Effect of process parameters on SPRC35 spot welded automotive panels

Transcription

1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No.3 pp June 2010 Effect of process parameters on SPRC35 spot welded automotive panels M. Balasubramanian Department of Mechanical Engineering, Velammal Engineering College, Surapet, Chennai, , India V. Balasubramanian Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, , India Manuscript received 14 October 2009; in revised form 24 March 2010 This paper presents an experimental study on the resistance spot weldability of SPRC35 (steel plate re-phosphorised cold rolled) sheets. The sheets were joined by using resistance spot welding as lap joint. The weld nugget diameter and tensile shear force were investigated. Tensile-shear tests were applied to the welded specimens to understand the tensile shear force that the joint can withstand. Empirical relationships were developed for both nugget diameter and tensile shear force. Use of automatic mode, will increase the dependence on the use of equations to predict the nugget diameter. The developed models have been checked for their adequacy and significance by the F test and t test respectively. The results obtained from the empirical relationships have been optimized and also tested using conformity test runs. KEY WORDS Resistance spot welding; SPRC35 steel 1 Introduction Resistance spot welding is an inexpensive and effective way to join metal sheets. It is one of the oldest of the electric welding processes in use by industry today. The first usage of this welding technique had been seen at the end of the 19th century, and it was started to use in combination of sheets after 1920s. The welding is effected by a combination of heat, pressure and time. As the name implies, it is the resistance offered by the material to be welded to current flow that causes a localized heating in the joining area. The time of current flow in the joint is determined by the material thickness and type, amount of the current and cross-sectional area of the welding tips and contact surfaces [1]. The resistance spot welding is especially used in automobile industry. One advantage of it than other techniques is its higher speed with respect to thickness of workpiece which will be welded and another is its susceptibility to automation [2]. In the spot welding process, two or three overlapped work pieces are held together under pressure between two electrodes and welded together as a result of the heat created Corresponding author. Professor; Tel.: ; Fax: address: manianmb@rediffmail.com (M. Balasubramanian)

2 186 by electrical resistance. The welding current passes from the electrodes to the work and its continuity is assured by forces applied to the electrodes. The sequence of operation is to develop sufficient heat to raise a confined volume of metal to the molten state. This metal is then allowed to cool while under pressure until it has adequate strength to hold the parts together [3]. The current density and pressure must be such that a nugget is formed, but not so high that molten metal is expelled from the weld zone. The duration of weld current must be sufficiently short to prevent excessive heating of the electrode faces. Because of the short electric current path in the work and limited weld time, relatively high welding currents are required to develop the necessary heat [4]. The presence of manganese contributes markedly to strength and hardness but to a lesser degree than carbon. The high strength is derived from the precipitation of carbides by micro alloying elements and pinning down of grain boundaries. The effects of welding current and time on the tensile-peel strength and tensile-shear strength of welding joint in electrical resistance spot welding of chromided micro-alloyed steel sheets having 0.8 mm thickness and galvanized chromided micro-alloyed steel sheets having 1.0 mm thickness were investigated. A timer and current controlled electrical resistance spot welding machine having 120 kva capacity and pneumatic application mechanism with a single lever was used to prepare the specimens. Welding current periods of 5, 10, 15, 20 and 25 were selected and it was adjusted by increasing 1 ka from 3.5 to 10 ka during welding process. The electrode pressure was fixed at 6 kn. The obtained welding joints were exposed to tensile-peel and tensile-shear tests and their core sizes, heights and nucleus size ratios were calculated by means of an optical microscope [5]. 2 Welding Parameters Achieving good weld quality starts with a good process design that minimizes the variables encountered in welding. Electrode force, current intensity and time are the most important welding parameters in electrical resistance spot welding. An electronically control unit is used in welding machines to pursuit the welding variables. The desired nugget diameter can only be obtained by adjusting welding current intensity versus welding time properly. When time is held short, the nugget diameter decreases. On the contrary, when it is held long the amount of molten metal increases and fused metal spurts out and as a result the strength of welding joint decreases [6]. The purpose of the electrode force is to squeeze the metal sheets to be joined together. This requires a large electrode force; else the weld quality will not be good enough. However, when the electrode force is increased the heat energy will decrease. This means that the higher electrode force requires a higher weld current. When weld current becomes too high, spatter will occur between electrodes and sheets. This will cause the electrodes to get stuck to the sheet [7]. 3 Experimental Studies and Optimization The material studied is SPRC35 (steel plate re-phosphorised cold rolled) sheet of 0.7 mm in thickness, which are widely used in automobile bodies. The dimension of the sheets is 100 mm 30 mm and the dimension of the spot welded specimen is shown in Fig.1. The electrode force used during spot welding operations was selected as 2000 N [6].

3 187 The welding conditions are given in Table 1. The experimental design is shown in Table 2 using a spot welding facility. The mechanical properties and chemical composition (wt pct) of the SPRC35 steel reported by the materials supplier (Hwashin Automotive Company) are as follows [8] : Young s modulus (204.2 GN/m 2 ), yield strength (233.2 GN/m 2 ), ultimate tensile strength (335.2 GN/m 2 ), elongation (40.1%) and C (0.022), Si (0.022), Mn (0.140), P (0.070), T-Al (0.019), S-Al (0.017), Ti (trace), Nb (0.005). The welding current was changed between 7 to 9 ka. The as welded joint samples are displayed in Fig.2. After the joint was established, the weld nugget diameters were measured and shearing tests were also performed on the specimen. The specimen under shear test is shown in Fig.3. The sheared spot welded samples are displayed in Fig.4. Regression analysis was done to develop empirical relationships. Table 3 and Table 4 depict the ANOVA performed to understand the significance of the factors for both tensile shear force and nugget diameter respectively. To validate the significance of the experimentation, confirmation runs were conducted as presented in Table 5. Empirical relationships developed are given below: Fig.1 Dimension of specimen after spot welding (in mm). Table 1 Welding conditions Welding parameter Electrode force Electrode tip diameter Welding time Squeezing time Hold time Weld rate Value 2000 N 6 mm 6-7 cycles 2-3 cycles 1 cycle 10 spots/min Fig.2 Spot welded specimens for tensile shear test. Table 2 Experimental design Natural variables Tensile shear force/n Nugget diameter/mm Weld current x 1 /ka Weld time x 2 /cycles Y 1 Y

4 188 Tensile shear force = x x 2 (1) Weld nugget diameter = x x 2 (2) where x 1 and x 2 are weld current and weld cycle respectively. The empirical equations were optimized for minimum nugget diameter and maximum tensile shear force. 4 Results and Discussion The results as shown in Table 2 indicate that the nugget diameter varies between 4.0 to 4.2 mm and tensile shear force varies between 360 to 403 N. Optimization of welding current and weld time were done keeping in mind a multi objective of having a Fig.3 Photograph of specimen under shear test. lesser nugget diameter as well as higher tensile shear force. Priority based optimization was done giving first priority to nugget diameter and then to tensile shear force. It was found that tensile shear force increased with increase in welding current. The curves depicted in Fig.5 and Fig.6 clearly show an increasing trend within the investigation range of both nugget diameter and tensile Fig.4 Photographs of specimens after shear test. shear force for an increase in welding current. The nugget diameter is a critical response in determining the quality of the spot weld. Hence, a relationship was developed graphically between welding current and nugget diameter as exhibited in Fig.5, which brings out the effect of the welding current on the nugget diameter. The increase in tensile shear force is attributed to increase of nugget diameter Table 3 Regression statistics for tensile shear force Multiple R 0.99 R Square 0.98 Adjusted R square 0.97 Standard error 2.62 Observations ANOVA df SS MS F Regression Residual Total Standard Lower Upper Lower Upper Coefficients t Stat P-value error 95% 95% 95.0% 95.0% Intercept Current Weld cycle

5 189 Table 4 Regression statistics for nugget diameter Multiple R R square Adjusted R square Standard error Observations 10 ANOVA df SS MS F Significance F Regression Residual Total Standard Lower Upper Lower Upper Coefficients t Stat P-value error 95% 95% 95.0% 95.0% Intercept Current Weld cycle Table 5 Confirmation runs Natural variables Tensile shear force/kn Nugget diameter/mm Weld current x 1/kA Weld time x 2/cycles Y 1 Y Fig.5 Effect of welding current on nugget diameter. Fig.7 Relationship between nugget diameter and tensile shear force. Fig.6 Effect of welding current on tensile shear force. as exhibited in Fig.7. When there is an increase in nugget diameter, of course increase in cross sectional area, the load carrying capacity also increases leading to increase in tensile shear force. 5 Conclusion In this study, SPRC35 steel sheets were joined by using resistance spot welding. It was observed that increase in welding current leads to increase in nugget diameter as well as tensile shear force. The optimum val-

6 190 ues were obtained at 7 cycles of weld time and 7 ka welding current. The nugget diameter obtained at these values is 4.01 mm and tensile shear force of kn. Confirmation runs showed improved results and almost adhered to the optimum identified during the experimental runs. Acknowledgements This project was done at M/s Hwashin Automotive India Pvt. Ltd. The authors wish to thank the organization for technical support and providing the facilities. The authors are also grateful to Mr.Karthik, section officer, M/s Hwashin Automotive India Pvt. Ltd. and Mr.Ramanathan of Maamallan Institute of Technology for their contributions. REFERENCES [1] M. Vural and A. Akkus, J Mater Process Technol (1-3) (2004) 1. [2] T. Satoh, H. Abe, T. Nakaoka and Y. Hayata, Weld World (1996) p.12. [3] Y.S. Yang and S.H. Lee, J Mater Process Technol 94(2) (1999) 151. [4] W.L. Chuko and J.E. Gould, Weld J 81(1) (2002) 1S. [5] S. Aslanlar, Mater Design 27(2) (2006) 125. [6] S. Agashe and H. Zhang, Weld J 82(7) (2003) 179S. [7] Welding Processes. AWS Welding Handbook, 7th ed., Vol.3 (London: American Welding Society, Macmillan Press Ltd, 1980). [8] K.M. Zhao, B.K. Chun and J.K. Lee, Finite Elem Anal Design 37 (2001) 117.