2016 International Conference on Advanced Materials Science and Technology (AMST 2016) ISBN: 978-1-60595-397-7

Mechanical Performance of Resistance Spot Welding Joint of Dissimilar High Strength Steels

Shang-gong ZHOU, Kai-wei LIU, Mian WANG and Jiang-wei REN*

School of Materials Engineering, Shanghai University of Engineering Science, Shanghai, China

*Corresponding author

Keywords: High strength steel, Resistance spot welding, microstructure, mechanical properties.

Abstract. Ultra-high strength hot stamped steel B1500HS and cold rolled high strength steel B250P1 with unequal thickness were resistance spot welded. The macrostructure and microstructure of welding joints were observed through optical microscope and scanning electron microscope. The mechanical performances of welding joints were studied by means of shear and cross tension test. The results show a single nugget about 7.6mm in size form and nugget shifts towards B1500HS side. Martensite form in nugget and the heat affected zone on B1500HS side, which provides the both parts higher strength than B250P1. All welding joints are pulled out in both shear and cross tension test. Bigger nugget is the most important controlling factor for shear strength. The shape of nugget has distinct effect on the properties of joints under cross tension condition.

Introduction

Lightweight automobile is one of the focuses in automobile industry in recent years. Lighting weight can reduce fuel consumption and exhaust pollution [1]. Experiments show that if 10% of automobile weight reduced, fuel efficiency can be increased by 6%–8%. Vehicle curb weight was reduced by 100 kg, fuel consumption in one hundred kilometers can be reduced 0.3–0.6 liters [2]. So, lightweight car has become a trend in the world. Meanwhile, the safety need from passengers is booming. To balance the two contradictory sides, ultra-high strength steel is widely used in automobile industry [3].

However, ultra-high strength steel, especially hot stamped steel, has specific characters in physical and chemical properties [4]. These characters of hot stamped steel lead to some complicated welding problems when it is resistance spot welded which is the most important welding technology in automobile industry. For example, greater electrode pressure is requested and shrink defects form easily. Moreover, the wear of electrode is intensified, which leads to the instable welding quality [5-7]. In this paper, ultra-high strength hot stamped steel and cold rolled high strength steel are resistance spot welded. The mechanical performance of resistance spot welding joint during shear and cross tension test is evaluated.

Experimental

Ultra-high strength hot stamped steel B1500HS and cold rolled high strength steel B250P1 were used as raw materials. The chemical composition and mechanical properties of two raw materials were shown in Table1-2 [8].

The ultra-high strength steel with the thickness of 1.6mm was resistance spot welded with the high strength steel which was 1.0mm in thickness. Single welding point was conducted at the center of overlapped area. The resistance spot welding was conducted on the ZDB-180 energy storage spot welding machine. The welding cycles consisted of pre-heat cycle and two-pulse welding cycles. The specimens were preload with welding force for 40ms firstly. Then 5kA pre-heat current last 60ms. Two-pulse welding cycles which had same welding current and welding time followed. The detail welding current and time were shown in Table 3. Moreover, 20ms cooling times were set among the pre-heat current and welding cycles.

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Table 1. Chemical composition of B1500HS and B250P1 (wt.%).

Materials C Si Mn Cr Mo B P S Ait Ti

B1500HS 0.23 0.25 1.35 0.19 0.04 0.0032 - - - -

B250P1 ≤0.008 - ≤1.20 - - ≤0.020 ≤0.12 ≤0.025 ≥0.015 ≤0.20

Table 2. Mechanical properties of B1500HS and B250P1.

Materials Yield Strength/MPa Tensile Strength/MPa Elongation/%

B1500HS (after quenching) 1500 ≥1400 ≥13

B250P1 250-360 ≥440 ≥32

Table 3. Resistance spot welding parameters.

Specimen Welding Current /kA Welding Time /ms Welding Force /kN

A 6 300 3.5

B 8.5 300 3.5

C 10 300 3.5

After spot welding, all joints were cut along the axis of nugget on electric discharge machine. The macrostructure and microstructure of joints were observed on VHX-600K optical microscope and S-3400 scanning electron microscope. The samples used for observation were prepared according to standard procedure and etched in alcoholic solution with 4 vol.% nitric acid.

Mechanical performance of spot welding joints under the condition of shear was tested according to ISO 14273-2001 and that under cross tension was tested according to ISO 14272-2001. The shear and cross tension test were performed on a AG-25TA type universal testing machine at a constant speed 2mm/min under ambient temperature. The given data of mechanical properties was the average value of five tests.

Results and Discussion

The macrostructure of spot welding joints are shown in Figure 1. It can be seen that reasonable nuggets form and are mostly located on the side of thicker plate in three cases. The phenomenon attributes to the difference of two metal plates on the thickness and mechanical properties. The upper B250P1 plate has less thickness and weaker yield strength, so it is easy to deform under welding force, which push the center of heat distribution moving to the side of thicker plate.

In addition, the size of nugget, which is 4.8mm, 6.8mm and 7.6mm respective in three cases, increases with the increasing of welding current. The penetration rate, the thickness ratio of nugget to welded point, increases with welding current also. In specimen C, the welding current is too strong to cause the splash. Moreover, no obvious macro defects are found in nuggets in all cases.

a) Specimen A b) Specimen B c) Specimen C

Figure 1. Macrostructures of resistance spot welding joints.

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The microstructure of spot welded joints has similar characters in all cases. The typical microstructure is shown in Figure 2. It can be seen that the nugget consists of columnar grains which grow along the direction of heat flow (in Figure 2a). Lath martensite blocks are found inside of columnar grains (in Figure 2b). The temperature gradient existed in the sample contributes to the formation of columnar grains. Interface of layers has highest temperature and the temperatures at the top and bottom surface are lowest because of the cooling of electrode. Moreover, the rapid cooling provided by electrode contributes to the formation of lath martensite.

a) nugget

b) SEM picture of nugget

c) HAZ on B1500HS side

d) near nugget zone in c)

e) intermediate zone in c)

f) near raw material zone in c)

Figure 2. Typical microstructures of resistance spot welding joint.

The heat affected zone (HAZ) on B1500HS side can be divided into three zones, as shown in Figure 2d-e, near nugget zone, intermediate zone and near raw material zone. Lath martensites are observed

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in three zones. The size of martensites becomes finer with the increasing distance from nugget. The martensites in nugget and HAZ of B1500HS provide higher strength than B250P1.

a) pull out mode in shear test b) pull out mode in cross tension test

Figure 3. Typical fracture locations of spot welding joints.

The typical fracture locations of spot welding joints after shear and cross tension are shown in Figure 3. It should be pointed out that all specimens are pulled out and the welded spots retain on B1500HS side in both shear test and cross tension test. It can be deduced from the fracture locations that the quality of welded spots is good enough and the mechanical properties of welded spots are better than those of B250P1 raw material under the condition of shear and cross tension stress.

Table 4. Mechanical properties of spot welding joints.

Specimen Welding Current

/kA Welding Time

/ms Welding Force

/kN Shear Force

/kN Cross tension Force

/kN

A 6 300 3.5 7.8 7.1

B 8.5 300 3.5 9.0 6.1

C 10 300 3.5 9.5 7.1

The detail mechanical properties of spot welding joints after shear and cross tension test are shown in Table 4. It can be seen that the shear force of joint increases with the increasing of welding current. The bigger nugget contributes to the improvement of shear strength. The fracture appearance in Figure 3a indicates that the fracture originates from the tip of two plats because of stress concentration and extends along the interface between nugget and B250P1 in shear test.

However, the cross tension force shows a different trend. Specimen B gets the lowest cross tension force and Specimen A has a similar cross tension force to specimen C. The phenomenon indicates the size of nugget is not the only factor influencing the cross tension strength of welded spots. It can be seen from Figure 3b that the welded spots are kept well and B250P1 plats endure high degree deformation after cross tension. So, it is reasonable to deduce that the fracture originates and extends as the fracture procedure in shear test. Compared with that in specimen A and C, the extending path in specimen B is shortest because of the relative flat shape of nugget. So, specimen B gets the lowest cross tension force. The thermal-physical, mechanical properties, thickness ratio of raw materials are important factors influencing the shape of nugget [9]. Moreover, joints in cross tension test have lower facture force than that in shear test. The results can be explained by the harder restrain provided by cross tension.

Conclusion

Based on the study on the microstructure and mechanical performance of resistance spot welding joints consisted of ultra-high strength steel B1500HS and high strength steel B250P1, conclusions below are drawn.

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(1) A single nugget without macro defects forms during resistance spot welding. The nugget shifts

towards B1500HS side because of the unequal thickness of raw materials. The size of nugget reaches 7.6mm.

(2) Columnar grains in nugget consist of lath martensite. Lath martensites are observed in the heat affected zone on B1500HS side and the size of martensites becomes finer with the increasing distance from nugget.

(3) All joints are pulled out in both shear and cross tension test. Bigger nugget is the most important controlling factor for shear strength. The shape of nugget has distinct effect on the properties of joints under cross tension condition.

Acknowledgements

This research was financially supported by Shanghai University of Engineering Science under contact No.cs1605008.

References

[1] T. Senuma,. Physical metallurgy of iron and modern high strength steel sheets, ISIJ International, 41 (2001) 520-532.

[2] X.Y. Zhang. Improvement of resistance spot weldability for dual-phase (DP600) steels using servo gun, Journal of materials processing technology, 209 (2009) 2671-2675.

[3] S.P. Tewari, Resistance spot weldability of low carbon and HSLA steels, Internation Journal of science and technology, 15 (2010) 1-11.

[4] L.J. Wang, Q.W. Cai, W. Yu, H.B. Wu, A.D. Lei, Microstructure and mechanical properties of 1500MPa grade ultra-high strength low alloy steel,. Acta metal. Sinica, 46 (2010) 687-694.

[5] L.J. Han, Z.W. Gu, Z.L. Zhang,. Microstructure and fractography of intermediate frequency spot welding of precision ultra-high strength quenching steel, Transactions of the China welding institution, 31 (2010) 101-104.

[6] H.P. YU, W.W. Wang, X.Y. Li, Y.H. Liu, Experiments and numerical simulation on spot-welded structure of ultra-high strength steel, Transactions of the China welding institution, 34 (2013) 9-12.

[7] S. Xu, J. Zhi, Y.Q. Sun, K.H. Hu, T. Gong, Resistance spot welding test on ultra-high strength hot stamping steel, Research on Iron & Steel, 40 (2012) 36-40.

[8] Y. Wang, X.X. Qin, W.X. Jiang, M.Y. Li, J.W. Ren, Microstructure and mechanical properties of spot welding joint of dissimilar high strength steel, Proceedings of the 2014 international conference on materials science and energy engineering, (2015): 269-274.

[9] L. Zhang, Study on nugget form effect factors of resistance spot welding for multilayer unequal-thickness low-carbon steel sheet, Master dissertation of Jilin University, 2014.

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