Structural integrity of additively manufactured aluminum alloys: Effects of build orientation on microstructure, porosity, and fatigue behavior

Highlights

• Build orientation effect on the fatigue behavior of LB-PBF Al alloys is studied.

• Horizontal specimens have a higher defect density than the vertical counterparts.

• Lack of fusion defects cause anisotropy in the fatigue behavior of QuesTek Al.

• AlF357, despite its higher porosity, exhibits similar fatigue behavior to AlSi10Mg.

• Scalmalloy has comparable fatigue behavior to AD1 regardless of its higher porosity.

Abstract

The use of aluminum (Al) alloys for additive manufacturing (AM) has recently gained significant attention, specifically in the aerospace industry. This has been resulting in introducing new high strength Al alloys that are more compatible with the AM processes. However, it is critical to investigate the structural integrity of these newly developed Al alloys prior to being deployed in safety-critical load-bearing applications. This study investigates and compares the microstructure, porosity, and fatigue behavior of five different contemporary Al alloys fabricated via a laser beam powder bed fusion (LB-PBF) AM process. Vertically and horizontally built specimens out of AlSi10Mg, Scalmalloy, QuesTek Al, AD1, and AlF357 are fabricated to capture any effects of build orientation on the structural integrity of these alloys. Despite the variation in micro-/defect-structure of vertical and horizontal specimens, no significant build orientation dependency is observed on fatigue behavior of AlSi10Mg, Scalmalloy, and AD1 alloys. However, AlF357 and QuesTek Al show some anisotropic behavior in the high cycle fatigue regime. Among the LB-PBF Al alloys investigated, Scalmalloy and AD1 are found to have the highest fatigue resistance ascribed to their superfine microstructure.

Introduction

Additive manufacturing (AM) is rapidly becoming a strategic technology generating revenues in various industries such as defense, aerospace, automotive, and biomedical. These industries can derive value from the profound benefits of AM, such as the manufacture of near-net-shape parts with complex geometries, reduction in lead times, reduction of component weight due to freedom in design, potential cost reductions, and environmentally friendly production. Along with various advantages, there are also many challenges associated with the AM techniques [1]. The unique thermal history of the AM processes (i.e., high cooling/solidification rate, thermal gradient, etc.) typically induces residual stresses, microstructural anisotropy, surface roughness and leads to the formation of volumetric defects (e.g., pores and lack of fusions (LoFs)) [2], [3].

Although the influence of defects (e.g., surface roughness, pore, and LoF) may be minimal on the static properties, these flaws can deteriorate the fatigue performance of additively manufactured (AM) parts significantly [4]. Recent studies [5], [6] reported that the defects are the source of scatter in fatigue results of AM metallic materials. The detrimental effect of process-induced defects (i.e., pore, LoF) is not limited to the room temperature fatigue behavior of AM materials. Bao et al. [7], [8] reported that the pores and LoF defects can elongate during cyclic deformation at elevated temperatures and result in fatigue failure by microvoid formation.

Many studies have been conducted to investigate the fatigue behavior of various AM metallic materials; a great deal of these works has been focused on titanium alloys [9], [10], [11], stainless steels [12], [13], [14], and Ni-super alloys [15], [16], [17]. Recently, Aluminum (Al) alloys have drawn much attention for fabricating parts via AM processes. Al alloys are known for their high strength-to-weight ratio, adequate hardenability, good corrosion resistance, and excellent weldability, making them suitable for being deployed in a wide range of applications, specifically in the aerospace industry [18], [19]. However, as compared to other materials, AM of Al alloys is even more challenging; Al powder particles are inherently light, have poor flowability, attract moisture due to the hydrophilic nature of Al, and have high thermal conductivity [18]. In addition, Al powder particles are highly susceptible to oxidation, and their low melt viscosity promotes porosity formation [20]. Such issues in the manufacture of Al alloys may induce defects in the parts, consequently influencing their mechanical properties, especially their fatigue performance.

Al-Si cast alloys (e.g., AlSi10Mg) are typically easier to process by AM as compared to the wrought Al alloys (e.g., AA6061 [21], A7075 [22]). There are quite a few papers in the literature that present various parameters influencing the fatigue performance of AM AlSi10Mg [23], [24], [25], [26], [27]. Jian et al. [28] studied the effect of powder particle size on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) behavior of laser beam powder bed fused (LB-PBF) AlSi10Mg. They showed that using smaller powder particles reduces the porosity, and consequently, enhances the fatigue strength of LB-PBF AlSi10Mg in non-heat treated (NHT) condition. Wu et al. [6] investigated the effect of volumetric defect size and population, as well as the orientation of LoF defects with respect to the loading direction, on the fatigue anisotropy behavior of LB-PBF AlSi10Mg. Lower ductility and fatigue strength were reported when the loading direction was parallel to the build direction as compared to the case where the loading direction was perpendicular to the build direction. This was attributed to size of the projected area of LoF defects on the plane perpendicular to the loading direction, which was larger when the loading direction was parallel to the build direction.

Maskery et al. [25] studied the effect of heat treatment and reported that performing T6 heat treatment (i.e., a solution treatment for 1 h at 520 °C, water quenched, followed by aging for 6 h at 160 °C, air-cooled) enhances the ductility as well as fatigue resistance of LB-PBF AlSi10Mg. In addition, Ngnekou et al. [29] found that while a T6-type heat treatment (i.e., a solution treatment for 8 h at 540 °C, followed by aging for 10 h at 160 °C, air-cooled) enhances the fatigue resistance of the material, it may result in build orientation dependency in fatigue performance of LB-PBF AlSi10Mg. This has been associated with the defect characteristics (i.e., size, shape, frequency, etc.) in different build orientations and the increased material sensitivity to the presence of defects after applying T6-type heat treatment. The EOS datasheet for the LB-PBF AlSi10Mg suggests that T6-type heat treatment may not be the best option for this material [30]. The traditional T6 heat treatment is proven to be challenging for certain sand-casting Al alloys (e.g., AlSi7Mg0.6) due to the possibility of blistering hydrogen porosity formation [31]. Therefore, EOS recommends only stress relieving instead of T6 heat treatment [30].

There are also limited studies on the fatigue behavior of AM AlSi7Mg in the literature; Lesperance et al. [32] investigated the VHCF behavior of LB-PBF AlSi7Mg using ultrasonic testing, and compared the results with those of the cast A356 alloy, and reported similar VHCF behaviors. Another AM Al alloy is A357, which is the modified version of A356 alloy with higher strength; although there are several studies on process optimization and microstructure characterization of LB-PBF A357, there are not many studies on the fatigue performance of this alloy in the literature. In one study, Bassoli et al. [33] investigated the fatigue behavior of LB-PBF AlA357 in NHT condition and reported somewhat similar fatigue behavior to LB-PBF AlSi10Mg alloy. Recently, AlF357 alloy, beryllium free derivative of the AlA357 alloy, has also been introduced to AM community [34]. However, there is no fatigue data available for this alloy in the literature, and it will be investigated in the present study.

Scalmalloy, introduced by the Airbus APWorks, is one of the recently developed Al alloys for AM [35]. This alloy possesses high yield and ultimate tensile strengths combined with acceptable ductility due to its unique microstructure comprised of nano-size grains and nano-size Al₃(Sc,Zr) precipitates [36]. There are only a few studies on the fatigue behavior of Scalmalloy in the literature which show superior fatigue performance of LB-PBF Scalmalloy to that of LB-PBF AlSi10Mg [37]. Muhammad et al. [38] recently compared the fatigue performance of LB-PBF AlSi10Mg, Scalmalloy, and a new Al alloy developed by QuesTek Innovations LLC. They reported higher fatigue resistance for Scalmalloy as compared to other Al alloys, which was attributed to its higher toughness (i.e., high tensile strength and ductility) ascribed to its unique nano-size microstructure.

Considering the ongoing alloy development for AM, especially the Al alloys with different microstructural features (i.e., grain morphology and size, precipitates, etc.), it is crucial to study their fatigue performance to fill the Gap FMP1 on “materials properties” according to the AM standardization road map compiled by America Makes & ANSI additive manufacturing standardization collaborative (AMSC) [39]. In line with this goal, the current study aims to investigate the potential anisotropy in fatigue performance of some contemporary LB-PBF Al alloys and correlate them with their micro-/defect-structure (i.e., microstructure and the defect structure in micro scale). These alloys with different microstructural characteristics (i.e., grain structure, precipitates, etc.), and often better tensile strength, may or may not possess better fatigue behavior due to the presence of volumetric defects. In addition, it has been shown that the thermal history experienced by the parts is varied in different build orientations, which may result in anisotropy in the mechanical behavior as a result of variation in defect type, size, and population [6], [40]. Therefore, generating data and evaluating the fatigue behavior of the LB-PBF Al specimens fabricated in different build orientations seems necessary.

This article is organized in the following order: in Section 2, the materials and methods are presented in detail. In Section 3, the experimental results, including the micro-/defect-structure analysis, and fatigue data, are presented. In Section 4, the fatigue behavior of the LB-PBF Al alloys is discussed and correlated to their micro-/defect-structure. Finally, some conclusions are drawn based on the experimental observations in this study and listed in Section 5.