The quality of the interface bonding performance of explosion welded composite plates is the main factor affecting the overall performance and service safety of composite plates. Currently, research on titanium steel explosion welded composite plates at home and abroad mainly focuses on the level, structure, mechanical properties, electrochemical properties, and other aspects of the bonding interface. The interface non-uniformity caused by various factors leads to differences in tensile, shear, impact, electrochemical, fatigue and other properties between explosion welded composite plates and homogeneous cladding and substrates.
Research has shown that flat or wavy interfaces can be obtained under different process parameters, and typical wavy interfaces can be obtained by welding dissimilar metal composite plates within appropriate welding windows; The high explosive load intensifies the plastic deformation at the bonding interface, causing the grains to elongate to varying degrees along the explosion direction and accompanied by the formation of some new brittle intermetallic phases. The wave shaped interface formed by the intense metal flow at the interface of the titanium steel explosion welded composite plate improves the bonding condition of the interface and enhances the shear performance of the bonding interface along the explosion welding direction. The grains near the interface of titanium steel composite plates are very small and uneven in size and shape; There is a clear regional distribution of grains from the interface on the substrate side to the area far away from the interface, and the grains on the cladding side also undergo deformation; The microhardness value is generally highest at the bonding interface, as the increase in microhardness value is influenced by grain size, plastic deformation, and even phase transformation. There are adiabatic shear lines, also known as flying lines, at the titanium interface on the cladding side. This structure changes under annealing treatment at different temperatures until it disappears. The purpose of conducting mechanical property testing on titanium steel composite plates is to evaluate and meet the indicators of tensile strength, yield strength, and shear strength of titanium steel explosive composite plates, in order to meet the standard requirements. This article takes the titanium steel explosion composite plate with industrial pure titanium and substrate carbon steel as the research object, and studies the microstructure, structure, hierarchy, and mechanical properties of the bonding interface of titanium steel composite plate. The influence of the non-uniformity of the bonding interface of titanium steel composite plate on the mechanical properties of the material is analyzed, providing theoretical basis for the design calculation and engineering application of pressure vessels, weapons and equipment, etc.
1. Selection and preparation of materials
Using ASTM B265 Gr.1 titanium as the cladding, with a thickness of 5mm; ASTM A516 Gr.70 carbon steel with a thickness of 35 mm is used as the substrate. According to ASTM B898-11 (2016), titanium steel composite plates were prepared using explosive welding method. The chemical composition and mechanical properties of the substrate and cladding are shown in Tables 1-3, respectively.
Tab． 1 Chemical composition of clad plate B265Gr． 1
2.Microstructure and morphology of the bonding interface of titanium steel composite plate
The typical morphology of the bonding interface of titanium steel explosion composite plate is shown in Figure 1. The bonding interface is a typical wavy shape with a wavelength of about 1723.5 μ m and a wave height of about 300 μ m, as shown in Figure 1 (a). The microstructure from the cladding to the substrate along the direction perpendicular to the interface thickness can be subdivided into deformation microstructure on the cladding side+local melting zone → equiaxed fine-grained zone on the substrate side (about 21 μ m) → fibrous deformation microstructure zone (about 200 μ m) → bending and twisting microstructure zone (about 108 μ m) → original microstructure zone (ferrite and pearlite stripes).
During explosive welding, the energy of the explosive is propagated in the form of waves along the direction of the explosion. The metal surface undergoes severe plastic deformation under the action of the shock wave, and a large amount of heat is generated at the contact surface, causing the metal to melt. The metal flows and generates a jet, forming a vortex (see Figure 1 (b)), and the final interface combines in a wave like shape. A vortex is generally a mechanical mixture composed of various substances such as metal debris carried by a jet, melted cooling materials, intermetallic compounds, and metal grains of the coating or substrate at the original position. There are defects such as trapped pores and loose or cracked structures formed by rapid solidification inside the vortex. Figure 2 shows the SEM morphology of different hierarchical regions at the bonding interface of titanium steel composite plates.
(1) The levels of transition from the cladding to the substrate at the interface of the titanium steel explosion welded composite plate are as follows: the deformation zone of the cladding side structure, the local melting zone, the equiaxed fine grain zone, the strip-shaped fiber zone, and the torsion zone on the substrate side.
(2) Compared with the original substrate, the hardness of the fine-grained region on the substrate side (3.60 GPa) and the locally melted region at the vortex (11.73 GPa) increased the most significantly, while the modulus of the two regions did not increase significantly; The hardness and modulus in other areas show obvious uneven distribution, and the uneven structure of the titanium steel bonding interface at various levels results in uneven hardness and elastic modulus.
(3) Influenced by interface ripples, the tensile strength (578 GPa) of the bonding layer along the detonation direction is between the substrate and the cladding, with the lowest elongation at section (31.5%) and shrinkage at section (40%); The tensile strength of the bonding layer perpendicular to the detonation direction (472 GPa) is lower than that of the substrate and cladding, with the lowest elongation at section (31.12%) and a shrinkage rate after fracture (58%) between the substrate and cladding. The tensile properties of this titanium steel composite plate are anisotropic.
