Breakthrough detection ensures high efficiency of the laser cutting machine and reduced cutting times.
Fiber lasers have now become accepted for industrial laser applications around the world. Global sales of all metal cutting lasers exceed $ 1.1 billion and are growing at 3-4% annually as they are incorporated into an estimated 7,000 sheet metal cutting machines installed in 2017. Fiber lasers have consistently replaced carbon dioxide (CO2) lasers in this market segment due to benefits that include increased efficiency, high beam quality and the ability to process highly reflective materials such as copper and copper alloys. Therefore, the sales of fiber lasers in this sector now exceed those of CO2 lasers. Performance improvements for this application will have a significant impact on global economies, as there is an increasing dependence on manufactured metal products. New generations of fiber lasers are starting to incorporate additional sensors and diagnostics that allow them to perform more advanced functions. Industrial laser cutting systems typically pierce and cut in separate operations, with fixed parameter sets for each material and thickness. The piercing procedure is often scheduled for a fixed dwell time, which can result in a loss of machine efficiency and hole quality, particularly on thick specimens with many holes. However, these processes can be improved by detecting, in real time, the light returning from the workpiece to the laser. This reflected light is usually considered a nuisance, but it contains information about the cutting process and can be used to detect the end of the pierce phase. There are already pierce detection systems based on the cutting workstation. However, next generation fiber lasers, such as the RedPOWER QUBE lasers, have an integrated pierce detection system that eliminates the need for any costly additional sensors inside the cutting head.
Detection Of The Piercing
There are many types of piercing detection systems that provide feedback to the cutting machine controller as soon as a pierce is reached. This allows the controller to move to the mowing stage with the minimum dwell time required. Usually, perforation detection systems are based in the cutting station in the vicinity of the focusing head, but this involves a more complex and expensive cutting head and an optical system with additional optical surfaces that can degrade the laser beam. They are also susceptible to damage due to the dusty environment often experienced when working near the cutting process. For example, for a skeleton cut on a 3 * 2 m sheet, 2,500 piercings may be needed, thus the time saving of 100ms per hole reduces the processing time per sheet by more than 4 minutes. The study of a wide range of materials and thicknesses has shown a typical time saving of 10-15% using a piercing detection system, so the economics of investing in automatic pierce detection are clear and considered necessary for all high performance cutting systems.
A laser cutting process is structured in several stages. Piercing is the first stage, which produces an almost vertical face across the sheet metal, forming the starting point of each cut. Depending on the material and sheet thickness, a complex pulse shape and power ramp may be required to achieve reduced pierce times, as well as to reduce splashing and swelling on the workpiece surface. And, depending on the temperature, surface roughness, and material quality of the part, pierce times can vary greatly for the same drilling program. For industrial processes, a certain safety factor (typically up to 3X) is added to the average residence time, which is obviously not needed for most holes,
A sheet metal perforation with a fiber laser and gas assisted cutting head is similar to a laser perforation process. Initially, the focused beam will be absorbed by the top surface of the sheet, causing a localized temperature rise leading to melting and potentially vaporization, depending on the intensity of the beam. The pressure of the coaxial gas jet and the vapor pressure of the evaporating metal will create a blind hole with droplets of molten metal sprayed upwards and outwards. The drilling process continues as the laser beam passes through the thickness of the sheet. While the perforation hole is blind (i.e. not yet fully drilled through the thickness of the sheet metal), there will be a higher level of laser light reflected upwards as the whole beam will hit the sheet metal. This is in contrast to the steady state cutting process, where the angled front of the cut will allow most of the unused portion of the beam to exit the back of the sheet.
During the drilling process, a variable time BR signal will occur, which can be used by the laser control system to determine the end of the pierce. This information is presented as a clear digital flag to the cutting workstation to stop the piercing cycle. Figure 2 shows the typical results obtained for a sheet metal perforation using a perpendicular focused beam. The process can be divided into several distinct phases:
• From the start of drilling at t1, there is a large BR signal. This is
consistent with the laser hitting the intact surface and knocking down the initial reflectivity.
• From t1 to t2, the pierce is a blind hole as the laser passes through the thickness of the sheet. The BR is unstable as the shape and position of the weld pool changes, but at higher levels than the cutting process.
• From t2 to t3, there is a transition as the pierce breaks through the back side of the sheet.
• After t3, there are relatively low (but not zero) BR levels until the laser turns off at t4.
Figure 3 shows a trace of the actual BR signal from a piercing cycle (with the detector initially saturated). This was achieved using a 1.5 kW laser with a 50 μm process fiber (BPP = 2.0) which cuts a 6 mm stainless steel sheet with a focal point of ~ 100 μm, with the beam focused under the surface of the piece. In this experiment, an additional detector was placed under the workpiece to confirm the crossing. The pierce digital flag can be clearly seen as high when the supplemental detector goes into saturation, indicating full breakthrough. Importantly, there is an automatic reset of the drill flag at the end of the laser activation period.
The piercing detection system works by identifying where t3 was reached. This triggers a programmable delay time, which is important to ensure that the piercing process is finished. Once the BR signal is below the threshold for the duration of the delay time, the drilling is considered finished and the laser provides a digital Pierce Flag signal that can be used in the production machine. Thresholds and dwell times are programmable, so the user can adjust settings to ensure Pierce Flag behavior is optimized for their process, with Pierce Flag going high just as the pierce is completed.
In many operations, pierce is achieved using a pulsed mode of operation to allow the laser to achieve a cleaner and more controlled pierce hole. By choosing suitable parameters for the detection algorithm, the pierce detection flag will still be activated at the appropriate point.
Pierce detection systems are becoming a required component of modern flat cutting workstations to ensure high performance and efficiency.For example, with the piercing detection sensors integrated into the redPOWER laser, the customer may decide to use a head cheaper than an expensive product.
Being able to monitor the amount of BR from the cutting process is a huge benefit, both for setting the correct process parameters and for monitoring the in-process quality. Piercing and cutting operations can be examined and optimized, and high levels of BR from highly reflective materials can be avoided.
Ultimately, the productivity of a laser cutting system can be increased by using a puncture detection system to ensure high machine efficiency and a reduction in wasted time during piercing. Simulations have shown that process time can be reduced by up to 15% on a 3 × 1.5m sheet with realistic part layout. Depending on the complexity and size of the part, the actual time and cost savings could be even higher.