Laser Powder Bed Fusion of Crack-Proof Al-Cu-Mg-Y Alloys

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jul 6 2022

In an article recently published in the journal Additive Manufacturing, researchers discussed the mechanical characteristics and microstructure of a laser-fabricated, crack-free Al-Cu-Mg-Y alloy.

Study: Gel Microstructure characterization and mechanical properties of crack-free Al-Cu-Mg-Y alloy fabricated by laser powder bed fusion.Image Credit: Ingo Bartussek/Shutterstock.com 

Background

Laser powder bed fusion (LPBF), a powder bed-based additive manufacturing (AM) technology, has been used to create complicated geometric metallic components. Due to the increased need for producing high-strength lightweight components, the application of LPBF to Al alloys has recently received a lot of attention.

As previously mentioned, the Al-Si system is the most popular Al alloy for LPBF. Although the post-heat treatment can increase the ductility of Al-Si alloys produced by LPBF, the strength gradually begins to decline due to the coarsening of the fine cellular eutectic Si phases. Recently, attempts have been made to use the LPBF process to create heat-treatable wrought aluminum alloys.

Preheating must be used in order to prevent solidification fractures in wrought aluminum alloys produced by LPBF. However, it is challenging to induce the refined microstructure during the LPBF process with a preheating treatment at a fairly high temperature. In the LPBF-fabricated wrought Al alloys, the inoculation treatment has proven to be a more practical method of preventing the formation of solidification fractures. Recent investigations on the inoculation treatment of the crack-free wrought Al alloys made by LPBF have been widely publicized. The LPBF process's quick cooling rate and significant thermal gradient, however, severely restrict the beneficial effects of the inoculation treatment on grain refining. The rare earth element yttrium (Y) is a promising microalloying ingredient for Al alloys in terms of castability and mechanical characteristics.

About the Study

In this study, the authors discussed the development of a unique, high-strength Al-Cu-Mg-Y alloy by LPBF through the employment of powders of the 2024 alloy that were amended with rare-earth yttrium (Y). The rare earth element Y was found to be an efficient alloying element in the Al-Cu-Mg-Y alloy produced by LPBF. This was mostly due to the combination of the refined grains, the reduced solidification crack susceptibility index, and the restricted brittle temperature range. During the LPBF process, element Y could also react with Al and Cu to generate the Al₈Cu₄Y phases.

The team used more Al₈Cu₄Y phases and finer grains in the LPBF-produced Al-Cu-Mg-Y alloy to it a better compressive yield strength than the majority of Al alloys. The Al-Cu-Mg-Y alloy produced by LPBF was severely deformed without collapsing up to a significant compressive strain of about 70%. The Al-Cu-Mg-Y alloy produced by LPBF demonstrated a noticeable increase in compressive stress beyond the yield point after T6 heat treatment, which was consistent with the uniform distribution of the Ω precipitate in the Al matrix. In addition, for the Al-Cu-Mg-Y alloys produced by LPBF and heat-treated with T6, the tensile strength was lower than the compressive strength. The interior pores of the two alloys had a role in this variation.

The researchers used Y-modified 2024 alloy powders to create a new Al-Cu-Mg-Y alloy by LPBF. The brittle temperature range, sensitivity to solidification cracks, and the fine grain structure were reviewed in relation to the crack-elimination mechanism of the Al-Cu-Mg-Y alloy produced by LPBF. During the LPBF process and T6 heat treatment, the microstructure evolution and its impact on the mechanical characteristics of the crack-free Al-Cu-Mg-Y alloy were thoroughly examined. This work served as a foundation for the creation of innovative, high-strength Al alloys appropriate for LPBF.

Observations

The microstructure and compressive properties of the Al-Cu-Mg-Y alloy's microstructure were investigated and contrasted with the Y-free LPBF-produced Al-Cu-Mg alloy. It was shown that the inclusion of element Y was a successful method for the removal of solidification fractures and the enhancement of compressive characteristics. The LPBF-produced Al-Cu-Mg-Y alloy was crack-free with a porosity value of 1.27 ± 0.12 vol. %, in contrast to the Al-Cu-Mg alloy, which had almost straight solidification fractures along the building direction. The solidification crack susceptibility index, the brittle temperature range, and refined grains were related to the eradication of solidification cracks.

The presence of element Y caused the brittle temperature range to become more constrained, which reduced the likelihood of solidification cracks developing. The element Y could improve crack resistance by lowering the crack susceptibility index. The element Y refined the grains to increase their capacity for liquid feeding and to make semi-solid materials tougher. This prevented the solidification crack from forming between adjacent grains. The element Y addition caused the Al₂Cu phases in the LPBF-produced Al-Cu-Mg-Y alloy to be replaced by the Al₈Cu₄Y phases, which differed from the microstructure in the LPBF-produced Al-Cu-Mg alloy.

Al₈Cu₄Y phase formation was primarily caused by the greater negative formation enthalpy of such phases and the chemically active element Y. Additionally, element Y refined the grains in the Al-Cu-Mg-Y alloy produced by the LPBF by raising the grain growth restriction factor. The T6 heat treatment resulted in a change in the Al-Cu-Mg-Y alloy from Al₈Cu₄Y dendrites to Al₈Cu₄Y particles. The bigger Al₈Cu₄Y particles then coarsened at the expense of the smaller ones. Al₂CuMg and Al_xMn_y particle production both take place simultaneously.

Conclusions

In conclusion, this study elucidated that after T6 heat treatment, the overall grain structure in the LPBF-fabricated Al-Cu-Mg-Y alloy remains unchanged when taking into account the pinning of the grain boundary by the distributed particles in the Al matrix. The compressive yield strength of the Al-Cu-Mg-Y alloy, which was produced by LPBF, was 267 ± 10 MPa, more than that of the majority of Al alloys.

Due to the production of the precipitates, which resulted in high-level resistance to the shearing during plastic deformation, the Al-Cu-Mg-Y alloy produced by LPBF fabrication exhibited a quick increment in the compressive stress following the T6 heat treatment.

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