October 20, 2009
New developments in laser-based inspection systems offer tube and pipe mills real-time inspection of laser-welded seams. These systems can help to shorten setup time, improve weld quality, and reduce scrap.
Advances in laser welding processes for tube and pipe mills bring many advantages to companies that use this technology. Laser welding minimizes heat-affected zones, reduces distortion of the tube, eliminates slag or spatter, increases the weld's strength, and has more capability to handle dissimilar materials than traditional welding methods. In addition, filler material is unnecessary because the parent material is liquefied and then solidified during the laser welding process. With its quality and precision, it is particularly useful for welding stainless steel, aluminum, nickel alloys, and titanium.
With these advantages come the higher capital costs of laser welding equipment. This in turn leads to a need for real-time inspection to get the maximum possible return on the laser investment.
Many of the requirements of advanced monitoring needed for laser welding are related to measurement precision and timeliness of the results. If the mill process is set up to capture weld properties and metrics in real time, it can catch discontinuities early on, before they become faults and waste expensive feedstock. It can also reduce any potential inspection bottleneck at the end of the mill.
A laser-based inspection sensor is
Measurements are calculated using triangulation theory, whereby measurements between the laser emitter, the projected laser line, and the camera form a triangle.
Modern software uses mathematical algorithms that describe desired profiles for various parameters for a range of ODs and capture the information in real time. Standard math coprocessors and modern PCs are robust enough to handle data from hundreds of frames per second and can detect defects less than 0.5 mm in size.
Measurements presented on the operator screen have tolerances that are defined during mill setup.
Parameters measured cover common welding faults:
The system compares measurements from the images to those of an ideal weld profile. If the measurement approaches the tolerance limit, a warning alerts the operator. Similarly, the system indicates, visually and audibly, when the threshold for a parameter is breached.
Consider the activities that could be made more efficient with an inspection system that presents metrics within ±0.025 mm (0.001 in.). For example, after changing the mill to run a new size of tube or pipe, the operators run some of the new material through the mill, then inspect and test it. Skilled operators rely on manual inspection to find basic discontinuities and use test equipment such as eddy current or ultrasonic test systems to detect subtle flaws.
When a fault is found at the end of the line, the product must be repaired or scrapped. The expense and time spent on repairs and wasted materials add up. With an inspection system presenting data on several variables on-screen, an operator can modify equipment settings or weld parameters to correct a minor problem before it turns into a defect.
When a flaw detection system is used in combination with a laser marking system, the mill can run without halting while the operator makes adjustments to bring the weld parameters back into tolerance. The operator doesn't have to stop the mill, thereby increasing the mill's efficiency.
When a discontinuity does become a fault, the system displays an error message on-screen and can send an optional audible alarm and error signal to the PLC. An optional command can stop the mill.
Conceptualizing all of the mill's equipment as an integrated system reveals how a laser-based inspection system can fit in.
An Eddy Current Complement. Laser-based inspection complements other technologies used for weld inspection, such as eddy current testing. While laser-based inspection monitors the weld, eddy current inspection looks at the tube or pipe's cross section. Eddy current testing is optimized for surface-breaking conditions and flaws just beneath the surface, ones produced from sudden changes in the weld seam. The additional set of "eyes" laser-based inspection provides can quantify a discontinuity that the eddy current system may not be able to interpret.
Sharing an Environment. Part of the expense of a laser mill is the cost of environmental conditions that must be maintained for the welding process. Requirements include a clean room to protect the laser unit; shielding to protect the staff from the laser beam; and sufficient cooling of the welding area. These are more than enough to fulfill the requirements of the lower-intensity laser inspection system.
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