Microstructure and mechanical behaviors of Gasar porous Cu/G4335V steel joint brazed by Ag-28Cu-0.75Ni alloy

Cu/steel composites have the advantages of low cost and high heat dissipation performance, which make them ideal materials for applications in the industrial heat dissipation field. Because of the unique pore structure, the Gasar porous Cu is more excellent in heat transfer performance. However, systematic research still needs to be done on the joining technologies of Gasar porous Cu/steel. In this paper, Gasar porous Cu was joined to G4335V steel using Ag-28Cu-0.75Ni. The microstructure, shear strength, and fracture behavior of the Gasar porous Cu/G4335V steel joint were investigated. The results show that a clear interface of the brazed joint and no brazing defects were found. The joint microstructure mainly comprises α-Cu (ss.), β-Ag (ss.), and Ag-Cu eutectic phase. As the pore diameter of Gasar porous Cu increased, the joining area of the Gasar porous Cu/G4335V steel joint became larger, thereby improving the shear strength of the joint. For the same pore diameter, the shear strength of the mode 1 joint (The load direction and the pore direction are parallel to each other. The pore direction refers to the growth direction of the pore.) was higher than that of the mode 2 joint (The pore direction and the load direction are perpendicular to each other). The fracture analysis indicated that the joint crack was initiated in α-Cu (ss.) and propagated along the banded α-Cu (ss.). The joint fracture was a mixed fracture mechanism that combined ductile-brittle fractures.


Introduction
Gasar porous Cu is a new type of microchannel structure with cylindrical pores arranged along the solidification direction.It has received extensive attention due to its unique functional properties, such as electrical conductivity [1], sound absorption [2], and thermal conductivity [3].Compared with solid Cu, Gasar porous Cu not only retains high thermal conductivity but also realizes low density, high specific surface area, and lightweight.These excellent properties have attracted the attention of industries with high heat dissipation needs, such as electronic components and nuclear power.At present, many scholars have studied the heat dissipation performance of Gasar porous Cu [4][5][6].It was found that the heat transfer performance of the Gasar porous Cu heat sink was much higher than that of the conventional groove fins and the microchannel heat sink [7,8].Ogushi et al [7] found that the heat transfer performance of the Gasar porous Cu heat sink was 4 times that of a conventional groove heat sink and 1.3 times that of the microchannel heat sink at the same pumping power.Researches have been done on the joining of Gasar porous Cu and Cu, as well as its joining technologies and heat dissipation performance [5,9,10].
Gasar porous Cu/G4335V steel joints are expected to be widely used in nuclear power and heat exchangers because of their excellent mechanical properties and thermal conductivity.However, there are few types of research on the joining technologies and application of Gasar porous Cu with dissimilar metals.Before applying Gasar porous Cu/G4335V steel in the heat exchange field, the joining technologies must be studied to ensure joint strength.In addition, the mechanical properties and fracture behaviors also need to be analyzed [11].At present, brazing [12,13], fusion welding [14], pressure welding [15] and accumulative roll bonding are common joining technologies [16].For Cu/steel systems, an excellent joint can be achieved by fusion welding and brazing.However, due to significant differences in thermophysical properties such as melting point, thermal conductivity, thermal expansion coefficient between Cu and steel, and low solid solubility, defects such as thermal stress and solidification cracks easily occur in the weld [17,18].Therefore, brazing is considered the preferred technology for joining dissimilar metals such as Cu and steel due to its advantages of uniform heating, high production efficiency, small workpiece deformation, and no need for secondary processing [19].Induction brazing is a welding technique that uses high, intermediate, or power-frequency electromagnetic induction to generate surface heating.Bhogendro et al [20] achieved a good joining between low-carbon steel (AISI 1015) and Cu by using the non-solder induction brazing with no cracks in the joint area.Bhogendro et al [21] successfully joined Cu with stainless steel by high-frequency induction brazing and found that the joint tensile failure was a brittle-ductile composite fracture mechanism.Shi et al [22] used a high-frequency induction technique to weld pure Cu to 304 stainless steel and found that the tensile strength increased with increasing brazing temperature.Furthermore, the tensile fracture surface of the joint shifted from α -Cu (ss.) to the heat-affected zone of the Cu side.
Poor wettability is a common problem in Cu/steel joining.Nickel plating on the base material [12] and selecting suitable brazing filler metal [23] are common methods to improve wettability.The most effective way to improve joint wettability is by alloying elements into the filler metal.Currently, many brazing materials such as Ag, Cu, and Ni-base have been used for Cu/steel joining.According to the research of Mu et al [23], it can be solved by using Ag-Cu filler metal containing Ni element.Due to the strong affinity of Ni with Cu and Fe, adding Ni to Ag-based brazing materials can effectively improve joint wettability during Cu-steel welding [24,25].Shi et al [22] also reported that Ag-base filler metal containing Ni benefits the wettability of Cu/304SS joints, thereby improving joint strength.Therefore, an Ag-based brazing alloy containing Ni was chosen as the filler metal in this paper.
In addition, it was found that the interface morphologies and its joining area also affect the joint properties [26,27].When Gasar porous Cu is joined to G4335V steel, its interface morphology and joining area differ from a flat interface.Therefore, the effect of pore structure on the mechanical behavior of the Gasar porous Cu/ G4335V steel joint needs to be investigated.In this work, Ag-28Cu-0.75Niwas used for high-frequency induction brazing of Gasar porous Cu/G4335V steel.The microstructure of the joint interface and the effects of different joining modes and pore diameters on the joining strength and fracture behaviors of Gasar porous Cu/ G4335V steel were analyzed.

Materials and methods
The base materials used were Gasar porous Cu and G4335V steel, and the filler metal was Ag-28Cu-0.75Ni,whose composition is shown in table 1.The solid-liquid phase of filler metal is 787 °C-796 °C.To prevent oxidation of the soldering surface, QJ102 (the chemical composition: 33.0-37.0wt% H 3 BO 3 , 21.0-253.0wt% KBF 4 , 40.0-44.0wt% KF) was used as a brazing flux.Its melting point is about 450 °C.Gasar porous Cu with a size of Φ30 × 600 mm was manufactured by a continuous casting technology under the hydrogen atmosphere.The Gasar porous Cu and G4335V steel were cut into 10 mm × 10 mm × 5 mm and 15 mm × 10 mm × 5 mm substrates by an electrical discharge machine, respectively.The porosity of Gasar porous Cu was expressed as P = (V 0 -V )/V 0 × 100%, where V and V 0 are the volume of Gasar porous Cu and non-porous Cu, respectively.The average pore diameter of Gasar porous Cu was measured by image analysis software (Image-Pro, Media Cybernetics).In this paper, two kinds of Gasar porous Cu with different pore diameters were selected for highfrequency induction brazing with G4335V steel.The average pore diameter of Gasar porous Cu were 1.23 ± 0.07 mm (porosity: 35.46 ± 1.56%) and 0.20 ± 0.03 mm (porosity: 34.06 ± 0.32%), respectively.In this paper, the microstructure and mechanical properties of Gasar porous Cu and G4335V steel under different joining modes were studied.In mode 1, the load direction and the pore direction are parallel to each other.In mode 2, the pore direction and the load direction are perpendicular to each other.The different joining modes of the Gasar porous Cu/G4335V steel joint are illustrated in figure 1.Before brazing, the Gasar porous Cu and G4335V steel were polished with SiC paper, and then ultrasonically cleaned with ethanol for 2 min.The Ag-28Cu-0.75Ni and QJ102 were placed on the surface to be brazed between Gasar porous Cu and G4335V steel.During the high-frequency induction experiment, specimens were heated to 840 °C at a rate of 10 K s −1 and held for 15 s. Figure 2 shows the welded samples of Gasar porous Cu/ G4335V steel under different joining modes.The sample was sectioned using an electrical discharge machine after brazing.The joint specimens were mechanically polished to #1000 and then coarse and fine polished with W0.25 diamond suspension and silica polishing suspension, respectively.The joints were then chemically etched with 5 g FeCl 3 +85 ml C 2 H 6 O+15 ml HCl mixture solution.The joint microstructure was observed by an optical microscope (OM) and scanning electron microscopy (SEM5000) with an energy spectrometer (EDS).For the mechanical properties of the joints, shear tests were carried out using the electronic universal testing machine (DCL-50, CIMACH) at a constant speed of 0.5 mm min −1 and shear height of 50 μm.The shear test of Gasar porous Cu/G4335V steel joint is shown in figure 3. Then the fracture surface of the Gasar porous Cu/ G4335V steel joint was observed by a scanning electron microscope (SEM5000).Shear strength was used to characterize the joining quality of Gasar porous Cu with G4335V steel.The effective adhesion range of Gasar porous Cu/G4335V steel was 10 × 10 mm 2 .The shear strength is calculated by formula (1): Where σ is the shear strength of the joint (MPa), F max is the maximum applied load (N), and S is the bonding area of the joint (mm 2 ).

Joint microstructure
The cross-sectional morphology of the Gasar porous Cu/G4335V steel joints under different joining modes is shown in figure 4. It was observed that Ag-28Cu-0.75Nihad fully penetrated into the pores of the Gasar porous Cu, and no obvious brazing defects were found in the Gasar porous Cu/G4335V steel joint area.Figure 5 shows the structure and element distribution of the Gasar porous Cu/G4335V steel joint.As shown in table 2, EDS results illustrate that the Gasar porous Cu/G4335V steel joint mainly comprised α-Cu (ss.), β-Ag (ss.), and Ag-Cu eutectic phases, where 'ss.' denotes a solid solution.The α-Cu (ss.) was distributed in an irregular and unevenly dispersed island pattern.Ni primarily exists in the α-Cu (ss.), and almost no Ni is dissolved in the β-Ag (ss.), as was previously reported [28].The phase diagram of the Cu-Ni binary alloy shows that the Cu-Ni system is an unlimited solubility system [29].Moreover, Cu and Ni possess the same facecentered cubic lattice type, which makes for a good affinity between them [30].There are few mutual solubilities between Ag and Ni at a low temperature.A locally enlarged view of the filler layer/G4335V steel interface in figure 5(a) is shown in figure 5(b).A reaction layer of approximately 1.0 μm thickness was found at the filler layer/G4335V steel interface.The reaction layer was mainly composed of β-Ag (ss.), which indicates the interdiffusion and metallurgical reaction between G4335V steel and filler layer.Thus, a reliable joint between Gasar porous Cu and G4335V steel can be achieved using Ag-28Cu-0.75Ni.
Figure 5(c) shows the EDS line scanning of elements in the Gasar porous Cu/steel joint and its energy spectrum, as shown in figure 5(d).According to the line scanning results, the contents of Ag and Cu elements showed an opposite trend, which was consistent with the results of energy dispersion spectroscopy in points 1-5.The EDS element mapping of Ag, Cu, Ni, C, Fe, Cr, Mo, Mn, Si, and V in the Gasar porous Cu/G4335V steel joint, as shown in figure 6.It can be seen that there is a clear dividing line at the filler layer/G4335V steel interface.Almost no Fe diffuses into the filler layer, and this is due to the very low solubility of Cu in Fe and the limited solubility of Fe in Cu.Besides, Fe diffuses more in Cu than Cu in Fe, which is consistent with a previous report [31,32].However, there is no clear demarcation line at the Gasar porous Cu/filler layer interface, and this is due to the fact that Ag and Cu are eutectic systems with limited solubility.In addition, C primarily exists in the α-Cu (ss.), while C is rarely dissolved in the β-Ag (ss.), and this is due to the fact that the solubility between C and Ag is very low and almost immiscible.

Mechanical properties and fracture behaviors
The pore morphology of Gasar porous Cu causes differences in the interface morphology and joining area of the Gasar porous Cu/G4335V steel joint.Zhang et al [33] studied the joint mechanical properties of straight and conical C/C composites and found that the average bending strength of the conical interface was 3.7 times that of the straight interface.Wang et al [34] investigated the vacuum brazing of surface perforated C/C composites with TiAl alloys and found that the joint strength was significantly improved.Therefore, the interface morphology and joining area were considered to be the most factors in determining the mechanical properties of joints [33,34].Therefore, the effect of joining area and interface morphology on joining strength should be considered when discussing the shear behaviors of the joint.
Figure 7(b) shows the shear strength data of Gasar porous Cu/G4335V steel joints with Ag-28Cu-0.75Nisolder under different joining modes and pore diameters.At the same pore diameter, the shear strength of the mode 1 joint was higher than that of the mode 2 joint.When the pore diameter was 1.23 ± 0.07 mm, the shear strength of the mode 1 and mode 2 joints was 72.91 MPa and 28.23 MPa, respectively.When the pore diameter was 0.20 ± 0.03 mm, the mode 1 and mode 2 joints was 45.86 MPa and 25.05 MPa, respectively.Studies have shown that solder infiltration in joints with grooved interface produces a pinning effect [35].The pinning effect of infiltrated solder is much more effective in the mode1 joint than in the mode 2 joint.This leads to a difference  in shear strength between Gasar porous Cu/G4335V steel joints.Therefore, the shear strength of the mode 1 joint was higher than that of the mode 2 joint.
In cases where the joining modes remain consistent, it was observed that an increase in the pore diameter of the Gasar Cu/G4335V steel joint leads to a corresponding enhancement in shear strength.When the pore diameter of Gasar porous Cu increased from 0.20 ± 0.03 mm to 1.23 ± 0.07 mm, the shear strength of mode 1 joint increased from 45.86 MPa to 72.91 MPa, and the shear strength of mode 2 joint increased from 25.05 MPa to 28.23 MPa.According to previous studies [27,36], joint strength generally depends on the joining area.Shokati et al [27] reported that the shear strength of flat and grooved C/C composites was joined with Ti6Al4V and found that the grooved interface improved the joint strength.Due to its controllable pore structure, two kinds of Gasar porous Cu with different pore diameters were joined with G4335V steel.As the pore diameter of Gasar porous Cu increases, the joining area of the Gasar porous Cu/G4335V steel joint grows, resulting in increased shear strength.Therefore, the Gasar porous Cu/G4335V steel joint with a pore diameter of 1.23 ± 0.07 mm had a higher shear strength.of the Gasar porous Cu/G4335V steel joints, which was a typical ductile fracture.In addition, it can be seen from figures 8(a) and (d) that part of the solder remains at the bottom of the pores.Obvious cracks were found in the solder, as shown in figures 8(c) and (f).This was a clear brittle fracture.Therefore, the fracture mechanism of Gasar porous Cu/G4335V steel joints was a mixed fracture mechanism combining ductile-brittle fracture.
Figure 9 shows the crack source and fracture path of the Gasar porous Cu/G4335V steel joint.When the Gasar porous Cu/G4335V steel joint was subjected to shear force, stress concentration occurred on both sides of the pores at the Gasar porous Cu/filler layer interface due to low yield strength.This result is consistent with that of a previous study [10].Thus, it is considered that crack sources occur on both sides of pores at Gasar porous Cu.From the phase analysis, it can be seen that only α-Cu (ss.) and β-Ag (ss.) exist near the pores of Gasar porous Cu.As shown in table 2, EDS results indicated that the parabolic dimples were α-Cu (ss.) in figures 8(b) and (e).Therefore, cracks originated from α-Cu (ss.) on both sides of the pore and propagated along the banded α-Cu (ss.), which eventually led to the fracture of the Gasar porous Cu/G4335V steel joint.

Conclusions
In this work, Gasar porous Cu and G4335V steel were brazed by high-frequency induction technology.The microstructure and mechanical properties of the Gasar porous Cu/G4335V steel joint were studied.The effects  of joining modes and pore diameters on the shear strength and fracture behavior of Gasar porous Cu/G4335V steel joints were investigated.The results are as follows: (1) In this paper, Ag-28Cu-0.75Niwas used to join Gasar porous Cu with G4335V steel, and α-Cu (ss.), β-Ag (ss.), and Ag-Cu eutectic phases were formed in the joint.Among them, the α-Cu (ss.) showed an irregular and unevenly dispersed island distribution.Moreover, no obvious brazing defects were found in the joint area, and a reliable joint was formed.
(2) At the same pore diameter, the shear strength of the mode 1 joint was higher than that of the mode 2 joint due to the difference in the pinning effect of the infiltrated solder.In the same joining mode, with the pore diameter of the Gasar porous Cu increased from 0.20 ± 0.03 mm to 1.23 ± 0.07 mm, the joining area of the Gasar porous Cu/G4335V steel joints increased, thereby improving the shear strength.The joint shear strength increased from 45.86 MPa to 72.91 MPa (mode 1) and 25.05 MPa to 28.23 MPa (mode 2) respectively.
(3) Under the action of shear stress, stress concentration occurred on both sides of the pores at the Gasar porous Cu/filler layer interface, which caused cracks originating from α-Cu (ss.) and spreading along the band α-Cu (ss.) and finally led to the fracture of the Gasar porous Cu/G4335V steel joint.Obvious parabolic dimples and cracks were found in the joint, which indicated that the Gasar porous Cu/G4335V steel joints have a mixed fracture mechanism combining ductile-brittle fracture.

Figure 5 .
Figure 5. Microstructural images and EDS map of Gasar porous Cu/G4335V steel joint: (a) joint morphology, (b) filler layer/G4335V interfacial morphology, (c) line scanning of the joint and (d) EDS spectrum corresponding to the phase of line in (c).
Figure 7(a) shows the load-displacement curves of the joint under different joining modes and pore diameters.The shear behaviors of Gasar porous Cu/G4335V steel joints show excellent ductility.The micro-morphology of the fracture interface after the shear test for modes 1(a)-(c) and mode 2(d)-(f) is shown in figure 8. Figures 8(a) and (d) show the overall fracture surface of the mode 1 and mode 2 joints, respectively.It can be seen from figures 8(b) and (e) that many parabolic dimples were generated at the fracture

Figure 7 .
Figure 7. (a) load-displacement curves of joints and (b) shear strength with different joining modes and pore diameters.

Figure 9 .
Figure 9. Schematic diagram of joint crack initiation and fracture path.