Grain refining of Ti-6Al-4V alloy fabricated by laser and wire additive manufacturing assisted with ultrasonic vibration

The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.     

An overview and critical assessment of the mechanisms of microstructural refinement during ultrasonic solidification of metals

A refined, equiaxed grain structure and the formation of finer primary intermetallic phases are some of the notable benefits of ultrasonic processing of liquid/solidifying melts. Ultrasonic treatment (UST) has been widely explored in Al and Mg-based alloys due to its operational versatility and scalability. During UST, the refinement of grain and primary intermetallic phases occurs via cavitation-induced fragmentation mechanisms. In addition, UST improves the efficiency (activation of particles) of the conventional grain refinement process when potent particles are added through master alloys. Though the UST’s ability to produce refined as-cast structures is well recognized, the understanding of the refinement mechanisms is still debated and unresolved. Significant efforts have been devoted to understanding these mechanisms through the use of sophisticated techniques such as in-situ/ real-time observation, lab-scale and commercial-scale casting processes. All these studies aim to demonstrate the significance of cavitation, fragmentation modes, and alloy chemistry in microstructure refinement. Although the physical effects of cavitation and acoustic streaming (fluid flow) are primary factors influencing the refinement, the dominant grain refinement mechanisms are affected by several solidification variables and casting conditions. Some of these include melt volume, solute, cooling rate, potent particles, grain growth (equiaxed, columnar or dendritic), and the cold zones of the casting where the onset of nucleation occurs. This review aims to provide a better insight into solidification variables emphasizing the importance of cold zones in generating fine structures for small- and large-volume (direct chill) castings. Another important highlight of this review is to present the relatively less explored mechanism of (acoustic) vibration-induced crystallization and discuss the role of cavitation in achieving a refined ingot structure.     

Microstructure characteristics of laser-MIG hybrid welded mild steel

To deepen the understanding of laser-arc hybrid welding, the weld shape and microstructure characteristics of laser-metal inert gas hybrid welded mild steel were analyzed. The results showed typical hybrid weld could be classified as two parts: the wide upper zone and the narrow nether zone, which were defined as arc zone and laser zone, respectively. In the hybrid weld, the microstructure, alloy element distribution and microhardness all have evident difference between laser zone and arc zone. The microstructure of arc zone consists of coarse columnar dendrite and fine acicular dendrite between the columnar dendrites, but that of laser zone is composed of fine equiaxed dendrite in weld center and columnar dendrite around the equiaxed dendrite. Compared to arc zone, laser zone has finer grain size, higher microhardness, smaller alloy element content in the fusion zone and narrower heat affected zone. The discussions demonstrated that the observed difference was caused by the difference of temperature gradient, crystallizing and the effects of arc pressure on the molten pool between laser zone and arc zone.     

In-situ observations and acoustic measurements upon fragmentation of free-floating intermetallics under ultrasonic cavitation in water

Grain refinement in alloys is a well-known effect of ultrasonic melt processing. Fragmentation of primary crystals by cavitation-induced action in liquid metals is considered as one of the main driving mechanisms for producing finer and equiaxed grain structures. However, in-situ observations of the fragmentation process are generally complex and difficult to follow in opaque liquid metals, especially for the free-floating crystals. In the present study, we develop a transparent test rig to observe in real time the fragmentation potential of free-floating primary Al₃Zr particles under ultrasonic excitation in water (an established analogue medium to liquid aluminium for cavitation studies). An effective treatment domain was identified and fragmentation time determined using acoustic pressure field mapping. For the first time, real-time high-speed imaging captured the dynamic interaction of shock waves from the collapsing bubbles with floating intermetallic particles that led to their fragmentation. The breakage sequence as well as the cavitation erosion pattern were studied by means of post-treatment microscopic characterisation of the fragments. Fragment size distribution and crack patterns on the fractured surface were then analysed and quantified. Application of ultrasound is shown to rapidly (<10 s) reduce intermetallic size (from 5 mm down to 10 μm), thereby increasing the number of potential nucleation sites for the grain refinement of aluminium alloys during melt treatment.     

Effect of acoustic field parameters on arc acoustic binding during ultrasonic wave-assisted arc welding

As a newly developed arc welding method, power ultrasound has been successfully introduced into arc and weld pool during ultrasonic wave-assisted arc welding process. The advanced process for molten metals can be realized by utilizing additional ultrasonic field. Under the action of the acoustic wave, the plasma arc as weld heat source is regulated and its characteristics make an obvious change. Compared with the conventional arc, the ultrasonic wave-assisted arc plasma is bound significantly and becomes brighter. To reveal the dependence of the acoustic binding force on acoustic field parameters, a two-dimensional acoustic field model for ultrasonic wave-assisted arc welding device is established. The influences of the radiator height, the central pore radius, the radiator radius, and curvature radius or depth of concave radiator surface are discussed using the boundary element method. Then the authors analyze the resonant mode by this relationship curve between acoustic radiation power and radiator height. Furthermore, the best acoustic binding ability is obtained by optimizing the geometric parameters of acoustic radiator. In addition, three concave radiator surfaces including spherical cap surface, paraboloid of revolution, and rotating single curved surface are investigated systematically. Finally, both the calculation and experiment suggest that, to obtain the best acoustic binding ability, the ultrasonic wave-assisted arc welding setup should be operated under the first resonant mode using a radiator with a spherical cap surface, a small central pore, a large section radius and an appropriate curvature radius.     

Synthesis of MCM-22 zeolite by an ultrasonic-assisted aging procedure

The synthesis of MCM-22 zeolite under hydrothermal crystallization conditions has been performed by an ultrasonic-assisted aging procedure. The ultrasonic-assisted aging of the initial aluminosilicate gel can shorten the crystallization time of MCM-22, decrease the amount of hexamethyleneimine (HMI) used, and broaden the range of SiO(2)/Al(2)O(3) ratios. By using the ultrasonic aging, pure phase of high-silica MCM-22 products with SiO(2)/Al(2)O(3)>100 can be obtained. When SiO(2)/Al(2)O(3) 相似文献   

高温合金活性剂激光微焊接接头的组织性能研究

采用配方均匀设计法,配制了SiO2-MnO2-CaO-TiO2-CaF2-NaF多组元活性剂,利用微型脉冲激光器对500 m厚GH4169高温合金进行了活性剂激光焊接试验。分析并讨论了焊接接头的显微组织和力学性能。试验结果表明,与传统激光焊相比,所配制的20种活性剂均增加了焊缝熔深,并且其中F12系混合活性剂增加熔深能力最为显著,使焊缝深宽比增加了159%,证明通过使用活性剂来增加微激光焊焊缝熔深,降低高温合金板激光焊接的成本是可行的。在活性剂作用下,焊缝显微组织仍由柱状晶和等轴晶组成,接头抗拉强度达到927 MPa,为母材强度的92.7%。     

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al₃Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.     

5083铝合金光纤激光-TIG复合焊接工艺研究

采用IPG YLS-6000光纤激光器和Fronius MagicWave3000job数字化焊机,对4mm厚5083H116铝合金进行了复合焊接试验。研究了电源特性、电流大小和热源间距等工艺参数对光纤激光-钨极惰性气体保护焊(TIG)复合焊接焊缝成形的影响规律,并分析了焊接接头的缺陷、显微硬度及力学性能。结果表明,光纤激光-TIG复合焊接5083铝合金,能够明显改善焊缝成形,提高焊接过程稳定性,特别是与变极性TIG电弧复合效果更为显著;光纤激光与变极性TIG电弧复合焊接,采用激光在前的方式,电弧电流150A,且热源间距不大于4mm,可以得到具有明亮金属光泽和均匀鱼鳞纹的焊缝,焊缝无气孔和裂纹缺陷,其表面有少量的下凹;复合焊接接头抗拉强度为318MPa,达到母材强度的93%,延伸率为7.6%,高于单光纤激光焊接,断口分析为韧性断裂。     

Optimization of hybrid laser – TIG welding of 316LN steel using response surface methodology (RSM)

In the present study, the hybrid laser – TIG welding parameters for welding of 316LN austenitic stainless steel have been investigated by combining a pulsed laser beam with a TIG welding heat source at the weld pool. Laser power, pulse frequency, pulse duration, TIG current were presumed as the welding process parameters whereas weld bead width, weld cross-sectional area and depth of penetration (DOP) were considered as the process responses. Central composite design was used to complete the design matrix and welding experiments were conducted based on the design matrix. Weld bead measurements were then carried out to generate the dataset. Multiple regression models correlating the process parameters with the responses have been developed. The accuracy of the models were found to be good. Then, the desirability approach optimization technique was employed for determining the optimum process parameters to obtain the desired weld bead profile. Validation experiments were then carried out from the determined optimum process parameters. There was good agreement between the predicted and measured values.     

Acoustic cavitation mechanism: a nonlinear model

During acoustic cavitation process, bubbles appear when acoustic pressure reaches a threshold value in the liquid. The ultrasonic field is then submitted to the action of the bubbles. In this paper we develop a model to analyze the cavitation phenomenon in one-dimensional standing waves, based on the nonlinear code SNOW-BL. Bubbles are produced where the minimum rarefaction pressure peak exceeds the cavitation threshold. We show that cavitation bubbles appear at high amplitude and drastically affect (dissipation, dispersion, and nonlinearity) the ultrasonic field. This paper constitutes the first work that associates the nonlinear ultrasonic field to a bubble generation process.     

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.     

超声复合电弧声调控特性研究

超声复合电弧作为一种新的焊接热源, 在电弧焊接过程中可利用超声实现对熔融金属的深度处理, 但是超声与电弧等离子体间相互作用机理还不清晰, 这成为阻碍该技术工程应用的关键问题. 本文通过实验与相应理论针对外加超声场对焊接电弧调控特性进行了研究. 为说明电弧特性, 针对试验中高速摄像采集的电弧图片进行了处理. 对比未加超声情况, 超声复合电弧受内外声场共同作用等离子体拘束程度明显提高, 电弧亮度增强, 弧柱高温区范围扩展至阳极, 中间粒子出现团聚并以一定频率上下抖动. 通过改变超声激励电流大小和声发射端高度, 电弧结构产生显著变化, 在谐振点附近, 电弧挺直度最强, 脉动频率最大. 试验结果显示通过外加超声可以达到对焊接电弧热等离子体调控的目的. 最后结合波动方程和二维声边界元模型, 分析了电弧内部声传播过程以及声场结构对等离子体粒子的作用规律, 这为进一步理解超声对电弧的调控机理打下良好基础.     

超声辅助电镀的仿真与实验*

为了提高镀层的表面质量,提出了一种超声辅助电镀的方法。超声的空化与搅拌作用可以影响电镀的电沉积过程。设计了合理的换能器结构,并搭建了超声辅助电镀装置。通过对模型进行压电声学场与电镀场耦合仿真分析,结果表明：在硫酸铜电解液,镀铜时间60s,电压300V条件下,进行超声辅助电镀,镀层厚度分布均匀,镀层中部电流密度分布均匀,边缘最大电流密度是未加超声的2倍左右 。基于仿真结果,进行相同的实验测试,结果表明：超声辅助电镀可以提高镀层的均匀性,减少铜颗粒表面的杂质,提高表面的光洁度。阴极样品距离换能器头部位置在60mm左右,镀层质量良好。     

Energetic study of ultrasonic wettability enhancement

Many industrial and biological interfacial processes, such as welding and breathing depend directly on wettability and surface tension phenomena. The most common methods to control the wettability are based on modifying the properties of the fluid or the substrate. The present work focuses on the use of high-frequency acoustic waves (ultrasound) for the same purpose. It is well known that ultrasound can effectively clean a surface by acoustic cavitation, hence ultrasonic cleaning technology. Besides the cleaning process itself, many authors have observed an important wettability enhancement when liquids are exposed to low and high (ultrasonic) frequency vibration. Ultrasound goes one step further as it can instantly adjust the contact angle by tuning the vibration amplitude, but there is still a lack of comprehension about the physical principles that explain this phenomenon. To shed light on it, a thermodynamic model describing how ultrasound decreases the contact angle in a three-phase wetting system has been developed. Moreover, an analytical and experimental research has been carried out in order to demonstrate that ultrasound is an important competitor to surfactants in terms of energy efficiency and environmental friendliness.     

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.     

Microstructure evolution of laser solid forming of Ti-Al-V ternary system alloys from blended elemental powders

Morphology evolution of prior β grains of laser solid forming (LSF) Ti-xAl-yV (x 11,y 20) alloys from blended elemental powders is investigated. The formation mechanism of grain morphology is revealed by incorporating columnar to equiaxed transition (CET) mechanism during solidification. The morphology of prior β grains of LSF Ti-6Al-yV changes from columnar to equiaxed grains with increasing element V content from 4 to 20 wt.-%. This agrees well with CET theoretical prediction. Likewise, the grain morphology of LSF Ti-xAl-2V from blended elemental powders changes from large columnar to small equiaxed with increasing Al content from 2 to 11 wt.-%. The macro-morphologies of LSF Ti-8Al-2V and Ti-11Al-2V from blended elemental powders do not agree with CET predictions. This is caused by the increased disturbance effects of mixing enthalpy with increasing Al content, generated in the alloying process of Ti, Al, and V in the molten pool.     

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this “bubble streamers” is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.     

Single-transducer dual-frequency ultrasound generation to enhance acoustic cavitation

Dual- or multiple-frequency ultrasound stimulation is capable of effectively enhancing the acoustic cavitation effect over single-frequency ultrasound. Potential application of this sonoreactor design has been widely proposed such as on sonoluminescence, sonochemistry enhancement, and transdermal drug release enhancement. All currently available sonoreactor designs employed multiple piezoelectric transducers for generating single-frequency ultrasonic waves separately and then these waves were mixed and interfered in solutions. The purpose of this research is to propose a novel design of generating dual-frequency ultrasonic waves with single piezoelectric elements, thereby enhancing acoustic cavitation. Macroscopic bubbles were detected optically, and they were quantified at either a single-frequency or for different frequency combinations for determining their efficiency for enhancing acoustic cavitation. Visible bubbles were optically detected and hydrogen peroxide was measured to quantify acoustic cavitation. Test water samples with different gas concentrations and different power levels were used to determine the efficacy of enhancing acoustic cavitation of this design. The spectrum obtained from the backscattered signals was also recorded and examined to confirm the occurrence of stable cavitation. The results confirmed that single-element dual-frequency ultrasound stimulation can enhance acoustic cavitation. Under certain testing conditions, the generation of bubbles can be enhanced up to a level of five times higher than the generation of bubbles in single-frequency stimulation, and can increase the hydrogen peroxide production up to an increase of one fold. This design may serve as a useful alternative for future sonoreactor design owing to its simplicity to produce dual- or multiple-frequency ultrasound.     

Ultrasonic inspection of stainless steel butt welds using horizontally polarized shear waves

Inspection of austenitic stainless steel weldments by conventional ultrasonic means is fundamentally limited by the textured, columnar grain structure of the weld metal. It is shown that, for selected angles of incidence, shear waves normally polarized with respect to the columnar grains can pass through the weld metal-base metal interface without partial reflection. As a consequence, the inspectability of stainless steel weldments can be improved. The operation of a low frequency, ultrasonic system for stainless steel butt weldments is demonstrated.