TWI Frequently asked questions
The two imperfections which give rise to most problems in production are solidification cracking and porosity. There is no doubt that the imperfections are related to the steel composition and thickness, and to the laser welding process parameters.
Solidification cracking
Solidification cracking is not acceptable and occurs when the solidifying weld metal cannot sustain the applied thermal strain during the final stages of solidification. This arises when the last liquid to solidify is a thin film along the dendrite boundaries, solidifying at a depressed temperature compared to the bulk solidus temperature. This solidification temperature depression results from segregation of elements which form low melting point compounds or phases.
For structural steels, this occurs with sulphur and phosphorous, which combine with other elements in the steel to produce low melting points films. In addition, elements which promote austenite primary formation can lead to an increased tendency for solidification cracking, these are C, Mn, Ni etc. However, Mn can also be beneficial since its combines with sulphur to form higher melting point phases of a globular form which cannot form films at dendrite boundaries. The practical results of these considerations is that to avoid cracking in steels, it is desirable to have low levels of C, S and P, plus a high level of Mn.
The welding speed and weld shape 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, with a more beneficial complex central region solidification structure. The welding speed which is critical varies with the material thickness but, for 12mm thick structural steel, 1m/min is a reasonable maximum, for CO2 laser welded, good quality steels i.e. C< 0.1%, with both S and P individually < 0.010%.
Other factors which influence cracking such as plate thickness, joint type and fit-up, component restraint, joint surface contamination need to be taken into account when developing welding procedures to avoid cracking.
In some instances, filler wire can be used to minimise solidification cracking.
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 weld metal being rejected into the solidifying weld metal as gas bubbles, or result from unstable collapses of the laser welding keyhole trapping in gases. 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 are forced out of the plates by the weld metal, or can escape naturally. They no longer need to float up through the molten metal in order to escape from the weld pool.
The source of this gas can be surface contamination on the plate being welded such as grease, oil, oxide and absorbed water vapour, cutting fluid residue etc. All of these can be controlled by adequate edge and surface preparation and cleaning.
Dissolved gas in the base metal is more difficult to deal with. To minimise porosity, it is necessary to use steels of low gas content i.e. fully killed steels, preferably with aluminium. Filler wire can be used to control porosity by providing deoxidising elements such as Si and Al.
Effective inert gas shielding is important in minimising porosity, as can careful choice of a plume or plasma control system and its associated assist gas type and set up, 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 ease the escape of gas bubbles trapped inside the molten pool. In some situations the addition of oxygen to the assist gas has been shown to reduce porosity levels due to an increase in the weld pool fluidity and changes in the flow of the molten metal.
