**2. AFM process parameters and classifications**

#### **2.1 AFM process parameters**

Rhoades [2–4] describe the fundamental theory of AFM process and recognized its process parameters. He reported that to facilitate the depth of cut first and foremost depends upon extrusion pressure, relative hardness, sharpness and also abrasive grain size. The controllable AFM process parameters are classified as in **Table 1** [5–9]. Perry showed that AFM can be used in engineering applications such as deburring, improving surface finish, contouring, elimination of recast layers generated due to thermal processing, etc. [10].

#### **2.2 Classification of AFM machines**

 Abrasive flow machines are basically distributed into three categories: one way AFM, two way AFM, and orbital AFM. A short description of these types is provided below. In a one way AFM arrangement, the abrasive media enters forcefully into the work piece passage at an opening position and then exits on the other side, leaving a smooth internal passage to mark its passage [11]. For a more powerful polish, two way AFM arrangements might be employed. In a two way AFM arrangement, two hydraulic cylinders can control the abrasive media flow. These cylinders push and pull the media through the work piece [12]. It gives an extremely polished, smoother end result in less time compared to a one way AFM, which is shown in **Figure 1**. In the orbital AFM, orbital vibrations of a low amplitude are used to achieve the desired surface finish requirements (**Figure 2**) [13].


#### **Table 1.**

*Classification of AFM process parameters [5–9].* 

*A Critical Study and Review on Abrasive Flow Micro Machining Process for Parameter… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

**Figure 1.**  *Two-way AFM process.* 

AFM makes it feasible to polish and smooth the area that could not be easily reached by enabling the media to flow from beginning to end of the object internally. **Figure 3** shows the mechanism of material removal for different AFM processes. The following modes have been identified for metal deformation in abrasive machining process. First, elastic deformation related to abrasion; second, plastic deformation where a greater part of the object is displaced without being removed; and third, micro-cutting where material is eliminated in the form of chips of a micro size. The existence of any particular mode of deformation basically depends on the magnitude of cutting forces exerted on an individual grain, and the resistive force offered by work piece.

**Figure 3.** 

*Mechanism of material removal for AFM processes.* 

#### **3. Experimental investigation explored in abrasive flow machining**

 Investigations by experimentation have been approved by a variety of investigators and reported that the total time to reach the essential finish is longer if the material deletion rate is low. To improve the performance of the AFM process, a variety of investigators have developed the mixed-mode machining processes in which different machining processes are used in conjunction with the AFM process to obtain a superior material removal rate (MRR) and obligatory surface finish in a smaller amount of time. Several of the latest developments in mixed-mode AFM processes are described here. Loveless [14] stated that dissimilar types of machining operations used to prepare the object before carrying out AFM is necessary and affects surface finish obtained throughout the process. The outcome shows that extrusion pressure did not have an essential effect while the type of machining method affected both metal removal and surface finish results. It also has been reflected that the viscosity of media significantly affected surface roughness. Authors [15] introduced a newly developed hybrid process called R-AFF. In this process, they rotated the work piece to increase the interaction between the work piece and the abrasive particles, which leads to improvements in the material removal rate and surface finish. The result shows that R-AFF can produce a 44% improvement in surface smoothness and an 81.8% improvement in MRR when compared to the AFF process. Researchers [16] inserted a drill bit in the experimental set-up to control the path of medium flow. This has been called the Drill Bit Guided Abrasive Flow Finishing (DBG-AFF) by researchers. The experiments were conducted on AISI 1040 and AISI 4340 materials. The arbitrary motion of abrasive particles generated by this arrangement helped to increase the interaction of fresh abrasive particles with work piece surface. Superior finishing rate and texture improvement have been reported in DBG-AFF when compared to AFM and R-AFF. **Figure 4** shows the DBG-AFF process. Authors [17] also tried to introduce the action of a centrifugal force on the abrasive laden viscous material. This experiment was carried out by inserting a rotating rod through the passage of the work piece. It was reported that the centrifugal force created by this rod is useful for the improvement in surface finish and material removal rate.

*A Critical Study and Review on Abrasive Flow Micro Machining Process for Parameter… DOI: http://dx.doi.org/10.5772/intechopen.81083* 

**Figure 4.**  *Schematic diagram of DBG-AFF.* 

#### **3.1 Modeling and optimization**

It is essential to construct a mathematical model to understand the effect of various process parameters to achieve desired surface finish in a limited time period, that is, rate of material removal.

The neural network technique has been utilized for the purpose of developing a model of AFM process by researchers [18]. Five significant process parameters have been reported and are as follows. Parameters related to mechanical properties of the work piece, machining behavior, technical specification of set-up, process objectives, and viscosity of working media. This model primarily reduces the development time for new applications of the method and provides the data on effect of input variables on output parameters. Finite element model (FEM) has been used to forecast the stresses induced during AFM and forces exerted on the work piece surface [19]. Using this approach, they calculated the force exerted by each abrasive particle and depth of indentation produced on the work piece. Fletcher et al. [20] introduced a new relationship between media rheological properties and the AFM process. They investigated the shear rate of the polymer when it passes through the restricted area of the work piece. It was observed that the rate of shear increases in this case. They also concluded that better surface finish can be obtained by increasing the length of the piston stroke. Higher shear stress is generated on the wall and the coefficient of viscosity decreases because of the increase in piston stroke. The effect of the tangential force and related specific energy have also been investigated based on five process parameters such as hardness of the work piece, applied pressure, number of cycles, grain size, and number of active grains [21].

#### **3.2 Developments in the abrasive medium**

Abrasive medium is the most important and central ingredient of the AFM process. The abrasive medium is the viscoelastic polymers that act as mover media and abrasive particles that act as a wounding device to remove the rough material from the work piece. Numerous studies and investigations have been carried out in this area and they have tried to introduce a new substitute media for AFM process.

Some fresh advancement in the abrasive media by various researchers for AFM process is described in this part.

 Wang et al. [22] developed an abrasive gel that has silicone rubber (P-silicone) and silicone rubber with additives (A-silicone). They concluded that silicone rubber (P-silicone) and silicone rubber with additives (A-silicone) are more suitable as media because they are easily available and also cost-effective. They also concluded that the surface finish is improved with A-silicone abrasive media with the number of cycles being less than five. Sankar et al. [23] used styrenebutadiene rubber (SBR) as the media and silicon carbide—SiC as the abrasives to study the effect during finishing on three types of work piece materials in AFM process. They proposed that surface roughness increases step-by-step as weight fraction of oil increase up to 10% and then starts falling. They also concluded that rheological properties of the media affect the material removal rate (MRR) and surface finish.

#### **4. Various applications of AFM**

Many industries and investigators applied AFM process on a variety of components for achieving greater surface finish. Some of the applications of the AFM process are as follows.

#### **4.1 Machining of industrial and bio-medical components**

Li et al. [24] used AFM equipment to achieve better surface roughness for a nonlinear tube runner in the military and civil field. Result shows that an AFM tool is important to achieve better surface integrity. Xu et al. [25] showed that AFM achieved better quality in helical gears. Wang et al. [26] improved the quality of the nozzle that is used in the diesel engine using the AFM process. Kumar et al. [27] introduced rotational-magnetorheological abrasive flow finishing (R-MRAFF) to achieve nano level surface roughness while finishing freeform components.

#### **4.2 Machining of MEMS (micro-electro-mechanical systems)**

Using the electron discharge machining process, we produced the micro channels but this process is prone to produce a recast layer in the area of machining. Focused ion beam (FIB) milling is also used to machine micro channels [28]. Microchannels are fundamental structure blocks of micro fluidic technology. Yan et al. [29] used a media to eliminate the recast layer from the micro channels formed by wire-EDM. They showed that the quality of the channels is improved with the use of the AFM process by removing recast layer and burrs. A miniaturized part such as fuel injector, micro filter, and ink-jet printer nozzle has a micro bore. These micro bores are finished with the help of AFM to offer better surface finish by removing internal unevenness.

#### **5. Conclusion**

The following concluding remarks are concluded from the above evaluation. AFM is a well-recognized superior finishing process. It uses abrasive laden medium that has the viscoelastic polymer and acts as a mover medium. Abrasive particles act as a cutting tool and remove the material from the work piece. AFM is normally

#### *A Critical Study and Review on Abrasive Flow Micro Machining Process for Parameter… DOI: http://dx.doi.org/10.5772/intechopen.81083*

 useful to finish multifaceted shapes for improved surface smoothness values. To overcome the drawback of low finishing rate, researchers have projected a mixture of AFM machines like as Orbital AFM, MRAFF, R-AFF, DBG-AFF etc. These processes have been effectively useful to finish works with complicated profiles generally used in bio medical, aerospace, automotive, die mold industries and the power sector. There is a need for more hybrid processes that improve the surface finish significantly. Abrasive flow machining is a possible candidate for further developments.
