SUS321 joints at various rotation speeds

    Microstructure evolution and mechanical properties of rotary friction welded TC4/SUS321 joints at various rotation speeds Xun Li, Jinglong Li, Zhongxiang Liao, Feng Jin, Fusheng Zhang, Jiangtao Xiong PII: DOI: Reference:

S0264-1275(16)30319-7 doi: 10.1016/j.matdes.2016.03.037 JMADE 1515

To appear in: Received date: Revised date: Accepted date:

22 January 2016 26 February 2016 8 March 2016

Please cite this article as: Xun Li, Jinglong Li, Zhongxiang Liao, Feng Jin, Fusheng Zhang, Jiangtao Xiong, Microstructure evolution and mechanical properties of rotary friction welded TC4/SUS321 joints at various rotation speeds, (2016), doi: 10.1016/j.matdes.2016.03.037

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ACCEPTED MANUSCRIPT Microstructure evolution and mechanical properties of rotary friction welded TC4/SUS321 joints at various rotation speeds

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

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Xun Lia, b, Jinglong Lib, Zhongxiang Liaoa, b, Feng Jina, b , Fusheng Zhangb, *, Jiangtao Xiongb

Xi’an 710072, PR China

Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical

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b

University, Xi’an 710072, PR China

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* Corresponding author: Fusheng Zhang

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Fax: ++86-29-88491426

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Tel.: ++86-29-88460673

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E-mail: [email protected] (Fusheng Zhang)

ACCEPTED MANUSCRIPT Abstract The dissimilar joints between TC4 titanium alloy rod and SUS321 stainless steel rod were produced by rotary friction welding (RFW). The influence of rotation speed was investigated on

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the morphologic distribution of intermetallics (IMCs) formed at the interface and on the mechanical properties that were assessed by tensile test and microhardness. Results showed that

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the morphology of IMCs, mainly TiFe2 and TiFe formed on SUS321 side, transforms from dispersive pattern to laminar pattern as the rotation speed increases from 400 to 1800 rpm with a transition point around 600 rpm. The joint strength increases and then decreases after reaching a

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maximum value with increasing rotation speed. The highest tensile strength of 468 MPa, corresponding to the joint efficiency of 90%, was achieved at 600 rpm, where the valley value of

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equilibrium interfacial temperature was obtained. All of the dissimilar RFWed joints failed completely in cleavage fracture mode along the boundary between IMCs layers and TC4 matrix.

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Key words: Titanium alloy; Stainless steel; Rotary friction welding; Interfacial microstructure;

1.

Introduction

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Rotation speed; Fracture morphology

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Titanium and its alloy are well-known materials with the appealing characteristics of high specific strength and excellent corrosion resistance. However, the application of titanium alloy is limited by the cost. The sound joints between titanium alloys and stainless steel have become a great

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demand in chemical [1], nuclear [2] and cryogenic industry [3] because of a compromise between costs and lightweight in manufacturing. In order to obtain the joints, various methods, including fusion welding [4-6], solid-state welding [7-11], had been adopted. However, conventional fusion welding joints between titanium alloy and stainless steel often form excessive brittle intermetallics (IMCs, commonly TiFe and TiFe2) [1] at the interface, resulting in the degradation in joint strength. Therefore, conventional fusion welding is not feasible for this couple in most cases. Solid-state welding processes that possibly limit the extent of intermixing can be more suitable since metallurgical mismatch can be counteracted. Diffusion bonding is one of the most widely adopted solid-state welding to accomplish the dissimilar joining of this couple. Metal foils are usually inserted to inhibit mutual diffusion and then to suppress the formation of IMCs between titanium alloys and stainless steel [12]. Additionally, the highest strength for diffusion bonding of TC4/stainless steel couples is 520.1 MPa that reaches 78.8% of the base material strength [7], which

ACCEPTED MANUSCRIPT roughly satisfies the industrial applications. It was worth noting that efforts have been made on the hybrid combination between titanium and steel via rotary friction welding (RFW). The higher quality joints could be expected since RFW process has the characteristics of short welding cycle

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and extrusion of interfacial plasticized metal [13], which could effectively inhibit the IMCs formed at the interface.

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The factors on the strength of RFWed titanium/stainless steel joints are spindle rotation speed, friction pressure, upsetting pressure, burn-off length (or friction time), surface roughness. Futamata and Fuji et al. [14, 15] conducted dissimilar RFW on pure titanium /SUS304L, indicating that a

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higher friction pressure (> 196 MPa) and upsetting pressure (294MPa) , and a lower surface roughness (< 0.1 μm) of faying surface are beneficial to obtain sound joints. The highest joint

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strength reached 460 MPa with the failure occurring in the titanium base material. Akbarimousavi and GohariKia et al. [16] proposed a remedy for improving the tensile strength of RFWed pure

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titanium/AISI 316L, which is the preheating of tip end of stainless steel in accompany with a high

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upsetting pressure (350 MPa) to produce deformation from stainless steel and hence remove the oxide layer that is harmful for the quality of joints. The failure of the joints occurs in the titanium

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side as well. All the results show RFW as the prospective method in manufacturing titanium/steel joints. Yet, Li et al. [17] reported that a thicker intermetallic zone (IMZ), caused by a longer friction time, within the critical value of 3 μm proposed by Meshram et al. [18], results in a higher

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TC4/SUS321 dissimilar joints tensile strength. To inhibit the formation of Fe-Ti and Cr-Ti IMCs, Muralimohan et al. [19] obtained the titanium/SUS304 RFWed joints, which possess the highest strength of 352 MPa, via inserting the Ni interlayer. The formation of IMCs and mechanical properties are significantly affected by interfacial temperature in RFW process[16, 20], in which the rotation speeds emerge as an important operation parameter [21, 22]. Thus, the rotation speed consequently affects the joint strength. Futamata et al. [14] pointed out that a negative effect on the joint strength when the rotation speed is higher than 1560 rpm, corresponding to the periphery velocity of 1.06 m/s. In addition, the influence of rotation speeds on the mechanical properties of dissimilar RFWed joints between AA6061 and alumina–YSZ composite was investigated by Uday et.al. [23]. And the highest bending strength of joints and lower microhardness were achieved at 630 rpm, corresponding to the periphery velocity of 0.53 m/s, resulting from the less IMCs formed at interface caused by a lower friction temperature. As a result, the IMCs formed at the joint interface

ACCEPTED MANUSCRIPT plays a critical role in the strength of dissimilar couple. However, the effect of rotation speed on the morphology of IMCs and on the strength of this dissimilar couple was seldom reported. In this paper, RFW between TC4 and SUS321 was carried out and rotation speed was selected

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as key variable. Then the effects of rotation speed on equilibrium interfacial temperature, on the microstructure evolution and on tensile strength of the RFWed TC4/SUS321 joints were discussed

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in detail. The correlations between the rotation speed and equilibrium interfacial temperature, the morphologic distribution of IMCs at the interface and tensile strength were investigated. Additionally, reaction products, microhardness distribution of the joints and fracture morphology

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were discussed as well. Materials and methods

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The base metals are commercially available TC4 titanium alloy (Ti-6Al-4V) and SUS321 stainless steel (1Cr18Ni9Ti). Table 1 and 2 list their chemical compositions and part of mechanical

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properties at room temperature. The faying surfaces, prior to welding, were polished to eliminate the

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effect of surface roughness and ultrasonically cleaned in acetone and dried. Fig. 1 shows dimension and configuration of the samples to be welded and the samples for tensile tests. Both TC4 and

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SUS321 to be welded were machined into rods with  25mm×100mm, as shown in Fig. 1a. Whereas Fig. 1b shows the tensile samples prepared according to the GB2561-1989, with a gauge length of 70 mm and a diameter of 10 mm.

Metal

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Table 1

Al

C

TC4

5.5-6.75

≤0.08

SUS 321

—

Nominal chemical composition of base metals (wt.%.) Cr

≤0.08 17-19

Fe

Mn

Ni

Ti

Si

S

O

V

≤0.40

—

—

Bal

—

—

≤0.20

3.5-4.5

—

—

Bal

≤2.0 9.0-12 0.4 ≤1.0 ≤0.030

Table 2 Mechanical properties of base metal at room temperature Material

Tensile strength (MPa)

Yield strength (MPa)

Elongation (%)

Hardness (Hv)

TC4

900

830

10

320

SUS 321

520

205

40

220

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Fig. 1 Dimension (in mm) and configuration of (a) the samples to be welded and (b) tensile test

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samples (GB2561-1989) The welding experiment was conducted by a continuous drive rotary friction welding machine (C320, Hanzhong Shuangji Friction Welding Techniques Co. Ltd., China). Based on the

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conclusion in Refs.[15, 16], the parameters were set up as follows: friction pressure 150 MPa,

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burn-off length 5 mm, and upsetting pressure 300 MPa with upsetting time 10 s. Yet, the rotation speeds were designed as 400 rpm, 600 rpm, 1200 rpm, 1500 rpm and 1800 rpm, corresponding to

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the periphery linear velocity ranging from 0.53 m/s to 2.34 m/s. The interfacial temperature variation at the periphery of the joints was recorded using an infrared thermographic camera (InfraTec VarioCAM® hr head-HS) at a sampling frame rate of 50 fps, which focused on the

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specimen with a distance of 50 mm. After welding, the joints were cut to show cross-section. TC4 and SUS321 were etched using Kroll’s reagent (5 mL HF + 15 mL HNO3 + 80 mL water) and Curran’s reagent (5 g FeCl3 + 15 mL HCl + 60 mL water) respectively. Microstructure and elements distributions across the interface were evaluated by optical microscope (OM, Olympus–MPG3) and scanning electron microscope (SEM, MIRA 3) equipped with an energy dispersive spectrometer (EDS). Microhardness was measured along the perpendicular direction of the joints interface using a Vickers microhardness testing machine (Struers Duramin–A300) with a load of 100 g and dwell time of 15 s. Tensile tests were performed on a tensile tester (Instron 3382) with a crosshead speed of 1 mm/min. The fracture surfaces of the joints were observed by SEM and analyzed by SEM-EDS in spot scan mode and phases identified by XRD (DX2700) using CuKa radiation.

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Results and discussion

3.1 Joint morphology and interfacial temperature evolutions with rotation speed Fig. 2 shows the sectional morphology of the joints evolving with the rotation speed. The

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morphology presents a concave interface projected well into stainless steel. There is a single side flash bending towards titanium alloy side, which indicates a sufficient plastic flow of titanium

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alloy. Additionally, the rotation speed exerts little influence on the joint morphology, of which the concave interface depicts the feature of dissimilar metals welding. In fact, such asymmetrical deformation on both sides implies lower hot strength of titanium alloy that consequently bears

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large deformation at welding temperature. Fig.3 issues the temperature dependent yield strengths of both metals [24], of which titanium alloy shows a sharp decrease of yield strength as the

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temperature increases; whereas stainless steel possesses a relatively higher hot strength. Accordingly, titanium alloy becomes softer than stainless steel at a temperature higher than 600 ºC

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that is much lower than the welding temperature (around 1000 ºC as later discussed).

Fig. 2 Macrographs of the friction-welded joints at different rotation speeds

Fig. 3 Temperature dependent yield strength of TC4 and SUS321 [24]

ACCEPTED MANUSCRIPT Fig. 4 displays the evolution of temperature distribution along the perpendicular of interface during the early friction process. A gradual temperature gradient emerges on titanium alloy side during the welding process indicating a higher accumulation since the thermal conductivity of

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SUS321 (16.1 W·m-1·K-1) is much higher than that of TC4 (6.7 W·m-1·K-1). This result contributes to the asymmetrical deformation as well. The feature of concave interface implies that titanium

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alloy located at the center has lower temperature and plasticity, while that located at the periphery has a higher temperature, and has been so fully plasticized that the metal was extruded out of the interface to form single flash. Thus, it is reasonable to deduce that the interfacial temperature has

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the uneven distribution and increases gradually from center to the peripheral.

Fig. 4 Temperature distribution along perpendicular direction of interface at different friction time

In a welding cycle, the welding interface initially suffers friction heating and subsequently forms a joint at steady state, of which the interfacial temperature would affect the quality of metallurgical bonding. The welding cycle can be characterized by torque features. Fig. 5 displays the evolutions of friction torque and interfacial temperature during the friction welding process, in which the torque sharply rises to a peak torque and subsequently falls into steady state. Hence, the welding cycle can be classified as three stages, namely initial increase (phase 1), drop (phase 2) and steady state (phase 3). Additionally, a drastic friction heat is produced in phase 1 indicated by an abrupt increment of temperature. Thereafter, the temperature rises slowly in phase 2. In phase 3, the temperature rises gradually and even can be regarded as equilibrium state approximately.

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Fig. 5 Sampled torque and temperature evolution in a welding cycle at 600 rpm

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Fig. 6 Equilibrium interface temperature varied with rotation speed

Fig. 6 displays equilibrium interfacial temperature at phase 3 vs rotation speed, which presents a ―V-curve‖ with a minimum rotation speed of 600 rpm, namely the periphery velocity of 0.78 m/s. It has been expected that the heat generation is proportional to the rotation speed and thus temperature will increase, following the increment in rotation speed. However, equilibrium temperature of 600 rpm is lower than that of 400 rpm. It may be inferred that different mechanisms of heat generation and dissipation happen before and after 600 rpm. Besides, it is reported that the equilibrium interface temperature at the periphery of rods reaches maximum in the whole process[25, 26], as has been mentioned above. And the peak temperature exceeds the phase transition temperature of TC4 (990 °C) and varies with rotation speed. So the equilibrium interface temperature variation caused by rotation speed has a significant effect on the microstructure

ACCEPTED MANUSCRIPT evolution which will affect the strength of dissimilar joints. 3.2 Microstructure characterization Fig. 7 shows the interfacial microstructure of the joints welded at different rotation speeds.

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There are two regions observed on TC4 side around the interface: a dynamic recrystallization zone (DRZ) and thermo-mechanically affected zone (TMAZ). It is obvious that the width of DRZ on TC4

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side is much larger than that on SUS321 side, where there is almost no DRZ in all samples. The width of DRZ slightly decreases from 0.35 mm to 0.2 mm as the rotation speed increases from 400 rpm to 1800 rpm. Thus, it is concluded that a thin DRZ just appears on TC4 side indicating a severe

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deformation that only occurs on TC4 surface layer. However, the effect of DRZ on the strength of the joint can be negligible.

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In order to study the effect of rotation speed on the microstructure evolution, a series of micrographs are observed as shown in Figs. 8-12, which are acquired along the radial direction at

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rotation speeds of 400, 600, 1200, 1500 and 1800 rpm. The observed zone covers the interface from

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the center to the radius of 5 mm is divided into the center zone, the middle zone and the periphery zone. Figs. 8 and 12 show micrographs of the joints welded at 400 rpm and 1800 rpm respectively

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and corresponding line scanning analyses to show elements distribution along the direction of perpendicular to interface. The elements distribution of Fe or Ti has a small plateau part, implying that the formation of thin IMCs layers at the interface. It is known that the formation of IMCs layers

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are affected by the diffusion of elements. In principle, Fe atoms can easily diffuse into Ti matrix than the other way around [27] because the diffusion coefficient of Fe in Ti matrix is much larger than that of Ti in γ-Fe matrix at elevated temperature [28] and the compact structure of austenite structure (fcc) of SUS321 preventing the migration of Ti atoms. Consequently, the IMCs are formed on TC4 side [29]. However, the IMCs layers are formed on SUS321 side, which can be distinguished from the base material through backscattered electron (BSE) imaging contrast as shown in Figs. 8- 12 . The contradiction happens owing to the following factors. Firstly, the contact surface of TC4 , where the IMCs are expected to form, is continuously extruded out of the interface during the welding process, as has been discussed in Section 3.1. Moreover, the high strain rate on TC4 side accelerates the mirgration of Ti atoms into SUS321 side to some degree because of a decreasing of diffusion activation energy of Ti [30]. Lastly, the IMCs is often produced in the region where the concentration of Fe exceeds 50 wt.% on the basis of the anlysis of Fe-Ti binary phase diagram (Fig.

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13). Thus the interfacial region on SUS321 side is benefit for the formation of IMCs.

ACCEPTED MANUSCRIPT Fig. 7 Evolvement of microstructure of the joints welded at (a) 400 rpm, (b) 600 rpm, (c) 1200 rpm, (d) 1500 rpm and (e) 1800 rpm

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Fig. 14 summarizes the thickness distributions of IMCs layers at different rotation speeds. The thickness consequently increases with rotation speed and the uneven distribution of IMCs thickness

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along the radial direction occurs when the rotation speed is more than 1200 rpm. In addition, there is the maximum thickness of IMCs layer at the periphery. It is reported that the strain rate next to joints interface is proportionally related with the linear velocity [31]. The higher strain rate adjacent to the

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welding interface accelerates the elements diffusion, which is beneficial to the formation of IMCs. So a thicker IMCs layer should form at the position with a larger linear velocity. Neverthless, the

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width of IMCs layer does not exceed 1 μm at different rotation speeds. Thus it can be deduced that the effect of inhomogeneous distribution of IMCs thickness along the radial direction is negilible

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and rotation speed has little effect on IMCs thickness.

Fig. 8 Microstructure and line-scan analysis around (a) the center zone, (b) the middle zone and (c) the periphery zone of the joint welded at 400 rpm

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Fig. 9 Microstructure around (a) the center zone, (b) the middle zone and (c) the periphery

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zone of the joint welded at 600 rpm

Fig. 10 Microstructure around (a) the center zone, (b) the middle zone and (c) the periphery

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zone of the joint welded at 1200 rpm

Fig. 11 Microstructure around (a) the center zone, (b) the middle zone and (c) the periphery zone of the joint welded at 1500 rpm

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Fig. 12 Microstructure and line-scan analysis around (a) the center zone, (b) the middle zone

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and (c) the periphery zone of the joint welded at 1800 rpm

Fig. 13 Fe-Ti binary phase diagram[32]

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Fig. 14 Distribution of IMCs layers along the radial direction at different rotation speeds

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Table 3 The EDS results of different position at the joint interface (at. %) Location

Al

+1 +2 +3 +4 +5 +6 +7 +8 +9 +10 +11 +12

Fig. 8(b) Fig. 8(c) Fig. 9(a) Fig. 9(a) Fig. 10(b) Fig. 10(c) Fig. 10(c) Fig. 11(a) Fig. 11(c) Fig. 11(c) Fig. 12(c) Fig. 12(c)

5.52 2.76 1.90 3.73 1.72 2.58 5.62 2.27 2.96 4.42 6.07 2.26

Cr

Fe

Ni

Possible phase

39.15 20.17 21.40 36.70 13.49 27.34 49.35 26.29 29.98 51.82 51.52 31.43

12.14 18.61 17.67 12.09 21.13 14.85 6.85 13.77 11.77 6.24 6.33 12.91

38.03 52.92 52.88 41.17 57.55 49.29 32.25 51.13 48.98 32.06 30.76 48.11

3.76 4.65 5.23 5.37 5.19 4.91 4.67 5.89 5.70 4.57 4.11 4.68

TiFe TiFe2 TiFe2 TiFe TiFe2 TiFe2 TiFe TiFe2 TiFe2 TiFe TiFe TiFe2

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Ti

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Points

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Fig. 15 Gibbs free energy of the formation of intermetallics phase

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Fig. 16 Microstructure observation around the periphery zone welded at 1800 rpm It is worth noting that the distribution of IMCs presents different morphologies that can be divided into dispersive and laminar pattern at different rotation speeds. IMCs at the interface

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apparently emerge as dispersive pattern when the rotation speeds were set below 600 rpm, as

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shown by Figs. 8 and 9; whereas, the IMCs turn to laminar pattern when the rotation speeds were set more than 1200 rpm, as shown by Figs. 10- 12. To deduce the composition and species of the

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IMCs, EDS analysis of the feature points was carried out. According to Fe-Ti binary phase diagram, the reaction products at the interface mainly consist of TiFe and TiFe2. Table 3 lists the chemical composition and possible phases of the positions 1-12 in Figs. 8- 12. Results show that

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the IMCs may be TiFe at the positions of +1, +4, +7, +10, +11, and TiFe2 at the rest positions, as measured in Fig.8- Fig. 12. Thus, it can be concluded that the IMCs of TiFe are dispersed in the layer of TiFe2 and the volume fraction of TiFe increases as the rotation speed increases (Figs. 8 and 9). Yet, the laminar IMCs layers are composed of intermetallic phase TiFe and TiFe2 that forms near TC4 side when the rotation speed reaches 1200 rpm. The Gibbs free energy of formation of IMCs issues [32] )

-85500 410.041 -73.553 n )-0.01017

2

124212.42

-1

(1)

and )

-30028.003 4.495

(2)

where G is the Gibbs free energy and T is the reaction temperature. Fig. 15 illustrates the data derived from Eqs. (1) and (2). The data reveals that the free energy of the formation of TiFe is far

ACCEPTED MANUSCRIPT larger than that of TiFe2 at the welding temperature ranging from 1000 to 1250 °C. Thus TiFe2 is more probable to form at the interface in aspect of free energy. In addition, the strongest bond of TiFe2 is Fe-Fe (26.96 kJ/mol), which makes it easy for the aggregation of the same atoms, while

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that of TiFe is Fe-Ti (14.77 kJ/mol) resulting in the formation of ordering phase [33]. This difference leads to a wider range of phase composition of TiFe2 in Fe-Ti binary phase diagram

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(Fig. 13). Furthermore, TiFe2 has stronger covalent bond energy, that is to say, TiFe2 possesses a better thermal stability. As a result, TiFe2 firstly appears at the interface. The sequence of IMCs phase formation at interface is determined not only by the thermodynamic criteria but also the

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diffusion process of the elements. IMCs are formed on SUS321 side where possesses enough Fe atoms for the nucleation of TiFe2. The diffusion of titanium into steel contributes to the first

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formation of TiFe2. Thereafter, TiFe nucleates and grows in laminar shape when the thickness of TiFe2 reaches a critical value that alters the diffusion flux of Fe and Ti at the interface [34] and the

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accumulation of Ti atoms via diffusion process at the interface. The reasons concerning the

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dispersion of TiFe primarily includes two aspects when the rotation speeds are around 600 rpm. On one hand, the relatively lower temperature (Fig.6) and strain rate at the interface reduce the

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diffusion coefficient, which is unable to supply sufficient Ti atoms to participate in the production of TiFe. Hence, the thickness and morphologic distribution of IMCs can be altered, to some degree, by restricting the diffusion process appropriately. On the other hand, a higher formation energy of

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TiFe requires a large amount of energy input that is not enough at such low rotation speeds. So TiFe is dispersedly formed in the TiFe2 layers. As the rotation speed increases, the amount of TiFe increases owing to the higher friction heating. It is worth noting that the morphology of TiFe2 turn to irregular distribution around center zone at 1500 rpm (Fig. 11(a)). In addition, micro cavities with the diameter of approximately 4 μm were just formed around the periphery zone at 1800 rpm (Fig. 16). This defect can be attributed to overheating of TC4 due to the highest welding temperature (Fig. 6) and strain rate at the interface. As a result, the rotation speed apparently affects the morphological distribution of IMCs rather than the thickness of IMCs layer. 3.3 Mechanical property of the joints 3.3.1

Microhardness

Fig. 17 depicts the microhardness distribution along the direction of perpendicular to the joint interface. The microhardness profiles for all the rotation speeds is almost similar, i.e., the hardness

ACCEPTED MANUSCRIPT of the TC4 substrate immediately adjacent to the joint interface is higher than that in the bulk material. The phenomenon is ascribed to the grain refinement of DRZ on TC4 side and the formation of hard IMCs on SUS321 side. Owing to the major factor affecting dynamic

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recrystallization manner that is strain rate rather than temperature[35], the grain refinement adjacent to the joints interface is primarily restricted by strain rate associated with the rotation speeds during

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the welding process.

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Fig. 17 Microhardness distribution along the center line of the joints

According to Hall-Petch relationship, a higher maximum hardness values of TC4 substrate next to the interface resulting from grain refinement are obtained at higher rotation speeds as presented in Fig. 9. Thus, the values of maximum hardness on TC4 side are positively related with the rotation speed. Besides, the microhardness almost remains unchanged on SUS321 side in comparison to that on TC4 side. This indicates that thermal softening, resulting from the heat input, does not occur on SUS321 side. 3.3.2

Tensile strength analysis

Fig. 10 presents the tensile strength of the joints as a function of the rotation speed. Each value is obtained by averaging three samples. The tensile strength profile presents an inverted ―V-curve‖ and the maximum tensile strength, 468MPa (90% of SUS321 substrate strength), is achieved at the rotation speed of 600 rpm, indicating the same critical value of 600 rpm in temperature as discussed

ACCEPTED MANUSCRIPT in Section 3.2. As the rotation speed increases from 600 rpm, the strength degrades and the temperature increases accordingly. Furthermore, at 600 rpm, the measured strengths are much concentrated with a small deviation of 6 MPa as indicated in Fig. 18, which suggests a reliable

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operation parameter around 600 rpm. Such results imply that the strength of the joint strength is inversely proportional to the temperature that affects the formation of IMCs.

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It is reported that the tensile strength of dissimilar joints greatly lies on the characteristic of IMCs layers at the interface. The strength of TC4/SUS321 joint is sensitive to the IMCs layer thickness[15, 36]. Furthermore, Meshram et al. proposed a critical IMCs thickness of 3 μm [18],

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below which the effect of IMCs is negligible. Nonetheless, the thickness of IMCs layer has not consistently exceeded 1 μm as discussed in Section 3.2. Thus the width of IMCs layer is not the

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major factor on the tensile strength of this dissimilar couple.

The morphological distribution of IMCs at the interface should be responsible for the joint

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strength. When the rotation speeds do not exceed 600 rpm, the impact of TiFe that is dispersed in

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TiFe2 layers, on mechanical properties is similar to that on particle hardening, i.e., TiFe serves as a dispersive particles reinforcing the IMCs layers of TiFe2 at the interface. As a result, a relatively

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higher tensile strength of the joints is achieved around such low rotation speeds. According to the strengthening theory, the hardening increases with the volume fraction and strength of the dispersion [37]. Thus the tensile strength of joints welded at 600 rpm is moderately higher than

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that at 400 rpm. When rotation speeds were set over 1200 rpm, due to the existence of laminar IMCs, stress concentration would occur between layers of brittle IMCs, which results in crack initiation and propagation and then leading to the degradation in the joints strength. It should be pointed out that the irregular distribution of TiFe2 around center zone at 1500 rpm would be further responsible for the degradation of the joint strength. Additionally, the tensile strength of the joints welded at 1800 rpm drops sharply on account of the presence of micro cavities at the interface (Fig. 16). Results indicate that it is sufficient for improving the tensile strength of this dissimilar friction welded joints at a relatively lower rotation speed and the optimal rotation speed is 600 rpm corresponding to the periphery velocity of 0.78 m/s in this study.

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Fig. 18 Variation of average tensile strength as a function of rotation speed

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3.4 Fracture morphologies and phases analysis

The fracture surface of tensile samples was observed by SEM and EDS spot scanning to understand the failure mechanism. The failure of the joints occurs only by fracture along the

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interface. Fig. 19 shows the fracture surface morphology and EDS result acquired from the central

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zone on each side of tensile failure samples. The fracture morphologies at higher magnification (Fig.

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20) clearly indicate that it is a cleavage fracture because of the presence of cleavage steps and river pattern. It is clear that the fracture surfaces are relatively rough at low rotation speeds than that at high rotation speeds and this indicates that a high strength achieved at a low rotation speed as

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discussed in Section 3.3.2. In addition, the surface of TC4 side is more flat than that of SUS321 side. The results of EDS points scan acquired from the river pattern indicate that the cracks propagate along the boundary between IMCs layers and TC4 substrate. The percentage of Ti atoms on SUS321 side is higher than that of Fe atoms on TC4 side. In addition, the atomic ratio of Fe to Ti on SUS321 side is approximately 2, which is inferred as the intermetallic, such as TiFe2. It reconfirms the result that the IMCs are mainly form on SUS321 side.

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Fig. 19 Morphologies of the fracture surface at central part of TC4 side (left column) and SUS321 side (right column) and EDS results (at.%) at different rotation speeds: 400 rpm (a,

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b), 600 rpm (c, d), 1200 rpm (e, f), 1500 rpm (g, h), 1800 rpm (i, j)

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Fig. 20 High magnification of fracture surface of the joint welded at 600rpm on (a) TC4 side

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and (b) SUS321 side

Fig. 21 XRD patterns on fracture surface of the joints welded at rotation speed of 600 rpm on (a) TC4 side and (b) SUS321 side

The IMCs were also analyzed by XRD on both fracture surfaces as shown in Fig. 21. XRD spectra for the fracture surfaces of TC4 side and SUS321 side reveals phases as TiFe, TiFe2, AlFe2V and Cr2Ti, α-Ti, TiV, Fe0.1Ti0.135V0.765, NiTi, among which the main peaks only corresponding to TiFe2 or TiFe emerges on SUS321 side. The detected IMCs verifies the occurrence of IMCs at interface as discussed in Section 3.2. Based on the discussions above, the origination and propagation of cracks in tensile joints could be deduced. During the tensile test, the deformation firstly occurs on SUS321 substrate ascribed to its lower yield strength at room

ACCEPTED MANUSCRIPT temperature. The stress later concentrates at the boundary between IMCs and TC4 substrate, where cracks originate and propagates along this boundary. Some metals are torn from the substrates in the process of propagation.

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Conclusions

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The effect of rotation speed on rotary friction welding of TC4/SUS321 dissimilar couples have

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been investigated. Based on the analysis of temperature field, microstructure characterization and mechanical properties assessment, the following conclusions can be drawn: 1.

As the rotation speed increases from 400 rpm to 1800 rpm, the profile of equilibrium interface

achieved at rotation speed of 600 rpm.

The morphological distribution of IMCs transforms from dispersive pattern to laminar pattern.

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2.

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temperature depending on the rotation speed presents ―V-curve‖ and a minimum value is

When the rotation speed does not exceed 600rpm, the dispersive pattern that TiFe serves as the

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dispersion in the layer of TiFe2 exists and the volume fraction of TiFe increases as the rotation

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speed increases. When the rotation speed exceeds 1200 rpm, laminar pattern consisting of TiFe2 layer and TiFe layer exists. In addition, the thickness of IMCs does not exceed 1 μm at different

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rotation speeds. The rotation speed mainly affects the morphologic distribution of IMCs rather than their thickness. 3.

The microhardness of TC4 substrates immediately adjacent to the interface is larger than that

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of base material, while that of SUS321 substrates has almost no change under the welding conditions. The maximum hardness on TC4 side are positively related with the rotation speed. 4.

The difference in morphological distribution of IMCs is responsible for the variation of tensile

strength. The highest average tensile strength of 468 MPa reaches 90% of SUS321 base material at a rotation speed of 600 rpm, which is considered to be optimal rotation speed at such condition, just corresponding to the extremum of equilibrium interface temperature. 5.

All the joints failed in cleavage fracture mode along the interface, which results from brittle

IMCs like TiFe, TiFe2. The IMCs are mainly on the fracture surface of SUS321 side implying that the crack propagated along the boundary between IMCs and TC4 substrate. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grand No: 51475376, 51575451).

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights The effect of rotation speed on morphology of intermetallics, joints strength, microhardness and interfacial temperature was studied.

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The variation of joint efficiency, morphology of intermetallics and interfacial temperature with rotation speed has the same critical point.

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A maximum joint efficiency of 90% is achieved at 600 rpm. All dissimilar joints fractured along the boundary between intermetallics and TC4 matrix.

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