Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white

Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white

Accepted Manuscript Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white Lifang Mei, Dongbing Yan, ...

1MB Sizes 12 Downloads 91 Views

Accepted Manuscript Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white Lifang Mei, Dongbing Yan, Genyu Chen, Dang Xie, Mingjun Zhang, Xiaohong Ge PII: DOI: Reference:

S0261-3069(15)00213-7 http://dx.doi.org/10.1016/j.matdes.2015.04.031 JMAD 7212

To appear in:

Materials and Design

Received Date: Revised Date: Accepted Date:

2 December 2014 18 March 2015 19 April 2015

Please cite this article as: Mei, L., Yan, D., Chen, G., Xie, D., Zhang, M., Ge, X., Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white, Materials and Design (2015), doi: http:// dx.doi.org/10.1016/j.matdes.2015.04.031

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparative study on CO2 laser overlap welding and resistance spot welding for automotive body in white Lifang MEI1, Dongbing YAN1, 3, Genyu CHEN2, Dang XIE1, 3, Mingjun ZHANG4, Xiaohong GE1 1

College of Mechanical and Automotive Engineering, Xiamen University of Technology, Xiamen 361024, China

2

The State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China 3

Key Laboratory of Precision Actuation and Transmission, Fujian Province University, Xiamen 361024, China

4

College of Automotive and Mechanical Engineering, Changsha University of Science and Technology, Changsha 410114, China E-mail: [email protected], [email protected]; tel: +86 592 6291385; fax: +86 592 6291385

Abstract: Our previous study included a comparative analysis of the characteristics of CO2 laser overlap welding and resistance spot welding in terms of the weld fundamentals, tensile-shear performance, microstructure, hardness, and corrosion resistance of welding joints. In this study, the processing properties of the two types of welding methods were further investigated. First, experiments were conducted on dissimilar auto body sheets using resistance spot welding and laser welding with different weld lengths. Next, the layout of the laser welding joints was analyzed. Finally, experiments were performed on an auto door sub-assembly using resistance spot welding and laser welding respectively. Experimental results showed that both the sheet thickness and weld pool width were correlated with the laser weld length (LWL). When the LWL was increased to 20 mm, the tensile-shear performance of the specimen was better than that of one resistance-spot welding point under various conditions. When performing weld joint layout using the "short-length and multi-segment" scheme, it was possible to produce laser welding specimens with a preferable joint morphology and mechanical properties. The laser welded auto door with welding joints having an optimized weld length and layout had better quality and performance, and the laser welding process had higher flexibility. Keywords: Welding; weld length; layout; formation; mechanical property 1. Introduction In recent years, laser welding has found an increasingly wide utilization in the field of auto manufacturing. Compared with traditional resistance spot welding, laser welding can produce a lightweight auto body, enhance productivity, lower cost, and remarkably improve the impact resistance, fatigue resistance, and corrosion resistance of the auto body, thus promoting the quality of the auto [1, 2]. The common welding structures used in laser welding an auto body include butt joints, overlap joints, and fillet joints [3-5]. In practical production, numerous weld joints involve the overlap welding of multiple layers of galvanized steel sheets (2–3 layers). When welding two-sided galvanized steel sheets for an auto body using laser overlap welding, drawbacks exist, including vaporization of the zinc coating, welding pores, and spatters exist [6-8]. However, compared with resistance spot welding, laser beams have advantages such as a higher heating capacity, smaller thermal deformation, higher welding speed, narrower weld joints and heat-affected zones, higher weld strength than base metal, and better adaptability to complex structures. In addition, they make it easier to achieve remote welding and automation [9-11]. Therefore, in the auto industry in developed countries, the laser welding technique is progressively replacing traditional welding methods. For the resistance spot welding of an auto body, the appropriate spacing between welding points should be considered. A spacing that is too small would cause a shunt when welding subsequent points, thereby affecting the weld strength. When designing the locations of endpoints and marginal welding points, suitable distances to the margin of the parts should be considered to ensure the integrity of heat-affected zones [12, 13]. For the laser welding of an auto body, because of the high power density of the input laser, a weld joint that is too long tends to

1

induce weld defects due to thermal accumulation deformation, whereas one that is too short cannot meet the welded joint strength requirement of the auto body. Therefore, the setting principles of the laser weld length (LWL) and spacing are a key factor in affecting the appearance and safety performance of an auto body. However, to the best of our knowledge, the principle for designing and determining the LWL and spacing has not been reported in the literature. The setting principles of the LWL and spacing are a challenging task in modern auto design and manufacturing processes. In this study, based on the findings of our previous study [14], (i) laser welding experiments were conducted on auto body sheets using weld seams of different lengths for overlap joints; (ii) the mechanical properties of welded specimens with different weld lengths were analyzed and compared with specimens having one resistance-spot welding point (RSWP) with respect to the weld strength; (iii) the impacts of such factors as the sheet category, thickness, and weld pool width (WPW) on the LWL were analyzed; and (iv) the minimum laser weld length (MLWL) with a performance comparable to the performance of one RSWP under different influencing factors was obtained. Next, the layout of the laser welding joints was analyzed to determine the appropriate spacing between weld joints. Experiments were performed on auto door parts using resistance spot welding and laser welding, and a comparative analysis was performed to examine the overall performances of the two welding processes and weld assemblies, thereby validating the rationality and feasibility of the setting principles for the LWL and spacing. 2. Experimental Materials and Methods 2.1 Experimental equipment Different types of electric welding machines are needed for different sheet combinations in resistance spot welding experiments. The DN3-160 suspension spot welding machine was one of the pieces of resistance spot welding test equipment used in this study (Fig. 1). The most important feature of this machine is that it can be suspended on a dedicated rack, and can be moved horizontally or vertically, or rotated. Thus, it is convenient for welding large-scale and complicated workpieces. The machine has a power supply voltage of 380 V, rated load power of 160 kVA, rated primary current of 410 A, and rated welding thickness of 2+2 mm. During resistance spot experiments, the welding gun was set in accordance with the technological parameters, and the workpiece was welded after the setting was finished. After welding was completed, the welding quality was checked to avoid welding defects such as a lack of penetration or burr. The experimental equipment used for laser welding included a DC025 slab CO2 laser device and three-dimensional and five-axis laser processing machine (Fig. 2). The maximum power output of the laser device is 2.5 kW with a wavelength of 10.6 μm and an output mode of TEM00. The focal spot diameter is 0.4 mm.

Fig. 1 Resistance spot welding equipment.

Fig. 2 Laser welding equipment.

2.2 Experimental materials Experiments were conducted on auto door parts. Two kinds of sheets for the auto door parts with different materials and different thicknesses were used as welding specimens. The material types, chemical compositions, and mechanical properties are listed in Table 1. The sheets were made into specimens with the specifications of 100 mm × 30 mm and 150 mm × 110 mm. The former was used for investigating the weld length, and the latter for analyzing the weld joint layout. The sheet surface was cleaned and scrubbed with a cotton ball dipped in acetone before welding to remove stains and oils. The shielding gas for the laser welding is argon. 2

Table 1. Mechanical properties and chemical compositions of test sheets (mass fraction, %) No.

Material mark

1

DC56D+Z

2

H220YD+ZF

Material thickness (mm) 1.2 0.8 1.2

0.8

Yield strength (Mpa)

Tensile strength (Mpa)

C (≤)

Si (≤)

Mn (≤)

P (≤)

S (≤)

Other (≤)

120~180

270~350

0.01

0.01

0.30

0.025

0.020

0.215

220~280

340~410

0.01

0.10

0.90

0.080

0.025

0.14

Table 2. Technological parameters of resistance spot welding

No.

Combination

Welding

Welding

Work

Pressuring

current

time

pressure

time

(kA)

(cycle:cyc)

(Mpa)

(cycle:cyc)

Electrode size(mm)

1

1.2mmDC56D & 1.2mmDC56D

9.62

12

0.4

10

7.91

2

0.8mmDC56D & 0.8mmDC56D

8.45

12

0.35

10

7.29

3

1.2mmDC56D & 0.8mmDC56D

8.70

12

0.4

10

7.45

4

1.2mmH220YD & 1.2mmH220YD

9.82

12

0.4

10

7.91

5

0.8mmH220YD & 0.8mmDC56D

8.70

12

0.35

10

7.29

Table 3. Technological parameters of laser welding

No.

1

Combination

1.2mmDC56D & 1.2mmDC56D

2

0.8mmDC56D & 0.8mmDC56D

3

1.2mmDC56D & 0.8mmDC56D

4

1.2mmH220YD & 1.2mmH220YD

5

0.8mmH220YD & 0.8mmDC56D

WPW

Welding

Welding

Defocusing

Shielding

Inter-sheet

(mm)

power

speed

amount

gas flow

space

(kW)

(m/min)

(mm)

(L/min)

(mm)

1.0

2.0

1.7

-0.4

15

0.2

2.0

2.0

0.8

+0.4

15

0.3

1.0

2.0

1.7

+0.4

15

0.15

1.5

1.6

1.1

+0.4

15

0.2



2.0

1.7

-0.2

15

0.2

1.0

2.0

1.6

-0.4

10

0.2

1.5

1.6

1.0

+0.4

10

0.3

2.0

1.8

+0.2

13

0.15



2.3 Experimental methods First, experiments were conducted on overlap joints of the specimens using resistance spot welding and laser welding with different weld lengths. Resistance spot welding experiments were carried out directly at the workplace and auto door equipment, and the main process parameters are listed in Table 2. For the laser welding experiments, an orthogonal test had to be conducted first with respect to different sheet welding combination schemes in accordance with the self-prepared orthogonal parameter table. The technological parameters for the preferable welding quality were obtained, as listed in Table 3. Using these parameters, the weld length was altered for laser welding experiments. A microcomputer-controlled electronic universal testing machine was utilized to analyze the tensile-shear strength of the welding specimens and to investigate the bearing capacity of the joints. The LWL whose strength was comparable to the strength of one RSWP was obtained. Next, welding experiments with different weld joint layouts were conducted on DC56D sheet overlap joints with the specifications of 150 mm × 110 mm × 0.8 mm. These experiments involved different weld lengths coupled with different spacings, identical weld lengths coupled with different spacings, and different weld lengths coupled with identical spacings, as shown in Fig. 3. The preferable weld joint layouts were obtained based on a comparative analysis of the weld formation and mechanical properties for the laser welded joints with different weld lengths and spacing layouts. 3

This information was used to provide a technical reference for the replacement of the resistance spot welding with laser welding for an auto body. Finally, experimental studies were performed on auto door parts using resistance spot welding and laser welding. A static stiffness analysis was conducted for a welding specimen on a full-vehicle dynamic/static flexible test platform to compare the overall welding performances of the two welded specimens.

Fig. 3 Weld joint layout schemes.

3. Results and Discussion 3.1 Surface appearance of laser weld joints and RSWPs Experiments were conducted on overlap joints for various sheet combinations using resistance spot welding and laser welding with different weld lengths. The weld joint surface appearances of the welded specimens are shown in Figs. 4 and 5. The area of the RSWP was jointly determined by the size of the electrode clamp of the welding gun and the welding parameters. Because the shapes and sizes of the electrode clamps of the top and bottom welding guns were identical, the two electrodes applied voltage to the two sheets at the same time to produce the current for welding during the welding process. Therefore, the weld width of the joint surface was similar to that of the joint backface, as shown in Fig. 4. The weld nugget sizes of the esistance spot welding joints for different sheet combinations were measured. The average value of the longitudinal and transverse sizes was taken as the weld nugget diameter (Table 4).

2 mm

Surface

Backface

Surface

2 mm

Backface

Fig. 4 Macroscopic and microscopic surface appearances of resistance spot welding overlap joints.

4

.

10 mm

6 mm

10 mm

8 mm

10 mm

12 mm

10 mm

10 mm

14 mm

10 mm

18 mm

10 mm

16 mm

10 mm

20 mm

10 mm

10 mm

10 mm

22 mm

10 mm

10 mm

24 mm 26 mm 30 mm Fig. 5 Overlap welding joint surface appearance of different LWLs. Table 4. Weld nugget diameter of resistance-spot weld points No.

Combination

Longitudinal

Weld nugget size Transverse

Weld nugget

(mm)

(mm)

diameter (mm)

1

1.2 mm DC56D & 1.2 mm DC56D

6.96

6.83

6.90

2

0.8 mm DC56D & 0.8 mm DC56D

6.51

6.33

6.42

3

1.2 mm DC56D & 0.8 mm DC56D

6.96

6.51

6.74

4

1.2 mm H220YD & 1.2 mm H220YD

6.96

6.83

6.90

6

0.8 mm H220YD & 0.8 mm DC56D

6.51

6.33

6.42

The weld joint area of the laser penetration welding was jointly determined by the WPW and weld length. The WPW was determined by the laser beam focusing spot size and welding parameters, and the weld length could be set in the numerical control program. Because the laser beam propagated from the light source generator, through the transmission light path, to the welding specimen surface, a certain delay existed. Therefore, when setting the weld length, the vacancy course path caused by the laser beam delay needed to be considered. Fig. 5 shows the surface appearances of different LWLs (6–30 mm). To obtain the exact LWL whose performance was comparable to the performance of one RSWP, the weld length was increased by 2 mm each time. In addition to being related to the weld length, the laser welding performance was also correlated with the WPW [15]. When the WPW differed, the mechanical properties of the joints differed even if the weld length was identical. Fig. 6 shows the WPWs of DC56D sheet overlap joints under different laser welding parameters observed using a stereomicroscope. It can be seen that the WPW of the surface is larger than that of the backface, 5

and the WPW of the backface increases with that of the surface.

Surface

Backface

1mm

1mm

WPW of surface ≈ 1.0 mm

Surface

Backface

1mm

1mm

WPW of surface ≈ 1.5 mm

Surface

Backface

1mm

1mm

WPW of surface ≈ 2.0 mm

Fig. 6 Laser welding joints with different WPWs.

3.2 Comparison of mechanical properties of laser welding joints and resistance spot welding joints To compare the performances of the two welding methods for an auto body, a tensile-shear test was conducted on specimens for the resistance spot welding and laser welding with different weld lengths under various conditions. The purpose of this test was to compare the tensile-shear performances of the resistance spot welding and laser welding joints. If the specimen fractures in the base metal zone, the tensile-shear load of the joint cannot be measured accurately. Thus, the welding specimens are first prepared as standard specimens according to the GB/T228-2002 standard "Metallic Materials–Tensile Testing at Ambient Temperature," and then they undergo the tensile-shear test. Under a certain tensile-shear load, both the resistance spot welding specimens and laser welding specimens’ fracture at the base metal zone far away from the weld joint (Fig. 7). In this case, the measured tensile-shear capacity (TSC) of a specimen cannot indicate the maximum tensile-shear load of the welded joint, but only shows that the tensile-shear capability of the joint is higher than that of the base metal. Therefore, in the tensile-shear test, the welded specimens directly underwent the test without being prepared as standard specimens, i.e., using nonstandard specimens to enhance the width of the base metal. Consequently, the base metal could bear a larger force when the specimen underwent the tension-shear test, to avoid the occurrence of fractures at the base metal to obtain the TSC data of joints.

Fig. 7 Fracture locations of standard tensile-shear specimens for resistance spot welding and laser welding

First, the TSC values of 0.8-mm-thick DC56D steel sheet overlap joints under different LWLs were measured, and the impact of the WPW on a joint's mechanical properties was analyzed, as shown in Fig. 8 (a). It can be found that the bearing capacity of the joint increases with the weld length. Under the same weld length, the TSC of the joint with an approximately 1.5-mm WPW is larger than that with an approximately 1.0-mm WPW. For laser welding joints with an approximately 1.0-mm WPW, when the weld length increases to 16 mm, the TSC value of the joint is 4.617 kN, which is larger than the maximum bearing capacity (MBC) of one RSWP (4.488 kN) under the same sheet conditions. For laser welding joints with an approximately 1.5-mm WPW, when the weld length increases to 14 mm, the TSC value of the joint is 4.487 kN, which is comparable to the MBC of one RSWP 6

on the sheet overlap joint (4.488 kN). Second, based on the experimental results of 0.8-mm-thick DC56D steel sheets, laser welding experiments were carried out using 1.2-mm-thick DC56D steel overlap joints under different weld lengths. To simplify the experimental data, the range of weld lengths was set at 14–30 mm. Fig. 8 (b) shows the TSC values of specimens for 1.0-mm and 2.0-mm WPWs under different weld lengths. It can be seen that for 1.2-mm-thick DC56D steel sheets and for laser overlap welding joints with an approximately 1.0-mm WPW, only when the weld length increases to 20 mm can the TSC of the joint (5.573 kN) be comparable to the bearing capacity of one RSWP (5.302 kN) under the same sheet conditions. For laser welding joints with an approximately 2.0-mm WPW, when the weld length increases to 16 mm, the TSC of the joint (5.971 kN) has already become larger than the MBC of one RSWP (5.302 kN). The data analysis in Fig. 8 can thus indicate that for the same sheet with different thicknesses and different WPWs, the TSC value of the joint increases with the weld length (6–30 mm). When the weld length increases to a certain value, the TSC of the joint becomes comparable to that of one RSWP. In addition, both the thickness of the sheets and the WPWs has a certain impact on the weld length setting principles. When the WPW is constant, the minimum weld length (MWL) required for thick sheets should be larger than that for thin sheets. Under the same sheet thickness and allowable WPW range, the MWL required for welding specimens with small WPWs should be larger than that for welding specimens with large WPWs. This is because WPWs that are too small would result in insufficient joint strength, while those that are too large would affect the welding quality and performance of the specimens.

(a)

(b)

Fig. 8 Tensile-shear capability of (a) 0.8 mm & 0.8 mm and (b) 1.2 mm & 1.2 mm auto-body DC56D steel sheet welding specimens under different weld lengths.

Next, based on the mechanical property analysis of 1.2-mm-thick DC56D sheet joints under different LWLs, a corresponding experimental analysis was conducted using H220YD high-strength galvanized steel sheets with the same thickness. Fig. 9 shows the analysis results. It can be seen that for the two kinds of sheets with the same thickness, under the same WPW, they have similar LWLs with a tensile-shear performance comparable to that of one RSWP. For 1.2-mm-thick DC56D and H220YD sheets with an approximately 1.0-mm WPW, the LWL whose bearing capacity is comparable to the bearing capacity of one RSWP is approximately 20 mm. This may be because the materials of the two kinds of sheets are similar; under appropriate parameters, they have comparable welding performances. Therefore, when welding two kinds of auto body sheets made of similar materials using laser welding, the same LWL can be determined if the sheet thickness and WPW are similar. Moreover, a comparative analysis of the tensile-shear performances was performed on auto-body dissimilar sheet overlap joints using resistance spot welding and laser welding. The dissimilar sheets included sheets made of the same materials and different thicknesses (0.8-mm-thick and 1.2-mm-thick DC56D sheets), and sheets with the same thickness and different materials (0.8-mm-thick DC56D and 0.8-mm-thick H220YD sheets). The analysis

7

results are shown in Fig. 10.

Fig. 9 Tensile-shear capability of 1.2 mm & 1.2 mm H220YD welding specimens under different weld lengths.

(a)

(b)

Fig. 10 Tensile-shear capability of (a) 0.8 mm & 1.2 mm DC56D and (b) 0.8 mm DC56D & H220YD auto-body dissimilar sheet overlap joint welding specimens under different weld lengths.

It can be seen from Fig. 10 (a) that for 0.8-mm-thick and 1.2-mm-thick DC56D sheet overlap joints, when the 0.8-mm-thick sheet is used as the upper sheet, the TSC of the joint (4.793 kN) becomes larger than the MBC of one RSWP on the joint (4.636 kN) as long as the LWL increases to 18 mm; when the 1.2-mm-thick sheet is used as the upper sheet, the LWL whose TSC is comparable to that of one RSWP on the corresponding joint (4.862 kN) is also 18 mm. Therefore, for the laser welding of 0.8-mm-thick and 1.2-mm-thick DC56D sheet overlap joints, the MLWL is approximately 18 mm in both cases: with the 0.8-mm-thick sheet as the upper sheet and with the 1.2-mm-thick sheet as the upper sheet. In addition, in the case of using the 1.2-mm-thick sheet as the upper sheet, all the bearing capacities of the laser welding specimens under different weld lengths were higher than those in the case of using the 0.8-mm-thick sheet as the upper sheet. This was primarily because under the keyhole effect during the welding process, the upper sheet takes part in the filling of inter-sheet gap after being melted to increase the inter-sheet connection width. When the thin galvanized steel sheet was used as the upper sheet, there was a larger indentation at the upper surface of the weld joint, causing the worsening of stress conditions and a decrease in the tensile-shear strength of the weld joints [16, 17]. In addition, during high-power-density laser welding, using the thin galvanized steel sheet as the upper sheet tended to produce thermal deformation, thereby affecting the mechanical properties of the weld joints.

8

Fig. 10 (b) shows that for the 0.8-mm-thick DC56D and 0.8-mm-thick H220YD sheet overlap joints, when the LWL increases to approximately 16 mm, all the maximum TSCs of the joints are larger than the bearing capacity of one RSWP under the same conditions, whether the DC56D sheet or H220YD sheet is used as the upper sheet. In the case of using the DC56D sheet as the upper sheet, all the maximum TSCs of the laser welding and resistance spot welding joints are slightly higher than those in the case of using the H220YD sheet as the upper sheet. Fig. 11 shows the joint cross-sectional shapes under the same technological parameters when the relative locations of the two sheets are different. It can be seen that the entire weld joint surface is smooth and continuous with preferable shaping, while small differences exist for the WPW and inter-sheet welding width. In the case of using the DC56D sheet as the upper sheet, the WPWs and inter-sheet welding widths are all slightly larger than those in the case of using the H220YD sheet as the upper sheet. Therefore, for the laser welding of dissimilar sheets with the same thickness and similar materials, the relative locations of the two sheets had a slight impact on the formation of the weld joints, but had little impact on the determination of the MLWLs.

A

B

Fig. 11 Cross-sectional shapes of DC56D and H220YD sheet laser overlap welding joints for (A) DC56D sheet as upper sheet and (B) H220YDsheet as upper sheet.

Based on the analysis of the fracture locations of nonstandard tensile-shear specimens undergoing resistance spot welding and laser welding with different weld lengths, it could be found that when the welding specimen bore a certain tensile-shear force, failure and fracture generally occurred at the joint location. Most of the fractures of the welding specimens occurred at the joint in the forms of (i) splitting of the upper and lower sheets along the joint (Fig. 12 (a), (b), (c)), (ii) cracking along the heat-affected zone of the upper sheet ( Fig. 12 (d), (e), (f)), and (iii) cracking along the heat-affected zone of the lower sheet (as Fig. 12 (g), (h)). Only a few fractures of specimens with large weld lengths occurred at the base metal, as shown in Fig. 12 (i). Therefore, the data measured in the nonstandard tensile-shear test demonstrated the tensile-shear performance of the weld joints. In summary, the TSC of the weld joint increased with the weld length. When the weld length increased to a certain value, the TSC of the joint became comparable to that of one RSWP. Therefore, for the laser welding of auto body parts, a single MLWL should be no less than the weld length to obtain a performance comparable to one RSWP. In addition, for auto-body galvanized sheet laser overlap welding within an appropriate WPW range, the overall TSC of a joint with a large WPW is higher than that of a joint with a small WPW, along with an increase it the weld lengths. The sheet type, thickness, and combination mode of welding specimens also have a certain impact on the LWLs setting principles. Therefore, when determining the LWL for auto body welding positions, these factors should be considered.

9

(a)

(b)

(d)

(e)

(g)

(h)

(c)

(f)

(i)

Fig. 12 Fracture locations of nonstandard tensile-shear specimens undergoing resistance spot welding and laser welding under different weld lengths.

3.3 Impact of laser weld joint layout on specimen performance For laser welding, experiments have demonstrated that when using a high-power laser to produce a weld joint with a large length on an auto body, the heat generated by welding keeps propagating along the front direction of the welding and accumulating, finally, reaching a maximum at the rear half of a joint. The rear half of a joint is the area with the most serious deformation, which can cause welding defects. For 110-mm-wide sheet specimens, for instance, if full-length welding was utilized, i.e., arranging a 110-mm-long weld length, then in the rear half of the weld joints, the large amount of thermal accumulation during the welding process would cause thermal deformation of the rear half of the weld joints before welding was applied. The deformation of the sheets led to inconsistent inter-sheet gaps at both ends of the joint. As a consequence, the weld joint was indented, as shown in Fig. 13. Therefore, for a weld joint with a large length, a section of the weld joint near the rear tended to produce welding defects under the same technological parameters. To effectively prevent the thermal deformation of auto body parts and avoid the impact of defective weld joints on the appearance and strength of an auto body, the weld length and spacing should be determined in a rational manner.

10mm

Fig. 13 Photograph of surface appearance of long weld joints.

10

Fig. 14 Photographs of welding specimens using schemes A1–A7.

For laser welding, weld lengths that are too small cannot meet the mechanical property requirements, whereas spacings that are too short cannot effectively prevent thermal accumulation. Therefore, for the analysis of weld joint layout schemes, the weld length can be determined based on the experimental findings. By integrating all the factors and conditions, the joint performance becomes better than the performance of one RSWP when the LWL reaches 20 mm. Thus, an MLWL of 20 mm was used. The minimum spacing could be set at 10 mm. For 0.8-mm-thick DC56D sheet overlap joints, welding experiments were conducted for weld joint layouts using welding parameters corresponding to a 1.0-mm WPW, according to schemes A1–A7 (Fig. 3). From Fig. 14, it can be seen that the appearance of the weld joints using schemes A1–A7 is continuous and smooth without welding thermal deformation, thereby satisfying the appearance and performance requirements for auto body weld joints. To obtain the bearing stress of each joint, tensile-shear tests were performed on welding specimens using schemes A1–A7. The test results are listed in Table 5. Table 5. Mechanical properties of laser overlap welding specimens under different weld joint layout schemes. Single-segment length for weld joint

Spacing (mm)

layout (mm)

(Segment number of

(segment number of weld joint)

spacing)

50

25 (2)

20 (3)

14.43

80

80 (1)

15 (2)

20.51

A3

80

40 (2)

10 (3)

21.14

A4

80

40 (2)

30 (1)

20.07

A5

80

26 (1), 28 (2)

15 (2)

21.17

A6

60

20 (3)

25 (2)

16.43

A7

80

20 (4)

10 (3)

21.20

Weld joint layout

Total

length

schemes

weld joint (mm)

A1 A2

of

Maximum TSC of the joint (kN)

The layout of the weld joints on specimens with the same specifications and style was considered from two aspects: the influence of the weld length and the influence of the layout for the same weld length. The overall weld length in schemes A2, A3, A4, A5, and A7 was 80 mm. In scheme A6 it was 60 mm, and in scheme A1 it was 50 mm. As indicated by the measured results for the tensile-shear capacity (TSC) of the corresponding welding test specimens,as listed in Table 5, the TSC of a welded test specimen increased as the overall weld length increased. When the overall weld length was constant (80 mm), even though the variation trend of the mechanical property of the test specimen was roughly A7 >A5 >A3 >A2 >A4, the mechanical properties of the weld joints in schemes A7, A5, and A3 are rather close. Weld joints that are short in length can better protect the auto body parts from deformation resulting from heat accumulation, and can maintain the consistency of the size 11

of the inter-sheet gap to prevent weld defects from influence the auto body appearance and strength properties. Therefore, it can be deduced from the analysis results for the characteristics of the weld formation and the layout of the welds that when the specifications, dimensions, and overall weld length of a welded specimen are constant, to obtain a welded joint with the optimal mechanical properties, the layout of the welds should be as “short-length and multi-segment” as possible on the basis of satisfying the weld length setting principles. 3.4 Laser welding and resistance spot welding of auto doors Based on the findings of the previously mentioned experimental analysis, laser welding experiments were conducted on auto door sub-assembly using the weld length setting principle and layout method, and a comparative analysis was performed in terms of the auto door sub-assembly welded by resistance spot welding. When the auto door sub-assembly, which consists of an inner plate, a hinge reinforcing plate, an inner cummerbund reinforcing plate, and a door lock reinforcing plate, were welded using the two methods , efforts were made to make one laser weld correspond to a resistance spot weld. The length of a laser weld was 20~30 mm, and the size of the nugget of a resistance spot weld was determined by the corresponding type and thickness of the sheet and technical parameters. Some of the welding parts were too narrow and small, with a special structure, to make a one-to-one correspondence, and a 40 mm laser weld had to be designed to replace two resistance spot welds. On the two auto door sub-assemblies, the number of laser welds was 49, whereas that of the resistance spot welds was 54. Fig. 15 shows the auto doors welded by resistance spot welding and laser welding. During the welding process, we found that the overall speed of the laser welding was significantly higher than that of the resistance spot welding. In welding cases involving different sheet thicknesses, different materials, different morphologies, and welding at different locations with identical sheet thicknesses, laser welding only needs to change welding parameters, whereas resistance spot welding has to replace the electrode holder in addition to changing welding parameters. Therefore, compared with resistance spot welding, laser welding is more procedural, more controllable, more agile, and more flexible.

Fig. 15 Auto doors welded by resistance spot welding and laser welding.

Finally, a static stiffness analysis was conducted for the auto doors welded by resistance spot welding and laser welding on the full-vehicle dynamic/static flexible test platform. Suppose an auto experiences an impact from the side. Let the impact load act on the middle of the outside door panel. A stress load of 10.0 kN is applied in the middle of the auto door. Under the effect of the stress load, the auto doors welded by laser welding and resistance spot welding had displacements of 86.26 and 114.86 mm in the middle, respectively. The GB15743-1995 [18] standard stipulates that the displacement must be less than 152 mm, so the auto doors welded by the two welding methods both met the national standard. However, under the same stress load, the auto door welded by the laser welding had a smaller displacement than that welded by the resistance spot welding. Thus, it can be seen that the laser welded auto door had a greatly increased stiffness. Obviously, this is very important for traffic safety.

12

Conclusions (1) Different lengths of laser welding seams were designed for overlap joints with different materials. The mechanical properties of specimens with different weld lengths were compared. The LWL with a performance comparable to one RSWP under various conditions was obtained. For DC56D sheet overlap joints, the MLWL should be approximately 16 mm and 20 mm for 0.8-mm and 1.2-mm sheet thicknesses, respectively. For 1.2-mm-thick H220YD sheet overlap joints, the MLWL should be approximately 20 mm. (2) The setting principles for LWLs were investigated, and the influencing factors for LWLs were analyzed. Experimental results showed that weld length setting principles has a certain correlation with the sheet thicknesses and WPWs; for auto-body dissimilar sheet overlap joints, when the difference between the sheet thicknesses and materials was small, the up-down relative location of the two sheets had a slight impact on the formation of weld joints, but it had little impact on the setting principles of LWLs. (3) A comparative analysis was conducted on the layouts of laser welding joints. It was found from experiments that when the specification of the welding specimens and the total length of the welded joints are constant, the "short-length and multi-segment" layout scheme should be employed when possible to achieve a preferable joint morphology and mechanical properties. (4) The auto door sub-assembles of a certain vehicle were welded using resistance spot welding and laser welding. The results indicated that the overall static stiffness of the auto door welded by a laser with the weld length setting principles and layouts was significantly higher than the auto body welded by resistance spot welding. Compared with resistance spot welding, laser welding is more controllable and agile, and thus it can produce better welding quality and higher flexibility. Acknowledgment This work was supported by the National Natural Science Foundation of China (No.51405411), the Natural Science Foundation of Fujian Province (No. 2013J05085), the National Natural Science Foundation of China (No. 51475400), the research project of Xiamen University of Technology (No.XYK201425), and the Natural Science Foundation of Hunan province (No. 2015JJ3003). References [1] Ribolla A, Damoulis GL, Batalha GF. The use of Nd: YAG laser weld for large scale volume assembly of automotive body in white. J Mater Proces Technol 2005;164: 1120-27. [2] Weinmann G.Laser technology and its application in automobile manufacturing.Ind Laser Solut 2006;10. [3] Gao SF, Tang XH, Xi SY. The laser welding technology of three-layer zinc coated sheet. Weld & Join 2006; (1): 52-7. [4] Li TQ, Yao PJ. The design standard of welded auto body. Auto Technol & Mat 2005; 5: 37-9. [5] Kong F, Ma J, Carlson B, et al. Real-time monitoring of laser welding of galvanized high strength steel in lap joint configuration. Opt. Laser. Technol 2012; 44 (7): 2186–96. [6] Zhang MJ, Chen GY, Zhou Y, et al. Observation of spatter formation mechanisms in high-power fiber laser welding of thick plate. Appl Surf Sci 2013; 280: 868-75. [7] Mei LF, Chen GY, Jin XZ, et al. Research on laser welding of high-strength galvanized automobile steel sheets. Opt & Lasers in Eng 2009; 47 (11): 1117-24. [8] Oliveira AC, Siqueira RHM, Riva R. One-sided laser beam welding of autogenous T-joints for 6013-T4 aluminium alloy. Mater & Des 2015; 65:726-36. [9] Kim JD, Na I, Park CC. CO2 laser welding of zinc-coated steel sheets. KSME Int J 1998; 12(4): 606-14. [10] Weinmann G.Laser technology and its application in automobile manufacturing.China Auto Tech Forum 2006; 9: 26-7. 13

[11] Chen GY, Mei LF, Zhang MJ, et al. Application and research of laser processing automobile body manufacturing[J]. Laser Opt Prog 2009; 46:17-23. [12] Wang M.Application of resistance welding in automobile industry. Electr Weld Mach 2003; 33 (1): 1-6. [13] Aslanlar S. The effect of nucleus size on mechanical properties in electrical resistance spot welding of sheets used in automotive industry. Mater& Des 2006; 27:125-131. [14] Mei LF, Yi JM, Yan DB, et al. Comparative study on CO2 laser overlap welding and resistance spot welding for galvanized steel. Mater & Des 2012; 40:433-442. [15] Wang X, Gu YX, Qiu TB, et al. An experimental and numerical study of laser impact spot welding. Mater & Des 2015; 65:1143-52. [16] Chen GY, Mei LF, Zhang MJ, et al. Research on key influence factors f laser overlap welding of automobile body galvanized steel. Opt & Laser Technol 2013; 45:726-33. [17] Zhang MJ, Chen GY, Zhou Y, et al. Research on microstructure and mechanical properties of laser keyhole welding–brazing of automotive galvanized steel to aluminum alloy. Mater & Des 2013; 45: 24-3 0. [18] GB15743-1995. The Intensity of Car Door.

14

Graphical abstract

Highlights Different laser weld lengths were designed for overlap joints with different materials. The minimum laser weld length under various conditions was obtained. The laser weld length has a correlation with the sheet thickness and weld pool width The "short-length and multi-segment" layout should be adopted for laser weld joints. Laser welding method can produce better welding quality and higher flexibility.

15