Shock Strengthening Research Articles

Mechanical strengthening and recovery of C/C-Mo joints during thermal cycling

In order to prepare C/C-Mo joints that possess good thermal shock resistant performance between 1100 °C and room temperature, MoNiSi compounds and Ni-based solid solutions were introduced into the interlayer. The as-prepared C/C-Mo joints display good stability and reliability at high temperatures, in particular the unique strengthening and recovery phenomenon of mechanical strength during thermal cycling. After 5 thermal cycles, the joint strength increased by 28.4%, up to 59.19 MPa, showing a thermal shock strengthening phenomenon. When further increasing the cycles, the joint strength first decreases, then begins to recover. The joint strength has recovered to a peak of 52.39 MPa after 50 cycles, which is higher than the original strength. The thermal shock strengthening is ascribed to the release of the residual thermal stress caused by the formation of the microcracks (dominated by the rapid alternating temperature factor), while the recovery phenomenon is related to the filling of the defects caused by the flowing of the phases with low melting points like Ni-based solid solutions (dominated by the thermal treatment factor). This work provides a strategy to obtain the joints with good tolerance to multiple thermal cycles and excellent thermal shock resistant performance.

Shock induced gradient microstructure with hierarchical nanotwins to enhance mechanical properties of Ti6Al4V alloy

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.

An energy-efficient method of laser remanufacturing process

With the global energy shortages and the increasing concerns of environmental protection in various countries, energy consumption optimization in laser remanufacturing has attracted much attention. To address the problems of large energy consumption and high cost in the laser remanufacturing process, the energy consumption characteristics and mechanism of the laser remanufacturing process are analyzed in this work. Then an energy consumption model of the laser remanufacturing process is established. Moreover, an optimization model of laser remanufacturing process parameters with objectives of energy consumption, powder utilization rate, hardness and aspect ratio is formulated. A novel non-dominated sorting genetic algorithm-II (NSGA-II) based on adaptive crossover probability and multi-crossover operators is proposed to solve the model. In this way, the optimal process parameters are obtained by the improved Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method. Finally, some remanufacturing experiments are carried out on the LDM4030 laser remanufacturing equipment. The results show that the model and algorithm presented in this work can effectively reduce energy consumption, improve powder utilization rate and ensure the quality of cladding. The research results of this work can provide an effective way for an energy-efficient laser remanufacturing process and give reference to the energy consumption optimization of other laser surface engineering technologies, such as laser shock strengthening and laser cleaning.

Understanding the role of monolayer graphene during long range shock strengthening of metal-graphene heterostructure

Graphene has been extensively used in graphene reinforced metal composite, with strengthening effects due to its ability to block dislocation from propagation during the traditional low strain rate deformation process. Laser shock peening has been applied in graphene/metal composites with graphene concentration of 2–10% with a relatively close distance between the graphene layers. This work discovered a strong long-range coupling effects of graphene as a super-strong nanomaterial and shock-wave transmitter during laser shock processing under room temperature LSP and cryogenic temperature (cLSP), under extremely low graphene concentration (1.42 × 10−6% vol.). Compared with simple compressed graphene-copper heterostructure, the yield strength of LSP and cLSP processed samples increases by 40%, and 76% respectively. We found that under laser shock peening (LSP) process, the shock wave can pass through long-distance to generate dislocation transportation from one layer to another graphene with the shock wave interaction between graphene layers separated very far away. Graphene plays an important role not only as a transmitter of shock waves, but also as a strong wall to bounce back shock waves to generate high dislocation density around graphene layers. We have designed experiments to compare the deformation behavior of the laminates under three deformation conditions: compression, LSP, and cLSP, respectively. It was found that the compressed sample has very few parallel dislocation arrays beneath the graphene interface, indicating that graphene blocks the dislocation movement and has very limited strengthening effects. The LSP processed samples contain much high dislocation density, while even higher density dislocation and strength are found in cLSP due to faster shock transportation in graphene/metal layers under cryogenic conditions. Finite element modeling was used to investigate the shock wave interaction with the graphene and metal layer under various conditions, which is consistent with experiments. Molecular dynamics simulation is used to simulate the microstructure of the laminates under various conditions and validated by experiments. This work provides a starting point to understand the long-range strengthening effects of 2D nanomaterials of extremely low concentrations and provide new design strategies for manufacturing graphene-metal nanocomposite and their strengthening approaches.

A theoretical approach for transient shock strengthening in high-energy-density laser compression experiments

In high-energy-density shock compression experiments, the desired state of compression is typically achieved by passing shock waves through a sequence of different materials. In this study, a theoretical approach for transiently strengthening such shocks passing through interfaces of experimentally relevant impedance ratios is examined. A semi-analytical method based on characteristics analysis is used to solve the problem of a shock passing from one material to another through an intermediate region of non-uniform impedance; by appropriately designing this region, a greater shock strength can be achieved in the second material for a finite duration than in the absence of this region. When a shock passes into a material of higher impedance, the shock strengthening increases to a maximum before decreasing to an asymptotic value as the strength of the incident shock is increased. For shocks passing into materials of decreasing impedance, the shock strengthening increases monotonically as the strength of the incident shock is increased when the impedance ratio is above a critical threshold. Incorporating multiple intermediate materials can further increase the strength of the transmitted shock, with an exponential discretization of intermediate material impedances being the most effective distribution for strengthening strong shocks. The results suggest that up to 25% and 9% increases in pressure behind the leading shock can be achieved for materials of increasing and decreasing impedance, respectively. The technique is applied to the design of laser-driven dynamic compression experiments, and the results of the analysis are verified via comparison to simulations performed with the HYADES hydrodynamics code.

Converging near-elliptic shock waves

Abstract

Effect of Laser Shock Strengthening on Modal Frequency

The phenomenon of laser shock strengthening (LSP) changing the resonance frequency of a structure can be used to adjust the frequency of aero-engine blades, and has a broad application prospect. In this paper, through the combination of mechanical analysis, numerical simulation and vibration test, the effect of LSP technology on structural vibration characteristics is studied. The influence mechanism of local stiffness changes on the modal frequency of the structure is analyzed from the perspective of vibration and energy. It is found that when the laser shock is strengthened in a region with large bending deformation, the greater the contribution to the overall modal stiffness and elastic potential energy, the greater the modal frequency. The impact is also greater. Established a mapping relationship among laser shock strengthening, local stiffness and modal frequency, and finally proved that LSP technology can significantly change the local elastic modulus of TC4 titanium alloy, which provides a reliable application of the technology in the production department Theoretical support.

Effect of initial surface roughness on the actual intensity of laser shock processing

We prepared samples with different surface roughnesses, which were subjected to laser shock treatment using the same laser parameters. We then investigated the effect of initial surface roughness on the actual intensity of laser shock. The results show that, after laser shock treatment, the lower the initial surface roughness of the target, the higher the degree of plastic deformation of the material. A smoother target surface allowed laser-induced shock waves to introduce a higher residual compressive stress on the material’s surface. The proposed reason for the above phenomenon is the change in the absorption per unit area of the laser energy by materials of different surface states. Thus, these results provide a different method for improving the actual impact intensity of laser shock. In fact, laser shock can be a highly feasible and economical method to achieve a more obvious laser shock strengthening effect by reducing the initial surface roughness of the target surface.

Research on Formation Mechanism of Dynamic Response and Residual Stress of Sheet Metal Induced by Laser Shock Wave

In order to reveal the quantitative control of the residual stress on the surface of metal materials, the relevant theoretical and experimental studies were carried out to investigate the dynamic response of metal thin plates and the formation mechanism of residual stress induced by laser shock wave. In this paper, the latest research trends on the surface residual stress of laser shock processing technology were elaborated. The main progress of laser shock wave propagation mechanism and dynamic response, laser shock, and surface residual stress were discussed. It is pointed out that the multi-scale characterization of laser and material, surface residual stress and microstructure change is a new hotspot in laser shock strengthening technology.

MacCormack’s technique-based pressure reconstruction approach for PIV data in compressible flows with shocks

This paper proposes an improved approach for extraction of pressure fields from velocity data, such as obtained by particle image velocimetry (PIV), especially for steady compressible flows with strong shocks. The principle of this approach is derived from Navier–Stokes equations, assuming adiabatic condition and neglecting viscosity of flow field boundaries measured by PIV. The computing method is based on MacCormack’s technique in computational fluid dynamics. Thus, this approach is called the MacCormack method. Moreover, the MacCormack method is compared with several approaches proposed in previous literature, including the isentropic method, the spatial integration and the Poisson method. The effects of velocity error level and PIV spatial resolution on these approaches are also quantified by using artificial velocity data containing shock waves. The results demonstrate that the MacCormack method has higher reconstruction accuracy than other approaches, and its advantages become more remarkable with shock strengthening. Furthermore, the performance of the MacCormack method is also validated by using synthetic PIV images with an oblique shock wave, confirming the feasibility and advantage of this approach in real PIV experiments. This work is highly significant for the studies on aerospace engineering, especially the outer flow fields of supersonic aircraft and the internal flow fields of ramjets.

Converging cylindrical shocks in ideal magnetohydrodynamics

We consider a cylindrically symmetrical shock converging onto an axis within the framework of ideal, compressible-gas non-dissipative magnetohydrodynamics (MHD). In cylindrical polar co-ordinates we restrict attention to either constant axial magnetic field or to the azimuthal but singular magnetic field produced by a line current on the axis. Under the constraint of zero normal magnetic field and zero tangential fluid speed at the shock, a set of restricted shock-jump conditions are obtained as functions of the shock Mach number, defined as the ratio of the local shock speed to the unique magnetohydrodynamic wave speed ahead of the shock, and also of a parameter measuring the local strength of the magnetic field. For the line current case, two approaches are explored and the results compared in detail. The first is geometrical shock-dynamics where the restricted shock-jump conditions are applied directly to the equation on the characteristic entering the shock from behind. This gives an ordinary-differential equation for the shock Mach number as a function of radius which is integrated numerically to provide profiles of the shock implosion. Also, analytic, asymptotic results are obtained for the shock trajectory at small radius. The second approach is direct numerical solution of the radially symmetric MHD equations using a shock-capturing method. For the axial magnetic field case the shock implosion is of the Guderley power-law type with exponent that is not affected by the presence of a finite magnetic field. For the axial current case, however, the presence of a tangential magnetic field ahead of the shock with strength inversely proportional to radius introduces a length scale \documentclass[12pt]{minimal}\begin{document}$R=\sqrt{\mu _0/p_0}\,I/(2\,\pi )$\end{document}R=μ0/p0I/(2π) where I is the current, μ0 is the permeability, and p0 is the pressure ahead of the shock. For shocks initiated at r ≫ R, shock convergence is first accompanied by shock strengthening as for the strictly gas-dynamic implosion. The diverging magnetic field then slows the shock Mach number growth producing a maximum followed by monotonic reduction towards magnetosonic conditions, even as the shock accelerates toward the axis. A parameter space of initial shock Mach number at a given radius is explored and the implications of the present results for inertial confinement fusion are discussed.

Dislocation configuration evolution of FCC alloys induced by Laser Shock Processing

To investigate the relations between dislocation motion and microstructure evolution of FCC metals during Laser Shock Processing (LSP), and then improve the understanding of the mechanism of laser shock strengthening, aluminium alloy 2A02 and austenitic stainless steel 201 were selected as the representatives, and laser shocked by the Nd:YAG laser with 1064 nm output wavelength and 20 ns short pulse. The microstructure evolution of test materials induced by laser shock was analysed via Transmission Electronic Microscope (TEM) and Inverse Fast Fourier Transformation (IFFT), and the dislocation configurations and their function were discussed. The experimental results indicated that the increase in both the surface hardness and the residual compressive stress of the laser-shocked test materials can be attributed to the contribution of complex dislocation configurations and nano-crystallisation.

Simulation on Laser Shock Strengthening of the Crankshaft

The residual stress distribution of fillet of the diesel engine crankshaft on laser shock processing was analyzed by ANSYS software. The simulation results compared with the experimental data, to get results as follows: the simulation results were basically consistent with the experimental data. This fully proved that the finite element simulation method and related simulation parameters were correct. The parameters of laser impacting were optimized by ANSYS simulation, and the residual stress favorable distribution of the crankshaft fillet was obtained.

AZ31B镁合金激光冲击强化及抗应力腐蚀研究

AZ31B镁合金激光冲击强化及抗应力腐蚀研究

A study on super-speed forming of metal sheet by laser shock waves

In this paper, the authors explore a dynamic super-speed forming method driven by laser shock waves. The initial exploration of laser shock forming (LSF) is done through bulge testing with specimens of 430 sheet metal, using a neodymium–glass laser of pulse energy 10–30 J and duration 20 ns (FWHM). The investigation revealed that the plastic deformation during the LSF is characterized as ultra-high strain rate, and it is indicated that the plastic deformation increases non-linearly with the increase of the energy density of the laser. By investigating the hardness and residual stress of the surfaces, it is conclude that LSF is a combination technique of laser shock strengthening and metal forming for introducing strain hardening and a compressive residual stress on the surface of the workpiece. This technique can achieve forming with or without a mold.

Laser shock forming on coated metal sheets characterized by ultrahigh-strain-rate plastic deformation

We report on a dynamic ultrahigh-strain-rate forming method driven by laser impact. This investigation reveals that laser shock forming is a mechanical, not a thermal process, and the strain rate can go up to 107–109 s−1, two or more orders higher than that of all the existing forming methods. By investigating the hardness and residual stress of the surfaces, we conclude that laser shock forming is a technique with combining laser shock strengthening and metal forming, which introduces strain hardening and compressive residual stresses. We also discover some nonlinear plastic deformation characters in laser shock forming.

Radio Signatures of Coronal Mass Ejection Interaction: Coronal Mass Ejection Cannibalism?

We report the first detection at long radio wavelengths of interaction between coronal mass ejections (CMEs) in the interplanetary medium. The radio signature is in the form of intense continuum-like radio emission following an interplanetary type II burst. At the time of the radio enhancement, coronagraphic images show a fast CME overtaking a slow CME. We interpret the radio enhancement as a consequence of shock strengthening when the shock ahead of the fast CME plows through the core of the preceding slow CME. The duration of the radio enhancement is consistent with the transit time of the CME-driven shock through the core of the slow CME. As a consequence of the interaction, the core of the slow CME changed its trajectory significantly. Based on the emission characteristics of the radio enhancement, we estimate the density of the core of the slow CME to be ~4 × 104 cm-3. The CME interaction has important implications for space weather prediction based on halo CMEs: some of the false alarms could be accounted for by CME interactions. The observed CME interaction could also explain some of the complex ejecta at 1 AU, which have unusual composition.

A metallographic and quantitative analysis of the influence of stacking fault energy on shock-hardening in Cu and Cu–Al alloys

This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.

Water shock wave emanated from the roughened end surface of an optical fiber by pulse laser input

Abstract We have discovered an interesting effect of surface roughness of optical fiber on the generated shock pressure pulse in the case of an intense laser pulse input. The phenomenon of shock strengthening by roughening the end surface of the optical fiber has been found. Water shock waves emanated from the fiber end surface have been evidenced by the laser shadowgraph technique. Other interesting features are the use of aluminium coating to enhance the shock pressure, repeated but finite lifetime of the effect of the surface roughness, bubble formation, etc. The detailed physics of these phenomena is unknown.

Laser shock processing of materials, physical processes involved and examples of applications

Laser shock processing (LSP) consists of irradiating a metallic target with a short (about 20 ns) and intense (>1013W m−2) laser light in order to generate, through a high pressure surface plasma (>1 GPa), a plastic deformation and a surface strengthening within materials. This paper initially reviews the physical processes involved in the analytical modeling of the generation pressure mechanism in a confined plasma regime. Limiting factors such as the dielectric breakdown in the confining medium are also discussed together with current research directions aimed at improving the laser—material coupling such as using short rise time pulses instead of Gaussian ones or shorter wavelengths than the traditional λ = 1.06 μm. Surface mechanical effects are also theoretically and experimentally presented. They consist mainly of compressive residual stresses generated in the first 1–2 mm of depth that are the key to enhanced mechanical properties. The application of LSP to two new areas is presented. These areas are the improvement of the fatigue behavior in aluminum alloys and a thermomechanical treatment using both a CO2 laser for surface quenching and a pulsed Nd–glass laser for mechanical shock strengthening. Finally the attractive and detrimental features of LSP are discussed with the objective of defining an industrially convenient laser shock configuration.