Evaluation of Fatigue Strength and Spot Weldability of High Strength Steel Sheet for Light Weight Automobile Body

Transcription

1 Key Engineering Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Evaluation of Fatigue Strength and Spot Weldability of High Strength Steel Sheet for Light Weight Automobile Body D.H. Bae 1,a, W.S. Jung 2,b and J.B. Heo 2,b 1 Professor, School of Mechanical Engineering, Sungkyunkwan University, Suwon, Kyunggi-do, Korea 2 Graduate School, Mechanical Engineering Department, Sungkyunkwan University, Suwon, Kyunggi-do, Korea a bae@yurim.skku.ac.kr, b imjws@korea.com Keywords: Spot weldability, Light weight vehicle body, Spot welded joint, Spot welding, Nugget, Stress concentration, Fatigue strength, Maximum stress, Fatigue limit Abstract. An effective way to reduce the weight of vehicle body seems to be application of new materials, and such trend is remarkable. Among the various materials for vehicle body, stainless steel sheets and cold rolled steel sheets are under the interests. However, in order to guarantee reliability of new material and to establish the long life fatigue design criteria for body structure, it is necessary to assess spot weldability and fatigue strength of spot welded lap joints fabricated under optimized spot welding condition. In this paper, spot weldability of stainless steel sheets, STS304L and STS316L, and cold rolled steel sheets, SPCC and SPCD. Fatigue strength of lap joints spot welded between similar and dissimilar materials were also assessed. Introduction For applying high strength material for light weight vehicle body, reliability on possibility and utility as a substitute material must be guaranteed. In recent, since the design concept of spot welded thin sheet structure has been changed from the traditional safe life design concept to the failure life design concept, automobile body requiring long life must be designed so as to secure proper structural rigidity [1, 2]. Therefore, one of the most highly regarded in the process of design and manufacturing of automobile body is reliability for the static and dynamic durability of spot weld. Spot welding is an important manufacturing technology. However, since the shape of spot-welded region is circular having the diameter of several mm, it becomes a source of stress concentration as well as fatigue crack generation by fatigue load. Thus, the fatigue strength of spot-welded components is lower than those of base metals [3, 4]. Therefore, assessing spot weldability and fatigue strength for stainless steel sheet which is considered as a substitute material of light weighted automobile body is very important for securing reliability on the substitute material as well as determination of fatigue design criterion. In this paper, weldability of similar and dissimilar high strength materials and fatigue strength of spot welded lap joints fabricated under optimized spot welding condition were investigated. Weldability Assessment of High Strength Steel Sheet Material and Spot Welding Procedure. Investigated materials are austenitic stainless steel sheet, 301L, 304L and cold rolled steel sheet, SPCC, SPCD. Chemical composition and mechanical properties are illustrated in Table 1 and Table 2. The electrode for spot welding was used φ 6mm, Chromium-Copper (Cr+Cu). And spot welder is air type. Spot welding conditions were based on recommended RWMA Group A, Class 3 for stainless steel sheets and RWMA Class A for cold rolled steel sheets. In case of spot welding for dissimilar material, i.e., between stainless steel sheets and cold rolled steel sheets, spot welding condition was set with ones of SPCC or SPCD. Optimum spot welding conditions were determined by assessing static strength in addition to depth and size of indentation as well as nugget size. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,10:22:59)

2 2884 Advances in Fracture and Strength Material STS301L STS304L SPCC SPCD Material STS301L STS304L SPCC SPCD Table 1 Chemical composition (wt. %) of materials C Si Mn P S Ni Cr Table 2 Mechanical properties of materials Yield Strength (MPa) Tensile Strength (MPa) N Elongation (%) Fig. 1 Sectional condition of the nugget (a) STS301L+SPCC (b) STS301L+SPCD (c) STS304L+SPCC (d) STS304L+SPCD Fig. 2 Conditions at the border of spot welded lap joint by FESEM ( 500) Result of Weldability Assessment. Fig. 1 shows an example of the nugget section of spot weld. In case of spot welding between similar materials, i.e., STS301L+STS301L, and STS304L+STS304L, by similarity of electric, thermal, and mechanical properties, quantitative differences of welding conditions considering pressure (p), current (I), and current time (t) were not large. Although static tensile strength showed a little difference according to the thickness of the upper and lower plates of spot welded lap joint, it was mostly estimated in 60-67% of the base metal. And also, in case of spot welding between dissimilar materials, i.e., STS301L+STS304L, static failure was occurred on the side of STS304L that is relatively lower than STS301L, and static tensile strength of them was estimated in 67-74% of the base metal. In case of cold rolled steel sheets of SPCC, SPCD, it was confirmed that their spot weldability was very good. By similarity between

3 Key Engineering Materials Vols them, quantitative differences of welding conditions were nearly nothing. Static tensile strength of lap joint welded between similar or dissimilar SPCC and SPCD was estimated in 55-64% of the base metal. In case of spot welding between dissimilar materials, i.e., STS301L+SPCC, STS301L+SPCD, STS304L+ SPCC, STS304L+SPCD, it was confirmed that spot weldability between them was very satisfiable, as shown in Fig. 2. In spite of different electric, thermal and mechanical properties, the difference of optimum welding condition was a little. And also, welding defects such as porosities, micro-cracks and so on were not found. In the case of joints spot-welded between dissimilar materials, SPCC and SPCD of which static tensile strength by tensile shear force is relatively lower than that of STS301L and STS304L were failed. However, static tensile strength of their spot welded lap joints were assessed in % of the base metal. Fatigue Strength Assessment of Spot Welded Lap Joint Specimen and Test Equipment Fatigue specimen was fabricated in width of the plate (W)=30mm, lapped length (2L)=30mm, plate thickness (t)=1mm and 1.2mm, nugget diameter (Φ )=6mm, as shown in Fig. 3. They were spot welded by the optimized welding condition determined through weldability evaluation. Fatigue tester used was hydraulic material testing system. Fatigue cracks revealed on the outer surface of spot welded lap joint was measured with traverse microscope. Case 1) t 1 =1.0mm, t 2 =1.0mm, Case 2) t 1 =1.2mm, t 2 =1.2mm, Case 3) t 1 =1.0mm, t 2 =1.2mm Fig. 3 Fatigue test specimen to asses fatigue strength of spot welded lap joints Test Condition and Procedure. Conducting fatigue test in air, since the influence of loading frequency is negligible, fatigue tests are generally conducted in 20 30Hz. Therefore, in this paper, loading frequency did apply 30Hz, and loading form used sine wave of load ratio (R= P min /P max ) = 0 (P min =0). Fatigue tests by the load decreasing method were conducted until 10 7 cycles that is fatigue limit of spot welded lap joint. Fatigue life was determined as the number of cycles when the length of fatigue crack revealed around the nugget edge on the loading side of spot welded lap joint became the same with nugget diameter. Result and Discussion. When tensile shear fatigue load acts on spot welded lap joint, the position of fatigue crack revealed on the outer surface of plate is variable according to magnitude of the load range. However, failure was generally occurred around heat affected zone of the nugget edge where the maximum stress by stress concentration is generated. Although strain behavior of spot welded lap joint by tensile shear load remarkably showed the different results according to applied load range and the shape of specimen, the position of fatigue crack initiation and fatigue crack growth pattern were nearly the same. Fig. 4 shows the relationship between the fatigue load range applied to each spot-welded lap joint and fatigue cycles (Δ P-N f relation) for the various joints welded similar and dissimilar materials, i.e. STS301L, STS304L, SPCC, SPCD. Fatigue strength of lap joints spot-welded similar and dissimilar STS301L, STS304L is relatively larger than those of SPCC and

4 2886 Advances in Fracture and Strength SPCD. Fatigue strength of the joints spot-welded between dissimilar materials, i.e., STS304L, STS316L, SPCC, and SPCD is lower than those of STS304L and STS316L, but is assessed in nearly equivalent level with SPCC and SPCD. Fig. 4 Δ P-N f relation of various specimens Conclusion Summarized conclusions are as follows; 1) The optimized condition for spot welding of similar and dissimilar materials; STS301L, STS304L, SPCC, and SPCD. 2) It was confirmed that spot weldability between similar and dissimilar materials; STS301L, STS304L, SPCC, and SPCD is good. 3) Fatigue design criterion for light weighted automobile body using STS301L, STS304L, SPCC, SPCD could determine using the Δ P -N f relation. 4) Fatigue strength of the joints spot-welded between dissimilar materials, i.e., STS304L, STS316L, SPCC, and SPCD is lower than those of STS304L and STS316L, but is assessed in nearly equivalent level with SPCC and SPCD. Acknowledgement The authors are grateful for the support provided by a grant from the Hyundai Motor Company (HMC) and Structural Integrity Research Center (SAFE) at the Sungkyunkwan University. Reference [1] C.K. Woo and C.S. Shin: Automobile Structure, Dong Shin Co. (1989), pp [2] D.M. Shin: Materials for Vehicle, Young Seol Co. (1994), pp [3] D.H. Bae, J.E. Niisawa and N. Tomioka: Theoretical Analysis of Stress Distribution of Single Spot Welded Lap Joint under Tension Shear Load, Journal of JSAE 39 (1988)

5 Key Engineering Materials Vols [4] M.E.M. El-sayed: Fatigue Analysis of Spot Welded Joints Under Variable Amplitude Load History, Fatigue Fracture Engineering Material Structure, Vol. 55 No. 3 (1996), pp [5] D.H. Bae: Evaluation of Fracture Mechanical Fatigue Strength of a Spot Welded Lap Joint Subjected to Tensile Shear Load, Journal of KSAE 13 (5) (1991), pp

6 Advances in Fracture and Strength / Evaluation of Fatigue Strength and Spot Weldability of High Strength Steel Sheet for Light Weight Automobile Body / DOI References [4] M.E.M. El-sayed: Fatigue Analysis of Spot Welded Joints Under Variable Amplitude Load History, Fatigue Fracture Engineering Material Structure, Vol. 55 No. 3 (1996), pp / (95)