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In the field of ultrasonic-assisted micro-forming, in addition to acoustic softening, impact effects also play a significant role, especially in terms of influencing the deformation behavior of surfaces, such as by generating more deformation on surface asperity. In this study, to understand the mechanisms involved in the effect of an impact, ultrasonic-assisted micro-forging tests were conducted on commercially pure copper, pure aluminum, and pure titanium. A method that can measure the increment in the temperature during ultrasonic vibration was developed. As a result, changes in the surface temperature of the material under the impact effect and acoustic softening were measured. It is indicated that, during ultrasonic vibration, the heat generated through acoustic softening is very limited and the main heat increase occurs after the impact effect. Once the impact effect occurs, the surface temperature increases with increasing amplitude. Nevertheless, for materials with different crystal structures, the influences of the impact effect are also different. The surfaces of copper and aluminum soften, creating more surface deformation, but the exact opposite effect is seen on a titanium surface. Observing the evolution of the microstructure on the material surface with EBSD demonstrates that the impact effect on FCC materials can reach deeper below the surface in terms of temperature diffusion compared to titanium. Meanwhile, the impact effect in the case of titanium causes the regeneration of twinning, which is reduced under the influence of the acoustic softening effect, consequently resulting in strain hardening. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) particle-reinforced aluminum matrix (SiCp/Al) composites are increasingly finding applications across a range of industries due to their exceptional mechanical properties. However, these composites often face challenges in achieving optimal machinability and meeting quality standards, primarily due to the presence of SiC particles, which can hinder their full potential. This study focuses on addressing these challenges through the application of ultrasonic vibration-assisted helical grinding (UVHG) to enhance the efficiency and quality of SiCp/Al composites. To achieve the desired quality and efficiency improvements, a comprehensive methodology involving ultrasonic vibration-assisted helical grinding (UVHG) has been implemented for SiCp/Al composites. A mechanical cutting force model was then developed to accurately predict grinding forces. The grinding force was deconstructed into distinct components: friction force, plastic deformation force, and fracture force, taking into account the underlying material removal mechanisms. To better understand the grinding force associated with a single diamond abrasive grit, parameters such as undeformed chip thickness and cross-sectional area were calculated and subsequently extrapolated to the entire tool. Introducing an innovative approach, the model incorporates the concept of an acoustic softening coefficient, which quantifies the reduction in deformation stress resulting from ultrasonic vibration. This coefficient serves to correct and account for the influence of ultrasonic vibration on the properties of SiCp/Al composites. Experimental machining using the UVHG technique was conducted across various test groups. The experimental outcomes aligned closely with the predicted cutting force values, with a deviation of only 5.07%. This alignment underscores the accuracy of the proposed model and its potential as a guiding framework for optimizing the grinding process of SiCp/Al composites. The newly established predictive cutting force model, coupled with the innovative UVHG machining approach, offers promising avenues for effectively machining SiCp/Al composites on an industrial scale. This advancement not only enhances the application potential of SiCp/Al composites but also paves the way for future research and development in this field. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration (UV)-assisted technology has been widely investigated in metal forming in the past decade. However, the research on ultrasonic-assisted forming of Ti2AlNb alloy has not been reported. To study the flow behavior of Ti2AlNb alloy during ultrasonic-assisted forming and set up the constitutive model that can accurately describe its flow behavior, the room temperature compression test of Ti2AlNb alloy with ultrasonic amplitude range of 0-31 µm and strain rate of 0.001-0.125 s−1 were carried out. Considering the influence of yield strength, strain hardening and strain rate hardening effect, a modified Johnson–Cook (J–C) constitutive model is proposed, which can accurately describe the flow behavior of Ti2AlNb alloy during ultrasonic-assisted compression. The results show that under the studied deformation conditions, the yield strength and strain hardening coefficient of Ti2AlNb alloy decrease with the increase of ultrasonic the amplitude. When the amplitude increases to 31 µm. the yield strength decreases by 42%. The ultrasound provides larger dislocation motion energy, increases dislocation activity, and alleviates dislocation stacking, thus reduce the yield strength. The strain hardening exponent and strain rate hardening coefficient increase with the increase of amplitude. The results, obtained from the quantitative analysis of the optimized J–C model, show that the prediction results are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Metallic bipolar plates (BPPs) are key components in the proton-exchange membrane fuel cell (PEMFC), which can replace traditional fossil fuels as a kind of clean energy. However, these kinds of plates, characterized by micro-channels with a high ratio between depth and width, are difficult to fabricate with an ultra-thin metallic sheet. Then, ultrasonic-vibration-assisted stamping is performed considering the acoustic softening effect. Additionally, the influence of various vibration parameters on the forming quality is analyzed. The experimental results show that ultrasonic vibration can obviously increase the channel depth. Among the vibration parameters, the vibration power has the maximum influence on the depth, the vibration interval time is the second, and the vibration duration time is the last. In addition, the rolling direction will affect the channel depth. When the micro-channels are parallel to the rolling direction, the depth of a micro-channel is the largest. This means that the developed ultrasonic-vibration-assisted stamping process is helpful for improving the forming limitation of micro-channels used for the bipolar plates in PEMFC. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration-assisted deformation (UVAD) technology has been widely utilized in the metallic materials forming and manufacturing process owing to its advantages of effectively reducing deformation stress. However, the intrinsic mechanism of UVAD involved in Ti3Al intermetallics has not yet been clarified. In this research, an ultrasonic vibration tension device for Ti3Al was designed. The mechanical properties under ultrasonic vibration-assisted tension (UVAT) were investigated. Subsequently, the rheological behavior under the effect of ultrasound was analyzed. The influences of the vibration amplitudes and vibration modes on the microhardness and microstructure were explored. The results indicated that Ti3Al behaved obviously an acoustic softening after introducing ultrasonic energy, and the degree of softening was positively correlated with amplitude. There was a “competitive” relationship between the acoustic softening and acoustic residual hardening effects. The maximum stress and microhardness reductions were 206.13 MPa and 6 % when the amplitude was 6.55 μm and 1.31 μm, respectively. The fracture mode evolved from ductility dimple fracture to a mixed-mode of cleavage pattern and brittle fracture. The metallographic organization revealed a “defective phase” structure in the α2 phase and B2 matrix. The amounts of defective phases enlarged with amplitude increasing and extended away from the vicinities of the fracture cross-section.

Tangential ultrasonic vibration and vertical ultrasonic vibration are applied in the surface burnishing process (SBP) to study the improvement mechanism of both on the machining performance. The force in the machining process is collected by a Kistler force sensor. The surface roughness (Sa, Sq, Ssk, and Sku) and cross-section hardness of the machined samples are characterized. A molecular dynamic model is established to discuss the acoustic softening effect. Experimental results show that the vertical ultrasonic can reduce the normal force (Fz) value in the machining process. Meanwhile, the tangential ultrasonic vibration can suppress the volatility of the X-axis direction force (Fx) and Y-axis direction force (Fy) while reducing the tangential force (Ft) value. Compared with SBP treated samples, the two-dimensional ultrasonic vibration-assisted surface burnishing process (2D-USBP) can better improve the surface properties of the sample, such as improving the surface roughness, the surface hardness, and the thickness of the strengthening layer. Molecular dynamic results show the introduction of ultrasonic vibration to change the microstructure in the sample. Under the excitation of ultrasonic vibration, the dislocation density and potential energy are increased, and the plastic deformation force is decreased.

Ultrasonic vibration assisted machining (UVAM) is an attractive option to achieve high-quality and low-wear machining of the advanced composites. The scope of this paper is to evaluate the role of ultrasonic vibration on the microstructure and material removal mechanism for SiCp/Al composites. Firstly, an ultrasonic vibration assisted tension (UVAT) molecular dynamics (MD) simulation method for SiCp/Al composites is proposed. The simulation results verify the existence of acoustic softening effect for SiCp/Al composites under ultrasonic vibration loads. Furthermore, it is found that the acoustic softening originates from the dynamic evolution of the dislocations in the Al matrix. However, the acoustic softening is hardly mentioned in conventional finite element (FE) simulations depicting the microscopic removal mechanism of materials. In this paper, the constitutive correction method is adopted to realize it. The stress reduction of the Al matrix caused by acoustic softening is reverse-identified, and the maximum is 102 MPa. Finally, a novel FE model for UVAM of SiCp/Al composites considering acoustic softening is constructed, and the microscopic removal mechanism of SiCp/Al composites is revealed by the FE simulation and microscopic experimental results. On the one hand, the ultrasonic vibration enhances the stress relaxation of the Al matrix by reducing the dislocation density, and further enhances the deformation ability of SiCp/Al composites. On the other hand, the matrix tearing dominates the generation and propagation of shear band cracks in conventional machining (CM), while the dominant factor in UVAM is the finely broken SiC particles inside the shear band. This study enhances the understanding of the microscopic removal mechanism in UVAM for SiCp/Al composites. [Display omitted] • Acoustic softening effect in SiCp/Al composites under ultrasonic vibration is verified by MD tension simulations. • A novel FE cutting model considering acoustic softening is developed for analyzing the microscopic removal mechanism. • Ultrasonic vibration promotes plastic deformation of SiCp/Al composites by reducing dislocations in the Al matrix. • Ultrasonic vibration changes the failure form of SiC particles inside the shear zone from fracture to crushing. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration (UV) assisted plastic forming technology is an ideal way to improve the formability of titanium and its alloys at room temperature, due to the softening effect which is more effective than thermal input. Thus, UV assisted compression test of commercially pure titanium is carried out and the deformation behavior is focused in this study. For an accurate prediction of the flow behavior and further application in UV forming process, the original Johnson–Cook (JC) model and its modified formulae have been adopted to construct the constitutive relation of pure titanium deformed with high-frequency oscillation. The yield stress reduction is related to both the amplitude and its square. The other material constants in JC model are established as the function of the amplitude. The comparative analysis of the predicted stress through the built JC model with the measured one is performed, showing the improved JC model can precisely describe the stress-strain relation in a large deformation range at different vibration amplitudes and strain rates.

Ultrasonic vibration has been widely utilized in the forming and processing of metallic materials due to its advantages of reducing forming force and improving deformation ability and surface quality. However, the effect of ultrasonic vibration on the deformation mechanism of metallic material is still unclear. Based on the method of segmented resonant design, an ultrasonic vibration tensile device was developed, and its vibration performance was evaluated in this work. Furthermore, the ultrasonic vibration tensile tests were carried out to investigate the influence of ultrasonic vibration parameters on the properties of the 6061-T6 Al material. The results showed that the design error of 3.3% and ultrasonic vibration amplitude of 8.7 μm were achieved. Ultrasonic vibration reduced the stress required for 6061-T6 Al material deformation, but did not change Young's modulus and strain hardening rate. The stress reduction was proportional to the square of the ultrasonic vibration amplitude, indicating that ultrasonic softening was attributed to the changed dislocation of 6061-T6 Al material. Meanwhile, the ultrasonic vibration has no permanent effects on the tensile behavior of 6061-T6 Al material. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration is a promising assistant technology to improve the microforming processes, for example, decreasing friction, enhancing surface finishing, and reducing forming load/stress. However, the mechanism behind the forming stress reduction due to ultrasonic vibration, so-called acoustic softening, is still not totally understood, because in most previous research studies, a simplified force sensing technology, which could not measure the real dynamic ultrasonic force during deformation, was used. In order to solve this issue, an ultrasound-assisted microcompression test system with a die-embedded dynamic force sensor was developed for studying the evolution of acoustic softening. With this system, the stress reduction by acoustic softening can be obtained by separating the stress reduction by stress superposition, which is just an apparent stress reduction due to the averaging of the dynamic oscillatory stress. Then, the stress reduction by stress superposition is verified using a theoretical model, confirming the reliability of the test system. Thus, the evolution of material deformation characteristics can be analyzed. By comparing the stress superposition to acoustic softening, it is found that stress superposition is greater than acoustic softening at the beginning of deformation, especially with a smaller ultrasonic amplitude. Even as the ratio of stress superposition to acoustic softening gradually decreases to some extent with increasing strain, stress superposition still accounts for nearly half of the total stress reduction. The results emphasize the importance of the dynamic force sensing technology in ultrasound-assisted microforming and provide some instructive understanding of the mechanism of acoustic softening. [ABSTRACT FROM AUTHOR]

The application of high-frequency vibration on the incremental forming process could cause changes in the plasticity of material which may contribute to the reduction of forming force, the increase of formability, and the improvement of surface finish. The present work aims to deepen the understanding of the softening effect to facilitate the accurate prediction of the ultrasonic-assisted forming process. First, a theoretical model describing the relationship between the stress and strain during the ultrasonic-assisted incremental sheet forming (UISF) was established based on the theory of crystal plasticity. In particular, the acoustic softening effect was reflected by adjusting the thermal activation process and the dislocation density evolution process. Then, the constitutive model parameters were identified through the back propagation (BP) neural network based on the experimental results. In addition, the developed model was used to simulate the UISF process by ANSYS/LS-DYNA software, and the effect of ultrasonic vibration on the deformation behavior was revealed. The results show that the FE model with the modified constitutive model considering the softening effect can improve the prediction accuracy. [ABSTRACT FROM AUTHOR]

• Ultrasonic vibration has both softening and hardening effects on materials. • Ultrasonic vibration has little effect on the coefficient of strain-rate hardening. • The new constitutive model is accurate and the maximum error of which is 5.71%. Establishing an accurate constitutive relation in ultrasonic vibration assisted metal forming, can provides a reliable theoretical basis for analyzing the mechanism of the ultrasonic vibration on materials. A constitutive model of 6063 aluminum alloy under the ultrasonic vibration upsetting at room temperature was constructed based on Johnson-Cook Model and experimental results. The influence of amplitude and frequency on the yield strength, hardening coefficient and exponent were analyzed quantitatively. Results showed that the yield strength was reduced due to the softening effect induced by the ultrasonic vibration. The maximum decreasing amount was 68.8% when imposing the maximum ultrasonic energy in this study. The hardening coefficient and exponent increased by 10.9% and 16.6% in maximum, respectively. However, the ultrasonic vibration has little impact on the strain rate hardening. The modified Johnson-Cook constitutive model under the ultrasonic vibration upsetting was established and was in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

• The ductility of Inconel 718 ultrathin sheet increases notably with assistance of ultrasonic vibration. • The softening effect is unapparent due to the distinct properties of superalloy. • The grain misorientations are significantly changed by the ultrasonic vibration. • A hybrid constitutive model is developed considering stress superposition and acoustic softening separately. Ultrasonic vibration (UV) has been widely applied in metal forming process due to its ability to improve the material properties. However, the mechanical response of difficult-to-deform materials with the assistance of UV is unclear, especially for the ultrathin sheet metals. Meanwhile, the current constitutive models are deficient for the description of UV assisted deformation behavior of the ultrathin material. In this research, UV assisted tensile tests with different amplitudes were performed on the ultrathin superalloy sheet, and the influences on the mechanical characteristic and microstructural evolution were explored. The experimental results indicated that the UV induced softening effect is not evident for high strength material, which is attributed to the tensile deformation characteristic of ultrathin sheet and the distinct mechanical property of superalloy. Nevertheless, the ductility of the superalloy sheet is notably enhanced under the ultrasonic amplitude of 3.18 μm, featured with the increase of elongation from 20.7% to 25.5%. Moreover, the texture is enhanced with increasing ultrasonic amplitude, and the grain misorientations are significantly changed under different UV conditions. To accurately describe the constitutive behavior of superalloy under the assistance of UV, a hybrid constitutive model separately considering stress superposition and acoustic softening effect was developed based on the dislocation evolution theory. The predicted results are in good agreement with the experimental data. These findings provide a more in-depth understanding on the mechanism of UV assisted forming and promote the application of UV assisted manufacturing processes for difficult-to-deform alloys. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The process of micro-coining has been widely applied in micro-forming as it is easy to realize the positioning and assembling of micro-parts. In this study, ultrasonic vibration-assisted micro-forming system was established whereby conventional coining (C-coining) and ultrasonic vibration-assisted coining (US-coining) of micro-cylinder of pure copper were carried out. In case of C-coining, the height-to-diameter ratio of the formed micro-cylinder essentially remained unchanged when the grain size and forming load were kept constant. There is an optimum height of the specimen was noted to obtain the highest micro-cylinder by keeping other conditions remain the same. On the other hand, in case of US-coining, the height of micro-cylinder increased after applying ultrasonic vibration under different grain sizes and diameters of micro-cylinder. The rate of growth in the height of micro-cylinder is directly proportional to the ultrasonic amplitude under different grain sizes. The application of ultrasonic vibration in US-coining has a great impact in the micro-coining of micro-cylinder when the ratio of the height of specimen to the diameter of micro-hole was less than 2, and the largest rate of growth is more than three times as compared to the process of C-coining. [ABSTRACT FROM AUTHOR]

The deformation capacity of foils deteriorates rapidly due to the appearance of size effects. In this study, the mechanism of ultrasonic vibration affecting the tensile deformation behavior of pure titanium foil was investigated using the developed ultrasonic-assisted tensile forming platform. The results showed that the energy of ultrasonic vibration during foil tensile deformation was mainly consumed by acoustic softening and mechanical vibration, and the energy consumed by acoustic softening was less than 20% of the total energy. When the percentage of grain boundary region is less than the critical value, increasing the ultrasonic amplitude results in a slight increase in the percentage of acoustic softening energy, but a sharp decrease in elongation. The digital image correlation results showed that the acoustic softening effect promoted the dispersion of local concentrated deformation of foils, resulting in more uniform deformation. Mechanical vibration could accelerate the formation and propagation of microcracks, which makes the fracture morphology of foils exhibit brittle fracture characteristics. Foil grain refinement is a useful method to enhance the acoustic softening effect during the tensile deformation process of foils. This work provides a basis for investigating the mechanism of ultrasonic vibration affecting the tensile deformation behavior of foils. [Display omitted] • The energy of ultrasonic vibration in the foils tensile process was mainly consumed by acoustic softening and mechanical vibration. • Ultrasonic amplitude hardly affects the absorption efficiency of acoustic softening energy when the grain boundary region proportion is small. • Acoustic softening can promote the dispersion of local concentrated deformation of foils for more uniform deformation. • Mechanical vibration can accelerate the formation and propagation of micro-cracks, resulting in the brittle fracture characteristics of fracture morphology. • Grain refinement is a useful method to enhance the acoustic softening effect during the tensile deformation process of foils. [ABSTRACT FROM AUTHOR]

Conventional high power ultrasonic vibration has been widely used to improve manufacturing processes like surface treatment and metal forming. Ultrasonic vibration affects material properties, leading to a flow stress reduction, which is called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition due to oscillatory stress, acoustic softening by easier dislocation motion and dynamic impact leading to extra surface plastic deformation. However, most researches ignored the stress superposition for the convenience of measurement, and few studies considered ultrasonic dynamic impact since the relatively low ultrasonic energy in macro scale. The purpose of this study is to investigate the characteristics and mechanisms of different ultrasonic volume effects in micro-forming. A 60 kHz longitudinal ultrasonic-assisted compression test system was developed and a series of ultrasonic-assisted compression tests at different amplitudes on commercially pure aluminum A1100 in micro-scale were carried out combining the surface analysis by SEM, EDX and micro-hardness test. Three different ultrasonic volume effects, stress superposition, acoustic softening and dynamic impact, were confirmed in the ultrasonic-assisted compression tests. In order to quantitatively predict stress superposition, a hybrid model for stress superposition is developed considering the elastic deformation of experimental apparatus in practice, the evolution of the modeling results fitted well with the experimental results. With low ultrasonic amplitude, stress superposition and acoustic softening occurred because vibrated punch contacted with the specimen all the time during compression. However, with higher amplitude, due to the extra surface plastic deformation by larger ultrasonic energy, forming stress was further reduced by the ultrasonic dynamic impact. A possible method to distinguish the effects of dynamic impact and acoustic softening is to analyze the waveform of the oscillatory stress in the process. In the case of ultrasonic dynamic impact effect, a higher amount of oxidation was observed on the specimen surface, which could be the result of local heating by surface plastic deformation and surface friction when the vibrated punch detached from the specimen. The findings of this study provide an instructive understanding of the underlying mechanisms of volume effects in ultrasonic-assisted micro-forming. [ABSTRACT FROM AUTHOR]

Ultrasonic-vibration assisted milling (UAM) process is one of the most recent advancements in the area of milling. In UAM, milling cutter vibrates with ultrasonic frequency and small amplitude during cutting. The assistance of ultrasonic-vibration during milling operation improves surface roughness, dimensional accuracy and tool life. Due to these benefits UAM finds application in precision manufacturing. Thermal aspect in machining plays a significant role in the dimensional accuracy of the products. Therefore, in this work analytical and experimental studies were carried out to model the temperature rise in workpiece during UAM process (with axial vibration). For analytical modelling, a process physics based equation was developed to predict the temperature rise in workpiece, considering acoustic softening. A setup was developed for performing UAM experiments. A statistical model was developed to estimate the temperature rise in workpiece during UAM. The predictions from physics-based and statistical models were in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Ultrasonic-vibration assisted milling (UAM) process is one of the most recent advancements in the area of milling. In axial UAM process, milling cutter is rotated and vibrated in axial direction simultaneously with high frequency and small amplitude. Experimental studies showed that superposition of axial ultrasonic vibrations in milling operation improved the process by reducing cutting forces and enhancing surface finish. This study is focused on analytical study of the UAM process with axial vibration of end milling tool. A process physics-based model was developed to evaluate the cutting forces in axial ultrasonic assisted milling process. The developed model included acoustic softening, intermittent cutting and end mill helix angle effects. For validation of the developed model an experimental setup was developed. Axial UAM of Al6063 alloy workpiece was carried out with a high speed steel end milling tool. The predicted values and trend were found in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Acoustic softening is a phenomenon where metals exhibit a lower flow stress when excited by a vibration at ultrasonic frequencies. This softening effect has been well documented in the literature, however, there are two areas that require further in-depth analysis. The first relates to the wide range of reported softening responses for the same alloy. The second relates to the use of coefficients and compensation factors when modeling the acoustic softening phenomenon in a complex forming processes. This study investigates the changes in softening response when altering specimen dimensions in ultrasonic assisted compression tests. Aluminum alloys (AA) 2024-O and AA7075-O were machined to different dimensions and compressed with ultrasonic assistance, to investigate the relationship between specimen volume and softening response. An inverse relationship between softening magnitude and sample volume was found. When relating this to acoustic energy density and stress, the values are invariant because the standard acoustic equations cannot account for changes in specimen height. The amplitude strain parameter, amplitude divided by the current specimen height, is introduced, and shown to account for changes in specimen dimensions. In addition, the softening effect diminishes relative to the state of strain. The results suggest acoustic softening is a function of the amplitude strain parameter and strain history. [ABSTRACT FROM AUTHOR]

High power ultrasonic vibration is widely used for improving manufacturing processes such as machining and metal forming. High frequency mechanical vibration affects material properties and friction forces in contacting surfaces. Flow stress reduction under superimposed ultrasonic vibration is called as acoustic softening. The amount of this parameter should be determined for ultrasonic assisted metal forming processes. For determination of this parameter for workhorse Ti-6Al-4V alloy, experimental setup was designed and fabricated. Then tensile test under longitudinal ultrasonic vibration was performed for different ultrasonic powers. Results show that ultrasonic vibration has considerable effect on plastic behavior of the alloy and decreases flow stress. Also, increasing ultrasonic power leads to higher acoustic softening. Yield stress reduction up to 9.52%, ultimate stress reduction up to 4.55% and elongation up to 13% were obtained at 340 W ultrasonic power. After applying ultrasonic vibrations and its termination, hardness of specimens were measured in which increase up to 9% was observed. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration is widely utilized in manufacturing processes mainly because acoustic field could significantly affect the metal plasticity leading to stress reduction. However, viewpoints on the influence mechanism have not reached a consensus yet. In this paper, an ultrasonic vibration assisted uniaxial tension experiment with copper foils is carried out using a specially-developed device. The results show that the extent of stress reduction increases with the increase of the vibration amplitude. Acoustic softening and stress superposition are both considered in a developed model to describe the stress reduction due to ultrasonic excitation during metal forming process. Considering that ultrasonic vibration provides the energy for dislocation sliding, acoustic softening is analyzed based on crystal plasticity theory considering ultrasonic intensity. Stress superposition, mostly induced by the additional periodic strain, is included by taking account of its proportional relationship with vibration amplitude. The calculation results from the numerical model show a good agreement with those from the experiment. These findings provide an instructive understanding of mechanism of stress reduction in ultrasonic vibration assisted metal deformation and are especially helpful for pro-actively designing ultrasonic vibration assisted metal forming processes. [ABSTRACT FROM AUTHOR]

High-frequency vibration is capable of modifying metallic surface topography significantly, while the underlying mechanisms are still unclear. In this study, the acoustic softening effect is considered to explain and model the effects of high-frequency normal vibration on surface topography. The surface asperities can be softened by the high-frequency vibration due to acoustic softening, leading to the enhancement of surface topography modification. A theoretical model for metallic surface topography modification by high-frequency vibration is proposed based on the acoustic plasticity. Numerical predictions of surface roughness evolution were conducted under various working conditions based on the model developed. It was found that the reduction of surface roughness (RSR) after vibration-assisted forming was affected by static stress, vibration amplitude, material properties and initial specimen surface roughness. The predictions using the developed model were compared with experimental data. Results showed that the predicted RSR agreed well with the experimental results, indicating that the analytical model is able to accurately capture surface topography evolution during vibration-assisted metal forming. This study provides a basis for understanding the underlying mechanisms of surface topography modification in vibration-assisted manufacturing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Based on the wave equation and the resonance theory of non-uniform material, a uniaxial tension device with ultrasonic vibration assistance for Ti 3 Al intermetallic compounds was designed. • The acoustic residual hardening induced by a small amplitude was dominant, while a large amplitude led to acoustic softening. • As ultrasonic vibration was applied, the fracture mode of Ti 3 Al evolved from the ductility dimple fracture to the mixed mode of cleavage pattern and brittle fracture. • The "defective phase" structure in the α 2 phase and B2 matrix was discovered and it extended from the vicinities of the fracture cross-section to the area away from the fracture cross-section as vibration amplitude was enlarged. Ultrasonic vibration-assisted deformation (UVAD) technology has been widely utilized in the metallic materials forming and manufacturing process owing to its advantages of effectively reducing deformation stress. However, the intrinsic mechanism of UVAD involved in Ti 3 Al intermetallics has not yet been clarified. In this research, an ultrasonic vibration tension device for Ti 3 Al was designed. The mechanical properties under ultrasonic vibration-assisted tension (UVAT) were investigated. Subsequently, the rheological behavior under the effect of ultrasound was analyzed. The influences of the vibration amplitudes and vibration modes on the microhardness and microstructure were explored. The results indicated that Ti 3 Al behaved obviously an acoustic softening after introducing ultrasonic energy, and the degree of softening was positively correlated with amplitude. There was a "competitive" relationship between the acoustic softening and acoustic residual hardening effects. The maximum stress and microhardness reductions were 206.13 MPa and 6 % when the amplitude was 6.55 μm and 1.31 μm, respectively. The fracture mode evolved from ductility dimple fracture to a mixed-mode of cleavage pattern and brittle fracture. The metallographic organization revealed a "defective phase" structure in the α 2 phase and B2 matrix. The amounts of defective phases enlarged with amplitude increasing and extended away from the vicinities of the fracture cross-section. [ABSTRACT FROM AUTHOR]

The objective of this paper is to reveal the main cause of volume effects of ultrasonic vibrations on the plastic behavior of pure aluminum specimens. For this purpose, specimens with different grain sizes were made by ECAP. An experimental tensile test system was designed and made, in which the specimens could be excited by ultrasonic vibrations with a frequency of 20 kHz and amplitude of 5 μm. Five specimens with grain sizes of 109, 38, 15, 7 and 0.97 μm were prepared. Tensile tests of the specimens were performed at room temperature and at constant speed of 0.2 mm/min under static load and superimposed ultrasonic excitations. It was found that ultrasonic vibrations had a remarkable influence on the plastic behavior of pure aluminum and after applying ultrasonic vibrations, flow stress of the all specimens reduced. Reduction of flow stress was dependent on grain size. The specimens with the largest grain size of 109 μm showed a flow stress reduction of 66% while finest grain size of 0.97 μm, a reduction of 11.3% was observed. The result of the current study can help to understand the underlying mechanisms of ultrasonic softening. [ABSTRACT FROM AUTHOR]

It is well known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. Recent experiments show that the simultaneous application of oscillatory stresses from audible to ultrasonic frequency ranges can lead to not only softening but also significant dislocation annihilation and subgrain formation in metal samples from the nano- to macro-size range. These findings indicate that the existing understanding of ultrasound softening – that the vibrations either impose additional stress waves to augment the quasi-static applied load, or cause heating of the metal, whereas the metal’s intrinsic deformation resistance or mechanism remains unaltered – is far from complete. To understand the softening and the associated enhanced subgrain formation and dislocation annihilation, a new simulator based on the dynamics of dislocation-density functions is employed. This new simulator considers the flux, production and annihilation, as well as the Taylor and elastic interactions between dislocation densities. Softening during vibrations as well as enhanced cell formation is predicted. The simulations reveal the main mechanism for subcell formation under oscillatory loadings to be the enhanced elimination of statistically stored dislocations (SSDs) by the oscillatory stress, leaving behind geometrically necessary dislocations with low Schmid factors which then form the subgrain walls. The oscillatory stress helps the depletion of the SSDs, because the chance for them to meet up and annihilate is increased with reversals of dislocation motions. This is the first simulation effort to successfully predict the cell formation phenomenon under vibratory loadings. [ABSTRACT FROM PUBLISHER]

Ultrasonic vibration assisted metal forming is a promising technology, but the microscopic mechanism of the ultrasonic effect on metal deformation has not been fully understood yet. In this paper, by molecular dynamics simulation, the microscopic mechanism behind the acoustic softening effect of ultrasonic vibration was studied. The stress superposition and acoustic softening can be simulated simultaneously by molecular dynamics method, and the stress–strain response of the polycrystalline copper simulations agrees well with the experimental results. Based on the analysis of atomic kinetic energy and dislocation density, it is found that ultrasonic vibration increases the atomic kinetic energy and promotes the generation, multiplication, motion, and annihilation of dislocations in the plastic deformation process, resulting in a reduction of dislocation tangles and pile-ups. Therefore, the hindrance to crystal sliding is significantly decreased, which promotes the plastic deformation accompanied by a macroscopic reduction in flow stress of copper. As for different ultrasonic parameters, the influence laws of ultrasonic amplitude and frequency on the deformation behaviors of coppers are different. These findings play an essential role in revealing the mechanism of ultrasonic vibration assisted plastic forming. [Display omitted] • Based on analysis of atomic kinetic energy and dislocation density, the micro-mechanism of acoustic softening was revealed. • The stress superposition and acoustic softening can be simulated simultaneously by molecular dynamics method. • The stress–strain response of the polycrystalline copper simulations agrees well with the experimental results. • The influence laws of ultrasonic amplitude and frequency on the deformation behaviors of coppers are different. [ABSTRACT FROM AUTHOR]

Highlights: [•] Acoustic softening during UC is shown to reduce the yield stress of Al 1100-0 foils by up to 82%. [•] Thermal softening during UC is relatively minor, less than 5% compared to acoustic softening. [•] Frictional heat generation is shown to be the dominant source of heat during UC. [Copyright &y& Elsevier]

Abstract: In conventional stress relieving by vibration, the residual stress reduces by induction of low frequency, high amplitude dynamic stress into an object. This method is confined to large pieces since the high amplitude of vibration deforms the thin or small objects. In this paper, a new stress relieving method based on ultrasonic vibrations is introduced which is applicable on the small or thin parts. The effectiveness of the method was verified by comparing it with thermal stress reliving. The stainless steel 316 was selected for residual stress reduction. The effects of ultrasonic vibration amplitude, relief time and preload parameters are evaluated. Acoustic softening of the metal is also evaluated since this phenomenon and dislocation activation by means of the acoustic waves are the main mechanisms behind this process. Experiments show that the residual stress of the small Almen strips can be reduced about 40% by thermal stress relief, while the ultrasonic stress relief is about 36%. It means the amount of stress reduction by ultrasonic method is comparable with thermal stress relieving. Statistical analyses of the experimental results show that the amount of stress reduction is directly proportional to the vibration amplitude and the stress relief time. The vibration frequency and the amplitude of the experiments were 24,500Hz and 23–46μm, respectively. Acoustic softening results showed that the tensile strength of the metal strongly decreases by superimposing of ultrasonic vibration in universal tensile test. The amount of this reduction is proportional to the ultrasonic intensity. Metallographic tests showed that there are no changes in grain size during the ultrasonic stress relieving process, and the only effect may be the dislocations movement. [Copyright &y& Elsevier]

Abstract: Micro/meso forming is an economically competitive process for the fabrication of miniature metallic parts. However, the size effects observed when scaling down the process leads to challenges such as tribological problems at the interface and premature tool wear due to localized stress concentration regions. In this study, a hybrid micro/meso forming assisted by high-frequency vibration and the contributing mechanisms of the high-frequency vibration were investigated. The effects of high-frequency vibration on the improvement of surface finish, decrease of the friction at the die–specimen interface, and reduction of forming stress were analyzed and discussed based on the vibration-assisted upsetting experiments. In addition, finite element analysis was conducted to help understand the significance of the vibration. Results show that a transverse vibration of 9.3kHz led to surface roughness (Ra) reduction from 1.5μm to 0.9μm at the top surface, friction coefficient decrease from 0.14 to 0.07 at the punch/specimen interface, and nearly 50% reduction in the forming stress. The findings of this study confirmed the significant benefits of high-frequency vibration applied in micro/meso forming of metallic materials and provided a basis to understand the underlying mechanisms of vibration-assisted forming. [Copyright &y& Elsevier]

• The effect of ultrasonic treatment on the 316 stainless steel was studied. • The de-twinning process under the ultrasonic treatment was observed. • A new mechanism for the de-twinning process was proposed. • Possible mechanisms for the observed acoustic softening were discussed. The effect of ultrasonic treatment on the microstructural evolution and the related softening process in tensile pre-deformed 316 stainless steel was studied by means of electron backscatter diffraction method, optical microscopy, and microhardness measurement. It was observed that different levels of ultrasonic energy induced complex microstructural changes in the treated samples. A large decrease in twin boundaries was observed, which is an indication of the de-twinning process under ultrasonic treatment. A new mechanism for the de-twinning process under oscillatory stress of ultrasonic vibration was proposed. It was shown that de-twinning under ultrasonic treatment led to dislocation production from twin boundaries. Inverse pole figures investigation revealed strong grain rotation following ultrasonic treatment in tensile pre-deformed samples. Subgrain formation in the ultrasonic treated austenitic stainless steel samples indicated that considerable ultrasonic energy was induced by the ultrasonic vibration, which provided the activation energy needed for dislocation climb and cross-slip. The ultrasonic induced subgrain formation, dislocation annihilation, and de-twinning, which resulted in a decrease of the microhardness in the samples, can be considered as possible mechanisms for the acoustic softening in the austenitic stainless steels. [ABSTRACT FROM AUTHOR]

• Acoustic softening is separated from stress superposition by dynamic force analysis. • Deformation strain plays an important role on the effect of acoustic softening. • Acoustic softening causes reduced dislocation density and random distribution. • Ultrasonic-induced dynamic recovery can be the reason for acoustic softening. • Acoustic hardening gradually weakens the effect of acoustic softening in deformation. The application of ultrasonic vibration is an effective method to overcome the processing problems in micro/meso-forming. Previously it was observed that ultrasonic vibration could reduce flowing stress in the forming process, called ultrasonic volume effect. The volume effect contains multi-mechanisms such as stress superposition leading to apparent average stress reduction, acoustic softening and ultrasonic impact leading to real stress reductin. However, the evolutional characteristics and the mechanism of acoustic softening on material deformation is still not clear. And in most previous studies only the average stress but not the oscillatory stress was measured due to the convenience of dynamic force sensing system, which confused the different ultrasonic volume effects, acoustic softening, stress superposition and ultrasonic impact. The purpose of this study is to investigate the effects of acoustic softening on micro/meso-compression behavior and microstructure evolution. An ultrasonic-assisted compression test system with dynamic force sensing technology was developed. And a series of ultrasonic-assisted micro/meso-compression tests at different amplitudes were carried out on pure copper C1100O combining the microstructure analysis by EBSD technique. By analyzing the waveform of the oscillatory stress in the process, acoustic softening was successfully separated from the stress superposition and it was found that the deformation strain plays an important role on the effect of acoustic softening. The stress reduction by acoustic softening increases with the flowing strain or ultrasonic amplitude increasing. Besides, there is an evolutionary transition of acoustic softening ratio between small strain and large strain. When acoustic softening occurs, the low-angle grain boundaries distribute randomly in grains, compared to the piled distribution without ultrasonic assistance, implying motions of the low-angle grain boundaries or dislocation is improved by acoustic softening, resulting in the real stress reduction. In addition, with small deformation strain, the elongated grain becomes equiaxed and dislocation density is significantly reduced, which may be the result of the increased dislocation annihilation due to ultrasonic-induced dynamic recovery. However, with the deformation strain increasing to some extent, acoustic hardening gradually becomes significant, leading to much less effectiveness of acoustic softening on dislocation density reduction. The findings of this study provide an instructive understanding of the underlying mechanisms of acoustic softening in ultrasonic-assisted micro/meso-forming. [ABSTRACT FROM AUTHOR]

• Acoustic softening and residual effects in aluminum are experimentally studied. • Effects of ultrasound in elastic and plastic regions are compared. • Effects of ultrasound of different energy intensities are compared. • Effects of ultrasound of different time duration are compared. • Underlying mechanisms of acoustic softening and residual effects are discussed. Ultrasonic vibration has been observed to lower the flow stress necessary to initiate plastic deformation, a phenomenon known as "acoustic softening". This unique effect of ultrasound has been extensively applied in welding, machining, forming of metals, and ultrasonic additive manufacturing to lower the yield stress necessary to initiate plastic deformation, it nevertheless lacks fundamental investigation. Some prior studies showed experimental errors due to the design of experimental setups and the associated testing methods that have been introduced, leading to questions about their observations and conclusions. Therefore, an experimental setup described in this paper is designed to minimize the constraints identified from the setups in prior studies. Three types of aluminum are studied: Al 1100-O a commercially pure aluminum, Al 6061-O an aluminum alloy without precipitate strengthening, and Al 6061-T6 a precipitate-strengthened aluminum alloy. The acoustic softening and residual effect are compared based on the similarities and differences in microstructures of the three types of aluminum. In both acoustic softening and residual effect, linear relations are obtained between stress change and ultrasound intensities. The slope defined by the linear relations, i.e. the acoustic softening factor, depends on the microstructure of the specific material. The underlying mechanism of acoustic softening is associated with the activation of dislocations by ultrasonic energy and subsequently their interactions with other dislocations and precipitates, whereas the residual effects are attributed to the permanent changes in dislocation density due to dislocation annihilation, dynamic annealing, and dislocation-precipitate interaction. [ABSTRACT FROM AUTHOR]

Ultrasonic vibration (UV) is widely used in the forming, joining, machining process, etc. for the acoustic softening effect. For parts with small dimensions, UV with limited output energy is very suitable for the microforming process and has been gaininf more and more attention. In this investigation, UV-assisted uniaxial tensile experiments were carried out utilizing GB 5052 thin sheets of different thicknesses and grain sizes, respectively. The coupling effects of UV and the specimen dimension on the properties of the material were analyzed from the viewpoint of acoustic energy in activating dislocations. A reduction of flow stress was found for the existing acoustic softening effects of UV. Additionally, the residual effects of UV were demonstrated when UV was turned off. The uniform deformation ability of thin sheet could be improved by increasing the hardening exponent with UV. The experimental results indicate that UV is very helpful in improving the forming limit in microsheet forming, e.g., microbulging and deep drawing processes.