2017 Volume 58 Issue 5 Pages 711-715
The microstructures and mechanical properties of lightweight friction-welded dissimilar materials such as A6063 and A2017 alloy rods were investigated in this study. Friction welding was performed at a rotation speed of 2,000 RPM, friction load of 12 kgf/cm2, and upset force of 25 kgf/cm2. After welding, grain boundary characteristic distributions and the formation of intermetallic compounds were analyzed by electron backscattering diffraction and transmission electron microscopy, respectively, while the mechanical properties of the welded materials were studied by Vickers microhardness and tensile testing. The obtained results revealed that the friction welding of the two alloys led to significant grain refinement from around 50 μm for the base materials to 2 μm for the welded zone, while the Vickers microhardness and tensile strength of the welded area were equal to 81% and 96% of the corresponding values for the base materials, respectively, owing to the formation and growth of intermetallic compounds. However, the fracture initiated in the A6063 base material during tensile testing indicated superior quality of the welded joint. Therefore, friction welding of dissimilar materials can be effectively used to produce joints with high durability.
Al alloys are widely used in aircraft, automobiles, and electronic parts due to their excellent formability, corrosion resistance, and welding properties1–3). In addition, their density is approximately one-third of that of steel, which results in a higher specific strength4,5). In order to increase fuel efficiency, lightweight Al alloys have recently started to replace steel in various motor vehicle components, including airplanes, automobiles, and ships6–8); thus, some parts of car chassis, interior materials of chairs and steering systems, and engine block components manufactured from Al alloys have been successfully commercialized9,10). Additional applications of Al alloys in the auto industry were also considered; however, the reported research studies on the Al alloy utilization in automobile steering systems are very scarce.
Friction welding (FW), which is used to weld two materials in the solid state, can significantly affect their microstructures and mechanical properties due to its lower heat input as compared to fusion welding techniques11–13). In particular, FW suppresses the generation of harmful gases and is characterized by a lower consumption of electrical power during the welding procedure, which made limelight a green process14). In addition, the microstructure and mechanical properties of the obtained welds can be effectively controlled by varying welding parameters such as rotation speed, friction force, upset force, and upset length15,16).
In this study, we investigated the microstructure and mechanical properties of friction-welded A6063 and A2017 alloys and systematically explained the relationship between their corresponding parameters.
The chemical compositions of A6063 (extruded and recrystallized state) and A2017 (extruded state) alloys utilized in this study are listed in Table 1. In order to maximize surface contact during welding, two specimens with sizes of ϕ12 × 80 mm (A6063) and ϕ20 × 100 mm (A2017) were prepared and subsequently cleaned in acetone to remove any impurities (see Fig. 1). Afterwards, the specimens were friction-welded at a rotation speed of 2,000 RPM, friction force of 1.2 MPa, upset force of 2.5 MPa, and upset length of 2.3 mm using a Nitto-Seiko FW machine (model FF−30II−C).
| Material | Chemical composition (mass%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| A6063 | Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti | Al |
| 0.50 | 0.17 | 0.012 | 0.036 | 0.45 | 0.01 | 0.003 | 0.01 | Bal. | |
| A2017 | Si | Fe | Cu | Mn | Mg | Cr | Zn | Zr+Ti | Al |
| 0.75 | 0.27 | 3.86 | 0.63 | 0.50 | 0.01 | 0.05 | 0.03 | Bal. | |
Configurations of the A6063 and A2017 alloy specimens utilized in this study. ND, TD and WD in this figure indicate normal direction, transverse direction and welding direction, respectively.
The macro- and microstructure of the obtained weld were evaluated by optical microscopy (OM), while weld grain shapes, sizes, and misorientation angles were studied by electron backscattering diffraction (EBSD). For this purpose, specimens with dimensions of 2 mm × 20 mm were machined, mechanically ground, and electro-polished at an applied voltage of 20 V and temperature of −40℃ using a solution containing 100 ml of perchloric acid and 900 ml of methanol. The surface analysis for the obtained specimens was performed by an orientation image mapping technique incorporated into an SEM instrument.
Precipitates dispersed in the welded zone were studied by transmission electron microscopy (TEM) at an applied voltage of 200 kV using discs with diameters of 3 mm that were mechanically polished until the surface roughness of 80 µm was achieved and then thinned to <10 μm by ion milling. Mechanical properties of the welded specimens were evaluated by using Vickers microhardness and tensile testing procedures. The Vickers microhardness test was performed by applying a load of 1.96 N to a cross-section of the weld zone for a dwell time of 10 s. Tensile test specimens, having the width of 3 mm, gauge length of 13 mm and thickness of 2 mm, were used to evaluate the transverse tensile strength of the obtained friction welds, and tensile testing was conducted at room temperature and at 1 × 10−2 S−1.
The top view and macrostructure of the friction-welded A6063 and A2017 alloys are shown in Fig. 2. The obtained weld bead sizes were greater for A6063 alloy than for A2017 due to its ductility (see Fig. 2(a)); however, the resulting joint was characterized by sound welding without any defects such as voids, cracks, or holes (Fig. 2(b)).
(a) Top view and (b) cross-sectional macrostructure of the friction-welded A6063 and A2017 alloys.
Grain boundary character distributions for the initial materials were analyzed by EBSD, (see Fig. 3). The initial A6063 alloy was composed of grains with sizes ranging between 5 μm and 100 μm and relatively homogeneous grain size distribution, as shown in Fig. 3(a), while elongated grains with widths of 50–100 μm as well as fine grains with sizes of 5–10 μm were heterogeneously distributed across the A2017 microstructure (Fig. 3(c)). The produced A6063 and A2017 textures corresponded to <001>//ND and <112>//ND orientations with intensities of 14.18 and 2.97, respectively (Figs. 3(b) and 3(d)).
Orientation image maps (OIMs) and inverse pole figures (IPFs) for the (a–b) A6063 and (c–d) A2017 initial materials.
The microstructures of the friction-welded zone were analyzed by EBSD, and the resulting orientation image maps (OIMs) are shown in Fig. 4. The initial grain sizes were significantly decreased for both sides of the friction welded zone, and a heat-affected zone (HAZ) was not observed after welding due to the absence significant grain growth (Fig. 4(a)). However, a plastic flow phenomenon (corresponding to an elongation of the grain shapes) was observed in the A2017 zone due to the difference in plasticity between the two alloys. As a result, the grains in magnified zone A were refined to 2.2 μm for A6063 and to 3.4 μm for A2017 (Fig. 4(b)), while the grains at the weld interface near the A2017 side exhibited more refined sizes as compared to those located near the A6063 side.
OIMs for the friction-welded A6063 and A2017 alloys. (a) A cross-sectional microstructure of the welded area. (b) A magnified microstructure of the red dotted zone in (a).
The grain size distributions for the initial and welded materials are shown in Fig. 5. As was shown earlier, the initial alloy grains were heterogeneously distributed (Fig. 3), and their average sizes were equal to 54.9 μm for A6063 and 65.2 μm for A2017 (see Figs. 5(a) and (b)). However, the friction-welded materials were characterized by more homogeneous size distributions as compared to those for the initial materials (Figs. 5(c) and 5(d)). The A6063 grain sizes in the welding zone ranged from 0.5 μm to 4.5 μm (with an average value of 2.2 μm), while those for A2017 varied between 0.8 μm and 6 μm, and their average size was equal to 3.4 μm.
Grain size distributions for the (a–b) initial and (c–d) friction-welded A6063 and A2017 alloy materials.
The misorientation angle distributions for the initial and welded materials are shown in Fig. 6. The fractions of the high-angle grain boundaries for the initial A6063 and A2017 alloys were equal to 81% and 92%, respectively (Figs. 6(a) (b)); however, after FW, the corresponding values became 86% and 79% (Figs. 6(c) and (d)), indicating the formation of a dynamically recrystallized state. A slightly lower fraction value observed for A2017 alloy could be attributed to the plastic flow phenomenon, owing to the existence of elongated grains.
Misorientation angle distributions for the (a–b) initial and (c–d) friction-welded A6063 and A2017 alloy materials.
The Vickers microhardness distribution for the friction-welded material is shown in Fig. 7. The measured values ranged between 68 Hv and 87 Hv for base A6063 alloy and between 120 Hv and 128 Hv for base A2017. In the friction-welded zone, the Vickers microhardness value for A6063 alloy slightly decreased from 68 Hv to 58 Hv, while that for A2017 was reduced from 128 Hv to 90 Hv.
A cross-sectional profile for the Vickers microhardness of the friction-welded A6063 and A2017 alloys.
The tensile properties of the friction-welded materials are shown in Fig. 8. According to the top view (Fig. 8(a)), the tested specimen was first deformed and then fractured in the base material zone of A6063. The initial yield and tensile strengths were equal to 185.2 MPa and 214.5 MPa for A6063 and 277.0 MPa and 434.7 MPa for A2017, respectively, while their corresponding elongations were 13.8% and 23.6% (Fig. 8(b)). After FW, the welded material exhibited yield and tensile strengths of 179.5 MPa and 203.4 MPa, respectively, which were almost equal to the corresponding values for base A6063 alloy. However, the elongation of the friction-welded material was significantly reduced to 7%.
(a) Top view and (b) tensile properties of the friction-welded A6063 and A2017 alloys.
Intermetallic compounds (IMCs) produced by FW of A6063 and A2017 alloys were analyzed by TEM, and the obtained results are shown in Figs. 9 and 10. The initial A6063 material contained distributed needle-shaped β-phase and AlMnSi IMCs with sizes of 100 nm and 120 nm, respectively (Figs. 9(a) and (b)). However, the number of these species decreased with aging, while AlMnSi IMCs with sizes of 100 nm and Al2Cu IMCs with sizes of 80 nm were formed in the welded zone, owing to the friction heat produced during welding (Figs. 9(c) and (d)). The initial A2017 alloy contained Al2Cu and AlMnSi IMCs with sizes of 15 nm and 70 nm, respectively (Figs. 10(a) and (b)); however, the Al2Cu particles significantly grew after welding, while the AlMnSi IMCs retained their original sizes (see Figs. 10(c) and 10(d)).
Bright field TEM images for the (a–b) initial and (c–d) friction-welded A6063 alloy. The white circles denote the actually analyzed microstructure points.
Bright field TEM images for the (a–b) initial and (c–d) friction-welded A2017. The white circles denote the actually analyzed microstructure points.
The obtained results reveal that A6063 and A2017 alloys were soundly welded without producing any defects such as distortions, cracks, voids, and holes both at the weld surface and on the inner side (Fig. 2). FW is characterized by a lower heat input than the one required for various fusion-welding techniques such as gas tungsten arc welding, laser welding, and electron beam welding, and thus suppresses the formation of potential weld defects as discussed above11–13). In addition, welding joint defects such as voids and cracks are rarely formed due to the friction and upset forces, while oxide and carbide species are almost never produced during welding because FW is a solid-state process17). The joint microstructure can be controlled by varying welding parameters, such as rotation speed, friction force, and upset force, thus making it possible to suppress the HAZ formation at the welded joint (Fig. 4). Therefore, FW can be effectively used for sound welding of dissimilar materials.
The weld grain sizes were significantly reduced after FW. The initial average grain sizes were 54.9 μm for A6063 and 65.2 μm for A2017, which noticeably decreased to 2.2 μm and 3.4 μm, respectively (see Figs. 3 and 4), while the grain size distribution was significantly homogenized (Fig. 5), which could be explained by FW-induced dynamic recrystallization. In general, the FW process is accompanied by plastic flow, which makes the welded material potentially capable of accumulating and storing energy through simultaneous recrystallization of nuclei18,19). In addition, the values of the friction heat released during welding (around 0.5–0.6 Tm) were high enough to initiate the recrystallization of the welded material directly observed in this study (as indicated by the fractions of high-angle grain boundaries of over 79% for both materials; see Fig. 6). Therefore, FW is efficient in promoting grain refinement through dynamic recrystallization.
The mechanical properties of the welded materials, such as yield and tensile strengths, were affected by the formation of Al2Cu and AlMnSi IMCs during FW, in spite of the decrease in Vickers microhardness (Fig. 7). In general, the formation of IMCs has an effect on material tensile properties (in other words, the densities and sizes of IMCs produced during welding directly affect the yield and tensile strengths of the welded area)20). After welding, A6063 IMCs with sizes of 80–100 nm were more densely distributed across the resulting microstructure than in the initial material (Fig. 9), while the density of the A2017 IMCs increased, and their sizes were bigger than those in the initial material, as shown in Fig. 10. The observed increase in IMCs density helped to maintain the high yield and tensile strengths of the weld, though its elongation was significantly decreased compared to those for the initial materials, as shown in Fig. 8(b). As a result, the tested specimen was first deformed and then fractured in the A6063 base material zone, which was relatively weak in comparison with the welded area between A6063 and A2017 (Fig. 8(a)). Therefore, the formation of IMCs during welding helps to preserve the mechanical properties of the welded materials such as yield and tensile strengths.
FW of A6063 and A2017 alloys was soundly performed without producing any defects such as distortions, cracks, voids, and fractures, while significant grain refinement accompanied by dynamic recrystallization was observed in the welded zone. In addition, the formation of IMCs helped to preserve the material mechanical properties, such as yield and tensile strengths; as a result, the welded specimen was originally deformed and fractured in the area of the A6063 base material during tensile testing. Therefore, FW of dissimilar materials can effectively produce soundly welded joints, as indicated by the investigation of the microstructure and mechanical properties of the welded area performed in this study.
This study was supported by research fund from Chosun University, 2016.
