They used the well-known technique of laser cladding. But the laser beam not only makes it possible to melt the material in question. It can also be used to heat the top layer of the already resolidified metal. The research team used this to specifically change the crystal structure of the steel in individual metal layers - and thus influence the mechanical properties without changing the chemical composition.
To do this, they developed an alloy of iron, nickel and titanium. Initially, this alloy is relatively soft. But under certain conditions, small nickel-titanium microstructures are formed, which then provide a special hardness.
To be able to create the nickel-titanium structures, the researchers interrupted the printing process after each newly applied layer and allowed the metal to cool to below 195 degrees Celsius. This is when the crystal structure begins to change. They then heated the material again. To do this, the researchers use the laser energy with which the next layer is printed.
Layers that have been directly coated with the next layer without a break remain softer because they are not yet present as martensite at this point. The tests confirmed an excellent combination of strength and ductility, according to the researchers.
A number of process adjustment screws are suitable for influencing the microstructures during 3D printing. Instead of alternating between heating and cooling, the crystal formation and subsequent hardening can also be controlled by varying the laser energy, laser focus or printing speed or by using external heating and cooling techniques.