Elsevier

Additive Manufacturing

Volume 47, November 2021, 102324
Additive Manufacturing

Research Paper
Effect of hot isostatic pressing of laser powder bed fused Inconel 625 with purposely induced defects on the residual porosity and fatigue crack propagation behavior

https://doi.org/10.1016/j.addma.2021.102324Get rights and content

Highlights

  • Manufactured LPBF IN625 specimens with intentionally seeded porosity.

  • Characterized the effect of HIP on the porosity.

  • Evaluated the tensile properties of HIPed IN625 with intentionally seeded porosity.

  • Measured the fatigue crack growth rates of HIPed IN625.

Abstract

Inconel 625 specimens with different levels of purposely induced defects (up to ~3% of specimen volume) were manufactured by laser powder bed fusion. They were subjected to a stress relief annealing followed by hot isostatic pressing. After each step of post-processing, the residual porosity and the pore size distribution were measured using the computed tomography technique. Tensile and fatigue crack growth testing were then carried out to evaluate the impact of residual defects on the mechanical resistance and damage tolerance of the manufactured specimens. Hot isostatic pressing was effective in reducing the processing-induced defect count and size, but near-the-surface defects were still observed. Large clusters of residual defects were also observed for specimens manufactured with ~3% porosity. The elongation at break, and, to a lesser extent, the tensile strength were impacted by an as-manufactured porosity as small as 0.25%. Fatigue crack growth testing showed that HIP increases the threshold stress intensity factor for long fatigue crack propagation from ~7 to ~9 MPa*m1/2 and that irrespective of the level of as-manufactured porosity. An as-manufactured porosity smaller that 1% did not impact the fatigue crack growth rate of hot isostatic pressed specimens significantly, whereas occasional crack retardation was observed for specimens with an as-manufactured porosity of ~3%.

Introduction

Additive manufacturing (AM) technologies are emerging as a cost-efficient approach for the toolless manufacturing of complex metallic components with a reduced lead time [1], [2]. Thanks to this capability, AM processes are now being seriously considered for the manufacture of structural components for aerospace applications such as replacement parts for military aircraft, for example [3], [4], [5]. Laser powder bed fusion (LPBF) is one of the most promising AM processes, due in part to the good resolution and dimensional accuracy achieved with this process [1], [6]. One of the serious challenges that must be overcome in broadening the implementation of the LPBF technology in the aerospace industry is to prove its capacity to manufacture metallic components capable of meeting stringent structural integrity requirements [7], [8], [9]. This challenge stems from the fact that many investigations focusing on the fatigue response of AM-built components have reported a higher scatter in their fatigue life than is seen in their conventionally-processed equivalents [10], [11].

The particular response of AM-built metallic materials to cyclic loading can be attributed, notably, to their specific microstructure. For laser powder bed fusion process, the cooling rates and the thermal gradients accompanying metal solidification are typically very high (respectively in the range of 1–40 K/μs and 5–20 K/μm) [12], [13], [14], and these characteristics are highly sensitive to processing conditions [15], [16], [17]. The temperature fields and thermal gradients are also highly directional, which causes the as-processed material to be highly anisotropic [18]. Therefore, the resulting grain size, shape, distribution, and material texture can vary significantly for a given process. Furthermore, LPBF components are generally produced using a set of processing parameters that are optimized using simple, standard geometry specimens [19]. However, when processing complex components with different geometric features, these parameters can be suboptimal in some locations, thus resulting in a heterogeneous microstructure at the component level.

In addition to specific microstructure features, a variety of processing-induced defects can be observed in AM-built parts, with their rate of occurrence, distribution, and morphology strongly impacting their fatigue performance [20], [21]. In the case of laser powder bed fusion, these defects can be pores of various origins, inclusions, surface irregularities, spattering particles, unfused powder, or cracks [21], [22], [23]. The presence of a certain level of processing-induced defects is unavoidable and can even be exacerbated by the same decisions that are made to improve the process efficiency, such as using multi-laser processing [24], [25], or maximizing the metal deposition rate [26]. Note also that pores could already be present in powder particles, especially in those produced by gas atomization [27]. Moreover, spatial distribution of pores in printed parts is not always homogeneous. For example, in LPBF-built parts, sub-surface pore distribution is often different from the pore distribution in the bulk material. This is often observed at the junction of the region filled with hatching laser exposure parameters and the contouring region where different parameters are employed [10], [28], [29]. Spatial discrepancies in defect distributions across the build plate or within a single component are also reported [30], [31].

In many publications, fractographic examinations of LPBF-built specimens identify processing-induced defects to be the fatigue crack nucleation sites [32], [33], [34]. More specifically, near-the-surface defects, such as spherical or lack-of-fusion (LOF) defects, are often considered to be the principal factors contributing to the fatigue life of LPBF-built parts being shorter and more scattered than that of their conventionally-manufactured equivalents [35], [36], [37].

To alleviate the negative impact of processing-induced porosity, hot isostatic pressing (HIP) has been extensively employed by the powder metallurgy and casting industries over the last 70 years [38], [39], [40]. This process has become one of the most recommended post-processing treatments of metal AM parts since the very invention of this technology [41], [42]. During this treatment, parts are exposed simultaneously to high temperatures and isostatic compression applied by a pressurized gaseous atmosphere (often argon). High temperatures promote recrystallization and favor material homogenization at the micro-, meso- and macro-levels, while the simultaneously applied isostatic pressure promotes the closure of internal voids. As a result, HIP can increase the fatigue strength of various LPBF-processed alloys [43]. Notwithstanding the positive effects of HIP, some limitations have been identified for a variety of LPBF-induced defects. For example, it was reported that surface-connected pores cannot be successfully eliminated by HIP, since these defects are permeable to process gas [44]. Moreover, the presence of near-the-surface closed pores reported after HIP [45], [46] has been linked to argon pick-up during long-term part exposure to the pressurized processing atmosphere [47]. It was also shown that HIP may not entirely close large and irregularly-shaped pores, such as lack-of-fusion defects [48], [49]. The limitations in this case being related to the limited solubility of argon entrapped in such pores in large quantities [50], [51], [52]. Therefore, HIP reduces the size of these pores and compresses the gas they contain [52], [53], which could result in porosity regrowth during a subsequent exposure of HIP-ed parts to high temperatures [54]. A so-called blistering effect of near-surface pores was also observed when performing post-HIP annealing of Ti6Al4V AM components [48].

For the aforementioned reasons, it appears nearly impossible to fully eliminate processing-induced defects in LPBF parts [27] and the fatigue lives of such parts cannot be predicted without considering the level of this porosity [55], [56], [57]. To predict a component life with respect to the level of processing-induced defects (i.e. initial discontinuity state) [58], [59], it is necessary to establish a relevant flaw-related metric, such as the equivalent initial flaw size [60], and to measure how these flaws impact the fatigue resistance of printed materials [61].

The aim of this paper is to investigate the ability of HIP to improve the static strength and resistance to fatigue crack growth of LPBF Inconel 625 (IN625) components. To this end, IN625 specimens were printed using laser exposure parameters developed to generate targeted volumes of processing induced defects composed of gas pores and lack-of-fusion defects. In this work, the term “porosity” is used to refer to the volume occupied by these two types of defects. The specimens were subjected to a two-step post-processing treatment consisting of stress relief (SR) annealing and HIP treatments. Experiments were carried out to characterize the processing-induced defects as well as the microstructure, the mechanical properties, and the fatigue crack growth rates (FCGR) of the printed and post-processed specimens. To isolate the impact of HIP on the aforementioned characteristics, the results obtained are compared to those collected from identical specimens subjected to the SR annealing only [62].

Section snippets

Specimen manufacturing

All specimens of this study were manufactured using an EOSINT M280 system equipped with a 400 W fibre laser under argon protective atmosphere. The composition of IN625 powder used in this work is given in Table 1. The powder was supplied by EOS and complies with the ASTM B443–00 specification [63].

The specimens were manufactured using the powder supplier recommended 40 µm powder layer thickness and four distinct laser exposure parameter sets, which were established to induce four distinct

Porosity and pore size distributions

The CT-measured porosity of specimens P0, P1, P2, and P3 after the HIP treatment are given in Fig. 2. For comparison, the porosity corresponding to the SR conditions (before HIP) are also provided in Fig. 2 (SR values are taken from [62]).

Before HIP, in the SR conditions, the porosity level increases exponentially for specimens P0, P1, P2 and P3 due to a progressively decreasing laser energy density (Table 2), which is expected [64]. After HIP, the porosity of all the specimens decreases

Limitations

The findings reported in this study should be regarded in the light of some limitations. First, defects were purposely induced in our specimens by controlling solely a set of filling (hatching) laser exposure parameters. This facilitated the control of porosity but in the case of real-life printed components, different processing parameters are typically used to build the contours, upward, and downward facing surfaces. The use of such a complete set of processing parameters reduces the

Conclusions

Experiments were conducted to evaluate the impact of the HIP treatment on the size and distribution of residual defects, microstructure, and mechanical properties of LPBF-built IN625 specimens with purposely induced defects ranging from ≤ 0.1% to ~3%. The results obtained confirmed the efficiency of HIP in diminishing the processing-induced defects by reducing both the defect count and size. After HIP, the residual defects in the specimens with an initial porosity of up to 1% was reduced below

CRediT authorship contribution statement

Jean-Rene Poulin: Conceptualization, Methodology, Visualization, Writing – original draft. Alena Kreitcberg: Investigation, Formal analysis, Visualization. Vladimir Brailovski: Supervision, Funding acquisition, Writing – review & editing.

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Vladimir Brailovski reports financial support was provided by Quebec Research Fund Nature and Technology. Vladimir Brailovski reports financial support was provided by Natural Sciences and Engineering Research Council of Canada.

Acknowledgements

The authors would like to thank the Fonds de Recherche du Québec - Nature et Technologie (FRQNT), Canada (Grant No. 198902) and the National Science and Engineering Research Council of Canada (NSERC), Canada (Grant No. RGPIN-2019-04088) for funding the present study. The authors would also like to thank Yan Bombardier for providing insightful comments during the revision of this manuscript.

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