New research iden­ti­fies causes for defects in 3D print­ing and paves way for bet­ter results

Addit­ive manufacturing’s prom­ise to revo­lu­tion­ize industry is con­strained by a wide­spread prob­lem: tiny gas pock­ets in the final product, which can lead to cracks and oth­er fail­ures. New research pub­lished today in Sci­ence, led by teams from Carne­gie Mel­lon Uni­ver­sity and the U.S. Depart­ment of Energy’s (DOE) Argonne Nation­al Labor­at­ory, has iden­ti­fied how and when these gas pock­ets form, as well as a meth­od­o­logy to pre­dict their form­a­tion — inform­a­tion that could dra­mat­ic­ally improve the 3D print­ing pro­cess. ​“The research in this paper will trans­late into bet­ter qual­ity and bet­ter con­trol in work­ing with the machines,” said Anthony Rol­lett, a Pro­fess­or of Mater­i­als Sci­ence and Engin­eer­ing at Carne­gie Mel­lon Uni­ver­sity and an author on the paper.​“For addit­ive man­u­fac­tur­ing to really take off for the major­ity of com­pan­ies, we need to improve the con­sist­ency of the fin­ished products. This research is a major step in that dir­ec­tion.” The sci­ent­ists used the extremely bright, high-energy X‑rays at Argonne’s Advanced Photon Source (APS), a DOE Office of Sci­ence User Facil­ity, to take super-fast video and images of a pro­cess called Laser Power Bed Fusion (LPBF), in which lasers are used to melt and fuse mater­i­al powder togeth­er. The lasers, which scan over each lay­er of powder to fuse met­al where it is needed, lit­er­ally cre­ate the fin­ished product from the ground up. Defects can form when pock­ets of gas become trapped into these lay­ers, caus­ing imper­fec­tions that could lead to cracks or oth­er break­downs in the final product. Until now, man­u­fac­tur­ers and research­ers did not know much about how the laser drills into the met­al, pro­du­cing cav­it­ies called​“vapor depres­sions,” but they assumed that the type of met­al powder or strength of laser were to blame. As a res­ult, man­u­fac­tur­ers have been using a tri­al and error approach with dif­fer­ent types of metals and lasers to seek to reduce the defects. In fact, the research shows that these vapor depres­sions exist under nearly all con­di­tions in the pro­cess, no mat­ter the laser or met­al. Even more import­ant, the research shows how to pre­dict when a small depres­sion will grow into a big and unstable one that can poten­tially cre­ate a defect. ​“We’re draw­ing back the veil and reveal­ing what’s really going on,” Rol­lett said.​“Most people think you shine a laser light on the sur­face of a met­al powder, the light is absorbed by the mater­i­al, and it melts the met­al into a melt pool. In actu­al­ity, you’re really drilling a hole into the met­al.” By using highly spe­cial­ized equip­ment at Argonne’s APS, one of the most power­ful syn­chro­tron facil­it­ies in the world, research­ers watched what hap­pens as the laser moves across the met­al powder bed to cre­ate each lay­er of the product. Under per­fect con­di­tions, the melt pool shape is shal­low and semi­cir­cu­lar, called the​“con­duc­tion mode.” But dur­ing the actu­al print­ing pro­cess, the high-power laser, often mov­ing at a low speed, can change the melt pool shape to some­thing like a key­hole in a war­ded lock: round and large on top, with a nar­row spike at bot­tom. Such​“key­hole mode” melt­ing can poten­tially lead to defects in the final product. ​“Based on this research, we now know that the key­hole phe­nomen­on is more import­ant, in many ways, than the powder being used in addit­ive man­u­fac­tur­ing,” said Ross Cun­ning­ham, a recent gradu­ate from Carne­gie Mel­lon Uni­ver­sity and one of the co-first authors of this paper.​“Our research shows that you can pre­dict the factors that lead to a key­hole — which means you can also isol­ate those factors for bet­ter res­ults.” The research shows that key­holes form when a cer­tain laser power dens­ity is reached that is suf­fi­cient to boil the met­al. This, in turn, reveals the crit­ic­al import­ance of the laser focus in the addit­ive man­u­fac­tur­ing pro­cess, an ele­ment that has received scant atten­tion so far, accord­ing to the research team. ​“The key­hole phe­nomen­on was able to be viewed for the first time with such details because of the spe­cial­ized cap­ab­il­ity developed at Argonne,” said Tao Sun, an Argonne phys­i­cist and an author on the paper.​“Of course, the intense high-energy X‑ray beam at the APS is the key.” The exper­i­ment plat­form that sup­ports study of addit­ive man­u­fac­tur­ing includes a laser appar­at­us, spe­cial­ized detect­ors, and ded­ic­ated beam­line instru­ments. In 2016, the Argonne team, togeth­er with their research part­ners, cap­tured the first-ever X‑ray video of laser addit­ive man­u­fac­tur­ing at micro­met­er and micro­second scales. That study increased interest in the tech­niques and the kinds of prob­lems that could be researched at Argonne’s APS. ​“We are really study­ing the most basic sci­ence prob­lem, which is what hap­pens to met­al when you heat it up with a high-power laser,” said Cang Zhao, an Argonne postdoc and the oth­er co-first author of the paper.​“At the same time, because of our unique exper­i­ment­al cap­ab­il­ity, we are able to work with our col­lab­or­at­ors on exper­i­ments that are really valu­able to man­u­fac­tur­ers.” The research team believes this research could motiv­ate makers of addit­ive man­u­fac­tur­ing machines to offer more flex­ib­il­ity when con­trolling the machines and that the improved use of the machines could lead to a sig­ni­fic­ant improve­ment in the final product. In addi­tion, if these insights are acted upon, the pro­cess for 3D print­ing could get faster. ​“It’s import­ant because 3D print­ing in gen­er­al is rather slow,” Rol­lett said.​“It takes hours to print a part that is a few inches high. That’s OK if you can afford to pay for the tech­nique, but we need to do bet­ter.” Source: Argonne Nation­al Labor­at­ory, press release, FEB­RU­ARY 22, 2019