Research On Interface Reaction Mechanism And Property Of Laser Cladded Transition Layer Of TC4 On The Surface Of CFRP

For the carbon fiber reinforced thermoplastics(CFRP)and titanium alloy Ti–6Al–4V(TC4)that are widely used in the aviation field,it is of great significance to use lasers to achieve a high-strength joining between the two materials.In this work,the results showed that the traditional CFRP/TC4 laser joining technology relied on the secondary remelting of the resin to form the bonding,and its joint interface performance was poor,which limited further industrial applications.Thus,this paper proposed a method of preparing a TC4 transition layer on the CFRP surface by using laser cladding technology,the metallurgical bonding between carbon fiber of CFRP and TC4 transition layer is realized,and improve the interface bonding performance.This work focused on the preparation of TC4 transition layer on CFRP surface,the microstructure and properties of laser cladding of TC4 transition layer on the CFRP surface were studied,and the relationship between laser process parameters,interface morphology,mechanical properties was established.The bonding mechanism of the interface between the CFRP sheet and the TC4 transition layer was revealed,and the interface bonding strength was improved by regulating the alloy elements of the TC4 transition layer.Firstly,laser directly joining of CFRP/TC4 sheet was carried out,the influence of laser heat input on the joint morphology,tensile shear properties was studied,and the formation mechanism of the interface pores was analyzed.It was found that heating the TC4 sheet by laser resulted in melting and decomposition of the resin at the interface of CFRP/TC4,thus the two materials achieved a reliable bonding through the physical adsorption mechanism.With the increase of heat input,the width of the resin melting zone became greater,the pore size at the interface became larger,the original structure of the CFRP matrix was destroyed.The tensile shear strength gradually decreased,the maximum strength of the joint iwas 8.15 MPa.It confirmed that the TC4 sheet was heated by laser,which caused the resin to melt and decompose at the interface of CFRP/TC4,and finally the bonding was achieved through the physical adsorption mechanism.In order to improve the bonding strength of CFRP/TC4 metal,TC4 powder was cladded on the surface of CFRP by laser cladding to form a metal transition layer.The effect of process parameters on the cladded layer quality was studied.Based on the analysis of thermal demposition product of CFRP by STA-MS,and observation of interface morphology and microstructure of CFRP/TC4 transition layer by SEM/TEM,it was found that the process parameters used for laser cladding of the TC4 transition layer on the CFRP surface were as follows: the thickness of the pre-built transition layer of h,spot size of R,laser defocusing distance of z,laser power of P,scanning speed of v were 0.7mm,3.7mm,60 mm,250W and 1.2m/min.The main thermal decomposition products of CFRP were hydrogen sulfide,carbon sulfide,benzene,and benzenethiol.The main microstructure of the transition layer was ?' martensite,Ti C and a tiny amount of Ti S2,the transition between the carbon fiber-resin-transition layer was continuous,and the TC4 powder was chemically metallurgically reacted with the resin and carbon fiber of the substrate to achieve the joining.The maximum shear strength of the TC4 transition layer interface was 18.5MPa.The interface reaction mechanism of CFRP/TC4 transition layer was explained by the thermodynamic view,and the interface reaction and the formation and growth process of intermetallic compounds were also analyzed.Under the common action of both laser energy and reaction heat,the TC4 powder pre-built on the surface of the CFRP material melted,and chemically reacted with the carbon fiber inside the CFRP and the residual carbon after resin decomposition to form Ti C.The formation process of Ti C at the interface can be divided into two types.One was to grow along the carbon fiber: Ti C will preferentially nucleate on the surface of the carbon fiber and gradually grow around the carbon fiber until it contacted the adjacent grain boundary to form a complete layered structure;The other was the growth within the TC4 liquid phase: the primary crystal Ti C was formed by the carbide produced by pyrolysis of the resin as the core to grow into dendrites,and the eutectic Ti C grew with nearly equiaxed particles.The final morphology of Ti C growth at the interface of CFRP/TC4 transition layer was layered,dendritic and granular generally.A certain amount of Al Si10 Mg powder was added to the TC4 powder,and the interface microstructure and stress state of the CFRP/TC4 transition layer were adjusted by alloying elements to improve the interface bonding strength.The results showed that with the addition of Al Si10 Mg powder,the amount of CFRP decomposition at the interface increased,and the distance of the transition layer penetrating the CFRP material side increased.The microstructure and properties of TC4 transition layer with the addition of 10 wt.% Al Si10 Mg was optimal,the main structure were ?' martensite,Ti C,Ti3 Al and Ti3 Al C.The addition of Al Si10 Mg had an effect on the solidification process of the TC4 transition layer.Ti3 Al C will nucleate on the surface of Ti C to form a cladding layer,hinder the growth of primary crystal Ti C,inhibit the formation of dendritic Ti C,and prevent the Ti C grains from being smaller at the interface.The final morphology of Ti C growth at the interface was primarily layered and granular.Through the residual stress calculation and geometric phase analysis technology on the interface of CFRP/TC4 transition layer,the stress and strain field at the interface was analyzed,and the influence of the addition of Al Si10 Mg on the internal stress-strain at the interface was studied to establish the relationship between stress and interface property.The results showed that with the addition of 10 wt.% Al Si10 Mg powder,the interfacial stress decreased from 739.69 MPa to 428.22 MPa,and the intragranular xx of Ti C decreased from 856.4MPa to 310.4MPa.The addition of Al element effectively relieved the residual stress at the interface,reduced the intracrystalline stress concentration,and promoted that the interface shear strength of the CFRP/TC4 transition layer increased from 18.5MPa to 26.8MPa.