Materials Transactions, Vol. 51, No. 5 (2010) pp. 1044 to 1050 #2010 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Microstructure and Mechanical Properties of Friction Stir Spot Welded Galvanized Steel Seung-Wook Baek1; *1 , Don-Hyun Choi1 , Chang-Yong Lee2 , Byung-Wook Ahn1 , Yun-Mo Yeon3 , Keun Song3 and Seung-Boo Jung1; *2 1 School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Gyeonggi-do 440-746, Korea 2 Automotive Applications Research Team, Technical Research Center, Hyundai Steel, 167-32 Kodae-Ri, Songak-Myon, Dangjin, Chungchungnam-do 343-823, Korea 3 Department of Advanced Materials Application, Suwon Science College 9-10, Botong-li, Jeongnam-myeon, Hwasung, Gyeonggi-do 445-742, Korea
Joints of galvanized steel were obtained by friction stir spot welding (FSSW) with lap configuration using CPS design tool. No mechanically mixed layer was formed between the top and bottom plates at the weld nugget due to the limited tool penetration and the lower pin height of the welding tool than the steel plate thickness. A deformed region, in which ZnO particles were detected, was observed in the joint. The formation of this deformed region was attributed to the explosion of the Zn coating layer due to friction heating and tool compression. With increasing tool penetration depth, the tensile shear strength of the joint increased to a maximum value of 3.07 kN at a tool penetration depth of 0.52 mm. [doi:10.2320/matertrans.M2009337] (Received October 5, 2009; Accepted March 4, 2010; Published April 25, 2010) Keywords: friction stir spot welding, galvanized steel, WC-Co alloy tool, explosion, tensile shear strength
1.
Introduction
Friction stir spot welding (FSSW) is a new process that has recently received considerable attention from the automotive and other industries.1) A novel variant of the friction stir welding (FSW) process, FSSW creates a spot, lap-joint without material melting. The appearance of the resulting joint resembles that of an electric resistance spot weld commonly used for auto-body assembly. The solid-state bonding and other features of the process make it inherently attractive for body assembly and other similar applications. The primary welding process for auto body structure assembly—the electric resistance spot welding (ERSW) process—can be problematic for many new, high performance, structural materials such as Al and Mg alloys, advanced high-strength steels.2,3) So far, the majority of the research and development efforts on FSW have been on aluminum alloys.4–12) Because Al alloys are easy to deform at relatively low temperatures (below about 550 C), they undergo FSW easily. Nevertheless, steel remains the primary material for body structures of all high-volume, mass-produced cars. The strong, customer-driven emphasis on safety and corrosion protection has been driving the increased use of galvanized steel in automobile body construction. However, the welding of galvanized steel presents some unique technical challenges to both the steel suppliers and the auto end-users due to the marked differences between welding galvanized steels and welding the bar or uncoated versions of the same steel products. The different physical and electrical properties of the coating, compare to those of low-carbon steel, lead to its quite different weld formation and welding behavior. *1Graduate
Student, Sungkyunkwan University author, E-mail:
[email protected]
*2Corresponding
The plating of the coating onto the copper alloy electrodes, in addition to its alloying with these electrodes, greatly accelerates the electrode wear.13–16) Recently, Research about FSSW of steel alloys is increased and shown successful result including low carbon steel,17,18) DP600,19) DP80020) and M-190,21) an effort was commenced to evaluate this joining process for more problematic steel alloys. However, these steel alloys had uncoated surface and FSSW of galvanized steel has not been examined. Also, in general, a tool with a threaded pin is used for severe plastic deformation in FSSW. In a previous report,22) Y. Tozaki et al. indicated that the length of threaded probe strongly affected on the strength of FSSW joints. However, there have been reports of the threaded pin tool causing a decrease in the joint’s volume and upper plate’s thickness, owing to the deep tool penetration.23,24) Moreover, using the conventional tool, pin hole inevitably remains at the centre of the weld nugget. It is believed that corrosion could take place preferentially at the pin hole because rainwater remains in the hole, where body paint barely reaches the bottom. And Uematsu et al. reported that shear strength and fatigue strength of joint were improved using the re-filling process due to wide joint area and pin hole was not remained after welding.25) Considering these results, it seems that reliability of the joint will be increased by reducing of the pin hole depth and enlarge the joint area. The present study applies FSSW to galvanized steel plate using WC-Co alloy tool as CPS (Cylindrical Pin with Shoulder) design with the following two goals. The first is to investigate the influence of one parameter—tool penetration depth—on the mechanical shear strength and failure mode of the welded joints. We chose to vary tool penetration depth because it has been previously suggested that this has an important effect on joint properties. The variation of the tool penetration depth also results in the variation
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of the depth with which the rotating tool shoulder presses onto the top sheet, thereby affecting that region’s microstructure. The second goal of this work is to analyze in detail the microstructure of a representative joint (0.52 mm insertion depth), in order to explain the mechanisms of this new joining process on a more fundamental and general level. 2.
Experimental Procedure
The base material used for welding in this study was a 0.6 mm-thick steel plate with 0.025 mm-thick zinc coat. The chemical composition was Fe-0.002C-0.005Si-0.127Mn0.0099P-0.008S (in mass%), with an ultimate tensile strength of 314 MPa and elongation of 43%. The shear joints were made in a lap configuration, with each plate being 30 mm wide by 100 mm long. Two sheets were overlapped by 30 mm, and then affixed into the welding instrument using a fixture in preparation for joining. The WC-Co alloy CPS tool was shown in Fig. 1. Tool had a shoulder diameter of 13.5 mm and pin diameter of 8.5 mm and height of the pin was 0.5 mm. FSSW was performed at a tool rotation speed of 1600 rpm and over a range of tool penetration depth of 0:240:52 mm. Temperature profile during FSSW was measured using the thermo graphic camera. Optical microscopy (OM), Hitachi S-3000H scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and JEOL JXA-8500F electron probe micro analyzer (EPMA) observations were carried out for joint. The metallurgical inspections were performed on a cross-section of the joint after polishing and etching with a nital solution.
Fig. 1
WC-Co alloy tool with CPS design used in this study.
Detailed microstructural observations were performed by JEOL 300 kV transmission electron microscopy (TEM). The TEM samples were prepared by electrolytically polishing with a 10% nitric acid + 90% ethanol solution. Temperature profile during FSSW was measured using the thermo graphic camera. The mechanical properties of the spot welds were characterized using tensile shear test. Tensile shear test was conducted at room temperature at a crosshead speed of 1 mmmin 1 . 3.
Result and Discussion
Macroscopic overviews of the cross-section of each joint are shown in Fig. 2. In all samples, the top and bottom steel sheets were compressed together to form a joint interface. The gap at the joint edge region was decreased with increasing tool penetration depth, suggesting that the
Fig. 2 Optical macro cross-sectional images at the following tool penetration depths: (a) 0.24 mm, (b) 0.30 mm, (c) 0.48 mm, and (d) 0.52 mm.
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Fig. 3 Optical microstructure of the FSSW joint: (a) base metal, (b) discolored region, (c) top region, (d) middle region.
Fig. 4
Microstructure of the middle region in the FSSW joint: (a) OM micrograph and (b) high magnification SEM micrograph.
decreasing gap at the joint edge region affected the joint strength and fracture location. A white layer, with a shape of bilateral symmetry, was observed at the top sheet in all samples. It was assumed that these layers were related with the phase transformation of the steel sheets or were a result of the reaction between the tool and steel sheets. A line was shown across the middle region of the joint, and was inferred to be the interface between the top and bottom steel sheets. Figure 3 shows the representative grain distribution in the base metal (a), side region under the pin (b), and center region under the pin (c)–(d). The material analyzed in Fig. 3 was the 0.52 mm-tool penetration depth sample. In the case of the base metal, microstructure was comprised of elongated
ferrite phase grains, with an average grain size of 25 mm. In the side region under the pin, fine and discolored grains were observed, which were attributed to the effect of the zinccoating layer on this region. In the center region under pin, the grains were larger than those of the base metal, we assumed that recrystalization occurred in this region due to stirring, and grain growth occurred due to the friction heating induced during the welding.17) In the middle region (Fig. 3(d)), a fully metallurgical, bonded region was observed, while very fine microstructures were revealed between the top and bottom sheets (Fig. 4). In this study, as the CPS tool did not penetrate the bottom sheet, the two sheets were not bonded by mechanical stirring, in contrast with conventional tool. However diffusion was occurred
Microstructure and Mechanical Properties of Friction Stir Spot Welded Galvanized Steel
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SEM micrographs of the discolored region in the FSSW joints: (a) low magnification, (b) high magnification.
Fig. 6
Result of EPMA analysis in trimmed region.
between the top and bottom sheets by friction heating and tool compression, they were bonded like diffusion bonding. Moreover this diffusion between top and bottom sheet was occurred during short time, it seems that interface between top and bottom sheet was disappeared, but grain growth was not fully occurred due to rapid cooling. Figure 5 shows a SEM and EDS analysis results of the microstructure of the joint. An interface was clearly evident between the top and bottom sheets, and defects were
observed in the bottom sheet. With its trimmed shape, the top sheet presented a different shape from the bottom sheet. Figure 5(b) shows a high magnification SEM micrograph of the trimmed shape region. In the EDS results, Zn was detected in this region but was not detected in the other regions. Also EPMA result was shown that Zn was existed in trimmed shape region (Fig. 6). Considering these result, it seems that Zn was assumed to have come from the Zncoated layer.
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TEM micrographs of the FSSW joint: (a) Zn-affected zone, (b) unaffected zones.
Fig. 8
Result of measured maximum peak temperature during FSSW.
To analyze the Zn behavior in the joint, the joint microstructure was analyzed with TEM. Figure 7 shows TEM micrographs of the Zn-affected and unaffected regions. About 20 nm-sized black particles were dispersed in the microstructure in the Zn-affected zone (Fig. 7(a)), but were not observed in the unaffected zone (Fig. 7(b)). In the EDS analysis results, Zn and O were detected in this region, suggesting that these black particles were ZnO. Maximum peak temperature during FSSW was shown in Fig. 8. It shows that maximum peak temperature between tool and workipiece was 1293 K. Considering this result, Fe was not melted during FSSW, but it suggested that the welding temperature exceeded the melting and boiling point of Zn during the welding, it was known that the melting and the boiling points of Zn are 693 and 1180 K, respectively. These result supported the following explanation for the Zn behavior during the welding. The friction heat generated between the tool and workpiece during the welding. This
result indicated that the Zn was melted and evaporated due to the high temperature. However, we assumed that evaporated Zn was not out of joint, because the outflow of any gas was blocked by the penetrated tool. This evaporated Zn that could not escape was exploded by the friction heat and tool compression, and this explosion easily deformed the joint, which was already softened by the friction heating (Fig. 5). As the joint was cooled after welding, the evaporated Zn turned to liquid and finally solidified in the joint. However, the center region in the joint was not affected by Zn, suggesting that the evaporated and melted Zn was spread from the center region to the side region due to the centrifugal force of the tool rotation. Figure 9 shows the effect of tool penetration depth on the shear strength of the joints. The error bars in the figure represent the standard deviation of the test data. The shear strength was maximized at a penetration depth of 0.52 mm. The shear strength peaks at the 0.24 mm penetration depth
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occurred in the top sheet, midway through the section which was under the tool shoulder. The bonded area of the joint was the same at both 0.24 mm- and 0.30 mm-tool penetration depths. However, as the tool penetration depth exceeded 0.30 mm, the bonding area was slightly increased to a maximum bonding area of 176.15 mm2 at the 0.52 mm-tool penetration depth. These results suggested that the increase of the lap-shear strength between these samples is related with the bonded area under the tool shoulder region. 4.
Fig. 9
Tensile shear test results.
and goes up for the 0.30 mm insertion depth. However over the 0.30 mm insertion depth, shear strength goes slightly goes up for the 0.52 mm insertion depth. Figure 10 shows photographs of the failure surfaces and cross section of the lap shear specimens joined at four different tool penetration depths. The figures also show the exact shear strength of the samples. While the difference in the lap-shear strength between these specimens is relatively small, there is a marked change in the failure mode with increasing tool penetration depth. In the 0.24 mm-tool penetration depth sample, fracture occurred at the interface between the top and bottom surfaces. However, as the tool penetration depth increased beyond 0.30 mm, the failure location shifted further away from the base metal. Failure
Conclusion
FSSW of galvanized steel was carried out as a function of tool penetration depth with a welding tool made from WC-Co alloy. The study results are summarized as follows: (1) Sound FSSW joints were obtained without any melting of the workpiece and the mechanically mixed zone, under the all welding conditions. (2) The deformation region that was observed in the joint and ZnO was detected in this region. This result implied that deformation region was related with behavior Zn during FSSW. (3) Maximum temperature during FSSW was about 1293 K, this temperature was lower than melting point of Fe but higher than boiling point of Zn. It suggested that Zn was evaporated and exploded due to high temperature and tool compression and this explosion affected on joint. (4) With increasing tool penetration depth, the tensile shear fracture load gradually increased to a maximum of 3.07 kN at the deepest tool penetration depth of 0.52 mm.
Fig. 10 Fracture images after tensile shear testing at the following tool penetration depths: (a) 0.24 mm, (b) 0.30 mm, (c) 0.48 mm, and (d) 0.52 mm.
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