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Ni electrical contact materials
Materials and Design 85 (2015) 511–519
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Arc erosion behavior of Ag/Ni electrical contact materials Chunping Wu a,b,c,⁎, Danqing Yi b, Wei Weng c, Suhua Li c, Jiemin Zhou a, Feng Zheng b a b c
School of Energy Science and Engineering, Central South University, Changsha, 410083 China Key Laboratory for Nonferrous Metal Materials of the Ministry of Education, School of Materials Science and Engineering, Central South University, Changsha, 410083 China FuDa Alloy Materials Co., LTD, Wenzhou, 325025 China
a r t i c l e
i n f o
Article history: Received 27 December 2014 Received in revised form 23 June 2015 Accepted 26 June 2015 Available online 3 July 2015 Keywords: Ag/Ni electrical contact material Arc erosion Erosion morphology Element mapping
a b s t r a c t Ag/Ni electrical contact materials tend to be weld together under high current and/or high temperature, which was a key problem to restrict the usage of Ag/Ni electric contact materials. Arc erosion characteristics of Ag/12Ni electrical contact material after 50,000 operations under direct current 19 V, 20 A and resistive load conditions were investigated. The result indicated that the probability distribution and change trend of arc energy and arc time during 50,000 operations were similar and the relationship between arc time and arc energy followed exponential function. On the one hand, “Crater” type erosion pit, island-like melted silver, pore, crack and coral-like structure spitting were observed on erosion surface of Ag/Ni contact materials. On the other hand, distribution of Ag and Ni element on molten pool of movable contact was different from that of stationary contact. For movable contact, element Ni mainly distributed on melted pool root, whereas element Ag mainly distributed inside of melted pool. For stationary contact, however, element Ni and Ag distributed layer by layer. Furthermore, arc erosion of stationary contact is more serious than that of movable contact. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Electrical contacts have found a variety of applications due to their high conductivity, low contact resistance and good arc erosion resistance [1–4]. For example, Ag/Ni electrical contact materials are used for household applications, contactors, miniature circuit breakers and automobile relays due to their low and stable contact resistance, excellent machinability, high resistance of electrical loss and non-toxcity [5]. However, Ag/Ni electrical contact materials tend to be weld together under high current and/or high temperature. As a result, a large number of researchers have focused on arc erosion properties of Ag/Ni electrical contact materials in order to improve their welding resistance. For example, Yoshida and coauthors clarified the influence of voltage on arc characteristics [6], arc duration [7] and electrode mass change [8] of Ag/Ni contacts for electromagnetic contactors. Morin et al. [9] found that Ag/Ni electrical contact materials had high local materials transfer under lamp and resistive loads at 14VDC and current range from 10 to 70 A. Kawakami et al. [10] discussed that the possibilities of lifetime predictions in terms of cathode losses and arc energy with the data of Ag/Ni contacts for electromagnetic contactor. Doublet et al. [11] investigated the arc erosion, welding tendency and welding forces of Ag/Ni electrical contact materials under resistive load at current range from 10 to 90 A. Luo et al. [12] investigated the arc erosion characteristics of Ag/Ni electrical contact materials ⁎ Corresponding author at: Key Laboratory for Nonferrous Metal Materials of the Ministry of Education, School of Materials Science and Engineering, Central South University, Changsha, 410083 China. E-mail address: [email protected] (C. Wu).
http://dx.doi.org/10.1016/j.matdes.2015.06.142 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
fabricated by mechanical alloying. Liu et al. [13] found that the welding resistance of Ag/10Ni was relatively bad under high arc energy. Li found [14] that the arc erosion resistance of Ag/10Ni electrical contact materials fabricated by chemical coating method was better than that fabricated by powder metallurgy method. Huang et al. [15] indicated that Ag/10Ni electrical contact materials fabricated by chemical co-deposition method had strong welding resistance. Yan et al. [16] suggested that the addition of brittle materials could improve the electrical performance of Ag/Ni contact. Chen et al. [17] indicated that RE (rare earth) could improve the electrical performance of Ag/Ni contact. Li et al. [18] found that Ag/Ni contact had serious material transfer. They indicated that the improvement of material transfer between Ag/Ni contact not only considers material intrinsic nature but also matching among them. Tan et al. [19] studied the arcing mechanism and electric corrosion patterns of Ag/10Ni contact under low voltage and direct current after single breaking operation. Their results indicated that the arc erosion spot area is a linear function of load current. Li et al. [20] investigated the arc erosion of Ag/Ni contact at 50 Hz and 400 Hz. They found that the welding resistance of Ag/10Ni contact was better at 400 Hz while the arc erosion resistance was better at 50 Hz. Li et al. [21] analyzed the arc energy, arc time and welding force of Ag/Ni contact. They indicated that the arc time of Ag/10Ni contact fabricated by chemical co-deposition method was longer with higher arc energy. Zheng and coauthor [22] studied the arc erosion morphology and formation mechanism of Ag/Ni contact. Eight distinct types of surface morphologies were observed after arc erosion. However, little information is available on element distribution and formation mechanism of molten pool of Ag/Ni contact after arc erosion in literature. Information on element distribution and formation
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C. Wu et al. / Materials and Design 85 (2015) 511–519 Table 1 Properties of Ag/12Ni materials fabricated by powder metallurgy technology.
Movable contact Cathode Side view
0.82mm
Density (g/cm3) Electrical contact resistance (μ Ω · cm) Strength of extension (MPa) Elongation after fracture (%) Hardness (Hv0.3)
Cu
0.82mm Anode
Ag/Ni
Stationary contact 5mm
Top view
10.36 1.92 312 19 83
3. Results
4mm
3.1. Electrical contact physical phenomena Fig. 1. CAD drawing of contacts.
mechanism of molten pool is helpful to improve the welding resistance and electrical performance of Ag/Ni contact. The aim of this work is therefore to investigate arc erosion behavior of Ag/12Ni contact and discuss relationship between electrical contact physical phenomena and formation mechanism of molten pool.
2. Experimental procedures 2.1. Tested contacts The form of contacts is shown in Fig. 1. The movable and stationary contacts have a curved surface. Ag/12Ni (Ag: 88 wt.%; Ni: 12 wt.%) contact material fabricated by powder metallurgy technology is selected as a contact material, which manufacture technology flow diagram is represented in Fig. 2 and properties are represented in the Table 1.
2.2. Experimental apparatus Apparatus used in this experiment are shown in Fig. 3. The system consists of measuring and test equipment such as industrial PC, signal collecting, measuring and protection device, contact motion simulator and steady flow test power system. The data of arc energy, arc time and welding force can be collected used by this apparatus. Three contact samples were tested in this article considering the repeatability of experimental results.
Electrical contact physical phenomena, such as arc energy, arc time, welding force, etc., will change due to the changing of contact surface microstructure and composition under arc erosion. An average every 100 operations of arc energy, arc time and welding force of Ag/12Ni electrical contact materials during 50,000 operations is represented in Fig. 4. Change trend of arc time and arc energy during 50,000 operations is similar, which is different from welding force. When operation numbers are less than 8000, arc time and arc energy increase with the increase of operation numbers. But they decrease when the operation numbers increase from 8000 to 15,000, and then they basically keep stable when operation numbers increase from 15,000 to 50,000. As well as arc time and arc energy, when operation numbers are less than 8000, welding force increases with the increasing of operation numbers. Welding force decreases when the operation numbers increase from 8000 to 10,000. It basically keeps stable when operation numbers increase from 10,000 to 30,000. But it first increases and then decreases when the operations increase from 30,000 to 40,000, and then it basically keeps stable when the operation numbers increase from 40,000 to 50,000. Consequently, electrical contact physical phenomena (arc time, arc energy and welding force) every operation change under the action of arc erosion. Mass of movable and stationary contact will change under the action of arc erosion due to evaporation and splash erosion. In addition, material transfer occurs between movable and stationary contact during arc operation and also will result in their mass change. In this experiment, mass on movable and stationary contact decreases (movable contact mass loss: 2.7 mg; stationary contact mass loss: 4 mg) after 50,000 operations. Mass loss of stationary contact is larger than that of movable contact, which indicates that arc erosion of stationary contact is more serious than that of movable contact.
2.3. Experimental conditions
3.2. Erosion morphology of Ag/Ni contact materials
The experimental conditions are shown in Table 2. The load current was set at DC 19 V 20 A, and current-carrying time was about 0.3 s. The arc energy, the arc time and the welding force were measured during the operation under the above conditions. In this paper, the mass losses were measured using an electrical scale. By using the scale, mass changes of more than 0.1 mg can be measured with accuracy. The microstructure was characterized by both an optical microscope (POLYVAR-MET) and a scanning electron microscope (Sirion200) equipped with an energy dispersive energy diffraction spectroscope (Gensis60). The element map analysis and composition quantitative analysis were used by electron probe micro-analyzer (JXA-8230).
3.2.1. Macro-morphology Macro-morphology of Ag/12Ni contact material after 50,000 operations under the above conditions is represented in Fig. 5, where surface morphology of movable and stationary contact is changed due to the arc erosion. On the one hand, serious deformation has been observed on both movable and stationary contact surface due to action of arc erosion and contact force. On the other hand, splash erosion also has been observed on both movable and stationary contact surface (see red circle in Fig. 5a and b). Furthermore, arc erosion on stationary contact surface is more serious than that on movable contact surface, which is consistent with the result of mass loss in the above.
Ag powders Mixing
Compress
Sintering
Extrusion
Ni powders
Fig. 2. Flow diagram of manufacture technology of Ag/12Ni electrical contact material.
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Vernier caliper Pull /pressure sensors
PC
PC19812
PC19118
Control box
Computer Soft platform
Movable contact
R
Steady flow test power system
Resistive load
Working power
Electromagnetic vibrator
Stationary contact
Contact motion simulator
Fig. 3. Experimental apparatus.
Arc erosion not only causes surface morphology change but also leads to composition change due to the physical metallurgy reaction on the contact surface. Composition of different zones on contact surface (see Fig. 5a and b) measured by EDS is shown in Table 3. The result on Table 3 indicates that silver content is less than 88 wt.% and nickel content is more than 12 wt.% when Ag/12Ni electrical contact material is eroded by electric arc. In addition, a large number of oxygen and small amount of carbon content are detected on contact surface, which indicates that oxygen and carbon in air dissolve in silver matrix
Table 2 Experimental conditions. Contact material
Ag/Ni (Ag88 wt.%:Ni12 wt.%)
Circuit condition Frequency Number of operations Switching mode Contact force Surrounding gas Electrode spacing
DC19V, 20 A, resistive load 0.67 Hz (ON 0.3 s) 50,000 DC mode 0.98 N Air 2 mm
Fig. 4. Average every 100 operations on arc energy, arc time and welding force of Ag/12Ni contact materials during 50,000 operations.
during arc erosion. Temperature on arc root zone is very high due to the action of electric arc. Solubility of nickel and oxygen in silver will increase with the temperature increasing. Part of nickel particles also will dissolve in molten pool when temperature is higher than nickel melted point (1453°C). These nickel particles are melting and cooling in a short time and then forming secondary crystallization. In addition, silver will melt and evaporation due to its low melting point (961°C) during arc erosion. On the one hand, silver melting increases oxygen solubility in silver, which results in oxygen content increasing on contact surface. On the other hand, silver evaporation decreases silver content on contact surface. Furthermore, nickel particles aggregates and floats on the melted silver due to its low density (Ni: 8.9 g/cm3, Ag: 10.5 g/cm3), which results in nickel content increasing on contact surface. Consequently, oxygen and nickel content increases whereas silver content decreases when Ag/12Ni contact material is eroded by electric arc.
3.2.2. Micro-morphology Arc erosion morphology characteristics of movable and stationary contact after 50,000 operations under the above conditions are clearly represented in Figs. 6 and 7, respectively. The surface morphology characteristics of movable and stationary contact after arc erosion are similar. “Crater” type erosion pits are observed on movable and stationary contact surface (see Figs. 6a and 7a). Silver will melt and evaporate due to the action of arc erosion. Under certain conditions, especially in large current, silver steam will outflow from arc root. “Crater” type of erosion pit will leave on contact surface due to large droplet splashing during the silver steam outflow from arc root. Island-like rich silver zone is observed on movable and stationary contact surface (see Figs. 6b and 7b). Silver will melt due to action of arc energy. Rapid solidification results in the formation of island-like structure on movable and stationary contact surface because the melted silver cannot spread out in time on contact surface. Pores and cracks are observed on movable and stationary contact surface (see Figs. 6c and 7c). Heat melting molten metals will absorb large amount of gas from air under action of electric arc. In addition, solubility of oxygen in liquid silver (0.3%) is 40 times larger than that in solid silver (0.008%). So the melted silver contains a large amount of oxygen during action of electric arc. One part of oxygen dissolved in melted silver will escape to air due to change of oxygen pressure after electric arc extinguishing. The other part of oxygen has no time to escape from melted silver due to the rapid solidification and then results in the formation of pore on contact surface and inner. Cracks on stationary contact surface are more serious than that on movable contact surface, which means arc erosion of stationary contact is more serious than that of movable contact. Crack is a kind of dangerous arc erosion morphology. Formation reason of crack is very complicated,
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a
b
007 006
002 001
005 003
004
008
Fig. 5. The macro-morphology of Ag/Ni contact materials after 50,000 operations. (a) movable contact; (b) stationary contact.
which mainly depended on structure and property of electrical contact material, arc energy and outside working conditions. It is inevitable that there are some defects (micro hole, micro crack, inclusion, grain boundary and interfacial dislocation group, etc.) in the material inner or surface. These defects are the root cause of the surface crack formation. On the one hand, silver on contact surface will melt under high temperature action of electric arc. However, the melted layer on surface will sharply cool and solidify due to short duration of the electric arc (less than 10 ms in this experiment). The rapid solidification of melted layer leads to the increasing of vacancy and dislocation density in melted layer microstructure. Increase of vacancy and dislocation density will decrease grain boundary intensity, which increases the possibility of grain boundary crack formation under stress action (see white circle in Fig. 7c). On the other hand, pore will decrease material's mechanical intensity, which is easy to cause the formation of crack or prompt the development of crack. For example, pore in Fig. 6b results in the formation and development of crack (see white circle in Fig. 6b). Coral-like structure spitting is observed on movable and stationary contact surface (see Figs. 6d and 7d). Coral-like structure spitting is a kind of accumulation of particles with size 200–500 nm, which mainly appears on the edge of contact surface due to splashing erosion. Gasification and liquid evaporation splash during electric arc erosion is the main reason of coral-like structure particles formation. On the one hand, surface material changes into liquid from solid under arc energy action, and then changes into gas and escapes from contact material surface, finally the
Table 3 Composition of different zone in Fig. 5a and b measured by EDS.
Element (wt%) Zone C
O
Ni
W
Ag
001
1.11
9.12
34.95
2.49
52.33
002
1.76
6.67
24.83
1.6
65.15
003
1.37
7.3
15.94
1
74.39
004
1.54
9.56
23.64
1.81
63.45
005
1.05
12.67
37.20
2.88
46.21
006
2.26
8.61
23.62
1.81
63.69
007
1.12
9.72
31.65
2.13
55.38
008
2.95
5.07
21.58
0.9
69.51
C and O was detected
Ni>12 wt %
Ag < 88 wt %
gaseous silver absorbed large amount of oxygen in air rapidly solidifies on contact surface and forms coral-like structure particles. On the other hand, silver molten pool forms on contact surface under arc energy action. Tiny droplet in the liquid pond splashes from the molten pool the under all kinds of force action (such as electrostatic field force, electromagnetic force and reacting force of material movement, surface tension, etc.). Consequently, coral-like structure particle is a product of the evaporation and splash erosion under electric arc action. 3.3. Cross section microstructure of Ag/Ni contact materials after arcing erosion 3.3.1. Optical microstructure Optical microstructure on cross section of Ag/Ni contact material after 50,000 operations under the above experimental conditions is represented in Fig. 8. Curved surface of movable contact changes into flat surface under arc erosion action (see Fig. 8a1) and material transfer (material breaking away from movable contact surface) is observed on movable contact surface (see white circle in Fig. 8a1). While curved surface of stationary contact changes into concave surface under arc erosion action (see Fig. 8b1) and material transfer and splashing is observed on stationary contact surface (see white circle in Fig. 8b1). There is some little erosion pits (the molten pool) on movable and stationary contact surface (see Fig. 8a2 and 8b2). 3.3.2. Element mapping of the molten pool Element mapping of molten pool of Ag/Ni movable and stationary contact material after 50,000 operations under the above experimental conditions is represented in Figs. 9 and 10, respectively. The element distribution of molten pool on movable and stationary contact materials is different. Area of molten pool on movable contact is smaller than that on stationary contact (see Figs. 9a and 10a), which indicates that arc erosion on stationary contact is more serious than that on movable contact. Composition image indicates that there are four different phases (dark phase; white phase, dark gray phase and light gray phase) on Ag/Ni contact material (see Figs. 9a and 10a). Composition of different phase in Figs. 9a and 10a is shown on Table 4. The result on Table 4 indicates that white phase contains 87–97 wt.% element W, dark phase contains about 98 wt.% element Ni, light gray phase contains about 99 wt.% element Ag and deep gray phase contains about 80 wt.% element Ni and 18 wt.% element W. In addition, the result of element mapping also indicates that dark phase mainly contains Ni element, white phase mainly contains W element, dark gray phase mainly contains Ni and W element and light gray phase mainly contains Ag element (see Figs. 9b, c, d and 10b, c, d). Shape of molten pool on movable and stationary contact is like wizard magic map. Distribution of Ag and Ni
C. Wu et al. / Materials and Design 85 (2015) 511–519
515
b
a
Melted silver
d
c
Pore
Crack
Fig. 6. The micro-morphology of Ag/Ni movable contact materials after 50,000 operations. (a) erosion pit; (b) island-like melted silver; (c) pore and crack; (d) coral-like structure.
a
b
Melted silver
c
d
Crack
Pore
Fig. 7. The micro-morphology of Ag/Ni stationary contact materials after 50,000 operations. (a) erosion pit; (b) island-like melted silver; (c) pore and crack; (d) coral-like structure.
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C. Wu et al. / Materials and Design 85 (2015) 511–519
a1
a2
b1
b2
Fig. 8. Optical microstructure on cross section of Ag/Ni contacts after 50,000 operations. (a1) movable contact; (a2) molten pool of the movable contact; (b1) stationary contact; (b2) molten pool of the stationary contact.
element on molten pool of movable contact is different from that of stationary contact. For movable contact, Ni element mainly distributes on the bottom of molten pool (see red circle in Fig. 9c), while Ag element mainly distributes inside of molten pool (see white circle in Fig. 9b). However, distribution of Ag and Ni element is layer by layer in molten pool on stationary contact (see Fig. 10b and c).
4. Discussions 4.1. Relationship among arc energy, arc time and welding force The degree of arc erosion depends on arc energy. Arc energy is larger and arc erosion more serious. Whereas arc energy increase with the increasing of arc time. Furthermore, welding force has an important effect on weld and arc erosion of electrical contact material. So arc energy, arc time and welding force are very important influence factors during arc erosion. It is very essential that discussion on the relationship among arc energy, arc time and wielding force during arc erosion. In order to clarify the relationship among arc energy, arc time and welding force, we plot the probability distribution curves on arc energy, arc time and welding force of Ag/12Ni electrical contact material during 50,000 operations (see Fig. 11). The results in Fig. 11 indicate that probability distribution curves of arc energy and arc time are similar, which is in accordance with the result in Fig. 4. However, the probability distribution curves of arc energy and arc time is different from that of welding force. Ninety-five percent of welding force is less than 25 g during 50,000 operations. The average value of arc energy, arc time and welding force are 1172 mJ, 4.987 ms and 9.041 g during 50,000 operations, respectively.
The results in Figs. 4 and 11 indicate that arc time has an influence on arc energy, but arc time has no direct influence on welding force. Kubo and his coauthor found that the relationship between arc time and arc energy can be described with the following equation [23]: E ¼ ∑U I t
ð1Þ
where E is arc energy, U is arc voltage, I is arc current and t is arc time. The relationship between arc erosion (mass loss) and arc energy can be described with the following equation [23]: W ¼ c Ed
ð2Þ
where W is arc erosion, E is arc energy, c and d are coefficients related to material. Wang established the relationship between arc erosion and arc time according to experiment result [24]. The relationship between arc erosion and arc current can be expressed by the following equation: W ¼kIt
ð3Þ
where W is arc erosion, I is arc current, t is arc time and k is coefficient related to material. According to Eqs. (2) and (3), we can get the following equation: E ¼aIt
ð4Þ
where E is arc energy, I is arc current, t is arc time and a is coefficient related to material. The previous studies have shown that arc energy and arc time have an important influence for arc erosion of material and arc time also has an important influence for arc energy. In this paper, the relationship
C. Wu et al. / Materials and Design 85 (2015) 511–519
517
b
a
3 1 4
CP
20µm
2
Ag
20µm
d
c
Ni
20µm
W
20µm
Fig. 9. Element mapping on molten pool of AgNi movable contact after 50,000 operations. (a) Composition image; (b) Ag distribution; (c) Ni distribution; (d) W distribution.
b
a
5 8
6 7
CP
50µm
c
Ni
Ag
50µm
d
50µm
W
50µm
Fig. 10. Element mapping on molten pool of Ag/Ni stationary contact after 50,000 operations. (a) Composition image; (b) Ag distribution; (c) Ni distribution; (d) W distribution.
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3000
Table 4 Composition of different zone in Figs. 9 and 10 measured by WDS.
2500
Element (wt %) Zone C
Ag
Ni
W
1
2.559
0.373
98.825
0.116
2
5.048
0.412
6.898
87.642
3
1.999
0.205
79.678
18.118
4
0.912
98.819
0.269
0.000
5
1.287
0.255
98.458
0.000
6
0.887
0.110
1.079
97.924
7
1.037
0.270
81.113
17.581
8
0.564
99.166
0.270
0.000
Movable
Stationary
Note:
Ni particles (dark phase); Niw phase (dark gray phase);
Arc nenergy (mJ)
Contacts
E=2569.88-2754.48/[1+exp(t-5.13)/1.84] R 2 =0.99515
2000 1500 1000 500 0 0
2
4
6
8
10
Arc time (ms)
W phase (white phase); Fig. 12. The relationship between arc energy and arc time of Ag/Ni contact materials during 50,000 operations.
Ag matrix (light gray phase).
between arc time and arc energy of Ag/12Ni electrical contact material under DC19V, 20 A and resistive load conditions during 50,000 operations is discussed. The relationship curves between arc energy and arc time are clearly represented in Fig. 12, where the relationship between arc energy (E) and arc time (t) is well expressed by an exponential law. So we can get the following equation by data fitting: E ¼ 2569:88−2754:48=½1 þ expðt−5:13Þ=1:84
ð5Þ
where E is arc energy and t is arc time. The correlation of data fitting (R2) on Eq. (5) is 0.99515, which indicates that the relationship between arc energy and arc time of Ag/12Ni electrical contact materials in this experimental conditions can be described used by Eq. (5). As a result, arc energy is exponentially increases with the arc time increasing in this experiment. 4.2. Element distribution of molten pool The above result indicates that the distribution of Ag and Ni elements on molten pool of movable contact is different from that of stationary contact. On movable contact, Ni element mainly distributes on the bottom of molten pool, while Ag element mainly distributes inside of molten pool. However, the distribution of Ni and Ag element is
layer upon layer in molten pool of stationary contact. So the formation mechanism of molten pool on movable and stationary contact is different. Fig. 13 shows the schematic diagram of molten pool formation of Ag/Ni contact materials after 50,000 operations. Under arc energy action, temperature on movable and stationary contact material will increase, silver will melt when temperature reaches the melting point of silver (961°C), while nickel particles are still in solid state due to its higher melted point (1453°C). A molten pool containing Ni particles is formed due to the melting of silver (see Fig. 13b). Under the action of gravity, Ni particles in molten pool will move from down to up because the density of nickel (8.9 g/cm3) is smaller than that of silver (10.5 g/cm3), while silver will move from up to down due to its high density (see Fig. 13c). So Ni particles are mainly distributed above silver layer when the melted silver rapidly cools. On movable contact material, when Ni particles in molten pool move from down to up, they will move to the bottom of molten pool because the molten pool bottom is located at the molten pool above. As a result, Ni particles mainly distributes in the bottom of molten pool, while silver mainly distributes inside the molten pool (see Figs. 9c and 13d). On stationary contact material, when the Ni particles of molten pool move from down to up, they will move to the surface of molten pool because the surface of molten pool is located at the molten pool above. As a result, Ni particles are mainly distributed on the surface of molten pool, while silver is mainly
Fig. 11. Probability distribution curves on arc energy, arc time and welding force of AgNi contact materials during 50,000 operations.
C. Wu et al. / Materials and Design 85 (2015) 511–519
Movable contact
Movable contact
Movable contact
519
Movable contact
Movable contact
W
Ag
Ni layer Ag layer
Molten pool
Ni
Stationary contact
(a)
Stationary contact
Stationary contact
Stationary contact
Stationary contact
(c)
(d)
(e)
(b)
Fig. 13. Schematic diagram of molten pool formation of Ag/Ni contacts after arc erosion. (a) connect; (b) silver melting; (c) Ni particles moving up; (d) silver rapid solidification; (e) Ni and Ag layer formation.
distributed inside molten pool. However, the result of element mapping indicates that the distribution of Ni and Ag elements on stationary contact is layer upon layer (see Figs. 10b, c and 13e). It is possibly attributed to the action of contact pressure under the cycle operation. 5. Conclusions • The change trend and probability distribution of arc energy and arc time of Ag/12Ni contacts during 50,000 operations are similar and the relationship between them can be expressed using by the followed exponential function: E = 2569.88 − 2754.48/[1 + exp(t-5.13) / 1.84]. • Arc erosion plays an important role for the composition and morphology of electrical contact materials' surface. Large amount of oxygen in air was dissolved on contact surface and the Ni content was increased and the Ag content was decreased. In addition, there are five types of arc erosion morphology, such as “crater” type erosion pit, island-like melted silver, pore, crack and coral-like structure spitting, when Ag/Ni contact material after 50,000 operations under DC19V, 20A and resistive load condition. • Material's density and contact electrodes (cathode or anode) have an important influence for the element distribution on the molten pool. On movable contact (cathode), Ni element mainly distributes on the bottom of molten pool, while Ag element mainly distributes inside of molten pool. However, the distribution of Ni and Ag element is layer upon layer in the molten pool on stationary contact (anode).
Acknowledgment This work is supported by the Fundamental Research Funds for the Central Universities (2012QNZT003), China Postdoctoral Science Foundation Funded Project (2012M521542), Hunan Provincial Natural Science Foundation of China (14JJ3014) and by Zhejiang Provincial Postdoctoral Scientific Research Funded Project (BSh1202). Reference [1] F. Fehim, U. Huseyin, Microstructure, hardness and electrical properties of silverbased refractory contact materials, Mater. Des. 24 (2003) 489–492. [2] P. Frederic, Electrical Contact Materials arc Erosion: Experimental and Modeling Towards the Design of an Ag/CdO Substrate, Georgia Institute of Technology, 2010. [3] M. Hasegawa, Break arc behaviors of Ag and Ag/SnO2 contact pairs under different contact opening speeds in DC load circuits, The 27th International Conference on Electrical Contacts, Dresden, Germany, June. 2014, 1–6, 2014.
[4] J. Swingler, A. Sumption, Arc erosion of AgSnO2 electrical contacts at different stages of a break operation, Rare Metals 29 (3) (2010) 248–254. [5] H. Wang, S.H. Li, W. Weng, X.P. Bai, L.Q. Liu, G.W. Li, Development of preparation process for Ag/Ni electrical contact materials, Electr. Alloy 4 (2013) 7–10. [6] K. Yoshida, K. Sawa, K. Suzuki, M. Watanabe, Arc characteristics and electrode mass change of Ag/Ni contacts for electromagnetic contactors, IEICE Trans. Electron. 94 (2011) 1395–1401. [7] K. Yoshida, K. Sawa, K. Suzuki, M. Watanabe, H. Daijima, Influence of voltage on arc duration and electrode mass change of Ag/Ni contacts for electromagnetic contactors, IEICE Tech. Report, Xi'an Jiaotong Univ November 2010, pp. 219–222. [8] K. Yoshida, K. Sawa, K. Suzuki, M. Watanabe, H. Daijima, Electrode mass change of Ag/Ni contacts for electromagnetic contactor, IEICE Tech. Report, Akita Univ November 2011, pp. 185–188. [9] L. Morin, N.B. Jemaa, D. Jeannot, Make arc erosion and welding in the automotive area, IEEE Trans. Components Packing Technol. 2 (2000) 240–246. [10] Y. Kawakami, M. Hasegawa, Y. Watanabe, K. Sawa, An investigation for the method of lifetime prediction of Ag/Ni contacts for electromagnetic contact, The 51th IEEE Holm Conference on Electrical Contacts, U.S.A.,September.2005 2005, pp. 151–154. [11] L. Doublet, N.B. Jemaa, F. Hauner, D. Jeannot, Make arc erosion and welding tendency under 42 VDC in automotive area, The 49th IEEE Holm Conference on Electrical Contacts, Washington, September. 2003 2003, pp. 158–163. [12] Q.F. Luo, Y.P. Wang, B.J. Ding, Microstructure and arc erosion characteristics of Ag/Ni contacts by mechanical alloying, IEEE Trans. Components Packing Technol. 28 (2005) 785–788. [13] X.J. Liu, X. Feng, H.J. Fei, Welding characteristics of Ag-based contact material under automobile lamp Load, The 49th IEEE Holm Conference on Electrical Contacts. wasshington, September, 2003 2003, pp. 145–149. [14] Y.T. Li, Arc erosion characteristics of Ag/Ni10 electrical contact materials fabricated by Chemical coating, Electr. Alloy 1 (2009) 10–13. [15] G.L. Huang, X.F. Yan, G.W. Li, Y. Li, Fabrication and properties analysis of Ag/Ni10 electrical contact materials by chemical vapor deposition, Electr. Alloy 1 (2010) 12–16. [16] X.F. Yan, X.P. Bai, L.Q. Liu, S.H. Li, D. Wan, Investigation on Ag/Ni electrical contact materials used in alternating current contactor for an air conditioning, Electr. Alloy 2 (2013) 13–17. [17] L. Chen, Xie M. Ning DK, H.Z. Cai, The welding resistance properties of Ag/Ni contact materials under direct current condition, Precious Met. 29 (2008) 6–10. [18] H. Li, Y.B. Hou, Arc erosion characteristics of Ag/Ni electrical contact materials under direct current and lamp load condition, Electr. Technol. 2 (2006) 60–62. [19] Z.L. Tan, S. Chen, W.M. Guan, J.M. Guo, Experiment analysis of arcing mechanism and electric corrosion patterns of Ag/Ni under low voltage and direct current after single breaking operation, The Eighth Academic Essays of China Nonferrous Metals Society, Beijing, September, 2010 2010, pp. 128–131. [20] J. LI, Z.Y. Ma, S.P. Huang, J.M. Li, Research on arc erosion of Ag/Ni alloy contact materials at 50 Hz and 400 Hz, New Technol. Electr. Power 29 (2010) 4–9. [21] S.H. Li, X. Yu, G.W. Li, X.P. Bai, Study on the electrical property of Ag/Ni contacts, Electr. Alloy 2 (2011) 7–13. [22] X.J. Zheng, Q.P. Wang, The type and formation mechanisms of Ag/Ni contacts morphology due to breaking arc erosion, J. Xi'an Jiaotong Univ. 27 (1993) 1–9. [23] S. Kubo, K. Kato, Effect of arc discharge on the wear rate and wear mode of a copper in prognoses metallized carbon contact strip sliding against a copper disk, Tribol. Int. 2 (1999) 367–378. [24] Q.P. Wang, Arc Theory of Electrical Appliance, Beijing Mechanical Industry Press, Beijing, 1992. 100–103.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Arc erosion behavior of Ag/Ni electrical contact materials Chunping Wu a,b,c,⁎, Danqing Yi b, Wei Weng c, Suhua Li c, Jiemin Zhou a, Feng Zheng b a b c
School of Energy Science and Engineering, Central South University, Changsha, 410083 China Key Laboratory for Nonferrous Metal Materials of the Ministry of Education, School of Materials Science and Engineering, Central South University, Changsha, 410083 China FuDa Alloy Materials Co., LTD, Wenzhou, 325025 China
a r t i c l e
i n f o
Article history: Received 27 December 2014 Received in revised form 23 June 2015 Accepted 26 June 2015 Available online 3 July 2015 Keywords: Ag/Ni electrical contact material Arc erosion Erosion morphology Element mapping
a b s t r a c t Ag/Ni electrical contact materials tend to be weld together under high current and/or high temperature, which was a key problem to restrict the usage of Ag/Ni electric contact materials. Arc erosion characteristics of Ag/12Ni electrical contact material after 50,000 operations under direct current 19 V, 20 A and resistive load conditions were investigated. The result indicated that the probability distribution and change trend of arc energy and arc time during 50,000 operations were similar and the relationship between arc time and arc energy followed exponential function. On the one hand, “Crater” type erosion pit, island-like melted silver, pore, crack and coral-like structure spitting were observed on erosion surface of Ag/Ni contact materials. On the other hand, distribution of Ag and Ni element on molten pool of movable contact was different from that of stationary contact. For movable contact, element Ni mainly distributed on melted pool root, whereas element Ag mainly distributed inside of melted pool. For stationary contact, however, element Ni and Ag distributed layer by layer. Furthermore, arc erosion of stationary contact is more serious than that of movable contact. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Electrical contacts have found a variety of applications due to their high conductivity, low contact resistance and good arc erosion resistance [1–4]. For example, Ag/Ni electrical contact materials are used for household applications, contactors, miniature circuit breakers and automobile relays due to their low and stable contact resistance, excellent machinability, high resistance of electrical loss and non-toxcity [5]. However, Ag/Ni electrical contact materials tend to be weld together under high current and/or high temperature. As a result, a large number of researchers have focused on arc erosion properties of Ag/Ni electrical contact materials in order to improve their welding resistance. For example, Yoshida and coauthors clarified the influence of voltage on arc characteristics [6], arc duration [7] and electrode mass change [8] of Ag/Ni contacts for electromagnetic contactors. Morin et al. [9] found that Ag/Ni electrical contact materials had high local materials transfer under lamp and resistive loads at 14VDC and current range from 10 to 70 A. Kawakami et al. [10] discussed that the possibilities of lifetime predictions in terms of cathode losses and arc energy with the data of Ag/Ni contacts for electromagnetic contactor. Doublet et al. [11] investigated the arc erosion, welding tendency and welding forces of Ag/Ni electrical contact materials under resistive load at current range from 10 to 90 A. Luo et al. [12] investigated the arc erosion characteristics of Ag/Ni electrical contact materials ⁎ Corresponding author at: Key Laboratory for Nonferrous Metal Materials of the Ministry of Education, School of Materials Science and Engineering, Central South University, Changsha, 410083 China. E-mail address: [email protected] (C. Wu).
http://dx.doi.org/10.1016/j.matdes.2015.06.142 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
fabricated by mechanical alloying. Liu et al. [13] found that the welding resistance of Ag/10Ni was relatively bad under high arc energy. Li found [14] that the arc erosion resistance of Ag/10Ni electrical contact materials fabricated by chemical coating method was better than that fabricated by powder metallurgy method. Huang et al. [15] indicated that Ag/10Ni electrical contact materials fabricated by chemical co-deposition method had strong welding resistance. Yan et al. [16] suggested that the addition of brittle materials could improve the electrical performance of Ag/Ni contact. Chen et al. [17] indicated that RE (rare earth) could improve the electrical performance of Ag/Ni contact. Li et al. [18] found that Ag/Ni contact had serious material transfer. They indicated that the improvement of material transfer between Ag/Ni contact not only considers material intrinsic nature but also matching among them. Tan et al. [19] studied the arcing mechanism and electric corrosion patterns of Ag/10Ni contact under low voltage and direct current after single breaking operation. Their results indicated that the arc erosion spot area is a linear function of load current. Li et al. [20] investigated the arc erosion of Ag/Ni contact at 50 Hz and 400 Hz. They found that the welding resistance of Ag/10Ni contact was better at 400 Hz while the arc erosion resistance was better at 50 Hz. Li et al. [21] analyzed the arc energy, arc time and welding force of Ag/Ni contact. They indicated that the arc time of Ag/10Ni contact fabricated by chemical co-deposition method was longer with higher arc energy. Zheng and coauthor [22] studied the arc erosion morphology and formation mechanism of Ag/Ni contact. Eight distinct types of surface morphologies were observed after arc erosion. However, little information is available on element distribution and formation mechanism of molten pool of Ag/Ni contact after arc erosion in literature. Information on element distribution and formation
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C. Wu et al. / Materials and Design 85 (2015) 511–519 Table 1 Properties of Ag/12Ni materials fabricated by powder metallurgy technology.
Movable contact Cathode Side view
0.82mm
Density (g/cm3) Electrical contact resistance (μ Ω · cm) Strength of extension (MPa) Elongation after fracture (%) Hardness (Hv0.3)
Cu
0.82mm Anode
Ag/Ni
Stationary contact 5mm
Top view
10.36 1.92 312 19 83
3. Results
4mm
3.1. Electrical contact physical phenomena Fig. 1. CAD drawing of contacts.
mechanism of molten pool is helpful to improve the welding resistance and electrical performance of Ag/Ni contact. The aim of this work is therefore to investigate arc erosion behavior of Ag/12Ni contact and discuss relationship between electrical contact physical phenomena and formation mechanism of molten pool.
2. Experimental procedures 2.1. Tested contacts The form of contacts is shown in Fig. 1. The movable and stationary contacts have a curved surface. Ag/12Ni (Ag: 88 wt.%; Ni: 12 wt.%) contact material fabricated by powder metallurgy technology is selected as a contact material, which manufacture technology flow diagram is represented in Fig. 2 and properties are represented in the Table 1.
2.2. Experimental apparatus Apparatus used in this experiment are shown in Fig. 3. The system consists of measuring and test equipment such as industrial PC, signal collecting, measuring and protection device, contact motion simulator and steady flow test power system. The data of arc energy, arc time and welding force can be collected used by this apparatus. Three contact samples were tested in this article considering the repeatability of experimental results.
Electrical contact physical phenomena, such as arc energy, arc time, welding force, etc., will change due to the changing of contact surface microstructure and composition under arc erosion. An average every 100 operations of arc energy, arc time and welding force of Ag/12Ni electrical contact materials during 50,000 operations is represented in Fig. 4. Change trend of arc time and arc energy during 50,000 operations is similar, which is different from welding force. When operation numbers are less than 8000, arc time and arc energy increase with the increase of operation numbers. But they decrease when the operation numbers increase from 8000 to 15,000, and then they basically keep stable when operation numbers increase from 15,000 to 50,000. As well as arc time and arc energy, when operation numbers are less than 8000, welding force increases with the increasing of operation numbers. Welding force decreases when the operation numbers increase from 8000 to 10,000. It basically keeps stable when operation numbers increase from 10,000 to 30,000. But it first increases and then decreases when the operations increase from 30,000 to 40,000, and then it basically keeps stable when the operation numbers increase from 40,000 to 50,000. Consequently, electrical contact physical phenomena (arc time, arc energy and welding force) every operation change under the action of arc erosion. Mass of movable and stationary contact will change under the action of arc erosion due to evaporation and splash erosion. In addition, material transfer occurs between movable and stationary contact during arc operation and also will result in their mass change. In this experiment, mass on movable and stationary contact decreases (movable contact mass loss: 2.7 mg; stationary contact mass loss: 4 mg) after 50,000 operations. Mass loss of stationary contact is larger than that of movable contact, which indicates that arc erosion of stationary contact is more serious than that of movable contact.
2.3. Experimental conditions
3.2. Erosion morphology of Ag/Ni contact materials
The experimental conditions are shown in Table 2. The load current was set at DC 19 V 20 A, and current-carrying time was about 0.3 s. The arc energy, the arc time and the welding force were measured during the operation under the above conditions. In this paper, the mass losses were measured using an electrical scale. By using the scale, mass changes of more than 0.1 mg can be measured with accuracy. The microstructure was characterized by both an optical microscope (POLYVAR-MET) and a scanning electron microscope (Sirion200) equipped with an energy dispersive energy diffraction spectroscope (Gensis60). The element map analysis and composition quantitative analysis were used by electron probe micro-analyzer (JXA-8230).
3.2.1. Macro-morphology Macro-morphology of Ag/12Ni contact material after 50,000 operations under the above conditions is represented in Fig. 5, where surface morphology of movable and stationary contact is changed due to the arc erosion. On the one hand, serious deformation has been observed on both movable and stationary contact surface due to action of arc erosion and contact force. On the other hand, splash erosion also has been observed on both movable and stationary contact surface (see red circle in Fig. 5a and b). Furthermore, arc erosion on stationary contact surface is more serious than that on movable contact surface, which is consistent with the result of mass loss in the above.
Ag powders Mixing
Compress
Sintering
Extrusion
Ni powders
Fig. 2. Flow diagram of manufacture technology of Ag/12Ni electrical contact material.
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Vernier caliper Pull /pressure sensors
PC
PC19812
PC19118
Control box
Computer Soft platform
Movable contact
R
Steady flow test power system
Resistive load
Working power
Electromagnetic vibrator
Stationary contact
Contact motion simulator
Fig. 3. Experimental apparatus.
Arc erosion not only causes surface morphology change but also leads to composition change due to the physical metallurgy reaction on the contact surface. Composition of different zones on contact surface (see Fig. 5a and b) measured by EDS is shown in Table 3. The result on Table 3 indicates that silver content is less than 88 wt.% and nickel content is more than 12 wt.% when Ag/12Ni electrical contact material is eroded by electric arc. In addition, a large number of oxygen and small amount of carbon content are detected on contact surface, which indicates that oxygen and carbon in air dissolve in silver matrix
Table 2 Experimental conditions. Contact material
Ag/Ni (Ag88 wt.%:Ni12 wt.%)
Circuit condition Frequency Number of operations Switching mode Contact force Surrounding gas Electrode spacing
DC19V, 20 A, resistive load 0.67 Hz (ON 0.3 s) 50,000 DC mode 0.98 N Air 2 mm
Fig. 4. Average every 100 operations on arc energy, arc time and welding force of Ag/12Ni contact materials during 50,000 operations.
during arc erosion. Temperature on arc root zone is very high due to the action of electric arc. Solubility of nickel and oxygen in silver will increase with the temperature increasing. Part of nickel particles also will dissolve in molten pool when temperature is higher than nickel melted point (1453°C). These nickel particles are melting and cooling in a short time and then forming secondary crystallization. In addition, silver will melt and evaporation due to its low melting point (961°C) during arc erosion. On the one hand, silver melting increases oxygen solubility in silver, which results in oxygen content increasing on contact surface. On the other hand, silver evaporation decreases silver content on contact surface. Furthermore, nickel particles aggregates and floats on the melted silver due to its low density (Ni: 8.9 g/cm3, Ag: 10.5 g/cm3), which results in nickel content increasing on contact surface. Consequently, oxygen and nickel content increases whereas silver content decreases when Ag/12Ni contact material is eroded by electric arc.
3.2.2. Micro-morphology Arc erosion morphology characteristics of movable and stationary contact after 50,000 operations under the above conditions are clearly represented in Figs. 6 and 7, respectively. The surface morphology characteristics of movable and stationary contact after arc erosion are similar. “Crater” type erosion pits are observed on movable and stationary contact surface (see Figs. 6a and 7a). Silver will melt and evaporate due to the action of arc erosion. Under certain conditions, especially in large current, silver steam will outflow from arc root. “Crater” type of erosion pit will leave on contact surface due to large droplet splashing during the silver steam outflow from arc root. Island-like rich silver zone is observed on movable and stationary contact surface (see Figs. 6b and 7b). Silver will melt due to action of arc energy. Rapid solidification results in the formation of island-like structure on movable and stationary contact surface because the melted silver cannot spread out in time on contact surface. Pores and cracks are observed on movable and stationary contact surface (see Figs. 6c and 7c). Heat melting molten metals will absorb large amount of gas from air under action of electric arc. In addition, solubility of oxygen in liquid silver (0.3%) is 40 times larger than that in solid silver (0.008%). So the melted silver contains a large amount of oxygen during action of electric arc. One part of oxygen dissolved in melted silver will escape to air due to change of oxygen pressure after electric arc extinguishing. The other part of oxygen has no time to escape from melted silver due to the rapid solidification and then results in the formation of pore on contact surface and inner. Cracks on stationary contact surface are more serious than that on movable contact surface, which means arc erosion of stationary contact is more serious than that of movable contact. Crack is a kind of dangerous arc erosion morphology. Formation reason of crack is very complicated,
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a
b
007 006
002 001
005 003
004
008
Fig. 5. The macro-morphology of Ag/Ni contact materials after 50,000 operations. (a) movable contact; (b) stationary contact.
which mainly depended on structure and property of electrical contact material, arc energy and outside working conditions. It is inevitable that there are some defects (micro hole, micro crack, inclusion, grain boundary and interfacial dislocation group, etc.) in the material inner or surface. These defects are the root cause of the surface crack formation. On the one hand, silver on contact surface will melt under high temperature action of electric arc. However, the melted layer on surface will sharply cool and solidify due to short duration of the electric arc (less than 10 ms in this experiment). The rapid solidification of melted layer leads to the increasing of vacancy and dislocation density in melted layer microstructure. Increase of vacancy and dislocation density will decrease grain boundary intensity, which increases the possibility of grain boundary crack formation under stress action (see white circle in Fig. 7c). On the other hand, pore will decrease material's mechanical intensity, which is easy to cause the formation of crack or prompt the development of crack. For example, pore in Fig. 6b results in the formation and development of crack (see white circle in Fig. 6b). Coral-like structure spitting is observed on movable and stationary contact surface (see Figs. 6d and 7d). Coral-like structure spitting is a kind of accumulation of particles with size 200–500 nm, which mainly appears on the edge of contact surface due to splashing erosion. Gasification and liquid evaporation splash during electric arc erosion is the main reason of coral-like structure particles formation. On the one hand, surface material changes into liquid from solid under arc energy action, and then changes into gas and escapes from contact material surface, finally the
Table 3 Composition of different zone in Fig. 5a and b measured by EDS.
Element (wt%) Zone C
O
Ni
W
Ag
001
1.11
9.12
34.95
2.49
52.33
002
1.76
6.67
24.83
1.6
65.15
003
1.37
7.3
15.94
1
74.39
004
1.54
9.56
23.64
1.81
63.45
005
1.05
12.67
37.20
2.88
46.21
006
2.26
8.61
23.62
1.81
63.69
007
1.12
9.72
31.65
2.13
55.38
008
2.95
5.07
21.58
0.9
69.51
C and O was detected
Ni>12 wt %
Ag < 88 wt %
gaseous silver absorbed large amount of oxygen in air rapidly solidifies on contact surface and forms coral-like structure particles. On the other hand, silver molten pool forms on contact surface under arc energy action. Tiny droplet in the liquid pond splashes from the molten pool the under all kinds of force action (such as electrostatic field force, electromagnetic force and reacting force of material movement, surface tension, etc.). Consequently, coral-like structure particle is a product of the evaporation and splash erosion under electric arc action. 3.3. Cross section microstructure of Ag/Ni contact materials after arcing erosion 3.3.1. Optical microstructure Optical microstructure on cross section of Ag/Ni contact material after 50,000 operations under the above experimental conditions is represented in Fig. 8. Curved surface of movable contact changes into flat surface under arc erosion action (see Fig. 8a1) and material transfer (material breaking away from movable contact surface) is observed on movable contact surface (see white circle in Fig. 8a1). While curved surface of stationary contact changes into concave surface under arc erosion action (see Fig. 8b1) and material transfer and splashing is observed on stationary contact surface (see white circle in Fig. 8b1). There is some little erosion pits (the molten pool) on movable and stationary contact surface (see Fig. 8a2 and 8b2). 3.3.2. Element mapping of the molten pool Element mapping of molten pool of Ag/Ni movable and stationary contact material after 50,000 operations under the above experimental conditions is represented in Figs. 9 and 10, respectively. The element distribution of molten pool on movable and stationary contact materials is different. Area of molten pool on movable contact is smaller than that on stationary contact (see Figs. 9a and 10a), which indicates that arc erosion on stationary contact is more serious than that on movable contact. Composition image indicates that there are four different phases (dark phase; white phase, dark gray phase and light gray phase) on Ag/Ni contact material (see Figs. 9a and 10a). Composition of different phase in Figs. 9a and 10a is shown on Table 4. The result on Table 4 indicates that white phase contains 87–97 wt.% element W, dark phase contains about 98 wt.% element Ni, light gray phase contains about 99 wt.% element Ag and deep gray phase contains about 80 wt.% element Ni and 18 wt.% element W. In addition, the result of element mapping also indicates that dark phase mainly contains Ni element, white phase mainly contains W element, dark gray phase mainly contains Ni and W element and light gray phase mainly contains Ag element (see Figs. 9b, c, d and 10b, c, d). Shape of molten pool on movable and stationary contact is like wizard magic map. Distribution of Ag and Ni
C. Wu et al. / Materials and Design 85 (2015) 511–519
515
b
a
Melted silver
d
c
Pore
Crack
Fig. 6. The micro-morphology of Ag/Ni movable contact materials after 50,000 operations. (a) erosion pit; (b) island-like melted silver; (c) pore and crack; (d) coral-like structure.
a
b
Melted silver
c
d
Crack
Pore
Fig. 7. The micro-morphology of Ag/Ni stationary contact materials after 50,000 operations. (a) erosion pit; (b) island-like melted silver; (c) pore and crack; (d) coral-like structure.
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C. Wu et al. / Materials and Design 85 (2015) 511–519
a1
a2
b1
b2
Fig. 8. Optical microstructure on cross section of Ag/Ni contacts after 50,000 operations. (a1) movable contact; (a2) molten pool of the movable contact; (b1) stationary contact; (b2) molten pool of the stationary contact.
element on molten pool of movable contact is different from that of stationary contact. For movable contact, Ni element mainly distributes on the bottom of molten pool (see red circle in Fig. 9c), while Ag element mainly distributes inside of molten pool (see white circle in Fig. 9b). However, distribution of Ag and Ni element is layer by layer in molten pool on stationary contact (see Fig. 10b and c).
4. Discussions 4.1. Relationship among arc energy, arc time and welding force The degree of arc erosion depends on arc energy. Arc energy is larger and arc erosion more serious. Whereas arc energy increase with the increasing of arc time. Furthermore, welding force has an important effect on weld and arc erosion of electrical contact material. So arc energy, arc time and welding force are very important influence factors during arc erosion. It is very essential that discussion on the relationship among arc energy, arc time and wielding force during arc erosion. In order to clarify the relationship among arc energy, arc time and welding force, we plot the probability distribution curves on arc energy, arc time and welding force of Ag/12Ni electrical contact material during 50,000 operations (see Fig. 11). The results in Fig. 11 indicate that probability distribution curves of arc energy and arc time are similar, which is in accordance with the result in Fig. 4. However, the probability distribution curves of arc energy and arc time is different from that of welding force. Ninety-five percent of welding force is less than 25 g during 50,000 operations. The average value of arc energy, arc time and welding force are 1172 mJ, 4.987 ms and 9.041 g during 50,000 operations, respectively.
The results in Figs. 4 and 11 indicate that arc time has an influence on arc energy, but arc time has no direct influence on welding force. Kubo and his coauthor found that the relationship between arc time and arc energy can be described with the following equation [23]: E ¼ ∑U I t
ð1Þ
where E is arc energy, U is arc voltage, I is arc current and t is arc time. The relationship between arc erosion (mass loss) and arc energy can be described with the following equation [23]: W ¼ c Ed
ð2Þ
where W is arc erosion, E is arc energy, c and d are coefficients related to material. Wang established the relationship between arc erosion and arc time according to experiment result [24]. The relationship between arc erosion and arc current can be expressed by the following equation: W ¼kIt
ð3Þ
where W is arc erosion, I is arc current, t is arc time and k is coefficient related to material. According to Eqs. (2) and (3), we can get the following equation: E ¼aIt
ð4Þ
where E is arc energy, I is arc current, t is arc time and a is coefficient related to material. The previous studies have shown that arc energy and arc time have an important influence for arc erosion of material and arc time also has an important influence for arc energy. In this paper, the relationship
C. Wu et al. / Materials and Design 85 (2015) 511–519
517
b
a
3 1 4
CP
20µm
2
Ag
20µm
d
c
Ni
20µm
W
20µm
Fig. 9. Element mapping on molten pool of AgNi movable contact after 50,000 operations. (a) Composition image; (b) Ag distribution; (c) Ni distribution; (d) W distribution.
b
a
5 8
6 7
CP
50µm
c
Ni
Ag
50µm
d
50µm
W
50µm
Fig. 10. Element mapping on molten pool of Ag/Ni stationary contact after 50,000 operations. (a) Composition image; (b) Ag distribution; (c) Ni distribution; (d) W distribution.
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C. Wu et al. / Materials and Design 85 (2015) 511–519
3000
Table 4 Composition of different zone in Figs. 9 and 10 measured by WDS.
2500
Element (wt %) Zone C
Ag
Ni
W
1
2.559
0.373
98.825
0.116
2
5.048
0.412
6.898
87.642
3
1.999
0.205
79.678
18.118
4
0.912
98.819
0.269
0.000
5
1.287
0.255
98.458
0.000
6
0.887
0.110
1.079
97.924
7
1.037
0.270
81.113
17.581
8
0.564
99.166
0.270
0.000
Movable
Stationary
Note:
Ni particles (dark phase); Niw phase (dark gray phase);
Arc nenergy (mJ)
Contacts
E=2569.88-2754.48/[1+exp(t-5.13)/1.84] R 2 =0.99515
2000 1500 1000 500 0 0
2
4
6
8
10
Arc time (ms)
W phase (white phase); Fig. 12. The relationship between arc energy and arc time of Ag/Ni contact materials during 50,000 operations.
Ag matrix (light gray phase).
between arc time and arc energy of Ag/12Ni electrical contact material under DC19V, 20 A and resistive load conditions during 50,000 operations is discussed. The relationship curves between arc energy and arc time are clearly represented in Fig. 12, where the relationship between arc energy (E) and arc time (t) is well expressed by an exponential law. So we can get the following equation by data fitting: E ¼ 2569:88−2754:48=½1 þ expðt−5:13Þ=1:84
ð5Þ
where E is arc energy and t is arc time. The correlation of data fitting (R2) on Eq. (5) is 0.99515, which indicates that the relationship between arc energy and arc time of Ag/12Ni electrical contact materials in this experimental conditions can be described used by Eq. (5). As a result, arc energy is exponentially increases with the arc time increasing in this experiment. 4.2. Element distribution of molten pool The above result indicates that the distribution of Ag and Ni elements on molten pool of movable contact is different from that of stationary contact. On movable contact, Ni element mainly distributes on the bottom of molten pool, while Ag element mainly distributes inside of molten pool. However, the distribution of Ni and Ag element is
layer upon layer in molten pool of stationary contact. So the formation mechanism of molten pool on movable and stationary contact is different. Fig. 13 shows the schematic diagram of molten pool formation of Ag/Ni contact materials after 50,000 operations. Under arc energy action, temperature on movable and stationary contact material will increase, silver will melt when temperature reaches the melting point of silver (961°C), while nickel particles are still in solid state due to its higher melted point (1453°C). A molten pool containing Ni particles is formed due to the melting of silver (see Fig. 13b). Under the action of gravity, Ni particles in molten pool will move from down to up because the density of nickel (8.9 g/cm3) is smaller than that of silver (10.5 g/cm3), while silver will move from up to down due to its high density (see Fig. 13c). So Ni particles are mainly distributed above silver layer when the melted silver rapidly cools. On movable contact material, when Ni particles in molten pool move from down to up, they will move to the bottom of molten pool because the molten pool bottom is located at the molten pool above. As a result, Ni particles mainly distributes in the bottom of molten pool, while silver mainly distributes inside the molten pool (see Figs. 9c and 13d). On stationary contact material, when the Ni particles of molten pool move from down to up, they will move to the surface of molten pool because the surface of molten pool is located at the molten pool above. As a result, Ni particles are mainly distributed on the surface of molten pool, while silver is mainly
Fig. 11. Probability distribution curves on arc energy, arc time and welding force of AgNi contact materials during 50,000 operations.
C. Wu et al. / Materials and Design 85 (2015) 511–519
Movable contact
Movable contact
Movable contact
519
Movable contact
Movable contact
W
Ag
Ni layer Ag layer
Molten pool
Ni
Stationary contact
(a)
Stationary contact
Stationary contact
Stationary contact
Stationary contact
(c)
(d)
(e)
(b)
Fig. 13. Schematic diagram of molten pool formation of Ag/Ni contacts after arc erosion. (a) connect; (b) silver melting; (c) Ni particles moving up; (d) silver rapid solidification; (e) Ni and Ag layer formation.
distributed inside molten pool. However, the result of element mapping indicates that the distribution of Ni and Ag elements on stationary contact is layer upon layer (see Figs. 10b, c and 13e). It is possibly attributed to the action of contact pressure under the cycle operation. 5. Conclusions • The change trend and probability distribution of arc energy and arc time of Ag/12Ni contacts during 50,000 operations are similar and the relationship between them can be expressed using by the followed exponential function: E = 2569.88 − 2754.48/[1 + exp(t-5.13) / 1.84]. • Arc erosion plays an important role for the composition and morphology of electrical contact materials' surface. Large amount of oxygen in air was dissolved on contact surface and the Ni content was increased and the Ag content was decreased. In addition, there are five types of arc erosion morphology, such as “crater” type erosion pit, island-like melted silver, pore, crack and coral-like structure spitting, when Ag/Ni contact material after 50,000 operations under DC19V, 20A and resistive load condition. • Material's density and contact electrodes (cathode or anode) have an important influence for the element distribution on the molten pool. On movable contact (cathode), Ni element mainly distributes on the bottom of molten pool, while Ag element mainly distributes inside of molten pool. However, the distribution of Ni and Ag element is layer upon layer in the molten pool on stationary contact (anode).
Acknowledgment This work is supported by the Fundamental Research Funds for the Central Universities (2012QNZT003), China Postdoctoral Science Foundation Funded Project (2012M521542), Hunan Provincial Natural Science Foundation of China (14JJ3014) and by Zhejiang Provincial Postdoctoral Scientific Research Funded Project (BSh1202). Reference [1] F. Fehim, U. Huseyin, Microstructure, hardness and electrical properties of silverbased refractory contact materials, Mater. Des. 24 (2003) 489–492. [2] P. Frederic, Electrical Contact Materials arc Erosion: Experimental and Modeling Towards the Design of an Ag/CdO Substrate, Georgia Institute of Technology, 2010. [3] M. Hasegawa, Break arc behaviors of Ag and Ag/SnO2 contact pairs under different contact opening speeds in DC load circuits, The 27th International Conference on Electrical Contacts, Dresden, Germany, June. 2014, 1–6, 2014.
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