Advancing next-generation weld monitoring

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Paul Webster and Chris Galbraith, of IPG Photonics, report on the development of a new inline weld monitoring system based on technology from Laser Depth Dynamics

As fibre lasers continue to enable faster and higher-quality welding applications for increasingly sophisticated part designs, manufacturers are pushing for more complete data regarding weld quality in order to maximise the performance and efficiency from their joining processes. Today’s laser welds commonly include complex geometries or mixed materials. Processing speed and adoption of new applications are increasing quickly as the cost per kilowatt of laser power steadily declines. As these factors place increased demand on traditional quality assurance approaches, such as destructive testing, manufacturers require more advanced monitoring solutions to maximise productivity. 

The accurate measurement of laser welding processes is not an easy task, laser welds produce intense light and heat, which blinds traditional cameras without specialised filtering and illumination. Additionally, important aspects of the laser weld process take place deep beneath the surface of the material, making the complete picture difficult to see. There are many process parameters that affect weld quality, and until recently several different types of quality sensor had to be employed to keep an eye on different aspects of a single process. This left manufacturers to cope with the bulk, expense and complexity of multiple tools pointed at the welding site, while in many cases still not providing enough certainty about the finished product. 

The quality assurance holy grail for many manufacturers in the aerospace, automotive and medical device sectors (among others) would be a single system that can monitor all relevant aspects of the welding process, with tight integration into the welding head, smart software to consolidate results into concise and actionable quality information, and the ability to inspect every weld in a production environment. Recent technological advances bring us closer to this. 

Seeing potential in start-ups 

IPG has developed a new inline weld monitoring system, based on technology originally developed by Laser Depth Dynamics – a Canadian start-up launched in 2012, acquired by IPG in late 2017 – that takes full advantage of these recent advances.

The potential of Laser Depth Dynamics’ technology was recognised early on by several industrial players, particularly in the automotive sector, who quickly began to adopt it for production use. Focusing on a single product allowed the company to grow rapidly, culminating in its acquisition by IPG Photonics. Freshly infused with new development resources, the new Canadian branch of IPG has been working with colleagues from our facilities in Massachusetts over the past year to develop the system – the LDD-700.

The new system uses inline coherent imaging (ICI) to provide an increased level of detail and accuracy for laser weld monitoring. The regions of interest for monitoring can be categorised into pre-, in-, and post-weld measurements. The LDD-700 pre-weld monitoring modes include tracking of the seam position, checking the working distance to the material, and looking for incorrect gap or bad fit-up of the joint. In-weld monitoring records the penetration depth of the weld as it’s formed, and post-weld inspection measures properties such as the height of the finished weld surface, the width of the weld bead, or the presence of surface defects such as blowouts.

The different monitoring modes of the new LDD system. Inline weld monitoring technology allows a single tool to perform multiple quality assurance tasks during welding. (Image: IPG Photonics)

To accomplish weld monitoring in multiple regions surrounding the process, the system fires a low-power measurement beam through the same optics that deliver the welding laser. This beam rapidly switches between different measurement locations on the part, and is capable of recording a swath of metrics, including weld penetration depth, joint position, surface quality and more. By the time the weld step is finished, the system has produced a multi-factor report on the weld quality which then automatically combines the relevant data to make a pass/fail decision. The machine controller is then signalled as to whether to scrap the part or not. 

In addition to reducing the need for several different previous-generation monitoring technologies cluttering the laser head and working area, this weld monitoring technology has real potential to lower operating costs through reduced destructive testing.

Smart monitoring for sophisticated welds

Inline weld process monitoring was initially introduced for welding beams with fixed optics. IPG has added functionality for applications that require joining of dissimilar materials and challenging non-ferrous alloys, where wobble-head welding is used to improve quality dramatically. Recent iterations of the LDD inline weld monitoring system are able to measure these increasingly sophisticated welding processes with exceptional detail. This technology provides the data manufacturers need to react to issues instantly, and save costly scrap. More recently, inline monitoring on scanner-based products has been introduced, enabling welding in a larger field of view for multiple parts or multiple welds per part. This combination is expected to have a huge impact on the fast-growing e-mobility sector. 

 An LDD-equipped IPG mid-power scanner, used for single-mode wobble-spot welding of mixed materials (aluminium on copper). The LDD depth measurement (right, inset) shows the exact penetration of the weld. A short region of shallow penetration is marked in red. In production, this would cause an NOK message to be sent to the controller, and the part could be scrapped. (Image: IPG Photonics)

The technology employed by the LDD weld monitor is extremely versatile. The system is able to resolve and measure pretty much anything that’s visible through the welding optics. This allows the same instrument to flexibly monitor different aspects of a process, ignoring those that aren’t relevant to a particular welding step and focusing on the critical metrics. Dramatically different processes can be run in quick succession, using the same head, and the different measurement modes, along with their respective quality limits, are activated and controlled using a single software utility. New monitoring modes can be continually added in software to advance the functionality of the system as production needs arise. The result is a very complete picture of the weld quality for every part, which empowers manufacturers to make better decisions more quickly. 

The newly developed LDD weld monitor is helping manufacturers by reducing scrap, by facilitating early detection of defective sub-assemblies, by providing more complete and accurate records for safety-critical applications than was previously possible, and by allowing QA procedures to be streamlined, saving time and operating costs. 

Paul Webster founded Laser Depth Dynamics in 2012 and is CTO and general manager of IPG Photonics Canada. Chris Galbraith, originally an applications specialist at Laser Depth Dynamics, is applications manager of IPG Photonics Canada.

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