January_AMP_Digital

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 0 currently used, only about 10 can be readily produced via AM, the most com- mon including Inconel 718, Ti6Al4V, a CoCr-base alloy, and AlSi10Mg [2,4] . Welding, and by equivalency AM, produces unique microstructures, gen- erally comprised of columnar grains oriented parallel to the direction of the relatively high thermal gradient. In the additive process, the accumulation of “welds” through the build direction can lead to preferential grain selection, ulti- mately producing a component with a highly textured microstructure. In typ- ical additive alloys, such as AlSi10Mg, this preferential columnar grain selec- tion can lead to accumulation of ther- mal stresses and anisotropic material properties [5] . UNWELDABLE ALLOYS TEAR AND CRACK In comparison, when subjecting unweldable alloys such as Al7075 or Al6061 to AM processes, hot tearing and cracking occur, resulting in print- ed components that have little to no retained strength. During solidification, solute rejection occurs over the freezing range, which may be as much as 100°C below the initial solidification tempera- tures of the primary alpha-aluminum phase. In combination with a columnar growth mode, a thin layer of solute-rich interdendritic liquid is left between the growing dendrites (Fig. 3). This is par- ticularly problematic when process- ing high strength wrought aluminum compositions, as many aluminum alloy compositions can undergo >6% volu- metric contraction upon solidification. As a solid, these alloy compositions have one of the higher coefficients of thermal expansion of common struc- tural metals. The combination of these effects leads to deformation of the semi-solid structure during solidifica- tion and high levels of stored strain energy in the solidified material. De- forming the semi-solid skeleton re- quires some liquid backfill to avoid cavitation. However, with large colum- nar grains, dendrite coherency occurs early in the solidification process, lock- ing the structure in place and prevent- ing backfill. This ultimately leads to a cavitation or tearing event that allows the strain energy to be released in the form of a crack. This phenomenon has been mathematically described in the RDG model and is the apparent mech- anism of crack formation in additive- ly produced Al7075 and Al6061, where cracks oriented along boundaries of adjacent columnar grains are observed (Fig. 3) [6] . It has beenhypothesized that dele- terious hot tearing and cracking effects couldbemitigatedby transitioning from columnar growth (Fig. 3) to uniform fine equiaxed growth during solidification. Decreasing the grain size and avoiding columnar growth can delay the onset of coherency allowing the semi-solid skeleton to deform like a granular sol- id as opposed to a rigid structure prone to cracking and tearing [7] . Multiple at- tempts at controlling microstructure during AM to achieve equiaxed struc- tures have been attempted with mixed levels of success. Manipulating laser pa- rameters and scan strategies to control the thermal gradients and solidification velocities can aid in pushing the solidi- fication through the columnar to equi- axed transition [8,9] . This requires thermal modeling and iterative verification, and may not be applicable to every alloy and accompanying component geome- try. A common issue with AM is the vary- ing microstructures that may develop on a single build. This can be affect- ed by both height within the build and geometry of the component as resid- ual heat build-up or paths for heat ex- traction force variations in the thermal gradients and solidification velocities of complex geometry components. As such, new parameters and scan strate- gies may be required for different part geometries or build orientations to achieve consistent microstructure and properties for a given material. NANOFUNCTIONALIZATION USED TO CONTROL SOLIDIFICATION With this in mind, the authors sought to investigate alternate meth- odologies to control solidification in- Fig. 3 — Different microstructures in nanofunctionalized and non-functionalized Al7075. Top, non-functionalizedmaterial indicates cracking occurring at intercolumnar regions due to solid- ification behavior; bottom, nanofunctionalized Al7075 with a uniform, crack-free microstructure due to high strain tolerance during solidification.