The main defects in laser welds in both steel and aluminium are solidification cracking (also called hot cracking or centreline cracking) and porosity. 
In steels, the chemical composition of the parent steel will have an effect on the crack-susceptibility. Elements such as sulphur and phosphorous cause solidification cracking to occur,  but manganese ties up this sulphur and has an effect to prevent solidification cracking. Some aluminium alloys have a wide solidification temperature range which makes them particularly susceptible to solidification cracking when welding.
For laser welds in thick section steel, the main cause of solidification cracking is the weld profile shape.  The high depth to width ratio of laser welds causes restraint and means that high thermal stress acts across the weld where the solidification fronts meet, causing centreline cracking.  Solidification cracking susceptibility is exacerbated if there is mid-section bulging in the weld cross-section profile due to locally increased strain.  Therefore, weld shape must be carefully controlled when laser welding.  Full-penetration laser welds are less prone to solidification cracking than partial penetration laser welds since the former do not suffer from the high restraint at the weld root. The likelihood of laser welds to suffer solidification cracking can limit the welding speed, parent material composition or weld penetration depth that is possible when laser welding both steels and aluminium.
Porosity is a less critical defect in laser welds than solidification cracking. Porosity in laser welds can be caused by plate or surface contamination or inadequate pre-clean.  However, in laser welds, porosity can also result from laser keyhole instability and collapse during partial penetration high-power laser welding.  Laser welding is shielded using an inert gas shield, and if this shielding is not adequate (due to either too high or too low gas flow rate) then porosity can result. Porosity can be a particular problem for laser welds in aluminium caused by hydrogen evolution in the weld metal. Hydrogen can come from moisture in the air or surface oxide, or result from inadequate pre-cleaning. 
High power laser weld showing solidification cracking due to mid-section bulging, and porosity
There are, of course, other defects that can be associated with laser welds. In particular, laser welded lap joints in zinc-coated steels can suffer from blow-holes, a severe defect that results in through-thickness holes formed from explosive evaporation of the zinc interlayer.
What is hot cracking (solidification cracking)?
What is porosity and how can it be prevented?
What is the influence of porosity on the mechanical properties of aluminium welds?
Email TWI's Defect Assessment Group for assistance in this technical area.
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