As a fusion welding process, in common with arc welding, laser welds in structural steels can contain defects. Without proper preparation of materials and selection of welding parameters, certain defects can be more likely (solidification cracks, pores, loss of toughness), and certain other defects less likely (e.g. excessive distortion).
Solidification cracking
Solidification cracking is not acceptable and occurs when the solidifying weld metal cannot sustain the strains acting on it during the final stages of solidification as the weld solidifies and cools. This arises particularly when the final liquid to solidify is present as a thin film covering the dendrite boundaries, commonly along the centreline of the weld, solidifying at a depressed temperature compared with the bulk solidus temperature. This temperature depression in turn results from segregation of elements to the dendrite boundaries which then form low melting point compounds or phases.
In structural steels, unacceptably high levels of sulphur and/or phosphorous are the main culprits. In addition, elements which promote primary austenite formation can also lead to an increased tendency for solidification cracking, these being C, Mn, Ni etc. However, Mn can also have a beneficial effect as it combines with sulphur to form phases with a globular form, which do not therefore form brittle films at the dendrite boundaries. The practical results of these considerations is that to avoid cracking in steels, it is desirable to have low levels of elements such as C, S and P, and a high level of Mn, in the weld metal. Therefore, in some instances, an appropriate addition of filler wire can be used to minimise solidification cracking.
Welding speed and the shape (aspect ratio) of the weld are also important. High welding speeds tend to produce deep, narrow welds, with a single centre line boundary, whereas lower welding speeds give a wider, shallower weld, which may also have a more beneficial, complex, solidification structure in its centre. The weld shape can be influenced by increasing heat input and/or using a laser beam with a larger focused spot diameter, both of which can be used to broaden the weld.
Other factors which influence cracking such as plate thickness, joint type and fit-up, component restraint and joint surface contamination need to be taken into account when developing welding procedures to avoid cracking. Thicker, poorly fitting, improperly cleaned and excessively restrained joints will be more likely to crack than clean, well fitting, less fixtured joints in thinner materials.
Porosity
In laser welds in structural steels, porosity can be a problem, but can be tolerated up to levels defined in laser welding standards. Weld porosity can originate from excessive gas, in the form of bubbles in the weld metal, becoming entrapped in the solidifying weld metal. It can also result from collapses of the laser welding keyhole, if that keyhole is unstable, trapping in gases.
The source of this gas can often be surface contamination on the workpieces being welded such as grease, oil, oxide and absorbed water vapour, cutting fluid residues etc. All of these can be controlled by adequate edge and surface preparation and cleaning.
Dissolved gases in the base metal itself are more difficult to deal with. To minimise porosity, it is necessary to use steels of a low gas content i.e. fully killed steels, preferably with aluminium. Filler wire can be used to control porosity, by adding deoxidising elements such as Si and Al in to the weld metal.
Effective inert gas shielding is important in minimising porosity, to avoid any inadvertent inclusions of atmospheric gases in to the weld.
Any steps that can be taken to enlarge the weld pool (e.g. welding with a higher heat input, or twin spot or hybrid laser-arc welding), or increase the time available for outgassing (e.g. reducing the welding speed) can also be beneficial in reducing porosity levels.
Careful choice of a plume or plasma control system and its associated assist gas type and set up can also be important, depending on the wavelength of the laser source and the welding parameters being used. Adequate control of the plume or plasma will result in a more stable keyhole, which will avoid unstable collapses of that keyhole.
Partial penetration welds are more prone to porosity than fully penetrating welds, as routes for the escape of gas bubbles then become more limited. Conversely, once the welds start penetrating, the pores which normally get trapped at the bottom of the weld can escape out of the root of the weld, and no longer need to float up through the molten metal in order to escape out through the weld face.
Changes in material properties
Although not technically a weld defect the high cooling rates experienced by the weld metal and HAZ immediately after laser welding can result in the formation of brittle, low toughness microstructures in structural steels. Monitoring hardness levels, particularly during the specification of a welding procedure, can serve as a useful guide to determining the minimum acceptable heat input. Nevertheless, it is advisable that this is complemented by impact toughness testing and, as required, CTOD testing. Commonly, the results of these measurements and tests will indicate whether increases in heat input, e.g. by reducing the welding speed, are necessary, to meet the required toughness. This is particularly the case for the HAZ toughness, where filler wire additions cannot alter the toughness values that result. Because of this, in some applications, lower hardenability ‘laser weldable’ steel grades may have to be considered.
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