Laser and hybrid welding of high-strength structural steels

Abstract

Laser and laser-arc hybrid welding processes are recognized as key joining methods in many industrial fields such as light and heavy machinery, ship-building and the automotive industry. In the early days of laser welding, the process was known as a special joining process. The term special is still valid to some extent but the process soon came to be placed in the group of normal welding processes without any speciality. The same and often named list of the advantages and disadvantages of the laser welding process still exists although the situation has partially changed and the list has seen some changes or at least the emphasis is different. The operating and investment costs of laser welding systems have decreased. The number of laser welding system suppliers and integrators has increased and the prices of the main components have dramatically decreased as well. One component of note is the laser resonator. Lasers of one micron wavelength dominate the laser market. The euro/kilowatt ratio has dropped to a level that makes investment possible for smaller companies too. This is especially true in laser cutting. The Covid pandemic had just a temporary influence on the laser technology market and now it is back on a growing curve, as shown by the market analysis reports.

Steelmaking is facing its biggest change since the whole mass production of steelmaking was born. First, the pressure of imported steel mainly from Asia and Eastern Europe forced European steel mills to focus more and more on the production of specialized steels and the reduction of production costs. This has led to the closure of unprofitable steel mills. In addition, one major change is imminent. Steelmaking is moving towards fossilfree production. The path will be a long and winding one but it will happen and most likely sooner rather than later. There is already a growing demand from customers for fossil-free steel. However, manufacturers have already managed to decrease the weight of products and reduce fuel consumption and are able to carry higher payloads by utilizing modern higher strength structural steels in structures.

The joining of high-strength and ultra-high-strength steel can be demanding with conventional welding processes, such as gas-metal-arc welding. The efficient utilization of steels in a welded condition requires the accurate following of recommendations set for example for the cooling rate and welding energy. Modern steels typically have limits set for a minimum and maximum value for the previously named characteristics. As precisely controlled and automated joining processes, laser welding and laser-arc hybrid welding processes are suitable joining methods for joining modern steels and later for the joining of fossil-free steel as well. The total lifetime cost for a new product can be extremely favourable when combining a high-strength fossil-free structural steel and a laser-based joining process.

This doctoral dissertation contains the published key findings of several laser and hybrid welding studies made for selected high-strength structural steels of several thicknesses which can utilized in many demanding industrial applications. The welded joints have been tested by non-destructive and destructive testing methods. Some non-standardized measuring methods have been used to some extent to produce scientifically valuable information for the publications. The results clearly show the connections between the welding parameters used and the achieved properties of the laser and hybrid welds. Equations to calculate some variables of welding, such as welding energy, heat input or cooling time were originally created to be used in conventional welding. Therefore, the special properties of laser welding or laser beam welding are not taken into account in these equations. One of these special properties is laser beam absorption, which is still a relatively demanding variable to measure absolutely correctly; for example it varies according to the welding speed and temperature. The doctoral dissertation addresses this topic and presents the utilization of the equations developed especially for laser welding, taking into account the previously mentioned special properties of the laser beam at least to some extent.