Research On Microstructure And Evolution Of TiNi Alloy/Q235 Steel Explosive Composite Interface

As a new type of functional material,TiNi shape memory alloy has excellent functional properties(shape memory effect,pseudo-elastic effect),high cavitation resistance and biocompatibility.It is widely used in aerospace,marine development,mechanical instruments biomedical and other fields.Due to the high price of materials and the needs of application scenarios,TiNi alloy is usually used as a functional component to composite with other metal matrix to prepare composite materials combining the advantages of the two materials.When preparing memory alloy composites by conventional fusion welding processes,extremely high heat input will significantly affect the martensitic transformation characteristics of TiNi alloys.At the same time,a wide melting zone is formed at the welded joint,and a large number of brittle phases(TiFe2,TiFe,etc.)existing inside seriously affect the mechanical properties of the joint and hinder the application of composite materials.The explosive welding method can realize large-area welding of dissimilar metals and has a small heat-affected area,which is very suitable for manufacturing memory alloy composite materials.This article uses explosive welding technology to prepare TiNi alloy/Q235 steel clad plate.The microstructure and nano hardness distribution of the TiNi alloy/Q235 steel composite interface were characterized by various characterization techniques such as metallographic microscope(OM),scanning electron microscope(SEM),energy spectroscopy(EDS),and nanoindentation.The results show that the composite interface presents a regular wave combination,but there are cracks in the melting zone.EDS analysis shows that the strong circular motion inside the vortex zone makes the three elements Ti,Ni and Fe evenly distributed in it.There is no obvious thermal diffusion of atoms at the interface between the melting zone and the base metal.In the explosive welding process,when the temperature of the composite interface decreases,the difference in thermal expansion coefficient between Q235 steel and martensite and the reverse phase transformation of martensite cause huge shear stress at the interface,which provides stress conditions for the generation of internal cracks in the melting zone.The formation of hard and brittle intermetallic compounds in the melting zone provides a material basis for it.The results of the nanoindentation test show that the hardness of the matrix material on both sides of the interface is higher,which is the result of the combined effect of work hardening and grain refinement,while the hardness of the matrix material is low at a distance from the interface,which is mainly due to softening effect caused by recrystallization and dynamics recovery.In order to study the evolution process of interfacial ripples and vortices,the SPH method was used to simulate the explosive recombination process of TiNi alloy/Q235 steel.The simulation results reproduced the characteristic structures of metal jets,wavy interfaces and vortices,and the interface morphology and corrugation characteristic sizes were in good agreement with the experimental results.The simulation results show that the metal jet is composed of substrate and composite material.The molten metal was distributed unevenly along the interface,mainly concentrated in the vortex area,and a small amount existed in other positions of the interface.The deformation of the composite board material is concentrated in a thin layer area near the composite interface,and a plastic strain zone is formed at the interface.The pressure at the collision point reached 8GPa,and the nearby metal materials showed fluid-like behavior.The wave forming process of the interface wave is basically consistent with the description of Bahrani's engraving mechanism.The cyclic penetration and extrusion of the metal jet on the base composite plate led to the formation of a continuous wave structure.The initial disturbance of the interface is an important factor in the formation of wavy interfaces.When the metal jet penetrates the substrate on one side,it is divided into two new jets,one forward forms a new re-injection stream,and the other intrudes into the composite interface backward.At this time,the molten material on the corrugated side slope closest to the collision point still had a high moving speed,and moved toward the new jet that invaded the interface backwards,forming a vortex zone.The ripples continue to develop and grow within a period of time after the collision point left.The molten material at the peaks and troughs was squeezed to the sides and injected into the vortex area.This process makes the molten material at the interface more concentrated in the vortex area,and the vortex area grew accordingly.