebted to Prof. Nima Shamsaei (Auburn University, AL, USA) for the useful comments and remarks for this revised version.

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