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Qualification of the stream finishing process for surface modification
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CIRP-1637; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Qualification of the stream finishing process for surface modification Volker Schulze (2)a,b,*, Jens Gibmeier b, Andreas Kacaras a a b
Institute of Production Science, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany Institute for Applied Materials, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface modification Residual stress Finishing
The stream finishing process represents an established and efficient production process for surface smoothing and edge rounding. In addition to the targeted setting of a defined surface topography the process features a high potential for mechanical surface modification that has not been realized yet. In this work the stream finishing process is carried out on normalised AISI4140 plane specimen with the aim of efficiently determining optimal processing time for surface modification (micro hardness, residual stresses, surface topography). In this context, the suitability of the Almen system [1] as an efficient method for characterizing change in residual stress during stream finishing is investigated. © 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction
achieved through vibropeening by high cycle fatigue results. Sangid et al. [9] demonstrated that vibrostrengthening allows an induction of compressive residual stresses with a magnitude of 290 MPa and an affected depth of approximately 300 mm into an aluminum alloy while specimens showed reduced surface roughness. Thus, compared to the shot peening process a superior fatigue life is reached. Furthermore, it was shown that shadowing effects during the vibrostrengthening caused reduced processing effects and must be considered. A computational model for the prediction of the fatigue life of a treated component based on experimentally measured residual stress profiles and surface characteristics was developed [10] showing an error of 12% compared to experimental results. Regarding surface modification Sangid et al. and Feldmann et al. mainly focus on surface roughness and residual stress states not considering the effect of work hardening on the processed surface playing a major role for tribological applications. Not addressing residual stress states Dehbi and Hamouda [11], Baghbanan et al. [12] and Wang et al. [13] proved a significant increase of surface hardness in a workpiece through vibratory finishing. To the best of the authors’ knowledge currently no comprehensive correlation between surface states and processing time for a mass finishing treatment is available. The stream finishing process shows higher relative flow velocities between workpiece and abrasive media compared to the vibratory finishing process. For this reason, the stream finishing process has great potential for mechanical surface modification with high surface quality and higher productivity due to reduced processing time. Aiming to close the above mentioned gaps, in this work a comprehensive correlation between processing time of the stream finishing treatment and the residual stress state was established to qualify minimal processing time with optimized surface states looking at micro hardness and surface topography on normalized AISI4140 specimen. Additionally, the Almen system is adapted to the stream finishing process in order to characterize the processing intensity depending on the processing time.
A wide variety of mass finishing processes for a defined setting of surface topography of complex shaped workpieces have been established earlier. However, mass finishing processes are still underrepresented in research. The majority of published articles focusses on the development of material removal models for vibratory finishing [2] or drag finishing processes [3–5]. Highly loaded component regions are often strengthened in the near surface region by mechanical surface modification processes [6]. Here, the main objective lies in the improvement of fatigue life, wear and corrosion properties of mechanical components [7]. The shot peening process allows a defined induction of surface layer states but causes increased roughness in the surface of processed regions [7–9]. In order to meet the high demands of fatigue life and very fine surface finishes of cyclically loaded mechanical components as used in the aerospace industry [2] more than one surface modification process is needed [8]. Hence, an alternative process capable of introducing compressive residual stresses equivalent to the shot peening process and improving surface roughness like a mass finishing process is needed. Feldmann et al. [8] and Sangid et al. [9,10] both investigated a modified vibratory finishing process for fatigue life enhancement and optimization of surface roughness. The vibropeening [8] of a nickel based alloy lead to a surface state with significantly reduced surface roughness (Ra < 0.25 mm) and induced compressive residual stresses almost equivalent to shot peening with a slightly higher influenced depth of about 150 mm. An increase of fatigue strength of 35% using soft media and 61% using hard media was proved to be
* Corresponding author at: Institute of Production Science, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany. E-mail address: [email protected] (V. Schulze). http://dx.doi.org/10.1016/j.cirp.2017.04.079 0007-8506/© 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
G Model
CIRP-1637; No. of Pages 4 V. Schulze et al. / CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
2
2. Experimental For experiments a stream finishing machine SF 1 68 (Otec GmbH) was used. A high density sintered aluminum oxide ceramic KXMA 16 with an average grain size of 1.7–2.4 mm with water and compound SC15 served as abrasive medium. The workpiece was immersed into the abrasive medium (Fig. 1). Rotational movement of the bowl caused medium flow around the workpiece leading to the processing effect. Besides the processing parameters the processing effect is highly influenced by part geometry and related flow characteristics. Especially for complex shaped parts shadowing and impoundment effects need to be considered. In this study simple plane specimen were used in order to reduce the flow effects mentioned. This allows parameter identification. Influences on part geometry will be topic of future studies.
Fig. 1. Schematic view of the stream finishing machine.
Three specimens were prepared, processed and analysed for each parameter configuration. AISI4140 (EN steel 1.7225) in a normalized state (austenized at 870 C for 40 min and slowly cooled under vacuum) was chosen as model workpiece material because of its high potential to increase hardness within processing. The Almen system [1] constitutes a well-established method for the characterization of process intensity within the shot peening process and was found to be also suitable for the processes burnishing and machine hammer peening [6,14] as well as for the vibratory finishing [12,15,16]. In order to adapt the Almen system for the stream finishing process strips with the mini Almen-geometry Type N (dimensions 25.4 mm 3.175 mm 0.79 mm) were made out of the target material AISI4140 (further named “adapted Almen strips”). Heat treatment was performed after surface grinding leading to an almost balanced residual stress state with a value of 60 8 MPa in the specimen. The grinding process caused longitudinal grooves on the workpiece’s surface. For the experiments, only the processing time was varied within the range of 0.25 min–120 min. Other processing parameters were predefined as follows. It is known, that greater impact velocities lead to more plastic deformation resulting in greater arc heights H [15,16]. Therefore, the rotational speed n of the bowl and the radius of immersion r were chosen to achieve the maximum relative flow velocity between workpiece and abrasive possible (n = 70 min1, r = 270 mm). A highly simplified fractional factorial design of experiments was implemented to identify reasonable processing parameters for depth of immersion h, angle of immersion ’ and angle of workpiece orientation g with a high processing effect. In this study, the processing time t was 16 min. Two factor levels were chosen for the three processing parameters named before. The resulting arc height of processed Almen strips was analyzed in order to evaluate the processing effect. According to Fig. 2 the most reasonable parameter configuration showing the greatest arc height was ’ = 25 , g = 0 and h = 125 mm. This was further used for studying the influence of time on the surface states. After finishing treatment arc height H, surface topography, mass change Dm and surface micro hardness were evaluated for each adapted Almen strip. The arc height was measured with an Almen gage TSP-M with a 0.001 mm resolution. The arithmetic
Fig. 2. Comparison of resulting arc heights H after a finishing time of t = 16 min for different parameter configurations.
average surface roughnesses Sa and texture aspect ratios Str (alternatively isotropy index [17]) as well as texture directions Std were determined by confocal light microscopy with a field of view of 160 mm 160 mm (magnification of 100) using the msoft analysis software by Nanofocus AG according to ISO 25178 [18]. All surface topography parameters were determined applying a robust Gaussian filter with lS = 0.8 mm and a Gaussian filter with lL = 80 mm. To evaluate the finishing treatment’s cold work effect the Vickers micro hardness on the adapted Almen strip’s surface was measured using the Qness’s Q10A+ hardness testing system applying a load of 50 g (HV 0.05). For the evaluation of material removal due to the processing the mass change Dm of the workpieces was measured with an analytical balance by MettlerToledo Ltd. showing a resolution of 0.1 mg. Residual stresses s RS were analysed by X-ray stress analysis in samples with same length and width as the adapted Almen strips according to the wellknown sin2c-technique [19] using a four circle diffractometer of type Huber and CrKa-radiation. To avoid any distortion induced by the stream finishing process or by sublayer removal a similar sample but with a thickness of 10 mm was chosen. For residual stress analyses in longitudinal and transverse direction of the sample, a primary aperture with a nominal diameter of 0.1 mm combined with a 4 mm symmetrizing slit [20] and a scintillation counter on the secondary side was used. Analyses of the residual stress depth distributions were carried out only for the stress component in longitudinal direction. Here, a larger primary aperture with a nominal diameter of 1 mm was applied. For analysis of the residual stress depth distribution a sublayer removal by means of electropolishing was carried out combined with the reapplication of the sin2c-measurement at the newly created surface. The {211}-ferrite interference lines were measured in the 2u-range between 146 to 165.8 in c-mode at 15 inclination angles between 60 c 60 . Line positions were determined using the Pearson VII function. The diffraction elastic constants 1 /2s2 = 5.82 106 MPa1 and s1 = 1.27 106 MPa1 for ferrite were used for residual stress evaluation. Moreover, the average integral breadths IB of the diffraction lines served to assess the stream finishing process’ cold working effect. 3. Results and discussion For qualitative characterization of changes in residual stresses due to plastic deformation during the stream finishing treatment the arc heights of the processed adapted Almen strips were measured. Fig. 3 shows the arc height versus the processing time. Analogous to Kirk investigating shot peening [21] and Ciampini et al. researching on vibratory finishing [15] the saturation curves were fitted using an exponential function H(t) HðtÞ ¼ A ð1 expðB tC ÞÞ þ D t
ð1Þ
where H describes the arc height, t is the processing time and A, B, C, D are fit parameters. Based on the curve fit (Table 1) the effective arc height Heff and the corresponding effective time teff were
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
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CIRP-1637; No. of Pages 4 V. Schulze et al. / CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
Fig. 3. Arc height vs. processing time.
3
Fig. 5. Texture aspect ratio vs. processing time.
t describing the processing time and E a fit parameter (Table 1). For surface texture evaluation the texture aspect ratio Str and texture direction Std were set and analysed. Fig. 5 depicts the texture aspect ratios compared with the processing time. Initially the value for texture aspect ratio is low, Str = 0.04, suggesting spatial anisotropic properties in surface topography with a
preferred direction of Std = 90 related to the grooves caused by surface grinding. According to Stout [17] there is no pronounced texture direction Std if the texture direction Str exceeds 0.5. After a finishing time of 0.25 min a significantly increased Str value can be seen showing almost constant values and no preferred texture direction in the course of further processing. By implication a finishing time of only 0.25 min can lead to a surface topography without oriented structures. Due to the accumulated impacts of abrasive particles and related mechanical work the micro hardness depends on time, increasing until saturation value is reached (Fig. 6). Starting from a micro hardness value of about 200 HV0.05 the saturation value of about 400 HV0.05 is reached after a finishing time of 3.75 min which is quite higher than the time at the end of roughness change, which is only 1 min according to Fig. 4. Since Fig. 3 indicates that the arc height still increases after 3.75 min it is obvious that even higher processing time is needed to reach a stationary situation. This can be traced back to lateral assimilation of stresses and work hardening effects needing high coverage. This is well known from shot peening [23]. In this stationary situation the material removal and the increase of work hardening in previously non-affected areas below the surface are becoming equal. The surface residual stresses in longitudinal and transverse direction as well as the corresponding mean integral breadths of the X-ray interference lines versus processing time are shown in Fig. 7. The initially almost balanced residual stress state and corresponding mean integral breadths are not considered in Fig. 7 due to slightly varying measuring conditions but are shown in Fig. 8 instead. Insignificant change of integral breadths by longer stream finishing times can be observed in Fig. 7 indicating that no further cold working was induced. This is consistent with the micro hardness values in Fig. 6. After a stream finishing period of 3.75 min significant compressive residual stresses of about 400 MPa are induced on the surface with comparable magnitudes in the longitudinal and transverse direction. A slight increase of the compressive residual stresses can be observed on the surface for processing times larger than 3.75 min resulting in a maximum magnitude of about 500 MPa after 16 min. Depth distributions of the residual stresses and integral breadths of the interference lines for analyses of the longitudinal direction are shown in Fig. 8 as an example for processing times of 3.75 and 16 min. The distributions of the residual stresses showing an almost linear decrease from 400 or 440 MPa at the surface to about zero at the depth of 25 mm clearly indicate a depth range of the stream
Fig. 4. Roughness and mass difference vs. processing time.
Fig. 6. Surface micro hardness vs. processing time.
Table 1 Values of parameters A, B, C, D and E for the curve fits. Curve fit
A [mm]
B [minC]
C [–]
D [mm]
E [min1]
H (t) Sa (t)
0.057 –
0.770 –
0.246 –
6.012 E-5 –
– 2.972
calculated by satisfying the following equation [15]. Hð2 tef f Þ Hðtef f Þ ¼ 0:1 Hðtef f Þ
ð2Þ
After a finishing time of 3.75 min the effective arc height Heff is reached by stochastically impinging abrasive particles causing plastic deformation of the surface. For further processing only a slight increase of the arc height can be observed indicating that no additional significant work hardening effects occur. Fig. 4 displays the measured arithmetic average roughness Sa and the mass difference Dm versus processing time t. Hashimoto et al. [22] established the basic fundamentals for surface roughness prediction in the vibratory finishing further proposing a corresponding mathematical model. According to them [22] the removal mechanisms “roughness change” and “micro cutting” of the vibratory finishing can be utilized for the stream finishing process, respectively. Starting from the initial arithmetic average roughness of Sa,0 = 0.24 mm “roughness change” occurs up to a finishing time of 1 min along with a higher progression of mass difference in continued processing. Sa clearly reaches a saturation value around Sa,1 = 0.13 mm for finishing times greater than 1 min, when micro cutting mechanisms occur with a linear mass change. These findings are in accordance with those of Hashimoto et al. [22]. Thus, the surface roughness prediction model (Eq. (3)) was applied to the curve fit of the measured arithmetic average roughnesses Sa of stream finished specimen showing an adequate fit. Sa ðtÞ ¼ ðSa;0 Sa;1 Þ expðE tÞ þ Sa;1
ð3Þ
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
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Fig. 7. Surface residual stresses in longitudinal and transverse direction and corresponding mean integral breadths vs. processing time.
Initially the workpiece surface topography showed an oriented texture due to previous grinding process. “Roughness change” occurs within a finishing time of 1 min where the roughness approaches a saturation value of Sa = 0.13 mm leading to an almost isotropic surface topography. The effective time teff, based on the temporal evolution of arc heights, represents a benchmark for minimal processing time generating significant compressive residual stresses and mechanical work hardening effects as a result of cold work in the near surface region. It is 3.75 min and therefore much higher than the time necessary for surface smoothing. Compressive residual stresses of approximately 400 MPa and an increase of micro hardness of approximately 200 HV0.05 on the surface were observed after teff of finishing. Measured surface residual stresses, integral breadths and micro hardnesses of processed specimen revealed almost stationary residual stress states and work hardening states in the near surface region for finishing times greater than teff. Depth distributions of residual stresses and integral breadths of processed specimen prompt the depth range of the stream finishing process to be 20–25 mm for induced residual stresses and 6–10 mm for cold working effects. References
Fig. 8. Depth distributions of the residual stresses and the integral breadths for the processing times of 3.75 and 16 min combined with the initial values (0 min).
finishing process of about 20–25 mm. The depth distributions of the integral breadths almost coincide, illustrating that no significant extra cold work is induced by longer processing times. Considering the comparable values of the initial integral breadth (t = 0 min) on the surface and the integral breadths in higher depths a significant change of the integral breadths can be demonstrated up to a depth of 6–10 mm indicating a depth range of the stream finishing process for the cold working effect. It may be postulated that after a certain finishing time almost stationary residual stress states and work hardening states occur in the near surface region. 4. Conclusion A comprehensive correlation between processing time of the stream finishing treatment and the residual stress state, micro hardness and surface topography on normalized AISI4140 plane specimen was established in order to qualify minimal processing time with an optimized surface state. It was demonstrated that the stream finishing process represents an effective mass finishing process for mechanical surface modification. This might be highly suitable for improvement of fatigue, wear and corrosion properties of mechanical components. Concluding, the key findings of this work are: The adapted Almen system showed to be an efficient method for characterizing changes in residual stress state during stream finishing.
[1] Almen J.O. (1944) Shot blasting test, Patent No. US2350440 A. [2] Mediratta R, Ahluwalia K, Yeo SH (2016) State-of-the-art on Vibratory Finishing in the Aviation Industry: An Industrial and Academic Perspective. The International Journal of Advanced Manufacturing Technology 85:415–429. [3] Barletta M, Pietrobono F, Rubino G, Tagliaferri V (2014) Drag Finishing of Sensitive Workpieces with Fluidized Abrasives. Journal of Manufacturing Processes 16:494–502. [4] Barletta M, Gisario A, Venettacci S, Rubino G (2014) A Comparative Evaluation of Fluidized Bed Assisted Drag Finishing and Centrifugal Finishing. Engineering Science and Technology an International Journal 17:63–72. [5] Uhlmann E, Eulitz A, Dethlefs A (2015) Discrete Element Modelling of Drag Finishing. Procedia CIRP 31:369–374. [6] Schulze V, Bleicher F, Groche P, Guo YB, Pyun YS (2016) Surface Modification by Machine Hammer Peening and Burnishing. CIRP Annals—Manufacturing Technology 65:809–832. [7] Schulze V (2005) Modern Mechanical Surface Treatment: States, Stability, Effects, Wiley-VCH Verlag GmbH & Co KGaA. [8] Feldmann G, Wong CC, Wei W, Haubold T (2014) Application of Vibropeening on Aero-engine Component. Procedia CIRP 13:423–428. [9] Sangid MD, Stori JA, Ferriera PM (2011) Process Characterization of Vibrostrenghtening and Application to Fatigue Enhancement of Aluminium Aerospace Components—Part I Experimental Study of Process Parameters. The International Journal of Advanced Manufacturing Technology 53:545–560. [10] Sangid MD, Stori JA, Ferriera PM (2011) Process Characterization of Vibrostrenghtening and Application to Fatigue Enhancement of Aluminium Aerospace Components—Part II Process Visualization and Modelling. The International Journal of Advanced Manufacturing Technology 53:561–575. [11] Dehbi S, Hamouda K (2015) Improved Surface Quality by the Process Vibratory Grinding. Defect and Diffusion Forum 365:183–187. [12] Baghbanan MR, Yabuki A, Timsit RS, Spelt JK (2003) Tribological Behaviour of Aluminum Alloys in a Vibratory Finishing Process. Wear 255:1369–1379. [13] Wang S, Timsit RS, Spelt JK (2000) Experimental Investigation of Vibratory Finishing of Aluminium. Wear 243:147–156. [14] Lienert F, Gerstenmeyer M, Krall S, Lechner C, Trauth D, Bleicher F, Schulze V (2016) Experimental Study on Comparing Intensities of Burnishing and Machine Hammer Peening Processes. Procedia CIRP 45:371–374. [15] Ciampini D, Papini M, Spelt JK (2008) Characterization of Vibratory Finishing Using the Almen System. Wear 264:671–678. [16] Ciampini D (2008) Impact Velocity, Almen Strip Curvature and Residual Stress Modelling in Vibratory Finishing, Department of Mechanical and Industrial Engineering, University of Toronto. [17] Stout KJ (2006) Development of Methods for the Characterisation of Roughness in Three Dimensions. Penton Press. [18] DIN EN ISO 25178, 2012, Geometrical product specifications (GPS) – Surface texture: Areal – Part 2: Terms, definitions and surface texture parameters. [19] Macherauch E, Müller P (1961) Das sin2C-Verfahren der röntgenographischen Spannungsmessung. Zeitschrift für angewandte Mathematik und Physik 13:305– 312. [20] Wolfstieg U (1976) Die Symmetrisierung unsymmetrischer Interferenzlinien mit Hilfe von Spezialblenden. HTM 31:23–27. [21] Kirk D (2005) Computer-based saturation curve analysis. The Shot Peener 19 (4):16–21. [22] Hashimoto F, DeBra DB (1996) Modelling and Optimization of Vibratory Finishing Process. Annals of the CIRP 45:303–306. [23] Schwarzer J, Schulze V, Vöhringer O (2002) Finite Element Simulation of Shot Peening—A Method to Evaluate the Influence of Peening Parameters on Surface Characteristics. in Wagner L, (Ed.) Proceedings of the 8th International Conference on Shot Peening ICSP 8, 507–515.
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
CIRP-1637; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Qualification of the stream finishing process for surface modification Volker Schulze (2)a,b,*, Jens Gibmeier b, Andreas Kacaras a a b
Institute of Production Science, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany Institute for Applied Materials, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Surface modification Residual stress Finishing
The stream finishing process represents an established and efficient production process for surface smoothing and edge rounding. In addition to the targeted setting of a defined surface topography the process features a high potential for mechanical surface modification that has not been realized yet. In this work the stream finishing process is carried out on normalised AISI4140 plane specimen with the aim of efficiently determining optimal processing time for surface modification (micro hardness, residual stresses, surface topography). In this context, the suitability of the Almen system [1] as an efficient method for characterizing change in residual stress during stream finishing is investigated. © 2017 Published by Elsevier Ltd on behalf of CIRP.
1. Introduction
achieved through vibropeening by high cycle fatigue results. Sangid et al. [9] demonstrated that vibrostrengthening allows an induction of compressive residual stresses with a magnitude of 290 MPa and an affected depth of approximately 300 mm into an aluminum alloy while specimens showed reduced surface roughness. Thus, compared to the shot peening process a superior fatigue life is reached. Furthermore, it was shown that shadowing effects during the vibrostrengthening caused reduced processing effects and must be considered. A computational model for the prediction of the fatigue life of a treated component based on experimentally measured residual stress profiles and surface characteristics was developed [10] showing an error of 12% compared to experimental results. Regarding surface modification Sangid et al. and Feldmann et al. mainly focus on surface roughness and residual stress states not considering the effect of work hardening on the processed surface playing a major role for tribological applications. Not addressing residual stress states Dehbi and Hamouda [11], Baghbanan et al. [12] and Wang et al. [13] proved a significant increase of surface hardness in a workpiece through vibratory finishing. To the best of the authors’ knowledge currently no comprehensive correlation between surface states and processing time for a mass finishing treatment is available. The stream finishing process shows higher relative flow velocities between workpiece and abrasive media compared to the vibratory finishing process. For this reason, the stream finishing process has great potential for mechanical surface modification with high surface quality and higher productivity due to reduced processing time. Aiming to close the above mentioned gaps, in this work a comprehensive correlation between processing time of the stream finishing treatment and the residual stress state was established to qualify minimal processing time with optimized surface states looking at micro hardness and surface topography on normalized AISI4140 specimen. Additionally, the Almen system is adapted to the stream finishing process in order to characterize the processing intensity depending on the processing time.
A wide variety of mass finishing processes for a defined setting of surface topography of complex shaped workpieces have been established earlier. However, mass finishing processes are still underrepresented in research. The majority of published articles focusses on the development of material removal models for vibratory finishing [2] or drag finishing processes [3–5]. Highly loaded component regions are often strengthened in the near surface region by mechanical surface modification processes [6]. Here, the main objective lies in the improvement of fatigue life, wear and corrosion properties of mechanical components [7]. The shot peening process allows a defined induction of surface layer states but causes increased roughness in the surface of processed regions [7–9]. In order to meet the high demands of fatigue life and very fine surface finishes of cyclically loaded mechanical components as used in the aerospace industry [2] more than one surface modification process is needed [8]. Hence, an alternative process capable of introducing compressive residual stresses equivalent to the shot peening process and improving surface roughness like a mass finishing process is needed. Feldmann et al. [8] and Sangid et al. [9,10] both investigated a modified vibratory finishing process for fatigue life enhancement and optimization of surface roughness. The vibropeening [8] of a nickel based alloy lead to a surface state with significantly reduced surface roughness (Ra < 0.25 mm) and induced compressive residual stresses almost equivalent to shot peening with a slightly higher influenced depth of about 150 mm. An increase of fatigue strength of 35% using soft media and 61% using hard media was proved to be
* Corresponding author at: Institute of Production Science, Karlsruhe Institute of Technology, Kaiserstr. 12, 76131 Karlsruhe, Germany. E-mail address: [email protected] (V. Schulze). http://dx.doi.org/10.1016/j.cirp.2017.04.079 0007-8506/© 2017 Published by Elsevier Ltd on behalf of CIRP.
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
G Model
CIRP-1637; No. of Pages 4 V. Schulze et al. / CIRP Annals - Manufacturing Technology xxx (2017) xxx–xxx
2
2. Experimental For experiments a stream finishing machine SF 1 68 (Otec GmbH) was used. A high density sintered aluminum oxide ceramic KXMA 16 with an average grain size of 1.7–2.4 mm with water and compound SC15 served as abrasive medium. The workpiece was immersed into the abrasive medium (Fig. 1). Rotational movement of the bowl caused medium flow around the workpiece leading to the processing effect. Besides the processing parameters the processing effect is highly influenced by part geometry and related flow characteristics. Especially for complex shaped parts shadowing and impoundment effects need to be considered. In this study simple plane specimen were used in order to reduce the flow effects mentioned. This allows parameter identification. Influences on part geometry will be topic of future studies.
Fig. 1. Schematic view of the stream finishing machine.
Three specimens were prepared, processed and analysed for each parameter configuration. AISI4140 (EN steel 1.7225) in a normalized state (austenized at 870 C for 40 min and slowly cooled under vacuum) was chosen as model workpiece material because of its high potential to increase hardness within processing. The Almen system [1] constitutes a well-established method for the characterization of process intensity within the shot peening process and was found to be also suitable for the processes burnishing and machine hammer peening [6,14] as well as for the vibratory finishing [12,15,16]. In order to adapt the Almen system for the stream finishing process strips with the mini Almen-geometry Type N (dimensions 25.4 mm 3.175 mm 0.79 mm) were made out of the target material AISI4140 (further named “adapted Almen strips”). Heat treatment was performed after surface grinding leading to an almost balanced residual stress state with a value of 60 8 MPa in the specimen. The grinding process caused longitudinal grooves on the workpiece’s surface. For the experiments, only the processing time was varied within the range of 0.25 min–120 min. Other processing parameters were predefined as follows. It is known, that greater impact velocities lead to more plastic deformation resulting in greater arc heights H [15,16]. Therefore, the rotational speed n of the bowl and the radius of immersion r were chosen to achieve the maximum relative flow velocity between workpiece and abrasive possible (n = 70 min1, r = 270 mm). A highly simplified fractional factorial design of experiments was implemented to identify reasonable processing parameters for depth of immersion h, angle of immersion ’ and angle of workpiece orientation g with a high processing effect. In this study, the processing time t was 16 min. Two factor levels were chosen for the three processing parameters named before. The resulting arc height of processed Almen strips was analyzed in order to evaluate the processing effect. According to Fig. 2 the most reasonable parameter configuration showing the greatest arc height was ’ = 25 , g = 0 and h = 125 mm. This was further used for studying the influence of time on the surface states. After finishing treatment arc height H, surface topography, mass change Dm and surface micro hardness were evaluated for each adapted Almen strip. The arc height was measured with an Almen gage TSP-M with a 0.001 mm resolution. The arithmetic
Fig. 2. Comparison of resulting arc heights H after a finishing time of t = 16 min for different parameter configurations.
average surface roughnesses Sa and texture aspect ratios Str (alternatively isotropy index [17]) as well as texture directions Std were determined by confocal light microscopy with a field of view of 160 mm 160 mm (magnification of 100) using the msoft analysis software by Nanofocus AG according to ISO 25178 [18]. All surface topography parameters were determined applying a robust Gaussian filter with lS = 0.8 mm and a Gaussian filter with lL = 80 mm. To evaluate the finishing treatment’s cold work effect the Vickers micro hardness on the adapted Almen strip’s surface was measured using the Qness’s Q10A+ hardness testing system applying a load of 50 g (HV 0.05). For the evaluation of material removal due to the processing the mass change Dm of the workpieces was measured with an analytical balance by MettlerToledo Ltd. showing a resolution of 0.1 mg. Residual stresses s RS were analysed by X-ray stress analysis in samples with same length and width as the adapted Almen strips according to the wellknown sin2c-technique [19] using a four circle diffractometer of type Huber and CrKa-radiation. To avoid any distortion induced by the stream finishing process or by sublayer removal a similar sample but with a thickness of 10 mm was chosen. For residual stress analyses in longitudinal and transverse direction of the sample, a primary aperture with a nominal diameter of 0.1 mm combined with a 4 mm symmetrizing slit [20] and a scintillation counter on the secondary side was used. Analyses of the residual stress depth distributions were carried out only for the stress component in longitudinal direction. Here, a larger primary aperture with a nominal diameter of 1 mm was applied. For analysis of the residual stress depth distribution a sublayer removal by means of electropolishing was carried out combined with the reapplication of the sin2c-measurement at the newly created surface. The {211}-ferrite interference lines were measured in the 2u-range between 146 to 165.8 in c-mode at 15 inclination angles between 60 c 60 . Line positions were determined using the Pearson VII function. The diffraction elastic constants 1 /2s2 = 5.82 106 MPa1 and s1 = 1.27 106 MPa1 for ferrite were used for residual stress evaluation. Moreover, the average integral breadths IB of the diffraction lines served to assess the stream finishing process’ cold working effect. 3. Results and discussion For qualitative characterization of changes in residual stresses due to plastic deformation during the stream finishing treatment the arc heights of the processed adapted Almen strips were measured. Fig. 3 shows the arc height versus the processing time. Analogous to Kirk investigating shot peening [21] and Ciampini et al. researching on vibratory finishing [15] the saturation curves were fitted using an exponential function H(t) HðtÞ ¼ A ð1 expðB tC ÞÞ þ D t
ð1Þ
where H describes the arc height, t is the processing time and A, B, C, D are fit parameters. Based on the curve fit (Table 1) the effective arc height Heff and the corresponding effective time teff were
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Fig. 3. Arc height vs. processing time.
3
Fig. 5. Texture aspect ratio vs. processing time.
t describing the processing time and E a fit parameter (Table 1). For surface texture evaluation the texture aspect ratio Str and texture direction Std were set and analysed. Fig. 5 depicts the texture aspect ratios compared with the processing time. Initially the value for texture aspect ratio is low, Str = 0.04, suggesting spatial anisotropic properties in surface topography with a
preferred direction of Std = 90 related to the grooves caused by surface grinding. According to Stout [17] there is no pronounced texture direction Std if the texture direction Str exceeds 0.5. After a finishing time of 0.25 min a significantly increased Str value can be seen showing almost constant values and no preferred texture direction in the course of further processing. By implication a finishing time of only 0.25 min can lead to a surface topography without oriented structures. Due to the accumulated impacts of abrasive particles and related mechanical work the micro hardness depends on time, increasing until saturation value is reached (Fig. 6). Starting from a micro hardness value of about 200 HV0.05 the saturation value of about 400 HV0.05 is reached after a finishing time of 3.75 min which is quite higher than the time at the end of roughness change, which is only 1 min according to Fig. 4. Since Fig. 3 indicates that the arc height still increases after 3.75 min it is obvious that even higher processing time is needed to reach a stationary situation. This can be traced back to lateral assimilation of stresses and work hardening effects needing high coverage. This is well known from shot peening [23]. In this stationary situation the material removal and the increase of work hardening in previously non-affected areas below the surface are becoming equal. The surface residual stresses in longitudinal and transverse direction as well as the corresponding mean integral breadths of the X-ray interference lines versus processing time are shown in Fig. 7. The initially almost balanced residual stress state and corresponding mean integral breadths are not considered in Fig. 7 due to slightly varying measuring conditions but are shown in Fig. 8 instead. Insignificant change of integral breadths by longer stream finishing times can be observed in Fig. 7 indicating that no further cold working was induced. This is consistent with the micro hardness values in Fig. 6. After a stream finishing period of 3.75 min significant compressive residual stresses of about 400 MPa are induced on the surface with comparable magnitudes in the longitudinal and transverse direction. A slight increase of the compressive residual stresses can be observed on the surface for processing times larger than 3.75 min resulting in a maximum magnitude of about 500 MPa after 16 min. Depth distributions of the residual stresses and integral breadths of the interference lines for analyses of the longitudinal direction are shown in Fig. 8 as an example for processing times of 3.75 and 16 min. The distributions of the residual stresses showing an almost linear decrease from 400 or 440 MPa at the surface to about zero at the depth of 25 mm clearly indicate a depth range of the stream
Fig. 4. Roughness and mass difference vs. processing time.
Fig. 6. Surface micro hardness vs. processing time.
Table 1 Values of parameters A, B, C, D and E for the curve fits. Curve fit
A [mm]
B [minC]
C [–]
D [mm]
E [min1]
H (t) Sa (t)
0.057 –
0.770 –
0.246 –
6.012 E-5 –
– 2.972
calculated by satisfying the following equation [15]. Hð2 tef f Þ Hðtef f Þ ¼ 0:1 Hðtef f Þ
ð2Þ
After a finishing time of 3.75 min the effective arc height Heff is reached by stochastically impinging abrasive particles causing plastic deformation of the surface. For further processing only a slight increase of the arc height can be observed indicating that no additional significant work hardening effects occur. Fig. 4 displays the measured arithmetic average roughness Sa and the mass difference Dm versus processing time t. Hashimoto et al. [22] established the basic fundamentals for surface roughness prediction in the vibratory finishing further proposing a corresponding mathematical model. According to them [22] the removal mechanisms “roughness change” and “micro cutting” of the vibratory finishing can be utilized for the stream finishing process, respectively. Starting from the initial arithmetic average roughness of Sa,0 = 0.24 mm “roughness change” occurs up to a finishing time of 1 min along with a higher progression of mass difference in continued processing. Sa clearly reaches a saturation value around Sa,1 = 0.13 mm for finishing times greater than 1 min, when micro cutting mechanisms occur with a linear mass change. These findings are in accordance with those of Hashimoto et al. [22]. Thus, the surface roughness prediction model (Eq. (3)) was applied to the curve fit of the measured arithmetic average roughnesses Sa of stream finished specimen showing an adequate fit. Sa ðtÞ ¼ ðSa;0 Sa;1 Þ expðE tÞ þ Sa;1
ð3Þ
Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079
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Fig. 7. Surface residual stresses in longitudinal and transverse direction and corresponding mean integral breadths vs. processing time.
Initially the workpiece surface topography showed an oriented texture due to previous grinding process. “Roughness change” occurs within a finishing time of 1 min where the roughness approaches a saturation value of Sa = 0.13 mm leading to an almost isotropic surface topography. The effective time teff, based on the temporal evolution of arc heights, represents a benchmark for minimal processing time generating significant compressive residual stresses and mechanical work hardening effects as a result of cold work in the near surface region. It is 3.75 min and therefore much higher than the time necessary for surface smoothing. Compressive residual stresses of approximately 400 MPa and an increase of micro hardness of approximately 200 HV0.05 on the surface were observed after teff of finishing. Measured surface residual stresses, integral breadths and micro hardnesses of processed specimen revealed almost stationary residual stress states and work hardening states in the near surface region for finishing times greater than teff. Depth distributions of residual stresses and integral breadths of processed specimen prompt the depth range of the stream finishing process to be 20–25 mm for induced residual stresses and 6–10 mm for cold working effects. References
Fig. 8. Depth distributions of the residual stresses and the integral breadths for the processing times of 3.75 and 16 min combined with the initial values (0 min).
finishing process of about 20–25 mm. The depth distributions of the integral breadths almost coincide, illustrating that no significant extra cold work is induced by longer processing times. Considering the comparable values of the initial integral breadth (t = 0 min) on the surface and the integral breadths in higher depths a significant change of the integral breadths can be demonstrated up to a depth of 6–10 mm indicating a depth range of the stream finishing process for the cold working effect. It may be postulated that after a certain finishing time almost stationary residual stress states and work hardening states occur in the near surface region. 4. Conclusion A comprehensive correlation between processing time of the stream finishing treatment and the residual stress state, micro hardness and surface topography on normalized AISI4140 plane specimen was established in order to qualify minimal processing time with an optimized surface state. It was demonstrated that the stream finishing process represents an effective mass finishing process for mechanical surface modification. This might be highly suitable for improvement of fatigue, wear and corrosion properties of mechanical components. Concluding, the key findings of this work are: The adapted Almen system showed to be an efficient method for characterizing changes in residual stress state during stream finishing.
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Please cite this article in press as: Schulze V, et al. Qualification of the stream finishing process for surface modification. CIRP Annals Manufacturing Technology (2017), http://dx.doi.org/10.1016/j.cirp.2017.04.079