an investigation into surface roughness of burnished hypereutectic al-si alloy based on the taguchi...
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Materials Science Forum Vols. *** (2004) pp.790-794
online at http://scientific.net
2004 Trans Tech Publications, Switzerland
An Investigation into Surface Roughness of Burnished Hypereutectic Al-Si Alloy Based on the Taguchi Techniques
L.F. Han1,a, W. Xia1,b, Y.Y. Li1,c and W.P. Chen1,c 1College of Mechanical Engineering, South China University of Technology, Guangzhou, P.R. China
Keywords: Burnishing, Surface roughness, Hypereutectic Al-Si alloy, Taguchi technique, ANOVA
Abstract. This paper presents an investigation on the surface roughness of burnished hypereutectic
Al-Si alloy a widely used light-weight and wear resistant material in automobile, electric and
aircraft industries. Based on the techniques of Taguchi, an orthogonal experiment plan with the
analysis of variance (ANOVA) is performed and a second-order regressive mathematical model is
established. Meanwhile, the influence of process parameters on surface roughness and its mechanism
are discussed. From the experiments, it is found that burnishing process is effective to decrease
surface roughness of hypereutectic Al-Si alloy components, in which, all input parameters have a
significant effect on the surface roughness. To achieve a small surface roughness, the optimum
process parameters are recommended.
Introduction
Burnishing is a no-chip surface finishing process. It is carried out by applying an appropriate pressure
through a highly polished and hardened ball or roller on a flat or cylindrical components surface, in
which the peaks of the metallic surface are pressed down and spread out to fill the valleys by plastic
deformation and no chip is removed [1]. Improvements of components in surface finish, surface
hardness, wear resistance, fatigue and corrosion resistance can be achieved by the application of this
process [1-3].
Most published works have dealt with the application of this process on steel and non-ferrous
metallic components [1-5]. Unfortunately, little attention has been paid to determine the effect of the
burnishing parameters on hypereutectic Al-Si alloy which has large quantity of hard silicon
reinforcing particles in soft aluminum matrix, although it has been widely used as light-weight and
wear resistant material in automobile, electric and aircraft industries [6,7].
In this paper, the four major burnishing parameters influencing the surface roughness of
hypereutectic Al-Si alloys are investigated and optimized using Taguchi’s techniques [8,9,11], since
no available work has been done on this subject by other researchers. The factors under investigation
in the experiments are the surface roughness after burnishing (Ra), the burnishing speed (V), the feed
rate (f), the burnishing force (F) and the number of passes (N).
Experimental
Specimen Preparation. Hypereutectic Al-Si alloy bars were processed via rapid solidification and
powder hot extrusion route (RS/PM). They were roughly turned to an initial surface roughness of
3.8µm to 4.5µm (Ra), and divided into regions according to the parameters to be studied. The
chemical composition of the components (wt.%) is shown in Table 1.
Table 1 Composition of hypereutectic Al-Si alloy used for experiment[10]
%Si %Cu %Mg %Fe %Ni %Mn %Al Composition:
28.5 1.1 0.95 1.1 <1.0 <0.5 balance
Materials Science Forum Vols. 471-472 (2004) pp 790-794Online available since 2004/Dec/15 at www.scientific.net© (2004) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.471-472.790
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Burnishing Tool. Referring to previous research [2], A modified ball-burnishing tool system was
designed and manufactured as shown in Fig.1. The shank of this tool can be mounted onto the tool
holder of an ordinary lathe. The 10mm diameter ball is made of bearing steel with a hardness of
60HRC and a surface roughness approximately Ra0.05. When the ball is pressed against the surface of
specimen, a spring will be compressed and the displacement is displayed in the dial gauge. This spring
calibrated by a Kistler® piezoelectric dynamometer of type9441 is used to measure the applied
burnishing force.
①adapter cover; ②adapter; ③dial gauge; ④guiding rod; ⑤shank; ⑥spring; ⑦burnishing ball
Fig.1 Assembly drawings of the ball-burnishing tool
Experimental Conditions. Burnishing was performed on a Chinese made ordinary lathe of type
C6132A1. The machined and burnished surfaces were measured by a handheld digital TIME®
R200
surface profilometer respectively. No lubricant was used throughout the experimental work.
Plan of Experiments. According to the Taguchi’s techniques [8,9,11], a rotary and combined
second-order orthogonal design was established for four independent factors at five levels, in which
the interaction actions were considered. The total number of experiments was determined to be 36
[11]. The factors to be studied and the assignment of the corresponding levels are indicated in Table 2,
and the experimental design is shown in Table 3.
Table 2 Factors and levels used in the experiments
Level (Code) Factors
-2 -1 0 1 2
Speed [m/min] -X1 8 16.5 25 33.5 42
Feed [mm/rev] -X2 0.05 0.09 0.13 0.17 0.21
Force [N] - X3 50 125 200 275 350
Number of pass – X4 1 2 3 4 5
Results and Discussion
The results of 36 experiments are shown in Table 3, from which it can be observed that the surface
roughness of hypereutectic Al-Si alloy components can be reduced significantly by ball-burnishing
process on an ordinary lathe. By selecting appropriate process parameters, the arithmetical average
value of roughness decreases below 0.25µm, which indicates that the smooth surface can be obtained
by burnishing instead of grinding process.
Analysis of Variance (ANOVA) of Process Parameters. Table 4 shows the results of the
ANOVA with the surface roughness (Ra) of burnished components. This analysis is carried out for a
level of significance of 1%, namely for a level of confidence of 99%. The last column of the table
shows the significance (S) of each factor, which indicates whether or not significantly influential on
the surface roughness.
From Table3 and Table4, it is found that the four investigated process parameters have statistical
significance on the surface roughness of burnished components, in which the burnishing force and
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feed rate are the most prominent factors, followed by the burnishing speed and the number of pass. To
achieve a small surface roughness, it is better to preferentially determinate the burnishing force and
feed rate. Meanwhile, there are many interactions between these parameters.
Table 3 Experiment matrix and result for Ra
No. X1 X2 X3 X4 Ra [µm]
1 33.5 0.17 275 4 1.04
2 33.5 0.17 275 2 0.63 3 33.5 0.17 125 4 0.47 4 33.5 0.17 125 2 0.49 5 33.5 0.09 275 4 0.89 6 33.5 0.09 275 2 0.29 7 33.5 0.09 125 4 0.36 8 33.5 0.09 125 2 0.25 9 16.5 0.17 275 4 0.61
10 16.5 0.17 275 2 0.59 11 16.5 0.17 125 4 0.43 12 16.5 0.17 125 2 0.46 13 16.5 0.09 275 4 0.24 14 16.5 0.09 275 2 0.23 15 16.5 0.09 125 4 0.23 16 16.5 0.09 125 2 0.27 17 42 0.13 200 3 0.65 18 8 0.13 200 3 0.31
No. X1 X2 X3 X4 Ra [µm]
19 25 0.21 200 3 0.47 20 25 0.05 200 3 0.26 21 25 0.13 350 3 0.92 22 25 0.13 50 3 0.38 23 25 0.13 200 5 0.66 24 25 0.13 200 1 0.25 25 25 0.13 200 3 0.26 26 25 0.13 200 3 0.34 27 25 0.13 200 3 0.22 28 25 0.13 200 3 0.39 29 25 0.13 200 3 0.38 30 25 0.13 200 3 0.33 31 25 0.13 200 3 0.28 32 25 0.13 200 3 0.24 33 25 0.13 200 3 0.33 34 25 0.13 200 3 0.25 35 25 0.13 200 3 0.23 36 25 0.13 200 3 0.26
Table 4 ANOVA table for the surface roughness (Ra)
Source 1SDQ
2df Variance Test F Fα=0.01
3S
X1 (V) 0.172 1 0.172 79.554 8.02 ★★
X2 (f ) 0.232 1 0.232 106.92 8.02 ★★★
X3 (F) 0.288 1 0.288 132.804 8.02 ★★★
X4 (N) 0.149 1 0.149 68.689 8.02 ★★
X1×X2 0.005 1 0.005 2.392 8.02
X1×X3 0.064 1 0.064 29.307 8.02 ★
X1×X4 0.078 1 0.078 35.924 8.02 ★
X2×X3 0.015 1 0.015 6.813 8.02
X2×X4 0.006 1 0.006 2.701 8.02
Source 1SDQ
2df Variance Test F Fα=0.01
3S
X3×X4 0.066 1 0.066 30.363 8.02 ★
X12 0.059 1 0.059 27.268 8.02 ★
X22 0.007 1 0.007 3.255 8.02
X32 0.235 1 0.235 108.52 8.02 ★★★
X42 0.045 1 0.045 20.597 8.02 ★
4Reg. 1.420 14 0.101 46.793 3.07 ★★
Error 0.046 21 0.002
Total 1.466 35
Remarks:
1SDQ: sum of squares;
2df: degree of freedom;
3S: significance;
4Reg.: regression
In this study, the regressive test F=46.793>Fα=0.01 (14,21). It indicates that the correlative model of
process parameters established based on the ANOVA will be highly statistically significant.
Mathematical Model. To find the relationship between the surface roughness (Ra) and the four
significant process parameters, based on the ANOVA, a second-order regressive mathematical model
was established as follows. Herein, a MATLAB programming tool[12] was employed to conduct the
multiple nonlinear regression and the optimization of the computed model.
Ra=0.29+0.08X1+0.10X2+0.11X3+0.08X4+0.06X1X3+0.07X1X4+0.06X3X4+0.04X12
+0.09X32+0.04X4
2 (1)
where, X1, X2, X3 and X4 are coded level with a range from –2 to 2.
Discussion on Influence of Process Parameters. According to the mathematical model, the 3D
figures of the surface roughness with relation to these process parameters are obtained in Figs.2-4.
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It can be seen from Fig.2 that the surface roughness obviously decreases to a small value, after
which, it starts to increase slowly. With increase in burnishing force and the number of pass, the
plastic deformation on workpiece surface increases, which causes the surface roughness to decrease
relatively. But when the two process parameters exceeds a certain limit, the repeating plastic
deformation acted on the workpiece surface will lead to the deterioration of the surface layer as a
result of overmuch work-hardening of the material. Furthermore, excessive burnishing force might
cause chattering of burnishing system[3] and even the micro-fragmentation and flaking of brittle
silicon particles. As a result, the surface roughness of the specimen starts to increase subsequently.
Also from Fig.2, a burnishing force from 125 to 275N with 2 or 3 passes is preferable in burnishing
hypereutectic Al-Si alloy components.
Fig.2 The surface roughness vs. Fig.3 The surface roughness vs. Fig.4 The surface roughness vs.
burnishing force and Number burnishing speed under different feed rate under different
of pass burnishing force burnishing force
The surface roughness decreases with the increase of burnishing speed until a minimum is reached,
then it goes up gradually as shown in Fig.3. If the burnishing speed raises beyond an appropriate
range, the resultant low deforming action of the ball as well as possible chattering at higher speeds[3]
will increase the instability of burnishing. Moreover, It is found when the burnishing speed exceeds
40m/min with the applied burnishing force of 350N, a rapid rise in temperature of the workpiece
occurs, which may increase the possibilities of material transformation between the burnishing ball
and workpiece interface, All these factors will lead to an increase in surface roughness. As from Fig.3,
low speeds from 16.5 to 33.5m/min for burnishing is preferable.
The surface roughness of burnished specimens increases with the raise of feed rate as shown in
Fig.4, where, no concave curve is observed for feed rate. In burnishing process, the elastic
deformation occurs in hard silicon particles, along with the plastic deformation in aluminum matrix,
which makes the material upheaved towards two sides of the burnishing ball. With small feed rate, the
pitch of burnishing trace is small. There has more chance to overlap the burnishing trace on the same
area, to smooth out the upheaved edges of the precious trace, and to press the silicon particles beneath
the surface of workpiece, which will cause a decrease in surface roughness. But with larger feed rate,
the pitch of burnishing trace increases relatively, which will cause a raise in surface roughness.
Therefore, to achieve a smooth surface, low feed rate from 0.05 to 0.15mm/rev for burnishing is
preferable.
Conclusions
Some useful conclusions in studying of burnishing hypereutectic Al-Si alloy based on Taguchi
techniques are obtained as follows:
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1.The surface roughness of hypereutectic Al-Si alloy components can be reduced significantly by
ball-burnishing process on an ordinary lathe, which makes it possible to obtain smooth surface by
burnishing instead of grinding process.
2.The four investigated process parameters have significant influence on the surface roughness, in
which, the burnishing force and feed rate are the most prominent ones.
3.A second-order regressive model of surface roughness is established, which is highly significant
for correlating the process parameters.
4.The surface roughness has a minimum value lower than Ra0.25µm, which corresponds to a best
combination of process parameters. To achieve a small roughness, a burnishing force from 125 to
275N, a low burnishing speed from 16.5 to 33.5m/min, a low feed rate from 0.05 to 0.15mm/rev and
with 2 or 3 passes for burnishing are recommended.
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794 Advances in Materials Manufacturing Science and Technology
Advances in Materials Manufacturing Science and Technology 10.4028/www.scientific.net/MSF.471-472 An Investigation into Surface Roughness of Burnished Hypereutectic Al-Si Alloy Based on the Taguchi
Techniques 10.4028/www.scientific.net/MSF.471-472.790
DOI References
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doi:10.1016/S0924-0136(03)00750-7 [5] M.H. El-Axir and M.M. El-Khabeery: J. Mater. Process. Technol. Vol. 132 (2003), p. 82
doi:10.1016/S0924-0136(02)00269-8 [6] G. Timmermans and L.Froyen: Wear Vol. 230 (1999), p. 105
doi:10.1016/S0043-1648(98)00336-6 [9] W. Yang and Y. tarng: J. Mater. Process Technol Vol. 84 (1998), p. 122
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