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Improving Joint Properties of Friction Welded Joint of High Tensile Steel∗ Masaaki KIMURA∗∗ , Masahiro KUSAKA∗∗ , Kenji SEO∗∗ and Akiyoshi FUJI∗∗∗ This report describes the improvements in the joint properties of friction welded joint of 780 MPa class high tensile steel. Welded joint made by a continuous drive friction welding machine, the conventional method, had not obtained 100% joint efficiency despite applying forge pressure. This was due to the softening of the welded interface zone for heat input during braking times. Therefore, we developed a continuous drive friction welding machine with an electromagnetic clutch to prevent heat input during braking time. We proposed the process as “The Low Heat Input Friction Welding Method (the LHI method).” In this case, the joint had the same tensile strength as the base metal at friction time when the friction torque reached the initial peak torque. That is, the welded joint obtained 100% joint efficiency by using only the friction stage up to the initial peak torque without the forge (upsetting) stage, despite the existence of a slightly softened region adjacent to the welded interface. Furthermore, the softened region was hardly generated when this joint was made by applying forge pressure at the initial peak torque. In conclusion, a welded joint of high tensile steel made by only the friction stage of the LHI method had excellent joint properties. The LHI method has a lot of advantages for joining such materials as super fine grain steel with which conventional fusion welding processes have difficulty.
Key Words: Welded Joint, Tensile Properties, Friction Welding, High Tensile Steel, Low Heat Input, Softening
1.
Introduction
Generally speaking, high tensile steel has an ultimate tensile strength of approximately 500 MPa or more. It is widely used for such important structural components as frames and bodies for buildings and automobiles, and so on. Most high tensile steel is made by adding another various elements besides carbon, manganese, or silicone to keep a low carbon equivalent to maintain high tensile strength and ductility. In addition, high tensile steel is basically heat-treated. It is softened by high heat input welding regardless of its weldability. It also needs some pre- or post-welding treatment. Hence, it is difficult to weld high ∗ ∗∗
∗∗∗
Received 16th May, 2005 (No. 05-4098) Department of Mechanical and System Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671–2201, Japan. E-mail:
[email protected] Department of Mechanical Engineering, Faculty of Engineering, National University Corporation-Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090– 8507, Japan
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tensile steel. A friction welding method, a solid state joining process, is suitable to minimize heat input for welding high tensile steel. In previous reports(1) – (3) , we clarified the joining mechanism during the friction welding process for carbon steel. Furthermore, we showed that low carbon steel friction welded joints obtained 100% joint efficiency and similar fatigue strength to the base metals by using only the first stage (up to the initial peak torque) of the friction welding process without forge pressure. We proposed “The Low Heat Input Friction Welding Method” (from now on, it is referred to as the LHI method)(4) – (7) . The LHI method provides more advantages than that of the conventional method, i.e., less axial shortening (burn-off), less flash (burr or collar) and so on. If high tensile steel is joined by the LHI method, the welded joint will have such superior properties as the same tensile strength as the base metal to fusion welds without heat treatment. The welded joint of the non-circular section by the LHI method had the same tensile strength as the base metal, and it fractured at the base metal(8) . Hence, the LHI method can be expected to be a joining method of high tensile steel. Series A, Vol. 48, No. 4, 2005
400 This study presents the friction welding of high tensile steel and the improvement of a joining method for joint tensile strength. In this report, we present the joint mechanical properties by the LHI method under various friction welding conditions, especially the relationship between the friction time and the joint strength of welded joint in contrast with that by the conventional method. We also show that the welded joint barely obtained a softened region by the LHI method with forge pressure. Furthermore, we propose that the LHI method can be applied to such materials as super fine grain steel, which has a welding heat input problem. 2.
Table 1 Chemical compositions and mechanical properties of material used (a) Chemical composition (mass%)
(b) Mechanical properties
Experimental Procedures
The material used was a 780 MPa class high tensile steel plate 16 mm thick (from now on, it is referred to as HT780). The chemical composition and mechanical properties of HT780 are given in Table 1. The dimension and configuration of the friction welding test specimen is shown in Fig. 1. Before joining, the weld faying (contacting) surface of the specimen was polished from 0.05 to 0.14 µm in roughness as the center line average height by a surface grinding machine. We used two friction welding methods: the conventional method and LHI method. A continuous (direct) drive friction welding machine was employed for joining. During friction welding operations, friction speed and friction pressure were set to the following combinations: 27.5 revolutions per second (s−1 ) and 90 MPa, or 27.5 s−1 and 30 MPa. The friction torque during the friction stage was measured with a load-cell and recorded with a personal computer through an A/D converter at a sampling time of 0.015 s. Figure 2 shows the schematic diagrams of the conventional and LHI methods. The rotation of specimen did not instantly stop by the conventional method. Therefore, the relative speed between both specimens was not instantly decreased to zero as shown in Fig. 2 (a), and the welded joint was deformed during braking time. Forge pressure was applied to the joint at twice the friction pressure. In contrast, the LHI method was used with an electromagnetic clutch to prevent braking deformation during rotation stop. When the clutch was released, the relative speed between both specimens instantly decreased to zero, as shown in Fig. 2 (b). In this case, friction pressure could be maintained (loaded), so that the braking time effect on joint deformation was negligible. Forge pressure was not applied to the joint. The detailed characteristics of the LHI method were precisely described in previous reports (4) – (7) . The joint tensile test was carried out with the test specimens after the flash (burr or collar) was machined out. Vickers hardness distribution at the half-radius location of the welded interface region was measured with a load of 9.81 N. Measuring range was 8 mm from the welded interface, and the measuring interval was 200 µm. Series A, Vol. 48, No. 4, 2005
Fig. 1 Shape and dimension of friction welding specimen
(a) Conventional method Fig. 2
(b) LHI method
Schematic diagrams of conventional method and LHI method
3. Results 3. 1 Properties of welded joint by conventional method Figure 3 shows the axial shortening and the joint JSME International Journal
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Fig. 3 Axial shortening and joint tensile strength of conventional joint: friction pressure of 90 MPa and forge pressure of 180 MPa
Fig. 4 Appearances of welded joints and joint tensile test specimens after tensile testing of conventional joint: friction pressure of 90 MPa and forge pressure of 180 MPa
tensile strength of the welded joint by the conventional method. Figure 4 shows the appearances of the welded joints and the joint tensile test specimens after tensile testing. From now on, we call the welded joint as the conventional joint. The conventional joints were made by friction pressure of 90 MPa with a forge pressure of 180 MPa. One of the conventional joints was made at a friction time of 3.0 s and applied in accordance with the axial shortening of the Japan Industrial Standard(9) . The other joint was made at 0.5 s without adjusting. Braking time was approximately 0.4 s. The axial shortening of the convenJSME International Journal
Fig. 5 Vickers hardness distribution across welded interfaces of conventional joint: friction pressure of 90 MPa and forge pressure of 180 MPa
tional joint at 3.0 s was extremely large (Fig. 3). The tensile strength of the conventional joint at 3.0 s was approximately 715 MPa, which fractured at the welded interface, as shown in Fig. 4 (a). On the other hand, the axial shortening of the conventional joint at 0.5 s was shorter than 3.0 s. However, the conventional joint at 0.5 s had approximately 825 MPa of joint tensile strength, and this joint also fractured at the welded interface (Fig. 4 (b)). That is, the joint tensile strengths of both conventional joints were lower than the base metal. Figure 5 shows the Vickers hardness distribution across the welded interfaces of these joints. The welded interface and its adjacent region of both conventional joints softened. Forge pressure was simultaneously added to the joint when the brake was applied. Then, the softened region during the friction process was pushed out as flash from the outer surface of the welded interface. However, the softened region of the conventional joint was not completely pushed out, despite applying forge pressure, because it was generated by friction heat during braking. This was because the rotation of the base metal did not instantly stop at braking. In addition, cooling time to room temperature after joining of the conventional joint was long, and the joint was annealed during the cooling stage. Therefore, the conventional joint could not obtain the same tensile strength as the base metal. 3. 2 Properties of welded joint by the LHI method Figure 6 shows the relationship between friction time and joint tensile strength plotted on a friction torque curve by friction pressure of 90 MPa. Figure 7 shows the appearances of the joint tensile test specimens after tensile testing. Those joints were made by the LHI method, and the forge pressure was not applied to them. From now on, such welded joints are called the LHI joint. At a friction time of 0.04 s, the joint tensile strengths were low (Fig. 6), and fractures occurred at the welded interSeries A, Vol. 48, No. 4, 2005
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Fig. 6 Relationship between friction time and joint tensile strength corresponding to friction torque curve of LHI joint: friction pressure of 90 MPa
Fig. 8 Vickers hardness distribution across welded interfaces of LHI joint: friction pressure of 90 MPa
Fig. 7 Example for appearances of joint tensile test specimens after tensile testing of LHI joint
Fig. 9 Relationship between friction time and joint tensile strength corresponding to friction torque curve of LHI joint: friction pressure of 30 MPa
face (Fig. 7 (a)). Joint tensile strength increased with increasing friction time, and the fracture occurred at the welded interface and the base metal (mixed mode fracture as shown in Fig. 7 (b)). Then, the LHI joint had the same tensile strength as the base metal when friction time was 0.3 s (close to the initial peak torque) or longer. All LHI joints fractured at the base metal (not at the welded interface) after the initial peak torque, as shown in Fig. 7 (c). However, joint tensile strength slightly decreased with increasing friction time after the initial peak torque. Figure 8 shows the Vickers hardness distribution across the welded interface at friction times of 0.5 and 3.0 s by friction pressure of 90 MPa. The LHI joint of 0.5 s, i.e., just after the initial peak torque, had a hardened region at the welded interface about 2.0 mm in longitudinal direction across the welded interface. It was approximately 160% of the hardness of the base metal. This joint
also had a softened region about 1.0 mm in longitudinal direction from the welded interface. On the other hand, the LHI joint at 3.0 s had a hardened region approximately 130% of the hardness of the base metal. This joint had also a softened region about 1.0 to 4.0 mm in longitudinal direction from the welded interface approximately 70% of the hardness of the base metal, and it was lower than 0.5 s. Figure 9 shows the relationship between friction time and joint tensile strength plotted on a friction torque curve by friction pressure of 30 MPa. Those joints were made by the LHI method without forge pressure. At a friction time of 0.7 s, joint tensile strengths scattered, and fractures occurred at the welded interface. When friction times were 0.9 and 1.1 s, joint tensile strength increased and scatter decreased. The joint tensile strengths were close to the base metal, and those joints had mixed mode fractures. The LHI joints at 1.3 and 1.5 s (close to the initial peak
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Fig. 10 Vickers hardness distribution across welded interfaces of LHI joint: friction pressure of 30 MPa
torque) had the same tensile strength as the base metal. The fractures occurred in the base metal in all LHI joints at 1.3 and 1.5 s. However, when friction time was 2.0 s or longer, the LHI joints did not obtain the same tensile strength as the base metal, and they had mixed mode fractures. Figure 10 shows the Vickers hardness distribution across the welded interface at friction times of 1.5 and 3.0 s by friction pressure of 30 MPa. The LHI joint of 1.5 s, i.e., just after the initial peak torque, had a hardened region about 3.0 mm in longitudinal direction across the welded interface. In addition, this joint also had a softened region about 1.5 mm in longitudinal direction from the welded interface. On the other hand, the LHI joint at 3.0 s had a hardened region similar to 1.5 s. This joint also had a softened region about 1.0 to 4.0 mm in longitudinal direction from the welded interface. The softened region of this joint was approximately 70% of the hardness of the base metal, and it was lower than 1.5 s. Incidentally, the LHI joint at 3.0 s by 30 MPa (Fig. 10) had similar hardness to the softened region of a joint at 3.0 s by 90 MPa (Fig. 8). However, the LHI joint at 3.0 s by 30 MPa fractured at the welded interface including the base metal as shown in Fig. 9. Figure 11 shows the crosssectional appearances of the welded interface region at 1.5 and 3.0 s by 30 MPa. The welded interface at 3.0 s had the not-joined region at the peripheral region, indicated by arrows (Fig. 11 (b)). The not-joined region at the peripheral region was produced at the welded interface during the friction process after the initial peak torque (steady state). The occurrence of the not-joined region was due to repeated joining and separating cycles at the welded interface during the steady state. Hasui et al.(10) clarified the joining and the separating at the welded interface, and we(5) also observed the not-joined region on the crosssectional. Thus, when friction time was longer, the LHI JSME International Journal
Fig. 11 Cross-sectional appearances of welded interface region of LHI joint: friction pressure of 30 MPa
Fig. 12 Microstructures of HT780 base metal and hardened region at welded interface of LHI joint: friction pressure of 90 MPa, friction time of 0.5 s and forge pressure of 180 MPa
joint had a mixed mode fracture. On the other hand, the welded interface at 1.5 s was completely joined toward the peripheral region (Fig. 11 (a)). That is, the LHI joint was not fractured at the welded interface when friction time was close to the initial peak torque. The LHI joint at the initial peak torque had the same tensile strength as the base metal, regardless of the friction pressure, because the narrow softened region of the LHI joint at the initial peak torque was constricted by the plastic constraint between the base metal and the hardened region of the welded interface. Figure 12 shows the microstructures of the base metal and a hardened region at the welded interface of the LHI joint at 0.5 s by 90 MPa. The hardened region at the welded interface showed a martensite structure (Fig. 12 (b)). This structure differed from the base metal (Fig. 12 (a)). Thus, the hardened region at the welded Series A, Vol. 48, No. 4, 2005
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Fig. 13 Vickers hardness distribution across welded interface of improved LHI joint: friction pressure of 90 MPa, friction time of 0.5 s and forge pressure of 180 MPa
interface was due to the structure of this region, which became martensite, as shown in Fig. 12 (b). 4. Discussion If high tensile steel joints obtain the same tensile strength as base metal and fractures at base metal, friction time may be the time at which the friction torque reaches the initial peak. That is, theoretically forge pressure is not necessary for the LHI method. The LHI joint had the same tensile strength as the base metal by plastic constraint as shown in Figs. 6 and 9, although they had a softened region as shown in Figs. 8 and 10. It is more desirable that the joint for practical use does not have a softened region even if the joint with a softened region has the same tensile strength as the base metal. That is, it is necessary to remove a softened region from the LHI joint. In an attempt to push out a softened region, we made a joint by the LHI method with applying forge pressure. This joint was made by friction pressure of 90 MPa at a friction time of 0.5 s by applying a forge pressure of 180 MPa and a forge time of 6.0 s. From now on, we call it the improved LHI joint. Figure 13 shows the Vickers hardness distribution across the welded interface of the improved LHI joint. Figure 14 shows the improved LHI joint and joint tensile test specimen after tensile testing. The improved LHI joint had barely generated a softened region at the welded interface as shown in Fig. 13. That is, a softened region was pushed out as flash by applying forge pressure. The tensile strength of the improved LHI joint was approximately 875 MPa, much higher than the base metal. In addition, the improved LHI joint fractured at the base metal, as shown in Fig. 14. The axial shortening of the improved LHI joint was about 1.8 mm, which was a little bit larger than the LHI joint (about 1.2 mm). As a conclusion, it was clarified that a welded joint of high tensile Series A, Vol. 48, No. 4, 2005
Fig. 14 Appearances of welded joint and joint tensile test specimen after tensile testing of improved LHI joint: friction pressure of 90 MPa, friction time of 0.5 s and forge pressure of 180 MPa
steel made by the LHI method had excellent joint properties. These results show that the LHI method is useful for welding high tensile steel softened by high heat input. In particular, the LHI method has a lot of advantages for joining such materials as super fine grain steel that have many problems in conventional fusion welding processes. 5.
Conclusions
This report described the mechanical properties of a friction welded joint of 780 MPa class high tensile steel and a method that improved the mechanical properties of welded joints. In particular, we investigated joint mechanical properties by the LHI method, whose relative speed instantly decreased to zero, under various friction welding conditions. The following conclusions are provided. ( 1 ) Welded joint by the conventional method had less tensile strength than the base metal. They softened at the welded interface and its adjacent region. ( 2 ) Welded joint by the LHI method obtained 100% joint efficiency by using only the friction stage up to the initial peak torque without the forge (upsetting) stage. The same results could be obtained in both cases of friction pressure of 90 or 30 MPa. However, those joints slightly softened at the adjacent region of the welded interface. ( 3 ) The welded joint by the LHI method with forge pressure (improved LHI method) also had the same tensile strength as the base metal. This joint barely had a softened region. ( 4 ) Welded joint of high tensile steel had excellent joint properties made only by the friction stage of the LHI method. The LHI method has a lot of advantages for joining such materials as super fine grain steel that have many problems in conventional fusion welding processes. Acknowledgements The authors acknowledge the financial support of the Hyogo Science and Technology Association, Japan. We wish to thank the staff members of the Machine and JSME International Journal
405 Workshop Engineering at Graduate School of Engineering of University of Hyogo (formerly, Himeji Institute of Technology). We also wish to thank the alumnus of Mr. Yoshikazu Izumi for his devoted contribution to this research project.
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References (1)
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ship between the Friction Time, Friction Torque, and Joint Properties of Friction Welding for the Low Heat Input Friction Welding Method, Quarterly Journal of the Japan Welding Society, (in Japanese), Vol.20, No.4 (2002), pp.559–565. Kimura, M., Kusaka, M., Seo, K. and Fuji, A., Properties of Low Carbon Steel Joint by Low Heat Input Friction Welding Method, Proceedings of IIW Osaka, (2004), pp.139–149. Kimura, M., Choji, M., Kusaka, M., Seo, K. and Fuji, A., Effect of Friction Welding Conditions and Aging Treatment on Mechanical Properties of A7075-T6 Aluminum Alloy Friction Joints, Science and Technology of Welding and Joining, Vol.10, No.4 (2005), pp.406– 412. Kimura, M., An, G.-B., Kusaka, M., Seo, K. and Fuji, A., Quality and Its Improvement of Friction Welded Joint between Rectangular Section Bar and Circular Section Bar, Quarterly Journal of the Japan Welding Society, (in Japanese), Vol.22, No.3 (2004), pp.403– 410. Japanese Industrial Standards Committee, JIS Z 3607 Recommended Practice for Friction Welding of Carbon Steel, (in Japanese), (1994). Hasui, A. and Fukushima, S., On the Torque in Friction Welding, Journal of the Japan Welding Society, (in Japanese), Vol.44, No.12 (1975), pp.1005–1010.
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