Investigation on ultrasonic volume effects: Stress superposition,acoustic softening and dynamic impact

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.     

类声子晶体结构对超声塑料焊接工具横向振动的抑制

利用声子晶体的带隙理论以及耦合振动理论对大尺寸矩形超声塑料焊接工具的耦合振动进行了研究.在实际工程应用中,大尺寸工具的横向振动将严重导致工具头辐射面位移不均匀,影响系统的焊接质量及工作效率.为提高其工作效率,改善工具头辐射面位移的均匀程度,利用类声子晶体结构对大尺寸超声塑料焊接工具的横向振动进行抑制,分析并得出了类声子晶体结构的横向振动带隙,同时分析了工具头横向振动未抑制与抑制后其辐射面位移的大小与均匀程度.研究表明,通过合理设计类声子晶体的结构及尺寸,可以有效抑制超声塑料焊接工具的横向振动.不但改善了焊接工具辐射面纵向振动位移的均匀程度,而且提高了焊接工具的纵向振动位移幅度.     

Acceleration of Mass Transfer under Dynamic Loading

Under dynamic loading, the interaction of unloading waves with each other and with the faces causes the deformation of the sample due to the formation of standing waves, at the nodes of which the localized- deformation bands arise (if the high-speed tension does not exceed the spall strength, so the material in the zone of interference of unloading waves preserves its continuity). A consequence of deformation localization is the mass transfer of atoms and the doping phase particles to nascent localized-deformation bands. The duration of stress oscillations in standing waves in the ultrasonic frequency range exceeds the duration of the initial impulse by at least two orders of magnitude, which ensures an increase in the distance traveled by atoms and dispersed particles during deformation in standing waves.     

压电振子阵列型超声喷丸强化*

为了提高超声喷丸强化加工的面积、效率及加工质量,对传统单振子超声喷丸强化技术进行了改进,提出了压电振子阵列型超声喷丸强化加工方法。分析了超声喷丸强化的机理,进行了压电振子的优化设计,分析了激振片的谐振响应,实验测试了系统样机在不同超声电源功率下弹丸撞击工件的区域分布;探究了超声电源功率、喷丸距离、弹丸直径和喷丸时间对加工样件显微硬度的影响,结果表明：以超声电源功率作为唯一变量,弹丸冲击工件表面的区域面积随超声电源功率增大而增大;7075铝合金样件的表面显微硬度及其提高率,随超声电源功率的增大、喷丸时间的增加、喷丸距离的减小及弹丸直径的增大而增大;超声电源功率为50W、喷丸时间为20min、喷丸距离为5mm、弹丸直径为1.5mm时喷丸效果最好,此时工件表面显微硬度增加量为65.1HV,表面显微硬度提高率为36.7%。压电振子阵列型超声喷丸强化技术可以有效增加喷丸冲击工件的面积,提高工件的表面显微硬度和加工效率。     

Ultrasonic excitation affects friction interactions between food materials and cutting tools

In the food industry, ultrasonic cutting is used to improve separation by a reduction of the cutting force. This reduction can be attributed to the modification of tool–workpiece interactions at the cutting edge and along the tool flanks because of the superposition of the cutting movement with ultrasonic vibration of the cutting tool. In this study, model experiments were used to analyze friction between the flanks of a cutting tool and the material to be cut. Friction force at a commercial cutting sonotrode was quantified using combined cutting–friction experiments, and sliding friction tests were carried out by adapting a standard draw-off assembly and using an ultrasonic welding sonotrode as sliding surface. The impact of material parameters, ultrasonic amplitude, and the texture of the contacting food surface on friction force was investigated. The results show that ultrasonic vibration significantly reduces the sliding friction force. While the amplitude showed no influence within the tested range, the texture of the contact surface of the food affects the intensity of ultrasonic transportation effects. These effects are a result of mechanical interactions and of changes in material properties of the contact layer, which are induced by the deformation of contact points, friction heating and absorption heating because of the dissipation of mechanical vibration energy.     

Forced vibrations of two nonlinearly connected solid waveguides under static load

Forced impact oscillations in nonlinearly coupled solid waveguides with nearly equal natural frequencies are examined using the harmonic balance method. A dynamic model is used to describe the process of ultrasonic micro-forging where the material to be worked is considered as a nonlinear connecting-link between the ultrasonic transducer and passive waveguide-reflector. The influence of the material properties (yield stress, stiffness of tool-workpiece contact, striker and blank geometries) and the processing conditions (amplitude of vibration, static compressive force, work rate) on the resonance characteristics of the vibratory system is taken into account. The resonance and antiresonance frequencies, boundaries of response stability, ranges of inphase and antiphase oscillations of impacting waveguides, and some other features of strongly coupled vibrators under impact loading are determined.It is shown also that the largest amplitude of impact oscillations can be attained only if the natural frequency of the ultrasonic transducer exceeds that of the waveguide-reflector. By measuring the dynamic drift in the unfastened ultrasonic unit, it is possible to control, directly in the course of ultrasonic micro-forging, the thickness of the metal workpiece. Calculated data are compared with the experimental results.     

Analytical description of ultrasonic hardening and softening

In the presented work, a modeling of ultrasonic hardening and softening has been carried out. The analytical model is constructed by the generalization of the synthetic theory of plastic deformation. A new term, ultrasonic defect intensity, is being introduced so that the phenomenon of both hardening and softening is described by the uniform system of constitutive equations.     

The influence of the initial structural state of armco iron on the ultrasonic treatment effect

Methods of diffraction electron microscopy have been used to examine the types of dislocation substructures formed in the surface layer of armco-iron specimens subjected to ultrasonic treatment. It is shown that banded or equiaxed ultrafine-grained structure can be generated in the material depending on its initial structural state. The special features of the plastic deformation and fracture of the ultrafine-grained surface layers of the specimens under uniaxial tension are described. The extent to which the mechanical properties of the examined material are improved by ultrasonic treatment has been found to depend on its initial state. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 74–82, January, 2009.     

Effect of Low-Amplitude Load Oscillations on Stiffness and Hardness for Al and W in Loaded Nanocontacts

The effect low-amplitude oscillations have on the mechanical properties of Al and W in nanocontacts is studied by means of continuous stiffness measurements (CSM). It is established that the additional superpositioning of oscillations until a critical amplitude is reached has no effect on the mechanisms, kinetics, or plastic deformation of a material. The threshold amplitudes and frequencies of such oscillations are determined for Al (2.5 nm) and W (3.5 nm).     

基于Jiles-Atherton理论的铁磁材料塑性变形磁化模型修正

Jiles-Atherton(J-A)模型在磁化建模领域应用广泛,但不同文献给出的J-A模型并不一致,致使采用不同表达式建立的塑性变形磁化模型存在多种版本,其正确性难以甄别.通过对无磁滞磁化方程、能量守恒方程和等效磁场强度方程的梳理与比较,发现原有模型中存在将磁化强度和无磁滞磁化强度混用、将不可逆磁化能量等效于全部的磁化能量、等效磁场强度中应力磁化项界定不清等问题.在此基础上,对上述方程进行了修正,推导了基于J-A模型的塑性变形磁化修正模型.将修正模型计算结果与原模型计算结果、相关文献中的试验结果进行对比,结果表明:与原有计算模型相比,修正模型计算结果的饱和磁化强度和剩余磁化强度随塑性变形增加而减小,矫顽力随塑性变形增大而增大,达到饱和磁化强度时的外磁场强度随塑性变形增大而增大的趋势有所减弱,更符合试验结果,可更准确地反映塑性变形对材料磁化的影响.     

Experimental and numerical studies of nonlinear ultrasonic responses on plastic deformation in weld joints

The experimental measurements and numerical simulations are performed to study ultrasonic nonlinear responses from the plastic deformation in weld joints. The ultrasonic nonlinear signals are measured in the plastic deformed30Cr2Ni4 Mo V specimens, and the results show that the nonlinear parameter monotonically increases with the plastic strain, and that the variation of nonlinear parameter in the weld region is maximal compared with those in the heat-affected zone and base regions. Microscopic images relating to the microstructure evolution of the weld region are studied to reveal that the change of nonlinear parameter is mainly attributed to dislocation evolutions in the process of plastic deformation loading. Meanwhile, the finite element model is developed to investigate nonlinear behaviors of ultrasonic waves propagating in a plastic deformed material based on the nonlinear stress–strain constitutive relationship in a medium. Moreover, a pinned string model is adopted to simulate dislocation evolution during plastic damages. The simulation and experimental results show that they are in good consistency with each other, and reveal a rising acoustic nonlinearity due to the variations of dislocation length and density and the resulting stress concentration.     

Turbulence and dissipative structures in shock-loaded copper

Shock-loading tests of polycrystalline copper M3 under conditions of uniaxial deformation at impact velocities of 100 to 700 m/s were performed. It was established that a threshold deformation rate exists above which dissipative structures in the dynamically deformed material arise in the form of local regions of cellular type, with a size of 15–25 μm, separated by shear plastic bands. The basic size of cellular structure domains is on the nanometer scale. The microhardness of the material within the cellular structures is somewhat higher than in the bands of plastic deformation that separate these structures. At threshold deformation rates and above it, the defect of the mass velocity, the difference between the impactor velocity for symmetrical collision and the free surface velocity at the plateau of the compression pulse, increases sharply as does the spall strength of the material.     

Conditions under which a thermal instability arises in friction

We investigate the one-dimensional, stationary problem of the temperature distribution and rate of plastic deformation in a uniform material under the influence of friction on the surface. The temperature distribution in the material is found by numerically solving the nonlinear heat conduction equation taking into account heat released both at the surface and in the sample interior. We modeled the rate of plastic deformation as a power-law function of temperature. We observed a phenomenon in the solution that might be called a thermal instability wherein stable solution exist only for loads lighter than some critical load. The temperature and plastic deformation distribution have a macroscopic character, and cannot be explained by the formation of a thin, highly deformed surface layer that is usually observed experimentally.Institute of Strength and Materials Research Physics, Russian Academy of Sciences, Siberian Branch. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 23–27, February, 1994.     

Plastic deformation and quasi-periodic vibrations in a tribological system

A one-dimensional macroscopic model is used to analyze the plastic deformation of materials without coating and with a plastic hardening coating or a plastic nonhardening coating at friction. The calculations show that mechanical vibrations can be excited in a tribological system and that their frequency decreases sharply when going from elastic to plastic deformation. One of the causes of the development of plastic deformation in the surface layer and in the sublayer of the material under a hard coating is found to be a decrease in the elastic properties of the material because of frictional heating. An intense plastic shear in the material under the hard coating can cause its failure due to incompatible strains of the coating and the base.     

Ultrasonic study on the change in elastic anisotropy with plastic compressive deformation

The macroscopic elasticity of a polycrystalline material is more or less anisotropic due to the presence of texture. In this Paper, the change in such elastic anisotropy with plastic deformation was studied ultrasonically. Test specimens prepared from a rolled plate of mild steel were deformed plastically by uni-axial compression, and the relative retardation and polarization directions of two independent transverse waves were measured by analysing frequency spectra of ultrasonic waves. It was confirmed from the results that the plastic deformation changes not only the degree of the elastic anisotropy but also its principal axes towards those of the plastic strain. A preliminary consideration was presented concerning the relation between the elastic stiffness and small plastic strain.     

Physical and mathematical simulation of the formation of mesostructure-ordered plasma coatings

Peculiarities in the structure formation of mesostructure-ordered thermal spray plasma coatings are considered. The role of this structure in ensuring high service parameters of particles under varying mechanical and thermal loading of the surface is demonstrated. An analytic relation connecting the strain of a particle being deposited and its bulk transformation factor with the particle velocity and mechanical characteristics is derived using physical and mathematical simulation of the impact followed by plastic deformation of the particle for a plastic hardenable material.     

A combined study of surface roughness in polycrystalline aluminium during uniaxial deformation using laser-induced photoemission and confocal microscopy

A new technique integrating measurements obtained by photo-stimulated electron (PSE) emission and scanning laser confocal microscopy (SLCM) has been developed to characterize the deformation of commercially pure aluminium in uniaxial plastic strain. Real time, in situ PSE signals provide details about the evolution and propagation rates of surface events during the deformation process. High-resolution SLCM measurements yield details describing the relative magnitude and spatial distribution of the surface features. During homogeneous deformation, uniform generation of surface area produces a monotonic increase in PSE intensity whereas necking induces a saturation condition. Analysis of the surface area created by the deformation revealed that the rate of generation correlates well with the PSE intensity. The consistencies in the data acquired with these two techniques signify that they yield complementary information and that the combination provides essential details about the deformation process in a material with low hardness such as an aluminium alloy.     

粗糙海面高功率微波传输特性矩量法分析

应用蒙特卡罗方法实现了粗糙海面的仿真与模拟,建立了基于双积分方程的高功率微波（HPM）近海面传输特性矩量法计算模型。模型采用光滑窗函数对均匀平面波进行调制,把均匀平面波入射调制为锥形波,消除了粗糙海面突然被截断而引起的边缘效应的影响;重新推导了锥形波入射下的基尔霍夫近似公式,并在满足基尔霍夫近似的条件下,通过对比分析,验证了模型的正确性;采用模型计算分析了不同海面几何参数和海水媒质参数对HPM近海面传输系数的影响。结果表明:粗糙海面的均方根高度对HPM传输系数影响明显,均方根高度越大,传输系数越小,能量分布越均匀;另外随着海水介电常数实部和虚部的增加,传输系数均有所增加,并且实部的影响更明显。     

Variation of the ultrasonic velocity in al under plastic deformation

Variation of the velocity of ultrasound propagation in polycrystalline aluminum under plastic deformation is studied. The dependences of the velocity of ultrasound on the strain and the actual stress are found to consist of three distinct stages. The study of the complex shapes of these dependences allows one to reveal additional stages in the parabolic stress-strain curve of the plastic flow, these features being impossible to observe by conventional methods. The behavior of the ultrasonic velocity observed in the experiment is explained by the changes in the defect structure of the material under deformation.