Mechanical properties of wire arc additive manufactured carbon steel cylindrical component made by gas metal arc welding process

2.1 Materials and fabrication process

Table 1 shows a typical composition of low carbon steel ER70S-6 wire with 1.2 mm diameter that was used for the current study, as well as mechanical properties are presented in Table 2. Layer by layer deposition was performed on a mild steel substrate with dimensions of 250×250×10 mm. A welding machine (CMT Advanced 4000R) (Figure 1) was used to fabricate the cylindrical component on the steel plate based on the AM principle with constant current GMAW process. The motion of the welding torch were performed by three-axis automatic motion system shown in (Figure 1), utilizing the welding machine setup with rotating table. The optimized welding parameters used to fabricate the cylindrical component are presented in Table 3. The substrate should be ground and cleaned before deposition and the arc torch kept constant perpendicular to the substrate of the surface during the deposition process.

Figure 1

GMAW-WAAM platform used for fabrication.

The one-pass deposition has a width of 7.8±2 mm and a height of 2.2 mm. A pause of 120 s was imposed between each layer deposition, in order to enable a partial cooling of the deposited material. The welding wire extended 15 mm from the nozzle (contact tip to workpiece distance). The used WAAM platform, the fabricated wall, and its graphical representation are shown in Figure 2. The measurements of the fabricated cylindrical component, including the thickness, height and diameter were presented in Table 4. The diameter, height of manufactured cylindrical component was about 127 mm and 160 mm, respectively. Figure 3 shows fabricated cylindrical wall component side view a) and top view b). Following the fabrication process, the deposited material (5 mm) removed at the end of component. The base plate with deposited material (5 mm) was removed from the built component to prevent the effect of dilution on mechanical and microstructural characteristics. The fabricated component was separated to in two regions: Bottom region (from substrate to 75mm height) and Top region (from 75 mm to end of component) and schematically shown in Figure 2. The fabricated component was machined by a CNC lathe and specimens were extracted by an automatic shaper machine. The component was machined to inspect the quality of depositions. The fabricated cylinder wall is free from macro level defects and cracks. The photographs of the machined cylindrical components (with 4 mm wall thickness) are displayed in Figure 4.

Figure 2

Schematic illustration of the steel cylindrical component a) indicating direction of deposition, b) showing separation of component in two regions (b-1(bottom region) and b-2(top region)) and c). extraction of samples from two regions.

Figure 3

Photograph of the Low carbon steel straight cylindrical component (as fabricated) a) side view and b) top view.

Figure 4

Photograph of low carbon steel cylindrical component (after machining), a) side view, b) top view and c) showing wall thickness.

2.2 Mechanical properties evaluation

Figure 5 a) and b) show the dimensions of the smooth and notch tensile specimens. As per ASTM E8M standard, the tensile testing was done using a 50 kN servo controlled testing system (H50KL). At room temperature, the tensile tests were done and the transverse head displacement speed was 2 mm/min. In the 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. Three specimens were tested from each region and the average value was reported for analysis purpose. The charpy impact specimens were tested using pendulum type Impact Testing Machine (Make & Model: FIE-BLUE STAR, India & 1E-IT-30(ASTM)). The charpy impact samples were prepared as per ASTM A370 sub-size standard and dimensions are shown in Figure 5 c). Three impact specimens were tested from top region and bottom region and the average of three was reported for analysis purpose. The tested tensile and impact specimen fracture surfaces were analyzed by scanning electron microscopy (model: JSM 6610LV). Microhardness was measured as per the procedure prescribed by ASTM E384-17 standard on the fine polished surface of the metallographic samples with a testing load of 500 gms and 15 s dwell time. Vickers Microhardness Testing Machine (SHIMADZU HMV-2T) was used to record microhardness at different regions and along the building direction.

Figure 5

Dimensions of Test Specimens a) smooth tensile specimen, b) notch tensile specimen and c) impact toughness specimen.

2.3 Macrostructure and microstructural characterization

The microstructural analysis was carried out at the center portion of top region and bottom region of the mirror polished microstructure samples etched by 2% Nital reagent. In order to examine the macrostructure of the manufactured component, the stereozoom microscope was used. The etched metallographic samples were then analysed under an optical microscope (MIL-7100) and the microstructure was recorded at different magnifications.

3.1 Tensile properties

The tensile properties of the fabricated cylindrical component evaluated at different regions (top and bottom) are presented in Table 5. The tensile properties of two regions are lower than the filler metal (all weld metal). Figure 6 a) and b) show the image of notch and smooth tensile specimens after testing. From the tensile test results, it was found that the average values of yield strength (YS) and ultimate tensile strength (UTS) in the bottom region was lower than the top region. The mean values of YS and UTS of the specimens extracted from bottom region are 333 MPa and 429 MPa, respectively, whereas the average values of YS and UTS of specimens extracted from top region are 350 MPa and 446 MPa. Stress-strain curves of the top and bottom region samples are shown in Figure 7. The samples extracted from top region showed higher strength with lower elongation than bottom region. It is due to the heterogeneous microstructure of deposited cylindrical wall. The notch tensile strength (NTS) and notch strength ratio (NSR) are determined by performing a tensile test on notch tensile specimens. The notch strength ratio (NSR) was calculated using the Eq. (1).

Figure 6

The photographs of the tested specimens, a) smooth tensile specimens, b) notch tensile specimens and c) impact toughness specimens.

Figure 7

Stress–strain curves of the tensile specimen.

It is observed that the top region samples were showed maximum NTS of 589 MPa than bottom region. The NSR is more than one in all samples. It suggests that the samples under the class of ductile and are less susceptible to notches. So, the GMAW-WAAM cylindrical wall components are ductile in nature.

3.2 Impact toughness

Table 5 shows the impact toughness in bottom and top regions of GMAW-WAAM cylindrical component. The photograph of the impact specimens after testing is shown in the Figure 6 c). The impact toughness of cylindrical wall component decreased from bottom region to top region. The impact toughness values were opposite to the tensile properties of bottom region and top region. It is observed that the bottom region samples were recorded highest impact toughness of 69 J, which is higher than top region. It is due to the variation in ductility of top region and bottom region. The variation in ductility is common in WAAM components along the building direction. The variation in ductility is mainly due to the different type of microstructures formed in two regions. In comparison with ER70S-6 filler metal, the value of the impact toughness of deposited weld metal is lower.

3.3 Microhardness survey

The Vickers microhardness variation in the deposited component from the bottom to the top region along the vertical direction is shown in Figure 8. The overall average micro-hardness was 161 ± 3 (Hv_0.5) across the entire component, with minimal fluctuation in the building direction due to changes in the microstructure. The evenly distributed microhardness values have a minor deviation (± 3 Hv_0.5) from the average microhardness value (161 HV), attributed to the presence of various micro-constituents as described in the microstructure characterization section. The average microhardness values are taken in center portion of bottom and top regions presented in Table 6. The highest value of microhardness (166 Hv_0.5) was recorded in the top region and bottom region recorded the lowest value of microhardness (157 Hv_0.5). The difference in microhardness is due to the different microstructures evolved in top region and bottom region.

Figure 8

Vickers microhardness variations on the surface of the component along the building direction.

3.4 Macrostructure and microstructural characterization

The macrostructure analysis was done on the specimen extracted from center portion of the top region and bottom region of cylinder component. Figure 9 a) and b) show the macrostructure images of bottom and top regions, respectively. The different 3 weld layers are clearly visible in the macro photographs and the photographs show the weld beads are free from major level defects and cracks and the layers are properly fused together. The macro photograph shows that the weld is free from cracks and other visual defects.

Figure 9

Macrostructures of the cylindrical component a) bottom region and b) top region.

Figure 10 a) displays the microstructure of the cylindrical component. From the micrograph, it is confirmed the presence of pearlite in ferrite matrix. The white regions are ferrite and dark areas are pearlite formed with in the grain at boundaries. The microstructure of two regions is the as-built cylindrical component is shown in Figure 10 b), c) and d). It is clear that the microstructure of the top region significantly differs from the microstructure of the bottom region. In fact, the cylindrical component of bottom region was characterized by almost equivalent microstructure, while the lamellar structures were observed in top region. The bottom region micrograph is shown in Figure 10 b). The bottom region is made of pure ferrite with roughly equal grains. Given the initial thermal shock caused by contact with the substrate, it is possible to see in the micrograph that this microstructure is characterized by complete ferrite grains. In Figure 10 c), the top region revealed the lamellar structures composed with polygonal ferrite (PF), as well as three kinds of ferrite grains: ferrite side plates with real plate like structures (α_w), the presence of brittle pearlite structures along the boundaries (GBF), and α_a shown in Figure 10 d). It is due to the effect of the constant cooling of the natural air and heat transfer toward the bottom of the built component.

Figure 10

Optical micrographs of WAAM component, a) primary microstructure of ER70S-6, b) in bottom region, c) in top region with lower magnification and d) in top region in higher magnification.

3.5 Fractography

Figure 11 and 12 show the fractographs of the bottom and top regions of smooth, notch and impact specimens at different magnifications. In response to fracture, all of the tensile and impact specimens had a ductile morphology with many deep and elongated dimples. There is no major difference in the fracture characteristics of all the specimens that have been taken from different regions (bottom and top). However, the fracture surface of the smooth tensile specimen differs from the bottom to the top region. The fracture surface of the top region specimen presents small micro voids, cleavage faces indicated in Figure 12 b). Figure 12 c) shows the high magnification image of the smooth tensile fracture surface of the top region. The top region of the smooth tensile fracture surface consists of small dimples, indicating that the fracture is ductile. The high strength and lower elongation are due to the fine dimples of the top region of the WAAM carbon steel cylindrical component. Similarly, the bottom region of the smooth tensile specimen presents large dimples indicating that the fracture is ductile in Figure 11 b). Figure 11 c) shows the high magnification image of the smooth tensile fracture surface of the bottom region. The bottom region of the smooth tensile fracture surface consists of more elongated dimples compared to the top region due to the microvoid coalescence effect. The high ductility is due to the more elongated dimples in the bottom region of the manufactured cylindrical component. Similar behaviour was observed by Zidong Lin et al. [17] in GMAW based WAAM medium carbon steel parts. The tensile strength of the horizontal sample showed high strength and lower elongation due to fine dimples, and more elongation and low strength was observed in vertical direction samples due to more elongated large dimples. The author concluded that the dimple size in the horizontal samples was smaller compared to the vertical samples due to the micro-void coalescence effect.

Figure 11

SEM fractographs of the specimen extracted from bottom region, (a–c) smooth tensile, (d–f) notch tensile and (g–i) charpy impact.

Figure 12

SEM fractographs of the specimen extracted from top region tested specimens, (a–c) smooth tensile, (d–f) notch tensile and (g–i) charpy impact.

The fracture surfaces of the bottom and top regions of the notch tensile specimens present cleavage and dimples as indicated in Figure 11 and 12. Hence, the mode of failure of the notch tensile specimens happens in a qausi-cleavage mode regardless of regions (bottom and top) similar to the results reported by Addanki Ramaswamy et al. [18]. The author observed that both cleavage and dimples had minor voids in the tested notch tensile samples.

Figure 11 (g–i) and 12 (g–i) show the fracture surfaces of the bottom and top regions of impact specimens. The top region of the tested impact specimen presents dimples with cleavage faces and dimples with small secondary cracks are formed in the bottom region of the manufactured carbon steel cylindrical component. All the impact specimens present ductile morphology with dimples in response to fracture. The high toughness in the bottom region of the component is due to the shallower dimples with secondary cracks. The creation and motion of dislocations in the crystal lattice are responsible for the plastic deformation. The component dissipates energy during the dislocation movements and crack tip dislocation leads to intrinsic ductility. So, the secondary cracks act as a crack divider impact loading [19].

3.6 Discussion

The stress versus strain curves recorded during tensile testing of the specimens extracted from the two regions (bottom and top) of the cylindrical components made by GMAW-WAAM are depicted in Figure 7. Additionally, Table. 4 presents the extracted tensile properties, including the YS and UTS of WAAAM cylindrical component along building direction in two regions. The average values of YS and UTS showed small difference, but not so much. Similarly Sridharan N et al. [11] also observed changes in YS and UTS along the horizontal and vertical direction of WAAM steel part. Figure 7 clearly shows that the elongation was reduced in the top region than the bottom region, confirming the change in ductility. In WAAM components, the change in ductility along the building direction is common. [20]. Beth E. Caroll et al. fabricated Ti-6Al-4V linear wall part by direct energy deposition technique. The author stated that the enhancement in ductility is partially due to the absence of pores in the component, and the presence of grain boundary α leading to fracture and responsible for the anisotropy in ductility. The change in the tensile strength of the bottom and top region samples is mostly attributed by the heterogeneous of the microstructure, as confirmed by the different microhardness values from the bottom region to the top region of the WAAM-ER70S-6 cylindrical component.

On the other hand, the reduction in the toughness from the bottom region to the top region is due to the formation of brittle α_w, GBF and PF in top region. Due to hard phase of α_w, GBF and PF, the hardness was increased in top region is one of the reasons for higher strength also; due to high rate of cooling in top region, it causes the microstructural transformation from austenite to α_w and GBF. Haselhuhn et al. [21] documented that the change of PF to α_w is due to the change in cooling rate from the center of melt pool to melt pool boundaries in a WAAM-ER70S-6 wall resulted in variation in tensile properties and hardness along the building direction. The differences observed in the microstructures are due to the variations in thermal history experienced by the multiple welding beads deposited, in which the top region is affected by the stronger thermal shock.

Wilson Heid et al. [22] observed the relation between change in ductility and microstructure of WAAM-Ti-6Al-4V alloy and reported that the% of elongation was higher in deposition direction than in building direction. In the WAAM Ti-6Al-4V alloy, Wang et al. [23] also observed the anisotropic in mechanical properties, showing high ductility and low strength in building direction than in the horizontal direction. The authors reported that the difference in columnar growth of Ti grains during solidification might be reason for variation in mechanical properties along deposition direction and building direction. The highest toughness was observed in bottom region, PF occurred in the form of coarse ferrite islands inside the prior austenite grains in top region. The presence of brittle pearlite structures along grain boundaries is often referred to as GBF and ferrite side plates are also referred to as was α_w were detrimental to toughness with its coarse grain structure in top region. Similar behaviour was observed Salimi A et al. [24] in banded steel structures. The author documented the change in impact toughness in the deposition direction and the building direction as a result of the built part's heterogeneous microstructures.