Diode laser used to reduce residual stress in metals during additive manufacturing
Scientists are using a high-power diode laser to reduce the residual stresses that build up in parts during metal additive manufacturing. The new technique, described in a paper
released online by the journal Additive Manufacturing, resulted in the reduction of effective residual stress in metal 3D-printed test parts by 90 per cent. In additive manufacturing (AM), residual
stress can build up in parts during the printing process due to expansion of heated material and contraction of cold material. This can generate forces that can distort the part and cause cracks that may weaken or even tear it to pieces, especially in metals. The researchers, from Lawrence
Livermore National Laboratory (LLNL) and the University of California, Davis, are using a high-power diode laser projected over a larger area to rapidly heat the printed metal layers during a build, which enables them to reduce temperature gradients and control cooling rates in the material. Using the approach, they can effectively get rid of residual stresses to the point where part failures no longer occur in the build. For the study, LLNL engineer and co-lead author Will Smith built small, bridge-like structures from 316L stainless steel using laser powder bed fusion (LPBF). Smith let each layer solidify before illuminating their surfaces with a secondary diode laser, initially at full power and immediately ramping down the intensity over 20 seconds. The result was akin to putting the part in a furnace after each layer, as surface temperatures reached about 1,000°C. The structure of the finished parts, with their thick legs and thin overhang section,
allowed researchers to measure how much residual stress was relieved by cutting one of the legs off and analysing how much the weaker overhang section moved. The research team said when the diode laser was used, the bridge no longer deflected. The researchers claim their technique is
more effective than other common methods used for reducing residual stress in metal parts during AM, such as altering the laser scanning strategy or using a heated build plate. In addition, because their approach heats from the top, there’s no limit on the height of the parts being built. The researchers will next perform a more
in-depth study, turning their attention to increasing the number of layers per heating cycle to see if they can reduce residual stress to the same degree, as well as attempting more complex parts and using more quantitative techniques to gain a more in-depth understanding of their process. ‘This technology is something that could
be scaled up, because we’re projecting over a relatively small area and there’s still a lot of room for improvement,’ Smith said. ‘By adding more diode lasers, we could add more heating area if someone wanted to integrate this into a system with a larger printing area.’ The researchers will explore controlling
phase transformations in titanium alloy (Ti64). 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 for aerospace standards.
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