Wire Arc Additive Manufactured CuMn13Al7 High-Manganese Aluminium Bronze

In this work, high-manganese aluminium bronze CuMn13Al7 samples were prepared by arc additive manufacturing technology. The phase composition, microstructure, and crystal structure of the high-manganese aluminium bronze CuMn13Al7 arc additive manufactured samples were analysed using direct-reading spectrometer, metallographic microscope, scanning electron microscope, and transmission electron microscope. The micro-hardness tester, tensile tester, impact tester, and electrochemical workstation were also used to test the performance of the CuMn13Al7 samples. By studying the microstructure and properties of the CuMn13Al7 samples, it was found that preparation of the samples by the arc additive manufacturing technology ensured good forming quality, almost no defects, and good metallurgical bonding inside the sample. The metallographic structure (α + β + point phase) mainly comprises the following: the metallographic structure in the equiaxed grain region has an obvious grain boundary α; the metallographic structure in the remelting region has no obvious grain boundary α; the thermal influence on the metallographic structure produced a weaker grain boundary α than the equiaxed grain region. The transverse and longitudinal cross sections of the sample had uniform microhardness distributions, and the average microhardness values were 190.5 HV0.1 and 192.7 HV0.1, respectively. The sample also had excellent mechanical properties: yield strength of 301 MPa, tensile strength of 633 MPa, elongation of 43.5%, reduction of area by 58%, Charpy impact value of 68 J/cm2 at – 20 ℃, and dynamic potential polarisation curve test results. Further, it was shown that the average corrosion potential of the sample was – 284.5 mV, and the average corrosion current density was 4.1×10–3 mA/cm2.


Introduction
Additive manufacturing is also called as three-dimensional (3D) printing and is different from traditional subtractive (cutting) processing technologies as it uses a 3D data model of the parts [1][2][3][4]. Based on the principle of discrete layering or stacking, different heat sources (laser beam, ion beam, electron beam, arc, ultraviolet light, etc.) are used to apply raw materials (powder, wire, liquid materials, etc.) in a layer by layer manner to manufacture parts. Compared with traditional processing technologies, additive manufacturing has outstanding features, including short manufacturing cycle, lower production costs for some parts, high material utilisation, high degree of design/manufacturing integration, and design of component structure. Optimisation techniques available in this process are especially suitable for manufacturing parts and components with complex structures and high added value of raw materials. Additive manufacturing thus has broad prospects in aviation, aerospace, automotive, biomedical, educational, and mould manufacturing applications, among others [5][6][7][8][9].
Additive manufacturing technology is mainly divided into metal and non-metallic additive manufacturing based on the types of materials used. Metal additive manufacturing technologies mainly include laser, electron beam, plasma, and electric arc additive manufacturing based on the heat source [10][11][12][13]. Compared with other additive manufacturing technologies, arc additive manufacturing has high production efficiency and is suitable for large-scale component manufacturing [14]. The disadvantage of using electric arc as the heat source is that the manufacturing accuracy is not as good as that obtained with lasers and electron beams [15][16][17][18]. Arc additive manufacturing technology has received widespread attention in aerospace, mechanical equipment production and other fields in recent years [19][20][21].
High-manganese aluminium bronze is a high-strength copper alloy that is based on ordinary aluminium bronze. By adding multiple elements, such as manganese, aluminium, iron, nickel, and titanium and with rare earth modification treatment, the matrix structure is strengthened. The phases are uniformly distributed in a tough matrix, and a type of alloy is formed with enhanced strength without reduction of plasticity. High-manganese aluminium bronze materials have high strength, hardness, toughness, and wear resistance, and their comprehensive mechanical properties are reported to be good [22,23]. The high-manganese aluminium bronze material has good corrosion resistance when exposed to the atmosphere, fresh water, and seawater. It resists high-speed seawater erosion and has high corrosion fatigue strength. Because the high-manganese aluminium bronze material has a higher electrode potential, the surface energy is important. A strong alumina film is formed, and if this film is damaged, it can be self-healed as well as re-formed and covered, so that the corrosion resistance is improved [24]. Further, high-manganese aluminium bronze alloys have good casting properties and can be welded. Therefore, high-manganese aluminium bronze materials are one of the main materials used in large propellers in various countries around the world. Manganese aluminium bronze materials are also widely used in steel rolling equipment and other metallurgical machinery to manufacture sliders, slides, bearings, bushings, nuts, etc. (generally referred to as metallurgical spare parts), which can adequately meet the requirements of production conditions [25,26]. However, cast high-manganese aluminium bronze suffers from reduced seawater corrosion resistance owing to the coarse structure, component segregation, and casting defects [27]. Therefore, it is of great significance to research new manufacturing processes for high-manganese aluminium bronze parts.
To the best of the authors' knowledge, there are no reports on research involving additive manufacturing of high-manganese aluminium bronze. Therefore, this study uses the arc additive manufacturing technology and CuMn 13 Al 7 welding wire to investigate high-manganese aluminium bronze arc additive manufacturing technology. It is expected that this study can provide theoretical and data support for popularisation and application of the additive manufacturing technology.

Experimental Setup and Analysis Procedure
The wire used in these experiments was a high-manganese aluminium bronze CuMn 13 Al 7 supporting welding material produced by Newland (Tianjin) Welding Material Co., Ltd., with a grade of CuMn 13 Al 7 M and diameter of 1.2 mm. The chemical composition (wt%) of the CuMn 13 A l7 wire is shown in Table 1. The arc additive manufacturing equipment uses TPS4000 cold metal transfer (CMT) advanced power supply, KUKA six-axis robot KR 10R1420 and IungoPNT robot arc additive manufacturing software. The experimental substrate was Q235 steel of dimensions 300 mm × 100 mm × 12 mm. Before the experiment, the substrate was polished with a hand-held grinding wheel to remove rust and scale on the surface of the steel plate. In order to reduce the amount of deformation of the substrate during the additive manufacturing process, the steel plate was fixed to the base steel plate using a designed fixture. In this research, the arc additive manufacturing process adopts the CMT mode, and single-layer single-pass, singlelayer multi-pass, and multi-layer multi-pass tests are performed in the early stages of testing; further, the preferred additive manufacturing process parameters are selected according to the shape of the bead. In order to control the forming accuracy of the additive manufacturing sample, a Fluke F59 infrared thermometer was used to detect the interlayer temperature during the manufacturing process. The process parameters selected for this research are as follows: current 115 A, voltage 11.4 V, wire feed speed 4.5 m/min, shielding gas 100% Ar, shielding gas flow 18 L/min, and interlayer temperature not exceeding 150 ℃. The specific process parameters are summarised in Table 2.
The size of the printed sample (thin wall) using arc additive manufacturing in this research is approximately 150 mm × 20 mm × 60 mm. Using a band saw and wire cutters, samples were obtained from the middle of the manufactured sample for metallographic analysis (hardness test, scanning electron microscopy (SEM) analysis), transmission electron microscopy (TEM), and chemical composition analysis. Tensile tests were conducted on both sides of the sample in directions parallel and perpendicular to the deposition direction and on impact specimens. The chemical composition analysis test was carried out according to GB/T 5121.27-2008, using a PRODIGY XP ICP-OES. The tensile tests of the sample were carried out according to the standard GB/ T228.1-2010. The room temperature tensile performance was evaluated using a 100 kN material testing machine (SINTECH20 / G). Displacement control was used during the stretching, and the initial strain rate of the sample was 0.005 min -1 . The sample impact test was performed according to GB/T229-2007 using a pendulum impact tester (ZBC2302-C); the test temperature was -50 °C, and three samples were tested in each group. The metallographic and SEM samples were cut, ground, and polished, and then etched with a saturated iron nitrate alcohol solution. The OLYMPUS GX71 metallographic microscope was used to observe the microstructure of the metallographic samples based on the standard GB/T 13298-2015 "Metal microstructure inspection method". ZEISS EVO-18 and Quanta650 scanning electron microscope were used to observe the microstructure of the metallographic sample at high magnification and corrosion morphology after the electrochemical corrosion test, respectively. The Genesis Apex2 X-ray spectrometer attached to the scanning electron microscope was used for component analysis. The TEM sample was cut from a 0.5 mm slice from the block sample, and ground to a thickness of about 120 μm; three wafers were punched out, and finely ground to about 50 μm. A 4% perchloric acid solution was used as the electrolyte, and a perforated film sample was obtained by double-spray electrolysis at − 20 °C. The electrolysis voltage was set to 75 V, and the Gatan691 ion thinner was used on the argon ions for 0.5 h to reduce their thickness. The TEM sample was used to observe the crystal structure with a JEM-2100 transmission electron microscope, and the acceleration voltage was 200 kV. According to the GB/T 4340.  "Vickers hardness test for metallic materials-Part 1: Test method" standard, the micro Vickers hardness is measured with a test force of 100 g, and the tests are conducted from the upper part of the weld (2 mm) to the lower part. Ten points were measured at equal distances (1 mm apart), and a microhardness tester VMH-104 was used. The Gill AC Bi-STAT electrochemical workstation was used to test the polarisation curves of the samples; the test conditions were room temperature (~25 °C) and 3.5% NaCl aqueous solution (neutral). After the sample was connected to the wire, it was sealed with a sealant, and a surface of 10 mm × 10 mm was reserved as the test surface. Before the test, the surface was polished with sandpaper until there were no obvious scratches on the surface. The three-electrode system was used for the test: the reference electrode was a saturated calomel electrode, a platinum electrode was the auxiliary electrode, and the sample was the working electrode (the potentials in the report are relative to the saturated calomel electrode unless specified otherwise). First the open-circuit potential was measured for 30 min, and then the potentiodynamic polarisation curve test was performed. When the polarisation curve was tested, the scanning range used was -150 to + 600 mV (relative to the open-circuit potential), and the scanning rate was 20 mV/min. After completion of the test, the results were processed with software to obtain the final data. Figure 1 shows a sample of the CuMn 13 Al 7 thin wall printed by arc additive manufacturing technology. It can be seen from the sample picture that the thinwalled CuMn 13 Al 7 printed by the CMT power supply is adequately formed without collapse, and the welding wire splashing is small during the forming process. In addition, from the sample cross-sectional view (metallographic sample) given in Figure 2, it can be seen that there are almost no defects inside the sample and that the metallurgical bonding is good, which further Printing speed (m/s) 12

CuMn 13 Al 7 Additive Manufacturing Forming Characteristics
Contact tip to work distance (mm) 15 Shielding gas flow (L/min) 18 Interpass temperature (℃) ≤150 Overlap rate (%) 50 Figure 1 Sample of CuMn 13 Al 7 high-manganese aluminium bronze prepared by wire arc additive manufacturing illustrates that CuMn 13 Al 7 has good forming performance by arc additive manufacturing. Table 3 shows the chemical composition analysis results of the high-manganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing samples. Compared to the chemical composition of the raw silk materials given in Table 1, it can be seen that the main alloying element Mn in the arc additive manufacturing sample has a burning loss of 6.1%. The decrease in relative content is more pronounced, which indicates that the arc additive manufacturing process has better protection and better deoxidation performance in the molten pool. Figure 3 is an X-ray diffraction (XRD) pattern of a highmanganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample, and the main phase composition of the sample is Cu 0.69 Al 0.28 Ni 0.02 and Cu. Figure 4 shows the metallographic pictures of the highmanganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing samples at different locations. The low magnification metallographic structure is shown in Figure 4a, and the equiaxed crystal region is shown below that. The entire sample is mainly composed of equiaxed crystal regions, and there are fewer columnar crystals. The middle strip is the remelting zone, and the heataffected zone is above the remelting zone. From Figures 4b and c, it can be seen that the metallographic structure of the equiaxed grain region of the highmanganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample is α + β + point phase, and the grain boundary α is obvious. The sample metallographic structure in the remelting zone is α + β + point phase with no obvious grain boundary α (see Figures 4d and e). The metallographic structure of the sample heat-affected zone is α + β + dot-like phase, and the grain boundary α is weaker than that in the equiaxed grain region (see upper part in Figures 4d and f ). In addition, it can be seen from the low magnification metallographic photograph in Figure 4a that no cracks, holes, solid inclusions, unfused, unwelded, poorly shaped or sized, and other defects are found, indicating that the internal quality of the CuMn 13 Al 7 steel arc additive manufacturing is good. Figure 5 shows a high magnification SEM image of the metallographic sample. No small defects were found in the high magnification SEM image. The sample mainly had a fine needle-like structure inside the equiaxed crystal region under high magnification SEM.

High-Manganese Aluminium Bronze CuMn 13 Al 7 Additive Manufacturing Composition and Microstructure Characteristics
To further analyse the crystal structure of the highmanganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample, Figure 6 shows the TEM image of the sample and the EDS composition of the    Figure 7 shows the crystal structure and corresponding selected area electron diffraction pattern of the highmanganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample. Figure 7a is a high-magnification image of the large particle enhanced phase in Figure 6a and the corresponding selected area electron diffraction pattern. From the analysis of the pattern, it can be seen that the corresponding phase is the Fe 2 MnAl phase. The EDS elemental composition results of the large particle enhanced phase are consistent, but there is no obvious Fe 2 MnAl phase diffraction peak in the XRD pattern of the sample in Figure 3. This may be attributable to the fact that XRD analysis is generally considered for phases with a relative content of less than 5%. The diffraction peak is not seen clearly in the entire spectrum, so no Fe 2 MnAl phase was found in the XRD pattern of the sample. Figure 7b shows the morphology of the matrix phase and the corresponding selected area electron diffraction pattern. From the analysis of the results, it can be seen that the corresponding phase is Cu 0.69 Al 0.28 Ni 0.02 , which is in accordance with the XRD (Figure 3) results and  In addition, TEM analysis revealed that particle-reinforced phases + twins ( Figure 8a) and particle-reinforced phases + dislocations (Figure 8b), particlereinforced phases, twins, and dislocations also existed in the sample. The appearance of such components is beneficial for increasing the strength of the sample.  The test results of the mechanical properties of the high-manganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample are shown in Table 4. Figure 10 shows the tensile curve of the CuMn 13 A l7 sample made by wire arc additive manufacturing. From the results, it can be seen that the sample has excellent mechanical properties, yield strength of 301 MPa, tensile strength of 633 MPa, elongation of 43.5%, reduction in area of 58%, and Charpy impact value of 68 J/ cm 2 at − 20 °C.

Performance of High-Manganese Aluminium Bronze
In order to analyse the fracture mechanism of the sample, Figure 11 shows the tensile fracture morphology of the high-manganese aluminium bronze CuMn 13 Al 7 arc additive manufacturing sample. It can be clearly seen from the overall morphology of the tensile fracture in Figure 11a that there are no obvious defects in the tensile specimen, which is in accordance with the experimental results of the tensile elongation and section shrinkage of the tensile specimen, as shown in Table 4. Figures 11b, c, and d are the morphologies of the tensile fracture fibre zone, radiation zone, and secondary fibre zone, respectively. From the figure, it can be seen that there is a large number of dimples in the fracture, indicating that the sample is stretched. The fracture mechanism is mainly ductile fracturing, and the radiation area has a river-like dissociation surface morphology. Figure 12 shows the kinetic potential polarisation curve of the sample. It can be seen from the figure that the anodic polarisation curve of the sample is divided into two distinct stages. The anode current density rapidly increases in the interval between the corrosion potential (EC) and point A, indicating that no obvious passivation behaviour has occurred on the sample surface. Controlled by activation and mass transfer processes, the maximum current and slower increase of current in the interval A ~ B indicate that the reaction in this interval is controlled by the mass transfer process. The corrosion potential and corrosion current density of the samples obtained by software fitting are shown in Table 5. Figure 13 shows the corrosion morphology of the sample after the potentiostatic polarisation test. It can be seen from the figure that the electrochemical corrosion mechanism of the sample is mainly intergranular corrosion.

Figure 13
Corroded morphologies of the CuMn 13 Al 7 sample formed by wire arc additive manufacturing in 3.5 wt% NaCl solution