Fabrication of metal parts by wire and Arc Additive Manufacturing (WAAM) has received an increased interest in recent years, as it allows high design flexibility and reduction of material wastage compared to other traditional manufacturing routes. This article compares the effect of heat input on metallurgical characteristics of wire arc additive manufactured low carbon steel cylindrical components fabricated by Gas Metal Arc Welding (GMAW) and Cold Metal Transferred Arc Welding (CMTAW) processes. Firstly, the influence of heat input on the grain size was analyzed. Subsequently, the effect of heat input on the metallurgical characteristics of the cylindrical components was studied along the building direction. The microstructure of the built cylindrical-walled component varies from the top to the bottom regions and can be distinguished into two regions: lamellar structures (widmanstatten ferrite and grain boundary ferrite) in the top regions; and equated grains of fully ferrite in the bottom region in GMAW. In the CMT-WAAM cylindrical component samples have been noted two different regions: the bottom region characterized by a ferritic structure with thin strips of pearlite and the top region characterized by a lamellar structure typically bainite with acicular ferrite.

Keywords: Wire arc additive manufacturing, gas metal arc welding, cold metal transfer arc welding, low carbon steel and microstructural characteristics.

Additive Manufacturing (AM) is relatively a new manufacturing technique that has created a lot of attention in the past decade. The ability to create structurally complicated components with unparalleled design flexibility has undergone a change. Every AM technique has a unique combination of heat source, feedstock, and movement mechanism, making it suitable for a wide range of applications [1]. AM methods for metal components have been categorized into four categories by ASTM (ASTM F2792), one of which is Directed Energy Deposition (DED). By definition, DED is "an additive manufacturing process in which concentrated heat energy is utilized to melt materials as they are deposited"[2].

Wire-feed and powder-feed techniques are variations of DED technology. When compared to powder, the ability to use wire as a feedstock results in a lower cost per kg and a higher material utilization rate. As a result, the manufacturing of large components is excellent for wire-feedstock technologies, which are the most effective additive processes for this purpose [3]. Wire arc additive manufacturing (WAAM) is a low-cost and efficient technology for fabricating large and medium-scale components and structures. WAAM uses a common arc-based welding equipment to manufacture large structures and components for multilayer deposition in a layered technique. Gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and Plasma arc welding (PAW) techniques can all be used in the WAAM process [4]. Various materials, ranging from titanium, aluminum, steel, and nickel alloys, have proved the feasibility of the WAAM method. The author [5] documented that the Cold Metal Transfer (CMT) process is suitable for large-scale stainless steel components with medium-high mechanical properties; Gas tungsten arc welding (GTAW) process is recommended for small-medium size titanium and stainless steel components with medium-high mechanical requirements; Plasma arc welding (PAW) process is appropriate for medium-large size steel and titanium components with medium-high mechanical characteristics.

CMTAW is a variant of GMAW process. The fundamental CMT method is based on wire movement, which aids in molten droplet detachment. This enables the arc to be extinguished at regular intervals, reducing the process's heat input [6]. The filler wire is pushed into the welding pool formed by the arc in the first phase or arcing period. The arc becomes extinguished and the welding current is reduced when the filler metal reaches the weld pool. The droplet detachment is followed during the short circuit by the rearward motion of the wire. The current is kept low in the short-circuit phase. After that, the cycle starts again when the arc is ignited and the wire is pushed into the weld pool. The current waveforms of GMAW and CMTAW processes. During the deposition, the current is constant in GMAW and the actual value fluctuates between the peak (in peak phase) and the base (in base phase), resulting zero current (in short circuit phase) in CMTAW [7]. In conventional spray transfer and pulsed spray transfers, the cooling rate is very slow compared to the CMTAW short circuit process [8]. Therefore, a low heat input process such as CMTAW can improve significantly the mechanical properties of WAAM carbon-steel components. The possibilities of this CMTAW process have not been studied till now for the WAAM of metal cylindrical components.

In the WAAM, heat input is the key factor affecting the morphology of the components, the microstructure and the mechanical properties. Another difficulty with WAAM in steels is the possible mixture of multiple microstructures (e.g., ferrite, Grain boundary ferrite (GBF), Widmanstätten ferrites (αw), bainite (B), martensite, and acicular ferrite (αa)) depending on % carbon, alloy elements, and cooling time [9]. Tiago The author [10] investigated the mechanical properties and microstructural characteristics of high strength low alloy steel manufactured by WAAM. The different microstructural features of ferrite, marten site, bainite and reverted austenite were found at different heat inputs. In reference [11] the author studied the microstructure and mechanical properties of multi directional pipe joints using WAAM and revealed that the microstructure comprised 28.2% pearlite and 71.8% ferrite, while the average size of grain did not surpass 15µm. Similarly, in recent research [12] the author observed anisotropy in mechanical properties and microstructural changes in Al-7Si-0.6 Mg alloy parts. This microstructural changes affectively impacts the mechanical properties of manufactured components. As a result, researchers have focused even more on microstructure evolution, mechanical properties, and fracture behavior in the WAAM process. However, heat input variations arise when different modes of metal transfers are used even if the feed rate for wire is maintained constantly.

In recent studies [13,14] it is reported that the high heat input and thermal histories experienced by the parts during AM technologies affects the direction and shape of grains results anisotropy in mechanical properties as well as changes in microstructures features. But low heat input WAAM-based techniques can lead to uniform microstructure and good mechanical properties [15]. In recent article [16] the authors scientifically investigated the effect on the porosity characteristics of additively manufactured Al-6.3% Cu alloys of different metal transfer modes during a CMT process, and their findings showed that heat input is one of the critical factors for the advanced CMT pulse process (CMT-PADV) to control porosity rates. In recent work [17], the authors observed a uniform microstructure with less heat input, resulting in greater tensile strength than those made in the component by higher heat input. The authors stated that, as compared to the bottom of the part, the top of the part resulted in a coarse-grained microstructure with a decreased tensile strength because of the higher temperature gradient at the top of the wall.

Based on the current literature survey, no systematic studies have been reported on comparing GMAW and CMTAW processes for the manufacturing of low carbon steel cylindrical components have been conducted. The correlation between the heat input used in GMAW and CMTAW and the process impacting the microstructural and mechanical properties of low carbon steel have yet to be documented. Low carbon steel (ER70S-6) is a steel with a low carbon content of from 0.05 to 0.25% and, depending on the weight percentage of its alloying elements and used processes, its microstructure is defined. Carbon steel is widely used in the construction of automobile parts and structures, pipes and food cane industries. This study provides a deeper understanding of the manufacturing of ER70S-6 low carbon steel cylindrical components by WAAM techniques using GMAW and CMTAW processes. The influence of heat input on microstructures was examined at various regions of the cylindrical components. There has also been an analysis of changes in microstructural characteristics and phases at different regions of the cylindrical components

Materials and Manufacturing

The cylindrical components were manufactured on a substrate plate of mild steel with dimensions of 250×250×10 mm. As filler material, the solid wire ER70S-6 (AWS A5.18 standard) with 1.2 mm diameter was used. The chemical composition of the filler wire used in this investigation is presented in [Table 1]. The welding machine CMT Advanced 4000 R [Figure 1] was used as a welding power source during the deposition process and the filler wire was supplied to the welding torch, which was kept stationary using a rotating table for each layer. The welding torch was kept constant perpendicular to the substrate. Meanwhile, the substrate will rotate with the arrangement of a rotating table system. The carbon steel cylindrical parts were built with GMAW and CMTAW using optimized process parameters presented in [Table 2]. A pause of 120 sec was imposed between each layer deposition, in order to enable a partial cooling of the deposited material. The cylindrical components were separated into two regions from base plate to 75mm bottom region in [Figure 2] and the middle of the component from 75mm to 150 mm top region in [Figure 2 and 3] shows the photographs of cylindrical components built via GMAW and CMTAW processes, and their dimensions are presented in [Table 3]. The heat input (kJ/mm) was calculated using Eq.

Where S is the travel speed in millimeters per minute, I am the average arc current in amperes, V is the arc voltage in volts and ? is the process efficiency in % which is assumed as 0.8.

Figure 1: CMTAW-WAAM setup used to manufacture cylindrical components.

Figure 2: Schematic illustration of the WAAM carbon steel cylinder indicating direction of deposition and showing separation of component in two regions.

Figure 3: Photographs of the low carbon steel straight cylindrical components, a) GMAW process and b) CMTAW process.

Table 1: Chemical composition (wt. %) of filler metal (all weld metal).

Table 2: Optimized WAAM process parameters used to fabricate the components.

Table 3: Dimensions of manufactured cylindrical components.

Macrostructure and Microstructure Analysis

The surface of metallographic samples was polished with different grades of emery papers and applied diamond paste for mirror finishing. For revealing bright microstructure from the center portions of bottom and top regions of components, 2% Nital reagent was used as an etchant. The macrostructures of the bottom and top regions of the GMAW and CMTAW cylindrical components were examined with a stereo zoom microscope. The microstructure of the bottom and top regions of the cylindrical components was examined with a light optical microscope. Image software was used to measure the grain size of cylindrical components and chemical elements were confirmed by Optical Emission Spectrometer. The obtained microstructural features and grain size were correlated to the heat input and subsequently to the mechanical properties of WAAM cylindrical components.

Macrostructure

[Figure 4a-4d] shows the macrostructures of the bottom and top regions of the GMAW component in Figure 4a, b and the CMTAW component in Figure 4c, d. The weld layers are clearly visible in the macrostructures, and the deposited beads are properly fused. The weld is free from flaws or other noticeable defects, as evidenced by the macrostructure.

Figure 4: Macrostructures of bottom and top regions of the cylindrical components, a) and b) GMAW and c) and d) CMTAW.

Microstructure Analysis

[Figure 5a] shows the low magnification micrograph of the ER70S-6 carbon steel revealing ferrite and pearlite at grain boundaries. The manufactured WAAM based GMAW and CMTAW carbon steel components almost revealed similar microstructure [Figure 5b and c]. However, the welding processes have an effect on the grain size of the manufactured components, particularly in the bottom region. The component manufactured by the GMAW process revealed larger grains (16.23 ± 0.71μm). In fact, high current and voltage used in the GMAW process led to an increase in the heat input. The solidification time and the cooling time increase with higher heat input. This increases the size of the grains. The fine grains (12.04 ± 0.43 μm) were formed in the cylindrical wall component manufactured by the CMTAW process shown in [Figure 5c]. The solidification time and cooling time decrease with low heat input. This decreases the size of the grains. In recent article [18] the authors also observed similar behavior in the middle and bottom regions of the ER70S-6 low carbon steel thin wall parts. The authors reported that the thin wall part of sample 4 revealed coarse grains due to high input at high current value and sample 1 revealed fine grain due to low heat input at low current value.

Figure 5: Optical photographs, a) primary microstructure of ER70S-6, b) average grain size of GMAW component= 16.23±0.71μm and c) average grain size of CMTAW component = 12.04±0.43 μm.

The microstructure of the GMAW component varied from the bottom to the top region shown in [Figure 6]. Figure 6b shows the bottom region of the GMAW component microstructure composed of a fully ferritin structure, indicated by the green color. The top region of the microstructure was composed of PF, GBF, αw and αa as shown in [Figure 6c and d]. In another investigation [19] the authors also documented similar results in low carbon steel walls. The author observed that the widmanstatten ferrite, acicular ferrite and allotriomorphic ferrite in upper region and equated ferrite in the bottom region. The differences in microstructure from lower to upper zone due to the effect of air cooling and heat transfer from the upper to lower zone of the wall.

Figure 6: Optical photographs of GMAW cylindrical component a) and b) bottom region, c) top region at lower magnification and d) top region at higher magnification.

[Figure 7b] shows the microstructure of the bottom region of the CMTAW cylindrical component composed of a ferrite and small strips of pearlite (indicated by the red color). The top region mainly consists of αa and B as shown in [Figure 7c] at low magnification. [Figure 7d] at higher magnification, revealing the formation of B (laths formed in the grain) and αa due to the fast cooling of the CMTAW process. The aforementioned heterogeneous in the microstructure during WAAM of low carbon AH36 steel filler wire is also documented in reference [20]. The author observed αa and B near the fusion line. The presence of αa and B in the microstructure of steel can improve the mechanical characteristics of the component [21,22]. This is mainly due to the finer structure of the two phases and a more even distribution of carbide and greater dislocation density, as well as the internal stresses during the B phase, which help to increase hardness/strength and alloy ductility [23,24].

Figure 7: Optical photographs of CMTAW cylindrical component, a) and b) bottom region, b) top region at lower magnification and c) top region at higher magnification.

[Figure 8] shows the SEM micrograph of the bottom region of the GMAW cylindrical component composed of fully ferritic structure and top regions consists of GBF and αw confirmed at higher magnification in SEM analysis. [Figure 9] shows the SEM micrograph of the bottom region of the CMTAW cylindrical component composed of a ferrite and small strips of pearlite (confirmed high %C in EDS). [Figure 9] shows the top region of the SEM micrograph, revealing the formation of B (laths formed in the grain) and αa due to the fast cooling of the CMTAW process.

Figure 8: Micrographs of GMAW cylindrical component.

Figure 9: Micrographs of CMTAW cylindrical component.

Table 4: Chemical composition (wt.%) of as built components.

The chemical analysis of the built carbon steel cylindrical components was confirmed by Optical Emission Spectroscopy (OES). The wt% of chemical elements of the manufactured components is presented in [Table 4]. The chemical composition of the WAAM cylindrical components was fairly similar to that of the ER70S-6 filler wire. The mixture of αw and GBF (at medium cooling rates) formed in the GMAW cylindrical component due to high heat input (0.502 kJ/mm). The low heat input (0.384 kJ/mm) of the CMTAW cylindrical component resulted B and αa (at fast cooling rates). However, the different types of microstructures like PF, αw, GBF, αa and B were formed due to different thermal histories. The transition of austenite to ferrite takes place between 500 and 800°C, and the microstructure generated following this transformation is controlled by time and cooling time.

XRD Analysis

The XRD pattern of the WAAM carbon steel components was carried out for phase identification from the bottom and top regions along the building direction of GMAW and CMTAW cylindrical components, and recorded spectra’s were presented in [Figure 10]. As clearly revealed, the WAAM cylindrical components primarily consist of α-Iron located at 2θ of approximately 46.22°, 66.34° and 82.14° in the GMAW component and 46.16°, 66.10° and 82.16° in the CMTAW component according to the JCPDS patterns of 98-000-9982. They have a strong preferred orientation along the (110) plane at 2θ = 46.16° and 46.22° in GMAW and CMTAW components. Other diffraction peaks, (200) and (211), with less intensity were also found. Further, the α-Iron (110) peaks have the same position in bottom and top regions of both the components. However, when comparing with the GMAW and CMTAW components, the relative intensity of α-Iron (110) and (211) peaks vary with the heat input increasing (in GMAW component) or the heat input decreasing (in CMTAW component). It indicates that while the decrease of heat input in CMTAW component promotes the (110) orientation, the increase of heat input in GMAW component has the opposite effect on (211) orientation. The XRD measures have also indicated that the austenite (Iron-FCC) phase is not present, and confirm that either the retained austenite is not produced or its volume is too low below the XRD detection limit. Also, because of the significantly reduced volume fraction of the XRD spectrum relative to the ferrite phase, the precipitated cement phase was not identified. The similar XRD patterns confirmed from bottom and top regions along the building direction is strong evidence for the uniform and homogeneous microstructure along the building direction of the cylindrical components.

Figure 10: XRD pattern of the GMAW and CMTAW cylindrical components at bottom and top regions.

In this study, ER70S-6 low-carbon steel cylindrical wall components were wire-arc additively manufactured utilizing GMAW and CMTAW-WAAM processes. The microstructural characteristics of the manufactured components were characterized in different regions along the building direction. The following important findings are drawn from this study:

• The variation in heat input levels has significant effects on the grain size, but does not significantly influence the microstructure evolution and the microstructure type of the built cylindrical components.

• The grain size is finer in the CMT-WAAM cylindrical component than in the GMAW-WAAM cylindrical component because it experienced a higher value of thermal shock than the CMT-WAAM cylindrical component that has a coarse grain size.

• The microstructure of the built cylindrical-walled component varies from the top to the bottom regions and can be distinguished into two regions: lamellar structures (αw and GBF) in the top regions; and equated grains of fully ferrite in the bottom region in GMAW.

• In the CMT-WAAM cylindrical component samples have been noted two different regions: the bottom region characterized by a ferritic structure with thin strips of pearlite and the top region characterized by a lamellar structure typically B with αa.

The first author is grateful to the Department of Science and Technology (DST), Ministry of Science and Technology, Government of India, New Delhi for the financial support rendered through Fellowship under PURSE-Phase-2 scheme. The authors wish to record sincere thanks to M/s. Fronius India Pvt. Limited, Chennai for technical support.

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Corresponding Author: Bellamkonda Prasanna Nagasai, Department of Manufacturing Engineering, Annamalai University, India. E-mail: nagasaibellamkonda143@gmail.com.

Copyright: © 2022 All copyrights are reserved by Bellamkonda Prasanna Nagasai, published by Coalesce Research Group. This This work is licensed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.