Study On Microstructure,Mechanical Properties And Formability Of Ti/Al Laminated Composites

Ti/Al laminated composites possess both excellent properties of Ti and Al,and can realize the complementary advantages of Ti and Al.The stamping parts of Ti/Al laminated composites（LMCs）have great development prospects in the fields of automobile,shipbuilding,aerospace,electronics,medical treatment and so on.However,the preparation of complex components has extremely strict requirements on the mechanical properties and formability of Ti/Al laminated composites.At present,the study on formability of LMCs is very limited.And different from a single sheet,the formability of LMCs is greatly affected by the interface and its structure and property evolution,which needs to be further studied.In the study,hot-pressing,rolling and annealing are used to prepare Ti/Al laminated composites.The interfacial structure of Ti/Al laminated composites is adjusted by optimizing the processing technology to make its mechanical and formability meet the requirements of production and application.Then the stamping tests are carried out.The effects of interface on crack initiation and propagation are discussed,and the forming failure mechanism is analyzed.The results show that the interface of Ti/Al/Ti laminated composites prepared by hot-pressing is well bonded,and there are no obvious holes or other defects.The Ti/Al interface becomes more curved with the increased hot-pressing temperature,and micron-scale TiAl₃phases are formed at the interface.Both interfacial bonding strength and micro-hardness enhance,however,the ultimate tensile strength（UTS）gradually reduces,and the elongation（EL）first reduces and then increases slightly.The reduction of UTS is related to the shear fracture tendency at the curved interface and the softening effect at high temperature with the increased hot-pressing temperature.The FE and cupping formability of LMCs is also related to the debonding of the weak interface and the constraint effect of the strong interface.The local debonding of the weak interface leads to the weakening of the restraint effect between the Ti and Al layers when hot-pressed at low temperature.So the continued deformation of LMCs is not affected by Al fracture.As the load continued,Ti with higher ductility fractures,and LMCs fail as a whole.Therefore,LMCs possess a high ductility and Erishen value closing to Ti.While the interfacial constraint is enhanced with the increased temperature.On the one hand,the deformation of LMCs can be coordinated by stress-strain transfer via interface.On the other hand,the coordination may be deteriorated by the hard and brittle intermetallic phase at the interface.Therefore,LMCs exhibit complex mechanical behavior due to the two competing mechanisms.Hot-rolling was performed for hot-pressed LMCs.It was found that the defects of LMCs after hot-rolling were reduced,the interface bonding strength was improved,and the strength was enhanced.However,the elongation and formability of LMCs were poor,which limited their engineering application.Therefore,the subsequent annealing was carried out.Then the Ti/Al interfacial structure at various annealing temperatures was studied,and the effect of interface on the mechanical properties and formability of LMCs was explored.The results indicate that the interfacial bonding strength gradually enhances with the increase of annealing temperature,accompanied by the generation of TiAl₃phase.However,the interfacial bonding is deteriorated by the thickened and coarsened TiAl₃phase layer when annealing above 400℃,which greatly affects the mechanical properties and formability of LMCs.The strength of LMCs gradually decreases,while FE and Erichsen value are closely related to the interfacial bonding strength with the increase of annealing temperature.As the temperature increases from 200℃to 400℃,the increase of Ti/Al interfacial bonding strength is beneficial to delay the premature necking and fracture of LMCs,thereby improving their ductility.As the temperature increases further,the coarsened TiAl₃phase is easy to cause interfacial debonding.The crack deflection and passivation at the debonding can effectively suppress the crack propagation,which improves the ductility and formability of LMCs unexpectedly.Especially when annealing at 550℃,although thick and coarse TiAl₃ phase is generated,LMCs show excellent elongation and formability,which is attributed to the energy consumption by crack initiation at Ti/Al interface.The Ti/Al multi-layer composites with different layers were prepared by two-step thermal deformation of hot-pressing+hot-rolling and annealing at 550℃for 2 h.It is found that the wavy morphology of Ti/Al interface becomes increasingly obvious,the TiAl₃phase layer at the interface gradually thickens,FE of LMCs enhances at the expense of UTS with the increased layers.The deterioration of UTS is mainly attributed to recovery and recrystallization softening effects of component metals,the hard and brittle TiAl₃phase at the interface and the tendency of shear fracture induced by the increasingly curved interface.The gradually enhanced FE mainly comes from multilayer microstructure.The TiAl₃phases at the interface easily induce more microcracks under the condition of more layers,which absorbs a lot of energy.And the relatively dispersed microcracks can reduce the impact of stress concentration.Therefore the ductility of LMCs improve.The cupping formability of LMCs is closely related to the texture of component metals and the interface.The texture of the component metal basal surface weakens with the increase of the number of layers of LMCs,which leads to a smaller plastic strain ratio（r）and a larger work hardening index（n）,enhancing the uniform deformation ability of LMCs.The thickness of Ti and Al layers decreases with the increase of the number of layers of LMCs,and the increased interfacial pressure in the thin metal component layers is conducive to interfacial bonding,which promotes the coordinated deformation of LMCs and improves the formability of LMCs.Also,more obstacles are suffered during crack propagation in LMCs with more layers,and the propagation path is longer and tortuous.In addition,part energy of crack propagation is released at the interface,which increases the difficulty of crack propagation,thus improving the formability.Deep drawing of Ti/Al multi-layer composites at room temperature is performed at room temperature.It was found that the peak drawing force of LMCs has a linear relationship with the initial blank diameter.LMCs with more layers has a higher limit drawing ratio（LDR）and can suffer from higher ultimate drawing force.The material at the tangent part between the wall and the corner of drawing cylindrical part is subjected to bending and violent friction,which undergoes the greatest thinning and has a high degree of work hardening.The area is easily necked and ruptured,and is considered to be deep drawing danger zone.The degree of thinning of component metals at the corner decreases as the number of layers increases,because the interfacial load transfer effect makes the interlayer stress distribution more uniform,which helps coordinate deformation between layers.In addition,the increased interface pressure in the thinner component metal layer promotes the formation of flat,small wavy interlock structure and zigzag-like structure interface,respectively.The increasingly strong overlap structure is beneficial to coordinate the deformation of the multi-layer composites.As the number of layers increases,the number of interfaces increases,the load transfer effect via the interface makes the stress distribution more uniform and helps coordinate the deformation between the layers.The crack deflects and tears due to the complex stress state when the deep drawing crack propagates to the interface,which hinders and delays the penetration of the crack.Therefore the drawing formability of LMCs is improved.