Metal cutting is considered the most significant procedure where the material removal process is performed by more than one edge to fulfill some of the specific criteria needed to shape the final workpiece in the form of particular geometrical dimensions and surface finish. Even though there has occurred a lot of advancement in the development of machining tools and process controlling and monitoring, there still exists room for improvement in metal cutting processes and procedures for which significant research can be conducted by various metal-forming industries [1]. This may occur due to the optimizing procedures’ intricacy, particularly when the materials with higher “mechanical characteristics” are developed and the machinability has not yet been inspected fully [2].

In addition to the material removal and metal cutting process, there is a fine requirement that product surface quality is considerably better since they play a major role in aerospace, automotive, and biomedical industries [3]. Even a small scratch or an uneven burr can cause huge damage to the engine. It may fail the aerospace gadgets or malfunction any of its components, etc. These industries are trying their best to spend a huge amount on the finishing of such components in order to make components that are cutting/burrs marks-free [4]. For the last 40 years, various manufacturing industries have been using conventional procedures for finishing the components, such as grinding, honing, lapping, etc., to get the machining components’ desired finishing. However, these conventional procedures of finishing are restricted to very few geometries and cannot work on complex and intricate geometries as well as complicated profiles for finishing of high level, which is required while the operation of the component is in process. These limitations and restrictions in the finishing process have led the industries to develop advanced finishing procedures, known as “Abrasive flow machining (AFM)” [5]. AFM process is used to perform the finishing process of the machining components and its internal features in several engineering materials such as alloys, ceramics, non-ferrous, super-alloys, refractory materials, semiconductors, carbides, quartz, and various other composites, etc., that are considered to be difficult to be done using the traditional processes economically and proficiently [6]. The significance of this AFM process is that it generates the nano-level finishing of the machining components that are essentially desirable at this time. The concept of the AFM finishing procedure was first given by the “Extrude Hone corporation of the USA” in 1960 for the finishing of aerospace components to achieve the desired accuracy. These days, the AFM process is the best technique for finishing intricate geometries, which were never achievable by any traditional finishing tools. Even a lot of study has been conducted by researchers in order to improve the finishing process performed by the AFM [7]. The AFM process efficiently improves the surface finish of the complicated geometry like gears, trim-dies, turbine blades, bio-medical implants, etc. [8,9]. Many researchers have highlighted the potential of the AFM process to finish any complex and freeform surfaces [10,11]. Researchers have also developed a hybrid form of AFM processes, i.e., rotational type abrasive flow machining (R-AFM), to finish the asymmetrical complicated workpiece [12]. Magnetorheological abrasive flow finishing (MRAFF) has also been developed under the umbrella of the hybrid AFM process to finish optical surfaces [13]. Many researchers have employed the AFM and its hybrid variants for surface finishing of metal matrix composite materials, i.e., Al/SiC MMCs [14,15]. Hybrid manufacturing processes, i.e., ultrasonic-assisted machining processes, are also a good solution due to less cutting force required during the machining operation [16]. Ultrasonic-assisted magnetic abrasive finishing (UAMAF) significantly improves the surface roughness as well as the hardness of the workpiece [17,18]. Centrifugal force-assisted abrasive flow machining (CFAFM) is an efficient hybrid machining process to finish cylindrical surfaces with better surface roughness [19].

The loose AFM process is considered the most significant approach to finishing the intricate and complex geometries and advanced materials used within industrial practices [20,21]. It is considered to be the recently developed “surface finishing process” that has been used widely in various industrial applications such as defense, aerospace, bio-medical, tool and die, etc. [22]. In loose AFM, no structure exists that connects or links the grains together in the matrix in bonded “abrasive process.” The various standard processes in Loose abrasive processes include AFM, polishing, lapping honing, and abrasive blasting. AFM is also one of the finishing techniques that entail the oil for the particle of abrasive and carrier polymer and assists in eliminating the barriers to acquiring the desired size and shape [14]. Thus, any complicated shape can be easily finished by Abrasive Flow Machining abbreviated AFM. Abrasive flow machining helped us to tackle the shortcomings of conventional processes like “grinding,” “lapping,” and “honing” [20]. The problem associated with AFM is its high cost and “low material removal rate MRR,” which makes it laborious and strenuous. AFM processes of finishing techniques have also emerged recently to tackle the challenges and issues such as processes requiring high machining time [23,24,25,26]. Figure 1 below illustrates the “loose abrasive-based machining” (LAM) process.

Figure 2 below signifies the various categories and sub-categories of the loose abrasive-based machining process.

The machining accuracy of different traditional and advanced machining processes is described in Figure 3.

A summary of loose abrasive-based machining processes about their basic principle and process parameters is shown in Table 1.

The comparison of surface roughness of traditional finishing techniques s shown in Figure 4 below.

The comparison of surface roughness of advanced finishing techniques, i.e., nano finishing techniques, is shown in Figure 5 below.

AFM is considered to be the non-conventional process of finishing that deburrs, polishes, alleviates the recast layers, and has the potential to generate the “compressive residual stresses” at the part where it is difficult to reach with the conventional finishing process. Few of the major applications of AFM include the finishing of various aerospace and medical components, automotive parts, and high-volume generation of electronic parts. Several materials such as soft aluminum, ceramics, tough nickel, and carbides have been micro-machined successfully through this process [36]. The process of AFM gives the high-level surface finish as well as close geometric tolerance at reasonable and economical rates for a significant range of industrial parts and components [37]. One of the basic reasons behind implementing the AFM process is the ability of its media to finish the intricate points in the components where it is difficult to reach using conventional methods. Also, it can follow the “complex contours” and work simultaneously on several edges and surfaces, thus making it more desirable in comparison to the other finishing processes [38]. In the manufacturing process, several mechanical parts undergo one machining operation at least once in order to fulfill a certain function within the environment [39]. Some criteria for the quality of such pieces are desired, such as dimensional, roughness, as well as geometric specifications, etc.

AFM alleviates small quantities of the material through “semi-solid abrasive laden media” across the workpiece. In this process, two vertical cylinders opposing each other extrude the media in both directions (back and forth) across the passage in the workpiece as well as tooling, as shown in Figure 6. Here, the media comprises the abrasive grains and that of the semi-solid carrier, where the media acts as a “self-deformable stone” possessing protruding abrasive particles that act as the cutting tools.

The abrasive flow machining process can provide good results for the machining components that need the removal of the defects created by the mechanical or manufacturing processes. The AFM process makes ideal for relief of surface stress, radiusing, polishing, deburring and geometric optimization [41]. Being an essential requirement of industry these days, a large amount of research work has been reported in this domain in literature.

AFM is considered to generate the “self-deforming tool” that accurately removes the extra workpiece material and then finishes the surface from the areas where it is difficult to do so with the conventional machining process [42].

The material removal process comprises three modes of deformation, which are as follows:

• “Elastic deformation”: it correlates with rubbing
• “Plastic deformation (ploughing)”: it correlates with the material being displaced without being alleviated.
• “Micro-cutting” [43,44,45]

Initially, in [46], the authors developed the one-way process of AFM where the medium traveled in a single direction and termed it to be the simplest type of AFM that consumes the least time during the finishing process [47,48,49,50]. It was observed that surface roughness could be minimized to 90% on the machined, cast, or EDM surface having the “dimensional tolerance” up to or ±0.005 mm [51,52].

In order to get a better radiusing and finishing action not only in the inner but outer surface as well as the components, two way AFM process of finishing was developed [53,54]. Its basic operation involves two horizontal or vertical hydraulic cylinders that lie opposite each other and are placed in between the work piece with the help of suitable fixtures, as illustrated in Figure 7. The pistons in the cylinders are used to make the “abrasive laden medium” move back and forth on the surface that needs to be finished. In this way, the finishing process is carried out in a two-way AFM operation.

In [55], another development was made regarding the oscillatory motion of the workpiece. With oscillations to keep going, the work piece hits the “abrasive medium” with the “eccentric path” that causes the intricate and complex structure to make an interaction fully and completely with that of the abrasive medium, which results in the same (equal) abrasion on each side of the components. In [56], an AFM setup has been developed on the lathe to carry the experiment on brass as well as aluminum. It was observed that the dominant factors are the “abrasive concentration” in the medium, followed by the mesh size, speed of the medium flow, and the number of cycles. A lot of improvement could be seen in the case of materials that were soft, considering the surface roughness enhancement. In [57], the temperature has been considered to be the most influential parameter in the AFM process regarding work efficiency; with the escalation in no. of cycles, the medium temperature increases, which means the decrease in the medium viscosity occurs with the increase in no. of cycles. In work, it has been presented that in AFM testing, an increase in no. of cycles significantly decreases the material removal rate (MRR) as well as surface roughness, thus reducing the efficiency [58].

In order to enhance the process performance and control the rheological properties, scientists and researchers have designed and developed several AFM process variants [59,60] by combining and incorporating the basic AFM operation with that of the traditional/non-traditional flow machining operation. The process of AFM has been classified as per the energy and forces used within the process [61].

Other than the above-mentioned four types of AFM process, there have also been developed some other AFM variants in order to compensate for the MRR and also for a longer finishing time. The variants of the AFM process have few external assistants, such as ultrasonic vibrations [62,63,64], the rotational effect [65,66,67], magnetic field assistance [39,68,69], and also the hybrid variant of the AFM process [54,70,71,72]. The external assistance is found to show the increase in the finishing force [73,74,75], abrasive velocity, enhancement in the contact length among the work surface as well as abrasive, active density of abrasive, or utilization of the synergic effect of two processes in order to eradicate the material [76]. These effects result in enhancing the finishing time, MRR [62,63], and surface roughness. The further classification of the developed AFM variants has been shown in Figure 8 below.

A summary of the differences and similarities of the different AFM process variants is shown in Table 2.

In order to give the best control on the AFM processes, the selection of the parameters of the process is necessarily required. This review paper has segmented the AFM process parameters into three main categories. The classification of these three main AFM parameters, including machine, medium, and workpiece [77,78,79]. Here, the machine decides the level to which the abrasion can be processed through “viable extrusion pressure,” flow rate, flow volume, and also the number of cycles [80,81]. “Rheological properties” of “abrasive laden medium” are the ones that signify the amount of abrasion. The grit size, viscosity, temperature, and polymer to dilute are considered to be the parameters for comprehending the rheological properties of the MR fluid. Next comes the workpiece that is finished by the AFM process. The properties of the workpiece, such as material type, roughness value, and also geometry, tell the machining time and abrasive type to be used [82].

Summary of the performance analysis of the AFM process regarding their process parameters is shown in Table 3.

The typical defects during different manufacturing processes for electronic components, pipes, welding parts, and textile materials can be improved using the abrasive-based machining process, i.e., the AFM process and its variant processes, as shown in Figure 9.