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Characterisation of aggregate notch cavity formation properties on abrasive waterjet cut surfaces
Journal of Manufacturing Processes 15 (2013) 355–363
Contents lists available at SciVerse ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Characterisation of aggregate notch cavity formation properties on abrasive waterjet cut surfaces D.J. Thomas ∗ Centre for Nano Health, College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, United Kingdom
a r t i c l e
i n f o
Article history: Received 12 September 2012 Received in revised form 13 December 2012 Accepted 4 February 2013 Available online 13 March 2013 Keywords: AWJ Waterjet cutting Notch cavities Aggregate formations Erosion cutting processes
a b s t r a c t Aggregate induced notch cavity formations were observed to have formed on the surface of abrasive waterjet cut edges. The secondary micro-machining effect of the abrasive waterjet has been observed to plastically deform grains in proximity to the cut-edge. This interaction governs the operative mechanism of material removal and resulted in the formation of notch cavities on the cut surface. Controlling the traverse cutting speed was found to be critical towards influencing kerf perpendicularity which influences the characteristics and quantity of notch cavity features and the deposition of nanometer sized particles of aggregate. It is shown that the surface waviness properties are reduced as the traverse speed is increased and there is an association between the surface properties and the plastic deformation of the microstructure. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Abrasive waterjet (AWJ) cutting is an accelerated erosion technology, in which particles of abrasive aggregate are introduced into a jet of water, which is accelerated to a velocity of up to 1.2 km s−1 using pressures in excess of 4000 bar. A resulting aspect of this cutting process is that the microstructure of the steel close to the cut-edge is plastically deformed and is harder than the parent microstructure. The rate of material removal, surface finish and tolerances of the cut-edge depend critically on numerous processing variables, the most significant of which are the waterjet traverse cutting speed and the abrasive grit size [1]. AWJ cutting has distinct advantages of; no thermal or mechanical distortion, flexibility and high precision [2], and offers a means of precision processing metallic materials [3–5]. A wide range of work has been conducted to study the mechanism of AWJ cutting and to develop kerf geometry and surface roughness models for process control and optimisation [6–15]. Early investigations [8,9] determined that different cutting zones exist in the processing of ductile and brittle materials after AWJ cutting. There are distinctive topographically zones, namely the initiation zone, smooth zone, transition zone and rough zone which each have different Ra , Rq and Rz surface parameters [16–19].
∗ Tel.: +44 01656737349. E-mail addresses: [email protected], [email protected]
As the waterjet penetrates through the workpiece, plastic deformation of the microstructure is a key effect when cutting alloy materials. As the waterjet penetrates further through the workpiece, the cutting process becomes an erosive cutting mechanism and this forms striations at greater angles of attack further through the thickness of the material. This mechanism is responsible for producing a rougher surface at the bottom of the kerf and this becomes aggravated when cutting thicker materials or when cutting at a higher traverse speed. An augmented effect of this is that the kerf is wider at the top than at the bottom due to the decrease in water pressure, in which a taper is produced. More recent investigations by [20–23] into the surface topography generated by the AWJ cutting process has made it possible to gain a complete understanding of the process behind the formation of notch cavities as part of the AWJ material removal mechanism. 2. Experimental 2.1. Material properties The three high strength steel grades used in this study were; XF350 which is a high strength low alloy (HSLA) steel and has a ferrite and pearlite microstructure, DP600 which is a complex phase, ferrite, retained austenite and martensite microstructure, hot rolled high strength steel (HSS) and S355 which is a structural ferrite and pearlite steel grade. The microstructures of these steels are shown in Fig. 1 and Table 1 presents their chemistry and mechanical properties.
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.02.003
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D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363 Table 2 AWJ cutting parameters used during the cutting trials.
Nomenclature A AWJ HSS HSLA Hv Ra
elongation to failure abrasive waterjet high strength steel high strength low alloy Vickers hardness roughness arithmetic mean of departures from the mean line roughness maximum depth of profile below the mean line waviness arithmetic mean of departures from the mean line waviness maximum depth of profile below the mean line weight
Rv Wa Wv Wt
Process parameter
Process setting
Pump pressure (bar) Water nozzle diameter (mm) Nozzle stand-off (mm) Focusing nozzle diameter (mm) Water-jet speed (m s−1 ) Type of abrasive Quantity of abrasive (g min−1 )
3600 0.28 1.2 0.8 800 Australian garnet (80 mesh) 500
Table 3 Traverse cutting speeds (mm min−1 ) parameters used to generate AWJ cut-edges. Steel
Cutting speed (mm min−1 )
XF350 XF350 XF350 DP600 DP600 DP600 S355 S355 S355
250 500 750 250 500 750 100 200 300
Table 3. When cutting thicker gauge steels as was the case with S355, a reduced traverse cutting speed was required in order to erode the material and achieve a through cut without necessitating a reduction in cut-edge surface quality. 2.3. Characterisation of cut-edges
Fig. 1. Microstructures of XF350, DP600 and S355 after etching with 2% Nital.
Surface and microstructural analysis of cut-edges was performed using a Leica optical light microscope and JEOL JSM-840A Scanning Electron Microscope (SEM). The microstructural characterisation of the near-edge region were analysed through observing the specimens transverse to the cut-edge. Metallographic analysis of the near edge region microstructure was carried out by etching specimens using a 2% Nital reagent for 15 s. In order to record and quantify notch cavities, a Taylor–Hobson form 2 Talysurf was employed with scans being carried out across a two-dimensional surface area, providing an accurate representation of the cut-edge roughness and waviness data parameters and for producing 3D axonometric plots. The degree to which the cut-edge surfaces had hardened during the cutting process was measured as Vickers microhardness (Hv ), and measurements were taken using a Leco M-400-G2 hardness tester with a 100 g load.
2.2. Abrasive waterjet cutting trials
3. Results and discussion
Cutting trials were carried out using a Bystronic Waterjet Byjet model Type 3015 at the Engineering Centre for Manufacturing and Materials. Table 2 shows the cutting parameters that were used to generate AWJ cut specimens. The three traverse cutting speeds used for generating AWJ cutedges of different qualities across an effective range are shown in
3.1. Abrasive waterjet cut-edge surface characteristics The difference between the surface qualities of cut-edges across the range of AWJ traverse cutting speeds is shown in Fig. 2. Towards the bottom of the cut, there was the presence of angular draglines that were more dominant at higher traverse cutting speeds, as a
Table 1 Steel chemistry as a weight percentage and mechanical properties of XF350, DP600 and S355 steel grades. Determined from XRD chemical analysis and mechanical testing. Grade
XF350 DP600 S355
Element composition (%)
Mechanical properties
C
Mn
Si
P
S
Al
Cr
Gauge (mm)
Hardness (Hv )
Yield strength Rp (MPa)
Tensile strength Rm (MPa)
Elongation A80 (%)
0.067 0.087 0.141
0.591 1.089 0.978
0.020 0.189 0.006
0.019 0.043 0.010
0.007 0.005 0.015
0.043 0.037 0.050
0.022 0.569 0.018
3.0 3.0 8.0
157 237 202
394 475 425
473 690 533
30.8 21.5 23.6
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Fig. 2. Surface micrographs of cut-edges generated across the range of traverse cutting speeds.
result of deceleration of the jet throughout the thickness of the workpiece. It is this property which is the limiting factor in the generation of through cuts, in which as the critical angle of the draglines is exceeded then the generation of a through cut is no longer achieved. At lower traverse cutting speeds there was the presence of a localised area of surface striations at the cut-edges top initial damage region. At high cutting speeds the predominance of the waterjet sweeping action is shown clearly in the rough cutting region. It was observed that particularly at 750 mm min−1 the draglines are vertical at the top and become angular towards the bottom of the cut-edge. This effect was also observed to have occurred increasingly on the surfaces of S355, but at lower traverse cutting speeds due to the fact that the waterjet has an increased depth of material to cut through. Levelled surface profiles of AWJ cut-edges are shown in Fig. 3 in which it was observed that the softer XF350 has an increased susceptibility to the formation of notch cavities. This is particularly the case at lower traverse cutting speeds which form an increased number of cavities in the smooth cutting region. Across the range of traverse cutting speeds and steel types the notch cavities present were between 50 and 75 m in depth. Fragmented garnet particles as shown in Fig. 4 were found across the surface of AWJ cut-edges. A secondary effect of this was that large notches were formed as a result of garnet aggregate becoming accumulated together forming a notch cavity on the surface. In the steady cyclic cutting stage, the waterjets changes in attack angle of the waterjet, between a vertical direction and becomes
deflected as the kinetic energy is reduced. This results in the formation of a reduction in impact velocity and increases particle fragmentation. Under this condition, material is removed by cutting as well as deformation wear processes, in which particles push the material into a plastic state until it is removed. Therefore, as the traverse cutting speed is increased then the waterjet hits the steel at a higher angle. This effect not only influences its microstructure as much but also forms notch cavities. As the traverse cutting speed is increased then so is the angle of the striation draglines. This indicates that more of the supersonic waterjet is deflected at higher angles of attack leading to the formation of a greater number of surface notches as observed on the surface line scans as shown in Fig. 3. Therefore, the inner contoured regions of the jet, which have higher velocities and are convergent, can result in tapered cuts on the workpiece. The mechanism behind the process of producing notch cavities was better understood when observed under higher magnification as shown in Fig. 5. It was identified that there was a cluster of garnet aggregate particles embedded into the cavity which had formed these notches. The microstructure in proximity of a notch cavity as shown in Fig. 6 was also observed to have plastically deformed grains around these regions. The top of the cut-edge was established to be significantly altered with the presence of compressed grains in the near edge region and this feature resulted in an increase in the cut-edge hardness. This same trend was found across the full range of traverse cutting speeds, which reduced in predominance as the cutting speed was increased.
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D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363
Fig. 3. Levelled talyscan profiles of XF350 and DP600 cut-edges showing notch cavities.
3.2. Perpendicularity profiles Across the range of traverse cutting speeds, it was found that the perpendicularity increased. The manipulation of the traverse speed appeared to be a critical factor towards the formation of the AWJ cut-edge surface formations, which influenced the perpendicularity of the cut-edge as shown in Fig. 7. As a result of the increase in the traverse cutting speed the kerf became thinner towards the bottom of the cut-edge and once the traverse cutting speed is too high then a through cut is no longer achievable. A transverse view of the cut-edge revealed a detailed impression of the burr found on the underside of the water-jet cut-edge. In every case the burr although only being minor was slightly more predominant on softer XF350 and S355 in comparison to DP600.
Fig. 8. The angular appearance of the surface draglines is clearly visible and appeared to produce more of a sweeping action towards the lower portion of the cut-edge surface. This angularity of the surface draglines increased as the traverse cutting speed was increased.
3.3. Surface topographical properties Topographical profiles used to determine secondary patterns that have formed on the surface of AWJ cut-edges are shown in
Fig. 4. Scanning electron micrograph of particles of garnet embedded into the surfaces of a XF350 cut-edge surface.
Fig. 5. Scanning electron micrograph of a cluster garnet aggregate inside a cavity on the AWJ cut surface of DP600.
D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363
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Fig. 6. Transverse profile of a notch cavity produced on the cut-edge surface of XF350 at a traverse cutting speed of 750 mm min−1 .
Surface roughness results confirm that there is a gradual increase in the surface roughness as the traverse cutting speed was increased as shown in Fig. 9. There was observed to be a trend that as the traverse cutting speed was increased then both the surface roughness and waviness parameters also increased. The depth of the notch cavities as induicated by the Rv parameter was influenced by traverse cutting speed. As the traverse cutting speed was increased it was found that the formation of AWJ surface draglines became distributed further apart forming separate features. In the case of S355 there was a trend that as the traverse cutting speed was increased, then so did surface roughness features, in which the Ra increased by 0.91 m between 100 and 300 mm min−1 . It was only when analysing the surface waviness properties that it was more pronounced that the traverse cutting speeds influence on the cut-edge surface properties became more apparent. Fig. 10 presents the surface waviness parameters Wa and Wq across the entire surface of the cut-edge. There was a decrease of 20 m in the surface waviness of DP600 between the traverse cutting speeds of 500 and 750 mm min−1 .This indicates that there is no significant difference in the surface features produced on XF350 and DP600 AWJ cut-edges. The outcome of this is that indeed slower traverse cutting speeds produce the smoothest surface due to the micro-machining effect, however, the effect of garnet aggregate
Fig. 7. Perpendicularity properties across the range of traverse cutting speeds.
particles and the development of notch cavities resulted in producing rougher surfaces. These results indicate that as the traverse cutting speed is increased then the depth of cut-edge surface notches also increases. Across the range of traverse cutting speeds the maximum peak above the mean line Wp increased by 33.3 m. Further to this the maximum depth below the mean line Wv increased by 38.2 m between 100 and 300 mm min−1 . Therefore there is a linear relationship between the surface waviness properties and traverse cutting speed. As the surface becomes more discontinuous indicated by the Ra and Wa parameters, so does the depth of the notches indicated by the Rv and Wv parameters as shown when comparing Figs. 9 and 10. As shown in Fig. 11 is an example of the rough surfaces formed on the surfaces of S355, which were observed to become progresively more discontinuious as the cutting speed is increased. It was this discontunuity factor that can cause an increased likelihood of notch cavities being formed. As shown previously in Fig. 5, surface notches were formed as the particles of garnet aggregate carved out a cavity in the cut-edge surface. These hard particles bombarding the surface were established as the only factor, which can cause the combined surface notch defects and mechanism of work hardening. 3.4. Surface hardness and microstructural characteristics The influence of garnet aggregate was observed to compress the grain microstructure close to the cut-edge surface. There is also a relationship between traverse cutting speed and the edge hardness as shown in Fig. 12. The AWJ cut hardening effect was more evident in XF350 than S355 and DP600 due to the fact that the latter two steels consist of a harder microstructure. Due to the taper of the cut-edge there is an increased spreading of the waterjet which apart from altering the kerf characteristics as measured by perpendicularity as shown in Fig. 7. Further to the influence of traverse cutting speed on surface properties there was also a decrease in cut-edge surface hardness as shown in Fig. 12 as a result of the increased deflection of the waterjet. As the traverse cutting speed is increased then the waterjet hits the steel at a higher angle and this does not influence its microstructure as greatly but does form a greater concentration of notch cavities on the surface. This is also dependent on the hardness properties of the workpiece being cut. When taking surface measurements it could be seen that as the traverse cutting speed parameter was increased the surface hardness reduced. At a traverse cutting speed of 250 mm min−1 XF350 and DP600 had a higher level of cut-edge surface hardening. For XF350 this relationship was apparent at the top of the cut-edge,
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D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363
Fig. 8. Levelled surface talyscans of cut-edges resolved using a Gaussian filter.
Fig. 9. Ra and Rv surface roughness measurements of cut-edges across a range of traverse cutting speeds.
Fig. 10. Wa and Wv surface waviness measurements of cut-edges across a range of traverse cutting speeds.
D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363
Fig. 11. Axonometric plots of S355 cut surfaces of across the range of traverse cutting speeds.
in which there was a measured hardness of 180 Hv and this was lower at the higher traverse cutting speed of 750 mm min−1 with a decrease in hardness to 168 Hv . There was measured to be the trend that the hardness level decreased throughout the thickness of the cut-edge before it increased again at the bottom. Results recorded from DP600 displayed only a slight change in the cut-edge surface hardness across the range of traverse cutting speeds and the top of the edge was measured to be the hardest region. However, at the bottom of the cut-edge a traverse cutting
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speed of 750 mm min−1 produced only a 12 Hv increase in the hardness at the top of the cut-edge surface before the hardness returned to normal levels. As was expected, a traverse cutting speed of 250 mm min−1 produced a harder cut-edge at 257 Hv , however, the level of work hardening in DP600 was not of the same magnitude as XF350. For S355 there was recorded to be an increase in hardness at the top of the cut-edge and this reduced through the depth of the steel until at the center of the workpiece there was a cut-edge softening below that of the parent material hardness. After this deep area of edge softening the cut-edge hardness increased once again to 232 Hv . Although the microstructural phase of the cut-edge is unaltered by the AWJ cutting process, the grains in the near edge region are plastically deformed as a mechanism of compressed as shown in Fig. 13. This is signified by the presence of highly compressed grains at the cut-edge surface. Observations taken from the near edge region were examined to reveal areas of grain deformation. It was immediately apparent that the grains had been extensively compressed at the top of the cut-edge at the low cutting speed of 250 mm min−1 in comparison to that of the bottom. The deposition of a high concentration of garnet aggregate particles of 80 nm in size as shown in Fig. 14 was observed to have been embedded inside notch cavities. Close to this region was also the presence of highly compressed regions of the cut-edge measured to be 50 Hv harder than the parent hardness of XF350. These observations indicate that the region around a notch defect has a high degree of localised plastic deformation resulting in highly compressed grains and a harder cut-edge surfaces. Using SEM analysis the clusters of surface particles were observed to be more predominant at a traverse cutting speed of 250 mm min−1 . The influence of garnet aggregate on the workpiece was observed to affect the microstructure close to the cut-edge and harden the material in this region, which decreased as the traverse cutting speed was increased. It would therefore be reasonable to advocate that the traverse cutting speed is responsible for cut-edge hardening by compressing the near edge microstructure through a mechanism of shot peening the grains, as a secondary effect to the cutting process. Because of the increased deflection of the waterjet at a traverse cutting speed of 750 mm min−1 the surface waviness is at its highest. As it was observed during cut-edge characterisation, notch cavities were also associated by elongated grains as shown in Fig. 6 formed momentarily as the abrasive waterjet exerts a pressure on the material. Externally these features are characterised by the presence of a cluster of nano sized garnet aggregate particles.
Fig. 12. Surface hardness profiles of cut-edges across the range of traverse cutting speeds. 3% error bars.
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D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363
Fig. 13. Micrograph of the distorted microstructure of an XF350 cut-edge generated at a traverse cutting speed of 250 mm min−1 .
• Notch cavities formed from a surface inconsistency that opened up during the cutting process, resulting in a microburst of fragmented particles becoming embedded inside. The impact speed of the particles being up to 800 m s−1 causes the garnet to fragment and formed a notch cavity which was filled with nano scale garnet particles. • When using an abrasive mass flow rate of 500 g min−1 the speed abrasive waterjet means that the kinetic energy of water must be spread over more particles. The momentum of the waterjet slows down through the depth of the kerf and more particles the less speed of abrasive waterjet but with a high kinetic energy. This is due to the fact that the mass in the stream increases. Acknowledgements
Fig. 14. Scanning electron micrograph of a cluster of garnet aggregate particles deposited inside a notch cavity.
4. Conclusions The following conclusions can be drawn from the present work: • Through a mechanism of garnet aggregate shot peening this induced cut-edge surface notch cavities. As the traverse cutting speed was increased there was a wider angle deflection of the waterjet. At higher traverse cutting speeds, this produced a cut which had angular draglines resulting in the formation of an increased incidence of notch cavities. The concentration and profile of these cavities influenced the hardness of the cut-edge, in which lower traverse cutting speeds resulted in the generation of the hardest cut-edge surfaces. • It was observed that the cutting process influenced the cut perpendicularity to a significant degree. As the traverse cutting speed is increased there is a wider angle cut which produces angular draglines resulting in the formation of deeper surface notches. This was as a result of the deflection on the supersonic waterjet being deflected as it slows down but the increased cutting depth of the workpiece. • Increasing the traverse cutting speed also influenced the localised near-edge hardness formed, as a result of the peening effect. Therefore, a balance can be made in order to combine the level of work hardening while minimising the surface roughness properties produced during the cutting process. • As the surface became more discontinuous at increasing cutting speed as indicated by the Ra parameter then so did the depth of the notches indicated by the Rv parameter. Controlling the traverse cutting speed was determined to influence the roughness properties of cut-edges in which a traverse cutting speed of 250 mm min−1 resulted in the most uniform surfaces being produced.
The present research was funded by a grant from the Engineering and Physical Sciences Research Council (EPSRC). The author wishes to thank the support of Swansea University College of Engineering and the Engineering Centre for Manufacturing and Materials during the pursuit of this research. References [1] Hashish M. Milling with abrasive-waterjets: a preliminary investigation. In: Proceedings of the Fourth U.S. Water Jet Conference. 1987. p. 1–10. [2] Van Luttervelt CA. On the selection of manufacturing methods illustrated by an overview of separation techniques for sheet materials. Annals of CIRP 1989;38(2):587–607. [3] Hashish M. A modeling study of metal cutting with abrasive waterjets. Journal of Engineering Materials and Technology 1984;106:221–8. [4] Guo NS, Louis G, Meier G. Surface structure and kerf geometry in abrasive waterjet cutting: formation and optimization. In: 7th American Waterjet Conf. 1993. p. 1–25. [5] Hashish M. A model for abrasive waterjet performance optimization. Journal of Engineering Materials and Technology 1989;111:154–62. [6] Matsui S, Matsumura H, Ikemoto Y, Tsujita K, Shimizu H. High precision cutting methods for metallic materials by abrasive waterjet. In: Proc. 10th Int. Symp. on Jet Cutting Technology. 1990. p. 263–78. [7] Hashish M. On the modeling surface waviness produced by abrasive waterjets. In: Proc. 11th Int. Symp. on Jet Cutting Technology. 1992. p. 17–34. [8] Finnie I, Wolak J, Kabil Y. Erosion of metals by solid particles. Journal of Materials 1967;2:682–700. [9] Chen L, Siores E, Wong WCK. Kerf characteristics in abrasive waterjet cutting of ceramic materials. International Journal of Machine Tools and Manufacture 1996;36(11):1201–6. [10] Siores E, Wong WCK, Chen L, Wager JG. Enhancing abrasive waterjet cutting of ceramics by head oscillation techniques. Annals of CIRP 1996;45(1):215–8. [11] Zeng J, Kim TJ. Development of an abrasive waterjet kerf cutting model for brittle materials. In: Proc. 11th Int. Symp. on Jet Cutting Technology. 1992. p. 483–503. [12] Wilkins RJ, Graham EE. An erosion model for waterjet cutting. Journal of Engineering for Industry 1993;115:57–61. [13] Tan DKM. A model for the surface finish in abrasive-waterjet cutting. In: Proc. 8th Int. Symp. on Jet Cutting Technology. 1986. p. 309–13. [14] Chao J, Geskin ES, Chung Y. Investigation of the dynamics of surface topography formation during abrasive waterjet cutting. In: Proc. 11th Int. Symp. on Jet Cutting Technology. 1992. p. 593–603. [15] Kovacevic R. Surface texture in abrasive waterjet cutting. Journal of Manufacturing Systems 1991;10:32–40. [16] Valiˇcek J, Hloch S, Kozak D. Surface geometric parameters proposal for the advanced control of abrasive waterjet technology. International Journal of Advanced Manufacturing 2009;41:323–8.
D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363 [17] Valiˇcek J, Hloch S. Optical measurement of surface and topographical parameters investigation created by abrasive waterjet. International Journal of Surface Science and Engineering 2009;4:360–73. [18] Kuˇsnerová M, Foldyna J, Sitek L, Valíˇcek J, Hloch S, Harniˇcárová M, et al. An investigation of surfaces generated by abrasive waterjets using optical detection. Strojniski Vestnik: Journal of Mechanical Engineering 2007;53: 224–32. [19] Hloch S, Valiˇcek J. Topographical anomaly on surfaces created by abrasive waterjet. International Journal of Surface Science and Engineering 2011;5:152–68.
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[20] Arola D, Ramulu M. Material removal in abrasive waterjet machining of metals surface integrity and texture. Wear 1997;210:50–8. [21] Thomas DJ. Characteristics of abrasive waterjet cut-edges and the affect on formability and fatigue performance of high strength steels. Journal of Manufacturing Processes 2009;11:97–105. [22] Ma C, Deam RT. A correlation for predicting the kerf profile from abrasive water jet cutting. Experimental Thermal and Fluid Science 2006;30:337–43. [23] Kong MC, Axinte D, Voice W. Aspects of material removal mechanism in plain waterjet milling on gamma titanium aluminide. Journal of Materials Processing Technology 2010;210:573–84.
Contents lists available at SciVerse ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical paper
Characterisation of aggregate notch cavity formation properties on abrasive waterjet cut surfaces D.J. Thomas ∗ Centre for Nano Health, College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, United Kingdom
a r t i c l e
i n f o
Article history: Received 12 September 2012 Received in revised form 13 December 2012 Accepted 4 February 2013 Available online 13 March 2013 Keywords: AWJ Waterjet cutting Notch cavities Aggregate formations Erosion cutting processes
a b s t r a c t Aggregate induced notch cavity formations were observed to have formed on the surface of abrasive waterjet cut edges. The secondary micro-machining effect of the abrasive waterjet has been observed to plastically deform grains in proximity to the cut-edge. This interaction governs the operative mechanism of material removal and resulted in the formation of notch cavities on the cut surface. Controlling the traverse cutting speed was found to be critical towards influencing kerf perpendicularity which influences the characteristics and quantity of notch cavity features and the deposition of nanometer sized particles of aggregate. It is shown that the surface waviness properties are reduced as the traverse speed is increased and there is an association between the surface properties and the plastic deformation of the microstructure. © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Abrasive waterjet (AWJ) cutting is an accelerated erosion technology, in which particles of abrasive aggregate are introduced into a jet of water, which is accelerated to a velocity of up to 1.2 km s−1 using pressures in excess of 4000 bar. A resulting aspect of this cutting process is that the microstructure of the steel close to the cut-edge is plastically deformed and is harder than the parent microstructure. The rate of material removal, surface finish and tolerances of the cut-edge depend critically on numerous processing variables, the most significant of which are the waterjet traverse cutting speed and the abrasive grit size [1]. AWJ cutting has distinct advantages of; no thermal or mechanical distortion, flexibility and high precision [2], and offers a means of precision processing metallic materials [3–5]. A wide range of work has been conducted to study the mechanism of AWJ cutting and to develop kerf geometry and surface roughness models for process control and optimisation [6–15]. Early investigations [8,9] determined that different cutting zones exist in the processing of ductile and brittle materials after AWJ cutting. There are distinctive topographically zones, namely the initiation zone, smooth zone, transition zone and rough zone which each have different Ra , Rq and Rz surface parameters [16–19].
∗ Tel.: +44 01656737349. E-mail addresses: [email protected], [email protected]
As the waterjet penetrates through the workpiece, plastic deformation of the microstructure is a key effect when cutting alloy materials. As the waterjet penetrates further through the workpiece, the cutting process becomes an erosive cutting mechanism and this forms striations at greater angles of attack further through the thickness of the material. This mechanism is responsible for producing a rougher surface at the bottom of the kerf and this becomes aggravated when cutting thicker materials or when cutting at a higher traverse speed. An augmented effect of this is that the kerf is wider at the top than at the bottom due to the decrease in water pressure, in which a taper is produced. More recent investigations by [20–23] into the surface topography generated by the AWJ cutting process has made it possible to gain a complete understanding of the process behind the formation of notch cavities as part of the AWJ material removal mechanism. 2. Experimental 2.1. Material properties The three high strength steel grades used in this study were; XF350 which is a high strength low alloy (HSLA) steel and has a ferrite and pearlite microstructure, DP600 which is a complex phase, ferrite, retained austenite and martensite microstructure, hot rolled high strength steel (HSS) and S355 which is a structural ferrite and pearlite steel grade. The microstructures of these steels are shown in Fig. 1 and Table 1 presents their chemistry and mechanical properties.
1526-6125/$ – see front matter © 2013 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmapro.2013.02.003
356
D.J. Thomas / Journal of Manufacturing Processes 15 (2013) 355–363 Table 2 AWJ cutting parameters used during the cutting trials.
Nomenclature A AWJ HSS HSLA Hv Ra
elongation to failure abrasive waterjet high strength steel high strength low alloy Vickers hardness roughness arithmetic mean of departures from the mean line roughness maximum depth of profile below the mean line waviness arithmetic mean of departures from the mean line waviness maximum depth of profile below the mean line weight
Rv Wa Wv Wt
Process parameter
Process setting
Pump pressure (bar) Water nozzle diameter (mm) Nozzle stand-off (mm) Focusing nozzle diameter (mm) Water-jet speed (m s−1 ) Type of abrasive Quantity of abrasive (g min−1 )
3600 0.28 1.2 0.8 800 Australian garnet (80 mesh) 500
Table 3 Traverse cutting speeds (mm min−1 ) parameters used to generate AWJ cut-edges. Steel
Cutting speed (mm min−1 )
XF350 XF350 XF350 DP600 DP600 DP600 S355 S355 S355
250 500 750 250 500 750 100 200 300
Table 3. When cutting thicker gauge steels as was the case with S355, a reduced traverse cutting speed was required in order to erode the material and achieve a through cut without necessitating a reduction in cut-edge surface quality. 2.3. Characterisation of cut-edges
Fig. 1. Microstructures of XF350, DP600 and S355 after etching with 2% Nital.
Surface and microstructural analysis of cut-edges was performed using a Leica optical light microscope and JEOL JSM-840A Scanning Electron Microscope (SEM). The microstructural characterisation of the near-edge region were analysed through observing the specimens transverse to the cut-edge. Metallographic analysis of the near edge region microstructure was carried out by etching specimens using a 2% Nital reagent for 15 s. In order to record and quantify notch cavities, a Taylor–Hobson form 2 Talysurf was employed with scans being carried out across a two-dimensional surface area, providing an accurate representation of the cut-edge roughness and waviness data parameters and for producing 3D axonometric plots. The degree to which the cut-edge surfaces had hardened during the cutting process was measured as Vickers microhardness (Hv ), and measurements were taken using a Leco M-400-G2 hardness tester with a 100 g load.
2.2. Abrasive waterjet cutting trials
3. Results and discussion
Cutting trials were carried out using a Bystronic Waterjet Byjet model Type 3015 at the Engineering Centre for Manufacturing and Materials. Table 2 shows the cutting parameters that were used to generate AWJ cut specimens. The three traverse cutting speeds used for generating AWJ cutedges of different qualities across an effective range are shown in
3.1. Abrasive waterjet cut-edge surface characteristics The difference between the surface qualities of cut-edges across the range of AWJ traverse cutting speeds is shown in Fig. 2. Towards the bottom of the cut, there was the presence of angular draglines that were more dominant at higher traverse cutting speeds, as a
Table 1 Steel chemistry as a weight percentage and mechanical properties of XF350, DP600 and S355 steel grades. Determined from XRD chemical analysis and mechanical testing. Grade
XF350 DP600 S355
Element composition (%)
Mechanical properties
C
Mn
Si
P
S
Al
Cr
Gauge (mm)
Hardness (Hv )
Yield strength Rp (MPa)
Tensile strength Rm (MPa)
Elongation A80 (%)
0.067 0.087 0.141
0.591 1.089 0.978
0.020 0.189 0.006
0.019 0.043 0.010
0.007 0.005 0.015
0.043 0.037 0.050
0.022 0.569 0.018
3.0 3.0 8.0
157 237 202
394 475 425
473 690 533
30.8 21.5 23.6
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Fig. 2. Surface micrographs of cut-edges generated across the range of traverse cutting speeds.
result of deceleration of the jet throughout the thickness of the workpiece. It is this property which is the limiting factor in the generation of through cuts, in which as the critical angle of the draglines is exceeded then the generation of a through cut is no longer achieved. At lower traverse cutting speeds there was the presence of a localised area of surface striations at the cut-edges top initial damage region. At high cutting speeds the predominance of the waterjet sweeping action is shown clearly in the rough cutting region. It was observed that particularly at 750 mm min−1 the draglines are vertical at the top and become angular towards the bottom of the cut-edge. This effect was also observed to have occurred increasingly on the surfaces of S355, but at lower traverse cutting speeds due to the fact that the waterjet has an increased depth of material to cut through. Levelled surface profiles of AWJ cut-edges are shown in Fig. 3 in which it was observed that the softer XF350 has an increased susceptibility to the formation of notch cavities. This is particularly the case at lower traverse cutting speeds which form an increased number of cavities in the smooth cutting region. Across the range of traverse cutting speeds and steel types the notch cavities present were between 50 and 75 m in depth. Fragmented garnet particles as shown in Fig. 4 were found across the surface of AWJ cut-edges. A secondary effect of this was that large notches were formed as a result of garnet aggregate becoming accumulated together forming a notch cavity on the surface. In the steady cyclic cutting stage, the waterjets changes in attack angle of the waterjet, between a vertical direction and becomes
deflected as the kinetic energy is reduced. This results in the formation of a reduction in impact velocity and increases particle fragmentation. Under this condition, material is removed by cutting as well as deformation wear processes, in which particles push the material into a plastic state until it is removed. Therefore, as the traverse cutting speed is increased then the waterjet hits the steel at a higher angle. This effect not only influences its microstructure as much but also forms notch cavities. As the traverse cutting speed is increased then so is the angle of the striation draglines. This indicates that more of the supersonic waterjet is deflected at higher angles of attack leading to the formation of a greater number of surface notches as observed on the surface line scans as shown in Fig. 3. Therefore, the inner contoured regions of the jet, which have higher velocities and are convergent, can result in tapered cuts on the workpiece. The mechanism behind the process of producing notch cavities was better understood when observed under higher magnification as shown in Fig. 5. It was identified that there was a cluster of garnet aggregate particles embedded into the cavity which had formed these notches. The microstructure in proximity of a notch cavity as shown in Fig. 6 was also observed to have plastically deformed grains around these regions. The top of the cut-edge was established to be significantly altered with the presence of compressed grains in the near edge region and this feature resulted in an increase in the cut-edge hardness. This same trend was found across the full range of traverse cutting speeds, which reduced in predominance as the cutting speed was increased.
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Fig. 3. Levelled talyscan profiles of XF350 and DP600 cut-edges showing notch cavities.
3.2. Perpendicularity profiles Across the range of traverse cutting speeds, it was found that the perpendicularity increased. The manipulation of the traverse speed appeared to be a critical factor towards the formation of the AWJ cut-edge surface formations, which influenced the perpendicularity of the cut-edge as shown in Fig. 7. As a result of the increase in the traverse cutting speed the kerf became thinner towards the bottom of the cut-edge and once the traverse cutting speed is too high then a through cut is no longer achievable. A transverse view of the cut-edge revealed a detailed impression of the burr found on the underside of the water-jet cut-edge. In every case the burr although only being minor was slightly more predominant on softer XF350 and S355 in comparison to DP600.
Fig. 8. The angular appearance of the surface draglines is clearly visible and appeared to produce more of a sweeping action towards the lower portion of the cut-edge surface. This angularity of the surface draglines increased as the traverse cutting speed was increased.
3.3. Surface topographical properties Topographical profiles used to determine secondary patterns that have formed on the surface of AWJ cut-edges are shown in
Fig. 4. Scanning electron micrograph of particles of garnet embedded into the surfaces of a XF350 cut-edge surface.
Fig. 5. Scanning electron micrograph of a cluster garnet aggregate inside a cavity on the AWJ cut surface of DP600.
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Fig. 6. Transverse profile of a notch cavity produced on the cut-edge surface of XF350 at a traverse cutting speed of 750 mm min−1 .
Surface roughness results confirm that there is a gradual increase in the surface roughness as the traverse cutting speed was increased as shown in Fig. 9. There was observed to be a trend that as the traverse cutting speed was increased then both the surface roughness and waviness parameters also increased. The depth of the notch cavities as induicated by the Rv parameter was influenced by traverse cutting speed. As the traverse cutting speed was increased it was found that the formation of AWJ surface draglines became distributed further apart forming separate features. In the case of S355 there was a trend that as the traverse cutting speed was increased, then so did surface roughness features, in which the Ra increased by 0.91 m between 100 and 300 mm min−1 . It was only when analysing the surface waviness properties that it was more pronounced that the traverse cutting speeds influence on the cut-edge surface properties became more apparent. Fig. 10 presents the surface waviness parameters Wa and Wq across the entire surface of the cut-edge. There was a decrease of 20 m in the surface waviness of DP600 between the traverse cutting speeds of 500 and 750 mm min−1 .This indicates that there is no significant difference in the surface features produced on XF350 and DP600 AWJ cut-edges. The outcome of this is that indeed slower traverse cutting speeds produce the smoothest surface due to the micro-machining effect, however, the effect of garnet aggregate
Fig. 7. Perpendicularity properties across the range of traverse cutting speeds.
particles and the development of notch cavities resulted in producing rougher surfaces. These results indicate that as the traverse cutting speed is increased then the depth of cut-edge surface notches also increases. Across the range of traverse cutting speeds the maximum peak above the mean line Wp increased by 33.3 m. Further to this the maximum depth below the mean line Wv increased by 38.2 m between 100 and 300 mm min−1 . Therefore there is a linear relationship between the surface waviness properties and traverse cutting speed. As the surface becomes more discontinuous indicated by the Ra and Wa parameters, so does the depth of the notches indicated by the Rv and Wv parameters as shown when comparing Figs. 9 and 10. As shown in Fig. 11 is an example of the rough surfaces formed on the surfaces of S355, which were observed to become progresively more discontinuious as the cutting speed is increased. It was this discontunuity factor that can cause an increased likelihood of notch cavities being formed. As shown previously in Fig. 5, surface notches were formed as the particles of garnet aggregate carved out a cavity in the cut-edge surface. These hard particles bombarding the surface were established as the only factor, which can cause the combined surface notch defects and mechanism of work hardening. 3.4. Surface hardness and microstructural characteristics The influence of garnet aggregate was observed to compress the grain microstructure close to the cut-edge surface. There is also a relationship between traverse cutting speed and the edge hardness as shown in Fig. 12. The AWJ cut hardening effect was more evident in XF350 than S355 and DP600 due to the fact that the latter two steels consist of a harder microstructure. Due to the taper of the cut-edge there is an increased spreading of the waterjet which apart from altering the kerf characteristics as measured by perpendicularity as shown in Fig. 7. Further to the influence of traverse cutting speed on surface properties there was also a decrease in cut-edge surface hardness as shown in Fig. 12 as a result of the increased deflection of the waterjet. As the traverse cutting speed is increased then the waterjet hits the steel at a higher angle and this does not influence its microstructure as greatly but does form a greater concentration of notch cavities on the surface. This is also dependent on the hardness properties of the workpiece being cut. When taking surface measurements it could be seen that as the traverse cutting speed parameter was increased the surface hardness reduced. At a traverse cutting speed of 250 mm min−1 XF350 and DP600 had a higher level of cut-edge surface hardening. For XF350 this relationship was apparent at the top of the cut-edge,
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Fig. 8. Levelled surface talyscans of cut-edges resolved using a Gaussian filter.
Fig. 9. Ra and Rv surface roughness measurements of cut-edges across a range of traverse cutting speeds.
Fig. 10. Wa and Wv surface waviness measurements of cut-edges across a range of traverse cutting speeds.
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Fig. 11. Axonometric plots of S355 cut surfaces of across the range of traverse cutting speeds.
in which there was a measured hardness of 180 Hv and this was lower at the higher traverse cutting speed of 750 mm min−1 with a decrease in hardness to 168 Hv . There was measured to be the trend that the hardness level decreased throughout the thickness of the cut-edge before it increased again at the bottom. Results recorded from DP600 displayed only a slight change in the cut-edge surface hardness across the range of traverse cutting speeds and the top of the edge was measured to be the hardest region. However, at the bottom of the cut-edge a traverse cutting
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speed of 750 mm min−1 produced only a 12 Hv increase in the hardness at the top of the cut-edge surface before the hardness returned to normal levels. As was expected, a traverse cutting speed of 250 mm min−1 produced a harder cut-edge at 257 Hv , however, the level of work hardening in DP600 was not of the same magnitude as XF350. For S355 there was recorded to be an increase in hardness at the top of the cut-edge and this reduced through the depth of the steel until at the center of the workpiece there was a cut-edge softening below that of the parent material hardness. After this deep area of edge softening the cut-edge hardness increased once again to 232 Hv . Although the microstructural phase of the cut-edge is unaltered by the AWJ cutting process, the grains in the near edge region are plastically deformed as a mechanism of compressed as shown in Fig. 13. This is signified by the presence of highly compressed grains at the cut-edge surface. Observations taken from the near edge region were examined to reveal areas of grain deformation. It was immediately apparent that the grains had been extensively compressed at the top of the cut-edge at the low cutting speed of 250 mm min−1 in comparison to that of the bottom. The deposition of a high concentration of garnet aggregate particles of 80 nm in size as shown in Fig. 14 was observed to have been embedded inside notch cavities. Close to this region was also the presence of highly compressed regions of the cut-edge measured to be 50 Hv harder than the parent hardness of XF350. These observations indicate that the region around a notch defect has a high degree of localised plastic deformation resulting in highly compressed grains and a harder cut-edge surfaces. Using SEM analysis the clusters of surface particles were observed to be more predominant at a traverse cutting speed of 250 mm min−1 . The influence of garnet aggregate on the workpiece was observed to affect the microstructure close to the cut-edge and harden the material in this region, which decreased as the traverse cutting speed was increased. It would therefore be reasonable to advocate that the traverse cutting speed is responsible for cut-edge hardening by compressing the near edge microstructure through a mechanism of shot peening the grains, as a secondary effect to the cutting process. Because of the increased deflection of the waterjet at a traverse cutting speed of 750 mm min−1 the surface waviness is at its highest. As it was observed during cut-edge characterisation, notch cavities were also associated by elongated grains as shown in Fig. 6 formed momentarily as the abrasive waterjet exerts a pressure on the material. Externally these features are characterised by the presence of a cluster of nano sized garnet aggregate particles.
Fig. 12. Surface hardness profiles of cut-edges across the range of traverse cutting speeds. 3% error bars.
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Fig. 13. Micrograph of the distorted microstructure of an XF350 cut-edge generated at a traverse cutting speed of 250 mm min−1 .
• Notch cavities formed from a surface inconsistency that opened up during the cutting process, resulting in a microburst of fragmented particles becoming embedded inside. The impact speed of the particles being up to 800 m s−1 causes the garnet to fragment and formed a notch cavity which was filled with nano scale garnet particles. • When using an abrasive mass flow rate of 500 g min−1 the speed abrasive waterjet means that the kinetic energy of water must be spread over more particles. The momentum of the waterjet slows down through the depth of the kerf and more particles the less speed of abrasive waterjet but with a high kinetic energy. This is due to the fact that the mass in the stream increases. Acknowledgements
Fig. 14. Scanning electron micrograph of a cluster of garnet aggregate particles deposited inside a notch cavity.
4. Conclusions The following conclusions can be drawn from the present work: • Through a mechanism of garnet aggregate shot peening this induced cut-edge surface notch cavities. As the traverse cutting speed was increased there was a wider angle deflection of the waterjet. At higher traverse cutting speeds, this produced a cut which had angular draglines resulting in the formation of an increased incidence of notch cavities. The concentration and profile of these cavities influenced the hardness of the cut-edge, in which lower traverse cutting speeds resulted in the generation of the hardest cut-edge surfaces. • It was observed that the cutting process influenced the cut perpendicularity to a significant degree. As the traverse cutting speed is increased there is a wider angle cut which produces angular draglines resulting in the formation of deeper surface notches. This was as a result of the deflection on the supersonic waterjet being deflected as it slows down but the increased cutting depth of the workpiece. • Increasing the traverse cutting speed also influenced the localised near-edge hardness formed, as a result of the peening effect. Therefore, a balance can be made in order to combine the level of work hardening while minimising the surface roughness properties produced during the cutting process. • As the surface became more discontinuous at increasing cutting speed as indicated by the Ra parameter then so did the depth of the notches indicated by the Rv parameter. Controlling the traverse cutting speed was determined to influence the roughness properties of cut-edges in which a traverse cutting speed of 250 mm min−1 resulted in the most uniform surfaces being produced.
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