- Email: [email protected]
Resistance ceramic-filled annular welding of thin steel sheets
Journal of Manufacturing Processes 45 (2019) 588–594
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Resistance ceramic-filled annular welding of thin steel sheets Daxin Ren
a,b
b,c
d
, Dewang Zhao , Chenbin Li , Liming Liu
b,c,⁎
, Kunmin Zhao
T
a
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 School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China d Beijing Xinghang Electro-mechanical Equipment Co., Ltd, Beijing, 100074 People’s Republic of China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic-filled annular electrode Resistance welding High strength steel
To meet the requirements of automobiles for high-strength steel welding and thin-sheet welding, a composite ceramic-filled annular electrode (A-electrode) was designed by controlling welding energy distribution. By embedding a circular non-conductive ceramic rod in the center of a conventional copper electrode, the welding heat changed from centralization in the center of the nugget of traditional spot welding to a more dispersed annular distribution. As a result, an annular nugget that corresponds to the annular copper end face was formed. Given the energy dispersion, a higher current was required to bond the sheets by the A-electrode than that by a traditional electrode. However, the indentation depth on the surface of the nugget decreased effectively. The stress distribution indicated that the stress was concentrated at the edge of the nugget under the load, and the unconnected part in the middle did not affect the strength. The size of the annular nugget was designed by adjusting the structure of the A-electrode. The strength of the nugget could be improved by forming a large nugget area.
1. Introduction Resistance spot welding is currently one of the most widely used welding methods in the automobile manufacturing industry [1–3]. Reducing sheet thickness is an effective way to reduce the quality of car bodies [4–6]. However, some difficulties are still experienced with regard to thin-sheet and ultra-thin-sheet welding. Thin-steel sheets (0.7 and 0.6 mm, even 0.5 mm) are often used as the automobile panel [7,8]. However, these sheets are sensitive to welding heat input. As a result, deep indentation and even burn-through can easily occur with a narrow welding window. The failure load of the nugget is closely related to the diameter of the nugget. Thus, obtaining a large diameter nugget with only a small indentation on the surface of the nugget remains a challenge [9,10]. Some methods have been used to improve resistance spot welding, such as parameters optimization, welding process simulation, microstructure controlling, adding cover sheet, and hybrid clinchingwelding process [11–15]. Through the above methods, the strength has been effectively increased. The energy distribution of the welding heat source is the key to form nuggets and weld seam. Regardless of fusion welding or resistance spot welding, energy density has a great impact on weld seams and welding nuggets [16–19]. Once the current of resistance spot welding is switched on, heat is initially generated at the edge of the electrode and then ⁎
rapidly transferred to the center of the spot welding [20,21]. The diameter of the nugget increases with the welding time. In this process, the highest energy density occurs at the center of the nugget. The increase of the nugget size caused by increasing the welding current or the welding interval may easily lead to great indentation, splash, and other defects [22,23]. To regulate the distribution of the welding current, the surface shaping of nuggets can be controlled by increasing the diameter of the nuggets. In the present work, a composite ceramic-filled annular electrode (A-electrode for short) was developed. The design idea is to change the distribution of resistance heat in the resistance spot welding process to control the nugget formation. In the welding with this kind of electrode, the ceramics in the electrode center are not conductive. The resistance heat generated only through the surrounding annular area. Finally, the annular nucleus is formed after welding. For resistance spot welding using ceramic-filled A-electrode, the shaping of the nuggets can be designed in accordance with the diameter of the ceramics in the center and dimension of the annular region in the outer copper. Nugget formation can meet the need for welding strength under different conditions. A thin sheet (0.7 mm) for a car body panel, mild steel sheet (1.0 mm) were used to study the impact of welding parameters on mechanical properties and formation. Resistance spot welding using ceramic-filled annular electrode (AW) and traditional spot welding
Corresponding author at: 2 Ling Gong Road, Dalian, 116024, People’s Republic of China. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.jmapro.2019.07.043 Received 15 April 2019; Received in revised form 1 July 2019; Accepted 30 July 2019 1526-6125/ © 2019 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 45 (2019) 588–594
D. Ren, et al.
Table 1 Material properties of welded sheets. Material
DC06 Q235
Material properties
Chemical composition wt.%
Yield Strength (MPa)
Tensile strength (MPa)
Elongation (%)
C
Mn
Si
S
P
138 235
302 375
24 26
0.006 0.14
0.28 0.35
0.02 0.3
0.01 0.04
0.01 0.03
(SW) were compared. The microstructure was then observed. Stress in tensile shear test was analyzed by numerical simulation. Finally, a tensile shear test peel test was performed to measure the mechanical strength
Table 2 Parameters of resistance welding.
2. Experimental materials and methods Two typical automobile steels were selected in the welding experiments. Here, 0.7 mm DC06 steel and 1.0 mm Q235 (ASTM A283-C) mild refer to the automobile steel. These steels are primarily used in manufacturing car body stamping parts, such as door and fender. The properties of the two types of steel materials are shown in Table 1. The particular ceramic-filled A-electrode used in this experiment was prepared as follows. First, a hole was drilled in the center of the copper electrode. A small amount of epoxy resin adhesives for automobile body was placed in the hole. Then, a ceramic rod with the same diameter as the hole was filled. Afterward, the adhesive was cured at 180 °C for 20 min in the resistance furnace. Finally, the electrode face was machined in accordance with the design size. When welding sheets of different thicknesses the electrodes with adequate diameters are needed. Because of lack of any previous experience with the welding using the A-electrode, the size of the electrode suitable for extreme conditions was adopted intuitively, see Fig. 1, to investigate the welding phenomenon. The white area in the center of the electrode was ceramic with a diameter of 4 mm, and the outer diameter of the annular copper area was 8 mm. The Circular face electrode with an end diameter of 8 mm was used to compare with the A-electrode. Mediumfrequency DC welder was used for welding, and the welding parameters are shown in Table 2. Sheets were welded as a lap joint with 100mm*25 mm size. Tensile shear strength and peel strength were evaluated at rates of 1.0 mm min−1 and 5 mm min−1, respectively. The tensile shear tests were operated on the basis of Chinese Standard GB2651-89. The geometry and dimensions of the test specimens are shown in Fig. 2. The mean values of the three specimens were obtained to evaluate the mechanical properties. In previous work, the resistance ceramic-filled annular welding
Parameters
Value
Welding current (kA) Welding time (ms) Electrode Force (kN)
6.4 100 150
7.2
8
8.8
9.6
10.4
11.2
12
Fig. 2. Test specimens’ geometry.
process was simulated by a thermal–electrical–structural coupling FE model. The Johns-Cook model was selected to simulate the constitutive relation of materials. The electrical resistance and thermal conductivity across the contact interfaces were obtained from the literatures [24–26]. C3D8R element was picked to model the work-pieces in thermal–electrical–structural coupled analysis. In this work, numerical simulation static analysis was conducted on the stress state in tensile shear test of two kinds of nuggets. The nugget regions are coupled by tie connection, which are consistent with the test. Other regions are established face contact, which are consistent with the previous work. The constitutive relation model is obtained from the literature [27]. C3D8T element was picked to model the work-pieces in static analysis. 3. Results and discussion 3.1. Nugget formation Fig. 3 shows the surface formation after the A-electrode was used to weld the DC06 sheet. The surface of the nugget corresponded to the face of the electrode, which was divided into two regions: a circular region in the middle and an annular region around the area. One of the most important characteristics of the spot nugget was small indentation on the surface even when achieving a large diameter. The cross sections of the Q235 spot nugget and annular nugget are shown in Fig. 4. The cross section of the annular nugget was divided into two parts. The annular width increased with the welding current increase. However, the depth of indentation on the surface changes slightly. By contrast, in welding using a circular electrode, The
Fig. 1. Appearance of the composite ceramic-filled annular electrode. 589
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Fig. 3. Surface morphologies of nugget using ceramic-filled annular electrode.
change in the microstructure compared with the base metal at the center of the annular nugget where the sheets cannot be bonded. Table 3 and Fig. 8 show the diameter and area comparison of the annular nugget and spot nugget area with the increase of the welding current. When an A-electrode was adopted, the inner diameter of the annular connection region gradually decreased, and its outer diameter gradually increased. The use of circular electrode can lead to a rapid increase in the size of nuggets when the current was small. with the current further increase, the size of the nuggets slowly increased. At a current of 12 kA, the nugget diameter was 7.1 mm and the bonding area 39.5 mm2. By contrast, although the joint formed by the annular nugget had a higher welding current threshold value than other joints, the area of the nugget rapidly improved as the current increases once the joint formed. At current of 12 kA, the area was 55.6 mm2. Once an effective bonding is formed using A-electrode, its outer diameter was 7 mm (the diameter of the annular center of copper materials,) and the large joint diameter guarantees failure load. 3.2. Mechanical properties An annular nugget has a particular shape. Its stress state under load is different from that of spot nugget. The space in the annular nugget may have an impact on the strength. Therefore, numerical simulation was used to analyze the stress state in tensile shear test of two kinds of nuggets. In the simulation, the outer diameter of annular nugget was same as that of the spot nugget. The stress distribution on the lower surface of the upper sheet is shown in Fig. 9. In terms of shear stress analysis, the stress distribution in the spot nugget was more uniform. The stress concentration positions of the two kinds of nuggets were similar, mainly occurring at the edge of the nugget. However, the maximum stress at the edge of the annular nugget was lower than that of the spot nugget. Area of high stress area in annular nugget was larger than that in spot nugget. This result is because joining did not form in the middle of the annular nugget, and the sheets at the nugget center were less constrained. Therefore, deformation may have occurred. This played a positive role in conduction and dispersion of stress. The maximum principal stresses at the edges of two kinds of nuggets were almost the same, but the internal stress of the annular nugget was slightly lower. As can be seen from the simulation results, the space between sheets at the annular center did not have negative impact on performance in tensile shear test. Shear stress of the annular nugget (a) and circular nugget (b),
Fig. 4. Cross-sections of the 1mm-Q235 nugget using annular electrode and circular electrode.
indentation depth increased rapidly with the increase of current. For the DC06 sheet, a similar result was observed, as shown in Fig. 5. The 0.7 mm sheet is ultra-thin. Hence, it is more sensitive to heat input in the welding process than the other sheets. It also has a higher ratio of the same indentation depth to the thickness of the sheet than the thicker sheets. Fig. 6 lists the ratios of indentation depth to the thickness of different sheets with the increase in current. The surface indentation of the nugget is inevitable. To ensure the performance and appearance of the nugget, the indentation depth is usually required to be less than 15% of the thickness of sheet. The current of the annular nugget on 0.7 mm DC06 was 11.2 kA, whereas the depth of indentation was only 8%. When the current is up to 12 kA, the indentation rate reached 11% of the thickness due to the occurrence of splash. The microstructures of DC06 nugget was similar to a traditional spot nugget as shown in Fig. 7. The molten zone was located at middle of the copper area of annular electrode. The DC06 base metal was composed of ferrite and a small amount of pearlite, and there was no significant 590
Journal of Manufacturing Processes 45 (2019) 588–594
D. Ren, et al.
Fig. 5. Cross-sections of the 0.7 mm-DC06 nugget using annular electrode and circular electrode. Table 3 Diameter of annular and spot nuggets. Welding current (kA) 6.4 RAW internal diameter (mm) RAW external diameter (mm) RSW diameter (mm)
5.0
7.2
5.4
8.0
8.8
9.6
10.4
11.2
12.0
6.3
5.9 8.1 6.7
5.2 8.8 6.8
4.9 9.0 6.9
4.7 9.2 7.0
4.4 9.5 7.1
Principal stress of the annular nugget (c) and circular nugget (d) The annular nuggets can provide higher tensile shear than the circular nuggets in a large welding parameter window. A comparative welding test of DC06 and Q235 was conducted by adopting A-electrode and circular electrode. The tensile test results are shown in Fig. 10. When the current was 8.8 kA, the DC06 nugget of weld ruptured in less than 2 kN. The fracture mode was interface fracture. When the current was larger than or equal to 9.6 kA, the failure load rapidly increased, and the fracture mode was pull-out fracture (as shown in Fig. 11). The starting position of the fracture was the edge of the nugget. In terms of strength, the maximum failure load obtained by the A-electrode was
Fig. 6. Comparison of indentation depth using annular electrode and circular electrode.
Fig. 7. Microstructures of DC06 annular nugget. 591
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Fig. 8. Comparison of Q235 nugget size using annular electrode and circular electrode.
diameter of 8 mm was adopted. Nonetheless, the diameter of the nugget was only 7.1 mm even if the welding current reached 12 kA. A great indentation also occurred on the circular nugget surface. When an annular joint was formed by an A-electrode, the larger radius of the annular nugget can help provide higher failure load than that provided by peeling with the use of a circular nugget. In terms of welding stability, the A-electrode produced fewer splashes than the circular electrode during the welding process. If the specified current value is exceeded, the welding splash by the circular electrode significantly increase. When 0.7 mm-thick DC06 sheet and 1 mm-thick Q235 were welded, the minimum current values of the produced splash were 9.6 and 10.4kA respectively by using circular electrode,; nevertheless, the minimum current were 12.0 and 12.6 kA respectively by using A-electrode. With the constant increase of the current, the splash considerably increased by the circular electrode. Particularly, for welding a 0.7 mm-thin sheet, the splash led to a great indentation depth on the surface of the nugget (as shown in Fig. 5). This
larger than that by using the circular electrode. For the relationship between welding current and failure load, a similar trend was observed in Q235. When the welding current was small, the failure load was low. However, when the current increased beyond a specific threshold value, the failure load rapidly increases, and the increasing trend was slow (Fig. 10). The A-electrode can provide higher peeling strength than the circular electrode. The peeling experiments for DC06 and Q235 are shown in Fig. 12. The results show that the peel failure load of DC06 spot nugget increased as the current increases. This finding is similar to the tensile test result. After the current reached 9.6 kA, the peeling strength accordingly decreased. Q235 exhibited a similar strength change. The peeling failure load initially increased and later decreased. The peeling strength of the A-electrode was relatively stable. As the current increased to a relatively high value, it changed within a small range. The size of the nugget had a relatively great impact on the peeling strength. On the basis of the results, a circular electrode with an electrode face
Fig. 9. Simulation of stress distribution in X diction on the lower surface of the upper sheet. Shear stress of the annular nugget (a) and circular nugget (b), Principal stress of the annular nugget (c) and circular nugget (d). 592
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Fig. 10. Comparison of tensile strength by using annular electrode and circular electrode.
corresponded to the face of the electrode, which was divided into two regions: a circular region in the middle and an annular region around the area. 2) Resistance heat was generated in the annular region and the energy density was further dispersed. Splash was reduced because of the energy dispersion distribution. The lowest current required to bond sheets by A-electrode is higher than that required by the circular electrode. A small indentation on the surface can be produced, while the connection area that was larger than that using a traditional electrode can be obtained. 3) The nucleation position of the nugget was at the center of the copper material. Furthermore, the annular width of the nugget increased with the welding current. Under the same end diameter, the Aelectrode can provide a larger area of the nugget than the circular electrode. 4) In welding with the A-electrode, a stable annular nugget was formed with the increase of the current. Afterward, a stable tensile strength and peeling-off strength can be obtained by forming a large nugget area in a large welding window.
condition is the underlying reason why the welding strength decreases after the welding current reached 11.2 kA. However, the welding strength remained high when A-electrode was used. The energy distribution of the welding heat source is the key to good nugget formation, high strength and welding stability. Circular electrode welding mainly produces resistance heat in the center, and causes a large concentration. The advantage is that sheets can be bonded with low heat input. However, increased heat accumulation in the center leads to excessive surface indentation and spatter. Given the isolation of ceramics, the center of the A-electrode does not produce resistance heat. Welding energy is generated in the annular region. In this way, the centralization of the traditional electrode energy at the center of the nugget is avoided. The energy density is relatively low because of the large conduction area, which means that the energy distribution is “soft”. Therefore, with the same external diameter as the traditional circular electrode, higher welding current is required to form a sheet bond with A-electrode. A conventional electrode can realize DC06-sheet connection at 8.0 kA; for the annular nugget, the lowest current value is required to reach 8.8 kA. Meanwhile, avoiding local energy concentration also decreases indentation depth and splash, so stable welding process and high strength are obtained.
Declaration of Competing Interest 4. Conclusions The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
A ceramic-filled A-electrode for resistance welding was designed in this study to address the difficulties in producing thin sheets for automobiles. Nugget formation and mechanical properties were also analyzed. Finally, the following conclusions are obtained: 1) When an A-electrode was used, the surface of the nugget
Fig. 11. Fractured joints in the tensile shear test and peel test of the annular nugget. 593
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Fig. 12. Comparison of the peeling strength using annular electrode and circular electrode.
Acknowledgement
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This work was supported by the National Natural Science Foundation of China (U1764251, 11472072, 51775160). References [1] Nannan C, Min W, Hui-Ping W, Zixuan W, Carlson BE. Microstructural and mechanical evolution of Al/steel interface with Fe2Al5 growth in resistance spot welding of aluminum to steel. J Manuf Process 2018;34:424–34. [2] Satpathy MP, Mishra SB, Sahoo SK. Ultrasonic spot welding of aluminum-copper dissimilar metals: a study on joint strength by experimentation and machine learning techniques. J Manuf Process 2018;33:96–110. [3] Vargas VH, Mejía I, Baltazar-Hernández VH, Maldonado C. Characterization of resistance spot welded transformation induced plasticity (TRIP) steels with different silicon and carbon contents. J Manuf Process 2018;32:307–17. [4] Spena PR, Maddis MD, Lombardi F, Rossini M. X dissimilar resistance spot welding of Q&P and TWIP steel sheets. Mater Manuf Process 2016;31:291–9. [5] Srithananan P, Keawtatatip P, Uthaisangsuk V. Micromechanics-based modeling of stress-strain and fracture behavior of heat-treated boron steels for hot stamping process. Mater Sci Eng A 2016;667:61–76. [6] Hernandez BVH, Kuntz ML, Khan MI, Zhou Y. Influence of microstructure and weld size on the mechanical behavior of dissimilar AHSS resistance spot welds. Sci Technol Weld Join 2008;13:769–76. [7] Ling Z, Li Y, Luo Z. Resistance element welding of 6061 aluminum alloy to uncoated 22MnMoB boron steel. Mater Manuf Process 2016;31:2174–80. [8] Kong JP, Han TK, Chin KG, Park BG, Kang CY. Effect of boron content and welding current on the mechanical properties of electrical resistance spot welds in complexphase steels. Mater Des 2014;54:598–609. [9] Zhang Y, Wang C, Shan H, Li Y, Luo Z. High-toughness joining of aluminum alloy 5754 and DQSK steel using hybrid clinching–welding process. J Mater Process Tech 2018;259:33–44. [10] Liting S, Jidong K, Sigler DR, Haselhuhn AS, Carlson BE. Microstructure and fatigue behavior of novel multi-ring domed resistance spot welds for thin x626-t4 aluminum sheets. Int J Fail 2018:S0142112318303967-. [11] Beni SS, Atapour M, Salmani MR, Ashiri R. Resistance spot welding metallurgy of thin sheets of zinc-coated interstitial-free steel. Metal Mater Trans A 2019;50:2218–34. [12] Zivkovic M, Vukovic M, Lazic V. Experimental and FE modeling investigation of
594
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Resistance ceramic-filled annular welding of thin steel sheets Daxin Ren
a,b
b,c
d
, Dewang Zhao , Chenbin Li , Liming Liu
b,c,⁎
, Kunmin Zhao
T
a
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 School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China d Beijing Xinghang Electro-mechanical Equipment Co., Ltd, Beijing, 100074 People’s Republic of China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic-filled annular electrode Resistance welding High strength steel
To meet the requirements of automobiles for high-strength steel welding and thin-sheet welding, a composite ceramic-filled annular electrode (A-electrode) was designed by controlling welding energy distribution. By embedding a circular non-conductive ceramic rod in the center of a conventional copper electrode, the welding heat changed from centralization in the center of the nugget of traditional spot welding to a more dispersed annular distribution. As a result, an annular nugget that corresponds to the annular copper end face was formed. Given the energy dispersion, a higher current was required to bond the sheets by the A-electrode than that by a traditional electrode. However, the indentation depth on the surface of the nugget decreased effectively. The stress distribution indicated that the stress was concentrated at the edge of the nugget under the load, and the unconnected part in the middle did not affect the strength. The size of the annular nugget was designed by adjusting the structure of the A-electrode. The strength of the nugget could be improved by forming a large nugget area.
1. Introduction Resistance spot welding is currently one of the most widely used welding methods in the automobile manufacturing industry [1–3]. Reducing sheet thickness is an effective way to reduce the quality of car bodies [4–6]. However, some difficulties are still experienced with regard to thin-sheet and ultra-thin-sheet welding. Thin-steel sheets (0.7 and 0.6 mm, even 0.5 mm) are often used as the automobile panel [7,8]. However, these sheets are sensitive to welding heat input. As a result, deep indentation and even burn-through can easily occur with a narrow welding window. The failure load of the nugget is closely related to the diameter of the nugget. Thus, obtaining a large diameter nugget with only a small indentation on the surface of the nugget remains a challenge [9,10]. Some methods have been used to improve resistance spot welding, such as parameters optimization, welding process simulation, microstructure controlling, adding cover sheet, and hybrid clinchingwelding process [11–15]. Through the above methods, the strength has been effectively increased. The energy distribution of the welding heat source is the key to form nuggets and weld seam. Regardless of fusion welding or resistance spot welding, energy density has a great impact on weld seams and welding nuggets [16–19]. Once the current of resistance spot welding is switched on, heat is initially generated at the edge of the electrode and then ⁎
rapidly transferred to the center of the spot welding [20,21]. The diameter of the nugget increases with the welding time. In this process, the highest energy density occurs at the center of the nugget. The increase of the nugget size caused by increasing the welding current or the welding interval may easily lead to great indentation, splash, and other defects [22,23]. To regulate the distribution of the welding current, the surface shaping of nuggets can be controlled by increasing the diameter of the nuggets. In the present work, a composite ceramic-filled annular electrode (A-electrode for short) was developed. The design idea is to change the distribution of resistance heat in the resistance spot welding process to control the nugget formation. In the welding with this kind of electrode, the ceramics in the electrode center are not conductive. The resistance heat generated only through the surrounding annular area. Finally, the annular nucleus is formed after welding. For resistance spot welding using ceramic-filled A-electrode, the shaping of the nuggets can be designed in accordance with the diameter of the ceramics in the center and dimension of the annular region in the outer copper. Nugget formation can meet the need for welding strength under different conditions. A thin sheet (0.7 mm) for a car body panel, mild steel sheet (1.0 mm) were used to study the impact of welding parameters on mechanical properties and formation. Resistance spot welding using ceramic-filled annular electrode (AW) and traditional spot welding
Corresponding author at: 2 Ling Gong Road, Dalian, 116024, People’s Republic of China. E-mail address: [email protected] (L. Liu).
https://doi.org/10.1016/j.jmapro.2019.07.043 Received 15 April 2019; Received in revised form 1 July 2019; Accepted 30 July 2019 1526-6125/ © 2019 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 45 (2019) 588–594
D. Ren, et al.
Table 1 Material properties of welded sheets. Material
DC06 Q235
Material properties
Chemical composition wt.%
Yield Strength (MPa)
Tensile strength (MPa)
Elongation (%)
C
Mn
Si
S
P
138 235
302 375
24 26
0.006 0.14
0.28 0.35
0.02 0.3
0.01 0.04
0.01 0.03
(SW) were compared. The microstructure was then observed. Stress in tensile shear test was analyzed by numerical simulation. Finally, a tensile shear test peel test was performed to measure the mechanical strength
Table 2 Parameters of resistance welding.
2. Experimental materials and methods Two typical automobile steels were selected in the welding experiments. Here, 0.7 mm DC06 steel and 1.0 mm Q235 (ASTM A283-C) mild refer to the automobile steel. These steels are primarily used in manufacturing car body stamping parts, such as door and fender. The properties of the two types of steel materials are shown in Table 1. The particular ceramic-filled A-electrode used in this experiment was prepared as follows. First, a hole was drilled in the center of the copper electrode. A small amount of epoxy resin adhesives for automobile body was placed in the hole. Then, a ceramic rod with the same diameter as the hole was filled. Afterward, the adhesive was cured at 180 °C for 20 min in the resistance furnace. Finally, the electrode face was machined in accordance with the design size. When welding sheets of different thicknesses the electrodes with adequate diameters are needed. Because of lack of any previous experience with the welding using the A-electrode, the size of the electrode suitable for extreme conditions was adopted intuitively, see Fig. 1, to investigate the welding phenomenon. The white area in the center of the electrode was ceramic with a diameter of 4 mm, and the outer diameter of the annular copper area was 8 mm. The Circular face electrode with an end diameter of 8 mm was used to compare with the A-electrode. Mediumfrequency DC welder was used for welding, and the welding parameters are shown in Table 2. Sheets were welded as a lap joint with 100mm*25 mm size. Tensile shear strength and peel strength were evaluated at rates of 1.0 mm min−1 and 5 mm min−1, respectively. The tensile shear tests were operated on the basis of Chinese Standard GB2651-89. The geometry and dimensions of the test specimens are shown in Fig. 2. The mean values of the three specimens were obtained to evaluate the mechanical properties. In previous work, the resistance ceramic-filled annular welding
Parameters
Value
Welding current (kA) Welding time (ms) Electrode Force (kN)
6.4 100 150
7.2
8
8.8
9.6
10.4
11.2
12
Fig. 2. Test specimens’ geometry.
process was simulated by a thermal–electrical–structural coupling FE model. The Johns-Cook model was selected to simulate the constitutive relation of materials. The electrical resistance and thermal conductivity across the contact interfaces were obtained from the literatures [24–26]. C3D8R element was picked to model the work-pieces in thermal–electrical–structural coupled analysis. In this work, numerical simulation static analysis was conducted on the stress state in tensile shear test of two kinds of nuggets. The nugget regions are coupled by tie connection, which are consistent with the test. Other regions are established face contact, which are consistent with the previous work. The constitutive relation model is obtained from the literature [27]. C3D8T element was picked to model the work-pieces in static analysis. 3. Results and discussion 3.1. Nugget formation Fig. 3 shows the surface formation after the A-electrode was used to weld the DC06 sheet. The surface of the nugget corresponded to the face of the electrode, which was divided into two regions: a circular region in the middle and an annular region around the area. One of the most important characteristics of the spot nugget was small indentation on the surface even when achieving a large diameter. The cross sections of the Q235 spot nugget and annular nugget are shown in Fig. 4. The cross section of the annular nugget was divided into two parts. The annular width increased with the welding current increase. However, the depth of indentation on the surface changes slightly. By contrast, in welding using a circular electrode, The
Fig. 1. Appearance of the composite ceramic-filled annular electrode. 589
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Fig. 3. Surface morphologies of nugget using ceramic-filled annular electrode.
change in the microstructure compared with the base metal at the center of the annular nugget where the sheets cannot be bonded. Table 3 and Fig. 8 show the diameter and area comparison of the annular nugget and spot nugget area with the increase of the welding current. When an A-electrode was adopted, the inner diameter of the annular connection region gradually decreased, and its outer diameter gradually increased. The use of circular electrode can lead to a rapid increase in the size of nuggets when the current was small. with the current further increase, the size of the nuggets slowly increased. At a current of 12 kA, the nugget diameter was 7.1 mm and the bonding area 39.5 mm2. By contrast, although the joint formed by the annular nugget had a higher welding current threshold value than other joints, the area of the nugget rapidly improved as the current increases once the joint formed. At current of 12 kA, the area was 55.6 mm2. Once an effective bonding is formed using A-electrode, its outer diameter was 7 mm (the diameter of the annular center of copper materials,) and the large joint diameter guarantees failure load. 3.2. Mechanical properties An annular nugget has a particular shape. Its stress state under load is different from that of spot nugget. The space in the annular nugget may have an impact on the strength. Therefore, numerical simulation was used to analyze the stress state in tensile shear test of two kinds of nuggets. In the simulation, the outer diameter of annular nugget was same as that of the spot nugget. The stress distribution on the lower surface of the upper sheet is shown in Fig. 9. In terms of shear stress analysis, the stress distribution in the spot nugget was more uniform. The stress concentration positions of the two kinds of nuggets were similar, mainly occurring at the edge of the nugget. However, the maximum stress at the edge of the annular nugget was lower than that of the spot nugget. Area of high stress area in annular nugget was larger than that in spot nugget. This result is because joining did not form in the middle of the annular nugget, and the sheets at the nugget center were less constrained. Therefore, deformation may have occurred. This played a positive role in conduction and dispersion of stress. The maximum principal stresses at the edges of two kinds of nuggets were almost the same, but the internal stress of the annular nugget was slightly lower. As can be seen from the simulation results, the space between sheets at the annular center did not have negative impact on performance in tensile shear test. Shear stress of the annular nugget (a) and circular nugget (b),
Fig. 4. Cross-sections of the 1mm-Q235 nugget using annular electrode and circular electrode.
indentation depth increased rapidly with the increase of current. For the DC06 sheet, a similar result was observed, as shown in Fig. 5. The 0.7 mm sheet is ultra-thin. Hence, it is more sensitive to heat input in the welding process than the other sheets. It also has a higher ratio of the same indentation depth to the thickness of the sheet than the thicker sheets. Fig. 6 lists the ratios of indentation depth to the thickness of different sheets with the increase in current. The surface indentation of the nugget is inevitable. To ensure the performance and appearance of the nugget, the indentation depth is usually required to be less than 15% of the thickness of sheet. The current of the annular nugget on 0.7 mm DC06 was 11.2 kA, whereas the depth of indentation was only 8%. When the current is up to 12 kA, the indentation rate reached 11% of the thickness due to the occurrence of splash. The microstructures of DC06 nugget was similar to a traditional spot nugget as shown in Fig. 7. The molten zone was located at middle of the copper area of annular electrode. The DC06 base metal was composed of ferrite and a small amount of pearlite, and there was no significant 590
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Fig. 5. Cross-sections of the 0.7 mm-DC06 nugget using annular electrode and circular electrode. Table 3 Diameter of annular and spot nuggets. Welding current (kA) 6.4 RAW internal diameter (mm) RAW external diameter (mm) RSW diameter (mm)
5.0
7.2
5.4
8.0
8.8
9.6
10.4
11.2
12.0
6.3
5.9 8.1 6.7
5.2 8.8 6.8
4.9 9.0 6.9
4.7 9.2 7.0
4.4 9.5 7.1
Principal stress of the annular nugget (c) and circular nugget (d) The annular nuggets can provide higher tensile shear than the circular nuggets in a large welding parameter window. A comparative welding test of DC06 and Q235 was conducted by adopting A-electrode and circular electrode. The tensile test results are shown in Fig. 10. When the current was 8.8 kA, the DC06 nugget of weld ruptured in less than 2 kN. The fracture mode was interface fracture. When the current was larger than or equal to 9.6 kA, the failure load rapidly increased, and the fracture mode was pull-out fracture (as shown in Fig. 11). The starting position of the fracture was the edge of the nugget. In terms of strength, the maximum failure load obtained by the A-electrode was
Fig. 6. Comparison of indentation depth using annular electrode and circular electrode.
Fig. 7. Microstructures of DC06 annular nugget. 591
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Fig. 8. Comparison of Q235 nugget size using annular electrode and circular electrode.
diameter of 8 mm was adopted. Nonetheless, the diameter of the nugget was only 7.1 mm even if the welding current reached 12 kA. A great indentation also occurred on the circular nugget surface. When an annular joint was formed by an A-electrode, the larger radius of the annular nugget can help provide higher failure load than that provided by peeling with the use of a circular nugget. In terms of welding stability, the A-electrode produced fewer splashes than the circular electrode during the welding process. If the specified current value is exceeded, the welding splash by the circular electrode significantly increase. When 0.7 mm-thick DC06 sheet and 1 mm-thick Q235 were welded, the minimum current values of the produced splash were 9.6 and 10.4kA respectively by using circular electrode,; nevertheless, the minimum current were 12.0 and 12.6 kA respectively by using A-electrode. With the constant increase of the current, the splash considerably increased by the circular electrode. Particularly, for welding a 0.7 mm-thin sheet, the splash led to a great indentation depth on the surface of the nugget (as shown in Fig. 5). This
larger than that by using the circular electrode. For the relationship between welding current and failure load, a similar trend was observed in Q235. When the welding current was small, the failure load was low. However, when the current increased beyond a specific threshold value, the failure load rapidly increases, and the increasing trend was slow (Fig. 10). The A-electrode can provide higher peeling strength than the circular electrode. The peeling experiments for DC06 and Q235 are shown in Fig. 12. The results show that the peel failure load of DC06 spot nugget increased as the current increases. This finding is similar to the tensile test result. After the current reached 9.6 kA, the peeling strength accordingly decreased. Q235 exhibited a similar strength change. The peeling failure load initially increased and later decreased. The peeling strength of the A-electrode was relatively stable. As the current increased to a relatively high value, it changed within a small range. The size of the nugget had a relatively great impact on the peeling strength. On the basis of the results, a circular electrode with an electrode face
Fig. 9. Simulation of stress distribution in X diction on the lower surface of the upper sheet. Shear stress of the annular nugget (a) and circular nugget (b), Principal stress of the annular nugget (c) and circular nugget (d). 592
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Fig. 10. Comparison of tensile strength by using annular electrode and circular electrode.
corresponded to the face of the electrode, which was divided into two regions: a circular region in the middle and an annular region around the area. 2) Resistance heat was generated in the annular region and the energy density was further dispersed. Splash was reduced because of the energy dispersion distribution. The lowest current required to bond sheets by A-electrode is higher than that required by the circular electrode. A small indentation on the surface can be produced, while the connection area that was larger than that using a traditional electrode can be obtained. 3) The nucleation position of the nugget was at the center of the copper material. Furthermore, the annular width of the nugget increased with the welding current. Under the same end diameter, the Aelectrode can provide a larger area of the nugget than the circular electrode. 4) In welding with the A-electrode, a stable annular nugget was formed with the increase of the current. Afterward, a stable tensile strength and peeling-off strength can be obtained by forming a large nugget area in a large welding window.
condition is the underlying reason why the welding strength decreases after the welding current reached 11.2 kA. However, the welding strength remained high when A-electrode was used. The energy distribution of the welding heat source is the key to good nugget formation, high strength and welding stability. Circular electrode welding mainly produces resistance heat in the center, and causes a large concentration. The advantage is that sheets can be bonded with low heat input. However, increased heat accumulation in the center leads to excessive surface indentation and spatter. Given the isolation of ceramics, the center of the A-electrode does not produce resistance heat. Welding energy is generated in the annular region. In this way, the centralization of the traditional electrode energy at the center of the nugget is avoided. The energy density is relatively low because of the large conduction area, which means that the energy distribution is “soft”. Therefore, with the same external diameter as the traditional circular electrode, higher welding current is required to form a sheet bond with A-electrode. A conventional electrode can realize DC06-sheet connection at 8.0 kA; for the annular nugget, the lowest current value is required to reach 8.8 kA. Meanwhile, avoiding local energy concentration also decreases indentation depth and splash, so stable welding process and high strength are obtained.
Declaration of Competing Interest 4. Conclusions The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
A ceramic-filled A-electrode for resistance welding was designed in this study to address the difficulties in producing thin sheets for automobiles. Nugget formation and mechanical properties were also analyzed. Finally, the following conclusions are obtained: 1) When an A-electrode was used, the surface of the nugget
Fig. 11. Fractured joints in the tensile shear test and peel test of the annular nugget. 593
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Fig. 12. Comparison of the peeling strength using annular electrode and circular electrode.
Acknowledgement
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