2.1 Base Metal and Welding Consumables

The base material used in this study was 1.0 mm thick cold-rolled CP1180 MPa AHSS (0.2C-2.4Mn-1.3Si). This steel was chosen for its high yield strength (886 MPa) and tensile strength (1185 MPa), critical for automotive structural components such as chassis and seat frames. Nital etching and observation under optical microscope and SEM revealed the microstructure of the BM primarily consists of martensite, along with ferrite matrix with second phase islands such as bainite shown in Fig. 1. BM microstructure of 1180MPa steel used in this study. (a) Optical microstructure (b) SEM microstructure (a) and (b) and retained austenite in small amounts (~<5%) is also reported as per the reported literature^[28].

Fig. 1. BM microstructure of 1180MPa steel used in this study. (a) Optical microstructure (b) SEM microstructure

Two commercially available welding consumables were selected to assess the influence of consumable strength level on the microstructure and mechanical properties of the welds: ER70S-6 and ER110S-G. The chemical compositions and tensile strengths reported in this study correspond to the actual consumables used, rather than only the minimum nominal requirements of the American Welding Society (AWS) classifications. The ER70S-6 consumable (AWS A5.18) contained 0.08 wt.% C, 1.45 wt.% Mn, and 0.90 wt.% Si, with a tensile strength of 530 MPa. The ER110S-G consumable (AWS A5.28) contained 0.10 wt.% C, 1.60 wt.% Mn, 0.60 wt.% Si, 1.40 wt.% Ni, 0.30 wt.% Cr, and 0.30 wt.% Mo, with a tensile strength of 900 MPa. Since the “G” classification in AWS A5.28 does not imply a single fixed chemistry, the actual alloying elements of the wire used in this study are explicitly reported here. These alloying additions are important because they influence hardenability, bainitic transformation, and fracture behavior. Both consumables were used with a wire diameter of 1.0 mm. For clarity, welds made using ER70S-6 are referred to as ER70 welds, while those made using ER110S-G are designated as ER110 welds throughout this study.

2.2 Welding Procedure

The design of the APWs involved creating a 6.0 mm diameter hole in the top sheet, as illustrated in Fig. 2a. Generally, RSW welding of steel requires a minimum of $4\sqrt{t}$ (t-thickness) to ensure pull-out failure^{[29, 30]}. Subsequently, plug welding is compared with RSW and found that APW hole diameter of minimum 6.0 mm is the minimum required size to ensure $4\sqrt{t}$ effective weld length (EWL) to ensure pullout failure. The sheets were then arranged in a lap joint configuration, preparing them for welding. In the next stage, the placement of the welding consumable was precisely controlled. Fig. 2b shows the alignment of the contact tip-to-work distance (CTWD), set at 15 mm. The actual welding procedure, as depicted in Fig. 2c-e, was carried out using a GMAW technique. The process utilized a Fronius power source (Transpuls synergic 3200) in conjunction with a YASKAWA robotic arm welding machine, which ensured consistent and reproducible results across all samples. Welding was performed in the 1G position with a fixed welding time of 1 second. This controlled time frame facilitated the complete filling of the plug hole, resulting in an EWL of approximately 6.0 mm at the sheet/sheet interface. A shielding gas mixture of 80% Argon and 20% CO₂ was used to protect the weld pool from oxidation, with a flow rate of 20 L/min. Optimization of the parameters have been ensured by plug welding different samples by varying the current, voltage and wire feed rate as shown in Fig. 3. A weld window has been created showing the no bonding condition, good joining and burning through regions. The optimized parameters for each consumable type were selected to achieve uniform weld deposition along with an EWL above 6 mm, as outlined in Table 1.

Fig. 2. Schematic representation and experimental setup of the APW methodology. (a) Diagram of the specimen dimensions, highlighting a 6.0 mm circular hole for welding. (b) Illustration of the Contact Tip to Work Distance (CTWD) and placement of the welding torch above the circular hole before welding. (c-e) Step-by-step APW process: (f) Micro-Vickers hardness measurement approach. Specimen dimensions for (g) Tensile shear testing and (h) cross tension testing.

Fig. 3. (a) Weld window indicating no bonding region, plug welds and burn through region for ER70 and ER110 weld conditions (b) Incomplete fill (c) Complete fill (d) Burn through

2.3 Metallographic Examination

After welding, the samples were sectioned through the center of the weld for metallographic analysis. The cross-sections were mechanically polished using standard procedures described as per ASTM E3-11(2017) Standard Guide for Preparation of Metallographic Specimens^[31], followed by etching with 3% Nital to reveal the macro and microstructural features. Initial examinations were performed using an optical microscope (Olympus BX51M) to assess the effective weld length and overall geometrical features of the welds, followed by microstructural identification. For a more detailed microstructural analysis of the WZ and CGHAZ, scanning electron microscopy (SEM; JEOL JSM-7200F) and electron backscatter diffraction (EBSD; Oxford Instruments) were employed at the Converging Materials Core Facility of Dong-Eui University. This analysis allowed for phase identification, while EBSD confirmed the phases present, offering more profound insights into the microstructural characteristics of the welds^[32]. In the CGHAZ, the distinction between lath bainite (LB) and granular bainite (GB) was made using a combined criterion based on SEM morphology and EBSD features, rather than a single parameter. LB was identified by elongated/lath-like morphology with locally aligned packets and a high density of parallel lath boundaries (typically expressed as frequent low-angle boundary segments in EBSD misorientation maps), whereas GB was identified by a more irregular/equiaxed granular morphology with less directional alignment and more heterogeneous local orientation gradients.

Quantitative phase fraction analysis was carried out on the WZ and CGHAZ using scanning electron microscopy (SEM) images combined with digital image analysis. For each weld condition (ER70 and ER110), a minimum of five representative SEM micrographs were acquired at ×2000 magnification from different regions of the WZ and CGHAZ to ensure statistical reliability. The images were analyzed using ImageJ software, where a thresholding and segmentation procedure was applied to distinguish between AF, GB, LB, and GBF. The gray-level contrast of the microstructural constituents was calibrated against reference micrographs and confirmed by morphology. The area fraction of each phase was determined by pixel counting, which, under stereological assumptions, corresponds to the volume fraction.

2.4 Microhardness and Mechanical Testing

Microhardness profiles across the weldments were determined using a Vickers hardness tester with a 300 g load applied for 10 seconds. Measurements were systematically taken across the weld cross-section to capture hardness variations. The Vickers microhardness measurements were taken at intervals of 0.2 mm to ensure accurate representation while minimizing any potential effects of strain fields around the indentations, as shown in Fig. 2f.

For mechanical testing, tensile-shear samples and cross-tension testing samples were prepared by AWS standards^{[27, 28]}, as shown in Fig. 2g and Fig. 2h, respectively. These tests were conducted at a constant crosshead speed of 10 mm/min. For each welding condition three specimens were tested to ensure repeatability. The reported load–displacement curves represent the average behavior, and error bars or shaded regions in the plots indicate the standard deviation across the three repetitions. The fracture load and corresponding load-displacement curves were recorded for each sample to assess the tensile shear performance of the different welds.

2.5 Fractography Analysis

Fractography analysis was performed on the fractured samples after mechanical testing to comprehensively understand failure mechanisms. The initial examination involved surface inspections using optical microscopy to identify macroscopic fracture features. Subsequently, a more detailed analysis was carried out using SEM (JEOL JSM-7200F electron microscope) Oxford EBSD software at the Converging materials core facility at Dong-Eui University to examine the microstructural details associated with crack initiation and propagation. For a thorough assessment, samples were cross-sectioned to expose internal crack paths, while other specimens were analyzed without cross-sectioning to examine surface fractures directly. This approach provided insights into the crack initiation sites, propagation paths, and their correlation with the microstructural features observed in the WZ and CGHAZ. Additionally, cross-tension testing samples were subjected to similar analysis to determine fracture modes, correlating fracture behavior with mechanical properties and microstructure.

3.1 Macroscopic analysis of Arc-Plug welds

Fig. 4 presents the surface and cross-sectional images of APWs created using both ER70 and ER110 welding consumables. Fig. 4a and Fig. 4d show the top surfaces of the welds made with ER70 and ER110, respectively, while Fig. 4b and Fig. 4e display the corresponding bottom surfaces. These images show that both welds produced circular shapes on both the top and bottom surfaces, indicating consistent deposition and weld formation. A small cavity is observed on the top surface of both welds, which could be attributed to the shrinkage cavities formed towards the end of the welding. Pouranvari et.al. in his work reported porosity factor which has inverse proportionality with critical weld size^[33].

Fig. 4. Surface and cross-sectional views of APWs: Comparative images of ER70 and ER110 welds showing (a, d) Top sheet surface, (b, e) Bottom sheet surface, and (c, f) Cross-section at low magnification.

However, the porosity size below 10% as in cross sections shown in Fig. 4c and Fig. 4f can be permitted from the manufacturer’s guideline since cavity driven porosity is inevitable in Zn coated steels.

Although small top-surface shrinkage cavities and micro-porosity were observed in both weld conditions, their size remained within the manufacturer guideline range and no marked difference in cavity severity was identified between ER70 and ER110 welds. Therefore, their influence on the comparative tensile-shear and cross-tension performance is considered limited in the present study, and the observed differences in load-displacement response and fracture behavior are primarily attributed to microstructural/hardness variations rather than geometric pore defects. This interpretation is also supported by the consistent pullout-type failure morphology and notch-driven crack initiation observed in the fractography analysis.

The cross-sectional images, depicted in Fig. 4c and Fig. 4f, provide a more detailed view of the weld morphology, mainly focusing on three critical geometrical features: the Top Reinforcement Length (TRL), EWL, and Bottom Penetration Length (BPL). These dimensions are crucial for understanding the overall integrity and mechanical performance of the welds. For the ER70 welds, the TRL was measured at 8.5 ± 0.4 mm, the EWL at 6.1 ± 0.2 mm, and the BPL at 6.8 ± 0.3 mm. Similarly, the ER110 welds exhibited a TRL of 8.3 ± 0.3 mm, an EWL of 6.0 ± 0.3 mm, and a BPL of 7.0 ± 0.3 mm. These measurements show that the welds produced with both types of welding consumables show similar lengths across the top, middle and bottom sections of the welds with only slight variations. A key observation from these measurements is that the TRL is consistently longer than both the EWL and BPL for both ER70 and ER110 welds. This trend can be explained by the deposition behavior of the welds, which aligns with findings from previous literature^{[16, 23, 34]}. The initial deposition during welding tends to occur at the center of the hole, where the welding consumable first contacts the bottom sheet. As the welding progresses, heat energy generated by the process aids in melting both the welding consumable and the bottom sheet, leading to material deposition primarily at the top sheet in the later stages of welding. This results in a greater accumulation of material at the top, contributing to the longer TRL.

These findings are consistent with previous studies, which have similarly reported the relationship between TRL, EWL, and BPL in welds produced under comparable conditions^{[37, 38]}. The analysis of the weld morphology, as illustrated in Fig. 4, highlights the effectiveness of the welding parameters in achieving consistent weld dimensions across different weld sections. The observed dimension similarity between the welds produced with ER70 and ER110 consumables underscores the reliability of the optimized welding conditions. The longer TRL compared to EWL and BPL is indicative of the deposition patterns during welding, which agrees with existing literature on the subject^[17].

3.2 Microstructural characterization after welding

The microstructural characteristics of the APWs fabricated with ER70 and ER110 welding consumables were analyzed using optical microscopy and SEM to identify differences in the WZ and CGHAZ. As illustrated in Fig. 5a and Fig. 5b, the cross-sectional view highlights the areas of interest marked for detailed microstructural analysis. WZ and CGHAZ are particularly examined due to their importance in determining the failure mode as per previous reports^[25].

In the WZ, Fig. 5e for ER70 welds reveals the presence of GBF, AF, and quasi-polygonal ferrite (QPF). These phases contribute to a more ductile weld, enhancing toughness. In contrast, the ER110 WZ, shown in Fig. 5f, predominantly consists of LB and GB, with a smaller fraction of AF. This microstructural composition in ER110 indicates a more complex, more brittle weld, aligning with the higher strength properties of the ER110 consumable^[25].

The CGHAZ microstructures are depicted in Fig. 5g for ER70 and Fig. 5h for ER110. Both welds exhibit bainitic structures; however, the ER110 welds demonstrate a finer and more uniform distribution of LB and GB than the ER70 welds. This refined microstructure will likely contribute to the increased hardness and strength observed in mechanical testing for the ER110 welds. These observations are consistent with previous studies, which reported similar phase distributions in the CGHAZ of welds made with high-strength steels^[25].

Multiple SEM micrographs were analyzed to determine the phase fractions within the WZ and CGHAZ to further quantify these microstructural differences. The quantitative data is represented in bar charts in Fig. 5a-b. For the ER70 WZ, the microstructure predominantly comprises 65% AF and 35% GBF, suggesting a weld with enhanced ductility due to the higher fraction of AF. In contrast, the ER110 WZ comprises approximately 48% AF, 20% GB, and 32% LB, indicating a more robust but less ductile weld structure. In the CGHAZ, ER70 and ER110 welds predominantly feature LB and GB phases, although the ER110 welds show a more refined phase distribution. This refined microstructure contributes to the increased strength and hardness of the ER110 welds^[27].

Fig. 5. Cross sectional images of APW with investigation area highlighted (a) ER70 (b) ER110 Welds. (c-d) Optical micrograph of the region of interest (c) ER70 and (d) ER110 welds. SEM Micrographs: (e)ER70 WZ (f) ER110 WZ (g) ER70 CGHAZ (h) ER110 CGHAZ.

Fig. 6. (a-b) Quantified phase fractions in the WZ and CGHAZ for ER70 and ER110 welds

For clarity, the LB/GB classification in the CGHAZ was interpreted from the combined appearance of IQ/IPF maps and line-misorientation behavior: regions showing elongated bainitic units with repeated fine misorientation peaks along aligned boundaries were classified as LB, while regions with non-lath, granular bainitic morphology and comparatively discontinuous/heterogeneous misorientation variations were classified as GB. This combined interpretation was used consistently for both ER70 and ER110 welds. To confirm the phases identified in the optical and SEM images, additional analysis was conducted using EBSD. This step was crucial due to the complexities involved in confirming bainitic structures solely based on morphology.^{[32, 36]}. The EBSD images, presented in Fig. 7 and Fig. 8, provide a comparative analysis between the WZ and CGHAZ for ER70 and ER110 welds respectively.

For the ER70 WZ, misorientation mapping was conducted along lines AA' and BB'. The data from line AA' (Fig. 7e) displayed two significant peaks corresponding to primary grain boundaries and multiple smaller peaks indicative of sub-boundaries. These features are characteristic of GBFs. In contrast, line BB' (Fig. 7f) exhibited more frequent and pronounced peaks, which are typical of AF due to its distinct substructural morphology^[37]. These observations confirm the presence of GBF and AF phases, suggesting a relatively ductile microstructure in the ER70 WZ. In the ER110 WZ, misorientation analysis was performed along lines CC' and DD'. The results along line CC' revealed two distinct zones: the first with a large misorientation angle exceeding 50°, which is indicative of LB, while the second zone along line DD' (Fig. 7g-h) showed higher peaks within the grains, confirming the presence of GB. Additionally, AF was noted, exhibiting similar characteristics to those observed in the ER70 WZ but in lower proportions. The overall structure of the ER110 WZ is more complex, with a combination of LB, GB, and AF phases contributing to its increased hardness and reduced ductility^{[37, 38]}.

The EBSD analysis revealed that the ER70 WZ is predominantly characterized by GBF and AF phases, which contribute to a more ductile weld structure with enhanced toughness. In contrast, the ER110 WZ displayed a combination of LB, GB, and AF, resulting in a harder but more brittle structure. The higher strain localization observed in the ER70 WZ, as indicated by the misorientation angles, suggests better crack propagation resistance than the more intricate grain structure of the ER110 WZ.

In the CGHAZ, the EBSD IQ maps and IPF images (Fig. 8) showed a generally uniform grain structure for both welds but with notable grain size and morphology differences. For the ER70 CGHAZ, misorientation mapping confirmed the presence of LB and GB phases with relatively lower misorientation angles, suggesting a more relaxed microstructure. The larger grain boundaries in this region are consistent with softer and more ductile behaviour. For the ER110 CGHAZ, misorientation mapping along lines AA' and BB' (Fig. 8b, e, f) confirmed the presence of GB and lath boundaries, which were identified by frequent peaks with low misorientation angles. The analysis along lines CC' and DD' (Fig. 8g-h) revealed a more refined and uniform grain structure, particularly with LB and GB phases, indicating higher hardness and strength. These refined grains are likely contributing to the enhanced performance of the ER110 welds under mechanical testing^[38].

A mix of LB and GB characterizes the CGHAZ of both ER70 and ER110 welds but with significant differences in the grain structure. The ER70 CGHAZ exhibited larger grain sizes, promoting ductility, whereas the ER110 CGHAZ showed a finer and more uniform grain distribution, which correlates with its higher hardness. The misorientation mapping further highlighted the complex grain structure in the ER110 CGHAZ, aligning with the observations from the SEM analysis.

Fig. 7. Combined EBSD analysis of WZs: (a-b) IQ and IPF images for the ER70 weld; (c-d) IQ and IPF images for the ER110 weld. Misorientation line maps: (e) AA' showing GBF, (f) BB' showing AF, (g) CC' showing GB, and (h) DD' showing AF.

Fig. 8. CGHAZ EBSD comparison: ER70: (a) IQ, (b) IPF; ER110: (c) IQ, (d) IPF. Misorientation: (e) AA' showing GB, (f) BB' showing LB, (g) CC' showing GB, (h) DD' showing LB.

3.3 Microhardness

Fig. 9 illustrates the microhardness distribution across the cross-section of the APWs for both the ER70 and ER110 welding consumables. The BM exhibited an average hardness value of approximately 365 HV, as determined from readings taken symmetrically on either side of the weld. This baseline is a reference point for comparing the hardness variations observed in the WZ and the HAZ of both welds.

Fig. 9. Microhardness distribution across the welds.

A distinct hardness difference was observed between the two consumables in WZ. The ER70 welds showed a lower hardness of around 300 HV, whereas the ER110 welds exhibited a significantly higher hardness, averaging around 450 HV. This variation in hardness can be directly correlated to the phase fractions identified in each WZ. As detailed in Fig. 6a and Fig. 6b, the ER110 WZ consists of a complex microstructure comprising 48% AF, 20% GB, and 32% LB. The presence of LB and GB, known for their high hardness and strength, accounts for the increased hardness in ER110 WZ. Conversely, the ER70 WZ predominantly comprises AF and GBF, with 65% and 35% phase fractions, respectively. The higher proportion of AF, typically less hard than LB and GB, results in lower hardness in the ER70 WZ.

The hardness profiles in the HAZ were consistent across both welds, with minimal variation. However, within the CGHAZ, which is adjacent to the WZ, a notable increase in hardness was detected. The CGHAZ of the ER110 welds demonstrated a slightly higher hardness than the ER70 welds. This difference can again be attributed to the phase distribution within the CGHAZ, as represented in Fig 6b. Quantitative analysis revealed that the CGHAZ of the ER110 welds comprised 55% LB and 45% GB, whereas the ER70 welds had a CGHAZ composed of 42% LB and 58% GB. The higher fraction of LB in the ER110 CGHAZ contributes to its increased hardness, aligning with the microstructural observations from the optical and SEM analyses.

In addition to the differences in C, Mn, and Si contents, the higher hardness and bainitic microstructure observed in the ER110 welds are also interpreted in relation to the additional Ni-Cr-Mo alloying of the ER110S-G consumable, which increases hardenability and promotes low-temperature transformation products. This alloying effect is considered to contribute to the higher weld zone hardness and the relatively brittle fracture tendency observed in the ER110 welds^{[25, 27]}.

3.4 Tensile shear testing

Figure 10 (a) compares the load–displacement responses of ER70 and ER110 welds. The plotted curves correspond to the average of three repeated tests per condition. The average maximum load for ER70 welds was 13.8 ± 0.4 kN, while that for ER110 welds was 15.2 ± 0.3 kN, where the values after “±” represent the standard deviation. The relatively small scatter indicates good repeatability of the tensile shear test results. The peak load of ER110 welds has marginally superior mechanical properties compared to ER70 welds.

Both types of welds experienced bottom pullout failure, as depicted in Fig. 10b-g. This failure mode is characterized by the complete detachment of the weld button from the bottom sheet, which may remain attached to the top sheet. This phenomenon is visible in the fracture surface images in Fig. 10b and Fig. 10e. The detailed fracture surfaces of the ER70 welds are presented in Fig.10c and Fig. 10d, where the complete pullout of the button and the resulting hole in the bottom sheet can be observed. Similarly, Fig. 10f and Fig. 10g depict the fracture surfaces of the ER110 welds, where the button is equally detached from the bottom sheet, indicating a consistent failure mode across both weld types.

The surface images from Fig. 10b and Fig. 10e confirm that in both welds, the weld button was completely pulled out from the bottom sheet and attached to the top sheet. The bottom pullout failure is further corroborated by the images in Fig. 10c and Fig. 10d for the ER70 welds and Fig. 10f and Fig. 10g for the ER110 welds. In both cases, the button remains attached to the top sheet, while a hole is observed in the bottom sheet, confirming the nature of the failure.

Additionally, signs of yielding can be observed on the fractured surfaces of the ER70 welds in Fig. 10c and Fig. 10d, and similar observations are made for the ER110 welds in Fig. 10f and Fig. 10g. The consistent bottom pullout failure in both welds may be attributed to differences in weld geometrical features, particularly the disparity between the TRL and BPL. The longer TRL compared to the BPL in all cases suggests that cracks originating from the notch propagate more readily into the bottom sheet than the top sheet, contributing to the observed failure mode.

Despite having similar nugget sizes and geometrical attributes, the slight differences in TSS between the ER70 and ER110 welds suggest that the microstructural characteristics play a crucial role in determining mechanical performance. The load-displacement curves for both welds share similar characteristics, with a linear increase in load up to the peak value, followed by two distinct drops in the curves.

Fig. 10 (a) Load-displacement curves from the TST; (b) and (c) show the fracture surfaces of ER70 and ER110 welds, respectively, with pullout failure; (d) and (e) display the bottom sheet and interface at the top sheet of the ER70 fracture sample; (f) and (g) show the interface and top sheet view of the ER110 fracture sample.

3.5 Fractography analysis

To gain detailed insights into the fracture behavior of ER70 and ER110 welds, a comprehensive fractography analysis was conducted using two complementary techniques: cross-sectional analysis and SEM observation of non-sectioned samples. The primary objective was identifying the crack initiation sites, tracing the crack propagation paths, and correlating the observed fracture features with the underlying microstructural characteristics and hardness distribution.

Cross-sectional views of the fractured samples (Fig. 11a and Fig. 11c) for both ER70 and ER110 welds reveal the primary crack locations responsible for the observed pullout failures. The fracture samples were sectioned along the loading axis, focusing on the bottom sheet where fracture predominantly occurred. In both welds, the bottom sheet fractured along the loading direction, with the top sheet exhibiting fractures towards the end of the tensile test due to the bending moment.

In ER70 welds, as shown in Fig. 11b, the crack path is initiated at the notch and propagated through the WZ or fusion boundary. In contrast, in ER110 welds (Fig. 11d), the crack also originated at the notch but propagated through the CGHAZ. The notch-induced crack initiation is a characteristic feature of such welds, as documented in previous studies^{[16, 20, 21]}.

The differences in crack paths can be attributed to the variations in hardness and microstructural features across WZ. As highlighted by^{[30, 39-42]}., the hardness distribution is critical in influencing the crack path during TST of spot welds. The lower hardness in the ER70 WZ (~300 HV) compared to the CGHAZ (~450 HV) made the WZ more susceptible to fracture, whereas the more uniform hardness in ER110 welds led to crack propagation through the CGHAZ. The microstructural differences, particularly the higher presence of LB in the CGHAZ of ER110 welds compared to the AF in the WZ, further explain this behavior.

SEM analysis provided a more detailed examination of the fractured surfaces, with Fig. 12 and Fig. 13 presenting the fracture features of ER70 and ER110 welds, respectively. For ER70 welds (Fig. 12), the SEM images reveal a bottom pullout failure, with the primary crack initiating at the notch (Fig. 12c) and propagating through the WZ. The presence of intergranular cracking is evident in Fig. 12d, while Fig. 12e and Fig. 12f display smaller cleavage facets and dimple features, indicative of ductile fracture. These observations suggest that the crack propagated through the AF and GBF phases, consistent with the microstructural features identified in the WZ.

In the case of ER110 welds (Fig. 13), the SEM images similarly reveal a bottom pullout failure, with crack initiation at the notch (Fig. 13c) and propagation through the CGHAZ. Intergranular cracking is observed in Fig. 13d, with large cleavage facets in Fig. 13f, characteristic of brittle fracture. These features are associated with LB and GB in the CGHAZ, as identified in the microstructural analysis.

The fractography analysis confirms the significant influence of microstructural features on the crack initiation and propagation behavior in both ER70 and ER110 welds. In ER70 welds, the lower hardness and the presence of AF in the WZ contributed to a more ductile fracture, with cracks propagating through the WZ. Conversely, in ER110 welds, the uniform hardness distribution and brittle phases like LB and GB in the CGHAZ led to a more brittle fracture, with cracks propagating through the CGHAZ.

The observation of intergranular fractures in both welds is particularly noteworthy, as such fractures are often associated with lower toughness and increased susceptibility to failure. This emphasizes the importance of controlling microstructural features during welding to optimize the mechanical performance of the welds under tensile shear loading. The findings align with existing literature, highlighting the critical role of microstructure in determining the fracture behavior of welds^{[30, 42, 43]}.

Fig. 11 (a) Representative image of the cross-section of the fracture sample highlighting the crack location in the bottom sheet responsible for pullout fracture, with corresponding crack locations for (b) ER70 welds and (d) ER110 welds.

Fig. 12. Fractography of ER70 welds: (a) SEM image of the button showing fracture morphology, (b) Overview of crack initiation and propagation, (c-f) Detailed analysis of intergranular cracking and fracture paths in the WZ, confirming ductile fracture behavior

Fig. 13. Fractography of ER110 welds: (a) SEM image of the button showing fracture morphology, (b) Crack initiation and propagation path, (c-e) Detailed analysis of intergranular cracking and cleavage facets in the CGHAZ, indicating brittle fracture behavior

3.6 Fracture Mechanism of APWs with Different Welding Consumables

The fracture mechanism of APWs, fabricated using ER70 and ER110 welding consumables, exhibits distinct differences that are critical to understanding their tensile shear performance. This section synthesizes the observations from microscopy, tensile shear testing, and fractography to provide a comprehensive analysis of these differences, as illustrated in Fig. 14.

Fig. 14(a) presents a schematic that delineates the tensile shear performance of the welds into three distinct stages, superimposed on the representative load-displacement curves. These stages are crucial in understanding the fracture mechanisms of the APWs.

In Stage 1, the load increases with displacement, showing similar behavior for both ER70 and ER110 welds. This stage corresponds to the elastic deformation of the welds and the surrounding material, where the structure remains intact, and no visible cracks are present. As discussed in the tensile shear performance section, the load continues to increase until the peak load is reached. This stage is characterized by the elastic-plastic transition, which occurs uniformly in both weld types due to the similar material properties and weld geometry at this point.

Stage 2 Crack Initiation and Propagation: it begins once the peak load is attained, where the primary crack initiates at the notch, marking the transition from elastic to plastic deformation. This stage is where critical differences in crack propagation between ER70 and ER110 welds become evident, as illustrated in Fig. 14b. The crack initiation at the notch leads to a drop in load-bearing capacity as the crack begins to propagate towards the bottom sheet, influenced by the geometric features of the weld.

The fractography analysis provided in Figs. 12 and 13 revealed that in ER70 welds, the crack predominantly propagates through the WZ. This behavior is largely attributed to the lower hardness of the WZ (~300 HV) compared to the CGHAZ (~450 HV), as highlighted in the microstructural analysis. The presence of AF in the WZ contributes to a lower resistance to crack propagation, making it the path of least resistance. In contrast, for ER110 welds, the crack propagates through the CGHAZ, where the microstructure is dominated by LB, which is harder, and more brittle compared to the WZ. The higher hardness and the presence of bainitic microstructures in the CGHAZ (~450 HV) create a preferred path for crack propagation in ER110 welds, leading to different fracture characteristics as the crack progresses. During this stage, circumferential growth of the crack occurs, further compromising the load-bearing capacity of the weld. As the crack extends, the structural integrity is further reduced, and towards the end of Stage 2, the bending moment generated during tensile shear testing initiates a secondary crack on the opposite side of the weld, which moves toward the top sheet.

Stage 3 is characterized by the progression of the secondary crack towards the top sheet, primarily within the WZ. This stage occurs due to the significantly reduced load-bearing area, leading to the final fracture. The bottom pullout failure, accompanied by a secondary crack in the top sheet, as shown in Fig.7f and Fig. 8g, is the ultimate result of this stage. The difference in fracture mechanisms between the two weld types is a direct consequence of their microstructural features and hardness distributions. The lower hardness of the ER70 WZ makes it more susceptible to crack initiation and propagation, resulting in a WZ-dominated fracture. Conversely, the higher hardness and more brittle microstructure of the CGHAZ in ER110 welds drive the crack through this region, leading to a different fracture path and failure mode.

Fig. 14. Schematic of fracture stages in APWs: (a) Load-displacement curve overlaid with schematic showing three fracture stages during tensile shear testing, (b) Schematic illustrating crack propagation differences between ER70 and ER110 welds, correlating with microstructural features.

3.7 Cross tension testing of APWs

Figure 15(a) shows the representative load–displacement curves obtained from the cross-tension tests of ER70 and ER110 welds. The curves correspond to the average of three repeated tests per condition. The average maximum CTS load for ER70 welds was 4.8 ± 0.18 kN, while for ER110 welds it was 5.35 ± 0.2 kN, where the values following “±” represent the standard deviation. The relatively small scatter confirms the good repeatability of the CTS results. Despite this similarity in peak load, the load-displacement curves for the two weld types reveal significant differences in their mechanical responses. The ER70 welds displayed a much smoother curve, characterized by gradual yielding, indicative of ductile behavior. In contrast, the ER110 welds demonstrated a sudden drop in the load-displacement curve, signifying a brittle failure mechanism.

Fig. 15. Cross tension testing (CTT) and fracture analysis: (a) Load-displacement curves from CTT, (b-c) Fracture surfaces of ER70 welds indicating ductile failure, (d-e) ER110 welds showing partial pullout and brittle fracture, highlighting the mechanical response under cross tension conditions.

The fracture surfaces provide further insights into these mechanical responses. Fig. 15b and Fig. 15(c) depict the fracture images for the ER70 welds, with Fig. 15b showing the interface side of the top sheet and Fig. 15c showing the bottom sheet, where a hole is observed on the bottom side. The fracture morphology indicates that a bottom pullout failure occurred in the ER70 welds, with the complete weld button remaining attached to the top sheet. This type of failure is consistent with the ductile nature suggested by the smooth load-displacement curve.

On the other hand, Fig. 15d and Fig.15(e) shows the fracture images for the ER110 welds, where the button remained attached to the bottom sheet (Fig. 15d), and a hole was observed on the top sheet (Fig. 15e). This fracture pattern indicates that during testing, the crack propagated towards the top sheet, following the path along the WZ, which aligns with the brittle failure mechanism observed in the load-displacement curve. The observed differences in fracture behavior between the ER70 and ER110 welds can be attributed to the microstructural variations in the WZ and the CGHAZ of each weld, as discussed earlier in Section 3.5. In the ER70 welds, the presence of GBF and AF within the WZ likely contributed to the increased ductility, allowing for a smoother load-displacement curve and ensuring a complete pullout failure. In contrast, the ER110 welds exhibited a more brittle fracture behavior, which can be linked to the presence of LB and GB in the WZ. These microstructural constituents tend to exhibit lower ductility, leading to the sudden drop in the load-displacement curve and the observed brittle fracture mode during cross tension testing.