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Optimization in resistance spot welding of CR3 sheets by embedding WC powder at the lap joint
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 28071–28079
www.materialstoday.com/proceedings
ICCMMEMS_2018
Optimization in resistance spot welding of CR3 sheets by embedding WC powder at the lap joint Amiya Ranjan Malika* , Bibhuti Bhusan Pania, Sushant Kumar Badjenab a
b
Department of Mechanical Engineering, VSS University Of Technology, Burla, Sambalpur, Odisha, India-768018 Department of Metallurgy and Material Engineering, VSS University Of Technology, Burla, Sambalpur, Odisha, India-768018
Abstract In this work Tungsten Carbide (WC) powder has been embedded at the interface of Lap joint through Resistance Spot Welding (RSW). The idea of using WC powder at the interface is to make composite in the nugget as WC has good wetting ability with Iron(Fe). The factors that could influence the Tensile Shear Load(TSL) are welding current, Electrode pressure, weld time and number of pulse. All other factors like squeeze time, hold time, off time are kept constant. The influence of current, electrode pressure, weld time and number of pulse on the nugget diameter and TSL were studied using L9 orthogonal array and Taguchi method. From the results we could infer that the influence of weld current, weld time and number of pulse are significant. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018). Keywords: Resistance Spot Welding; Ferrous Metal Matrix Composite; Taguchi method.
1. Introduction The welding is a process where two similar or dissimilar metals are joined by melting and fusion at high temperature.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
Nomenclature N t IF PF OA CR RSW TSL HAZ MSD S/N Yopt Ym Yi
Number of pulse Sheet thickness Interfacial Failure Peel Off Failure Orthogonal array Cold rolled steel sheet Resistance spot welding Tensile shear load Heat affected zone Mean square deviation Signal to noise ratio Optimum response Average response of total experiment Average response of optimum factor label
They are called Fusion welding. On the other hand, the low temperature welding processes are there in which there is no melting of base metal, they are Soldering and Brazing. There are also solid-state welding processes in which there is no melting of base metals such as friction welding. They are named based on source of heat generation such as Oxy-Acetylene welding, Tungsten Inert gas welding, Metal Inert gas welding, Submerged Arc welding, Shielded Metal Arc welding, Electroslag welding and Resistance Spot welding. The Resistance Spot Welding process is a joining process commonly used in automotive industry. It utilizes the resistance of interface and the joining material to generate heat by localized current flow. The heat generated that is enough for fusion of metals is given by (1) below. =
(1)
Where H is the heat generated (J), I is the applied current (KA), R is the resistance of the workpiece (Ω), T is the weld time (mS or Cycles). Normally the spot weld equipment consists of transformer. The primary side is connected to the power supply line and secondary side to the welding side. The R is the combined resistance of the electrode and workpiece in ready to weld state. These resistances include resistance of the two electrodes, resistance of workpieces, resistance between electrode and workpieces during applied electrode pressure and resistance at the interface of lap. This combined resistance is the only factor for heat generation due to the applied high current and low voltage for a weld time. An automobile body may contain thousands of spots which make the body part to withstand heavy load. The cooling rate of the RSW is in the order of 1000 to 10000º C/S. this prevents the undesired grain growth in the weld zone. RSW has been studied for optimum parameter towards Tensile Shear Strength (TSS), Nugget Diameter, microstructure, hardness and Failure Characteristics by many authors. It has been found that Welding current, electrode force, Weld time and hold time are the factors influencing the above-mentioned output parameters [14, 16, 20]. S. Aslanlar et al. [1] had studied RSW of galvanized chromided steel sheet. They reported improvement of tensile peel strength (TPS) increased with weld time and Welding current up to a point then decreased due to more heat and reduction of nugget thickness by electrode pressure. Dursun Ozyurek [2] studied weldability of 304L austenitic stainless steel. They compared welding in ambient as well as nitrogen environment. They reported with increasing current there is increase in weld nugget diameter, ultimately improvement of TSS. They also reported nugget diameter was less in case of nitrogen atmosphere. This was due to formation of carbides and nitrides as solid solution which prevent diffusion and nugget get smaller. This also resulted into improved TSS. P. Marashi et al. [3] studied modes of failure in dissimilar metal SW. The different metals have different thermal conductivity, this would lead to different cooling rate. This would ultimately result into uneven nugget and phase distribution. They also reported, with increase in weld current the failure mode changes from interfacial to peel off type. Wei Liu et al. [4] had studied failure and fatigue behaviour of spot welded CR high strength austenitic stainless steel(CRHS
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
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301LN). They reported, though the nugget diameter has followed D>5√t formula the failure by tensile shear occurred at the nugget interface (The point of higher hardness). The fatigue study they reported that crack initiated at the HAZ and propagated through thickness along the HAZ. This was due to the stress concentration at folding location. Xinsheng Liao et al. [5] had studied microstructure of spot welded dual phase steel. They reported, by modified pulse (Applying tempering pulse after welding pulse) the specific type of microstructure and phase was formed. In the welding pulse, the fusion zone contains needle like and leaf like martensite laths. In tempering pulse there was formation of acicular ferrites from residual austenite and forming sandwich arrangement. F. Khodabakhshi et al. [6, 7] had improved RSW by increasing resistance of low carbon steel through constrained plastic deformation. Increased resistance means increased heat input for welding. So, the weldability improved. Similarly, different steel sheets have been studied for microstructure, phase and mechanical properties [8, 9, 10]. Hongqiang Zhang et al. [11] had studied RSW of dissimilar thickness dual phase steel (DP600/DP780). They reported that irrespective of sheet thickness the solidification line would be at the geometrical center line of the joint, as well as asymmetrical nugget would be formed in higher melting rate, lower resistance and higher thermal conductivity metal side. Similar works have been reported by many authors [12, 13, 15, 17, 19, 20]. Luo Yi et al. [21] had studied Acoustic emission signals of expulsions in spot welding of Zinc-coated steel sheets. They studied it because expulsion is the phenomenon affecting the strength of spot weld. All the above works have been studied on single spot weld. Wei Liu et al. [18] had studied and said with increasing number of spot the static strength increases as well as the fatigue limit increases nonlinearly. A. Chabok et al. [22] had reported that due to double pulse spot welding in DP1000 steel the columnar martensitic grain was surrounded by equiaxed austenite grain. This ultimately improved the toughness of the weld by softening of martensite. A lot of work has been done, but using carbide powder at the interface hasn’t been studied. In our work, hard Tungsten Carbide(WC) powder has been embedded in between two flat pieces and welding was performed. There has been numerous work in ferrous composites where WC was embedded in ferrous matrices (by casting as well as powder metallurgy technique) due to low wetting angle of WC with Fe [23]. In this work we have studied how TSL varies with I, P, T and N. The microstructure was observed too. 2. Experimental Detail 2.1. Tools and Setup A Jig Saw was used to cut sample of 100×25 flat piece. A 17 KVA spot welder (Mechelonic SS-35-450) was used for welding. A digital Vernier calliper was used to measure the Nugget Diameter. The TSL test was conducted in computerised UTM. An optical microscope (Leica Make) for microstructure analysis. 2.2. Material and Method First CR3 steel sheets of 1.5 mm and elemental analysis were collected from the market. The compositions are given below in Table.1. Table 1. Composition of AIAI 1010 CR3 steel sheet Iron Fe 99.18-96.2 %
Manganese Mn 0.30-0.60 %
Sulphur S ≤0.05 %
Phosphorous P ≤0.04 %
Carbon C 0.08- 0.13 %
They were cut into flat samples of 100×25 using Jig saw. The flat samples were polished with emery paper to remove undesired rust layer. Then they were cleaned with ethanol. For this thickness, 0.05gm of WC powder were put at the interface and lap welded. The experimental factor and labels are as given in Table.2.
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079 Table 2. Experimental Factor Factor label I (KA) P (KG/Cm2) T (ms) N
1 5.49 3 200 1
2 6.27 4 400 2
3 6.88 5 600 3
The welding was done using L9 OA in Table.3 and exactly at the desired spot where WC powders were kept. This reduces the number of experiments required to understand influence of each factor. The parameter e.g. squeeze time, hold time and off time were kept constant. Their D was measured too. Table 3. Experimental design and response SPECIMEN NAME 15SWFEWC1111 15SWFEWC1222 15SWFEWC1333 15SWFEWC2123 15SWFEWC2231 15SWFEWC2312 15SWFEWC3132 15SWFEWC3213 15SWFEWC3321
I (KA) 1 1 1 2 2 2 3 3 3
P (Kg/cm2) 1 2 3 1 2 3 1 2 3
T (mS) 1 2 3 2 3 1 3 1 2
N
TSL (KN) 5.4 10.94 11.71 14.76 10.51 10.86 15.55 11.74 12.52
1 2 3 3 1 2 2 3 1
Failure Type IF IF IF PF IF IF PF IF IF
D (mm) 2 3.7 4.9 5.9 4.1 3 7.7 4.5 4.2
The Taguchi method is a design of experiment technique which help in improving process and product quality with minimum cost [24]. By this method all the TSL values are transformed into Mean Square Deviation (MSD) and S/N ratio values for analysis. As higher TSL value is desired, so Bigger is Better Quality Characteristics (QC=B) is adopted and the MSD formula is given below in (2). Always the maximum S/N ratio (3) is desired irrespective of the desired QC. = ∑ /
(2)
=−
(3) Table 4. MSD and S/N ratio of TSL SAMPLE NAME
TSL(KN)
MSD
S/N
15SWFEWC1111
5.4
0.03429
14.6479
15SWFEWC1222
10.94
0.00836
20.7803
15SWFEWC1333
11.71
0.00729
21.3711
15SWFEWC2123
14.76
0.00459
23.3817
15SWFEWC2231
10.51
0.00905
20.4321
15SWFEWC2312
10.86
0.00848
20.7166
15SWFEWC3132
15.55
0.00414
23.8346
15SWFEWC3213
11.74
0.00726
21.3934
15SWFEWC3321
12.52
0.00638
21.9521
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
(a)
(b)
(c)
(d)
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Fig. 1. Main Effect of Factors. (a) Weld current vs TSL; (b) Electrode Pressure vs TSL; (c) Weld time vs TSL; (d) Number of pulse vs TSL.
By observing the above main effect graph in Figure.1 we concluded the optimum parameters as in Table.5. for highest TSL. In Table.6 we can observe percent contributions of each factors. To validate the results of ANOVA one confirmation test was performed with parameters given in Table.7. Table 5. Optimum factor I (KA)
P (Kg/cm2)
T (ms)
N
6.88
5
400
3
Table 6. ANOVA DOF (f)
SUM OF SQUARE (S)
VARIANCE (V)
WELD CURRENT (I) ELECTRODE PRESSURE (P)
2
19.394257
2
WELD TIME (T) NO OF PULSE (N)
FACTOR
PURE SUM OF SQUARE (P.S.)
PERCENT CONTRIBUTION (%)
9.697128
19.394257
34.83133
0.815522831
0.407761
0.815522831
1.464647
2
18.51350768
9.256754
18.51350768
33.24954
2
16.95720156
8.478601
16.95720156
30.45448
0
0
ERROR
0
0
TOTAL
8
55.6804886
FRATIO
99.999997
The predicted optimum TSL would be given by the equation (4) given below. =
∑
−
(4)
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079 Table 7. Final run I (KA)
P (Kg/cm2)
T (ms)
N
7.3
5
400
3
TSL(KN)
D (mm)
Failure Type
TSL(predicted) (KN)
16.73
8.6
PF
17.76
3. Results and discussion 3.1. ANOVA Analysis Observing the average effects of each factors in Figure.1 we concluded that with increasing welding current the TSL goes on increasing. The same trend was observed in electrode pressure as well as number of pulse. On the other hand, the weld time shows an inverted V trend. But its reducing effect on TSL is very less and can be neglected. The influence of electrode pressure on TSL is very less which may be neglected during practical application. Based on the percentage contribution results for higher TSL in Table.5, the order of significance of factors are welding current, weld time, number of pulse and electrode pressure. The final run in Table.6, we observe that the resulted TSL is well close to the predicted. The difference could be due to some error in conducting the experiment. This result indicate that the WC has been properly bonded with Fe during spot welding. By comparing D vs TSL we can conclude that with increase in Nugget diameter the TSL increases. The failure type also changes from IF to PF type. In the below Fig.2 there is slight reduction in TSL with increase in nugget diameter. This can be inferred as error in conducting the experiments, but over ally the trend is increasing.
Region of Interfacial Failure Region of Peel Off Failure
Fig. 2. Nugget Diameter vs TSL.
3.2. Microstructure The base metal was ferrite (white area) and martensite plate (brown area). Observing the structure of sample 15SWFEWC1111 (with all the factor at lowest label) in Fig.3(a), we could see that the nugget wasn’t properly formed. Only small amount of WC melted and embedded with Fe as golden area, the remaining WC as silver area. With increase in current, pressure and weld time the nugget formed. But with medium current the nugget contains all WC
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
(a)
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(b)
All WC
(c)
(e)
(d) WC + Martensite + Ferrite
(f)
Fig. 3. Microstructure of SW samples (a) 15SWFEWC1111; (b) 15SWFEWC2231; (c) 15SWFEWC3321; (d) 15SWFEWC1222; (e)15SWFEWC3132; (f) 15SWFEWC2312.
melted in Fig.3(b) and with high current nugget contains composite structure containing WC, martensite and ferrite in Fig.3(c). With increase in N the structure in nugget become more columnar and intermixing of WC and martensite and ferrite in Fig.3 (d)-(i). Out of all 7 samples failed by interfacial failure due to brittleness of WC. But the embedment of WC with Fe is good due to low wetting angle with Fe. One thing we must control during welding
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
is that with successive number of pulse we have to weld at original weld position otherwise the nugget would be formed as given in Fig.3(e).
(h)
(g)
(i)
Fig. 4. Microstructure of SW samples (g) 15SWFEWC1333; (h) 15SWFEWC2123; (i) 15SWFEWC3213.
4. Conclusion With increase in weld current the nugget diameter goes on increasing. Ultimately the TSL increases. The failure type too changes from interfacial failure to peel off type failure. The weld current, weld time and number of pulse are the most significant factors which influence TSL as well as D. This is a clear indication that WC get bonded properly with Fe during RSW which is observed in microstructure in Fig.3 and Fig.4. By the use of Taguchi method, the RSW can be studied with minimum number of experiments. Acknowledgements We are very grateful to Prof. Sarat Kumar Swain of VSSUT Burla for subscribing journals. We are very grateful to Mr. Pravakar Samal for assisting in operating the spot welding machine. We are grateful to Mr. Prashant Kumar Samal for operating the UTM. We are sincerely thankful to Mr. Ratnakar Sahoo for helping in operating optical microscope.
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
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References [1] J. S. Aslanlar, A. Ogur, U. Ozsarac, E. Ilhan, Z. Demir, “Effect of welding current on mechanical properties of galvanized chromided steel sheets in electrical resistance spot welding” Materials and Design 28 (2007) 2–7. [2] Dursun O¨ zyu¨rek, “An effect of weld current and weld atmosphere on the resistance spot weldability of 304L austenitic stainless steel” Materials and Design 29 (2008) 597–603. [3] P. Marashi, M. Pouranvari, S. Amirabdollahian, A. Abedi, M. Goodarzi, “Microstructure and failure behavior of dissimilar resistance spot welds between low carbon galvanized and austenitic stainless steels” Materials Science and Engineering A 480 (2008) 175–180. [4] Wei Liu∗, Runjiao Wang, Junlei Han, Xiangyang Xu, Qiang Li, “Microstructure and mechanical performance of resistance spot-welded coldrolled high strength austenitic stainless steel” Journal of Materials Processing Technology 210 (2010) 1956–1961. [5] Xinsheng Liaoa, Xiaodong Wanga, Zhenghong Guoa, Min Wangb, Yixiong Wub, Yonghua Ronga, “Microstructures in a resistance spot welded high strength dual phase steel” M A T E R I A L S C H A R A C T E R I Z A T I O N 6 1 ( 2 0 1 0 ) 3 4 1 – 3 4 6. [6] F. Khodabakhshi, M. Kazeminezhad, A.H. Kokabi, “Mechanical properties and microstructure of resistance spot welded severely deformed low carbon steel” Materials Science and Engineering A 529 (2011) 237– 245. [7] F. Khodabakhshi, M. Kazeminezhad, A.H. Kokabi, “Resistance spot welding of ultra-fine grained steel sheets produced by constrained groove pressing: Optimization and characterization” M A T E R I A L S C H A R A C T E R I Z A T I O N 6 9 ( 2 0 1 2 ) 7 1 – 8 3. [8] D.W. Zhao, Y.X. Wanga, L. Zhang, P. Zhang, “Effects of electrode force on microstructure and mechanical behaviour of the resistance spot welded DP600 joint” Materials and Design 50 (2013) 72–77. [9] M. Alizadeh-Sh, S.P.H. Marashi, M. Pouranvari, “Resistance spot welding of AISI 430 ferritic stainless steel: Phase transformations and mechanical properties” Materials and Design 56 (2014) 258–263. [10] Danial Kianersi, Amir Mostafaei, Ahmad Ali Amadeh, “Resistance spot welding joints of AISI 316L austenitic stainless-steel sheets: Phase transformations, mechanical properties and microstructure characterizations” Materials and Design 61 (2014) 251–263. [11] Hongqiang Zhang, Xiaoming Qiu, Yang Bai, Fei Xing, Haiyan Yu, Yanan Shi, “Resistance spot welding macro characteristics of the dissimilar thickness dual phase steels” Materials and Design 63 (2014) 151–158. [12] Nachimani Charde, Farazila Yusof, Rajprasad Rajkumar, “Material characterizations of mild steels, stainless steels, and both steel mixed joints under resistance spot welding (2-mm sheets)” Int J Adv Manuf Technol (2014) 75:373–384. [13] M. R. A. Shawon, F. Gulshan, A. S. W. Kurny, “Effect of Welding Current on the Structure and Properties of Resistance Spot Welded Dissimilar (Austenitic Stainless Steel and Low Carbon Steel) Metal Joints” J. Inst. Eng. India Ser. D (January–June 2015) 96(1):29–36. [14] A. G. Thakur, V. M. Nandedkar, “Optimization of the Resistance SpotWelding Process of Galvanized Steel Sheet Using the Taguchi Method” Arab J Sci Eng (2014) 39:1171–1176. [15] F. Khodabakhshi, M. Kazeminezhad, A. H. Kokabi, “Metallurgical characteristics and failure mode transition for dissimilar resistance spot welds between ultra-fine grained and coarse-grained low carbon steel sheets” Materials Science&EngineeringA637(2015)12–22. [16] Shaik Shafee, B. Balu Naik, K. Sammaiah, “Resistance Spot Weld Quality Characteristics Improvement By Taguchi Method” Materials Today: Proceedings 2 (2015) 2595 – 2604. [17] Mohammad Hosein Bina, Mostafa Jamali, Morteza Shamanian, Hamed Sabet, “Effect of Welding Time in the Resistance Spot Welded Dissimilar Stainless Steels” Trans Indian Inst Met (2015) 68(2):247–255. [18] Wei Liu, Hailong Fan, Xiangzhong Guo, Zhihong Huang, Xiaohui Han, “Mechanical Properties of Resistance SpotWelded Components of High Strength Austenitic Stainless Steel” Journal of Materials Science & Technology 32 (2016) 561–565. [19] Xinjian Yuana, Ci Lia, Jianbin Chena, Xiuyang Lia, Xuebo Lianga, Xueyu Pan, “Resistance spot welding of dissimilar DP600 and DC54D steels” Journal of Materials Processing Technology 239 (2017) 31–41. [20] Haiqiang Long, Yumei Hu, Xiaoqing Jin, Jinhua Shao, Hao Zhu, “Effect of holding time on microstructure and mechanical properties of resistance spot welds between low carbon steel and advanced high strength steel” Computational Materials Science 117 (2016) 556–563. [21] Luo Yi, Wan Rui, Xie Xiaojian, Zhu Yang, “Expulsion analysis of resistance spot welding on zinc-coated steel by detection of structureborne acoustic emission signals” Int J Adv Manuf Technol (2016) 84:1995–2002. [22] A. Chabok a, E. van der Aa b, J.T.M. De Hossonc, Y.T. Pei, “Mechanical behavior and failure mechanism of resistance spot welded DP1000 dual phase steel” Materials and Design 124 (2017) 171–182. [23] F. Akhtar, “Ceramic reinforced high modulus steel composites” , Canadian Metallurgical Quarterly 2014 VOL 53 NO 3. [24] Ranjit K. Roy, “Design of experiment using Taguchi Approach” , Wiley India : 2013.
ScienceDirect Materials Today: Proceedings 5 (2018) 28071–28079
www.materialstoday.com/proceedings
ICCMMEMS_2018
Optimization in resistance spot welding of CR3 sheets by embedding WC powder at the lap joint Amiya Ranjan Malika* , Bibhuti Bhusan Pania, Sushant Kumar Badjenab a
b
Department of Mechanical Engineering, VSS University Of Technology, Burla, Sambalpur, Odisha, India-768018 Department of Metallurgy and Material Engineering, VSS University Of Technology, Burla, Sambalpur, Odisha, India-768018
Abstract In this work Tungsten Carbide (WC) powder has been embedded at the interface of Lap joint through Resistance Spot Welding (RSW). The idea of using WC powder at the interface is to make composite in the nugget as WC has good wetting ability with Iron(Fe). The factors that could influence the Tensile Shear Load(TSL) are welding current, Electrode pressure, weld time and number of pulse. All other factors like squeeze time, hold time, off time are kept constant. The influence of current, electrode pressure, weld time and number of pulse on the nugget diameter and TSL were studied using L9 orthogonal array and Taguchi method. From the results we could infer that the influence of weld current, weld time and number of pulse are significant. © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018). Keywords: Resistance Spot Welding; Ferrous Metal Matrix Composite; Taguchi method.
1. Introduction The welding is a process where two similar or dissimilar metals are joined by melting and fusion at high temperature.
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International Conference on Composite Materials: Manufacturing, Experimental Techniques, Modeling and Simulation (ICCMMEMS-2018).
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
Nomenclature N t IF PF OA CR RSW TSL HAZ MSD S/N Yopt Ym Yi
Number of pulse Sheet thickness Interfacial Failure Peel Off Failure Orthogonal array Cold rolled steel sheet Resistance spot welding Tensile shear load Heat affected zone Mean square deviation Signal to noise ratio Optimum response Average response of total experiment Average response of optimum factor label
They are called Fusion welding. On the other hand, the low temperature welding processes are there in which there is no melting of base metal, they are Soldering and Brazing. There are also solid-state welding processes in which there is no melting of base metals such as friction welding. They are named based on source of heat generation such as Oxy-Acetylene welding, Tungsten Inert gas welding, Metal Inert gas welding, Submerged Arc welding, Shielded Metal Arc welding, Electroslag welding and Resistance Spot welding. The Resistance Spot Welding process is a joining process commonly used in automotive industry. It utilizes the resistance of interface and the joining material to generate heat by localized current flow. The heat generated that is enough for fusion of metals is given by (1) below. =
(1)
Where H is the heat generated (J), I is the applied current (KA), R is the resistance of the workpiece (Ω), T is the weld time (mS or Cycles). Normally the spot weld equipment consists of transformer. The primary side is connected to the power supply line and secondary side to the welding side. The R is the combined resistance of the electrode and workpiece in ready to weld state. These resistances include resistance of the two electrodes, resistance of workpieces, resistance between electrode and workpieces during applied electrode pressure and resistance at the interface of lap. This combined resistance is the only factor for heat generation due to the applied high current and low voltage for a weld time. An automobile body may contain thousands of spots which make the body part to withstand heavy load. The cooling rate of the RSW is in the order of 1000 to 10000º C/S. this prevents the undesired grain growth in the weld zone. RSW has been studied for optimum parameter towards Tensile Shear Strength (TSS), Nugget Diameter, microstructure, hardness and Failure Characteristics by many authors. It has been found that Welding current, electrode force, Weld time and hold time are the factors influencing the above-mentioned output parameters [14, 16, 20]. S. Aslanlar et al. [1] had studied RSW of galvanized chromided steel sheet. They reported improvement of tensile peel strength (TPS) increased with weld time and Welding current up to a point then decreased due to more heat and reduction of nugget thickness by electrode pressure. Dursun Ozyurek [2] studied weldability of 304L austenitic stainless steel. They compared welding in ambient as well as nitrogen environment. They reported with increasing current there is increase in weld nugget diameter, ultimately improvement of TSS. They also reported nugget diameter was less in case of nitrogen atmosphere. This was due to formation of carbides and nitrides as solid solution which prevent diffusion and nugget get smaller. This also resulted into improved TSS. P. Marashi et al. [3] studied modes of failure in dissimilar metal SW. The different metals have different thermal conductivity, this would lead to different cooling rate. This would ultimately result into uneven nugget and phase distribution. They also reported, with increase in weld current the failure mode changes from interfacial to peel off type. Wei Liu et al. [4] had studied failure and fatigue behaviour of spot welded CR high strength austenitic stainless steel(CRHS
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
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301LN). They reported, though the nugget diameter has followed D>5√t formula the failure by tensile shear occurred at the nugget interface (The point of higher hardness). The fatigue study they reported that crack initiated at the HAZ and propagated through thickness along the HAZ. This was due to the stress concentration at folding location. Xinsheng Liao et al. [5] had studied microstructure of spot welded dual phase steel. They reported, by modified pulse (Applying tempering pulse after welding pulse) the specific type of microstructure and phase was formed. In the welding pulse, the fusion zone contains needle like and leaf like martensite laths. In tempering pulse there was formation of acicular ferrites from residual austenite and forming sandwich arrangement. F. Khodabakhshi et al. [6, 7] had improved RSW by increasing resistance of low carbon steel through constrained plastic deformation. Increased resistance means increased heat input for welding. So, the weldability improved. Similarly, different steel sheets have been studied for microstructure, phase and mechanical properties [8, 9, 10]. Hongqiang Zhang et al. [11] had studied RSW of dissimilar thickness dual phase steel (DP600/DP780). They reported that irrespective of sheet thickness the solidification line would be at the geometrical center line of the joint, as well as asymmetrical nugget would be formed in higher melting rate, lower resistance and higher thermal conductivity metal side. Similar works have been reported by many authors [12, 13, 15, 17, 19, 20]. Luo Yi et al. [21] had studied Acoustic emission signals of expulsions in spot welding of Zinc-coated steel sheets. They studied it because expulsion is the phenomenon affecting the strength of spot weld. All the above works have been studied on single spot weld. Wei Liu et al. [18] had studied and said with increasing number of spot the static strength increases as well as the fatigue limit increases nonlinearly. A. Chabok et al. [22] had reported that due to double pulse spot welding in DP1000 steel the columnar martensitic grain was surrounded by equiaxed austenite grain. This ultimately improved the toughness of the weld by softening of martensite. A lot of work has been done, but using carbide powder at the interface hasn’t been studied. In our work, hard Tungsten Carbide(WC) powder has been embedded in between two flat pieces and welding was performed. There has been numerous work in ferrous composites where WC was embedded in ferrous matrices (by casting as well as powder metallurgy technique) due to low wetting angle of WC with Fe [23]. In this work we have studied how TSL varies with I, P, T and N. The microstructure was observed too. 2. Experimental Detail 2.1. Tools and Setup A Jig Saw was used to cut sample of 100×25 flat piece. A 17 KVA spot welder (Mechelonic SS-35-450) was used for welding. A digital Vernier calliper was used to measure the Nugget Diameter. The TSL test was conducted in computerised UTM. An optical microscope (Leica Make) for microstructure analysis. 2.2. Material and Method First CR3 steel sheets of 1.5 mm and elemental analysis were collected from the market. The compositions are given below in Table.1. Table 1. Composition of AIAI 1010 CR3 steel sheet Iron Fe 99.18-96.2 %
Manganese Mn 0.30-0.60 %
Sulphur S ≤0.05 %
Phosphorous P ≤0.04 %
Carbon C 0.08- 0.13 %
They were cut into flat samples of 100×25 using Jig saw. The flat samples were polished with emery paper to remove undesired rust layer. Then they were cleaned with ethanol. For this thickness, 0.05gm of WC powder were put at the interface and lap welded. The experimental factor and labels are as given in Table.2.
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079 Table 2. Experimental Factor Factor label I (KA) P (KG/Cm2) T (ms) N
1 5.49 3 200 1
2 6.27 4 400 2
3 6.88 5 600 3
The welding was done using L9 OA in Table.3 and exactly at the desired spot where WC powders were kept. This reduces the number of experiments required to understand influence of each factor. The parameter e.g. squeeze time, hold time and off time were kept constant. Their D was measured too. Table 3. Experimental design and response SPECIMEN NAME 15SWFEWC1111 15SWFEWC1222 15SWFEWC1333 15SWFEWC2123 15SWFEWC2231 15SWFEWC2312 15SWFEWC3132 15SWFEWC3213 15SWFEWC3321
I (KA) 1 1 1 2 2 2 3 3 3
P (Kg/cm2) 1 2 3 1 2 3 1 2 3
T (mS) 1 2 3 2 3 1 3 1 2
N
TSL (KN) 5.4 10.94 11.71 14.76 10.51 10.86 15.55 11.74 12.52
1 2 3 3 1 2 2 3 1
Failure Type IF IF IF PF IF IF PF IF IF
D (mm) 2 3.7 4.9 5.9 4.1 3 7.7 4.5 4.2
The Taguchi method is a design of experiment technique which help in improving process and product quality with minimum cost [24]. By this method all the TSL values are transformed into Mean Square Deviation (MSD) and S/N ratio values for analysis. As higher TSL value is desired, so Bigger is Better Quality Characteristics (QC=B) is adopted and the MSD formula is given below in (2). Always the maximum S/N ratio (3) is desired irrespective of the desired QC. = ∑ /
(2)
=−
(3) Table 4. MSD and S/N ratio of TSL SAMPLE NAME
TSL(KN)
MSD
S/N
15SWFEWC1111
5.4
0.03429
14.6479
15SWFEWC1222
10.94
0.00836
20.7803
15SWFEWC1333
11.71
0.00729
21.3711
15SWFEWC2123
14.76
0.00459
23.3817
15SWFEWC2231
10.51
0.00905
20.4321
15SWFEWC2312
10.86
0.00848
20.7166
15SWFEWC3132
15.55
0.00414
23.8346
15SWFEWC3213
11.74
0.00726
21.3934
15SWFEWC3321
12.52
0.00638
21.9521
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
(a)
(b)
(c)
(d)
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Fig. 1. Main Effect of Factors. (a) Weld current vs TSL; (b) Electrode Pressure vs TSL; (c) Weld time vs TSL; (d) Number of pulse vs TSL.
By observing the above main effect graph in Figure.1 we concluded the optimum parameters as in Table.5. for highest TSL. In Table.6 we can observe percent contributions of each factors. To validate the results of ANOVA one confirmation test was performed with parameters given in Table.7. Table 5. Optimum factor I (KA)
P (Kg/cm2)
T (ms)
N
6.88
5
400
3
Table 6. ANOVA DOF (f)
SUM OF SQUARE (S)
VARIANCE (V)
WELD CURRENT (I) ELECTRODE PRESSURE (P)
2
19.394257
2
WELD TIME (T) NO OF PULSE (N)
FACTOR
PURE SUM OF SQUARE (P.S.)
PERCENT CONTRIBUTION (%)
9.697128
19.394257
34.83133
0.815522831
0.407761
0.815522831
1.464647
2
18.51350768
9.256754
18.51350768
33.24954
2
16.95720156
8.478601
16.95720156
30.45448
0
0
ERROR
0
0
TOTAL
8
55.6804886
FRATIO
99.999997
The predicted optimum TSL would be given by the equation (4) given below. =
∑
−
(4)
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079 Table 7. Final run I (KA)
P (Kg/cm2)
T (ms)
N
7.3
5
400
3
TSL(KN)
D (mm)
Failure Type
TSL(predicted) (KN)
16.73
8.6
PF
17.76
3. Results and discussion 3.1. ANOVA Analysis Observing the average effects of each factors in Figure.1 we concluded that with increasing welding current the TSL goes on increasing. The same trend was observed in electrode pressure as well as number of pulse. On the other hand, the weld time shows an inverted V trend. But its reducing effect on TSL is very less and can be neglected. The influence of electrode pressure on TSL is very less which may be neglected during practical application. Based on the percentage contribution results for higher TSL in Table.5, the order of significance of factors are welding current, weld time, number of pulse and electrode pressure. The final run in Table.6, we observe that the resulted TSL is well close to the predicted. The difference could be due to some error in conducting the experiment. This result indicate that the WC has been properly bonded with Fe during spot welding. By comparing D vs TSL we can conclude that with increase in Nugget diameter the TSL increases. The failure type also changes from IF to PF type. In the below Fig.2 there is slight reduction in TSL with increase in nugget diameter. This can be inferred as error in conducting the experiments, but over ally the trend is increasing.
Region of Interfacial Failure Region of Peel Off Failure
Fig. 2. Nugget Diameter vs TSL.
3.2. Microstructure The base metal was ferrite (white area) and martensite plate (brown area). Observing the structure of sample 15SWFEWC1111 (with all the factor at lowest label) in Fig.3(a), we could see that the nugget wasn’t properly formed. Only small amount of WC melted and embedded with Fe as golden area, the remaining WC as silver area. With increase in current, pressure and weld time the nugget formed. But with medium current the nugget contains all WC
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
(a)
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(b)
All WC
(c)
(e)
(d) WC + Martensite + Ferrite
(f)
Fig. 3. Microstructure of SW samples (a) 15SWFEWC1111; (b) 15SWFEWC2231; (c) 15SWFEWC3321; (d) 15SWFEWC1222; (e)15SWFEWC3132; (f) 15SWFEWC2312.
melted in Fig.3(b) and with high current nugget contains composite structure containing WC, martensite and ferrite in Fig.3(c). With increase in N the structure in nugget become more columnar and intermixing of WC and martensite and ferrite in Fig.3 (d)-(i). Out of all 7 samples failed by interfacial failure due to brittleness of WC. But the embedment of WC with Fe is good due to low wetting angle with Fe. One thing we must control during welding
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Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
is that with successive number of pulse we have to weld at original weld position otherwise the nugget would be formed as given in Fig.3(e).
(h)
(g)
(i)
Fig. 4. Microstructure of SW samples (g) 15SWFEWC1333; (h) 15SWFEWC2123; (i) 15SWFEWC3213.
4. Conclusion With increase in weld current the nugget diameter goes on increasing. Ultimately the TSL increases. The failure type too changes from interfacial failure to peel off type failure. The weld current, weld time and number of pulse are the most significant factors which influence TSL as well as D. This is a clear indication that WC get bonded properly with Fe during RSW which is observed in microstructure in Fig.3 and Fig.4. By the use of Taguchi method, the RSW can be studied with minimum number of experiments. Acknowledgements We are very grateful to Prof. Sarat Kumar Swain of VSSUT Burla for subscribing journals. We are very grateful to Mr. Pravakar Samal for assisting in operating the spot welding machine. We are grateful to Mr. Prashant Kumar Samal for operating the UTM. We are sincerely thankful to Mr. Ratnakar Sahoo for helping in operating optical microscope.
Amiya Ranjan Malik et al./ Materials Today: Proceedings 5 (2018) 28071–28079
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