简介概要

Interface evolution of Cu-Ni-Si/Al-Mg-Si clad composite wires after annealing

来源期刊：Rare Metals2020年第12期

论文作者：Zhen Yang Xu-Jun Mi Xue Feng Hao-Feng Xie Li-Jun Peng Guo-Jie Huang Yan-Feng Li Xiang-Qian Yin

摘要：Interface micros tructures of Cu-Ni-Si/Al-MgSi clad composite wires during isothermal annealing from623 to 773 K were investigated.The composite wires were fabricated by a drawing process.The evolution of intermetallic compounds（IMCs） was analyzed.A continuous IMCs layer forms only after annealing for 1 min,which may be due to more IMCs nucleation points generated by deep drawing process.IMCs consist of Al₄Cu₉,AlCu and Al₂Cu identified by energy-dispersive spectroscopy（EDS）and transmission electron microscopy（TEM）.The growth activation energies of total IMCs,Al₂ Cu,AlCu and Al₄Cu₉,are 98.8,69.4,101.3 and 137.1 kJ·mol^-1,respectively.The higher growth activation energy of Al₄Cu₉ results in the higher growth rate under high temperature.However,the average interdiffusion coefficient for each IMC calculated by Wagner method shows that interdiffusion in A12 Cu and AlCu is more active than that in Al₄Cu₉.The higher growth rate of Al₄Cu₉ may be caused by the long concentration range.

详情信息展示

稀有金属(英文版) 2020,39(12),1419-1424

Interface evolution of Cu-Ni-Si/Al-Mg-Si clad composite wires after annealing

Zhen Yang Xu-Jun Mi Xue Feng Hao-Feng Xie Li-Jun Peng Guo-Jie Huang Yan-Feng Li Xiang-Qian Yin

Key Laboratory of Nonferrous Metals and Processes,GRIMAT Engineering Institute Co.,Ltd

作者简介：Zhen Yang,e-mail:yangzhen@grinm.com;*Xu-Jun Mi,e-mail:yzcopper@163.com;

收稿日期：15 January 2017

基金：financially supported by the National Key Research and Development Plan (No. 2016YFB0301405);

Interface evolution of Cu-Ni-Si/Al-Mg-Si clad composite wires after annealing

Zhen Yang Xu-Jun Mi Xue Feng Hao-Feng Xie Li-Jun Peng Guo-Jie Huang Yan-Feng Li Xiang-Qian Yin

Key Laboratory of Nonferrous Metals and Processes,GRIMAT Engineering Institute Co.,Ltd

Abstract：

Interface micros tructures of Cu-Ni-Si/Al-MgSi clad composite wires during isothermal annealing from623 to 773 K were investigated.The composite wires were fabricated by a drawing process.The evolution of intermetallic compounds(IMCs) was analyzed.A continuous IMCs layer forms only after annealing for 1 min,which may be due to more IMCs nucleation points generated by deep drawing process.IMCs consist of Al₄Cu₉,AlCu and Al₂Cu identified by energy-dispersive spectroscopy(EDS)and transmission electron microscopy(TEM).The growth activation energies of total IMCs,Al₂ Cu,AlCu and Al₄Cu₉,are 98.8,69.4,101.3 and 137.1 kJ·mol^-1,respectively.The higher growth activation energy of Al₄Cu₉ results in the higher growth rate under high temperature.However,the average interdiffusion coefficient for each IMC calculated by Wagner method shows that interdiffusion in A12 Cu and AlCu is more active than that in Al₄Cu₉.The higher growth rate of Al₄Cu₉ may be caused by the long concentration range.

Keyword：

Intermetallic compounds; Clad composite wires; Diffusion; Activation energy;

Received： 15 January 2017

1 Introduction

Copper-clad aluminum (CCA) wires,which are the copper layer concentrically coated on the surface of aluminum core conductor,are kinds of composite wires [ 1] .CCA wires combine the advantages of excellent conductivity and low weight,which have been widely used in automobile and aerospace [ 2, 3, 4, 5, 6, 7] .In order to increase the strength and pressure welding property,the CCA wires combined with various copper alloy and aluminum alloy have obtained more and more attention [ 8] .

Generally,the CCA wires are produced by annealing after drawing from bars [ 9] .The annealing process aims to improve plasticity and conductivity which were deteriorated during cold working [ 10] .However,during annealing,the heat accelerates the interface reaction of forming intermetallic compounds (IMCs) between Cu and Al [ 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22] .These IMCs are hard and brittle,and they could be fractured firstly,thus deteriorating the performance of whole wires [ 23, 24, 25] .

A lot of work was done on studying interface evolution of CCA wires during annealing.Funamizu and Watanabe [ 26] studied the diffusion couples of Cu and Al in the temperature range of 673-808 K,and five phases (Al₂Cu,AlCu,Al₂Cu₃,Al₃Cu₄ and Al₄Cu₉) were observed.However,Chen and Hwang [ 27] obtained only four phases(Al₂Cu,AlCu,Al₃Cu₄ and Al₄Cu₉).Gibbs [ 28] considered that the lack of some IMCs is due to the slow nucleation rates or effectively zero growth rates.In the actual conditions,the species of IMCs are various with the original materials,preparation technology and heat treatments.Although a few researches have been done on pure Cu combined with pure Al CCA wires,litter work on the interface evolution of Cu alloy and Al alloy CCA wires was studied.

In this work,the CCA wires combined with Cu-Ni-Si and Al-Mg-Si were prepared,and the interface evolution after annealing was studied.The growth kinetics of IMCs was also calculated to analyze the growth behavior of IMCs.

2 Experimental

Cu-Ni-Si/Al-Mg-Si clad composite wires were fabricated using Al-0.88 Mg-0.88Si (wt%) bar with a diameter of20 mm and Cu-2.8Ni-0.68Si-0.19 Mg (wt%) tube with an outer diameter of 21.7 mm.In order to get better aging strengthening effect,the Cu-Ni-Si tube was solutiontreated at 1223 K for 1 h.The as-extruded Al-Mg-Si bar was crammed into Cu-Ni-Si tube,and then,the clad bar was drawn to 0.5 mm in diameter.The drawing was carried out with a speed of 10 m·min^-1 at room temperature.The volume ratio of Cu-Ni-Si maintains at approximately 15%.

The wires were isothermally annealed at 623,673,723and 773 K.The annealing time went from 30 s to 16 h.The observations of cross-sectional interface were carried out by scanning electron microscope (SEM,JSM-7001F)and transmission electron microscope (TEM,JEOL JEM-2010).TEM specimen was prepared with focus ion beam scanning electron microscopes (FIB-SEM,Zeiss Auriga).

3 Results and discussion

Interface micros true tures of Cu-N i-S i/Al-Mg-S i clad composite wires after annealing with various temperatures and time are shown in Fig.1.It can be seen that with the increase in annealing time from 1 min to 16 h,the thickness of IMCs layers increases from 0.43 to 8.31μm at623 K,and the interface structure with three layers becomes obvious after annealing for 4 h (Fig.1a-e).To distinguish the three layers,EDS quantitative analysis is used (Table 1).It shows that the composition of A,B and C layers in Fig.1e corresponds to Al₄Cu₉,AlCu and Al₂Cu.It is also needed to point out that a continuous layer forms after annealing for 1 min which is shorter than other results [ 29] .It may be due to more IMCs nucleation points generated by deep drawing process,and the IMCs nucleate and link together rapidly,resulting in the formation of continuous layer.

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Table 1 EDS quantitative analyses of each layer in Fig.1f (at%)

Fig.1 Interfacial SEM images of Cu-Ni-Si/Al-Mg-Si clad composite wires after annealing:a 623 K for 30 s,b 623 K for 1 min,c 623 K for5 min,d 623 K for 4 h,e 623 K for 16 h and i thickness proportion of each layer for 4 h

Fig.2 a TEM bright-field image of Cu-Ni-Si/Al-Mg-Si interface after annealing at 623 K for 1 h and SAED patterns of b Al₄Cu₉,c AlCu and d Al₂Cu

The microstructure of interface evolution with various temperatures is shown in Fig.1f-h,and thickness proportion of each layer is also supplied in Fig.1i.It can be seen that the interface thickness increases from 5.6 to 33.9μm with the temperature rising from 623 to 773 K.And the proportion of Al₄Cu₉ increases rapidly with the temperature;however,the proportion of Al₂Cu decreases and the proportion of AlCu keeps still.The reason was discussed though calculating the activation energy of IMCs which will show in later.It is also needed to mention that a number of coarse black particles distribute around the interface zone,which are identified as Mg₂Si by EDS.These Mg₂Si particles mainly arrange in a line in the IMCs layers nearby Al-Mg-Si alloy,which may be due to the immobility and diffusionless of Mg₂Si,and the left side of the line could be considered as the original interface of CuNi-Si/Al-Mg-Si before annealing.

In order to further confirm the phases of IMCs,TEM bright-field images of the interface and the selected area electron diffraction (SAED) pattern of each IMC layer are shown in Fig.2.SAED pattern of the IMC layer marked by Arrow 1 which shows the structure of cubic is indexed as Al₄Cu₉ phase from the zone axis of .SAED pattern of the IMC layer marked by Arrow 2 is indexed as AICu phase from the zone axis of [ 11] .The structure of AlCu phase is orthorhombic.SAED pattern of the IMC layer marked by Arrow 3 is indexed as Al₂Cu phase from the zone axis of .The pattern shows the structure of monoclinic.

To analyze the evolution of IMCs,the growth rate constants and activation energies are calculated.The relationship between thickness of IMCs and annealing time at different temperatures is shown in Fig.3.At a certain temperature,the thickness of IMCs increases with the time.With the elongation of time,the driving force of diffusion comes down and the diffusion distance gets longer,resulting in the decrease in the growth speed of thickness.Thus,the thickness (d) of IMCs and the time can be expressed by a parabolic law [ 16] .

where K is the growth rate constant and t is the annealing time.The growth rate constants of IMCs are listed in Table 2.According to the classical kinetic theory,Arrhenius equation can be used to calculate the activation energy of IMCs [ 30] .

Fig.3 IMCs growth with different annealing time and temperatures in Cu-Ni-Si/Al-Mg-Si clad composite wires:a total thickness,b Al₂Cu,c AlCu and d Al₄Cu₉

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Table 2 Growth rate constants of IMCs in Cu-Ni-Si/Al-Mg-Si clad composite wires and average interdiffusion coefficients for different IMCs

where K₀ is the growth factor,Q is activation energy,R is the molar gas constant (8.314 J·K^-1·mol^-1) and T is absolute temperature.The Arrhenius plots lnK versus 1000/T (Fig.4) are used to fit out K₀ and Q.The calculated growth activation energies of total IMCs,Al₂Cu,AlCu and Al₄Cu₉ are 98.8,69.4,101.3 and 137.1 kJ·mol^-1,respectively.

As seen, which means that the growth rate constant (K) of Al₄Cu₉ increases more rapidly than that of Al₂Cu;thus,Al₄Cu₉ has higher growth rate under high temperature.

The growth of IMCs is controlled by the diffusion of Cu and Al atoms.The study on interdiffusion coefficients for IMCs is significant to explain the growth mechanism;thus,the different interdiffusion coefficients were calculated.Wagner [ 31] presented a simplified derivation of the principal equation to define the average interdiffusion coefficient in multiphase systems.In this case,the average interdiffusion coefficient for i IMC can be written as:

Fig.4 Arrhenius plots of lnK versus 1000/T for a total thickness and b different IMCs

where is the mole fraction of component Cu in phase i coexisting with phase (i-1), is the mole fraction of component Cu in phase i coexisting with phase (i+1) and K is the growth rate constant of phase i.

The calculated average interdiffusion coefficients for different IMCs are listed in Table 2.The values are related to the growth rate constants (K).The results show that the average interdiffusion coefficient of Al₄Cu₉ is less than that of Al₂Cu and AlCu,meaning that the interdiffusion in Al₂Cu and AlCu is more active than that in Al₄Cu₉.However,the trend is different from the result of growth rate.The diffusion rate is also affected by concentration gradient,and the long concentration range of Al₄Cu₉ (Cu:62.5 at%-69.0 at%) may play more important role in interface forming process.

4 Conclusion

In this study,the CCA wires combined with Cu-Ni-Si and Al-Mg-Si were prepared,and the interface evolution after annealing was studied.Because of more IMCs nucleation points generated by deep drawing process,a continuous IMCs layer forms only after annealing for 1 min.IMCs are identified to consist of Al₄Cu₉,AlCu and Al₂Cu by EDS and TEM.The growth activation energies of total IMCs,Al₂Cu,AlCu and Al₄Cu₉ are 98.8,69.4,101.3 and137.1 kJ·mol^-1,respectively.The higher growth rate of Al₄Cu₉ under high temperature is due to the higher growth activation energy.