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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12523-6
10.1016/j.heliyon.2024.e36492
e36492
Research Article
Research on hot cracking in laser welding of Al-contaminated CoCrFeMnNi high-entropy alloys
Cao Xia caox@czu.cn
a⁎
Tian Songya b
Xu Fan b
Zhang Genyuan b
a Changzhou Institute of Technology, 213032, Changzhou, Jiangsu, PR China
b Hohai University, 213002, Changzhou, Jiangsu, PR China
⁎ Corresponding author. Department of Mechanics and Surface Engineering, Changzhou Institute of Technology, Changzhou 213002, PR China. caox@czu.cn
17 8 2024
30 8 2024
17 8 2024
10 16 e3649217 5 2024
15 8 2024
16 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
To study the thermal cracking susceptibility of laser-welded CoCrFeMnNi high-entropy alloys, stainless steel and aluminum alloy plates were each used as backing for welding. The microstructure of the weld and morphology of the fracture were examined. In addition, the chemical compositions of the fractures, the interfacial tension between the CoCrFeMnNi high-entropy alloy and liquid aluminum alloy, and the linear expansion coefficient of the CoCrFeMnNi high-entropy alloy were determined. The results show that when stainless steel is used as the base plate, no cracking is apparent in the weld, and the microstructure is made up of dendrites and equiaxed crystals. Conversely, when an aluminum alloy plate is adopted, solidification cracks are seen at the center of the weld, and the microstructure consists of bright polygonal dendrites scattered in a dark gray matrix. In the later stage of solidification, the contact angle between the Al-dominated low-melt liquid metal and the CoCrFeMnNi high-entropy alloy is about 14.6°, which is distributed in the form of a liquid film between the dendrite of the CoCrFeMnNi high-entropy alloy weld, and the linear expansion coefficient of the high-entropy alloy is 23 × 10−6-25 × 10−6 K−1 in the temperature range of 900–1100 K, which is higher than the thermal expansion coefficient of austenitic stainless steel in this range, and the solidification temperature range is 1000–1400K. Therefore, thermal cracks tend to occur during the solidification process.

Keywords

High-entropy alloy
Laser welding
Solidification cracking
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pmc1 Introduction

The take-off of carrier-based aircraft includes vertical take-off, catapult take-off and glide jump take-off, and vertical take-off only needs a small deck area to take off and land, the aircraft can be dispersed and configured, which is convenient for flexible sorties, transfers and camouflage. It is not easy to be discovered by the enemy, the attendance rate has also been improved, and the strike on the enemy has a high suddenness, which greatly improves the battlefield survival rate of the aircraft.

The aircraft take-off can overcome gravity only by jet reaction during vertical takeoff, while the other end of the jet acts on the deck of AM-2 aluminum alloy, which has a melting of 529 °C; Even a short period of exposure to an environment above 700 °C will seriously damage its mechanical properties; At the same time, under the erosion of the engine exhaust, it is easy to produce fragments, which in turn leads to FOD (damage to foreign objects) of the engine, which is fatal to the fighter. Therefore, it is necessary to cover the 6061-T6 aluminum alloy plate with a layer of high-entropy alloy CoCrFeMnNi, which has high temperature resistance, corrosion resistance, wear resistance and heat insulation, while the aluminum plate has the performance of light weight and fast thermal conduction, and the performance of the two is compounded to meet the performance requirements of the vertical take-off and landing deck.

High-entropy alloys (HEAs) are a new type of metallic material that have emerged in recent years with very promising applications. Yeh et al. [1,2] defined them as alloys consisting of at least five principal elements with approximately equal atomic percentages. HEAs have high mixing entropy, which promotes the formation of solid solution phases and reduces the probability of intermetallic compound formation. Thus, the microstructures of HEAs are mostly simple face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close-packed (HCP) solid solution phases [3]. These unique microstructures give HEAs superior comprehensive properties compared to traditional alloys, such as good mechanical properties [4,5], corrosion resistance [6], and wear resistance [7]. Therefore, HEAs are expected to be used as structural materials in industrial production.

The CoCrFeMnNi HEA is a single-phase solid solution with an FCC structure [8], which is similar to austenitic stainless steel and has a greater tendency to hot cracking. However, regarding CoCrFeMnNi HEA, there have been no reports on thermal cracking in existing research on its weldability. In this study, stainless steel-HEA joints and aluminum alloy-HEA joints were prepared by laser welding, and the microstructure and crack tendency of the joints were analyzed, and the mechanism of crack formation was studied.This study investigated the formation mechanisms of joint microstructures and cracks to provide experimental evidence for the laser weldability of such alloys.

2 Materials and method

2.1 Materials and welding processes

The experimental base material was a CoCrFeMnNi HEA casting. After homogenization at 1000 °C for 24 h, followed by cold rolling, experimental specimens of 40 mm × 18 mm × 2 mm were obtained through wire cutting and annealed at 800 °C. The chemical composition of the alloy is described in Table 1.Table 1 Chemical composition of the CoCrFeMnNi HEA (wt.%).

Table 1Co	Cr	Fe	Mn	Ni	
21.61	19.18	20.81	19.17	19.23	

A HLW-F1500 portable fiber laser welding machine was used to weld the specimen plates, with 99.99 % argon gas as the shielding gas and a 304 stainless steel or 6061 aluminum alloy base plate. Their chemical compositions are listed in Table 2, Table 3, respectively. Before welding, the surface of the specimen surface was ground and polished with sandpaper and ultrasonically cleaned and dried. No filler material was added during welding. The welding parameters of the specimens selected for this study are summarized in Table 4.Table 2 Chemical composition of 304 stainless steel (wt.%).

Table 2C	Mn	Si	S	P	Cr	Ni	Fe	
0.05	1.60	0.36	0.002	0.01	18.50	8.82	70.658	

Table 3 Chemical composition of 6061 aluminum alloy (wt.%).

Table 3Si	Mg	Fe	Mn	Cu	Cr	Al	
0.58	0.94	0.16	0.03	0.23	0.07	97.99	

Table 4 Welding parameters.

Table 4Specimen	Laser power (W)	Welding speed (mm·s−1)	Gas flow rate (L·min−1)	Base plate	
1#	900	10	20	304	
2#	900	5	20	6061	

2.2 Microstructure characterization

After welding, the specimen was wire cut perpendicular to the weld direction to prepare a metallographic specimen, and it was corroded after grinding and polishing. Electrolytic corrosion was performed using an aqueous oxalic acid solution (10 %) with an electrolysis voltage of 6 V. The specimen was connected to the positive pole of the power supply, and the auxiliary electrode was connected to the negative pole. The corrosion lasted for 15 s. The microstructure of the weld was observed with the help of an XJG-05 metallurgical microscope (OM), while the morphology of the fracture was examined and the micro-area composition of the crack fracture were determined using a JSM-6360LA scanning electron microscope (SEM) coupled with an Oxford energy dispersive spectrometer (EDS). A DIL 402 linear expansion coefficient dilatometer (manufactured by Netzsch) was used to determine the linear expansion coefficient of the CoCrFeMnNi HEA, and JMat was adopted to calculate the phases precipitated during weld solidification under different aluminum contents.

2.3 Measurement of wetting angle

The experimental procedures were as follows. An HEA specimen with a diameter of 10 mm and height of 2 mm was obtained by EDM (electrical discharge machining) cutting, and its surface was polished to make it smooth. Next, 1 g of aluminum grains were placed on the surface of the specimen, and the surface of the grains was covered with a brazing flux (QJ401) to minimize oxidation. Subsequently, the specimen, together with the aluminum grains and brazing flux, were put into QSH-VF-1200T furnace and heated to 670 °C at a rate of 10 °C/min in argon and holding for 15 min. After the specimen was cooled in the furnace, it was taken out to examine the aluminum surface. Aluminum grains were added continuously by repeating the procedures described until the maximum contact angle was reached, i.e., the angle at which the aluminum liquid is just about to but has not yet been lost.

3 Results

3.1 Weld microstructures

After welding, no cracks were detected in the weld of stainless steel-HEA joints, whereas obvious cracking was observed in the center of the weld of aluminum alloy-HEA joints. Fig. 1 shows the microstructures of the welds of the two specimens. According to Fig. 1(a), when stainless steel is used as the base plate, the weld microstructure consists of dendrites growing perpendicular to the direction of the fusion line and equiaxed crystals at the center of the weld. Fig. 1(b) illustrates that the crack propagates along the grain boundaries, exhibiting intergranular fracture characteristics. The micro-structure morphology of the weld of aluminum alloy-HEA joints undergoes significant changes. Single-phase equiaxial crystals transformed into bright polygonal dendrites scattered throughout the dark gray matrix.Fig. 1 Microstructures of the welds.

Fig. 1

Table 5 shows the EDS analysis results at different positions of the two specimens (C and D denote the locations inside dendrites and at grain boundaries of stainless steel-HEA joints, respect-tively, and L and M denote those within the bright and dark gray regions in aluminum alloy-HEA joints, respectively). The following can be observed. Firstly, the weld of stainless steel-HEA joints does not contain Al. Secondly, the atomic percentage of Al in the bright region of the weld of aluminum alloy-HEA joints is 18.58 %, which is slightly higher than that in the dark gray region (17.50 %). Furthermore, the distributions of elements in the two regions are notably uneven. There are higher Cr and Fe contents in the bright region, while the Ni content in the dark gray region is comparatively higher.Table 5 EDS analysis results of the welds.

Table 5	Cr	Mn	Fe	Co	Ni	Al	
C	21.10	18.57	20.75	21.55	18.03	0	
D	18.52	22.73	18.78	18.28	21.49	0	
L	14.59	13.29	16.07	17.63	18.28	18.58	
M	8.73	15.99	11.00	19.25	25.01	17.50	

3.2 Fractographic analysis

The macroscopic morphology of the weld crack in aluminum alloy-HEA joints is shown in Fig. 2, where the crack is located at the center of the weld. Fig. 3 shows the microscopic fracture morphology of the crack. Fig. 3(a) and (b) illustrate equiaxial crystal cracking along the center of the weld. The crystals have smooth surfaces, in their natural crystalline forms. These are typical features of longitudinal solidification cracks at the center of a weld. At the same time, evident secondary transgranular cracking is noted on the fracture along the equiaxial crystal crack, as indicated in Fig. 3(b) and (e). In Fig. 3(c) and (f), cracking is observed along the coarse columnar dendrites in the weld, and the cellular dendrites on the fracture are arranged orderly, without any signs of deformation. Furthermore, the dendritic morphology is clearly visible. Because of the rapid cooling of the weld metal during laser welding, dendrites are strongly oriented. As crystallization proceeds, crystal grains continue to grow. At a certain stage of cooling, they come into contact with each other and constantly roll together. Meanwhile, strong solid-phase connection has not yet formed between the cellular dendrites, and there is no interaction between them. At this time, the volume fraction of the low-melting-point liquid metal in the molten pool is small, and it is difficult for the liquid metal to flow. Cracking of liquid films occurs along the grain boundaries under the action of shrinkage stresses. Therefore, solidification cracks propagate along boundaries of the orderly arranged cellular dendrites, and the residual liquid metal between the dendrites cools rapidly to give a natural, smooth crystalline form after solidification cracking.Fig. 2 Macroscopic morphology of cracks in aluminum alloy-HEA joints.

Fig. 2

Fig. 3 Fracture morphology of aluminum alloy-HEA joints.

Fig. 3

Table 6 lists the EDS results at locations A and B in Fig. 3(d). According to these results, in addition to the five elements originally present in the base metal, the main constituent elements on the fracture surface of the weld crack include Al. More specifically, the atomic percentages of Al at A and B are 23.33 % and 25.05 %, respectively. Both values are higher than those in the weld near the crack.Table 6 EDS analysis results of the fracture.

Table 6	Cr	Mn	Fe	Co	Ni	Al	
A	15.19	15.00	16.33	15.55	14.59	23.33	
B	15.50	14.98	15.29	14.94	14.24	25.05	

Fig. 4(a) shows the weld crack morphology of aluminum alloy-HEA joints observed under a scanning electron microscope. Before SEM observation, the specimen was only ground and polished but not corroded. Nevertheless, the general outline of the weld is clearly visible in the figure, and the crack is located at the center of the weld. EDS line scans were performed on the weld cracks, and the results for Al at different locations of the weld are plotted in Fig. 4(b) and (c). From the figure, it is clear that the intensity measured in the CoCrFeMnNi HEA base material area is basically 0, indicating that the base material does not contain Al. Contrarily, Al starts to appear in the proximity of the fusion line. Furthermore, from the fusion line to the center of the weld, the Al content slowly increases. At the center of the weld, i.e., the boundary of the thermal crack, the Al content rises abruptly and significantly and peaks.Fig. 4 EDS line scan results of the weld cracks of aluminum alloy-HEA joints.

Fig. 4

When a 6061 aluminum alloy base plate is used in laser welding, due to the high energy density of the heat source, the energy of the laser beam penetrates the HEA specimen so that the surface temperature of the base plate rises rapidly. Because the melting point of the aluminum alloy is low, Al in the base plate evaporates and diffuses into the HEA weld. As a result, Al is found on the fracture surface. As the molten metal in the weld solidifies, Al continuously diffuses from the fusion line to the center of the weld. In the late stages of solidification of the weld metal, Al is enriched at the grain boundaries at the center of the weld, while a thin Al-rich low-melting liquid film is formed at the center of the dendrite junction.

When a 304 stainless steel base plate is used in laser welding, compared to 304 stainless steel backing plate, cracks appear at the center of the weld seam, and the Al content is enriched at the center of the weld seam. Previous studies have shown that when the Al content exceeds 16 %, the BCC structures have been discovered in the microstructure of HEA [9]. Therefore, it is deduced that the formation of solidification cracks is caused by Al contamination in the weld.

4 Discussion

4.1 Effects of solidification temperature range of the HEA

Solidification cracks are produced in the late stages of weld solidification and crystallization. A larger solidification temperature range indicates a wider brittle temperature range and greater tendency for crystalline cracks to form in the weld [10]. The solidification processes of CoCrFeMnNi HEAs with different Al contents were calculated with the help of JMat by assuming that the atomic percentages of Al are 0 %, 20 %, 40 %, and 60 % and that the remaining elements are in equal atomic proportions. The calculated solidification temperature ranges are shown in Fig. 5. According to the figure, the solidification temperature range of the alloy is 163 K when the Al content is 0, and it reaches 721 K when the Al content increases to 60 %. In other words, as the Al content increases, the solidification temperature range of the weld gradually increases, and therefore, thermal cracking in the weld becomes more likely to occur.Fig. 5 Solidification processes of CoCrFeMnNi HEAs with different Al contents.

Fig. 5

4.2 Effects of linear expansion coefficient of the HEA

The formation of solidification cracks is the combined result of the intergranular low-melting-point liquid film and large tensile stress acting on the weld. Fig. 6 illustrates the experimental thermal expansion coefficient of the CoCrFeMnNi HEA. According to the figure, the thermal expansion coefficient of the alloy at 900–1100 K varies between 23 × 10−6 and 25 × 10−6, which is higher than that of austenitic stainless steel (16 × 10−6–17 × 10−6 K−1) in the same temperature range. Hence, the welded joints of the alloy will result in greater tensile stresses during solidification, and they are more susceptible to thermal cracking. Meanwhile, compared to the stainless steel plate, the aluminum alloy base plate has higher thermal conductivity. This indicates that the heat dissipation is more rapid and the weld cooling rate is faster, leading to a larger tensile strain rate. In other words, in the late stages of weld solidification, the proportion of solid phase increases, so the residual liquid metal spreads over the grain boundaries of dendrites to form a thin liquid film. The solidification shrinkage stress causes significant deformation of dendrites, which exceeds the critical deformation of the liquid film. Hence, the liquid film is torn apart, and cracking in the weld occurs.Fig. 6 Thermal expansion coefficient of CoCrFeMnNi HEA.

Fig. 6

4.3 Effects of morphology of the intergranular low melting liquid metal in the HEA

The formation of crystalline cracks is closely related to the state of existence of the thin liquid film at the grain boundaries, while its form of existence is influenced by the grain boundary energy between dendrites and the solid–liquid interfacial tension between the crystals and the liquid metal [10], as demonstrated in Fig. 7.Fig. 7 Grain boundary energy balance at a three-grain junction.

Fig. 7

According to the figure, the relationship between the grain boundary energy and solid–liquid interfacial tension is given by(1) γα/α=2γα/βcosθ2

where γα/α denotes the grain boundary energy, γα/β is the solid–liquid interfacial tension, and θ is the contact angle. When θ = 0°, the liquid spreads all over the intergranular space, forming a thin liquid film. At this point, the tendency for thermal cracking in the weld is the highest. Meanwhile, when θ = 180°, the liquid exists between crystals in spherical shapes, making it less likely to form a thin liquid film, and the tendency for thermal cracking decreases [11].

Based on the above analysis, it is clear that there is significant enrichment of Al in the proximity of cracks. It can be inferred that, within the brittle temperature range, the liquid metal in the weld is mainly composed of low-melting-point liquid aluminum alloy. In other words, the main constituent element of the intergranular liquid film is Al.

The semi-empirical relationship between the grain boundary energy and self-diffusion coefficient in pure metals and alloys was obtained according to first principles [12] and is expressed by Equation (2):(2) γα/α=RT2a02NAIn(D0gbD0v)+12a02NA(Qv−Qgb)

where a0 is the lattice constant, NA is the Avogadro constant, Qgb denotes the grain boundary diffusion energy, and Qv is the bulk diffusion energy. Vaidya et al. [13,14] experimentally measured the diffu-sion coefficient of Ni in the CoCrFeMnNi HEA. Through calculation, the authors discovered that the relationship between the grain boundary energy of the CoCrFeMnNi HEA and temperature satisfies the following relationship by Equation (3):(3) γα/α=0.553+(3.37×10−4)×T

According to the phase diagram of the CoCrFeMnNi alloy [15], its melting point is approximately 1563 K. Therefore, its grain boundary energy is approximately 1.08 J/m2.

Young proposed the well-known Young's equation based on research on surface tension and capillary effects:(4) γα/g=γα/l+γl/g

where γα/g, γα/l, and γl/g are the solid–gas, solid–liquid, and liquid–gas surface tension, respectively, and α is the wetting angle. In the calculation process of Equation (4), because it is difficult to determine solid–liquid surface tension, that between the liquid Al and solid CoCrFeMnNi alloy could not be obtained. Thus, the experiment was redesigned. A system for characterizing wettability at finite liquid–solid interfaces was chosen to determine the surface tension between the CoCrFeMnNi HEA and liquid Al, and the model is shown schematically in Fig. 7[16,17].

From Fig. 8, it can be seen that the following equilibrium relationship between surface tensions exists in the plane of the solid–liquid interface as Equation (5):(5) γs/l=−γl/gcosα

Fig. 8 Schematic diagram of the surface tension measurement model for finite solid–liquid interfaces.

Fig. 8

At this point, the solid–liquid surface tension γs/l is simply a function of the liquid–gas surface tension γl/g and contact angle α. Kou et al. [18] measured the liquid-phase surface tension of pure liquid Al in an argon atmosphere at 1563 K to be 0.9 J/m2. Hence, only the wetting angle α between the liquid Al alloy and solid CoCrFeMnNi alloy needs to be determined experimentally.

Fig. 9 shows the experimentally obtained maximum wetting state of the Al liquid on the finite surface of the CoCrFeMnNi HEA (after the furnace was opened, both the HEA and AI were oxidized in the air and they become black and grayish white, respectively). The maximum wetting angle α was measured to be 127.2° with the help of 3D scanning.Fig. 9 Maximum wetting state of the Al liquid on the finite surface of the CoCrFeMnNi alloy.

Fig. 9

Therefore, the solid–liquid interfacial tension γs/l between the pure Al liquid and CoCrFeMnNi HEA was calculated to be approximately 0.54 J/m2. By substituting the grain boundary energy and solid–liquid surface tension into Equation (1), the resulting contact angle θ was approximately 14.6°. It is noted that the contact angle between the Al liquid and CoCrFeMnNi HEA crystals is small. Thus, in the late stages of weld solidification, the residual liquid metal between dendrites has already formed intergranular contact angles that are close to zero, suggesting a higher tendency for thermal cracking in the weld.

5 Conclusion

(1) The use of a stainless steel base plate in the laser welding of CoCrFeMnNi HEA results in a crack-free weld. On the contrary, when an aluminum alloy base plate is adopted, solidification cracks are formed in the center of the weld. More specifically, the fracture runs along the grain boundaries.

(2) During the welding process, Al evaporates into the HEA weld and diffuses from the fusion line to the center of the weld. The Al content peaks at the center of the weld. The contact angle of the Al liquid between dendrites of the CoCrFeMnNi HEA is approximately 14.6°, and it spreads all over the intergranular space.

(3) In the late stages of weld solidification, the solidification interval increases with the increase of aluminum content in the residual liquid metal. Due to the large linear expansion coefficient of CoCrFeMnNi HEA, the shrinkage stress and tensile strain also increase. The residual liquid metal is distributed between dendrites in the form of a thin liquid film, and thermal cracks are formed in the weld under the action.

Data availability statement

Has data associated with your study been deposited into a publicly available repository?

Response: No. Data will be made available on request.

CRediT authorship contribution statement

Xia Cao: Writing – review & editing, Methodology, Conceptualization. Songya Tian: Writing – original draft, Data curation. Fan Xu: Supervision, Software. Genyuan Zhang: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Xia Cao(1973-),female, associate professor, research direction: welding and forming processing of metal materials.
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