Study on the parameters optimization and the microstructure of spot welding joints of 304 stainless steel
-
Hui Liu
,
Lei Yu
Abstract
Resistance spot welding experiments of 304 stainless steel were performed in this study. Orthogonal test and analysis method were used to carry out the experiments and optimize process parameters based on their effects on tensile shear load and nugget diameter of the spot-welded joint. The microstructure and the defects of spot-welded joints were studied. The highest tensile shear load of joint was 9.529 kN produced at optimum parameters, i.e., electrode force of 3.5 kN, welding current of 6.5 kA, and welding time of eight cycles. Microstructure of the nugget of welded spot joint was columnar dendrite grains which contains austenite and δ-ferrite, and the microstructure of the HAZ region was coarse equiaxed grains of austenite that were induced by different heat inputs. The solidification cracks and shrinkage cavities were observed in resistance spot-welded joints which were produced at higher heat input. These defects seriously reduced the strength of spot-welded joint.
1 Introduction
304 austenitic stainless steel has excellent properties, good corrosion resistance, high tensile strength, low yield strength, high ductility, and easy to fabricate, which has been widely used in rail vehicles fabrication and equipment manufacturing for common chemical and food industries [1,2,3]. 304 stainless steel can also be used in furnace and jet engines, from cryogenic to high temperature application [4]. When stainless steel is used for the construction of a structure, welding and joining procedures are required. According to recent studies, some methods for austenitic stainless steel welding are studied such as laser-arc hybrid welding [5], laser beam welding [6], gas tungsten arc welding [7], and electron beam welding [8]. Resistance spot welding is one of the most important methods in manufacturing industry among the joining methods for structural components. At present, there are only few research works that have focused on the spot welding process of 304 austenitic steel and few results available about the microstructure and defect of welded spots. To explore the influence of parameters on the tensile shear load and microstructure evolution is very important. In this study, orthogonal test and analysis method has been applied to optimal resistance spot welding process parameters for 304 stainless steel. And its weldability for resistance spot welding has been studied by comparing the metallurgical properties and microstructure of the base metal and the welded spot nugget.
2 Experimental materials and methods
The original rolled sheet of 304 stainless steel used in this experiment was 1 mm thick. Major chemical compositions and mechanical properties of experimental materials are presented in Tables 1 and 2, respectively. Parallel to the rolling direction of the plate, the specimens were cut in dimensions of 80 × 20 × 1 mm. The geometry and configuration of the specimens are shown in Figure 1. In order to clean the surfaces of the specimens, alcohol and acetone were used to wash the greasy dirt before the experiment. Specimens were subjected to weld by DN-100 spot welding machine. The tensile tests were performed using a universal testing machine WDW-200, whose strain rate is controlled by a microprocessor system. Spot welding tests were carried out with the Cu–Cr alloy electrode tips with a diameter of 5 mm.
Chemical composition of base metal
| Element | C | Cr | Ni | Mo | Mn | Si | P | S | Fe |
|---|---|---|---|---|---|---|---|---|---|
| Wt% | ≤0.08 | 18～20 | 8～11 | 2～3 | ≤2.0 | ≤0.75 | ≤0.03 | ≤0.035 | Balance |
Mechanical properties of base metal
| Tensile strength σ b/MPa | Yield strength σ s/MPa | Elongation δ (%) | Vickers hardness HV |
|---|---|---|---|
| ≥520 | ≥205 | ≥40 | ≤200 |
Geometry and configuration of the specimens.
3 Optimization methods
3.1 Orthogonal tests
The welding experiments were designed by the orthogonal test methods in this study in order to obtain optimal process parameters of resistance spot welding. The optimal parameters set can be selected and the main effects of these parameters in order of priority can be obtained by appropriate methods [9,10]. There were three controllable variable factors and which factor has three levels. Electrode force was selected as A, welding current was selected as B, welding time was selected as C in the end. It was assumed that there was no interaction among these factors. Three variable factors and their three levels are shown in Table 3. The welding tests were implemented using the orthogonal array L9 (34). As shown as in Table 4, the tensile shear load of welded joints and the nugget diameter of welded spots were selected as experiment index y. Table 4 also shows the arrangement and data of these experiments. Each row in Table 4 represents a set of experiments, and each set of experiments was repeated eight times. Four welded joints were used for the tensile shear test, the others were used for measuring the welded spot nugget diameter and analysis of microstructure and welding defects. The values for analysis were the mean value of four experimental results.
Variable parameters and their levels
| Parameter | Electrode force A/kN | Welding current B/kA | Welding time C/cycle |
|---|---|---|---|
| Level 1 | 3.0 | 5.5 | 6 |
| Level 2 | 3.5 | 6.0 | 7 |
| Level 3 | 4.0 | 6.5 | 8 |
Orthogonal experimental design and results
| Number | Electrode force/kN | Experiment parameters | Error | Results (y) | ||
|---|---|---|---|---|---|---|
| Welding current/kA | Welding time/cycle | Tensile shear load/kN | Nugget diameter/mm | |||
| 1 | 1 | 1 | 1 | 1 | 7.272 | 5.1 |
| 2 | 1 | 2 | 2 | 2 | 7.465 | 5.1 |
| 3 | 1 | 3 | 3 | 3 | 7.270 | 4.9 |
| 4 | 2 | 1 | 2 | 3 | 8.229 | 5.2 |
| 5 | 2 | 2 | 3 | 1 | 8.030 | 5.2 |
| 6 | 2 | 3 | 1 | 2 | 9.529 | 5.4 |
| 7 | 3 | 1 | 3 | 2 | 8.659 | 5.2 |
| 8 | 3 | 2 | 1 | 3 | 6.991 | 4.9 |
| 9 | 3 | 3 | 2 | 1 | 7.863 | 5.1 |
The experiment results and analysis are shown in Table 5. Section numbers K 1, K 2, and K 3 represent the average values of factors A, B, and C under levels 1, 2, and 3. Among K 1, K 2, and K 3, the difference value between the maximum and the minimum is expressed by the symbol R. The larger the R value is, the greater the influence of this parameter on nugget diameter and tensile shear load is.
Analysis of orthogonal experiment
| Source | K 1 | K 2 | K 3 | R | f | S | V | F | |
|---|---|---|---|---|---|---|---|---|---|
| Tensile shear load | A | 7.336 | 8.596 | 7.838 | 1.260 | 2 | 2.416 | 1.208 | 1.306 |
| B | 8.053 | 7.498 | 8.221 | 0.723 | 2 | 0.866 | 0.433 | 0.468 | |
| C | 7.931 | 7.852 | 7.965 | 0.113 | 2 | 0.027 | 0.0135 | 0.015 | |
| Error | 7.721 | 8.551 | 7.497 | 1.066 | 2 | 1.850 | 0.925 | — | |
| Total | — | — | — | — | 8 | 5.159 | — | — | |
| Nugget diameter | A | 5.03 | 5.27 | 5.07 | 0.24 | 2 | 0.096 | 0.048 | 1.157 |
| B | 5.17 | 5.07 | 5.13 | 0.10 | 2 | 0.016 | 0.008 | 0.193 | |
| C | 5.13 | 5.13 | 5.10 | 0.03 | 2 | 0.003 | 0.0015 | 0.036 | |
| Error | 5.13 | 5.23 | 5.00 | 0.23 | 2 | 0.083 | 0.0415 | — | |
| Total | — | — | — | — | 8 | 0.198 | — | — | |
From the following equations, the degree of freedom f, the sum of square S, the mean square V and F were obtained, which are listed in Table 5.
The number of each level r = 3, the number of levels m = 3, the total number of tests n = 9, and T is the sum value of the experiment results.
3.2 Results and discussion
As can be seen, the peak tensile shear load of the spot-welded joint was 9.529 kN and the largest nugget diameter was 5.4 mm, respectively. Figure 2(a) shows K 1, K 2, and K 3 of each factor, and gives the effects of test levels on the tensile shear load. Figure 2(b) shows K 1, K 2, and K 3 of each factor, and gives the effects of test levels on the nugget diameter. It was shown that the tensile shear load of spot-welded joints changed with the test level of the electrode force rapidly, while they changed with the test level of the welding current and welding time slowly. The same result was obtained for the nugget diameter. Therefore, the tensile shear load and nugget diameter are changed with electrode force sensitively. Table 5 shows the values of R of factor A, B, and C. As shown as the values of R, the order of the effect of three factors on tensile shear load was R A > R B > R C, similar to that on nugget diameter. It is shown in Figure 2 that the optimum processing parameter set was electrode force of 3.5 kN, welding current of 6.5 kA and welding time of eight cycles. In order to reveal which factor had a more significant effect on the response, the sum of squares S was used to describe the fluctuation of data in this study. V is the mean square of the factors, and the larger the V value, the more significant the influence of this factor on the tensile shear load and nugget diameter of spot-welded joint. The value of V indicates that the order of the effect of three factors is S A > S B > S C , as shown in Table 5. That means influence of electrode force is the greatest, followed by the welding current, and welding time is the smallest. This result is the same as that of the R value. The obtained result shows that the most dominant factor affecting the nugget diameter and tensile shear load of spot-welded joint is the electrode force.
Influence of test levels on tensile shear load and nugget diameter: (a) Tensile shear load of joints and (b) nugget diameter of joints.
4 Microstructure of joints
The microstructure of 304 stainless steel joint produced by resistance spot welding in different regions is presented in Figure 3. It is a typical equiaxed crystal of austenitic grains in the base metal as shown in Figure 3(a). Coarse austenitic grains are detected in the HAZ region induced by weld heat input, which is shown in Figure 3(b). The spot-welded joint contains a narrow and fine grain region around the periphery and casting columnar dendrite grain region in the center, which is shown in Figure 3(c). This narrow and fine grain region is a plastic ring, which is a plastic deformation and recrystallization area formed due to high temperature with high electrode pressure in the welding process. It can eliminate the welding defects and improve the mechanical properties. Figure 3(d) shows that obviously directional columnar dendrite grain structure has been produced in the nugget center. As a result, the crystallization begins from the partial melting crystalline grains around the edge of the nugget.
Microstructure of spot-welded joint: (a) Region of base metal, (b) region of HAZ, (c) edge of the nugget, and (d) center of the nugget.
Due to the relatively narrow crystallization temperature range of stainless steel, its dendrites can fully grow. The columnar dendrite grows toward the center core along the opposite direction of heat dissipation. During the solidification process of welding stage, cooling rates and temperature gradients of the nugget region are obviously different from the HAZ region and the base metal, so columnar dendrite grain size of the nugget is much bigger than that of the HAZ region and the base metal. The solidification structure of the nugget is also determined by the relationship between the temperature gradient of the liquid phase and the cooling rate during the process of spot welding. Due to the high resistance rate and the poor thermal conductivity, the latent heat of crystallization released during the solidification process will lower the temperature gradient and the cooling rate near the nugget center, leading to a slowdown in the growth rate of the dendrites. Because solidification in the nugget of 304 stainless steel is a non-equilibrium freezing procedure, according to the Fe–Cr–Ni binary phase diagram, the phase precipitated on γ-ferrite boundaries is δ-ferrite.
5 Weld defects analysis
Similar to fusion welding of 304 stainless steel, welding defects formed in the resistance spot-welded joints are hot cracks and shrinkage cavities. It is determined by the thermophysical properties of the austenitic stainless steel. Previous studies showed that the effect of process parameters on welding defects are obviously different. As shown in Figure 4(a), result of the experimental research indicates that cracks in the 304 stainless steel resistance spot-welded joints are mainly solidification cracks. Based on the analysis, it was proved that cracks were mainly determined by electrode force and welding current, since stress condition and component segregation caused by the electrode force and welding current lead to solidification cracks. Due to the increase in heat input when the welding current was up to 6.5 kA, larger columnar and dendritic grains were formed. Because of the formation of continuous intergranular liquid films in the last stage of solidification for the liquid, voids, i.e., solidification cracks will be left at the grain boundaries in the region of the solidus. As shown in Figure 4(b), shrinkage porosity and shrinkage cavity are significantly related to the expansion and contraction of metals and the forging pressure during the melting and solidification stages. In spot welding, the internal pressure is caused by volume expansion caused by metal liquefaction. When the electrode force is lower than 3.5 kN and welding current is higher than 6.5 kA, the plastic ring opens since the internal pressure is too large, and the splash will be produced. The volume of liquid metal decreases, and the nugget cannot be completely filled with the liquid metal, then cracks and shrinkage cavities will occur during solidification stage. The tensile and the fatigue properties of the spot-welded joints would be seriously reduced by these welding defects.
Defects of 304 stainless steel in spot welded joint: (a) Solidification crack and (b) cavity of shrinkage.
6 Conclusion
Spot resistance welding tests were performed for 304 stainless steel, mechanical properties and microstructure of spot-welded joints produced at different process parameters were investigated and the conclusions are summarized as follows:
The optimum spot welding process parameters for 304 stainless steel are electrode force of 3.5 kN, welding current of 6.5 kA and welding time of eight cycles. The peak tensile shear load of the resistance spot welded joints was 9.529 kN under the optimum welding parameters.
Austenite with coarse grains was obtained in HAZ region of 304 stainless steel spot-welded joints, austenite and δ-ferrite with columnar dendrite grains were obtained in the nugget center, and a narrow plastic ring with fine grains was formed around the periphery of the nugget. Dissimilarity of grain morphology distribution in the joint occurred due to the difference in pressures, cooling rates, and temperature gradients at each region during the solidification of the welded metal.
Solidification cracks and shrinkage cavities were typical defects in the 304 stainless steel spot-welded joints. These defects were caused by lower electrode force and higher welding current during welding process, which seriously reduced the tensile and the fatigue properties of the joints.
Acknowledgements
The research was financially supported by Innovation Team Project Funding of Changchun Institute of Technology (Grant No. 320230001).
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Funding information: The research was financially supported by Innovation Team Project Funding of Changchun Institute of Technology (Grant No. 320230001).
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Author contributions: Hui Liu: methodology, conceptualization, writing – original draft, and formal analysis. Lei Yu: conceptualization, writing – review and editing, funding acquisition, resources, and supervision. Dayu Wang: formal analysis, investigation, and writing – review and editing.
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Conflict of interest: The authors declare no conflict of interest.
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Data availability statement: Not applicable.
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