“Stainless Steel/Copper” Fabricated with Wire-Feed Electron Beam Additive Manufacturing

Article Characterization of a Bimetallic Multilayered Composite “Stainless Steel/Copper” Fabricated with Wire-Feed Electron Beam Additive Manufacturing

Kseniya Osipovich *, Andrey Vorontsov , Andrey Chumaevskii, Denis Gurianov , Nikolai Shamarin, Nikolai Savchenko and Evgeny Kolubaev

Institute of Strength Physics and Materials Science, Siberian Branch of Russian Academy of Sciences, 634055 Tomsk, Russia; [email protected] (A.V.); [email protected] (A.C.); [email protected] (D.G.); [email protected] (N.S.); [email protected] (N.S.); [email protected] (E.K.) * Correspondence: [email protected]; Tel.: +7-3822-286-863

Abstract: The results of investigating the structure and properties of multilayered bimetallic “steel– copper” macrocomposite systems, obtained by wire-feed electron beam additive manufacturing, are presented in the paper. The features of boundary formation during 3D printing are revealed when changing the ﬁlaments of stainless steel and copper. Inhomogeneities in the distribution of steel and copper in the boundary zone were detected. Interphase interaction occurs both in the steel and copper parts of the structural boundary: Cu particles with an average size of 5 µm are formed in the iron matrix; Fe particles with an average size of 10 µm are formed in the copper matrix. It was revealed that such structural elements, as solid solutions of both copper and iron, are formed in the Citation: Osipovich, K.; Vorontsov, boundary zone, with additional mutual dissolution of alloying elements and mechanical mixtures of A.; Chumaevskii, A.; Gurianov, D.; system components. The presence of the disc-shaped precipitations randomly located in the matrix Shamarin, N.; Savchenko, N.; was revealed in the structure of the “copper–steel” boundary by transmission electron microscopy; Kolubaev, E. Characterization of a this is associated with rapid cooling of alloys and the subsequent thermal effect at lower temperatures Bimetallic Multilayered Composite “Stainless Steel/Copper” Fabricated during the application of subsequent layers. The existence of these disc-shaped precipitations of steel, with Wire-Feed Electron Beam arranged randomly in the Cu matrix, allows us to draw conclusions on the spinodal decomposition Additive Manufacturing. Metals 2021, of alloying elements of steel. The characteristics of mechanical and micromechanical properties of a 11, 1151. https://doi.org/10.3390/ bimetallic multilayered composite with a complex formed structure lie in the range of characteristics met11081151 inherent in additive steel and additive copper.

Academic Editor: Aleksander Lisiecki Keywords: electron beam additive manufacturing; microstructure; mechanical properties; stainless steel; copper alloys; bimetallic multilayered composite Received: 16 June 2021 Accepted: 19 July 2021 Published: 21 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral The technology of obtaining polymetallic materials has been widely studied in the with regard to jurisdictional claims in published maps and institutional afﬁl- last decade. Polymetallic materials are made using two or more metals or alloys. These iations. materials are designed to provide an optimal combination of physical, mechanical and chemical properties unattainable with any of their individual components. As has been described [1], high thermal conductivity, strength and corrosion resistance are essential for materials used in electricity generation and its transportation processes. Currently, many technical and industrial problems cannot be solved using only single materials based on Copyright: © 2021 by the authors. one metal or alloy (monometallic materials). This is due to the fact that in these operating Licensee MDPI, Basel, Switzerland. conditions the material must have a high rate of heat transfer with minimal thermal losses, This article is an open access article distributed under the terms and as well as resist any form of corrosion in the working environment. As a compromise, a conditions of the Creative Commons heterogeneous combination of materials can be used, where one superior property can Attribution (CC BY) license (https:// be provided locally in the structure, while at the same time using the advantages of the creativecommons.org/licenses/by/ primary material elsewhere. As proposed [2] in addition to this, the emerging requirements 4.0/). for lightweight structures with optimized and variable characteristics and functionality

Metals 2021, 11, 1151. https://doi.org/10.3390/met11081151 https://www.mdpi.com/journal/metals Metals 2021, 11, 1151 2 of 19

have increased the scope of applications for multilayer structures. This necessitates the coupling of materials with different physical/thermal properties. Multilayered composites are widely used in many industries, such as the military, automotive and aerospace industries due to their outstanding mechanical properties, as previously described [3]. Multilayered composites can consist of different types of layers, such as ceramic/metal, metal/metal, metal/composite or ceramic/composite. The most common metals and alloys, such as steel, cast iron, copper, aluminum and magnesium alloys, can be used to prepare polymetallic metal/metal composites [4]. Plasma spraying [5], pressing [6], welding [7] and friction stir welding [8] are used to produce polymetallic materials. However, the quality of polymetals using such methods directly depends on the quality of the polymetallic plate’s weld. Heterogeneous materials should form a strong con- nection with each other without internal cavities and cracks, because due to discontinuities, the polymetallic product loses the required values of performance properties. This has led to even greater interest in additive manufacturing (AM). AM methods, such as electron beam 3D printing, can provide spatial process control directly during 3D printing of the product and reduce the production process, compared to conventional methods of product manufacturing. Distinctive features of electron beam 3D printing are: the combination of wire technology and a vacuum chamber with an electron beam significantly reduces the cost of printing, an increase in the productivity of the process, high physical and mechanical properties and reducing the possibility of defect formation in the printed products. The combination of different mechanical and physical–chemical properties implies not only a multifunctional finished product, but also the complexity and, in some cases, the impossibility, of combining metals and alloys. One such example discussed in this study is the combination of C11000 copper and SS 321 stainless steel. Copper has a face-centered cubic (FCC) crystal lattice and has high thermal conductivity characteristics, but has less corrosion resistance compared to stainless steel. Thus, products based on copper have many applications in electrical engineering due to their high characteristics and excellent formability. Usually, stainless steels have been used as structural materials in the nuclear, petrochemical and chemical industries because of their excellent corrosion resistance, but they do not conduct heat as well as copper. Copper (Cu)–stainless steel (SS) compounds are required in the nuclear power industry as an important component of vacuum chambers for particle accelerators [9], and in cryogenics [10], providing the high electrical conductivity of copper and high mechanical strength of steel. To combine the advantages of stainless steel with copper, a suitable welding process must be applied. However, the large differences in the properties of Cu and SS, such as melting temperature (Cu: 1085 ◦C, SS: 1400–1500 ◦C) and thermal conductivity (Cu: 401 W/mK, SS: 17–19 W/mK) make the joining task more difficult for any fusion welding processes, and the formation of intermetallic compounds when joining Cu alloys to other materials may also occur [11]. As has been shown [12], copper penetration along the heat-affected zone (HAZ) of steel can lead to hot cracking because copper has very limited solubility with steel in the liquid state. Meanwhile, the Fe–Cu phase diagram does not contain intermetallic phases except for a small region of limited solubility between Cu and Fe. The significant difference in thermal expansion and thermal conductivity coefficients for copper and steel inevitably causes large deformation and residual stresses when the material cools from the melting temperature, which leads to cracking during solidification of bimetallic samples. Hydrogen readily dissolves in liquid copper and promotes pore formation in the bimetallic junction area [12]. Liquid-phase separation between Cu and Fe is commonly observed in laser, electron beam and arc welding processes [13]. In this case, the presence of liquid-phase separation leads to the formation of pores and cracks, which negatively affects the mechanical properties of the finished product. It is known that the layer bonding in printed samples results in weaker parts than what is found in the bulk material and that print settings and raster orientation can affect the material characteristics [14]. The literature contains a number of studies on copper/steel welding using different methods, such as diffusion welding [15], radial friction welding [16], inert gas arc welding Metals 2021, 11, 1151 3 of 19

with a non-consumable tungsten electrode [17], electron beam welding (EBW) [18], laser welding (LBW) [19], explosion welding [20] and friction stir welding processes [21], hot- pressing [22], the selective laser welding method [23] and the laser direct deposition method [24]. Magnabosco et al. [18] demonstrated the presence of defects such as voids in electron beam welding of copper with austenitic stainless steels. Chen et al. [19] observed cracks in a stainless steel/copper disc, in a similar joint made by laser welding. However, it is clear that all high-energy fusion welding processes (EBW, LBW) suffer from major drawbacks, and more experimental difﬁculties are associated with solid-state welding processes for the Cu–SS system joint. As demonstrated, there are few works on studying the properties of copper–steel bimetals made by additive manufacturing methods. Shu et al. [25] describe bimetallic samples fabricated by selective laser melting (SLM) and consisting of 316L stainless steel and C18400 copper alloy. These steel and copper samples had a developed stainless steel/copper interface, but the transition zone was quite narrow and porous, which affected the mechanical properties of these bimetallic samples. Tan et al. [12] were able to partially avoid the previously mentioned problems by using SLM AT to form a bimetallic martensitic steel sample. Good metallurgical contacts at the interface between martensitic copper steels and copper were obtained and an interfacial bonding mechanism was proposed. Imran et al. [24] demonstrated that H13 tool steel is successfully deposited on a copper substrate by laser direct metal deposition. No cracks or pores were observed at the interfaces of this material, but tensile cracks were formed in the heat-affected zones or joints. The electron beam free form fabrication (EBF3) method was used to produce bimetallic samples by depositing two layers of AISI 304 wire and then T2 copper wire [12]. The presence of dendrites and spherical Fe-particles in copper-rich areas was shown, as well as cracks at the copper/steel interface. As has been shown [26], bimetal-based C11000 and SS 304 were fabricated by electron beam additive manufacturing with simultaneous feeding of two wires. In the present work, a bimetallic multilayered composite “stainless steel/copper” was fabricated by wire-feed electron beam additive manufacturing to study the regularities of the formation of the steel–copper and copper– steel transition zone. Investigations are connected with the phenomena which occur during solidiﬁcation from the liquid and solid-phase transformations.

2. Materials and Methods The samples were obtained in an experimental facility for additive electron beam fabrication of metal products. AISI 321 stainless steel wire and C11000 copper wire were used for 3D printing of the samples (Table1). The diameter of the steel wire was 1.2 mm and the diameter of the copper wire was 1 mm. The nominal chemical composition of the materials is shown in Table1.

Table 1. Chemical composition of the substrate and ﬁlaments used in the wire-feed electron beam additive manufacturing.

Material Fe Cu Ni Cr Mn Ti Si C AISI 321 Bal. to 0.3 9–11 17–19 to 2 - to 0.8 to 0.08 C11000 to 0.005 Bal. to 0.002 - - - - -

The steel substrate was ﬁxed on a three-axis water-cooled table. An electron beam was used to form a molten pool. A wire was fed into the molten pool and the vertical wall was 3D printed by layer-by-layer deposition of the material. The building direction was per- pendicular to the table surface. An alternating strategy of depositing heterogeneous wires was chosen to manufacture the vertical wall. The printing parameters of manufacturing the vertical wall are shown in Table2. Metals 2021, 11, x FOR PEER REVIEW 4 of 19

wires was chosen to manufacture the vertical wall. The printing parameters of manu-

Metals 2021, 11, 1151 facturing the vertical wall are shown in Table 2. 4 of 19

Table 2. The 3D printing parameters for manufacturing the vertical wall from dissimilar materials based on steel and copper. Table 2. The 3D printing parameters for manufacturing the vertical wall from dissimilar materials based onMaterial steel and copper. Steel Copper Steel Copper Steel Layer number 5 5 5 5 5 Material Steel Copper Steel Copper Steel I, А 65 ÷ 58 55 ÷ 50 55 ÷ 50 55 ÷ 50 55 ÷ 50 Layer,number mm/min 5280 5300 5300 5300 5 300 I, A 65 ÷ 58 55 ÷ 50 55 ÷ 50 55 ÷ 50 55 ÷ 50 U, V 30 30 30 30 30 υ, mm/min 280 300 300 300 300 U, V 30 30 30 30 30 A multilayered macrocomposite was formed on a rectangular 4.5 mm thick AISI 304 steelA substrate. multilayered First, macrocomposite 5 layers of SS was321 formedsteel were on adeposited rectangular on 4.5to the mm substrate; thick AISI after the 3045th steellayer substrate. had been First, deposited, 5 layers ofthe SS steel 321 steelwire were feed deposited was stopped. onto theThen, substrate; 5 layers after of C11000 thecopper 5th layerwere haddeposited been deposited, on the formed the steel layers wire of feed steel was and stopped., after the Then, deposit 5 layersion ofof the 5th C11000layer was copper completed, were deposited the feeding on the of formedthe copper layers wire of steel was and, stopped. after theThen, deposition the application ofof the5 layers 5th layer of steel was on completed, the previously the feeding deposited of the copperlayers wireof copper was stopped. and deposition Then, the of copper applicationon the previously of 5 layers deposited of steel on steel the previously layers were deposited repeated. layers After of copper the “vertical and deposition wall” building ofprocess copper was on the completed, previously the deposited last 5 steel layerslayers were were repeated. deposited. After Thus, the “vertical the finished wall” multi- building process was completed, the last 5 steel layers were deposited. Thus, the ﬁnished layered macrocomposite was a “wall” with alternating interlayers of SS–Cu–SS–Cu–SS multilayered macrocomposite was a “wall” with alternating interlayers of SS–Cu–SS–Cu– SSwith with length length ≈ ≈100100 mm, mm, thickness thickness ≈ 55 mm mm and heightheight 30 30 mm. mm. FromFrom the the vertical vertical walls walls grown grown according according to the to scheme the scheme of Figure of 1Figure, samples 1, samples were cut were cut outout with with a DK7750 a DK7750 electrical electrical discharge discharge machine machine (Suzhou Longkai(Suzhou Electromechanical Longkai Electromechanical Tech- nologyTechnology Co., Ltd., Co., Suzhou, Ltd., Suzhou, China) to China study the) to macro-study andthe microstructure,macro- and microstructure, and mechanical and me- andchanical thermophysical and thermophysical properties. properties.

FigureFigure 1. 1.Scheme Scheme of cuttingof cutting samples samples for research: for research: 1—samples 1—samples for macrostructural for macrostructural studies, measure- studies, meas- menturement of microhardness of microhardness and elemental and elemental composition; composition; 2—areas for2— transmissionareas for transmission electron microscopy; electron micros- 3—areascopy; 3— forareas X-ray for diffraction X-ray diffraction analysis; 4—areas analysis; for 4— measurementareas for measurement of thermal linear of thermal expansion linear coefﬁ- expansion cient;coefficient; 5—samples 5—samples in the form in the of double form of blades double for mechanicalblades for mechanical tensile tests. tensile tests.

SamplesSamples for for macrostructural macrostructural studies studies were were cut cut in the in longitudinalthe longitudinal direction direction relative relative to tothe the cladding cladding layer layer and and hadhad dimensionsdimensions of of 30 30 mm mm in heightin height and and 10 mm 10 inmm width. in width. Met- Metal- allographic studies of the structure of the samples were carried out using an OLYMPUS lographic studies of the structure of the samples were carried out using an OLYMPUS LEXT confocal microscope (Olympus NDT, Inc., Waltham, MA, USA). Microhardness wasLEXT measured confocal on microscope the same samples (Olympus by the Vickers NDT, Inc., method Waltham, on a Duramin-5 MA, USA (Struers). Microhardness A/S, Ballerup,was measured Denmark) on the at a same load of samples 50 g for 10by s.th Alonge Vickers the indentationmethod on traces, a Duramin the elemental-5 (Struers A/S, compositionBallerup, Denmark of the bimetallic) at a load layered of 50 macrocomposite g for 10 s. Along of the the samples indentation was analyzed traces, usingthe elemental acomposition Zeiss LEO EVO of the 50 bimetallic scanning electron layered microscope macrocomposite (Carl Zeiss, of the Oberkochen, samples was Germany), analyzed using witha Zeiss an energyLEO EVO dispersive 50 scanning elemental electron microanalysis microscope attachment. (Carl Zeiss, After mechanical Oberkochen, tensile Germany), testing,with an the energy fractographic dispersive analysis elemental was performed microanalysis using aattachment. scanning electron After microscope.mechanical tensile × × 3 Atesting tensile, the sample fractographic with a working analysis part was size ofperformed 12 1.5 using2.7 mm a scanningwas used electron to study microscope. the mechanical characteristics. The tests were performed on a UTS 110M universal testing A tensile sample with a working part size of 12 × 1.5 × 2.7 mm3 was used to study the machine (Testsystems, Ivanovo, Russia). The strain rate was 1.4 × 10−3 s−1. A JEOL JEM-2100mechanical transmission characteristics. electron The microscope tests were (JEOL performed Ltd., Akishima, on a UTS Japan) 110M was universal used for testing −3 −1 microstructuralmachine (Testsystems, studies. Disc Ivanovo, plates with Russia a diameter). The ofstrain 3 mm rate and was a thickness 1.4  of10 300 sµm. A JEOL JEM-2100 transmission electron microscope (JEOL Ltd., Akishima, Japan) was used for

Metals 2021, 11, x FOR PEER REVIEW 5 of 19

Metals 2021, 11, 1151 5 of 19 microstructural studies. Disc plates with a diameter of 3 mm and a thickness of 300 μm were cut from area 2 (Figure 1). The discs were thinned to the first hole in the central part of the sample for transmission electron microscopy (TEM) using the Micron-M facility by were cut from area 2 (Figure1). The discs were thinned to the ﬁrst hole in the central part single-sided jet electrolytic polishing. X-ray structural analysis was performed on an of the sample for transmission electron microscopy (TEM) using the Micron-M facility byX-ray single-sided diffractometer jet electrolytic (DRON-7 polishing., Innovation X-ray center structural “Burevestnik analysis”, St. was Petersburg, performed Russia on an) X-rayusing diffractometerCoKα radiation (DRON-7, at 0.05° increments Innovation and center an “Burevestnik”,exposure time of St. 10 Petersburg, s per step. Russia) A DIL using402 PC/4 CoK dilatometerα radiation ( atNetzsch 0.05◦ increments-Gerätebau andGmbH, an exposureSelb, Germany time of) was 10 s used per step. to measure A DIL 402the PC/4coefficient dilatometer of thermal (Netzsch-Gerätebau linear expansion (CTE). GmbH, Samples Selb, Germany) for the LET was measurement used to measure had 3 3 thethe coefﬁcientfollowing dimensions: of thermal linear steel, expansion 1.63 × 1.94 (CTE). × 24.77 Samples mm ; copper, for the 1.82 LET × measurement 1.72 × 24.91 mm had; 3 theand following polymetal, dimensions: 1.85 × 1.71 steel, × 24.92 1.63 mm× 1.94. The× samples24.77 mm were3; copper, heated 1.82 to ×between1.72 × 24.9120 and mm 6003; and°C. The polymetal, samples 1.85in the× form1.71 ×of 24.92rectangular mm3. parallelepipeds The samples were were heated cut in tothe between direction 20 of and the 600polymetallic◦C. The samples sample ingrowth the form (Figure of rectangular 1, position parallelepipeds 4). were cut in the direction of the polymetallic sample growth (Figure1, position 4). 3. Results 3.3.1. Results Selection of Parameters 3.1. Selection of Parameters The appearance of vertical walls grown by electron beam additive manufacturing usingThe filaments appearance of heterogeneous of vertical walls materials grown by of electron steel (SS)beam and additive copper manufacturing (Cu) is shown using in filamentsFigure 2. of heterogeneous materials of steel (SS) and copper (Cu) is shown in Figure2.

FigureFigure 2.2. VerticalVertical wallswalls ofof “SS–Cu–SS–Cu–SS”“SS–Cu–SS–Cu–SS” werewere manufacturedmanufactured usingusing coppercopper andand steelsteel wireswires byby thethe methodmethod ofof electronelectron beambeam additiveadditive manufacturing.manufacturing.

DuringDuring thethe meltingmelting ofof fedfed wirewire byby thethe electronelectron beambeam withwith thethe formationformation ofof aa moltenmolten pool,pool, therethere isis a a local local increase increase in in the the substrate substrate temperature temperature near near the the molten molten pool. pool. As aAs result, a re- asult, temperature a temperature gradient gradient occurs, occurs, which whic leadsh toleads the presenceto the presence of internal of internal stresses. stresses. After a num-After bera number of thermal of thermal cycles, thecycles, substrate the substrate deforms deforms under the under influence the influence of temperature of temperature stresses. stresses.In the course of growing the vertical wall, macrodefects are formed. This occurs due to insufficientIn the heatingcourse of or growing overheating the vertical of the fed wall, filament, macrodefects which entails are formed. a difference This occurs in the sizedue ofto theinsufficient formed layer.heating In or the overheating case of overheating, of the fed material filament, spreads, which entails droplets a difference are formed in and the thesize shape of the of formed the vertical layer. wall In the is distorted case of overheating, across the width. material In the spreads, case of droplets insufficient are heating, formed unmeltedand the shape parts of of the the vertical wire remain, wall is which distorted leads across to non-melting the width. of In the the first case layers of insufficient with the substrateheating, unmelted and layers parts with of eachthe wire other, remain, and pores which are leads formed to non in-melting the areas of not the filledfirst layers with unmeltedwith the substrate filament. and In the layers present with work, each theseother, factors and pores were are taken formed into account,in the areas and not optimal filled parameterswith unmelted were filament selected. In for the vertical present wall work printing, these from factors heterogeneous were taken materialsinto account, in order and tooptimal avoid theparameters influence were of insufficient selected for heating vertical or wall overheating. printing Thisfrom is heterogeneous affected by a significant materials difference,in order to ofavoid 21–24 the times, influence in the of thermalinsufficient conductivity heating or values overheating. of theused This materials.is affected Theby a alternationsignificant difference, of copper andof 21 steel–24 times, wire material in the thermal during con printingductivity creates values conditions of the used of high ma- heatterials. dissipation The alternation of previously of copper applied and steel copper wire layers, material in whichduring the printing steel wirecreates does condi- not havetions timeof high to melt.heat dissipation At the same of previously time, during applied the application copper layers, of copper, in which it spills the steel out wire due todoes the not insufficiently have time to high melt. thermal At the conductivity same time, during of steel. the For application printing withof copper, steel wire, it spill ins thisout case,due to the the selection insufficiently of parameters high thermal was based conductivity on already of known steel. For data printing [22]. When with using steel heterogeneouswire, in this case materials, the selection with different of parameters thermal physical was based properties on already as filaments known for data printing, [22]. itWhen is necessary using heterogeneous to vary the low materials and high with values different of heat thermal inputs depending physical properties on the material. as fila- In this work, the value of the heat input was calculated using the formula [27]:

E = (60 × U × I)/(1000 × v), (1)

Metals 2021, 11, x FOR PEER REVIEW 6 of 19

ments for printing, it is necessary to vary the low and high values of heat inputs depending on the material. mentsIn for this printing, work, the it value is necessary of the heat to varyinput the was low calculated and high using values the offormula heat inputs[27]: depending on the material. E = (60  U  I)/(1000 × v), (1) Metals 2021, 11, 1151 In this work, the value of the heat input was calculated using the formula6 of 19 [27]: where U and I are voltage and current of the electron beam, and v is the printing speed. Using the data on printing parametersE = (60 (Table  U  2)I)/(1000 and Form × v),u la (1), the dependence of lay-(1) er-by-layer heat deposition is obtained (Figure 3). where U U and and I areI are voltage voltage and and current current of the of electron the electron beam, and beam, v is theand printing v is the speed. printing speed. Using thethe datadata on on printing printing parameters parameters (Table (Table2) and 2) Formula and Form (1),u thela (1) dependence, the dependence of of lay- erlayer-by-layer-by-layer heat heat deposition deposition is is obtained obtained (Figure (Figure3). 3).

Figure 3. Dependence of heat input values on the layer number in the manufacturing of bimetallic layered macrocomposite. Black dots show values for steel; red dots show values for copper. Figure 3. 3.Dependence Dependence of heatof heat input input values values on the on layer the number layer innumber the manufacturing in the manufacturing of bimetallic of bimetallic For convenience, the bimetallic layered macrocomposite was divided into five areas: layered macrocomposite. macrocomposite. Black Black dots dots show show values values for steel; for red steel; dots showred dots values show for copper.values for copper. area I—printing the first five layers of stainless steel onto the substrate; II and For convenience, the bimetallic layered macrocomposite was divided into five areas: IV—Forprinting convenience, five layers the of bimetallic copper onto layered the macrocompositealready deposited was steel; divided III and into V —fivepr areas:inting areastainless I—printing steel thelayers first onto five layers the already of stainless deposited steel onto copper the substrate; layers. II and IV—printing areafive layers I—printing of copper theonto firstthe already five depositedlayers of steel; stainless III and V—printing steel onto stainless the substrate; steel II and A gradual decrease from 0.42 kJ/mm to 0.38 kJ/mm occurs during the printing pro- IVlayers—printing onto the alreadyfive layers deposited of copper copper onto layers. the already deposited steel; III and V—printing stainlesscessA as gradual one steel move decrease layerss away onto from 0.42fromthe kJ/mmalready the cooled to deposited 0.38 kJ/mmsubstrate. copper occurs Due during layers. to the printingheat accumulation process in the assample, oneA moves gradual the awaycreatio decrease fromn of the the cooledfrom molten 0.42 substrate. kJ/mmpool Due and to to 0.38the the melting heatkJ/mm accumulation occursof the filamentduring in the sample, the occur printing with pro-a re- cesstheduced creation as electronone ofmove the beam moltens away power. pool from andAfter the the cooledprinting melting substrate. the of the first filament fiveDue steel to occur the layers, withheat atheaccumulation reduced heat input valuesin the electron beam power. After printing the first five steel layers, the heat input values of zones sample,of zones the II– Vcreatio occurredn of inthe the molten range poolof 0.33 and–0.30 the kJ/mm. melting This of theprinting filament mode occur was with chosen a re- to II–Vensure occurred the contact in the range of layers of 0.33–0.30 of dissimilar kJ/mm. This materials. printing The mode printing was chosen parameters to ensure used were ducedthe contact electron of layers beam of dissimilarpower. After materials. printing The the printing first parametersfive steel layers, used were the aimedheat input values aimed at adjusting the values of thermal investment, influencing the temperature gra- ofat adjustingzones II– theV occurred values of thermalin the range investment, of 0.33 influencing–0.30 kJ/mm. the temperature This printing gradient mode and was chosen to ensuretakingdient intoand the accounttaking contact theinto of difference accountlayers of of the chemicaldissimilar difference and materials. thermophysical of chemical The and propertiesprinting thermophysical parameters of dissimilar properties used were of aimedmaterials.dissimilar at adjusting materials. the values of thermal investment, influencing the temperature gra- dientFigureFigure and 4taking shows 4 shows into the morphologythe account morphology the of difference the layeredof the layeredof polymetallic chemical polymetallic sample.and thermophysical sample. properties of dissimilar materials. Figure 4 shows the morphology of the layered polymetallic sample.

FigureFigure 4. 4.Optical Optical microscopy microscopy of the oflongitudinal the longitudinal section of sectiona sample of of bimetala sample layered of bimetal macrocom- layered macro- posite.composite. The white The dashedwhite dashed lines show lines the boundariesshow the boundaries of the sample of by the zone. sample The boundaries by zone. ofThe the boundaries of meltthe melt pool arepool marked are marked with yellow with lines. yellow The samplelines. The was sample divided intowas ﬁve divided areas: areainto I—thefive areas: ﬁrst area I—the Figure 4. Optical microscopy of the longitudinal section of a sample of bimetal layered macro- 5first layers 5 layers of stainless of stainless steel on steel the substrate; on the substrate; II and IV—5 II layersand IV of— copper5 layers on theof copper already depositedon the already depos- composite. The white dashed lines show the boundaries of the sample by zone. The boundaries of steel;ited steel; III and III V—stainless and V—stainless steel layers stee onl thelayers already on depositedthe already copper deposited layers. copper layers. the melt pool are marked with yellow lines. The sample was divided into five areas: area I—the first 5Surface layers macrodefectsof stainless steel formed on the during substrate; vertical II walland IV growth—5 layers do not of affectcopper the on internal the already depos- Surface macrodefects formed during vertical wall growth do not affect the internal itedmacrostructure steel; III and of V the—stainless sample, exceptsteel layers for the on disturbance the already of deposited layer thicknesses. copper layers. Due to the differencemacrostructure in melting of temperaturethe sample, of except the materials, for the a disturbance of of the layer uniform thicknesses. thickness Due to the ofdifference theSurface copper in layer macrodefectsmelting in the temperature longitudinal formed sectionofduring the materials, can vertical be observed walla disturbance ingrowth Figure 4do .of The notthe average affectuniform the thickness internal macrostructure of the sample, except for the disturbance of layer thicknesses. Due to the difference in melting temperature of the materials, a disturbance of the uniform thickness

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of the copper layer in the longitudinal section can be observed in Figure 4. The average Metals 2021, 11, 1151 value of the thickness of the copper layers is 4.1 ± 0.5 mm. Typical characteristics7 of 19 for additively manufactured products are observed, where overlapping molten pools with a semi-ellipse shape are formed. A middle-ground approach uses geometry of the melt valuepool of and the simulates thickness of a the series copper of layers overlapping is 4.1 ± 0.5 semi mm.-circles Typical or characteristics semi-ovals. forLack-of-fusion additivelyporosity manufactureddepends on both products the aremelt observed, pool size where as well overlapping as the moltenscanning pools pattern. with a For a given semi-ellipse shape are formed. A middle-ground approach uses geometry of the melt pool melt pool size, the minimum depth of overlap is defined [28]. This quantity is affected by and simulates a series of overlapping semi-circles or semi-ovals. Lack-of-fusion porosity dependsthe hatch on distance both the meltof the pool melt size pool, as well which as the then scanning dictates pattern. the Formaximum a given meltlayer thickness. poolThe size, formation the minimum of a depthmolten of overlappool with is defined a semi [28-ellipse]. This quantity shape is affected characteristic by the of various hatchmethods distance of additive of the melt manufacturing, pool, which then both dictates for ferrous the maximum [29] and layer for thickness. non-ferrous The metals [30], formationdue to the of apeculiarities molten pool withof the a semi-ellipse process. In shape terms is characteristic of their size, of variousmelt pool methodss are statistically of1.1 additive–1.3 mm manufacturing, in width and both 0.5 for–0 ferrous.7 mm [in29 ]depth. and for The non-ferrous molten metals pool [boundaries30], due to the in the sample peculiarities of the process. In terms of their size, melt pools are statistically 1.1–1.3 mm in widthare visible and 0.5–0.7 as “fish mm-scale in depth.s” [31], The filled molten with pool a boundaries cellular structure in the sample [32]. are visible as “fish-scales”A detailed [31], filled discussion with a cellular of the structure microstructure [32]. of each zone separately (I, II, III, IV and V) ofA th detailede bimetallic discussion layered of the microstructuremacrocomposite of each is given zone separately below. (I, II, III, IV and V) of the bimetallic layered macrocomposite is given below. 3.2. Microstructure of Copper Interlayers of Zones II and IV 3.2. Microstructure of Copper Interlayers of Zones II and IV CopperCopper interlayers interlayers of the of sample the sample are of particular are of particular scientific interest scientific due tointerest the effect due of to the effect grainof grain size, which size, iswhich described is described by the Hall–Petch by the ratio. Hall In–Petch this case ra (Figuretio. In5 ), this a fine-grained case (Figure 5), a fi- structurene-grained is observed structure in the is observed copper interlayers in the copper (zone II andinterlayers IV). (zone II and IV).

FigureFigure 5. 5.Optical Optical microscopy microscopy of copper of copper interlayers: interlayers: (a) zone ( II;a) ( bzone) zone II; IV. (b Copper) zone interlayersIV. Copper have interlayers have onlyonly equiaxed equiaxed grains grains with with an average an average size of size 28.5 ±of0.2 28.5µm. ± 0.2 μm.

In a previous study, when C11000 copper was deposited on an AISI 321 steel substrate, a regionIn witha previous a fine-grained study, structure, when C11000 of not morecopper than was 1 mm deposited in length, on was an observed AISI 321 steel sub- bystrate, Osipovich a region et al. with [33]. a Since fine the-grain growthed structure, process of theof not sample more in zonesthan 1 II mm and IVin waslength, was ob- continuous,served by aOsipovich homogeneous et al. growth [33]. ofSince coarse the grains growth over process the entire of wall the height sample would in zones be II and IV expected.was continuous, However, microstructural a homogeneous analysis growth reveals of only coarse equiaxed grains grains over with the an average entire wall height sizewould of 28.5 be± expected.0.2 µm. However, microstructural analysis reveals only equiaxed grains with an averageWhen C11000 size of copper 28.5 ± layers 0.2 μm. are deposited onto already consolidated 321 stainless steel layers, some grains grow, due to the adjacent ones, by migration of high-angular boundaries,When i.e., C11000 collective copper recrystallization layers are dep takesosited place. on Into this already case, grain consolidated boundaries 321 stainless migrate,steel layers, and an some equiaxial grains structure grow with, due minimal to the surface adjacent energy ones and, by grains migration of equal of size high-angular andboundaries, shape is formed. i.e., collective One of the factorsrecrystallization contributing takes to the formation place. In of this an equiaxed case, grain fine- boundaries grainedmigrate, copper and structurean equiaxial can be struc the alloyingture with of copper minimal by iron, surface as well energy as elements and grains present of equal size inand the shape composition is formed. of austenitic One of steel. the Particlesfactors ofcontributing more refractory to the metals formation will contribute of an equiaxed fi- to the formation of a large number of crystallization nuclei during solidification of the ne-grained copper structure can be the alloying of copper by iron, as well as elements product. At the same time, impurity atoms and second phase inclusions (in the transition present in the composition of austenitic steel. Particles of more refractory metals will contribute to the formation of a large number of crystallization nuclei during solidification of the product. At the same time, impurity atoms and second phase inclusions (in the

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Metals 2021, 11, 1151 8 of 19 transition zone) are the factors that prevent the migration of grain boundaries. As the driving force of collective recrystallization decreases as it develops, the grain growth stops when it reaches a certain value. However, first of all, the difference between direc- zone) are the factors that prevent the migration of grain boundaries. As the driving force of collectivetional solidification recrystallization (constrained decreases asit growth) develops, resultthe grains in growth the formation stops when it of reaches oriented cells or aoriented certain value. dendrites However, and first free of all, solidification the difference (unconstrained between directional growth) solidification, which results in (constrainedequiaxed dendrite growth) results(equiaxed in the grain) formation formation of oriented [34]. cells The or columnar oriented dendrites structure and is formed due freeto the solidification so-called (unconstrained constrained solidification growth), which whereas results in the equiaxed equiaxed dendrite grains (equiaxed appear when the grain)unconstrained formation [34solidification]. The columnar takes structure place. is Since formed only due tofive the layers so-called of constrainedcopper have been de- solidification whereas the equiaxed grains appear when the unconstrained solidification takesposited, place. the Since influence only five of layersthese offactors copper is have not reduced been deposited, and there the influence is no direct of theseed grain growth factorsof the is elongated not reduced shape. and there Directed is no directed grain grain growth growth is offacilitated the elongated by shape.the slow Directed heat dissipation grainand high growth temperature is facilitated gradient by the slow generated heat dissipation by the additive and high temperaturemanufacturing gradient process [35]. generated by the additive manufacturing process [35]. 3.3. Microstructure of Steel I, III and V Zones 3.3. Microstructure of Steel I, III and V Zones FigureFigure6 shows 6 shows the morphology the morphology of the grain–dendrite of the grain–dendrite structure of structure stainless of steel, stainless steel, zoneszones I andI and III. III. This This morphology morphology includes includes both equiaxial both equiaxial and columnar and columnar grains described grains described earlierearlier [36 [36].].

FigureFigure 6. 6.Optical Optical microscopy microscopy of steel of areas: steel ( aareas:) zone I;(a ()b )zone zone I; III; (b (c)) zone V.III; The (с morphology) zone V. The of the morphology of grain–dendritethe grain–dendrite structure structure of stainless of steelstainless areas. steel areas.

A common feature of additive steel macroimages is the ﬁne-grained microstructure, A common feature of additive steel macroimages is the fine-grained microstructure, typical for AM [32]. Elongated and oriented grains depending on manufacturing process parameterstypical for have AM been[32]. discussedElongated previously and oriented [37]. Itgrains has been depending noted that on the manufacturing evolution process ofparameters crystallization have cells be isen related discussed to the segregationpreviously of[37]. alloying It has elements been noted at the interfaces,that the evolution of leadingcrystallization to microsegregation. cells is related It should to bethe noted segregation that solute of microsegregation alloying elements transforms at the interfaces, intoleading solute to redistribution microsegregation. after back-diffusion It should be into noted the solid that [solute38]. Equiaxial microsegregation cells can be transforms eitherinto s smallolute or redistribution large with sizes ofafter 0.2–0.6 backµm-diffusion and 1–2 µ m,into respectively. the solid In[38]. this case,Equiaxial ferrite cells can be is located between the primary arms of this cell structure (dendrites) [39]. Heating ferrite toeither 912 ◦ Csmall leads or to large the formation with sizes of tinyof 0.2 austenite–0.6 μm grains and 1 at–2 the μm, boundaries respectively. of the In ferrite this case, ferrite grains.is located Further between heating the leads primary to the growth arms of of this these cell new structure austenitic (dendrites) grains with complete[39]. Heating ferrite replacementto 912 °C leads of the ferriteto the grains formation by austenitic of tiny grains—iron austenite transformations grains at the occur.boundaries Since the of the ferrite electrongrains. beamFurther process heating of additive leads manufacturing to the growth is aof layer-by-layer these new austenitic deposition ofgrains material with complete duringreplacement sample of manufacturing, the ferrite grains there isby a austenitic constant re-melting grains— ofiro then transformations already deposited occur. Since the electron beam process of additive manufacturing is a layer-by-layer deposition of material during sample manufacturing, there is a constant re-melting of the already deposited layers, i.e., thermal cycling. It seems that solidification which appears after re-melting is, in reality, rapid solidification. In the case of the rapid solidification, the

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layers, i.e., thermal cycling. It seems that solidification which appears after re-melting is,phenomenon in reality, rapid of microsegregation solidification. In the (as case well of as the redistribution) rapid solidification, is slightly the phenomenon modified by an of microsegregation (as well as redistribution) is slightly modified by an increase in the increase in the partition ratio, k, as explained previously [40]. However, the proper be- partition ratio, k, as explained previously [40]. However, the proper behavior of the havior of the partition ratio during rapid solidification has been defined [41]. Thermal partition ratio during rapid solidification has been defined [41]. Thermal cycling leads to an evolutioncycling leads of the to structure an evolution by height of the in individual structure zones—I,by height III in and individual V—from smallzones to— largeI, III and dendrites.V—from Optical small to microscopy large dendrites. showed thatOptical the structures microscopy of stainless showed steel that in zones the structure I and V s of arestainless similar steel (Figure in zones6a,c). ThisI and means V are an similar increase (Figure in dendrite 6a,c). grainThis means size. The an explanationincrease in den- fordrite this grain is the size. thermal The explanation energy provided for this by is the the powerful thermal electronenergy provided beam, which by the to somepowerful extentelectron can beam, reduce which the rate to of som thee solidification extent can reduce process, the thereby rate givingof the the solidification dendrite more process, timethereby to become giving coarse-grainedthe dendrite more during time the to solidification become coarse process.-grained The during dendritic the structuresolidification mainlyprocess. developed The dendritic with a structure deviation frommainly the developed building direction with a (z) deviation due to a from change the in building the temperaturedirection (z) gradient due to a during change solidification. in the temperature gradient during solidification. TheThe microstructures microstructures for for EBAM-manufactured EBAM-manufactured steel steel 321 in 321 different in different parts ofparts zone of I, IIIzone I, andIII and V are V shownare shown in Figure in Figure7. 7.

FigureFigure 7. 7.SAED SAED pattern pattern with with interpretation interpretation and and dark-ﬁeld dark-field TEM TEM image image for steel for steel areas. areas. The elementalThe elemental compositionscompositions of of the theγ -γ and- andδ-Fe δ-Fe regions regions in wt.in wt. % revealed% revealed by EDSby EDS analysis. analysis. The typicalThe typical composition composition ofof thethe ferrite ferrite phase phase is Fe-(27.2–29.3)Cr-(3.2–3.8)Niis Fe-(27.2–29.3)Cr-(3.2–3.8)Ni (wt. %). (wt. The %). red The rectangle red rectangle marks the marks enlarged the zone enlarged ofzoneδ-ferrite of δ-ferrite lamellae, lamellae, the boundaries the bound of whicharies of are which partially are partially curved. curved.

TEMTEM images images clearly clearly show thatshow the that steel microstructure the steel microstructure contains two phases—Austenite contains two phas- andes— acicularAusteniteδ-ferrite. and acicular The morphology δ-ferrite. The of morphologyδ-ferrite dendrites of δ-ferrite does notdendrites depend does on thenot de- positionpend on in the the position steel blank in the (Figure steel 7blank). According (Figure 7). to theAccording EDS analysis, to the EDSδ-ferrite analysis, lamellae δ-ferrite arelamellae enriched are inenriched chromium in chromium and depleted and in depleted nickel. The in nickel. typical The composition typical composition of the ferrite of the phaseferrite is phase Fe-(27.2–29.3)Cr-(3.2–3.8)Ni is Fe-(27.2–29.3)Cr-(3. (wt.2–3 %)..8)Ni The (wt. elemental %). The composition elemental of composition the austenitic of the phaseaustenitic is proportional phase is proportional to the initial wire to the composition, initial wire but composition, the chromium but and the nickel chromium content and δ isnickel somewhat content excessive—Fe-(18.3–19.1)Cr-(9.4–10.3)Ni is somewhat excessive—Fe-(18.3–19.1)Cr (wt.-(9. %).4–10-ferrite.3)Ni (wt. in the %). form δ-ferrite of in lamellae 300–500 nm wide is uniformly distributed in the steel structure. The boundaries the form of lamellae 300–500 nm wide is uniformly distributed in the steel structure. The between phases are partially curved (Figure7) and partially faceted by the Kurdjumov– boundaries between phases are partially curved (Figure 7) and partially faceted by the Sachs orientation relationship between austenite and ferrite. However, δ-ferrite is a high- temperatureKurdjumov phase–Sachs and orientation cannot be observed relationship as it undergoes between phaseaustenite transformations and ferrite. at roomHowever, temperature.δ-ferrite is a However, high-temperature based on the phase literature and cannot analysis be [42 observed], the above as features it undergoes indicate phase thattransformationsδ-ferrite formed at room at high temperature. temperature Howeve was ﬁxedr, bybased quenching, on the literature and it became analysis possible [42], the toabove see it features at room indicate temperature. that δ-ferrite formed at high temperature was fixed by quenching, and Thus,it became the morphologypossible to see of it zones at room I, III temperature. and V is characterized by a grain–dendrite structure,Thus, with the the morphology growth of elongated of zones dendrites, I, III and with V ferrite is characterized located between by a the grain primary–dendrite dendritestructure, arm with spacing, the growth associated of withelongated thermal dendrites, cycling, and with the ferrite direction located of dendrite between growth the pri- inheritedmary dendrite by the directionarm spacing, of the associated fabrication with process. thermal cycling, and the direction of den- driteTo growth better understandinherited by the the formation direction of ofδ-Fe the and fabrication its different process. volume fraction in zones I, III andTo V better of the sample,understand quantitative the formation studies of were δ-Fe carried and its out different using several volume methods: fraction X-ray, in zones I, III and V of the sample, quantitative studies were carried out using several methods: X-ray, metallographic and local-type ferritometer measurements (Figures 8 and 9). The relative intensities of the γ-Fe and δ-Fe phases were calculated using Formula (2) [43]:

퐼훿 푅훿 = ( ) × 100% (2) ∑ 퐼퐴

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metallographic and local-type ferritometer measurements (Figures8 and9). The relative intensities of the γ-Fe and δ-Fe phases were calculated using Formula (2) [43]: wherewhere I δI δis is the the integral integral intensity intensity of of the the peaks peaks corresponding corresponding to to the the δ δ-Fe-Fe phase; phase; ∑ ∑IAI Ais is the the sum of all analyzed integral intensities. sum of all analyzed integral intensities. Iδ Moreover, the volume fractionR ofδ = δ-Fe in different× 100% parts of the sample is different: for (2) Moreover, the volume fraction of δ-Fe∑ inIA different parts of the sample is different: for zoneszones I I and and V V of of steel steel sections sections, , thethe volume volume fraction fraction of of δ δ-Fe-Fe varies varies within within the the range range fI,VwherefI,V(δ(δ-Fe)-Fe)I δ= = is(16.3 (16.3 the– integral–17.2);17.2); for for intensity zo zonene III III of, ,a a thedecrease decrease peaks in correspondingin f IIf(δII(δ-Fe)-Fe) of of 1. 1.7 to7––1 the1.9.9 times δtimes-Fe phase;as as compared compared∑IA is theto to thethesum volume volume of all analyzedfraction fraction for integralfor zones zones intensities.I Iand and V V of of steel steel sections sections is is typical. typical.

FigureFigure 8. 8. X XX-ray-ray-ray diffraction diffraction of of steel steel areas areas of of bimetallic bimetallic layered layered layered macrocomposite macrocomposite macrocomposite in in in zone zone zoness sI, I, I,III III III and and and V. V. V. TheThe enlarged enlarged image image shows shows X X-rayX-ray-ray peaks peaks in in a asection section of of angles angles from from 52 5252 to toto 62. 62.62.

Figure 9. Dependence of δ-Fe volume fraction on distance of substrate using a local type ferritometer. FigureFigure 9. 9. Dependence Dependence of of δ δ-Fe-Fe volume volume fraction fraction on on distance distance of of substrate substrate using using a alocal local type type ferritom- ferritom- eter. eter. Moreover, the volume fraction of δ-Fe in different parts of the sample is different: δ for zonesIt is seen I and that V all of steelexperimental sections, methods the volume of determining fraction of -Fethe variesvolume within fraction the of range the δIt is seen that all experimental methods of determiningδ the volume fraction of the δf-I,VFe( phase-Fe) = a (16.3–17.2);re consistent for with zone each III, aother. decrease There in fisII( a-Fe) general of 1.7–1.9 tendency times for as comparedthe volume to δthe-Fe volume phase a fractionre consistent for zones with I andeach V other. of steel There sections is a is general typical. tendency for the volume fractionfraction to to decrease decrease in in the the middle middle of of the the sample sample and and when when approaching approaching the the copper copper inin- terlayers,terlayers, zones zones II II and and IV. IV. This This is is due due to to the the fact fact that that copper, copper, like like nickel, nickel, is is a a stabilizer stabilizer of of γγ-Fe-Fe, ,i nin this this case, case, the the ferrite ferrite that that occurs occurs during during the the last last stages stages of of solidification solidification [44]. [44]. The The

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fraction to decrease in the middle of the sample and when approaching the copper interlayers,It is zones seen that II and all experimental IV. This is methods due to ofthe determining fact that copper, the volume like fraction nickel, of the is aδ-Fe stabilizer of phase are consistent with each other. There is a general tendency for the volume fraction to γ-Fe, in this case, the ferrite that occurs during the last stages of solidification [44]. The decrease in the middle of the sample and when approaching the copper interlayers, zones liquidII projection and IV. This begins is due to in the the fact peritectic that copper, region like nickel, in the is a Fe stabilizer–Ni system of γ-Fe, and in this proceeds case, to the eutecticthe reaction ferrite that in occurs the Cr during–Ni system the last stagesin the of Fe solidiﬁcation–Cr–Ni ternary [44]. Thesystem. liquid The projection eutectic ferrite is thenbegins located in the along peritectic the regionboundaries in the Fe–Ni of the system austenite and proceeds grains, to in the contrast eutectic reactionto Widmanstatten in patternthe morphologies, Cr–Ni system in the in Fe–Cr–Ni which the ternary ferrite system. is Thecontained eutectic ferritemainly is then in locatedthe core along grains. The the boundaries of the austenite grains, in contrast to Widmanstatten pattern morphologies, microstructure,in which the as ferrite shown is contained in Figure mainly 6, in in this the core alloy, grains. can The be microstructure,characterized as by shown regions in of both eutecticFigure ferrite6, in this and alloy, skeletal can be characterized ferrite. Th bye regions characteristics of both eutectic of ferriteWidmanstatten and skeletal pattern morphologiesferrite. The characteristicsof alloy 321 of were Widmanstatten also reveal patterned. morphologies Values for of the alloy alloying 321 were alsoelements to calculaterevealed. the ValuesNi–Cr for equivalents the alloying elementsratio were to calculate obtained the Ni–Cr by equivalentsSEM. Figure ratio 10 were shows the obtained by SEM. Figure 10 shows the variation in the basic and alloying elements of variationthe material. in the basic and alloying elements of the material.

(a) (b) Figure 10.Figure Changes 10. Changes in the elemental in the elemental composition composition (SEM (SEM EDS EDS analysis)analysis with) with the the distance distance from thefrom substrate the substrate in bimetal in bimetal laminate macrocompositelaminate macrocomposite (a) produced (a) produced by electron by electron beam beam additive additive manufacturing manufacturing and and changes changes elements elements necessary necessary for the for the analysis ofanalysis equivalents of equivalents (b). 3 (b 2020). 2021 00A7 2016 2020 2021 00A7 2016 &#x ☐ □ The element composition in the 2 to 4 mm and 10 to 14 mm areas corresponds to Thethe chemistry element ofcomposition C11000 copper. in Inthe the 2 remaining to 4 mm heightand 10 areas, to 14 the mm chemical areas composition corresponds to the chemistrycorresponds of C11000 to the compositioncopper. In ofthe stainless remainin steelg 321. height In the areas, graph of the the chemical dependence composition of correspondselemental to composition the composition on the distance of stainless to the zones steel of material 321. In supply the graph change of from the stainless dependence of steel to copper and vice versa, there is an area in which the composition of austenitic steel elementalis different composition from that shownon the in Tabledistance1 and thereto the is azones fairly highof material concentration supply of copper. change from stainlessAnalysis steel of to the copper data allows and us vice to conclude versa, that ther ine the is transitionan area zone,in which the austenitic the composition steel of austeniticis alloyed steel with is copperdifferent and therefrom isthat the formationshown ofin aTable two-phase 1 and region—copper there is a and fairly high concentrationaustenitic stainlessof copper. steel Analysis alloyed with of copper.the data This allows does not us contradictto conclude theavailable that in the data transition zone,on the the austenitic Fe–Cu phase steel diagram is alloyed [45], according with copper to which and intermetallic there is the compounds formation based of a on two-phase iron and copper are not formed, but a limited amount of copper (≈5.8% at 1083 ◦C) can regiondissolve—copper in γ -Fe.and As theaustenitic number ofstainless spherical inclusionssteel alloyed of copper with decreases copper. with distanceThis does not contradictfrom the the interface, available the concentration data on the of copper Fe–Cu also phase becomes diagram lower. The [45], presence according of a small to which intermetallicconcentration compounds of iron in based the copper on iron part ofand the copper bimetal sampleare not near formed, the interface but a maylimited be amount due to the formation of a solid solution of iron in copper, although the solubility of iron in of copper (≈5.8% at 1083 °C) can dissolve◦ in γ-Fe. As the number of spherical inclusions of copper is rather low (≈2.8% at 1083 C). Creq/Nieq ratios [46] were used for the theoretical coppercalculation, decreases using with formulas distance for equivalents from the ((3) interface, and (4)): the concentration of copper also becomes lower. The presence of a small concentration of iron in the copper part of the bimetal sample near theCreq interface= Cr + 1.37 may× Mo be + 1.5due× toSi +2the× formationNb + 3 × Ti of a solid solution (3) of iron in copper, although the solubility of iron in copper is rather low (≈2.8% at 1083 °С). Creq/Nieq ratios [46] were used for the theoretical calculation, using formulas for equivalents ((3) and (4)):

Creq = Cr + 1.37 × Mo + 1.5 × Si +2 × Nb + 3 × Ti (3)

Nieq = Ni + 0.31 × Mn + 22 × C + 14.2 × N + Cu (4) The graph of the equivalent dependence is shown in Figure 11.

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Metals 2021, 11, x MetalsFOR PEER 2021 ,REVIEW 11, x FOR PEER REVIEW Nieq = Ni + 0.31 × Mn + 22 × C + 14.2 × N + Cu12 of (4) 19 12 of 19

The graph of the equivalent dependence is shown in Figure 11.

Figure 11. Change in the equivalent Creq and Nieq ratio to the distance from the substrate in bimetal Figure 11. Change in thethe equivalentequivalent CrCreqeq and Nieq ratioratio to to the the distance distance from the substrate inin bimetalbimetal laminatelaminate macrocompositemacrocompositelaminate macrocomposite producedproduced byby electronelectron produced beambeam by additiveadditive electron manufacturing. manufacturing.beam additive manufacturing.

The results of Thestudies results [4 [477 ]]of showed showed studies that that[47 ]if ifshowed the the content content that ofif of theCr Cr andcontent and Ni Ni in of in stainless Cr stainless and Ni steel steelin stainlessis steel is eq eq isthe the same same and and the the sameCr Creq/Nieq and/Nieq value theeq valueCr is less/Ni is than less value than1.5, is then 1.5,less thenthanthe solidification the1.5, solidiﬁcationthen the mainlysolidification mainly starts starts frommainly starts from eq eq fromthe γ- thephase.γ-phase. Itthe is seenγ It-phase. is that seen Itthe that is valueseen the valuethat of the the of equivalent thevalue equivalent of the Cr eqequivalent and Creq Niandeq ratioCr Nieq andinratio zone Ni in zoneIIIratio is es- IIIin zone III is es- issentially essentially lower, lower,sentially which which lower, agrees agrees which with with the agrees the results results with of of thethe the resultsmeasurement measurement of the measurement of of the the δ-Fe-Fe volume volume of the δ-Fe volume fractionfraction presentedpresentedfraction above.above. presented In other above. words, In theother studiedstudied words, zonezone the IIIIIIstudied isis providedprovided zone III byby is thethe provided processprocess by the process conditions underunderconditions whichwhichδ δunder-Fe-Fe at at the whichthe crystallization crystallization δ-Fe at the stagecrystallization stage is is formed formed stage to to a lessera is lesser form extent edextent to than a than lesser in extent than zonesin zones I and I and V.in Just V. zones Just as with asI and with further V. further Just cooling, as cooling,with the further least the amountleast cooling, amount of the residual leastof residual δamount-Fe will δ- Feof remain residualwill re- in δ-Fe will re- thismain zone. in this zone.main in this zone. 3.4. Microstructure of “SS–Cu” and “Cu–SS” Boundaries 3.4. Microstructure3.4. Microstructure of “SS–Cu” and of “ “CuSS––SSCu”” B andoundaries “Cu–SS ” Boundaries During the copper ﬁlament deposition step, a sharp boundary between Cu and Fe During the copperDuring filament the copper deposition filament step, deposition a sharp boundary step, a sharp betwee boundaryn Cu and betwee Fe isn Cu and Fe is is formed on the already deposited steel layers (Figure 12a). It could be compared to the formed on theformed already on deposited the already steel deposited layers (Figure steel layers 12a). It(Figure could 12a).be compared It could beto thecompared to the formation of the oriented cells within the diffusion joint (interconnection) formation, as has formation of theformation oriented of cells the orientedwithin the cells diffusion within jointthe diffusion (interconnection) joint (interconnection) formation, as formation, as been described [48]. has been describedhas been [48]. described [48].

(a) (a) (b) (b) Figure 12. OpticalFigure microscopy 12. OpticalFigure of boundary 12.microscopyOptical of microscopyof “ SSboundary–Cu” (a of) ofand boundary “SS “Cu–Cu–SS” of(”a )(“SS–Cu” band) in “bimetalCu (–aSS) and” laminate(b) “Cu–SS” in bimetal macrocomposite (b )laminate in bimetal macrocomposite pro- laminate produced by electronduced beam by additive electronmacrocomposite manufacturing.beam additive produced manufacturing. by electron beam additive manufacturing.

Interfacial interactionInterfacial occurs interaction in both occurs the copper in both and the iron copper parts. and Cu iron particles, parts. with Cu particles, an with an average size ofaverage 5 µm, aresize formed of 5 µm, directly are formed near thedirectly sharp near boundary the sharp with boundary the iron withmatrix. the iron matrix. Fe particles withFe particlesan average with size an of average 10 µm sizeare formedof 10 µm in arethe formedcopper inmatrix. the copper Since thematrix. in- Since the in-

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Metals 2021, 11, 1151 13 of 19 terfacial interaction occurs at the Fe and Cu boundary at equal values of the component concentrations, structural elements such as solid solutions of copper in iron and of iron in copper withInterfacial an additional interaction mutual occurs in dissolution both the copper of and the iron alloying parts. Cu elements particles, withand anmechanical mixturesaverage of the size ofsystem 5 µm, arecomponents formed directly are nearformed the sharp in this boundary region. with The the mechanical iron matrix. mixture Fe particles with an average size of 10 µm are formed in the copper matrix. Since the near theinterfacial boundary interaction region occurs is formed at the Fe by and re Cu-melting boundary the at already equal values applied of the stainless component steel layer whenconcentrations, the copper layer structural is applied. elements An such electron as solid solutionsbeam forms of copper a melt in iron pool and in of which, iron under the influencein copper withof the an additionaltemperature mutual gradient, dissolution with of the different alloying elementsdensity andvalues mechanical of copper and steel, mixturestheir distinctive of the system feature components as a non are-mixing formed system, in this region. emerges The mechanicala narrow mixtureregion up to 200 near the boundary region is formed by re-melting the already applied stainless steel layer µm with formed inclusions of these materials. when the copper layer is applied. An electron beam forms a melt pool in which, under the Theinfluence lack ofof the ther temperaturemal stability, gradient, which with was different discussed density valuesabove, of is copper associated and steel, with sharp thermaltheir gradients distinctive due feature to the as a different non-mixing thermal system, conductivity emerges a narrow of regionmaterials. up to This 200 µ leadsm to the appearancewith formed of a inclusionscurved interface of these materials. between the first steel layer and the copper layers already depoThesited lack (Figure of thermal 12b). stability, During which the was deposition discussed above,of the istop associated layer, it with is confined sharp to the thermal gradients due to the different thermal conductivity of materials. This leads to muchthe cooler appearance bottom of a layer, curved causing interface betweenelastic compressive the first steel layer deformation. and the copper However, layers at ele- vatedalready temperatures, deposited (Figure the yield 12b). strength During the of deposition the top of layer the top decreases, layer, it is confined allowing to the for plastic compmuchressive cooler deformation. bottom layer, Coolingcausing elastic of the compressive plastically deformation. compressed However, top at layer elevated causes it to shrink,temperatures, causing a thebending yield strength angle ofwith the toprespect layer decreases, to the direction allowing forof plasticthe layer compressive deposition. This deformation. Cooling of the plastically compressed top layer causes it to shrink, causing a results in a tensile stress in the direction of growth. It is important to note that this bending angle with respect to the direction of the layer deposition. This results in a tensile mechanismstress in occurs the direction in the of solid growth. phase It is important [44]. to note that this mechanism occurs in the Basedsolid phase on microstructural[44]. studies, we can conclude that zones II and IV are FCC copper; zonesBased onI, II microstructural and V are γ studies, + δ-Fe we with can conclude different that δ zones-ferrite II and content IV are FCCs (Figure copper; 13) in the γ δ δ boundaryzones I, zones II and fromV are copper+ -Fe with to different steel and-ferrite steel contents to copper (Figure (Figure 13) in the 14a). boundary The different zones from copper to steel and steel to copper (Figure 14a). The different structure is due structureto the is fact due that to copper the fact hardens that copper iron (Figure hardens 14b). iron (Figure 14b).

Figure 13.Figure X-ray 13. diffractionX-ray diffraction of bimetal of bimetal laminate laminate macrocomposite macrocomposite in in zones zone I,s III I, andIII and IV–V IV boundary–V boundary of “SS–Cu”. of “SS–Cu”.

One of the most important types of solid solution decays is spinodal decomposition [44]. Spinodal decomposition occurs most often during rapid cooling or quenching with tempering.

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(a) (b) FigureFigure 14. 14.TEMTEM image image of of bimetal bimetal laminatelaminate macrocomposite macrocomposite boundary: boundary: (a) “SS–Cu” (a) “SS shows–Cu” hardening shows hardening of steel with of copper;steel with copper; (b) “(bCu) “Cu–SS”–SS” show showss spinodal spinodal decomposition.decomposition. The The elemental elemental compositions compositions of the Cu-based of the Cu and-based Fe-based and samples Fe-based in wt. samples % as in wt. % as revealedrevealed byby TEMTEM EDS EDS analysis. analysis.

One of the most important types of solid solution decays is spinodal decomposition [Many44]. Spinodal phenomena decomposition are associated occurs most with often the duringdecomposition rapid cooling of orsolid quenching solutions. The ki- neticswith tempering. of decomposition can be significantly affected by the presence of defects in the crystalMany lattice phenomena and the are elastic associated stress withfields the associated decomposition with of them. solid solutions.Elastic fields The can change significantlykinetics of decomposition during decomposition, can be significantly which affected may bycause the presencesuch phenomena of defects in as the cold brittleness,crystal temper lattice and embrittlement the elastic stress of fieldssteels, associated embrittlement with them. of nanocrystalline Elastic fields can changematerials during significantly during decomposition, which may cause such phenomena as cold brittleness, electrolytic vaporization and other types of ductile–brittle transitions occurring in the temper embrittlement of steels, embrittlement of nanocrystalline materials during elec- graintrolytic boundary vaporization region. and other A study types of ductile–brittlethe microstructure transitions morphology occurring in in the the grain Cu–Fe transi- tionboundary region region. revealed A study one of of the two microstructure characteristic morphology patterns: in thedisc Cu–Fe-shaped transition precipitations region formed randomlyrevealed one and of twoarranged characteristic over the patterns: matrix. disc-shaped The existence precipitations of this disintegration formed randomly pattern leads toand the arranged conclusion over thethat matrix. the nucleation The existence and ofgrowth this disintegration mechanism pattern corresponds leads to to the disc-shaped precipitationsconclusion that the arranged nucleation randomly and growth [44]. mechanism It is corresponds possible that to disc-shaped chaotically pre- distributed cipitations arranged randomly [44]. It is possible that chaotically distributed disc-shaped disc-shaped precipitations can further lead to the formation of a modulated structure. In precipitations can further lead to the formation of a modulated structure. In most ex- mostperiments, experiments, spinodal decompositionspinodal decomposition is obtained after is obtained rapid quenching after rapid of the alloysquenching and of the al- loyssubsequent and subsequent annealing at annealing lower temperatures. at lower temperatures. This heat treatment This sequence heat treatment can include sequence can includesequential sequential layer deposition layer in deposition wire-feed electron in wire beam-feed additive electron manufacturing. beam additive Rapid manufacturing. heat Rapiddissipation heat occurs dissipation due to the occurs previously due depositedto the previously copper layers, de which,posited in copper this case, layers, act as which, in thisa substrate case, act on whichas a substrate successive on layers which of stainless successive steel wirelayers are of successively stainless deposited.steel wire are succes- sively3.5. Mechanical deposited. Testing The identified features of the microstructure affect the characteristics of the mechanical 3.5.properties. Mechanical Microhardness Testing measurements were carried out along the height of a bimetallic sampleThe (Figure identified 15) in the features work. of the microstructure affect the characteristics of the me- chanicalThe changeproperties. in microhardness Microhardness along measurements the height of the multilayeredwere carried sample out along is similar the height of a to the change in chemical composition along the height (Figure 10a). The analysis of bimetallicthe distribution sample of microhardness (Figure 15) in at transitionsthe work. through copper layers testifies to a sharp change in mechanical properties in a material from a value of microhardness of 1.85 GPa, corresponding to austenite steel, to a value of 0.91 GPa in the copper part of a sample. The gradual change in microhardness on the boundary of Fe–Cu zones is connected with partial inclusion of copper in steel, and on the boundary of Cu–Fe zones—inclusions of steel in copper layers. Basically, such changes are connected with the formation of mechanical mixtures of components of the system at various structural-scale levels and are not caused by the formation of new phases.

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MetalsMetals 20212021,, 1111,, 1151x FOR PEER REVIEW 1515 ofof 1919

Figure 15. Dependence of the microhardness values on distance from substrate of the bimetallic layered macrocomposite sample. Block diagrams of microhardness measurements in the steel– copper boundary zone without jumps in values beyond the error limits.

The change in microhardness along the height of the multilayered sample is similar to the change in chemical composition along the height (Figure 10a). The analysis of the distribution of microhardness at transitions through copper layers testifies to a sharp change in mechanical properties in a material from a value of microhardness of 1.85 GPa, corresponding to austenite steel, to a value of 0.91 GPa in the copper part of a sample. The gradual change in microhardness on the boundary of Fe–Cu zones is connected with partial inclusion of copper in steel, and on the boundary of Cu–Fe zones—inclusions of Figure 15.15. Dependence of the microhardness values on distancedistance fromfrom substratesubstrate ofof thethe bimetallicbimetallic steel in copper layers. Basically, such changes are connected with the formation of me- layeredlayered macrocomposite macrocomposite sample. sample. Block Bloc diagramsk diagrams of microhardness of microhardness measurements measurements in the insteel–copper the steel– chanical mixtures of components of the system at various structural-scale levels and are boundarycopper boundary zone without zone without jumps in jumps values inbeyond values beyond the error the limits. error limits. not caused by the formation of new phases. TheThe measurementmeasurchange ementin microhardness resultresult of of the the along coefﬁcient coefficient the height of thermal of of thermal the linear multilayered linear expansion expansion sample (CTE) is (C similar shownTE) is showninto the Figure change in Figure16 .in chemical The 16. The CTE composition CTE values values for along for copper, copper, the h steeleight steel and (Figure and polymetallic polymetallic 10a). The analysis samples sample ofs arethe αT(Cu)αdistributionT(Cu) = = 1.83992 1.83992 of microhardness × 1010−5,− αT(SS)5, αT(SS) at= 1.80035 transition = 1.80035  s10 through×−5 and10− 5αT(SS and copperα-Cu)T(SS-Cu) layers = 1.74819 testifies = 1.74819  10 to−5, ×arespec- sharp10−5, tively.respectively.change The in mechanicaladditively The additively grownproperties grown copper in coppera and material steel and fromsamples steel a samples value were of found were microhardness found to have to a have higherof 1.85 a higher CTEGPa, comparedCTEcorresponding compared to the to polymetallic theaustenite polymetallic steel, sample. to sample. a value of 0.91 GPa in the copper part of a sample. The gradual change in microhardness on the boundary of Fe–Cu zones is connected with partial inclusion of copper in steel, and on the boundary of Cu–Fe zones—inclusions of steel in copper layers. Basically, such changes are connected with the formation of mechanical mixtures of components of the system at various structural-scale levels and are not caused by the formation of new phases. The measurement result of the coefficient of thermal linear expansion (CTE) is shown in Figure 16. The CTE values for copper, steel and polymetallic samples are αT(Cu) = 1.83992  10−5, αT(SS) = 1.80035  10−5 and αT(SS-Cu) = 1.74819  10−5, respectively. The additively grown copper and steel samples were found to have a higher CTE compared to the polymetallic sample.

FigureFigure 16. 16. MeasurementMeasurement result ofof thethe thermalthermal linearlinear expansion expansion coefﬁcient. coefficient. Typical Typical result result for for stainless stain- lesssteel steel (black (black curves) curves) and and copper copper (red (red curves). curves). Solid Solid curves curves are are the the result result for thefor the bimetallic bimetallic layered lay- eredmacrocomposite macrocomposite sample. sample.

AsAs can can be be seen seen from from Figure 12,12, the Fe Fe–Cu–Cu and Cu Cu–Fe–Fe transition boundaries have a complexcomplex shape. shape. At At the the same same time, time, as aswritten written above, above, the thealloying alloying of copper of copper with with steel steeland and the mixing of copper into the steel layers takes place. If we take into account the total length of the transition boundaries up to 1 mm, the CTE value is just in the range of αT(Cu) > αT(SS-Cu) > αT(SS). With the difference in CTE values, the components of the

polymetallic product create stresses at the component boundaries when heated and thereby prevent the expansion of the polymetallic sample. As a result, the polymetallic zone Figure 16. Measurement result of the thermal linear expansion coefficient. Typical result for stain- boundariesless steel (black are curves) stressed and and copper mutually (red curves). limit component Solid curves expansion are the result when for heated.the bimetallic This canlay- haveered m aacrocomposite positive effect sample. in the fabrication of structural parts operated in a large temperature range, when it is necessary to consider gaps in the mating of parts. TheAs can stress–strain be seen from plots Figure are shown 12, the as Fe the–Cu extreme and Cu values–Fe transition of the mechanicalboundaries charac-have a teristicscomplex in shape. Figure At 17 the. The same green time, shaded as written area above, highlights the alloying the values of thatcopper include with allsteel of and the

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the mixing of copper into the steel layers takes place. If we take into account the total

Metals 2021, 11length, x FOR PEER of theREVIEW transition boundaries up to 1 mm, the CTE value is just in the range of16 of 19 αT(Cu) > αT(SS-Cu) > αT(SS). With the difference in CTE values, the components of the polymetallic product create stresses at the component boundaries when heated and thereby preventthe the mixing expansion of copper of the into polymetallic the steel layers sample. takes place. As a Ifresult, we take the into polymetallic account the total zone boundarieslength are stressed of the transition and mutually boundaries limit up tocomponent 1 mm, the expansion CTE value is when just in heated. the range of This can have aαT(Cu) positive > αT(SS effect-Cu) in >the αT(SS). fabrication With the of difference structur inal CTEparts values, operated the components in a large of the temperature range,polymetallic when it productis necessary create to stresses consider at gaps the component in the mating boundaries of parts. when heated and The stress–therebystrain plots prevent are the shown expansion as the of theextreme polymetallic values sample. of the Asmechanical a result, the charac- polymetallic teristics in Figurezone 17. boundariesThe green areshaded stressed area and highlights mutually the limit value components that include expansion all when of the heated. This can have a positive effect in the fabrication of structural parts operated in a large obtained values of ultimate strength, yield strength and strain to failure. As can be seen, temperature range, when it is necessary to consider gaps in the mating of parts. Metals 2021, 11, 1151 16 of 19 the test result of sampleThe stress # 1, –cutstrain in theplots extreme are shown position as the extremeof the polymetallic values of the sample,mechanical has charac- the worst performance.teristics in ThisFigure is 17. due The to green a defect shaded in area the highlights form of the a discontinuity, values that include which all of the caused the sampleobtained to fracture. values ofThe ultimate remaining strength, tensile yield samplestrengths and # 2 –strain9 have to failure.similar As charac- can be seen, obtained values of ultimate strength, yield strength and strain to failure. As can be seen, teristics and havethe tensile test result strength of sample σUS #= 1, 314 cut ± in 23 the MPa, extreme yield position strength of the σYS polymetallic = 194 ± 15 sample,MPa has the test result of sample #1, cut in the extreme position of the polymetallic sample, has the the worst performance. This is due to a defect in the form of a discontinuity, which and strain to failureworst ε = performance.0.16 ± 0.01 This(Fig isure due 17b). to a defect in the form of a discontinuity, which caused the causedsample the to sample fracture. to The fracture. remaining The tensile remaining samples tensile #2–9 sample have similars # 2– characteristics9 have similar and characteristics and have tensile strength σUS = 314 ± 23 MPa, yield strength σYS = 194 ± 15 MPa have tensile strength σUS = 314 ± 23 MPa, yield strength σYS = 194 ± 15 MPa and strain to andfailure strainε =to 0.16 failure± 0.01 ε = (Figure 0.16 ± 170.01b). (Figure 17b).

(a) (b) (a) (b) Figure 17. Results of uniaxial tension of bimetallic samples: (a) dependence of σ(ε) with minimum (red curve) and maximum (black curve)Figure tensileFigure 17. Results 17. strength;Results of uniaxial of (b uniaxial) experimental tension tension of of bimetallic bimetallic yield strength samples: YS, ((aa)) dependence ultimatedependence strength of ofσ( ε σ(ε)) with US with minimum and minimum deformation (red (redcurve) curve) and ε and values of the samples.maximum maximum (black (black curve) curve) tensile tensile strength; strength; (b(b) experimental) experimental yield yield strength strength YS, ultimate YS, ultimate strength strength US and deformation US and deformationε values ε valuesof of the the samples. samples.

Figure 18 showsFigure fractographyFigure 18 18 shows shows fractographyof fractography the sample of of the theafter sample sample fracture. after after fracture. fracture.

Figure 18. Fractographic images after uniaxial tension of bimetallic layered macrocomposite. Figure 18.Figure Fractographic 18. Fractographic images images after after uniaxial uniaxial tension of of bimetallic bimetallic layered layered macrocomposite. macrocomposite. Fracture analysis showed that intergrain fracture occurs. At the same time, the surface of Fracturethe fracture analysis does showed not have that chipping intergrain fractureareas, but occurs. there At are the facets same time,of individual the surface grains Fracture analysis showed that intergrain fracture occurs. At the same time, the sur- andof small the fracture “cups does”. The not above have chipping signs indicate areas, but that there ductile are facets–brittle of individual fracture occurred grains and in the face of the fracturesamples.small does “cups”. not have The above chipping signs indicate areas, that but ductile–brittle there are facets fracture of occurred individual in the samples.grains and small “cups”.4. The Conclusions above signs indicate that ductile–brittle fracture occurred in the samples. 1. A bimetallic layered macrocomposite was successfully fabricated by electron beam additive manufacturing with alternating ﬁlament feeding of stainless steel and copper. 2. Macrostructural analysis of the macrocomposite showed complete deposition of the layers and the absence of pore- and crack-type defects. 3. Microstructural studies of stainless steel layers of zones I, III and V by X-ray phase analysis and transmission electron microscopy determined the presence of two phases—austenite γ-Fe and coarse acicular δ-Fe in the form of lamellae with a width of 300–500 nm.

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4. The effect of copper stabilization of γ-Fe in stainless steel layers of zone III between copper interlayers was revealed, manifesting itself in a reduction of 1.7–1.9 times of the δ-Fe volume fraction as compared to stainless steel layers of zones I and V, not bounded by copper interlayers on both sides. For I and V zones of steel sections, according to close microstructure, the value of the δ-Fe volume fraction varies in the range of fI,V (δ-Fe) = 16.3–17.2%. 5. In the copper layers of zones II and IV, collective recrystallization with formation of equiaxed grains of size 28.5 ± 0.2 µm takes place. 6. Different mechanisms of boundary formation during 3D printing of stainless steel and copper are established: during formation of the SS–Cu boundary, austenite hardening by copper takes place; during formation of the Cu–SS boundary, spinodal decomposition is observed. 7. Interfacial interaction occurs in the boundary regions, which is observed in both copper and iron: Cu particles with an average size of 5 µm are formed immediately near the sharp boundary of the boundary zone in the region of the iron matrix, while Fe particles with an average size of 10 µm are formed in the copper matrix. 8. The values of mechanical properties of the layered bimetal lie in the range of values of mechanical properties of monometallic products based on Fe and Cu, manufactured by wire-feed electron beam additive manufacturing. The strength of the sample is lower than that of pure additive steel and higher than that of pure additive copper.

Author Contributions: Conceptualization, Funding Acquisition, Investigation, Writing—Original Draft, K.O.; Methodology, Investigation, Data Curation, Visualization, A.V.; Methodology, Resources, Investigation, A.C.; Investigation, Data Curation, Formal analysis, D.G.; Methodology, Resources, N.S. (Nikolai Shamarin); Investigation, Data Curation, Formal analysis, N.S. (Nikolai Savchenko); Conceptualization, Writing—Review and Editing, E.K. All authors have read and agreed to the published version of the manuscript. Funding: The work was performed according to the government research assignment for ISPMS SB RAS, project FWRW-2019-0034. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Results data can be obtained upon request to the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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