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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 8 2 2 The former is a natural effect of the high laser energy density parameters and directly impacts the latter by decreas- ing the amount of zinc and magnesium available for precipitation strength- ening. It was therefore proposed that by decreasing the laser energy densi- ty and performing a hot isostatic press (HIP) treatment after the build, strength and ductility could be improved. Each of these hypotheses were tested inde- pendently and the results are shown in Fig. 4. Decreasing the laser energy den- sity resulted in a >75% retention of the strengthening elements and a 10% in- crease in yield and ultimate tensile strength over the nanofunctionalized Al7075 with the ALSi10Mg parameters. While retaining additional alloying ele- ments may lead to higher crack suscep- tibility in conventional processing, the nanofunctionalized material still com- pletely avoids any hot cracking. Addi- tionally, utilizing nanofunctionalized Al7075 processed with AlSi10Mg param- eters and applying an industry standard HIP treatment, elongation increased by 3X and the elastic modulus increased by 5%, both indicating an elimination of residual porosity that promotes ear- ly fracture and decreases the effective cross-section area during tensile test- ing. The results indicate that crack-free high strength aluminum alloys can be processed via AM with equivalent prop- erties to the wrought counterpart with- out significant manipulation of laser parameters or scan strategies. SUMMARY To summarize, nanofunctional- ization has been successfully used to control the solidifying microstructure in AM and produce crack-free 7000 and 6000 series aluminum alloys for the first time. This approach can also be ap- plied to other difficult to process and crack-susceptible alloy systems; re- search across a broader spectrum of materials systems is currently under- way. Parameter optimization has been demonstrated for reducing the impact of elemental vaporization and this can be decoupled from the microstructure control aspect due to the overwhelming effect of introducing a high density of heterogeneous nucleation sites. While strength and ductility have been shown to be wrought equivalent, other mate- rial properties such as stress corrosion cracking and fatigue life will be required to meet production requirements of ex- isting components. While such investi- gations are still needed, the decreased grain size and eliminated porosity are good indications that these materi- al properties may be met. Additional- ly, the nanofunctionalization approach may be applicable to other non-pow- der based processing methods includ- ing wire-fed AM and analogous welding operations. This is a fundamentally new way to control microstructure during material processing and opens the door for additional material and process functionality. ~AM&P For more information: John H. Mar- tin is a research staff member at HRL Laboratories LLC, 3011 Malibu Canyon Rd., Malibu, CA 90265, 310.317.5000, jhmartin@hrl.com, www.hrl.com. Acknowledgments This work was funded by HRL Lab- oratories LLC. Special thanks to the HRL media team and Dana Martin for their aid in producing the figures. References 1. J.H. Martin, et al., 3D Printing of High-Strength Aluminium Alloys, Nature, Vol 549, 2017. 2. J.J. Lewandowski and M. Seifi, Met- al Additive Manufacturing: A Review of Mechanical Properties, Annu. Rev. Ma- ter. Res., Vol 46, p 151-186, 2016. 3. S. Kou, Welding Metallurgy, Wiley- Interscience, 2003.doi:10.1016/j.theo chem.2007.07.017. 4. W.E. Frazier, Metal Additive Manu- facturing: A Review, J. Mater. Eng. Per- form., Vol 23, 2014. 5. A. Basak and S. Das, Epitaxy and Mi- crostructure Evolution in Metal Additive Manufacturing, Annu. Rev. Mater. Res., Vol 46, annurev-matsci-070115-031728, 2015. 6. M. Rappaz, J. Drezet, and M. Gre- maud, A New Hot-Tearing Criterion, Metall. and Mat. Trans. A, Vol 30, p 449- 455, 1999. 7. L. Yuan, C. O’Sullivan, and C.M. Gourlay, Exploring Dendrite Coheren- cy with the Discrete Element Method, Acta. Mater., Vol 60, p 1334-1345, 2012. 8. R.R. Dehoff, et al., Site Specific Control of Crystallographic Grain Ori- entation through Electron Beam Addi- tive Manufacturing, Mater. Sci. Technol., Vol 31, p 931-938, 2015. 9. N. Raghavan, et al., Numerical Mod- eling of Heat-Transfer and the Influence of Process Parameters on Tailoring the Grain Morphology of IN718 in Electron Beam Additive Manufacturing, Acta Mater., Vol 112, p 303-314, 2016. 10. C. Laser and H.I. Group, CL 30AL/CL 31AL Aluminum Alloy, www.concept-la- ser.de, 2012. 11. H.E. Boyer and T.L. Gail, Materials Handbook Desk Edition, ASM Interna- tional, 1985. Fig. 5 — Zinc andmagnesium vaporization impacts with the standard AlSi10Mg parameters. Left, residual porosity seen in as-built nanofunctionalized Al7075 indicates a mixture of pore siz- es and shapes; right, composition change of the main strengthening elements in Al7075 during laser melting.