One problem with 3D-printed metal parts is that stresses due to the expansion of heated material and contraction of cold material can build up in parts while they are being printed, generating forces that distort the parts and causing cracks that weaken or tear them to pieces.
Researchers at Lawrence Livermore National Laboratory (LLNL) and the University of California, Davis are addressing this problem by using laser diodes. These are high-powered lasers made using the same process used to build LLNL’s National Ignition Facility (NIF). In 3D printing the diodes rapidly heat the printed layers during a build. Using diodes for printing reduced residual stress in metal 3D-printed test parts by 90%. They did this by letting researchers reduce the temperature gradients (the difference between hot and cold extremes) and control cooling rates.
“In metals it’s hard to overcome these stresses,” says researcher John Roehling. “There’s been a lot of work on trying to do thing such as changing the scanning strategy to redistribute the residual stresses, but basically our approach was to get rid of them as we’re building the part, so you don’t have any of those problems. Using this approach, we can effectively get rid of residual stresses to the point you don’t have part failures during the build anymore.”
For the purpose of the study, LLNL engineer Will Smith built small, bridge-like structures from 316L stainless steel using laser powder bed fusion (LPBF). He let each layer solidify before illuminating their surfaces with the diodes, initially at full power and then ramping down the intensity over a period of 20 sec. The result was akin to putting the part in a furnace after each layer, as surface temperatures reached about 1,832°F (1,000°C).
This image shows the building and annealing of a rectangular block of stainless steel 316L. The first and second panels are the focused scanning laser melting the powder layer into the underlying part. The third panel is the diode turning on and illuminating the surface of the part to heat and anneal it. The last panel is right after the diode turns off, showing the block is at high temperature (>950°C).
The finished parts, with their thick legs and thin overhang section, let researchers measure how much residual stress was relieved by cutting off one of the legs and analyzing how much the weaker overhang section moved. When diodes were used, the bridge didn’t deflect anymore, researchers said.
“Building the parts was similar to how a normal metal 3D printer works, but the novel part of our machine is we use a secondary laser that projects over a larger area to heat the part afterward each layer. It raises the temperature rapidly and slowly cools it in a controlled fashion,” Smith says. “When we used the diodes, we saw a trend in the reduction of residual stress, and it compared to what is done traditionally by annealing a part in an oven afterwards.”
The approach is an offshoot of a previous project in which laser diodes, developed to smooth out lasers in NIF, were used to 3D print entire metal layers in one shot. It is better than other common methods for reducing residual stress in metal parts, such as altering the scanning strategy or using a heated build plate, Roehling says. And because the approach heats from the top, there’s no limit on how tall the parts can be.
Researchers will try to increase the number of layers per heating cycle to see if they can reduce residual stress to the same degree, attempt more complex parts, and use more quantitative techniques to gain a more in-depth understanding of the process.
“This technology is something that could be scaled up, because right now we’re projecting over a relatively small area and there’s still a lot of room for improvement,” Smith says. “By adding more of the diode lasers, we could add more heating area if someone wanted to integrate this into a system with a larger printing area.”
More importantly, Roehling says, researchers will explore controlling phase transformations in titanium alloy (Ti64). Typically, when building with Ti64, phase transformation causes the metal to become extremely brittle, causing parts to crack. If researchers could avoid the transformation by cooling the part slowly, it could make the material ductile enough to meet aerospace standards.