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Process monitoring systems usher in the future of laser welding quality control

In today’s economy, the old adage “time is money” seems more apt than ever. Being able to repeatedly produce high-quality products can determine a company’s success within the manufacturing sector. Having to scrap a product due to poor quality welds, for example, incurs significant costs in both time and money. Recalling a product can be even more costly and can damage a company’s reputation.

The thermal-time signature of a spiral weld is plotted as signal [arb. units] versus time [ms], as recorded by Amada Weld Tech’s MM-L300A laser welding process monitor. Two spiral welds on steel were recorded: one performed with cover gas in the weld zone and the second without. The inset pictures show the visual appearance after welding for each condition.

Therefore, catching weld errors, in this case laser welding errors, as they occur instead of reacting to defects downstream ensures seamless weld quality control. In recent years, a number of laser welding process monitors have been developed with the goal to increase yield, improve quality, save labor and increase savings. These monitors can help a manufacturer continuously meet the demands within the market.

Weld success criteria

Laser welding is a manufacturing process that uses the output energy from a laser to heat and melt two parts that are then joined together in a fusion bond as they resolidify. A number of factors – such as material and plating selection, equipment capability and process settings – determine whether the process has the potential to be successful.

Weld success criteria depends on the purpose of the weld and surrounding environmental elements. A successful weld can be defined by tensile strength (how well the part holds together), hermeticity (how well the weld seals a package), conductivity and other tests. While some tests are non-destructive, many require destroying the parts to determine quality.

But the ultimate question is how to know if the laser welding process was successful?

Typically, a design of experiments study is performed to determine the range of output based on the variation of input parameters. The result is then used to set boundaries around the measurable input parameters to infer whether the process is successful. After implementation, the welds are periodically checked via testing to confirm success. While this increases the likelihood of success and certain inferences can be made for adjacent welds, it does not provide 100 percent certainty.

The MM-L300A detects production errors in real time, including gaps between parts, missing parts, over-penetration, incorrect focus and cover gas absence.

The interaction of the laser with a material produces a visible and audible event. An experienced welding engineer or operator might instinctively know whether a good or bad weld was produced. These welding engineers look at the height or brightness of the plume, listen for the sound of the weld and visually inspect the weld afterward. Furthermore, quality control may take a few samples to destructively test whether it is a good or bad weld.

However, this type of monitoring misses a lot of available measurements and is not scientific or traceable, which is why products designed to measure during the laser welding process are entering the market.

New monitoring method

With the advent of these laser welding process monitors, there is now a way to observe the process as it’s happening to determine success. During welding, a number of measurements can be recorded to provide information about the weld. These include information about the laser radiation, the heating process and the weld penetration. Different radiation wavelengths and frequencies of sound can be collected, as well.

One important measurement to record is the thermal characteristics during the laser welding process. The material is locally heated and emits radiation that is temperature dependent. This signal can be detected and recorded versus time.

Similar to a night vision scope, which can extend the spectral range and intensity range viewers can observe, some laser welding process monitors use thermal imaging to collect radiation from the weld zone. The 1,300-nm to 2,500-nm range corresponds to the melting point of common metal materials.

During the welding process, like the hairpin welding seen here, the quality of the weld improves when the process can be monitored in real time.

Each weld will have a unique temperature-time profile. Once this waveform is defined through multiple measurements, it is possible to set upper and lower limits for variation and comparative measurements can be made.

Temperature increases as the laser pumps energy into the part. The increase in temperature depends on the material, part geometry and weld parameters. Once the laser stops emitting, the temperature decrease manifests itself as a visible cooling pattern. A monitoring system enables the user to monitor the whole waveform of a weld versus time. Accounting for certain known variations, boundaries are set up around that profile. When the monitoring system records that the profile heated up too slowly or quickly or cooled too slowly or quickly, issues with a weld are revealed.

Detecting issues

A common fallacy is the belief that once the weld parameters are dialed in, the laser welding process will always be successful. However, a number of things can go wrong during manufacturing, which can originate from material, process or equipment variations. Common examples include:

  • Damaged parts from mishandling
  • Improper loading of parts
  • Parts being out of focus
  • Insufficient cover gas
  • Low laser power
  • Change in material composition or plating
  • Gap between materials

Each of these scenarios can detrimentally affect the weld quality. The goal is to catch and address these issues early. The more production that occurs with faulty parameters, the greater the chance a production recall can occur. Amada Weld Tech’s MM-L300A, which measures the laser welding process in real time, can prevent costly errors from becoming a reality.

This high-resolution laser welding process monitor is ideal for detecting these production errors in laser welding for spot or seam welds. In addition to detection of gaps between parts and incorrect focus, the MM-L300A also detects missing parts, over-penetration and cover gas absence in order to provide operators real-time feedback on weld quality.

As described, the MM-L300A determines weld success by detecting and recording a thermal signal from the area of laser interaction and provides the user an output waveform around which maximum/minimum or envelope limits can be set. Once these are in place, the unit compares a new weld waveform in real time to identify a weld that is good or not. These qualities make this high-resolution monitor ideal for process development and quality control laser welding applications.

Other errors that can be detected include incorrect positioning or bent pins, which might not always be catchable by visual inspection alone. The smaller and thinner materials are, the more likely they are to get bent or positioned incorrectly. Thus, monitoring is a major asset to the micro-welding sector as monitoring systems can capture very fine defects that might be too minute for visual inspection to detect.

Without real-time process monitoring, users cannot see errors until after laser welding is complete and quality inspection has begun. The amount of parts a manufacturer may have to discard depends on how often the weld is being checked. For example, if welds are only monitored at the end of the day, a company has reason to suspect the quality of all welds made that day may be compromised if any inconsistency is detected. As such, more welds than fewer have to be inspected to ensure consistent weld quality efficiently.

Monitoring benefits

There are a number of benefits to monitoring a weld in real time, including failure detection, increased product throughput and traceability. This ability streamlines production and quality assurance and is a great asset to any company.

Failure detection: First and foremost, the function of a laser welding process monitor is to measure the signal and compare it to a known good reference. This is a first line of defense that can indicate whether a weld was successful or not. The earlier this is caught, the greater the cost savings.

Increased throughput: Laser welding monitoring can reveal weld errors as they occur, instead of after production. This can increase throughput and lower scrap. In production, it is necessary to test a number of samples to ensure quality. This might be done every day or potentially more frequently. If one of the weld tests fails, the lot of parts may need to be scrapped.

By measuring the thermal-time waveform as the weld occurs and comparing it to a known good reference, instantaneous detection of errors can be made for each and every part. If something goes wrong, then a signal can be sent and the offending part can be separated out and flagged to stop production until the issue is resolved and production of good parts can continue.

Additionally, being able to tell whether a weld is going to be good or not as the welding process takes place greatly enhances production line efficiency. Relying on a high-precision monitoring system reduces the number of additional cross sections and destructive testing a manufacturer must do to ensure a weld is good quality.

Traceability: Monitoring provides the opportunity to record measurements for 100 percent traceability.

In today’s networked world, data and data analysis are becoming increasingly essential in the manufacturing sector. Laser welding process monitoring systems provide an extra layer in which data can be collected and stored, whether locally or on a network, to be accessed later for tracking, tracing and analyzing purposes. The data becomes invaluable if there is ever a product recall and there is a need to source the root cause.

Implementing a laser welding process monitor into a production line can significantly increase a manufacturer’s return on investment. High-precision monitoring systems, such as the MM-L300A, can help customers understand and target problems in order to get successful welds each time to achieve improved yield.

The MM-L300A is easy to integrate into a laser welding workstation. The sensor can be mounted in both on-axis (through a focus head) and off-axis configurations to monitor the signal. The unit offers Ethernet and RS-232 output signals that can be connected to a PLC or a network.

Amada Weld Tech

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