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Vol. 8. Issue 5.
Pages 4333-4346 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4333-4346 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.044
Open Access
Surface layer microhardness and roughness after applying a vibroburnishing process
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Gheorghe Nagîţ, Laurenţiu Slătineanu, Oana Dodun
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oanad@tcm.tuiasi.ro

Corresponding author.
, Marius Ionuţ Rîpanu, Andrei Marius Mihalache
Department of Machine Manufacturing Technology, “Gheorghe Asachi” Technical University of Iași, Iași, Blvd. D. Mangeron, 59 A, 700050, Romania
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Tables (2)
Table 1. Experimental conditions and results.
Table 2. Variation of microhardness HV in the surface layer.
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Abstract

The ball vibroburnishing process of external cylindrical surfaces involves workpiece rotation, while a ball is being pressed and forced to roll on the cylindrical surface, simultaneously, achieving an axial vibration. Experimental research was designed in accordance with the principles of a full factorial experiment with six independent variables at two levels on the test pieces made of two distinct steels, the medium-carbon steel 1C45 and the low-carbon alloy steel 18CrMn4-4. The frequency and amplitude of the vibration motion, ball diameter, radial rolling force, workpiece peripheral speed, and feed rate were considered as process input factors. The process output parameters were the surface roughness parameter Ra, Vickers microhardness of the surface layer measured at a certain distance from the hardened surface, and the hardened layer thickness. Through mathematical processing of the experimental results, power type functions were determined as empirical models. Worth mentioning is the fact that significant influence is exerted by the vibration amplitude on the size of the surface roughness parameter Ra, as well as the vibration motion frequency on the surface layer Vickers microhardness.

Keywords:
Ball vibroburnishing
Asperity generation
Process input factors
Surface roughness
Surface layer Vickers microhardness
Hardened layer thickness
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1Introduction

In order to obtain improved behaviour during the operation of mechanical equipment, the part surface layers must exhibit specific properties. There are various means to change the properties of the part surface layer. Thus, there are certain processes available for alloying or applying thermo-chemical and heat treatments to the part surface layer, but an additional way of changing the surface layer properties could involve applying certain mechanical treatments, in order to generate the modification of the shapes of the crystalline grains and the connections among grains. Hence it is possible to obtain an increase in the surface layer microhardness/strength. In the case of materials that cannot change their properties by means of heat treatments, applying a burnishing process may serve as one of the few low-cost methods of obtaining more convenient material mechanical properties [1–6]. Essentially, the burnishing process ensures pressing of the surface layer material by means of a hard, rounded tool that presses the workpiece material, without appropriate material removal and determining the decrease in the surface asperity heights as well as superficial hardening.

If the burnishing tool revolution surface is analysed, one may observe and distinguish between the use of spherical, conical, cylindrical, and barrel rolls. In the case of conical, cylindrical, and barrel rolls, only tool rotation movement around a single axis is obtained (overlapped over the other usual processing movements), while the spherical rolls can achieve a more complex rolling movement on the workpiece surface.

Over the years, various machining techniques based on rollers or balls, and the results obtained from their application, have been investigated by researchers [1–6].

Following this, other researchers developed scientific investigations in the field of rollers or ball burnishing, and the theoretical and/or experimental results of these investigations were published. In the past several decades, in order to increase the efficiency of rollers or ball burnishing techniques, an additional vibration motion along the surface to be processed was introduced for the roll/ball support, paving the way for a new research direction, concerning the so-called vibroburnishing process. Vast studies were initially developed in the former Soviet Union; for example, Shneider [1,2] initiated significant research in the field of rollers or ball vibroburnishing processes in the 1960s.

The geometrical characteristics of the microscopic and submicroscopic reliefs generated by vibroburnishing represented an objective on which the researchers would focus their preoccupations [3–5]. The investigations concerning the influence exerted by the vibroburnishing process parameters on the surface layer roughness and microhardness were developed inclusively to optimize the process results [7–14]. The experimental tests were conducted on samples made of various metallic materials [15–18]. New or improved equipment for applying the vibroburnishing processes was proposed and used [4,5,19–21]. The experimental research proves that the use of the vibroburnishing process can significantly contribute to the improvement of the mechanical properties of the surfaces layers and thus to an improvement of the parts service properties [4,6,22–25]. The increase of the vibrations frequency up to a level corresponding to the ultrasonic field was taken into consideration as a way of improving the performances of the vibroburnishing process [19,26–37].

Some of the main results concerning the investigation of vibroburnishing processes and applications are summarized below.

Matalin and Svinitskaya [3] analysed the so-called microscopic and submicroscopic reliefs obtained by certain distinct finishing methods, using the electron microscope. They observed the similarities among the surface layer plastic deformations generated by certain finishing methods, including vibroburnishing.

Pande and Patel investigated the influence of the burnishing speed, ball force, frequency, and amplitude of vibration on the surface layer roughness and microhardness by means of response surface methodology [7]. They proposed two mathematical models to offer an image regarding the correlation between the size of the surface roughness parameter Ra and surface layer microhardness. The empirical mathematical models were second-degree polynomials, in which the ball diameter was not considered as a factor that exerts an influence on the surface roughness parameter value and the hardened layer microhardness.

Barski and Gajek proposed the use of an electromagnetic device for the ball vibroburnishing process [33]. They took used test pieces made of 1C45 steel and conducted experimental research aimed at demonstrating the influence exerted by four process input factors (rolling force, test piece peripheral speed, work feed, and vibration frequency) on the value of the surface roughness parameter Ra. A second-degree polynomial was determined as a mathematical empirical model.

As a result of experimental research, Nagîţ et al. [21] proposed and patented an improved device for applying the ball vibroburnishing process on a universal lathe. This device used an oscillating slide and eccentric bolt as essential components for changing the rotation motion in linear vibration. Nagîţ et al. considered that such a solution for the ball vibroburnishing device could be established inclusively, using a method for the technical creativity stimulation. Nagîţ et al. proposed also the use of exponential empirical mathematical models to highlight the influence exerted by certain process input factors on the value of the surface roughness parameter Ra and the surface layer microhardness when using the ball vibroburnishing process [9,10,21].

Korzynski and Lubas analysed oscillatory (vibratory) burnishing when using this process to generate a certain texture on the workpiece surface [5]. They considered that oscillatory motion could be generated by means of a mechanical, pneumatic, hydraulic or electromagnetic drive. Furthermore, Korzynski [4] noted oscillating burnishing as a means of producing oil pockets and certain surface textures in such pockets.

Ogiński et al. explored the problem of oscillatory burnishing optimization by considering a certain surface coverage value with microgroove amounts. As an optimization criterion, they used the oscillatory burnishing time corresponding to the processing of an internal cylindrical surface [11].

In a doctoral thesis published in 2015 that refers to the surface properties of ball burnished brass components, Dabeer considered the generation of controlled homogeneous surfaces, good contact stiffness, higher oil retention capacity and low friction and wear as main advantages of the vibratory and ultrasonic burnishing in comparison with the common burnishing [38].

At the Technical University of Varna, various studies on the vibroburnishing process were performed by Slavov and his collaborators. They conducted research that highlighted the influence of the vibroburnishing process input factors on the parameters of technological interest and proposed the theoretical and empirical mathematical models corresponding to the different aspects of the investigated process [34–36].

It is observed that, while vast experimental and theoretical investigations exist in the case of roller or ball burnishing, published results on ball vibroburnishing are scarce. No studies or mathematical models have been identified that highlight the influence exerted in the same experimental set of tests using more than five process input factors on the parameters of technological interest for vibroburnishing, such as the surface roughness parameter Ra and microhardness HV of the hardened surface layer. It is possible that, among the above-mentioned parameters of technological interest (surface roughness, microhardness, and thickness of hardened surface layer), certain correlations may exist; however, this aspect has not been determined in previously published works. Furthermore, improvement may be possible in the design of the ball vibroburnishing equipment. Experimental research could also consider the influence exerted by the ball vibroburnishing process input factors on the hardened layer thickness. Such issues are the objectives of the research presented in this paper.

2Materials and methods

Essentially, in the case of applying ball vibroburnishing to an external cylindrical surface, one could consider the test piece peripheral speed vp and feed rate f of the device that supports and presses the ball on the test piece surface (Fig. 1). In order to materialize the ball vibroburnishing process, additional ball support vibration motion is necessary. This additional motion involves the use of a device for ball vibroburnishing that must offer vibration characterized by certain values of frequency ν and amplitude A. The pressure exerted on the ball and distinct motions involved in the ball vibroburnishing will determine the ball achieving complex rotation around its distinct axes on the workpiece surface.

Fig. 1.

Ball vibroburnishing scheme: nwp – workpiece rotation; f – longitudinal feed, mv – vibratory motion; F – rolling force exerted on ball.

(0.2MB).

It is expected that, owing to the ball pressure and work movements, the initial surface asperities would be plastically deformed, and a decrease in the asperity heights may occur. As a result of the movements achieved during the ball vibroburnishing process, a certain distance exists between two consecutive ball rectilinear strokes on the workpiece surface, and this distance provides an image regarding the level of overlapping of the ball stroke traces. To a certain extent, this level of traces overlapping could determine the roughness parameter values of the machined surface.

Furthermore, as a result of the test piece surface plastic deformation affected by the ball vibroburnishing process, it is expected that the asperity profile would change in the manner highlighted in Fig. 2. Under the action of the workpiece material plastic deformation, the asperity may exhibit a lower height, and enlargement of its base may be observed.

Fig. 2.

Expected change of the asperity profile as a consequence of the ball burnishing process.

(0.13MB).

In order to highlight the behaviour of the surface asperities under the ball pressure action during the ball vibroburnishing process, a simple experiment was developed and conducted, taking into consideration a regular profile obtained by means of turning with a constant feed rate and high turning speed. It is known that, before applying the vibroburnishing process, the workpiece surface must be prepared so that its asperity height value is as low as possible. Usually, a grinding process can be used to diminish the surface roughness parameter values that characterize the asperity heights. However, as the asperity profile obtained by applying the cylindrical grinding process is an irregular one, which does not allow for a direct evaluation of the asperity profile changes after applying the ball burnishing process, it is preferable to take into consideration a known regular profile resulting, for example, from applying turning using a cutting tool with a known corner radius rε. If the turning speed vturning is high enough so that the built-up edge does not appear and the cutting tool clearance angle is close to zero degrees, the turned surface profile in an axial section through the test piece can be approximated by a chain of arcs of a circle, with a pitch equal to the feed f (expressed in mm/rev) and radius equal to the turning tool corner radius rε (mm). In such a case, one can better observe the changes in the asperity profiles under the action of the pressure exerted by the burnishing tool and its movements (Fig. 2).

Thereafter, considering the machining scheme that is valid for ball vibroburnishing, a single manual stroke of the ball pressed against the test piece cylindrical surface was achieved only along the horizontal trajectory. The final stroke zone was investigated by means of two images: one taken using a camera and the second with the scanning electron microscope. The images thereby obtained can be observed in Fig. 3, where the laterally pushed material under the action of ball rolling can be observed.

Fig. 3.

End of trace achieved by single manual stroke of ball on test piece surface previously prepared by turning (using a turning tool with corner radius rε = 0.8 mm, vturning = 143 m/min, fturning = 0.36 mm/rev, Raturning = 6.53 µm, db = 11.32 mm, F = 235 N): image obtained by means of camera (a); image obtained by means of scanning electron microscope Tescan Vega II LMH; magnification: 100× (b).

(0.22MB).

The lateral pushing of the workpiece material can also be observed in the simulation of the ball stroke by means of the finite element method. In Fig. 4, one can see an image of the groove generated by the ball movement that determines the material deformation as a consequence of the plastic rolling process.

Fig. 4.

Simulations of the behaviour of the workpiece material in the case of a ball single stroke under the action of the pressure exerted by burnishing tool by using the finite element method: total deformation cross section view of the channel imprinted by the burnishing ball (a); von-Mises equivalent stress full view of the channel imprinted by the burnishing ball (b).

(1.4MB).

The modelling was achieved in the Engineering Data for the 1C45 steel, taking into consideration the Young’s modulus equal to 200000 MPa, a value for the Poisson’s ratio equal to 0.3, and the Yield strength equal to 250 MPa. The ball and the workpiece were introduced as parasolid .x_t, since Ansys preserves even the shell surfaces of such 3D models. For computational speed purposes, only a part of the ball and the workpiece were modelled, since the focus area is very small in comparison on with the complete models. For meshing purposes, it was introduced a Body Sizing method of 0.5 mm for both parts which was found most suitable in terms of accuracy over computing power. The setup supposed a fixed support applied to the bottom face of the workpiece and two displacements along the Z and X axes. The reason for this setting was the fact that we took into account the results obtained for the HV microhardness measured at a depth of h0.02 = 0.02 mm from the processed surface. The ball exits the workpiece after a complete revolution (f = 0.024 mm/rev) (see Table 2). The analysis was performed in five time steps with no Large Deflection or Inertia Relief. Thus, the displacements are increments of the above presented values. The analysis was performed at environmental temperature of 22 °C. Solution was focused on what happens to the workpiece. Z and X axes directional deformation related values were extracted. One considered the equivalent von-Mises stress after a full revolution of the vibroburnishing ball. The maximum values were appreciated as illustrative, but the setup could be yet perfectible. Nevertheless on a macro level, the trace resembles the one presented in Fig. 3 without the sharp edges. This happens since the setup was performed in time steps as the ball moves gradually in depth and ahead and then exits in about a second. The time frames may be reduced, but the above mentioned working station won’t cope with the new setup and the software will crush. An explanation could be based on the fact that the 1C45 steel is a hard material and many iterations must be performed. If the resolution and the accuracy of the results are of no interest, than the sharp edges will appear after the proper setup.

Additional information concerning the behaviour of the asperity profiles as a consequence of the ball action can be observed in the profilograms presented in Fig. 5. In Fig. 5 (a), one can see the regular profile as a result of turning with a high cutting speed, while Fig. 5 (b) highlights the aspect of the asperities resulting from turning, found in the bottom of the groove generated by the manual single ball stroke and affected by the plastic deformation process. An increase in the asperity base widths as a consequence of their plastic deformation can clearly be observed.

Fig. 5.

Profilograms corresponding to (a) turned surface and (b) groove bottom surface, obtained by single work stroke (test piece material: 1C45): surface turned using tool characterized by corner radius rε = 0.8 mm, vturning = 143 m/min, fturning = 0.36 mm/rev, Raturning = 6.53 µm (a); surface profile of groove bottom, obtained using single manual stroke with ball diameter db = 11.32 mm, F = 235 N, Ra = 2.68 µm; profilograms obtained by means of surface roughness meter Taylor Hobson Surtronic 25 (b).

(0.3MB).

In order to test some of the above-mentioned hypotheses, experimental research was designed, considering the possibility of developing a ball vibroburnishing process for external cylindrical surfaces on a universal lathe. A ball vibroburnishing device was placed on the universal lathe instead of the classical slide supporting the cutting tool. In the case of the ball vibroburnishing device used, at the end of the rotating shaft, there was a disc on which a threaded bolt could be set in order to ensure an imposed eccentricity e; this is an essential difference compared to the solutions presented by Shneider [1] and Korzynski and Lubas [5]. Following the development of the experimental research, and taking into consideration certain possibilities to improve the above-mentioned device, a second version (Fig. 6) was conceived and patented [21]. In this case, the trained shaft rotates into an eccentric hole existing in a sleeve, which in turn is placed in an eccentric hole existing in a second sleeve.

Fig. 6.

Second version of device based on use of oscillating slide and eccentric bolt.

(0.36MB).

The concept of developing a planned factorial experiment with six process input factors at two experimental levels was considered. The hypothesis that a monotone variation exists in the influence exerted by each process input factor on the output sizes was accepted. The vibration frequency ν (Hz), diameter db (mm) of the burnishing ball, vibration amplitude A (mm), radial burnishing force F (N), test piece peripheral speed vp (m/min), and feed rate f (mm/rev) of the burnishing tool (in fact, the longitudinal feed rate of the lathe carrier) were considered as input factors. The experimental conditions are mentioned in column nos. 2–7 in Tables 1–2.

Table 1.

Experimental conditions and results.

Exp. no.Experimental conditionsExperimental results in the case of steel 1C45Experimental results in the case of steel 18CrMn4-4
Vibration frequency ν, Hz  Ball diameter db, mm  Vibration amplitude A, mm  Force F, N  Test piece peri-pheral speed vp, m/min  Feed rate f, mm/rev  Surface roughness parameter Ra, µm  Micro-hard-ness HV, at h0.02 = 0.02 mm  Thick-ness of hardened layer h, mm  Surface rough-ness para-meter Ra, µm  Micro-hardness HV, at h0.02 = 0.02 mm  Thickness of hardned layer h, mm 
Column no. 1  10  11  12  13 
5.83  6.75  0.6  100  18.84  0.024  0.07  307  0.32  0.04  288  0.42 
5.83  6.75  0.6  100  18.84  0.132  0.25  286  0.22  0.28  263  0.32 
5.83  6.75  0.6  100  75.36  0.024  0.21  308  0.32  0.15  290  0.42 
5.83  6.75  0.6  100  75.36  0.132  0.47  287  0.22  0.43  265  0.32 
5.83  6.75  0.6  600  18.84  0.024  0.02  330  0.37  0.05  308  0.47 
5.83  6.75  0.6  600  18.84  0.132  0.09  310  0.32  0.12  287  0.42 
5.83  6.75  0.6  600  75.36  0.024  0.10  332  0.37  0.18  310  0.47 
5.83  6.75  0.6  600  75.36  0.132  0.20  312  0.32  0.27  290  0.42 
5.83  6.75  1.6  100  18.84  0.024  0.65  295  0.27  0.55  273  0.37 
10  5.83  6.75  1.6  100  18.84  0.132  0.85  278  0.22  0.71  258  0.32 
11  5.83  6.75  1.6  100  75.36  0.024  0.90  299  0.27  0.80  275  0.37 
12  5.83  6.75  1.6  100  75.36  0.132  1.40  380  0.22  1.20  258  0.32 
13  5.83  6.75  1.6  600  18.84  0.024  0.25  315  0.32  0.40  293  0.42 
14  5.83  6.75  1.6  600  18.84  0.132  0.40  295  0.22  0.49  273  0.37 
15  5.83  6.75  1.6  600  75.36  0.024  0.03  318  0.32  0.07  295  0.42 
16  5.83  6.75  1.6  600  75.36  0.132  0.05  297  0.22  0.09  275  0.37 
17  5.83  15.85  0.6  100  18.84  0.024  0.03  310  0.32  0.02  288  0.42 
18  5.83  15.85  0.6  100  18.84  0.132  0.10  295  0.27  0.08  272  0.37 
19  5.83  15.85  0.6  100  75.36  0.024  0.07  315  0.32  0.07  300  0.42 
20  5.83  15.85  0.6  100  75.36  0.132  0.20  300  0.27  0.15  280  0.37 
21  5.83  15.85  0.6  600  18.84  0.024  0.01  343  0.42  0.02  318  0.47 
22  5.83  15.85  0.6  600  18.84  0.132  0.02  310  0.32  0.06  298  0.42 
23  5.83  15.85  0.6  600  75.36  0.024  0.03  350  0.42  0.07  324  0.47 
24  5.83  15.85  0.6  600  75.36  0.132  0.09  318  0.32  0.12  303  0.42 
25  5.83  15.85  1.6  100  18.84  0.024  0.24  308  0.32  0.20  287  0.42 
26  5.83  15.85  1.6  100  18.84  0.132  0.31  285  0.22  0.27  267  0.37 
27  5.83  15.85  1.6  100  75.36  0.024  0.36  311  0.32  0.30  290  0.42 
28  5.83  15.85  1.6  100  75.36  0.132  0.58  290  0.22  0.50  270  0.37 
29  5.83  15.85  1.6  600  18.84  0.024  0.10  328  0.37  0.20  306  0.42 
30  5.83  15.85  1.6  600  18.84  0.132  0.15  307  0.32  0.20  288  0.37 
31  5.83  15.85  1.6  600  75.36  0.024  0.18  331  0.37  0.26  308  0.42 
32  5.83  15.85  1.6  600  75.36  0.132  0.36  311  0.32  0.47  290  0.37 
33  15.66  6.75  0.6  100  18.84  0.024  0.04  317  0.32  0.02  297  0.42 
34  15.66  6.75  0.6  100  18.84  0.132  0.26  299  0.27  0.20  277  0.37 
35  15.66  6.75  0.6  100  75.36  0.024  0.15  320  0.32  0.11  300  0.42 
36  15.66  6.75  0.6  100  75.36  0.132  0.23  302  0.27  0.17  280  0.37 
37  15.66  6.75  0.6  600  18.84  0.024  0.02  355  0.37  0.03  335  0.47 
38  15.66  6.75  0.6  600  18.84  0.132  0.09  328  0.37  0.15  315  0.47 
39  15.66  6.75  0.6  600  75.36  0.024  0.06  358  0.37  0.11  337  0.47 
40  15.66  6.75  0.6  600  75.36  0.132  0.12  330  0.37  0.20  318  0.47 
41  15.66  6.75  1.6  100  18.84  0.024  0.42  304  0.27  0.46  284  0.42 
42  15.66  6.75  1.6  100  18.84  0.132  0.72  287  0.22  0.76  265  0.37 
43  15.66  6.75  1.6  100  75.36  0.024  0.15  307  0.27  0.15  285  0.42 
44  15.66  6.75  1.6  100  75.36  0.132  0.33  290  0.22  0.32  267  0.37 
45  15.66  6.75  1.6  600  18.84  0.024  0.11  340  0.37  0.18  317  0.47 
46  15.66  6.75  1.6  600  18.84  0.132  0.21  313  0.32  0.29  293  0.42 
47  15.66  6.75  1.6  600  75.36  0.024  0.28  343  0.37  0.38  320  0.47 
48  15.66  6.75  1.6  600  75.36  0.132  0.41  316  0.32  0.53  295  0.42 
49  15.66  15.85  0.6  100  18.84  0.024  0.02  326  0.37  0.01  306  0.47 
50  15.66  15.85  0.6  100  18.84  0.132  0.09  312  0.22  0.06  291  0.42 
51  15.66  15.85  0.6  100  75.36  0.024  0.08  331  0.37  0.05  310  0.47 
52  15.66  15.85  0.6  100  75.36  0.132  0.10  380  0.47  0.07  295  0.42 
53  15.66  15.85  0.6  600  18.84  0.024  0.01  315  0.32  0.01  345  0.47 
54  15.66  15.85  0.6  600  18.84  0.132  0.46  376  0.47  0.58  316  0.47 
55  15.66  15.85  0.6  600  75.36  0.024  0.02  340  0.37  0.05  350  0.47 
56  15.66  15.85  0.6  600  75.36  0.132  0.90  342  0.37  1.10  320  0.47 
57  15.66  15.85  1.6  100  18.84  0.024  0.12  320  0.37  0.08  298  0.42 
58  15.66  15.85  1.6  100  18.84  0.132  0.20  300  0.27  0.14  277  0.37 
59  15.66  15.85  1.6  100  75.36  0.024  0.25  322  0.37  0.19  300  0.42 
60  15.66  15.85  1.6  100  75.36  0.132  0.35  303  0.27  0.30  280  0.37 
61  15.66  15.85  1.6  600  18.84  0.024  0.06  357  0.42  0.10  338  0.47 
62  15.66  15.85  1.6  600  18.84  0.132  0.10  360  0.42  0.15  310  0.47 
63  15.66  15.85  1.6  600  75.36  0.024  0.12  330  0.37  0.21  340  0.47 
64  15.66  15.85  1.6  600  75.36  0.132  0.19  333  0.37  0.25  311  0.47 
Table 2.

Variation of microhardness HV in the surface layer.

Experimental conditionsTest piece materialDistance from vibroburnished surface h, mm
0.02  0.07  0.12  0.17  0.22  0.27  0.32  0.37  0.42  0.47  0.52  0.57 
ν = 5.83 Hz, db = 15.85 mm, A = 0.6 mm, F = 600 N, v = 18.84 m/min, f = 0.024 mm/rev (experiments no. 21)  1C45  343  318  299  280  261  247  240  235  231  230  230  230 

Sleeves with an external diameter of 60 mm were used as test pieces. These sleeves were made from two distinct steels: medium-carbon steel 1C45 (initial surface Vickers microhardness HV = 230) and alloyed steel for cementation 18CrMn4-4 (HV = 210). Prior to applying the burnishing process, the test pieces were finished by a grinding process (in the case of sleeves made from 1C45 steel Ra = 1.27–1.54 µm).

Moreover, the Vickers microhardness beginning from the vibroburnished surface to the test piece interior was measured, at distances of 0.05 mm from the initial measurement made at 0.02 mm, up to the zone in which the microhardness became equal to that of the base material. An example of the microhardness values measured in the case of the two experimental materials is presented in Table 2 and Fig. 7. In order to evaluate the thickness of the layer hardened by applying the ball vibroburnishing process, the value of the distance h from the test piece surface up to the zone in which a different microhardness to that corresponding to the base material was considered, and these values were included, for example, in columns 10 and 13 in Table 1.

Fig. 7.

Change in microhardness size from ball vibroburnished surfaces in the case of test pieces made from two steels (1C45 and 18CrMn4-4, with ν = 5.83 Hz, db = 15.85 mm, A = 0.6 mm, F = 60 daN, vp = 18.84 m/min, f = 0.024 mm/rev; microhardness measured by means of microhardness tester PMT3).

(0.15MB).
3Results and discussion

The experimental results were mathematically processed by means of specialized software, based on the least squares method [39]. The software analyses the possibility of considering certain mathematical empirical models, such as first- and second-degree polynomials, power-type function, exponential function, and hyperbolic function. The adequacy of a certain empirical mathematical model for the experimental results could be verified by means of the so-called Gauss criterion, which takes into account the sum of the squares of the differences between the ordinates of the points corresponding to the proposed empirical model and experimental values, respectively. A lower value of the Gauss criterion indicates a superior adopted empirical mathematical model.

In machine manufacturing processes, power-type functions are frequently preferred owing to the fact that the values of the exponents attached to the independent variables offer a direct image in what regards the intensity of the influence exerted by a certain size considered as an input factor on the dependent variable (output parameter). Therefore, by means of mathematical processing of the experimental results included in Table 2, the following power-type functions were determined for the surface roughness parameter Ra, microhardness HV, and thickness h of the hardened surface layers obtained by applying the ball vibroburnishing process:

Ra1C45 = 11.665∙ν−0.11 ∙db−0.59∙A1.12 ∙F−0.44 ∙νp0.34∙f 0.54,
for which the Gauss criterion has the value SG = 0.03897444;
Ra18MnCr10 = 1.834∙ν−0.23db−0.65A1.17F−0.045νp0.37f 0.52,
(Gauss criterion value: SG = 0.04810425);
HV1C45 = 191.603ν0.04db0.03A−0.02F0.03νp0.01f −0.02,
(Gauss criterion value: SG = 319.354);
HV18MnCr10 = 149.049∙ν0.06db0.04A−0.04F0.05νpf −0.04,
(Gauss criterion value: SG = 17.08551);
h1C45 = 0.061ν0.11db0.15A−0.10F0.11νp0.009f -0.10,
(Gauss criterion value: SG = 0.001695116);
h18MnCr10 = 0.168ν0.09db0.05A−0.06F0.06νpf −0.06.
(Gauss criterion value: SG = 0.0003958187).

The graphical representations in Figs. 8–10 was elaborated based on the equations (5) and (6).

Fig. 8.

Influence exerted by frequency f and amplitude A of vibratory motion on the value of surface roughness parameter Ra, in the case of test pieces made from steels 1C45 and 18CrMn4-4 (F = 350 N, v = 18.84 m/min).

(0.24MB).
Fig. 9.

Influence exerted by frequency f and amplitude A of vibratory motion on surface layer microhardness HV, measured at distance of 0.02 mm from ball vibroburnished surface, in the case of test pieces made from steels 1C45 and 18CrMn4-4 (db = 15.66 mm, vp = 75.36 m/min, f = 0.024 mm/rev F = 350 N).

(0.25MB).
Fig. 10.

Influence exerted by frequency f and amplitude A of vibratory motion on the hardened layer thickness, in the case of test pieces made from steels 1C45 and 18CrMn4-4 on thickness h of hardened layer (ν = 15,66 Hz, vp = 75.36 m/min, F = 350 N).

(0.3MB).

An examination of the equations corresponding to sizes of technological interest and the graphical representations in Figs. 7–10 allows one to make certain particular and general observations.

Thus, it is noted that in the case of both steels (1C45 and 18CrMn4-4), as expected, the increase of the vibration frequency ν, of the ball diameter db, and rolling force F determines the decrease (improvement) in the surface roughness parameter Ra, as the exponents attached to these sizes have negative values in empirical models (1) and (2), while the increase of the vibration amplitude A, of the test piece peripheral speed vp and feed rate f leads to an increase in the value of the surface roughness parameter Ra. The vibration amplitude A exerts the greatest influence on the parameter Ra, as the exponent attached to this size has the maximum absolute value compared to the exponent values established for the other process input factors.

In the experimental interval investigated, the influence exerted by the rolling force F appears to be very low in the case of the steel 18CrMn4-4, and this may have a connection to the higher plasticity of this steel, owing to its lower carbon content.

The mathematical empirical models (3) and (4), established for the microhardness HV, demonstrate that the influences exerted by the process input factors appear to be rather low, as all of the exponents have low subunit values.

For this reason, one could appreciate that in the case of the obtained experimental results, the following relations could also be taken into consideration:

HV1C45 = 191.6;
HV18MnCr10 = 149

Among the process input factors considered, in the case of both steels, the most intense influence is exerted by the vibration frequency ν. This could be explained by the fact that a higher frequency results in numerous passes in the same surface zone, which may determine an increase in the affected zone microhardness. For both steels, the increased vibration amplitude A and longitudinal feed rate f led to a decrease in the microhardness HV, as the exponents attached to these sizes were negative. An explanation for this could be the reduced time of the ball action on the test piece surface when the vibration amplitude A and longitudinal feed rate f have higher values. Furthermore, it is observed that the increase in the other process input factors considered determines the increase in the surface layer microhardness.

In the case of the hardened layer thickness h, it is observed that the directions of the influences exerted by the process input factors are the same in the cases of both steels, as all the exponents attached to the process input factors have the same sign (plus or minus) in empirical mathematical models (5) and (6), although the intensities of their influences are different. Therefore, it can be concluded that, in the case of the steel 1C45, the highest influence is exerted by the ball diameter db, the exponent of which has the maximum absolute value in mathematical empirical model (5). In the next size order positions, the rolling force F and vibration frequency ν can be found. The increase in the sizes of ν and F leads to an increased hardened layer thickness h. In the case of the steel 18CrMn4-4, all influences are lower than in the case of the steel 1C45 and, as in the above-mentioned situation, this could be explained by the higher plasticity of the steel 18CrMn4-4. An interesting aspect is the fact that, while the hardened surface layer hardness is essentially higher in the case of the steel 1C45, the hardened surface layer thickness is greater in the case of the steel 18CrMn4-4, which could also be a result of the higher plasticity of the second steel, containing less carbon.

The changes in the surface roughness parameter Ra and microhardness HV values of the hardened surface layer are, to a certain extent, similar to those obtained by Pande and Patel [7]. Thus, it is confirmed that the surface roughness parameter Ra values increase when the peripheral speed v and feed rate f diminish, and the rolling force F increases.

The experimental results included in the Table 1 facilitates a comparison of the vibroburnishing process with other processing methods. Thus, one could notice that an optimal selection of the values that correspond to the vibroburnishing process input factors could ensure a significant diminishing of the surface roughness parameter Ra (values of the surface roughness parameter Ra = 0.02 µm were obtained), associated also with a certain increase of the surface layer microhardness. One could notice that even other subtractive machining processes could lead to values of the surface roughness parameter Ra≥0.01 µm, as the cases of the superfinishing and lapping processes are, the increase of the surface layer microhardness is much lower in such situations.

As expected and confirmed by the experimental results, the decrease in the surface roughness is more intense when the rolling force exerted by the deformation element is higher. However, another result of the rolling force action is the hardening effect, which could be evaluated by means of the HV microhardness measured at a depth of h0.02 = 0.02 mm from the processed surface, and the hardened layer thickness h.

An image of the surface initially characterized by a regular profile obtained by turning, and affected by the ball burnishing process, can be seen in Fig. 11. In this image, the traces left by the ball action have a high inclination owing to the high ratio between the peripheral speed vp of the test piece and average vibration speed. One can observe the existence of grooves that were not affected by the ball vibroburnishing process and areas smoothed under the ball action.

Fig. 11.

Traces existing on vibroburnished surface that highlight inclined ball movement as a result of combining test piece rotation and vibration motion (fturning = 0.2 mm/rev, magnification 200× ).

(0.26MB).

The above-mentioned arguments lead to the question of whether a certain correlation exists among the decrease in the surface roughness parameter Ra and the increase in the microhardness HV and hardened layer thickness h. An answer to this question may be provided by the value of the Pearson’s correlation coefficient.

The correlation coefficient values were determined by using the function CORRELATION from the Excel software for the two-by-two columns of results from the columns 8 to 13 in Table 1 (column nos. 8 to 10 for steel 1C45 and column nos. 11 to 13 for the steel 18CrMn4-4). Examining the correlation coefficient values, it is firstly noted that, for both steels, the correlation coefficients exhibit similar values when using a certain evaluation criterion among the three criteria considered (surface roughness parameter Ra, microhardness HV, and hardened layer thickness h). Thus, if the values of the surface roughness Ra and surface layer microhardness HV are considered, it is found that the correlation coefficient values are -0.427 in the case of steel 1C45 and −0.391 in the case of steel 18CrMn4-4. This means that a certain correlation exists between the output sizes considered, but it is not excessively strong. A similar conclusion can be formulated if the correlation between the surface roughness Ra and thickness h is considered; the correlation coefficient values are −0.441 in the case of steel 1C45 and −0.408 in the case of steel 18CrMn4-4.

A stronger correlation is observed between the hardened layer microhardness HV and thickness h. The correlation coefficient values are 0.932 in the case of steel 1C45 and 0.899 in the case of steel 18CrMn4-4. This correlation may be explained by the fact that an increase in the microhardness HV measured at 0.02 mm from the vibroburnished surface is more strongly connected with the increase in surface layer thickness h.

The empirical mathematical models (1), (2), (3), (4), (5) and (6) could be directly used in the industrial practice to estimate the roughness, hardness and thickness of the surface layer affected by the vibroburnishing process or to optimize this process by the adequate selection of the values of process input factors.

4Conclusions

Over the past several decades, various aspects of the vibroburnishing process have been investigated, considering its application in the cases of external and internal cylindrical surfaces, as well as flat surfaces. Taking into consideration such aspects, as well as theoretical and experimental research concerning the vibroburnishing process, the following conclusions may be elaborated:

  • An examination of the machined micro-zone using images obtained by means of a scanning electron microscope highlighted the material movement as a result of the ball vibroburnishing process.

  • In order to obtain a more complete image concerning the manner in which the process input factors may influence the values of the surface roughness parameter Ra, surface layer microhardness HV, and hardened layer thickness h, experimental research was designed and implemented. The vibration frequency and amplitude, ball diameter, radial rolling force, peripheral speed, and longitudinal feed rate were considered as process input factors. The experiments were conducted on test pieces made from two distinct steels: medium-carbon steel (1C45) and low-carbon alloy steel (18CrMn4-4).

  • Power-type empirical mathematical models were determined by means of mathematical processing of the experimental results. Examination of these empirical mathematical models showed that, in the case of the surface roughness parameter Ra, the vibration amplitude is the most significant factor, while in the case of the surface layer microhardness, the vibration frequency exerts the strongest influence.

  • If the hardened layer thickness is considered, it is found that the main significant influencing factors are the rolling force in the case of the steel 1C45 and the vibration frequency in the case of the steel 18CrMn4-4.

  • When examining the correlation between two process output parameters, values closer to 1.00 were identified for the correlation coefficient in the case of the surface layer microhardness and hardened layer thickness.

In the future, one intends to evaluate the behaviour of other metallic materials under the action of the ball vibroburnishing process.

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Journal of Materials Research and Technology

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