FEATURE
Issue: 

Laser welding needs ‘sound scientific understanding’

After speaking at the Association of Laser Users’ Industrial Laser Applications Symposium in March in Kenilworth, UK, Professor Stewart Williams at Cranfield University argues that a more scientific approach to laser welding would increase uptake of the technology

Since joining the Welding Engineering Centre at Cranfield University, one thing that struck me was that the majority of industrial welding is carried out with conventional arc processes. It is impossible to be exact, but I would estimate that laser welding accounts for only about 20 per cent of industrial welding. Furthermore, many of the applications are in precision areas such as medical and electronic. In the heavier areas laser welding is seldom used. Considering that laser welding (or laser hybrid welding) has many advantages compared to conventional welding processes this seems surprising. So why is this?

It is my belief that it is the often adopted engineering approach to laser welding, and indeed many other laser processes, which is the root cause of this. This is manifested in the following types of activities:

  • Development of laser processes using system parameters such as power and travel speed;
  • Development of a new laser source and then seeing what it can do;
  • Investigation of different types of laser with different materials;
  • l Use of a specific laser system to obtain a desired result – laser supplier process development; and
  • Fiddling with system parameters and observing the changes.

The consequences of this approach are that laser processes are perceived as hard or needing specialised knowledge (a large amount of ‘black art’); processes become laser system specific and it is impossible or difficult to transfer from one system to another; and phenomenological effects are difficult to explain or understand.

A simple laser welding process development example will highlight the engineering approach. Say the requirement is for a 5mm-deep weld in a specific material. If the user has a laser available, the (usually maximum) power and travel speed are adjusted until full penetration is achieved. If a different laser or optical set-up is used then this process will need to be repeated. Alternatively if the request is sent to laser suppliers then a variety of laser processes will be offered depending on the specific set-up used by the supplier. These welds will all have different characteristics.

To overcome these issues, an approach using a sound scientific understanding should be adopted for laser application development and process specification. This approach needs to be based on the fundamentals of laser material interaction and not laser system parameters. How is this done?

There are three primary parameters that control laser material interaction: power density, interaction time, and specific point energy[1]. If a process is specified using these parameters then this process will be unique and repeatable. It should be noted that the three parameters are all dependent on the beam diameter in different ways. This means that when the beam diameter changes, for example by using a different optical set-up or if there is a change for some other reason, then all these parameters will simultaneously change. Given three particular values for these parameters there is only one unique beam diameter that will satisfy these values. So we need to identify which of these parameters controls particular aspects of the process being used.

In the case of laser welding it has been shown that the penetration depth is controlled by the power density and specific point energy. These can be combined into one parameter – power factor – which is the laser power divided by the beam diameter. The weld width is controlled by the interaction time[2]. Defining a laser welding process using these parameters gives a specification that is totally independent of the beam diameter and therefore the laser or optical system. This means that the process can be transferred easily between laser systems. A user can put in the required penetration depth and the power/travel speed combination will automatically be calculated for that particular system based on the known beam diameter.

Using this science-based approach also helps in understanding phenomenological effects such as the large depth of field in laser welding. As you move away from the focus, the beam diameter increases resulting in the power density decreasing. If you decreased the power density by the same amount by reducing the laser power there would be a significant reduction in penetration depth, but this is not observed when defocusing. This is because the result of increasing the beam diameter is to increase the specific point energy which also controls penetration depth. There is an offsetting effect of these parameters leading to a much greater depth of focus than might be expected.

This scientific approach to laser application development is one of the main planks for the EPSRC Centre for Innovative Manufacturing in Laser-Based Production Processes. Within the Centre we believe this is the way forward to get an increased uptake of laser-based production processes in the UK. This is being applied to a range of other laser processes such as cutting, selective laser melting and laser peening. 

Stewart Williams is Professor of Welding Science and Technology in the School of Aerospace, Transport and Manufacturing at Cranfield University in the UK. His main research activities are focused on additive manufacture and laser processing

 

REFERENCES 

[1] Suder WJ and Williams SW (2012), ‘Investigation of the effects of basic laser material interaction parameters in laser welding’, J. of Laser Appl., vol. 24, ISSN 1042-346X.

[2] Suder WJ and Williams SW (2012), ‘Power factor model for selection of welding parameters in CW laser welding’, Optics & Laser Technology 56 (2014) 223–229.

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