Comparative analysis on overlap welding properties of fiber laser and CO2 laser for body-in-white sheets

Comparative analysis on overlap welding properties of fiber laser and CO2 laser for body-in-white sheets

Materials and Design 49 (2013) 905–912 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/lo...

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Materials and Design 49 (2013) 905–912

Contents lists available at SciVerse ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Comparative analysis on overlap welding properties of fiber laser and CO2 laser for body-in-white sheets Mei Lifang a,⇑, Yan Dongbing a, Yi Jiming a, Chen Genyu b, Ge Xiaohong a a b

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

a r t i c l e

i n f o

Article history: Received 7 October 2012 Accepted 2 February 2013 Available online 16 February 2013 Keywords: Fiber laser Carbon dioxide laser Welding Mechanical properties

a b s t r a c t A systematic study on overlap welding properties of autobody galvanized steel is conducted by using the late-model fiber laser and CO2 laser respectively. The welding joints surface formation, cross-sectional weld shape and mechanical properties of the two types of lasers are compared, and it is analyzed whether dissimilar laser sources would impact the welding quality, so as to control and optimize the craft quality of laser welding automobile body. The results show that under the test conditions, the weld fusion width presents wide at the top and narrow at the bottom and the cross-sections of overlap joints take a ‘‘Y’’ shape by using CO2 laser welding, while those by using fiber laser welding have a shape like ‘‘I’’ which was mean the fusion widths of the upper and lower surface are almost the same. And the anti-shearing force of fiber laser welding joints was stronger due to its larger inter-sheet joint connection width. In addition, under certain conditions, the fiber laser welding could result in more uniform and finer joints, higher strength and hardness, and better mechanical properties. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction With the development of automotive technologies, there is an increasing demand for autobody material, joint structure, joint performance, etc., and the welding of autobody parts is becoming increasingly difficult [1]. Traditional welding technologies are gradually replaced by latest welding technologies, as they cannot meet the requirements of autobody changes [2]. Laser welding technologies can improve autobody strength, stiffness, assembly accuracy as well as safety performance, and consequently have been eagerly adopted by major automobile manufacturers [3,4]. Laser overlap welding, as the most widely applied welding type in the autobody laser welding, its welding quality directly affects the autobody performance. With the emergence of new highpower laser sources, a variety of laser sources have been used in welding research fields. The studies on galvanized steel welding properties of various laser sources have all been reported, such as scholar Chen et al. studied the welding property of galvanized steel using CO2 laser [5]; scholar Schmidt and their teams researched the welding characteristic of galvanized steel by YAG laser [6], and Quintino et al. reported the fiber lasers welding preliminary [7]. However, no scholars or researchers have systematically conducted comparative analysis on welding properties of different kinds of laser sources, and certain books and papers just ⇑ Corresponding author. Tel./fax: +86 592 6291385. E-mail addresses: [email protected], [email protected] (L. Mei). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.02.003

vaguely point out that the gas laser source is suitable for cutting and that the solid laser source is more suitable for welding, without providing sufficient basis or detailed reasons. In addition, Zn evaporation and burning loss and other issues would cause shrinkages, overburning, and other defects to the high energy density laser welding of the galvanized steel. Besides, there are significant differences in the welding quality of the galvanized steel because the lasers of different wavelengths have different coupling properties with the materials [8]. When different laser sources are used for welding, there are significant differences on joint molding properties and mechanical properties. Among a series of new laser sources, which one has a superior welding performance? All These issues need to be further studied. In recent years, the fiber laser has become the focus in the field of laser welding and is widely favored by the industrial processing circles all over the world. Compared with the traditional CO2 laser and solid-state laser, the fiber laser has a smaller size, higher efficiency, and lower cost, and is easier for system integration; the flexible processing is easier to realize for the fiber optic transmission [9]. However, there are few researches on the fiber laser welding of the galvanized steel [10], especially on the comparative study with the CO2 laser welding of the galvanized steel. Moreover, the investigation of the impacts of different lasers on the welding characteristics has rarely been reported. Therefore, this paper focuses on overlap welding studies by using fiber laser and CO2 laser respectively based on materials such as new type of autobody galvanized steel and common cold rolled

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steel. When using the two laser sources for welding, the weld forming characteristics, joint mechanical properties and main factors affecting joint welding performance are analyzed respectively. Then comparative analysis is systematically carried out on overlap welding properties of fiber laser and CO2 laser to find the one with better welding performance. This provided the experimental and theoretical basis for the application of laser welding in galvanized steel. 2. Test conditions and methods

responding to the two laser sources, and then the study on welding performance of the two laser sources is conducted based on the above-mentioned parameters. 2.2.3. Welding process To improve welding quality, the test pieces are cleaned with acetone before welding to remove the surface oil stains. In the welding process of galvanized steel sheets, argon gas is used for protecting the weld pool through the Laval nozzle, and the standard feeler gauge is used to adjust the inter-sheet space so as to study the impact of the space on overlap welding performance.

2.1. Test material and equipment The door parts sheets of an autobody are used as test material: DC56D (ultra-low carbon galvanized steel) and DC03 (low carbon steel), with the sheet thicknesses of 1.0 mm and 1.2 mm respectively. Their respective chemical compositions and mechanical properties are shown in Table 1. First, the sheets are cut to 60 mm  30 mm test samples using CO2 laser. The test equipment used includes a YLR-4000-ST2 fiber laser device, a DC025 slat CO2 laser device, and a laser process machine as shown in Figs. 1 and 2. The maximum output power of the fiber laser device is 4.0 kW (continuous output with wavelength of 1.07 lm and mode of TEM00). The maximum output power of the CO2 laser device is 2.5 kW (continuous or pulse output with wavelength of 10.6 lm and mode of TEM00). The fiber laser device uses optical fiber with 400 lm core diameter for beam transmission, and meanwhile uses the YW50 welding head as the processing head. The collimating and focusing system comprises collimating lens with 150 mm focal length and focusing lens with 50 mm focal length; it is transmission-type focusing with focal spot diameter of 0.67 mm. The CO2 laser device uses optical transmission system of lens mode, with the reflector for steering and transmission; it is reflection-type focusing with focal spot diameter of 0.4 mm. 2.2. Test methods 2.2.1. Joint form Based on welding characteristics of body structural members, the DC56D and DC03 sheets are combined and clamped by a clamping device to conduct dissimilar sheet laser overlap welding test, with the overlap joint form as shown in Fig. 3. The laser beam is incident to the upper sheet surface vertically or after steering an angle, forming continuous laser weld at overlap locations along with the movement of the laser beam when using appropriate welding parameters. 2.2.2. Technological parameter design As there are big differences in laser source and processing equipment between fiber laser welding and CO2 laser welding, including laser source wavelength, laser beam transmission and focused spot sizes, and gas protection mode, different technological parameters are required to be designed for welding methods of the two laser sources. In this case, the orthogonal welding test is made by using fiber laser and CO2 laser respectively, from which two groups of optimal technological parameters are obtained cor-

2.2.4. Post-welding test piece treatment and analysis The stereo microscope is used to observe the weld appearance and analyze the weld formability and surface appearance characteristics; the tensile-shear test sample is made with electrical discharge wire cutting according to the welding joint tensile-shear test method under the metal materials room-temperature mechanical experiment national standard GB/T228.1-2010 [11], and the tension-shear strength of the welding test piece is analyzed by using the computer-controlled electronic universal testing machine to study the bearing capacity of joints; the welding test piece is sampled through wire cutting, polished, etched by the nital, cleaned and dried, and then prepared as metallographic specimen, and the weld joint structures and cross-sectional shapes are observed through the optical microscope; the weld cross-sectional hardness distribution is measured with the electronic micro-hardness tester; the scanning electron microscopy is used to analyze the shear fracture appearance and the fracture mechanism of the welding test piece. 3. Test results and analysis 3.1. Surface formation of weld The surface formation of weld is an important factor for laser welding performance, therefore analysis of weld formation will help to improve the laser welding process, weld reliability as well as welding quality. The weld surface appearance mainly involves the weld smoothness, continuity, macro-pores, spatter, surface oxidation, etc. [12]. First, comparative analysis is conducted on the weld appearance of the two laser sources based on the DC56D and DC03 dissimilar sheet overlap joints when using optimal technological parameters. When the fiber laser welding parameter power P = 2.5 kW, welding speed v = 1.8 m/min, defocus amount Df = 0 mm, inter-sheet space d = 0.2 mm, and side-blown protective gas flow q = 25 L/min, the upper and lower surface appearances of the overlap welding test piece are as shown in Fig. 4. When the CO2 laser welding technological parameter power P = 2.0 kW, welding speed v = 1.6 m/min, defocus amount Df = +0.2 mm, inter-sheet space d = 0.2 mm, and coaxial protective gas flow q = 1.6 bar, the upper and lower surface appearances of the overlap welding test piece are as shown in Fig. 5. As shown in Figs. 4 and 5, the weld appearances of the upper and lower surfaces of the joints are smooth and continuous, without obvious defects such as pores, cracks, Slag and lack of fusion,

Table 1 Chemical composition and mechanical properties of sheets. Name

DC03 DC56D + Z

Chemical compositions (mass fractions (%))

Tensile strength (MPa)

C (6)

Si (6)

Mn (6)

P (6)

S (6)

Alt (P)

Ti (6)

0.08 0.01

– 0.01

0.45 0.30

0.030 0.030

0.025 0.025

0.020 0.020

– 0.01

Note: Z – Zinc coating weight: 45/45 g/m2.

280.94 300.68

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Fig. 1. Fiber laser and welding head.

Fig. 2. CO2 laser and welding head.

Fig. 3. Overlap joint and force analysis.

and the joints have little deformation, taking the shape of scaly ripples. Compared with the CO2 laser, the fiber laser welding appearance presents much more compact and well-distributed scaly ripples, and the weld formation is more beautiful, and the color of the weld is silver white. While the surface of the CO2 laser weld has large fluctuations, many pores and spatters, and molten metals splashed to both sides of the weld. In addition, the two laser sources have different weld fusion widths. When using fiber laser for welding, there is little difference between fusion widths of the upper and lower surfaces; when using CO2 laser for welding,

the fusion width of the lower surface is smaller than that of the upper surface. The main reason is that the fiber laser has good adaptability to the fluctuations on the surface and to the gap between the plates, while the CO2 laser has poor adaptability to the weld joint [13]. Some white and brown materials are observed with the stereo microscope at the weld pool edges of fiber laser welding joints. The analysis shows that the side-blown shielding gas is used for fiber laser welding; the side-blown shielding gas is mainly used to remove the plasma on the upper surfaces of steel sheets, as well as to stabilize the weld pool instability caused by the high-pressure zinc steam. Stable holes provide exits for highpressure zinc steam to acquire a stable welding process [14]. Under the effect of holes tension and weld pool expansion force, part of the zinc steam or zinc element is pushed to the edge of the weld pool. The zinc steam or zinc element at the edge of the weld pool is extremely easy to react with the remained oxygen element in sheets or oxygen in the air to generate oxides, and the oxides together with trace amount spatter stagnate around the weld fusion line and accumulate as a layer during the solidification of the weld pool, as shown in Fig. 4a. The coaxial protection mode is used for CO2 laser welding, which could protect welding joints from oxidation, but with much spatter around the joint fusion line. In addition, when welding the galvanized sheets using fiber laser and CO2 laser, the galvanized layers at the surface of the welding test pieces all have a small amount of burning loss. The hot-temperature liquid metal spatter in the weld pool falls on both sides of the weld, resulting in zinc burning loss areas at the edge of the laser weld. The emergence of zinc burning loss regions of the weld joint will weaken its anticorrosion ability [15]. When investigating the fiber laser welding performance of the overlap joint on the DC56D and DC03 plates, we analyze the welding influencing factors when the zinc coating sheet is placed at the top and at the bottom, respectively. Although the chemical compositions of the two plates are similar, the DC56D plate has a zinc coating on the surface but the DC03 plate does not, and they have different absorption rates of the fiber laser [16]. Hence, the joint welding performances are slightly different between the two situations when the coated sheet is used for the top plate and the bottom plate. Fig. 6 shows the appearances of the upper and lower

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Fig. 4. Weld appearance of fiber laser overlap welding joint.

Fig. 5. Weld appearance of CO2 laser overlap welding joint.

surfaces of the weld when the relative positions of the DC56D and DC03 plates are swapped, under the same welding parameters. As shown in Fig. 6, the weld pool width with the DC56D plate being the top plate is smaller than the one with the DC03 plate being the top plate. The reason is that the DC56D plate has a zinc coating on the surface, and the boiling point of the zinc (906 °C) is much lower than the melting point of the steel (1530 °C). When the high power density laser beam is irradiated on the surface of the workpiece, the zinc will be vaporized quickly to form zinc vapor, which tends to be ionized into plasma [17]. The plasma inclines to refract, scatter, and absorb the incident laser, with significant shielding effect on it. That would affect the energy transfer efficiency of the laser and reduce greatly the energy density of the laser acting on the workpiece [16]. Thus, the region of the workpiece on which the laser acts is changed, and the weld formation is influenced. As a consequence, the weld pool width with the DC56D plate being the top plate becomes smaller. When the coated sheet is placed at the bottom, the welding process will not be protected directly by the inert gas. Hence, the degree of oxidation would be increased and a certain amount of spatter would be accumulated in the vicinity of the weld fusion line. In the same

way, the oxidation degree of the surface of the uncoated sheet when it is at the bottom is higher than that when it is at the top. 3.2. Cross-sectional weld shape The type of laser source has a certain impact on the cross-sectional weld shape. The cross-sectional shape of joint will be different when using different types of laser sources for welding in the appropriate technological parameters. When using CO2 laser for welding, the weld cross-section takes the shape of ‘‘Y’’ (wide at the top and narrow at the bottom); when using fiber laser for welding, the cross-sectional shape is similar to ‘‘I’’ with almost the same upper and lower part width, as shown in Fig. 7. The fiber laser wavelength (1.07 lm) is shorter than the CO2 laser wavelength (10.6 lm), and its incident beam energy can be better absorbed by the sheet initial surface and holes. Literature shows that, lasers of different wavelengths require different threshold power densities for producing laser-induced plasma, and the threshold power density required by fiber laser with wavelength of 1.07 lm to produce laser-induced plasma is approximately two orders of magnitude higher than that of CO2 laser with wavelength of 10.6 lm

Fig. 6. The upper and lower surface topography of weld joint. (a) Upper surface of DC56D and DC03; (b) Lower surface of DC56D and DC03; (c) Upper surface of DC03 and DC56D; (d) Lower surface of DC03 and DC56D.

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Fig. 7. Cross-sectional shapes of CO2 laser and fiber laser weld.

[18]. Therefore, when using CO2 laser for welding, laser-induced plasma is easily produced and then will affect the welding process; the welding process will be less affected by the plasma when using fiber laser that has shorter wavelength than CO2 laser. In addition, the plasma’s absorption for laser enhances with the growth of power density and holding time, and is directly proportional to the square of wavelength. The same plasma’s absorption for CO2 laser is two orders of magnitude higher than that for fiber laser [19]. Therefore, when using CO2 laser for welding, the plasma’s refraction, scattering and absorption for incident laser weaken the beam energy density and penetrating power, accordingly making the cross-section of welding joints wide at the top and narrow at the bottom. The cross-sectional weld shapes for different laser sources with the same inter-sheet space is shown in Fig. 8. The Figure shows, when the space is large, molten materials fill the space, the inter-sheet joint width becomes larger, and the inter-sheet width L of fiber laser welding joint is larger than that of CO2 laser welding joint. The upper surface center of both welds has sinking, and the sinking is worse for fiber laser welding; the lower surface of weld is smooth for CO2 laser welding, while it sinks for fiber laser welding. The main reason is that, when using fiber laser for welding the overlap joint with large space, the upper and lower parts of the sheet melt rapidly, and most of the molten materials flow into both sides of the inter-sheet space to fill the space under the interaction forces of these molten materials, making the upper and lower surfaces of the weld sunk [20,21]. Secondly, when the gap between the plates is too large, the gravity of the molten metal bath at the top plate will be too big. Then, it will be difficult for the surface tension to sustain the molten bath inside the weld, and the molten

L

(a) (a) CO2 laser welding δ=0.3mm

(b) (b) Fiber laser welding δ=0.3mm

Fig. 8. Cross-sectional shapes of CO2 laser and fiber laser weld under the same inter-sheet space.

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bath will drip or sink from the weld. It will fill the gap, finally causing certain sags on the surface of the weld [22]. The weld of the overlap joint mainly bears tensile-shear forces of the upper and lower sheets, as shown in Fig. 3. When using fiber laser for welding the overlap joint with large space, the inter-sheet joint connection width L is relatively large, although the upper and lower surfaces of the weld look sunk. Therefore, with the appropriate technological parameters, fiber laser welding test pieces have a better tensileshear performance than CO2 laser welding test pieces. During the process of the laser welding of the galvanized steel, inappropriate selection of the welding parameters would result in a variety of defects, among which the pores and sags are the main problems. The quantity of pores and the degree of sags of the galvanized steel are closely related to the heat input and the interplate gap. Larger heat inputs would cause more pores and sags, while larger inter-plate gaps would cause more sags. There problems can be controlled by reducing the laser power, increasing the welding speed, controlling the remelting of the weld and the inter-plate gap, or other methods [23]. That is because Zn has a low melting point, and is more likely to evaporate compared with other metals. Besides, it has small surface tension and low viscosity. When it is irradiated by the high power laser, the vapor and melt are likely to be produced and splashed. Therefore, the heat input control is particularly important to solve the above problems during the process of the laser welding of the galvanized steel. The key parameters which determine the welding formation include the laser power P, the welding speed v, the defocusing amount Df, and the inter-plate gap d, etc. 3.3. Mechanical properties of weld 3.3.1. Hardness of welding joint The HXD-10007 digital and intelligent micro-hardness tester is used to test the hardness of the welding test piece joint. The hardness of various parts such as weld zone, heat-affected zone (HAZ) and base metal zone (BMZ) is tested respectively starting from the weld center (load – 4.9 N, holding time – 15 s). The hardness value of each zone is obtained by testing 3–5 points in different locations of each zone and then averaging the values of these points. In the hardness testing, the hardness of fiber laser and CO2 laser welding joints is compared, taking DC56D and DC03 steel overlap welding test pieces as the test sample. As shown in Fig. 9, the hardness of fiber laser welding joint (weld zone and heat-affected zone) is slightly higher than that of CO2 laser welding joint without exception, and this is mainly concerned with the joint structure and will be discussed in the follow-up analysis. The great hardness of fiber laser welding joints effectively avoids the softening of the heat-affected zone, and also shows its high strength to some extent. 3.3.2. Tensile-shear test of welding joint The common sheets overlap joint type for autobody is partial overlap joint. The overlap joint mainly bears tensile-shear load according to the force analysis for weld joints. Therefore tensileshear tests are conducted for welding test pieces to assess the tensile-shear strength in this paper. Prepare the test piece as the standard test sample using the wire cutting electric discharge machine, and conduct a tensile-shear test on the computer-controlled electronic universal testing machine with the force loading rate of 1.0 mm/min, and then output data on mechanical properties from the computer. For the overlap joint bearing a pair of tensile-shear loads, shear strength exists due to the uncoaxiality of tensile forces, and the edge of overlap joint have serious stress concentration. Tensile-shear mechanical tests are respectively conducted on the CO2 laser and fiber laser welding test pieces of DC56D and DC03 steel overlap joints. The results show that, when using CO2

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Fiber laser welding joint

Hardness /HV

CO2 laser welding joint

Weld zone

DC52D-HAZ

DC03-HAZ

DC52D-BMZ

DC03-BMZ

Fig. 9. Hardness comparisons of fiber laser and CO2 laser welding joints.

laser for welding, the tensile-shear strength of DC56D and DC03 steel welding test pieces is 150 MPa; while when using fiber laser for welding, it is 170 MPa. This shows, with the appropriate technological parameters, the tensile-shear capacity is better when using fiber laser welding test pieces, and this mainly results from the larger joint connection width (L) between the two sheets of fiber laser welding test piece. It is found from this test that, the tensile-shear force borne by overlap joints mainly depends on the inter-sheet joint connection width L. In addition, it is also concerned with the joint microstructure. To find the reason to the loss of the tensile strength and shear strength of the CO2 laser, we first observe the appearance of the fractures of the two on the macroscopic level. As shown in Fig. 10a, in the macroscopic cross-section, the fracture surface of welding joint under the two laser sources is rough, with gray color and necking before fracture, i.e. ductile fracture; the test piece fracture and the forced direction form an angle of approximate 45° in the shape of shearing-type ductile fracture by initial judgment [24]. To better understand the joint fracture mechanism, SEM (Scanning Electron Microscope) analysis is further conducted on fractograph, as shown in Fig. 10b. It can be seen from the

Figure that the fracture dimple of the fiber laser welding specimen is large and deep. It is typical ductile fracture. Whereas, the fracture dimple of the CO2 laser welding specimen is relatively small and shallow, and part of it belongs to brittle fracture; the overall has a mixed type of fracture characteristics. 3.3.3. Metallographic structure of welding joint After a series of metallurgy chemical reactions, the molten pool metal’s temperature falls rapidly along with the removing of heat source, resulting in the formation of weld through solidification as well as occurrence of solid phase transition in the subsequent cooling process. Different structural changes will also take place in the fusion zone and heat-affected zone under the effect of welding heat source. The solidification of molten pool and the solid phase transition of weld determine the crystalline structure, microstructure and properties of weld metal [25]. In addition, under the effect of different welding heat sources, the chemical composition and structure of weld will also have varying degrees of unevenness phenomena, and even produce a variety of welding defects. In this case, analyzing the microstructure of welding joints is of great significance.

Fig. 10. Fractograph of test piece.

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Fig. 11. Metallographic structures of DC03 and DC56D base metal.

DC03 Heat-affected Zone

Molten Pool Zone

DC56D Heat-affected Zone

Metallographic structure of CO2 laser welding joint

DC03 Heat-affected Zone

Molten Pool Zone

DC56D Heat-affected Zone

Metallographic structure of fiber laser welding joint Fig. 12. Metallographic structure of DC03 and DC56D sheet overlap joint.

Fig. 13. Higher magnification micrographs in molten pool zone for both types of lasers.

The microstructure of laser welding joints is analyzed by using MM-6 horizontal metallurgical microscope. The metallographic structures in DC03 and DC56D base metals are shown in Fig. 11. The Figure shows that, base metal structure is mainly composed of ferrites because of the low carbon content of sheets. Moreover,

the crystalline grain of DC56D steel is much finer and more uniform than that of DC03 steel at the same magnification; therefore DC56D sheets have better mechanical property than DC03 sheets. The metallographic structure appearances of DC03 and DC56D sheet overlap joints respectively using fiber laser and CO2 laser

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welding are shown in Fig. 12. The Figure shows that, CO2 laser welding joint’s crystalline grain in the heat-affected zone is much coarser than fiber laser welding joint’s crystalline grain, and meanwhile its structural uniformity in the heat-affected zone is also worse than fiber laser welding joint’s structural uniformity. When using fiber laser for welding, there is a high energy density and rapid welding speed, and consequently a small heat-affected zone. In addition, the fiber laser can quickly melt weldments with a fast cooling speed, and as a result, the crystalline grain is much finer than CO2 laser welding. The structure of heat-affected zone is under the effect of welding thermal cycle, therefore the structure changes gradually as well. The heat-affected zone is in the middle of the molten pool zone and base metal zone. It can be seem from the above analysis that the welding joint’s hardness value drops obviously around the fusion line, and coarse columnar grain structure is in a relatively weak structure status, so the weld zone has a higher performance than the heat-affected zone. Seen from the higher magnification micrographs in molten pool zone, a certain number of small lath-shaped low carbon martensites exist in both types of laser welding joints, with acicular ferrites and Widmanstatten structures as well (see Fig. 13), which are produced under the condition of high temperature changes and continuous and rapid cooling of weld metals [26]. But for the fiber laser welding joint, the weld zone includes a large number of lath-shaped and acicular structures that are roughly parallel and even are in the same direction, this structure not only has higher strength, but also has some toughness, therefore the property of fiber laser welding joints is better than that of CO2 laser welding joints. 4. Conclusions Fiber laser welding and CO2 laser welding tests are respectively conducted for autobody steel. Following conclusions are drawn by means of systematical comparative study on the welding characteristics of the two types of lasers. (1) In this study, the cross-section of the overlap joint takes the shape of ‘‘Y’’ when using CO2 laser for welding, i.e. the upper surface has a larger weld fusion width than the lower surface; the cross-section of the overlap joint takes the shape of ‘‘I’’ with a relatively large inter-sheet joint width when using fiber laser for welding, i.e. the weld fusion width of the upper and lower surface is almost the same. In addition, the squamous ripple in the fractograph of fiber laser welding is much finer and more uniform. (2) When the space between the upper and lower sheet of overlap joints is large, the weld surface of both laser types has sinking, but the sinking is worse for fiber laser welding; the weld backside is relatively smooth for CO2 laser welding, while it sinks for fiber laser welding. In addition, the intersheet joint weld width L of fiber laser welding joint is larger than that of CO2 laser welding joint. (3) With the appropriate technological parameters, the tensileshear capacity is better when using fiber laser welding test pieces (compared with CO2 laser welding test pieces), and this mainly results from the larger joint width between the two sheets of fiber laser welding test piece joints. In addition, the hardness of fiber laser welding joint is slightly higher than that of CO2 laser welding joint without

exception, and the heat-affected zone of fiber laser welding joints is much narrower than that of CO2 laser welding joints, with much finer and more uniform crystalline grains.

Acknowledgment This research is supported by a science and technology project of Fujian Provincial Department of Education (No. JA11235), a research project of Xiamen University of Technology (No. YKJ10022R), two projects of National Natural Science Foundation of China (Nos. 51175165, 51105321 and 61104225), and the National Science and Technology Major Project of China (No. 2012ZX04003101). References [1] Chao L, Zheng AB. Material and joint technology of AUDI A6L white-body. Automot Access 2007;12:34–5. [2] Wu Q, Gong JK, Chen GY, Xu LY. Research on laser welding of vehicle body. Opt Laser Technol 2008;40:420–6. [3] 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 Process Technol 2005;164–165:1120–7. [4] Xi SY. Robust analysis and experiment of laser welding process parameters on overlap joint. Appl Laser 2008;28:297–300. [5] Chen W, Ackerson P, Molian P. CO2 laser welding of galvanized steel sheets using vent holes. Mater Des 2009;30:245–51. [6] Schmidt M, Otto A, Kägeler C. Analysis of YAG laser lap-welding of zinc coated steel sheets. CIRP Annals – Manuf Technol 2008;57:213–6. [7] Quintino L, Costa A, Miranda R, et al. Welding with high power fiber lasers – a preliminary study. Mater Des 2007;28:1231–7. [8] Hitoshi H, Takashi I, Shigeharu K, et al. Effect of shielding gas and laser wavelength in laser welding of magnesium alloy sheet. Quart J Jpn Weld Soc 2001;19:591–9. [9] Peng B. All-round performance of fiber laser applying in the entire processing industry. Laser Technol Appl 2008;2:5–7. [10] Mei LF, Chen GY, Jin XZ, et al. Study on fiber laser overlap welding of automobile aluminum alloy. Chinese J Lasers 2010;37:2091–7. [11] GB/T228.1-2010. Metallic materials-tensile testing – Part 1: Method of test at room temperature. Beijing: China Standard Press; 2011. [12] Wei W, Yao Y, Chen M. Research on welding formation and performance of lap welding joint of galvanized automobile steel steels. Automob Technol Mater 2009;23:12–7. [13] Tan CW, Li LQ, Chen YB, et al. Characteristics of fiber laser and CO2 laser welding of magnesium alloys. Chinese J Lasers 2011;38:1–7. [14] Zhang Y, Chen GY, Wei HY. A novel ‘‘sandwich’’ method for observation of the keyhole in deep penetration laser welding. Opt Lasers Eng 2008;46:133–9. [15] 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;40:433–42. [16] Li LJ. Modern laser processing and equipment. Beijing: Beijing Institute of Technology Press; 1993. [17] Zhang Y, Li SC, Jin XZ, Chen GY, Mei LF. Key technology of laser welding of galvanized steel. Laser Optoelectron Prog 2010;7:1–6. [18] Chen YB. Modern laser welding technology. Beijing: Science Press; 2005. [19] Zymanski ZS, Hoffman J, Kurzyna J. Oscillations of keyhole pressure and plasma radiation during CW CO2 laser welding. Proc. SPIE: Laser Technol 2000;4238:7–19. [20] Chen W, Ackerson P, Molian P. CO2 laser welding of galvanized steel sheets using vent holes. Mater Des 2009;30:245–51. [21] Huang HJ, Xi SY, Ding JJ. The research on laser welding process of zinc-coated steel of car body for improving the influence of Zinking layer. Appl Laser 2005;25:306–8. [22] Chen YB. Modern laser welding technology. Beijing: Science Press; 2005. [23] Chen GY, Mei LF, Zhang MJ, et al. Research on key influence factors of laser overlap welding of automobile body galvanized steel. Opt Laser Technol 2013;45:726–33. [24] Derek H, Li XG, Dong CF, et al. Fractography observing measuring and interpreting fracture surface topography. Beijing: Science Press; 2009. [25] Wu CS. The heat process of welding and the morphology of molten pool. Beijing: Machinery Industry Press; 2004. p. 192–3. [26] Shanghai Jiaotong University Writing Group. Metallographic analysis. Beijing: National Defense Industry Press; 1982.