THE ALM APPLIED TO A ROCKET ENGINE INJECTION HEAD – DEVELOPMENT PLAN AND MATERIAL CHARACTERIZATION

Recently, the worldwide rocket propulsion attention has been focusing on innovative manufacturing technologies alternative to the traditional subtractive ones, due to the intrinsic difficulties concerning the welding and brazing of numerous components. The Electron Beam Melting (EBM) is one of the most challenging additive manufacturing technique, since it works with metallic powders at high temperature and in vacuum conditions, so guaranteeing reduced residual stresses and impurities in the final product. CIRA has been equipped with an ALM laboratory made-up of an EBM ARCAM A2x machine and all the auxiliary systems useful to manufacture complex components using the titanium alloy. The single injector thrust chamber breadboard, named “Subscale Breadboard Heat Sink” (SSBB-HS), already developed and fire tested, has been chosen as the reference breadboard to assess the manufacturing technology effectiveness. The SSBB-HS injection head back plate, originally made in Inconel 718, is redesigned to be manufactured as a unique part by using the EBM technology with the Ti-6Al-4V alloy. The current design of the back plate assembly foresees a main body with manifolds to be welded, and the single injector to be brazed, thus the possibility to make all the parts in a single “machine” run, as a unique part, allows gaining benefits in terms of weight and cost reductions. Once manufactured, the ALM SSBB-HS injection head will be first tested against leakage and pressure proof for the acceptance, and then integrated with the SSBB-HS thrust chamber for a firing test campaign. In this paper, activities concerning a preliminary thermo-structural analysis of the back plate in Ti-6Al-4V alloy and its re-design tailored on the ALM manufacturing process are presented together with a preliminary test campaign aimed at characterizing the microstructure and the mechanical behaviour of the Ti-6Al-4V alloy processed by EBM. The preliminary mechanical test campaign has been aimed at evaluating the influence of “layer thickness”, “skin-microstructure”, and “temperature” on modules of elasticity, yield and ultimate strengths and elongations. Outcomes highlight the feasibility of using the Ti-6Al-4V instead of the Inconel 718 alloy, keeping almost unchanged the component mechanical performance, and at the same time by achieving a strong benefit in terms of weight saving. Some warnings regarding the manufacturing process arising from the re-design activity are analysed and properly addressed. 1.0 SCENARIO AND MOTIVATION Additive Layer Manufacturing (ALM) is an emerging technology by which functional solid parts are made directly from electronic data, generally files from computer-aided design (CAD) software, starting from metal powder. It is based on “layer-by-layer” fabrications and offers many advantages such as short lead time, complex geometry capability, tooling free and very low waste material [1]. Aerospace firms are increasingly turning to the additive manufacturing technology to reduce the costs of developing models and prototypes and of creating components. In a constant effort to reduce aircraft weight, the industry is developing a growing proportion of its parts from titanium, plastic, and other lightweight THE ALM APPLIED TO A ROCKET ENGINE INJECTION HEAD – DEVELOPMENT PLAN AND MATERIAL CHARACTERIZATION STO-MP-AVT-264 2 STO-MP-AVT-264 NATO UNCLASSIFIED+SWEDEN, NEW ZEALAND AND AUSTRALIA NATO UNCLASSIFIED+SWEDEN, NEW ZEALAND AND AUSTRALIA materials. Many of these materials are costly, and additive manufacturing can make it possible to keep the amount used to a minimum. Aircraft landing gear, for example, can be additively manufactured layer by layer, rather than cut from a raw material block, thereby greatly reducing material waste and costs [2]. Some aerospace applications are jet nozzles, hot structural components up to 500°C. Light Alloys like titanium alloys are mainly employed for space application where thanks to the absence of oxygen the mechanical performances at elevated temperature are exploited. The increasing breadth and sophistication of these applications are, in turn, driving needs for improvements in process control, materials, and inspection to ensure quality and safety ([3], [4]). Despite these advances, limits on the size of goods produced by additive manufacturing together with issues concerning materials, accuracy, surface finish, and certification standards have limited its use. Recently, the worldwide attention in the aerospace field has been focusing on innovative manufacturing technologies, such as the ALM, alternative to the traditional subtractive ones. Especially in the rocket propulsion field, where the intrinsic difficulties, concerning the welding and brazing of numerous components, are crucial issues to be addressed. The NASA Glenn Research Centre is applying state of the art characterization techniques to interrogate microstructure and mechanical properties of additively manufactured materials and components at various steps in their processing [5]. The materials investigated for upper stage rocket engines include titanium, copper, and nickel alloys. Additive manufacturing processes include laser and electron beam powder bed, and electron beam wire fed processes. Various post build thermal treatments, including Hot Isostatic Pressure (HIP), have been studied to understand their influence on microstructure, mechanical properties, and build density. In Particular, An in-depth characterization of Electron Beam Melted (EBM) Ti-6Al-4V material has been completed. The mechanical properties of HIP’ed EBM Ti-6Al-4V turned out to be equivalent or superior to conventionally manufactured material, probably due to the refined, lamellar microstructure. Inclusions, both low and high density, were present in the EBM Ti-6Al-4V but generally did not affect the mechanical properties of the alloy. Airbus Defence and Space pursues a comprehensive approach to apply additive manufacturing to liquid rocket engine injectors [6]. The research and technology activities, performed within the National technology program TARES, sponsored by the German Space Agency, and DLR Bonn, address the entire manufacturing process, from material properties and design concepts to non-destructive inspection technologies to allow for the adequate quality assurance by means of a stepwise approach. The ALM also enables the creation of 3D structures that cannot be manufactured with conventional production methods, for example more complex geometries of optimized cooling channels in the combustion chamber walls of rocket engines. Today, channels are typically milled and closed with electrical deposition, or alternatively welded or brazed from tubes. In the frame of the DLR-study LAMP (Laser Additive Manufacturing for Propulsion) at the DLR Institute of Space Propulsion, the applicability of ALM Powderbed Selective Laser Melting (SLM) for production of rocket propulsion components was investigated [7]. The influence of hot firing testing on the P8 test bench at DLR-Lampoldshausen was a mechanical fatigue effect, manifesting in developing local leakage locations in the chamber wall. The tests with water cooling had shown a negligible influence on the leakage rate. During testing with the regeneratively cooled configuration, a rapid increase in the leakage rate was observed. As concerns CIRA activities, the aim of the materials and processes research line in the HYPROB program is to assess the feasibility and the effectiveness of Additive Manufacturing techniques applied to rocket engine parts already developed in the framework of the same program. The main goal of the program is to design, manufacture and test a liquid oxygen-liquid methane regenerative rocket engine demonstrator. To achieve this final objective, a number of intermediate breadboards have been designed and tested to investigate some critical aspects [8]. Among these breadboards, Single-injector LOX/GCH4 Sub-Scale Breadboard Heat Sink (SSBB-HS) has been chosen for assessing the manufacturing capabilities of Additive Layer Manufacturing (ALM) techniques. Details about design and testing of SSBB-HS are given in [9]. CIRA-TR-16-0121 Rev. 0 P. 2/15 THE ALM APPLIED TO A ROCKET ENGINE INJECTION HEAD – DEVELOPMENT PLAN AND MATERIAL CHARACTERIZATION STO-MP-AVT-264 STO-MP-AVT-264 3 NATO UNCLASSIFIED+SWEDEN, NEW ZEALAND AND AUSTRALIA NATO UNCLASSIFIED+SWEDEN, NEW ZEALAND AND AUSTRALIA The components to be manufactured by using the EBM technology are: the injectors post, and the Injection Head (IH) back-plate assembly (part #1 in Figure 1-1). The injector post shall be preliminary manufactured in order to understand the process features and limits, and necessary post-processing activities. The injector head, to be manufactured in a subsequent phase, is an assembly made of three different parts, including also the injector post. In particular, IH is made of a support, and a GCH4 inlet plus the injector post integrated to the support (see Figure 1-2). The idea is to manufacture the IH by Electron beam manufacturing (EBM) as a unique piece. Figure 1-1: View of the SSBB-HS. Figure 1-2: SSBB-HS Injection Head Assembly. Electron beam manufacturing (EBM) is a relatively new ALM technology. A high-energy electron beam, as a moving heat source, locally melts and fuses metal powders and produces parts in a layer-building fashion. EBM is able to make full-density metallic parts, drastically extending AM applications, and significantly accelerating product designs and developments in a wide variety of metallic-part applications, especially for complex components ([10], [11]). One of the main advantages of this technology is the ability to process very hard machinable materials like Titanium, Ti-6Al-4V, Titanium Aluminide, CrCb, Inconel 718 ([12], [13]), materials that have a large spread of application in the aerospace industry, especially in the production of rocket engine parts. Another advantage is th