The combination of steel strip edge preparation via laser cutting, accurate strip positioning systems, and laser welding in a single unit provides the optimum coil-to-coil joining technology for continuous production plants.

This technology has convinced the management team of Yichang Three Gorges Quantong Colored and Galvanized Plate Co. Ltd. to implement four IDEAL Laser welding machines of the Type LAS300 in their rolling mill. The end to end joining of coils of strip has increased in popularity over the years as a method of increasing productivity through continuous operation in many steel processing operations, such as pickling, rolling or galvanising. Initially, this was done with welding machines which are integrated into the production lines, using flash-butt welding for thick sheet areas and overlapped welding of thinner sheets. These coil joining methods are, however, considered problematic for processing high-strength Dual phase and Trip steels with tensile strengths of approximately 800N/mm² and higher.

Laser welding has established itself as a safe and reliable joining process in many applications, including the steels mentioned above, as the steel itself has almost no influence on the process due to the highly concentrated laser beam. Furthermore, with the ever increasing demands in the market for better coil quality, the demands on the geometry of the welded seam in terms of bumps and depressions in the weld area are also increasing and laser welding has led to an improvement in product quality in this area too.

Laser Welding

The very high power density of the laser beam – 4 MW/cm² – vaporises the steel and creates a keyhole. This cavity is a high pressure region surrounded by walls of molten metal. As the beam moves relative to the work piece the molten metal flows backwards around the cavity edges at the trailing edge, forming a narrow fusion zone or weld. The vapour or plasma formed absorbs laser energy so must be minimised. This is done usually using helium to blow it out of the way, and which also helps prevent oxidation of the liquid steel. Coil joining with a laser beam offers several advantages as compared to the resistance welding process:

Targeted heat concentration in a small area (narrow heat affected zone)

High strength weld at low seam volume
High process speeds ensure short processing times
The laser beam does not exercise any force on the work piece and minimises residual stresses and distortion
No consumable electrode needed
No wear of mechanical knives, thus less maintenance and downtime

In order to minimise the power requirement for welding and to maximise weld quality a zero gap between the two pieces to be joined is ideally required. This can be guaranteed only by means of Laser cutting.

LASER cutting

It is a form of sublimation cutting in which cutting gas is fed coaxially with the laser beam onto the steel surface which blows the vaporised steel out of the keyhole. Nitrogen is usually used, but in some cases oxygen is preferred. The transition of the material from solid to the gaseous state – sublimation – takes place almost directly, i.e., without going through the liquid phase. This gives rise to a vapour capillary which enables deep penetration of the laser beam into the material. The process gas does not only blow out the vapour from the cut joint, it also prevents the condensation of the vapour in the cut joint.

Cutting with laser beam offers several advantages compared to mechanical cutting of strip:

Rectangular cutting edge over the full sheet thickness and sheet width
Oxidation and hardness-free cutting edge over the full width and thickness
Uniform cutting quality and cutting force for all material strengths (up to 1,500 N/mm²)
No wear and tear because there is no mechanical contact and only minimal gas on the work piece

COIL TO COIL JOINING

The field of application for a laser coil joining machine requires the processing of:

Wide range of steel grades
Sheet thickness from 0.3 mm to 6.5 mm
Sheet widths of up to 2,200 mm
Sheet tensile strengths up to 1,500 N/mm²
Coated sheets
Sheets with excellent surface quality
Sheets of differing thicknesses
In addition the welded product must exhibit:
No damage on the steel surfaces.
The welded seam should not cause any depressions in the neighbouring positions.
Several welded seam detection holes can be created so that the joints can be detected easily and subsequently removed.
Different steel strip widths need to be accommodated. This can be achieved by laser cutting of the wider coil at an angle so there is a smooth transition between coils.

(Author: Mr. Michael Tieze,
Sales Manager