Laser beam welding is a process where a focused laser beam is used as a moving heat source to join pieces of metal. The focused laser beam has a high power density that allows high speed welding, a deep penetration and a narrow heat affect zone (HAZ). There are two distinct types of laser welding modes: a conduction welding mode and a keyhole welding mode. When the laser beam intensity reaches 109 W/m2, the molten pool starts to evaporate. As the laser intensity increases above 1010 W/m2, the recoil pressure of the metal vapor pushes the molten metal downward and aside and a deep capillary called the “keyhole” is generated (Dawes, 1992). In the keyhole mode welding process, the keyhole maintains open due to the dynamic balance between the liquid metal surface tension and the pressure of the metal vapor and laser-induced plasma (Bakowski et al., 1984). When the laser radiates on the wall of the keyhole, the laser reflects multiple times on the wall of the keyhole. The laser beam energy is absorbed by Fresnel absorption directly by the walls of the keyhole, and a fraction of the laser energy is absorbed during each reflection (Dowden, 2009). Due to the multiple reflections of the laser beam, the keyhole behaves like an optical black body, making the keyhole mode welding process a highly energy efficient one (Steen, 2003).
Lap joint is the most common type of joint in the automotive assembly application; the traditional car body assembly method in a lap joint configuration uses resistance spot welding. However, the heavy and big spot guns limit the flexibility and accessibility of the welding process (Park et al., 2010); moreover, the localized joints are not particularly strong compared to those acquired by laser welding. On the other hand, the laser welding provides several benefits including a high scanning speed, high strength and low distortion of the joints, and the flexible implementation of the system for the automotive industry. Because of these advantages, laser welding shows immense potential over the conventional resistance spot welding and has been widely used in the automotive industry in the fabrication of different auto bodies parts (Forrest et al., 2004).
In order to reduce the weight of the vehicles and improve fuel efficiency and safety, the development of lightweight, and high-strength vehicles has prompted an increased use of advanced high strength steels (AHSS) in the automotive industry. These new steel grades include dual phase (DP) steels, transformation-induced plasticity (TRIP) steels, high hole expansion (HHE) steels, complex-phase (CP) steels, martensitic steels (MS), and twining induced plasticity (TWIP) steels (UltraLight Steel Auto Body- Advanced Vehicle Concepts, 2001). Additionally, these steels are galvanized in order to improve the surface corrosion resistance for automotive parts. However, it is still a great challenge to laser weld of galvanized steels in a zero-gap lap joint configuration. When laser welding of galvanized steels in a zero-gap lap-joint configuration, the zinc coating at the contact interface will vaporize; due to the lower boiling point (906 ºC) of zinc as compared to the melting temperature of steel (above 1500 ºC), the highly pressured zinc vapor expels the liquid metal out of the weld pool, resulting in blowholes and pores which dramatically decrease the mechanical properties of the weld (Akhter et al., 1988) (Fig. 1).
2. Review of the laser welding of galvanized steels in a lap joint configuration
Over the past several decades, industry and academic researchers have been seeking new technologies that will successfully join galvanized steels in a lap joint configuration. Many techniques were proposed, and some are listed in Table 1.
As mentioned above, all of these techniques have some drawbacks that limit their application in the industry. So far, there is no report on a cost-effective, efficient, and easy-to-apply welding procedure that is capable of joining galvanized high-strength steels in a zero-gap lap joint configuration without the material addition or without the assistance of a second heat source. In order to avoid highly pressurized zinc vapor caused weld defects like spatters and blowholes, either the zinc coating should be removed before the steel melts or the vaporized zinc needs to be vented out properly during the welding process. The most direct way of accomplishing this is to mitigate the presence of high pressure zinc vapor during the welding process. In order to achieve this goal several techniques are proposed which will be described in the following sections of this chapter.
3. Low power / low speed laser welding of galvanized steels in a zero-gap lap joint configuration
3.1. Experimental procedure
Ribic et al. (2009) concluded that the generated zinc vapor escapes from the weld pool if the solidification time is longer. A lower welding speed will generate an enlarged weld pool that will require a longer solidification time. An experimental work is presented to show that the effect of zinc vapor on the quality of a weld in a zero-gap lap joint configuration may be successfully mitigated. A fiber laser of 4 kW in power with a focused spot of 0.6 mm in diameter was used as the welding heat source, and a 6-axis high precision robot was used to implement the welding procedure of galvanized steels (see Fig. 2). Pure argon with a flow rate of 30 standard cubic feet per hour (SCFH) was employed as side shielding gas to suppress the laser-induced plasma and to protect the molten material against corrosion. The base material used in this work was galvanized high strength dual phase (DP) steel DP980, whose nominal chemical composition is listed in Table 2 (Burns, 2009). The coupons of galvanized DP980 steel sheets were 1.2 mm and 1.6 mm in thickness, with a zinc coating weight of about 60 g/m2.
3.2. Experimental results for low power / low speed laser welding
The top and bottom views of the weld obtained under a laser power of 2.0 kW and a welding speed of 5 mm/s show an acceptable weld surface quality (see in Fig. 3). A high-speed CCD camera with a frame rate of 4000 fps combined with a green laser with a band pass filter wavelength of 532 nm as the illumination source were used for real time monitoring of the dynamic behavior of the molten pool under different laser welding conditions. The weld pool formed under a relatively low welding speed was larger and relatively stable (see Fig.4a). On the other hand, the molten pool acquired under a higher welding speed shows sever fluctuation, and the high pressured zinc vapor generated at the faying surface jetted into the molten pool causing blowholes (see Fig. 4b). According to Ribic’s work, an enlarged weld pool has a longer solidification time which obviously decreased the probability that the zinc vapor would be trapped in the molten pool under a relatively low welding speed (around 5 mm/s), and a visually acceptable weld quality could be acquired. However, if the welding speed is exceedingly low, the sagging may be generated, which also reduces joint strength. Fig. 5 shows the tensile shear test results of the joints acquired under different welding speeds. A higher failure load was acquired under a lower welding speed. The trapped zinc vapor may result in pores inside the joints which could decrease the failure load with respect to the joints acquired under the same welding conditions but without zinc at the faying surface (see Fig. 5). Although an acceptable quality of joints could be achieved by this low power / low welding speed procedure, this procedure is not accepted by the industry because of a low productivity.
4. Two-pass laser welding of galvanized steels in a zero-gap lap joint configuration
4.1. Experimental procedure
In order to improve the production efficiency, the automotive industry requires a welding technique that can join of overlapped galvanized high strength steels successfully under a higher working speed. As discussed previously, if the zinc coating is removed before the steel starts to melt, a much higher welding speed can be achieved. Therefore, a two-pass laser welding process that is capable of successfully joining galvanized steel sheets in a zero-gap lap joint configuration is presented. Fig. 6 shows the schematic view of the two-pass laser welding process. The defocused laser beam shown in Fig. 6a is used to preheat the two overlapped sheets during the preheating pass. Only when the width of the area where the zinc coating is vaporized by preheating is larger than the distance between the zinc boiling isotherms (906 ºC), a sound weld could be acquired (see Fig. 6c). The laser power was set at its maximum value of 4.0 kW in order to allow a higher scanning speed. The laser welding speed was set at 60 mm/s in order to acquire a partial penetrated joint that was determined by the preliminary executed experimental trails. The preheating parameters, like the defocused position of the laser beam and the laser scanning speed, are critical in achieving a final weld quality. As shown in Table 3, three levels of defocused off-set distance (26 mm, 44 mm, and 62 mm which corresponds to the defocused diameters of laser beam of about 3 mm, 5 mm, and 7mm, respectively) and four levels of scanning speeds (20 mm/s, 30 mm/s, 40 mm/s, and 50 mm/s) were chosen to optimize the preheating procedure.
4.2. Experimental results for two-pass laser welding
As the defocused off-set distance increased, the defocused laser beam spot became larger, and the laser energy distribution was dispersed. A defocused off-set distance combined with a lower scanning speed generated too much energy that penetrated the top sheet resulting in spatters and permanent defects which could not be mitigated by the following laser welding pass (see Fig. 7a). A longer defocused off-set distance combined with a higher scanning speed could not vaporize a sufficient amount of zinc coating; the remaining zinc coating at the contact interface caused weld defects (see Fig. 7b). Thus, only for the optimized laser defocused off-set distance and the scanning speed, will a reasonable width of the zinc coating be vaporized (see Fig. 7c). Fig. 8 shows the experimental results for the selected preheating parameters (shown in Table. 3). The optimized preheating parameters that allowed a sound weld are shown in Fig. 8, area A.
Figs. 9 and 10 show the preheated interfaces of the coupons and the cross-sections corresponding to the different locations marked on the preheated interfaces. The zinc coatings far from the preheated zones at the top and bottom sheets are not affected (see Figs. 9b and 10b); the zinc coatings are melted and deformed at the edges of the preheated zone (see Figs. 9c and 10c); and the zinc coatings are vaporized at the center of the preheated zone (see Figs. 9d and 10d).
During the laser preheating pass, the defocused laser beam burns the zinc at the top surface, and melts and partially vaporizes the zinc coatings at the interface of the two overlapped steel sheets, and improves the absorption of the laser beam which results in the formation of a stable keyhole through which any zinc vapor formed at the interface will be vented out (Yang et al., 2009). Fig. 11 shows the top and bottom views of the weld obtained under preheating and welding with a laser power of 4.0 kW, a defocused off-set distance of 62 mm, a scanning speed of 30 mm/s, and a welding speed of 60 mm/s. Fig. 12 shows the weld cross-section of the weld shown in Fig. 11.
The tensile shear test was carried out in order to determine the mechanical strength of the welded joints obtained by the two-pass laser welding procedure. The experimental results demonstrated that the two-pass welded joints were broken in the HAZ of the bottom steel sheet. One of the tensile shear results is shown in Fig. 13. The tensile shear test for the welded coupons without a zinc coating at the interface was also performed. In order to use the data as a reference, the welded coupons without a zinc coating at the interface had a average failure load value of 5295.88 N which was lower than that of the two-pass welded coupons (6127.58 N). The reason for this difference in results is explained by the fact that the preheating process increased the laser beam absorption of the coupons, which contributed to a stronger (wider) partially penetrated weld joint.
5. Laser welding of galvanized steels in a lap joint configuration with a pressure wheel
5.1. Experimental procedure
Based on the experimental study performed, it was found that the stability of the laser welding process was sensitive to the clamping conditions. A relatively loose clamp condition resulted in a better weld than a very tight clamp condition. The gap ahead of the weld pool is the key to performing the laser welding of galvanized steel in a lap joint configuration successfully. Moeckel et al. (2003) developed a device for controlling the gap at the faying surface of the overlapped galvanized steel sheets in order to degas the generated zinc vapor during the welding process. The Fraunhofer Institute developed a pressure wheel system which could control the roller clamping force that allows for the controlling of the gap at the faying surface (Fraunhofer Institute website). In order to achieve an over lapped galvanized steel joint with a single laser beam without a pre- and/or post-weld process, a force-controllable pressure wheel (ZM YW50 PW P300 II) is used to control the gap near the laser focused spot during the laser welding. The laser welding of galvanized steels for a lap joint configuration with a pressure wheel control system is shown in Fig. 14. Fig. 15a shows the close-up of the pressure wheel set-up. The laser head is set-up under a 30 °decline with respect to the pressure wheel, and Fig. 15b shows the pressure wheel controller.
5.2. Experimental results for laser welding with a pressure wheel
The feasibility of welding galvanized steel sheets in a lap joint configuration by controlling the pressure wheel force during the fiber laser welding process is discussed. Fig. 16 shows the welds obtained by various levels of pressure wheel force under a laser power of 4.0 kW and a welding speed of 50 mm/s. The corresponding weld cross-sections are shown in Fig. 17. The cross-sections of the welds show that the weld beads are under the angle because the laser head is set-up with an angle of 30º with respect to the pressure wheel (see Fig. 15a). A sound weld was obtained by using a single laser beam with a force-controllable pressure wheel under the optimized force. As shown in Figs. 16 and 17, with an increased pressure wheel force, the weld quality decreased, and lots of spatters and blowholes were generated (see Fig. 16d). An increased pressure wheel force larger than 12N resulted in a decreased gap between the overlapped sheets near the laser focused area; the gap became too narrow to evacuate the generated high pressured zinc vapor. The jet of high pressured zinc vapor generated spatters and blowholes during the welding process.
Kong et al. (2012) reported that there is a correlation between the optical emission of the plasma and zinc vapor induced welding defects in the laser welding of galvanized steel for an overlapped joint configuration. Therefore, the spectroscopy was used to on-line monitor the laser welding of galvanized steel with a pressure wheel for an overlap joint configuration. The set-up to monitor the optical emission of the plasma in laser welding is shown in Fig. 18.
The emission line intensities detected from plasma during the laser welding of galvanized steel under various pressure wheel forces are shown in Fig. 19. The intensities of the emission lines above the weld pool were much lower when the pressure wheel force is larger than 12 N; while, the intensities of the emission lines were relatively higher when the pressure wheel force is set at 0.3, 6 or 12 N. The reason for this stems from the fact that when welding of galvanized steel under a higher pressure wheel force (18 N), the gap near the focal laser spot became too narrow to evacuate the high pressured zinc vapor; the zinc vapor caused spatters that disturbed the stability of the plasma which affected the intensity of the detected spectrum (Kong et al. 2012). The evolution of iron electron temperature within the laser-induced plasma along the weld bead length is shown in Fig. 20. The iron electron temperature was calculated by using the Boltzmann Plot method expressed by Equation (1) (Kong et al., 2012, Griem, 1997 and Marotta, 1994):
where Te is the plasma electron temperature, Em is the energy of the upper state, k is the Boltzmann constant, Im is the emission line relative intensity, Am is the transition probability, gm is the statistical weight, and λm is the wavelength.
As shown in Fig. 20, under lower pressure wheel forces (0.3, 6, and 12 N), the electron temperature showed lower intensity and less fluctuation compared to a higher pressure wheel force (18 N). The presence of the zinc vapor induced spatters in the plasma which increased the iron electron concentration which, in turn, increased the iron electron temperature value (Kong et al., 2012).
Thus, there is a correlation between the optical emission of the plasma and zinc vapor induced welding defects during the laser welding; and this optical signal could be further used as feedback for the closed-loop control of the laser welding of galvanized steel with a pressure wheel, which is shown in Fig. 21.
In this chapter, issues related to the laser welding of galvanized steels in a zero-gap lap joint configuration are discussed. The authors’ recent research results on the laser welding of galvanized steel in a lap joint configuration are reviewed. The different welding procedures, namely, low power / low speed welding, two-pass laser welding and laser welding with a pressure wheel are introduced for the laser welding of galvanized steels in a lap joint configuration. It was found that acceptable weld quality could be achieved by a low power / low welding speed procedure; however, the relatively low welding speed limits its application in the industrial environment. A high quality weld could be obtained by introducing a preheating pass with a defocused laser beam. By using the optical signal acquired during the laser welding process as a feedback signal in the pressure wheel force control, it is possible that a controllable clamping force could be a solution for achieving a good weld quality without using a pre- and post- welding procedure.
This work was financially supported by the NSF’s Grant No. IIP-1034652.