Vacuum 174 (2020) 109221

Available online 24 January 20200042-207X/© 2020 Elsevier Ltd. All rights reserved.

Joining AlCoCrFeNi high entropy alloys and Al-6061 by explosive welding method

Ali Arab *, Yansong Guo, Qiang Zhou, Pengwan Chen **

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, PR China

A R T I C L E I N F O

Keywords: High entropy alloys AlCoCrFeNi Explosive welding Microstructure Interface bonding

A B S T R A C T

High entropy alloy (HEA) is an emerging class of materials that shows promising potential for many applications. Numerous HEAs have been synthesized and characterized over the past twenty years, however only limited researches have been done on the welding process of HEAs. In this research, AlCoCrFeNi High Entropy Alloys and Al-6061 plates are welded by using the explosive welding technique. In order to characterize the morphology and the composition of the compound, a scanning electron microscope was utilized. The weldability window was calculated in order to verify the weldabilty of the AlCoCrFeNi to Al-6061, and three different conditions were selected to perform the experiments. In all the experiments the AlCoCrFeNi and Al-6061 were properly welded together. However, some cracks were observed in the welded samples obtained in high velocity collision. The results suggest that the explosive welding technique is potentially a good method for joining high entropy alloys to other metals.

1. Introduction

Usually alloys have been developed according to the base element model. This strategy begins with one and rarely two principal elements. In 2002, a new class of the alloy system known as High Entropy Alloys (HEAs) was introduced [1]. HEAs are defined as concentrated solid so-lutions, containing equiatomic or non-equiatomic quantities of five or more principal elements [2–5]. The HEAs present excellent oxidation and corrosion fatigue and wear resistance, high hardness and strength with reasonably good ductility. Also, because of their sluggish defect formation kinetics under heavy radiation doses, HEAs can replace the currently available structural materials in nuclear and high efficiency thermal power plants in the future. Numerous HEAs have been synthe-sized and characterized over the past twenty years [6,7]. Meanwhile, these excellent mechanical properties of HEAs are not the only impor-tant characteristic for a potential usage as structural material; the suit-ability of HEAs to technological processing and fabrication is also essential [8,9].

Welding is a one of the critical processes which is used in a most of the industries, such as military, aerospace, building construction and energy. Joining of HEAs in similar or dissimilar material systems is increasingly important for expanding the applications of HEAs.

Therefore, it is important to understand the behavior and mechanical properties of HEAs during and after the welding process [10]. Different welding techniques are used for joining the HEAs such as Arc Welding, Electron Beam Welding, Laser Welding and Friction stir welding [11]. Arc welding is one of the first welding method that has been used for joining the HEAs [12]. Kashaev et al. [13] used the Fiber laser beam welding method to join the CoCrFeNiMn with FCC structure. They observed how the welding process changed the microstructure of the sample and how this change led to the increase of the hardness in the fusion zone. Zhu et al. [14] used the Friction stir welding method to join the FCC structure HEA (CoCrFeNiAl0.3). The stir zone exhibited a refined equiaxed microstructure due to recrystallization and this zone shows the highest hardness with grain refinement. Same observation is reported by other researchers [8,15,16]. Most of these researches only focus on joining two HEAs together. Shu et al. [17] investigated the cladding process of the FeCrCoNiSiB HEA to low carbon steel, they improved the wear resistance of the sample in high-temperature service environment. Recently, Liu et al. [18] fabricated the soft HEA ((FeCoNi)25 (AlTiZr)75) on Si (100) substrate using an RF magnetron sputtering system. Chao et al. [19] studied the cladding of the AlxCoCrFeNi (x ¼ 0.3, 0.6 and 0.85) to the stainless steel by the direct laser deposition method. Yue et al. [20] fabricated the AlCoCrCuFeNi HEA on pure magnesium

* Corresponding author. ** Corresponding author.

E-mail addresses: arabali83@yahoo.com, a.arab@bit.edu.cn (A. Arab), pwchen@bit.edu.cn (P. Chen).

Contents lists available at ScienceDirect

Vacuum

journal homepage: http://www.elsevier.com/locate/vacuum

https://doi.org/10.1016/j.vacuum.2020.109221 Received 22 October 2019; Received in revised form 21 January 2020; Accepted 22 January 2020

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substrates using laser cladding method. They observed the interlayer consists of some partially melted AlCoCrCuFeNi powders in an Mg based matrix. However still limited researches have been done on the welding of the HEAs.

As compared with other welding methods, explosive welding has great capability to join a wide varieties of similar or dissimilar metals that cannot be joined by any other conventional joining methods, such as Al/Cu, Ti/Al, steel/Al and Ti alloys [21,22]. Explosive welding is a solid-state process, in which two or more layers of metals are joined under the high pressure generated by the impact of a flyer plate accel-erated by explosion. Various parameters that affect the quality of explosive-welded have been extensively investigated, and the process of the explosive welding has been fully investigated experimentally and theoretically. Various metal plates have been joined by explosive welding method [8], including amorphous and brittle alloys and multilayer metal foils. Moreover, various bilayer materials are fabri-cated by explosive welding such as ceramics [23,24] and metals [25–27], and their mechanical properties have been extensively studied. However, cracks in the brittle material is the serious problem during explosive welding. These cracks introduced with high-speed

deformation. For solving this problem Hokamoto et al. [23] used the underwater shock wave to weld the Al/ZrO2.

AlxCoCrFeNi has great mechanical properties. This class of HEAs exhibits a range of microstructures and properties [28]. The AlxCoCr-FeNi microstructure can be tailored from single FCC structure, duplex FCC plus BCC structure to single BCC structure by increasing the amount of aluminum, meanwhile the hardness raised from 116 HV to 509 HV. AlCoCrFeNi has BCC structure. AlCrFeCoNi has polygonal grains with intragranular dendritic segregation microstructure [29]. Further, AlCoCrFeNi solidified with dendritic and interdendritic microstructures due to elemental segregation [30].

AlxCoCrFeNi is fabricated by different methods such as arc melting and microwave sintering [6,31–33]. These researches show this HEA has a great potential in a wide range of applications. However, no research has been done yet to investigate the usage of the explosive welding technique for joining the HEAs to other metals.

AlCoCrFeNi has BCC structure and due to its brittle behavior. In this study, the AlCoCrFeNi HEA is welded to the Al plate through the explosive welding technique. Stand off distance between the flyer and base plate as one of the important parameters in the explosive welding is

Fig. 1. a)XRD result of fabricated HEA b) EBSD analysis of fabricated HEA.

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changed in this study to investigate the effect of these parameters on the quality of the welding.

2. Methodology

The HEA was fabricated using the arc melting process. The AlCoCr-FeNi alloy was prepared by arc-melting a mixture of constituent ele-ments (in molar amount of 20% per each element) with higher than 99.9 wt% purity ratio. The melting process was repeated at least five times in order to improve the chemical homogeneity of the alloys. HEAs microstructure was confirmed by X-ray diffraction (XRD). The obtained results are presented in Fig. 1 (a). XRD analysis shows, AlCoCrFeNi has body-centred cubic (BCC) structure. Fig. 1(b) shows the EBSD analysis of the AlCoCrFeNi, The analysis verified the XRD results indicating that the fabricated HEA has a BCC structure, Al, Cr, Fe, Co, Ni elements are distributed in the crystal lattices and form quinary BCC solid solution. EBSD analysis also reveals that the average grin size in the fabricated HEA is 50 μm. The hardness of the fabricated AlCoCrFeNi is 450 HV, compressive test shows, σy is 1380 GPa and σUTS is 2008 GPa for casting AlCoCrFeNi. The density of fabricated HEAs is 7015 kg/m3. The high hardness of this HEAs is attributed to the spinodal decomposition of the BCC solid solution and the precipitation of NiAl nano-particles, the detail of mechanical properties of AlCoCrFeNi can be find in the liter-atures [29,34,35].

Thin circular plates of AlCoCrFeNi used as base plate by diameter of 70 mm and thickness of 2 mm. The Al-6061 alloy plate dimension was 85 mm � 150 mm � 3 mm. Al-6061 plate was used as a flyer plate, and an explosive charge was placed on its surface. ANFO with a detonation velocity of 2100 m/s and density of 0.65 g/cm3 was selected as the explosive material. The base plate was supported by an Al-6061 plate of 10 mm thickness. This process is illustrated in Fig. 2. Table 1 shows the parameters of the explosive welding method that is utilized in this study. Three different conditions were designed and tested in order to study the weldability of high entropy alloys and Al-6061, the thickness of explo-sive material was kept constant in all tests (10 mm). The stand of dis-tance between the flyer and base plate was changed (from 1 mm to 3 mm) to achieve a different collision velocity. This experimental condi-tion is presented in Table 1.

The microstructures of the sample were examined using an optical microscope and a scanning electron microscope. Scanning electron mi-croscopy (SEM) was carried out on Hitachi- S-4800. To study the microstructure, rectangular specimen was cut from welded sample by wire cut machine. The specimen was grounded, then polished, and finally etched with an Aqua Regia solution. A sample with thickness of 60 μm was prepared for TEM characterization and was thinned using a cryo-focused ion beam (FIB) instrument at liquid nitrogen temperature. Vickers microhardness test (Shimadzu Vickers hardness tester HSV-20, Japan) was conducted along the line perpendicular to the interface of the welded sample, (the measurement repeated in the 5 different areas of the specimen and average of them is reported as hardness of sample).

3. Results

During the process of the explosive welding, explosive detonation drove the flyer plate to a certain speed, and the accelerated flyer plate at a velocity VP collided with the base plate at a collision angle. The rela-tionship between the collision velocity VP and the collision angle is through the following formula [36].

VP¼ 2VC sin�β

2

�(1)

Vc is horizontal collision velocity. And β (collision angle) is calcu-lated through the following equation [37].

β¼

ffiffiffiffiffiffiffiffiffiffiffiffiK þ 1K � 1

r

� 1

!

:π2:

r

r þ 2:71þ 0:184�

te=S� (2)

Where K is a constant parameter, te is the thickness of the explosive material, r is the ratio of the explosive mass to mass of flyer, and S is stand of distance.

The k in equation (2) can be measured by the following equations [38,39].

k¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiV2

C

V2gþ 1

s

(3)

Vg¼ 600þ 0:52VCffiffiffiffiffiffiffiffiffiffiffiγ þ 1p (4)

where γ is a ratio of specific heat constant. For ANFO, the value of γ is 2.881 [39], by the above equation the value of the K was calculated as 2.

The amount of kinetic energy lost through collision, is calculated by the following equation [36,40].

ΔKE¼mf mcV2

P

2�mf þ mc

� (5)

Where the mf is the mass of the flyer plater per the area, and mb is the mass of the collided plate per the area. The collision velocity, collision angle and amount kinetic energy lost through collision for each test is calculated by equations (1)–(5) and presented in Table 2.

A collision between metallic plates is accompanied by various

Fig. 2. Scheme of explosive welding set up.

Table 1 Parameters of the explosive welding experiments.

Test name

HEA Thickness (mm)

Al-6061 Thickness (mm)

Explosive Thickness (mm)

Stand-off Distance (mm)

HA1 2 3 10 1 HA2 2 3 10 2 HA3 2 3 10 3

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physical phenomena at the collision surface of the plates. For a better understanding the phenomena, the weldabilty window of the Al/ AlCoCrFeNi is calculated as shown in Fig. 3. During the explosive welding process, initially flyer plate is accelerated by shock wave pro-duce by the detonation pressure, and then by the expanding gaseous. If the stand of distance between the flyer and base plate is sufficient, the flyer can reach to the terminal velocity that is require for the welding. As shown in Fig. 3, all tests are within the welding window and the HA1 test is located on the border of weldability window. The following equations were used in order to define the weldability windows in this study. The right limit of the weldability windows (highest collision velocity (Vc)) determines the conditions of jet formation at the collision point. To achieve a good welding interface, collision velocity should be lower than the sound speed in the bulk material [41], Abrahamsen [42] found that the highest collision is determined by weak function of the collision angle(β):

VC ¼β10þ 5:5 (6)

However, the metallic plate could also be welded by a low collision velocity. Lower boundary of the weldability windows is related to the amount of pressure at the collision point. The impact pressure should be arise sufficiently to overcome to the yield stress to lead the plastic deformation in the interface of plates. Equation below is used to deter-mine this boundary [43]:

sin β¼ k

ffiffiffiffiffiffiffiffiHv

ρV2c

s

(7)

Where β is the collision angle, Hv is hardness of the flyer, ρ is density of the flyer, Vc is the collision velocity and k is constant value. By increasing the collision velocity or collision angle from this line, the type of the welding interface turns from straight into wavy. Crossland and Williams [44] proposed the equation for transition boundary from the straight to the wavy interface.

tan β¼ 1:14

ffiffiffiffiffiffiffiffiHv

ρV2c

s

(8)

As shown in Fig. 3. All these tests are below the transition boundary and test HA1 is on the lower boundary, base on the weldability window it is expected the all samples have the straight interface (all the sample is below of the transition line that suggested by the Crossland and Williams [44]). Fig. 4 (a) shows the AlCoCrFeNi/Al explosive welded sample. In all of the three conditions, the Al and AlCoCrFeNi are tightly welded together. As it was expected according to the weldability windows, the interface type was observed to be straight throughout all of the test conditions. However in the HA3 sample, several cracks and peelings were visible on the rear side of the AlCoCrFeNi alloy. It might be due to the reflection of the shock wave generated from the impact of Al plate to the AlCoCrFeNi alloy. Zhou et al. [41] reported the similar observation in joining the copper and W. Fig. 4 (b, c and d) show the SEM image of interface of the these 3 samples. Several transverse cracks were observed in the welded interface of the samples HA3 and HA2. The impact energy in HA2 and HA3 samples is higher than that of HA1. During the explo-sive welding process, the shock wave was generated on the interface and propagated throughout the HEAs plate. When the shock wave reached the free surface, a tensile wave was reflected, leading to transverse cracking. The intensity of the reflected wave depends on the amount of

kinetic energy lost on the interface during the collision as calculated by equation (3), kinetic energy loss was deposited in the form of plastic work and shock wave. The transverse cracks in most of the case in the HA2 and HA3 samples initiated the tip of the jet. In the AlCoCrFeNi, cracks usually initiated at the junction of the BCC grains as weakest point and they will propagated transgranularly [32]. It is important to mentioned the BCC structure is harder than the FCC structure. As cast AlCoCrFeNi has lamellar structure and this structure dislocation motion is very difficult and it leads to the low ductility of this HEA [35], this brittle behavior of the casted AlCoCrFeNi is observed even at the high temperature (700 �C). Ghasemali et al. [32] observed the brittle fracture in the AlCoCrFeNi high entropy alloys due to their BCC structure. Chao et al. [19] found vertical cracks in the Al0.3CoCrFeNi cladding on the stainless steel by the direct laser deposition, Al0.3CoCrFeNi has FCC structure that is softer than the BCC. During the explosive welding by high velocity collision (HA2 and HA3) Al jet penetrated into the AlCoCrFeNi. Jet phenomenon is an essential condition for explosive welding. However, due to the brittle behavior of the AlCoCrFeNi during the welding process, the jet phenomenon leads to the formation of the cracks. In the sample HA1, SEM observation indicates that the formed interface was almost flat and there were no visible cracks in the sample HA1. It is because of the lower impact velocity in this condition that leads to the lower intensity of tensile wave. Durgutlu et al. [45] mentioned that the minimum stand of distance for the explosive welding should be the half of thickness of the flyer, however in this research with the stand of distance lower than this value, good welding interface is achieved. It is observed in the sample HA1, the impact velocity and collision angle is sufficient for good welding.

The interface of HA1 sample was also characterized by elemental mappings. Fig. 5 shows the elements mapping in the cross section of the HA1 explosive welded sample. As shown in Fig. 5, there was no change in the distribution of the elements in the AlCoCrFeNi alloy, and all el-ements are distributed homogeneously in the HEA. For further study on the bonding interface of the HA1sample, line scan analysis was con-ducted on this sample. Line scan analysis can describe the possibility of the formation of intermetallic compounds in the welding interface. Fig. 6 shows the line mapping of the elements trough the line 1 that shows in Fig. 6. The line scanning analysis shows after around 1 μm from the bonding interface the amount of Al and elements that excited in the HEAs are almost unchanged. At the bonding interface from the AlCoCrFeN side to the Al, the concentration of Al element increases linearly, however, the other elements are vanished after the bonding interface. This analysis indicated that the transition layer width is 1.8 μm which is almost consistent all along bonding interface. The short diffusion area can limit the formation of intermetallic compounds [46]. In the middle of this transition layer, the point element analysis was conducted and it indicated that the atomic percentage of Al, Co, Cr, Fe, and Ni are 99.43%, 0.28%, 0.11%, 0.11%, and 0.07%, respectively. The

Table 2 Dynamic parameters of explosive welding experiments.

Test name Impact Velocity (m/s) Collision Angle (�) Energy (MJ/m2)

HA1 360.4581 9.85174 0.41 HA2 435.0788 11.89793 0.60 HA3 467.3114 12.78292 0.69

Fig. 3. The calculated weldability window of AlCoCrFeNi/Al.

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Fig. 4. a) Explosive welded sample & optical microscope image, b) SEM image of sample HA3, c) SEM image of sample HA2 d) SEM image of sample HA1.

Fig. 5. Elemental mappings of the welded AlCoCrFeNi/Al.

Fig. 6. Line scan analysis across the bonding interface.

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chemical components in this narrow diffusion layer show there is no intermetallic compound occurring in this zone. For explosive welding, interdiffusion is one of the key mechanisms in the welding process for forming composites made of dissimilar metals, and quality of joining depends crucially on the atomic diffusion across the interface. Liu et al. [47] found out that the transition layer between the Al-6061 and AlCoCrFeN can be formed at temperature higher than the 560 �C, and they observed that this transition layers has the FCC structure. During the explosive welding process, due to high velocity impact, the tem-perature generated at the interface of the plates is higher than the melting points of the metals. As a result, the atomic diffusion coefficient will increase because of the accumulated high temperature, thus leading to the formation of diffusion layer [48]. In the HA1 sample, the width of diffusion layer is small because of the low velocity of the impact. The width of the diffusion layer increases by elevating the impact velocity in the tests HA2 and HA3. However, due to the brittle nature of the AlCoCrFeN alloy, increasing the impact velocity led to the appearance of a lot of cracks in the samples.

To inspect the phase nature of the welding interface, HRTEM analysis of the interface was carried out. The HRTEM observation of the interface confirmed that the transition layer is formed between Al-6061 and AlCoCrFeN (Fig. 7). It is observed that the Al-6061, transition layer and AlCoCrFeN are joined together in the atomic scale without any cracks and defects. To join the materials together, atoms at the bonding surface should bring at a certain distance apart where attractive and repulsive forces are in equilibrium [48]. During explosive welding, this is happening due to the drastic collision of Al-6061 and AlCoCrFeN. Fig. 7 c,d and e, show the Electron diffraction pattern of the sample in three different zones: Al-6061 (Fig. 7 (c)), transition layer (Fig. 7 (d)) and AlCoCrFeN (Fig. 7 (e)). Fig. 7, c and e, show typical crystalline lattice structure of Al phase and BCC phase, respectively. Fig. 7 (c) shows a mixed phase consisting of lattice structure and inapparent rings, which is different from the Al-6061 and AlCoCrFeN. Electron diffraction pattern at the selected zone indicates that the grain in the transition zone is smaller than the original grain size of materials. Li et al. [24] observed similar phenomenon in the interlayer of explosive welded of Si3N4/Al.

To evaluate the mechanical property of the AlCoCrFeNi/Al welding, the hardness of the welded samples and the fabricated HEAs and

received Al-6061 were tested using a Vickers Indentation technique. Fig. 8 shows that the hardness profile of the explosive welded sample near to the interface. Hardness of the Al is increased near to the inter-face, as moving away from the interface, hardness becomes approaching to the typical values of the Al. The higher hardness near to the interface might be explained by shock pressure with collision or the intensive plastic deformation [46,49]. It may due to the grain refinement that occurred in the transition layer. The hardness of the AlCoCrFeNi has not been any changed near to the interface, it is confirmed that the micro-structure of the HEAs did not change due to the shock pressure after welding. Li et al. [50] investigated that dynamic recrystallization occurred in the interface of AlCoCrFeNi2.1 welded by the rotary welding technique. In the process of welding the HEAs to other metals, the ele-ments in HEA could enter the crystal lattice and change the mechanical properties of metals [50], although this phenomena is not observed in the explosive welding of AlCoCrFeNi and Al.

4. Conclusion

In the current work, AlCoCrFeNi and Al-6061 plates were success-fully joined by the explosive welding method. For better understanding the effect of the different parameters on the quality of the welding, weldebility windows of the AlCoCrFeNi and Al-6061 was calculated. To achieve the different collision velocity, stand of distance between the flyer and basement was changed from 3 to 1 mm. It was observed that cracks were initiated in the welding interface and propagated throughout the AlCoCrFeNi alloy in sample with the stand of distance of 2 mm (HA2) and in sample with stand of distance of 3 mm (HA3). In the sample with stand of distance of 3 mm (HA3), some cracks were observed on the other side of AlCoCrFeNi, due to the reflected wave. The Sample with the stand of distance of 1 mm showed a straight interface and no cracks was observed in the welding interface. The HRTEM observation of the interface confirms that the transition layer is formed between Al-6061 and AlCoCrFeN (width of transition layer is around 1.8 μm). Electron diffraction pattern at the selected zone shows that the grain in this layer is smaller than the original grain size of Al-6061 and AlCoCrFeN. Microhardness test reveals the microstructure of the AlCoCrFeNi after welding did not change due to the explosion shock

Fig. 7. (a and b) HRTEM image of the HA1 sample, c) Electron diffraction pattern at Al d) Electron diffraction pattern at diffusion layer e) Electron diffraction pattern at AlCoCrFeN.

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wave. However, the hardness of the Al is increased near the interface, because of the plastic deformation and grain refinement that occurred in the transition layer.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgements

This research was financially supported by the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology) with Grant No. QNKT17-01.

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