Comparative study on CO2 laser overlap welding and resistance spot welding for galvanized steel

Comparative study on CO2 laser overlap welding and resistance spot welding for galvanized steel

Materials and Design 40 (2012) 433–442 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/lo...

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Materials and Design 40 (2012) 433–442

Contents lists available at SciVerse ScienceDirect

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

Comparative study on CO2 laser overlap welding and resistance spot welding for galvanized steel Mei Lifang a,⇑, Yi Jiming a, Yan Dongbing a, Liu Jinwu a, Chen Genyu b 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 18 January 2012 Accepted 7 April 2012 Available online 17 April 2012 Keywords: Welding Galvanized steel Mechanical properties

a b s t r a c t The CO2 laser overlap welding and the resistance spot welding are respectively investigated on DC56D galvanized steel used for auto body. The characteristics of the two types of welding methods are systematically analyzed in terms of the weld molding, tensile-shear performance, microstructure, hardness, and corrosion resistance of welding joint. The results show that, the fusion widths of the upper and lower surface are almost the same for the resistance welding joint, and the weld nugget is surrounded by the heataffected zone. While the laser welding joint belongs to deep penetration welding, the weld fusion width presents wide at the top and narrow at the bottom, and the heat-affected zone is situated on both sides of the weld pool. Compared with resistance spot welding joint, laser welding joints have much more ultrafine microstructures, much smaller heat-affected zones, as well as greater resistance to deformation and corrosion. In addition, the tensile-shear performance of laser weld joints is superior to that of resistance welding joints under certain conditions. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, galvanized steel sheets with excellent corrosion resistance have been widely used in automobile industry, replacing cold rolled steel sheets as auto body materials to improve the corrosion resistance and overall service life of vehicles [1,2]. Common auto body sheets mainly adopted resistance spot welding technology for good welding quality. When welding galvanized steel sheets, it is required to use greater welding current and electrode pressure as well as longer welding time (approximately increase by 1/3 or more) due to the shunting effect of zinc coating compared with welding of common steel sheets with the same thickness, and this means electrodes need to withstand more pressure and higher temperature in the process of welding, resulting in a sharp decline of electrode service life [3,4]. In addition, the electrode is easily alloyed with the zinc coating on its working face, which defiling the electrode and exacerbating its failure [5,6]. Especially for twosided galvanized steel sheets, if resistance spot welding is used, zinc could increase the resistance and enlarge the welding spot, bringing follow-up treatment difficulties. The following difficulties may also turn up for resistance spot welding of galvanized steel sheets: it is prone to have cracks, pores and soft tissues in the internal areas of the welding spot; it is easy to produce welding spatters. The difficulty of resistance spot welding for galvanized steel

⇑ Corresponding author. Tel./fax: +86 592 6291385. E-mail addresses: [email protected], [email protected] (L.F. Mei). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.04.014

sheets rests with narrow or rigor process specifications in terms of the welding production line [7]. Furthermore, due to the large weld width and heat input during resistance spot welding, a great deal of zinc coating vaporization and burning loss reduced the corrosion resistance of galvanized steel weld. Compared with resistance spot welding, laser beam has extremely high heating capability, less unit heat input, little heat distortion, high depth-to-width ratio of weld, high welding speed, narrow heat-affected zone, and low zinc loss, and so on. In addition, laser welding belongs to unilateral processing, and therefore it has good adaptability to complex structures and is easy to achieve remote welding and automation [8–10]. Furthermore, laser welding can help realize lightweight automobiles, improve the strength, stiffness, assembly accuracy and safety performance of auto body, and reduce the manufacturing cost. For this reason, the traditional welding method is gradually replaced by laser welding technology in the automobile industry of the developed countries to start a new welding era [11]. However, when welding galvanized steel with laser, drawbacks such as vaporization of zinc coating, welding pores, spatters, poor fusion and cracks also exist. In the existing literature, resistance spot welding and laser welding for galvanized steel are all separately studied, and most of the literature is only limited to the welding process, difficulties, applications and development advantages of one welding technology [12–16]. Some literature puts particular stress on resistance spot welding of galvanized steel, while some other literature is more optimistic about the application of laser welding. No literature has systematically analyzed and reported the differences between

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resistance spot welding and laser welding for auto body galvanized steel in respect of principle, weld molding, joint mechanical properties, joint microstructure appearance, and corrosion resistance. This brings certain difficulties to technical personnel or researchers on how to correctly select the welding method based on the existing conditions and requirements. In this case, this paper has analyzed their respective welding performance features by systematically studying the differences of resistance spot welding and laser welding technologies in galvanized steel, and will provide a theoretical reference for selection of this welding method under different purposes and requirements.

2. Welding principles Resistance spot welding is a welding method that using electrode to exert some pressure on the workpiece and meanwhile power on it, heating part of the welding metal to a plastic or molten state by using the Joule heat generated from the contact resistance between the electrodes, and then forming the welding joint under pressure [17]. Good metal plasticity is required for resistance spot welding. When welding, the weldment surface must first be cleaned, and then the overlap joints of the sheets to be welded should be correctly assembled and placed between two cylindrical copper electrodes and clamped with pressure. When enough current passes through, a large amount of resistance heat will be produced at the joint of two sheets, and then the metal in the central region of two electrodes will be quickly heated to a high plastic or molten state, forming a lenticular liquid weld pool. When disconnecting the current and continuing to keep the pressure, the metal will be cooled and solidified to form a welding spot, as shown in Fig. 1. Laser welding is a welding method of using the principle of atomic stimulated radiation, stimulating the operation materials to produce a highly monochromatic, directional and bright beam, gathering the beam to the focal point through focusing lens to obtain an extremely high energy density, and then using the energy density to interact with the workpiece to result in metal vaporization, fusion, crystallization and solidification and finally form weld. When the power density on the laser spot is large enough (>106 W/ cm2), the metal will be quickly melted under laser irradiation, its surface temperature will rise to the boiling point in a very short period of time, and metal vaporization occurs [18]. Then the metal vapor leaves the metal pool surface at a certain speed, and produces an additional stress to react with the molten metal, make it sag down and form a small pit under the spot. Following the heating process, the laser can be directly radiated to the pit bottom, forming a small elongated hole. The hole moves forward along

Fig. 1. Resistance spot welding principle.

with the beam in the welding direction relative to the workpiece. The metal melts around the hole to form a weld pool, and the weld pool re-solidifies to form the weld, as shown in Fig. 2. 3. Experimental details 3.1. Experimental materials and equipments Both resistance spot welding and laser welding use the following type of auto body sheet as the test material: DC56D + Z (ultralow carbon and high-strength galvanized steel), with the sheet thickness of 1.2 mm. The chemical compositions and mechanical properties are shown in Table 1. First, the sheets are cut to test samples with specification of 100 mm  30 mm using CO2 laser. The sheet surface is cleaned with a cotton ball dipped in acetone before welding to remove oil stains on the surface. The DN3-160 suspension spot welding machine is used for resistance spot welding test, as shown in Fig. 3. This machine has power supply voltage of 380 V, rated load power of 160 kVA, rated primary current of 410 A and rated welding thickness of 4 mm. When welding, the electrode is directly connected with the workpiece to be welded, and meanwhile certain pressure is required to be exerted on the workpiece to make it powered on. After welding is completed, the welding quality will be checked to avoid welding defects such as impervious welding, too deep indentation and burr. The test equipment used for laser welding includes a DC025 slab CO2 laser device, a three-dimensional and five-axis laser processing machine, etc., as shown in Fig. 4. The maximum power output of the laser device is 2.5 kW (continuous or pulse output with wavelength of 10.6 lm and output mode of TEM00). The CO2 laser device uses lens group way as its optical transmission system, with the reflector for steering and transmission; it is reflection-type focusing, with focal spot diameter of 0.4 mm. The laser beam is incident to the upper sheet surface vertically, forming continuous weld in the overlap area along with the movement of the laser beam when using appropriate welding parameters. 3.2. Experimental methods 3.2.1. Joint type Based on welding characteristics of auto body parts, two DC56D sheets are clamped by a clamping fixture for overlap welding test. The types of welding joint under different welding methods (i.e. resistance spot welding and laser overlap welding) are shown in Fig. 5. 3.2.2. Technological parameter design Since there are big differences in welding source and processing equipment between resistance spot welding and laser welding, different welding technologies and parameters shall be used respectively for these two welding methods. The automobile with DC56D galvanized steel sheets used has been put into mass pro-

Fig. 2. Laser welding principle.

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LF. Mei et al. / Materials and Design 40 (2012) 433–442 Table 1 Chemical compositions and mechanical properties of test sheet (mass fraction,%). Material mark

Material thickness (mm)

Yield strength (Mpa)

Tensile strength (Mpa)

C (6)

Si (6)

Mn (6)

P (6)

S (6)

Other (6)

DC56D + Z 45/45-FD-O

1.2

120–180

270–350

0.01

0.01

0.30

0.025

0.020

0.215

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

Fig. 3. Resistance spot welding equipment.

Fig. 4. Three-dimensional laser processing equipment.

Fig. 5. Schematic of resistance welding and laser welding.

duction in an auto company, the resistance welding technological parameters for these auto body material have already been standardized. In this case, these standardized parameters will be directly used for the resistance spot welding test in this paper. In the laser welding test, an orthogonal parameter table has been mapped out, the orthogonal test has been conducted, and the optimal technological parameters have been obtained. This part of work has already been completed in the previous research work [19]. Therefore the process optimization of laser welding is no longer studied in this paper, and appropriate welding parameters can be given directly, as shown in Table 2. Finally, comparative study on the welding performance of specimens is carried out based on the respective optimal technological parameters of the two welding methods.

3.2.3. Treatment and analysis methods The stereo microscope is used to observe the weld joint appearance and cross section shape. The weld is processed to the standard tensile-shear specimen by the wire cut electro discharge machine. In addition, the tensile-shear strength of the welding specimen is analyzed by using the computer-controlled electronic universal testing machine to study the bearing capacity of joints. After welding specimens are sampled using wire cutting, polished, corroded by the nital, cleaned and dried, and prepared as metallographic specimen, and then the weld joint structures and cross-sectional shapes are observed through the optical microscopy. Then, the weld cross-sectional hardness distribution is measured with the electronic micro-hardness tester. Finally, the salt spray test is carried out to analyze the corrosion resistance of welding specimens.

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Table 2 Welding technological parameters. Combination

Welding method

Welding technological parameters

1.2 mmDC56D + 1.2 mmDC56D

Resistance spot welding

Welding current (kA) 9.62 Welding power (kW) 2.0

Laser welding

Welding time (Cycle: cyc) 12 Welding speed (m/ min) 1.7

4. Results and discussion 4.1. Weld appearance Resistance spot welding and laser welding tests are respectively conducted for DC56D galvanized steel, and the surface appearances of the welding specimen joints are shown in Figs. 6 and 7. As shown in Fig. 6, there is no defect such as cracks, burning loss or spatters in the weld zone surface of the resistance spot welding joint. The nugget surface is slightly depressed, but the pit is shallow, which still meets the quality standards. As shown in Fig. 7, the laser weld surface is balanced, smooth and continuous, without obvious defects such as pores, cracks and spatters, and the weld joints have little deformation, taking the shape of delicate scaly ripples. For resistance spot welding, the welding spot area is determined by both the welding torch electrode size and welding parameters, while for deep penetration laser welding, the weld area depends on both weld fusion width and weld length, of which the weld fusion width is mainly determined by the laser beam focusing spot size and welding parameters, and the weld length can be set in the welding procedure. As shown in Figs. 8 and 9, under the appropriate technological parameters, the surface and back appearances of resistance welding spot and laser weld joint are observed with a stereo microscope. It shows that the surface and back fusion widths are almost the same for resistance welding spot. When welding, two electrodes simultaneously press the two sheets and power them on for welding, the upper and lower welding torch electrode chucks have the same shape and size, therefore the upper and lower weld nuggets have almost the same size. While, the weld back fusion width is narrower than the surface fusion width for laser welding. Because during laser welding, the laser beam is directly incident to the upper sheet surface, the upper sheet quickly melts down and even vaporizes after absorbing a great deal of heat, then transfers the heat to the lower sheet through molten materials or the welding ‘‘hole’’, there is a great speed for moving forward in the heating process, and the lower sheet could absorb part of the heat before the heat source moves away. In addition, when using CO2 laser for welding, the plasma’s refraction, scattering and absorption for the incident laser weaken the energy density and penetrability of the laser beam [20]. Thus making the cross section of the weld joint wide at the top and narrow at the bottom.

Operating pressure (Mpa) 0.4 Defocusing amount (mm) 0.4

Pressure time (Cycle: cyc) 10 Shielding gas flow (L/ min) 15

Electrode size (mm) 7.91 Inter-sheet space (mm) 0.2

The cross sections for resistance spot welding and laser welding joints are also observed and analyzed with a stereo microscope, and the cross-sectional shapes with the two welding methods are shown in Fig. 10. It can be clearly seen that the welding joints are without exception composed of heat-affected zone and weld nugget zone. For resistance spot welding joints, the heat-affected zone is located around the nugget. While for laser welding joint, the upper and lower sheets are completely penetrated, and the heat-affected zone is located on both sides of the nugget zone. In this test, the average nugget diameter is 6.42 mm and the weld penetration depth is 2.17 mm for resistance spot welding; but the surface and back fusion widths are respectively 1.5 mm and 0.6 mm for laser weld and the weld penetration depth is the sum of thickness and space of two sheets. 4.2. Weld mechanical property The tensile-shear test is respectively performed on the resistance spot welding and laser welding specimens so as to compare their respective mechanical property characteristics under the two welding methods. If a fracture occurred in the base metal zone of the specimen in the tensile-shear process, the tensileshear load bearing ability of the weld joint will not be accurately measured since this test only aims to compare the tensile-shear capacity of resistance spot welding and laser welding joints. In this case, standard and non-standard tensile-shear test samples are respectively made, as shown in Figs. 11 and 12. The base metal width is enlarged for the non-standard test sample, which enables the base metal to bear greater forces during tensile-shear, and consequently avoids fracture in the base metal. If a fracture occurred in the base metal when using the standard test sample for tensile-shear test, then the non-standard test sample will be used for analysis. The tensile-shear test for standard and non-standard specimens is carried out by using the computer-controlled electronic universal testing machine, with the respective tensile-shear speed of 1.0 mm/min and 3 mm/min and the chuck spacing of 60 mm for the non-standard specimen, and the mechanical property data is output through computer. The test results for standard and non-standard tensile-shear specimens are shown in Figs. 11 and 12, from which we can see that the tensile-shear strength of

Fig. 6. Appearance for resistance spot welding joint.

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Fig. 7. Appearance for laser overlap welding joint.

1mm

(a) Surface

1mm

(b) Back

Fig. 8. Micro-appearance for resistance spot welding joint.

1mm

1mm

(a) Surface

(b) Back

Fig. 9. Micro-appearance for laser welding joint.

1mm

Resistance spot welding

1mm

Laser welding

Fig. 10. Cross-sectional shape of welding joint.

the non-standard tensile-shear specimen before the specimen joint loses efficacy is greater than the maximum tensile-shear

strength of the corresponding standard tensile-shear specimen. The maximum tensile-shear strengths of the standard tensile-

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Fig. 11. Standard and non-standard tensile-shear specimen of laser welding joint.

Fig. 12. Standard and non-standard tensile-shear specimen of resistance spot welding joint.

shear specimens before fracturing are similar under the two welding methods, and the fractures are both located in the base metal zone that is far away from the weld, as shown in Fig. 13. The tensile-shear strength here obtained for specimens is not the maximum tensile-shear load bearing ability of the welding joint, but only indicates that the tensile-shear strength of the welding joint is greater than that of the base metal under this welding condition. Therefore, non-standard tensile-shear specimens are taken as the analysis objects when comparing the mechanical properties of welding specimens under the two welding methods. As shown in Fig. 14, under the effect of certain tensile-shear load, the fracture for the non-standard resistance spot welding tensile-shear specimen is located in the heat-affected zone of the welding joint, while the fracture for the non-standard laser welding tensile-shear specimen is located in the base metal zone that is far away from the weld. The maximum tensile-shear load bearing capability of the resistance spot welding specimen is the tensile-shear test results. While the tensile-shear test results for laser welding specimen only indicate that the bearing capacity of the welding joint is greater than that of the base metal, but cannot reflect the welding joint’s ultimate tensile-shear capacity. It can be seen from the measuring results for non-standard tensile-shear specimens in Table 3: the tensile-shear bearing capacity of a deep penetration laser welding specimen with surface fusion width of 1.5 mm and weld length of 30 mm is far greater than that of a resistance welding spot with average nugget diameter of 6.42 mm and weld penetration depth of 2.17 mm. It can also be further predicted that the tensile-shear performance of the deep penetration laser welding specimen is related to the weld fusion width and weld length, while the bearing capacity of the resistance welding spot mainly depends on the nugget size and weld penetration depth.

4.3. Weld microstructure Metallographic test samples are prepared through sampling, mounting, grinding and polishing, and then the microstructure is analyzed by using metallurgical microscope. The crystalline grain size and grain boundary of the weld zone, heat-affected zone and base metal zone, the structural patterns of these zones, as well as the cross-sectional shape of the weld joint are observed, defects such as micro-pores, micro-cracks, dry joint are checked, and the weld penetration depth and fusion width are measured. The cross-sectional of weld joint observed with metalloscope for resistance spot welding and laser welding specimens are shown in Figs. 15 and 16, which is consistent with the shapes observed with stereo microscope; however, the weld junction between the weld zone and the heat-affected zone can be clearly seen from the above two figures. The structural pattern differences between the nugget zone and the heat-affected zone of resistance spot welding and laser welding are analyzed through microstructure enlarged 32–200 times. Metallographic microstructures of resistance spot welding and laser welding joints are respectively shown in Figs. 17 and 18. As shown in Figs. 17 and 18, the weld structure for laser welding joints is relatively fine and uniform, without defects such as pores and cracks. The tiny isometric crystal zone and columnar crystal zone in the center are formed mainly because of large degree of supercooling in the weld central part during welding, high nucleation rate, much crystalline nucleation, and grain refinement after crystallization. The formation of columnar crystal zone is mainly related to temperature gradient. When crystallizing, there has a high degree of supercooling, and at the same time as great base metal thermal conductivity, there has large temperature gradient between the base metal and the weld pool, the crystalline grain forms directionality during crystallization [18]. In addition,

Fig. 13. Fracture location of standard tensile-shear specimen.

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Fig. 14. Fracture location of non-standard tensile-shear specimen.

Table 3 Tensile-shear performance of welding specimens. Combination

Welding method

Maximum tensile-shear strength of standard specimen (kN)

Maximum tensile-shear strength of non-standard specimen (kN)

1.2 mmDC56D + 1.2 mmDC56D

Resistance spot welding Laser welding

3.541

5.302

3.537

9.989

Fig. 15. Cross-section of resistance spot welding joint (32  ).

Compared with laser welding joint, the resistance spot welding joint has much coarser, looser and more uneven weld zone structure, irregular shape of crystalline grain can easily result in conspicuous stress concentration, and the crystalline grain in the heat-affected zone is also much coarser. According to ferro carbon equilibrium diagram, the transition (a ? c) temperature of ultra-low carbon steel is 723–911 °C (depending on the free carbon content [21]), which is much higher than that of the traditional carbon steel. In the area next to the weld nugget, the crystalline grain grows up obviously under the effect of a high temperature of over 800 °C. Coarse crystalline grain of resistance spot welding joint is shown in Fig. 17. Coarse elongated grain structure is located nearby the heat-affected zone. According to the Hall–Petch formula, coarse grain structure has lower strength than base metal of fine crystalline grain, thus making this zone as the weak phase. When bearing mechanical load, the welding spot will crack nearby the heat-affected zone under the effect of low stress. The phenomenon of crystalline grain growth will occur as well in the heat-affected zone for laser welding of DC56D galvanized steel sheet. However, due to the great heat input during resistance spot welding, and he time of cooling around the weld junction is extended. Therefore, the crystalline grain in the weld and heat-affected zones of resistance spot welding joint is coarser than that of laser welding joint. 4.4. Weld hardness

Fig. 16. Cross-section of laser welding joint.

there are fine ferrolites and high dislocation density in the weld zone of laser welding joint, and the joint strength and toughness are improved under the comprehensive effects of fine-grain strengthening and dislocation strengthening.

The digital and intelligent micro-hardness tester is used to test the micro-hardness of the welding specimen joint. The microhardness of various parts such as weld zone, heat-affected zone and base metal zone is tested (breadthwise) respectively starting from the weld center (load 4.9 N, holding time 15 s). The hardness value of each zone is obtained by testing 5 points in different locations of each zone and then averaging the values of these points. In addition, 6 points in the weld cross section are tested starting from the position that is 0.1 mm from the upper sheet surface and moving along the vertical direction at an interval of 0.4 mm to get the hardness value of each point. The hardness test locations are shown in Fig. 19.

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Fig. 17. Microstructure of resistance spot welding joint.

Fig. 18. Microstructure of laser welding joint.

The micro-hardness distribution is analyzed for laser welding and resistance spot welding specimen joints in order to study the micro-hardness distribution regularities of welding joints. Microhardness measurements are conducted in the vertical and lateral zones of resistance spot welding and laser overlap welding specimen joints, and the test results are shown in Fig. 20. As shown in the figure, for DC56D galvanized steel sheets, the micro-hardness values for the weld zone and heat-affected zone of the laser welding joint are both slightly greater than that of resistance spot welding. In addition, the micro-hardness values for the weld zone and heat-affected zone of the laser welding and resistance spot welding joints are both greater than that of the base metal zone. Within the vertical area of the resistance spot welding joint, the hardness for locations A and F is reduced significantly since they are located in the heat-affected zone. During resistance spot welding, due to

the great heat input, extended time of cooling around the weld junction, high weld temperature and long residence time at high temperature, the cooling speed slows down, quench hardening tendency reduces, and then both the hardness and tensile-shear strength drop accordingly. While for laser welding, both the welding speed and weld cooling speed are quite high, the microstructure is fine and uniform, and therefore the weld hardness is higher. 4.5. Weld corrosion resistance Neutral salt spray test is used to detect and analyze the corrosion resistance of resistance spot welding and laser welding specimens, and corrosion resistance is assessed based on the time duration for the salt spray test as well as appearance changes of the welding specimens. In the test, the mass concentration of NaCl

Fig. 19. Hardness test diagram.

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Fig. 20. Analysis on welding joint hardness.

Before corrosion

48h

24h

120h

144h

120h

144h

Laser welding specimen

Before corrosion

24h

48h

Resistance spot welding specimen Fig. 21. Appearances of welding specimens after corrosion.

is 5%, the PH value is 6.6, the temperature of the test chamber is set as 35 °C, the saturator temperature is 47 °C, and a continuous spray mode is used, with spray pressure of 0.09 MPa. The test samples need to be thoroughly cleaned with clean soft brush and all-round clean water, and then dried before putting into the salt spray test chamber for testing (test samples placed at an angle of 45°). The corrosion resistance of resistance spot welding and laser overlap welding specimen joints is analyzed. Appearance changes of these specimens in the salt spray test buckets during different corrosion time are shown in Fig. 21. Fig. 21 shows that after 24 h a certain amount of white rust emerges at the surface of both resistance spot welding and laser welding specimens of DC56D steel, while yellow pitting emerges at the edge of the resistance welding spot; after 48 h there is no rust at the laser weld zone, while a small amount of rust spots emerge at the zinc burnout zone nearby, and a small area of yellow rust begins to emerge at the base metal of the specimen. But, a large area of light yellow rust emerges at the base metal of the resistance spot welding specimen, and red corrosion spots begin to emerge at the edge of welding spots; after 120 h a small area of red block rust emerges at the base metal of the laser welding specimen and the heat affected zone, while a large area of red rust emerges at both the base metal and the welding spot of the resistance spot welding specimen; after 144 h the surface of the base metal of the laser welding specimen is covered with light yellow corrosion spots, and a large area of red rust emerges locally, and part of the weld surface is also covered

with a layer of corrosion products, whereas they are not produced by the weld, but by leaching of base metal corrosion products at the adjacent areas or by that the rust liquid stays at and covers their surfaces after dropping. While there is a large area of red rust at the base metal of the resistance spot welding specimen, and a large number of brown corrosion products is formed at the welding spots and the surrounding heat affected zones. Therefore, for DC56D galvanized sheets used for auto body, the corrosion resistance of laser welding joints is better than that of resistance spot welding joints. That is mainly due to the surface of laser weld exists a uniform gray oxide film, which prevents further oxidation and corrosion of the weld. In addition, from the corrosion electrochemistry point of view, because galvanized steel sheets are used as a wide range of zinc anode, corrosion generally does not occur at the narrow laser weld, so the laser welding has little impact on the corrosion resistance of galvanized sheets. It can also be seen from the figure that, the heat-affected zone on both sides of the weld has a weak corrosion resistance for laser welding specimens. During laser welding, zinc around the weld pool could easily evaporate at high temperature, resulting in a zinc burning zone on both sides of the laser weld. On the other hand, molten materials in the weld pool spatter under the effects of metal vapor expansion, resulting in metal spatters filled up on both sides of the laser weld (shown as burn marks). Therefore, coating burning and spatter burn marks on both sides of the weld are main reasons influencing the corrosion resistance.

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5. Conclusions

References

Resistance spot welding and laser welding tests are respectively conducted for DC56D galvanized steel. Following conclusions are drawn by means of systematical comparative study on the characteristics of the two welding methods.

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(1) The nugget surface of the resistance spot welding joint is slightly depressed, the nugget surface fusion width and back fusion width are almost the same, and the nugget zone is surrounded by the heat-affected zone. While the laser welding joint surface is uniform, smooth and continuous, the back fusion width of the weld is narrower than the surface fusion width, the upper and lower sheets are completely penetrated, and the heat-affected zone is located on both sides of the weld zone. (2) The tensile-shear test results of resistance spot welding and laser welding specimens show that, the tensile-shear bearing capacity of a deep penetration laser weld is far greater than that of a resistance welding spot under certain condition. The tensile-shear performance of deep-penetration laser welding specimens relates to the weld length and fusion width, while the bearing capacity of the resistance welding joint mainly depends on the nugget size and weld penetration depth. (3) Compared with resistance spot welding specimen, the weld joint for laser welding is more even; the weld zone and heataffected zone have much finer and more compact microstructure; and the hardness is higher as well; in addition, the corrosion resistance of the weld joint is also better.

Acknowledgments 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), a project of National Natural Science Foundation of China (No. 50575070), and a state key project (No. 2007AA042006).