1 State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China 2 Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China
Cite this article:
Wei FU,Xiaoguo SONG,Long LONG,Jianhang CHAI,Jicai FENG,Guodong WANG. INTERFACIAL MICROSTRUCTURE AND MECHANI-CAL PROPERTIES OF INDIRECT BRAZED GRAPHITE/COPPER JOINT. Acta Metall Sin, 2016, 52(6): 734-740.
Graphite and metal composite structures were widely used in aerospace, electrical engineering and electronics. Because of its conveniences and less cost, brazing was widely used to bond graphite and metals. Due to the differences in microstructure, graphite was difficult to be wetted by traditional braze alloys. To improve the wettability of traditional brazing alloys on graphite, active brazing process and indirect brazing process were developed to braze graphite to metals. As to active brazing process, active elements (such as Ti, Cr, Zr) were added into traditional brazing alloys, a high brazing temperature, was essential to guarantee the reaction of active elements with graphite. However, the mechanical properties of metals will degrade under high temperature. Electroplating and chemical plating were the general techniques for indirect brazing process. The covered coating had a mechanical combination which decreased the joint strength, rather than metallurgical bonding with graphite. Therefore, in this work, a new metallization method was proposed. On the one hand, a metallurgical bonding was formed between metallization layer and graphite substrate. On the other hand, graphite could be brazed to metal at a relatively low temperature. Firstly, graphite was metalized by Ti-containing Sn0.3Ag0.7Cu metallization powder at 950 ℃ for 30 min. Then metalized graphite was brazed with copper by Sn0.3Ag0.7Cu successfully. The typical interfacial structure of brazed joint was copper/Cu3Sn/Cu6Sn5/β-Sn/TiC/graphite. Element Ti of metallization powder played an important role in metallization process for a reaction layer TiC was formed on the interface of graphite and metallization layer. Nevertheless, Ti contents had no effect on interfacial structure and shear strength of brazed joint. With the increase of brazing temperature, more and more element Cu dissolved into molten solder and formed Cu-Sn compounds by reacting with Sn. Furthermore, shear strength was improved slightly. Fracture analysis reveals that cracks extended along β-Sn layer and presented ductile fracture. When Cu-Sn compounds occupied the entire brazing seam (joint brazed at 600 ℃), shear strength improved remarkably and reached 30 MPa. Additionally, the joint was fractured in graphite entirely.
Fund: Supported by National Natural Science Foundation of China (No.51405099) and Aerospace Science and Technology Innovation Foundation of Shanghai (No.SAST2015045)
Fig.1 Schematic of assembling brazing joint (unit: mm)
Fig.2 SEM image of metallization layer with Ti content of 1.4%
Fig.3 XRD spectra of graphite, metallization layer and reaction layer
(1)
Fig.4 Effect of Ti contents on the interfacial structure of metallization layer with Ti contents of 1.0% (a) and 1.8% (b)
Point in Fig.2
Sn
Ag
Cu
Ti
C
Possible phase
A
98.34
0.52
1.07
0.07
-
β-Sn
B
2.51
0.01
0.06
49.56
47.86
TiC
C
93.72
0.47
0.85
0.38
4.58
β-Sn
Table 1 EDS results of the metallization layer in Fig.2
Fig.5 Low (a) and locally high (b) magnified SEM images of copper/graphite joint brazed at 520 ℃ for 5 min with Ti content of 1.6%
(2)
Point in Fig.5a
Sn
Ag
Cu
Ti
Possible phase
A
24.65
0.30
75.05
0
Cu3Sn
B
44.92
0.15
54.88
0.05
Cu6Sn5
C
97.93
0.47
0.85
0.75
β-Sn
Table 2 EDS results of points marked in Fig.5a
Fig.6 Interfacial microstructures of copper/graphite joints brazed at 520 ℃ for 5 min with Ti contents of 1.2% (a) and 1.8% (b)
Fig.7 Effect of Ti content on the shear strength of brazed joints
Fig.8 SEM-SE image (a) and XRD spectrum (b) of fracture of copper/graphite joint brazed at 520 ℃ for 5 min with Ti content of 1.6%
Fig.9 Effects of brazing temperature on the interfacial microstructure of copper/graphite joints brazed at 540 ℃ (a) and 600 ℃ (b) with Ti content of 1.6%
Fig.10 Effect of brazing temperature on the shear strength of copper/ graphite joints with Ti content of 1.6%
Fig.11 Fracture morphology of copper/graphite joint brazed at 600 ℃
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