Magnetic Field-assisted Finishing Research Articles

Characterization of oscillatory magnetic field-assisted finishing of directed energy deposition NASA HR-1 integral channels

Additive manufacturing (AM), such as directed energy deposition (DED), enables fabrication of complex geometries for critical parts at near-net shape, but creates a need for post-processing to achieve desired geometry and performance. In particular, parts made using DED are sometimes printed with a high initial surface roughness, requiring post-processing to meet application-dependent requirements. Magnetic field-assisted finishing (MAF), in which a magnetic polishing tool is manipulated by magnetic force and generates relative motion against a target surface, has been applied to smooth AM parts. An advantage of MAF is that the magnetically manipulated polishing tools can finish both external part surfaces and part interiors. In this paper, an oscillating magnetic polishing tool is proposed to smooth the inner surfaces of rectangular NASA HR-1 alloy channels made using DED. Because effective tool motion allows reduction of surface roughness and waviness, parameters that control polishing-tool motion are of great interest. This paper describes three parameters that control polishing-tool motion: number of polishing tools, magnetic field, and abrasive slurry. The effects of tool motion on the polishing characteristics are demonstrated, showing that the roughness of the interior channel surface can be reduced from several tens of micron to a sub-micron level.

Achieving high accuracy of Co-Cr-Mo femoral head for improving tribological properties of hip joint prosthesis via a three-axial MFAF process

Rough surface and low precision roundness of femoral head can significantly affect tribological properties of hip joint prosthesis such as wear, von Mises stress, friction coefficient, and scratches, resulting in failure of hip joint prosthesis. In this work, firstly, an ultra-smooth surface with a high precision roundness was achieved for Co-Cr-Mo femoral heads via a three-axial Magnetic Field Assisted Finishing (MFAF) process. Secondly, the effects of a rough and smooth surface of Co-Cr-Mo femoral head on wear rate of acetabular cup were evaluated via a wear simulator test. Finally, the von Mises stresses of both femoral heads and acetabular cup were evaluated via ABAQUS finite element (FE) model. Using this process, surface roughness (Sa) and roundness (RONt) of femoral head were reduced from 0.330 to 0.032 μm and from 26.15 to 2.17 μm, respectively. A high accuracy of Co-Cr-Mo femoral head was achieved by reducing its surface roughness and roundness, which reduced the wear rate of the acetabular cup by more than 68 % compared to an unfinished femoral head. In addition, an ultra-smooth surface of femoral head dramatically reduced the von Mises stresses at the contact area of hip joint prosthesis. Results of this work demonstrate that a three-axial MFAF process is an effective method to successfully achieve high accuracy of Co-Cr-Mo femoral head with advantages of simultaneously improving tribological properties of hip joint prosthesis.

Magnetic field-assisted finishing: mechanism, application, and outlook

Magnetic field-assisted finishing: mechanism, application, and outlook

Novel Process Modeling of Magnetic-Field Assisted Finishing (MAF) with Rheological Properties

The performance of a magnetic-field-assisted finishing (MAF) process, an advanced surface finishing process, is severely affected by the rheological properties of an MAF brush. The yield stress and viscosity of the MAF brush, comprising iron particles and abrasives mixed in a liquid carrier medium, change depending on the brush’s constituents and the applied magnetic field, which in turn affect the material removal mechanism and the corresponding final surface roughness after the MAF. A series of experiments was conducted to delineate the effect of MAF processing conditions on the yield stress of the MAF brush. The experimental data were fitted into commonly used rheology models. The Herschel–Bulkley (HB) model was found to be the most suitable fit (lowest sum of square errors (SSE)) for the shear stress–shear rate data obtained from the rheology tests and used to calculate the yield stress of the MAF brush. Processing parameters, such as magnetic flux density, weight ratio of iron and abrasives, and abrasive (black ceramic in this study) size, with p-values of 0.031, 0.001 and 0.037, respectively, (each of them lower than the significance level of 0.05), were all found to be statistically significant parameters that affected the yield stress of the MAF brush. Yield stress increased with magnetic flux density and the weight ratio of iron to abrasives in MAF brush and decreased with abrasive size. A new process model, a rheology-integrated model (RM), was formulated using the yield stress data from HB model to determine the indentation depth of individual abrasives in the workpiece during the MAF process. The calculated indentation depth enabled us to predict the material removal rate (MRR) and the instantaneous surface roughness. The predicted MRR and surface roughness from the RM model were found to be a better fit with the experimental data than the pre-existing contact mechanics model (CMM) and wear model (WM) with a R2 of 0.91 for RM as compared to 0.76 and 0.78 for CMM and WM. Finally, the RM, under parametric variations, showed that MRR increases and roughness decreases as magnetic flux density, rotational speed, weight ratio of iron to abrasive particles in MAF brush, and initial roughness increase, and abrasive size decreases.

Selective laser melting of oxide dispersion strengthened MA956 alloy and its surface finishing by magnetic field assisted finishing

Selective laser melting (SLM) is a prominent metal additive manufacturing (AM) capable of processing a myriad of engineering materials with high precision and design freedom. However, similar to other AM processes, poor surface finishing has been an omnipresent problem in SLM technology. In this study, magnetic field assisted finishing (MAF) was used to finish SLM fabricated oxide dispersion strengthened (ODS) MA956, an iron‑chromium‑aluminum alloy. The effect of laser processing parameters on part density and surface roughness was first studied. Using a multi-objective optimization technique, the optimal parameters to obtain the highest density and lowest surface roughness were determined. Finally, MAF was applied to the parts built with the obtained optimal SLM parameters, but without significant improvement on the surfaces of as-printed samples. Hence, the surfaces of as-printed samples were post-processed (ground) to yield better initial surface conditions prior to MAF. The effect of initial roughness, iron particles size, and abrasive size on MAF performance was studied. Initial roughness had the most dominant effect followed by abrasive size. The underlying mechanism behind the dependency on initial roughness on final surface quality was analyzed by studying the change on the surface profiles with different starting initial roughness. The initial roughness required for MAF to be effective was determined. Using the optimal processing conditions, MAF was applied to the post-processed samples to attain the final average surface roughness (Ra) as little as 0.36 μm starting from initial average roughness (Ra) of 1.53 μm.

Application of magnetic field assisted finishing process for nanofinishing of freeform femoral knee joint and performance analysis of finished surface

Finishing of femoral knee joint implant is very cumbersome due to its freeform nature. In the present study, a magnetic field-assisted finishing process is employed to uniformly finish the femoral knee implant over its surface curvature at the nanometer level. Experiments are conducted to analyze the freeform surface finishing capability of magnetic field-assisted finishing tool, which proves the tool’s capability to achieve the required surface properties of a femoral knee implant. A comparative analysis between the performance of the magnetic field assisted finishing process and manually polished surface relative to tibial bearing material, that is, ultra-high molecular weight polyethylene with the help of pin-on-disc tribometer, is carried out. It will help to understand the in vivo performance of a finished femoral component of the knee joint by a magnetic field-assisted finishing process. The wear tests prove that the volume loss of the disc material while considering magnetic field-assisted finished pins (11.79 mm3) are better in all cases than the manually polished pins (27.97 mm3). The tribology test also demonstrates that femoral knee implant polished using magnetic field-assisted finishing process will face fewer problems related to wear debris. It will also reduce the problems that occur due to the mixing of debris in the bloodstream. The polishing tool provides almost uniform nanometer-level surface finishing with a standard deviation of 19.23 along the surface curvature. The minimum achieved surface roughness of the femoral knee joint is 20 nm.

Iron-particle-polishing-tool behavior during magnetic field-assisted finishing of fused silica

Commercial processing of fused silica components leaves behind contaminants and damage that can hinder the component performance in a lasing system. Magnetic field-assisted finishing (MAF) has been applied to remove the contamination from fused silica using a polishing cloth-based tool, which was hindered by tool degradation. This paper proposes replacing the polishing cloth with iron particles, which align in a brush around the tool magnet. These experiments explore the behavior of the iron particle tool and the resultant polishing characteristics. A 43% improvement over the baseline laser induced damage threshold demonstrates the feasibility of the tool for removing contamination.

Magnetic field-assisted finishing processes: from bibliometric analysis to future trends

The need for ultraprecision finishing has grown, and magnetic field-assisted finishing has shown potential for overcoming some challenges. This study evaluates the scientific production and identifies future directions of magnetic field-assisted finishing based on a bibliometric analysis. Using Bibliometrix, network mapping and descriptive analysis were performed on 1558 documents related to magnetic field-assisted finishing-related research published over the past 51 years. The results of the comprehensive literature reviewed showed that the theme exhibits a rising trend of 56% in the last 10 years, being mainly conducted by Chinese, Indian, and American researchers. Different geometries and materials can be finished and which had surface roughness ranges from sub-nanometer- to micrometer-scale. Surface finishing of freeform dies and molds, optical components, and medical devices have been standing out as current process applications in tooling, aerospace, and biomedical industries. AISI 304 stainless steel was the most tested metal. Finally, potential areas of research were identified in the coming years, which could lead to new fields of application for magnetic field-assisted finishing in industry.

Experimental Investigation on the Effect of Surface Shape and Orientation in Magnetic Field Assisted Mass Polishing.

Magnetic field assisted finishing (MFAF) technology has been widely used in industries such as aerospace, biomedical, and the optical field for both external and internal surface finishing due to its high conformability to complex surfaces and nanometric surface finishing. However, most of the MFAF methods only allow polishing piece-by-piece, leading to high post-processing costs and long processing times with the increasing demand for high precision products. Hence, a magnetic field-assisted mass polishing (MAMP) method was recently proposed, and an experimental investigation on the effect of surface posture is presented in this paper. Two groups of experiments were conducted with different workpiece shapes, including the square bar and roller bar, to examine the effect of surface orientation and polishing performance on different regions. A simulation of magnetic field distribution and computational fluid dynamics was also performed to support the results. Experimental results show that areas near the chamber wall experience better polishing performance, and the surface parallel or inclined to polishing direction generally allows better shearing and thus higher polishing efficiency. Both types of workpieces show notable polishing performance where an 80% surface roughness improvement was achieved after 20-min of rough polishing and 20-min of fine polishing reaching approximately 20 nm.

Magnetic field assisted finishing (MFAF) process for internal finishing of alumina ceramic tube

Nowadays, fine surface finish is in great demand for many industrial applications. An MFAF process is discussed in this conference paper to achieve a highly finished internal surface of non-ferromagnetic Alumina ceramic tube. Conventional grinding and polishing techniques cannot be used to finish this material because they cannot achieve high surface precision and produce surface defects like micro cracks. Therefore, MFAF process is an alternative method as it minimizes the surface damage because of flexible finishing conditions. MFAF Process, has flexible abrasive brush which can easily finish complex geometry of the workpiece. The workpiece material is alumina ceramic which is in higher demand as a substrate in electronic device applications. The MFAF process was done on alumina ceramic tube and 3 parameters, each having 2 different levels was selected. The parameters considered in the study are chuck speed (rpm), quantity of Diamond abrasive and iron power, machining time. 4 experiments with different levels of parameters were conducted. After finishing experiment using MINITAB 19 software, optimum parameters were determined. From the results of experiment, it was concluded that quantity of diamond abrasive and iron powder did not have much influence on finishing time compare to the chuck speed (r.p.m).

Optimization of magnetic field assisted finishing process during nanofinishing of titanium alloy (grade-5) implant using soft computing approaches

ABSTRACT The present study explores the potential of magnetic field assisted finishing (MFAF) in precise finishing of femoral knee implant made of bio-titanium alloy at the nanometric level. The process parameters, viz. rotational speed of tool, feed rate and carbonyl iron particle (CIP) concentration of MFAF have significant influence on the final surface topography of implants and thus selection of optimal set of their values is a difficult but an important task. In the present work, central composite rotatable design of response surface methodology is considered for nanofinishing of titanium alloy based femoral knee implants. Based on the experimental results, a polynomial regression prediction model for surface roughness is established in terms of MFAF process parameters. The fuzzy logic model with the help of ANOVA analysis is employed to analyze the effect of individual and interaction of parameters on surface roughness of implants. Finally, for further optimizing the final surface roughness of knee implant and determining the best settings of MFAF parameters, an improved salp swarm algorithm (ISSA) is proposed by introducing two important enhancements in basic SSA. The results reveal that SSA and ISSA metaheuristics achieved an improvement in final surface finish of knee implant by 4.04% and 14.24%, respectively, in comparison to default experimental trials during finishing.

Effects of build- and scan-directions on magnetic field-assisted finishing of 316L stainless steel disks produced with selective laser melting

Selective laser melting (SLM) is an additive manufacturing process, which allows for producing parts of very complex geometry. While SLM may produce the desired net-shape geometry, the surface conditions of as-built SLM parts are often insufficient. Also, the as-built state may yield undesired tensile stresses at the surface. Post processes such as magnetic field-assisted finishing (MAF) processes can yield the desired surface roughness and improve the residual stress state. However, in a part made by SLM, these processes will act on surfaces with various different orientations to the built direction, and, in particular, to the laser scanning direction. This paper analyzes the influence of laser scanning direction with respect to the surface to be finished by MAF using 316L steel disks produced with different orientations and scan strategies. The build plane of an as-printed disk has greater surface roughness and hardness but lower tensile residual stresses than the plane perpendicular to the build plane. These characteristics create difficulties in material removal and smoothing but do not impede MAF’s ability to increase hardness and impart compressive residual stresses to the surface scanned by the laser. The results indicate that material removal rate and hence optimal finishing time depend on the orientation of the surface with respect to the scanning direction, which should be accounted for in process planning.

Fused silica contamination layer removal using magnetic field‐assisted finishing

AbstractFused silica optics used in lasing systems requires a high laser‐induced damage resistance. Processes typically used to polish fused silica lenses induce subsurface and surface damage that collect ceria abrasive, creating a layer of contamination. The contamination can be a precursor to laser damage during use. A preliminary study showed the feasibility of magnetic field‐assisted finishing (MAF) for polishing fused silica and suggested possible beneficial effects of the MAF‐polished surface on the laser‐induced damage threshold (LIDT). This paper proposes a method to examine the fundamental polishing characteristics of MAF for fused silica. Using the proposed method, this paper explores the material removal characteristics of the MAF process and improves the understanding of the MAF polishing mechanism. The 45% improvement of LIDT shows the efficacy of MAF for removing the contamination layer of fused silica surfaces with minimal changes in the surface roughness.

Investigation of a novel finishing tool in magnetic field assisted finishing for titanium alloy Ti-6Al-4V

Titanium alloys have numerous applications in field of medical industries, aerospace and biology owing to its excellent mechanical properties and corrosion resistance. A novel magnetic field-assisted finishing (MFAF) tool is developed to carry out the finishing operation of Ti-6Al-4 V workpieces. Accordingly, the use of the designed magnetic field generator integrated with four permanent magnets as a tool to achieve alternating magnetic regions. The size of magnetic pole is calculated via theoretical analysis. Two kinds of distribution of four magnetic poles (N-N-S-S and N-S-N-S) are performed by using simulation models, and experimental measurements are conducted to validate the simulation results which show good agreement between them. Experiments were carried out to verify the performance of the designed tool for Ti-6Al-4 V workpieces finishing based on an established platform. The results show that, with the assistance of the finishing parameters control, the surface quality is dominated by varying the particle sizes, working gaps, feed rates and spindle rotational speeds. The surface roughness of 46 nm was achieved after finishing process based on the initial value of 1.121 μm, which improved by over 95%. The scratches on the surface are significantly reduced by metalloscope and field-emission scanning electron microscopy (FE-SEM) observations.

Magnetic field assisted finishing process for super-finished Ti alloy implant and its 3D surface characterization

Titanium alloy is used in medical industries due to its biocompatibility. Requirement of implant’s surface roughness and surface topography depends mainly upon its application. In the present study, application of titanium alloy is considered as femoral knee joint implant. The capability of magnetic field assisted finishing (MFAF) process and the polishing tool to provide implant worthy surface is analyzed here. In MFAF process, magnetorheological fluid mixed with abrasive powder in acidic base medium is used as the finishing medium. Characterization of the finished surface is carried out by analyzing 3D surface roughness parameters. The selected 3D surface parameters ( Sa, Spk, Sk and Svk) are considered due to their importance concerning load-bearing articulating surface of knee joint implant. Statistical design of experiment is used for experimental study and subsequently process parameters are optimized. From experimental investigation, the values of Sa, Spk, Sk and Svk are obtained as 11.32 nm, 15.82 nm, 6.51 nm and 41.15 nm, respectively, at optimum process parameter condition. The optimum process parameter values are 901 rpm of the tool, 0.60-mm working gap and 4.30 hrs of finishing time. The obtained values of 3D surface roughness parameters are in the nanometer range and the surface topography will render better wear properties, performance and longer implant life. Further confirmation experiments support the optimized values. The effect of individual process parameter on output responses is also analyzed.

A novel magnetically driven polishing technique for internal surface finishing

This paper presents a novel magnetically driven polishing tool for internal surface finishing. Differing from the existing magnetic abrasive finishing or magnetic field assisted finishing (MAF) methods; this new technique performs material removal inside a tube using a spherical magnet with abrasives driven by external bar magnets. This configuration of magnets not only simplifies the design but also allows more flexibility in the potential application to internal finishing of manifolds. The physical principle was introduced and polishing experiments were conducted to investigate the effect of magnetic abrasive particles and abrasive size based on the two major criteria of surface roughness and material removal. The results unequivocally substantiated the capability of the developed method with a more than 98% improvement in surface roughness for ring polishing and an 86% improvement for section polishing. An analytical model was devised to illustrate the material removal mechanism of the proposed internal polishing technique based on the experimental results. The technique developed in the paper shows signs of future success in finishing internal surface of complex components in a wide range of industrial applications.

A novel media properties-based material removal rate model for magnetic field-assisted finishing

Magnetic field assisted finishing (MFAF) is a category of non-conventional finishing processes that use magnetic field to manipulate finishing media typically consisting of magnetic particles and non-magnetic abrasives suspended in a carrier fluid. In order to better control the process, an improved understanding of the actual removal process is needed. This paper introduces a new material removal rate model for magnetic field-assisted finishing (MFAF) that aims to do so. The model considers the complexity of finishing media used in MFAF processes, where two different types of particles are present and interact with each other. The proposed material removal rate expression is based on contact mechanics and is a function of number of active magnetic particles, number of active abrasives, force per magnetic particle, and force per abrasive. Expressions for particle numbers were developed by considering an ideal face-centred cubic configuration for the magnetic particle network, while expressions for forces were developed based on a proposed framework for the particle interactions. The model was verified experimentally for a double-magnet MFAF process by varying the abrasive size and abrasive concentration. When the abrasive size was increased from 0.6 μm to 15 μm, the material removal rate decreased, consistent with the theoretical trend given by the model. Then, when abrasive concentration, given by the abrasives-to-carbonyl-iron volumetric ratio, was increased from 0 to 0.768, the material removal rate initially increased and then reached a maximum when the volume ratio is 0.259 before decreasing with further increase of the volume ratio. This is also in agreement with the theoretical trend given by the model.

Investigation on Surface Integrity of Rapidly Solidified Aluminum RSA 905 by Magnetic Field-Assisted Finishing.

RSA 905, a rapidly solidified aluminum alloy, has been widely utilized in optical, automotive, and aerospace industries owing to its superior mechanical properties such as hardness and strength compared to conventional aluminum alloys. However, the surface finishing of RSA 905 to achieve submicron surface roughness is quite challenging and was rarely addressed. This paper presents an experimental and analytical study on magnetic field-assisted finishing (MFAF) of RSA 905 through a systematic investigation on surface integrity in relation to the MFAF process parameters. The effect of abrasive and polishing speed conditions on material removal and surface roughness was quantitatively investigated. The surface and subsurface quality were evaluated by optical microscope and scanning electron microscope (SEM) observations, residual stress measurement, surface microhardness and tribology test. The results show that relatively high material removal and low surface roughness were obtained under conditions using the SiC abrasive with a grit size of 12 µm at polishing speed of 400 rpm or using the Al2O3 abrasive with a grit size of 5 µm at polishing speed of 800 rpm. Heat melt layer caused by wire electrical discharge machining (EDM) during the sample preparation was removed by MFAF without inducing new subsurface damage. The MFAF process also helps release the surface residual stress and improve the tribological performance although the surface microhardness was slightly reduced.

Surface quality improvement of selective laser sintered polyamide 12 by precision grinding and magnetic field-assisted finishing

Surface quality is essential for additive manufactured components due to the growing demand in various industries. This paper presents an experimental and analytical study on post-processing of selective laser sintered Polyamide 12 (PA12), aiming at improving surface quality and clarifying interrelations between surface quality and process parameters. The effects of post-processes on surface and subsurface characteristics regarding material removal, surface morphology and roughness, hardness, tribology performance were quantitatively evaluated. The results show that after post-processing, the surface roughness of the PA12 components were reduced obviously from over 15μm Ra to 2.85μm Ra and 0.89μm Ra by precision grinding and magnetic field-assisted finishing (MFAF), respectively. The un-melted powder surface layer was effectively removed although the surface hardness was slightly reduced. The MFAF processed surface showed a better tribology performance represented by lower coefficient of fiction and higher wear resistance. Moreover, the results of laser Raman analysis and X-ray photoelectron spectroscopy (XPS) indicated that there were no obvious chemical changes induced on the sub-surface level within 10μm by the post-processes.

Toolpath generation and finishing of bio-titanium alloy using novel polishing tool in MFAF process

Nanofinishing of biomaterials is a necessary process to lengthen the prosthetic life and performance. Here, Magnetic Field Assisted Finishing (MFAF) process is used to finish biomaterials in the nanometer level. MFAF process uses Magnetorheological (MR) fluid mixed with abrasive particles as the polishing medium. In this study, titanium is used as the workpiece material. A specially designed tool is used to carry out the nanofinishing process. Two types of path planning i.e. parallel and spiral tool path are adopted during finishing. The surface roughness and surface texture differ for each generated tool path. The surface roughness generated from each of the path planning processes is analyzed and it is found that parallel toolpath generates lowest surface roughness (~10 nm) with better surface texture than spiral toolpath. Hence, the parallel toolpath is considered as the optimum toolpath for finishing biomaterials in MFAF process. Experimental investigations using parallel tool path are carried out to explore the capability of the developed novel tool along with the determination of the optimum range of the process parameters to explore finishing capability. Preliminary experimental study shows that best surface finish and surface topography is achieved using 1200 rpm tool rotational speed, 1 mm working gap, and 6.30 h. finishing time.