Parametric Studies and Finite Element Analysis of Welded Steel in Resistance Spot Welding Process

Transcription

1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Parametric Studies and Finite Element Analysis of Welded Steel in Resistance Spot Welding Process Kishore, N. 1*, Sreenu, S. 2, Ramachandran, N. 3, Allesu, K. 4, 1* NIT Calicut, , 2 NIT Calicut, , 3 Professor, NIT Calicut, , 4 Professor, NIT Calicut, , Abstract Resistance Spot Welding (RSW) is a widely used joining process for fabricating sheet assemblies in the automotive, marine and aerospace industries. Modern vehicles contain spot welded points. The main requirements of these automobile parts are corrosion resistance of chassis, the high strength values of sheets used, the stiffness of welded joints when exposed to an external force and the ability to absorb impacts, along with low cost and comfort. In RSW, electrodes travel on a predetermined path and make contact with the sheets at selected weld points to apply force. Electric current is then turned on and flows through the sheets clamped between the electrodes. Due to the contact resistance to current flow, the temperature rises from resistance heating. When the temperature reaches the desired fusing temperature, a molten nugget is formed, the current is shut off after a designated time to allow the nugget to cool down and solidification of the weld spot occurs under pressure. It is proposed to study the effect of various parameters on nugget dimensions and strength of the weld joint in RSW and concentrating on different material and thicknesses to get good weld joint at low cost and compared with analytical part done in Ansys. Keywords: Weld nugget, Tensile strength, Temperature distribution. 1. Introduction: Resistance spot welding (RSW) is mostly used in the automobile industry for joining thin sheet metal plates. While compared with other welding processes such as arc welding processes, resistance spot welding is quick process, easily automated and easily maintained. It involves interaction of electrical, thermal, mechanical and metallurgical properties.the spot welding process joins two or more metal sheets together at the interaction point. It is a simpleprocess that uses two copper electrodes toapply the force on work sheets together and allow high current to pass through it to form the weld nugget. The growthof the weld nugget depends upondifferent parameters such as electrode force, diameter of the electrode contact surface, squeeze time, weld time, welding current and internal heat generation between plates due to contact resistance. Quality and strength of weld joint depends upon shape and size of nugget. However, the joining between two dissimilar metals have some difficulties because of different thermal and physical properties. Lots of research works were done to avoid difficulties in the joining of dissimilar metals. JieShen et al (2011) discussed about modelling of multiple stacks of sheet metals in RSW and conclude that in the DP 600 steel the diameter of weld nugget is more compared to GI sheets. Qiu et al (2009) done the experiments on steel and aluminum alloys by using a stainless steel cover plate. Houet al (2007) discussed about the stress, strain and temperature distribution of mild steel plates by using FEM analysis in ansys. Triyono et al (2013) discussed about critical nugget diameter formed between dissimilar metals and conclude that nugget was in asymmetric shape. Kolarik et al (2012) discussed about the nugget formation between low carbon steel and AISI 304 austenitic stainless steel. Zhao et al (2013) discussed about the usage of flexible strips in RSW of ultra-thin automotive steel sheets. Lum et al (2004) discussed about the electrode pitting in RSW of aluminum alloys that causes electrode degradation. Liu et al (2013) aimed at evaluating the microstructural change and fatigue resistance of Mg/steel resistance spot welds, in comparison with Mg/Mg welds. In this study, the focus is on RSW between mild steel and austenitic stainless steel 304 and comparison of the variations in weld nugget size and shape between 186-1

2 Parametric Studies and Finite Element Analysis of Welded Steel in Resistance Spot Welding Process similar materials and dissimilar materials and thus concluding on the strength of joints. Figure 1. Schematic RSW setup 2. Experimental Setup: The tests were carried out using 10 kva Resistance Spot Welding machine pneumatically operated having thyristorised electronic timer. The work piece materials were cut in the dimension of mm (Figure 2). Figure 2. Schematic of the test sample Figure 3. RSW machine 2.1Experiments: The specimens are prepared as shown in Figure 2. Then, specimens are overlapped with 25 mm and welded. The electrode force was fixed at 8.32 N. Squeeze time was kept constant at 40 cycles(1 cycle = 0.02 sec). The welding time was applied as 10, 20 and 30 cycles. The welding current was increased from 3 to 6 ka by 1 ka increments. The heat input was taken as 30% of total heat input. Two set of sample experiments were done for finding nugget size and tensile strength. Nugget dimensions were measured on OLYMPUS BX-51 microscope with maximum 1000 X magnification. Tensile tests were done on the sophisticated universal testing machine with maximum capacity 10 kn

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Table 1: Chemical composition of materials Element C Mn P S Cr Ni Si Mild steel (wt %) austenitic stainless steel (wt %) Material Density (kg/m 3 ) Young s modulus (GPa) Table 2: Physical properties of materials Coefficient of thermal expansion (µm/m/ 0 C) Thermal conductivity (W/m.K) Specific heat (J/kg.K) Electrical resistivity (Ωm) 304 SS MS copper Results and discussions: Hamedi, and Pashazadeh (2008) in their paper on 2D axisymmetric modeling of finite element method developed to simulate RSW process using ANSYS, highlighted the advantages of getting temperature distributions at different weld stages, through these temperature distributions study of weld nugget formations and thus on the size prediction too. Also effects of welding current, welding time and electrode force on nugget formation can be investigated through this modeling. Considering these advantages, Ansys was effectively utilized here too. Structural and thermal-electric analysis was thus done in Ansys workbench In structural analysis, equivalent stress, equivalent elastic strain and total deformation at various points was found. In thermalelectric analysis, temperature distribution was found.the tensile tests were done on the sophisticated universal testing machine. Nugget dimensions was measured on OLYMPUS BX-51 microscope with maximum 100 X magnification. 3.1Equivalent Stress: It can be seen that there is mainly compressive stress in the area of electrode and workpiece interaction. The maximum stress about MPa at the electrode and workpiece interacted contact surface. The stress field became more complex due to the generation of thermal stress. Figure 4. Distribution of equivalent stress at squeeze step 3.2Equivalent Elastic Strain: Figure 5. Distribution of equivalent elastic strain 186-3

4 Parametric Studies and Finite Element Analysis of Welded Steel in Resistance Spot Welding Process The equivalent elastic strain occurred at electrode and workpiece interaction surface is very less. The maximum strain about It creates annulus around the nugget and prevents liquid metal expulsion during welding. 3.3 Total deformation: Figure 7. Temperature distribution 3.4 Tensile strength: It was observed that increase in current and welding time makes the strong welding joint.variation of tensile strength with respect to welding current and welding time is shown in Figure 7. Various failures that occurred during tensile test are shown in Figure 8. Figure 6. Deformation of weldment during squeeze step During squeeze step, the deformation occurred in the weldment is very low. The total deformation occurred is about 1.97 nm is very less as the basic purpose of applying force is to hold the work piece only. 3.5 Temperature Distribution: The temperature at the centre of the work pieces increases very fast manner at the starting of welding. The highest temperature keeps at the center of the workpiece throughout the whole welding process. During holding process, the molten material solidifies to form weld nugget. The temperature distribution at the end of welding process was shown in Figure 7. Tensile strength, MPa tensile strength Welding time, cycles Welding current, ka Figure 8. Variation of Tensile strength with respect to welding current and welding time 186-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India The comparison of tensile strength for mild steel, stainless steel and dissimilar metals is as shown in figure 9. magnification factor to get exact results. The nugget diameters at 5 ka current and 20 cycles weld time were mm, mm and mm for stainless steel, mild steel and dissimilar materials respectively, as depicted in figures 11 through 13. Tensile strength, MPa Mild steel Stainless steel Dissimilar materials Welding current, ka Figure 11. Image of stainless steel nugget Figure 9. Comparison of Tensile strength for mild steel, stainless steel and dissimilar materials The following modes of failure, namely, button pullout, tear at the edge of one side, separation and tear at the edge of both sides, were observed during tensile test. The failure modes are shown in figure 10. Figure 12. Image of mild steel nugget Figure 13. Image of dissimilar materials nugget Figure10. Failure of Tensile tested specimens 3.6 Microstructure of weld nugget: Images of nugget was taken by using OLYMPUS BX-51 microscope with 5X magnification. The dimensions mentioned in the image was multiplied with 4 Conclusions: Tensile tests are done for the resistance spot welded sheets of mild steel, stainless steel and dissimilar materials. It was observed that the tensile strength is maximum for stainless steel and minimum for mild steel. It is also observed that tensile strength increases with welding current and weld time irrespective of the materials. Nugget diameter was observed to be maximum in stainless steel and minimum in dissimilar materials. It was observed that the variation of nugget diameter occurred in dissimilar material due to different mechanical and thermal properties of materials

6 Parametric Studies and Finite Element Analysis of Welded Steel in Resistance Spot Welding Process 5 References: Arumugam, A., Charde, N. (2011), A Mechanical Study of Spot Weld Growth in Mild Steel, 302 Austenitic Stainless Steel and Both Materials Joined, Journal of Materials Science and Engineering, A 1, pp Chigurupati, P., Chun, B.K., Bandar, A., Wu, W.T. (2010), Finite Element Modelling of Resistance Spot Welding Process, Int J Mater Form, 3, pp Hou, Z., Kim, I., Wang, Y., Li, C., Chen, C. (2007), Finite element analysis for the mechanical features of resistance spot welding process, Journal of Materials Processing Technology, 185, pp Kahraman, N. (2007), The influence of welding parameters on the joint strength of resistance spotwelded titanium sheets, Materials and Design, 28, pp Kolarik, L., Sahul, M., Kolarikova, M., Sahul, M., Turna, M., Felix, M. (2012), Resistance Spot Welding of dissimilar Steels, ActaPolytechnica, 52, pp Liu, L., Xiao, L., Feng, J.C., Kim, S., Zhou, Y. (2013), Microstructure and fatigue properties of Mg-to-steel dissimilar resistance spot welds, Materials and Design, 45, pp Lum, I., Fukumoto, S., Biro, E., Boomer, D.R., Zhou, Y. (2004), Electrode Pitting in Resistance Spot Welding of Aluminum Alloy 5182, Metallurgical and Materials Transactions, 35A, pp Qiu, R., Iwamoto, C., Satonaka, S. (2009), Interfacial microstructure and strength of steel/aluminum alloy joints welded by resistance spot welding with cover plate, Journal of Materials Processing Technology, 209, pp Shen, J., Zhang, Y., Lai, X., Wang, P.C. (2011), Modeling of resistance spot welding of multiple stacks of steel sheets, Materials and Design, 32, pp Thakur, A.G., Rasane, A.R., Nandedkar, V.M. (2010), Finite Element Analysis of resistance spot welding to study nugget formation, International Journal of Applied Engineering Research, 1, pp Triyono.,Purwaningrum, Y., Chamid, I. (2013), Critical Nugget Diameter of Resistance Spot Welded Stiffened Thin Plate Structure, Modern Applied Science, 7, pp Zhao, Y., Zhang, Y., Lai, X., Wang, P.C. (2013), Resistance Spot Welding of Ultra-Thin Automotive Steel, Journal of Manufacturing Science and Engineering, 135, pp M Hamedi, H Pashazadeh (2008), Numerical study of nugget formation in resistance spot welding. Intl Jl of Mechanics, 1, Vol 2, pp