Growth Mechanism And Microstructure Regulation Of Cuw/AI Interface

Integrated material comprised of CuW and copper alloy has been widely applied in high-voltage electr-ical appliances.It is of great significance to investigate the bonding of CuW and Al by replacing Cu,driven by the ever increasing demand for copper and the need for cost reduction.The microstructure of interface which bonds the heterogeneous metals affects the interface strength and comprehensive properties.Therefore,this work focuses on the formation and regulatory mechanism of CuW/Al interface.In present work,a CuW/Al composite was fabricated by infiltrating molten Al into a CuW alloy with porous W surface,which aimed at the characteristics of CuW alloy.The microstructures of the CuW/Al interface were characterized by field-emission scanning electron microscope(SEM),transmission electron microscopy(TEM)and electron backscattering diffraction(EBSD).The phase composition,interface microstructure,morphology and formation were mainly studied.The formation sequence of intermetallic compounds(IMCs)was determined,the interfacial growth kinetic equation was established and the interfacial growth mechanism of CuW/Al was revealed.The properties of IMCs were discussed by the first-principles calculations.Based on this investigation,the strategy of regulation the interface microstructure is put forward,including the variation of infiltration parameter and introduction of interlayer.Through experimental analysis and theoretical calculation,the main conclusions were as follows:(1)CuW/Al interface with multiple zones and morphologies have a complex structure.Five transition zones formed,which are layer-like,hypereutectic,eutectic,hypoeutectic and needle-like zones from the CuW side to the Al side.Three kinds of intermetallic compounds(IMCs)formed at the transition zones,which were identified as AlCu,Al2Cu and Al4W.With the variation of the temperature and time,It can be found that not all typical transition zones appeared at the interface at a given holding time or temperature,probably because changing these parameters caused variations in atom concentration at each transition zone,resulting in different interfacial morphologies.The analysis of formation mechanism of CuW/Al interface shows that interfacial metallurgical bonding was obtained by the reaction diffusion of Cu,W and Al atoms,with the variation of the concentration gradient,solid state diffusion and natural convection.Kinetic analysis shows that the growth of CuW/Al interface follows linear growth behavior,it can be expressed as:y=3.27×108 exp(-163/RT)t,indicating the interface formation is controlled by reactive diffusion.First-principles calculations showed that Al2Cu formed first because of the low formation enthalpy.The maximum shear strength of the interface was 51.6 MPa.(2)Based on the results of first-principles calculations,AlCu,Al2Cu and Al4W were mechanically stable under pressure.For Al4W,shear elastic constant C44 decreased as pressure increased from 15 to 20 GPa,indicating that the Al4W was prone to slip along the(100)plane under shear deformation at elevated pressure.According to the results of B/G ratio and the Cauchy pressure(C12-C44)calculations,AlCu,Al2Cu and Al4W changed from brittle to ductile behavior with the increase of pressure.The microhardness of IMCs was reduced in following sequence:Al4W(14.26 GPa),AlCu(13.76 GPa)and AM2Cu(7.78 GPa).The results were consistent with the experimental work,the microhardness of IMCs was reduced in following sequence:Al4W(10.48 GPa),AlCu(9.19 GPa)and Al2Cu(7.83 GPa).The results showed that the plasticity of IMCs can be changed by the introduction of pressure,and the quantities of Al4W with relatively large microhardness was reduced,which provides theoretical guidance for the interfacial regulation and will be conductive to the improvement of the interfacial strength.(3)By introducing pressure,the interfacial microstructure was changed and interfacial strength was enhanced.Firstly,the interfacial thickness was greatly reduced.The thickness of interface layer is 1.048 mm at 20 MPa,710℃ and 60 min,which was greatly reduced by 89.1%compared with the original CuW/Al.Secondly,the distribution of zones were changed.Three zones,i.e.Al-Cu IMCs layer,eutectic and Al-W IMCs layer formed at interface,including five kinds of IMCs,which were identified as Al4Cu9、AlCu、Al2Cu、Al4W and Al12W.The microhardness of IMCs was reduced in following sequence:Al4W(14.26 GPa),AlCu(13.76 GPa),Al4Cu9(11.58 GPa),Al12W(10.39 GPa)and Al2Cu(7.78 GPa).Lastly,Al4W with higher hardness was significantly reduced and mainly distributed in the zone adjacent to Al side.The maximum shear strength of the interface was 59.83 MPa.The strength of the optimized interface was increased by 16%compared with the original CuW/Al.(4)By adding Ni or Ti interlayers,the interface microstructure was optimized and the interfacial strength was effectively improved.Firstly,the thickness of interface layer was decreased.The interfacial thickness of CuW/Ni(5 μm)/Al was 13μm at 20 MPa,710℃ and 40 min.Secondly,the types of IMCs were changed.The IMCs type changed from Al-Cu and Al-W to Al-Ni or Al-Ti IMCs,CuW/Ni(30 μm)/Al interface was comprised of Al3Ni2,and CuW/Ti(10 μm)/Al interface mainly consist of Al3Ti.Lastly,the microhardness of IMCs was reduced.At 20 MPa,710℃ and 40 min,CuW/Ni(30 μm)/Al interface merely consists of Al3Ni2,a relatively low hardness(6.37 GPa)IMCs,which played a key role in improving the interfacial strength.The strength of the optimized interface was increased by 76%compared with the original CuW/Al.