Elsevier

Wear

Volumes 342–343, 15 November 2015, Pages 44-51
Wear

Wear of abrasive media and its effect on abrasive flow machining results

https://doi.org/10.1016/j.wear.2015.08.013Get rights and content

Highlights

  • Wear of abrasive media occurs due to application in abrasive flow machining.

  • Wear of carrier and particles influences process efficiency and machining results.

  • Viscosity and elasticity rise due to entry of debris and increased finest fraction.

  • Increased viscosity and elasticity cause up to 20% worse surface quality.

  • Blunting of abrasive particles leads up to 30% less material removal.

Abstract

Abrasive media were examined before and after application in abrasive flow machining to evaluate the wear of media due to the machining process. Both media were tested on workpieces under the same working conditions to study the effect of abrasive media wear on the results of the machining process. With the help of rheological and granulometric characterisation methods, it has been shown that alterations of rheological behaviour and composition of the abrasive medium as well as particle shape and size are responsible for the degradation of the abrasive efficiency. The increasing viscosity of the abrasive medium and progressing rounding of the large abrasive particles as a result of the machining process are the main factors that cause a decreased material removal rate and reduced surface quality.

Introduction

A multitude of industry branches (e.g., manufacturing of medical, automotive and aerospace components or mechanical engineering) utilise abrasive suspensions for cutting or finishing high-performance elements. A detailed description of the applications of abrasive flow machining was provided by Yadav et al. [1]. Abrasive flow machining (AFM) is an advanced finishing process applied to deburr, polish or radius edges and surfaces of internal, difficult-to-reach workpiece geometries, and it employs special viscoelastic abrasive media. One of the first studies of this advanced machining process was conducted by Rhoades, who explained the process principle in detail [2]. In abrasive flow machining, two opposing cylinders clamp the workpiece between them and seal the machining passage. Hydraulically operated pistons inside the cylinders repeatedly extrude the abrasive medium back and forth through or across the workpiece to be finished (see Fig. 1). One up-and-down motion of the pistons amounts to a working cycle.

Spur et al. [3] also described the process in general as well as the abrasive medium that is utilised in abrasive flow machining. The semisolid abrasive medium consists of abrasive particles with a concentration of up to 40 vol% and a polymeric carrier medium. The viscosity of the abrasive medium can be adjusted by the addition of oil and other additives. Usually, silicon carbide, aluminium oxide or boron carbide are applied as abrasive materials.

Roughness or burrs are removed by the alternating movement of the abrasive suspension along the workpiece surface and edges. The rheological behaviour of the abrasive medium supports the material removal. On one hand, the carrier medium transfers the pressure of the pistons to the abrasive particles. On the other hand, the viscoelastic material hardens at mechanical strain-for example, at a constricted opening-and steadies the abrasives. The abrasive medium absorbs the abraded workpiece material and transports it away from the location of removal.

A large variety of parameters influence the process abrasive flow machining and results in complexity and difficulty in predicting process results. The input parameters can be assigned to three different groups-machine, workpiece and workpiece fixture, abrasive medium-which are described by Mali and Manna [4] and are listed in Fig. 2.

The process and medium parameters significantly affect the results of the machining process-namely, material removal rate and surface quality. The results are controllable by the variation of these parameters. Numerous studies examine the influence of the input parameters on the machining results. For example, Sankar et al. [5] found in their study that the viscosity of the abrasive medium and the size of the abrasive particles directly determine the removal rate of the workpiece material. A high viscosity and large particles lead to an increased material removal rate but also a reduced surface quality. Rajesha et al. [6] stated that the rheological behaviour of the abrasive medium is the main input parameter, whereas Kar et al. [7] found that the number of cycles and the concentration of abrasives are the basic input parameters. Some studies conduct empirical modelling of the machining process to predict the process results as presented by Jain et al. [8]. In other studies, essential elements of the machining process are theoretically modelled and validated by experimental results. For instance, Walia et al. [9] examined the forces acting on the particles and the number of active abrasive particles in the process, and Uhlmann et al. [10] addressed the modelling of the viscoelastic materials' behaviour.

Consistent process efficiency and quality of results are ensured only by invariable input parameters. However, the properties of the abrasive medium change because of wear in the machining process. Until now, these alterations of medium properties have not been systematically examined or documented except in a previous study of our own [11].

In this study, the rheological properties of new and used (for 420 h) abrasive media are characterized and compared. In addition, further features are quantified that generally affect the rheological behaviour such as composition of the abrasive medium, particle size and shape. Furthermore, the performance of both media in the machining process is tested by utilising them under the same process conditions on test workpieces with the same initial surface qualities. Finally, a conclusion is drawn about the process results with reference to the changed medium properties.

Section snippets

Abrasive media

In this study, an abrasive medium by the company Micro Technica Technologies GmbH is characterized before and after utilisation in an abrasive flow machining process. The abrasive medium was applied to the machining of surfaces and rounding of edges with a total machining time of 420 h.

The carrier medium consists of polyborosiloxane with addition of hydrocarbon oil and metal soap to adjust the viscosity of the mixture. The carrier medium contains three different sizes of silicon carbide

Rheological properties of the abrasive medium

The abrasive medium in process abrasive flow machining exhibits a special rheological behaviour-namely, viscoelasticity. A viscoelastic medium possesses viscous as well as elastic properties. At rest or under very small stress, the medium flows and deforms like a viscous fluid, whereas it reacts elastically at rapid and high stress. The viscoelastic behaviour can be characterized by frequency sweeps in which the rheological properties are measured at high frequencies (which represent rapid

Conclusions

The property alterations of an abrasive medium due to utilisation in abrasive flow machining and their influence on the efficiency and results of the finishing process have been studied. The major conclusions obtained from this work are as follows:

  • The viscosity and elastic percentage of the viscoelastic medium rise due to usage of abrasive media in the machining process. This is caused by both the entry of wear debris and the increasing fraction of fine abrasive particles, which lead to an

Acknowledgements

The authors give thanks to the Fraunhofer Gesellschaft for the financial support of the project “AbraSus” (Grant number WISA 823 260).

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