High Strain Rate Plastic Deformation Research Articles

Phase transformation mechanisms of NiTi shape memory alloy during electromagnetic pulse welding of Al/NiTi dissimilar joints

The phase transformation mechanisms of NiTi shape memory alloy (SMA) during electromagnetic pulse welding (EMPW) of Al/NiTi dissimilar alloy joints were studied. Two types of amorphization behaviors were observed in the NiTi SMA of the Al/NiTi dissimilar joint. These are: i) fast cooling-induced amorphization of the joint interfacial reaction layer; and ii) severe deformation-induced amorphization. Additionally, the martensitic transformation was observed in the NiTi SMA during the high strain rate plastic deformation imposed by the EMPW process. The study shows that the formation of B19' martensite during high strain rate plastic deformation of NiTi SMA is influenced by both the strain rate and the peak pressure of the deformation during welding.

Surface crack analysis of 8Cr4Mo4V steel based on Crystal/Phase microstructure model by grinding

Metal polycrystalline materials tend to be homogeneous and isotropic at the macroscopic level, which leads it difficult to visually characterize the sprouting and expansion of cracks by traditional grinding simulation. In this study, the Johnson–Cook constitutive model was used for the finite element method (FEM) based on grinding characteristics with a high strain rate and large plastic deformation. The models are focused on the actual microstructure distribution, one of them contains grains and grain boundaries, and another contains phase organization (martensite and acicular martensite) and carbides. The study pinpoints the changes in microstructure under different grinding parameters during the grinding process and the influences on the formation of microscopic cracks. Ultimately, it was found that the shedding of large primary carbide is a critical factor for inducing surface cracks. The uncoordinated deformation between brittle carbides with the other phases is the main cause of carbide shedding. Therefore, this article visualized shows the influence of grinding parameters on surface cracks from the perspective of microstructure modeling, which is of great significance in guiding practical processing.

Microstructure and wear behaviors of 17-4 PH stainless steel fabricated by laser cladding with post laser shock peening treatment

Laser cladding (LC) technology provides an attractive and cost-effective way to prepare wear-resistant coating on high-value engineering components. However, the thermal effect of coating will lead to a large amount of residual tensile stress in the coating, which seriously affects the wear performance. This paper investigated the influence of laser shock peening (LSP) on the microstructure and wear behaviors of wire-based LC 17-4 PH coating. Results showed that there was no laser remelting phenomenon during LSP process, and all residual austenite in cladding layer transformed into martensite after LSP treatment, accompanied by a more compact and uniform microstructure. LSP can introduce severe plastic deformation to form dislocation structures on the top surface of the coating, which lead to grain refinement through dynamic recrystallization (DRX), and the average grain size decreased from 4.15 μm to 1.81 μm when the laser energy is 9J. Meanwhile, the high strain rate plastic deformation induced by LSP generated high residual compressive stress on the material surface, with a maximum value of 500 MPa. The surface microhardness of the cladding layer increased by 100HV after LSP treatment, reaching to a peak of 458HV at a laser power of 9J, and gradually decreased along the depth direction, presenting a gradient distribution. After LSP treatment, GCr15 steel counter ball was used to simulate the sliding friction of solid particles under real working conditions. Results showed that the friction coefficient (COF) and wear rate of 17-4 PH coating were significantly reduced. Furthermore, the wear type changed from dominative adhesive to slight abrasive wear. The improvement of microhardness and wear properties were attributed to the combined effects of Hall-Petch and Taylor hardening strengthening.

High strain rate plastic deformation driven microstructure evolution in AlCoCrFeNi dual-phase high entropy alloy

High strain rate plastic deformation under laser shock processing (LSP) has been applied to the AlCoCrFeNi dual-phase high entropy alloy (HEA) for optimizing the microstructure and properties. Phase constituents, microstructure evolution of the HEA before and after LSP were investigated. Microstructures of the HEA samples were examined using scanning electron microscopy and transmission electron microscopy. Results showed that the microstructure in the LSPed region was obviously refined. High density dislocations were produced in the alloy and dislocation slip was the main plastic deformation mode during LSP treatment. When the laser energy increased to 5 J and 7 J, nano-precipitates, twins and stacking faults appeared, and the plastic deformation mode was a mixture of deformation twinning and dislocation slip. The refined grains, dislocation structures, deformation-induced twins and stacking faults were analyzed and associated with the microhardness. The microhardness of the HEA was significantly improved after LSP, which can be attributed to the synergistic strengthening of grain refining strengthening, precipitation strengthening and dislocation strengthening.

Intermetallic Reaction of the Bonding Interface of TA2/Q235 Explosive Welding Composite

During the explosive welding, the bonding interface of welded materials was fast heated due to high strain rate and drastic plastic deformation. The periodical wave interface, with an amplitude of ~300 μm and a period wavelength of ~800 μm, was identifiable as a uniform wave interface formed in the bonding interface. The details of the formation of melting zone and mixing zone of welding materials at the interface were observed. Combined with the Ti-Fe binary phase diagram and the principle of diffusion welding, the phase composition and evolution process of the melting and mixing zone of the bonding interface were investigated by transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). Significance of the intermetallic compound was found in the mixing zone and melting zone, which was mainly TiFe, TiFe2, TiO2, Fe2O3 and some other intermetallic oxides. Meanwhile, the phenomenon of the titanium agglomeration and oxygen precipitation was observed in the melting zone. The bonding interface could be determined as a mixing welding of mechanical mixing, melting, diffusion and solidification that occurred in the mixing zone, and melting welding and diffusion welding mainly occurred in the melting region.

Microstructure evolution and mechanical property strengthening mechanisms of Cu/Cu NPs/Cu joint fabricated by ultrasonic spot welding

Ultrasonic spot welding, as a high strain rate plastic deformation process, utilizes the high frequency sliding between the metal sheets to generate a solid-state joining. For the ultrasonic spot welded Cu/Cu joint, even with the good metallurgical bonding at the welding interface, it couldn't obtain high T-peel strength and maintain quite high T-peel load bearing capacity before failure. In order to enhance the T-peel strength and bearing capacity, Cu nanopaticles (NPs) interlayer was screen-printed on the Cu sheet surface that was to be welded. It was beneficial to enhance the temperature at the welding interface and the degree of dynamic recrystallization, obtain the nano-scaled equiaxed grains with random orientations at the welding interface, and change the microstructure evolution pattern along the weld thickness. These results are key factors to the improvement of the joint mechanical strength. In comparison with the Cu/Cu joint, the T-peel strength of the Cu/Cu NPs/Cu joint (471.7 N) was increased by 69.4%, the displacement (0.97 mm) corresponding to the T-peel strength was enhanced by 42.7%, the obviously increased fracture energy was achieved, and quite high T-peel load bearing capacity could be maintained after exceeded the T-peel strength. The mixed failure modes of brittle cleavage and quasi-cleavage as well as shallow dimples fracture occurred in the Cu/Cu joint. However, the failure mode for the Cu/Cu NPs/Cu joint was turn into the micro-void coalescence.

Functional gradation of aluminum alloy by impact of ballistics as severe plastic deformation process

In the present effort, impact of bullets fired at high speeds on a stationary target was used as a high strain-rate plastic deformation method to generate Functionally Graded Materials (FGMs). The bullet-shaped Aluminum alloy Al5052 specimens impacted the UHMWPE target at different projectile velocities ranging from 100 m/s to 750 m/s, to study the effect of the impact speed. Moreover, few of the projectiles impacted along the longitudinal axis whereas the remaining projectiles impacted with an obliquity to investigate the effect of leading-edge shape. The metallography of the projectile specimens fired at 750 m/s shows grain refinement from 70 ± 3 μm at the rear/ un-deformed point to 10 ± 1 μm at the front/ severely deformed point which is in the impact zone. Similar but of less magnitude variations were observed at other impact speeds also. The hardness variation was 45% (70 ± 3 HV at the rear surface to 102 ± 2 HV at the front face) at 750 m/s. Moreover, least variation in hardness and grain size was observed for projectile impacted at lowest velocity tested (100 m/s). Further, functionality in hardness and grain refinement was seen in graded direction due to shape of the projectile specimens, impact velocity, and target material. Depending upon the impact angles, axisymmetric (impacted normal to the longitudinal axis) or unsymmetrical (impacted at an angle) variation of grain refinement and hardness was observed.

Strengthening of pretreated aluminum during ultrasonic additive manufacturing

Strengthening effects in materials bonded by the high strain rate plastic deformation process, Ultrasonic Additive Manufacturing (UAM), were investigated. Aluminum (Al 6061) was pretreated by tempering and annealing prior to bonding through UAM. Following UAM, multiscale material characterization was performed. Tensile testing in the rolling (x) direction demonstrated the material became harder after the UAM process, and nanoindentation demonstrated the foil-foil interfaces became harder than the bulk foil material. The strengthening effects are a result of microstructure changes at the interfaces and in the bulk foil regions which were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. These microstructure changes result from dynamic recrystallization, dynamic recovery, adiabatic heating, and precipitate dissolution. This study signifies the metallurgical features creating mechanical strength increases, rather than decreases, in UAM builds.

Ultrahigh strength in lightweight steel via avalanche multiplication of intermetallic phases and dislocation

Lightweight high-strength steel has been an important topic for structural materials. Recent studies show that B2 intermetallic phase, nucleated around crystal defects such as grain boundaries and dislocations, can be used to further strengthen mechanical properties of metal alloys. Due to the limited density of nucleation sites, however, the current thermomechanical processing leads to formation of low density and coarse B2 phase. In this study, a lightweight steel Fe-Mn-Al-C (70 wt.% Fe) is processed by a thermal-controlled high strain rate plastic deformation process, warm laser shock peening (wLSP), in which high-density dislocations can be generated and high mobility of solute atoms at elevated temperature can facilitate the precipitation of B2 intermetallic phase. Simulations based on coupled phase-field and dislocation dynamics model are performed to investigate the intermetallic precipitation and dislocation formation during wLSP process. It is found that high density dislocations can be obtained in wLSP due to the pinning effect of B2 intermetallic phase on dislocations. The interaction between B2 intermetallic precipitates and shock wave facilitates the generation of high-density dislocations, which in turn serve as nucleation sites for B2 phase formation. This strong coupling effect results in avalanche multiplication of ultrahigh density and ultrafine B2 precipitates, and significant improvements in both strength and ductility. The yield strength of the lightweight steel reaches 2030Mpa with ultimate strength of 2850 MPa, among the highest for lightweight mid-carbon steel. This work provides a new insight to produce lightweight metals with ultrahigh density and ultrafine intermetallic phases for high strength and ductility.

Strengthening Effects at Dissimilar Metal Interfaces Created by Ultrasonic Additive Manufacturing

Ultrasonic additive manufacturing (UAM) technology can produce structural materials with complex geometries and embedded sensors. Under UAM-processing conditions, complex interfaces are created between these materials due to high-strain rate (> 105 s−1) plastic deformation. Fundamental understanding of strengthening mechanisms at these interfaces are needed for engineering deployment. In this research, we correlated the interface microstructure to their strengthening mechanisms using nanoindentation and electron microscopy techniques. Strengthening at interface regions is correlated to microstructural features at different length scales including refined grains (200 nm), increase in vacancy clusters (1016 cm−3), and dislocation networks (1010 cm−2). This conclusion was arrived through superposition principles to describe the combination of many strengthening mechanisms and their respective microstructure feature sensitivity.

Mechanisms of Cracking in Pure Magnesium During High Strain Rate Plastic Deformation

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Formation of ultrafine grains in magnesium alloy during dynamic extrusion at room temperature

Magnesium alloy is a class of lightweight structural materials with promise for many applications. Here, the authors report a significant grain refinement, high deformation twin density and obvious nanograined structures in magnesium alloy through a novel high strain rate plastic deformation method—dynamic extrusion. By using finite element simulation and transmission electron microscopy (TEM), the authors find that the stain rate during plastic deformation affects the microstructure evolution process, which results in the dynamic deformation mechanisms transition. The results are of interests not only for understanding the deformation mechanisms under dynamic plastic deformation but also for nanostructures formation of the magnesium alloy.

Improving mechanical property and microstucture evolution of 7075 aluminum panel formed by unequal alternate double-sided laser shock forming

With thrust weight ratio increasing, integral panel is an important component to reduce the aircraft weight, and it is a great challenge to ensure the forming accuracy and mechanical property in the large-scale panel. Laser shock forming has a great development prospect in realizing integral panel forming and improving its mechanical properties. This work investigated unequal alternate double-sided laser shock forming, which can make 7075 aluminum panel form and induce the hardened layers in both panel sides. The improvements were analyzed in mechanical properties and microstructure evolution of 7075 aluminum panels after unequal alternate double-sided laser shock forming. In the surface and subsurface layer, the residual stress and microhardness were verified to be enhanced by the laser shock wave. The results of XRD and EBSD provided the evidence of grain refinement. The strengthening mechanism of unequal alternate double-sided laser shock forming was analyzed in this work. The grains are distorted and refined during high strain rate plastic deformation due to dislocation slip and accumulation. The mechanical properties were enhanced by unequal alternate double-sided laser shock forming due to the hardened layers in both panel sides. The hardened layers and grain refinement have a great significance in inhibiting the crack generation and growth.

Deformation Behavior and Microstructure Evolution of CoCrNi Medium-Entropy Alloy Shaped Charge Liners

To investigate the microstructure evolution and dynamic deformation behavior of a CoCrNi medium entropy alloy under an ultrahigh explosive loading rate, CoCrNi-shaped charged liners were fabricated and fired into steel targets. Targets with residual jet fragments were recovered for detailed microstructural analysis using scanning electrical microscopy, X-ray diffraction, electron backscattered diffraction, and transmission electron microscopy. The results indicate that the grain size was reduced by more than 3 times and grains were found to be equiaxed in the residual jet, which indicates that dynamic recrystallization (DRX) occurred during this extremely high strain rate and large plastic deformation. Furthermore, the content of Cr element in CoCrNi at the grain boundaries increased significantly after detonation deformation. The reduced grain sizes are believed to reduce the bulk diffusion path of Cr from grain interior into grain boundaries with the support of sufficient energy provided by the ultrahigh temperature. The enrichment of Cr at grain boundaries promotes the formation of nanosized Cr-rich precipitates with body-centered cubic (BCC) structures, which were found to be widely distributed along grain boundaries in the residual jet. These precipitates were considered obstacles for grain boundary movement and promotion of crack initiation along the grain boundaries, which might cause ductility loss of the CoCrNi-shaped charge jet and loss of penetration capability.

The Taylor–Quinney coefficients and strain hardening of commercially pure titanium, iron, copper, and tin in high rate compression

This work presents an investigation on the effects of adiabatic heating and strain rate on the dynamic compressive response of titanium, iron, copper, and tin. The high strain rate tests were carried out with a Split Hopkinson Pressure Bar (SHPB) and the low strain rate tests with a servohydraulic testing machine. The temperature increase of the specimens during deformation was measured with high speed infrared thermography (IRT). The results show that all the investigated materials have positive strain rate sensitivity and temperature increases of up to 65 °C were observed in the high strain rate experiments (500–3100 s−1). Adiabatic heating in all investigated materials increased with strain rate. The temperature increase at the strain rate of 1 s−1 clearly diminished the strain hardening rate of iron and titanium but was seemingly insufficient to impact the mechanical behavior of copper and tin. The Taylor–Quinney coefficients (βint and βdiff) were found to be strain and strain rate dependent. At higher strain rates (1200–3100 s−1), the integral βint was smaller in the beginning of the test (0.2 to 0.7) and increased to approximately 0.8–0.9 at larger plastic strains. The differential βdiff comprised gaussian curves as a function of strain whose maximum values were from 0.9 to 1.2 for the investigated materials. Tin had lower βint and βdiff with higher strain hardening rates, while copper had a higher βint and βdiff with a low strain hardening rate throughout the high strain rate tests. These results indicate that copper had a more stable microstructure during deformation and converted most of the applied plastic work into heat, while tin had a faster evolving microstructure which stored more plastic work in its microstructure during plastic deformation. Furthermore, this suggests that βint and βdiff can be used as parameters to investigate the stability and the microstructural evolution of materials under high strain rate plastic deformation. βdiff is more appropriate to describe the instantaneous thermomechanical behavior of a material and βint is more appropriate for applications which benefit from a single parameter to characterize how efficiently a material converts plastic work into heat up to a given strain level.

Classification and variation of fracture modes in laser shock hole-clinching

Laser shock hole-clinching is a high–strain rate mechanical joining process in which the metal foils are joined together based on plastic deformation generated by a laser-induced shock wave. In the process, fracture is a typical defect and seriously influences the clinching quality and production efficiency of joints. However, the classification and variation of fracture modes in laser shock hole-clinching are of less concern. In this study, the fracture mode of Cu-Fe joints in laser shock hole-clinching was experimentally investigated. Both optical microscopy and scanning electron microscopy were adopted to analyze the fracture surface morphology of clinched joints. The influence of laser power density, laser spot diameter, the initial grain size and thickness of metal foil, and spacer height on the fracture mode of joints was evaluated. It is revealed that the temperature increase caused by high–strain rate plastic deformation has no impact on the fracture behavior of the joining partners. Fracture always occurs on joining partner I, and it can be divided into four modes, including bottom surface fracture, bottom corner fracture, neck fracture, and mixed fracture. It is found that fracture on the bottom surface is seldom seen but mixed fracture accounts for most of the cracked specimens. Neck tensile fracture rarely appears alone, and it usually exists accompanied by fracture on the bottom corner. The fracture mode varies from a tensile fracture mode on the bottom corner to a mixed fracture mode and then to a shear fracture mode on the neck with the enhancement of laser power density. In addition, the initial grain size of joining partner I has a significant impact on the fracture mode of clinched joints. The fracture mode varies from bottom corner fracture to mixed fracture in relation to the change of mechanical properties of metal foil with an enlarged grain size. However, the mixed fracture mode always appears with the enlargement of both laser spot diameter and spacer height.

Mechanical properties and strengthening mechanisms of shock-modified titanium alloy

High strain-rate plastic deformation treatment is applied to modify the mechanical properties of a titanium alloy via gas gun loading. The yield strength, ultimate tensile strength (UTS) and fracture strain derived from stress-strain curves are 1.02–1.18 GPa, 1.23–1.41 GPa and 7.4–10.9%, respectively, for the impact velocities explored. Compared to the untreated titanium alloy, the specimen shocked at 609 m/s exhibits the best strength-toughness tradeoff: the UTS (1.41 GPa) is increased by 14.6%, and the fracture strain (9.7%) is decreased by 11.2%; the relatively small reduction in ductility is related to the {101¯1} contraction twinning and high dislocation density.

Magnetic pulse welding of copper to steel tubes–Experimental investigation and process modelling

Abstract Magnetic pulse welding allows joining of metallic tubes by a short electromagnetic pulse and the resulting high strain rate impact and plastic deformation. Since melting of materials is avoided, magnetic pulse welding is an efficient method for joining of dissimilar materials. However, the process occurs very fast with little opportunity for real-time monitoring and control, which is also difficult due to the presence of a high amplitude electromagnetic field. A novel attempt is presented here to examine the underlying phenomena for magnetic pulse welding of copper flyer tubes to steel target tubes by computational process modeling and a focused experimental investigation. A coupled electromagnetic and mechanical finite-element model was created to compute the electromagnetic field and pressure distribution, and the progressive nature of flyer tube impact velocity on the target tube. The consequent progress of the angle of impact and contact length are examined for a range of standoff distances between the flyer and target tubes, the target tube wall thickness. The computed results are validated extensively with corresponding experimentally measured results. Overall, the coupled theoretical and experimental investigation provided a useful quantitative insight of magnetic pulse welding of copper and steel tubes, which is expected to help in the advancement of practical application of the process.

Microstructure evolution and adiabatic heating during dynamic biaxial deformation of a 304 stainless steel

This study focuses on the microstructure evolution and adiabatic heating during high strain rate biaxial deformation of a 304 stainless steel. Adiabatic heating is measured in situ during drop weight impact testing of the studied material. The considerable temperature rise induced by adiabatic heating (up to 184.3 °C when impacted with 140 J) is detected. The microstructure was analyzed by X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM) techniques. Dislocation glide, twinning and austenite-martensite phase transformation are operating during high strain rate plastic deformation. The XRD analysis shows a steady increase of α′-martensite fraction (up to 36%) with plastic strain (up to 0.8), whereas the fraction of intermediate ε-martensite remains negligible (<0.5%). The biaxial stress compensated the possible impediment of martensitic transformation caused by adiabatic heating and the increasing stacking fault energy (SFE). Nanotwins having a width of ~5 nm–~20 nm are inhomogeneously distributed in the microstructure. Interrelationship between adiabatic heating, deformation mechanisms and microstructure evolution is discussed.

Understanding the Antipathogenic Performance of Nanostructured and Conventional Copper Cold Spray Material Consolidations and Coated Surfaces

The role of high strain rate and severe plastic deformation, microstructure, electrochemical behavior, surface chemistry and surface roughness were characterized for two copper cold spray material consolidations, which were produced from conventionally gas-atomized copper powder as well as spray-dried copper feedstock, during the course of this work. The motivation underpinning this work centers upon the development of a more robust understanding of the microstructural features and properties of the conventional copper and nanostructured copper coatings as they relate to antipathogenic contact killing and inactivation applications. Prior work has demonstrated greater antipathogenic efficacy with respect to the nanostructured coating versus the conventional coating. Thus, microstructural analysis was performed in order to establish differences between the two coatings that their respective pathogen kill rates could be attributed to. Results from advanced laser-induced projectile impact testing, X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, scanning transmission microscopy, nanoindentation, energy-dispersive X-ray spectroscopy, nanoindentation, confocal microscopy, atomic force microscopy, linear polarization, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and copper ion release assaying were performed during the course of this research.