Issn : 1597-5835 a Review on the Application of Resistance

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

A REVIEW ON THE APPLICATION OF RESISTANCE SPOT WELDING OF AUTOMOTIVE SHEETS S. M. Manladan1, I. Abdullahi2, M.F. Hamza3

1,2Department of Mechanical Engineering, Bayero University, Kano 3Department of Mechatronics Engineering, Bayero University, Kano

ABSTRACT The automotive industry is now one of the most important sectors of all developed economies. It provides job opportunities to many people at various levels of the economy. Resistance spot welding is the dominant welding process in the automotive industry. The structural integrity of a vehicle depends largely on the quality of resistance spot welds. The process is fast, effective and also complicated due to complex interactions between electrical, mechanical, thermal and metallurgical processes. Due to increasing demand for improved fuel efficiency, reduced

CO2 emission and superior crashworthiness, significant effort is continuously being applied to develop lightweight materials with excellent strength-ductility combination. This paper reviews automotive sheet materials and mechanical performance of resistance spot welds in terms of weld nugget size, load bearing capacity and failure mode, under quasi static loading conditions. It also reviews the effect of process parameters such as welding current, welding time and electrode force on the mechanical performance of spot welds.

Keywords: Automotive sheets; resistance spot welds; weld nugget size; failure mode; mechanical properties.

INTRODUCTION

1.1 Fundamentals of Resistance Spot Welding is achieved by heat conduction via the two water-cooled electrodes, which serve as efficient heat sinks, and also Resistance spot welding (RSW) is a welding process in radially outwards through the sheets. The weld is which two or more similar or dissimilar overlapping normally formed in a fraction of a second and the metal sheets are placed between two water-cooled copper electrodes are retracted after each weld is formed alloy electrodes and large electrical current is passed (Charde et al., 2014; Kocabekir et al., 2008; Liu et al., through them for a controlled period of time under 2010). controlled pressure. The electrodes compress the base Figure 1 and Figure 2 show schematic diagrams for metals together and the electrical resistance at the metals resistance spot welding of two and three sheets interface causes a localized heating. When the flow of respectively current ceases, the electrode force is maintained while the weld metal rapidly cools and solidifies. The cooling .

www.bayerojet.com 20

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Electrode force

Top electrode

To electric Top sheet current source Bottom sheet

Bottom electrode

Electrode force Figure 1: Schematic diagram for RSW of two sheets Electrode force

Top electrode

Top sheet Middle sheet To electric power source Bottom Sheet

Bottom electrode

Electrode force Figure 2: Schematic diagram for RSW of three sheets 1.1.1 Heat Generation in RSW current and time increases the heat input. The heat generation in RSW is due to the resistance of the parts being welded to the flow of a localised electric As shown in Figure 3, two types of resistances exist in current, based on Joule’s law which can be expressed as RSW processes, namely, bulk resistance (R2 and R4) and follows (Pouranvari & Marashi, 2013; Razmpoosh et al., contact resistance, which is found at the electrode-sheet 2015; Wang et al., 2007): interfaces (R1 and R5) and at the faying interface (R3). The total electrical resistance is due to these two Q = I2Rt --- (1) resistances. Both the contact resistance and bulk resistance are usually not constant. The contact resistance Where: Q (joules) is heat, I (ampere) is welding current, is a strong function of both temperature and pressure. R (ohms) is electrical resistance, t (seconds) is time of The bulk resistance is sensitive to temperature and current application. Generally, increasing the welding independent of pressure (Zhang & Senkara, 2011)

www.bayerojet.com 21

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Figure 3: Illustration of the electrical resistances in a sheet stack-up during RSW(Zhang & Senkara, 2011)

1.1.2 RSW Process Parameters suitability for automation or robotization and inclusion in high-production assembly lines. Moreover, the process The quality of the joint in RSW is influenced by the can be used to join most metals provided suitable welding parameters. These parameters mainly include welding conditions are applied (Qiu et al., 2011; Vural & welding current, welding time, electrode force and Akkus, 2004). electrode geometry (Hongqiang et al., 2014). Typically, there are about 2000–5000 spot welds in a

modern vehicle. The quality, structural performance, 1.2 Significance of RSW in Automotive Industry durability, safety design, stiffness, strength and integrity

of a vehicle depend not only on the mechanical RSW is the dominant metal sheet joining process in the properties of the sheets, but also on the quality of automotive industry because of minimum skill resistance spot welds (Alizadeh-Sh et al., 2014; Dancette requirements, inexpensive equipment, ease of control, its et al., 2011; Zhang et al., 2014). versatility, high operating speeds, repeatability,

2. AUTOMOTIVE SHEET MATERIALS AND RESISTANCE SPOT WELDING

2.1 Automotive Sheet Materials (Kuziak et al., 2008; Tamarelli, 2011). The main materials used in automobile are as follows: Materials selection is a key stage of vehicle design. As 2.1.1 Mild Steel the motivation to reduce the mass of vehicles continues Conventional mild steel has a relatively simple ferritic to grow, more attention is being given to materials microstructure; with low carbon content and minimal selection in the automotive industry(Pouranvari & alloying elements. Mild steels are inexpensive and have Marashi, 2013). Some of the properties required of relatively low strength with excellent formability. They automotive materials are high strength-to-weight ratio, have long been used for many applications in vehicles, toughness, formability, weldability, corrosion resistance, including the body structure, closures, and other ancillary fatigue resistance, crashworthiness and cost effectiveness parts (Tamarelli, 2011)

www.bayerojet.com 22

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

carbon content and no other additional interstitial solutes 2.1.2 Interstitial Free (IF) IF is essentially a single phase bcc steel, with very low (Hadzima et al., 2007). IF steels are extensively used for AHSS can be divided into first generation, second manufacturing car bodies and other different parts (Dutta generation and third generation. The microstructure of & Ray, 2013). first generation AHSS is based on ferrite and they include: Dual phase (DP), complex phase (CP), 2.1.3 High-Strength Low-Alloy (HSLA) martensitic (MS) and transformation-induced plasticity High-strength low-alloy (HSLA) steels, also called (TRIP) steels. The microstructure of second generation micro-alloyed steels, are carbon steels in which micro- AHSS is based on austenite. Typical example of second alloying is done with small amounts of elements such as generation AHSS is twinning-induced plasticity (TWIP). Nb and V (Zhang et al., 2013). They are designed to The third generation AHSS possess strength–ductility meet specific mechanical properties. They have better combinations significantly better than first and second mechanical performance and resistance to atmospheric generations. They are still being researched and they corrosion than conventional carbon steels. They possess have the potential of further reducing the mass of vehicle high strength and good toughness and are widely used in and at the same time offer improved crash automotive industry (Han et al., 2014; Zhang et al., worthiness(Pouranvari & Marashi, 2013; Tamarelli, 2013). 2011). 2.1.4 Advanced High Strength Steels (AHSS) Advanced high strength steels (AHSS) are a new Dual phase (DP) steel generation of steel grades developed to meet light weight The microstructure of DP steel consists of a mixture of requirements without compromising passenger safety. ferrite phase and the martensitic phase. It is an ideal They possess an excellent combination of strength and material for automotive application due to its high tensile ductility. The high strength enables vehicle strength (up to 1.2 GPa), ductility (10% or higher) and manufacturers to realize weight reduction by using toughness (Han et al., 2014; Koyama et al., 2014). thinner gages while the increased ductility enhances crashworthiness and production of components with Complex Phase (CP) Steel more complex geometry. In the future, these steels are The microstructure of CP steels consists of a expected to be the dominant structural steels in an ferrite/bainite matrix, containing bits of martensite, automobile, especially in body-in-white applications (Li retained austenite and pearlite. They have fine et al., 2015; Rossini et al., 2015). The main reasons for microstructure and possess high strength, good wear the increased use of AHSSs include (Pouranvari & characteristics, fatigue strength, high energy absorption Marashi, 2013) : (i) the reduction in passenger car weight and crash worthiness(Tamarelli, 2011). by the increased use of high strength thinner gauge sheet, leading to reduced fuel consumption and emissions (ii) Martensitic (MS) Steels the improvement of the passive safety of vehicles, which In these steels, the microstructure consists of martensitic leads to a better passenger safety by an improved matrix with a small amount of very fine ferrite and/or crashworthiness (iii) the strong competition from the bainite phases. They have extremely high strength. lightweight materials, in particular Al and Mg alloys, and Although, they have low ductility, quenching and plastics. tempering can be used to improve their ductility. They www.bayerojet.com 23

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 are used where high strength is critical, such as door 2.1.5 Stainless Steels intrusion beams, rocker panel inners and reinforcements, Stainless steel has recently been identified as an ideal side sill and belt line reinforcements, springs and clips. candidate for car body structural applications. It offers weight savings, enhanced crashworthiness, corrosion and Transformation-Induced Plasticity (TRIP) Steels oxidation resistance and excellent manufacturability. TRIP steels possess a multi-phase microstructure of Under impact, it offers excellent energy absorption in retained austenite and finely dispersed bainite in a ferrite relation to strain rate. The next generation vehicle matrix. The retained austenite is the most important programme demonstrated that stainless steels can be used constituent. It is metastable and during deformation, it to reduce weight and to increase safety and sustainability undergoes a stress-induced transformation into of the structural automotive systems. The main types of martensite, thus improving strength. TRIP steels possess stainless steels are: Austenitic, ferritic, martensitic and high strength, good ductility and formability (Khan et duplex stainless steels. Stainless steels can be used in al., 2008; Kuziak et al., 2008). door pillars Figure 4 schematically illustrates the microstructures of including A and B pillar reinforcement parts, bumper DP, TRIP, CP and MS steels. beams, rollover bars, crash boxes, suspension/wheel

housings and sub frames(Alizadeh-Sh et al., 2015; Twinning Induced Plasticity (TWIP) Steels Pouranvari et al., 2015). TWIP steel is a new type of AHSS with high manganese content (usually between 15 to 25 wt%), an austenitic 2.1.6 Aluminum Alloys microstructure at room temperature and low stacking In recent years, there is a growing interest in the use of fault energy (SFE). TWIP steels possess both high aluminum alloys for automobile body structures, closures strength and superior plasticity. During plastic and chassis. Aluminum alloys continue to attract deformation, mechanical twins are formed. These attention in the automotive industry due to their low mechanical twins impede dislocation glide and localized density (approximately one third that of steel), high necking (dynamical Hall-Petch effect). The extraordinary strength-to-weight ratio, excellent corrosion resistance, combination of high strength and ductility is due to high ease of being recycled, excellent formability and good strain hardening rate associated with the deformation crashworthiness. Aluminum alloys are expected to be twinning phenomenon. Tensile strengths greater than extensively used in the future to partially replace steel 1000 MPa and elongation at fractures above 50% could that is primary construction material in an automobile be obtained (Kim et al., 2015; Rossini et al., 2015; Spena (Cui et al., 2014; Qiu et al., 2011; Zheng et al., 2015). et al., 2014). The occurrence of TWIP effect depends strongly on SFE. Generally, intense twinning occurs 2.1.7 Magnesium Alloys during deformation when the SFE is between 20 to 45 Magnesium has been described as a green engineering MJ/m2. Automotive sheet components made with TWIP material and one of the most promising material steels are still under development (Spena et al., 2014) . categories in the twenty-first century. It possesses low The industrial focus is now mainly on TWIP steels of Fe- density, approximately one-fourth that of steel and two- Mn-C-Al and Fe-Mn-Si-Al alloy systems. Figure 5 thirds that of aluminum (Palanivel et al., 2015), high compares the strength-ductility combinations of various specific strength, high elastic modulus, strong ability to automotive sheet materials. withstand shock loads and hot formability. Other remarkable properties which make magnesium an www.bayerojet.com 24

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 attractive structural material include excellent aluminum alloys as the primary structural material in the electromagnetic interference shielding, high heat automotive and aerospace industry(Babu et al., 2012). dissipation capability, good castability, damping capacity According to “Magnesium Vision 2020”, the average and recyclability(Zhao et al., 2015). In the future, magnesium content in an automobile could increase to as magnesium alloys are expected to replace steel and much as 772kg by the year 2020(Cole, 2007).

Figure 4:

Schematic representation of first generation of AHSSs(Pouranvari & Marashi, 2013)

Figure 5: Relationship between the tensile strength and elongation in various automobile steels(Chen et al., 2013)

2.2 Resistance Spot Welding RSW lead to the formation of temperature gradient across the sheets assembly. Upon cooling, three distinct 2.2.1 Metallurgical Characteristics of Resistance Spot zones can be identified, as shown in Plate 1: (i) Fusion Welds Zone (FZ) or weld nugget which experiences melting The rapid heating and cooling cycles encountered in and solidification and normally shows a cast structure. www.bayerojet.com 25

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

The Fusion zone size is controlled by heat input and it experience melting but undergoes microstructural determines bonding area of the joint changes. (ii) Heat Affected Zone (HAZ) which does not (iii) Base Metal (BM).

Plate 1: A typical macrostructure of resistance spot weld in a galvanized low carbon steel (Goodarzi et al., 2009)

Hardness variation could be observed across the sheets temperatures encountered and cooling rate. Figure 6 assembly after welding. This variation depends on the shows hardness profile for some resistance spot welded chemical composition, initial microstructure of the BM, AHSSs.

Figure 6: Hardness profile for resistance spot welded DP780, DP600 and 590R steels (Khan et al., 2008)

2.2.2. Resistance Spot Welding of Multiple Sheets emission by developing new materials with higher strength-to-weight ratio and better performance. Another Resistance spot welding of multiple (three or more) reason is design complexity. As the design of modern sheets is now inevitable in vehicle manufacturing and automobile becomes more complex, multiple sheet other engineering fields. In the present manufacturing of joining becomes inevitable (Choi et al., 2007; Ma & automobiles, resistance spot welding of three steel sheets Murakawa, 2010). Some of the complex structures in comes up to one third of total welded joints (Lei et al., which multiple sheet spot welding is applied include 2011; Shen et al., 2011). front longitudinal rails, A, B, and C pillars, and the One of the reasons for the need of multiple sheet welding bulkhead to inner wing and at cross-member intersection. is the continuous attempt to reduce the weight of vehicles The multiple sheets welding leads to improvement of in order to improve fuel efficiency and to reduce CO 2 design flexibility, lower cost and higher productivity www.bayerojet.com 26

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 compared to two sheet joining of automobile body panels study showed a scatter in the plot of the maximum (Mori et al., 2014; Tavasolizadeh et al., 2011). applied loads versus cycles to failure of spot welds and this is attributed to the weld sizes and also the strength of However, RSW of multiple sheets is very challenging. the base metal. Lei et al. (2011) built a two dimensional Compared to two-sheet spot welding, joining three sheets finite element (FE) model for RSW process of mild steel is significantly more complicated due to the addition of with three sheets assemblies. The temperature extra interfaces. Welding current flow throughout the distribution changes of the three sheets assemblies were joints is also complicated. The use of different material obtained and the thermal characteristics were analyzed. combinations and different sheet thicknesses further The results of their work revealed that the highest complicates the process (Nielsen et al., 2011). The weld temperature starts at two faying surfaces between the nugget quality has become a major concern. There is workpiece-workpiece symmetric to the centre of the insufficient growth of the weld nugget, which has a sheets. This was due to contact resistance and equal detrimental effect on the crashworthiness of the vehicle. thickness of the metal sheet used which differs from Moreover, the failure mechanism of three-sheet temperature distribution of the assembly of two metal resistance spot welds is not well understood (Kang et al., sheets. Tavasolizadeh et al. (2011) studied the weld 2010; Pouranvari & Marashi, 2012c; Shen et al., 2011). nugget growth, mechanical performance and failure Despite the importance of multiple sheet spot welding, behavior of three thickness low carbon steel resistance reports in the literature dealing with their welding spot welds. They concluded that increasing welding time behavior and mechanical behavior are limited. leads to increase in peak load and energy absorption of Pouranvari and Marashi (2012c) investigated the failure the joints. Shen et al. (2011) developed a model to behavior of 1.25/1.25/1.25 mm three-sheet low carbon predict the weld nugget formation process of resistance steel resistance spot welds. It was found that fusion zone spot welding of a sheet stack made of 0.6 mm thick size along sheet/sheet interface is the most important galvanized SAE1004, 1.8 mm thick galvanized SAE1004 controlling factor of peak load and energy absorption. and 1.4 mm thick galvanized dual-phase (DP600) steel. Kang et al. (2010) studied the fatigue characteristics of The results of their work revealed that the model spot welds for three equal thickness sheet stack-ups of a underestimated the weld sizes in all interfaces. Dual Phase (DP600) and mild steel. The results of the

www.bayerojet.com 27

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Figure 7: Specimen geometry for (a) CT, (b) TS and (c) CP tests (Pouranvari & Marashi, 2013)

2.2.3 Mechanical Properties of Resistance Spot Welds tests are examples of tests conducted under dynamic loading conditions (Pouranvari & Marashi, 2013).Figure In an automotive structure, spot welds experience both 8 schematically illustrates sample geometry for TS, CT shear loading due to the relative displacement or rotation and CP tests. of the adjacent sheets and tensile loading due to the Due to simplicity in preparing samples for tensile–shear separating forces applied between adjacent sheets in a (TS) test , it is widely used to determine the strength of direction normal to the sheets (Rathbun et al., 2003). resistance spot welds (Babu et al., 2012). In this test, 2.2.4 Testing the Mechanical Performance of Resistance load bearing capacity (peak load) and failure energy are Spot Welds the two most important parameters used to describe the The mechanical performance of spot welds is normally performance of the joint. The peak load and failure considered under quasi-static and dynamic loading energy are extracted from the load–displacement curve conditions. Tensile-shear (TS), cross-tension (CT) and obtained from the test. (Pouranvari & Marashi, 2013). coach peel (CP) tests are examples of tests conducted Figure 8 shows a typical load–displacement curve under quasi-static loading conditions. Impact and fatigue obtained TS test

Figure 8: Typical load displacement-curve obtained from TS test (Kianersi et al., 2014)

www.bayerojet.com 28

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

2.2.5 Failure Mode of Resistance Spot Welds Interfacial Failure Mode (IF) Failure mode of resistance spot welds is a qualitative In this failure mode, the joint fails through the weld measure of mechanical properties and an indicator of nugget centerline and the fracture surface is relatively their load-bearing capacity and energy absorption smooth. Cracks usually initiate from a sharp notch and capability. There are three types of failure mode: then propagate through the weld nugget. IF mode is Interfacial, partial interfacial and pull out failure mode accompanied by little plastic deformation and has a (Pouranvari & Marashi, 2012a; Zhang et al., 2014). detrimental effect on the crashworthiness of the vehicles Figure 10 shows a schematic diagram of these failure (Zhang et al., 2014). Plate 2 shows the surfaces of a modes. resistance spot weld in an austenitic stainless steel which failed in IF mode .

www.bayerojet.com 29

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Figure 9: Schematic representation of three failure modes observed in tensile shear test: (a) interfacial, (b) partial interfacial, and (c) pullout (Yao et al., 2014).

Plate 2: Interfacal failure mode resistance spot-welded metastable austenitic stainless steel (Liu et al. 2012)

Partial Interfacial Failure (PIF) Mode pullout and interfacial failure modes, As shown in Figure 10, in PIF mode a fraction of respectively (Radakovic & Tumuluru, 2008): the weld nugget is removed. The crack first 퐹푃푂 = 푘푃푂 . 휎푈푇 . 푑. 푡 --- (2) 2 propagates in the weld nugget, then redirects 퐹퐼퐹 = 푘퐼퐹. 휎푈푇 . 푑 --- (3) perpendicularly to the centerline towards one of where the sheets. 푘푃푂 ~2.2 and 푘퐼퐹 ~0.6 푎푟푒 constants; 휎푈푇 is the ultimate tensile shear strength of the base Pullout Failure (PF) Mode material, d is nugget diameter, t is the sheet PF occurs by withdrawal of the weld nugget thickness. from one sheet, as shown in Figure 10 and Figure 12. Fracture may initiate in the base metal 2.2.6 Critical Weld Nugget Diameter (Fusion (BM), HAZ or HAZ/FZ, depending on the Zone Size) metallurgical and geometrical characteristics of The weld nugget diameter controls the the weld zone and the loading conditions. Spot mechanical performance and failure mode of welds that fail by PF mode have higher peak resistance spot weld. Small nugget diameter loads and energy absorption levels than those often results in interfacial failure (IF) mode that fail through IF or PIF failure modes. PF is while a large one normally leads to pullout the most preferred failure mode and hence failure (PF) mode (Zhang et al., 2014) process parameters should be adjusted so that the pullout failure mode is established(Babu et al., The transition from IF mode to PF mode is 2012; Pouranvari & Marashi, 2013) generally related to the increase in the size of the The amount of force required to cause failure is FZ above a minimum value. The minimum FZ equal to the product of the strength of the size is a function of sheet thickness, material and the failed area of cross section. BM/HAZ/FZ material properties and loading Based on this, the following equations were conditions (Dancette et al., 2011) derived to predict failure loads, FPO and FIF, for

Plate 3: Pull out failure mode of resistance spot weld in dissimilar DP780 and DP600 steels (Zhang et al., 2014) www.bayerojet.com 30

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

A critical nugget size exists which determines the parameter and it is essential for heat generation. transition from IF to PF modes. If the welding current is too small, the heat input Chao (2003) derived a relationship for critical is inadequate to form the weld nugget and the weld nugget size (DC) in the cross-tension (TS) strength of the weld joints is low. If the welding test: current is too large, expulsion occurs. Expulsion 2 4 refers to the drifting away or loss of molten τHAZ 3 3 Dc = 0.86 FZ t --- (4) metal from the fusion zone (Zhang et al., 2014). Kc Where t is the sheet thickness, τHAZ is the shear Figure 13 shows a defect-free and a surface in FZ which expulsion occurred. strength of the HAZ and Kc is the fracture toughness of the fusion zone. For the tensile shear loading condition, Chao Podržaj et al. (2006) observed that increasing (2003) proposed the following equation: welding current generally indicated better weld 4 quality for resistance spot welded mild steel Dc = 3.41t3 --- (5) sheets. However, very high welding currents lead Where t is the sheet thickness. to expulsion and subsequent loss of mechanical Pouranvari and Marashi (2012a) developed the properties. Kianersi et al. (2014) optimized the following equation to predict the critical FZ size welding current and time in RSW of the (FZs) required to ensure pullout failure mode austenitic stainless steel sheets grade AISI during the cross-tension test of AISI 304 spot 316L.The welding current was varied from 4kA welds: to 9kA, in steps of 1kA, at constant welding t ×HFL FZs CT = 4t --- (6) time. As illustrated in Plate 4, it was observed HFZ that increasing welding current lead to increase Where t is sheet thickness, HFL and HFZ are hardness of failure location and fusion zone in weld nugget diameter. Moreover, it was respectively. observed that the load bearing capacity also increased with increased welding current from 2.2.7 Effect of Welding Parameters on 4kA to 8kA. However, it was noted that Mechanical Performance of Resistance Spot increasing the current from 8kA to 9kA led to Welds reduction in load bearing capacity (Figure 7). This was attributed to expulsion. Effect of Welding Current Welding current is the most important process

Plate 4: Spot welded specimens (a) at lower heat inputs (defectless), (b) at higher heat inputs (expulsion phenomenon) (Razmpoosh et al., 2015)

www.bayerojet.com 31

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Figure 6: Variation of weld nugget diameter with welding current(Kianersi et al., 2014)

Figure 7: Variation of load bearing capacity with welding current(Kianersi et al., 2014)

Aslanlar et al. (2007) found that welding current element modelling. had a significant influence on tensile-shear and tensile-peel failure behavior for galvanized Effect of Welding Time chromate steel sheets. Small weld nugget Welding time has a significant influence on the diameters and lower tensile strengths were mechanical performance of spot welded joint. obtained with small welding currents. Decrease Increasing welding time leads to increase in heat in load bearing capacity and nugget diameter input and larger weld nugget diameter(Kocabekir were observed by increasing the current above et al., 2008). 10kA. Tavasolizadeh et al. (2011) observed that, for three thickness resistance spot welded low Similarly, Lang et al. (2008) investigated the carbon steel, increasing the welding time leads to effect of welding parameters on the better weld nugget formation (Plate 5). This microstructure and mechanical properties of leads to an increase in peak load and energy AZ31 magnesium alloy. The results showed that absorption of the joint and transition of increasing the welding time (1–16 cycles) and interfacial failure mode to pullout failure mode, welding current (15–23 kA) led to an increase in primarily due to the enlargement of weld nugget nugget diameter and consequently joint strength. size along sheet/sheet interface. In another study, It was, however, noted that welding currents however, it was noted that longer welding time higher than 25kA resulted in excessive heat is required to ensure sufficient weld nugget generation, leading to metal expulsion and growth along the sheet/sheet interface of sheets consequently reduction in joint strength. compared to the required time for nugget growth However, Moshayedi and Sattari-Far (2014) along geometrical centre of the joint(Harlin et noted that welding current has slight effect on al., 2003; Pouranvari & Marashi, 2012c). micro-properties of DP600 spot welds. Expulsion and unsatisfactory partial interfacial Effect of Electrode Force failure mode were observed at 12kA. The Electrode force influences the properties of experimental results were found to be in good resistance spot welded joints (Qiu et al., 2010), agreement with results predicted by finite primarily because of its effects on contact www.bayerojet.com 32

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 resistance and contact area. Generally, increasing caused expulsion. the electrode force leads to reduction in the Zhao et al. (2013) investigated the effects of contact resistance(Liu et al., 2010). Very high electrode force on microstructure and mechanical electrode force could lead to severe indentation, behavior of the resistance spot welded DP600 sheet separation and distortion (Liu et al., 2010). joint. The results indicated that varying the On the other hand, very low electrode force electrode force has effect on weld nugget sizes. would result in high contact resistance, excessive They noted that there is a critical electrode force heat generation and consequently expulsion, that gives maximum weld nugget size and best surface marking and poor joint quality (Liu et al., mechanical properties. However, it was noted 2010). Lang et al. (2008) investigated the effect that the electrode force has little effect on the of electrode force on mechanical properties of micro-hardness and martensite content of the AZ31 magnesium alloy (in the range of 1.5– 4.5 welded joint, although it alters the microstructure kN) at a welding current and welding time of and grain size. 23kA and 8 cycles, respectively. The results Harlin et al. (2003) found that increasing the showed that the electrode force had a manifest electrode force from 2.1kN to 6kN leads to a effect on the weld nugget size and thus the shift in the position of weld nugget formation strength of the joint. Lowering the electrode from sheet/sheet interface to the center of the force within this range enhanced the weld nugget middle sheet in three thickness zinc-coated steel diameter and improved the joint strength. resistance spot welds. However, very low electrode force (1.5 kN)

Plate 5: Macrostructure of weld nugget for spot weld made at welding time of a 0.18 s, b 0.22 s and c

3. ANALYSIS OF FINDINGS Analysis of Findings have great potential for vehicle weight reduction Vehicle crashworthiness, environmental and enhanced crashworthiness and should pollution and fuel economy are the major therefore be used extensively in vehicle concerns in the automotive industry. Vehicle manufacturing. manufacturers are constantly under pressure to Light alloys, with superior specific strength, improve fuel efficiency and thus reduce excellent recyclability and other remarkable greenhouse gas emissions. Therefore, a lot of properties, such as aluminum alloys and research efforts and investments are directed especially magnesium alloys are ideal candidates towards developing lightweight materials with for next generation vehicles. However, due to excellent strength-ductility combination for the their low bulk resistivity and high thermal automotive industry. Although conventional conductivity, their RSW is significantly more mild steel is the least expensive of all the challenging than RSW of steels. Large welding automotive sheet materials, weight saving current and therefore electrode degradation, requirements does not favour its use. AHSS porosity and formation of columnar dendritic which possess high strength-ductility zones in the FZ and HZ are major concerns combination, especially DP and TWIP steels, during RSW of these alloys. Although RSW has www.bayerojet.com 33

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 been perfected to produce good quality joints in steels, to successfully incorporate magnesium Nugget diameter is the most critical factor that and aluminum alloys into the automotive determines the overall mechanical performance structure, significant research efforts are required and failure mode of spot welds. Large nugget to perfect the RSW of these alloys. diameter leads to high strength and better energy absorption. Resistance spot welding process Cost effectiveness is also a major consideration parameters such as welding current, welding in vehicle construction. AHSS and other newly time and electrode force greatly influence the developed light alloys, like magnesium alloys, weld nugget size, load bearing capacity, energy are more expensive than conventional mild steel. absorption and transition from interfacial failure Therefore, an effective strategy to improve mode to pullout failure of resistance spot welds. overall crashworthiness and fuel economy would For both similar and dissimilar materials be the dissimilar welding of the various combination, the welding parameters should be automotive sheet materials to form hybrid optimized to produce large nugget size and pull structures. For example, dissimilar joining of out failure mode in order to ensure higher peak AHSS to conventional mild steel or to load and energy absorption. magnesium alloys.

3 CONCLUSION

Significant progress has been made in TWIP has shown a very good ductility – strength developing suitable materials for the automotive property. industry. Materials with high strength to weight Process parameters (welding current, welding ratio, improved formability and better time and electrode force) are very important in performance such as advanced high strength determining the quality of weld produced. The steels, aluminum and magnesium alloys are process parameters of RSW should therefore be increasingly being used in the automotive optimized to get optimum nugget diameter size industry to offer weight savings, improved fuel to avoid interfacial failure and also to ensure efficiency and passenger safety. However, among quality the materials been used in automotive industry .

REFERENCES

Alizadeh-Sh, M., Marashi, S., & Pouranvari, M. (2014). Resistance spot welding of AISI 430 Babu, N. K., Brauser, S., Rethmeier, M., & ferritic stainless steel: phase transformations and Cross, C. (2012). Characterization of mechanical properties. Materials & Design, Vol. microstructure and deformation behaviour of 56, pp. 258-263. resistance spot welded AZ31 magnesium alloy. Materials Science and Engineering: A, Vol. 549, pp. 149-156. Alizadeh-Sh, M., Pouranvari, M., & Marashi, S. (2015). Welding metallurgy of stainless steels during resistance spot welding Part II-heat Chao, Y. (2003). Failure mode of spot welds: affected zone and mechanical performance. interfacial versus pullout. Science and Science and Technology of welding and joining, Technology of welding and joining, Vol. 82, pp. pp. 1-10. 133-137.

Aslanlar, S., Ogur, A., Ozsarac, U., Ilhan, E., & Charde, N., Yusof, F., & Rajkumar, R. (2014). Demir, Z. (2007). Effect of welding current on Material characterizations of mild steels, mechanical properties of galvanized chromided stainless steels, and both steel mixed joints under steel sheets in electrical resistance spot welding. resistance spot welding (2-mm sheets). The Materials & Design, Vol. 281, pp. 2-7. International Journal of Advanced

www.bayerojet.com 34

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Manufacturing Technology, Vol. 751-4, pp. 373- Han, Q., Asgari, A., Hodgson, P. D., & Stanford, 384. N. (2014). Strain partitioning in dual-phase steels containing tempered martensite. Materials Chen, L., Zhao, Y., & Qin, X. (2013). Some Science and Engineering: A, Vol. 611, pp. 90-99. aspects of high manganese twinning-induced plasticity (TWIP) steel, a review. Acta Harlin, N., Jones, T., & Parker, J. (2003). Weld Metallurgica Sinica (English Letters), Vol. 261, growth mechanism of resistance spot welds in pp. 1-15. zinc coated steel. Journal of materials processing technology, Vol. 143, pp. 448-453.

Choi, B. H., Joo, D. H., & Song, S. H. (2007). Observation and prediction of fatigue behavior Kang, H.-T., Accorsi, I., Patel, B., & Pakalnins, of spot welded joints with triple thin steel plates E. (2010). Fatigue performance of resistance spot under tensile-shear loading. International journal welds in three sheet stack-ups. Procedia of fatigue, Vol. 294, pp. 620-627. Engineering, Vol. 21, pp. 129-138.

Cole, G. (2007). Magnesium vision 2020-a north Khan, M., Kuntz, M., Biro, E., & Zhou, Y. american automotive strategic vision for (2008). Microstructure and mechanical magnesium. Paper presented at the IMA properties of resistance spot welded advanced Proceedings, pp. 13 high strength steels. Materials Transactions, Vol. 497, pp. 1629-1637.

Cui, L. H., Qiu, R. F., Shi, H. X., & Zhu, Y. M. (2014). Resistance Spot Welding between Kianersi, D., Mostafaei, A., & Amadeh, A. A. Copper Coated Steel and Aluminum Alloy. (2014). Resistance spot welding joints of AISI Paper presented at the Applied Mechanics and 316L austenitic stainless steel sheets: Phase Materials, pp. 19-22 transformations, mechanical properties and microstructure characterizations. Materials & Design, Vol. 61, pp. 251-263. Dancette, S., Fabrègue, D., Massardier, V., Merlin, J., Dupuy, T., & Bouzekri, M. (2011). Experimental and modeling investigation of the Kim, J., Hong, S., Anjabin, N., Park, B., Kim, S., failure resistance of advanced high strength Chin, K., Lee, S., & Kim, H. (2015). Dynamic steels spot welds. Engineering Fracture strain aging of twinning-induced plasticity Mechanics, Vol. 7810, pp. 2259-2272. (TWIP) steel in tensile testing and deep drawing. Materials Science and Engineering: A, Vol. 633, pp. 136-143. Dutta, K., & Ray, K. (2013). Ratcheting strain in interstitial free steel. Materials Science and Engineering: A, Vol. 575, pp. 127-135. Kocabekir, B., Kacar, R., Gündüz, S., & Hayat, F. (2008). An effect of heat input, weld atmosphere and weld cooling conditions on the Goodarzi, M., Marashi, S., & Pouranvari, M. resistance spot weldability of 316L austenitic (2009). Dependence of overload performance on stainless steel. Journal of materials processing weld attributes for resistance spot welded technology, Vol. 1951, pp. 327-335. galvanized low carbon steel. Journal of materials processing technology, Vol. 2099, pp. 4379- 4384. Koyama, M., Tasan, C. C., Akiyama, E., Tsuzaki, K., & Raabe, D. (2014). Hydrogen- assisted decohesion and localized plasticity in Hadzima, B., Janeček, M., Estrin, Y., & Kim, H. dual-phase steel. Acta Materialia, Vol. 70, pp. S. (2007). Microstructure and corrosion 174-187. properties of ultrafine-grained interstitial free steel. Materials Science and Engineering: A, Vol. 4621, pp. 243-247. Kuziak, R., Kawalla, R., & Waengler, S. (2008). Advanced high strength steels for automotive industry. Archives of civil and mechanical engineering, Vol. 82, pp. 103-117. www.bayerojet.com 35

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37

Lang, B., Sun, D., Li, G., & Qin, X. (2008). Podržaj, P., Polajnar, I., Diaci, J., & Kariž, Z. Effects of welding parameters on microstructure (2006). Influence of welding current shape on and mechanical properties of resistance spot expulsion and weld strength of resistance spot welded magnesium alloy joints. Science and welds. Science and Technology of Welding & Technology of Welding & joining, Vol. 138, pp. Joining, Vol. 113, pp. 250-254. 698-704.

Pouranvari, M., Alizadeh-Sh, M., & Marashi, S. Lei, Z., Kang, H., & Liu, Y. (2011). Finite (2015). Welding metallurgy of stainless steels element analysis for transient thermal during resistance spot welding Part I: fusion characteristics of resistance spot welding process zone. Science and Technology of welding and with three sheets assemblies. Procedia joining, pp. 1-4. Engineering, Vol. 16, pp. 622-631.

Pouranvari, M., & Marashi, S. (2012a). Failure Li, D., Feng, Y., Song, S., Liu, Q., Bai, Q., Ren, mode transition in AISI 304 resistance spot F., & Shangguan, F. (2015). Influences of silicon welds. Welding Journal, Vol. 91, pp. 303-309. on the work hardening behavior and hot deformation behavior of Fe–25wt% Mn–(Si, Al) TWIP steel. Journal of Alloys and Compounds, Pouranvari, M., & Marashi, S. (2012c). Weld Vol. 618, pp. 768-775. nugget formation and mechanical properties of three-sheet resistance spot welded low carbon steel. Canadian Metallurgical Quarterly, Vol. Liu, L., Feng, J., & Zhou, Y. (2010). 18 - 511, pp. 105-110. Resistance spot welding of magnesium alloys. In L. Liu (Ed.), Welding and Joining of Magnesium Alloys: Woodhead Publishing, pp. 351-367e Pouranvari, M., & Marashi, S. (2013). Critical review of automotive steels spot welding: process, structure and properties. Science and Ma, N., & Murakawa, H. (2010). Numerical and Technology of welding and joining, Vol. 185, experimental study on nugget formation in pp. 361-403. resistance spot welding for three pieces of high strength steel sheets. Journal of materials processing technology, Vol. 21014, pp. 2045- Qiu, R., Shi, H., Yu, H., Zhang, K., Tu, Y., & 2052. Satonaka, S. (2010). Effects of electrode force on the characteristic of magnesium alloy joint welded by resistance spot welding with cover Mori, K., Abe, Y., & Kato, T. (2014). Self-pierce plates. Materials and Manufacturing Processes, riveting of multiple steel and aluminium alloy Vol. 2511, pp. 1304-1308. sheets. Journal of materials processing technology, Vol. 21410, pp. 2002-2008. Qiu, R., Zhang, Z., Zhang, K., Shi, H., & Ding, G. (2011). Influence of welding parameters on Moshayedi, H., & Sattari-Far, I. (2014). the tensile shear strength of aluminum alloy joint Resistance spot welding and the effects of welded by resistance spot welding. Journal of welding time and current on residual stresses. Materials Engineering and Performance, Vol. Journal of materials processing technology, Vol. 203, pp. 355-358. 21411, pp. 2545-2552.

Radakovic, D., & Tumuluru, M. (2008). Predicting resistance spot weld failure modes in Palanivel, S., Nelaturu, P., Glass, B., & Mishra, shear tension tests of advanced high-strength R. (2015). Friction stir additive manufacturing automotive steels. Welding Journal, Vol. 874, for high structural performance through pp. 96. microstructural control in an Mg based WE43 alloy. Materials & Design, Vol. 65, pp. 934-952. Rathbun, R. W., Matlock, D., & Speer, J. (2003). Fatigue behavior of spot welded high-strength www.bayerojet.com 36

ISSN : 1597-5835 Manladan, S.M et al /(JET) Journal of Engineering and Technology Vol. 10 (2) 2015 ,20-37 sheet steels. Welding Journal, Vol. 828, pp. 207- tensile shear load in resistance spot welded joints 218. of AZ31 Mg alloy. Science and Technology of Welding & joining, Vol. 128, pp. 671-676.

Razmpoosh, M., Shamanian, M., & Esmailzadeh, M. (2015). The microstructural Yao, Q., Luo, Z., Li, Y., Yan, F., & Duan, R. evolution and mechanical properties of resistance (2014). Effect of electromagnetic stirring on the spot welded Fe–31Mn–3Al–3Si TWIP steel. microstructures and mechanical properties of Materials & Design, Vol. 67, pp. 571-576. magnesium alloy resistance spot weld. Materials & Design, Vol. 63, pp. 200-207.

Rossini, M., Spena, P. R., Cortese, L., Matteis, P., & Firrao, D. (2015). Investigation on Zhang, C., Hu, X., Lu, P., & Zhang, G. (2013). dissimilar laser welding of advanced High Tensile overload-induced plastic deformation strength steel sheets for the automotive industry. and fatigue behavior in weld-repaired high- Materials Science and Engineering: A. strength low-alloy steel. Journal of materials processing technology, Vol. 21311, pp. 2005- 2014. Shen, J., Zhang, Y., Lai, X., & Wang, P. (2011). Modeling of resistance spot welding of multiple stacks of steel sheets. Materials & Design, Vol. Zhang, H., Qiu, X., Xing, F., Bai, J., & Chen, J. 322, pp. 550-560. (2014). Failure analysis of dissimilar thickness resistance spot welded joints in dual-phase steels during tensile shear test. Materials & Design, Spena, P. R., D’Aiuto, F., Matteis, P., & Vol. 55, pp. 366-372. Scavino, G. (2014). Dissimilar Arc Welding of Advanced High-Strength Car-Body Steel Sheets. Journal of Materials Engineering and Zhang, H., & Senkara, J. (2011). Resistance Performance, Vol. 2311, pp. 3949-3956. welding: fundamentals and applications: CRC press.

Tamarelli, C. (2011). AHSS 101: the evolving use of advanced high strength steels for Zhao, D., Wang, Y., Zhang, L., & Zhang, P. automotive applications. Steel Market (2013). Effects of electrode force on Development Institute (AUTOSTEEL), microstructure and mechanical behavior of the Washington, DC, USA. resistance spot welded DP600 joint. Materials & Design, Vol. 50, pp. 72-77.

Tavasolizadeh, A., Marashi, S., & Pouranvari, M. (2011). Mechanical performance of three Zhao, Y., Lu, Z., Yan, K., & Huang, L. (2015). thickness resistance spot welded low carbon Microstructural characterizations and mechanical steel. Materials Science and Technology, Vol. properties in underwater friction stir welding of 271, pp. 219-224. aluminum and magnesium dissimilar alloys. Materials & Design, Vol. 65, pp. 675-681.

Vural, M., & Akkus, A. (2004). On the resistance spot weldability of galvanized Zheng, R., Lin, J., Wang, P.-C., Zhu, C., & Wu, interstitial free steel sheets with austenitic Y. (2015). Effect of adhesive characteristics on stainless steel sheets. Journal of materials static strength of adhesive-bonded aluminum processing technology, Vol. 153, pp. 1-6. alloys. International Journal of Adhesion and Adhesives, Vol. 57, pp. 85-94.

Wang, Y., Mo, Z., Feng, J., & Zhang, Z. (2007). Effect of welding time on microstructure and