Contribution In Determining The Fatigue Endurance of Slide Diamond Burnishing AISI52100 Steel Components

The present paper is an investigation on the effect of slide diamond burnishing on the fatigue endurance of a component made of AISI52100 steel. Burnishing operation has been performed on cylindrical specimens using optimal parameters statistically selected such as burnishing force, burnishing feed and number of tool passes. Bending fatigue tests in air at R= -1 and S-N curves have been plotted by incrementing the applied stress from a maximum stress level of about 66% of the ultimate tensile stress to a stress value below which fatigue does not occur. Results show that slide diamond burnishing has increased the fatigue resistance comparing to unburnished specimens. The fatigue endurance is respectively 222 MPa and 190 MPa. At high stress levels, the fatigue resistance improvement is clearly observed and the fatigue trends are in good agreement with those reported in literature. However, the present results are slightly lower and that is attributed to the shoulder llet value of the specimen. For small llet radius, fatigue resistance is lowered. of testing cycles and the change in shoulder llets. The contribution of the present work then consists in determining the effect of slide diamond burnishing on the fatigue endurance of a component made of AISI52100 steel. S-N curve are carried out on cylindrical specimens for untreated and ball burnished specimens. The applied pressure and slide diamond burnishing regimes are extrapolated from statistical experimental data. Results are compared to literature data.


Introduction
Nowadays, ball burnishing surface nish process is considered as one of the best alternative to increase fatigue lifespan of engineering components [1,2]. Recent research works have demonstrated its effectiveness through three main parameters, decreasing surface roughness, increasing surface hardness and particularly generating surface compressive residual stresses [3,4]. Usually, the burnishing process consists in sliding carbide or diamond tool in the form of roller or ball surface by applying pressure su cient to create surface plastic deformation improving the surface integrity of the component. However, the performance of the burnishing process depends on the material and the geometry of the part, and also on the material and geometry of the burnishing tool together with the conditions of burnishing process that should be carefully selected [5,6].
Recently Maximov et al [7] have devoted a review paper on state-of-the-art, achievements and perspectives of the slide burnishing of metal components. They have concluded that the majority of slide burnishing studies are focused on the study of surface integrity, where the attention is focused mostly on the roughness and microhardness, whereas, signi cantly less attention is paid to wear resistance and fatigue strength.
For instance, Rodríguez et al [8] in their investigation on surface improvement of shafts by the deep ballburnishing technique have shown that ball-burnishing performs both physical and mechanical properties of turned parts by improving surface quality, increasing the hardness of the workpiece and introducing compressive residual stresses which are favorable for increasing the fatigue life of the piece and improve the wear resistance of the component. Moreover, they have pointed out that the most in uencing parameter in the burnishing pressure which should be carefully selected rather than the burnishing speed and feed rate have little effect therefore maximum values should be used as the processing time can be signi cantly reduced.
Meanwhile, the interest to fatigue resistance is related to the bene cial effect of ball burnishing in generating compressive residual stress at the surface layers of a component [9,10]. Amador et al [11] have carried investigation on ball burnishing effect on deep residual stress on AISI 1038 steel. Using hole drilling technique, they have shown that ball burnishing has proved to introduce relevant residual stress up to 0.6 mm depth which is very relevant in using the process as an industrial nishing process to improve the surface nish parts subjected to fatigue working regimes.
One the research work that is coherent with the effect ball burnishing on fatigue resistant is that reported by Sadeler et al [12]. They have investigated the in uence of ball burnishing on fatigue behaviour of AISI 1045 steel and have demonstrated that the fatigue limit and fatigue life are improved by the burnishing nish process. R. Avilés et al [13] have studied the In uence of low-plasticity ball burnishing on the high-  Tables 1 and 2. σ ut, σ y σ yh σ yl and e are respectively, the ultimate stress, the yield stress, the Upper yield stress, the lower yield stress and the elongation. Fatigue specimens, are prepared from a parent bar of 400 mm diameter according to the speci cation given in the operating instruction of the fatigue machine GUNT WP 140.01 Two series of specimen (as machined and slide diamond burnished) have been prepared following the procedure described in Fig. 1.

Slide diamond burnishing process
In order to investigate the effect of mechanical surface treatment, a series of fatigue specimens have been subjected to slide diamond burnishing on a 16k20 lathe, Fig. 2. A purpose made diamond burnisher has been designed to achieve the burnishing operation (Details of the burnishing tool is given in literature [15]. The procedure of slide diamond burnishing operation is illustrated in Fig. 3

Roughness measurement :
Roughness measurements have been carried out on as-machined specimens and on burnished specimens using Mitutoyo model SJ-301. For each specimen, three measurements of Ra are made along the specimen diameter at three angles of 0, 120 and 240°.

Microhardness measurement
Microhardness have been measured using Matzuzawa model MXT70 indenter under a load of 3 kgf. Two type of measurements have been conducted. The rst type concerned the microhardness along the external diameter. The second type permitted to identify the microhardness pro le on the external layer of the cross section of the specimen through incremental radial distance of 0.025 mm.

Fatigue test
Fatigue test have been conducted on a GUNT WP140.1 fatigue machine offering 0.37kW of power and dynamic loads up to 300N. All fatigue tests were carried out at R= -1 in room temperature. S-N curves have been obtained by starting from a stress level 1.03 to 1.04 times the upper yield strength of the material that is 66% of the ultimate stress. The respective value of the starting stress in both specimen conditions was 468 MPa. Then the stress has been decreased by increment of 30 MPa a stress value until fracture of the specimens does not occur after 1 million cycles. Figure 4 shows fatigue test settlement.
Results And Discussions

Effect of SDB on specimen roughness
As expected, slide diamond burnishing has generated good surface roughness. Figure 5 illustrates the effect of slide diamond burnishing on the surface roughness of the fatigue specimens. The mean value roughness given in Fig. 5, corresponded to a series of 9 measurements on each specimen. Turning and grinding operations resulted in a surface roughness value of 0.450 µm, which is in good agreement with that recommended in literature [17]. Meanwhile slide diamond burnishing improved the surface roughness by compressing the external layer to a state of matt surface where a mean value of 0.12 µm is obtained.

Effect of SDB on specimen microhardness
From the microhardness point of view, burnishing allows the surface layers to be consolidated by the phenomenon of work hardening, which is manifested by an increase in microhardness (Fig. 6). The value of microhardness on the outer layer of SDB surface, increased from 301 HV 3 to 535 HV 3 that is 1.77 times. An exploratory measurements of microhardness on the external layers of the SDB specimens revealed that have been subjected to hardening over 250 µm depth from the outer layer. Figure  In fact, down to 100 µm, microhardness dropped to the value given by the turning and grinding process at the outer surface. When going deeper down to 200µm, the microhardness continues to decrease until it becomes constant at a depth of 250 µm. Thence the SDB treatment generate a hardening process that performance the quality of the outer layer and reduces the detrimental effect of the turning and grinding process. Which is in good agreement with that recommended in literature [18].
3.3 Effect of SDB on Wohler curves of AISI 52100 steel. Figure 8 illustrates the stress-strain curves of two types of specimens: the as machined (AM) and the slide diamond burnished (SDB). At least 3 fatigues tests at each level of applied stress have been reproduced at a stress ratio of R= -1 in order to get best SN tting curves. As the stress decreases from a stress value 3% above the upper yield strength of the material (568 MPa) by increment of 30 MPa, to a stress level of 308 MPa, the fatigue life increases slowly at rst when fracture appears after 60000 cycles. Then the fatigue life increases quite rapidly as the applied stress level is decreased when a fatigue limit is observed after 1 million cycle for a stress level of 222 MPa for the slide diamond burnished specimens and 190 MPa for the as machined specimens. Reference tests have been left to end up to 10 million cycles without observing fracture. A mean value of the scatter in results is represented because the fatigue sensitivity to a number of test and material parameters are impossible to control.
From Fig. 8, it can be clearly seen that comparing to machined and grinded specimens; slide diamond burnishing has improved the fatigue life resistance of the material since the fatigue limit has increased from 190 MPa to 222 MPa. In fact, hardening, generated by slide diamond burnishing, permitted to compress the external layers obtained after machining, grinding, and therefore reduced stress concentration zone. Figure 9 shows the fracture surface of both types of specimens. In the case of the slide diamond burnished specimen, the center rapid fracture zone is clearly de ned from the propagation zone. In the case of turned and grinded specimens, it is very di cult to separate the fracture zone from the propagation zone. The corresponding engineering models for both types of specimens are respectively expressed by power laws of the form given in Equ (2)

Effect of shoulder llet radius on SN curves in AISI 52100 steel
It is interesting to examine the behaviour of fatigue life at high stress levels where the value of applied stress is 75% above the material yield stress. The non-burnished specimens show lower life at all stress levels comparing to the slide burnished specimens. Thus SDB process improves the fatigue life and the improvement can be quanti ed by using equations (2) and (3). The results are in good agreement with those reported in literature [13,14,19]. Figure 10 illustrate comparative SN curves plots with literature data [14] for similar materials. Basically, similar trend are observed however, the present results show lower fatigue life than those reported by Travieso for AISI 1038 material. In fact the decrease of the fatigue life is attributed to the radius of the specimen shoulder which smaller than that used by Travieso et al. They have used 4 mm ball burnishing tool where as in the present work, a 2.5 mm diameter diamond burnisher generated excessive stress concentration at the specimen shoulder. Therefore, even though that the slide diamond burnishing improves the surface roughness of the component, the fatigue life is very sensitive to shoulder radius.

Conclusion
The present work is a contribution in determining the fatigue endurance of slide diamond burnishing on of AISI 52100 steel components. SDB process improved the roughness of the material to reach a value of Ra = 0.12 µm and generated hardening on 250 µm deep outer surfaces of the components. The microhardness increased from 301 to 541 HV 3 . Hardening generated a decrease of the microhardness down to 220 HV 3 . The fatigue endurance represented 50% of the material yield stress for the SDB components and 40% for the non-burnished components. Thus SDB process improves the fatigue life.
The improvement is quanti ed through engineering power law.
Results are in good agreement with those given in literature. In fact, a comparative analyses with SN curve of similar material AISI 1038 steel revealed similar trend behaviour. In addition, it permitted to sort out that fatigue life of cylindrical components are highly sensitive to the radius of the components shoulder radius.

. Con icts of interest/Competing interests
This present work has been carried out during the last 3 years in order to meet an industrial need of the mechanical manufacturing company Ateliers Maghrébins de Mécanique in Annaba in Algeria, which concerns the study of the opportunity to improve the roughness surface of machined cylindrical parts by using burnishing operation. The company has supplied the material and prepared over 100 fatigue test pieces. A literature review has been oriented towards in order to identify the work carried out in this direction, particularly the work of Travieso-Rodriguez et al 2019 [14] and Hamadache H [15]. Slide diamond burnishing process has been carried out at the research laboratory of advanced technology in production engineering using a purpose design burnishing tool. Roughness and hardness measurements were necessary to better understand the Wöhler curves that were plotted. This work will be integrated into an industrial context.   Slide diamond burnishing operation on 16k20 lathe.  Fatigue testing machine fatigue testing machine Type GUNT WP 140.01: 1 switch box, 2 electric motor, 3 bearing, 4 clamped specimen, 5 load application device with spring balance and hand wheel, 6 protective cover, 7 measurement ampli er.

Figure 7
Hardening of the external layers of fatigue specimen through SDB.

Figure 8
Page 16/17 SDB effect on the fatigue endurance of AISI5210 steel.

Figure 9
Fatigue fracture surface of a) machined and grinded specimen and b) Slide diamond burnished specimen