Over the last couple of decades the possibilities of what additive manufacture, more commonly known as 3D printing, can achieve has spread into the medical, aerospace and military industries. The most effective way of performing these processes for metals would be power bed fusion. This is where a layer of the metal powder, less than a fifth of a millimetre thick, is distributed in the correct shape. A laser fuses this powder together and second layer is applied. This continues until the design is realised. For more detail I strongly recommend looking here at this page made by Loughborough University which goes into a lot more of the details.
This process is preferred because more complicated metal parts can be created with more fine and detailed shapes. When views on a microscale the idea of the laser “just” melting the powder together becomes a lot more complicated. The powder can melt or vaporise based on the scan time; the molten metal can flow into its surroundings; thermal agitation and vaporisation can eject powder from the surface; and various other complex phase changes that can occur. If these effects cannot be controlled than the result can be course surfaces between moving parts, residual stress between the metal lattice and weakening due to unwanted impurities or even just vacancies in the lattice.
The Advanced Photon Source in Illinois has been used to produce X-ray imaging to analyse the dynamics of the laser power bed fusion in the powder layer as well as above it. It was found that features such as the melt pools, powder ejections and phase transitions could all be examined with resolutions previously unseen for both distance and time intervals. For example it was found that a keyhole pore (pore in the metal formed from the surface down) only requires about 50 microseconds to close the keyhole that created it. The solidification zone quickly encompasses the vacancy and at that point it is locked in place. Many of the other detrimental effects were elucidated and hopefully the laser process can be adapted to avoid now they are understood.