2018 Volume 59 Issue 3 Pages 503-506
This study evaluates the microstructure development and mechanical properties of friction-welded dissimilar steels. Rod type steel materials of S20C and SCM415H with a size of 12 mm diameter and 100 mm length were friction welded at a rotation speed of 2000 rpm and an upset force of 30 kg/cm2. Electron backscattering diffraction method was used to study the grain boundary characteristics. The mechanical properties of the welds were evaluated by Vickers microhardness and tensile tests. The application of friction welding led to grain refinement of the welds, and the average grain size at the welded zone significantly refined to 4.9 μm comparing with those of the base material zone (66.8 μm at S20C and 19.8 μm at SCM415H). The grain refinement contributed to an increase in the mechanical properties such as hardness and strength. Consequently, the Vickers microhardness increased by 20 and 15%, respectively, when compared to those of the base material. Moreover, fracture occurred at the base material zone and not at the welded zone, which confirmed the soundly welded state of the steel materials.
Steel as an industrial material is widely used in automobiles, machines, and chemical and power plants owing to their excellent formability, machinability, and welding properties1,2). Moreover, their lower cost relative to non-ferrous metals makes them attractive for use in several applications. Recently, lightweight metals, such as Al, Mg, and Ti have been used in automobile systems3–6); however, safety-related components are still manufactured using steels of high strength and durability. For example, universal joints and tie rod in automobile steering systems are manufactured by fusion welding of high strength steel. However, the application of fusion welding in steels adversely affects the microstructure and the mechanical properties of the welds7–9).
Solid state welding can be used to obtain welds with excellent mechanical properties through microstructure development, which is attributed to a lower heat input relative to that in fusion welding using arc, laser, and electron beam9,10). In particular, a lower heat input suppresses hot cracking and distortion of materials and production of harmful gas11–13). As a type of solid state welding, friction welding (FW) applies a friction force, rotation speed, and upset force at the material joints, and the microstructures and the mechanical properties of the welds are determined by these parameters13). Besides, the friction heat and the metallic plastic flow that occurs during FW are sufficient for dynamic recrystallization of welds which results in grain refinement of the welds14,15). Studies on welding of dissimilar materials by several techniques have been reported16–18). However, FW of dissimilar steels has been rarely reported. Therefore, this study was carried out to evaluate the microstructure development and mechanical properties of friction-welded dissimilar steels.
The materials used in this study were S20C and SCM415H steels, and their chemical compositions are shown in Table 1. For the FW of the two materials, specimens were prepared with a size of 12 mm diameter and 100 mm length. The prepared specimens were cleaned by acetone to remove surface contamination. Subsequently, the specimens were welded using the FW machine of Nitto-Seiki TM (FF-30II-C Model) at a rotation speed of 2000 rpm, friction force of 22 kgf/cm2, upset force of 30 kgf/cm2, and upset length of 3 mm.
| Material | Chemical composition (mass%) | |||||||
|---|---|---|---|---|---|---|---|---|
| S20C | C | Si | Mn | P | S | Cr | Ni | Cu |
| 0.21 | 0.25 | 0.42 | 0.03 | 0.03 | 0.20 | 0.20 | 0.30 | |
| SCM415H | C | Si | Mn | P | S | Cr | Ni | Mo |
| 0.16 | 0.26 | 0.75 | 0.03 | 0.03 | 1.02 | 0.15 | 0.18 | |
In order to evaluate the macro- and the microstructures of the welds, optical microscopy (OM) was employed, and the soundness of the welds was evaluated. Evaluations of the grain shape, grain size, and grain misorientation of the welds were carried out using the electron backscattering diffraction technique. For this work, specimens were machined to a size of 2 mm × 20 mm and then mechanically ground. Then, the surfaces of the specimens were electro polished at 20 V and −40℃ using a solution comprising 100 ml perchloric acid and 900 ml methanol. The sample surfaces were then analyzed using an orientation image mapping system incorporated with a scanning electron microscope. For the evaluation of mechanical properties, Vickers microhardness and tensile tests were employed. Vickers microhardness tests were carried out on the cross section of the weld zone at a load of 1.96 N and a dwell time of 10 s. Tensile test specimens were used to evaluate the transverse tensile strength of the friction welds.
The photograph and the macrostructure of the friction-welded S20C and SCM415H steels are shown in Fig. 1. The top view in Fig. 1(a) shows a soundly welded shape with a smaller weld flash at the SCM415H side relative to the S20C side. The macrostructure of the friction weld zone in Fig. 1(b) indicates a soundly welded state without any defects, such as voids, holes, cracks, and so on. In particular, the grain-refined zone ([1] 1346 μm among the weld zone indicated at figure) at the SCM415H side of the weld zone ([2] 2927 μm indicated at figure) was notably narrow relative to the S20C side owing to its heat-resisting property. In addition, a thermo-mechanically-affected zone (TMAZ) with metallic flow was observed at both the sides without the formation of a heat-affected zone (HAZ).
(a) Photograph and (b) cross-sectional macrostructure of the friction-welded S20C and SCM415H steels.
The grain boundary maps of the initial and the welded steel materials are shown in Fig. 2. Before welding, the S20C steel (hot extruded) material consists of elongated grains ranging from 10 to 110 μm in size and heterogeneous grains with an average size of 66.8 μm, as shown in Fig. 2(a). However, SCM415H steel was composed of comparatively homogeneous grains of sizes ranging from 5 to 30 μm with an average size of 19.8 μm, as shown in Fig. 2(b). At the welded zone, the grains were significantly refined to a size of 4.9 μm relative to the initial materials. In particular, the SCM415H side showed a finer grain size of 3.7 μm compared to the S20C side (6.0 μm), as shown in Fig. 2(c).
Grain boundary maps of the initial materials ((a) S20C and (b) SCM415H) and the (c) friction welded material.
The misorientation angle distributions of the initial and the welded steel materials acquired at grain boundary maps of Fig. 2 are shown in Fig. 3. The S20C steel material showed a notably high fraction of low angle grain boundaries, approximately 61%, owing to the hot extruded state without sufficient recrystallization, as shown in Fig. 3(a). On the contrary, SCM415 steel exhibited a high fraction of high angle grain boundaries, greater than 80%, owing to the fully recrystallized state as shown in Fig. 3(b). In the case of the welded material, the fraction of high angle grain boundaries was approximately 80%, as shown in Fig. 3(c).
Misorientation angle distributions of the initial materials ((a) S20C and (b) SCM415H) and the (c) friction-welded material.
The Vickers microhardness distribution of the friction-welded material is shown in Fig. 4. The base material zone has a microhardness value ranging from 195 to 223 Hv at the S20C steel side and 250 to 278 Hv at the SCM415H steel side, as shown in Fig. 4. The application of friction welding significantly increases the microhardness of the weld zone; the Vickers microhardness increase to 243–261 Hv at the S20C side and 330–360 Hv at the SCM415H side, respectively. In particular, the aspect of microhardness development was more prominent in SCM415H steel (22–32% increase from that of the base material) than S20C steel (17–24% increase from that of the base material).
Variation of Vickers microhardness of the friction-welded S20C and SCM415H steels along the cross-section.
The tensile properties of the friction-welded materials are shown in Fig. 5. As shown in the optical micrograph in Fig. 5(a), the tested specimen was first deformed and then fractured in the base material zone of S20C steel. The yield and the tensile strengths of the initial material were 245 and 402 MPa for S20C and 421 and 663 MPa for SCM415H, respectively, while their elongations were 28 and 18%, as shown in Fig. 5(b). After FW, the welded material exhibited yield and tensile strengths of 386 and 601 MPa, respectively, with an elongation of 15%, which were approximately 10% lower than those of the SCM415H base material.
(a) Optical micrograph and (b) tensile properties of the friction welded S20C and SCM415H steels.
The FW on dissimilar S20C and SCM415H steels led to significant grain refinement in the welds without the formation of a HAZ owing to the absence of coarsened grains near the welded zone, as shown in Fig. 1(b). In general, frictional heating (approximately 0.5–0.6 Tm) and metallic plastic flow that take place during FW are sufficient to produce dynamic recrystallization during welding14,15). Furthermore, severe metallic plastic flow during FW also contributes to simultaneous dynamic recrystallization, consequently leading to the formation of refined grains in the friction-welded zone relative to the base material14,19,20). In this study, the friction-welded zone (average 4.9 μm) showed significantly refined grains when compared to the base material (average 66.8 μm at S20C and 19.8 μm at SCM415H) as shown in Fig. 2. In addition, the high angle grain boundaries in the welded zone occupied more than 80% of the whole grain boundaries, which indicates the formation of a perfectly recrystallized state by the FW process, as shown in Fig. 3.
The grain refinement at the weld zone leads to an increase in the mechanical properties such as microhardness and tensile strength. Consequently, the Vickers microhardness of the friction-welded zone increased by more than 15% at the S20C side and 20% at the SCM415H side when compared to the base material zones, as shown in Fig. 4. In addition, the yield and the tensile strengths are slightly lower, by approximately 10%, without notable decrease in the elongation relative to the SCM415H base material, as shown in Fig. 5(b). In particular, the optical micrograph in Fig. 5(a) of the tensile tested specimen showed fracture at the S20C base material zone owing to the higher strength at the welded zone, which is also attributed to the refined grain size. In addition, phase transformations to bainite and martensite were suppressed owing to the low heat input (approximately 0.5–0.6 Tm as mentioned above), which contributes to the absence of formation of a HAZ. Therefore, grain refinement and suppression of phase transformation in welds can be effective for obtaining friction-welded steels with excellent mechanical properties.
S20C and SCM415H steels were soundly welded without the formation of any defects, such as voids, cracks, distortion, etc., in the welds. The application of FW led to grain refinement of the welds owing to dynamic recrystallization, which resulted in significantly improved mechanical properties, such as microhardness and yield and tensile strengths when compared to the base material. Furthermore, the absence of HAZ and phase transformation in the welds contributed to fracture at the base material zone, and not at the welded zone. Therefore, FW of dissimilar steels improves the microstructures and the mechanical properties of the welds when compared to fusion welding.
This study was supported by research fund from Chosun University, 2017.
