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.