Characterization of Net Heat Input in Laser Processing

1
Characterization of Net Heat Input in Laser
Processing

• Guofang Zhou zhouguo1_at_iit.edu
• T. Calvin Tszeng tszeng_at_iit.edu
• Philip Nash nash_at_iit.edu

Oct. 17th, 2003
2
Objective

• To quantify the net heat input from the laser
  into the workpiece
• Laser power density profile
• Absorption rate of the sample

3
Approach
Back surface mounted thermocouples can avoid the
influence the absorption on the top surface.
Determine the surface heat flux which minimize
the differences between the calculated
temperature and measured ones from the
thermocouples
4
Approach Validation
Assume the heat flux at the surface to get the
temperature histories from the locations at the
back surface
25000 kW/m2
12500 kW/m2
12500 kW/m2
0.91 mm
1
4
5
6
2
3
22 mm
Material SS 410 Heating Period 0.31 second
5
Temperature histories of 6 points obtained from
HOTPOINT to act as the input for the inverse
calculation.
6
Calculated Surface Power Density
Very small RMS Error
25000 kW/m2
12500 kW/m2
12500 kW/m2
7
Experiments at Alion
Laser Source
1500 W Laser Beams
94 mm
22 mm
0.91 mm
12 mm
SS 410 strip
Thermocouple wires labeled 1,2,,7 from left to
right
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Temperature Histories
Sampling Rate 500 Hz
2 mm
5 mm
5 mm
2
3
4
5
6
7
1
Failed
Thermocouple Locations
9
Preliminary Results
10
Power Density History in Heating Period
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Observations

• The absorption rate is around 10.
• The absorbed heat flux into the sample is
  relatively steady.
• The fluctuation of the very first 0.04 second is
  to be investigated.
• Possible Reasons
• 1. Time lagging of thermocouple measurement
• 2. Step size in the calculation

12
Time Lagging of Thermocouples Measurement
h

• Given a heat transfer coefficient (h) on the top
  surface
• The other boundaries are insulated

2.5
2
Heat Transfer Coefficient (kW/m2-K)
Surface Temperature (C)
0
950 C
500
M.S. Thesis, K. Gummadam
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Time Lagging of Thermocouples Measurement
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Conclusion

• Inverse calculation is very sensitive to the
  fluctuation of the measured temperature history
  especially at very high heating rate.
• Time lagging of thermocouple measurement can be
  an factor.