Formation of Validating Experiment for Structure Simulation

4.3.1. Nd:YAG Disk Laser

The laser beam used in the second experiment is generated by a disk laser generator TruDisk 8002 from Trumpf as shown in Figure 4.11.

BIAS ID 162794

Figure 4.11: Nd:YAG disk laser generator TruDisk 8002, Co. Trumpf GmbH & Co. KG

The TruDisk 8002 disk laser generator uses Nd:YAG as its active medium. It combines the advantages of the active solid-state medium and diodes. The diodes, as the pumping source, provide excitation energy, ensuring high efficiency, while the disk-shaped active medium guarantees high beam quality. According to the manufacturer´s specifications, the reachable maximum nominal laser power is 8 kW with a laser wavelength of 1030 nm and a good beam parameter product of 8 mm·mrad. TruDisk 8002 operates in the continuous wave mode. The minimum cable diameter required to transfer the laser light is 200 µm.

4.3.2. Workpiece Dimensions and Fixation Workpiece Dimensions

The steel workpiece has a shape of a long thin rectangular plate whose dimensions are 50 mm wide, 200 mm long and 10 mm thick. The bottom surface of the workpiece is countersunk with a depth of 2 mm at three locations arranged as depicted in Figure 4.12.

The countersink angle is 120°. These three embosses are prepared for a specific supporting mechanism to be explained in the next subsection.

BIAS ID 162795

weld seam

200 10

2

12 12

30 30

A

A

2550 120°

A - A

66

Figure 4.12: Workpiece dimensions in the structure-simulation validating experiment

The weld starts 30 mm away from one edge and ends 30 mm away from the other. The total weld length is consequently 140 mm.

Workpiece Fixation

In order not to be influenced by unidentified effects on welding distortion, the workpiece-supporting mechanism must carefully be designed. Two criteria must be met. First of all, rigid fixation of the workpiece, introducing complex residual stress profiles, should be avoided. Secondly, welding-prompted temperature distributions developing on the workpiece should be as simple as possible. Based on these considerations, the workpiece holder as shown in Figure 4.13 has been developed.

Laser

y z x Welding direction

Workpiece

Ceramic ball

Holder

1

2 3

BIAS ID 162796

Figure 4.13: Workpiece supporting in the structure-simulation validating experiment

The workpiece holder is made up of aluminium embedded with three ceramic (ZrO2) spheres, having very low thermal conductivity and thermal expansion coefficient. When stationed on the fixture, the workpiece has no direct contact with the aluminium base but, instead, the supporting balls protruding above the base. Because of the ceramic´s weak thermal conductivity, heat can presumably be released from the workpiece surface solely via air convection. This leads to unsophisticated temperature contours on the workpiece during welding.

Regarding the mechanical load, no stress is applied on the workpiece during welding because of no fastening. However, the workpiece is not freely movable but restrained to the degree of movement freedom of each sphere, determined by the geometry of the milled slots in which the spheres stay. According to Figure 4.13, ball 1 can move only in the x direction, ball 2 cannot move at all and ball 3 can move in both x and y directions.

The workpiece will move in correspondence with those three balls during welding.

Installing this workpiece holder on the machine table during welding is simple just by clamping it on the table. This is also applied when the holder is installed on the displacement measuring machine.

4.3.3. Experiment Execution

The second experiment was carried out for the purpose of structure-simulation model validation. The time-dependent temperature profile and welding distortion were both expected experiment consequences. The applied values of the process parameters, e.g.

laser power, laser feed rate, focus position in relation with the workpiece surface, laser focal length, laser beam operating mode, used laser polarisation and shielding gas, in the structure-simulation validating experiment are listed in Table 4.4.

Table 4.4: Process parameters in the structure-simulation validating experiment

The experimental steps in this test were similar to those in the process-simulation validating experiment i.e. specimen preparation, measurement system setup and welding process conduction under process monitoring, consecutively. Nevertheless, details of each step from both experiments are totally different due to their distinctive objective. The former aimed at observing the weld pool geometry whereas the latter aimed at the component temperature distribution and distortion as mentioned above.

Preparation of the test specimen in this experiment was fairly complicated and carefully accomplished since the measured process data could be affected by this preparation. After completely machined, the steel plate was heat-treated to take out the residual stress, potentially influencing the pattern and degree of welding distortion. Figure 4.14 shows the temperature development in the annealing furnace measured by a thermocouple. The machined steel plate was placed in the furnace filled up with argon. The furnace was then heated up slowly to the temperature around 550°C and subsequently cooled down very slowly. Argon of 2 l/min was continuously fed to maintain the shielding gas atmosphere within the furnace. When the temperature had reached 100°C, the gas feeding was stopped and the furnace was opened to let the hot workpiece naturally cool down under

Laser power (W)

Feed rate (m/min)

Focus position (mm)

Focal length (mm)

Laser beam mode

Laser polarisation

Shielding gas

8000 3 -3 200 Multimode Circular Argon

the room temperature. In the end, the workpiece was coated with a thin film of graphite spray, needed for an accurate temperature measurement.

0 200 400 600

40

0 10 20 30

Time (h) BIAS ID 162797

Temperature (°C)

Figure 4.14: Temperature development in the stress-relief annealing

Prior to the experiment, the workpiece was measured its distortion and then set up on the welding machine table. Relative to the workpiece´s location, the thermographic camera, used to measure the surface temperature development during welding, was installed and calibrated. Figure 4.15 demonstrates how each experimental component was arranged.

Workpiece Clamp

Machine table Welding torch

Shielding gas jet

Workpiece holder

Thermographic camera

BIAS ID 162798

Figure 4.15: Experimental setup of the structure-simulation validating experiment

Finally, a single-pass laser welding operation was performed while the temperature development on the workpiece surface was being monitored and recorded by the thermographic camera.