output of the laser system itself may be monitored with power meters and beam profilers to ensure correct delivery of power to the workpiece. Some sophisticated laser systems tightly integrate laser power delivery with robot motion to further reduce potential process errors.


It’s a generally sound philosophy to try to predict and design out problems rather than react to them, but pre-process checks can’t stand alone. No pre-process measurement can capture all the variables that may influence results, so verification of the results of the process is essential.


Tried-and-true post-process inspection methods such as manual visual surface inspection and destructive testing are still favorites of many laser users who favor their intuitiveness and robustness. Challenges arise due to time, cost and expense, however. Visual inspection provides limited information, and the best measurements from destructive testing can only be practically obtained from a small fraction of finished welds; sometimes at an enormous cost in labor, scrap and lost production.


Automated post-process non-destructive testing exists in the form of x-ray CT, ultrasound and magnetic flux leakage. X-ray builds up highly detailed three-dimensional images of finished welds, including subsurface features, but is too expensive and time- consuming for all but the most specialized applications. EMAT ultrasound uses electromagnetic coupling to both produce and detect an ultrasound source inside ferrous materials. Magnetic flux leakage detects subsurface defects by measuring regions in which magnetic field lines “leak” out of the part as they skirt around voids in the material.


With downstream post-process checks, the most complete information (from sectioning and CT) is also the most difficult to obtain. Destructive testing can only be used on a small fraction of parts, none of which are serviceable after the fact. As an added complication, the further downstream the check is performed, the higher the value of the scrapped parts when defects are found.


A good balance of time and cost savings is found by concentrating checks in-process, using automated sensing equipment. In- process checks can also be sorted into pre- in- and post-weld groupings. In this case the leading (pre) and trailing (post) measurements happen close to the welding process, typically while the welding beam is on. Some in-weld measurements look directly at the point of contact between the welding beam and the material in order to directly sense process dynamics as they unfold.


Sensors that lead the process during welding carry an advantage over earlier pre-process checks in that they are placed at a confluence of weld quality pre-determinants. These sensors can catch errors caused by stock tolerances and fit-up, fixturing and motion control, often with the same measurement. Examples of this kind of sensor include laser triangulation and camera-based systems for seam following. The position of the seam is used in a feedback loop to correct the weld path on the fly.


Trailing sensors allow the finished weld to be assessed before any further value is added to the part, avoiding expensive scrap further downstream. Both ultrasound and magnetic flux leakage are good candidates for immediate, on-line inspection. Laser triangulation is also a popular choice to measure surface topography of the finished weld bead.


For measurement of the process itself, relatively few sensing options exist. The weld process produces intense light across a wide region of the spectrum, blinding traditional cameras without specialized filters. Photodiode sensors make use of these emissions by measuring different bands of optical radiation from the process zone; backscattered light from the welding laser, radiation from the weld plume, or blackbody emissions from the melt region can all be used to assess the weld process. The challenge when implementing these indirect measurements lies in determining which signals correspond to an in-spec weld. The teach-in process for such sensors typically involves lengthy comparisons with destructive testing, and once this stage is complete, the process conditions must remain stable for the sensor to function properly. The relationship between the light coming from process and the shape of the finished weld is complicated.


Thermography is another in-process sensing method that maps the distribution of heat on the surface of the melt pool and weld seam, in order to draw conclusions about subsurface features (e.g., fusion in a lap joint).


Indirect measurements are a useful litmus test for determining whether a process is behaving consistently, but they have their limits. The data often doesn’t provide enough to detail to point to a specific failure mode, or to control process parameters in response to measurements.


Inline coherent imaging (ICI), an emerging in-process measurement technology patented by Laser Depth Dynamics, measures weld penetration directly at the point where the process laser interacts with the workpiece. This technology is natively compatible with select modern welding heads (e.g., Laser Mechanisms FiberWELD), and retrofits are possible with


(Continued on page 18) www.lia.org 1.800.34.LASER 17


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