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Abstract

Poppet valves used in internal combustion engines have a high risk of failure due to significant temperature and pressure. These poppet valves need surface finishing at the nano-scale level to prolong their life during their working use. In the present research, the chosen poppet valve has narrow ridge profiles, which is difficult to nano-finish by conventional processes due to certain limitations. The magnetorheological fluid-based finishing method can be effectively used for this kind of complicated narrow profile. For the magnetorheological fluid-based finishing processing of the poppet valve, a novel magnet fixture and setup is used. For checking the efficiency of this setup, surface characterization and surface roughness for polished and unpolished surfaces are outlined using a field-emission scanning electron microscope, microscope and optical profilometer. The final surface roughness of S_a = 23.1 nm at poppet profiles were obtained. All manufacturing defects like burrs, dents, scratches and pits are almost removed. The study of finishing forces in the magnetorheological fluid-based finishing method is also carried out using magnetostatic fluid–solid interaction, experimental and theoretical analysis. This force analysis supports the development of the material dislodgement model to anticipate material removal rate while finishing. The gap (error = 12.87%) between the experimental and theoretical material removal rate is marginal. It has high accuracy and reliability for specific applications.

Keywords

Nanofinishing magnetorheological fluid material removal rate model magnetostatic fluid–solid interaction analysis poppet valve Nickel-aluminium-bronze (BS1400 Gr. AB2).

Introduction

The poppet valve is one of the classes of seating valves.¹ The poppet has some fundamental advantages over other types of valve because it is simple in construction, having the self-centring capability, the valve is free to rotate about the stem, and the sealing effectiveness and efficiency can be maintained easily than other types of valves. The intake and exhaust valves of the gas propulsion engine cylinder generally use poppet design. Poppet kind of valves is commonly fabricated in four primary configurations: semitulip, tulip, mushroom and ridge.¹ Nanofinishing of ridges of the new poppet is a challenging task due to its narrowness. Precise finishing of the narrow ridges will enable poppet valve to properly fit on the valve seat surface in the aerospace gas propulsion engine to make it leak proof. Poppet should have high-temperature resistance, mainly used as inlet and exhaust valves. Nickel aluminium bronze (BS1400 Gr.AB2) is used as a material for poppet valve due to its corrosion resistance and bearing properties. Excellent finishing up to 20 nm is required on these ridges. Surface roughness is an essential element in the estimation of part performance in a real environment.^2,3 A magnetorheological fluid-based finishing (MRFF) method, is used to render specifically easily manageable finishing forces with no sub-layer exposure.^4,5 In this method, magnetorheological (MR) fluid is the polishing media, a mixture of abrasive and iron grains in a carrier fluid. Under various magnetic fields, different finishing forces can be developed to finish the different workpiece components by altering the rheological properties of magnetorheological polishing (MRP) fluid. Also, by using electric or magnetic fields, better heat transfer enhancement using porous fin in the polishing system can be done.^6,7 This is because it is obvious that during the polishing process, a significant amount of heat is generated at the surface of the workpiece due to friction, which causes a change in the rheological properties of the MR fluid. As a result of this rheological change, the quality of the finishing surface deteriorates, and less quality control is achieved. Therefore, the polishing system needs to be able to better handle this heat for effective cooling during the polishing process.^8,9 The MRFF process also has the ability to finish the single crystal silicone blank.¹⁰ A rotational-MRFF procedure is utilized to finish the freeform surface of stainless steel knee implant.¹¹ The whole process of polishing with MRP fluid offers a nano-metre spectrum of extremely fine surface finish. This form of complex surfaces can be very effectively finished with the MR fluid-based polishing technique.

The performance of different operating parameters of the engine depends specifically on the position of initiation and duration of injecting air, manner of the beginning of the valve, air injection pressure, valve design and different control parameters.¹² Poppet valves are the usual method for regulating air/fuel combination in the cylinder and exhaust gas flow from the internal combustion (IC) engine.¹² Oxide film and tribo film can avoid direct metal connection and decrease the friction coefficient on the sitting region, decreasing adhesive wear and wear-regulated deformation.^13,14 Residual fuel-burning deposits on valve seats cause rapid corrosion due to a molten phase presence.¹³ The lamellar structure forms on valve head reduce the strength, matrix loading capacity, durability and resistance to gas corrosion.¹⁵ The valve seat leakage increases the percentage of hydrocarbon emissions. A good valve seat ensures the perfect mixture of air-fuel ratio. To ensure a proper seal, generally, the valve lapping process is used. So valve maintenance is a critical operation.^15,16

The lapping process is generally used for polishing the poppet valve and valve seats used in IC engines.^17,18 These lapping processes are done using a synthetic paste (mixture of oil and sand) on valve surfaces and rotating them on the head end in their respective seats/inserts.¹⁹ The lapping processes create circumferential scratches on the poppet valve, which can cause complete failure of the poppet value during its working use. However, in some cases, elaborate controlled lapping operations are used to remove tool marks or scratches caused by reaming tools or drills in the valve cavities.²⁰ Electrochemical honing (ECH) and micro-grinding were also used to get a higher degree of precise surface smoothness of the restoration surfaces of the engine valve face. ECH process has performed better than micro-grinding for the finishing of recovered surfaces.²¹ The grinding process damages the poppet valve by rigorous abrasion and creates intermittent scratch marks and dents. Reaming and polishing with abrasive paper are also used for polishing the poppet valve; however, they form a Beilby layer over the poppet valve surfaces. This Beilby layer reduces the heat transfer rate, which is prone to create huge thermal stress in the poppet valve and can cause early failure during its working use in IC engine.²² It is evident that when the surface is nano-finished and all defects are almost removed, poppet valve fatigue strength, wear and corrosion resistance get improved, which further enhances its life.^18,23

From the study of the previous works on the poppet valve, it was shown that no work on the magnetorheological process for the finishing of the poppet valve has yet been reported. The present work tries to achieve uniform nano-scale finishing along the poppet valve profiles, which may reduce the issues caused by previous finishing methods. This nano-scale surface finishing of the poppet valve will improve its wear and corrosion resistance and will further reduce the leakage problem of air/fuel combination to maintain the perfect air/fuel mixture ratio and will lower the hydrocarbon emissions.

To conquer these goals through MRFF process, in this work, a novel magnet fixture and setup are used for ultra-precision polishing along poppet valve working profiles. The current MRFF process also dislodges almost all manufacturing defects from the poppet valve profiles. The produced surfaces subsequent to MR polishing are investigated with various strategies to characterize the surface morphology of poppet valve profiles. Further, simulation of forces responsible for finishing is conducted by utilizing COMSOL® Multiphysics software focused on magnetostatic fluid–solid interaction finite element analysis (FEA) and theoretical analysis for the MRFF process. The finishing forces are also measured experimentally to verify the simulated and theoretical force values. Finally, a material removal rate (MRR) model is further formulated using outcomes of finishing forces analysis. After the MR polishing, the newly polished poppet valve may have better mechanical efficiency and more extended durability.

Experimental investigation

The magnet fixture consists of five permanent magnets at specified places. It is a requirement to design an ideal fixture to render enough magnetic flux in finishing zone. The magnet utilized is Sm₂Co₁₇ (Samarium-Cobalt) of grade 30 H. The magnet was chosen because of its good magnetic control and excellent corrosion resistance. Figure 1 shows the model of the poppet valve and their finishing profiles where polishing is required and the magnet fixture with an exact poppet valve profile. To polish the complex narrow ridge profiles of poppet workpiece, a similar profile on the magnet fixture is made. It was challenging to polish this kind of narrow ridge which has only a width of 0.32 mm (Figure 1) and a circular periphery. The major challenge was to polish only these ridges and make them leak-proof. To overcome this challenge, the same ridge profile has been created on the magnet fixture to prevent MRP fluid from entering the valley part (in between ridges) during rotation of the poppet valve in the computer numerical control (CNC) milling machine. During rotation of poppet valve, relative angular motion between these two ridge profiles (one on poppet and other on magnet fixture) will ensure that other parts are not impacted by MRP fluid.

Figure 1.

Poppet valve with their finishing ridge profiles and magnet fixture having exact poppet profile.

Normal indentation force (F_i) is responsible for indenting the roughness peak heights of workpiece material, and shear force (F_s) is responsible for shear-off the indented peak materials from the workpiece surface to make it flat profile, as shown in Supplemental Figure 1. The trapped abrasive in-between CIPs chain will rotate with the rotation of the poppet valve against the stationary magnet fixture. The finishing mechanism of the poppet valve MR finishing is demonstrated in Supplemental Figure 1.

The experimental setup for poppet valve MR finishing is demonstrated in Figure 2. The poppet valve is fitted to a precision 3-axis CNC milling machine head. Poppet valve is held with a precise vice during polishing, as demonstrated in Figure 2. MRP fluid is filled in-between the working distance of magnet fixture and poppet profile in MR fluid container (Figure 2). To start the finishing process, at first CNC spindle is rotated at 1200 r/min and guided the spindle to move 0.1 mm below at every time interval of 10 min during finishing. In the working gap of 2 mm between poppet and magnet fixture profiles (Supplemental Figure 2), the spindle is guided to move 1 mm below the starting position to excellent finish the poppet ridge profiles in 100 min. This vertical motion of poppet valve is regarded to make the process faster by considering that the sharp cutting edge of abrasives does not leave contact from poppet ridge profiles at any time interval. In Table 1, the initial study criteria during the finishing experiments are given.²⁴

Figure 2.

Experimental setup for magnetorheological finishing of poppet valve.

Table 1.

Experimental conditions during poppet valve magnetorheological (MR) finishing.

Parameter	Conditions
Finishing time	1 h. 40 min
Vertical feed rate	0.01 mm/min
Rotational speed of poppet	1200 r/min
Working gap	2 mm

Until now, no work has been reported for Nickel-Al-bronze (HRC 47) poppet valve MR polishing, so compositions of MRP fluid are selected based on the literature survey related to similar hard materials²⁵ and initial experimentations. Glycerol is applied to all media to improve MR viscosity. It works as stabilizer that supports to spread CIPs, consistently resisting accumulation. It also supports in preventing disintegration. Deionized water is utilized in place of distilled water to avoid corrosion. Stirrer is being utilized for preparing magnetorheological polishing fluid (MRPF) by blending CIPs and SiC abrasive grains in carrier media. HNO₃, H₂O₂ and deionized water are used as carrier media.²⁴ Table 2 lists the volume percentage contribution of every element in MRP fluid.

Table 2.

Compositions of MRP fluid.

Constituents	Vol. %
CIP (EN grade)	30
SiC abrasive (mesh size 800)	7.1
Glycerol	8
Nitric acid (HNO₃)	1.17
Hydrogen peroxide (H₂O₂)	1.3
Deionized water	52.43

It is not possible to have an abrasive percentage lower than 5%, nor can it be much higher. Lower abrasive content will cause more attraction (F) between CIPs in magnet field effect, as gap between CIPs will be reduced (as per equation (1)), which will further reduce the effectiveness of abrasives. More abrasive content will cause their separation from CIPs chain and will not do any polishing even though they will slide and roll. Hence, final R_a increases, and MRR decreases.²⁶

F = 0.35 μ_{0} {(\frac{d^{2} x M}{4 g})}^{2}

(1)

where µ₀ is permeability for the free space, d is the diameter of a CIP, g is gap among centres of two carbonyl iron particles, and x is constant.

Magnetostatic fluid–solid interaction FE analysis

Magnetostatic fluid–solid interaction FEA simulation utilizing COMSOL® Multiphysics was conducted to analyze the magnetic field and normal stress distribution on the poppet ridge profiles along with the MRP fluid. COMSOL® Multiphysics solves the magnetic field, fluid flow and solid mechanics problems of a specified model by allocating materials, boundary criteria and initial values. Supplemental Table 3 displays the simulation's boundary conditions. To simplify the simulation study, the following assumptions were considered:

The solver computes static magnetic fields.

The source of the static magnetic field is a permanent magnet.

The medium properties are considered independent of time and temperature.

The medium is assumed homogeneous, although it contains solid particles.

The simulation model for fluid–solid interaction analysis with the externally allocated magnetic field is shown in Supplemental Figure 2. The rectangular permanent magnet, Sm2Co17 (Samarium-Cobalt) of grade 30H, is considered to give a continuous fine polishing of the poppet ridge surfaces for the current analysis. In any finishing process, the forces exerted on the component profiles perform an important position in the surface integrity. The volume of material removal from the component profiles during finishing mostly relies on the forces responsible for finishing. These forces are exerted on the component surfaces via abrasive grains. After the use of outside magnetic field, CIPs in MRP fluid generate a cross-linked chain-like framework.

The magnetic force of the CIPs induces in the abrasive grains resulting in the indenting force on the component profiles. It helps in achieving the roughness of the desired value. From the literature survey, it has been found that there are two types of forces exerting on the component profiles, i.e., normal and tangential force. Normal force is modelled by assessing magnetic excitation forces of abrasives, spindle pressure on abrasives through poppet and their effect on component profiles at time of fluid–solid (poppet profiles) interactions. The fluid–solid contact phenomena yield tangential force as well. The fluid surface interactions including an operating magnetic field and external pressure generate shear stress and normal stress on the poppet profiles. In the finishing region, because of using the magnetic field, MRPF functions like Bingham plastic media. Zero velocity and displacement fields for every flow parameter are initially applied at the poppet profile and MR fluid surface interaction. The flow is steady. Rotation of the poppet valve is not considered.

The positions (EF and GH) (Supplemental Figure 2) on the finishing poppet profile are taken to evaluate the various forces working on abrasives in contact with poppet ridge surfaces; thus, it can be used to construct the analytical model for material removal on these ridge profiles. Only positions (EF & GH) (Supplemental Figure 2) is considered, and other parts of finishing profiles are leftover because they are symmetric to these positions and will give similar results. Supplemental Figure 2 also shows the working gap at positions A and A’ of finishing profiles of the poppet valve, which is filled with MRP fluid.

Mechanism of magnetorheological finishing of poppet valve ridge profiles

Roughness peak are eliminated by different finishing forces operating on active abrasive particles while MR poppet polishing. Active abrasives depress the work surfaces, and the amount of the depression is determined by the normal indentation force (F_i). In order to formulate the material removal mathematical model for poppet profile finishing, the precise estimation of resultant normal indentation force, total active abrasive grains and indentation depth on the poppet profile must be known. The following assumptions simplify the theoretical analysis of this mathematical model:

SiC abrasive grains are spherical in shape, having an average diameter of 19 μm.^4,11,27

Each abrasive has a single active cutting edge.

The load on each abrasive is constant, and therefore the workpiece surface has the same indentation depth.

Magnetic leaks and losses are ignored.

The nano-chips of the workpiece do not alter MR polishing fluid rheological properties.

Magnetic force's tangential component is neglected.

For the calculation of the forces due to magnet field, which acts upon CIPs and to study their consequences on the polishing performance, it is important to explore magnetic flux density (MFD) across gap among the poppet and magnet fixture profiles, loaded with MRPF. The value of MFD along their poles is calculated using equation (2).²⁸ The axial alignment of permanent magnets in a magnet fixture is depicted in Figure 3. Because the magnetic fixture's structure is built on same magnet pole locations, polar axis (X-axis) positioning stays constant irrespective of the magnet's direction (Figure 3). It is necessary to determine the MFD at various places throughout the work gap in order to calculate normal forces owing to the magnetic effect. Equation (2) is used to calculate the magnetic field across its polar axis.²⁹

\begin{aligned} B_{0} & = \frac{μ_{0} M}{π} [\tan^{- 1} \frac{a b}{(x - c) \sqrt{a^{2} + b^{2} + {(x - c)}^{2}}} - \tan^{- 1} \frac{a b}{(x + c) \sqrt{a^{2} + b^{2} + {(x + c)}^{2}}} \end{aligned}]

(2)

Figure 3.

Shape, size, and axial alignment of (a) horizontal and (b) vertical (fifth) magnets in magnet fixture for magnetorheological (MR) finishing poppet valve ridge profile.

Here a, b, and c are magnet dimensions (mm), in accordance with the axes of Y, Z, and X as demonstrated in Figure 3, x is axial distance (mm) measured from X-axis origin, and B is MFD in axial X-direction (A/m). µ₀M is permanent magnet remanence (1.03 T) for Sm2Co17 grade of magnet. When magnetic materials are is passed through a magnetic field area, total field produced inside magnetic materials is presented by equation (3).²⁸

B (x) = B_{0} + μ_{0} M_{C I P}

(3)

where B₀ is external magnetic field. CIPs magnetization (M_CIP) can be found from the MB curve (Supplemental Figure 3) of MRP fluid. Calculation of magnetic field (B₀) throughout work gap is done by using equation (2). equation (3) is used to calculate magnet field within MRP media across the working gap by any particular magnet. The CIPs chain that binds active abrasives is more in a ferromagnetic material than in free space.

Using FEA and mathematical expressions (equations. (2) and (3)), Supplemental Table 4 compares the changes of the MFD across the working gap for polishing the poppet valve profiles. In Supplemental Table 4, working gap is starting from position A' and B' to A and B, respectively from magnets origin. The resultant of MFD calculated for horizontal and vertical magnet is taken at 45°. There will be almost no effect of magnet fixture on analytical evaluation of MFD along working gaps, as magnetic fixture is fabricated from aluminium (relative permeability µ_r < 1.01) materials, non-magnetic in nature. As a result, CIPs magnetization in work gap is solely aided by MRPF and not by any alternate available materials, so, effects of magnet fixture is neglected on the magnetization of CIPs.

Active abrasive particles region along poppet valve ridge profiles

Due to the total developed normal stress from externally applied pressure and magnetic field along poppet valve profiles, an effective active abrasive surface area formed (Figure 4). The MFD and normal stress are almost uniform and higher at poppet valve finishing profiles. This makes the active abrasives smoothly indent into poppet surfaces. As the overall active abrasives increase and become more consistent, the MRR and surface polishing will improve. An effective surface is defined as the region where active abrasives depress into part surfaces using normal indent forces and polish using shear force.²⁶ The quantity of active abrasive grains required to polish the poppet valve profile is governed by the active surface area. The total surface area for active abrasive grains is ‘ $π r \times w$ ’, on the poppet ridge profiles, where ‘r’ is the radius of poppet finishing ridge profiles, ‘w’ is the width of poppet ridge profiles. The total abrasive grains acting on the poppet valve profiles are determined by the given below specific equation

N_{g} = \frac{% vol . fraction of SiC abrasive grains \times π r \times w \times D_{g}}{Volume ofsingle SiC abrasibe grain}

(4)

Figure 4.

Active abrasive grains zone on the ridge profiles of poppet during its magnetorheological (MR) surface finishing.

where, D_g (19 µm) is diameter of single active SiC abrasive grain. Volume of single SiC abrasive grain $= \frac{4}{3} π {(\frac{D_{g}}{2})}^{3} = 3.591 \times 10^{- 15} m^{3}$ , N_g is amount of active abrasive grains on poppet valve finishing profiles. The total number of active abrasive grains is calculated for both poppet ridge profiles is listed in Supplemental Table 5. Just the position of the active abrasive grains change during finishing, leaving the effective surface region untouched. The dormant abrasives get active when the poppet valve is rotated.

Results and discussion

MFD and normal stress distribution along poppet valve ridge profiles

The FEA outcomes for varying MFD and normal stress along poppet finishing profile (EF and GH) (Supplemental Figure 2) are demonstrated in Figure 5(a) to (c), respectively. The maximum MFD at EF and GH positions along the popper ridge profiles is obtained as 0.19 T and 0.18 T (Figure 5(a)), respectively, postmagnetostatic FEA, which is good enough for finishing the poppet ridge surfaces. This magnetic field effectively magnetizes CIPs to form a cross-linked chain-like framework and exerts forces on the abrasive grains against the poppet component in-between the working gap of the magnet fixture and poppet profiles to indent them into the poppet surface consistently. These magnetic fields are the reason for the development of normal finishing forces, which play an important role in achieving fine surface quality. Thus, controlling the volume of material removal via regulating the magnetic field helps in achieving the surface roughness of the desired value

Figure 5.

Distribution of normal stress at poppet finishing profiles (a) contour plot and (b) at positions EF and GH (Supplemental Figure 4); (c) magnetic flux density (MFD) distribution at EF and GH (Supplemental Figure 4) of poppet finishing profiles.

The FEA findings help in layout the magnet fixture profiles as the exact copy of the poppet profiles. Further, on the basis of magnetostatic, fluid flow and solid mechanics FEA, the appropriate profile region on the poppet ridge surfaces was studied where consistent normal stress is acting to finish poppet ridge profiles while the polishing process uniformly. This allows accomplishing the aim of the current investigation like enhancing the poppet valve form accuracy, surface finish and removing all surface defects.

Bingham-Papanastasiou model (equation 5) is considered as the viscosity model during simulation study.³⁰

η = μ + \frac{τ_{y}}{| \dot{γ} |} [1 - e^{(- m | \dot{γ} |)}]

(5)

where ‘m’ is regularizing parameter, which regulates the exponential stress growth, ‘η’ is apparent viscosity, ‘

\dot{γ}

’ is shear rate, ‘µ’ is plastic viscosity, and ‘τ_y’ is yield shear stress. Higher yield stress suggests a greater MR impact of the MRP fluid.^30,31

Before yielding, the MRP media behaves as a non-linear elastic solid and has an elastic modulus.³¹ In order to get the elastic modulus as a normal stress value, multiply it by the average compressive strain, that is induced by depressing the poppet into the MRP fluid ribbon.³² In the current experiment, the part is pressed by 0.11 mm and the overall MRPF height is 2 mm, i.e., the average strain is (0.11/2 mm). As a result, equation 6 provides the normal stress caused by an added magnetic field.

σ_{n} = 3 \times (3 ϕ μ_{0} M_{s} H_{0}) \times \frac{0.11}{2}

(6)

where

σ_{n}

is normal stress (Pa), H₀ is magnetic field strength (A/m),

ϕ

is CIPs volume fraction, and M_s is saturation magnetization (A-m²/Kg). FEA obtained results for varying normal stress distributions along the positions EF and GH of poppet profiles (Supplemental Figure 2) are demonstrated in Figure 5(a) and (b). As shown in Figure 5(b), the maximum normal stress obtained is 53 Pa and 44 Pa at positions EF and GH, respectively. It's a well-known fact that consistent normal stress causes abrasive grain depression of the same depth upon each poppet ridge surface, resulting in consistent removal of material from each poppet ridge surface and a uniform surface finishing.

Normal forces acting on active abrasives

The forces, which work upon the abrasive grains, are dependent on the surface of poppet profiles and of motion imposed on MRP media. Because of the magnetic forces, CIPs which hold the abrasives are pushed against the workpiece. The magnetic line of forces is supposed to be normal to every face of the poppet profiles. Although the magnetic lines of force may be slightly inclined to the poppet surface, their consequences (i.e., the magnetic force's tangential component) are neglected in the current analysis for simplicity of magnetic force modelling, and only magnetic forces normal component (F_m) is considered. Normal magnetic force F_m assists in penetrating poppet surfaces using abrasive grains. F_m, that act on CIPs in the magnetic field is presented as³³

F_{m} = m μ_{0} χ_{m} H \nabla H

(7)

where m is CIPs mass (kg),

χ_{m}

is CIPs magnetic susceptibility (m³/kg) inside MRP media, and

\nabla H

is gradient of magnetic field strength. Magnetic field strength (H) depends upon MFD (B), as per equation (8) for practical uses.

B = μ_{0} H

(8)

So, using equations (8), (7) can be written as equation (9).

F_{m} = m \frac{χ_{m}}{μ_{0}} B \nabla B

(9)

equation (9) is defined as equation (10) by considering the fluctuation of MFD (B) in the work gap (x) solely in the normal direction from the poppet ridge profile to the magnet fixture ridge profiles and disregarding the fluctuation of B in alternative direction.

F_{m} (x) = m \frac{χ_{m}}{μ_{0}} B (x) \frac{d B (x)}{d x}

(10)

where B(x) is MFD variation in working gap (x) between 0 and 2 mm. CIPs magnetic susceptibility

(χ_{m})

is defined by equation (11).

χ_{m} = \frac{M}{H} = \frac{μ_{0} M}{B}

(11)

where M is CIPs magnetization (A-m²/Kg), and we can find its value from CIPs M–B curve (Supplemental Figure 3).²⁶ B (x) is evaluated from FEA and theoretical analysis in working gap of BA and B'A' (Supplemental Figure 2). The MFD variation in the working gap BA and B'A' (Supplemental Figure 2) using FEA is demonstrated below in equation (12) and equation (13), respectively.

B_{B A} (x) = - 0.015 x + 0.2283

(12)

B_{B^{'} A^{'}} (x) = - 0.014 x + 0.2097

(13)

In equations (12) and (13), x is in the range of 0–0.002 m. Equations (14) and (15) are obtained by differentiating equations (12) and (13), with regard to x, as mentioned below.

\frac{d B_{B A} (x)}{d x} = - 0.015

(14)

\frac{d B_{B^{'} A^{'}} (x)}{d x} = - 0.014

(15)

The CIPs next to active abrasives have the main contribution in indenting the poppet profiles.²⁷ Equations (10)–(15) are used to determine the magnetic force acting on CIPs near active abrasives. The simulated and theoretical magnetic forces (F_m) that act on CIPs near to active abrasive at positions A and A (Supplemental Figure 2) of poppet ridge profile, are presented in Supplemental Table 6. Due to repelling lifting forces, these magnetic forces indent the abrasives into the ridge surfaces of the poppet component and finishes it. Material removal mechanism is abrasive wear which appears because of the micro-slicing of materials at peaks.

The vertical feed rate of the poppet is responsible for generating an additional normal force (F_n) created by spindle pressure on abrasives of MRP fluid, which in turn allows abrasives to penetrate more into the poppet ridge surfaces. Normal force (F_n) on abrasives is given by

F_{n} = σ_{n} \times π r \times w

(16)

where

σ_{n}

is the normal stress created by a spindle after additional deformation of MRP fluid ribbon to 0.01 mm. Thus, the total indentation force (F_i) which are responsible for the indentation of abrasive particle in poppet surface is given in equation 17.

{\vec{F}}_{i} = {\vec{F}}_{m} + {\vec{F}}_{n}

(17)

By using a magnet fixture, which is an exact copy of poppet profiles provides a nearly consistent distribution of active abrasives grains along poppet ridge surfaces. This reasons for the consistent material removal and surface finishing.

To verify the simulated and theoretically calculated normal force values acting on poppet valve profiles, the normal force is measured experimentally using a 3-axis dynamometer. The gap between normal force obtained from experimentation, simulation, and theoretical analysis is marginal (error < 10%). Dynoware software is used to process and analyze the acquired signal from the dynamometer. Force measurement is started just before the poppet valve comes in contact with the MR fluid. During finishing, feed is given to the valve in the Z direction. Hence, the main force component acting on the abrasive particles is the normal force along Z-axis (F_n). Also for verifying the simulated and theoretical MFD values along poppet valve profiles, MFD values are measured experimentally using a digital Gaussmeter. After measuring the MFD values (Supplemental Table 4), the magnetic force is calculated and presented in Supplemental Table 6. Supplemental Table 6 represents the magnetic force and normal force from the theoretical, experimental and FE analysis at positions A and A' (Supplemental Figure 2) of poppet ridge profiles.

Modeling of material removal

The total indentation force (equation (17)) which acts on active abrasive grains makes it to indent into poppet ridge surfaces. When these abrasive grains rotate because of the shear force of MRP media, they dislodge materials in form of chip. Every abrasive grain has a single active slicing tip. The load on every abrasive grain is constant thus develops the same indentation depth on every ridge profile of poppet. Post evaluating the normal indentation force (F_i) from equation (17), the penetration diameter (D_i) and depth of penetration (t)³⁴ of an active abrasive grain into poppet ridge profiles could be found using equations (18) and (19), respectively. The Brinell hardness number of the machined poppet ridge profiles has been evaluated by Brinell hardness testing equipment.

D_{i} = \sqrt{D_{g}^{2} - {(D_{g} - \frac{2 \times 10^{- 6} F_{i}}{9.81 H_{B H N} π D_{g}})}^{2}}

(18)

where D_g is the abrasive diameter (m), H_BHN is the Brinell hardness number of poppet valve material (H_BHN = 445) and normal indentation force (F_i) is in N.

t = \frac{D_{g}}{2} - \frac{1}{2} \sqrt{D_{g}^{2} - D_{i}^{2}}

(19)

Total depth of penetration is calculated from penetration of single SiC abrasive particle multiplied by total count of active SiC abrasive grains/poppet finishing profile. The total depth of penetration/poppet finishing spot/rotation created by N_g active abrasive grains/poppet finishing profile/rotation is presented in Supplemental Table 7.

Theoretically, MRR can be calculated from using equations (20) and (21).

MR R_{theoretical} = \frac{Total Volume of material removed times density}{finishing time}

(20)

Total volume of material dislodged is calculated from equation (21). Density of Nickel-Al-Bronze alloy material is 7600,000 g/m³. Finishing time is 100 min (Table 1).

Total Volume of material removed = N_{g} \times π (t^{2} \times \frac{D_{g}}{2} - \frac{t^{3}}{3}) \times 1200 \times 100

(21)

Here, 120,000 is multiplied in the above equation because this much amount of revaluation will poppet valve cover before completing the finishing cycle. t_total is the total depth of indentation per poppet finishing profile and per rotation, listed in Supplemental Table 7. The theoretically calculated MRR is presented in Supplemental Table 8.

The material removal for Nickel-Al-Bronze poppet valve was determined depending on the mesh size of abrasive grain and total rotations of poppet valve. Material removal is amount of material that is dislodged per minute. The required time in completion of the desired number of finishing rotations (Table 1) is used to found the MRR. The $MRR = \frac{Initial weight - Final weight}{finishing time}$ is presented in Supplemental Table 8.

Magnet fixture, which is an almost exact copy of the poppet component, has polished poppet valve up to nano-scale level uniformly.

Surface characterization of poppet valve finishing profiles

Experiments are carried out in order to assess the capability and usability of the new magnet fixture, as well as to achieve standardized uniform finishing of the complicated ridge profiles of poppet valve. There is an appropriate field of constant and optimal distributions of MFD and normal stresses on the ridge profiles of poppet valve are present. This assists in obtaining a fine surface finishing. In order to understand the fining surface properties, the initial and final characterization of the poppet valve was carried out before and after the experiment for the comparative analysis between the initial and final surface characteristics of poppet profiles. Macrographs were observed using an optical microscope, the surface roughness values were studied utilizing a non-contact optical profilometer (make: Taylor Hobson), material properties were checked using an energy dispersive X-Ray (EDX) analysis and also the presence of foreign materials were analyzed by means of a field-emission scanning electron microscope (FESEM) experiment.

Both surface roughness and surface topography are essential variables in the case of a poppet valve. Poppet valve efficiency and lifespan largely depend upon the poppet surface morphology. Poppet valve surfaces were finished with MRP media, which composition is given in Table 2. Before and after finishing with the novel setup, the surface roughness (S_a) for the poppet valve is measured using an optical profilometer at different positions on the poppet profiles (Figure 1). On these positions, S_a values are measured five times, and their average is considered. Based on these five measurements, a standard error ( $S E = S D / \sqrt{n}$ , $S D = \sqrt{\frac{\sum {(X - M)}^{2}}{n - 1}}$ , X = value in data distribution, M = mean of values, n = number of observations) analysis is performed and presented in Figure 6 with the error bar. After final finishing with the novel setup, the minimum average surface roughness (S_a) for the ridge profiles of the poppet was decreased to 0.023 μm from the original value of 0.318 μm (Figure 6). The uncertainty ( $\sqrt{\frac{\sum {(X - M)}^{2}}{n \times (n - 1)}}$ ) in measured S_a values is calculated, and the final S_a values for the poppet profiles are obtained as 0.0231 ± 0.004 μm. The various surface roughness parameters (S_a, S_sk, S_ku) at different selected positions (Figure 1) of poppet ridge profiles are presented in Figure 6.

Figure 6.

Surface roughness parameters at different selected positions of poppet valve finishing ridge profiles (Figure 1).

In the MRP fluid, HNO₃ is used to brighten the surface, and an extra agent H₂O₂ is used for dissolving the metallic surface to help abrasive grains. By the reaction of hydrogen peroxide (H₂O₂), nitric acid (HNO₃), and poppet material give the Copper (II) nitrate (CuNO₃₎, which is used as a polishing agent. The wt. % of oxygen has increased to 8.4%, and copper wt. % of copper has reduced from 79.9% to 76.1%, confirmed from EDX analysis (Figure 7). Increased oxygen content on the poppet surface forms an oxidation layer on the poppet component, which helps in reducing the wear and tear of the poppet valve component and further prevents corrosion.¹³

Figure 7.

EDX analysis of poppet valve profile (a) before and (b) after finishing.

Figure 8(b) shows a macrograph of the poppet valve after finishing with MRP fluid. The main requirement was to finish only the required finishing profile and does not touch any other region. Due to the use of fifth magnet, the mentioned requirement has been fulfilled, as the magnetic field is entirely concentrated at the required finishing profile of the poppet valve (Figure 5(a)). As observed from Figure 8(a) and (b), deep valleys and scratches have been removed after final finishing. Figure 8(d) shows the surface topography of the poppet valve component after finishing. From surface topography pictured from the optical profilometer also confirms that all manufacturing defects has been removed. The FESEM images of surfaces of the poppet workpiece at 500× magnification after MR finishing is demonstrated in Figure 8(f). After finishing, the smooth surface without pits, burr, and scratch marks is obtained on the poppet valve ridge profile.

Figure 8.

Macrograph of poppet valve surfaces (a) before and (b) after finishing; surface topography of poppet valve surfaces (c) before and (d) after finishing; field-emission scanning electron microscope (FESEM) image of poppet valve surfaces (e) before and (f) after finishing.

This significant uniform decrease of final surface roughness values and enhancement in surface characteristic of poppet valve profile indicates that the novel magnet fixture can produce a uniform fine surface finish with an optimum oxidation layer.

Testing of poppet valve

Nickel aluminium bronze (BS1400 Gr.AB2) alloy poppet valve is tested for wear and corrosion rate before and after polishing. Wear failures in mechanical parts is difficult to forecast due to the complexity of the physical process that regulates it. Valve wear is mainly induced by contact between the valve and its seat. However, micro-sliding between the valve and its seat created by combustion pressure also causes valve wear.³⁵ So, wear testing of the poppet valve is a functional requirement after polishing.

The wear rate of poppet valves was measured experimentally using Ducom pin-on-disc wear testing equipment. The finished valve's wear rate was found to be lower than that of the unfinished valve. A wear test was performed on a disc having diamond-like coatings as a counter surface. When conducting the wear test on the poppet valve, the ASTM G99 method was used. The wear test parameters are presented in Supplemental Table 9. A drop in wear rate from 8.54 × 10⁻⁴ mm³/m for an unfinished poppet valve to 0.45 × 10⁻⁴ mm³/m for a finished poppet valve is observed by Ducom equipment.

The poppet valve also requires corrosion resistance property during its working use in the combustion chamber. Corrosion test was carried out in potentiostat from which Tafel graph is obtained and shown in Supplemental Figure 4. Potentiostatic corrosion tests were carried out in a cylindrical container (320 mm diameter) serving as a test cell and filled with 5 L of electrolyte. A 3.5 wt% NaCl solution was prepared with analytical grade NaCl and distilled water to be used as a test medium to simulate seawater in the lab. A cylinder made from stainless steel mesh was placed inside along the container wall, serving as a counter electrode. The reference electrode (RE) was inserted from the top in the centre of the container. For corrosion test, poppet valve sample, serving as working electrodes, were contacted by clamping to suitably bent stainless steel wires and placed opposite to the RE. The corrosion resistance for the finished poppet valve is really high compared to the initial unfinished poppet valve. The polarization curves show the corrosion rate by the Tafel graph. The corrosion rate has been decreased for the finished poppet valve of 19.1 μm from the unfinished poppet valve of 65.2 μm. The surface characterization, surface roughness, wear and corrosion rate analysis show that the currently developed finishing method has proved its efficiency for poppet valve profile finishing.

Conclusions

For polishing the complicated ridge profile of the poppet valve, efficacy of novel setup is confirmed experimentally and theoretically. The FE analysis of magnetostatic fluid–solid interaction shows that novel magnet fixture is capable of finishing the ridge profiles of poppet valve uniformly. As a result, the newest MRFF method with unique magnet fixture is an intriguing solution for improving practical usage of poppet valves. The present research demonstrates the novel magnet fixture's dependability and efficiency. The following specific conclusions are drawn.

The uniform distribution of MFD and normal stress in the desired area means that the active abrasives will uniformly indent into the poppet surface, which results in a consistent cutting of the asperities levels in vertical and rotating directions with the poppet valve movement. It allows smooth, high-geometrical accuracy finishing.

The surface roughness (S_a) of poppet valve profiles is reduced significantly to 0.023 μm from 0.318 μm. The quality of the poppet surface is improved considerably.

The percentage error of 12.87% between experimental and simulated MRR is obtained for the poppet valve ridge profiles. The experimental investigation supports the theoretical outcomes as the gap among the found results of both studies is within acceptable limit.

As number of active abrasives per rotation is uniform, it resulted in an improved MRR and consistent surface finishing.

The maximum normal stress obtained is 53 and 44 Pa at finishing profile – 1 and finishing profile – 2 of the poppet valve, respectively.

The analytically evaluated MFD values at the poppet valve finishing profile are consistent with the FEA obtained and experimental MFD values.

The final MR finished poppet valve has an improved oxide film, which further helps in resisting corrosion and wear, improving lifespan by avoiding direct metal-to-metal contact.

The testing of the poppet valve shows that the corrosion and wear resistance property of the finished poppet valve has been improved compared to the unfinished poppet valve.

The continuous fine surface finish of the poppet valve ridge profiles will make it leak-proof by perfectly sealings with the valve sitting surface to minimize hydrocarbon emissions at elevated pressures and temperatures during its working use.

The present finishing method can also be further applied for other types of poppet valves (tulip, mushroom, rigid, etc.), spool and rotary valves, and any complex components with narrow profiles having a cylindrical head. The developed material removal model and simulation of finishing forces analysis can be used for various other MRFF processes for different applications. In the present simulation study, further, the change in rheological properties of MR fluid with the effect of temperature can also be studied, which can give more significant understanding of the MRFF process.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221139102 - Supplemental material for Experimental and theoretical analyses of material removal in poppet valve magnetorheological finishing

Supplemental material, sj-docx-1-pie-10.1177_09544089221139102 for Experimental and theoretical analyses of material removal in poppet valve magnetorheological finishing by Manjesh Kumar, Chandan Kumar, Amit Kumar, Debashish Gogoi and Manas Das in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

Supplemental Material

sj-docx-2-pie-10.1177_09544089221139102 - Supplemental material for Experimental and theoretical analyses of material removal in poppet valve magnetorheological finishing

Supplemental material, sj-docx-2-pie-10.1177_09544089221139102 for Experimental and theoretical analyses of material removal in poppet valve magnetorheological finishing by Manjesh Kumar, Chandan Kumar, Amit Kumar, Debashish Gogoi and Manas Das in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

Footnotes

Acknowledgments

Authors would like to acknowledge the Department of Mechanical Engineering at SRM University, Andhra Pradesh, India, for providing the facilities for carrying out research at their institute.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received the following financial support for the research, authorship, and/or publication of this article: Science & Engineering Research Board (SERB), New Delhi, India, for their financial assistance for project No. EEQ/2017/000597 titled ‘Fabrication of Prosthetic Im-plants and further Nanofinishing using Magnetic Field Assisted Finishing (MFAF) Process’.

ORCID iD

Manjesh Kumar

Supplemental material

Supplemental material for this article is available online.

Appendix 1

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Supplementary Material

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