Laser peening to treat cracks and voids in additive manufacturing
Laser peening might be one of the key next steps for additive manufacturing.
3-D printing, or additive manufacturing (AM), started as a popular modeling and prototyping tool. Today, Airbus has more than 1,000 3-D printed parts on its A-350 XWB jets, while Lockheed Martin and Boeing are aggressively incorporating the technology as well.
“From simple geometry parts with lots of cracks and voids that you risk breaking in your hand, they’ve come a long way toward robust parts that are actually being used in different industries,” said Micheal Kattoura, a Materials Sciences Engineer at LSP Technologies.
Mission-Critical AM Components
Can AM parts serve in mission-critical functions, facing high pressures, cracking and corrosion? Right now, that depends on the development of strategies for strengthening AM-manufactured parts to compensate for their weaknesses.
Kattoura worked on AM parts and laser peening during his postgraduate work at the University of Cincinnati, and he recently explored the potential of laser peening for improving metal alloys at the 2019 TMS Annual Meeting & Exhibition in San Antonio, Texas, one of the largest annual material science conferences with 4,000 attendees.
The conference featured a symposium on Additive Manufacturing of Metals, or 3-D printing using powdered metals. Kattoura presented his own academic research, entitled, “Effect of Laser Shock Peening Processing Parameters on the Microstructure, Residual Stress, and Fatigue Behavior of Additive Manufactured CoCr Alloy.”
Laser peening has a great deal of promise for enhancing the performance of parts created in additive manufacturing (AM), Kattoura concluded. Manufacturers should soon explore laser peening to provide parts with higher performance and safety, he believes.
“There is so much energy used in the additive manufacturing process that problems can arise,” Kattoura explained. “We’re seeing voids in the metal. We’re getting undesirable variables in the microstructures, like grain size, grain orientation, and precipitates. Sometimes we’re seeing high tensile residual stress, the result of high energy levels, and that can result in cracks and distortions, too.”
Remedies to enhance parts from additive manufacturing
AM materials scientists are trying several remedies. One avenue is adjusting the parameters or variables in the additive manufacturing process itself. But they have much to learn about how each parameter affects the ultimate integrity of AM-manufactured parts.
Kattoura surveyed some other strategies, including post-processing techniques used to enhance AM-printed metal parts:
• Hot isostatic pressing (HIP) can be used to reduce the porosity of metals that improves the material’s mechanical properties and workability. HIP adds to the density of powdered metal in a furnace at high temperatures and pressures.
• Heat treatments at various temperatures, such as debinding, carburizing, sintering, and annealing may help obtain the desired microstructure of AM-printed parts.
“But these treatments often pose additional challenges and problems, and AM researchers are well aware of the challenges,” Kattoura said. “The parts may crack during the heat treatment and cooling cycle, or they may become so distorted they cause tolerance issues.
Adding laser peening to the AM mix
Fortunately, Kattoura said, laser shock peening (LSP) can provide a ready and viable solution to some of these problems in multiple ways.
• LSP replaces the detrimental tensile stresses created in parts during the AM process with beneficial compressive stresses that enhance the material properties of parts.
• LSP reduces voids near the surface created by AM, just as LSP reduced voids in powdered metal parts manufactured through more conventional methods.
• LSP could eliminate the need for any additional processing of the entire parts (HIP or Heat treatment) and the accompanied challenges.
• Applying LSP to the critical area of the part enhances its material properties and mitigates different types of failures.
Kattoura researched how laser shock peening would affect the microstructure, residual stress, and fatigue behavior of cobalt-chromium (CoCr) alloy after additive manufacturing processes.
Synergy of Laser Peening and 3-D printing
Under a variety of conditions, laser peening led to an increase in the hardness and high magnitudes of surface compressive stresses for the alloy. “In addition, the study showed laser peening could increase the fatigue life of AM-produced parts by 10-15 times,” Kattoura said.
“Additive manufacturing has made much progress in recent years, with AM parts approved for some low risk applications in aerospace. Laser shock peening can add additional capability and confidence to the reliability of AM parts,” Kattoura said. “The promise of AM will continue to attract funding for basic and applied research, and we hope LSP Technologies will find technology collaborators interested in combining laser peening with AM.”