Introduction to Laser Welding

Laser welding has been used by the automotive industry for several years. Namely, Fiat installed a CO2 laser in 1975 to weld power train components. In the 1980’s, car builders began replacing resistance spot welding by laser welding for the production of tailored welded blanks. These assemblies of metal sheets of different thicknesses and metallurgical compositions can be stamped and provide, exactly where needed, specific mechanical and metallurgical properties.

Despite the fact that a large part of the manufacturers have a very limited knowledge of laser welding, it is a mature process that has already proven all the advantages it can bring to a production line.

The Process

Laser welding usually uses a laser source emitting a beam of infrared light. The beam is invisible to the human eye and can be a severe threat to workers’s safety if suitable security procedures are not applied. Consequently, laser welding is done by a robot (or other type of automated motion system) in a light tight enclosure.

Laser welding can be realized in two modes: conduction and keyhole. Conduction welding is done in the liquid phase and requires a power density of 105 W/cm2. Keyhole welding implies vaporizing the metal with a power density of 106 W/cm2. The capillary, small « tube » of metal vapor formed in the base metal, allows the formation of typical

Lorraine Blais,Project Managerlblais@cstpq.com

Figure 1 : Laser welding of a steel structure at CSTPQ

narrow deep welds (see Figure 2), while conduction welding presents a limited penetration.

Knowing that the laser beam focus diameter is usually around ½ mm, powers over 1 kW are very often needed to weld in keyhole mode.

The advantages of laser welding are :- Speed of the process ;- precision ;- possibility to join dissimilar materials ;- the small space required for the laser beam to access the parts to be welded ;- excellent control of projections ;- limited deformations ;- possibility to weld in overlap configuration ;- ability to produce welds that are perfectly tight to air and liquid.

Figure 2 Joint resulting in

stainless steel 304, 0.090’’

Equipment

Several types of high power laser sources are currently suitable for laser welding. Diode, CO2 and Nd:YAG sources have been available for many years and have proven to be reliable in a large number of applications.

More recently, fiber lasers and disk Yb:YAG lasers conquered large market shares due to their high efficiency and to the numerous advantages of the high optical quality of the beam they produce.

Each technology having its own forces and weaknesses and being adapted to specific applications, it does not seem possible to declare either of them absolutely superior to the other. The choice of a laser source is made according to the application and to the specific priorities of the end user.

Figure 3 : 3 kW fiber laser source

CSTPQ has chosen fiber laser as the « heart » of its laser welding cell. The main points that led to this choice were, among others:

- unequaled wallplug efficiency of fiber lasers ;- high optical quality of the laser beam ;- almost no maintenance required ;- very low operational cost ;- no preheating period needed ;- possibility to use very long optic fiber cables (up to 200 m!), with a low diameter between the laser source and the workpiece ;- possibility to upgrade the laser source to higher power by adding power modules ;- very small footprint.

Once generated, the laser beam can be split between several working points (several welding cells or several robots). This sharing can be done in time or in power. While power sharing allows making more than one weld at the same time, time sharing is mostly used to feed up to eight processing cells with a single laser source. Considering setup and unloading usually take a lot longer than processing, it is indeed a good way to optimize the investment…

In the case of diode laser, YAG laser and fiber laser, the laser beam can be transmitted to the working point using a fiber optic cable, whose length limitation depends on the type and power of the laser source.

The beam passes from the fiber optic cable to a collimator (a lens that « straightens » the diverging beam exiting the fiber optic cable) and enters the welding head itself. Inside this welding head, the beam is focalized on the parts to be welded, at a distance that can vary from 15 cm to 2 m.

Figure 4 : Welding at a 30 cm distance

Putting the laser at work

Laser welding opens the door to numerous innovations in term of conception. Remote welding, overlap welding, joining of dissimilar material, etc. bring possibilities for new products and manufacturing avenues.

Many materials are laser weldable: steel, titanium, stainless steel, thermoplastics, etc. Since a research center form NRC in Saguenay is already active in laser welding of aluminum, CSTPQ chose to concentrate on steel (galvanized, stainless, highly alloyed…). Development work on other metals (copper, brass, powder metallurgy based alloys, etc) have also been conducted.

Stainless steel is particularly suitable for laser welding. With proper parameters, one can obtain nice welds with very low deformation and no discoloration on the underside of an overlap weld (if the weld is not a full penetration one, of course). With a 3 kW fiber laser, CSTPQ easily welds two 0.060’’ thick 304 stainless steel plates at a 7 m/min process speed. With more powerful laser sources, speeds in the range of 20 m/min can be reached!

Overlap welding of galvanized steel brings some additional challenge. Zinc vapor quickly forms between the parts and can cause important lacks of material in the weld bead. Different techniques can prevent the zinc vapor to escape through the melt pool - taking away with it a fair amount of molten material. Unfortunately, use of these techniques often implies additional manufacturing operation and a high precision. But even then, the productivity gain is still worth it…

Figure 5 : Welding on galvanized steel

One of the main difficulties one faces when bringing laser welding to the production floor is its low gap bridgeability. As no filler material is used in most cases, the parts need to be in intimate contact to fuse.

For butt joints, the maximal gap between parts is around 0.004’’ for parts under 0.080’’ thick. Control and preparation of parts to be welded calls for particular attention... Happily enough some joint configurations make things easier than others.

To fully experience the benefits of laser welding, it is highly recommended to adapt the design of the parts to the process. Laser welding a part designed for another process usually limits the gains in term of productivity and quality.

Hybrid Welding

In some cases, the use of filler material is necessary, either to bridge gaps, to produce very high strength joints or to avoid cracking. Hybrid laser/arc welding can then be used.

This process combines an electric arc and a laser beam in the same weld pool. One can then benefit from the high tolerance to gaps (thanks to the filler material), from deep penetration of the laser and to the wide weld pool of the arc welding process. Additionally, because of high welding speeds leading to low heat input, the parts suffer less deformation and degradation of mechanical properties compared to parts welded by sole arc welding. Finally, no parts preparation is necessary (no chamfer), and this alone represents an important economy.

Figure 6 : Hybrid laser/GMAW welding at CSTPQ

Several processes can be combined to the laser : GMAW, GTAW, PAW… CSTPQ uses laser/GMAW, as shown on figure 6.

In Conclusion…

Because of its speed, precision and flexibility, laser welding can greatly improve productivity and quality of assemblies. After Europe and Asia, North America is now ready to adopt this process.

As the cost of the equipment has considerably come down over the last decade, it is now accessible and cost effective for many manufacturing companies.

CSTPQ offers information, development services and technical support on laser welding. For any question on this process, please contact Lorraine Blais at (418) 856-4350 # 105 or lblais@cstpq.com .

Figure 8 : Butt joint on ¼’’ thick steel, one pass, no preparation

Figure 7 : Hybrid weld, T joint on 5/16’’ thick steel, one pass, no preparation