PSI - Issue 38

Christophe Grosjean et al. / Procedia Structural Integrity 38 (2022) 94–108 C.Grosjean and al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Metallic additive manufacturing (AM) processes are now widely used in many sectors (aerospace, medical, tooling …) and are spreadi ng in the industry [1]. This article deals with a hydraulic block used on construction and mining machines. Those components are used with high internal and cyclic pressure of several hundred bars. Therefore, there is a strong need for a high-quality material and design to achieve these requirements. Firstly, a topology optimization was performed to reduce weight and take advantage of AM ’s ability to produce complex geometries. Next, three different AM process were evaluated to produce this component: Laser-Powder Bed Fusion (L-PBF), Metal Binder Jetting (MBJ) and Lost-Wax Casting (LWC). For the latter process, the wax model was produced using a polymer AM machine. Then, different mechanical exams (hardness, tensile tests) were carried out on samples and a pressure’s test was done on each component. A comparison between a conventional part and an AM parts enabled to demonstrate the interest of topology optimization to reduce pressure drops. Finally, fatigue tests were used to evaluate the cyclic pressure resistance of the parts. These tests were completed by a calculation phase allowing to better understand the origin of the various identified initiation sites. 2. Context and initial choices Hydraulic components are widely used in many sectors such as aerospace, transport, mechanics. In this study, the main objective was to demonstrate the possibility to produce by AM processes a hydraulic component with both high resistance to mechanical fatigue and low weight. This implies to succeed in obtaining a material with very few defects and to optimize the geometry. For ease of use, the chosen component contains only one pipe and has a simple geometry but most of the results can be used to produce more complex parts to better benefit from the advantage of AM processes. Metallic AM processes gather a wide range of production solutions, ISO/ASTM 52900 [2]. The most well-known process is L-PBF which enables to produce very complex part. Two other processes: MBJ and Lost-Wax Casting LWC were studied even if fatigue tests were only performed on the L-PBF parts. Each process has advantages, especially: • for L-PBF: very complex geometry, rather large material availability • for MBJ: very complex geometry and details, roughness (better than L-PBF), no support or plate to remove • for LWC: mass production, serial production, roughness and drawbacks: • for L-PBF: need of support-sometimes in the channel which are difficult to remove, roughness, cost and productivity • for MBJ: limitation of the size part, deformation during sintering, residual porosity, material availability • for LWC: limitation of the geometry complexity Previous studies [3] demonstrate that the alloy 17-4PH enables to reach high mechanical performance and this alloy is available for the three processes. 17-4PH is a martensitic precipitation-hardening stainless steel that provides a good combination of high strength, good corrosion resistance, good mechanical properties. All parts have seen the same heat treatment: solution treatment and ageing (Annealing 1030°C 1h + ageing 480°C 2h) excepted for LWC part (see below). Other treatments conducing to higher ductility could also be considered to fine-tune the strength/ductility ratio. Shotblasting has been carried out after heat treatment and ageing. 3. Topology optimization A topology optimization was performed using Optistruct software (see Fig. 1). The objective was to reduce the weight but also to reduce the pressure drop in the channel. The pipe diameter, the position of the inlet and outlet, and all fasteners were kept in the same position during the topology optimization. Then a calculation in the linear elastic domain of the material 17-4PH was performed with the software ANSYS Workbench V19.2 to validate the dimensioning. Following hypothesis have been taken: