Diode laser used to reduce residual stress in metals during additive manufacturing

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Building a rectangular block of 316L stainless steel with LPBF and then annealing it with a diode laser. (Image: LLNL)

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, residual stress can build up in parts during the printing process due to the expansion of heated material and contraction of cold material. This can generate forces that can distort the part and cause cracks that can weaken or even tear it to pieces, especially in metals.

‘In metals, it’s really hard to overcome these stresses,’ said materials scientist John Roehling, the paper’s lead author. ‘There’s been a lot of work on trying to do things like 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.’

The researchers, from Lawrence Livermore National Laboratory (LLNL) and the University of California, Davis, are therefore using a high-power diode laser projecting 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 within the material. Using the approach, they can effectively get rid of the residual stresses to the point where part failures no longer occur during 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 a period of 20 seconds. The result was akin to putting the part in a furnace after each layer as surface temperatures reached about 1,000ᵒC.

Bridge-like structures additively manufactured from 316L stainless steel were used to demonstrate the benefits of annealing with a diode laser to reduce residual stresses in the material. (Credit: LLNL)

The structure of the finished parts, with their thick legs and thin overhang section, allowed the 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. According to the researchers, when the diode laser was used, the bridge didn’t deflect anymore.

The researchers claim that their technique is more effective than other common methods used for reducing residual stress in metal parts during additive manufacturing, 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 right now we’re projecting over a relatively small area and there’s still a lot of room for improvement,’ Smith noted. ‘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.’

More importantly, Roehling added, the researchers will also 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 the researchers could avoid the transformation by cooling the part slowly, it could make the material ductile enough to meet aerospace standards. According to Roehling, preliminary results for this have been promising.

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