A Review of Magnetic Abrasive Finishing for the Internal Surfaces of Metal Additive Manufactured Parts

Abstract

With the rapid development of high-end manufacturing industries such as aerospace and national defense, the demand for metal additive manufactured parts with complex internal cavities has been steadily increasing. However, the finishing of complex internal surfaces, especially for irregularly shaped parts, remains a significant challenge due to their intricate geometries. Through a comparative analysis of common finishing methods, the distinctive characteristics and applicability of magnetic abrasive finishing (MAF) are highlighted. To meet the finishing needs of complex metal additive manufactured parts, this paper reviews the current research on magnetic abrasive finishing devices, processing mechanisms, the development of magnetic abrasives, and the MAF processes for intricate internal cavities. Future development trends in MAF for complex internal cavities in additive manufactured parts are also explored; these are (1) investigating multi-technology composite magnetic abrasive finishing equipment designed for complex internal surfaces; (2) studying the dynamic behavior of multiple magnetic abrasive particles in complex cavities and their material removal mechanisms; (3) developing high-performance magnetic abrasives suitable for demanding conditions; and (4) exploring the MAF process for intricate internal surfaces.

1. Introduction

The rise of additive manufacturing technology has driven innovation and progress in the manufacturing industry, revolutionizing production processes and design methods while significantly improving production efficiency and product quality. This technology, which integrates a highly interdisciplinary knowledge system, has become one of the most transformative and influential advanced manufacturing technologies of the 21st century [1]. Figure 1 illustrates the manufacturing principles of additive manufacturing technology. As shown in the figure, the technology is based on computer digital models and employs a layer-by-layer manufacturing and accumulation approach, enabling the near-net-shape formation of complex geometries. Compared to traditional manufacturing processes, additive manufacturing offers advantages such as shorter production cycles, higher material utilization, and greater tolerance and design flexibility [2,3,4]. It has been widely applied in the innovative design and structural repair of complex components [5,6,7].

Due to the use of the path-by-path overlap and layer-by-layer stacking manufacturing method, metal additive manufactured parts typically exhibit inherent defects such as a balling effect, staircase layering, micro-cracks, and powder adhesion [8,9], as shown in Figure 2. With the accumulation of these defects, the original surface quality of the as-printed parts gradually deteriorates, and the surface roughness (Ra) often exceeds 10 μm [10,11]. Therefore, to meet the requirements of industrial applications, finishing has become an essential post-processing step for additive manufactured parts [12].

Currently, common finishing methods for metal additive manufactured parts include manual polishing, laser polishing, chemical or electrochemical polishing, fluid cavitation abrasive polishing, abrasive flow polishing, and magnetic abrasive finishing. Due to geometric interference, manual polishing and laser polishing are primarily suitable for external surfaces, whereas polishing complex internal surfaces remains considerably challenging [13,14].

Chemical or electrochemical polishing can achieve surface finishing by selectively dissolving microscopic protrusions on the metal surface, which is particularly effective for removing adhered powder and spherical particles from additive manufactured parts. However, polishing liquids often contain acidic reagents, which pose hazards to both the environment and operators [15]. In electrochemical polishing, the structure and performance of the electrode directly determine the efficiency and quality of the polishing process. However, the limited accessibility of the electrode to the internal cavities of workpieces restricts its broad application for polishing complex internal surfaces [16].

Fluid cavitation abrasive polishing (FCAP) is a composite process that polishes internal surfaces by controlling the cavitation erosion effect to drive micro-abrasive particles [17]. The technical principles are illustrated in Figure 3 [18]. When high-pressure fluid passes through the cavitation chamber, cavitation occurs due to a sudden pressure drop. As cavitation bubbles are driven into the high-pressure region, micro-explosions occur near the surface of the workpiece, generating micro-jet impacts. These micro-jets accelerate the abrasive particles toward the inner wall of the workpiece (AlSi10Mg), producing effects of impact or micro-cutting. Through the combined action of cavitation erosion and abrasive wear, adhered powder and loose layers on the inner wall can be effectively removed. However, this polishing process typically takes several hours or even tens of hours, resulting in relatively low efficiency [19].

Figure 4 is a schematic diagram of abrasive flow manufacturing (AFM) [20]. Abrasive flow manufacturing applies appropriate pressure to viscoelastic abrasives, forcing them to flow through enclosed channels. During this process, hard particles exert a certain degree of extrusion and sliding action on the inner wall surfaces to achieve finishing. Due to the flowability and flexibility of the viscoelastic abrasives, the abrasives can adhere closely to the inner walls of the workpiece (H65 copper) for extended periods during the reciprocating flow through the enclosed channels, with relative motion occurring between them. Abrasive flow manufacturing can polish certain complex internal channels [21]. However, for internal channels with features such as blind cavities, blind grooves, thin walls, variable cross-sections, high length–diameter ratios, or complex geometries, the accessibility of the abrasives is relatively poor [22]. Additionally, due to the wall effect, over-polishing or the under-polishing is prone to occur near the inlet and outlet of the flow channels [23].

Figure 5 is a schematic diagram of magnetic abrasive finishing (MAF) for internal surfaces of AISI 316L stainless steel [24]. Magnetic abrasive finishing utilizes magnetic force to gather ferromagnetic abrasives inside the workpiece, forming a flexible magnetic brush that is pressed onto the internal surface with a specific pressure. When the magnetic field in the processing area changes, the magnetic brush moves along the direction of the magnetic field gradient, achieving internal surface finishing [25,26]. This technology has been widely applied in fields such as aerospace, precision medical devices, and the ceramics industry [27]. Due to the adaptive structure of the magnetic brush and the fact that both the normal grinding pressure and tangential relative motion are provided by the external time-varying magnetic field, it effectively overcomes the limitations imposed by the workpiece geometry. As a result, this technique shows great promise as a preferred method for polishing and finishing complex internal cavities [28,29].

Table 1. A characteristic comparison of common finishing technologies.

Method	Accessibility	Precision	Efficiency	Sustainability	Adaptability
Manual polishing	Weak	Low	Low	Good	External surfaces
Laser polishing [31]	Weak	High	High	Good	External surfaces
Chemical polishing [32]	Strong	Low	Low	Poor	Complex internal surfaces
Electrochemical polishing [33]	Relatively strong	Relatively high	Relatively high	Poor	External or regular internal surfaces
FCAP [18]	Relatively strong	Relatively high	Low	Relatively good	Regular internal surfaces
AFM [34]	Relatively strong	Relatively high	Relatively high	Good	Closed and constant cross-sectional channels
MAF [35]	Strong	High	High	Good	Complex channels with blind cavities or grooves

presents a characteristic comparison of common finishing technologies [30]. As shown in the table, manual polishing and laser polishing have poor accessibility for internal surfaces. Chemical or electrochemical polishing sacrifice environmental friendliness. Fluid cavitation abrasive polishing imposes strict requirements on the cross-sectional shape of internal channels, while abrasive flow manufacturing struggles to polish internal surfaces with blind cavities or blind grooves, and suffers from poor consistency in polishing quality. In contrast, magnetic abrasive finishing (MAF) uses magnetic energy as the power source for finishing, offering strong abrasive accessibility, good process controllability, high processing efficiency, and environmental friendliness. It is suitable for finishing various complex internal surfaces.

2. Research Progress on MAF

MAF, also known as magnetic finishing or magnetic polishing, is a surface finishing technology that uses magnetic force to drive ferromagnetic abrasives. As early as 1938, Kargolow introduced the magnetic force in finishing processes, marking the beginning of research on magnetic abrasive finishing technology. Subsequently, countries such as Singapore [36], Japan [37], China [38], India [39], and America [40] published a large number of related papers, which gradually led to the refinement of MAF technology [41]. With the rapid development of high-end manufacturing industries, MAF has shown promising application prospects. Several well-known universities both domestically and internationally, such as the University of Florida, National University of Singapore, Chonbuk National University, Utsunomiya University, Dalian University of Technology, Hunan University, Taiyuan University of Technology, Nanjing University of Aeronautics and Astronautics, Shandong University of Technology, and Liaoning University of Science and Technology, have invested significant efforts in promoting and applying MAF technology, yielding many research outcomes. This paper reviews the current research progress on MAF technology in four areas: device development, processing mechanisms, abrasive preparation, and process research.

2.1. Current Advances in the Research of MAF Devices

An MAF device typically consists of a magnetic field generation unit, either permanent magnets or electromagnets, and a mechanical structure that provides relative motion, such as reciprocating linear motion, rotational motion, or a combination of both. For workpieces with varying geometries, the device structure can be designed and adjusted to achieve the desired grinding or polishing outcomes. Among these methods, planar MAF is the simplest and most widely employed technique. Under the influence of the magnetic field, ferromagnetic abrasives are positioned between the magnetic poles and the workpiece, aligning themselves along the magnetic field lines. Through the combined effects of rotational and feed motions, the magnetic abrasives complete the finishing process across the entire surface [41].

Many researchers have made improvements to magnetic field generating devices, significantly enhancing the quality of MAF. As shown in Figure 6, Jiang et al. [38] increased the magnetic field gradient by introducing cross-shaped micro-grooves on the end face of the magnetic pole, thereby strengthening the magnetic grip on the abrasive particles. This modification effectively improved the efficiency of planar finishing.

To achieve a higher magnetic induction intensity in the processing area, Lee et al. [42] and Fan et al. [43] designed yoke structures in the magnetic circuit to constrain and guide the magnetic field lines. This design increased the magnetic flux density passing through the workpiece and reduced the divergence of the magnetic field lines, effectively enhancing the processing pressure.

Since electromagnets can adjust magnetic field intensity by controlling the current, some researchers have applied them to MAF and achieved favorable processing results [44]. Xie et al. [45] proposed an MAF scheme dominated by an alternating magnetic field. This scheme periodically adjusts the shape of the magnetic brush by controlling the magnitude and direction of the current in the coil, ensuring that the magnetic abrasives maintain good contact with the workpiece. Thamir et al. [46] developed a multi-coil collaborative control grinding device. This device powers the electromagnets in a specific sequence and direction, generating a rotating magnetic field in the processing area to drive the movement of the magnetic abrasives, thus eliminating the need for large mechanical movement systems.

According to the Preston equation [47], the relative motion between the abrasive and the workpiece is one of the necessary conditions for achieving surface finishing. Therefore, many researchers have conducted extensive studies on the mechanisms that generate relative motion. Barman [48] and Zou et al. [37] added a helical feed motion to rotating cylindrical magnetic poles, making the abrasive trajectories more dense and significantly improving the uniformity of polishing quality. Some researchers have introduced vibration devices into MAF to enhance processing efficiency and quality [49]. Guo et al. [50] applied high-frequency vibrations to the magnetic pole using a voice coil motor, significantly improving the surface quality of micro-grooves without damaging the microstructure. As shown in Figure 7a, Zhang et al. [36] used a vibrating cylinder to drive the magnetic pole, producing a linear reciprocating motion that effectively improved the surface integrity of 3D-printed parts. To address the challenge of polishing high-strength alloy steel, Guo et al. [51] introduced ultrasonic vibrations into the processing area through an amplitude transformer, as illustrated in Figure 7b. This device significantly improved grinding efficiency and enhanced the surface quality of both the bottom and side surfaces of annular grooves.

In internal surface MAF, the primary motion is generally achieved by rotating the workpiece or magnetic pole. A sufficient gap is usually maintained between the magnetic pole and the workpiece to avoid motion interference, and the workpiece itself has a certain wall thickness. Consequently, the distance between the magnetic abrasives and the magnetic pole increases, leading to insufficient processing pressure [52].

To address this issue, Zhou et al. [53] mixed ferromagnetic steel needles into the magnetic abrasives, utilizing the high magnetic permeability of the steel needles to enhance processing pressure. Chen et al. [29] and Han et al. [54] added V-shaped magnetic poles and spherical magnetic poles inside the workpiece, respectively. These auxiliary magnetic poles not only increased the processing pressure of the magnetic brush but also caused the magnetic abrasives to continuously be renewed, thereby extending the service life of the magnetic brush.

In summary, with the continued promotion and application of MAF technology by scholars, innovative designs for MAF devices have emerged, effectively solving grinding and polishing challenges for various workpieces of different shapes, materials, and sizes. These innovations provide valuable technical references for the development of MAF devices for complex internal surfaces. However, there is still significant room for improvement and optimization in the corresponding magnetic field and motion generation devices for the magnetic abrasive finishing of complex internal surfaces.

2.2. Current Advances in the Mechanisms of MAF

It is well known that mechanism research provides a theoretical basis for process control and quality optimization in processing operations, serving as the foundation for the application and development of MAF technology [55]. This has also been a key focus of researchers. Among these studies, the calculation of magnetic field parameters is a prerequisite for theoretical modeling. Based on Maxwell theory, Jayswal et al. [56] and Jain et al. [57] derived the governing equation for the scalar magnetic potential φ in the magnetic gap, as shown in Equation (1). After determining the appropriate boundary conditions, the magnetic field distribution in the processing area was obtained using numerical iteration methods:

$${\displaystyle \frac{1}{r}}{\displaystyle \frac{\partial }{\partial r}}\left[r{\mu }_r{\displaystyle \frac{\partial \phi }{\partial r}}\right]+{\displaystyle \frac{\partial }{\partial z}}\left[{\mu }_r{\displaystyle \frac{\partial \phi }{\partial z}}\right]=0$$

(1)

where μ_r is the relative permeability of the magnetic abrasive; r and z are the cylindrical coordinate components. Based on Ampere circuital law, Singh et al. [58] derived an expression for the magnetic flux density in the processing gap, further revealing the influence of ferromagnetic workpieces on the spatial magnetic distribution. In recent years, the widespread use of finite element software has also made the calculation of complex magnetic parameters more precise and efficient [59,60].

In a magnetic field, the magnetic abrasives become magnetized and are pressed onto the surface of the workpiece with a certain normal pressure. Typically, the pressure P exerted by a single magnetic abrasive on the workpiece surface can be expressed as [61,62]:

$$P={\displaystyle \frac{B^2}{4{\mu }_0}}\times {\displaystyle \frac{3\pi \left({\mu }_r-1\right)\omega }{3\left(2+{\mu }_r\right)+\pi \left({\mu }_r-1\right)\omega }}$$

(2)

where B is the magnetic flux density in the processing gap, μ_r is the permeability of the vacuum, and ω is the volume fraction of the ferromagnetic material within the magnetic abrasive. Based on this, Paswan et al. [63] used the principle of the Brinell hardness measurement to determine the indentation depth of a single magnetic abrasive on the workpiece surface. Gao et al. [64] considered the elastic deformation of the workpiece when calculating the cutting depth of the abrasive, making the results more consistent with real-world engineering applications.

When the magnetic field in the processing area changes, the magnetic abrasives move along the gradient of the magnetic field and perform micro-cutting on the workpiece surface with a certain cutting depth. Alam et al. [65] divided the mixture of ferromagnetic material and abrasives into cyclic units, with each unit simplified into a body-centered cubic (BCC) crystal cell model, as shown in Figure 8. Based on this model, the authors calculated the resistance torque experienced by the magnetic brush during the grinding process.

Based on the Johnson-Cook model, Shukla et al. [66] established a shear strength model of the workpiece material under high strain rates, revealing the mechanism of chip formation in ultrasonic-assisted MAF. Misra et al. [67] proposed a roughness prediction model for ultrasonic-assisted MAF. This model includes both steady-state and transient terms, establishing a functional relationship between the finishing quality and the initial roughness of the workpiece.

Kala et al. [68] characterized the arrangement pattern of magnetic abrasives within the magnetic field, as shown in Figure 9, and assumed that the distance between magnetic chains remains constant, meaning the shape of the magnetic brushes does not change during the MAF process. Based on this, the authors derived a functional expression for surface roughness as related to the particle size of the magnetic abrasives and their composition ratio.

As shown in Figure 10, He et al. [69] simplified the cross-section of the magnetic chains into a rectangular shape and established a material removal model. They investigated the effect of the compressive deformation of the magnetic brush on grinding pressure and experimentally validated the model’s accuracy.

In the field of internal surface MAF, Li et al. [70] established a material removal model for cylindrical internal surfaces based on the Archard wear theory, revealing the influence of process parameters on grinding efficiency. For a double-layer tubular structure, as shown in Figure 11, Guo et al. [28], using the Preston equation, derived a material removal model for the double-layer internal surfaces (Inconel 718) and revealed the evolution of surface roughness at different rotational speeds.

With the advancement of computer technology, some researchers have used simulation methods to reveal the processing mechanisms of MAF. For example, Mosavat et al. [71] used the Smooth Particle Hydrodynamic (SPH) method to simulate the evolution of surface morphology at different rotational speeds, as shown in Figure 12. Numerical simulations make mechanism exploration more efficient and the results more intuitive, which can accelerate the development of new technologies in MAF.

In conclusion, numerous researchers have conducted in-depth studies on MAF technology, making significant progress. However, there is still room for further exploration in the analysis of MAF mechanisms. For instance, much of the existing literature focuses on the mechanism analysis of simple workpieces, such as flat surfaces, circular cross-section tubes, and annular cross-section tubes, while there are few published studies on the mechanisms of MAF for complex internal surfaces. Additionally, the interactions between magnetic abrasives are often overlooked, especially the multi-abrasive interaction mechanisms within complex internal cavities, which have rarely been reported.

2.3. Current Advances in the Magnetic Abrasive Preparation

Magnetic abrasives are the primary tool in MAF, consisting mainly of ferromagnetic materials, hard abrasive particles, and a binder or thickening agent, providing both magnetic properties and micro-cutting capability. The preparation process of magnetic abrasives plays a decisive role in their quality, and the quality of the abrasives is a key factor that influences the performance of MAF [72]. Therefore, the preparation process of magnetic abrasives has consistently attracted attention from various research institutions.

Currently, common magnetic abrasive preparation methods include sintering, rapid solidification by atomization (RSA), chemical composite plating (CCP), bonding, and simple blending (SB) [73]. Among these, sintered magnetic abrasives are made by sintering ferromagnetic powder, hard abrasives, and metallic binders at high temperatures [39]. Chen et al. [74] used a scanning electron microscope (SEM) to characterize magnetic abrasives with different abrasive phases. The authors explored the effects of the sintering process on the performance of magnetic abrasives and optimized the parameters. Liu et al. [75] conducted a comparative analysis of the grinding performance of magnetic abrasives prepared by the sintering method and the simple blending method. The results indicated that sintered magnetic abrasives exhibited a higher material removal rate.

The rapid solidification by atomization method uses a high-pressure atomized gas flow containing abrasive particles to impact the molten iron matrix, followed by rapid cooling to form magnetic abrasives [76]. As shown in Figure 13a, Gao et al. [77] used the rapid solidification by atomization method to prepare magnetic abrasives with diamond abrasive phases, and investigated the intrinsic relationship between the feed rate of the abrasive particles and the sphericity of the magnetic abrasives. Jiang et al. [78] employed a two-stage atomization nozzle to prepare magnetic abrasives with cubic boron nitride (CBN) abrasive phases, as shown in Figure 13b. The morphological characterization results showed that the smaller the abrasive particle size, the better the encapsulation by the iron matrix.

Magnetic abrasives prepared by the chemical composite plating method are formed by co-depositing the abrasive particles and electroplated metal from the plating solution onto an iron matrix [79]. Yang et al. [80] studied the chemical adsorption effect between the iron matrix and diamond particles in the plating solution, determining the optimal composition ratio of the solution. As a result, they successfully prepared high-performance magnetic abrasives, with their morphology shown in Figure 14.

Magnetic abrasives prepared by the bonding method typically refer to composite material particles formed by binding ferromagnetic materials and abrasive particles together using an organic binder [81]. Zhao et al. [82] used resin glue, iron powder, and white corundum powder as the binder, ferromagnetic phase, and abrasive phase, respectively, to prepare quasi-spherical magnetic abrasives using the bonding method. These magnetic abrasives were produced through an extrusion molding process, avoiding the cracks often introduced by crushing methods. In MAF experiments on 45 steel, the abrasives achieved a material removal rate of 0.67 μm/min. Professor Sun Yuli’s group from Nanjing University of Aeronautics and Astronautics [83] used a solution blending method to prepare nano-particle-reinforced magnetic abrasives. MAF tests showed that compared to traditional bonded magnetic abrasives, these nano-reinforced abrasives exhibited higher polishing efficiency and a 50% longer lifespan.

The mechanical blending method is a simple process in which ferromagnetic materials, abrasives, and thickeners are mixed in a certain ratio to form abrasive clusters [84]. Heng et al. [59] used iron powder, carbon nanotubes, and diamond polishing paste to create mixed magnetic abrasives, successfully improving the surface quality of ZrO ceramic rods through magnetic abrasive finishing. Li et al. [85] innovatively added shear thickening agents to traditional mixed magnetic abrasives, significantly enhancing the material removal efficiency in MAF.

Table 2. Comparison of characteristics of common types of magnetic abrasives.

Magnetic Abrasive	Bonding Strength	Efficiency	Cost	Preparation Time	Processing Quality	Process Controllability
Sintered [90]	Strong	High	High	Long	Good	Poor
RSA [91]	Strong	High	High	Long	Good	Poor
CCP [80]	Sub-strong	Sub-high	Sub-high	Sub-long	Sub-good	Poor
Bonded [92]	Sub-strong	High	Low	Short	Good	Good
SB [84]	Poor	Low	Low	Short	Sub-good	Sub-good

presents a comparison of the characteristics of common types of magnetic abrasives [86,87,88,89]. Sintered magnetic abrasives offer strong bonding strength, high grinding efficiency, and a long service life, but their preparation process is highly dependent on sintering temperature, making the process complex and time-consuming. Magnetic abrasives prepared by rapid solidification by atomization exhibit high sphericity and good processing consistency, but require large-scale atomization equipment, leading to high preparation costs. Chemical composite plated magnetic abrasives avoid the micro-cracks typically introduced by crushing processes. However, the composition of the plating solution is complex and often corrosive, resulting in lower controllability and environmental friendliness. The simple blending method is simple to operate and has a low cost, but the ferromagnetic material’s grip on the abrasive particles is weak, leading to lower processing efficiency. Upon comparison, bonded magnetic abrasives, as consolidated tools, offer sufficient bonding strength while reducing preparation time and costs, making them suitable for small-batch production. Additionally, the mechanical properties of these abrasives can be flexibly adjusted by modifying the organic binder.

In conclusion, bonded magnetic abrasives offer a high cost–performance ratio. The increasing demand for magnetic abrasive finishing of complex internal surfaces in modern manufacturing is becoming more urgent. Under these circumstances, researchers are more inclined to choose the bonding method as the preparation process for magnetic abrasives, allowing for the rapid development of tailored MAF solutions for specific processing needs. Therefore, optimizing the preparation process parameters for bonded magnetic abrasives and improving their processing performance is of great importance.

2.4. Current Advances in the MAF Process for Internal Surfaces

Process experimental research plays an indispensable role in optimizing process parameters, refining fundamental theories, and expanding application fields. Currently, with the continuous development of MAF technology for internal surfaces, researchers have conducted extensive studies on key process parameters, such as the processing gap [93], magnetic field characteristics [94], relative motion speed [95], and abrasive specifications [96]. These studies have gradually revealed the influence of key parameters on the quality of internal surface finishing. Magnetic abrasive finishing for complex internal surfaces remains a highly challenging task, and many scholars are actively exploring this area, achieving some interim research results.

Paswan et al. [97] used radially magnetized cylindrical magnetic poles to perform magnetic abrasive finishing on the inner surface of a sleeve made of GCr15 steel. The authors applied the response surface methodology to optimize key factors such as magnetic pole rotational speed, ferromagnetic phase ratio, and feed rate, determining the optimal factor levels. After 40 min of processing under optimal conditions, the results indicated that the surface roughness (Ra) was reduced from the original 0.39 μm to 0.06 μm, significantly improving the surface quality.

Aggarwal et al. [98] used a combination of permanent magnets and electromagnetic coils to perform magnetic abrasive finishing on the internal surface (EN-31 steel) of a variable cross-section blind hole, including the conical sidewall and the flat bottom. The authors combined theoretical analysis with experimental research to investigate the effects of process parameters, such as finishing time, excitation current, and the magnetization of magnetic abrasives, on the processing quality. As illustrated, the surface morphology defects were effectively removed after magnetic abrasive finishing. The surface roughness of the blind hole’s sidewall and bottom was reduced from the original 390 nm to 90 nm and 50 nm, respectively.

Chen et al. [99] utilized a rotating magnetic field to perform magnetic abrasive finishing on the inner surface of a curved aluminum tube (6061 aluminum). The results showed that when the magnetic pole rotational speed was 800 r/min, the magnetic abrasive particle size was 150 μm, the feed rate was 6 mm/s, and the processing time was 45 min; the surface roughness (Ra) at the bend of the aluminum tube decreased from 0.67 μm to 0.13 μm. As illustrated, the wavy surface features were effectively removed, and the microscopic morphology was significantly improved.

To address the challenge of polishing the inner wall of fine tubes with a high length–diameter ratio (1 mm inner diameter, 1200 mm length), Deng et al. [100] proposed a method for self-rotating magnetic abrasive finishing of fine tubes, dominated by a strip array magnetic pole. Through single-factor experiments, the authors studied the effects of the magnetic abrasive particle size and filling amount on the surface roughness of the inner wall. The finishing results of the inner surface of the fine tube (Ni–Ti alloy) were as follows: after processing, the surface roughness (Ra) of the inner wall decreased from the original 0.75 μm to 0.08 μm, significantly improving the surface quality.

Heng et al. [101] used an automatically adjustable gap device to perform magnetic abrasive finishing on the inner surface of an elliptical cross-section tube (stainless steel). Through single-factor experiments, the authors revealed the effects of tube rotational speed, abrasive slurry content, and finishing time on the processing quality of the inner surface of the elliptical cross-section tube. After 24 min of magnetic abrasive finishing, deep scratches on the inner surface were effectively removed (as shown in Figure 15a,b), and the surface roughness (Ra) decreased from the original 0.20 μm to 0.05 μm. Additionally, the processed inner surface exhibited clear letter reflections (as shown in Figure 15c,d), indicating a significant improvement in surface morphology.

In conclusion, experimental research on magnetic abrasive finishing for complex internal surfaces has progressed rapidly, and several challenges in finishing complex internal surfaces have been successfully addressed. These research findings provide strong scientific support for expanding practical engineering applications. However, most of the internal surfaces processed in these studies typically have regular cross-sections, such as circular or elliptical shapes [102]. Finishing internal surfaces with irregular features, such as blind cavities or grooves, remains significantly challenging, and there is limited published literature on this topic. Therefore, there is an urgent need to propose a novel processing solution to address the difficulties of magnetic abrasive finishing for such workpieces.

3. Conclusions

Significant research progress has been made in the field of finishing metal additive manufactured parts. However, finishing complex internal surfaces remains a challenging engineering issue. Magnetic abrasive finishing for complex internal surfaces will face numerous challenges, including:

• Magnetic field and motion generation devices for MAF on complex internal surfaces still require significant improvement and optimization. Future research should focus on exploring innovative devices requiring assistance from multiple fields of physics.
• There is still room for further exploration in the mechanism analysis of MAF. Many studies focus on simple workpieces, such as flat surfaces, circular cross-section tubes, and annular cross-section tubes. However, there is limited published research on the mechanisms of MAF for complex internal surfaces. Additionally, the interactions between magnetic abrasives are often overlooked, particularly in complex internal cavities. Future research should focus on exploring the dynamics of magnetic abrasives under complex working conditions and their material removal mechanisms.
• In light of increasingly stringent internal surface finishing requirements, it is essential to develop new, efficient, and high-quality magnetic abrasives, optimize the preparation process parameters, and improve grinding performance.
• With the rapid development of additive manufacturing technology, the internal cavity structures of irregularly shaped parts requiring finishing are becoming increasingly complex, with higher precision requirements. In this context, single-method finishing solutions may have limitations. Therefore, composite finishing techniques that synergize multiple advanced processes are a crucial direction for further exploration in this field.