PSI - Issue 43

Tibor Varmus et al. / Procedia Structural Integrity 43 (2023) 184–189 Tibor Varmus / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction The wrought 2024 aluminum alloy is an established aluminum alloy with copper, magnesium and manganese as the main alloying elements that is commonly available in alclad sheet and plate forms as well as extruded profiles. Due to its high-strength and fatigue resistance, the wrought 2024 alloy is widely used mainly in aircraft construction. The advancement in additive manufacturing (AM) technologies involves an effort directed towards the development of new alloys specifically designed for Laser Powder Bed Fusion (L-PBF) application, Tan at al. (2020). Aluminum alloys are of particular interest for AM applications because of their high strength-to-weight ratio and good fatigue resistance. Many aluminum alloys are already widely used in critical applications in aircrafts, Gu at al. (2019). However, the printability of 2024 aluminum alloy starting from gas-atomized powder has been challenged by its tendency to hot tearing during the solidification process and related degradation of mechanical properties. Mair et al. (2021) used hexaboride CaB 6 ceramic nanoparticles as a structure-refining additive to eliminate hot cracking. Recently, Elementum 3D (2022) introduced the so-called Reactive Additive Manufacturing (RAM) patented technology that made this alloy compatible with L-PBF. RAM aluminum alloy powders take advantage of chemical reactions in the melt pool to form dispersion-strengthened metal matrix composite (MMC) aluminum alloys with superior performance, even in the most challenging environments. Al2024-RAM2 powder with addition of 2 % by weight of nucleation agents such are ceramic nanoparticles successfully prevents hot cracking and results in a high performance material. The fine equiaxed grain structure generated upon solidification is associated with mechanical properties comparable to traditionally wrought aluminum alloy. The novelty of this alloy, commercially A2024 RAM2, caught the attention of the motorsport sector and it has been specifically permitted for the fabrication of structural parts by 2022 Formula 1 Technical Regulations, FIA (2021). The qualification of new materials for part production requires characterization of mechanical properties and especially fatigue properties for the design of critical components, Kumar at al. (2012). The present contribution is devoted to the study of the fatigue strength of Al2024-RAM2 which has recently been introduced into industrial production by the service company BEAM IT (Fornovo Taro, Italy). Miniature specimens manufactured along three different building directions were produced with a SLM 280 HL Twin system (SLM Solutions GmbH, DE). The printed specimens underwent a solution-plus-aging heat treatment (T6). After production, the specimens have been characterized in terms of microstructure and surface quality. The effect of directionality on fatigue behavior under cyclic plane bending with a load ratio R =  min /  max = 0 at frequency of 25 Hz has been determined. 2. Material and methods The gas atomized Al2024-RAM2 powder (Elementum 3D, USA) was processed using a SLM 280 HL Twin system (SLM Solution Group AG, Germany) to fabricate fatigue mini specimens according to the process parameters listed in Table. 1. The chemical composition of Al2024-RAM2 alloy is given in Table. 2. Three sets of miniature specimens with different orientations with respect to the build direction, Fig. 1a, were manufactured. The advantage of the miniature specimens designed by Nicoletto (2017) is the reduction of material consumption and production costs. A comparison of the miniature specimen size and the normal sized specimen is shown in Fig. 1a. The method of cyclic loading of miniature specimens and specimen dimensions are shown in Fig. 1b. Testing of the flat specimen surface allows the determination of the fatigue life. The stressed location indicated in red in the middle of the flat part of specimens opposite to the notch, Fig. 1b, correspond to place where the fatigue crack initiates under cyclic plane bending loading with a stress ratio R = 0. Fatigue testing was performed in an electro mechanical fatigue testing machine operating at a frequency of 25 Hz in fixed rotation-control mode.

Table 1. L-PBF process parameters used in specimen fabrication. Parameter Layer thickness Laser power

Preheated build plate

Heat treatment

Value

60 µm

323 W

200 °C

T6 (solution annealing + ageing)