Experimental and statistical investigation of thermo-mechanical friction drilling process/Termomechaninio frikcinio grezimo proceso eksperimentinis ir statistinis tyrimas.

Krasauskas, P.

1. Introduction

One of the actual problems in the manufacturing engineering is related to the assembly of the sheet metals, thin-walled tubes or profiles. These tasks could be performed using friction drilling technology, which enable to simplify assembly process and to improve reliability of the joint.

Friction drilling is nontraditional metal treatment method, used to produce holes in the thin-walled sheet metal for assembly of various structural elements. This method enables to eliminate additional manufacturing like welding countless nuts or assembly using J-nuts.

A rotating punch-type tool is forced into the material, the heat generated by the friction, heats the surrounding area, the material become plastic and forms cylindrical hole without metal removal. The tool penetrated into the material pierce a hole and the excess of the material forms the neck on the underside and collar on the upside of the sheet, increasing the wall thickness and strength of a hole.

Typical friction drilling steps and the movements subjected to the tool are showed in Fig. 1.

[FIGURE 1 OMITTED]

Friction drilling process investigation overview has showed that during drilling workpiece temperature can increase up to 600[degrees]C and tool--up to 650-750[degrees]C [1-3], meanwhile tool penetration force depends on drilling regimes and shape of the tool and various in very large limits.

However, the influence of mechanical properties and chemical composition of the materials on drilling process, as complex, is not investigated.

The aim of this work was to investigate the influence of materials mechanical properties, drilling regimes and plate thickness on axial drilling force and torque in order to optimise drilling regimes.

2. Materials and workpieces

The experiment was performed using three various sheet materials:--hot rolled S235 steel, AISI 4301 stainless steel and Al 5652 aluminium alloy. The chemical composition, mechanical properties and dimensions of the workpieces are presented in Tables 1-3.

3. Experimental technique

The experiment was performed on a CNC milling machine "DMU-35M" with controller "Sinumerik 810D/840D" using tungsten carbide tool with diameter of 5.4 mm. The shape of the tool is showed in Fig. 2, dimensions --in Table 4.

Drilling program was written using "Shop Mill" software, which enable to simulate drilling time and to change drilling regimes in expeditiously manner.

During the experiment drilling force was measured using rearranged standard force dynamometer DOSM-1M, the measurements results were recorded to the computer via oscilloscope "PICO ADC-212 (Fig. 3).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

4. Experimental results

The experiment was planned according the course: spindle rotational speed set of 2000, 2500 and 3000 rpm was selected and for each ones drilling feed ratio set of 60, 100 and 140 mm/min was assigned.

The analysis of the experimental data showed that axial force, from the initial contact to the collar forming, varies in very large limits.

The example of force and temperature records and the same records presented in the force and temperature units are showed in Fig. 4.

[FIGURE 4 OMITTED]

It was defined that independently of cutting regimes, forming force reaches its maximal value when the conical section of the drill penetrates into the material ("c"--step, Fig. 1); when the sheet is pierced, the actual force drastically decreases ("d"--step) and increases again when the collar on the upper sheet surface is formed ("e"--step).

The experimental curves of the axial force variation during drilling for hot rolled S235 steel is presented in Fig. 5, for AISI 4301 stainless steel--in Figs. 6 and 8 and for Al 5652 aluminium alloy--in Fig. 9.

[FIGURE 5 OMITTED]

It was founded that maximal drilling force [F.sub.max] proportionally depends on feed ratio FR and sheet thickness t :--the bigger FR and t calls bigger forming force and conversely depends on rotational speed S, because higher drilling speed causes higher temperature in the contact zone between tool and workpiece, as a result the piercing force is needed lower.

The actual drilling torque was not measured, therefore for ones calculation, special experiment comprised step by step holes drilling in the plates with the thickness of 1, 1.5 and 2 mm, with the feed step of 0.5 mm was performed. Thereafter, the plates using wire electrodischarge machining technology (EDM) were cut throw the centres of the holes in order to define actual surface contact area between workpiece and tool (Fig. 7).

Referring to [1, 2], it was defined that maximal torque results when the tool conical section is fully pierced into the sheet, therefore drilling torque was calculated using truncated cone model (Fig. 10).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Equations for axial force F and torque T for truncated conical surface are expressed

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2)

where t is the plate thickness, mm; [mu] is friction coefficient; p is the pressure in the contact zone, MPa; r is the surface radius, mm; [theta] is the angle of truncated conical section ([theta] = 30[degrees]); A is the contact surface area between tool and workpiece.

The value of friction coefficient was set 0.4 for steel and 0.5--for aluminium alloy [2-5]; the pressure was calculated from the yield stress condition in the contact zone.

5. Design of experiment

In this stage of investigation, the influence of drilling regimes and mechanical properties of the materials to the maximal axial force [F.sub.max] and torque [T.sub.max] was performed.

In order to obtain the relationship of mechanical properties and drilling regimes on drilling parameters [F.sub.max] and [T.sub.max] and to obtain regression model which in the best way could explain mechanical properties of the materials and drilling parameters influence on axial force [F.sub.max] and torque [T.sub.max] variation, the multivariable regression analysis was carried out.

Experimental matrix, on which base regression analysis was performed, is presented in Table 5.

Statistical evaluation of the experimental data was performed using "Excel" function "Data Analysis", which performs error of estimate, average deviation, maximum deviation for any observation, explained proportion of variance ([R.sup.2]), adjusted coefficient of multiple determinations, F-value, Prob. (F), Prob. (t) and performs analysis of variances. Estimation of applicability of used models was based on the coefficient of maximum deviation [R.sup.2] and F-value, because these parameters are acceptability criteria of model adequacy to the experimental data.

If the intervals of factors variation are tenuous, iterations can be limited by linear approximation

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3)

where [a.sub.n] are unknown parameters of the model (regression coefficients); n = 1, 2, 3, ..., i are the factors of influence; [X.sub.1], [X.sub.2], [X.sub.3], ... [X.sub.1] are independent variables.

Referring to this, regression analysis was performed making presumption that drilling force and torque are stipulated as the entirety of mechanical properties of the materials--yield limit [[sigma].sub.y] and ultimate strength [[sigma].sub.u], drilling regimes--spindle rotational speed S and feed ratio FR and sheet thickness t as total action of them and could be expressed by five variable regression model for [F.sub.max] and [T.sub.max] respectively

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (5)

Summary output, analysis of variance, parameter values and comparative five variable linear regression analysis for maximal axial drilling force and torque are presented in Tables 6 and 7.

Regression analysis showed that five variable linear regression model with 96% probability describes experimental [F.sub.max] data and the hypothesis of influence of the factors, introduced into regression model with 5% significance level is accepted, because [F.sub.max] = 64.0 > [F.sub.0.05] = 2.901.

The same regression analysis with respect to drilling torque [T.sub.max] showed similar probability results: [R.sup.2] = 0.84 and F = 32.1 > [F.sub.0.05] = 2.901.

Research enabled to conclude that presented models Eqs. (6) and (7) with 95% and 92 % probability for [F.sub.max] and [T.sub.max] respectively (confidence coefficient [alpha] = 0.05), reasonably explain experimental data variation, so drilling force [F.sub.max] and torque [T.sub.max] dependence upon material mechanical properties, drilling regimes and thickness of the workpiece can be expressed

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7)

ANOVA results showed that sheet thickness, yield limit and feed ratio are significant parameters that most intensively affect [F.sub.max]; meanwhile [T.sub.max] significantly influences feed ratio and material mechanical properties--yield limit and ultimate strength. Contrary to expectation, spindle rotational speed has no valuable influence on drilling regimes variation.

The coincidence of the experimental and calculated [F.sub.max] and [T.sub.max] values enabled to conclude that regression models Eqs. (6) and (7) could be used to optimise friction drilling process for wide spectrum of the structural materials.

6. Conclusions

The investigation of friction holes drilling with various cutting regimes showed that biggest drilling force was given when conical section of the tool penetrates into the sheet; when the sheet is pierced force significantly decreases, but torque reaches its maximal value.

The analysis of spindle rotational speed influence on axial force variation showed that minimal spindle speed (2000 rpm) calls bigger drilling force in compare to the higher speed (2500 and 3000 rpm); drilling feed influence on axial force and torque variation analysis showed that than bigger feed--than bigger axial force. The experiment showed that drilling force considerably depends on sheet thickness; therefore it should be considered optimising friction drilling process.

Probabilistic investigation of the influence of mechanical properties of the materials {[[sigma].sub.y],[[sigma].sub.u]), drilling regimes --tool rotational speed S, feed ratio FR and sheet thickness t on drilling parameters [F.sub.max] and [T.sub.max] showed, that proposed five variable linear regression model reasonably explain axial force [F.sub.max] and torque [T.sub.max] variation. ANOVA showed that sheet thickness t, feed ratio FR and yield limit [[sigma].sub.y] are significant parameters that most intensively affect [F.sub.max] and [T.sub.max], however spindle rotational speed S has less valuable influence.

Received March 02, 2011

Accepted December 15, 2011

References

[1.] Miller, S. F.; Jia, T.; Shih, A. J. 2006. Friction drilling of cast metals, International Journal of Machine Tools and Manufacture 46: 1526-1535.

[2.] Miller, S. F.; Rui, L.; Wang, H.; Shih, A. J. 2006. Experimental and numerical analysis of the friction drilling process, Journal of Manufacturing Science and Engineering 128 (3): 802-811.

[3.] Miller, F S.; Shih J. A. 2007, Thermo-mechanical finite element modeling of the friction drilling process. Journal of Manufacturing Science and Engineering 129: 531-538.

[4.] Buffa, G.; Hua, J.; Shivpuri, R.; Fratini, L. 2006. A Continuum based FEM model for friction stir welding-model development, Materials Science and Engineering, A419: 389-396.

[5.] Soundararajan, V.; Zekovic, S.; Kovacevic, R. 2005. Thermo-mechanical model with adaptive boundary conditions for friction stir welding of Al 6061, International Journal of Machine Tools and Manufacture 45: 1577-1587.

P. Krasauskas

Kaunas University of Technology, Kestucio 27, 443123, Kaunas, Lithuania, E-mail: povilas.krasauskas@ktu.lt

Table 1

Chemical composition of as--received sheet metal

Element, wt %   S235   AISI4301   Al 5652

C               0.2      0.08        --
Si              1.0       --        0.25
Mn              1.0       --        0.011
Cr               --      0.17       0.2
Ni              0.5       --         --
Mg               --       --      2.2--2.8
Cu               --       --        0.04
Zn               --       --        0.25
P               0.04     0.04        --
Fe               --       --        0.4
Ti               --       --        0.2

Table 2

Mechanical properties of the metal

            Ultimate strength      Yield limit            Elongation
Material    [[sigma].sub.u], MPa   [[sigma].sub.y], MPa   [A.sub.5], %

S235        430                    245                    20
AISI 4301   395                    225                    26
Al 5652     195                     65                    19

Table 3

Workpieces dimension, mm

Material    Thickness   Length   Width

S235        2.5
AISI 4301   1.5         350      60
            2
A15652      1.5

Table 4

Dimensions of the friction drill, mm

D1    D2   D3   L1   L2   L3   L4   L5   R     [[theta].sup.
                                               [degrees]]

5.4   8    11   11   14   7    5    6    0.5   30

Table 5

Experiment matrix and results

Material    Thickness   Spindle    Feed     Axial       Torque
grade         t, mm      speed    ratio,    force    [T.sub.max],
                         S, rpm   mm/min   [F.sub.        Nm
                                           max], N

                                     60      4892         1.76
                          2000      100      4469         2.08
                                    140      4072         2.52
                                     60      3401         2.45
S235           2.5        2500      100      3877         2.52
                                    140      3741         2.39
                                     60      3122         2.49
                          3000      100      3712         2.53
                                    140      4010         2.42
                                     60      2319         2.32
                          2000      100      2401         2.53
                                    140      2422         2.50
AISI 4301                            60      2126         2.42
               1.5        2500      100      2172         2.53
                                    140      2305         2.53
                                     60      1951         2.40
                          3000      100      2187         2.51
                                    140      2477         2.53
                                     60      3140         2.24
                          2000      100      4092         1.89
                                    140      3752         1.30
AISI 4301                            60      2954         2.46
               2.0        2500      100      3156         2.12
                                    140      3497         1.70
                                     60      2629         2.45
                          3000      100      2898         2.50
                                    140      3149         2.29
                                     60      1202         1.07
                          2000      100      1181         1.17
                                    140      1344         1.17
Al 5652                              60      1128         1.07
               1.5        2500      100      1161         1.17
                                    140      1358         1.17
                                     60      1091         0.86
                          3000      100      1293         0.92
                                    140      1232         1.17

Table 6

Regression statistics of mechanical properties and drilling
regimes influence on axial force and torque

                    [F.sub.max]   [T.sub.max]

Multiple R              0.96           0.92
R Square                0.91           0.84
Adjusted R Square       0.90           0.82
Standard Error          345            0.26
Observations             36

Table 7

ANOVA for mechanical properties and drilling regimes

Factor       df   1 SS     MS       1 F    Significance F

For axial force [F.sub.max]

Regression   5    3.82E7   7.63E7   64.0   4.27E-15
Residual     30   3.58E6   1.19E5
Total        35   4.17E7

For torque [T.sub.max]

Regression   5    10.74    2.15     32.1   3.52E-11
Residual     30   2.01     0.067
Total        35   12.74