Study on rheological behaviors of media and material removal mechanism for abrasive flow machining (AFM) micro structures and corresponding simulations
Graphical abstract
Introduction
With the development of automotive, aerospace, energy, biomedical industries and so on, the finishing of complex surfaces of various workpieces, especially those with micro structures (e.g., micro holes), puts forward high requirement for the flexibility, machinability and precision of the common technologies [1], [2]. During such finishing processes, it is well known that considerable efforts and high costs are required for making a specific amount of materials free from workpieces [3]. Nevertheless, most traditional technologies, including turning, milling, grinding and polishing, can hardly meet the requirements of machining micro-scale complex surfaces [4], [5]. In consequence, one new finishing technology, named abrasive flow machining (AFM), which utilizes the semi-solid media mainly composed of soft matrix and hard abrasive grits as the “cutting tools”, was developed. The basic principle and processing route can be described as follows: the workpiece is placed between upper and lower hydraulic cylinders filled with the media. The workpiece and media are enclosed by some fixture, and the media can circulate within the confined space under a hydraulic extrusion pressure [6], [7]. The material removal rate (MRR) and the surface roughness reduction (ΔRa) can be controlled by adjusting the media or machining parameters [8], guaranteeing the machinability and precision. Especially, it is the fluidity of the media that ensures the great flexibility for machining complex surfaces. As a result, the AFM shows great potentials to break the limitations of traditional technologies and to tolerate extreme conditions [9]. However, the machining of some micro structures, for instance, the micro holes with diameters smaller than 0.1 mm, was still challenging.
The material removal mechanisms in the common AFM processes for machining macro-scale complex structures have been well studied. Lv et al. [10] demonstrated that the kinetic energy of abrasive grits, which was mainly provided by the hydraulic pressure and the radial force, had close relationships with the elastic components of the media. The hard abrasive grits were pressed into target surfaces and pushed to abrade the surfaces, leading to the removal of the desired materials or the reduction of the surface roughness. Jain et al. [11] hypothesized that the major material removal mechanism could be combined effects of micro-cutting and micro-plowing, while Singh et al. [12] indicated that for aluminum and brass workpieces, the material removal in the form of microchips and grooves dominated, which was driven by the plowing effect and the continuous abrasive flow. In other words, the material removal was driven by some micro-plastic deformation induced by the abrasive grits, with the help of the viscous flow.
The phenomenological relationships between key machining parameters and AFM processes, also for machining the macro-scale structures have been systematically investigated. Przylenk et al. [13] stated that the more abrasive grits could provide larger contact area, promoting the material removal. Jain et al. [14] established a model to describe the changes of the material removal and surface roughness with respect to the cycle (reciprocal) number, abrasive concentration, grit size and the grit velocity, proving that the most critical factor was the abrasive concentration. Gorana et al. [15] pointed out that the density of the active abrasive grits played a vital role in reducing the surface roughness. Moreover, some other machining parameters (e.g., extrusion pressure, cycle number, flow speed), and the properties of the workpieces, could also affect the machining effects [16]. As directly driving the abrasive grits to produce machining effect under extrusion, the media is indeed the most crucial element in the AFM process, the rheological behaviors of which determine the accessibility of the AFM, the material removal mechanism, the machining effects, and the probable machining limitations. Bremerstein et al. [17] indicated that several critical machining parameters could affect the final machining effect by modifying the rheological behaviors of the media. For instance, it was the viscoelastic behaviors of the media that could help to apply either the radial or the axial forces on the abrasive grits, generated by elastic and viscous components, respectively. The former could help to press the grits against the workpiece surfaces, and the latter could contribute to a certain degree of the flow velocity under the extrusion pressure, inducing relative sliding and material removal. Moreover, more complex rheological behaviors of the specific media, such as creep recovery, stress relaxation, shear thinning, storage modulus and loss modulus, also had direct or indirect effects on the machining process, which have also been systematically studied [18], [19]. However, the selected media were designed and only applied for machining macro-scale structures, which were probably not suitable for machining micro-scale ones. In addition, there was hardly any study that directly correlated the rheological behaviors of the media with the material removal phenomenon in the AFM process.
Simulations have been extensively applied in recent years in order to gain a deep insight into the influences of machining parameters, optimize corresponding parameters, and optimally design the fixtures (especially for obtaining uniform flow field distributions and material removals on the much complicated surfaces), mainly for the sake of reducing some experimental costs. A simple non-Newtonian constitutive model, namely the power law model, was constantly used to simulate the flow behaviors of the media, mainly including the distribution of the flow velocity, strain rate and the shear stress, with the purpose of obtaining uniform surface roughness [20], [21]. A slightly-modified wall slip model was commonly used for describing the typical wall slip behaviors [22]. Uhlmann et al. [23] analyzed the flow behaviors of some specific media (MF10-80S) based on power law model, showing high accordance between the measured and simulated results. Fu et al. [21] employed the power law model and Cox-Merz law to describe the non-Newtonian fluid flow, based on the rheological behaviors of the SBR media. It was noted that all above studies always concentrated on the machining of macro-scale structures, and the most-frequently-adopted power law model was a highly simplified model for describing the non-Newtonian fluid. To better simulate non-Newtonian media, especially those for machining the micro structures, it was essential to introduce some new models. Furthermore, many parameters in the new models should be elaborately defined according to the accurate characterizations on the rheological behaviors of the media.
In the present work, a type of media, which was specifically developed for machining micro structures, was selected as the research target. Components, structures, and rheological behaviors of the media were systematically characterized, based on which the material removal mechanism was firstly discussed. Besides, the shear viscosity curve was fitted by the Carreau-Yasuda model, and corresponding parameters were precisely defined and applied in computational fluid dynamics (CFD) simulations, the results of which were adopted for further explaining experimental results.
Section snippets
Media
The present media were especially developed for machining the micro structures (e.g., micro holes with diameters from 0.08 mm to 0.5 mm and depth-diameter ratios of above 5), which were mainly composed of some soft matrix and hard abrasive grits as usual. The abrasive grits were SiC abrasives with diameters from 15 μm to 20 μm. The soft matrix, mainly the base polymer melt, presented viscoelastic and shear thinning behaviors in nature. Some plasticizer oil was also added into the media in order
Analyses on infrared spectra of the media
As a typical type of polymer, the unique rheological behaviors had close relationships with the interactions between the molecular chains in the polymer melt of the media (e.g., coiling and twisting). Especially for machining the micro structure, the elaborately-prepared media must have even lower shear viscosity so as to enhance the fluidity, as a result, the plasticizers were added with the purpose of dispersing the molecular chains, and reducing the Van der Waals force between the adjacent
Conclusion
In summary, the workpieces with micro holes were taken as research objects in the present study. The components, structures and rheological behaviors of the abrasive media were analyzed in order to further and systematically study their influences on the material removal mechanism. Major conclusions were as follows:
- 1)
The similar components and structures made the plasticizer oil easily penetrate into the polymer melt, which can disperse the polymer chains to reduce the interaction force between
CRediT authorship contribution statement
Baocai Zhang: Conceptualization, Methodology, Resources, Investigation, Software, Writing - Original Draft preparation, Formal analysis, Data Curation, Experiment Validation.
Yu Qiao: Formal analysis, Data Curation, Experiment Validation.
Nasim Khiabani: Data Curation, Validation, Writing - Review & Editing.
Xinchang Wang: Data Curation, Validation, Writing - Review & Editing, Funding acquisition, Project administration, Supervision.
Declaration of competing interest
All of the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is sponsored by the National Natural Science Foundation of China (52175423), Key-Area Research and Development Program of Guangdong Province (Guangdong Science and Technology Department) (2020B010185001), National Natural Science Foundation of China (51705320), Huohua Project (20-163-00-TS-009-159-01), along with Shanghai Municipal Human Resources and Social Security Bureau-Pujiang Program (2019PJD021).
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