Study On Interfacial Structure Evolution And Bonding Property Of Copper/Aluminum Clad Sheet Produced By Twin-Roll Casting

Copper/aluminum（Cu/Al）laminated composite is a kind of metal composite with excellent properties.It combines low resistance and high thermal conductivity copper with light and inexpensive aluminum.It can make these two metals complement each other in cost and performance,which produces a very high cost-performance synergistic effect.Cu/Al laminated composites have been widely used in aerospace,power electronics,communications and new photovoltaic energy fields.Twin-roll casting is a new method to prepare Cu/Al laminated composites.This method combines continuous casting with semi-solid state,realizes high strength metallurgical bonding of dissimilar metals based on high temperature of casting and rolling compound pressure.It has the characteristics of high efficiency,energy saving and excellent product performance.In the pilot-scale production of Cu/Al clad sheet by horizontal twin-roll casting,problems such as poor product performance,unstable process and low metallurgical bonding appeared.In the processing stages,compound thickening,species increasing,cracking and separation are easy to occur at the interface of clad sheet,which can lead to delamination,cracking and performance reduction of Cu/Al clad sheet.In this paper,aiming at the problems in the pilot-scale preparation and processing of Cu/Al clad sheet,the parameters of the twin-roll casting were optimized by finite element simulation method,thus the Cu/Al clad sheet with excellent properties was prepared,and its interface structure was studied.Through the study of interface structure and bonding properties of Cu/Al clad sheet in the processing,the synergistic effect and deformation mechanism between interface and matrix were explored,the strengthening mechanism of bonding properties of Cu/Al clad sheet was analyzed.Finally,the relationships between processing technology,interface structure and bonding properties were established to realize the effective control of the interface and bonding properties in the production and processing of Cu/Al clad sheets.The research contents and main conclusions are as follows:A coupled heat-flow model for horizontal casting-rolling process of Cu/Al clad sheet was constructed.For the two important factors of the sump depth and interfacial melt length,the influence of parameters on temperature field in the casting zone was studied.The optimal sump depth ratio in the twin-roll casting process of Cu/Al clad sheet was obtained.The process parameters were optimized and the Cu/Al clad sheet was prepared.The main mechanical properties are as follows:average peel strength was 66.09 N/mm,tensile strength was 124 MPa and fracture elongation was 47.2%.The transmission specimens of as-cast and annealed strengthened Cu/Al clad sheets were prepared by focused ion beam（FIB）technique for the first time.The interface,phase boundary structure and composition distribution of the clad sheets were characterized and analysis.The results show that the as-cast intermetallic compound（IMC）layer was composed of Al₂Cu and Al₄Cu₉ with a total thickness of500-600 nm;the grain boundaries and phase boundaries were clear and nano-sized holes exist;the grain boundaries of the intermetallic layer have equiaxed grain characteristics,and the interfacial structure of IMC layer and Cu/Al matrix is coupled.The annealed strengthened interface consists of three layers of IMC grains.The growth of IMC grains was mainly perpendicular to the direction of the interface layer.A small number of grains showed columnar crystal characteristics with a total thickness of about 1.7μm.The phase interface structure of Al₂Cu and Al₄Cu₉ is coupled.The sheet shape and interface structure of Cu/Al clad sheet during rolling were studied.Based on the three characteristic stages of rolling process of as-cast clad sheet,the causes of special deformation of clad sheet were explained by finite element simulation.The evolution law of interface layer and matrix near interface during rolling process of clad sheet is as follows:IMC layer broke and separated gradually,copper and aluminum structure was extruded into the fracture zone to form a Cu/Al direct contact area.The average length and proportion of the broken section of IMC layer decreased linearly with the thickness of clad sheet;the matrix near IMC was not deformed,and the separation area of IMC layer produces large deformation.It was found that the pulling effect of clad sheet interface on the co-deformation of copper and aluminum mainly depended on the bonding force between IMC layer and matrix,the high strength of IMC layer itself and the structural effect of rolling process interface.The tensile properties and interfacial structure of Cu/Al clad sheets were studied.The reason for the high fracture strength and elongation of Cu/Al clad sheet was explained.The interfacial structure of Cu/Al as-cast clad sheet during annealing process was systematically studied.The results showed that the IMC layer increased slowly at first and then rapidly with the increase of annealing temperature.When the annealing temperature was higher than 350 ℃,a new phase of AlCu appeared in the interfacial layer.With the prolongation of annealing time,the thickness of IMC layer increased sharply at first,then slowly and tended to be stable.It was found that low temperature annealing can effectively control the thickness of total IMC layer and AlCu layer.Aiming at the growth law of IMC layers at the interface of Cu/Al clad sheet,based on diffusion kinetics,the nucleation time of new phase of AlCu was imported for the first time,and the growth kinetics equation of interface layer of Cu/Al clad sheet was constructed.After rolling and annealing,it was found that secondary IMC layer can be formed between the interfaces,which increased the metallurgical bonding ratio of the interface between the clad sheet and primary IMC layer,and has a pulling effect on the growth process,and gradually fused with the increase of annealing temperature.Low temperature annealing after rolling can significantly improve the peeling strength of clad sheet.The pilot-scale annealing process after rolling was proposed as follows:annealing temperature was 280-300 ℃,annealing time was 3 h.The strengthening mechanism of bonding strength was studied based on the peel strength and crack propagation behavior of Cu/Al clad sheet during processing.Combining with the mode of peeling crack diffusion of Cu/Al as-cast clad sheet,it was concluded that the bonding strength of clad sheet mainly depended on the bonding strength of aluminum matrix near the interface and the interlaminar bonding strength of IMC.It was found that the rolling process reduced the metallurgical bonding ratio at the interface of the clad sheet,but when the processing rate was more than 40%,the mechanical meshing force between copper and aluminum was formed.In view of the rolling process,it was found that the bonding strength of clad sheet mainly depended on the bonding strength of aluminum matrix,IMC interlaminar bonding force and mechanical meshing force.The reason for the decrease of peeling strength at high temperature can be explained as follows:peeling cracks were restrained to propagate between layers of IMC under the thick interface layer of three-layer IMC structure,and the main fracture mode of IMC layer was cleavage fracture,which was weak bonding force.