Experimental and statistical investigation of thermo-mechanical friction drilling process

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.


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.Friction drilling process investigation overview has showed that during drilling workpiece temperature can increase up to 600ºC and toolup to 650-750ºC [1][2][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.

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.

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.http://dx.doi.org/10.5755/j01.mech.17.6.1014During 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).

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.
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. x ma F 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).
where t is the plate thickness, mm; μ is friction coefficient; p is the pressure in the contact zone, MPa; r is the surface radius, mm; θ is the angle of truncated conical section (θ = 30 ) ; 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][3][4][5]; the pressure was calculated from the yield stress condition in the contact zone.

Design of experiment
In this stage of investigation, the influence of drilling regimes and mechanical properties of the materials to the maximal axial force x ma F and torque max T was per- formed.
In order to obtain the relationship of mechanical properties and drilling regimes on drilling parameters x ma F and max T and to obtain regression model which in the best way could explain mechanical properties of the materials and drilling parameters influence on axial force x ma F and torque max T 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 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 2 and Fvalue, 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 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 y  and ultimate strength 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 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 x ma F data and the hypothesis of influence of the factors, introduced into regression model with 5% significance level is accepted, because x ma F = 64.0> F 0.05 = 2.901.The same regression analysis with respect to drilling torque max T showed similar probability results: R 2 =0.84 and F = 32.1 > F 0.05 = 2.901.
Research enabled to conclude that presented models Eqs. ( 6) and (7) with 95% and 92 % probability for x ma F and max T respectively (confidence coefficient α=0.05), reasonably explain experimental data variation, so drilling force ANOVA results showed that sheet thickness, yield limit and feed ratio are significant parameters that most intensively affect x ma F ; meanwhile max T significantly influences feed ratio and material mechanical propertiesyield 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 x ma F and max T values enabled to conclude that regres- sion models Eqs. ( 6) and (7) could be used to optimise friction drilling process for wide spectrum of the structural materials.

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.

P. Krasauskas EXPERIMENTAL AND STATISTICAL INVESTIGATION OF THERMO-MECHANICAL FRICTION DRILLING PROCESS S u m m a r y
This paper deals with the experimental investigation and analysis of the thermo-mechanical friction drilling process.Experiment technique is presented and described; experimental results of the thermo-mechanical friction drilling for hot rolled S235 steel, AISI 4301 stainless steel and Al 5652 aluminium alloy are presented and discussed.Statistical five variable linear regression analysis was performed in order to evaluate the influence of mechanical properties of the materials, drilling regimes and workpiece thickness on maximal drilling force and torque variation.Proposed multivariable linear regression models to optimise drilling process.

Fig. 1
Fig. 1 Friction drilling steps: a -initial contact; b -tool-tip penetration to the material; c -material flow; d -collar forming; e -tool withdrawal

Fig. 2
Fig. 2 Shape of the friction drilling tool

a
are unknown parameters of the model (regres- sion coefficients);

Table 1
Chemical composition of as-received sheet metal

Table 4
Dimensions of the friction drill, mm

Table 6
Regression statistics of mechanical properties and drilling regimes influence on axial force and torque investigation of the influence of me-