A team of researchers led by Dr. Lin Liu at the Huazhong University of Science and Technology (HUST), China, have discovered a way to scale-up 3D printing of bulk metallic glass (BMG).
With a unique atomic structure, BMG alloys are highly resistant to wear and corrosion while maintaining the melted malleability of glass. However, this strong combination comes at a cost – micro-cracks that occur during 3D printing present a severe disadvantage to BMG utility.
At HUST, Dr. Liu, Jianji Zhang, Wei Xing, Di Ouyang and Ning Li have developed composite iron and iron-nickle BMG alloys that suppress these deal-breaking micro-cracks, with findings that provide general guidelines for processing BMGs via selective laser melting (SLM).
The creation of micro-cracks
Micro-cracks occur in metal 3D printed components due to the high thermal stresses imposed by the melting process. By applying finite element analysis (FEA) to this process, and conducting systematic experiments on 3D printed iron alloys, the HUST team have pinpointed the trigger the points of these stresses.
As explained by the researchers, “we revealed that micro-cracks in inherently brittle Fe-based metallic glass during selective laser melting (SLM) are triggered by highly concentrated thermal stress around micro-pores, which is difficult to avoid during SLM even by careful process optimization.”
Preventing flaws in BMGs
The HUST team’s solution to suppress the micro-cracks that occur in SLM is the addition of high-toughness copper and copper-nickel alloys to make BMG composites.
The high toughness of these alloys occurs in the second phase of transition. As such, “The results revealed that the generation of highdensity dislocations in second phases during SLM drastically reduce thermal stress by releasing strain energy,”
“and thus,” the study continues, “suppress micro-crack formation.”
The fracture toughness of these composite iron BMGs was proven to be approximately 20 times higher than standard iron BMG.
As Dr. Liu explains, “The most important finding in this work is not simply adding a toughening metals in Fe-based amorphous matrix (actually Cu and Cu-Ni are not tough enough), the most important issue is that Cu and Cu-Ni are immisible with Fe, so that the addition of the second phase could not severely deteriorate the glass forming ability of the Fe-based BMG in 3D printing process and simultaneously suppressed cracking.”
Real world application of BMGs
For their potential to withstand the sub-zero climate of Jupiter’s Moon Europa, NASA has also been investigating the 3D printed potential of BMGs. More recently, a paper co-authored by Desktop Metal engineers focuses on the extrudability of these alloys.
“3D printing of Fe-based bulk metallic glass composites with combined high strength and fracture toughness” is published online in Materials & Design journal. It is co-authored by Ning Li, Jianji Zhang, Wei Xing, Di Ouyang and Lin Liu.
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Featured image shows SEM imaging of a micro-crack that forms inside a copper-nickle BMG when 3D printed by SLM. Image via Materials & Design