Laser Dynamic Forming Research Articles

Reducing the Rebound Effect in Microscale Laser Dynamic Forming through Multi-Pulse Laser Shock Loading

Microscale laser dynamic forming, as a novel high-speed microforming technique, can overcome the shortcomings of traditional microforming methods. However, in practical applications, laser dynamic microforming technology is often affected by the rebound behavior of the workpiece, limiting the further improvement of processing quality and efficiency. This paper aims to reduce the rebound effect in laser dynamic forming by using multi-pulse laser shock loading. The forming results of workpieces under different laser energies and laser impact numbers were studied using experimental and numerical simulation methods. After multiple laser shocks, numerical simulations of the forming results were conducted using ANSYS/LS-DYNA software. These numerical simulation results were then experimentally validated and compared. The surface morphology of the workpieces was characterized using a confocal microscope and a scanning electron microscope (SEM). Energy-dispersive spectroscopy (EDS) was used to analyze the chemical element content changes in the collision regions at the bottoms of the workpieces after multi-pulse loading. The SEM and EDS results revealed the collision behavior patterns during the forming process. Finally, the forming laws of workpieces under multiple laser shocks were summarized.

Deformation and Fracture Behaviors in Microscale Laser Dynamic Flexible Forming: The Case of Fabricating Micro-Channel on Copper Foil

The case of fabricating micro-channel on copper foil using micro-scale laser dynamic flexible forming (μLDFF) is investigated in this work. To study the deformation and fracture behaviors, the copper foils are subjected to increasing laser power density until the occurrence of fracture. When laser power density ranges from 0.110 GW/cm2 to 0.441 GW/cm2, smooth micro-channel features are formed on copper foils, and the depth of the micro-channel gradually increases. Melting marks are observed at the micro-die entrance of the sample with 0.441 GW/cm2 laser power density. When laser power density is 0.544 GW/cm2, partial fractures are observed. The surface of fracture region is rougher than that of micro-cracks and no micro-cracks regions, which is caused by the higher strain in fracture region. And the fracture edge is irregular, which is caused by the soft effect of material. Complete fracture occurs when laser power density is 0.691 GW/cm2.

Experimental and numerical study on fragmentation mechanism of copper sheet in laser dynamic forming

The fragmentation mechanism of copper sheet in laser dynamic forming (LDF) process is investigated. The investigation of the fragmentation mechanism in the moving sample is quite difficult, so finite element method is adopted to provide detailed information on the stress state during micro-forming. The shock loading was generated using laser-shock-rubber loading technique in which a layer of rubber is inserted to improve laser shock efficiency. When laser power density is 0.491 GW cm−2, the obvious circular fragmentation is located at the center region of rear surface. When laser power density is 0.658 GW cm−2, the diameter of the fragmentation region increased, and what’s more, circumferential and radial cracks were formed in the fragmentation region. The void linkages and terrace-like pattern were also observed. Finite element model reveals that the fragmentation in the moving sheet is not caused by the initial rubber direct loading, but the deceleration at the last stage of forming. Because the curvature in the tip has the highest value at the stopping point, deceleration passes through its maximum value and then causes fragmentation at the last stage of forming. When laser power density is 0.658 GW cm−2, the single layer fragmentation, multiple layer fragmentation, circumferential crack, and radial cracks occur in that sequence.

Dynamic failure mechanism of copper foil in laser dynamic flexible forming

Abstract Laser dynamic flexible forming (LDFF) is a novel high velocity forming (HVF) technology, in which the foil metal is loaded by laser shock wave. Strain localization is readily to occur around the bulge edge, which will result in the ultimate dynamic failure. In this work, the microstructures before and after dynamic fracture are characterized by transmission electron microscopy (TEM) to investigate the dynamic failure mechanism. The plastic deformation regions of copper foil are composed of shock compression, strain localization and bulge. Microstructure refinement was observed in three different plastic deformation regions, particularly, dynamic recrystallization (DRX) occurs in the strain localization and bulge regions. In bulge region, extremely thin secondary twins in the twin/matrix (T/M) lamellae are formed. The microstructure features in the strain localization region show that superplastic flow of material exists until fracture, which may be due to DRX and subsequent grain boundary sliding (GBS) of the recrystallized grains. The grain coarsening in strain localization region may degrade the material flowing ability which results in the dynamic fracture.

DEFORMATION ANALYSIS OF H62 Cu ALLOY FOIL SUBJECTED TO MULTI-PULSED LASER DYNAMIC FORMING: SIMULATIONS

By utilizing the ABAQUS software, this paper simulates and analyzes the forming of the H62 foil subjected to multi-pulse laser dynamic forming (LDF). The Johnson–Cook failure mode is adopted to predict the fracture threshold value of the H62 foil. Compared with the single-pulsed LDF, the multi-pulsed LDF improves the limit depth and forming quality of the foil effectively. With the increase of impact number, the uniformity of foil is effectively improved. Appropriate peak pressure and impact number are important to increase limit forming depth and improve forming quality.

Surface degradation of micro-mold in micro-scale laser dynamic forming and its effects on workpiece

Micro-scale laser dynamic forming (μLDF) is a kind of high velocity forming processes, which uses laser shock wave to load the sheet metal. The surface degradation of micro-mold in μLDF and its effects on workpiece are first investigated in this work. The surface degradation is the gradual failure of micro-mold due to the regular repeated laser shock loadings, and some micro-channel defects with the dimensions of several micrometers are observed on the mold surface. During μLDF process, the material of workpiece flows into the cavities of micro-defects on mold surface when the laser power density increases to 6.46 GW/cm2, and the more obvious defect-like features are formed on the rear surface of workpiece as laser power density increases. This type of single-side forming is very interesting, and similar to coining. In order to investigate the surface degradation evolution process, laser shock target experiments are performed. The surfaces of the nodular cast iron targets were irradiated by different numbers of laser pulses, and the corresponding surface morphologies were observed to investigate the surface degradation evolution process. The experimental results show that, surface degradation of micro-mold is formed by void nucleation, growth, and coalescence under the repeated shock loadings. Numerical simulations considering micro-defect on mold surface are performed to investigate the effect of micro-defect on workpiece. Numerical results also show that laser shock wave can be used to replicate micro-coining feature on metal surface.

Reducing the rebound effect in micro-scale laser dynamic flexible forming through using plasticine as pressure-carrying medium

Abstract Micro-scale laser dynamic flexible forming (μLDFF) is a new kind of high velocity forming (HVF) process, which uses laser shock wave pressure to realize thin metal sheets micro-forming. Rebound effect usually occurs when forming deep complex micro-features, which has a significant effect on the forming precision. This work firstly investigates the impact and rebound behaviors of workpiece in μLDFF with rubber medium to probe the main affecting factors of rebound effect, then proposes a new μLDFF with plasticine medium to reduce the rebound effect. With the increase of laser energy, the average surface roughness of impact-affected zone (IAZ) reduces firstly by flattening behavior, and then increases afterward due to coining-like operation. The 3D deviation analyses between workpieces and micro-die are performed to characterize the fitability. When laser energy is 675 mJ, bottom region of workpiece fits well with the micro-die. When laser energy is increased to 1020 mJ, bottom region of workpiece separates from the micro-die due to rebound effect. A finite element model considering the machining marks on micro-die surface is built, and the predicted results are validated by experimental results. Coefficient of restitution (COR) and the rebound ratio (α) are used to investigate the impact and rebound process. The results show that rebound effect is dominated by laser energy, surface morphology of micro-die, and loading duration. To effectively reduce the rebound effect, the plasticine layer is first introduced as pressure-carrying medium in this work. The deformation behavior of plasticine medium is like a Reiner-Rivlin fluid. So the plasticine medium closely contacts with the sample during forming, this is equivalent to indirectly increasing the stiffness of sample and prolonging the loading duration. In addition, the plasticine layer can absorb impact-induced reflection wave. So plasticine medium can effectively improve the forming precision.

Deformation and fracture behaviors of copper sheet in laser dynamic flexible forming

Laser dynamic flexible forming (LDFF) is a novel high velocity forming (HVF) technology, in which the high-energy laser shock waves act on the workpiece via a flexible medium. Dynamic bulge tests of copper sheet are performed to investigate the deformation and fracture behaviors in LDFF in this paper. The laser ablation states on K9 glass surfaces under different laser energies were characterized. The samples after LDFF were characterized by optical microscopy and scanning electron microscopy. For laser energy ranging from 1020 mJ to1380 mJ, as laser energy increases, the average surface roughness (Ra) in bulge region rapidly increases due to surface free forming. However, the Ra in laser shock compression region slowly decreases due to shock flattening. Strain localization occurs around the bulge edge, and the composition of oxygen element in this region is increased with laser energy. When laser energy is equal to or greater than 1690 mJ, localized deformation evolved into the dynamic failure. Apart from the elongated micro-voids, three interesting phenomena were observed: the melting marks, the serrated fracture boundaries and filaments. As the laser energy increases, the fracture mode varies from a tensile fracture mode to a shear fracture mode, then to a filament fracture mode. The filament fracture mode indicates that superplastic flow of material may exist until fracture, and oxidation behaviors make a great contribution to the superplastic flow.

Investigation of Micro-Bending of Sheet Metal Laminates by Laser-Driven Soft Punch in Warm Conditions.

Microscale laser dynamic flexible forming (µLDFF) is a novel ultrahigh strain rate manufacturing technology with high efficiency and low cost. However, the µLDFF is just confined to single-layer foil at present. In this work, sheet metal laminates (Cu/Ni) were selected as the experimental material for its excellent mechanical and functional properties, and a new micro-bending method of sheet metal laminates by laser-driven soft punch was proposed in warm conditions. The micro-mold and warm platform were designed to investigate the effects of temperature and energy on formability, which were characterized by forming accuracy, surface quality, element diffusion, and so on. The experimental results show that the forming accuracy and quality increased first and then decreased with laser energy, but the hardness increased consistently. In warm conditions, the fluidity of material was improved. The forming depth and accuracy increased for the relieved springback, and the surface quality increased first and then decreased. The tensile fracture disappeared with temperature for the decreased hardness and thinning ratio, and the element diffusion occurred. Overall, this study indicates that the formability can be improved in warm conditions and provides a basis for the investigation of micro-bending of sheet metal laminates by µLDFF in warm conditions.

Experimental and Numerical Simulation Investigation on Laser Flexible Shock Micro-Bulging

Laser flexible shock micro-bulging (LFSB) is a novel micro fabrication technology, which combines laser dynamic forming and flexible die forming, which is a type of high strain rate micro-forming. The LFSB of 304 stainless steel foils was investigated in this paper. Experimental and simulated results indicated that the bulging depth and thickness thinning rate of bulging parts increased with an increase of laser energy and a decrease of workpiece thickness. Experimental results also showed the surface morphology of bulging parts. The hardness distribution in the cross section of bulging parts was revealed by nanoindentation experiments. The internal microstructure of micro bulging parts was observed by TEM. In addition, the equivalent stress and plastic strain distribution of bulging parts were shown in the numerical simulation under different workpiece thicknesses and laser energies.

Improving the forming capability of laser dynamic forming by using rubber as a forming medium

Laser dynamic forming (LDF) is a novel high velocity forming technique, which employs laser-generated shock wave to load the sample. The forming velocity induced by the high energy laser pulse may exceed the critical forming velocity, resulting in the occurrence of premature fracture. To avoid the above premature fracture, rubber is introduced in LDF as a forming medium to prolong the loading duration in this paper. Laser induced shock wave energy is transferred to the sample in different forming stages, so the forming velocity can be kept below the critical forming velocity when the initial laser energy is high for fracture. Bulge forming experiments with and without rubber were performed to study the effect of rubber on loading duration. The experimental results show that, the shock wave energy attenuates during the propagation through the rubber layer, the rubber can avoid the premature fracture. So the plastic deformation can continue, the forming capability of LDF is improved. Due to the severe plastic deformation under rubber compression, adiabatic shear bands (ASB) occur in LDF with rubber. The material softening in ASB leads to the irregular fracture, which is different from the premature fracture pattern (regular fracture) in LDF without rubber. To better understand this deformation behavior, Johnson–Cook model is used to simulate the dynamic response and the evolution of ASB of copper sample. The simulation results also indicate the rubber can prolong the loading duration.

Size effects on formability in microscale laser dynamic forming of copper foil

Unlike quasistatic loading of traditional microforming technology, microscale laser dynamic forming (μLDF) provides an efficient approach to fabricate microparts with enhanced mechanical properties. However, the size effects phenomenon is still inevitable in high strain rate microforming. It is thus necessary to investigate the size effects and dynamic deformation behaviour in μLDF. In this work, four different microchannels using annealed copper foils with four different grain sizes are manufactured simultaneously to investigate the size effects on formability in μLDF. The ratio of sheet thickness to grain size (N=t/d) is used to represent the interactive effects of grain size and specimen size, whereas the ratio of channel width to sheet thickness (M=W/t) is employed to characterize the feature size effect. Experimental results show that the N value and M value have an interactive effect on the normalized forming depth and the surface roughness value. When the M value decreases to the critical point of 4.5, the normalized forming depth is dramatically decreased and greatly influenced by the N value. Furthermore, the formability characterized by surface quality, thickness reduction and forming accuracy declines with the decrease of N value. The irregular thickness distribution and the inhomogeneous material flow of coarse-grained microchannel are attributed to the anisotropy and uneven grain distribution of microstructure. Despite the decreased formability, the coarse-grained microchannel with high necking ratio is still fabricated without failure and experiences more enhanced hardening effect than the fine-grained specimen, presenting the superplastic deformation behaviour in μLDF. When laser energy increases, the surface quality of the formed parts is improved by the intensive high speed sliding of asperities on the micro-die. The surface roughness value of the coarse-grained specimen is significantly reduced and therefore, the localized necking can be suppressed.

Forming Properties of a Microscale Laser Dynamic Flexible Forming Technique

Microscale laser dynamic flexible forming (µLDFF) is a novel micro-forming process. Laser-induced shockwaves act on the soft punch to deform the metal foils. In this article, the forming properties of the µLDFF process are studied from four angles, namely, the maximum deformation depth, accuracy, thickness thinning ratio, and surface quality. The maximum deformation depth of the samples formed under different laser energies was measured. The 2D profiles of the workpieces were measured to study the accuracy. The results show that the deformed samples replicated the mold features well. The cold-mounted technique was used to measure the thickness of the deformed metal foils along the cross section and to discuss the thickness thinning ratio. The results show that µLDFF can reduce localized necking and stress concentrations effectively. The surface roughness was characterized to study the surface quality of the workpieces, and the results indicated that the deterioration of the formed surface was weakened during µLDFF. The advantages of the ultrahigh strain rate and soft punch are both presented in the µLDFF process, which is favorable for fabricating micro components.

Direct Integration of Functional Structures on 3-D Microscale Surfaces by Laser Dynamic Forming

This paper demonstrates the scalable and fast-shaping top-down integration capability of laser dynamic forming (LDF), transferring functional structures conformal to three-dimensional (3-D) micro-to-mesoscale curvilinear features on various substrates by the laser-induced shockwave. The functional materials preserve their electrical resistance and temperature coefficient of resistance after the laser shock induced transfer. The ductile interconnections inherit 3-D microscale structures on various substrates without excessive necking and fracture. This process is realized by the lamination of functional materials with cushion layers and the shockwave controlled by laser pulse intensity. The ability of direct transfer is affected by the laser intensity, cushion layer thickness, and geometry of the 3-D substrates. The experiments and numerical study reveal that the cushion layer absorbs most of shockwave energy by large thickness reduction and extends the formability of ductile interconnections. Eventually, the thickness of ductile functional materials is distributed uniformly along the 3-D surfaces. The ranges of the processing conditions for direct integration of functional materials without property degradation are also investigated.

Investigation of microscale laser dynamic flexible forming process—simulation and experiments

This paper presents a novel microscale laser dynamic flexible forming (μLDFF) technique on plastic forming of thin metal sheets. This process utilizes flexible rubber material as a soft punch, under the pressure generated by laser-induced shock wave, to act on the thin metal sheet. This novel technique has many new features and the forming mechanism is complex, as well as it takes place in the scale of nanoseconds, which is too short to monitor the deformation behaviors during the process. In order to reduce the experiment times and cost, this paper firstly investigates the feasibility of the process numerically using finite element software ANSYS/LSDYNA, and some key process parameters, such as hardness and thickness of soft punch, thickness of thin metal sheet, aspect ratio of rigid die, as well as laser pulse energy are detailed studied to obtain reasonable parameters. Then a series of experiments based on these reasonable parameters are conducted, and good experimental results are obtained, which verifies the feasibility of the use of simulation to guide the experiment. The results of both numerical simulations and experiments show that the use of combination of soft punch and thin metal sheet can increase the deformation depth in comparison to focusing the beam directly onto thin metal sheet, which is due to the impedance mismatch at the soft punch–thin metal sheet interface.

3D microscale laser dynamic forming: Multiscale modeling and experimental validation

Microscale laser dynamic forming (μLDF) shows great potential in fabricating robust and high-aspect-ratio metallic microcomponents. Experiments revealed that strain rate and sample size play important roles in determining the dynamic plasticity and final results of μLDF. To further understand these effects, a multiscale modeling methodology is adopted to characterize the microscale dynamic plasticity considering the evolutions of nano-to-submicron dislocations avalanches under shock loading. In this methodology, 3D discrete dislocation dynamics simulations are implemented to derive the yield strength and the initial strain hardening dependence on size and strain rate. It is observed that there exist three dynamic stages during deformation process. The initial strain hardening rate in Stage II increases with strain rate. The mechanical threshold stress model, intrinsically equipped with strain-rate-dependent flow stress and initial hardening, is chosen and modified to incorporate size effect quantitatively. This scale-dependent model, implemented in abaqus/explicit, provides deformation depths and thickness variations in good agreement with experimental results in μLDF.

Effects of Temperature on Laser Shock Induced Plastic Deformation: The Case of Copper

Laser shock induced plastic deformation has been used widely, such as laser shock peening (LSP), laser dynamic forming (LDF), and laser peen forming. These processes have been extensively studied both numerically and experimentally at room temperature. Recently, it is found that at elevated temperature, laser shock induced plastic deformation can generate better formability in LDF and enhanced mechanical properties in LSP. For example, warm laser shock peening leads to improved residual stress stability and better fatigue performance in aluminum alloys. There is a need to investigate the effects of elevated temperature on deformation behavior of metallic materials during shock induced high strain rate deformation. In this study, LSP of copper are selected to systematically study the effects of elevated temperature in shock induced high strain rate deformation. Finite element modeling (FEM) is used to predict the deformation behavior. The FEM simulation results of surface profile and residual stress distribution after LSP are validated by experimental results. The validated FEM simulation is used to study the effects of temperature on the plastic deformation behaviors during LSP, such as plastic affected zone, stress/strain distribution, and energy absorption.

Forming Limit and Fracture Mode of Microscale Laser Dynamic Forming

The microscale laser dynamic forming (LDF) process is a high strain rate microfabrication technique, which uses a pulse laser to generate high pressure by vaporizing and ionizing an ablative coating, and thus produces complex 3D microstructures in thin foils. One of the most important features of this technique is ultrahigh strain rate (typically 106–7 s−1), which is theoretically favorable for increasing formability. However, due to the lack of measurement techniques in microscale and submicroscale, the formability of workpieces in LDF is hardly studied. In this article, experiments were carried out on aluminum foils to study the forming limits and fracture of thin films in LDF. The deformation depth was measured by an optical profilometer and the formed feature was observed using a focused ion beam and a scanning electron microscope. Meanwhile, a finite element model based on a modified Johnson–Cook constitutive model and a Johnson–Cook failure model was developed to simulate the mechanical and fracture behaviors of materials in LDF. Experimental results were used to verify the model. The verified model was used to predict the forming limit diagram of aluminum foil in LDF. The forming limit diagrams show a significant increase in formability compared with other metal forming processes.

Multiple-pulse laser dynamic forming of metallic thin films for microscale three dimensional shapes

Laser dynamic forming (LDF) is a novel high energy rate microfabrication technique, which makes use of the shock pressure induced by laser to generate dynamic high strain rate three dimensional (3D) forming of thin films. In LDF process, a high shock pressure accelerates the workpiece to a high velocity and deforms it into complex 3D shapes. The forming velocity of the workpiece imparted by a single laser pulse with high energy may exceed the critical forming velocity of the material, and thus causing it to fracture. This problem is more severe when 3D structure of large aspect ratio needs to be formed. To overcome this problem, multiple-pulse LDF is investigated in this study. The total laser energy is evenly distributed in different laser pulses to keep the forming velocity below the critical forming velocity of the material. The effects of the multiple-pulse LDF on the deformation behavior of ultrathin foils are investigated. The deformation depth and thickness variation distribution of the formed 3D features are characterized to reveal these effects. In addition, the effects of vacuum conditions on multiple-pulse LDF process are carried out. It is found that the bounce off of the foil can be effectively reduced by multiple-pulse LDF and the final shape could be controlled much more accurately. By extending single pulse LDF to multiple-pulse LDF, the forming capability of LDF is further enhanced, and thus enlarges the applicable range of this technique.

Laser-Induced High-Strain-Rate Superplastic 3-D Microforming of Metallic Thin Films

Microforming of metals has always been a challenge because of the limited formability of metals at the microscale. This paper investigates an innovative microforming technique: microscale laser dynamic forming (¿LDF), which induces 3-D superplastic forming in metallic thin films. This forming process proceeds in a sequence of laser irradiation and ionization of ablative coating, shockwave generation and propagation in metallic thin films, and conformation of metallic thin films to the shape of microscale molds. Because the deformation proceeds at ultrahigh strain rates, it is found that materials experience superplastic deformation at the microscale. In this paper, experiments are systematically carried out to understand the deformation characteristics of ¿LDF. The topologies and dimensions of the deformed samples are characterized by scanning electron microscopy and optical profilometry. The thickness variations are characterized by slicing the cross section of the deformed material using the focused ion beam. The magnitude of deformation depth in ¿LDF is determined primarily by three critical factors: film thickness, mold geometry, and laser intensity. The relationships between these factors are explored in process maps to find suitable processing conditions for ¿LDF. Nanoindentation tests are conducted to show that the strength of the thin films is increased significantly after ¿LDF.