Corrosion properties of CuAl9Mn2/ER 321 composites formed by dual-wire-feed electron beam additive manufacturing

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 significantly depend on the composition and cooling rate during the alloy production. Ordered structures, martensitic transformations and intermetallic compounds may form in this group of alloys [6–8]. Multiphase composition and microstructure of the copper alloys greatly affect both tribological and corrosion characteristics [3], therefore, increased attention is paid to studies of the microstructure and properties of these materials. One of the high-tech and extensively developed methods for producing composite copper- and steelbased materials is additive technologies, which make it possible to fabricate complex parts through a layerby-layer growth and have many advantages over traditional metallurgical technologies [1, 9–12]. In general, coatings or bimetallic alloys composed of aluminum bronze and steel are studied [10–12]. For example, in the paper [1], the tribocorrosion properties of additively produced stainless steel 316 (316SS), impregnated with bronze in seawater were studied. Tribological test showed that the resulting composite had yield strength and friction characteristics comparable to the conventional 316SS steel, while significant improvement in wear resistance was achieved at the test loads up to 80 N and reciprocating frequencies up to 20 Hz. It is assumed that the bronze smeared along the wear track serves as a solid lubricant, so the resulting passive oxide film seems to be a tribofilm that inhibits an abrasive wear at high loads. At the same time, an assessing the corrosion properties of various types of bronzes [1, 2] do not allow to unambiguously conclude about the nature of the corrosion damage, as well as the mechanisms of formation of corrosion products (CuO, Cu2O, etc.) in copper alloys during immersion into chloride-containing solutions. To date, the fabrication of composites based on aluminum bronze and steel could be performed using modern metallurgical methods of additive manufacturing, for example, electron beammelting. In particular, this technology is realized using a dual-wire-feed electron beam additive manufacturing (EBAM) method [13–15]. Earlier, the authors of this work produced composites based on aluminum bronze (CuAl9Mn2) and stainless steel (ER 321) using EBAM [13, 16]. It was found that depending on the “CuAl9Mn2 : ER 321” ratio the structural-phase states of the obtained composites were different and its mechanical characteristics could be improved. Our ongoing study aims to reveal the operating characteristics of the “CuAl9Mn2/ ER 321” composites. The kinetics of a charge transfer at the interfaces and the factors responsible for the resistance to the appearance of the corrosive currents between microgalvanic elements (copper- and ironbased phases) remain poorly studied issues. The purpose of the work is to study the corrosion resistance of the composites based on CuAl9Mn2 aluminum bronze and ER 321 stainless steel, produced by a dual-wire-feed electron beam additive manufacturing. The scientific objectives of this study include (i) a comprehensive assessment of the electrochemical behavior of CuAl9Mn2/ER 321 composites in marine solution (3.5 wt. % NaCl); (ii) a determination of the phase composition of corrosion products and type of corrosion damage; (iii) an identification of the predominant corrosion mechanism. Methodology In order to fabricate composites, the wires ∅1.6 mm of CuAl9Mn2 aluminum bronze and ER 321 stainless steel were used as raw materials. A stainless steel plate with a thickness of 10 mm was chosen as a substrate. To produce the bronze-steel composites, an EBAM installation, equipped with two wire feeders, was used (Fig. 1). The following parameters of the EBAM process were used: beam accelerating voltage 30 kV, beam current from 44 to 77 mA, print speed (movement of the working stage) was 400 mm/min. The required volume ratio of bronze and steel was maintained constant during processing by automatically adjusting the appropriate ratio of the wire feed speeds. As a result, the composites with a size of 80×120×8 mm3 and a volume ratio CuAl9Mn2 : ER 321 = 90 : 10, 75 : 25, 50 : 50 and 25 : 75 were obtained. A more detailed method for producing composites is presented in early works [13, 16]. For metallographic studies, the specimens were cut out by means of an electrical discharge machine, according to the Fig. 1. Then, the specimens were subjected to mechanical polishing using an abrasive sandpaper, followed by polishing with a diamond paste. To assess the mechanical properties, the specimens were tested for uniaxial static tension, and the microhardness was measured by the Vickers method [13, 16].

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