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Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique
Materials and Design 37 (2012) 384–392
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Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique Y.F. Sun a,⇑, H. Fujii a, N. Takaki b, Y. Okitsu b a b
Joining and Welding Research Institute, Osaka University, Ibaraki, 567-0047 Osaka, Japan Honda R&D Co. Ltd., Automobile R&D Ctr, Haga, 321-3393 Tochigi, Japan
a r t i c l e
i n f o
Article history: Received 4 November 2011 Accepted 12 January 2012 Available online 24 January 2012 Keywords: A. Ferrous metals and alloys D. Welding E. Mechanical F. Microstructure
a b s t r a c t With the successful application of the flat spot friction stir welding technology to aluminum alloys, this technique was expanded to the spot lap welding of 1 mm thick mild steel in this study. It reveals that sound joints can be successfully obtained with smooth surfaces and without any internal welding defects. Two welding strategies based on the welding parameter can be used to obtain the welds that fracture through plug failure mode at high shear tensile strength. One way is to weld the sheet at low heat input in the first step and the second step is used to generate large stir zone and flatten the sample surface. However, the microstructure in the stir zone is not homogeneous and a coarse columnar grain structure forms at the bottom of the stir zone. Another way is to make the stir zone penetrate into the lower sheet during the first step and the second step is only aimed to flatten the sample surface. In this case, the total heat input can be reduced and the microstructure of the stir zone can be remarkably refined. The sound joints fractured along the circumstance of the stir zone and reached about 6600 N during the shear tensile tests. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The spot friction stir welding (FSW) technique was invented by Kawasaki Heavy Company in 2000 as a variant of the linear FSW method [1]. Shortly after the invention, spot FSW was developed very quickly and its application to many kinds of aluminum and magnesium alloys were investigated [2–5]. As a relatively new process, it shows great potential to be a replacement for single point joining processes like resistance spot welding (RSW) and rivet technology. Recently, it has received considerable attention from the automotive and other industries. For example, spot FSW was first used in the Mazda RX-8 rear door panel in 2003. It was claimed that the use of energy significantly decreased and the investment cost was about 40% lower than before. Today a manufactured car requires 2000–5000 spot welds. To increase the fuel economy and improve the vehicle performance in the vehicle industry, spot FSW is being adopted more and more by the transportation industries. With the development of high wear-resistance rotating tools, the spot FSW technique has also been expanded to some other materials with higher strength and higher melting points, such as the spot welding of steel [6–8]. As for steel, the leading candidate for spot welding is still the RSW method. However, with the growing interest in the application of advance high strength steels within the automotive architecture, demands ⇑ Corresponding author. Tel.: +81 6 68798663. E-mail address: [email protected] (Y.F. Sun). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2012.01.027
for reliable spot welding methods have been significantly increasing over the past few years. This is because the high welding temperature and rapid cooling rate during the resistance spot welding process might result in the formation of a brittle microstructure and deteriorate the mechanical properties of the welds. In addition, the shortened electrode life is another major issue for the RSW of high strength steels. However, one of the disadvantages of spot FSW technique is that a keyhole generally remains at the center of the stir zone. Although the mechanical properties of the completed welds may be unaffected by the presence of the keyhole, the keyhole is aesthetically undesirable and the corrosion could take place preferentially at the keyhole [9]. Some techniques have been developed to prevent the formation of the keyhole, such as the refilling method with retractable tools, and the application of probe-less rotating tools or consumable tools [10–12]. Recently, a novel spot FSW technique containing two steps was introduced and has been successfully applied to the welding of 6061 and 5052 Al alloys [13]. In the first step, a specially designed back plate with a round dent on the plate surface was used. As a result, a keyhole is formed on the top side and a protuberance is formed on the bottom side of the welds. In the second step, a probe-less rotating tool is used to remove the protuberance and the keyhole as well. That is also the reason why it is called the flat spot FSW technique. Up to now, almost all of these methods are only applicable for aluminum alloys, which are easy to plastically deform. Although the spot FSW of low carbon steel sheets has been performed with a probe-less rotating
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tool [14,15], the use of probe-less tools is only suitable for the welding of thin sheets and much heat input must be introduced by using high rotation speed. The bonding between the two thin sheets was sometimes mainly achieved by the frictional heat and high pressure. In this study, the flat spot FSW technique was used for the welding of 1 mm-thick mild steel sheet, which is the most common material used in the car industry. It reveals that this technique is simple and can easily remove the keyhole in the high melting point materials like steel to obtain sound welds with smooth surface. After welding, the welding parameter independent microstructural evolution in the welds was investigated and the fracture behavior of the welds was analyzed.
2. Experimental procedure In this study, the flat spot FSW method was applied to the welding of 1 mm-thick sheet of mild steel, which has a composition shown in Table 1. The individual sheet is 100 mm in length and 30 mm in width and two sheets were spot lap welded on an overlap area of 30 30 mm2. Prior to the welding, the steel sheets were cleaned with acetone to remove the impurities on the surface such as dirt and oil, In the first step, the conventional spot FSW was conducted on a specially designed back plate containing a round dent. The diameter and the depth of the dent can be changed to optimize the mechanical properties of the joints. In the present study, the diameter and depth of the round dent were 8 mm and 1 mm, respectively. The rotating tools used in this study were made of Si3N4 ceramics, which have high strength at high temperature and have been proved to be very suitable for the FSW of steels [16]. In the first step, the rotating tools have a shoulder with 12 mm in diameter and a probe with 4 mm in diameter and 1 mm in length. After the first step, a keyhole is formed in the upper sheet due to the retraction of the probe and a protuberance formed on the lower sheet due to the flow of the materials into the dent. In the second step, a probe-less rotating tool and a smooth back plate are used to remove both the keyhole and the protuberance. The rotating tool is pressed on the keyhole directly under a certain applied load. Details of this flat spot FSW technique can be found elsewhere [13]. The rotating tool used in the second step has only a smooth surface of 15 mm diameter. The welding parameters are summarized in Table 2. For comparison, the flat spot FSW of mild steel was classified into two groups based on the welding parameters. In Group A, the welding condition varied in the first step, but only one welding parameter was used in the second step. On the contrary, only one welding condition was used in the first step in Group B and the welding condition varied in the second step. After welding, optical microscopy (OM) and electron backscattered diffraction (EBSD) were used to characterize the microstructure of the joints. The EBSD measurements were carried out using a JEM-7001FA field emission scanning electron microscope (FE-SEM) with a TSL (TexSemLaboratories, Inc.) orientation imaging system. The microstructures in the center of the stir zone after the second step as well as the base metal were also characterized by transmission electronic microscope (TEM). For the TEM sample preparation, thin sheets were first cut at the desired locations and then mechanically polished to 100 lm. The polished thin sheets were finally twin-jet electro-polished to make an electron-beam transparent thin film using a solution of HClO4:CH3COOH = 1:9 at 30 V and Table 1 The chemical composition of the mild steel. Element wt.%
C 0.002
Mn 0.12
P 0.013
S 0.008
Fe Bal.
385
10 °C. The thin films were observed by a JEOL 2100F TEM at 200 kV. The shear tensile tests were carried out using an Instron-type testing machine with a crosshead speed of 1 mm/min.
3. Experimental results 3.1. Microstructure evolution of Group A After welding, the appearances of the welds in both Group A and Group B are quite similar and independent of the welding parameters. As a typical example, Fig. 1 shows the appearance of the sample after the first step, which was welded at a rotation speed of 600 rpm. A keyhole can be observed on the top side of the upper sheet, showing a profile similar to that formed in the conventional spot FSW processed materials. However, on the bottom side of the lower sheet, a protuberance was formed just below the keyhole due to the flow of the materials into the dent on the back plate. Fig. 2 shows the appearance of the weld after the second step of the welding process. Using a rotation tool without a probe, the keyhole and the protuberance were completely removed and the surfaces on both sides of the weld were quite smooth. After retraction of the rotating tool, a little flash formed around the edge of the stir zone. Fig. 3 shows the typical cross-sectional macrostructure of the samples in Group A. For the samples after the first step as shown in Fig. 3a, a keyhole and a protuberance can be observed. Since the thermo-mechanically affected zone (TMAZ) has a much refined grain structure and appears black after chemical etching, the stir zone can therefore be easily distinguished from the base metal. According to the OM images, the area size of the stir zone increases and the bottom of the stir zone becomes closer to the joint interface with the increasing rotation speed. In this study, the depth of the stir zone was defined as the distance from the sample surface to the bottom of the stir zone. When the rotation speed was 600 rpm, the depth of the stir zone was about 0.68 mm. However, the depth of the stir zone increased to about 1.44 mm when the rotation speed was 900 rpm. When the rotation speed was higher than 800 rpm, the bottom of the stir zone can propagates through the joint interface and penetrates into the lower sheet. After the second step, both the upper and lower sheets are rather flat. No keyholes on the top surface and no protuberances on the bottom surface are observed. The stir zone is formed symmetrically along the central axis of the samples. However, no obvious stir zone can be observed in the center part of the sample. Although only one welding condition was used for the samples in Group A, the shape and size of the stir zone are relatively complicated and varied with the different rotation speeds. Fig. 4 shows the enlarged OM images of the stir zones in the welds after the second welding step, which were taken in the white square area as shown in Fig. 3b. Although only one rotation speed was used in the second welding step for all the samples, the microstructures are quite different due to the different welding parameters used in the first welding process. As for the samples welded at 600 rpm or 700 rpm in the first step, only one stir zone and TMAZ can be found after the second step. However, when the rotation speed was higher than 700 rpm in the first step, two kinds of stir zones and TMAZ can be seen in the joints after the second step. Obviously, the stir zone and TMAZ in the above area are formed in the second step and the lower stir zone and TMAZ are formed in the first step. In this figure, the stir zone and TMAZ formed in the first step and in the second step are shortly named as SZ-1, TMAZ-1 and SZ-2, TMAZ-2, respectively. Since the rotation speed in the second step is low, the SZ-1 and TMAZ-1 cannot be overlapped by those formed in the second step. The bottom of
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Table 2 List of the welding parameters. 1st Step
Group A Group B
2nd Step
Load (kg)
Rotation speed (rpm)
Dwell time (s)
Load (kg)
Rotation speed (rpm)
Dwell time (s)
1000 1000
600–900 600
2 2
2000 2000
600 600–1000
2 2
Fig. 1. Appearance of the welds after the first step welding process.
Fig. 2. Appearance of the welds after the second step welding process.
the SZ-1 and TMAZ-1 therefore can be remained after the second step. In addition, it was found that the shape and size of SZ-2 and TMAZ-2 are quite similar to each other and formed above the joint interface. In the SZ-2, the average grain size was about 20 lm for all the welds. Based on the above microstructure characterization, the microstructure of the stir zone after the second step is relatively complicated. For further investigation, the EBSD measurements were carried out in the mixed stir zone of the samples. As a typical
example, Fig. 5a presents the EBSD map showing the microstructural details in the stir zone, which was welded at a rotation speed of 900 rpm in the first step. Since the SZ-1 is larger than SZ-2, the bottom of the SZ-1 can still be observed. In addition, the joint interface cannot be observed anymore, which indicated the intense mixing of the materials in the stir zone. Fig. 5b shows the distribution of the grain size calculated from the EBSD measurements. Most of grains have a grain size of about 20 lm. The largest grain size in the stir zone is about 60 lm and formed at the bottom of
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Fig. 3. OM images showing the cross-sectional macrostructure of the welds after (a) first step and (b) second step welding process.
Fig. 4. Enlarged OM images showing the mixed microstructure of stir zone after the second step. The samples were welded at (a) 600 rpm; (b) 700 rpm; (c) 800 rpm and (d) 900 rpm.
SZ-1. However, the large grains have a very small fraction of less than 1%. In addition, the coarse grain structure formed in SZ-1 can be further refined by the second step of the welding process. 3.2. Microstructure evolution of Group B Fig. 6 shows the cross-sectional macrostructure of the samples in Group B. During the first welding step, only one welding condition was used. The stir zone is small and formed above the joint
interface due to the low heat input, as shown in Fig. 6a. After the second step, the SZ-1 and TMAZ-1 cannot be observed and only the SZ-2 and TMAZ-2 can be found. Similar to the stir zones formed in the samples of group A, the stir zones formed symmetrically along the central axis after the second step. In addition, the area size of the stir zones becomes larger with the increasing rotation speed. The depth of the stir zone increases from 0.62 mm for the sample welded at 700 rpm to 1.32 mm for the sample welded at 1000 rpm. When the rotation speed is equal or higher than
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Fig. 5. (a) IPF and IQ maps showing the microstructure of the stir zone and (b) distribution of the grain size calculated from the EBSD results.
Fig. 6. OM images showing the cross-sectional macrostructure of the welds after (a) first step and (b) second step of the welding process.
900 rpm, the stir zone propagates across the joint interface and penetrates into the lower sheet. Fig. 7 shows the enlarged OM images of the stir zone of the samples after the second step, which was taken in the area as indicated by the white squares in Fig. 6b. It can be found that the bottom of the stir zone became closer to the joint interface as the rotation speed increased. The joint interfaces for all the joints are indicated by the white arrows in the images. It was interesting to find that the joint interface can still be observed even in the stir zone. The joint interface in the stir zone is straight and exists at its original position, which seems to be unaffected by the strong stirring during the welding process. This phenomenon is quite different from the microstructure in the stir zone of the aluminum alloy welds, in which the joint interface cannot be observed [13]. In addition, it was found that the average grain size in the stir zone also increases with the increasing rotation speed, i.e., from 20.6 lm at 700 rpm to 46.6 lm at 1000 rpm. Fig. 8 presents the EBSD map showing the microstructure of the samples welded at 1000 rpm in the second step welding process. The EBSD map was scanned in the white rectangular area as indicated in the inserted OM image of the sample, which covers a large area from stir zone to the base metal. In the stir zone, the microstructure is not homogeneous and the grain size increases from about 20 lm at the top area to about 90 lm in the bottom area. Fig. 8b shows the distribution of the grain size calculated from the EBSD results. Most of the grains have a grain size of about 20 lm. However, the largest grain size is about 90 lm. At the bottom of the stir zone, a coarse columnar grain structure can be observed. The formation of the coarse column grain structure can be explained as follows. It is known that the top surface has the highest temperature during the welding process due to the severe
friction between the sample surface and the tool shoulder. However, the thermal conductivity is quite low for steel, which is about 55 W/(mK) and much lower than 255 W/(mK) for aluminum alloys. The top surface of the welds can be first cooled down to a lower temperature in air after the welding process, while the temperature is still relatively high inside the sample for a period of time. Therefore, a thermal gradient was formed within the stir zone. As a result, at the bottom of the stir zone a coarse columnar grain structure is formed due to the directional grain growth. Between the stir zone and the base metal, the TMAZ consisting of elongated grains can be observed. The TMAZ has an ultrafine grained structure with an average grain size of about 1 lm. However, the HAZ cannot be easily distinguished. Another feature of the microstructure is that the interface beyond the stir zone is not well bonded. A thin gap can still be observed and the grains along the interface show a different crystalline orientation. Fig. 9 shows the high magnification EBSD image quality (IQ) map and SEM images of the microstructure along the joint interface in the stir zone. The joint interface is still discernable mainly due to the distribution of the surface impurities. The joint interface is straight throughout the whole stir zone, indicating that the materials flow caused by the smooth tools is generally parallel with the sheet surface. In the IQ map shown in Fig. 9a, the crystalline orientations of the grains along the joint interface are also indicated. It reveals that the grains along the joint interface generally have same orientation, which confirmed that the surface impurities are actually distributed within the grains. From the high resolution SEM image as shown in Fig. 9b, the curved grain boundary is found around the impure particles. The possible formation mechanism for the interfacial microstructure is supposed to be caused by particle-triggered dynamic recrystallization and grain growth. The impure particles
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Fig. 7. Enlarged OM images showing the microstructure of the mixed stir zone after the second step.
Fig. 8. (a) IPF and IQ maps showing the microstructure of the stir zone and (b) distribution of the grain size calculated from the EBSD results.
can act as the nucleation site for the recrystallized grains and finally embedded inside the grains after its grown-up. Another possibility is that the surface impurities might increase the frictional heat during the welding process and therefore result in local melting along the sheet interface. After welding, some of the particles are swallowed by the advance of the solidification front, while some of the particles can block the solidification front which leads to the formation of curved grain boundary. Fig. 10 shows the TEM images taken in different areas of the welds, which is indicated as small square area in the OM image in this figure. The base metal shows an equiaxial grain structure as shown in Fig. 10a. The center of the stir zone also has an equiaxial structure as shown in Fig. 10c, which contains very low dislocation density after dynamic recrystallization. However, as for the area in the center of the sample but beyond the stir zone, it shows remarkably refined grain size and elongated grain structure as shown in Fig. 10b. It is supposed that the elongated grain structure
is caused by the pressing of the rotating tools at high applied load and therefore experienced much small plastic deformation in the center part of the samples. 3.3. Mechanical properties of the welds Fig. 11 shows the shear tensile load–displacement curves for the welds in Groups A and B, respectively. In both groups, the welds can be divided into two classes according to their fracture behavior. When the welds fracture through the interfacial mode, the load dropped to zero in a very short time after failure. However, when the welds fracture through the plug failure mode, the applied load starts to drop as the crack initiates. As the crack propagates along the circumference of the stir zone, the load continues to drop gradually. With the subsequent tearing of the base metal along the loading direction, a long tail will form on the load– displacement curve. As for the welds in Group A as shown in
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Fig. 9. (a) EBSD IQ image and (b) SEM image showing the microstructure of the joint interface in the stir zone after the second welding step of the sample in Group B.
Fig. 10. TEM images showing the microstructure of different locations of the sample after second welding step.
Fig. 11. Load–displacement curves for the spot welds obtained in (a) Group A and (b) Group B.
Fig. 11a, the different rotation speed used in the first step is indicated because only one rotation speed was used in the second step. When the rotation speed is less than 700 rpm, the joints show a low shear tensile strength and elongation and the joints fractured through the interfacial failure mode. However, when the rotation speed increases to 800 rpm or more, the joints fracture through the plug failure mode and show much higher shear tensile strength
and larger elongation. As for Group B shown in Fig. 11b, the different rotation speed used in the second step is indicated because only one rotation speed was used in the first step. Similar to that in Group A, the welds fractured through the interfacial failure mode when the rotation speed is less than 800 rpm. However, the welds fractured through a plug failure mode when the rotation speed is equal or higher than 800 rpm.
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As typical examples, Fig. 12 shows the appearances of some fractured tensile specimen, which fractured through different failure mode. In Fig. 12a, the specimen was welded at 600 rpm in the first step and 700 rpm in the second step. The specimen fractured through the interfacial mode, because the stir zone of the joints is formed above the joint interface and the bonding between the two plates is not strong. Fig. 12b shows the appearance of the specimen welded at 900 rpm in the first step and 700 rpm in the second step, which belongs to Group A. Fig. 12c shows the appearance of the specimen welded at 600 rpm in the first step and 1000 rpm in the second step, which belongs to Group B. For both specimens, the stir zone can penetrate into the lower sheet and the specimens fractured through a plug failure mode. Under tensile stress, the specimen first experienced uniform plastic elongation after the yielding point. Then, necking happened around the welded area in the top plate and finally the specimen fractured into two pieces.
4. Discussion The above results reveal that this flat spot friction stir welding technique is also suitable for the welding of high melting point materials like mild steel to make a smooth surface on both the top side and bottom side of the joints. The final joints may fracture through the interfacial failure mode or plug failure mode, depending on the microstructure of the welds, in other words, the welding conditions. However, spot welds that fracture through the interfacial mode are generally not expected to be used for the engineering applications, especially in the automobile industry. These welds that fail through the plug fracture mode are supposed to absorb more impact energy under an applied force and therefore are required for industrial application. In order to obtain sound welds that have high mechanical properties and fracture through plug mode, there are two kinds of welding strategies to fulfill this objective. The first method is used in Group A, a large stir zone is generated to penetrate into the lower sheet in the first step. The second step is only aimed to remove the keyhole and protuberance on the sample surface. The second method is used in Group B, the two sheets are just bonded with a small stir zone in the first step. The second step welding process need to generate a large stir zone penetrating into the lower sheet. Although both approaches can be used to produce smooth welds without keyhole and welding defects, the welding parameters such as rotation speed, applied load are quite different. These welding parameters are quite important for the welds quality and are challenging factors for the capacity of the FSW machine, especially when welding high melting point materials. In addition, the low energy required in the welding process can certainly reduce the investment cost.
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Based on these considerations, the welding strategy used in Group A seems to be more promising. Since the adjoining interface between the upper and lower sheets is bent due to the materials flow into the dent on the back plate, the materials flow caused by the rotating tool is nearly perpendicular but not parallel to the joint interface. Under the extrusive force of the probe, the metallurgical bonding can be easily complicated with relatively lower heat input. The second step welding process is used to flatten the sample surface. In addition, the coarse grain structure in the stir zone after the first step can be significantly refined in the second step due to the lower rotation speed. However, if the stir zone needs to penetrate into the lower sheet during the second step welding process, like the samples in Group B, a much higher rotation speed is required. As a result, the stir zone shows an extremely coarse column grain structure in the bottom of the stir zone. Although the coarse grain structure exert no influence to the shear tensile strength and the welds fracture along the circumstance of the welded area, it is said that the formation of coarse column grain structure might be harmful to the fatigue properties of the welds and will inevitably deteriorate its mechanical properties [17]. The flat spot FSW technique is a newly developed welding method and has only been applied to the joining of 6061 and 5052 aluminum alloys [13]. As for flat spot welded aluminum alloys, the microstructure evolution and mechanical properties are quite different with that of the steels in this study. It is because that the plastic deformation of aluminum alloys is very easy, the stir zone is much larger and easily propagates across the sheet interface before the completion of the welding process. This can be confirmed by the remarkably microstructural refinement at the bottom of the lower sheet below the rotating tools. Therefore, the different welding parameter in the first step and the second step does not show quite difference in the final microstructure and mechanical properties. Compared with the conventional spot FSW of steel, this flat spot welding method can produce joints without keyhole. The smooth surface appearance is considered more acceptable in automotive applications. On the contrary, the keyhole produced by the rotating tool may decrease the bonded area, which is a critical factor determining the strength of the weld [18–20]. The use of rotating tools without probe has also been tried for the spot welding of thin steel sheets to remove the keyhole. Since the plastic deformation resistance of steel is much higher than that of the aluminum alloys, much more heat input is required for sound welding of steel. The stir zone of the steel joints may not penetrate into the lower steel sheet and the joining of the two steel plates is generally achieved by diffusion bonding at high temperature. In this case, the peak temperature during the welding process seems much important
Fig. 12. Appearance and cross-sectional macrostructure of the fractured specimen after shear tensile tests with different failure mode. (a) Interfacial mode; (b) plug mode in Group A and (c) plug mode in Group B.
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and will dominate the microstructure formed at the welding interface [14]. If the stir zone penetrates into the lower steel sheet, much higher applied load is generally used and the thickness of the upper sheet was reduced greatly. Sometimes, the upper sheet might bend upward at the edge of the joint area and therein leads to the formation of a gap between the two sheets, which will affect the joints strength and the fraction location [21]. Up to now, faster cycle time in RSW result in greater productivity than spot FSW. Comparing with widespread implementation of RSW, the spot FSW process is not yet used extensively in the automotive industry. The invention of this flat spot FSW can produce joints with smooth surface and improved mechanical properties, which will make it more competitive in the future than other spot welding techniques.
5. Conclusions The flat spot FSW technique was applied to mild steel sheets in this study, which has a much higher melting point than that of aluminum alloys. It was revealed that sound welds with high mechanical properties can be obtained and the technique show high feasibility for the joining of mild steel. After welding, the keyhole usually formed during the conventional spot FSW can be successfully removed and the mechanical properties of the joints can be improved. The welds show two kinds of failure modes. When the stir zone is small and formed above the joint interface, the joints will fracture through the interfacial mode and have a low shear tensile strength. However, when the stir zone is large and penetrates into the lower sheet of the joints, the joints will fracture through the plug failure mode and show a higher shear tensile strength. To obtain sound welds that can fracture through the plug failure mode, it is better for the stir zone to penetrate into the lower sheet in the first step. In this case, the second step is only used to flatten the sample surface. Acknowledgements The authors wish to acknowledge the financial support of a Grant-in-Aid for the Cooperative Research Project of Nationwide Joint-Use Research Institutes, the Global COE Programs from the Ministry of Education, Sports, Culture, Science, and Collaborative Research Based on Industrial Demand and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science and Technology of Japan, ISIJ Research Promotion Grant.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique Y.F. Sun a,⇑, H. Fujii a, N. Takaki b, Y. Okitsu b a b
Joining and Welding Research Institute, Osaka University, Ibaraki, 567-0047 Osaka, Japan Honda R&D Co. Ltd., Automobile R&D Ctr, Haga, 321-3393 Tochigi, Japan
a r t i c l e
i n f o
Article history: Received 4 November 2011 Accepted 12 January 2012 Available online 24 January 2012 Keywords: A. Ferrous metals and alloys D. Welding E. Mechanical F. Microstructure
a b s t r a c t With the successful application of the flat spot friction stir welding technology to aluminum alloys, this technique was expanded to the spot lap welding of 1 mm thick mild steel in this study. It reveals that sound joints can be successfully obtained with smooth surfaces and without any internal welding defects. Two welding strategies based on the welding parameter can be used to obtain the welds that fracture through plug failure mode at high shear tensile strength. One way is to weld the sheet at low heat input in the first step and the second step is used to generate large stir zone and flatten the sample surface. However, the microstructure in the stir zone is not homogeneous and a coarse columnar grain structure forms at the bottom of the stir zone. Another way is to make the stir zone penetrate into the lower sheet during the first step and the second step is only aimed to flatten the sample surface. In this case, the total heat input can be reduced and the microstructure of the stir zone can be remarkably refined. The sound joints fractured along the circumstance of the stir zone and reached about 6600 N during the shear tensile tests. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The spot friction stir welding (FSW) technique was invented by Kawasaki Heavy Company in 2000 as a variant of the linear FSW method [1]. Shortly after the invention, spot FSW was developed very quickly and its application to many kinds of aluminum and magnesium alloys were investigated [2–5]. As a relatively new process, it shows great potential to be a replacement for single point joining processes like resistance spot welding (RSW) and rivet technology. Recently, it has received considerable attention from the automotive and other industries. For example, spot FSW was first used in the Mazda RX-8 rear door panel in 2003. It was claimed that the use of energy significantly decreased and the investment cost was about 40% lower than before. Today a manufactured car requires 2000–5000 spot welds. To increase the fuel economy and improve the vehicle performance in the vehicle industry, spot FSW is being adopted more and more by the transportation industries. With the development of high wear-resistance rotating tools, the spot FSW technique has also been expanded to some other materials with higher strength and higher melting points, such as the spot welding of steel [6–8]. As for steel, the leading candidate for spot welding is still the RSW method. However, with the growing interest in the application of advance high strength steels within the automotive architecture, demands ⇑ Corresponding author. Tel.: +81 6 68798663. E-mail address: [email protected] (Y.F. Sun). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2012.01.027
for reliable spot welding methods have been significantly increasing over the past few years. This is because the high welding temperature and rapid cooling rate during the resistance spot welding process might result in the formation of a brittle microstructure and deteriorate the mechanical properties of the welds. In addition, the shortened electrode life is another major issue for the RSW of high strength steels. However, one of the disadvantages of spot FSW technique is that a keyhole generally remains at the center of the stir zone. Although the mechanical properties of the completed welds may be unaffected by the presence of the keyhole, the keyhole is aesthetically undesirable and the corrosion could take place preferentially at the keyhole [9]. Some techniques have been developed to prevent the formation of the keyhole, such as the refilling method with retractable tools, and the application of probe-less rotating tools or consumable tools [10–12]. Recently, a novel spot FSW technique containing two steps was introduced and has been successfully applied to the welding of 6061 and 5052 Al alloys [13]. In the first step, a specially designed back plate with a round dent on the plate surface was used. As a result, a keyhole is formed on the top side and a protuberance is formed on the bottom side of the welds. In the second step, a probe-less rotating tool is used to remove the protuberance and the keyhole as well. That is also the reason why it is called the flat spot FSW technique. Up to now, almost all of these methods are only applicable for aluminum alloys, which are easy to plastically deform. Although the spot FSW of low carbon steel sheets has been performed with a probe-less rotating
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tool [14,15], the use of probe-less tools is only suitable for the welding of thin sheets and much heat input must be introduced by using high rotation speed. The bonding between the two thin sheets was sometimes mainly achieved by the frictional heat and high pressure. In this study, the flat spot FSW technique was used for the welding of 1 mm-thick mild steel sheet, which is the most common material used in the car industry. It reveals that this technique is simple and can easily remove the keyhole in the high melting point materials like steel to obtain sound welds with smooth surface. After welding, the welding parameter independent microstructural evolution in the welds was investigated and the fracture behavior of the welds was analyzed.
2. Experimental procedure In this study, the flat spot FSW method was applied to the welding of 1 mm-thick sheet of mild steel, which has a composition shown in Table 1. The individual sheet is 100 mm in length and 30 mm in width and two sheets were spot lap welded on an overlap area of 30 30 mm2. Prior to the welding, the steel sheets were cleaned with acetone to remove the impurities on the surface such as dirt and oil, In the first step, the conventional spot FSW was conducted on a specially designed back plate containing a round dent. The diameter and the depth of the dent can be changed to optimize the mechanical properties of the joints. In the present study, the diameter and depth of the round dent were 8 mm and 1 mm, respectively. The rotating tools used in this study were made of Si3N4 ceramics, which have high strength at high temperature and have been proved to be very suitable for the FSW of steels [16]. In the first step, the rotating tools have a shoulder with 12 mm in diameter and a probe with 4 mm in diameter and 1 mm in length. After the first step, a keyhole is formed in the upper sheet due to the retraction of the probe and a protuberance formed on the lower sheet due to the flow of the materials into the dent. In the second step, a probe-less rotating tool and a smooth back plate are used to remove both the keyhole and the protuberance. The rotating tool is pressed on the keyhole directly under a certain applied load. Details of this flat spot FSW technique can be found elsewhere [13]. The rotating tool used in the second step has only a smooth surface of 15 mm diameter. The welding parameters are summarized in Table 2. For comparison, the flat spot FSW of mild steel was classified into two groups based on the welding parameters. In Group A, the welding condition varied in the first step, but only one welding parameter was used in the second step. On the contrary, only one welding condition was used in the first step in Group B and the welding condition varied in the second step. After welding, optical microscopy (OM) and electron backscattered diffraction (EBSD) were used to characterize the microstructure of the joints. The EBSD measurements were carried out using a JEM-7001FA field emission scanning electron microscope (FE-SEM) with a TSL (TexSemLaboratories, Inc.) orientation imaging system. The microstructures in the center of the stir zone after the second step as well as the base metal were also characterized by transmission electronic microscope (TEM). For the TEM sample preparation, thin sheets were first cut at the desired locations and then mechanically polished to 100 lm. The polished thin sheets were finally twin-jet electro-polished to make an electron-beam transparent thin film using a solution of HClO4:CH3COOH = 1:9 at 30 V and Table 1 The chemical composition of the mild steel. Element wt.%
C 0.002
Mn 0.12
P 0.013
S 0.008
Fe Bal.
385
10 °C. The thin films were observed by a JEOL 2100F TEM at 200 kV. The shear tensile tests were carried out using an Instron-type testing machine with a crosshead speed of 1 mm/min.
3. Experimental results 3.1. Microstructure evolution of Group A After welding, the appearances of the welds in both Group A and Group B are quite similar and independent of the welding parameters. As a typical example, Fig. 1 shows the appearance of the sample after the first step, which was welded at a rotation speed of 600 rpm. A keyhole can be observed on the top side of the upper sheet, showing a profile similar to that formed in the conventional spot FSW processed materials. However, on the bottom side of the lower sheet, a protuberance was formed just below the keyhole due to the flow of the materials into the dent on the back plate. Fig. 2 shows the appearance of the weld after the second step of the welding process. Using a rotation tool without a probe, the keyhole and the protuberance were completely removed and the surfaces on both sides of the weld were quite smooth. After retraction of the rotating tool, a little flash formed around the edge of the stir zone. Fig. 3 shows the typical cross-sectional macrostructure of the samples in Group A. For the samples after the first step as shown in Fig. 3a, a keyhole and a protuberance can be observed. Since the thermo-mechanically affected zone (TMAZ) has a much refined grain structure and appears black after chemical etching, the stir zone can therefore be easily distinguished from the base metal. According to the OM images, the area size of the stir zone increases and the bottom of the stir zone becomes closer to the joint interface with the increasing rotation speed. In this study, the depth of the stir zone was defined as the distance from the sample surface to the bottom of the stir zone. When the rotation speed was 600 rpm, the depth of the stir zone was about 0.68 mm. However, the depth of the stir zone increased to about 1.44 mm when the rotation speed was 900 rpm. When the rotation speed was higher than 800 rpm, the bottom of the stir zone can propagates through the joint interface and penetrates into the lower sheet. After the second step, both the upper and lower sheets are rather flat. No keyholes on the top surface and no protuberances on the bottom surface are observed. The stir zone is formed symmetrically along the central axis of the samples. However, no obvious stir zone can be observed in the center part of the sample. Although only one welding condition was used for the samples in Group A, the shape and size of the stir zone are relatively complicated and varied with the different rotation speeds. Fig. 4 shows the enlarged OM images of the stir zones in the welds after the second welding step, which were taken in the white square area as shown in Fig. 3b. Although only one rotation speed was used in the second welding step for all the samples, the microstructures are quite different due to the different welding parameters used in the first welding process. As for the samples welded at 600 rpm or 700 rpm in the first step, only one stir zone and TMAZ can be found after the second step. However, when the rotation speed was higher than 700 rpm in the first step, two kinds of stir zones and TMAZ can be seen in the joints after the second step. Obviously, the stir zone and TMAZ in the above area are formed in the second step and the lower stir zone and TMAZ are formed in the first step. In this figure, the stir zone and TMAZ formed in the first step and in the second step are shortly named as SZ-1, TMAZ-1 and SZ-2, TMAZ-2, respectively. Since the rotation speed in the second step is low, the SZ-1 and TMAZ-1 cannot be overlapped by those formed in the second step. The bottom of
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Table 2 List of the welding parameters. 1st Step
Group A Group B
2nd Step
Load (kg)
Rotation speed (rpm)
Dwell time (s)
Load (kg)
Rotation speed (rpm)
Dwell time (s)
1000 1000
600–900 600
2 2
2000 2000
600 600–1000
2 2
Fig. 1. Appearance of the welds after the first step welding process.
Fig. 2. Appearance of the welds after the second step welding process.
the SZ-1 and TMAZ-1 therefore can be remained after the second step. In addition, it was found that the shape and size of SZ-2 and TMAZ-2 are quite similar to each other and formed above the joint interface. In the SZ-2, the average grain size was about 20 lm for all the welds. Based on the above microstructure characterization, the microstructure of the stir zone after the second step is relatively complicated. For further investigation, the EBSD measurements were carried out in the mixed stir zone of the samples. As a typical
example, Fig. 5a presents the EBSD map showing the microstructural details in the stir zone, which was welded at a rotation speed of 900 rpm in the first step. Since the SZ-1 is larger than SZ-2, the bottom of the SZ-1 can still be observed. In addition, the joint interface cannot be observed anymore, which indicated the intense mixing of the materials in the stir zone. Fig. 5b shows the distribution of the grain size calculated from the EBSD measurements. Most of grains have a grain size of about 20 lm. The largest grain size in the stir zone is about 60 lm and formed at the bottom of
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Fig. 3. OM images showing the cross-sectional macrostructure of the welds after (a) first step and (b) second step welding process.
Fig. 4. Enlarged OM images showing the mixed microstructure of stir zone after the second step. The samples were welded at (a) 600 rpm; (b) 700 rpm; (c) 800 rpm and (d) 900 rpm.
SZ-1. However, the large grains have a very small fraction of less than 1%. In addition, the coarse grain structure formed in SZ-1 can be further refined by the second step of the welding process. 3.2. Microstructure evolution of Group B Fig. 6 shows the cross-sectional macrostructure of the samples in Group B. During the first welding step, only one welding condition was used. The stir zone is small and formed above the joint
interface due to the low heat input, as shown in Fig. 6a. After the second step, the SZ-1 and TMAZ-1 cannot be observed and only the SZ-2 and TMAZ-2 can be found. Similar to the stir zones formed in the samples of group A, the stir zones formed symmetrically along the central axis after the second step. In addition, the area size of the stir zones becomes larger with the increasing rotation speed. The depth of the stir zone increases from 0.62 mm for the sample welded at 700 rpm to 1.32 mm for the sample welded at 1000 rpm. When the rotation speed is equal or higher than
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Fig. 5. (a) IPF and IQ maps showing the microstructure of the stir zone and (b) distribution of the grain size calculated from the EBSD results.
Fig. 6. OM images showing the cross-sectional macrostructure of the welds after (a) first step and (b) second step of the welding process.
900 rpm, the stir zone propagates across the joint interface and penetrates into the lower sheet. Fig. 7 shows the enlarged OM images of the stir zone of the samples after the second step, which was taken in the area as indicated by the white squares in Fig. 6b. It can be found that the bottom of the stir zone became closer to the joint interface as the rotation speed increased. The joint interfaces for all the joints are indicated by the white arrows in the images. It was interesting to find that the joint interface can still be observed even in the stir zone. The joint interface in the stir zone is straight and exists at its original position, which seems to be unaffected by the strong stirring during the welding process. This phenomenon is quite different from the microstructure in the stir zone of the aluminum alloy welds, in which the joint interface cannot be observed [13]. In addition, it was found that the average grain size in the stir zone also increases with the increasing rotation speed, i.e., from 20.6 lm at 700 rpm to 46.6 lm at 1000 rpm. Fig. 8 presents the EBSD map showing the microstructure of the samples welded at 1000 rpm in the second step welding process. The EBSD map was scanned in the white rectangular area as indicated in the inserted OM image of the sample, which covers a large area from stir zone to the base metal. In the stir zone, the microstructure is not homogeneous and the grain size increases from about 20 lm at the top area to about 90 lm in the bottom area. Fig. 8b shows the distribution of the grain size calculated from the EBSD results. Most of the grains have a grain size of about 20 lm. However, the largest grain size is about 90 lm. At the bottom of the stir zone, a coarse columnar grain structure can be observed. The formation of the coarse column grain structure can be explained as follows. It is known that the top surface has the highest temperature during the welding process due to the severe
friction between the sample surface and the tool shoulder. However, the thermal conductivity is quite low for steel, which is about 55 W/(mK) and much lower than 255 W/(mK) for aluminum alloys. The top surface of the welds can be first cooled down to a lower temperature in air after the welding process, while the temperature is still relatively high inside the sample for a period of time. Therefore, a thermal gradient was formed within the stir zone. As a result, at the bottom of the stir zone a coarse columnar grain structure is formed due to the directional grain growth. Between the stir zone and the base metal, the TMAZ consisting of elongated grains can be observed. The TMAZ has an ultrafine grained structure with an average grain size of about 1 lm. However, the HAZ cannot be easily distinguished. Another feature of the microstructure is that the interface beyond the stir zone is not well bonded. A thin gap can still be observed and the grains along the interface show a different crystalline orientation. Fig. 9 shows the high magnification EBSD image quality (IQ) map and SEM images of the microstructure along the joint interface in the stir zone. The joint interface is still discernable mainly due to the distribution of the surface impurities. The joint interface is straight throughout the whole stir zone, indicating that the materials flow caused by the smooth tools is generally parallel with the sheet surface. In the IQ map shown in Fig. 9a, the crystalline orientations of the grains along the joint interface are also indicated. It reveals that the grains along the joint interface generally have same orientation, which confirmed that the surface impurities are actually distributed within the grains. From the high resolution SEM image as shown in Fig. 9b, the curved grain boundary is found around the impure particles. The possible formation mechanism for the interfacial microstructure is supposed to be caused by particle-triggered dynamic recrystallization and grain growth. The impure particles
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Fig. 7. Enlarged OM images showing the microstructure of the mixed stir zone after the second step.
Fig. 8. (a) IPF and IQ maps showing the microstructure of the stir zone and (b) distribution of the grain size calculated from the EBSD results.
can act as the nucleation site for the recrystallized grains and finally embedded inside the grains after its grown-up. Another possibility is that the surface impurities might increase the frictional heat during the welding process and therefore result in local melting along the sheet interface. After welding, some of the particles are swallowed by the advance of the solidification front, while some of the particles can block the solidification front which leads to the formation of curved grain boundary. Fig. 10 shows the TEM images taken in different areas of the welds, which is indicated as small square area in the OM image in this figure. The base metal shows an equiaxial grain structure as shown in Fig. 10a. The center of the stir zone also has an equiaxial structure as shown in Fig. 10c, which contains very low dislocation density after dynamic recrystallization. However, as for the area in the center of the sample but beyond the stir zone, it shows remarkably refined grain size and elongated grain structure as shown in Fig. 10b. It is supposed that the elongated grain structure
is caused by the pressing of the rotating tools at high applied load and therefore experienced much small plastic deformation in the center part of the samples. 3.3. Mechanical properties of the welds Fig. 11 shows the shear tensile load–displacement curves for the welds in Groups A and B, respectively. In both groups, the welds can be divided into two classes according to their fracture behavior. When the welds fracture through the interfacial mode, the load dropped to zero in a very short time after failure. However, when the welds fracture through the plug failure mode, the applied load starts to drop as the crack initiates. As the crack propagates along the circumference of the stir zone, the load continues to drop gradually. With the subsequent tearing of the base metal along the loading direction, a long tail will form on the load– displacement curve. As for the welds in Group A as shown in
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Fig. 9. (a) EBSD IQ image and (b) SEM image showing the microstructure of the joint interface in the stir zone after the second welding step of the sample in Group B.
Fig. 10. TEM images showing the microstructure of different locations of the sample after second welding step.
Fig. 11. Load–displacement curves for the spot welds obtained in (a) Group A and (b) Group B.
Fig. 11a, the different rotation speed used in the first step is indicated because only one rotation speed was used in the second step. When the rotation speed is less than 700 rpm, the joints show a low shear tensile strength and elongation and the joints fractured through the interfacial failure mode. However, when the rotation speed increases to 800 rpm or more, the joints fracture through the plug failure mode and show much higher shear tensile strength
and larger elongation. As for Group B shown in Fig. 11b, the different rotation speed used in the second step is indicated because only one rotation speed was used in the first step. Similar to that in Group A, the welds fractured through the interfacial failure mode when the rotation speed is less than 800 rpm. However, the welds fractured through a plug failure mode when the rotation speed is equal or higher than 800 rpm.
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As typical examples, Fig. 12 shows the appearances of some fractured tensile specimen, which fractured through different failure mode. In Fig. 12a, the specimen was welded at 600 rpm in the first step and 700 rpm in the second step. The specimen fractured through the interfacial mode, because the stir zone of the joints is formed above the joint interface and the bonding between the two plates is not strong. Fig. 12b shows the appearance of the specimen welded at 900 rpm in the first step and 700 rpm in the second step, which belongs to Group A. Fig. 12c shows the appearance of the specimen welded at 600 rpm in the first step and 1000 rpm in the second step, which belongs to Group B. For both specimens, the stir zone can penetrate into the lower sheet and the specimens fractured through a plug failure mode. Under tensile stress, the specimen first experienced uniform plastic elongation after the yielding point. Then, necking happened around the welded area in the top plate and finally the specimen fractured into two pieces.
4. Discussion The above results reveal that this flat spot friction stir welding technique is also suitable for the welding of high melting point materials like mild steel to make a smooth surface on both the top side and bottom side of the joints. The final joints may fracture through the interfacial failure mode or plug failure mode, depending on the microstructure of the welds, in other words, the welding conditions. However, spot welds that fracture through the interfacial mode are generally not expected to be used for the engineering applications, especially in the automobile industry. These welds that fail through the plug fracture mode are supposed to absorb more impact energy under an applied force and therefore are required for industrial application. In order to obtain sound welds that have high mechanical properties and fracture through plug mode, there are two kinds of welding strategies to fulfill this objective. The first method is used in Group A, a large stir zone is generated to penetrate into the lower sheet in the first step. The second step is only aimed to remove the keyhole and protuberance on the sample surface. The second method is used in Group B, the two sheets are just bonded with a small stir zone in the first step. The second step welding process need to generate a large stir zone penetrating into the lower sheet. Although both approaches can be used to produce smooth welds without keyhole and welding defects, the welding parameters such as rotation speed, applied load are quite different. These welding parameters are quite important for the welds quality and are challenging factors for the capacity of the FSW machine, especially when welding high melting point materials. In addition, the low energy required in the welding process can certainly reduce the investment cost.
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Based on these considerations, the welding strategy used in Group A seems to be more promising. Since the adjoining interface between the upper and lower sheets is bent due to the materials flow into the dent on the back plate, the materials flow caused by the rotating tool is nearly perpendicular but not parallel to the joint interface. Under the extrusive force of the probe, the metallurgical bonding can be easily complicated with relatively lower heat input. The second step welding process is used to flatten the sample surface. In addition, the coarse grain structure in the stir zone after the first step can be significantly refined in the second step due to the lower rotation speed. However, if the stir zone needs to penetrate into the lower sheet during the second step welding process, like the samples in Group B, a much higher rotation speed is required. As a result, the stir zone shows an extremely coarse column grain structure in the bottom of the stir zone. Although the coarse grain structure exert no influence to the shear tensile strength and the welds fracture along the circumstance of the welded area, it is said that the formation of coarse column grain structure might be harmful to the fatigue properties of the welds and will inevitably deteriorate its mechanical properties [17]. The flat spot FSW technique is a newly developed welding method and has only been applied to the joining of 6061 and 5052 aluminum alloys [13]. As for flat spot welded aluminum alloys, the microstructure evolution and mechanical properties are quite different with that of the steels in this study. It is because that the plastic deformation of aluminum alloys is very easy, the stir zone is much larger and easily propagates across the sheet interface before the completion of the welding process. This can be confirmed by the remarkably microstructural refinement at the bottom of the lower sheet below the rotating tools. Therefore, the different welding parameter in the first step and the second step does not show quite difference in the final microstructure and mechanical properties. Compared with the conventional spot FSW of steel, this flat spot welding method can produce joints without keyhole. The smooth surface appearance is considered more acceptable in automotive applications. On the contrary, the keyhole produced by the rotating tool may decrease the bonded area, which is a critical factor determining the strength of the weld [18–20]. The use of rotating tools without probe has also been tried for the spot welding of thin steel sheets to remove the keyhole. Since the plastic deformation resistance of steel is much higher than that of the aluminum alloys, much more heat input is required for sound welding of steel. The stir zone of the steel joints may not penetrate into the lower steel sheet and the joining of the two steel plates is generally achieved by diffusion bonding at high temperature. In this case, the peak temperature during the welding process seems much important
Fig. 12. Appearance and cross-sectional macrostructure of the fractured specimen after shear tensile tests with different failure mode. (a) Interfacial mode; (b) plug mode in Group A and (c) plug mode in Group B.
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and will dominate the microstructure formed at the welding interface [14]. If the stir zone penetrates into the lower steel sheet, much higher applied load is generally used and the thickness of the upper sheet was reduced greatly. Sometimes, the upper sheet might bend upward at the edge of the joint area and therein leads to the formation of a gap between the two sheets, which will affect the joints strength and the fraction location [21]. Up to now, faster cycle time in RSW result in greater productivity than spot FSW. Comparing with widespread implementation of RSW, the spot FSW process is not yet used extensively in the automotive industry. The invention of this flat spot FSW can produce joints with smooth surface and improved mechanical properties, which will make it more competitive in the future than other spot welding techniques.
5. Conclusions The flat spot FSW technique was applied to mild steel sheets in this study, which has a much higher melting point than that of aluminum alloys. It was revealed that sound welds with high mechanical properties can be obtained and the technique show high feasibility for the joining of mild steel. After welding, the keyhole usually formed during the conventional spot FSW can be successfully removed and the mechanical properties of the joints can be improved. The welds show two kinds of failure modes. When the stir zone is small and formed above the joint interface, the joints will fracture through the interfacial mode and have a low shear tensile strength. However, when the stir zone is large and penetrates into the lower sheet of the joints, the joints will fracture through the plug failure mode and show a higher shear tensile strength. To obtain sound welds that can fracture through the plug failure mode, it is better for the stir zone to penetrate into the lower sheet in the first step. In this case, the second step is only used to flatten the sample surface. Acknowledgements The authors wish to acknowledge the financial support of a Grant-in-Aid for the Cooperative Research Project of Nationwide Joint-Use Research Institutes, the Global COE Programs from the Ministry of Education, Sports, Culture, Science, and Collaborative Research Based on Industrial Demand and a Grant-in-Aid for Science Research from the Japan Society for Promotion of Science and Technology of Japan, ISIJ Research Promotion Grant.
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