Functionally graded materials: using additive manufacturing to design and tailor metal properties

Functionally graded materials (FGM) are designed materials tailored to exhibit as-built inhomogeneous compositions and microstructures. Since Laser Metal Deposition (LMD) uses various hoppers to supply powder feedstock at the laser focus, it has the ability to produce FGMs by selectively depositing different elemental powders into the molten pool at specific locations in the structure during part buildup [1,2].

Schematic of a laser direct deposition system [5].

LMD is considered a quick and flexible strategy to tailor the compositions and microstructures during additive manufacturing of components.

Dissimilar powder materials can be placed into separate powder hoppers. The computer control system, which is integrated into the powder feed system, enables the user to vary the deposit composition of a function of position. The use of multiple powder feeders in the LMD system makes it possible to investigate new alloys’ composition potential mechanical properties such as those of high entropy alloys [4].

Composition variation in 1 layer of a cylindrical sample [5]

Fabricating three-dimensional heterogeneous FGMs objects requires an integrated design and fabrication planning system. [5]  A knowledge-based material design and a constructive representation are generally combined to discretise an heterogeneous object, which has continuous material variation, into a multi-material part with stepwise variations that can be used by the DMD process.

computer controlled hoppers for LDD system. [7]

Using this step-by-step technique, graded samples of Cu–xNi reveal how the material compositions change during deposition of FGMs.[5]

Binary Ti–xMo alloys samples were deposited from elemental Ti to Ti–25at-% Mo within a 25 mm length part using LMD. The microstructures across the graded alloy correspond to those typically observed in α/β Ti alloys, but the microstructural scale is significantly refined. Interesting microstructure gradients are tailored across the alloy.

Microstructures variation of Ti-xMo graded alloys built using LDD. [2,6]

The ability to achieve such substantial changes in composition/microstructure across a rather limited length makes LMD a highly attractive candidate for developing novel structured FGM components with unique properties. It is widely accepted that the ability to produce near net-shape components with graded compositions from elemental powders using LMD may potentially be a feasible route for manufacturing unitized structures for high demanding aerospace applications.

References
[1]. Schwendner KI, Banerjee R, Collins PC et al (2001) Direct laser deposition of alloys from elemental powder blends. Scr Mater 45(10):1123–1129
[2]. Banerjee R, Collins PC, Bhattacharyya D et al (2003) Microstructural evolution in laser deposited compositionally graded a/b titanium-vanadium alloys. Acta Mater 51(11):3277–3292
[3]. Liu WP, Dupont JN (2003) In-situ reactive processing of nickel aluminides by laser-engineered net shaping. Metall Mater Trans A 34(11):2633–2641
[4] I. Kunce, , M. Polanski, J. Bystrzycki, Structure and hydrogen storage properties of a high entropy ZrTiVCrFeNi alloy synthesized using Laser Engineered Net Shaping (LENS), International Journal of Hydrogen Energy Volume 38, Issue 27, 10 September 2013, Pages 12180–12189
[5]. Shin KH, Natu H, Dutta D et al (2003) A method for the design and fabrication of heterogeneous objects. Mater Des 24(5):339–353[6]. Collins PC, Banerjee R, Banerjee S et al (2003) Laser deposition of compositionally graded titanium-vanadium and titanium-molybdenum alloys. Mater Sci Eng A 352(1/2):118–128
[7] http://www.gtv-mbh.com/?changelang=2
[8] http://matter.media.mit.edu/assets/pdf/Publications_FGRP.pdf