Resistance Spot Welding of Ultra-Thin Automotive Steel

YangYang Zhao YanSong Zhang1 e-mail: [email protected] Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PRC

XinMin Lai State Key Laboratory of Mechanical, System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PRC

Pei-Chung Wang Manufacturing Systems Research Laboratory, General Motors R&D Center, Warren, MI 48090

1

Resistance Spot Welding of Ultra-Thin Automotive Steel One of the major challenges in spot welding of ultra-thin gage steel (e.g., 0.75 mm). In this study, the method of inserting flexible strips between the electrode and workpiece in resistance spot welding of 0.4 mm thick galvanized SAE1004 steel sheet has been adopted in order to reduce electrode tip temperature and improve weld quality. The effect of the inserted strips on the Joule heat generation and temperature distribution has been analyzed analytically. Then, because of the difficulties in measuring the experimental electrode tip temperature, a finite element model has been employed to estimate temperature distributions within the weld zone. The effects of the process variables (i.e., strip material and thickness) on the cap temperature and weld quality were modeled. Experiments were also conducted to validate the modeling results. Test data and modeling results showed that the presence of the strip significantly facilitated weld initiation and growth and decreased the rate of electrode degradation. Of the materials investigated, the desirable strip for resistance spot welding 0.4 mm thick galvanized SAE1004 steel was determined to be 0.12 mm thick Cu55Ni45 alloy. [DOI: 10.1115/1.4023367] Keywords: resistance spot welding (RSW), ultra-thin automotive steel, flexible strip, electrode tip temperature

Introduction

Two recent trends in the automotive industry are the improvement of corrosion resistance through the use of either aluminum or coated steel, and mass reduction either by the use of lighter materials or by optimization of sheet metal thicknesses throughout the vehicle structure. Thin sheets of coated steel have some advantages over the use of aluminum in terms of cost and stiffness [1]. Despite these advantages, there are limiting factors to the application of thin gage steels. One of the major challenges in spot welding of thin gage steel (e.g., 0.75 mm), which would increase the cycle time and production cost significantly. Electrode wear is inherent and plays an important role in resistance spot welding [2,3]. Previous studies [4–6] have shown that the rate and extent of the electrode wear were attributed to the excessively high temperature developed at the electrode/steel interface. An extraordinary short electrode life caused by the high temperature at the electrode surface makes resistance spot welding impractical for thin sheet welding. Factors influencing the electrode life include the welding process parameters, electrode designs, and efficiency of water cooling [7]. Various approaches, such as selecting the proper process variables, electrode designs and materials, coatings and improving the cooling system, have been used to improve the electrode life. In resistance spot welding of thicker sheets (i.e., >0.75 mm), a weld current stepping program is employed to compensate for electrode wear. Freytag [7] has shown that a current value toward the upper end of the available range will result in a longer electrode life. However, this protocol might not be suitable for thin gage sheets. A stronger current would result in higher temperature at the electrode/sheet interface and therefore increase the rate of electrode degradation. Extensive efforts have also been done on the

1 Corresponding author. Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received May 21, 2012; final manuscript received December 27, 2012; published online March 22, 2013. Assoc. Editor: Wei Li.

development of electrode materials and designs. Holiday et al. [8] claimed that certain electrode materials, such as Cu-Al2O3, can maintain a good mechanical performance when exposed to high temperatures and consequently can prolong the electrode life. Surface modification of the weld electrode by electro-spark deposited composite coatings provides another solution to minimize electrode wear and improve the electrode life [9,10]. Unfortunately, the aforementioned methods are costly for practical use. Furthermore, an optimal design of an electrode cooling system is also adopted to improve the cooling effect and prolong electrode life. However, according to the previous research [11] that cap cooling improvement might not be sufficient for welding of thin gage steel. Recently, resistance spot welding with a cover plate or flexible strip has attracted increasing attention. Qiu et al. [12] have already successfully welded aluminum to steel and magnesium alloy using this cover plate method. The inserted metals enhance the joule heat generation throughout the welding process and therefore facilitate weld formation and growth. Moreover, the presence of the strip increases the distance between the electrode tip and faying interface and consequently decreases the temperature at the electrode surface. The strip prevents the electrode from contacting the molten zinc directly, especially for resistance welding of zinc coated steel, which substantially decreases the extent of electrode surface alloying with zinc and thereby extends electrode life. In this study, the method of inserting the flexible metal strips, shown in Fig. 1, has been adopted for welding 0.4 mm thick galvanized SAE1004 steel. There are three main parts in this study. First, an analytical analysis has been carried out to study the difference between resistance spot welding with and without inserted strips, in terms of heat generation and temperature distribution. Furthermore, an incremental finite element model coupled with mechanical, electrical, and thermal fields was developed to investigate the feasibility of the method of inserting the strips on resistance welding of thin gage steel. The model was then employed to assess the effects of strip properties on the temperature distribution and electrode tip temperature. The preferable strip material for resistance welding of 0.4 mm thick galvanized SAE1004 steel was identified. Finally, the experiments were performed to validate the simulation results.

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Fig. 1 (a) Schematic and (b) experiment setup of resistance welding with inserted flexible strips

2

Analytical Analysis

2.1 Heat Generation. To analyze the heat generation, an analytical analysis of resistance welding of thin gage steel with inserted strip is developed. Figure 2 shows the schematics of resistance spot welding with inserted strips. As shown, due to the introduction of the metal strips, the joule heat generation would be different from traditional RSW process. To simplify the calculation of heat generation in resistance welding, the edge effect [13] and the weld current flowing out of the contact area, I’1, I’2, I’3, I’4 shown in Fig. 2 were neglected. And the weld current through the contact area can be assumed as I0 I1 I2 I3 I4

(1)

where I0 is the weld current applied through the electrodes, I1, I2, I3, I4 are the currents within the contact area through the top strip, top sheet, bottom sheet and bottom strip, respectively. As shown in Fig. 2, the joule heat generation during the welding can be divided into four parts: Q ¼ QSteel þ QStrip þ QInterface þ QE

(2)

Where QSteel and QStrip represent the joule heat generated by the sheet and strip within the contact area, respectively. QInterface indicates the heat generation at the five interfaces (top electrode/ top strip, top strip/top sheet, sheet/sheet, bottom sheet/bottom strip, and bottom strip/bottom electrode). QE is the joule heat pro-

duced by the two electrodes. Due to the superb electrical conductivity of copper, the resistance heat generated by the copper electrodes QE is negligibly small. According to Joule’s law, the heat generation of each part within a time increment, Dt can be expressed as follows: q2 l2 Dt prc2 q l1 ¼ Q1 þ Q4 ¼ 2I02 1 2 Dt prc

QSteel ¼ Q2 þ Q3 ¼ 2I02

(3)

QStrip

(4)

QInterface ¼ I02 ðRC1 þ RC2 þ RC3 þ RC4 þ RC5 ÞDt

(5)

where Rc1, Rc2, Rc3, Rc4, Rc5 are the resistances at the five interfaces, respectively, q1, q2 are the resistivity of the strip and sheet, and rc is the radius of the contact area. Compared with a traditional RSW process, QStrip is an additional part of heat generation caused by the resistivity of the inserted strip. According to Eq. (4), QStrip is strongly affected by the thickness and resistivity of the inserted strip. Furthermore, the inserted strips create two extra interfaces and consequently result in greater contact resistance and heat generation at the interfaces, QInterface. 2.2 Electrode Tip Temperature. To examine the effect of the presence of the inserted strip on the heat generation and temperature at the electrode-to-workpiece interface, the analytical analysis was performed based on the model developed by Gould et al. [14,15] under the following assumptions: (1) Heat flow from the workpiece into the electrodes is simplified to one dimension. (2) The peak temperature distribution in the resistance spot weld can be described by a sine wave half period, with the peak at the faying interface of the workpieces. (3) The top and bottom electrodes are essentially straight sided. The variation in temperature in the spot weld can be expressed as follows [14]: 2 kE Dx p cos x 1þ p kS DxE 2Dx (6) H ¼ HP 2 kE Dx 1þ p kS DxE

Fig. 2

Heat generation during RSW process

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where HP is the peak temperature in the spot weld, Dx is the thickness of the workpiece, DxE is the electrode face thickness and kE and kS are the thermal conductivities of the electrode material (Cu) and workpiece, and x is the distance from the weld faying surface toward the electrode face. Transactions of the ASME


According to Eq. (6), the temperature at the electrode tip HE of traditional RSW is estimated as HE ¼

H P 2 kE Dx 1þ p kS DxE

(7)

The calculation of HE should be carried out in two stages when the strip is inserted. The temperature at the steel/strip interface HS should be calculated first as HS ¼

H P 2 kStrip Dx 1þ p DxStrip kS

(8)

where kStrip and Dxstrip are the thermal conductivity and thickness of the inserted strip, respectively. Then, the HE of RSW with the inserted strip can be estimated as

HE ¼

H S DxStrip 2 kE 1þ p kStrip DxE

(9)

In sum, the temperature developed at the electrode surface HE can be expressed as:

8 ; HP > traditional RSW > > 2 kE Dx > > < 1þ p kS DxE HE ¼ HP ; RSW with strips > > > DxStrip 2 kStrip Dx 2 kE > > 1þ 1þ : p DxStrip p kStrip kS DxE

(10)

Then, the HE of traditional RSW of 0.4 mm thick SAE1004 steel and RSW with 0.10 mm thick AISI304 strips are as follows: 8 1725 > ¼ 972 o C ; traditional RSW > > > 2 0:941 0:4 > > > ¼ 690 o C with strips > > 2 0:036 0:4 2 0:941 0:1 > > : 1þ 1þ p 0:124 0:1 p 0:036 2:5

where the HP of traditional RSW is assumed to be 1725 C, and HP of RSW with 0.10 mm thick AISI304 strips is 2000 C. Dx ¼ 0.4 mm, Dxstrip ¼ 0.1 mm, DxE ¼ 2.5 mm, and thermal conductivities kE, kS, and kStrip are 0.941, 0.124, 0.036 Cal/cms C [16], respectively. As shown in Eq. (11), the temperature at the electrode tip, HE, is decreased by using the strips. However, it is difficult to estimate HP quantitatively by the analytical method due to the nonlinearity of the RSW process, and a series of assumptions and simplifications made in the formulations. Therefore, a finite element method was employed to estimate the electrode tip temperature.

3

Finite Element Modeling

To improve the electrode life and weld size, an understanding of heat generation and temperature distribution in the spot welding is imperative. In this study, finite element techniques [17–23] have been used to assess the effect of the metal strip on the temperature distribution in resistance welding of thin gage steel. A finite element model has been generated using ANSYS software [24]. Details of the geometric model and boundary conditions are described in the following. 3.1 Model 3.1.1 Geometric Model. The physical configuration of the resistance spot welding process can be simplified to a twodimensional axisymmetric model. Figure 3(a) shows the element grid used in the present analysis. The model was composed of 2966 nodes and 3174 elements, wherein 672 of them were contact elements [24] which were inserted between the electrode/strip, strip/sheet, and sheet/sheet interfaces. Refined meshes shown in the enlarged area of Fig. 3(a) were used to the capture the region where the temperature gradient would be large. Journal of Manufacturing Science and Engineering

(11)

3.1.2 Boundary Conditions. Two types of boundary conditions shown in Fig. 3(b) were applied to the finite element model as follows: The thermal-electrical boundary conditions are: (1) The electrical potential at the bottom end of the lower electrode was assumed to be zero, and the current was applied uniformly to the top of the upper electrode. (2) The convective heat transfer to the surrounding air was considered by using a convective heat transfer coefficient of 19.4 Wm2 K1 [20]. (3) Both the ambient air and initial water temperatures were assumed to be 21 C. Thermal-mechanical boundary conditions are: (1) Electrode force was applied evenly to each nodal point at the top end of the upper electrode. (2) The displacement of the nodes on lower surface of the electrode was set to zero. (3) The centerline of the model was constrained to extend only along the Y-axis, without displacement along the X-axis. 3.1.3 Material Properties and Welding Parameters. In order to estimate the RSW process properly, all of the relevant mechanical (i.e., modulus, Poisson ratio, and stress–strain relationship), physical (i.e., density, thermal conductivity, expansion coefficient, and specific heat) and electrical (i.e., resistivity) properties for the steel sheets and copper electrode are needed. Since mechanical, electrical, and physical properties under elevated temperature are not readily available, many of these values are estimated from literature and assumed to be homogeneous and isotropic [25,26]. Details on the determination of material properties are given in Refs. [16] and [18]. Contact resistivity for all the faying interfaces (sheet-to-sheet, electrode-to-strip, strip-to-sheet) was also APRIL 2013, Vol. 135 / 021012-3


Fig. 3 (a) Geometric model and (b) boundary conditions for modeling of resistance welding of thin gage steel Table 1 Welding parameters Welding parameters

Value

Electrode diameter Electrode force Welding current Squeeze time Weld time Hold time Cooling water flow rate

5 mm 1.8 kN 5.7 kA 200 ms 160 ms 40 ms 3 L/min

assumed to vary with temperature [27–29]. The welding parameters used in finite element analysis are given in Table 1, unless otherwise specified, the welding current is 5.7 kA. 3.2 Effect of the Inserted Strip on Resistance Spot Welding 3.2.1 Contact Status. The introduction of the flexible strip might alter the contact characteristics between the workpieces. To understand how the inserted strips would affect the contact behavior

between the workpieces, finite element modeling of the squeeze cycle in resistance welding of 0.4 mm thick SAE1004 steel was performed and the results are shown in Fig. 4. As shown in Fig. 4(a), the radius of the contact area decreased from 3.0 mm to 2.8 mm with the insertion of the strip. The decreased contact area would result in greater current density at the sheet/sheet interface shown in Fig. 4(b) for RSW with strips. Since the joule heat is directly proportional to the square of the current density, the heat generation would increase by making use of the inserted strips. 3.2.2 Temperature Distribution. The effect of introducing a flexible strip on the joule heat generation and temperature distribution was analyzed by the aforementioned model. The comparison of temperature distribution was based on the premise that the weld size was identical which necessitates unique welding parameters for traditional RSW and RSW with 0.10 mm thick AISI304 strips. For the condition with strips, the welding current was 5.7 kA, while the traditional RSW welding current was required to increase from 5.7 kA to 6.5 kA to achieve the same weld nugget size. Figures 5(a) and 5(b) present the calculated temperature contours for resistance welding of 0.4 mm thick SAE1004 steel sheets

Fig. 4 Distribution of (a) contact pressure and (b) current density at the sheet/sheet interface

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Fig. 5 Typical temperature contours at the instant that current is terminated (a) without strips under welding current of 6.5 kA, (b) with 0.10 mm thick AISI304 strips under welding current of 5.7 kA and (c) temperature at the electrode surface versus welding time, and (d) temperature distribution from the weld center to the electrode at the end of welding along the Y-axis

without and with the presence of 0.10 mm thick AISI304 strips, respectively. The calculated weld size, shown in gray color in the calculated contours of temperature distribution, is defined as the region where the temperature exceeds the melting point (1500 C) of the steel. As can be seen, the weld diameters of the two welding types were the same even the required welding current for RSW with the strips was 800 A lower than without the strips. This is an indication of the positive effect the inserted strips have upon heat generation. The temperature history at the electrode surface (T1) and sheet/strip interface (T2) during the welding process are presented in Fig. 5(c). As can be seen, the temperature at the sheet/strip interfaces (T2) increased from 856 C to 1330 C which is attributed to the joule heat contribution from the strips themselves and the additional sheet/strip interfaces which enhance the heat generation in the welding process (see Eqs. (3)–(5)). However, the calculated electrode tip temperature decreased from 856 C to 714 C. This temperature decrease can be explained by the results presented in Fig. 5(d). The temperature variation along the Y-axis can be divided into three parts, namely, steel sheet, strip, and electrode. While the temperature distribution of the electrode is comparatively uniform due to the extraordinary thermal conductivity of the copper alloy, there is a steep decrease in temperature where the strip is positioned. The presence of the strip increases the thermal resistance between the steel sheet and electrode thereby inhibiting the heat dissipation to the electrode surface. 3.2.3 Effect of the Electrode Force. While the effect of the welding current and time on heat generation is revealed, the influence of the electrode force is not clear. Figures 6(a) and 6(b) illustrate the effect of the electrode force on the distribution of contact pressure at the sheet/sheet interface and temperature distribution at the weld center. As can be seen, although the electrode force Journal of Manufacturing Science and Engineering

significantly affects the contact pressure, it exhibits little influence upon the contact radius and heat generation. These results suggest that the fluctuation of the electrode force in the current study has little effect upon the weld size. The question as to if the method of inserting the strips is effective in enhancing the heat generation and reducing the electrode tip temperature in resistance spot welding of 0.4 mm thick SAE1004 steel has been explained by both analytical and finite element analysis. Next, the influence of strip properties on the heat generation and temperature distribution is investigated. As Eqs.(4) and (10) illustrated, the heat generation during the welding process is strongly influenced by the strip resistivity and thickness, while the electrode tip temperature primarily depends on the thermal conductivity and thickness of the strip. Since the resistivity and thermal conductivity of a material are closely related [13], the resistivity and thermal conductivity of the strip are combined as one factor, namely strip material. 3.3 Effect of Material Properties of Inserted Strip. Because of the high temperature and electrode pressure during the resistance welding process, the selection of the strip material tends toward metals having a high electrical and thermal conductivity and a high melting point. Based upon these criteria, various metals were selected and listed in ascending order of resistivity, referred to Table 2. In this section, the thickness of strip was fixed at 0.1 mm. Finite element modeling of resistance welding of 0.4 mm thick SAE1004 steel with various strips shown in Table 2 was performed and the results are presented in Fig. 7. As shown, the temperature developed within the steel sheet differs widely, being higher for the strips with higher resistivity and consequently, the weld size increases with the resistivity of the strip material. These APRIL 2013, Vol. 135 / 021012-5


Fig. 6

(a) Contact pressure at sheet/sheet interface and (b) temperature at the weld center under different electrode force

Table 2 Strip materials and resistivities [16] No. 1 2 3 4 5

Material

Resistivity (lXm)

Copper SAE1004 steel Dual phase steel Cu55Ni45 alloy AISI304

0.022 0.142 0.284 0.498 0.730

results are attributed to the fact that joule heat generation is directly proportional to the resistivity of the material, as indicated in Eqs. (3)–(5). According to the American Welding Society Standard [30], the minimum acceptable weld size for a 0.4 mm thick sheet is 3.0 mm. However, the heat generation is not sufficient to form a decent weld by using the strips made of copper and SAE1004 steel. The materials with comparatively low resistivity have an insufficient heat generation and produce an undersized weld. Furthermore, the copper with good thermal conductivity dissipates the heat quickly away from the molten metal and consequently cannot produce a weld. Thus, copper and SAE1004 are excluded from consideration as a strip material for resistance welding of 0.4 mm thick galvanized SAE1004 steel sheet. For the conditions given here, a strip material with resistivity higher than the workpiece would be beneficial for enhancing the heat generation and weld size.

While the heat generation and weld size directly increases with the strip resistivity, its effect on the electrode tip temperature is complex. As presented in Fig. 7(a), the temperature distribution can be divided into three parts, namely, the sheet, strip, and electrode. For the strip part, the temperature gradients differ greatly, being steeper for the strip materials with lower thermal conductivity. From this point of view, the strips with lower thermal conductivity may be better for lowering electrode tip temperature. However, modeling results showed that the electrode tip temperature increased instead after a slight drop when the strips are changed from dual phase steel to AISI304. As the strip resistivity increases, the joule heat generation increases. But the thermal conductivity of the strip decreases simultaneously and this hinders the heat transfer from the sheet through the strip and to the electrode. At first, the descending thermal conductivity suppresses the heat transfer. After that, as the resistivity increases, the incremental heat generation overwhelms the inhibition of heat transfer and consequently results in an increase in tip temperature. Based on this investigation, strips made of Cu55Ni45 alloy which leads to moderate heat generation and electrode tip temperature is more suitable for RSW of 0.4 mm thick galvanized SAE1004 steel. 3.4 Effect of the Strip Thickness. According to the results above, strips made of Cu55Ni45 alloy which results in sufficient heat generation and comparatively lower electrode tip temperature was determined as the strip material for the RSW of 0.4 mm thick

Fig. 7 Calculated effect of the resistivity of the strip material on temperature distribution along the (a) Y-axis and (b) X-axis in resistance welding of 0.4 mm thick galvanized SAE1004 steel

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Fig. 8 (a) Calculated effect of Cu55Ni45 alloy strip thickness on temperature distribution and (b) electrode tip temperature/temperature at sheet/strip interface versus strip thickness relationship

galvanized SAE1004 steel. The effect of the strip thickness on the temperature distribution was analyzed and the results are presented in Fig. 8. Figure 8(a) shows the calculated effect of the strip thickness on the temperature distribution during resistance spot welding of 0.4 mm thick SAE1004 steel. An increase in strip thickness leads to greater resistance and consequently more joule heat generation. Thus, the temperature developed within the steel sheet increases with the strip thickness. However, the thickness of Cu55Ni45 strip had little effect upon the temperature developed at the surface of the electrode. Figure 8(b) depicts the correlations between the temperature at the electrode tip and sheet-to-strip interface versus strip thickness. As shown, the electrode tip temperature decreases slightly as the strip thickness increases. The electrode tip temperature is a function of the comprehensive function of heat generation and dissipation. On the one hand, the heat generation is enhanced as the strip thickens. On the other hand, an increase in strip thickness inhibits the heat transfer from the steel sheet through the strip to the electrode tip. As a result of the balance of heat generation and dissipation, the temperature at the electrode surface exhibits only slight variation within the parameters of the current study. However, one cannot draw the conclusion that the thicker the strip thickness the better. The temperature at the sheet/strip interface is another constraint for the usage of the strip. The strip cannot be allowed to melt during the welding process, otherwise a weld would form between the strip and steel sheet, which is undesirable. Given this constraint, the optimal thickness of Cu55Ni45 alloy strip for the current study is 0.12 mm. Modeling results showed that the insertion of the strips could enhance weld size and reduce the electrode tip temperature, and furthermore indicated the optimum strip thickness for welding 0.4 mm thick SAE1004 steel. To validate the modeling results, experiments were performed and the results are presented in Sec. 4.

4

Experimental Validation

Modeling results revealed that strips made of Cu55Ni45 alloy with a thickness of 0.12 mm are desirable for RSW of 0.4 mm thick SAE1004 steel. In this section, tests were performed to validate the modeling results. 4.1 Experimental Procedure 4.1.1 Material. The workpiece used here was a 0.4 mm thick galvanized (46/43 gm2) SAE1004 steel, with the following chemical compositions (wt. %): 0.04C, 0.01Si, 0.23Mn, 0.01P, 0.01S and Fe balance. 0.10 mm thick AISI304 and 0.12 mm thick Cu55Ni45 were also selected for strips in this study. Resistance Journal of Manufacturing Science and Engineering

spot welding without a strip was performed on two stacked sheets of the SAE1004 steel as a baseline. The strips were then inserted between the top electrode and upper sheet in addition to the bottom electrode and lower sheet. 4.1.2 Sample Fabrication. Spot welding was performed using a midfrequency DC welding machine, where current was conducted through a class I copper–chromium electrode with a face diameter of 5 mm. Lap-welded joints were made using test coupons cut to approximately 100 mm long and 38 mm wide. Unless it was explicitly stated, the welding current used in the following tests was 5.7 kA and the other welding parameters were as listed in Table 1. Weld nugget size is used as an indicator for weld quality. The minimum acceptable weld size is 3.0 mm for a 0.4 mm thick steel sheet according to American Welding Society Standard [30]. The actual weld nugget size was estimated by measuring the diameter of pullout buttons during the peel tests. Metallographic examinations were also conducted to inspect the weld discrepancies. The weld size was determined by the average value of three replicates for each welding condition. 4.1.3 Microscopic Analysis. In order to investigate the changes in the electrode face profile resulting from the degradation process, the electrodes were cross sectioned, mounted and polished after termination of the electrode wear tests. The obtained samples were then etched in a 4% Nital solution and examined by scanning electron microscope (SEM) examination and energy-dispersive spectroscopy (EDS) analysis. 4.2 Effect of Electrode Force. Since cosmetic quality is important for thin gage welding, tests were conducted to examine the effect of the electrode force on the weld indentation. Figures 9(a)–9(d) present the top view and cross section of spot welded 0.4 mm thick SAE1004 steel without and with the 0.10 mm thick AISI304 strip. As shown in Figs. 9(a) and 9(c), the joint without strips exhibited a brown appearance, which is attributed to an excessively high temperature at the electrode face, and very little copper residue on the sheet surface. Furthermore, the weld indentation was also measured and shown in Figs. 9(b) and 9(d). As shown, the weld indentation was shallower when the strips were inserted, which can be attributed to the fact that the strips bear a portion of the deformation during the welding process. The effect of electrode force on the weld indentation for resistance welding of 0.4 mm thick steel with inserted strips was also investigated and the results are shown in Figs. 9(c)–9(h). Test results showed that an increase of the electrode force slightly increased the weld indentation. As shown, the measured weld indentations still did not exceed the 20% of the sheet thickness limit [31]. Another thing to note is that the electrode force APRIL 2013, Vol. 135 / 021012-7


Fig. 9 Top view and typical cross section of spot welded galvanized 0.4 mm thick SAE1004 steel (a), (b) without strip under 5.7 kA, 160 ms, 1.8 kN, (c)–(h) with 0.10 mm thick AISI304 strip under 5.7 kA, 160 ms and (c), (d) 1.8 kN, (e), (f) 1.4 kN and (g), (h) 2.2 kN

Fig. 10 (a) Effect of strip on the weld formation and cross section of spot welded galvanized 0.4 mm thick SAE1004 steel, (b) with 0.10 mm thick AISI304 strips, and (c) without strips under the weld condition of 1.8 kN, 5.7 kA and 160 ms

exhibited little influence upon the weld size in either radial or depth directions. These results are consistent with the modeling results presented in Fig. 6. 4.3 Effect of Flexible Strip on the Weld Formation. To assess the effect of the flexible strip on the weld formation, two 021012-8 / Vol. 135, APRIL 2013

different strips were investigated; 0.10 mm thick AISI304 strip and 0.12 mm thick Cu55Ni45 strip. To determine the weld initiation and growth, the welding time was increased from 0 to 160 ms in increments of 20 ms, and the weld size versus weld time were recorded and are presented in Fig. 10(a). As can be seen, welding with strips resulted in not only a 20 ms earlier weld initiation but also an approximate 20% increase in weld size. Figures 10(b) and 10(c) Transactions of the ASME


Fig. 11 Effect of the inserted strip on the (a) electrode profiles and (b) growth in electrode surface diameter during electrode wear test under the weld condition of 1.8 kN, 5.7 kA and 160 ms

show the cross sections of the weld size for the cases with and without strips. As shown, the calculated weld sizes have good agreement with the measured results. In addition, the weld size of using 0.1 mm thick AISI304 strip is larger than that of with 0.12 mm thick Cu55Ni45 strip. This is primarily attributed to the high resistivity of AISI304 material. 4.4 Effect of Inserted Strip on Electrode Wear. The extent of the electrode degradation is primarily governed by the tempera-

ture developed at the electrode surface. The growth of the electrode tip diameter is closely related to the amount of electrode deterioration. Electrode tip diameter measured during the electrode wear test is often used as an indicator of the electrode deterioration. In this study, the electrode wear test was conducted under a single set of welding parameters (referred to Table 1 with the welding current of 5.7 kA) and at a welding rate of one weld per 2 s. The electrode face diameter was measured using the carbon imprint method at intervals of 100 continuous welds.

Fig. 12 SEM observation and EDS analysis of the electrode after 600 welds, (a), (b) without strip; (c), (d) with 0.10 mm thick AISI304 strip and (e), (f) with 0.12 mm thick Cu55Ni45 strip in resistance welding of 0.4 mm thick galvanized SAE 1004 steel

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Figures 11(a) and 11(b) are the compilation of the electrode profiles and electrode face diameters used in resistance welding of 0.4 mm thick galvanized SAE1004 steel with different strips. The surface profiles differ significantly between the electrodes tested with and without strips. In reference to the modeling results shown in Fig. 5(c), the electrode tip temperature without strips is about 850 C, which is too harsh for the copper–chromium electrode to bear. Intensive surface alloying and fast degradation may occur under such high temperatures. In fact, it can be observed that a silvery layer was formed on the electrode face, resulting in a rough surface. However, with the introduction of 0.10 mm thick AISI304 strip and 0.12 mm thick Cu55Ni45 strip, the electrode surface was in fact, still fairly smooth and shiny which can be attributed to the decrease of electrode tip temperature. Moreover, the rate of the growth in electrode surface diameter decreased significantly with the use of a thin strip. As shown in Fig. 11(b), the cap diameter grew to about 5.3 mm after 300 welds using traditional RSW yet the introduction of a 0.10 mm thick AISI304 strip meant that it tool at least 600 welds to reach the same level of degradation; a 100% improvement which is the result of the reduced electrode tip temperature in accordance with the modeling results. The extent of alloying is dependent on the temperature developed at the electrode surface. To examine the surface alloying of tested electrodes, SEM and EDS analyses were employed. Figure 12 shows the analysis results for the electrodes after 600 welds under three experimental conditions for the 0.4 mm thick galvanized SAE1004 steel: (1) without strip, (2) with 0.1 mm thick AISI304 strip and (3) with 0.12 mm thick Cu55Ni45 strip. As shown in Fig. 12(a), a 25 lm thick alloy layer accumulated on the electrode surface after 600 welds without strips. However, the average thickness of the alloy layer decreased to less than 10 lm when welding with 0.1 mm thick AISI304 strips, while the alloy layer was only 2 lm by making use of 0.12 mm thick Cu55Ni45 strip, as shown in Figs. 12(b) and 12(c). Besides, the distribution of alloy layer at the electrode face using the 0.12 mm thick Cu55Ni45 strips appeared to be more uniform than that of AISI304 strips. Furthermore, as shown in Fig. 12(b) the constituent of the alloy layers differed significantly. With the application of the two types of strips, the leading content of the alloy layer changed from iron to copper. The thinner thickness and different constituents of the alloy layers implies that the extent of surface alloying occurring at the electrode surface decreased. This can be attributed to the reduced electrode temperature caused by the presence of the strips. Although the electrode tip temperature has not been measured experimentally, these results indicate that the application of a strip in resistance welding of thin gage steel reduced the cap temperature and prolonged the electrode life.

5

Conclusions (1) A method of using a disposable flexible strip at the electrode/sheet interfaces has been developed for resistance welding 0.4 mm thick galvanized SAE1004 steel. This method not only improved the heat generation but effectively lowered the temperature developed at the electrodeto-sheet interface. (2) The application of the strips significantly facilitated weld initiation and growth. (3) The introduction of a strip between the electrode and workpiece, preferably 0.12 mm thick Cu55Ni45 alloy strip, significantly decreases electrode wear.

Acknowledgment This research was supported by the General Motors Collaborative Research Laboratory at Shanghai Jiao Tong University. This research was also sponsored by Shanghai Rising-Star Program (11QA1403600) and Project Nos. 50905111 and 51275304 supported by National Natural Science Foundation of China. 021012-10 / Vol. 135, APRIL 2013

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