### 1. Introduction

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^{)}. DPFs that have been mass-produced or developed thus far were designed to be detachable and attachable using the flange-up method that fastens each part with a clamp. Since most assembly processes in the muffler manufacturing process require welding, however, excellent mechanical properties, such as high tensile and fatigue properties, are required for the welded joint during the design of a muffler module considering the extreme environment and repeated loads of transport equipment

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^{)}. When dissimilar materials with different thicknesses are subjected to lap joint, the access of the welding torch is difficult due to the limited work environment, and it is also difficult to secure stability for the quality and mechanical properties of the welded joint due to the complex design structure with a large difference in thickness between the upper and lower materials. In particular, since welding defects including fracture in the welded joint, which cause poor tensile strength, are increasing, it is necessary to derive optimal welding conditions to address such defects

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^{)}. Therefore, this study aimed to derive the optimal welding conditions that satisfy the tensile properties of the single lap jointed dissimilar combination with a thickness ratio of 4:1 for muffler module welding as well as the critical bead geometry for securing excellent weld quality by conducting regression analysis and ANSYS simulation to identify bead geometry factors that affect tensile strength. First, correlations between major weld bead geometry factors and tensile strength were analyzed using regression analysis, and then ANSYS analysis was conducted to verify the relationships between the fracture location and the weld bead geometry factors during the tensile test.

### 2. Experimental Method

### 2.1 Experimental materials and equipment

### 2.2 Tensile strength evaluation and bead cross-section analysis

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^{)}, and then the test was repeated four times under each welding condition. The final fracture location was defined as the location where fracture occurred three times or more after the tensile test. For the weld bead cross-section analysis, the welded joint collected from the center of the specimen under each welding condition was precisely cut and polished with sandpaper, and then observed using a microscope after 3% nital etching (HNO

_{3}3 mL + ethic alcohol 97 mL). Fig. 3 shows geometry factors for the bead cross-section geometry that varies depending on the welding parameters in single lap joint

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^{)}. The dimensions of the selected geometry factors were captured using a microscope, and then the Image-Pro plus software was used.

### 2.3 Regression analysis

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^{)}. The regression analysis used in this study utilized MINITAB 19, a statistical analysis software program. The current, speed, and T.P were selected as independent variables that affect dependent variables, i.e. the weld bead geometry factors. Main effects plot analysis was also conducted to identify the main factors that affect the six geometry factors (dependent variables) according to the change in the levels of the independent variables

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^{)}. The main effects plot is frequently used to identify the effect of one or more categorical factors on the quantitative response. The main effects plot parallel to the x-axis means that there is no effect, and the size of the main effect increases as the slope of the plot increases

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^{)}. In addition, fitted line plot analysis was conducted among the regression analysis methods to derive the correlation between each dimension of the weld bead geometry and tensile strength. Fitted line plot analysis is suitable for expressing the relationship between a quantitative predictor and response, and its result is determined valid when R

^{2}is 95% or higher or when the P-value is 0.05 or less

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^{)}. Since the value of R

^{2}increases as the number of variables and the amount of data increase

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^{)}, it was excluded in terms of judgment in this study.

### 2.4 ANSYS simulation

### 3. Experiment Results and Discussion

### 3.1 Analysis of bead geometry factors for single lap joint

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^{)}. Table 4 shows the results of measuring the dimensions of the bead geometry factors for excellent welds shown in Fig. 3, excluding welding conditions that exhibited humping beads (measurement locations: L1-L5, θ).

##### Table 4

##### Table 5

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^{)}. The results of Fig. 5 and Table 5 indicate that the fracture location and tensile strength vary depending on T.P among the other welding parameters. In the case of the welding conditions that caused W.M fracture, fracture occurred at the interface between SS400 and filler. This indicates that L4 in Table 4, which represents the length of the bonded area between SS400 and filler, can have an influence among the other bead geometry factors. L5, which is the length of the bonded area between SUS439 and filler, is also judged to be a main factor that affects the tensile strength, but its value was mostly equal to or higher than that of L4, indicating that fracture mostly occurred at the interface between SS400 and filler. When the conditions that caused fracture and the L4 value were compared through the results in Tables 4 and 5, it was found that the length of L4 was higher than 5.5 mm. B.M fracture or W.M fracture occurred when L4 was higher than 4.8 mm, and W.M fracture occurred when it was lower than 4.8. In other words, it appears that B.M fracture may occur rather than W.M fracture when the length of L4 is higher than 68% of the thickness of the upper base metal (8 mm). From section 3.2, an attempt was made to identify welding parameters that determine the bead geometry factors through basic linear regression analysis using the values of the bead geometry factors obtained from the experiment and to examine the relationship among the tensile strength, fracture location, and bead geometry factors obtained through this study.

### 3.2 Correlation derivation through regression analysis

^{2}also showed a high value of more than 75%. This indicates that there is a general linear relationship between L4 and tensile strength. This appears to be because the length of L5 is equal to or higher than that of L4, which are factors that may affect the tensile strength, and thus the influence of L4 is larger as mentioned earlier on the actual weld bead geometry. When the relationship among the weld bead geometry factors, tensile strength, and fracture location was analyzed through regression analysis, it was found that there is a linear relationship between L4 and tensile strength compared to other geometry factors. Through the actual tensile strength results and regression analysis, it was verified that, for the L4, the fracture location started to change at the L4 of 5.6 mm, which corresponds to 68% of the thickness of the upper base metal (8 mm). ANSYS analysis was conducted to investigate the reason.

### 3.3 Verification of the correlation between the bead and tensile strength through ANSYS analysis

##### Table 6

### 4. Conclusion

1) When the teaching position (T.P) was 1 mm from the bottom plate, conditions that satisfied more than 400 MPa were derived. When T.P was zero, fracture occurred in the weld metal (W.M) and the tensile strength was not satisfied.

2) As for regression analysis to examine the correlation between the L4 length and tensile strength, it was confirmed through the main effects plot that the L4 length, among the bead geometry factors, is most affected by the change in T.P. The results of fitted line plot analysis also confirmed that the change in L4 has the largest impact on the change in tensile strength.

3) When the relationship among the L4 length, tensile strength, and fracture location was examined through ANSYS analysis, it was found that W.M fracture occurred when L4 was 4.8 mm or less and base metal (B.M) fracture when L4 was 5.6 mm or higher. Based on this, it is judged that the critical L4 length for the change in fracture location is between 4.8 and 5.6 mm.