www.ccsenet.org/mas Modern Applied Science Vol. 6, No. 5; May 2012

ISSN 1913-1844 E-ISSN 1913-1852 34

Fatigue Behavior of Resistance Spot-Welded Unequal Sheet Thickness Austenitic Stainless Steel

Triyono1, Jamasri2, M. N. Ilman2 & R. Soekrisno2 1 Mechanical Engineering Department, Sebelas Maret University, Indonesia 2 Mechanical and Industrial Engineering Department, Gadjah Mada University, Indonesia

Correspondence: Triyono, Mechanical Engineering Department, Sebelas Maret University, Jl. Ir. Sutami 36A Surakarta, Indonesia. Tel: 62-27-163-2163. E-mail: tyon_bila@yahoo.co.id

Received: December 5, 2011 Accepted: April 7, 2012 Online Published: May 1, 2012

doi:10.5539/mas.v6n5p34 URL: http://dx.doi.org/10.5539/mas.v6n5p34

The research is financed by the Ministry of Research and Technology of Indonesia and Indonesian Railway Industry

Abstract

This paper presents a comparative study on the fatigue strength of resistance spot-welded unequal and equal sheet thickness austenitic stainless steel. Lap joints of 3.0-1.0 mm and 1.0-1.0 mm thick austenitic stainless steel were made using the same resistance spot welding schedule with current, weld time and electrode force of 4.7 kA, 20 cycles and 6 kN respectively. The sinusoidal wave form with a constant stress amplitude was selected in the fatigue tests whereas the stress ratio and frequency used were 0.1 and 8 Hz respectively. Fatigue strength and tensile-shear load bearing capacity of 3.0-1.0 mm joint were higher than that of 1.0-1.0 mm joint, although its nugget diameter was smaller. The joint stiffness was the controlling factor of the fatigue strength of resistance spot-welded unequal sheet thickness austenitic stainless steel.

Keywords: resistance spot welding, unequal sheet thickness, fatigue, austenitic stainless steel

1. Introduction

Austenitic stainless steels are used for a very broad range of applications especially in automotive, railway vehicle, ship body, and airplane structures when an excellent combination of strength and corrosion resistance in aqueous solutions at ambient temperature is required. Stiffened thin plate construction where the thinner plate is reinforced by thicker plate called a frame, is generally applied to the structures. Gean et al. (1999) have claimed that it is a cost-effective way of achieving a high-performance vehicle structure because it remains suited to low-volume manufacture. This structure is typically joined by the resistance spot welding (RSW) process. The advantages of using RSW are that it is a quicker joining technique, suitable for automation, no filler material is required, and that the low heat input implies less risk for altered dimensions during welding.

Many standards and recommendations are developed by individual companies, such as Ford Motor Company and General Motors. Professional organizations such as the American Welding Society (AWS), Society of Automotive Engineering (SAE), the American National Standards Institute (ANSI), and International Organization for Standardization (ISO) also contribute to a significant portion of the standards. Because of the drastic differences in design, understanding and perception of weld quality, automobile manufacturers and others tend to have very different requirements on weld quality. Zhang and Hongyan (2006) have concluded that in general, spot weld size is enveloped between 3√ and 6√ (t is the thickness of the sheets in millimeters). This recommendation is very useful in finding good weld schedules for equal sheet thickness welding. However, in automotive body application, the majority of welds are between two dissimilar thicknesses. In this case, schedules for welding unequal sheet thickness are generally developed by and practiced within individual manufacturers. Some researchers also have proposed the spot welding unequal sheet thickness researches to evaluate these recommendations. The joint of unequal thickness of the same metal may produce a strength problem due to the heat unbalance (Hasanbasoglu & Kacar, 2007) and have the unique failure mechanism (Pouranvari & Marashi, 2010).

Despite various applications of spot welded unequal thickness in automotive body, reports in the literature dealing with its mechanical behaviors, especially the fatigue behaviors are limited. In fact, Gean et al. (1999)

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Vol. 6, No.

SSN 1913-1844

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5; May 2012

E-ISSN 1913-185

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Figure 10.

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After initiatiopropagating thin Figure 12(acircumferencebehavior is siKhanna, 2007

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Vol. 6, No.

SSN 1913-1844

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ngth of 3.0-1.0ring load bear

her stiffness, rwhich rotated oreas that of 1.0 mm joint spe

mm joint

the thickness tearing fracturoint propagateshown in Figuin a previous

(b) 1.0-1.0 mm

5; May 2012

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ne load range

0 mm joint waring capacity orotation of joinon both sheet0-1.0 mm joinecimens, crack

and continuere mode as seeed at the nuggeure 12(b). Thistudy (Long &

m joint

52

as of nt s. nt ks

ed en et is &

www.ccsenet.org/mas Modern Applied Science Vol. 6, No. 5; May 2012

Published by Canadian Center of Science and Education 41

Given SEM views in Figure 13 and Figure 14 are the comparison of the last fracture surfaces of the 3.0-1.0 mm and 1.0-1.0 mm joint specimens which were subjected to same load of 3.4 kN. As seen in Figure 13(a), crack of 3.0-1.0 mm joint initiated on the inside of thin sheet. On the more half of thickness, it propagated slowly due to low peel stress and induced ductile fracture characterized by intergranular cracking. Ductile fracture changed to brittle fracture characterized by transgranular cracking on the remaining thickness due to increased peel stress. The embrittlement of stainless steel was attributed to strain-induced martensite forming during the fatigue tests (Vural et al., 2006). Different view was given by fracture surface of 1.0-1.0 mm joint specimen as shown in Figure 13(b). It displayed brittle fracture characterized by transgranular cracking on the entire thickness due to high peel stress.

(a) (b) Figure 13. Initial crack of fatigue test samples: (a) 3.0-1.0 mm joint (b) 1.0-1.0 mm joint

(a) (b) Figure 14. Crack propagation zone: (a) 3.0-1.0 mm joint (b) 1.0-1.0 mm joint

On the crack propagation zone, brittle fracture characterized by transgranular cracking was observed on both 3.0-1.0 mm and 1.0-1.0 mm joint specimens. They displayed wave of plastic deformation as shown in Figure 14. However, plastic deformation intensity of 1.0-1.0 mm joint was higher than that of 3.0-1.0 mm joint. It was indicated by the number of waves in the same observation area as seen in Figure 14.

4. Conclusions

Fatigue of resistance spot welded unequal sheet thickness austenitic stainless steel has been studied. Due to significant thickness difference, the asymmetric weld nugget, high microhardness on the edge of nugget and tearing fatigue fracture mode were observed. The fatigue strength of 3.0-1.0 mm joint was higher than that of 1.0-1.0 mm joint, although its nugget diameter was smaller. The endurance limit of 3.0-1.0 mm and 1.0-1.0 mm joint were 2.9 kN and 0.7 kN respectively. Ductile and brittle fractures were observed on 3.0-1.0 mm fatigue specimens whereas 1.0-1.0 mm fatigue specimens failed to fully brittle fracture mode. The joint stiffness was the controlling factor of the fatigue strength of resistance spot-welded unequal sheet thickness austenitic stainless

www.ccsenet.org/mas Modern Applied Science Vol. 6, No. 5; May 2012

ISSN 1913-1844 E-ISSN 1913-1852 42

steel.

5. Acknowledgments

The authors would like to express their sincere gratitude for the financial support of the Ministry of Research and Technology of Indonesia and Indonesian Railway Industry.

References

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Hasanbasoglu, A., & Kacar, R. (2007). Resistance Spot Weldability of Dissimilar Materials (AISI 316L-DIN EN 10130-99 Steels). Material and Design, 28, 1794-1800. http://dx.doi.org/10.1016/j.matdes.2006.05.013

Long, X., & Khanna, S. K. (2007). Fatigue properties and failure characterization of spot welded high strength steel sheet. International Journal of Fatigue, 29, 879-886. http://dx.doi.org/10.1016/j.ijfatigue.2006.08.003

Nordberg, H. (2006). Fatigue Properties of Stainless Steel Lap Joints. SAE Transactions: Journal of Materials & Manufacturing, 114, 675s-690s.

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Pouranvari, M., & Marashi, P. (2010). Resistance Spot Welding of Unequal Thickness Low Carbon Steel Sheet. Advanced Materials Research, 83, 1205-1211. http://dx.doi.org/10.4028/www.scientific.net/AMR.83-86.1205

Vural, M., & Akkus, A. (2004). On the resistance spot weldability of galvanized interstitial free steel sheets with austenitic stainless steel sheets. J. Mater. Proc. Technol., 153-154, 1-6. http://dx.doi.org/10.1016/j.jmatprotec.2004.04.063

Vural, M., Akkuş A., & Eryürek, B. (2006). Effect of Welding Nugget Diameter on The Fatigue Strength of The Resistance Spot Welded Joints of Different Steel Sheets. J. Mater. Proc. Technol., 176(1-3), 127-132. http://dx.doi.org/10.1016/j.jmatprotec.2006.02.026

Wang, Y., Zhang, P., Wu Y., & Hou, Z. (2010). Analysis of the Welding Deformation of Resistance Spot Welding for Sheet Metal with Unequal Thickness. Journal of Solid Mechanics and Material Engineering, 4, 1214-1222. http://dx.doi.org/10.1299/jmmp.4.1214

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