- Email: [email protected]
Resistance ceramic-filled annular welding of DP980 high-strength steel
Materials and Design 183 (2019) 108118
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Resistance ceramic-filled annular welding of DP980 high-strength steel Daxin Ren a, b, Dewang Zhao c, Kunmin Zhao a, Liming Liu b, c, *, Zhubin He d a
School of Automotive Engineering, Dalian University of Technology, Dalian 116024, People's Republic of China Key Laboratory of Liaoning Advanced Welding and Joining Technology, Dalian University of Technology, Dalian 116024, People's Republic of China c School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People's Republic of China d Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, People's Republic of China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A ceramic-filled annular electrode for resistance spot welding was proposed. The annular electrode can produce large size of nugget and smaller indentation. Heat distribution and nugget growth mode were changed in relative to spot welding.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2019 Received in revised form 7 August 2019 Accepted 11 August 2019 Available online 12 August 2019
The extensive application of automobile high-strength steel requires high welding strength. A composite ceramic-filled annular electrode is designed based on the concept of controlling heat distribution and changing the nugget-growth mode. DP980 high-strength steel sheets were resistance welded to explore the effect of the annular electrode, and a traditional resistance spot electrode was also used for comparative analysis. The formation and mechanical properties of the nugget were analyzed. The results show that the resistance annular welding formed an annular nugget whose plan view is similar to the end face of the electrode. The annular nugget had a smaller surface indentation depth than the spotwelded nugget. Under the same external diameter of the end face of the electrode, applying the annular electrode produced a nugget area larger than that produced by a traditional circular end face electrode, thereby providing higher tensile and peeling strengths within a larger welding process window. The numerical simulation results of resistance annular welding (RAW) indicated that no current passed in the ceramic filled area; a small annular liquid nugget formed at the initial stage, and then grew inward and outward simultaneously during the welding process. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Ceramic-centered annular electrode Resistance welding High strength steel
1. Introduction The application of lightweight materials, including high- and * Corresponding author at: 2 Ling Gong Road, Dalian 116024, People's Republic of China. E-mail address: [email protected] (L. Liu).
ultra-high-strength steel, can reduce the body weights of automobiles [1]. The use of high-strength steel requires welding strength that satisfies design requirements. Certain difficulties are still encountered in high-strength steel welding. Several highstrength steel sheets exhibit excellent properties. Welding results in the formation of material softening at the Heat Affected Zone
https://doi.org/10.1016/j.matdes.2019.108118 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2
D. Ren et al. / Materials and Design 183 (2019) 108118
(HAZ) which is of decisive influence on the response of the structure, and the HAZ microstructure depends on the steel material, type of weld, heat input during welding, and the post welding conditions [2,3]. Thus, a high safety factor of welding strength is required in welding [4e6]. For resistance spot welding (RSW), the size of the nugget determines the failure load of the joint, and a large nugget can provide a relatively high safety factor [7,8]. In addition to ensuring joint strength, the welding process of high-strength steel must guarantee good nugget formation and a large welding process window. Traditional RSW mainly refers to welding using circular end face electrodes. After nucleating at the nugget center, the nugget gradually grows around the center [9,10]. The method for obtaining large nuggets under this condition is to increase the diameter of the end face of the electrode, welding current, and welding time. Considering the high energy density of the interface between the sheets at the center of the electrode, a high current or a long duration will easily lead to spatter, which causes the formation of internal defects in the nugget and a large indentation depth on the nugget surface [11e14]. A composite ceramic-filled annular electrode (A-electrode) has been recently developed to satisfy the requirements of high strength, large process window, and good nugget formation [15]. The primary thinking behind this design includes controlling the resistance heat distribution and changing the nugget growth mode. In terms of controlling resistance heat distribution, the energy becomes increasingly dispersed in the annular conducive region, thereby avoiding the energy concentration in the nugget center. As for the nugget growth mode, an annular nugget grows both inward and outward simultaneously from its initial ring formation, ultimately yielding a large nugget area. In the early stage of experiment, the above idea was realized by drilling a hole at the center of a circular electrode referring to some scholars' work before. However, the surface indentation of the annular area was quite deep, and an upward bulge was formed in the nugget center. In subsequent experiments, the process was improved by embedding a non-conducive ceramic rod into the center of the electrode, so the resistance ceramic-filled annular welding (RAW) came into being. The nugget formation tailored for different metal sheets can be realized by using different size of ceramic rod and copper annular area at the electrode tip to satisfy the welding strength requirement. In this work, 1.2mm-thick DP980 dual phase high-strength steel sheets for automobile were welded to explore the effects of the welding parameters on the mechanical properties and the formation mechanism of the nugget. The microstructure of the nugget was observed, and the nugget growth process was analyzed by numerical simulation.
2. Experimental materials and methods A 1.2 mm DP980 steel sheet was selected for the welding experiment. The steel sheets were cut to 100 mm 25 mm. The properties and main chemical compositions of this material are listed in Tables 1 and 2. A ceramic-filled annular electrode was adopted in this experiment. The minimum nugget diameter for 1.2 mm sheets is 3.9 mm according to the BMW Group Standard (GS96002). Given the lack of any previous experience with welding using the A-electrode, this study intuitively adopted an electrode size suitable for extreme conditions (photographs of the electrode and its size are shown in Fig. 1) to investigate the welding phenomenon. The white area at the
Table 2 Main chemical compositions of DP980 (in wt pct). Fe
Al
C
Cr
Cu
Mn
Mo
Balance
0.05
0.11
0.26
0.01
2.17
0.29
Fig. 1. Appearance of the composite ceramic-filled annular electrode.
Table 3 Factor levels for orthogonal experiment design. Factor levels
Welding current/kA
Welding time/ms
Electrode force/kN
1 2 3 4
7.2 8.0 8.8 9.6
50 100 150 200
1.2 1.5 1.8 2.1
center of the electrode comprised 4 mm diameter ceramic materials; the outer diameter of the annular copper area was 8 mm. The top and bottom electrodes had the same diameter and geometry in the experiments. A traditional resistance spot electrode (S-electrode) with a circular end face diameter of 8 mm was used to conduct a comparative experiment. A medium-frequency direct current welding machine was used for welding. The orthogonal experimental method was used to investigate the influence of parameters on tensile shear strength. The electrode force (F), welding time (T), and welding current (I) were selected as three factors, and four levels were selected for each factor. The details of the factors and levels are shown in Table 3. After welding, the cross sections of the nugget were prepared. Nugget microstructures were observed by sectioning through the center of the weld. The cross section was etched in 4% Nital reagent for 10e30 s to enhance the weld microstructure. The indentation depth, diameter, and area of RSW and RAW nuggets were measured according to the etched cross section, and the measurement method is shown in Fig. 2. The tensile shear tests were operated on the basis of GB2651-89, and the geometry and dimensions of the test specimens are shown in Fig. 3. Tensile shear and peeling tests were conducted on joints at rates of 1.0 and 5.0 mm min1, respectively. The failure load of the welded joints was measured by the average value of three samples per
Table 1 Material properties of DP980 steel.
DP980
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Elastic modulus (GPa)
Elongation (%)
Poisson's ratio
Thermal expansion coefficient (E06 (1/K))
980
1010
11.58
202.47
11.58
0.28
8.1
D. Ren et al. / Materials and Design 183 (2019) 108118
3
Fig. 2. Sketch of nugget measurement.
welding condition. Vickers hardness was tested every 0.2 mm along the interface using a load of 0.5 kg and a dwell time of 10 s. The RAW process was simulated by a thermaleelectricalestructural coupling finite element (FE) model. The focus of the model was to explain the distribution of energy and the formation of nugget. The material properties of DP980 steel and ceramic were consistent with the experimental part. Materials produce a wide range of temperature variations and plastic deformation upon
welding. Therefore, the JC model is applicable to this study as it describes flow stress as the product form of three functions, namely, strain hardening, strain rate reinforcement, and thermal softening. Cooling has significant effect on the mechanical properties of high strength DP980 steel [16] due to cooling after welding been considered in the finite element model. The model was incorporated in the commercial software Abaqus 6.16. The C3D8R element was picked to model the work pieces in the thermaleelectricalestructural coupled analysis. C3D8R is an eight-node trilinear displacement with a reduced integration point and hourglass control. “C” stands for continuum, “3” for trilinear, “D” for displacement, and “R” for reduced. The welding process involved two steps. The first step was directly applying vertical pressure to the upper electrode head, and the second step was applying electric potential to the upper and lower electrodes [17]. The pressure and current were consistent with the experiment. The schematic diagram of the FE analysis model is shown in Fig. 4. 3. Results and discussion 3.1. Nugget shaping
Fig. 3. Test specimens' geometry: (a) tensile shear test and (b) peel test.
Fig. 5 shows the surface morphology of a RAW nugget. The circular area in the nugget center surface corresponds to the ceramic part of the electrode. An annular indentation formed under the action of the annular copper end face of the electrode. The RAW nugget demonstrated a larger welding parameter
Fig. 4. Schematic of FE model.
Fig. 5. Surface morphology of a RAW nugget.
4
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 6. Cross section of a circular nugget subjected to different welding times (welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e110 ms).
Fig. 7. Cross section of an annular nugget subjected to different welding times (welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e210 ms).
(welding time) window than the RSW nugget in this experiment in terms of nugget formation. Traditional S- and A-electrodes were adopted to weld sheets. According to the experiment results
(Section 3.2), relatively low current (8 kA) and appropriate electrode force (1.5 kN) were adopted to conduct a comparative experiment for different welding times (40e210 ms). A maximum
Fig. 8. Comparison of diameters (a) and areas (b) of RSW and RAW nuggets. (Welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e210 ms).
D. Ren et al. / Materials and Design 183 (2019) 108118
5
Table 4 Results and analysis of the orthogonal experiment on RAW. Welding Welding Electrode current (I)/kA time (T)/ms force (F)/kN
Fig. 9. Cross section and microstructures of an annular nugget with welding times 80 ms (a), 130 ms (b), and 190 ms (c). (Welding current of 8 kA, electrode force of 1.5 kN).
210 ms welding time in RAW was used because welding spatter began to occur at this welding time. The cross sections of the RSW and RAW nuggets are depicted in Figs. 6 and 7, respectively. The macrostructures of the two types of joints were analyzed in terms of nugget size and indentation depth. Using an A-electrode yielded a larger nugget area than using an Selectrode in this experiment. The diameter and area results after measuring the size of the nuggets shown in Figs. 6 and 7 (the measurement method is shown in Fig. 2) are presented in Fig. 8. When two electrodes were adopted for welding, the nugget gradually grew with prolonged welding time. An S-electrode was adopted for welding for 100 ms, and the nugget reached a diameter of 7.7 mm and an area of 46.2 mm2. When an A-electrode was adopted, the inner diameter of the annular connection region gradually decreased, and its outer diameter gradually increased. At 200 ms, the RAW nugget area reached 69 mm2. The connection area of the RAW nugget was equal to that of the RSW nugget with a diameter of
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 K1 K2 K3 K4 k1 k2 k3 k4 R Important order
Average failure load/ kN
7.2 50 1.2 1.59 7.2 100 1.5 18.1 7.2 150 1.8 18.3 7.2 200 2.1 17.1 8.0 50 1.5 7.3 8.0 100 1.2 18.1 8.0 150 2.1 15.6 8.0 200 1.8 18.1 8.8 50 1.8 14.0 8.8 100 2.1 18.2 8.8 150 1.2 20.8 8.8 200 1.5 22.7 9.6 50 2.1 18.5 9.6 100 1.8 20.9 9.6 150 1.5 22.6 9.6 200 1.2 22.9 55.09 41.39 63.39 59.1 75.3 70.7 75.7 77.3 71.3 84.9 80.8 69.4 13.7725 10.3475 15.8475 14.775 18.825 17.675 18.925 19.325 17.825 21.225 20.2 17.35 7.452 9.852 1.977 Welding time > Welding current > Electrode force
Fracture mode IF PF PF PF IF PF PF PF IF PF PF PF PF PF PF PF
Note: Ki represents the sum of the testing data marked with level number “i” for each factor. ki is the average value of Ki (ki ¼ Ki / 9). R ¼ max{k1, k2, k3, k4} min{k1, k2, k3, k4}.
9.3 mm. The nugget size of the spot-welded joint was essential to determining the failure load. The A-electrode could effectively improve the nugget size in this experiment and thereby enhance the mechanical properties of the joint. The RAW nugget had a smaller surface indentation depth than the RSW nugget in this experiment. In ensuring the performance and appearance of the nugget, the indentation depth should
Fig. 10. Microstructures of annular nuggets: (a) to (f) corresponding to regions I to VI in Fig. 8.
6
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 11. Influence of parameters on the quality of a RAW joint.
generally be <15% of the sheet thickness. When the S-electrode was adopted with the welding time of 100 ms, a 0.35 mm indentation formed on the surface (Fig. 6). The indentation depth reached 29% of the sheet thickness. When the A-electrode was adopted, even if the welding time reached 200 ms, only a 0.12 mm indentation (10% of the sheet thickness) formed in the annular region (Fig. 7). The RAW nugget formed a wider welding time window than the RSW nugget, as indicated by the nugget formation. Connection between sheets formed when the S-electrode was used with a welding time of 40 ms. When the A-electrode was used, the sheets could not connect if the welding time was <70 ms despite the occurrence of local melting. Figs. 9 and 10 show the typical macrostructures of three DP980 RAW nuggets and the detailed microstructures. The annular connection completely formed at 80 ms, as shown in Fig. 9(a) and (b). Although a relatively high welding time for sheet bonding was required for RAW, welding spatter did not occur until the welding time reached 210 ms in the experiment. If indentation depth and nugget formation were considered, the welding time window using an S-electrode was in the range of 40e90 ms, and the time value was expanded to 80e210 ms when using an A-electrode. The RAW joint region is consisting of three structural zones: weld nugget, heat affected zone (HAZ), and base metal (BM) as shown in Figs. 9 and 10. The microstructure of the weld nugget is consisting of lath martensite. The formation of martensite was attributed to the high cooling rate of RSW. Similar microstructures were observed inside and outside the annular nugget (Fig. 10(e) and (f)). Two HAZs (inner HAZ and outer HAZ) were formed in the RAW joint. This phenomenon is markedly different from that of traditional RSW joint.
out fracture (PF). Table 4 shows that RT > RI > RF; thus, the most important impact factor was welding time, followed by welding current and electrode force. In RSW, the energy was concentrated at the center, so the nugget was formed for a short time. The RAW resistance heat energy was distributed more widely, and it was necessary to form an annular-shaped melting region. Therefore, so it took longer time to form a nugget without defect. This was the reason for RT > RI. The influence of factors on strength is shown in Fig. 11. The effects of welding current and time showed similar trends, that is, welding strength increased with the increase of these two factors. This situation was due to the extension of current and time enhancing the welding heat production, resulting in a large nugget size and improved joint strength. Fig. 11 shows that the strength first increased with the increase of the electrode force and then decreased when the electrode force reached 2.1 kN. In resistance welding, the
3.2. Mechanical properties The RAW results of tensile shear tests based on the orthogonal experiment method are provided in Table 3. The maximum average failure load in this experiment was 22.9 kN. The failure mode was interfacial fracture (IF) with only three parameters (1#, 5#, and 9#). When the failure load exceeded 14.0 kN, the failure mode was pull-
Fig. 12. Tensile shear test result (welding current of 4.8e11.2 kA, electrode force of 1.5 kN, and welding time of 100 ms).
D. Ren et al. / Materials and Design 183 (2019) 108118
7
Fig. 13. Fractured RAW joints in the tensile shear test: (a) IF, (b) PF from nugget edge, and (c) PF from HAZ.
electrode force affected the contact resistance. A greater force led to a decrease in contact resistance between the sheets and a low quantity of resistance heat. Ultimately, the size of the nugget and the joint strength were reduced [18,19]. The RAW nugget demonstrated a larger welding process window than RSW nugget based on the tensile shear and peel strength. The mechanical property test was conducted under the same welding time (100 ms) and electrode force (1.5 kN) and under different welding current conditions to compare the effects of the two types of electrodes. The tensile shear test result is shown in Fig. 12. With the use of an S-electrode, the nugget strength increased with increased current and decreased when the current exceeded 8.8 kA. This result was due to the initiation of spatter when the current reached 8 kA in the experiment. When the current reached 9.6 kA, the high welding energy caused a sharp increase in the spatter.
The maximum welding current used in RAW was 11.2 kA because welding spatter began to occur when the A-electrode was used. The joint strength of the RAW nugget increased quickly with increased current. When the current reached 8.8 kA, the failure load of RAW nugget exceeded the maximum of the RSW nugget. With a constant increase in current, continuous growth can be maintained. Typical fractured RAW joints are shown in Fig. 13. With currents of 6.4 and 7.2 kA, the mode of the joint was an interfacial fracture (Fig. 13a). As the current increased in this experiment, two types of pull-out failure modes were observed. When the current increased to 8 kA, the initial fracture was located at the nugget edge, and cracks propagated into the outer HAZ and BM (Fig. 13b). When the current reached 9.6 kA, failure appeared to be initiated at the outer HAZ (Fig. 13c). When the annular electrode was used, spatter did not occur until the current was 11.2 kA. The relatively
Fig. 14. Comparison of peeling test results (welding current of 4.8e11.2 kA, electrode force of 1.5 kN, and welding time of 100 ms).
Fig. 15. Hardness distribution of annular nuggets (welding current of 8 kA, electrode force of 1.5 kN, and welding time of 130 ms).
8
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 16. Diagram of RAW nugget growth.
stable welding process of the annular electrode enabled the strength to constantly increase with increased current. Compared with that of the RSW joint, the improved tensile shear performance of the RAW joints might have been due to the minimum level of expulsion and indentation depth. The relationship between strength and current in the peeling test was similar to that in the tensile shear test. The maximum failure load of the RSW nugget peeling initially increased and then decreased before peaking at 1542 N with the current of 8 kA. The failure load of the annular nugget increased quickly with the increased current, and the peak value was 2013 N. The RAW joint fracture mode was PF when the current reached 7.2 kA. The failure load of the RAW joint was higher than that of the RSW joint within the current range of 8e11.2 kA in this experiment (Fig. 14). The peeling performance of the spot-welded joint depends on the nugget size, and peeling stress is mainly distributed at the edge of the nugget. Therefore, once the RAW nugget was formed, its larger outer diameter ensured that the joint could withstand higher peeling loads than the RSW nugget. Connection between sheets formed quicker when S-electrode was used; However, the mechanical properties of the joint are enhanced and the welding parameter window were expanded when A-electrode was used. The welding time for optimized welding parameters was between 100 and 150 ms when A-electrode was used. Relatively speaking, the number of high strength steel (HSS) and ultra-high strength steel nugget is less than other steel nugget, so the cost of time does not increase much. The hardness distribution of the RAW nugget is plotted in Fig. 15. The hardness was higher in the annular nugget area than in the BM. Furthermore, two softened HAZs were formed inside and outside of the nugget. The hardness values of the HAZs were slightly less than that of DP980 BM. The lowest hardness in the outer HAZ was 233 HV, which was 5 mm from the nugget center. Martensite was formed (as shown in Fig. 10) under rapid cooling due to the formation of an annular-shaped melting zone, so the microhardness is significant higher than the BM. Under the action of welding thermal cycle, microstructure and performance changes occur in the BM of
the inner and outer sides of the annular nugget. The original strengthening mechanism of high-strength steel disappeared, so the soften inner HAZ and outer HAZ were formed. 3.3. Numerical simulation analysis The use of A-electrode changes the heat distribution and nugget growth mode relative to the case of traditional resistance. Traditional spot welding mainly produces heat at the center, and the heat diffuses to the surrounding area [18]. At the initial stage, small liquid nugget formed at the sheets interface. Then, the nugget grew along the radial and axial directions with prolonged time [20]. Numerical simulation was used to analyze the RAW nugget growth process, and the result is shown in Fig. 16. The calculated melting zone had nearly the same area as the experimental nugget. When the A-electrode was used, no current was generated in the ceramic-filled action area. Resistance heat was generated at the annular connection area between the sheets corresponding to the annular copper area of the electrode end face. As a result, annular liquid nuggets with small widths were produced at the initial stage. Then, the liquid nuggets grew inward and outward simultaneously with time. The ceramic-filled action area can provide a space for resistance heat to be transferred inward and for nuggets to grow inward, thereby avoiding a local concentration of heat. Thus, when the annular electrode was used, the nuggets grew extensively. The annular nuggets clearly have a larger area, smaller indentation, and less spatter than traditional circular nuggets. 4. Conclusions A ceramic-filled annular electrode for resistance welding was designed to address the problems in high-strength steel welding for automobile manufacturing. The A-electrode used in this experiment had a 4 mm inner diameter and an 8 mm outer diameter, and its center was filled with ceramic. The nugget formation, mechanical properties, and nugget growth process were analyzed. The findings are as follows.
D. Ren et al. / Materials and Design 183 (2019) 108118
(1) The RAW formed an annular nugget with shapes corresponding to the end face of the electrode. A larger nugget area and less indentation depth were obtained in RAW than in RSW. When the A-electrode was used, the nugget area reached 69 mm2, which was equivalent to that of the nugget with a diameter of 9.3 mm using a traditional S-electrode. (2) In terms of nugget formation, RAW nuggets can form a wider welding time window than RSW nuggets. With regard to sheet bonding and indentation depth, the welding time for RSW ranged from 40 to 70 ms. The time window expanded to 80e200 ms when an A-electrode was used, under which condition no splatter was produced. (3) The most important impact factor of RAW was welding time, followed by welding current and electrode clamping force. The maximum failure load of the RAW joint reached 22.9 kN in tensile shear test. With regard to tensile shear and peel strength, the RAW nugget demonstrated a larger welding process window than the RSW nugget. (4) The use of A-electrode changed the heat distribution and nugget growth mode relative to the traditional resistance spot welding. The numerical simulation result of RAW indicated that no current passed in the ceramic-filled area. An annular liquid nugget was produced between the sheets corresponding to the shape of the copper at the electrode tip. The liquid nugget grew inward and outward simultaneously during the welding process. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1764251, 51775160), and the Fundamental Research Funds for the Central Universities (DUT19LAB24). References [1] K. Chen, X. Liu, J. Ni, Friction stir resistance spot welding of aluminum alloy to advanced high strength steel, ASME J. Manuf. Sci. Eng. 140 (2018) 111007. [2] F. Javidan, A. Heidarpour, X.L. Zhao, J. Minkkinen, Effect of weld on the mechanical properties of high strength and ultra-high strength steel tubes in fabricate hybrid sections, Eng. Struct. 118 (2016) 16e27.
9
[3] M. Nassirnia, A. Heidarpour, X.L. Zhao, J. Minkkinen, Innovative hollow corrugated columns comprising corrugated plates and ultra high-strength steel tubes, Thin-Walled Struct. 101 (2016) 14e25. [4] L. Ying, P. Andrea, A. Tim, K. Menachem, Z. Wei, Subcritical heat affected zone softening in hot-stamped boron steel during resistance spot welding, Mater. Des. 155 (2018) 170e184. [5] P. Srithananan, P. Keawtatatip, V. Uthaisangsuk, Micromechanics-based modeling of stress-strain and fracture behavior of heat-treated boron steels for hot stamping process, Mater. Sci. Eng. A 667 (2016) 61e76. [6] B.V.H. Hernandez, M.L. Kuntz, M.I. Khan, Y. Zhou, Influence of microstructure and weld size on the mechanical behavior of dissimilar AHSS resistance spot welds, Sci. Technol. Weld. Join. 13 (2008) 769e776. [7] A. Chabok, D.A.E. Van, J.T.M. De Hosson, Y.T. Pei, Mechanical behavior and failure mechanism of resistance spot welded DP1000 dual phase steel, Mater. Des. 124 (2017) 171e182. [8] J.P. Kong, T.K. Han, K.G. Chin, B.G. Park, C.Y. Kang, Effect of boron content and welding current on the mechanical properties of electrical resistance spot welds in complex-phase steels, Mater. Des. 54 (2014) 598e609. [9] M. Rashid, J. Medley, Y. Zhou, Nugget formation and growth during resistance spot welding of aluminium alloy 5182, Can. Metall. Q. 50 (2011) 61e71. [10] Y. Li, F.Y. Yan, Z. Luo, Y.J. Chao, S.S. Ao, X.T. Cui, Weld growth mechanisms and failure behavior of three-sheet resistance spot welds made of 5052 aluminum alloy, J. Mater. Eng. Perform. 24 (2015) 2546e2555. [11] Y. Zhang, C. Wang, H. Shan, Y. Li, Z. Luo, High-toughness joining of aluminum alloy 5754 and DQSK steel using hybrid clinchingewelding process, J. Mater. Process. Tech. 259 (2018) 33e44. [12] Y. Zhao, Y. Zhang, X. Lai, Resistance spot welding of ultra-thin automotive steel, ASME J. Manuf. Sci. Eng. 135 (2013) 021012. [13] Z. Wan, H. Wang, N. Chen, M. Wang, B.E. Carlson, Characterization of intermetallic compound at the interfaces of Al-steel resistance spot welds, J. Mater. Process. Tech. 242 (2017) 12e23. [14] D.R. Sigler, J.G. Schroth, M.J. Karagoulis, D. Zuo, New electrode weld face geometries for spot welding aluminum, in: AWS. Sheet. Metal. Welding. Conference. XIV. Livonia, 2010, pp. 1e19. [15] Daxin Ren, Dewang Zhao, Chenbin Li, Liming Liu, Kunmin Zhao, Resistance ceramic-filled annular welding of thin steel sheets, J. Manuf. Process 45 (2019) 588e594. [16] F. Azhari, A. Heidarpour, X.L. Zhao, C. Hutchinson, Post-fire mechanical response of ultra-high strength (Grade 1200) steel under high temperatures: linking thermal stability and microstructure, Thin-Walled Struct. 119 (2017) 114e125. [17] N. Ma, H. Murakawa, Numerical and experimental study on nugget formation in resistance spot welding for three pieces of high strength steel sheets, J. Mater. Process. Tech. 210 (2010) 2045e2052. [18] M. Hamedi, M. Atashparva, A review of electrical contact resistance modeling in resistance spot welding, Welding in The World 61 (2017) 269e290. [19] S.M. Manladan, F. Yusof, S. Ramesh, et al., A review on resistance spot welding of aluminum alloys, Inter. J. Adv. Manu. Tech. 90 (2017) 605e634. [20] M. Rashid, J. Medley, Y. Zhou, Nugget formation and growth during resistance spot welding of aluminium alloy 5182, Can. Metall. Q. 50 (2011) 61e71.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Resistance ceramic-filled annular welding of DP980 high-strength steel Daxin Ren a, b, Dewang Zhao c, Kunmin Zhao a, Liming Liu b, c, *, Zhubin He d a
School of Automotive Engineering, Dalian University of Technology, Dalian 116024, People's Republic of China Key Laboratory of Liaoning Advanced Welding and Joining Technology, Dalian University of Technology, Dalian 116024, People's Republic of China c School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People's Republic of China d Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, People's Republic of China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A ceramic-filled annular electrode for resistance spot welding was proposed. The annular electrode can produce large size of nugget and smaller indentation. Heat distribution and nugget growth mode were changed in relative to spot welding.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2019 Received in revised form 7 August 2019 Accepted 11 August 2019 Available online 12 August 2019
The extensive application of automobile high-strength steel requires high welding strength. A composite ceramic-filled annular electrode is designed based on the concept of controlling heat distribution and changing the nugget-growth mode. DP980 high-strength steel sheets were resistance welded to explore the effect of the annular electrode, and a traditional resistance spot electrode was also used for comparative analysis. The formation and mechanical properties of the nugget were analyzed. The results show that the resistance annular welding formed an annular nugget whose plan view is similar to the end face of the electrode. The annular nugget had a smaller surface indentation depth than the spotwelded nugget. Under the same external diameter of the end face of the electrode, applying the annular electrode produced a nugget area larger than that produced by a traditional circular end face electrode, thereby providing higher tensile and peeling strengths within a larger welding process window. The numerical simulation results of resistance annular welding (RAW) indicated that no current passed in the ceramic filled area; a small annular liquid nugget formed at the initial stage, and then grew inward and outward simultaneously during the welding process. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Ceramic-centered annular electrode Resistance welding High strength steel
1. Introduction The application of lightweight materials, including high- and * Corresponding author at: 2 Ling Gong Road, Dalian 116024, People's Republic of China. E-mail address: [email protected] (L. Liu).
ultra-high-strength steel, can reduce the body weights of automobiles [1]. The use of high-strength steel requires welding strength that satisfies design requirements. Certain difficulties are still encountered in high-strength steel welding. Several highstrength steel sheets exhibit excellent properties. Welding results in the formation of material softening at the Heat Affected Zone
https://doi.org/10.1016/j.matdes.2019.108118 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2
D. Ren et al. / Materials and Design 183 (2019) 108118
(HAZ) which is of decisive influence on the response of the structure, and the HAZ microstructure depends on the steel material, type of weld, heat input during welding, and the post welding conditions [2,3]. Thus, a high safety factor of welding strength is required in welding [4e6]. For resistance spot welding (RSW), the size of the nugget determines the failure load of the joint, and a large nugget can provide a relatively high safety factor [7,8]. In addition to ensuring joint strength, the welding process of high-strength steel must guarantee good nugget formation and a large welding process window. Traditional RSW mainly refers to welding using circular end face electrodes. After nucleating at the nugget center, the nugget gradually grows around the center [9,10]. The method for obtaining large nuggets under this condition is to increase the diameter of the end face of the electrode, welding current, and welding time. Considering the high energy density of the interface between the sheets at the center of the electrode, a high current or a long duration will easily lead to spatter, which causes the formation of internal defects in the nugget and a large indentation depth on the nugget surface [11e14]. A composite ceramic-filled annular electrode (A-electrode) has been recently developed to satisfy the requirements of high strength, large process window, and good nugget formation [15]. The primary thinking behind this design includes controlling the resistance heat distribution and changing the nugget growth mode. In terms of controlling resistance heat distribution, the energy becomes increasingly dispersed in the annular conducive region, thereby avoiding the energy concentration in the nugget center. As for the nugget growth mode, an annular nugget grows both inward and outward simultaneously from its initial ring formation, ultimately yielding a large nugget area. In the early stage of experiment, the above idea was realized by drilling a hole at the center of a circular electrode referring to some scholars' work before. However, the surface indentation of the annular area was quite deep, and an upward bulge was formed in the nugget center. In subsequent experiments, the process was improved by embedding a non-conducive ceramic rod into the center of the electrode, so the resistance ceramic-filled annular welding (RAW) came into being. The nugget formation tailored for different metal sheets can be realized by using different size of ceramic rod and copper annular area at the electrode tip to satisfy the welding strength requirement. In this work, 1.2mm-thick DP980 dual phase high-strength steel sheets for automobile were welded to explore the effects of the welding parameters on the mechanical properties and the formation mechanism of the nugget. The microstructure of the nugget was observed, and the nugget growth process was analyzed by numerical simulation.
2. Experimental materials and methods A 1.2 mm DP980 steel sheet was selected for the welding experiment. The steel sheets were cut to 100 mm 25 mm. The properties and main chemical compositions of this material are listed in Tables 1 and 2. A ceramic-filled annular electrode was adopted in this experiment. The minimum nugget diameter for 1.2 mm sheets is 3.9 mm according to the BMW Group Standard (GS96002). Given the lack of any previous experience with welding using the A-electrode, this study intuitively adopted an electrode size suitable for extreme conditions (photographs of the electrode and its size are shown in Fig. 1) to investigate the welding phenomenon. The white area at the
Table 2 Main chemical compositions of DP980 (in wt pct). Fe
Al
C
Cr
Cu
Mn
Mo
Balance
0.05
0.11
0.26
0.01
2.17
0.29
Fig. 1. Appearance of the composite ceramic-filled annular electrode.
Table 3 Factor levels for orthogonal experiment design. Factor levels
Welding current/kA
Welding time/ms
Electrode force/kN
1 2 3 4
7.2 8.0 8.8 9.6
50 100 150 200
1.2 1.5 1.8 2.1
center of the electrode comprised 4 mm diameter ceramic materials; the outer diameter of the annular copper area was 8 mm. The top and bottom electrodes had the same diameter and geometry in the experiments. A traditional resistance spot electrode (S-electrode) with a circular end face diameter of 8 mm was used to conduct a comparative experiment. A medium-frequency direct current welding machine was used for welding. The orthogonal experimental method was used to investigate the influence of parameters on tensile shear strength. The electrode force (F), welding time (T), and welding current (I) were selected as three factors, and four levels were selected for each factor. The details of the factors and levels are shown in Table 3. After welding, the cross sections of the nugget were prepared. Nugget microstructures were observed by sectioning through the center of the weld. The cross section was etched in 4% Nital reagent for 10e30 s to enhance the weld microstructure. The indentation depth, diameter, and area of RSW and RAW nuggets were measured according to the etched cross section, and the measurement method is shown in Fig. 2. The tensile shear tests were operated on the basis of GB2651-89, and the geometry and dimensions of the test specimens are shown in Fig. 3. Tensile shear and peeling tests were conducted on joints at rates of 1.0 and 5.0 mm min1, respectively. The failure load of the welded joints was measured by the average value of three samples per
Table 1 Material properties of DP980 steel.
DP980
Yield strength (MPa)
Tensile strength (MPa)
Elongation (%)
Elastic modulus (GPa)
Elongation (%)
Poisson's ratio
Thermal expansion coefficient (E06 (1/K))
980
1010
11.58
202.47
11.58
0.28
8.1
D. Ren et al. / Materials and Design 183 (2019) 108118
3
Fig. 2. Sketch of nugget measurement.
welding condition. Vickers hardness was tested every 0.2 mm along the interface using a load of 0.5 kg and a dwell time of 10 s. The RAW process was simulated by a thermaleelectricalestructural coupling finite element (FE) model. The focus of the model was to explain the distribution of energy and the formation of nugget. The material properties of DP980 steel and ceramic were consistent with the experimental part. Materials produce a wide range of temperature variations and plastic deformation upon
welding. Therefore, the JC model is applicable to this study as it describes flow stress as the product form of three functions, namely, strain hardening, strain rate reinforcement, and thermal softening. Cooling has significant effect on the mechanical properties of high strength DP980 steel [16] due to cooling after welding been considered in the finite element model. The model was incorporated in the commercial software Abaqus 6.16. The C3D8R element was picked to model the work pieces in the thermaleelectricalestructural coupled analysis. C3D8R is an eight-node trilinear displacement with a reduced integration point and hourglass control. “C” stands for continuum, “3” for trilinear, “D” for displacement, and “R” for reduced. The welding process involved two steps. The first step was directly applying vertical pressure to the upper electrode head, and the second step was applying electric potential to the upper and lower electrodes [17]. The pressure and current were consistent with the experiment. The schematic diagram of the FE analysis model is shown in Fig. 4. 3. Results and discussion 3.1. Nugget shaping
Fig. 3. Test specimens' geometry: (a) tensile shear test and (b) peel test.
Fig. 5 shows the surface morphology of a RAW nugget. The circular area in the nugget center surface corresponds to the ceramic part of the electrode. An annular indentation formed under the action of the annular copper end face of the electrode. The RAW nugget demonstrated a larger welding parameter
Fig. 4. Schematic of FE model.
Fig. 5. Surface morphology of a RAW nugget.
4
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 6. Cross section of a circular nugget subjected to different welding times (welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e110 ms).
Fig. 7. Cross section of an annular nugget subjected to different welding times (welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e210 ms).
(welding time) window than the RSW nugget in this experiment in terms of nugget formation. Traditional S- and A-electrodes were adopted to weld sheets. According to the experiment results
(Section 3.2), relatively low current (8 kA) and appropriate electrode force (1.5 kN) were adopted to conduct a comparative experiment for different welding times (40e210 ms). A maximum
Fig. 8. Comparison of diameters (a) and areas (b) of RSW and RAW nuggets. (Welding current of 8 kA, electrode force of 1.5 kN, and welding time in the range of 40e210 ms).
D. Ren et al. / Materials and Design 183 (2019) 108118
5
Table 4 Results and analysis of the orthogonal experiment on RAW. Welding Welding Electrode current (I)/kA time (T)/ms force (F)/kN
Fig. 9. Cross section and microstructures of an annular nugget with welding times 80 ms (a), 130 ms (b), and 190 ms (c). (Welding current of 8 kA, electrode force of 1.5 kN).
210 ms welding time in RAW was used because welding spatter began to occur at this welding time. The cross sections of the RSW and RAW nuggets are depicted in Figs. 6 and 7, respectively. The macrostructures of the two types of joints were analyzed in terms of nugget size and indentation depth. Using an A-electrode yielded a larger nugget area than using an Selectrode in this experiment. The diameter and area results after measuring the size of the nuggets shown in Figs. 6 and 7 (the measurement method is shown in Fig. 2) are presented in Fig. 8. When two electrodes were adopted for welding, the nugget gradually grew with prolonged welding time. An S-electrode was adopted for welding for 100 ms, and the nugget reached a diameter of 7.7 mm and an area of 46.2 mm2. When an A-electrode was adopted, the inner diameter of the annular connection region gradually decreased, and its outer diameter gradually increased. At 200 ms, the RAW nugget area reached 69 mm2. The connection area of the RAW nugget was equal to that of the RSW nugget with a diameter of
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 K1 K2 K3 K4 k1 k2 k3 k4 R Important order
Average failure load/ kN
7.2 50 1.2 1.59 7.2 100 1.5 18.1 7.2 150 1.8 18.3 7.2 200 2.1 17.1 8.0 50 1.5 7.3 8.0 100 1.2 18.1 8.0 150 2.1 15.6 8.0 200 1.8 18.1 8.8 50 1.8 14.0 8.8 100 2.1 18.2 8.8 150 1.2 20.8 8.8 200 1.5 22.7 9.6 50 2.1 18.5 9.6 100 1.8 20.9 9.6 150 1.5 22.6 9.6 200 1.2 22.9 55.09 41.39 63.39 59.1 75.3 70.7 75.7 77.3 71.3 84.9 80.8 69.4 13.7725 10.3475 15.8475 14.775 18.825 17.675 18.925 19.325 17.825 21.225 20.2 17.35 7.452 9.852 1.977 Welding time > Welding current > Electrode force
Fracture mode IF PF PF PF IF PF PF PF IF PF PF PF PF PF PF PF
Note: Ki represents the sum of the testing data marked with level number “i” for each factor. ki is the average value of Ki (ki ¼ Ki / 9). R ¼ max{k1, k2, k3, k4} min{k1, k2, k3, k4}.
9.3 mm. The nugget size of the spot-welded joint was essential to determining the failure load. The A-electrode could effectively improve the nugget size in this experiment and thereby enhance the mechanical properties of the joint. The RAW nugget had a smaller surface indentation depth than the RSW nugget in this experiment. In ensuring the performance and appearance of the nugget, the indentation depth should
Fig. 10. Microstructures of annular nuggets: (a) to (f) corresponding to regions I to VI in Fig. 8.
6
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 11. Influence of parameters on the quality of a RAW joint.
generally be <15% of the sheet thickness. When the S-electrode was adopted with the welding time of 100 ms, a 0.35 mm indentation formed on the surface (Fig. 6). The indentation depth reached 29% of the sheet thickness. When the A-electrode was adopted, even if the welding time reached 200 ms, only a 0.12 mm indentation (10% of the sheet thickness) formed in the annular region (Fig. 7). The RAW nugget formed a wider welding time window than the RSW nugget, as indicated by the nugget formation. Connection between sheets formed when the S-electrode was used with a welding time of 40 ms. When the A-electrode was used, the sheets could not connect if the welding time was <70 ms despite the occurrence of local melting. Figs. 9 and 10 show the typical macrostructures of three DP980 RAW nuggets and the detailed microstructures. The annular connection completely formed at 80 ms, as shown in Fig. 9(a) and (b). Although a relatively high welding time for sheet bonding was required for RAW, welding spatter did not occur until the welding time reached 210 ms in the experiment. If indentation depth and nugget formation were considered, the welding time window using an S-electrode was in the range of 40e90 ms, and the time value was expanded to 80e210 ms when using an A-electrode. The RAW joint region is consisting of three structural zones: weld nugget, heat affected zone (HAZ), and base metal (BM) as shown in Figs. 9 and 10. The microstructure of the weld nugget is consisting of lath martensite. The formation of martensite was attributed to the high cooling rate of RSW. Similar microstructures were observed inside and outside the annular nugget (Fig. 10(e) and (f)). Two HAZs (inner HAZ and outer HAZ) were formed in the RAW joint. This phenomenon is markedly different from that of traditional RSW joint.
out fracture (PF). Table 4 shows that RT > RI > RF; thus, the most important impact factor was welding time, followed by welding current and electrode force. In RSW, the energy was concentrated at the center, so the nugget was formed for a short time. The RAW resistance heat energy was distributed more widely, and it was necessary to form an annular-shaped melting region. Therefore, so it took longer time to form a nugget without defect. This was the reason for RT > RI. The influence of factors on strength is shown in Fig. 11. The effects of welding current and time showed similar trends, that is, welding strength increased with the increase of these two factors. This situation was due to the extension of current and time enhancing the welding heat production, resulting in a large nugget size and improved joint strength. Fig. 11 shows that the strength first increased with the increase of the electrode force and then decreased when the electrode force reached 2.1 kN. In resistance welding, the
3.2. Mechanical properties The RAW results of tensile shear tests based on the orthogonal experiment method are provided in Table 3. The maximum average failure load in this experiment was 22.9 kN. The failure mode was interfacial fracture (IF) with only three parameters (1#, 5#, and 9#). When the failure load exceeded 14.0 kN, the failure mode was pull-
Fig. 12. Tensile shear test result (welding current of 4.8e11.2 kA, electrode force of 1.5 kN, and welding time of 100 ms).
D. Ren et al. / Materials and Design 183 (2019) 108118
7
Fig. 13. Fractured RAW joints in the tensile shear test: (a) IF, (b) PF from nugget edge, and (c) PF from HAZ.
electrode force affected the contact resistance. A greater force led to a decrease in contact resistance between the sheets and a low quantity of resistance heat. Ultimately, the size of the nugget and the joint strength were reduced [18,19]. The RAW nugget demonstrated a larger welding process window than RSW nugget based on the tensile shear and peel strength. The mechanical property test was conducted under the same welding time (100 ms) and electrode force (1.5 kN) and under different welding current conditions to compare the effects of the two types of electrodes. The tensile shear test result is shown in Fig. 12. With the use of an S-electrode, the nugget strength increased with increased current and decreased when the current exceeded 8.8 kA. This result was due to the initiation of spatter when the current reached 8 kA in the experiment. When the current reached 9.6 kA, the high welding energy caused a sharp increase in the spatter.
The maximum welding current used in RAW was 11.2 kA because welding spatter began to occur when the A-electrode was used. The joint strength of the RAW nugget increased quickly with increased current. When the current reached 8.8 kA, the failure load of RAW nugget exceeded the maximum of the RSW nugget. With a constant increase in current, continuous growth can be maintained. Typical fractured RAW joints are shown in Fig. 13. With currents of 6.4 and 7.2 kA, the mode of the joint was an interfacial fracture (Fig. 13a). As the current increased in this experiment, two types of pull-out failure modes were observed. When the current increased to 8 kA, the initial fracture was located at the nugget edge, and cracks propagated into the outer HAZ and BM (Fig. 13b). When the current reached 9.6 kA, failure appeared to be initiated at the outer HAZ (Fig. 13c). When the annular electrode was used, spatter did not occur until the current was 11.2 kA. The relatively
Fig. 14. Comparison of peeling test results (welding current of 4.8e11.2 kA, electrode force of 1.5 kN, and welding time of 100 ms).
Fig. 15. Hardness distribution of annular nuggets (welding current of 8 kA, electrode force of 1.5 kN, and welding time of 130 ms).
8
D. Ren et al. / Materials and Design 183 (2019) 108118
Fig. 16. Diagram of RAW nugget growth.
stable welding process of the annular electrode enabled the strength to constantly increase with increased current. Compared with that of the RSW joint, the improved tensile shear performance of the RAW joints might have been due to the minimum level of expulsion and indentation depth. The relationship between strength and current in the peeling test was similar to that in the tensile shear test. The maximum failure load of the RSW nugget peeling initially increased and then decreased before peaking at 1542 N with the current of 8 kA. The failure load of the annular nugget increased quickly with the increased current, and the peak value was 2013 N. The RAW joint fracture mode was PF when the current reached 7.2 kA. The failure load of the RAW joint was higher than that of the RSW joint within the current range of 8e11.2 kA in this experiment (Fig. 14). The peeling performance of the spot-welded joint depends on the nugget size, and peeling stress is mainly distributed at the edge of the nugget. Therefore, once the RAW nugget was formed, its larger outer diameter ensured that the joint could withstand higher peeling loads than the RSW nugget. Connection between sheets formed quicker when S-electrode was used; However, the mechanical properties of the joint are enhanced and the welding parameter window were expanded when A-electrode was used. The welding time for optimized welding parameters was between 100 and 150 ms when A-electrode was used. Relatively speaking, the number of high strength steel (HSS) and ultra-high strength steel nugget is less than other steel nugget, so the cost of time does not increase much. The hardness distribution of the RAW nugget is plotted in Fig. 15. The hardness was higher in the annular nugget area than in the BM. Furthermore, two softened HAZs were formed inside and outside of the nugget. The hardness values of the HAZs were slightly less than that of DP980 BM. The lowest hardness in the outer HAZ was 233 HV, which was 5 mm from the nugget center. Martensite was formed (as shown in Fig. 10) under rapid cooling due to the formation of an annular-shaped melting zone, so the microhardness is significant higher than the BM. Under the action of welding thermal cycle, microstructure and performance changes occur in the BM of
the inner and outer sides of the annular nugget. The original strengthening mechanism of high-strength steel disappeared, so the soften inner HAZ and outer HAZ were formed. 3.3. Numerical simulation analysis The use of A-electrode changes the heat distribution and nugget growth mode relative to the case of traditional resistance. Traditional spot welding mainly produces heat at the center, and the heat diffuses to the surrounding area [18]. At the initial stage, small liquid nugget formed at the sheets interface. Then, the nugget grew along the radial and axial directions with prolonged time [20]. Numerical simulation was used to analyze the RAW nugget growth process, and the result is shown in Fig. 16. The calculated melting zone had nearly the same area as the experimental nugget. When the A-electrode was used, no current was generated in the ceramic-filled action area. Resistance heat was generated at the annular connection area between the sheets corresponding to the annular copper area of the electrode end face. As a result, annular liquid nuggets with small widths were produced at the initial stage. Then, the liquid nuggets grew inward and outward simultaneously with time. The ceramic-filled action area can provide a space for resistance heat to be transferred inward and for nuggets to grow inward, thereby avoiding a local concentration of heat. Thus, when the annular electrode was used, the nuggets grew extensively. The annular nuggets clearly have a larger area, smaller indentation, and less spatter than traditional circular nuggets. 4. Conclusions A ceramic-filled annular electrode for resistance welding was designed to address the problems in high-strength steel welding for automobile manufacturing. The A-electrode used in this experiment had a 4 mm inner diameter and an 8 mm outer diameter, and its center was filled with ceramic. The nugget formation, mechanical properties, and nugget growth process were analyzed. The findings are as follows.
D. Ren et al. / Materials and Design 183 (2019) 108118
(1) The RAW formed an annular nugget with shapes corresponding to the end face of the electrode. A larger nugget area and less indentation depth were obtained in RAW than in RSW. When the A-electrode was used, the nugget area reached 69 mm2, which was equivalent to that of the nugget with a diameter of 9.3 mm using a traditional S-electrode. (2) In terms of nugget formation, RAW nuggets can form a wider welding time window than RSW nuggets. With regard to sheet bonding and indentation depth, the welding time for RSW ranged from 40 to 70 ms. The time window expanded to 80e200 ms when an A-electrode was used, under which condition no splatter was produced. (3) The most important impact factor of RAW was welding time, followed by welding current and electrode clamping force. The maximum failure load of the RAW joint reached 22.9 kN in tensile shear test. With regard to tensile shear and peel strength, the RAW nugget demonstrated a larger welding process window than the RSW nugget. (4) The use of A-electrode changed the heat distribution and nugget growth mode relative to the traditional resistance spot welding. The numerical simulation result of RAW indicated that no current passed in the ceramic-filled area. An annular liquid nugget was produced between the sheets corresponding to the shape of the copper at the electrode tip. The liquid nugget grew inward and outward simultaneously during the welding process. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1764251, 51775160), and the Fundamental Research Funds for the Central Universities (DUT19LAB24). References [1] K. Chen, X. Liu, J. Ni, Friction stir resistance spot welding of aluminum alloy to advanced high strength steel, ASME J. Manuf. Sci. Eng. 140 (2018) 111007. [2] F. Javidan, A. Heidarpour, X.L. Zhao, J. Minkkinen, Effect of weld on the mechanical properties of high strength and ultra-high strength steel tubes in fabricate hybrid sections, Eng. Struct. 118 (2016) 16e27.
9
[3] M. Nassirnia, A. Heidarpour, X.L. Zhao, J. Minkkinen, Innovative hollow corrugated columns comprising corrugated plates and ultra high-strength steel tubes, Thin-Walled Struct. 101 (2016) 14e25. [4] L. Ying, P. Andrea, A. Tim, K. Menachem, Z. Wei, Subcritical heat affected zone softening in hot-stamped boron steel during resistance spot welding, Mater. Des. 155 (2018) 170e184. [5] P. Srithananan, P. Keawtatatip, V. Uthaisangsuk, Micromechanics-based modeling of stress-strain and fracture behavior of heat-treated boron steels for hot stamping process, Mater. Sci. Eng. A 667 (2016) 61e76. [6] B.V.H. Hernandez, M.L. Kuntz, M.I. Khan, Y. Zhou, Influence of microstructure and weld size on the mechanical behavior of dissimilar AHSS resistance spot welds, Sci. Technol. Weld. Join. 13 (2008) 769e776. [7] A. Chabok, D.A.E. Van, J.T.M. De Hosson, Y.T. Pei, Mechanical behavior and failure mechanism of resistance spot welded DP1000 dual phase steel, Mater. Des. 124 (2017) 171e182. [8] J.P. Kong, T.K. Han, K.G. Chin, B.G. Park, C.Y. Kang, Effect of boron content and welding current on the mechanical properties of electrical resistance spot welds in complex-phase steels, Mater. Des. 54 (2014) 598e609. [9] M. Rashid, J. Medley, Y. Zhou, Nugget formation and growth during resistance spot welding of aluminium alloy 5182, Can. Metall. Q. 50 (2011) 61e71. [10] Y. Li, F.Y. Yan, Z. Luo, Y.J. Chao, S.S. Ao, X.T. Cui, Weld growth mechanisms and failure behavior of three-sheet resistance spot welds made of 5052 aluminum alloy, J. Mater. Eng. Perform. 24 (2015) 2546e2555. [11] Y. Zhang, C. Wang, H. Shan, Y. Li, Z. Luo, High-toughness joining of aluminum alloy 5754 and DQSK steel using hybrid clinchingewelding process, J. Mater. Process. Tech. 259 (2018) 33e44. [12] Y. Zhao, Y. Zhang, X. Lai, Resistance spot welding of ultra-thin automotive steel, ASME J. Manuf. Sci. Eng. 135 (2013) 021012. [13] Z. Wan, H. Wang, N. Chen, M. Wang, B.E. Carlson, Characterization of intermetallic compound at the interfaces of Al-steel resistance spot welds, J. Mater. Process. Tech. 242 (2017) 12e23. [14] D.R. Sigler, J.G. Schroth, M.J. Karagoulis, D. Zuo, New electrode weld face geometries for spot welding aluminum, in: AWS. Sheet. Metal. Welding. Conference. XIV. Livonia, 2010, pp. 1e19. [15] Daxin Ren, Dewang Zhao, Chenbin Li, Liming Liu, Kunmin Zhao, Resistance ceramic-filled annular welding of thin steel sheets, J. Manuf. Process 45 (2019) 588e594. [16] F. Azhari, A. Heidarpour, X.L. Zhao, C. Hutchinson, Post-fire mechanical response of ultra-high strength (Grade 1200) steel under high temperatures: linking thermal stability and microstructure, Thin-Walled Struct. 119 (2017) 114e125. [17] N. Ma, H. Murakawa, Numerical and experimental study on nugget formation in resistance spot welding for three pieces of high strength steel sheets, J. Mater. Process. Tech. 210 (2010) 2045e2052. [18] M. Hamedi, M. Atashparva, A review of electrical contact resistance modeling in resistance spot welding, Welding in The World 61 (2017) 269e290. [19] S.M. Manladan, F. Yusof, S. Ramesh, et al., A review on resistance spot welding of aluminum alloys, Inter. J. Adv. Manu. Tech. 90 (2017) 605e634. [20] M. Rashid, J. Medley, Y. Zhou, Nugget formation and growth during resistance spot welding of aluminium alloy 5182, Can. Metall. Q. 50 (2011) 61e71.