Wire Arc Additive Manufacturing (WAAM) offers high quality technology for producing large complex geometry structures in close proximity to near net form, using cost-effective manufacturing resources, such as welding and wiring materials. In this study, the Cold metal transfer based-WAAM system was utilised to manufacture the steel cylindrical component. The mechanical properties and microstructure analysis of the component were analysed at two regions (bottom and top region) along the building direction. The results showed that the microstructure of the part differed from the bottom to the top region, resulting in a hardness difference between 169 and 181 (Hv0.5) and impact toughness varied from 72 J to 80 J. There were also anisotropic features in the tensile properties: the yield strength and the ultimate tensile strength ranged from 401 to 457 MPa and between 492 and 543 MPa respectively.
Key words: Wire arc additive manufacturing, Cold metal transfer welding, Low carbon steel, Microstructural characteristics, Tensile properties and Impact toughness.
Introduction
Steel refers to iron alloys most widely utilised materials for engineering purposes. Different qualities may be produced depending on their composition, but steel is generally of high tensile strength and low cost. Low-carbon steels are often used in structural applications such as tanks, structural assembly, vessels and agronomic components, and in general components [1]. Low-carbon steel components are now mostly manufactured by traditional casting, forging and machining technologies. These techniques nonetheless have complicated production procedures and can lead to a longer cycle of production or a high buy-to-fly ratio. Hence, increased demand from today's industry requires novel manufacturing techniques, in particular for producing large scale low carbon steel components, to ensure greater production efficiency and material utilisation rate than conventional methods [2]. Wire arc additive manufacturing (WAAM) is the most advantageous method of producing components with mediumlarge dimensions among metallic additive manufacturing (AM) methods. A welding source between the electrode and the workpiece is employed by WAAM systems to melt the metal wire. Gas tungsten arc welding (GTAW) and Gas metal arc welding (GMAW) processes are the most commonly used welding techniques integrated with WAAM technology. In recent years, few researches have comprehensively studied GMAW-based WAAM’s potential with regard to technological difficulties and metallurgical characteristics for various alloys [3]. Yang et al. [4] reported the possible deposit of steel parts through the double electrode GMAW-WAAM technique, and the influence of parameters on surface topography and forming of ER70S-6 steel parts manufactured using GMAW-WAAM reported by Xiong et al. [5].
From a production standpoint, the traditional GMAW and WAAM technologies have multiple similarities, resulting in analogue issues and problems related to these two processes. As a result of thermal cycles encountered by multi-pass welding of WAAM, the strength-ductility combinations in steels could be adversely affected, either by changes in the microstructure [6] and/or changes in mechanical properties along the building direction and deposition direction that are commonly found during multi-pass welding processes [7]. Therefore, it seems to be beneficial for the WAAM to implement a low-heat input welding process. The WAAM of ferrous alloys can also be complicated in that the fabricated component may have a mixture of several microstructures, such as ferrite, Widmanstätten ferrite (αw), acicular ferrite (αa), bainite (B) and martensite (M), depending on the carbon percentage, alloy constituents, and the cooling rate of steel [8]. In the latest report, Ahsan et al. [9] fabricated a square box shaped bimetallic additively manufactured structure (BAMS) of ER70S-6 and 316L steel with WAAM. The authors found that the heterogeneous microstructures in manufactured components in the sequence of αa and polygonal ferrite (PF) at the outer surface and αw, grain boundary ferrite (GBF) and PF at the inner surface of the low carbon steel component. In stainless steel component, different microstructures are observed in the sequence of skeletal ferrite at the outer surface and skeletal ferrite with lathy morphology at the inner surface. It is attributed to the lack of convection and slow cooling rate caused by stationary air inside the square, as well as faster heat transfer in the surrounding environment, resulting in a fast cooling rate on the outer surface of components. Such types of heterogeneous microstructures lead to anisotropic mechanical properties.
Several studies [10, 13] revealed that the high heat input and temperature gradient encountered by the parts during the AM manufacturing process determine the direction and shape of grain formation and the resulting anisotropic mechanical characteristics and diverse microstructures. But less thermal WAAM-based techniques, like cold metal transferred arc welding (CMTAW), can lead to a more consistent microstructure and uniform profile of hardness reported by Prado Cerqueira JL [11]. Li et al. [12] also observed that heat input has an impact on the structure of the Al-7Si-0.6 Mg component. The heat consumption also impacts the production of internal defects, grain size, distortion and phase change. In stainless steel, Wang et al. [13] again revealed a uniform microstructure with less heat input, resulting in greater tensile strength than those made in the component by higher heat input. The authors observed that, as compared to the bottom of the part, the coarse-grained microstructure at the top of the part resulted in a decreased tensile strength because of the higher temperature gradient at the top of the wall. Therefore, adopting a fast cooling version of the CMT process would plausibly favour the mechanical properties of the WAAM components.
CMT is a variant of GMAW; it is a novel method for WAAM. It uses mechanism for wire retraction which delivers signal that retracts filler wire and offers weld time to cool back each drop. This process employs integrated wire motions into the overall control of the process and a two motor driven system that pushes the wire forward and once the short circuit happens, it pulls the wire back. The retraction of the wire in this process attributes to the detachment of the droplet during the short circuiting and helps the droplet to transfer into the weld pool without the assistance of electromagnetic force unlike conventional short circuiting-GMAW. Therefore, the heat input and spatter can be decreased significantly in this process. A cycle in the CMT waveform (Figure 1) can be defined as the time period required to deposit a molten droplet into the weld pool. As a result, a rapid cooling technique like CMT can significantly improve the mechanical characteristics of carbon steel WAAM components. This transfer mode’s capability for WAAM of metallic components have not been documented before.
Figure 1: Typical current-voltage waveform of the CMT process.
In relation with WAAM, many investigations have been done to date on the manufacturing of titanium and nickel-base super alloys, but not so much on carbon steels. Therefore, in the present study, the CMT based WAAM technology was used to build a carbon steel cylindrical component. This study
Table 1: Chemical composition of filler metal (all weld).
provides a detailed analysis of the evolution of mechanical and microstructure characteristics of the different regions of the carbon steel cylindrical component made by the WAAM technique using the CMT process.
Experimental Work
Materials and methodology.
For the current research, AWS A5.18 ER70S-6 wire was used, with typical composition and mechanical properties are presented in Table 1 and Table 2. The filler wire ER70S-6 has deoxidizers that enable improved material flow and faster welding speed, contributing to increased productivity and improvement of consumable life. In order to fabricate the WAAM cylindrical component in this study, the CMT based WAAM process (CMT Advanced 4000R machine) was used with the optimized parameters presented in Table 3. Figure 2 shows the setup and manufacturing of the WAAM component. The base plate selected for this study was mild steel with dimensions of 250 × 250 × 10 mm. The motions of the welding torch are performed by a three-axis automatic motion system, as shows in Figure 2, utilizing the welding machine setup with rotating table. The motion of the welding torch was kept constant perpendicular to the substrate during the deposition, and it can be performed by the three-axis automatic motion system shown in Figure 2, utilizing the welding machine setup with rotating table. The used CMT-WAAM platform, the
Table 2: Mechanical Properties filler metal (all weld).
Table 3: Optimized CMT-WAAM Process Parameters used to manufacture the component.
Figure 2: CMT-WAAM setup used to manufacture cylindrical component.
manufactured component, and its schematic representation are shown in Figure 3. The base plate and deposited material (5 mm) was cut off and removed from the manufactured component. The fabricated component was divided into two regions; the bottom region (from substrate to 75mm in height) and top region (from 75 mm to the end of component), as shown in Figure 3. The dimensions of the built cylindrical component are presented in Table 4 and the fabricated cylindrical component is shown in Figure 4. Following the fabrication process, the deposited material (5 mm) is removed at the end of the component. The fabricated cylindrical walls are free from defects and cracks. The photographs of the machined cylindrical wall component displayed in Figure 5 with 4mm wall thickness.
Figure 3: Schematic illustration of the WAAM ER70S-6 steel cylindrical wall, a) indicating direction of deposition, b) showing separation of component in two regions, c) extraction of samples from two regions.
Table 4: Dimensions of manufactured cylindrical component
Figure 4: Photograph of the low carbon steel straight cylindrical component, a) side view, b) top view, c) wall thickness of component.
Figure 5: Photograph of low carbon steel cylindrical component (after machining), a) side view, b) top view, c) showing wall thickness.
Mechanical testing.
Microhardness survey was done on the polished surface of the samples following the procedures prescribed by the ASTM E384-17 standard. Microhardness was measured using a Vickers Microhardness Testing Machine (Make & Model: SHIMADZU HMV-2T) with a applied load of 500 gms and a dwell period of 15 sec at different regions, including the centre part of the bottom and top region and along the building direction.
Figure 3 c) demonstrates the extraction of tensile specimens from the cylindrical components produced by CMT in the bottom and top regions. As per ASTM E8M standard, the tensile testing was done using a 50 kN servo controlled testing system. At room temperature, the tensile test was done and the transverse head displacement speed was 2 mm/min. In 0.2 % offset methodology, the yield strength was determined from the engineering stress curve. The strain was calculated from a gauge length of 25 mm. In the bottom and top regions, the average values for three tensile samples were presented as the final value of the property in the CMT part. The tested tensile and impact specimen fracture surfaces were analysed by scanning electron microscopy (model: JSM 6610LV).
The impact toughness of the specimens was evaluated using a pendulum type testing machine. The charpy impact samples were prepared according to the ASTM A370 subsize standard. Three impact specimens were examined each from the top and bottom regions, and the average of the three was given as the final toughness value.
Macro and microstructure analysis.
Different grades of emery sheets and diamond paste were used to mirror polish the metallographic samples. For obtaining clear structures from samples, 2% Nital reagent was used as an etchant. The macro-structural features of the machined cylindrical parts were observed under a stereozoom microscope. A light optical microscope (make: MEJI, Japan; model: MIL-7100) was used to examine the microstructures of different regions of cylindrical components.
Results
Tensile properties.
Table 5 shows the tensile properties of the specimen extracted from the two regions of the cylindrical component. Both the regions exhibited lower tensile properties than the filler metal (all weld). Figure 6 depicts the stress-strain curves of both the regions. The top region exhibited higher strength with more elongation than the bottom region. The variation in the
Table 5: Tensile properties of manufactured cylindrical component.
Figure 6: Stress–strain curves.
microstructural characteristics of both regions is the reason for this. The notch strength ratio (NSR) and notch tensile strength (NTS) were calculated by testing notch tensile specimens. When compared to the bottom region, the top region showed a higher NTS of 658 MPa. The NSR is greater than one for both the specimens. It denotes that the cylindrical component made by CMT-WAAM is notch ductile and less sensitive to notches. The average values of ultimate tensile strength (UTS) and yield strength (YS) of the bottom region are found to be lower than the top region. The average UTS and YS values of specimens extracted from the bottom region are 401 MPa and 492 MPa, whereas the average UTS and YS values of specimens extracted from the top region are 457 MPa and 543 MPa, respectively. The anisotropy in the microstructure of the bottom and top regions might explain the reasons for the variations in YS/UTS of the CMT-WAAM cylindrical component.
Charpy impact toughness.
The impact toughness of the cylindrical component is lower in the bottom region than in the top region. It is observed that the impact toughness of the top region is 80 J, which is greater than the impact toughness of the bottom region. Ductility varies along the building direction in WAAM components. The differences in ductility are mostly attributed to the various types of microstructures evolved in two regions.
Figure 7: Vickers microhardness distribution on a different points on the surface along the building direction.
Microhardness analysis.
Figure 7 represents the total variation in microhardness of the manufactured cylindrical component along the building direction. The microhardness measurement (Figure 7) shows that the component has uniform hardness throughout. The component overall average microhardness was 173 (Hv0.5) from bottom to top. A minimal variation was observed in the building direction and this may be due to minor changes in microstructures. Table 6 depicts the average microhardness values recorded in the central part of the bottom and top regions. The maximum value of microhardness (181 (Hv0.5)) is related to the top region, while the lowest value of microhardness (169 (Hv0.5)) is related to the bottom region.
Table 6: Average microhardness of cylindrical component.
Macro and microstructure analysis.
The macrostructure of the bottom and top regions was examined in the middle of the extracted specimen. The macrostructure of the bottom and top regions are shown in Figure 8. In the macro images, the distinct three weld layers are clearly visible, and the weld beads are free of faults and defects,
Figure 8: Macrostructures of a) Bottom region, b) Top region.
and the layers are correctly fused. The weld is devoid of flaws and other visible faults, as shown in the macrostructure. The microstructure of the CMT-WAAM ER70S-6 steel cylinder component is shown in Figure 9a. The white regions and black areas appeared within the grains at boundaries confirmed the presence of ferrite grains and pearlite. Figure 9 depicts the microstructure of two regions of the as-built cylindrical wall. The bottom region is characterised by its interaction with the cold substrate prior to deposition; this zone had a lower thermal shock since the substrate was warm when the weld bead was deposited; and lastly, the top region has a greater thermal shock due to its contact with calm air at room temperature. The microstructure of the top region is noticeably different from that of the bottom region. The bottom region has a nearly equiaxed microstructure, whereas the top region has a lamellar microstructure. A micrograph of the bottom region is shown in Figure 9b, and it contains a large volume fraction of ferrite grains with a small fraction of pearlite grains (highlighted by red colour). Equiaxed grains can be observed where thin lamellae have been dispersed. Given the initial thermal shock caused by contact with the cold substrate, this microstructure can be described as ferrite grains and pearlite lamellae. The ER70S-6 is a low-carbon steel, which justifies the presence of ferrite with thin pearlite strips in the manufactured WAAM cylindrical component. The top region (Figure 9c) contains lamellar structures, and the higher magnification of those structures is shown in Figure 9d. This is the area where thermal shock shows a higher value and hence the microstructure is entirely different and of the lamellar type, which is characterised by lamellar structures of b B and αa.
Discussion
The stress-strain curves of all tensile specimens in both regions are presented in Figure 7. All specimens' strain-stress curves in two regions show typical behaviour of low-carbon steels. The average UTS and YS values in the bottom region were lower than those in the top region presented in Table 5. The average values of YS and UTS showed a small difference, but not so much. In comparison with ER70S-6 low-carbon steel (YS and UTS values are mentioned in Table 2), the values of YS and UTS of all tensile specimens are near to those of ER70S-6. The difference in the microstructure of the manufactured cylindrical component is the reason for the anisotropy in tensile strength of the bottom and top regions. The difference in the UTS of the bottom and top regions is mostly due to the heterogeneous nature of the microstructure. The different microhardness values along the building direction of the WAAM cylindrical component from the bottom region (lower hardness) to the top region (higher hardness) proved this. Similar behaviour was observed Chen at al. [14] made research on the microstructure characteristics and mechanical properties of austenitic stainless steel manufactured by WAAM technique. The author said that the peak temperature reduces the γ phase temperature to 600°C by increasing the number of depositions. Under these different high temperatures, the δ-phase slowly melts into the γ-phase and σ-phase and microstructure differs from coarse morphology to fine morphology. From top to bottom, the microhardness in the forming component rises. Wang et al. [15] manufactured inconel 625 part by WAAM. They found that the previously deposited layers experience different thermal cycles, which effect the segregation of Mo and Nb. The formation of the laves-phase mainly depends on the content of Nb and Mo, which leads to changes in the hardness and tensile properties along vertical and horizontal direction.
Figure 9: Optical micrographs taken from a the WAAM-ER70S-6 steel cylindrical wall, a) primary microstructure of ER70S-6, b) in bottom region, c) in top region with lower magnification, d) in top region in higher magnification.
The increment in the impact toughness, hardness and tensile properties from the bottom region to the top region is due to the formation of B and αa in the top region. In comparison with ferrite with a low volume fraction of pearlite (formed in the bottom region), αa and B show better tensile properties, hardness and toughness. Similar microstructures were reported by Liberini et al. [16]. The microstructure of the fabricated WAAM thin wall showed lamellar structures of B in the upper zone, a fully ferritic structure in the middle zone, and ferrite with pearlite in the lower zone. Aldalur et al. [17] observed a mixture of allotriomorphic ferrite, αa and B in the upper zone and polygonal ferrite with acicular ferrite formed in the reaming area of the ER70S-6 WAAM part. The author found that the heterogeneous microstructure is due to the variable energy input produced by deposition strategy.
Conclusions
In this study, the mechanical properties and microstructural features of a carbon steel (ER70S-6) cylindrical component manufactured via the CMT process were evaluated. The main conclusions drawn from this investigation are:
1. The macro photographs show the weld beads are free from defects, cracks and other visual defects and the layers are properly fused together.
2. The top region of cylindrical component made by CMTWAAM technique exhibited higher ultimate tensile strength,yield strength, higher hardness and toughness than bottom region due to formation of αa and B.
3. The microstructure of the built cylindrical component varies from the bottom to the top region due to the differences in thermal histories. The bottom region is composed of ferrite and fine pearlite, and the top region consists of B and αa.
Acknowledgements
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.
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