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Explosive welding, as a well-known composite processing technology, can be used to weld two or more similar and dissimilar materials. Generally, periodic wavy interfaces are formed in the bonding zone during explosive welding process. The bonding strengths of the welding interfaces are largely influenced by the microstructure of the welding interface. In this paper, the effects of microstructure on mechanical properties of titanium-steel explosive welding interface were presented. The bonding interface with different microstructure at different locations of the titanium-steel welding interface can be characterized as three distinct zones due to different stages of detonation wave propagation including the detonation growth zone near the initiation point, steady detonation zone and reflection-affected zone near the far-end boundary. The microstructure of interfaces was investigated by optical microscopy, scanning electron microscopy, energy dispersive spectrometer. It was found that a nearly straight interface with a small wavy interface was created in the detonation growth zone near the initiation point, and an irregularly wavy interface was formed in the reflection-affected zone near the far-end boundary. While a periodic wavy interface with various defects (e. g. microcracks, voids and intermetallic compounds etc.) was formed in the steady detonation zone. A small-size H-shaped specimen was used to evaluate the tensile properties of interfaces with the bonding interface perpendicular or parallel to the tensile loading direction. The results indicated that the amplitude and wavelength of wavy interfaces showed a tendency of increasing, plateauing and decreasing, corresponding to the detonation growth zone, steady detonation zone and reflection-affected zone, respectively. The mechanical properties of the bonding interface along the detonation direction also displayed a similar tendency. The periodic wavy interface in the steady detonation zone exhibited the highest strength. Notably, the defects, such as microcracks, void and intermetallic compounds in the vortex area had significant effects on the mechanical properties of the bonding interface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Large-depth gradient hardness distribution with depth of 2.5 mm was produced in commercially pure titanium by explosive hardening. • Gradient α-ω phase transition, gradient grain size, gradient twining and gradient dislocation was occurred in commercially pure titanium by explosive hardening. • Two types of grain refinement modes, multi-directional twinning-twinning interaction and twinning-dislocation wall interaction, were revealed. Explosive hardening of commercial pure titanium plate was conducted by a thin-layer high-detonation-velocity explosive. The nanoindentation and Vickers hardness tests of the treated titanium plates along the depth of shock loading were performed. The microstructures along the depth were characterized by using different methods, including X-Ray Diffraction, Optical Microscopy, Electron Back-Scattered Diffraction and Transmission Electron Microscopy. Experimental results showed that large-depth gradient hardness distribution with depth of 2.5 mm was produced in CP titanium by explosive hardening. There are two gradient hardened layers, ultra-hardened layer with a depth of 0–40 μm and a hardness increase of ∼400%, sub-hardened layer with a depth of 40 μm-2500 μm and a hardness increase of 54.3%. Gradient microstructures with different characteristics were observed, including gradient α-ω phase transition, gradient grain size, gradient twining and gradient dislocation. The area of gradient microstructure is consistence with that of gradient hardness distribution. Grain refinement/α-ω phase transition mainly contributed to hardness sharply increasing in ultra-hardened layer. High-density dislocation/twining mainly contributed to hardness increasing in the sub-hardened layer. Finally, the formation mechanism of gradient microstructures and grain refinement mechanism was detailly discussed. [ABSTRACT FROM AUTHOR]

The dissimilar joining of TA2/Q235 had attracted great attention because of its important applications in the nuclear fusion, aerospace, petrochemical industry. The possibility of joining TA2 and Q235 irrespective of different thermal expansion coefficients, different melting temperatures, or different mechanical properties was restricted with the conventional welding methods. Explosive welding, as a well-known composite processing technology, could be used to weld two or more similar and dissimilar plates. Which had been used to successfully fabricate TA2/Q235 composite in this work. The failure often occurred near the bonding interface of transition joints and mechanical parts were fabricated by explosive welding in the industrial application and production. Then the tensile-shear test was conducted to investigate the tensile shear failure mechanism at the bonding interface with in-situ SEM, and also nanoindentation was used to identify the mechanical properties. The interface bonding shear strength was around 345 MPa. Further, the microstructure characterization of interfacial zone and the melting and mixing of welding materials were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results demonstrated that the failure of the bonding interface occurred along the interface wave direction. It was because that the defects, such as cavities, cracks and brittle intermetallic, in the melted zone of the interface wave. The discontinuities and stress concentration points would result in degraded mechanical properties. The fracture morphology showed microcracks, cleavage plane, dimples, fragmentation of brittle intermetallic compounds, which presented mainly brittle fracture, while ductile fracture existed in some zone. Meanwhile, there were the serious lattice distortion and high internal stress in Ti matrix, while the deformation and elongation of the grain and fine grains formed in Fe matrix. [ABSTRACT FROM AUTHOR]

In the present work, explosion hardening (EH) technique was used to produce gradient microstructure with martensitic transformation in commercially pure titanium (CP Ti) to improve its mechanical properties. After EH treatment, phase structure, grain size distribution, dislocation structure and twin structure in CP Ti were characterized. Subsequently, microhardness tests, tensile/compressive tests and three-point bending tests were conducted to investigate mechanical properties of EH treated CP Ti along different depth positions from EH treated surface. Experimental results show that gradient microstructure in terms of grain size, α-ω martensitic transformation, texture intensity, twin density, dislocation density and hierarchical nanotwins, are observed in EH treated CP Ti. According to microstructure characteristics, four layers including nanostructure surface layer, refined structure layer, deformed coarse grained layer and coarse grained matrix are divided from EH treated surface to matrix. Microhardness tests show a gradient hardness layer with a maximum hardness increase of 83% within 3.5 mm from EH treated surface. Tensile and compressive test results show that the most significant strengthening exists within 2 mm depth from EH treated surface with a maximum tensile yield stress increase of 71% and the total EH affected depth is more than 25 mm. In addition, the gradient microstructure is affected by the number of EH treatments. For example, bending yield stress increases by 41.3% in 1-EH treated specimens and 64.8% in 2-EH treated specimens. Combining microstructure characterization and mechanical properties testing, shock-induced strengthening mechanism of CP Ti is analyzed: for positions close to EH treated surface, formation of α-ω martensitic transformation and grain refinement is responsible for the significant strengthening phenomenon; whereas, for positions far away from EH treated surface, the moderate strengthening phenomenon is due to the generation of high-density dislocations/twins. • Explosion hardening (EH) was used to enhance mechanical properties of CP Ti. • Gradient microstructure with martensitic transformation formed in EH treated CP Ti. • A gradient hardness layer with a maximum hardness increase of 83% within 3.5 mm from EH treated surface. • Gradient microstructure contributed to gradient mechanical properties enhancement. [ABSTRACT FROM AUTHOR]

In the present work, Ti6Al4V alloy was treated by one/two passes explosion hardening (EH) technique to enhance mechanical properties. The phase structures, grain size distribution, dislocation and twin structures of EH treated Ti6Al4V alloy were characterized in detail. Mechanical properties of EH treated Ti6Al4V alloy were measured by tensile and compressive tests. The mechanism of shock-induced strengthening was subsequently analyzed. Experimental results showed that gradient grain microstructure with nanotwins/hierarchical nanotwins and β-α phase transformation occurred near the surface of EH treated Ti6Al4V alloy. Furthermore, the tensile and compressive strengths of Ti6Al4V alloy exhibited significant enhancement after EH treatment. The compressive strength of the materials increased from 1008 MPa in untreated condition to 1121 MPa in one pass EH treatment and 1365 MPa in two passes EH treatment, while the fracture strains always remained more than 0.15. The significant strengthening of Ti6Al4V alloy after two passes EH treatment was mainly attributed to the formation of hierarchical nanotwins, which can effectively impede dislocation motion. In summary, EH is an effective technique to improve mechanical properties of Ti6Al4V alloy by producing gradient microstructure with hierarchical nanotwins. The present research may have the potential use for metallic materials in the field of load-carrying capacity, tribological property and fatigue property. • Gradient microstructure with hierarchical nanotwins was produced by explosion hardening. • After two passes explosion hardening, compressive strength of Ti6Al4V increased by 35.4%. • Both twin-dislocation wall interactions and twin-twin interactions contributed to grain refinement. • Shock strengthening was mainly due to the formation of hierarchical nanotwins, which effectively impedes dislocation motion. [ABSTRACT FROM AUTHOR]

Ni–Al energetic structural materials (ESMs) has been widely applied in defense field, because of its good mechanical and energy release properties. The effects of the microstructure on mechanical and energy release properties of ESMs are not well understood. In this study, by designing the particle size Ni:Al = 1 μm:25 μm and Ni:Al = 20 μm:25 μm, two kinds of samples, Ni continuous phase (Ni cp) and Al continuous phase (Al cp), were prepared by explosive consolidation. The microstructure characteristics were analyzed by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscope. Quasi-static and dynamic compression tests were also conducted to analyze the mechanical properties. It showed that the Ni cp ESMs had higher quasi-static compressive strength of 325 MPa and fracture strain of 21.3%, compared with the Al cp ESMs having the strength 275 MPa and the fracture strain 17.5%. Also, the dynamic compressive strength of the Ni cp ESMs sample is higher than that of the Al cp ESMs sample. The reaction characteristics of the two ESMs in different atmospheres were discussed by differential scanning calorimetry. The result showed that the reaction temperature of the Ni cp ESMs was lower. Impact-induced energy release tests on the two ESMs were performed to understand their energy release properties. The results showed that the microstructure had significant effects on the ignition velocity threshold and reaction efficiency. The Ni cp ESMs has better reaction performance because of the high interface density. • High dense Ni–Al ESMs is prepared by explosive consolidation. • Two different microstructures defined as Ni continuous phase and Al continuous phase are obtained. • The effect of the microstructure on the mechanical properties is analyzed. • The effect of the microstructure on the impact-induced energy release is evaluated. [ABSTRACT FROM AUTHOR]

Heterogeneous gradient microstructure in commercially pure titanium (CP Ti) was fabricated through the explosion hardening (EH) technique followed by annealing treatment (AT). The heterogeneous gradient microstructure was characterized by a microhardness tester, optical microscope and transmission electron microscope. Subsequently, dynamic shear properties and microstructure evolution of the material were systematically studied by split Hopkinson pressure bar apparatus with hat-shaped shear specimens. Experimental results showed that various heterogeneous gradient microstructures were obtained by EH + AT at different annealing temperatures. Enhanced dynamic shear properties with special yield stress-fracture strain combinations were observed, exhibiting better mechanical properties than the other metallic materials. Such excellent mechanical properties originated from finer grain size, higher twin/dislocation density and gradient synergistic effect. During dynamic shear tests of gradient CP Ti, adiabatic shear bands (ASBs) were easy to nucleate, propagate and interact, especially on EH treated specimen surface due to the generation of high-density micro-defects. Formation of twins/dislocations during EH treatment promoted dynamic recovery during the ASBs nucleation stage, whereas AT delayed ASBs nucleation due to a decrease in micro-defects, which contributed to better ductility. During dynamic shear tests of gradient CP Ti, propagation of ASBs and cracks was delayed by the gradient microstructure. The adiabatic temperature rise of 2-EH treated CP Ti was calculated to be 108 °C when ASBs nucleated, which was so low that it was supposed to have no effect on ASBs nucleation. [ABSTRACT FROM AUTHOR]

The welding of thick W onto Cu, with good bonding, has been a big challenge due to the large differences in physical properties between W and Cu. Among various novel methods, explosive welding is the promising one to produce bimetals with large size and great thickness. However, the cracking of brittle W under high strain rate limits its application. In this work, hot-explosive welding technique was explored to overcome this problem. A 2 mm thick W plate was preheated to 500 ℃ and was successfully welded with pure Cu plate, without any cracks formed in W layer. The result suggests that preheating W to over its dynamic ductile-to-brittle transition temperature and decreasing the imported kinetic energy are two most important factors for the successful welding of thick W plate. The weldability window calculated using the parameters at 500 °C predicted the formation of a good wavy interface. The microstructures at W-Cu interface were characterized by optical microscope, SEM, EBSD and TEM. The mechanically mixed W-Cu phase and the 2∼6 nm thick amorphous layer along the interface created strong bonding between the immiscible W and Cu. The measured interfacial compressive shear strength reached 188 MPa, indicating a good bonding strength of the interface. [ABSTRACT FROM AUTHOR]

The commercially pure (CP) titanium has been processed by explosion hardening to produce a gradient microstructure. The hardness distribution of CP titanium after explosion hardening was measured. The microstructure of explosion hardened CP titanium was revealed by transmission electron microscopy and optical microscopy. The split Hopkinson pressure bar was used to study the effect of explosion hardening on dynamic behavior and adiabatic shearing formation of CP titanium. The experimental results showed that explosion hardening induced the gradient hardened microstructure with a depth of 2.6 mm in CP titanium. The explosion hardened specimens present enhanced static and dynamic compressive strengths, which was caused by grain refinement, the α-ω phase transition, and micro-defects' proliferation. The gradient deformation and gradient ASB occurred in explosion hardened specimen after dynamic loading, whereas the homogeneous deformation and a bipyramid-shaped adiabatic shear band were observed in untreated specimen. The ASBs were easier to nucleate, propagate, bifurcate and interact in the explosion hardened CP titanium during dynamic loading, especially in the shocked side due to the higher-density micro-defects. This was attributed that the ω phase particles and the proliferation of twinning/dislocations during explosion hardening promoted the dynamic recovery process during the nucleation stage of ASBs. The gradient microstructure in CP titanium with explosion hardening delayed the propagation of ASBs and cracks under dynamic loading. [ABSTRACT FROM AUTHOR]