EFFECT OF END LOAD AND TOOL SPEED ON TORQUE AND TEMPERATURE OF FRICTION DRILLING PROCESS: AN EXPERIMENTAL STUDY

BY: VARUN KUMAR

SHYAMAL SATODIA

IntroductionCurrent sheet metal joining techniques include:Arc weldingResistance spot welding

These operations have drawbacks since:Extra material is added, and quality issues are inevitableAluminum alloys, integral to automotive industry today can not be welded

easily

Introduction (Contd.)Friction drilling is a new alternative for joining sheet bodies.Also known as thermal or flow drilling, the heat generated due to friction between rotating tool and work piece is used to soften, penetrate and plastically deform the material to create a hole with a process generated sleeve, to be used for tapping later.A conical shaped Silicon Carbide tool is used for the process due to high working temperature.The height of the bush is almost twice the thickness of sheet metal

Friction Drilling process Step 1: Approaching conical tool Step 2: Penetration of tool tip

into softened work piece Step 3: Material deformation due

to heating Step 4: End of drilling process and

formation of collar by shoulder Step 5: Tool retraction

Research backgroundAuthors/year Parameters studied Limitation of the study

Streppel and Kal, 1983 Variations in end load, frictional moment and tool wear

A theoretical model for calculation of thrust force, torque, frictional moment and wear rates missing.

Kerkhofs et. al, 1994 Effect of tool coating of wear resistance, nature of tool failure

Absence of Tool wear prediction time and tool failure prediction model

Scott Miller et al. (2005-2007) Torque generation, thrust force model, heat generation rate, microstructural changes in work piece and tool, FEM modeling

Nature of coefficient of friction undefined, tool feed not considered in torque models, model for temperature field absent, relationship between end load and process output missing.

Qu and Blau, 2008 Model to calculate friction coefficient and shear stress

Relation between end load and process output parameters.

Research motivation Absence of a correlation between the applied end load and tool speed on dependent variables of the process like torque, heat generation and temperature.Lack of understanding about the nature of contact prevailing during drilling operations leaves doubts about the applicability of prior models developed.

Theoretical background Deformation of metal takes place due to development of shear zone at tool- workpiece interface Three different kinds of contact exist in thermal drilling process: sticking condition, sliding condition and partial sticking+sliding condition.The relationship between contact pressure and shear stress is as follows in sticking condition is

Where, p = contact pressure, μ = coefficient of friction

shear p

Theoretical background (cont.) Torque developed for a conical tool is given as:

Rate of heat generation in conical tool is given as:

Temperature field in workpiece can be evaluated using

3 3 22 12 (h h ) tan

2

3cos2

pT

3 3 32 1(h h ) tan2 2( )

3 sin 2

pq

( ) 20 0

2(( ( v) ).r)2

v y vtq HT T e Ykg g

FEM model To evaluate temperature generated in workpiece during thermal drilling, a FEM model was used.The boundary conditions for the model include:

Surface convection on top and bottom surfaces of plate Surface radiation from top of plate Dirichlet boundary conditions on side walls of the plate

Heat generated by the tool was applied in steps as the tool advances through the material to observe transient temperature field.

Deprag Flowform Screwdriving (FFS) • AISI 1020 steel tool was used and Al 6063 alloy was used

as work piece• ThermoVision A40 Infrared Camera was used for

temperature measurement• Two different samples of 3.4 mm and two 1.3 mm (used

together as 2.6 mm) were used• The top surface of the work piece was painted black to

increase the emissivity of surface for temperature detection from camera

Experimental Setup

Workpiece holder

Tool holder

Infrared camera

Experimental procedure 6 tests were conducted with for different conditions of speed and end load. Each condition was repeated with three samples to nullify process and material variations.Test conditions have been shown in Annex ANo lubricant was used during the processIR camera was used to generate the temperature variation while the servo motor mechanism in the drilling machine generated the torque variation during process.

Results from experimentCycle time is calculated between points A and BAlso, two peak torque locations were obtained for data points 1 and 2 corresponding to drilling and tapping operation respectivelyThe average cycle time (average of three samples tested for each condition) have been shown in table

A 

B

1

2

Speed, (rpm)

End load, (N) Thickness, (mm)

Time(Sec)

6000 575 2.6 2.106000 650 2.6 1.0885000 650 2.6 2.366000 700 3.4 1.7286000 950 3.4 2.7765000 950 3.4 3.87

Results (Contd.) Hypotheses H1a and H2a: Effect of end load and torque on cycle time.

The experimental conditions are shown in table below

The plot clearly shows that frictional torque experienced by the tool increases as end load is increased.

Speed, rpm End load, N Thickness, mm

6000 575 2.6

6000 650 2.6

Results (Contd.) Hypothesis H1b: Effect of rotational speed on torque

Test conditions are shown in table below.

It was observed that increasing speed indeed reduces the frictional moment experienced by the tool

Speed, (rpm) End load, (N) Thickness, (mm)

6000 950 3.4

5000 950 3.4

Results (Contd.) Hypothesis H3a: Effect of end load on temperature Test conditions are shown in the table below It can be seen that higher end loads reduce the

maximum temperature attained in the materialSpeed, (rpm) End load, (N) Thickness, (mm)

6000 575 2.6

6000 650 2.6

FEM ResultsAlthough the FEM model was not a complete representation of drilling process, its results could still predict the maximum temperatures

Test conditions were 650 N end load and 6000 rpm speed.

A maximum temperature of 197 can be seen in the ℃workpiece. However, the maximum temperature observed in experiment was 206 deg.C for test condition with speed = 6000 RPM and end load = 650 N.

Joint Quality Process parameters for drilling affect the quality of the drill to a great extent

The bushing of the left hole is comparatively smoother and of good quality as compared to the one on the right which shows the material of the bushing deformed in a very abnormal fashion

Future scopes Friction drilling process finds a huge scope in automotive industry for metal joining process in chassis and body

Joint quality is much better as compared to resistance spot welding as the plate itself has a threaded bushing for fastening

Its very clean process as its void of any lubrication which thereby do not contaminated the work piece

Annexure A- Experiment test conditions

Test no. End load(N)

Speed(RPM)

Sheet thickness(mm)

1 575 6000 2.6

2 575 6000 2.6

3 575 6000 2.6

4 650 6000 2.6

5 650 6000 2.6

6 650 6000 2.6

7 950 6000 3.4

8 950 6000 3.4

9 950 6000 3.4

10 700 6000 3.4

11 700 6000 3.4

12 700 6000 3.4

13 650 5000 2.6

14 650 5000 2.6

15 650 5000 2.6

16 950 5000 3.4

17 950 5000 3.4

18 950 5000 3.4

Friction drill Tool • The tool has 5 regions each playing a

significant role in the processCenter region: Tool penetrationConical region: Heat generating regionCylindrical region: Shapes the holeShoulder region: Collar formation in the bushShank region: Tool holding• There can be threaded tool which

relegates the need of separate tapping