Advanced Production processes Assessment
Manav Jain #7638831 BEng (HONS) Mechanical Engineering Date of Laboratory: 01-03-2013

Contents
Introduction ............................................................................................................................................ 2 System A.................................................................................................................................................. 2 Theory ................................................................................................................................................. 2 Procedure............................................................................................................................................ 3 Results and discussion ........................................................................................................................ 4 Effect of changing the number of pulse and how these results change with laser fluence, F ....... 4 Change of etch rate with increasing fluence .................................................................................. 4 Threshold fluence, FT and absorption coefficient, α ....................................................................... 5 System B.................................................................................................................................................. 6 Theory ................................................................................................................................................. 6 Results and discussion ........................................................................................................................ 7 Effect of changing the speed on the laser cutting process ............................................................. 7 Effect of changing power on the laser cutting process................................................................... 7 Change in heat affected zone width against power ....................................................................... 8 Conclusions ............................................................................................................................................. 9 System A.............................................................................................................................................. 9 System B.............................................................................................................................................. 9

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Introduction
The following report is an experimental demonstration of the working of two laser machining systems. System A is a UV excimer laser system which is a low power laser system with applications in manufacture of micro-chips and applications for human laser lens corrections. System B represents an IPG fibre laser system which has applications in manufacturing of a wide range of metal components in industry.

System A
Theory
System A is a UV excimer laser system (500W, 200Hz, 15ns, 248nm) and is shown by figure 1. The generalised layout of the UV excimer laser system is shown by Figure 2. The laser beam is formed by the laser power supply and diodes and is made to pass through the required aperture /mask. The beam is now in the required shape and is made to follow the required path with the help of mirrors such that the laser hits the lens and converge to form the image on the work piece in order to machine it in the required pattern.
Figure 1: Experimental setup Figure 2: Instrumentation layout

Start-up procedure for this system was simple. It was made sure that the work piece (polymer) was in place on the stage ready for micro machining. The stage (and hence the position of the laser on the work piece) was controlled using a numerical control program fed in using the computer. The number of pulses per second was also fed in this way. Required mask/aperture was placed in the path of the beam in order to get the required pattern of micro-machining. Finally, for the safety of the user, the machine setup was enclosed in a glass enclosure in order to protect the

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user from exposure to UV radiations from the laser. And the door to the enclosure was closed at all times during the machining process and opened only when the laser was turned off. Before the machining process required is carried out, the distance between work piece and lens must be set such that the work piece is on the image point of the lens at all times. This was carried out by the following method.

u

f

(v-f)

f=10cm

v

Figure 3: Finding the image point of the laser system

Firstly, the lens maker’s formula was used to find the u and v shown in figure 3. This was done using the magnification prescribed to be 1/7 and the focal length of the lens which was 10cm. Hence, the following system of equation gave the values of u and v. Magnification, And lens maker’s formula →

This value of v is the theoretical value. The stage is set at this value of v and a program is made to increase the value of v in step changes. The image point of the laser is now obtained by making the laser hit the sample at step changes of the stage moving down after each impression. The sample was then examined visually and the position of the impression closest to the expected image of the laser was noted and this was defined as the image point of the laser for the experiment. The experimental value of v was hence taken to be 14.1cm. Note: The examining of the impressions at the different steps was extremely rough as it didn’t make a difference over the purpose of this experiment. More precise measurements could be made by increasing the number of steps and decreasing the increment of each step.

Procedure
After initial set-up and safety methods, a program was run in order to direct the laser controllers to run the experiment to study the change in depth with increase in number of pulses and also the effect of the change in energy of laser (fluence). After this, the depth of impressions in each of the holes and the area of the impression were measured with the use of a digital microscope. The results were recorded and plotted in the form of graphs. 3|Page

Results and discussion
Effect of changing the number of pulse and how these results change with laser fluence, F Laser fluence, F is given by the following relationship:

Hence, it is seen that laser fluence is directly proportional to the energy of the laser as the area of the laser spot is assumed to remain constant throughout the experiment. The following graph shows the variation of depth with an increase in the number of pulses (time spent on the spot). This is expected to be linear for a constant energy laser and the graph demonstrates this baring experimental errors.

Graph 1: Depth of impression vs Number of pulses E1 (34mJ)
700 depth of impression ( µm) 600 500 400 300 200 100 0 0 20 40 60 80 100 number of pulses
E2 (32mJ)

y = 8.1933x y = 7.2017x y = 5.7483x y = 4.5683x y = 4.0833x
E3 (29mJ) E4 (27.5mJ) E5 (24mJ) Linear (E1 (34mJ)) Linear (E2 (32mJ)) Linear (E3 (29mJ)) Linear (E4 (27.5mJ)) Linear (E5 (24mJ))

Graph 1 also shows demonstrates that as the energy of laser is reduced the depth reduces and the graph becomes less steep (lowering the gradient).Hence, the depth of impression decreases with decreasing laser fluence. Change of etch rate with increasing fluence The Etch rate, X is defined as the gradient of the graph 1 for a given energy of laser.

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The Etch rate for each of the energy’s used is found by the slope of the linear approximation of the lines found experimentally. The linear lines must pass through zero as at zero pulses the depth of the impression must also be zero. Graph 2, shows the variation of etch rate against the natural log of fluence.
Etch rate (µm/pulse)
9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00

Graph 2: Etch rate vs Ln ( fluence )

Threshold fluence, FT and absorption coefficient, α The plot of Etch rate verses the natural log of fluence is expected to be linear and the interception of this linear (best fit) line on the x axis as shown in graph 2 gives the value of the natural log of the threshold fluence (ln (FT)). From the graph, ln (FT) = 0.11

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 ln (FLUENCE)

⁄

Absorption coefficient (α) is determined by the following equation: ( ) From the graph, Where X is the Etch rate and F is the corresponding laser fluence.

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System B
Theory
System B is an IPG fibre laser system (1kW, CW, 1070nm) shown by figure 4. Figure 5 is a generalised layout of the complete system and shows the various components involved. The fibre laser runs through the fibre bundles into the fibre collimator which then directs the laser beam into the lens. The lens focuses the laser onto the mild steel sample, melting it. The molten metal is then removed by the high pressure gas which follows the laser path from the nozzle hence leaving a kerf. The parameters of the laser can be changed to get the required minimisation of kerf. Similarly HAZ is also formed on the sample and these formations along with dross and spatter are shown by
Figure 4 : Experimental setup system B

figure 6. The mild steel sample sits on a high speed stage which is numerically controlled by the connected computer and the laser system operates on G codes. For the health and safety of the users and surroundings, two systems are employed. Firstly, the system was enclosed in a mild steel enclosure; this is done because mild steel is a good absorber of radiations. Secondly, a vent was placed near the work piece in order to suck in all the metal oxides and other toxic fumes and gases which could result depending on the work piece and gas used as some of these can be poisonous.

Figure 5: layout of IPG fibre laser system

Figure 6: Characteristics of the cut

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Results and discussion
Effect of changing the speed on the laser cutting process The speed of the laser is the speed at which the stage is moved in order to create the cut. This speed can be altered in order to get an optimum speed for the required cut based on all other parameters being constant. Theoretically, as the cutting speed is increased, the kerf width would decrease and should hence be a better result. Although, increasing the speed also results in a possibility of not forming a complete cut, wherein the bottom face would not get cut although the top would. The possibility of splatter would also increase with increasing cutting speed. Figure 7 shows the cuts formed due to the various cutting speeds used. It is seen that when the resultant cut was the best when the cutting speed was 20mm/s as this showed characteristics of true cut and minimum kerf width compared to the cuts at other speeds.
Figure 7: effect of changing the cutting speed

Effect of changing power on the laser cutting process As the power is increased the kerf width is expected to increase. Graph 3 shows the effect on the kerf width on the top and bottom surfaces as the power is increased for each of the gas mixtures used, namely, nitrogen and oxygen.

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Graph 3 : kerf width vs power
300 250 kerf width (µm) 200 oxygen-top face 150 100 50 0 0 200 400 600 800 1000 power (W) nitrogen-bottom face oxygen-bottom face nitrogen-top face

It is inferred from graph 3 that for oxygen, the kerf width of bottom and top are extremely close at the power of 300W and as the power is increased the difference between the kerf widths of these surfaces increases. Hence the cut starts to taper as the power is increased. However for the case of nitrogen there is considerable amount of taper at 300W but this reduces to a minimum at 500 W after which the two lines (top face and bottom face) continue to go apart hence increasing the taper. In general the values of kerf width when nitrogen is the gas used are lesser than those for the case with oxygen being the gas. This is because when the gas mixture is oxygen, the heat generated is due to two processes, the oxidisation of metal in addition to the power of laser. This leads to more metal melted in the case of oxygen as the mixing gas compared to nitrogen which is inert when it is supplied to a metal. Change in heat affected zone width against power Graph 4 shows the effect of increasing power and changing the gas used, on the heat affected zone. It is inferred from graph 4 that for the case with nitrogen, the top surface is not affected and has no heat affected zone although when the worked piece is flipped it is observed to have spatter and heat affected zone. This is much greater than the heat affected zone present in the case with oxygen as the gas used. In the case of oxygen both top and bottom surfaces have heat affected zones, spatter and the bottom surface also has dross formations. It is also seen that the heat affected zone is rather constant with minor fluctuations as the power is increased for the case of oxygen as the gas mixture used compared to a greater increase seen when nitrogen is the gas used.

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Graph 4: Heat affected zone width vs power
800 700 600 HAZ (µm) 500 400 300 200 100 0 0 200 400 600 800 1000 power (W) nitrogen - back face oxygen - front face oxygen back face

Conclusions
System A
 Linear relationship between the depths of impression to the number of pulses was captured and the gradient (etch rate) was found for each of the lines representing different energy of laser.  The etch rate was found to be linearly proportional to the natural log of fluence and the threshold fluence was found by the extension of the linear plot on the x axis and this was used to find the absorption coefficient .

System B
   The increase in cutting speed resulted in narrower kerf width, although as the speed was increased further the property of having a complete cut was lost. The increase in power resulted in a wider kerf width in general. In the case of oxygen as the gas chosen, the values of kerf width were higher than those found for the case with nitrogen as the gas chosen. This was accounted for by the exothermic reaction of metal oxidation which added heat to the metal.   In case of nitrogen, the angle of taper was a minimum at the power of 500W whereas in the case of oxygen the angle of taper was a minimum at a power of 300 W. The heat affected zone on the top face in the case of nitrogen as the gas was consistently zero as the power was increased whereas for the case with oxygen, this was at a constant of around 200µm through the increase in power.

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

The heat affected zone on the bottom face in the case of nitrogen was the highest of all other faces and increased as the power was increased whereas for the case of oxygen as the chosen gas the HAZ at the bottom surface was slightly higher than that on the top surface of the same with the value varying around 350µm as the power was increased.

References
1. Laboratory hand-out and lecture slides 2. http://www.sciencedirect.com/science/article/pii/S0379677911001147 3. http://www.sciencedirect.com/science/article/pii/S0924424704000901

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