Materials Physics
Microstructures and Properties of Ni–Cr–P Filler Metals and Brazed Joints Bearing CNTs
2022 Volume 63 Issue 10 Pages 1375-1379
Details
2022 Volume 63 Issue 10 Pages 1375-1379
In this paper, the effects of small amounts (0∼1.0 wt%) of carbon nanotubes (CNTs) on the microstructures and properties of Ni–Cr–P filler metal and brazed joints have been investigated. Various content of CNTs were successfully blended into Ni–Cr–P paste for preparation novel filler metals, the results indicate that small amount of CNTs can increase the melting temperature ∼3°C, enhance the wettability of filler metals on steel substrate, and improve the shear strength of brazed joints. Moreover, the microstructures can be refined obviously. However, excessive addition of CNTs will induce the nano-agglomeration, which degrade the wettability of filler metals and shear strength of brazed joints, coarsen the microstructures. Based on the optimization content of CNTs, 0.1% CNTs can maximize the properties of filler metals and brazed joints.
Ni-based filler metals have good corrosion resistance and oxidation resistance, are suitable for brazing high temperature alloy (such as stainless steel, heat-resistant steel, etc.), mainly used in the furnace brazing, resistance brazing and induction brazing, etc., the protective gas or vacuum are needed in brazing.1–3) These filler metals show high temperature strength of brazed joint and excellent corrosion resistance compared to other types of filler metals. Pure Ni has a high melting point (1452°C), in order to reduce the melting temperature, C, B, P, Cr, Mn, etc., have been added into Ni to form Ni-based filler metals. The addition of these elements has different effects on the properties of Ni-based filler metals. So how to select suitable additives to promote the performance of Ni-based filler metals has become an important research topic in the industry.
For Ni–Cr–P filler metal, Cu element was added into the alloy, which can induce the decrease of melting point, but the difference between liquidus temperature and solidus temperature become large, the wettability of filler metal can be enhanced significantly on 316L stainless steel, 9% Cu shows the superiority.4) Moreover, MoS2 decorated with can also improve the wettability of Ni–Cr based filler metals, can reduce the thermal damage of diamond, the linear expansion coefficient, strength and hardness of filer metal will be decreased.5) Rare earths are called as the “vitamins” for metals,6) which means a small amount of rare earths can improve the properties. Rare earth La and Ce were selected as additives into Ni–Cr filler metals,7) 0.1% La and 0.05% Ce can enhance the wettability of filler metals with base material, the brazing performance can also be improved, the good interface can be fabricated by this filler metal and infiltration brazing technology. C was added into Ni–Cr filler metal to modify the property and microstructure,8) 1.0% C can enhance the wettability, can react with Cr to form the carbide, in which part of Cr3C2 can grow in nucleus on the diamond surface, Cr7C3 can be formed in brazed joints, the micro-hardness of the brazed joints decreases from 630 HV0.2 to 580 HV0.2. Until now, for the enhancement of properties, little reference has been reported with the addition of particles and nanowires into Ni–Cr based filler metals, the results of adding particles or nanowires into lead-free solders can be used for Refs. 9)–12), I envision nanowires acting like steel bars in concrete to strengthen brazed joints, which can be utilized to further study new Ni–Cr based filler metals with high performance.
In this paper, CNTs were selected as additives to improve the properties of Ni–Cr–P filler metals. The wettability, melting temperature and microstructures of Ni–Cr–P–xCNTs filler metals, shear strength and fracture morphologies of brazed joints were studied systematically, which can provide a reference for Ni based filler metals.
In this part, commercial Ni–Cr–P paste and commercial CNTs were used as the raw materials for the preparation of new filler metals, mechanical agitation was utilized to stir the raw materials in crucible, and five different compositions of Ni–Cr–P–xCNTs paste (x = 0, 0.05%, 0.1%, 0.2%, 0.5% and 1.0%) can be obtained. Table 1 shows the com-position of Ni–Cr–P filler metals with different CNTs content. The Q235 steel with dimension of 40 mm × 40 mm × 2 mm was selected as the substrate and base material.
The melting characteristic of Ni–Cr–P–xCNTs paste was tested with STA449 F3 comprehensive thermal analyzer, which can determine the effect of CNTs on the melting temperature of Ni–Cr–P filler metal. 0∼1000°C temperature range for the test was conducted, and the heating rate was 10°C/min. According to the national standard GB/T113634-2008 “Test Method for Wettability of Filler Metals”, the wettability of Ni based filler metals on the surface of steel substrate was studied. For the wettability, oxides on the surface Q235 steel should be removed before brazing with hydrochloric acid and H2O solution, Ni–Cr–P–xCNTs pastes were put on the surface of Q235 steel substrates, and vacuum furnace was utilized to melt the pastes and to test the spreading testing with 945°C heating temperature. In order to determine the spreading area, Image-J software was selected to calculate the data of specimen. Six groups of experimental results were conducted and the average value was taken to analyze the effect of CNTs on wettability.
The mechanical property test of brazing joint was carried out according to the national standard GB/T 11363-2008 “Strength of Brazing Joint for Test Method”. Lap joints were formed with Ni–Cr–P–xCNTs filler metals, the brazing temperature is 945°C. SANS UTM (universal testing machine) was applied to determine the shear strength of Ni–Cr–P–xCNTs brazed joints. Every composition in this test should be done six experiments, the average value of the six data should be taken and utilized to investigate the effect of CNTs.
For the variation of properties of filler metals, the microstructures can be applied to analyze the properties change. In order to analyze the effect of CNTs on the microstructure of Ni–Cr–P–xCNTs brazed joints, the joint samples were prepared by section, grinding, polishing, and was corroded by (NH4)2S2O8+H2O solution. Scanning Electron Microscopy (SEM) was utilized to observe the microstructures of Ni–Cr–P–xCNTs filler metals and the fracture morphologies of brazed joints, and Energy Dispersive Spectrometer (EDS) can be used to determine the elements in the Ni–Cr–P–xCNTs brazed joints.
The brazing temperature of filler metals can be determined according to the melting temperature, so it is necessary to measure the melting temperature of Ni–Cr–P–xCNTs filler metals for testing. Figure 1 shows the results of the influence of CNTs on the melting temperature of Ni–Cr–P filler metal. It can be seen that with the addition of CNTs, the solid temperature and liquidus temperature of the filler metals increase slightly and vary within the range of ∼3°C. Therefore, the same peak brazing temperature can be used for Ni–Cr–P–xCNTs brazing. Similar phenomenon can be found in our previous work,13,14) 0.05%∼0.125% CNTs were added into Sn solder, the increase amplitude of melting temperature is in the range of 0.1°C∼2.06°C, which can be attributed to the small amount of CNTs addition and no reaction between CNTs and the elements of alloys. Therefore, the CNTs will show little influence on the melting temperature, the content of CNTs addition should be control within a small range.
Effect of CNTs on the melting temperature of Ni–Cr–P filler metal.
Brazed joints formed in the brazing process have a great impact on the reliability of products, and wettability is crucial to the formation of brazed joints, so wettability is one of the important indexes to evaluate filler metals.15) Figure 2 shows the influence of different CNTs contents on the spreading area of Ni–Cr–P filler metal, which proves the influence regularity of CNTs addition. It is found that the spreading areas of Ni–Cr–P filler metal can be significantly increased by adding a small amount of CNTs. When the content of CNTs is 0.1%, the wetting spreading area reaches the maximum value, which increases by more than 11.3%. When the content of CNTs is further increased, the spreading area of filler metal decreases obviously. When the addition of CNTs content is more than 0.1% (0.2%, 0.5% and 1.0%), the spreading area of filler metal can be decreased obviously. Compared with filler metal without CNT, the spreading area of Ni–Cr–P–1.0CNTs filler metal is still significantly higher, increasing by 3.0%. It can be seen from Fig. 2 that the influence trend of CNTs is “increase at first and then decrease”. When trace amount of CNTs is added, positive improvement effect can be produced.
Effect of CNTs on the spreading area of Ni–Cr–P filler metal.
The wettability of Ni–Cr–P filler metals bearing CNTs can be explained using Young equation, as shown in eq. (1). Due to the addition of CNTs, the balance of the wetting system will be broken, the γgl can be reduced, which will induce the decrease of the θ. Therefore, the addition of CNTs can increase the wetting spreading area of Ni–Cr–P filler metal. When the addition of CNTs is excessive, the agglomeration of CNTs will happen, and the improvement effect of CNTs will be weakened, so the spreading area will be reduced obviously.
| \begin{equation} \gamma_{\text{gs}} = \gamma_{\text{ls}} + \gamma_{\text{gl}}\cos \theta \end{equation} | (1) |
Figure 3 shows the microstructures of Ni–Cr–P filler metals with different content of CNTs, the microstructure of the Ni–Cr–P filler metal is mainly composed of Ni(Cr) solid solution, Ni3P intermetallic compound phase, Ni(Cr) and Ni3P eutectic structure. Due to the samples prepared with Ni–Cr–P–xCNT/Q235 steel, the Fe can diffuse into the filler metal to form Ni(Cr, Fe) solid solution. The microstructure morphology can also be reported in Qin’s work.16) Bulk Ni3P phase and small dot-shape eutectic particles can be observed in the matrix, bulk Ni3P phase will degrade the properties of filler metals or brazed joints, the bulk Ni3P phase observed was 60 µm long and 20 µm wide. With the addition of CNTs, the microstructure can be refined obviously, especially for Ni3P phase, the reason may be that CNTs promote a high nucleation density of the Ni3P phase and eutectic particles during solidification. When the content of CNTs is 0.1%, the matrix microstructure, Ni3P phase and eutectic particles are refined to the maximum extent among the series of Ni–Cr–P–xCNTs filler metals, the diameter of nubby Ni3P phase is around 8 µm. But when the content of CNTs addition is 0.2%, the microstructure and phases are obviously coarsened, the coarsened microstructure can also be observed in Ni–Cr–P–0.5CNTs filler metal, bulk Ni3P phase precipitates in the matrix microstructure. Moreover, 1.0% CNTs can also further coarsen the microstructures of Ni–Cr–P filler metal, the sizes of Ni3P is 60 µm × 12 µm respectively, which demonstrates that the excessive addition of CNTs (≥0.2%) can result in the coarsening microstructures, the main reason for this is that excessive CNTs tend to agglomerate, which can degrade the positive influence of CNTs.
Microstructures of Ni–Cr–P–xCNT filler metals (a) x = 0 (b) x = 0.05 (c) x = 0.1 (d) x = 0.2 (e) x = 0.5 (f) x = 1.0.
Brazed joints in equipment plays the role of mechanical connection and support, so mechanical properties is one of the important indexes of brazed joints. Figure 4 plots the shear strength of Ni–Cr–P–xCNTs brazed joint. It can be seen that the shear strength of brazed joint is significantly improved with the addition of trace CNTs. When the content of CNTs is around 0.1%, the shear strength of brazed joint reaches the peak value, and the shear strength increases 27% compared with Ni–Cr–P brazed joint without CNTs addition. In Sn–3.8Ag–0.7Cu solder, the strengthening behavior can also be found with small amount of CNTs, which has been reported by Zhu.17) When the addition content of CNTs exceeds 0.1%, the shear strength of brazed joint can be decreased obviously. When the amount of CNTs reaches 1.0%, the shear strength of brazed joint still increases by 7.8% compared with Ni–Cr–P brazed joint.
Effect of CNTs on the shear strength of Ni–Cr–P filler metal.
Due to the addition of CNTs into Ni–Cr filler metals, the theory of the dispersion strengthening can be used to explain the affect mechanism.18) Equation (2)∼(5) can be used to express the stress acting on the surface of particles using the movement of dislocations.
| \begin{equation} \tau = n\tau_{0} \end{equation} | (2) |
| \begin{equation} n = \frac{\pi (1 - \upsilon)L\tau_{0}}{Gb} \end{equation} | (3) |
| \begin{equation} \tau = \frac{\pi (1 - \upsilon) L\tau_{0}^{2}}{Gb} \end{equation} | (4) |
| \begin{equation} \tau_{0} = \sqrt{\frac{G\text{b}\tau}{\pi (1 - \upsilon)L}} \end{equation} | (5) |
Figure 5 shows the fracture morphologies of Ni–Cr–P–xCNTs brazed joints after shear testing, for Ni–Cr–P brazed joint without the addition of CNTs, the fracture mode is mainly brittle fracture, and there are a lot of intermetallic compound particles on the surface of the fracture, and a few dimples in some areas, intermetallic compound particles in the dimples. With the addition of 0.05% CNTs, the similar fracture mode can be observed, tiny intermetallic compound particles can be found on the surface of fracture microstructure. For Ni–Cr–P–0.1CNTs brazed joints, the fracture surface shows obvious laceration marks, which indicates that the ductile mode may be the main fracture mechanism, which is the reason that the shear strength of Ni–Cr–P–0.1CNT brazed joints is more excellent than original Ni–Cr–P brazed joints. When the addition of CNTs is 0.2% and 0.5%, there are significantly fewer laceration marks on the surface, and secondary crack can be observed. For Ni–Cr–P–1.0CNTs brazed joints, the mixed fracture mode with ductile and brittle can be found for the fracture morphology, moreover, two strong cracks can be found, which illustrates the degrade of mechanical properties. The fracture morphology can suitable to explain the variation regularity of shear strength of brazed joints. Figure 6 shows the agglomeration of CNTs on the fracture surface with 1.0% CNTs addition, large size CNTs agglomerated will play a negative effect on refinement of microstructure and enhancement of mechanical property. For Sn–57.6Bi–0.4Ag solder joints with different Ni-modified CNTs,19) at the fracture interface, loose phases containing CNTs have fewer quantities but large sizes can be found due to the excessive amount of CNTs, which could degrade the mechanical property of solder joints, which has a good agreement with the results in Ni–Cr–P–xCNTs brazed joints.
Fracture morphology of Ni–Cr–P–xCNT brazed joints (a) x = 0 (b) x = 0.05 (c) x = 0.1 (d) x = 0.2 (e) x = 0.5 (f) x = 1.0.
Fracture morphology of Ni–Cr–P–1.0CNT brazed joints and the CNTs agglomeration.
The present work was carried out with the support of Central Plains Science and technology innovation leader (PZYLJ20319270), the Distinguished Researcher project of Henan Province.
