Methods and systems for characterizing laser machining properties by measuring keyhole dynamics using interferometry
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Publication Number
US-9757817-B2
Publication Date
2017-09-12
Expiration Date
Abstract
A method, apparatus, and system are provided to monitor and characterize the dynamics of a phase change region (PCR) created during laser welding, specifically keyhole welding, and other material modification processes, using low-coherence interferometry. By directing a measurement beam to multiple locations within and overlapping with the PCR, the system, apparatus, and method are used to determine, in real time, spatial and temporal characteristics of the weld such as keyhole depth, length, width, shape and whether the keyhole is unstable, closes or collapses. This information is important in determining the quality and material properties of a completed finished weld. It can also be used with feedback to modify the material modification process in real time.
Core Innovation
The invention uses low-coherence interferometry to image a laser-machined phase change region (PCR) in real time. An imaging beam is directed to multiple positions relative to the processing beam, including leading, aligned, and trailing (offset/lag) positions within and around the PCR. Interferometry output is processed to determine PCR characteristics.
The PCR characteristics determined from the interferometry output include keyhole (capillary/vapor channel) depth, length, width, and shape. The approach also enables detection of keyhole collapse or instability and differentiation of liquid versus solid behavior. It further supports identification of weld-depth spiking linked to porosity and detection of transients such as zinc vapor explosions.
The document further describes system-level features that improve measurement quality and robustness, including calibration/compensation techniques based on top surface reference points (TSRP) and optical path-length compensation. These include determination of TSRP and compensation for gas-pressure-induced path-length changes and scanning-induced path-length changes. The implementation additionally includes multiplexing of interferometer/reference arms and, optionally, closed-loop feedback to laser process control.
Claims Coverage
Not explicitly described in patent.
Not explicitly described in patent.
Stated Advantages
Improved signal-to-noise ratio (SNR) and measurement stability when imaging beam positions are offset/lag relative to the processing beam.
Real-time determination of phase change region (PCR) characteristics including keyhole geometry and keyhole collapse/instability.
Capability to detect transients such as zinc vapor explosions and to link spiking to weld porosity.
Robust interferometry measurements supported by calibration/compensation, including TSRP determination and optical path-length compensation.
Optional closed-loop feedback to laser process control.
Documented Applications
Real-time weld quality assessment by imaging a laser-machined phase change region (PCR) and determining keyhole characteristics and related stability/instability behavior.
Optional closed-loop laser process control based on PCR characteristics determined from low-coherence interferometry outputs.
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