Residual stress profiles induced by abrasive flow machining (AFM) in 15-5PH stainless steel internal channel surfaces

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Abstract

Surface integrity is an important factor to select finish process of a part’s surface and evaluate its quality. In particular, residual stress is one among important parameters to affect its fatigue life. Abrasive flow machining (AFM) has long been considered to be an effective surface finish process of external surfaces with complicated geometries or conformal cooling channels. Surface topography and surface roughness produced by AFM were shown to be comparable to those by grinding. However, more research on residual stress induced by AFM needs to be done. Different surface topographies after AFM with abrasive media having different abrasive size and concentration have been reported. Three different wear mechanisms, such as rubbing, ploughing, and cutting, have been known to be responsible for creating those surface topographies and their material removal. Therefore, in this study, 15-5PH stainless steel internal channel surfaces were created by boring. Then, their surfaces were finished by AFM with different abrasive size and concentration. Their material removal and surface topographies showed a big difference. Surface topography analysis reveals their possible wear mechanisms and movements, which can result in residual stress profiles in both directions – parallel and perpendicular to AFM flow. Big abrasive grains (54 grit size) left frequent pile-ups and curved flow lines on the workpiece surface, indicating that significant ploughing mechanism and rolling movement occurred during AFM. As a result, its compressive residual stress in the direction perpendicular to AFM flow gradually decreased with respect to the workpiece depth. On the other hand, clear abrasion and straight flow line were observed on the workpiece surface after AFM with MV65%-150 having medium grit size and highest concentration, suggesting significant cutting mechanism during AFM and the least plastic deformation among AFM finished surfaces in this study. Consequently, its compressive residual stresses in the both directions decreased sharply as the workpiece depth increased. AFM is characterized by its low material removal rate, however, it can induce compressive residual stress on the finished surface because its process temperature rise is not significant compared to other finish processes, such as grinding and turning.

Introduction

Surface integrity is an important factor to select finish process of a part’s surface and evaluate its quality. In particular, residual stress is one among important parameters to affect its fatigue life. Abrasive flow machining (AFM) has been considered as a promising finish process for surfaces having complicated geometries, such as conformal cooling channel and free form surfaces. Loveless et al. (1994) performed AFM on surfaces created by various machining processes, such as milling, turning, grinding, and electrical-discharge machining (EDM). They found that surface roughness, Ra is reduced on those surfaces after AFM. Surface topography of those surfaces after AFM is comparable to that created by grinding. Thus, AFM is shown to be an effective way of improving surfaces produced by various machining processes.

Extensive reviews of past works on AFM to date have been done by Kumar and Hiremath (2016); Sambharia and Mali (2017); Petare and Jain (2018). From their reviews, it is noted that most of studies have been dedicated to material removal and/or surface roughness improvement on the workpiece surface after AFM. On the other hand, a few studies on residual stress induced by AFM on the workpiece surface have been conducted. Kenda et al. (2012) created internal channel in the workpiece of AISI D2 steel by EDM. High tensile residual stress was induced on its internal surface. Then, they performed AFM on EDM generated surfaces with different pressures of 3.5 and 6 MPa. High compressive residual stresses were induced after AFM. Higher compressive residual stress was induced on the surface at higher applied AFM pressure of 6 MPa than that at lower AFM pressure of 3.5 MPa. Uhlmann and Roßkamp (2018) found compressive residual stresses in the sub surface of boreholes having different diameters (of 4, 6, and 9 mm) after AFM. Walia et al. (2008) performed centrifugal force assisted abrasive flow machining (CFAFM) on the workpiece. They observed that compressive residual stress on the workpiece surface increased with increasing rotational speed of centrifugal force generating (CFG) rod. In these three studies, their AFM process parameters to affect residual stress on the workpiece are AFM extrusion pressure (Kenda et al., 2012), workpiece borehole diameter (Uhlmann and Roßkamp, 2018), and rotational speed of centrifugal force generating (CFG) rod (Walia et al., 2008). Very little work on residual stress induced by AFM media parameters, such as abrasive concentration, abrasive grain size, and media viscosity has been done.

Different surface topographies after AFM with abrasive media having different abrasive size and concentration have been reported. Three different wear mechanisms, such as rubbing, ploughing, and cutting, have been known to be responsible for creating those surface topographies and their material removal. Thus, it is necessary to investigate the residual stress and surface integrity on the workpiece surface after AFM with different media. We adopt 15-5PH stainless steel that is widely used in aerospace and automobile industries. We created internal channels in the bar by boring process. Then, we perform AFM in 150 cycles on the internal channel surface with different abrasive media having different abrasive concentration and grain size. Surface roughness, and material removal evolution during AFM is monitored during AFM. Scanning electron microscopy (SEM) studies to reveal workpiece surface topography after AFM are conducted. With these observations, we justify possible wear mechanisms between the abrasive grains and workpiece. Residual stress profiles on the surfaces after AFM with different abrasive media are measured. Then, we explain why and how their subsequent residual stress profiles were induced by AFM with different abrasives concentration and grain size by referring to their possible wear mechanisms.

Section snippets

AFM test conditions

It is known that AFM process parameters, such as AFM extrusion pressure, media abrasive concentration, abrasive grain size and media viscosity are major factors to influence on surface roughness and material removal. Thus, we chose different media having different wt% abrasive concentration (35, 50, and 65%) and abrasive grain size (54 and 150 grit) with medium viscosity (MV). Silicon-carbide and polyboroxane were employed as abrasive and polymeric carrier, respectively. AFM test conditions

Velocity of AFM media flow

(Jain et al. (1999)) used continuity equation to calculate velocity of media flow in the fixture tube. It can be expressed as (Eq. 1)A1×v1=A2×v2where A1 and A2 are cross section areas (mm2) of the cylinder and the fixture tube internal channel. v1 and v2 are their flowing velocities (mm/sec). In a two-way AFM, one cycle time is composed of up-stroke and down-stroke.

Each cycle time is displayed after AFM for one cycle in the control monitor in Fig. 1(a). Each stroke time was measured to be a

Conclusion

In this study, 15-5PH stainless steel internal surface was made by boring. Then, AFM tests with media, in which abrasive size and concentration differ, were performed on its surface. Their surface integrities, such as areal surface roughness, Sa and amount of material removal, residual stress profiles were measured. Their surface topographies were presented. These results can lead to the following conclusions.

  • (1)

    There was a big difference in material removal depending on abrasive grain size and

Acknowledgements

The authors are grateful to “Institut Carnot I@L” for financial support to this research. The authors are also grateful to Extrude Hone for providing AFM machine, media, and sensors. Special thanks to; Polly Patrick and Hervé Seux for producing fixture tube units. Herve Pascal for residual stress measurement using XRD. Jacquier Maryane for taking SEM micrographs.

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