- Email: [email protected]

S0924-0136(17)30120-6 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.03.027 PROTEC 15170

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

11-11-2016 24-3-2017 25-3-2017

Please cite this article as: Rao, Shravan Singh, Chhibber, Rahul, Arora, Kanwer Singh, Shome, Mahadev, Resistance spot welding of galvannealed high strength interstitial free steel.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.03.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Resistance spot welding of galvannealed high strength interstitial free steel

Shravan Singh Rao (a), Rahul Chhibber (a), Kanwer Singh Arora (b), Mahadev Shome (b)

a) Department of Mechanical Engineering, Indian Institute of Technology, Jodhpur b) R&D, Tata Steel Limited, Jamshedpur

Corresponding Author: Rahul Chhibber Mechanical Engineering Department, IIT Jodhpur, Jodhpur 342001, India. Email: [email protected] and [email protected]

ABSTRACT: Variation in dynamic contact resistance with the change in welding process parameters such as weld current, weld time and electrode force were taken into account for establishing the range of adequate nugget formation parameters. Influence of the welding process parameters on the shear – tensile strength, nugget diameter and the observed failure mode was analysed. The adequate resistance spot welding process parameters for galvannealed high strength interstitial free steel sheets of 1.6mm thickness were estimated as 8kA weld current, 250ms weld time and 3.5kN electrode force. Increase in the mean dynamic contact resistance led to a significant reduction in nugget diameter. A critical nugget diameter

1

distinguishing between the IF and PF mode was experimentally determined by failure mode analysis. Different numerical models for estimation of critical nugget diameter were evaluated.

KEYWORDS: Dynamic contact resistance; Nugget formation; Resistance spot welding; Galvannealed steel; Failure mode.

INTRODUCTION Resistance spot welding (RSW) is widely used in automotive Industries. It is comparatively a clean process as it does not involve any filler material. The joint is created with the application of pressure and heat. In the case of RSW, the flow of electric current causes heating. This heating further leads to an occurrence of localized melting and coalescence of a small volume of the material. This localized heat input is estimated as a product of squared value of weld current times the electrical resistance of material to be welded. The electrical contact resistance of the material plays an important role in the nugget formation during spot welding. It is difficult to monitor the nugget formation as the nugget is not directly exposed and exists between the electrodes. Aktas et al. (2012) used the RSW process to join DP600 steel sheets of different thicknesses and compared the strength of the joints under shear – tensile and tensile – peel loading and also studied the microstructure and hardness variations in the weld specimens. Tsai et al. (1992) used finite element methods and found the weld nugget to initiate in a ring shape at a certain distance from the electrode centre which expands both inward and outward during the welding process to form the nugget. For analysing the spot welding process, a system is needed which can evaluate the process dynamically and monitor the different process parameters. Dickinson et al. (1980) built a dynamic electrical monitoring system to non – discretely monitor the current, voltage, resistance and power during RSW and 2

related the dynamic resistance curves obtained to the nugget formation phenomenon during the spot welding process. Luo et al. (2016) monitored the change in welding current and electrode voltage in real time in the secondary circuit. Cho and Cho (1985) presented an analytical model to predict the resistance variation during the resistance spot welding and studied the complex phenomenon of thermo – electric interaction at the weld interface to analyse both temperature and voltage distributions in the weldment. Cho and Rhee (2003) analysed the effect of weld thickness, corona bond and resistivity on the dynamic contact resistance and observed the nugget formation phenomenon using high speed camera. Shome (2009) has analysed and compared the dynamic contact resistance (DCR) of DP590 and DP780 steel weld joints. Kianersi et al. (2014) studied the optimization of RSW welding parameters for joining of austenitic stainless steel sheets and defined the transition region between interfacial and failure modes as the optimum welding condition. Gedeon and Eager (1986) studied the material variations and process modifications in order to determine their effects on the adequate range of spot welding parameters for galvanized steel sheet and performed dynamic inspection monitoring of welding parameters on resistance spot welding of uncoated and coated steel sheet. Tumuluru (2007) in his study of Dual Phase (DP) steel found that welding current range for galvannealed steel is wider as compared to galvanized steel, though the type of coating does not affect shear – tensile strength. During the process of galvannealing, reconstruction of the cold rolled surfaces takes place that leads to the development of gamma phase. In the Interstitial Free (IF) steels no interstitial solute atoms are available to cause straining of solid iron lattice and so IF steel is very soft. Satyendra (2014) explained the production process of the IF steels. The carbon content of IF steels is extremely low which is obtained by vacuum degassing process for the removal of CO, H2, N2 and other gases during steel making. The minor actinides i.e. Ti and Nb are added after the process of vacuum degassing in order to form carbide and nitride precipitates and consequently making the matrix

3

almost pure. The yield strength of IF steels is normally low with high formability. Bhadeshia (2010) studied phase transformation during spot welding of IF steels and found that the central region forming the nugget comprises of equiaxed grains, followed by columnar grains in the heat affected zone. Hayat (2011) analysed and compared the resistance spot weldability of IF steel with bake hardening steel and austenitic stainless steel. It is evident from the above mentioned literature that various research investigations have been carried out on resistance spot welding of steels. It is also observed that studies related to nugget formation phenomenon with parametric variations for IF steels and their failure analysis has been rarely reported despite its high significance. The present study aimed at providing the adequate nugget growth parameters and studying the corresponding variations in the shear – tensile strength and monitoring the dynamic contact resistance during welding process with the corresponding variations in the resistance spot welding process parameters. The study also analysed the failure mode transition with the variations in different process parameters. EXPERIMENTATION The material used for the experimentation was Galvannealed High Strength Interstitial Free (HIF) steel sheets of thickness 1.6mm. Table 1 shows the mechanical properties and chemical composition of the base material used for experimentation. The mechanical properties were determined through tensile testing on Universal Testing Machine and the chemical composition was determined using optical emission spectroscopy. The mechanical characteristics of welding machines i.e. moving mass and machine stiffness have a significant influence on the weld quality. The machine used for the process of resistance spot welding was 150kVA pedestal Medium Frequency DC machine. The bottom and top electrode material was copper – chromium alloy with truncated cone electrode cap diameter of 16mm and electrode tip diameter of 6mm.

4

A rogowski coil was mounted on the lower electrode arm for the measurement of high speed current pulses and alligator clips were fixed on the electrodes in order to acquire the voltage data. The current and voltage data was recorded using Miyachi make Weld checker 370B. Gedeon et al. (1987) obtained the dynamic inspection information for galvannealed and uncoated steel sheets and explained the weld formation and growth mechanism conveying the importance of dynamic electrical resistance for both coated as well as uncoated steel. The dimensions of weld specimen also possess a very significant influence on shear – tensile test results. Zhou et al. (1999) in their study on identifying critical specimen sizes for shear – tensile tests suggested that the occurrence of undesirable failure modes directly signifies that the specimen width is not enough. In the present study, weld coupons measuring 105 mm x 45 mm were overlapped in such a way that the overlap length is 35mm as shown in figure 2. The coupons used were as per BS1140:1993 standards. A fixture was used for the welding of coupons with the required dimensions.

The welding electrode force is generally assumed as a constant during resistance spot welding process however in reality it changes during the process due to many factors i.e. weld schedules and machine characteristics. In the present study also the welding force was assumed constant despite the changes occurring due to weld schedules and machine characteristics.

Figure 3 shows the dynamic contact resistance (DCR) curve at a welding current of 8kA, weld time of 250ms and electrode force of 3.5kN, indicating the Rmin and β peak in the curve. A number of specimens were welded using one factor at a time (OFAT) approach by varying a particular weld parameter and keeping the other parameters constant for different investigations. The specimens welded at weld current of 7, 8 and 9kA with varying weld time at a con-

5

stant electrode force of 3.5kN. Investigations were also carried out by varying electrode force at constant weld current of 8kA and weld time of 250ms. Some other parameters were kept constant during the welding i.e. hold time was taken as 200ms and squeeze time as 470ms. The adequacy of nugget formed was studied at different weld parameters including 4 to 10kA of weld current. Shear – tensile testing of the welds was done in order to determine the shear - tensile strength and failure energy. The welded spots were sectioned across the centre line and polished. Etching of the section was done using Picral solution for 5 seconds, followed by 2% Nital solution for 15 seconds at room temperature. Nugget diameter of the welds was determined. The failure modes were also observed during shear – tensile tests to obtain a critical nugget diameter. RESULTS & DISCUSSION The initial tests were carried out to estimate the range of adequate nugget formation. Nugget growth can be estimated as a function of weld time and current. It comprises of four sequential stages: incubation period, rapid growth, reduced growth and expulsion. Figure 4 shows the DCR curves for variation in weld current values from 4 to 10kA with a constant weld time of 250ms and constant electrode force of 3.5kN. It was found that in the case of the current values up to 5kA, no sharp Rmin was observed and the joint obtained was also not acceptable. The proper nugget formation took place at a weld current value of 6kA. An increment in the nugget diameter is observed with the corresponding increase in weld current and an adequate nugget is observed from 6kA to 9kA value of weld current providing a well – defined β peak. On further increasing the current value, expulsion took place at 10kA which led to a sharp reduction in resistance as shown in the figure 4. Thus the adequate range of weld current can be given as 6 to 9kA. It is observed from literature that Luo et.al (2016) had found the adequate weld current range as 5 to 8kA for the Q235 steel sheets with thickness of 1.0mm.

6

Further, the DCR curves can be compared within the adequate weld nugget range as in the figure 5. The figure 5 shows the DCR curves for 6, 7, 8 and 9kA value of weld current at constant weld time of 250ms and the electrode force of 3.5kN. A reduction in the value of Rmin is observed with the increase in value of weld current. Luo et al. (2016) observed similar behaviour for Q235 steels.

This reduction in the value of Rmin with increase in value of weld current can be attributed to the increased surface breakdown and asperity softening. With increase in time, the resistance increases due to the increased resistivity caused due to rise in temperature. The sharp β peak obtained in case of 8kA shows a transition from nugget formation stage to nugget growth stage. The value of mean dynamic contact resistance decreases with the increase in value of weld current as shown in table 2. Thus, DCR influences the formation and growth of the weld nugget formed during the welding process. Figure 5 also shows the increase in nugget diameter with the increase in weld current within the adequate nugget range. The nugget diameters were measured using LEICA M165C stereomicroscope. Figure 6 shows the DCR curves for variation in electrode force of 3 kN, 3.5 kN and 4 kN and constant weld current of 8kA and constant weld time of 250ms. The value of Rmin decreased with the increase in the value of electrode force. This can be attributed to the increased contact area due to asperity breakdown. With the increase in electrode force, the mean dynamic contact resistance also decreased as shown in Table 2. Song et al. (2005) also found the decrease in contact resistance with the electrode force for mild steel, aluminium and stainless steel specimen. At an electrode force of 3.5kN, a sharp β peak is obtained. Figure 6 shows that the nugget formation starts earlier in case of 3.5kN of electrode force as compared to 3 kN and 4kN electrode force. Thus it can be observed from the figures 5 and 6

7

that a weld current of 8kA, electrode force of 3.5kN and a weld time of 250ms can be considered as the most desirable parameters for nugget formation.

Microhardness values at different zones i.e. base metal (BM), heat affected zone (HAZ) and weld zone were obtained for the weld current value of 8kA with weld time of 250ms and electrode force of 3.5kN. The hardness tester used was LECO microhardness tester with Amh43 software. Vickers hardness testing was carried out by applying 300 gmf load during indentation. Figure 7 shows the microhardness values in the three different zones. The microhardness values were observed to be maximum in the weld zone followed by heat affected zone and base metal respectively. Chakraborty et al. (2011) explained this behaviour being caused as a result of high dislocation density and precipitate formation during the process of joining through the resistance spot welding. Pouranvari et al. (2011) also observed the similar behaviour for resistance spot welds of DP600 and DQSK materials. Figure 8 shows the microstructural variations from the base metal to the weld zone obtained using LEICA DM6000 M microscope with 50X magnification. Since the weld / fusion zone cools slowly, large columnar grain structure was observed in the weld zone followed by the coarse grain structure in the heat affected zone and fine grain structure in the base metal. Higher strength steels resist the yielding of asperities and lead to the production of higher amount of heat for melting and thus causing the formation of large size nuggets. Studying the variation of nugget diameter with the dynamic contact resistance is important in understanding the phenomenon of nugget formation and nugget growth. It is clear from figure 9 that the nugget diameter (D) decreases with increase in the mean dynamic contact resistance (Mean DCR). The linear best fit description of the behaviour is shown in equation 1 and the R – Square value of 0.81 for the linear regression fit as shown in table 3.

8

D = -0.0823 * Mean DCR + 17.647

eq. (1)

In this way the nugget diameter and dynamic contact resistance can be related. Luo et al. (2016) observed similar behaviour for the resistance spot welding of Q235 steel sheets. Nugget growth depends on the value of weld current. The nugget diameter increased with the increase in weld current within the adequate nugget range but on further increasing the current i.e. in the case of expulsion, the nugget diameter size decreased. Figure 10 shows the change in nugget diameter with weld current at a constant weld time of 250ms and constant electrode force of 3.5kN. It was observed that at a weld current of 10kA, the nugget diameter decreased due to the occurrence of expulsion. Moshayedi and Sattari - Far (2012) also observed a similar reduction in nugget diameter in the case of expulsion for austenitic stainless steel in their experimental study of nugget size growth. The nugget growth depends on the weld time. During incubation period, the nugget was not formed. Figure 11 shows the nugget diameter variations with the corresponding variations in weld time from 150 to 350ms at a constant weld current of 8kA and electrode force of 3.5kN. The incubation period was 150ms for 8kA of constant weld current and 3.5kN of constant electrode force. The nugget diameter increased up to weld time of 250ms and does not change consequently with further change in the weld time. Gould (1987) observed similar behaviour for resistance spot welding of 1.09 mm AISI 1008 steel sheets using single phase resistance welding machine. This suggests that for adequate nugget formation, the weld time should be 250ms.

The application of electrode force leads to an occurrence of interfacial surface variations. This causes the variation in heat generation and subsequently the formation of nugget. The nugget diameter of the welds at constant weld current of 8kA and constant weld time of 250ms can be compared with the corresponding variations in electrode force. Figure 12

9

shows the sharp increase in nugget diameter up to electrode force of 3.5kN. On further increasing the electrode force, no significant changes are witnessed. The figures 8, 9 and10 also reveal that for adequate nugget formation, the resistance spot welding process parameters are 8kA weld current, 250ms weld time and 3.5kN electrode force.

The amount of heat input during a welding process is related with the dynamic contact resistance and input weld current. With the rise in heat input during welding process, the nugget diameter increases. This can be attributed to the increased temperature due to heat input that leads to a larger melting area causing significant increase in the nugget diameter. Figure 13 shows this behaviour using a second order polynomial fit with R – Squared value of 0.94 as shown in table 3. The polynomial best fit description of the behaviour is shown in equation 2. D = 9.288*Hi2– 1.436*Hi – 9.137

eq. (2)

Shear - tensile tests were carried out using the weld specimens in order to determine the shear - tensile strength, peak load and failure energy of the welds. The tests were performed at a cross-head displacement rate of 5mm/min. Figure 14 shows the measured shear – tensile strength (S) with the subsequent increase in nugget diameter. This shear – tensile strength data was analysed using curve fitting by a linear fit with R – Square value of 0.89 as shown in table 3. The linear best fit description of the behaviour is shown in equation 3. S = 2.778 * D – 4.973

eq. (3)

The larger nugget diameter leads to an increase in load bearing area and thus increases the shear – tensile strength of the joint.

10

Figure 15 shows the variations in shear - tensile strength values with increase in the weld time at different weld current values of 7, 8 and 9kA. The increase in weld time leads to an increase in the nugget diameter within the adequate nugget range. This increase in nugget diameter leads to increased shear – tensile strength. On increasing the weld time from 200 to 250ms, the weld with 7kA of current shows a higher rate of increase in shear – tensile strength as compared to the welds with 8 and 9kA. This difference in the rate reduces with further increase in the weld time. For higher current value i.e. 9kA, the strength decreased significantly with an increase in the weld time after a certain value. It can be inferred from the results that the weld time should be restricted up to a certain value in case of using higher current during resistance spot welding process.

Figure 16 shows the variation in shear - tensile strength values with weld current at different weld time values of 200, 250, 300 and 350ms with constant electrode force of 3.5 kN. Increase in the weld current from 7 to 8kA, with weld time of 200ms shows an increase of more than four times in shear – tensile strength. The increase in shear – tensile strength for higher values of weld time is comparatively less. A comparative decrease is observed in the shear – tensile strength increment in case of transition from 8 to 9 kA as compared to 7 to 8 kA of weld current. From the figures 15 and 16, it can be inferred that there is a reduction in the increase of shear – tensile strength with increasing weld time and weld current.

Figure 17 shows the variation in shear - tensile strength with the variation in weld current at constant weld time 250ms and constant electrode force 3.5kN. The strength shows an increment with increase in weld current within the adequate weld range i.e. up to 9kA of weld current. The strength decreased at a weld current of 10kA. This can be attributed to the occur-

11

rence of expulsion during welding. This is the reason for 10kA of weld current value to be excluded from the adequate weld range.

Figure 18 shows the trend of shear - tensile strength with variation in the heat input during welding process in the adequate weld range. It is clear from the figure that with an increase in weld heat input, the shear – tensile strength of the weld increases linearly with R square value of 0.91 as shown in table 3. The linear best fit description of the behaviour is shown in equation 4. S = 5.21* Hi – 5.49

eq. (4)

The increase in heat input causes the melting of adjacent surfaces, leading to an increase in nugget volume. This increased nugget volume increases the shear – tensile strength. Table 3 presents the description of the fit for figures 9, 13, 14 and 18 with R – squared values and the coefficients of the fit description. Failure mode analysis of resistance spot welds provides information on the quality of the weld joint. The spot welds can fail in three different modes: 1) Interfacial Failure (IF) 2) Pull – out Failure (PF) 3) Partial Interfacial Failure (PIF) In IF mode of failure, the crack propagates through the weld nugget and in PF mode of failure, the weld nugget pulls out from one sheet. In PIF mode, the crack first starts propagating through nugget and then is redirected in thickness. In IF mode, driving force for failure is the shear stress at interface of two sheets. In the PF mode, the resulting tensile stress around the nugget is the driving force for failure.

12

The occurrence of the type of failure mode mainly depends upon the nugget diameter. There is a minimum nugget diameter beyond which, the interfacial mode of failure is avoided. Figure 19 shows different failure modes. As the nugget diameter increases, the failure mode shows a transition from interfacial to pull – out. The nugget diameter exhibiting the transition from interfacial to pull – out failure mode is said to be the critical nugget diameter. It is to be assured that the critical diameter is large enough to make the weld to undergo pull – out failure. American welding society (AWS), American national standards institute (ANSI) and Society of Automotive Engineering (SAE) verified the critical nugget diameter for the safety purpose so that the weld nugget attains critical size to bear the anticipated load. The equation used to describe this is: eq. (5) Where, D is the critical nugget diameter and t is thickness of sheet. Using this equation, the critical nugget diameter D = 5.1mm was obtained. The experimentally determined critical nugget diameter at which the failure mode changes to pull – out mode is D = 5.5mm which is greater than that obtained using equation 5. The existing standard industrial criterion for weld nugget sizing is not adequate for ensuring the failure to occur as pull – out failure. Pouranvari and Marashi (2011) also found this equation to be invalid in their case of DP600 steel sheets. Vanden Bossche (2003) established an analytical model for critical nugget diameter. The weld sizing criterion was developed assuming that the fracture will occur in the location that yields first i.e. the weld nugget or the heat affected zone. The model is shown in equation 6. eq. (6) Where, σ is the yield strength of base metal and w is the width of weld specimen. Using the equation 6, the critical nugget diameter obtained for the present study is 7.9mm. This value is not in good agreement with the experimental value.

13

Chao (2003) derived an equation for critical nugget diameter for cross – tension test specimens but at the same time suggested the use of the analytically developed model for shear – tensile test specimens also. The limitation with the model is that it follows a model which is dependent upon the fracture toughness and strength of the material but it tries to show that the model is material independent. The model is shown as follows: eq. (7) Using equation 7, the critical nugget diameter obtained for the present study was 6.4mm. This value was also not in a good agreement with the experimentally obtained value but this can be used as a model since it estimates a little higher critical nugget diameter value. This will act as a very safe model for the present work. Zhao et al. (2016) developed an analytical model for the critical nugget diameter, which is shown in equation 8.

eq. (8)

Where, (H0)1 and (H0)2 are the Vickers micro – hardness values at heat affected zone (HAZ) and fusion zone (FZ) respectively. Also, x is given as

, where H is the penetration

depth and D is the nugget diameter as shown above in the figure 20. Hardness value of heat affected zone is assumed as half of the value of the fusion zone. This can be given as (H0)1 = 0.5 *(H0)2. This leads to the value of x equals to 0.228. eq. (9) The indentation depth is assumed as 20% of the sheet thickness thus the value of penetration is H = 0.8*t, critical nugget diameter can be determined as shown in equation 10.

14

eq. (10) Using equation 10, the critical nugget diameter found estimated as 5.6mm. This is in very good agreement with the experimental critical nugget diameter of 5.5mm. Thus, this model can be used with a good accuracy for resistance spot welding of galvannealed HIF steel sheets with thickness 1.6mm. Table 4 shows the comparison of the critical nugget diameter values obtained using different models with the experimentally obtained critical nugget diameter. This shows that the model developed by Zhao et al. is the most satisfactory considering the experimentation in the present study. The total failure energy is the area under load – displacement curve. Figure 21 shows the increase in failure energy with the corresponding increase in the nugget diameter. It is clear from the figure that in case of interfacial failure mode, the rate of increase in failure energy is not significant compared to the pull – out failure mode. There occurs a sharp rise in the failure energy for critical nugget diameter where a transition takes place from interfacial to pull – out failure. Fractography of the obtained fractured surfaces during shear – tensile test was carried out using scanning electron microscope (SEM) in order to visualise the different failure modes. The fractured specimens were first cleaned ultrasonically and then the SEM fractographs were obtained. Figure 22 shows the SEM images of fractured surface for observed interfacial failure in the specimen. Solidification shrinkage during resistance spot welding leading to development of cracks is the main cause behind the occurrence of interfacial failure. Torn bands at the location C in Figure 22 show the quasi cleavage failure with sharp cleavage planes at most of the locations while at some locations minute dimples were also observed, showing limited ductility. Location B in Figure 22 shows the part of the weld exhibiting the ductile failure. Yuan et

15

al. (2017) also observed a typical quasi cleavage in their work on spot welding of dissimilar DP600 and DC54D steels.

Figure 23 shows the SEM images of the observed pull – out failure in the specimen. Shear dimples can be observed in the SEM images at the different locations of the fractured surface. This shows the occurrence of ductile failure during the shear – tensile test. CONCLUSIONS 1. For galvannealed High strength Interstitial free steel sheets of 1.6mm thickness, adequate weld current range is 6kA to 9kA at constant weld time of 250ms and constant electrode force of 3.5kN. 2. With the increase in weld current and electrode force, Rmin value decreases. Also the mean dynamic contact resistance decreases, whereas there is an increase in nugget diameter and the shear – tensile strength. 3. The shear – tensile strength increases with the increase in weld current within the adequate weld range. Also, with an increase in weld time, the rate of increase in shear – tensile strength with weld current decreases. 4. With increase in heat input during welding, the shear – tensile strength increases within the adequate weld range. 5. As the nugget diameter increases, the failure mode shows a transition from interfacial failure to pull – out failure and the failure energy shows a sharp increase during the transition from interfacial failure mode to pull – out failure mode. 6. The failure mode observed is brittle at most of the fracture locations in case of interfacial failure and shear – ductile in case of pull – out failure.

16

FUNDING: This research did not receive any specific grant from funding agencies in the public, commercial or not – for – profit sectors.

ACKNOWLEDGEMENT: The authors are grateful to the management of IIT Jodhpur and Tata steel limited Jamshedpur for their support in carrying out the experiments.

REFERENCES: Aktas, S., Ozsarac, U. and Aslanlar, S., 2012. Effect of spot welding parameters on tensile properties of DP 600 steel sheet joints. Materials and manufacturing processes, 27: 756764. Bhadeshia, H.K.D.H., 2010. Phase transformation during spot welding of interstitial – free steel. International conference on interstitial – free steels, Jamshedpur, India. Chakraborty, G., Pal, T.K. and Shome, M., 2011. Microstructural development in resistance spot welded galvannealed IF steel sheet. Material science and technology, Vol 27, 1, 382 – 386. Chao, Y.J., 2003 Failure mode of resistance spot welds: interfacial vs pullout. Sci. Technol. Weld. Join. , 8, 133- 137. Cho, H.S. and Cho, Y.J., 1985. A theoretical model for the dynamic resistance in resistance spot welds. Journal of Engineering Manufacture199: 71. Cho, Y. and Rhee, S., 2003 Experimental study of nugget formation in resistance spot welding. Welding Journal. Dickinson, D.W., Franklin, J.E. and Stanya, A., 1980. Characterization of spot welding behaviour by dynamic electrical parameter monitoring. Welding Journal, 59(6), 170s – 176s.

17

Gedeon, S.A., Eager, T.W., 1986. Resistance spot welding of galvanized steel: Part 1 Material variations and process modifications. Metallurgical Transactions B, Volume 17B, 879 – 885. Gedeon, S.A., Eager, T.W., 1986. Resistance spot welding of galvanized steel: Part 2 Mechanisms of spot weld nugget formation. Metallurgical Transactions B, Volume 17B, 887 – 901. Gedeon, S.A., Sorensen, C.D., Ulrich, K.T., Eagar, T.W., 1986. Measurement of dynamic electrical and mechanical properties of resistance spot welds. Welding research supplement, 378 – 385. Gould, J.E., 1987. An examination of nugget development during spot welding, using both experimental and analytical techniques. Welding research supplement, 1 – 10. Hayat, F., 2011. Resistance Spot weldability of dissimilar materials: BH180-AISI304L steels and BH180-IF7123 Steels. J. Mater. Sci. Technol., 27(11), 1047 – 1058. Kianersi, D., Mostafaei, A., Mohammadi, J., 2014. Effect of welding current and time on microstructure, mechanical characterizations and fracture studies of resistance spot welding joints of AISI316L austenitic stainless steel. Metallurgical and materials transactions A., 45, 10, 4423 – 4442. Luo Yi, Rui Wan, Xie Xiaojian, Zhu Yang., 2016. Study on the nugget growth in single phase AC resistance spot welding based on the calculation of dynamic resistance. Journal of materials processing technology, 229, 492-500. Moshayedi, H., Sattari – Far, I., 2012. Numerical and experimental study of nugget size growth in resistance spot welding of austenitic stainless steels. Journal of materials processing technology 212, 347 – 354. Pouranvari, M. and Marashi, S.P.H., 2011. Failure mode transition in AHSS resistance spot welds, Part 1 Controlling factors. Material Science and Engineering A 528, 8337-8343.

18

Pouranvari, M. and Marashi, S.P.H., Safanama, D.S., 2011. Failure mode transition in AHSS resistance spot welds, Part 2 Experimental investigation and model validation. Material Science and Engineering A 528, 8344-8352. Satyendra, 2014. Interstitial free steels. Ispat Digest, Jun 13. Shome, M., 2009. Effect of dynamic contact resistance on advanced high strength steels under medium frequency DC spot welding conditions. Science and Technology of Welding and Joining, Vol 14, num 6, 533-541. Song, Q., Zhang, W., Bay, N., 2005. An experimental study determines the electrical contact resistance in resistance welding. Welding Journal, 73 – 76. Tsai, C.L., Jammal, O.A., Papritan, J.C., Dickinson, D.W., 1992. Modelling of resistance spot weld nugget growth. Welding research supplement, 47 – 54. Tumuluru, M., 2007. The effect of coatings on the resistance spot welding behaviour of 780MPa dual – phase steel. Welding journal, Vol. 86, 161 – 169. Vanden Bossche, J., 2003. Ultimate strength and failure mechanism of resistance spot weld subjected to tensile, shear or combined tensile/ shear loads. J. Eng. Mater. Technol., 125, 125 – 132. Yuan, X., Li, C., Chen, J., Li, X., Liang, X., Pan, X., 2017. Resistance spot welding of dissimilar DP600 and DC54D steels. Journal of Materials Processing Technology, 239, 31 – 41. Zhao, D., Wang, Y., Liang, D., Zhang, P., 2016. Modelling and process analysis of resistance spot welded DP600 joints based on regression analysis, Materials and design 110, 676 – 684. Zhou, M., Hu, S.J., Zhang, H., 1999. Critical specimen sizes for tensile – shear testing of steel sheets. Welding research supplement, 305 – 313.

19

Figure 1

Spot Weld Experimental Setup

Figure 2

Specifications of shear – tensile specimen (Figure not to scale)

20

Figure 3

DCR curve at weld parameters 8kA, 250ms & 3.5kN

Figure 4

Adequacy of weld nugget for weld time 250ms and electrode force 3.5kN with

variation in weld current (kA)

21

Figure 5 DCR curves and nugget diameters at constant weld time 250ms and electrode force 3.5kN with variation in weld current (kA)

Figure 6

DCR curves at constant weld current 8kA and weld time 250ms with variation

in electrode force as 3, 3.5 and 4kN

22

Figure 7

Microhardness values for the weld at 8kA weld current, 250ms weld time and

3.5kN electrode force.

Figure 8 Microstructure of the weld at 8kA weld current, 250ms weld time and 3.5kN electrode force

23

6.5 6.0

Nugget Diameter, D(mm)

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 130

140

150

160

170

180

190

Mean DCR (µ ohm)

Nugget Diameter (mm)

Figure 9

Nugget Diameter as a function of Mean Dynamic Contact Resistance

6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 6

7

8

9

10

Weld Current (kA)

Figure 10

Nugget Diameter as a function of Weld Current at constant weld time 250ms

and electrode force 3.5kN

24

6.0

Nugget Diameter (mm)

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 150

200

250

300

350

Weld Time (ms)

Figure 11

Nugget Diameter as a function of Weld Time at constant weld current 8kA

and constant electrode force 3.5kN

5.6

Nugget Diameter (mm)

5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 3.0

3.2

3.4

3.6

3.8

4.0

Electrode Force (kN)

Figure 12

Nugget Diameter as a function of Electrode Force at constant weld current

8kA and constant weld time 250ms

25

6.5

Nugget Diameter, D(mm)

6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

Heat Input, Hi(kJ)

Figure 13

Nugget Diameter as a function of Heat Input

Shear - tensile strength, S(kN)

14 12 10 8 6 4 2 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Nugget Diameter, D(mm)

Figure 14

Shear – tensile strength as a function of Nugget Diameter

26

Figure 15

Shear – tensile Strength as a function of Weld Time with variation in weld

current as 7, 8 and 9kA at an electrode force of 3.5kN

Figure 16

Shear – tensile Strength as a function of Weld Current with variation in weld

time as 200, 250, 300 and 350ms at constant electrode force of 3.5kN

27

Shear – tensile Strength as a function of Weld Current at constant weld time

Figure 17

250ms and constant electrode force of 3.5kN

Shear - tensile strength, S(kN)

14 12 10 8 6 4 2 0 1.5

2.0

2.5

3.0

3.5

4.0

Heat Input, Hi(kJ)

Figure 18

Shear – tensile Strength as a function of Heat Input within adequate weld

range at 3.5kN

28

Figure 19

Failure modes during shear - tensile testing

Figure 20

Nugget diameter and Penetration

Figure 21

Failure Energy as a function of Nugget diameter 29

Figure 22

Fractography of Interfacial failure specimen

Figure 23

Fractography of Pull – out failure specimen 30

Table 1

Chemical composition & Mechanical Properties of galvannealed HIF steel

Chemical Composition (%) Carbon

Manganese Phosphorus

Sulphur

Silicon

Minor Actinides

0.004

0.53

0.008

0.104

0.04(Ti) + 0.039(Nb)

0.029

Mechanical Properties Yield Strength (MPa)

Ultimate tensile Strength (MPa)

Elongation (%)

224

375

40

Table 2

Mean DCR values at different process parameters

S.No.

Weld Current(kA)

Weld Time(ms)

Electrode force(kN)

Mean DCR (µΩ)

1

6

250

3.5

184.2

2

7

250

3.5

160.4

3

8

250

3

164.9

4

8

250

3.5

156.1

5

8

250

4

155.5

6

9

250

3.5

145.0

Table 3 Type of Fit

Description of fits for different figures Figure No.

R – squared Value

Fit Description Data Intercept

Slope

31

Linear

Second Order

Table 4 S.No

Model

1.

Existing Industrial model Vanden – Bossche

2.

9

0.81

17.647

-0.082

14

0.89

-4.973

2.777

18

0.91

-5.490

5.211

Intercept

B1

B2

-9.137

9.288

-1.436

13

0.94

Comparison of different models for critical nugget diameter Model Equation

Critical Nugget Diameter Obtained using model (mm) 5.1

Difference between model value and experimental value (mm)

Remarks

-0.5

Unsafe

7.9

2.4

Very high factor of safety

3.

Chao

6.4

0.9

Good agreement

4.

Zhao et al.

5.6

0.1

Satisfactory

32