Research on key influence factors of laser overlap welding of automobile body galvanized steel

Research on key influence factors of laser overlap welding of automobile body galvanized steel

Optics & Laser Technology 45 (2013) 726–733 Contents lists available at SciVerse ScienceDirect Optics & Laser Technology journal homepage: www.elsev...

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Optics & Laser Technology 45 (2013) 726–733

Contents lists available at SciVerse ScienceDirect

Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec

Research on key influence factors of laser overlap welding of automobile body galvanized steel Genyu Chen a,n, Lifang Mei b, Mingjun Zhang a, Yi Zhang a, Zujian Wang a a b

The State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China Department of Mechanical Engineering, Xiamen University of Technology, Xiamen 361024, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 October 2011 Received in revised form 28 April 2012 Accepted 4 May 2012 Available online 30 June 2012

In views of structure characteristics of the auto-body parts, the influences of the beam incident angle and joint gap on the performance of laser overlap welded joints were investigated. The experimental results indicate that there were the critical values of beam incident angle and joint gap during laser overlap welding of galvanized steel. The thickness of sheet and the width of joint had a certain influence on the critical beam incident angle and the limit joint gap. With regard to thicker sheet, the limit joint gap can increase appropriately, but the critical beam incident angle should not be too big. With narrow weld width, the laser beam incident angle can increase appropriately, but the joint gap should not be bigger. Additionally, the critical beam incident angle and the limit joint gap were varied with the thickness of the upper sheet. The tensile-shear tests show that the maximum tensile-shear strength of the joint can be obtained with an optimized beam incident angle and joint gap. & 2012 Published by Elsevier Ltd.

Keywords: Laser overlap welding Galvanized steel Influence factors

1. Introduction The body-in-white is a complex shell structure to be made up of several hundred kinds of thin steel sheet pressing parts which are jointed by welding, riveting, mechanical joining, adhesive bonding and so on [1]. Statistics indicate that the body-in-white is consisted of more than 400 components and parts and more than 4000 welding spots. In particular, welding is one of the key processes in manufacturing of auto-body and similar welded components [2]. The main welding methods used in traditional auto-body manufacturing are resistance spot welding and arc welding. With the development of laser welding technology as well as emerge of high power laser, the laser welding technology was brought to fabricate the body-in-white in car manufacturing sector. The application of laser welding play an important role in reducing auto-body own weight, reducing costs, improving strength and rigidity of the body-in-white [3–5]. Due to the complex structure of auto-body parts, there are some narrow welding position, where the welding head is prone to interfered with the auto-body parts and fixtures when the laser beam incident vertically. Thus, the welding head should be inclined with certain angle to carry out the welding process. In this case, the influence of laser beam incident angle on the weld performance is very important in auto-body welding process [6,7]. In galvanized steel welding, zinc is prone to evaporate during the welding process due to the zinc’s relatively low boiling point

n

Corresponding author. Tel./fax: þ 86 731 88823899. E-mail address: [email protected] (G. Chen).

0030-3992/$ - see front matter & 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.optlastec.2012.05.002

(906 1C) as compared to the melting temperature of steel (1530 1C) [8]. Assuming that there is no joint gap in overlap configuration, the vaporized zinc is pressurized until it meets the keyhole. Zinc vapor causes damage to the weld zone and creates pores in the seam, resulting in poor surface quality, reduced strength, and inferior corrosion resistance [9,10]. With suitable joint gap, the welding process is stable and the weld surface quality is good without welding defects, such as blowholes, spatters and so on [11–14]. If the joint gap is small, it is unfavorable for the escape of zinc vapors, resulting in unstable welding process with bad weld appearance [15–17]. If the joint gap is oversized, two parts are unable to mutual melting welding together. Therefore, the joint gap is also one of the key influencing factors in automobile body welding. This problem is usually improved by introducing a joint gap between the sheets so as to offer escape route for zinc vapors. This paper aims to analyze the influences of the laser beam incident angle and the joint gap on laser overlap welding performance of auto-body galvanized steel, and will provide a technical instruction for laser welding body-in-white.

2. Experimental details Experiments were carried out with a DC025 slab CO2 laser, with output power up to 2500 W. The laser beam is a TEM00 mode with a small TEM01 component with 0.5 mrad divergence. Laser beam was focused on the work piece by mean of a gold copper alloy parabolic focusing mirror of 200 mm focal length with a spot size of 0.4 mm.

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Automobile steel sheets DC56DþZF with thicknesses of 0.8 mm and 1.2 mm were used as experimental material. The chemical composition and mechanical properties were listed in Table 1. The work pieces were sectioned into specification as 60  30 mm2 by CO2 laser. The experimental setup is shown in Fig. 1. The different incident angles were obtained through rotating the welding head and the degree can read from the computer. The joint gap was controlled by pre-positioned standard plug gauge between two sheets, as shown in Fig. 1b and d. Before welding, work pieces were cleaned by aceton in order to removing the superficial oil and dirt. Argon was used as shielding gas through coaxial nozzle to protect fusion zone in the welding process. First, a group of optimized welding parameters were obtained by orthogonal experiments, as shown in Table 2, and the influences of the different joint gaps and incident angles on welding performance were analyzed on the basis of parameters. After welding, the surface appearance of the joint was observed under stereo microscope, the tensile-shear strength of work piece was measured by microcomputer control electron universal testing machine with the force loading rate of 1.0 mm/min. The size of tensile-shear standard test sample as shown in Fig. 1e according to the China national standard of GB228-2002.

3. Results and discussion 3.1. Influence of the incident angle on the welding performance In practical welding, the welding manner and incident angle will change in different welding positions on the automobile Table 1 Chemical compositions and mechanical properties of the tested material. Grades

Chemical composition (wt%)

Mechanical properties (MPa)

C

Tensile strength

Si

Mn

P

S

DC56D þZF 0.014 0.008 0.118 0.014 0.030 300.68

Yield strength 155.29

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body. Because of the restrictions of body construction, the lens group can’t be keeping vertical to each welding position. Thus, the laser beam should irradiate on the sheet surface with an incident angle. In this case, the laser energy intensity on the work piece surface will be changed, and then the welding quality will be affected finally. Fig.2 shows the influence of the incident angle on the surface appearance of the welded joint with different sheet thickness. From Fig. 2, defects of the welding appearance, such as concave and shrinkage cavity, was occurred when the incident angle was greater than 301 with single-sheet thickness of 1.2 mm. The welding process deteriorated dramatically, resulting in lots of pores and concaves when the incident angle was greater than 401. However, the joint surface still even, smooth and continuous, even though the incident angle reached to 351 with single-sheet thickness of 0.8 mm. The welding process became unstable when the incident angle was greater than 401, as shown in Fig. 3. There had a critical beam incident angle for laser welding process and the critical incident angle was varied with the sheet thickness. In details, a bigger incident angle was available for thinner sheet, on the contrary, a smaller incident angle was allowed with thicker sheet. It was also known that when the incident angle was less than the critical angle, incident angle had little effect on weld shape, but when the incident angle bigger than the critical angle, welding quality deteriorated with generation of pore, spatter and partial penetration rapidly. Figs. 4 and 5 show the influence of the incident angle on the cross-sectional characteristic of the welded joint with the singlesheet thicknesses of 1.2 mm and 0.8 mm, respectively. The results indicate that the welded joints tilted in one direction gradually with increasing the incident angle from 01 to 401. Furthermore, the interface weld width increased, and the weld width on the sheet surface increased first and then decreased with increasing the incident angle less than 401. Particularly, the weld width increased with increasing incident angle from 01 to 251, and then decreased when incident angle increased from 251 to 401 with single-sheet thickness of 1.2 mm, as shown in Fig. 4. For the single-sheet thickness of 0.8 mm, the weld width increased with the incident angle increasing from 01 to 351, and then decreased when the incident angle was greater than 351, as shown in Fig. 5.

Fig. 1. Experimental setup (a) welding head inclined with an incident angle, (b) plug guage used to get contant gap, (c) schematic diagram of laser welding with different incident angles, (d) schematic diagram of laser welding with different gaps, (e) overlap joint and force analysis.

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Table 2 Technological parameters of laser welding experiments. Sequence no.

Combination

1

1.2 mm DC56D 1.2 mm DC56D

2

0.8 mm DC56D 0.8 mm DC56D

Weld width (mm)

1.0 1.5 2.0 1.0 1.5

Laser welding technological parameters Laser power (kW)

Welding speed (m/min)

Defocusing (mm)

Shielding gas flow (l/min)

Lap joint gap (mm)

2.0 1.6 2.0 2.0 1.6

1.7 1.1 0.8 1.7 1.1

 0.4 0 þ 0.4 þ 0.4 þ 0.4

15 10 15 15 15

0.2 0.3 0.3 0.15 0.2

Fig. 2. Surface morphology of overlap joints with different incident angles (single-sheet thickness was 1.2 mm).

Fig. 3. Surface morphology of overlap joints with different incident angles (single-sheet thickness was 0.8 mm).

It may be attribute to the laser spot on the metal surface changed from circle shape into ellipse shape when the incident angle increased, resulting in a bigger laser spot as well as an uneven energy distribution on the metal surface, and then influenced the welding quality. When the incident angle increased to a certain value, the bigger laser spot led to the lower laser beam energy density which was insufficient to melt the sheet material. Fig. 6 shows the relationship between the weld width and incident angle with two kinds of sheet thicknesses. With increasing the incident angle, the weld width was increased first and then decreased. In details, the weld width of thin steel sheets was larger than that of thick steel sheets with the same incident angle. Fig. 7 shows the influence of the incident angle on the strength of the welded joint under different conditions. With increasing the incident angle, the tensile-shear load of the joint was increased a little first and then decreased rapidly. Fig. 7a reveals the relationship between the incident angle and the tensile-shear load of the joint was varied with the joint gap. For the overlap joint with the minimum steel thickness of 0.8 mm, the maximum tensile-shear load of the joint decreased gradually when the incident angle was greater than 151 with the joint gap of 0.15 mm. However, when the incident angle increased to 351, the tensile-shear load of the joint was still large with the joint gap of 0.2 mm. When the incident angle was greater than 401, the tensile-shear load of the joint deteriorated dramatically. Therefore, in some welding positions of the auto body parts, the incident angle can not be too large with small joint gap. From Fig. 7b, the tensile-shear load of the joint decreased quickly as long as the incident angle was greater than 151 with weld width of 2.0 mm and sheet thickness of 1.2 mm. However, the strength of the

joint still good with the incident angle of 301 with weld width of 1.0 mm. Therefore, it prefers to keep a small weld width when laser welding is carried out with a great incident angle. In addition, the influence of incident angle on welding performance was varied with the sheet thickness, as shown in Fig. 7c. The tensile-shear load of the joint began to decrease when the beam incident angle was higher than 301 with sheet thickness of 1.2 mm. On the contrary, the tensile-shear load of the joint decreased until the incident angle up to 351 with sheet thickness of 0.8 mm. Based on the analysis of the influence of incident angle on welding morphology mentioned previously, the surface morphology began to deteriorate at 351 for the overlap welding with single-sheet thickness of 0.8 mm, and the surface morphology began to deteriorate at 301 for the overlap welding with single-sheet thickness of 1.2 mm. It can be summarized that, the critical incident angle was 351 for laser overlap welding with single-sheet thickness of 0.8 mm, while the critical incident angle was 301 for laser overlap welding with single-sheet thickness of 1.2 mm. Fig. 8 shows the macroscopic view of failure specimens of 0.8 mm thick sheets with different incident angles. Here, the welded joints all fractured at the base metal when the incident angle less than 351. When the incident angle was up to 401, the specimen fractured at the heat affected zone due to the generated welding defects, such as shrinkage cavity, collapse, and over burning. 3.2. Influence of the joint gap on the welding performance Fig. 9 shows surface morphology of the overlap joint of galvanized steel sheets with single-sheet thickness of 1.2 mm.

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Fig. 4. Cross-sectional morphology of overlap joint with different incident angles (single-sheet thickness was 1.2 mm).

Fig. 5. Cross-sectional morphology of overlap joint with different incident angles (single-sheet thickness was 0.8 mm).

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Fig. 6. The variation of weld width due to an increasing incident angle.

In Fig. 9, the weld surface appeared discontinuous, at the same time, the defects such as spatters, pores and concave were generated with the joint gap of 0.1 mm. When the joint gap increased to 0.15 mm, the quality of the weld surface was improved a little, but a few pores still occurred. With increasing the joint gap up to 0.35 mm, the weld surface appeared smooth and continuous. However, the defects such as concave and pores were occurred again with joint gap of 0.4 mm. When the joint gap was 0.5 mm, the weld surface appeared continuous concave and the two sheets can’t be melted together owing to the large joint gap between two sheets. So, it should be control the joint gap between 0.2 mm and 0.35 mm for laser overlap welding of galvanized steel with single-sheet thickness of 1.2 mm. Fig. 10 shows the cross-sectional shape of the overlap joint of galvanized steel sheets with single-sheet thickness of 0.8 mm.

Fig. 7. The influence of the incident angle on the strength of the welded joint for different values of: (a) joint gap, (b) weld width and (c) sheet thickness.

Fig. 8. Macroscopic view of failure specimens of 0.8 mm thick sheets with different incident angles.

Fig. 9. Surface morphology of overlapped joints with different joint gaps.

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Fig. 10. Cross-sectional morphology of the overlap joints with different joint gaps.

Fig. 11. The influence of the joint gap on the strength of the welded joint with different weld widths and sheet thicknesses: (a) 0.8 mm and (b) 1.2 mm.

Fig. 12. The variation of the joint strength due to an increasing joint gap with different welding speeds.

From Fig. 10, the welded seam appeared smooth when the joint gap was between 0.2 mm and 0.35 mm, however, the welded seam appeared concave with the joint gap of 0.4 mm, and only a few material between two sheets fused together though both of sheets can be melted when the joint gap reached to 0.45 mm. Extremely, two sheets can’t be fused together with the joint gap of 0.5 mm. Fig. 11 shows the influence of the joint gap on the strength of the welded joint with different weld widths and sheet thicknesses. As shown in Fig. 11a, for overlap joining of 0.8 mm thick sheet, the maximum tensile-shear load was achieved when the joint gap and weld width were 0.15 mm and 1.0 mm, respectively, or 0.25 mm and 1.5 mm, respectively. However, for overlap joint of 1.2 mm thick sheet, the maximum tensile-shear load was achieved when the joint gap and weld width were 0.2 mm and 1.0 mm, respectively, or 0.3 mm and 1.5 mm, respectively, as shown in Fig. 11b. The result indicates that there was a limit

Fig. 13. The variation of the joint strength due to an increasing joint gap with different sheets thicknesses.

joint gap for laser welding of galvanized steel. And the influence of the joint gap on the strength of the welded joint was varied with the weld width. In details, the smaller joint gap should be chosen for welding with smaller weld width. In other word, when the weld width was a little greater, it is better not to choose small joint gap. It can be attribute to the small joint gap between two sheets which was occupied by fused metal leading to plenty of zinc stream can’t be escaped from the welding pool to form the zinc gas hole and uneven weld seam. Consequently, the strength of the overlap joint deteriorated. So, when welding of auto-body with larger joint gap, it is feasible to obtain a wide welding pool to improve the joint strength of auto-body part. On the contrary, it is suitable to obtain a narrow welding pool to improve the joint strength with smaller joint gap.

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Fig. 14. Macroscopic view of failure specimens of with different joint gaps (single-sheet thickness was 0.8 mm).

The limit joint gap can be increased by decreasing welding speed with small joint gap while keeping other parameters constant, as shown in Fig. 12. The limit joint gap can be increased from 0.3 mm to 0.35 mm by decreasing the welding speed from 1.7 m/min to 1.4 m/min with sheet thickness of 0.8 mm and weld width of 1.0 mm. It may be ascribed to increase of the weld width which was achieved by decreasing welding speed. It must be noted that it would affect the surface appearance of welded seam when welding speed was too slow. Fig. 13 illustrates that the influence of the joint gap on the joint strength was varied with the sheet thickness. In order to meet mechanical requirement, the limit joint gap was 0.4 mm when the thickness of sheets was 0.8 mm. However, when the thickness of sheets was 1.2 mm, the limit joint gap was 0.45 mm. It should be point out that the limit joint gap increases with the increasing sheet thickness. Fig. 14 shows the macroscopic view of failure specimens of the overlap joint of 0.8 mm sheet with different joint gaps. When the joint gap was range from 0.1 mm to 0.15 mm, the fracture occurred at the heat affected zone. But when the joint gap was 0.2 mm, the test specimens fractured at base metal near the welded seam. With limiting the joint gap between 0.25 mm and 0.35 mm, the test specimens fractured at base metal far away from the welded seam. As expected, the test specimens fractured at base metal near the welded seam again with the joint gap of 0.4 mm.

4. Conclusion Based on welding characteristic of body-in-white, the key influence factors of overlap welding of automotive parts were analyzed. Major conclusions could be summarized as follows: (1) There was a critical beam incident angle for laser welding of automotive part. When the incident angle exceeded the critical incident angle, the appearance and strength of the joint began to deteriorate. (2) There was a limit joint gap for laser overlap welding of the galvanized steel. If there was smaller or no joint gap, welding defects, such as spatters and pores and so on, would be generated. On the contrary, the two sheets couldn’t be melted together when the joint gap was bigger than the limit joint gap. (3) The critical incident angle was varied with the joint gap, weld width and sheet thickness during laser welding of galvanized steel. The critical incident angle decreased with decreasing

the joint gap, or increasing the weld width. The critical incident angle was 351 for laser overlap welding with single-sheet thickness of 0.8 mm, while the critical incident angle was 301 for laser overlap welding with single-sheet thickness of 1.2 mm. (4) The limit joint gap was varied with the weld width and sheet thickness during laser overlap welding of galvanized steel. The limit joint gap increased with increasing of the weld width and sheet thickness.

Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51175165), the National Science and Technology Major Project of China (No. 2012ZX04003101), the science and technology project of Fujian Provincial Department of Education (No. JA11235) and research project of Xiamen University of Technology (No. YKJ10022R). References [1] Liang C, Zheng AB. Material and joint technology of AUDI A6L white-body. Automobile Parts 2007;12:34–5. [2] Chen GY, Mei LF, Zhang MJ, et al. Application and research of laser processing automobile body manufacturing. Laser & Optoelectronics Progress 2009;46: 17–23. [3] Ribolla A, Damoulis GL, Batalha GF. The use of Nd: YAG laser weld for large scale volume assembly of automotive body in white. Journal of Materials Processing Technology 2005;164-165:1120–7. [4] Han LJ. Application of laser welding technology in auto-body manufacture of FAW-Volkswagen steps leaps. Machinist Metal Forming 2008;8:32–5. [5] Wu Q, Gong JK, Chen GY, et al. Research on laser welding of vehicle body. Optics & Laser Technology 2008;40:420–6. [6] Chen GY, Wang ZJ, Mei LF, et al. Effect of laser incident angles on welding performance of galvanized sheet for vehicle body. Laser Technology 2010;34:729–32. [7] Kim C, Kim J, Lim H, et al. Investigation of laser remote welding using disc laser. Journal of Materials Processing Technology 2008;201:521–5. [8] Xie J, Denney P. Galvanized steel joined with lasers. Welding Journal 2001;80:59–61. [9] Akhter R, Steen W, Cruciani D. Laser welding of zinc coated steel. Proceedings of 5th International Conference on Lasers in Manufacturing 1988;13–14:195206. [10] Mei LF, Chen GY, Jin XZ, et al. Research on laser welding of high-strength galvanized automobile steel sheets. Optics and Lasers in Engineering 2009;47:1117–24. ¨ [11] Schmidt M, Otto A, Kageler C. Analysis of YAG laser lap-welding of zinc coated steel sheet. CIRP Annals—Manufacturing Technology 2008;57:213–6. [12] Katayama S, Kawahito Y. Interpretation of laser welding penetration and welding phenomena. Chinese Journal of Lasers 2009;36:72–80. [13] Iqbal S, Gualini MMS, Rehman A. Dual beam method for laser welding of galvanized steel: experimentation and prospects. Optics & Laser Technology 2010;42:93–8.

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