APPLICATION FOCUS: PROCESS MONITORING
ADVANCES IN PROCESS MONITORING BENEFIT LASER MICROWELDING
Coherent’s Florian Furger, Markus Danner and Roland Mayerhofer discuss how combining real-time readings from multiple sensors enables better control of high-precision welding
Accurate process monitoring is particularly critical in high- precision materials processing applications. This is because the cost impact of scrapping or reworking a part due to processing errors is more costly the more expensive a part is, the more complex the processing step involved, or the later the error is detected in the production chain. In addition, the probability that a part will fail to meet specifications increases as the task or specifications themselves become more challenging. Precision laser materials
processing applications, such as microwelding and microcutting, represents the ‘perfect storm’ of these conditions. This is because the tasks are invariably demanding, have a narrow process window, and the parts to which they are applied frequently have inherently high value or are
“Increasingly fast and powerful microprocessors, together with machine learning software, enables a more sophisticated approach for creating process fingerprints”
36 LASER SYSTEMS EUROPE AUTUMN 2021
Figure 1: The occurrence of a deviation in the weld seam position is identified in real-time data from the plasma, laser back reflection and temperature signals during this pulsed laser welding process
safety or health related – for example in medical device manufacturing. Another example is battery production for e-mobility, in which laser processes are applied in late stages of production – after significant value has been built into the product. This article explores how
recent advances in process monitoring technology promise better results in the most demanding applications.
Process monitoring phases Process monitoring can be applied at three distinct phases of a laser application. The first of these is pre-process inspection. At this stage, vision systems, sometimes coupled with automated pattern recognition software, are used to assess part fit, weld gap dimensions, overall part
position, and even to verify that the correct parts have been supplied. The next step is actual, real- time monitoring of the process itself. This can be accomplished in several different ways, including machine vision in the visible and infrared, and the use of pyrometers and acoustic sensors. Other analysis tools such as optical coherence tomography (OCT) can be used to assess weld depth as well. The third phase is post-
operation inspection. This can include measurement and assessment of the physical dimensions and characteristics of the laser processed area, which is usually performed using various camera and machine vision tools. Parameters assessed usually include seam width, and the identification of weld gaps or
spattered material. OCT and other tools are also used to determine weld dimensions and mechanical characteristics (porosity, the presence of voids, grain size, etc.).
Real-time process monitoring In-process monitoring is the most critical and complex of these three phases, so it’s worth examining this step in depth. Because there is so much going on during actual laser processing, numerous parameters need to be measured. Plus, data sampling rates must typically be very high, so as to be able to accurately identify any transient problems. And, the more quickly a problem can be detected, the better the opportunity to stop or correct the process before a bad part is produced. The most important areas
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