Research status of ultrasonic cutting of aerospace metal materials

This paper summarizes the current state of research on ultrasonic cutting of difficult-to-machine metal materials from four perspectives: cutting force, cutting temperature, tool wear, and surface quality. Additionally, key challenges in cutting are discussed.

 

1. Introduction

Difficult-to-machine metal materials, such as titanium alloys and high-temperature alloys, are essential raw materials for producing key aerospace engine components, including turbine disks, blades, and casings. With the rapid advancement of the aerospace industry, these critical components often operate under harsh conditions, including high temperatures, high pressure, cyclic loads, and humidity. This imposes stringent requirements on the service performance of these essential components. Therefore, improving the service life of these components and achieving high-quality, efficient machining of metals like titanium alloys is a critical research focus.

The precision of part manufacturing is crucial for ensuring service life, and adopting effective machining methods is essential for enhancing the longevity of critical components. Cutting, as a common machining method, plays a pivotal role in mechanical processing and is widely used for machining difficult-to-machine metal materials. However, due to the superior mechanical properties of these metals, traditional machining often results in high cutting forces, elevated cutting temperatures, severe tool wear, and poor surface integrity. As a result, this leads to low machining efficiency, reduced precision, and increased processing costs.

Ultrasonic vibration-assisted machining (UVAM) is an unconventional machining method that uses transducers (such as piezoelectric ceramics or magnetostrictive materials) to convert high-frequency electrical energy into high-frequency mechanical vibration energy, which is then applied to the machining process. This technique enables ultra-precision machining of materials and fundamentally changes the material removal mechanism compared to traditional methods. In ultrasonic-assisted machining, ultrasonic vibrations are applied between the workpiece and the tool during cutting, significantly reducing friction, cutting forces, and heat. Additionally, these ultrasonic vibrations increase chip fatigue damage, improving chip-breaking efficiency. The ultrasonic vibration acceleration at the tool’s cutting edge can reach tens of thousands of times greater than the acceleration due to gravity. This tremendous inertial force makes it difficult for chips to adhere to the cutting edge, preventing the formation of built-up edges and reducing adhesive wear of the tool. Consequently, UVAM offers unique advantages in machining aerospace metal materials.

The impact of various factors on cutting performance during ultrasonic cutting is illustrated in Figure 1. Currently, ultrasonic vibration-assisted cutting has been successfully applied to difficult-to-machine metal materials, yielding promising results.

 

2.1 Cutting force

In traditional metal cutting, there is significant interaction between the tool and the workpiece. This interaction generates substantial cutting forces, which can pose serious challenges to the stability of the machining process. To address this issue, some researchers have turned to ultrasonic cutting, which helps reduce cutting forces and enhances process stability.

Cutting force significantly influences tool wear and surface quality. Shen et al. studied the variation of milling force in ultrasonic vibration-assisted vertical milling. Their results indicated that the separation phenomenon caused by ultrasonic vibrations leads to pulsed fluctuations in the cutting force curve, which remains lower than the average cutting force observed in traditional milling.

Sofuoglu et al. utilized DEFORM-2D finite element software to create models for titanium alloy and Hastelloy alloy during three processes: ordinary turning, ultrasonic vibration-assisted turning, and thermal ultrasonic vibration-assisted turning. They analyzed the cutting forces involved in these various processes. The findings revealed that thermal ultrasonic vibration-assisted cutting resulted in the lowest cutting force and effective stress. Specifically, for Hastelloy alloy, thermal ultrasonic vibration-assisted turning reduced cutting forces by over 50%, while for titanium alloy, the reduction was approximately 70%. This reduction is attributed to the thermal softening effect of thermal ultrasonic vibration-assisted cutting, which alleviates cutting forces during machining. Additionally, the separation contact between the tool and the workpiece during ultrasonic machining contributes to further reductions in cutting forces.

To mitigate chatter during the machining of titanium alloy and improve process stability, Lanos et al. employed ultrasonic vibration-assisted turning for this material. They developed a cutting force model based on process parameters and targeted material removal rates, analyzing the effects of different machining parameters on cutting forces. The results demonstrated that, under identical parameters, ultrasonic vibration-assisted turning achieved lower cutting forces, effectively suppressing chatter during machining.

Niu et al. established a maximum undeformed cutting thickness model for longitudinal-torsional ultrasonic vibration-assisted milling, based on the tool motion trajectory. They derived a milling force model under longitudinal-torsional ultrasonic vibration conditions by utilizing an oblique cutting model and a cutting force geometry relationship model. Their analysis indicated that the predicted values were consistent with experimental patterns related to parameter variations. Among the ultrasonic machining parameters, the longitudinal-torsional phase difference had the greatest impact on cutting forces in both directions, followed by ultrasonic frequency and longitudinal vibration amplitude, with torsional amplitude having the least influence.

Wang et al. created a milling force prediction model under ultrasonic conditions by considering tool geometry and the cutting motion trajectory at the tool tip. Their findings revealed that longitudinal-torsional ultrasonic vibration could reduce radial cutting forces, while tangential and axial forces remained largely unchanged, primarily due to the direction of ultrasonic vibration.

To enhance the quality of surface machining, Xu et al. analyzed the generation mechanism of cutting forces under ultrasonic conditions and developed a cutting force model specifically for ultrasonic vibration-assisted turning of 304 stainless steel based on process parameters. Their findings indicated that the separation effect introduced by ultrasonic vibration can significantly reduce cutting forces during machining; however, increasing the amplitude does not necessarily lead to improved performance, as an optimal value exists. A proper balance of process parameters can lead to better machining outcomes.

Namlu et al. investigated the combination of ultrasonic vibration-assisted milling with minimal lubrication to evaluate the impact of various process parameters on cutting forces during the milling of titanium alloys. They found that regardless of the cooling method employed, the cutting force under ultrasonic conditions consistently remained lower than that of conventional cutting. Furthermore, they observed that the cutting force varies with the machining stage, exhibiting different effects during the roughing and finishing stages. Specifically, ultrasonic cutting is more effective at reducing cutting forces during the roughing stage, while conventional cutting forces tend to be lower in the finishing stage. Thus, the optimal strategy for minimizing cutting forces involves using minimal quantity lubrication (MQL) combined with conventional milling (CM) in the finishing phase, and MQL with ultrasonic-assisted milling (UAM) in the roughing phase.

Chen et al. studied the effects of ultrasonic vibration on the machining process through a fully transient cutting methodology. They considered how ultrasonic vibration affects the shear angle and flow stress in the shear zone, proposing a new model to determine both average and transient cutting forces. The results illustrated that the ultrasonic cutting process is influenced by multiple factors, including transient properties, acoustic softening, thermal softening, plowing, and friction. The discrepancies between experimental and predicted cutting forces for Ti6Al4V, AISI 1045, and Al6063 were approximately 7%, 10.2%, and 11%, respectively, demonstrating that ultrasonic vibration can effectively reduce cutting forces.

In considering the impact of vibration on the cutting performance of workpiece materials, Nik et al. optimized the design of an ultrasonic vibration device by integrating theoretical design with finite element simulation and genetic algorithms. They subsequently analyzed how varying machining parameters affect cutting forces and surface roughness, revealing that the application of ultrasonic vibration can lower cutting forces and enhance surface quality. Compared to conventional grinding, the normal and tangential grinding forces under ultrasonic conditions were reduced by 13.5% and 14.2%, respectively.

Wang Chenxu et al. explored the removal mechanism associated with ultrasonic vibration-assisted grinding of a specific superalloy and examined the effects of machining parameters on cutting performance. Their results confirmed that ultrasonic vibration significantly decreased cutting forces and grinding wheel wear during machining, while also greatly improving surface quality.

 

2.2 Cutting Temperature

In traditional metal cutting, the cutting process generates extremely high heat in the contact area, which negatively impacts both tool life and the quality of the surface finish. These excessive cutting temperatures can significantly shorten tool life and lead to issues such as adhesion and delamination on the machined surface. In contrast, ultrasonic cutting can effectively control the cutting temperature of titanium alloys, enhancing both grinding wheel longevity and surface finish quality. A comparison of cutting temperatures across various cutting methods is illustrated in Figure 2.

 

To investigate the effect of ultrasonic vibration on the turning process, KHAJEHZADEH et al. utilized Al2O3-coated tools for ultrasonic-assisted turning of aviation aluminum. They examined how ultrasonic amplitude, cutting speed, and feed rate influenced cutting temperature. The findings indicated that effective cooling was linked to the careful selection of process parameters. At lower feed rates, increasing the ultrasonic amplitude significantly reduced cutting temperature.

LOTFI et al. conducted a finite element simulation to analyze the impact of two-dimensional elliptical ultrasonic vibration on the cutting performance of nickel-based high-temperature alloys. Their results demonstrated that applying elliptical vibration increased the shear angle and lowered the cutting temperature on the tool’s front face, thus helping to inhibit chip nodule formation.

CHEN et al. developed a heat transfer model for ultrasonic-assisted cutting of titanium alloys based on a non-uniform moving heat source. They explored how ultrasonic amplitude and frequency affected the heat transfer process during machining. The results indicated that the center of the equivalent heat source tended to shift backward on the front face of the tool, and temperature gradients in the horizontal and vertical directions were not consistent.

The effects of varying vibration parameters on shear surface temperature during flat and curved surface machining were analyzed. It was found that increasing ultrasonic amplitude and frequency can reduce temperature gradients, leading to lower shear surface temperatures. Specifically, the machined surface temperature decreases with increasing ultrasonic amplitude but rises with increasing frequency.

LIN et al. discovered that combining minimum quantity lubrication (MQL) with ultrasonic vibration can further enhance the machining performance in single ultrasonic vibration-assisted turning, notably reducing both cutting forces and cutting temperatures. The effectiveness of MQL is also influenced by the nozzle angle. Finite element simulations examined how different nozzle angles affect cutting temperature and stress variations. The findings indicated that an optimal nozzle angle facilitates better cooling and lubrication, improving surface quality and extending tool life.

To explore the impact of the high-frequency sinusoidal motion of the tool tip on the machining process, GHOLAMZADEH et al. conducted two-dimensional finite element simulations of ultrasonic vibration-assisted cutting under dry conditions. They investigated how various machining-related parameters affect tool tip cutting temperature. Their results revealed that, under certain conditions, the instantaneous temperature of the tool tip can be high, yet the average temperature remains relatively low. The highest temperature occurs slightly above the tool tip.

In a study by CHEN et al., finite element simulations were employed to analyze the influence of the tool tip arc radius on the stress state in the plowing zone during ultrasonic vibration-assisted cutting of titanium alloys. The results demonstrated that, compared to traditional grinding, ultrasonic vibration reduces cutting temperatures. However, cutting temperatures still increase with higher cutting speeds and tool tip arc radii. The maximum cutting temperature is observed on the rake face because an increase in cutting speed and tool tip arc radius leads to higher cutting energy consumption. This results in excess heat accumulation on the rake face through the chips, causing the cutting temperature to rise.

Addressing the limitations of critical cutting speed in ultrasonic cutting, ZHANG et al. proposed a high-speed vibration cutting method to enhance surface quality and machining efficiency. They established a transient cutting temperature model to describe temperature variations at the cutting interface. Their results indicated that cutting speed and duty cycle are the primary factors influencing cutting temperature, which typically relates to the settings of machining parameters.

To predict the impact of processing parameters on the cutting performance of titanium alloy, MUHAMMAD et al. employed a fuzzy logic model to assess the effects of process parameters on cutting temperature in both ultrasonic vibration-assisted turning and traditional turning. Their findings showed that the developed model accurately predicts outcomes and reveals the underlying mechanism of cooling in ultrasonic vibration cutting.

 

2.3 Tool wear

Titanium alloys and other challenging-to-cut metals generate significant forces and heat loads during traditional cutting processes, which leads to rapid tool wear. This excessive wear not only shortens the tool’s lifespan but also adversely affects the quality of the machining process. In turn, this impacts the forces and heat loads generated during cutting. Figure 3 illustrates how different cutting methods influence tool wear.

 

 

Studies have demonstrated that ultrasonic vibration can effectively reduce tool wear, which is a significant challenge in the processing of titanium alloys. NI et al. investigated the wear mechanism of tools subjected to ultrasonic action and analyzed how different tool wear states affect surface quality. They discovered that applying ultrasonic vibration in the feed direction primarily led to defects such as tip fracture and cracks in the tools. This was largely due to the high-frequency vibration impact between the tool and the workpiece; however, the wear level was notably lower compared to traditional milling tools, resulting in improved surface quality of the processed parts. Additionally, it was found that incorporating micro-cooling and lubrication could further enhance processing effectiveness.

To extend tool life when machining titanium alloys, Tong Jinglin et al. examined the effects of longitudinal-torsional ultrasonic vibration on tool wear characteristics during milling. Their results indicated that this form of ultrasonic vibration altered the interaction between the tool and the workpiece, leading to reduced flank wear and improved surface quality.

LIU et al. also explored the wear mechanism of tools during ultrasonic vibration-assisted milling. Their study identified three types of wear experienced by the tools: oxidation wear, adhesive wear, and diffusion wear. Tools tend to achieve optimal surface processing quality during the normal wear stage. Compared to conventional milling, ultrasonic vibration-assisted milling results in longer tool life, superior surface quality, and smaller edge burrs, while significantly reducing cutting forces and temperatures.

Yu et al. examined the tool wear mechanism during elliptical ultrasonic vibration-assisted turning and analyzed the reasons behind lower tool wear under both separation- and non-separation-dependent conditions. Their findings indicated that, in non-separation-dependent elliptical vibration cutting, the acoustic softening effect reduces the material’s yield stress, leading to decreased tool wear. In contrast, during separation-dependent elliptical vibration cutting, the high-frequency impact of the tool on the workpiece reduces contact stress, thereby minimizing tool wear.

 

2.4 Surface Quality

Surface quality is a crucial factor that influences the fatigue life of a workpiece, and the quality of machining directly impacts its service life. In recent years, many researchers have focused on enhancing the surface integrity of challenging metals, such as titanium alloys, by employing ultrasonic vibration-assisted cutting techniques. This approach has yielded significant improvements in surface integrity. The effects of ultrasound on surface properties are illustrated in Figure 4.

 

Zou et al. conducted a study on ultrasonic vibration-assisted turning of 304 austenitic stainless steel, focusing on the impact of ultrasonic vibration on surface roughness. They determined that appropriately matching the process parameters could decrease surface roughness and enhance overall surface quality.

Sui et al. explored the formation mechanisms of surfaces produced by ultrasonic high-speed turning and developed a predictive model for surface topography and roughness using numerical and finite element simulations. Their findings indicated that ultrasonic high-speed turning achieves superior surface quality compared to conventional cutting methods, with surface roughness values (Ra) falling below 0.4 μm.

In a study of TC18 titanium alloy, Xie et al. utilized ultrasonic vibration-assisted milling to examine the effects of machining parameters on surface integrity. They found that rotational speed had a significant impact on both surface topography and residual stress. The surface morphology resulting from ultrasonic machining was more regular compared to that produced by conventional milling, and there was a 50.9% increase in residual compressive stress on the cut surface. Additionally, ultrasonic vibration was shown to create a plastic deformation layer on the machined surface, which in turn enhanced hardness and wear resistance. The thickness of this plastic deformation layer increased with higher ultrasonic vibration amplitudes.

To improve the wear resistance of titanium alloy surfaces, Peng et al. employed high-speed ultrasonic vibration-assisted turning (HAVAT) on these materials. Their characterization of the machined surface integrity revealed that the HAVAT process significantly reduced surface roughness compared to conventional turning. Moreover, they observed a substantial increase in the depth of plastic deformation in the surface layer, leading to the formation of a nano-grained gradient layer that enhanced surface microhardness. Residual compressive stresses reaching as high as 840 MPa were also noted on the machined surface.

 

2. Current Status of Ultrasonic Hybrid Cutting Research

Hybrid energy field-assisted cutting is a technique that merges single energy field-assisted cutting with additional energy fields to enhance machining collaboration. This approach capitalizes on the strengths of various energy fields to mitigate the limitations of single energy field-assisted cutting, ultimately improving machining stability, efficiency, and quality.

 

3.1 Ultrasonic Laser Processing

DESWAL et al. developed a novel ultrasonic hybrid laser-assisted cutting process that combines ultrasonic vibrations with laser processing. They conducted experiments using 3003 aluminum alloy to compare and analyze the effects of traditional cutting, ultrasonic vibration-assisted machining alone, and laser machining alone on the material’s processing performance. The results showed that the ultrasonic laser hybrid cutting method produced lower surface roughness, reduced cutting forces, and smoother chips compared to the other machining methods. This improvement is attributed to the hybrid process’s combination of the contact-separation phenomenon during ultrasonic machining and the thermal softening effect of laser machining, which together lead to lower cutting forces and smoother chips.

However, the ultrasonic laser hybrid process exhibited the highest cutting temperature during machining. This is due to the thermal effect of the laser, which concentrates heat energy in the machining area during the separation between the tool and the workpiece, resulting in elevated cutting temperatures.

In another study, Su Yongsheng et al. examined the impact of the ultrasonic laser composite energy field on the milling performance of titanium alloy. They analyzed how this hybrid approach affected various performance indicators such as surface morphology, cutting force, and tool wear. Their findings indicated that when performing conventional CNC precision milling with laser selective melting on titanium alloy, the cutting force decreased as the cutting speed increased but increased with higher feed rates. This method also resulted in noticeable tool marks on the surface. Conversely, using ultrasonic laser selective melting on titanium alloy resulted in relatively low cutting forces and smoother machined surfaces. This is attributed to the combined effects of the laser altering the metallographic structure of titanium alloy and the contact-separation characteristics present during ultrasonic machining.

Moreover, the study revealed that, within the selected parameter range, both the front and rear blades of the ultrasonic and conventional laser selective melting titanium alloy tools experienced significant adhesion. This suggests that the influence of ultrasound on improving the anti-adhesion properties of the tool blades is limited. The selection of appropriate process parameters markedly impacts machining outcomes.

DOMINGUEZ et al. investigated the effects of cutting parameters on cutting forces and surface roughness in laser-ultrasonic hybrid machining. They discovered that both single-laser and ultrasonic machining reduce cutting forces and temperatures compared to conventional cutting methods, but they are primarily constrained by lower cutting parameters. On the other hand, ultrasonic-laser hybrid machining combines the advantages of ultrasonic tool-workpiece separation and laser thermal softening, expanding the range of applicable cutting parameters and further decreasing cutting forces and surface roughness while enhancing machining efficiency.

 

3.2 Ultrasonic Magnetic Field Machining

YIP et al. combined ultrasonic energy fields with magnetic fields to machine titanium alloys. By introducing a magnetic field into ultrasonic-assisted diamond cutting, they aimed to address the limitations of ultrasonic machining. This approach minimized surface damage and side burrs caused by tool vibrations, ultimately improving the surface quality of the titanium alloy.

Experimental results indicated that the presence of a magnetic field significantly reduced the material swelling that is typically enhanced by ultrasonic tool motion. Additionally, the magnetic field minimized the area of the cutting scar resulting from the cyclic motion of the ultrasonic tool. The percentage errors in groove depth and width were significantly reduced to 1.69% and 1.77%, respectively, when a magnetic field was applied. Figure 5 illustrates the effects of different machining methods on the surface profile.

 

3.3 Ultrasonic Electrodischarge Machining

Dong et al. established a material removal model for the TC4 titanium alloy during ultrasonic electrodischarge machining (UEDM) by analyzing voltage variations and applying heat transfer theory. They examined the impact of different amplitudes on the material removal process and demonstrated that UEDM can achieve higher material removal rates and improved surface quality.

Kurniawan et al. conducted UEDM-assisted turning on titanium alloys and utilized finite element simulation to assess the influence of various factors on the CNC machining process. Their findings indicated that electrodischarge machining (EDM) softens the workpiece material and enhances the material removal rate, while ultrasonic vibration decreases cutting forces, reduces tool wear, and suppresses burr formation.

Xu et al. employed this technique to machine titanium alloys, comparing EDM with ultrasonic vibration-assisted milling. Their results showed that UEDM combines the advantages of both ultrasonic vibration and EDM. EDM softens the material’s surface, thereby lowering cutting forces, while ultrasonic vibration increases EDM’s discharge efficiency and further reduces cutting forces.

In comparison to conventional milling (CM), ultrasonic-assisted machining (USM), and electrodischarge machining (EDM), UEDM offers superior surface integrity and significantly reduces the occurrence of edge burrs. The effects of ultrasonic EDM on cutting performance are illustrated in Figure 6.

 

3. Conclusion

This paper provides an overview of the current state of research on ultrasonic cutting of difficult-to-cut metal materials. It examines how ultrasonic cutting affects the cutting performance of these materials. Research has demonstrated that ultrasonic cutting offers several advantages, including reduced cutting force, lower cutting temperatures, extended tool life, and improved surface integrity during machining. However, there are still some challenges and issues that need to be addressed. The paper also outlines the current problems and challenges associated with ultrasonic cutting of difficult-to-cut metal materials.

(1) Developing special high-precision vibration cutting machine tools
We are developing specialized vibration cutting machine tools that feature high spindle rotation accuracy. By fully utilizing the precision cutting capabilities of vibration cutting, we aim to apply these tools in the field of micro-nano machining. This approach will enhance the machining accuracy and surface quality of workpieces, thereby meeting the stringent requirements for high-precision parts.

(2) Promoting the numerical control and intelligentization of ultrasonic cutting
With advancements in numerical control technology, the integration of ultrasonic cutting techniques into CNC machine tools allows for more precise and consistent motion. This, in turn, enhances machining accuracy. Additionally, when combined with artificial intelligence, ultrasonic cutting can be intelligently controlled, enabling the optimization of cutting parameters. As a result, both machining efficiency and surface quality can be significantly improved. This progress will facilitate the broader application of ultrasonic cutting technology across various fields.

(3) Development and improvement of ultrasonic cutting standards.
To encourage the widespread use and standardized development of ultrasonic cutting technology, it is essential to develop and enhance relevant technical standards and specifications. These standards will address the performance requirements for cutting equipment, the selection and usage of cutting tools, and the optimization of cutting parameters. This comprehensive framework will provide substantial support for the standardization and large-scale application of ultrasonic cutting technology.

 

 

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