**1. Introduction**

Low-carbon high-strength steel has been applied in many fields such as shipbuilding, automobiles, mining instruments, and railways due to its unique characteristics of mechanical properties and weldability [1]. However, it is di fficult to manufacture large scale fabrications with a complex structure by means of conventional methods, and it is also time consuming with high costs. When facing these problems, additive manufacturing (AM) may be a better choice as a promising technology [2].

AM refers to a technology that usually joins materials together by layers, which has been developed rapidly due to its high material utilization and high geometric freedom [3]. Compared with conventional technologies, AM is good at manufacturing complex components, especially those that

can be performance-customized within a certain time [4,5]. When considering AM technique used for metallic alloys, wire and arc additive manufacturing (WAAM) o ffers better competitive advantages over other techniques such as a higher deposition rate, lower cost, and high buy-to-fly ratio of components that can be realized within a shorter delivery time. Meanwhile, WAAM can also overcome many di fficulties associated with manufacturing special alloys [6–8], such that it has increasingly shown the grea<sup>t</sup> potential in the manufacture of large metal parts by an arc-based process [9]. The typical WAAM can be divided into gas tungsten arc welding (GTAW), plasma arc welding (PAW), and gas metal arc welding (GMAW) as heat sources. As a modified GMAW process, cold metal transfer (CMT) has some advantages, such as low energy input, high deposition rate, no spattering, and extremely stable arc, therefore this AM technique has become a popular as well as widely used technique. The excellent characteristics of CMT make it to be an ideal process for fabricating a large-scale part, which can overcome common troubles encountered during conventional welding process [10,11]. In this study, it was adopted as the heat source during the deposition process.

Although WAAM is widely employed in AM due to its advantages, several challenges remain to be addressed such as poor surface quality, inhomogeneous microstructure, and the anisotropy of mechanical properties caused by di fferent thermal history [12–14]. As a result, researchers have paid even more attention to the analysis of microstructure evolution, mechanical properties, and fracture behavior in the process of WAAM. This analysis is vital for its application in the ship building industry. Tiago A. Rodrigues et al. [15] studied the microstructure and mechanical properties of a high-strength low-alloy (HSLA) steel fabricated by WAAM. The same microstructural constituents of ferrite, bainite, martensite, and retained austenite were obtained for all heat inputs. Average values for the ultimate tensile strength ranged between 700 MPa and 795 MPa. Dai Yili et al. [16] investigated the microstructure and mechanical properties of multi-directional pipe joints using WAAM and pointed out that the microstructure consisted of 71.8% ferrite and 28.2% pearlite, while the average grain size did not exceed 15 μm. The tensile strength of the forming part reached 562 MPa. Youheng Fu et al. [17] explored the microstructure and mechanical properties of the bainitic steel WAAM part post-treated by rolling, and illustrated that hybrid deposition and micro-rolling treatment provided a novel way for the full transformation of columnar dendrites to equiaxed grains in the production of multi-pass multi-layer specimens. The maximum tensile strength reached 1309 MPa after optimizing.

As aforementioned, numerous studies have been conducted on the microstructure and mechanical properties of high-strength steel. However, few researchers have addressed the fundamental aspects of the solidification behavior and microstructure evolution as well as local strain concentration near inter-layer zones using DIC technology. In this study, a low-carbon high-strength steel developed for ship building was deposited as a thin wall component. It also analyzed the surface quality, microstructure evolution, microhardness, and transversal and longitudinal tensile properties. Finally, the relationship between fractography and the anisotropy of tensile properties was revealed.

## **2. Materials and Methods**

The experiments were conducted on a fixed substrate plate of 907 shipbuilding steel with dimensions of 150 mm × 300 mm × 10 mm. The alloy wire, called A-Fe-W-86, was developed for specific projects and used as a welding material with a 1.2 mm diameter. The chemical compositions of the tested materials are listed in Table 1.


**Table 1.** Chemical compositions of substrate and wire (wt.%).

During the deposition process, the CMT RCU 5000i (Fronius, Vienna, Austria) was used as a welding power supply and the welding wire was fed to the welding torch, which was kept stationary for each layer. The process parameters are listed in Table 2.


**Table 2.** Process parameters for deposition.

The deposition started from the end point of the previous layer for each subsequent layer. The path strategy was chosen in order to ensure the thickness and width of the start and end portions similar to that of the central portion, thus avoiding significant deviation from the originally expected shape [18]. The schematic diagram is shown in Figure 1.

**Figure 1.** A schematic diagram for the experimental procedure.

The deposited component was prepared for the analysis of the microstructure and mechanical properties. For consistency, all specimens were adopted from the homogeneous and stable parts of the component. The specimens, which were used to analyze the surface quality, microstructure evolution, and Vickers hardness (HV 0.2) of the top and the bottom, were drawn from the cross section of the component by a ZwickRoell Indentec (ZwickRoell, Ulm, Germany) testing machine. The tensile experiments were carried out using a Shimadzu AG-X plus (Shimadzu Scientific Instruments, Shanghai, China) as the tester with a displacement rate of 0.02 mm/s. All tests were performed at room temperature. The specific sampling location is shown in Figure 2. Both transversal and longitudinal tensions were performed to obtain the data of strain evolution during the testing to reveal the anisotropy behavior. Before the tests, specimens were sprayed with a randomized speckle pattern that consisted of black micron size speckles on a white background to achieve high contrast. The size and surface treatment condition of the specimen is shown in Figure 3.

**Figure 2.** Schematic diagram for sampling location.

**Figure 3.** The size and surface treatment condition of the specimen.

During the testing, the strain and surface displacements were calculated by tracking the speckle pattern on the specimen surface. Images were captured using an iX i-SPEED 7 CCD camera (iX Cameras, Shanghai, China) at the rate of 10 fps (frames per second), with a pixel array of 350 × 750 pixels. Then, the data were analyzed by DIC software to obtain the strain field distribution [19]. The experimental setup is shown in Figure 4.

**Figure 4.** The tensile process and devices.

## **3. Results and Discussion**

## *3.1. Surface Quality Assessment*

It is well known that the components manufactured by WAAM have poor surface roughness and dimension accuracy. Additionally, cracks between the inter-layer adjacent region, lack of fusion and inclusions are usually visible. To meet industrial requirements, surface quality is a critical consideration in the manufacturing process [20].

Figure 5 shows the typical thin-wall cross section of the as-deposited component. It can be noted that the surface of the thin wall is uniform and well formed with good quality. The deposition strategy can improve the deposition quality by filling the arc craters that are frequently caused at the arc start and end points. The so-called step e ffect can be shown at two sides of the thin-wall, which results from the layer-by-layer deposition model. The cross section of the polished state is shown in Figure 5a, and it can be observed that the condition of the as-built deposition is steady, indicating that each part of the component shows the same size. Figure 5b,c demonstrate the surface conditions after the scanning electron microscope (SEM) analysis, which has a smooth surface and full fusion, although there are some oxides or impurities brought from the prepared specimen. At the same time, the juncture between layers is reported in Figure 5d,e. Optical microscopy (OM) analysis shows that cracks, inclusions, and pores are not found in the bonding areas. Generally, the deposited part is well formed without unfused effects, cracks, inclusions, and pores observed under the condition in the present experimental method.

**Figure 5.** Assessment of the surface quality of the component: (**a**) Overall condition on cross section; (**b**,**<sup>c</sup>**) surface condition analyzed by SEM; (**d**,**<sup>e</sup>**) inter-layer adjacent region condition analyzed by OM.
