Improving the mechanical properties of 2219-T6 aluminum alloy joints by ultrasonic vibrations during friction stir welding

Accepted Manuscript Title: Improving the Mechanical Properties of 2219-T6 Aluminum Alloy Joints by Ultrasonic Vibrations during Friction Stir Welding Authors: Yanying Hu, Huijie Liu, Hidetoshi Fujii PII: DOI: Reference:

S0924-0136(19)30102-5 https://doi.org/10.1016/j.jmatprotec.2019.03.013 PROTEC 16153

To appear in:

Journal of Materials Processing Technology

Received date: Accepted date:

21 February 2019 10 March 2019

Please cite this article as: Hu Y, Liu H, Fujii H, Improving the Mechanical Properties of 2219-T6 Aluminum Alloy Joints by Ultrasonic Vibrations during Friction Stir Welding, Journal of Materials Processing Tech. (2019), https://doi.org/10.1016/j.jmatprotec.2019.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improving the Mechanical Properties of 2219-T6 Aluminum Alloy Joints by

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Ultrasonic Vibrations during Friction Stir Welding

Yanying Hu1, Huijie Liu1,*, Hidetoshi Fujii2

1

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin

150001, P.R. China.

Joining and Welding Research Institute, Osaka University, Mihogaoka 11-1, Ibaraki 567-0047,

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2

Corresponding author. Tel.: +86 451 8641 3951; Fax: +86 451 8641 6186.

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Email address: [email protected] (H.J. Liu).

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*

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Japan

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Graphical abstract

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Abstract Friction stir welding assisted by ultrasonic vibrations on the bottom surface of workpieces (UVBSFSW) is proposed. The aim of this study is to reveal the effect of ultrasonic vibrations on the microstructure and mechanical properties of the joint. For comparative purposes, another joint without

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the effect of ultrasonic vibrations was obtained using the same welding parameters. The morphologies of onion skin on the top surface are changed and the distance from the wave crest to the trough is directly

reduced from 90±3 μm to 67±3 μm as a result of the ultrasonic vibrations. The macrostructures indicate

that more material around the probe is softened and takes part in the plastic deformation due to the Blaha

effect of the ultrasonic vibrations. A higher intensity of the texture and typical 𝐵/𝐵̅ texture component in the stir zone (SZ) were detected in the UVBS-FSW joint. The hardness distributions are more

homogeneous and the density of geometrically necessary dislocations, stored strain energy and residual

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stress in the joint are all reduced due to the application of the ultrasonic vibrations. Finally, the tensile

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property and fracture elongation of the joint are improved by UVBS-FSW.

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Keywords: Ultrasonic vibrations, friction stir welding, Aluminum alloys, Microstructure, Mechanical

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properties.

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1. Introduction

Applications of ultrasonic vibrations during material processing has been discussed since the early

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1950s. Blaha and Langenecker (1955) discovered that the mean stress level necessary to maintain plastic flow decreased for the aluminum and zinc single crystals. This work-softening phenomenon is called the

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Blaha effect or acoustoplatic effect. Further studies were conducted by many researchers and the Blaha effect was also detected in the polycrystals of copper, titanium and steel. It may be argued that the Blaha effect is similar to thermal softening. However, Langenecker et al. (1966) found that in the case of

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aluminum crystals an ultrasonic energy of 1015 eV/cm3 was sufficient to reduce the stress to nearly zero, while 1022 eV/cm3 of thermal energy was required to cause the same amount of stress reduction. Siu et

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al. (2011) reported that the strength of Ni3Al increased with the increasing temperature, while ultrasonic vibrations softened it just like other materials. Furthermore, after cessation of the insonation, the stress necessary to continue plastic flow attained a value that would have been reached if insonation had not occurred, indicating the difference between Blaha effect and thermal softening. Ultrasonic vibrations have been used as a means for the facilitation of the plastic flow of metals. Superimposing ultrasonic vibrations over plastic deformation, for example, wire drawing, upsetting and

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extrusion has evoked much attention. A series of results have indicated that the Blaha effect due to ultrasonic vibrations could only be seen in the plastic period rather than the elastic period. Rozner (1971) studied the effect of ultrasonic vibrations on the friction coefficient between a rigid steel die and plastic meal strips. It was concluded that the drawing force and die force were both reduced by the application

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of ultrasonic vibrations. Pohlman and Lehfeldt (1966) designed an experimental set-up which made it

possible to test the friction force between an oscillating ball and a turntable. They reported that at low

contact speeds, the friction force may be reduced by ultrasonic vibrations to very low percentages, while with the increasing contact speeds, the effect became smaller quickly. Ahmadi et al. (2015) investigated

the effect of grain size on the Blaha effect of pure aluminum and found that the specimens with the grain size of 109 μm showed a stress reduction of 66% while the grain size of 0.97 μm only demonstrated a reduction of 11.3%.

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Although friction stir welding (FSW) is one of the welding techniques, it has a much closer

connection to severe plastic deformation due to its characteristic of dynamic crystallization in the solid

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state. Hence, the application of ultrasonic vibrations during FSW has great potential for improving the

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joint quality and welding efficiency. Many investigators superimposed the ultrasonic vibrations on the tool or on the top surface of the workpieces. The experimental results of Park (2009) and simulation

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results of Montazerolghaem et al. (2012) confirmed that the axial force was reduced by superimposing the ultrasonic vibrations. Liu et al. (2015) stated that the application of ultrasonic vibrations during FSW

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can promote the plastic material flow in the joint. Shi et al. (2015) compared the thermal cycle during friction stir welding with and without the effect of ultrasonic vibrations. They found that the temperature

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difference between the two processes at the identical location was less than 20 K and concluded that the thermal effect of the ultrasonic vibration was negligible. These results indicated that the ultrasonic

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vibrations could not only reduce the axial force and thus enhance the material flow, but also avoid an increase in temperature. Therefore, superimposing the ultrasonic vibrations during FSW is extremely interesting and worthwhile process. The original temperature distribution during FSW results in a poor

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material flow in the root. Moreover, taking the damping of the ultrasonic energy into further consideration, it is more appropriate to apply the ultrasonic on the bottom surface of the workpieces

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since the strongest vibrations should be directly transferred to the region most in need. The backing plate in FSW, an indispensable support structure for the welding process, is the most troublesome aspect of this idea due to lack of space for containing the sonotrode below the workpieces. In the present study, friction stir welding assisted by ultrasonic vibrations on the bottom surface of the workpieces (UVBS- FSW) is proposed. To reveal the contribution of the ultrasonic vibrations, the microstructure characteristics and mechanical properties of the UVBS-FSW joint are compared with

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those of the joint obtained using the same welding parameters but without the effect of ultrasonic vibrations.

2. Experimental procedures

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2.1 Base material and welding process The base materials (BM) were 2219-T6 aluminum alloys (300 mm×100 mm×5 mm). The tensile

strength and fracture elongation of the BM were 440 MPa and 11.2%, respectively. A conical threaded tool with a length of 4.75 mm was used to weld the workpieces. The shoulder diameter and probe

diameter were 14 mm and 3.4-6.0 mm, respectively. The plunge depth of the shoulder was 0.2 mm and

the tool was tilted at an angle of 2.5° during the welding process. The welding process was performed at three welding speeds, i.e. 200 mm/min, 300 mm/min and 400 mm/min, whereas the rotation speed was

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constant at 800 rpm. However, to substantially reveal the microstructure characteristics and the intrinsic relationship between microstructure and mechanical properties, the detailed discussion in Section 3-5

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was mainly focused on the joints obtained at 300 mm/min because it was the optimal parameter in contrast. On the other hand, to make the positive effect of ultrasonic on mechanical property

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were completely represented in Section 5.

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improvement convincible and reliable, statistical data of tensile test results at diverse welding speeds In order to apply the ultrasonic vibrations on the bottom surface of the workpieces, the backing

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plate of the conventional FSW had to be abandoned, however, its support function must be undertaken by another structure. Otherwise, the plastic material in the weld zone under extrusion of the tool would

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inevitably collapse. Based on this idea, a support column was developed and installed underneath the workpieces to tolerate the axial force. The column was not rotated but only traveled at the same welding speed as the tool. Its diameter was 18 mm and a 2 mm fillet was machined around the periphery of the

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working plane. When the backing plate is replaced by the column, there is sufficient space for the sonotrode. In the present study, the sonotrode was placed in front of the column, as shown in Fig. 1.

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Hence, friction stir welding assisted by ultrasonic vibrations on the bottom surface of workpieces (UVBS- FSW) was achieved. The tip of the sonotrode was a round flat surface with a diameter of 4 mm. The contact force between the sonotrode and the workpiece was 250 N. During the welding process, the

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sonotrode was moved along the faying surface and the speed of it was the same as that of the tool and column. The vibration frequency and unloaded amplitude were 20 kHz and 40 μm, respectively. To reveal the contributions of the ultrasonic vibrations, another joint with the sonotrode power off was welded using the same welding parameters and the same machine. In an effort to avoid confusion, the welding process without the effect of ultrasonic vibrations is described as column supporting friction stir welding (CS-FSW). 4

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Fig. 1 The schematic drawing of friction stir welding assisted by ultrasonic vibrations on the bottom surface of workpieces

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(UVBS- FSW).

2.2 Testing instruments

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An optical microscope (OM, Olympus-BX51M, Japan) was used to observe the microstructures of the joints after etching by Kroll’s reagent (92 ml distilled water, 6 ml HNO3 and 2 ml HF) at room

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temperature. Scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) analyses were conducted using a S-4300SE field emission SEM equipped with a TSL OIMtm EBSD

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system at the accelerating voltage of 20 kV. The specimens for the EBSD analyses were sectioned, ground by sandpapers, mechanically polished and electrically polished. A solution of C2H5OH: HClO4=4:1 by volume was prepared for the electrical polishing at 20 V and 0 ℃. A digital image

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correlation (DIC) was used to record the strain histories during the tensile tests. The cross-sections of the joints were spray-painted with a white background, then black speckles providing a random speckle

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pattern for the DIC program to follow. During the tensile tests, the traces of the black speckles were captured by a high-speed camera. The positions of the black speckles in the first image were used to calculate the strain histories in the longitudinal and transverse directions. Details about the computational principles have been reported in the Ref. (Huh et al., 2013). The hardness distributions of the joints were measured along the lines in a distance of 0.5 mm, 2.5 mm and 4.5 mm from the top surface using a Vickers indenter with a load of 200 g and a dwell time of 10 s.

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3. Onion skin and fluctuating law The onion skin on the top surface is the most prominent feature of a friction stir welded joint, meanwhile, it is direct evidence of periodical plastic deformation. Figure 2a) and 2b) shows the three-

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dimensional periodic patterns of the onion skin in UVBS-FSW joint and CS-FSW joint. It has been well accepted that the intervals between neighboring wave crests or troughs on the top surface are determined by the following equation: 𝑑=

𝑣 𝜔

=

300 mm/min 800 r/min

= 375 μm/r

(1)

In fact, the average calculated value in the two joints are both 374±8 μm, nearly equal to the

distance traveled during one revolution of the tool. The outlines of the onion skin are shown in Fig. 2c). It can be seen that in the CS-FSW joint, the amplitude from wave crest to trough is 90±3 μm. However,

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after applying ultrasonic vibrations, the value of the amplitude is directly reduced to 67±3 μm.

Furthermore, the profile of the onion skin is significantly influenced by the ultrasonic vibrations. If the

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distance between neighboring wave troughs is defined as one revolution, it is divided into the ascent

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stage and descent stage for the sake of convenient descriptions. Since the proceeding velocity of the tool is constant, the time ratio is equal to the distance ratio. As can be seen in Fig. 2c), the ascent time and

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descent time in one revolution are represented by T1 (t1) and T2 (t2), respectively. Calculating the value of T1:T2, it is equal to 1:2 and t1:t2 is equal to 1:0.68. This phenomenon indicates that after application of

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the ultrasonic vibrations, the ascent stage of the onion skin in one revolution is shortened and the descent CS-FSW one.

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a)

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stage is prolonged. Furthermore, the descent part of the UVBS-FSW joint is not as smooth as that of the

90 μm

b)

110 μm

75 μm

90 μm

50 μm

60 μm

25 μm

30 μm

0 μm

0 μm

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Height, m

c)

140 120 100 80 60 40 20 0 -20

t2

t1

UVBS-FSW

T2

T1

67 μm

90 μm

CS-FSW

Fig. 2 The 3D periodical patterns of the onion skin on the top surface a) UVBS-FSW, b) CS-FSW and c) fluctuating outlines. 6

In order to understand the effect of ultrasonic vibrations on the fluctuating onion skin, it is better to first discuss the formation process of onion skin. Cui et al. (2008) proposed that the periodical variation in the stress state led to the periodical variation of the surface height thus producing the onion skin. The authors in the present study agree with this point by Cui et al. However, they stated that it was the

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rotation of the threads on the probe that resulted in the change of the stress state. In fact, whatever tools, materials and parameters are used, the onion skin is always formed on the top surface of the friction stir welded joints. Zhou et al.(2018) processed Mg alloy joint by double-sided FSW and also found the

formation of onion skin. Hence, the understanding of the formation mechanism of the onion skin is still in its early stage. A schematic drawing is demonstrated in Fig. 3. For the material stirred by the tool, it

has to be changed from rigid to plastic until deposited behind the tool. Fig. 3a) and 3e) show the position of the tool at t0, i.e., the last revolution has just finished and the shoulder is located at the trough. In the

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next moment, fresh rigid material on the advancing side (AS) will be extruded by the proceeding tool

and take part in a new revolution (Fig. 3b). At the beginning, the temperature of the fresh rigid material

c)

d)

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b)

a)

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is low. Since there is a strong negative correlation between the yield strength and temperature, the fresh

t0

f)

g)

h)

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e)

t3

t2

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t1

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t0

t2

t3

t1

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Fig. 3 The formation mechanism of the onion skin: a)-d) top view at different moments, and corresponding e)-h) longitudinal section.

rigid material is initially very difficult to be pressed down by the tool. Hence, the tool has to slightly

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move up, as shown in Fig. 3f). When the material travels from the AS to retreating side (RS), it is gradually softened due to the severe extrusion and friction between the tool and workpieces (Fig. 3c). Accordingly, the tool is able to fall back (Fig. 3g). With the welding process proceeding, the plastic material is rotated behind the probe and begins to deposit in the trailing edge. At this moment, the load between the plastic material and tool reaches its minimum value, leading to the lowest position of the shoulder (Fig. 3h). Because the alternate transformation from rigid materials to plastic materials in each

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evolution results in the fluctuation of the tool position, it leads to the formation of the onion skin on the top surface of the joint. With the application of ultrasonic vibrations during the welding process, the Blaha effect contributes to the faster softening of the extruded materials. As a result, the amplitude from the wave

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crest to trough is significantly reduced and the ascent stage of the onion skin in one revolution is also obviously shortened, as shown in Fig. 2.

4. Microstructure characteristics and formation mechanism

The optical cross-section macrostructure of the joints is represented in Fig. 4. The stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) are separated by the dash lines. As can be seen from Fig. 4, the widths of the SZ in the center and bottom regions are both

measured. The center and bottom widths of the SZ in the UVBS-FSW joint are 5.2 mm and 4.1 mm,

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respectively. However, without the effect of ultrasonic vibrations, both the center and bottom widths are

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only 4.8 mm and 3.7 mm, respectively. This phenomenon indicates that ultrasonic vibrations make more material around the probe be softened and take part in the plastic deformation. Similarly, they also

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broaden the TMAZ on the AS, as shown in Fig. 4a). In contrast, the ultrasonic vibrations have little

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influence on the TMAZ on the RS, except for a longer travelling distance toward the SZ in the upper region. Accordingly, it can be concluded that the ultrasonic vibrations could not only help the tool drive

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more material, but also promote the material travelling a longer distance.

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b)

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a)

5.2 mm

4.1 mm

2 mm

4.8 mm

3.7 mm

2 mm

Fig. 4 The cross-section macrostructure of the joints: a) UVBS-FSW, b) CS-FSW.

Figure 5 shows the EBSD maps taken from the center of the SZ and TMAZ on the AS, as indicated 8

by the rectangles in Fig. 4. Individual grains are colored according to their crystallographic orientation relative to the WD with an orientation code triangle being shown in the bottom right corner. The black lines and white lines in the maps indicate the high-angle boundaries (HABs) and low-angle boundaries (LABs), respectively. To avoid spurious boundaries caused by orientation noise, a lower limit boundary

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misorientation cut-off of 2º was employed. It can be seen from Fig. 5a) and 5b) that the SZ is

characterized by fine equiaxed grains blended with coarser irregular shaped grains containing significant LABs with various stages of misorientation development. Ly et al. (2018) found that the refinement of

the grains was attributed to the dynamic recovery and the continuous dynamic recrystallization (CDRX). The EBSD maps of the TMAZ on the AS of the joints are illustrated in Fig. 5c) and 5d). It can be seen

that the grains in the UVBS-FSW joint are elongated more drastically than those in CS-FSW joint due to

d )

c)

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a)

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b )

ND

TD

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WD

Fig. 5 EBSD maps illustrating grain structure at a) SZ, UVBS-FSW joint, b) SZ, CS-FSW joint, c) TMAZ on AS, UVBSFSW joint, and d) TMAZ on AS, CS-FSW joint.

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excellent flow ability of the plastic material. By comparison, the grains in UVBS-FSW joint are coarser than those without the effect of ultrasonic vibrations. Kaibyshev et al. (2005) stated that the microstructure evolution during the CDRX consists of two sequential processes; i.e., (i) the formation of

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three-dimensional arrays of the LABs, and (ii) the gradual transformation of the LABs into HABs. LABs with a low misorientation are continuously formed by dynamic recovery during deformation by rearrangement of the accumulating lattice dislocations. Mobile dislocations move across the LABs and are trapped by the sub-structures resulting in an increase in their misorientation. The rotation of subgrains also occurs due to the absorption of dislocations into the sub-grain boundaries until they develop a high misorientation. It is conjectured that the movement of the dislocations and rotation of the sub-grains 9

are presumably both enhanced when an oscillatory stress is superimposed. As a result, the grain size in the UVBS-FSW joint is coarser. The thermal cycle is one of the most important aspects for the understanding to microstructure characteristics. A known temperature distribution is also helpful to evaluate the effect of ultrasonic

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vibrations. In the present study, the thermal cycles have been measured during the welding process, as

represented in Fig. 6. Before the welding experiment, a hole with depth of 2 mm was drilled from the top surface of the workpiece in which a thermocouple was put into and connected to the hole bottom. To

protect the thermocouple from being broken by the rotating tool, the hole had to be drilled 2 mm from

the periphery of the shoulder. As can be seen in Fig. 6, the peak temperature of the UVBS-FSW joint in the tested region is 333 ºC which is nearly equal to that of the CS-FSW joint. Similar experimental results were also observed by Shi et al. (2015) and Lai et al. (2014). This result indicated that the

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application of ultrasonic vibrations would not lead to a significant thermal effect on the welding process. Schmidt et al. (2008) stated that the heat generation and material flow during the FSW were fully

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coupled, i.e., the heat generation is related to the material flow and frictional/contact conditions and vice

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versa. When the ultrasonic vibrations are superimposed, the average friction coefficient and yield shear stress are reduced due to the “Blaha effect”, indicating the heat generation rate should be lower. In fact,

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the ultrasonic energy absorbed by the plastic material is eventually transformed into thermal energy leading to the increase in the heat generation rate. The experimental observation in Fig. 6 confirms that

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extremely insignificant. 400 350

CS-FSW

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300

UVBS-FSW

250 200

A

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Temperature, C

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the two aspects of the ultrasonic vibrations compensate each other, thus the temperature variation is

150 100 50

0

0

50

100

150

200

Time, s Fig. 6 The measured thermal cycles in the UVBS-FSW joint and CS-FSW joint. 10

With respect to the effect of ultrasonic vibrations on the material flow behaviors, it is imperative to detect the shear textures in the SZ. Since the deformation close to the probe is predominantly by the simple shear, it is imperative to detect the shear textures in the SZ. The examined regions of the SZ are demonstrated in Fig. 7a) and 7b). In general, shear deformation of FCC metals occurs by gliding on the

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{111} planes in the <110> directions. The first fiber, called A fiber, contains a {111} pole in the shear

plane with the in-plane component randomly positioned; another fiber, called B fiber, is derived from the <110> shear direction where the shear plane component is randomly distributed about this axis (Field et al., 2001). For comparative purposes, the ideal simple shear orientations for FCC metals as they appear

in the {111} pole figure are represented in Fig. 7c). In a friction stir welded joint, Prangnell and Heason (2005) found that the shear plane (SP) and shear direction (SD) might be expected to be approximately

parallel to the probe surface or SZ flow lines. The pole figures (PFs) on the left half of Fig. 7d)-7g) have

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the same orientations as the cross-section plane of the joint with the normal direction vertical. If the PFs are rotated around the axes in order to align the assumed shear plane normal (SPN) vertical and SD

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horizontal, the textures can be much sharper, as shown in the right half of Fig. 7d)-7g). The rotation

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angles required to turn to the conventional alignment of the SP are provided. As illustrated in Fig. 7d) and 7f), the texture patterns in the center and bottom of the SZ in the

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UVBS-FSW joint do not change and are both dominated by the 𝐵/𝐵̅ texture component. Pettersen et al. (2003) found the dependence of the texture components on the strains and proposed that the C texture

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component was gradually replaced by the 𝐵/𝐵̅ texture component with the increasing strain. Furthermore, they observed that at a strain of 25, the C texture component had vanished and the 𝐵/𝐵̅

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texture component was seen to totally dominate the texture. Montheillet et al. (1984) stated that when the temperature was elevated, the C and 𝐴/𝐴̅ texture components disappeared progressively and the 𝐵/𝐵̅

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texture component became sharper. It has been widely accepted that the microstructure in the SZ suffers at a relatively high temperature. While it consists of nearly only the 𝐵/𝐵̅ texture component, indicating

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that the SZ in the UVBS-FSW joint probably achieves a strain of at least 20-25. Notably, the maximum intensity of the texture in the bottom of the SZ is almost equal to that in the center region (Fig. 7d and 7f). This phenomenon confirms that the materials in the bottom region experience an adequate

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deformation due to the application of ultrasonic vibrations. However, without the effect of ultrasonic vibrations, the maximum intensity of the 𝐵/𝐵̅ texture component in the center of SZ is only 4.9 (Fig. 7e). In addition, as can be seen in Fig. 7g), the texture pattern in the bottom of the SZ in the CS-FSW joint is dominated by the C texture component which prefers to be generated at a moderate strain. According to the comparison of the texture pattern and intensity in Fig. 7, it can be deduced that the material flow behaviors are significantly enhanced by the ultrasonic vibrations, especially for those in 11

the bottom region of the joint. Based on the rotation angles represented in Fig. 7d)-7g), the approximate profiles of the plastic materials around the tool are depicted in Fig. 8. The SP and SD have already been indicated by a rectangle and an arrowed line, respectively. Although the UVBS-FSW joint and CS-FSW joint are both

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obtained at the same welding parameters using the same tool, it is obvious that the volumes of the driven materials are significantly different. The probe could drive limited materials around it without the

ultrasonic vibrations so that the SP in the CS-FSW joint is approximately parallel to the probe surface. As a result, the SP in the bottom region of the SZ is nearly vertical to the welding direction (Fig. 8b).

However, the tool in the UVBS-FSW is able to drive more material to rotate due to the Blaha effect of the ultrasonic vibrations and thus the volume of the probe-affected zone increases. Hence, it is natural that the rotation angle for the SP to its conventional alignment is smaller.

a)

c)

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b) d

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e g

A

f

e)

72º TD, 2º WD

80º TD, 10º WD

g)

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f)

D

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d)

87º TD, 10º WD

45º TD, 15º ND

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Fig. 7 a) and b) schematic illustrations of examined regions, c) ideal orientations of FCC metals under simple shear in the {111} pole figure [45], d) and f) (111) PFs of the SZ in UVBS-FSW joint, e) and g) (111) PFs of the SZ in CS-FSW joint.

b)

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a)

Fig. 8 Schematic profile of the plastic material in the UVBS-FSW joint (a) and CS-FSW (b). 12

5. Mechanical properties and improving mechanism Figure 9 shows the tensile strength and fracture elongation of the joints. In contrast, 300 mm/min is the optimal welding parameter for both UVBS-FSW and CS-FSW. Most importantly, the tensile strength

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and fracture elongation achieved at diverse welding parameters are all improved by the UVBS-FSW. The following analyses are focused on the joints obtained at 300 mm/min, as the rectangle marked in Fig. 9. The maximum tensile strength of the UVBS-FSW joint is 359 MPa and the strength coefficient reaches 81.6%, which is obtained at welding speed of 300 mm/min. However, the tensile strength of the CS-

FSW joint obtained at the same parameter is only 346 MPa. Furthermore, the elongations of the joints with and without ultrasonic vibrations are 6.7% and 5.5%, respectively. Hence, the application of

ultrasonic vibrations has a positive influence on the improvement of the mechanical properties. The

hardness distributions of the joints are illustrated in Fig. 10. Obviously, compared to the BM, a general

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softening and reduction of the hardness in the weld zone is detected. The hardness value of the BM is

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around 140±5 HV. As can be seen in Fig. 10a), the width of the TMAZ of the UVBS-FSW joint is slightly broadened. The increasing volume of the driven material by the rotating tool accounts for this

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phenomenon. Furthermore, it is noted that the hardness values in the SZ from top to bottom are all above

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95 HV and the difference among the three lines is significantly reduced (Fig. 10a). However, without the effect of ultrasonic vibrations, the SZ hardness is lower and decreases successively from the top to bottom (Fig. 10b). Sato et al. (2001) claimed that the hardness of the joint was governed by the

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microstructure. Therefore, the results demonstrated in Fig. 10 indicate that the application of ultrasonic

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UVBS-FSW CS-FSW 344

9

UVBS-FSW CS-FSW

8

355

7

346

333

325

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Tensile strength, MPa

359

350

b)

300

Fracture elongation, %

a) 400

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vibrations during the welding process contributes to the homogeneity of the microstructure.

250

6.7

6.5

6

5.5

5.9

5

3.7

4

3.4

3

A

2 1 0

200

200

300

200

400

300

400

Welding speed, mm/min

Welding speed, mm/min

Fig. 9 The tensile strength and fracture elongation of the joints.

Figure 11 and Figure 12 show the strain histories during the tensile test. During the tensile test, the 13

local deformation of the joints is dramatically inhomogeneous due to the heterogeneous microstructure among the HAZ, TMAZ and SZ. In the UVBS-FSW joint, the plastic strain is initiated from the TMAZ. Furthermore, the deformation on the AS is more remarkable (Fig. 11a-11b). As the tensile test proceeds, the deformation in the SZ gradually makes more contributions to the global strain (Fig. 11c). Afterwards,

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the TMAZ on the AS is found to suffer a significant plastic deformation, leading to a clear necking (Fig. 11d-11e). The tensile specimen is finally fractured in the TMAZ on the AS. It can be seen from Fig. 12 that the strain histories of the CS-FSW joint exhibit a completely different tendency. Although the

obvious plastic deformation is also initiated from the TMAZ on the AS, it is evident that the largest local

strain region transforms into the TMAZ on the RS during the following period until the joint is fractured. Liu et al. (2003a, 2003b) claimed that the fracture path was dependent on the hardness and corresponded well with the lowest hardness distribution. It can be seen from Fig. 10 that the lowest hardness regions

180

Top

Middle

Bottom

b) 180

TMAZ

SZ

Microhardness

130 120 110 100

AS

RS

-20

-15

-10

TE

60 -25

-5

0

5

Bottom

TMAZ

SZ

TMAZ

10

15

130 120 110 100

20

AS

RS

80

TMAZ SZ TMAZ

70

SZ

140

90

D

80

TMAZ

150

140

90

TMAZ

160

TMAZ

M

Microhardness

150

TMAZ

A

160

SZ

Middle

170

170 TMAZ

Top

N

a)

U

for the two joints are both located at the TMAZ. The hardness difference of the TMAZ between the AS

TMAZ SZ TMAZ

70 60 -25

25

-20

-15

-10

-5

0

5

10

15

20

Distance from weld center, mm

Distance from weld center, mm

0.01 0.005

CC

a)

EP

Fig. 10 Hardness distributions of the joints: a) UVBS-FSW, b) CS-FSW.

0 0.02

b)

0.01 0 0.1

A

c)

0.05 0 0.2

d)

0.1 0 0.23

e)

0.11 0

14

25

Fig. 11 The extensional strain distributions of the UVBS-FSW joint at the strain of (a) 5.6%, (b) 6.6%, (c) 11.6%, (d) 14.9% and (e) 15.6% (immediately before fracture).

and RS of the UVBS-FSW joint are minor. According to Fig. 10b), the TMAZ hardness on the AS of the CS-FSW joint is obviously higher than that on the RS, especially in the top and center regions, leading

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to the final fracture on the RS. However, surprisingly, the strain clusters in this joint are initiated on the AS, as shown in Fig. 12a) and 12b). This unusual deformation phenomenon should be of particular

concern. Prime et al. (2006) and Sun et al. (2018) proposed that the residual stress was an important factor regarding the mechanics of the materials as it was a key quantity for the mechanical behavior.

They found that the weld zone was under tension and the BM far away from the weld zone was under

compression. Furthermore, a higher peak residual tensile stress was detected on the AS compared with the RS. Wade et al. (2010) attributed this to the higher peak temperature on the AS due to the greater

U

level of the thermomechanical effect and strain rate. In essence, the material is compressed during the

hardness test and thus the residual tensile stress counteracts a part of the compressive force, leading to

N

shallow indentations and higher hardness values. However, during the tensile test, the residual tensile

A

stress adds to the tensile stress results in the early softening of the TMAZ on the AS. Ultrasonic treatment has been widely used to eliminate or partially reduce the residual stress of joints for many

M

years. The effect of residual stress on the flow behavior of the CS-FSW joint is validated by the experimental results in Fig. 10b) and Fig. 12. In contrast, the consistency of the hardness distribution and

TE

ultrasonic vibrations.

0.005 0 0.02 0.01 0 0.14 0.07

CC

c)

0.01

EP

a) b)

D

strain cluster indicates the residual stress in the UVBS-FSW joint has been significantly reduced by the

0 0.17

d)

0.08

A

0 0.2

e)

0.1 0

Fig. 12 The extensional strain distributions of the CS-FSW joint at the strain of (a) 5.6%, (b) 6.6%, (c) 11.6% (d) 12.6% and (e) 13.4% (immediately before fracture).

To assist in the understanding of the strain patterns, the global strain and local strain of the joints 15

need to be analyzed. The extensometer of the global strain is determined as the maximum possible gauge length not exceeding the uniform elongation of the specimen. It is observed from Fig. 13a) and 13b) that the center region of the joint experiences a dramatic plastic strain compared with the AS region and RS region. The lower hardness in the center region accounts for this phenomenon (Fig. 10). Hence, the

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deformation in different locations of the joint is inhomogeneous and mainly concentrates in the weld

zone. Fig. 13c) and 13d) show the comparison of the local strains in the center region of the joints from top to bottom. The almost overlapping blue and black curves in Fig. 13c) represent the coincident

deformation in the top and bottom regions. However, in the CS-FSW joint, the deformation in the

bottom region is consistently the lowest (Fig. 13d), indicating the asymmetrical and inhomogeneous deformation in the thickness direction.

Center AS RS Global strain

18 16

Center AS RS Global strain

18 16

Local strain, %

14 12

A

12 10 8 6

M

Local strain, %

14

20

U

b)

20

N

a)

4 2

-2 0

1

2

3

D

0 4

5

6

10

8 6 4 2 0

-2 7

0

1

2

Global Strain, %

TE

18 16 14

16

4

5

6

7

Center-up Center-mid Center-bottom Global strain

14 12

EP

12 10

8 6

CC

Local strain, %

d)

Center-up Center-mid Center-bottom Global strain

Local strain, %

c)

3

Global Strain, %

10 8 6 4

4

2

2

0

0

-2

-2

A

0

1

2

3

4

5

6

0

7

1

2

3

4

5

Global Strain, %

Global Strain, %

Fig. 13 The global strain and local strain in the selected regions: a) and c) UVBS-FSW joint, b) and d) CS-FSW.

Local changes in the lattice orientation reflect the lattice curvature and can be used to calculate the geometrically necessary dislocation (GND) density. As a first-order approach, the kernel average misorientation (KAM) is chosen as a measure for the local misorientations. This method to retrieve the 16

GND densities from the EBSD data follows the report of Takayama and Szpunar (2004). The KAM distributions in the center of the SZ and TMAZ of the UVBS-FSW joint and CS-FSW joint are presented in Fig. 14. The orientation gradients in the grains in the SZ are significantly smaller, whereas those in the TMAZ are larger due to incomplete dynamic recrystallization. Furthermore, the ultrasonic vibrations

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have positive effects on the reduction of the misorientations in the grains. From the KAM value, the

GND density ρ can be derived using the following equation (Liu et al., 1998; Takayama and Szpunar, 2004): 𝛼𝜃

𝜌 ≈ 𝑏𝑢

(2)

where θ (rad) is the average misorientation angle across the dislocation boundaries, b is the magnitude of the Burgers vector, 𝑢 is the distance between the misoriented points (namely the “step size” of the

EBSD map), and α is a constant dependent on the geometry of the boundaries. In the present study, b is

U

0.286 nm for aluminum, 𝑢 is fixed at 0.5 μm and α is chosen to be 3 for boundaries of mixed character.

N

Notably, the real dislocation density is likely to be higher than the calculated value from Eq. (2) due to the existence of dislocations that do not contribute to developing the misorientation (Dutta et al., 2013).

A

However, the GND density obtained using Eq. (2) retains a certain reference value and similar trends

M

relative to the real dislocation density. The stored strain energy (E) per unit volume can be obtained by the following equation (Dutta et al., 2013; Takayama and Szpunar, 2004): 1

𝐸 = 2 𝐺𝜌𝑏 2

D

(3)

where G is the shear modulus and 26.1 GPa for aluminum. Eq. (3) reveals that the stored strain energy is presented in Table 1.

TE

proportional to the GND density. For a more quantitative comparison, the values of ρ and E are

EP

Table 1 The GND density (ρ) and stored strain energy (E) values calculated from Eq. (2) and Eq. (3).

CC

ρ (×1014 /m2)

A

E (J/mol)

Joint

SZ

TMAZ on AS

TMAZ on RS

UVBS-FSW

3.22

4.14

4.10

CS-FSW

3.55

5.16

4.87

UVBS-FSW

3.44

4.41

4.38

CS-FSW

3.79

5.51

5.20

According to Table 1, the GND density and stored strain energy in the UVBS-FSW joint are

significantly lower than those in the same region of the CS-FSW joint. The mobility of dislocations is enhanced by the effect of the ultrasonic vibrations and thus the collision and elimination of opposite sign dislocations occur more frequently during the migration of the LABs. As a result, compared with the joint without the effect of ultrasonic vibrations, the GND density in the UVBS-FSW joint decreases. The greater level of the thermomechanical effect and strain rate on the AS produces higher values of ρ and E 17

in the TMAZ. Although the values in the TMAZ are drastically higher than those in the SZ, the differences between the AS and RS decrease due to the application of the ultrasonic vibrations. This result shows a good agreement with the hardness distribution in Fig. 10. The relationship between the residual strain (ε) and residual stress (σ) can be described by the following equation (Sun et al., 2018): 𝐸

𝑣

ℎ𝑘𝑙 𝜎𝑖𝑖 = 1+𝑣ℎ𝑘𝑙 [𝜀𝑖𝑖 + 1−2𝑣 (𝜀𝑥𝑥 + 𝜀𝑦𝑦 + 𝜀𝑧𝑧 )] ℎ𝑘𝑙

(4)

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hk𝑙

where i=x, y, z correspond to TD, ND and WD, respectively, Ehkl is the Al(hkl) specific elastic modulus, and vhkl is the Poisson’s ratio. As shown in Fig. (4), it is obvious that the residual stress (σ) is

proportional to the residual strain (ε). Hence, the higher stored strain energy (E) in the joint indicates a

higher residual stress. The hardness distribution in Fig. 10b) and strain histories in Fig. 12 have provided undisputable evidence for the effect of residual stress on the deformation process of the CS-FSW joint.

b)

KAMav:1.13 6º

c)

KAMav:1.12 º

d)

e)

KAMav:1.41 º

f)

KAMav:1.33 º

EP

TE

D

KAMav:0.97 º

M

A

N

KAMav:0.88 º

U

a)

CC

Fig. 14 Maps of the kernel average misorientation in: a) SZ, UVBS-FSW joint, b) TMAZ on AS, UVBS-FSW joint, c) TMAZ on RS, UVBS-FSW joint, d) SZ, CS-FSW joint, e) TMAZ on AS, CS-FSW, and f) TMAZ on RS, CS-FSW joint.

A

6. Conclusions (1) The onion skin on the top surface of joint is significantly influenced by the ultrasonic vibrations.

The amplitude from the wave crest to trough is directly reduced from 90±3 μm to 67±3 μm due to the contribution of the Blaha effect. (2) More material around the probe is softened and takes part in the plastic deformation due to the Blaha effect by the ultrasonic vibrations. Furthermore, the material flow behaviors in the bottom region

18

of UVBS-FSW joint are significantly enhanced, leading to the formation of typical shear texture 𝐵/𝐵̅ component. (3) The hardness values in the SZ of UVBS-FSW joint from top to bottom are all above 95 HV and the hardness distributions in the thickness direction are more homogeneous. Furthermore, compared with

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the CS-FSW joint, the tensile strength and fracture elongation of the UVBS-FSW joint are improved to 359 MPa and 6.7%, respectively.

(4) With the application of ultrasonic vibrations, the density of geometrically necessary

dislocations, stored strain energy and residual stress are all reduced in the UVBS-FSW joint, which is attributed to the improvement of the mechanical properties.

Acknowledgement

U

The authors acknowledged the financial support from the National Natural Science Foundation of

N

China (No. 51775143 and No. 51435004), Defense Industrial Technology Development Program (JCKY2017203B066) and the Innovation Program of Chinese Welding Society. This work was partly

A

financially supported by Project to Create Research and Educational Hubs for Innovative Manufacturing

M

in Asia, Osaka University of Special Budget Project of the Ministry of Education, Culture, Sports,

A

CC

EP

TE

D

Science and Technology.

19

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