Laser welding puts a very large amount of energy into a very small area, which produces a narrow deep weld with a small heat affected zone and very little distortion. That combination is why it dominates in places where a part must stay dimensionally stable after joining, and it is also why it is unforgiving about how well the parts fit together before the beam arrives.
Conduction and keyhole, two different welds
At lower power density the beam heats the surface and heat conducts inward, producing a shallow wide weld with a good cosmetic finish. Above a threshold the metal vaporises and the beam drills a keyhole, producing a deep narrow weld with a high depth to width ratio. Most structural laser welding is keyhole welding, and most cosmetic laser welding is conduction welding.
Fit up is the whole game
Because the beam is small, a gap the beam can fall through is a gap that produces no weld. Joint preparation, fixturing and part tolerance therefore matter far more than with a process that lays down filler. Many laser welding failures are not welding problems at all; they are stamping or machining tolerance problems that only became visible at the weld station.
What it is good for
Thin sheet assemblies that must not distort. Hermetic seals on enclosures and medical devices. Joints in tight spaces where a torch will not fit, since the beam only needs line of sight. Dissimilar thickness joints. And high volume work, because cycle times are short and the process automates cleanly with no electrode to dress or consumable to feed.
Where a conventional process wins
Thick section structural work where filler is needed to fill a real gap. One off and repair work where fixturing cost cannot be amortised. Materials that are highly reflective or crack sensitive without careful parameter development. And any job where the parts arrive with the variation that a filler process is designed to absorb.
Questions people ask about laser welding
Does laser welding need filler wire?
Often not, which is one of its advantages. Filler is added where a gap must be bridged or where the weld metal chemistry needs adjusting, but autogenous welding is the common case.
What gap can it tolerate?
Very little. As a rule the acceptable gap is a fraction of the material thickness, which is why fixturing and upstream tolerance control matter so much.
Is it safe to have in a general shop?
It requires proper enclosure, interlocks, eye protection appropriate to the wavelength and fume extraction. Those are engineering controls that have to be in place before the machine runs, not afterwards.
Can it weld dissimilar metals?
Some combinations, with care. The metallurgy governs: some pairs form brittle intermetallics regardless of how well the process is controlled. Test the specific pair before designing around it.