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 1 Fig. 4 — Tensile results of common additive aluminum and Al7075. Top left, tensile results indicate an increased tensile strength over the original nanofunctionalized Al70751 utilizing lower laser energy density and HIP post treatment; top right, principle strain contour map produced from digital image correlation indicates high levels of strain immediately before fracture in the HIP’d sample; bottom, table of specific results and ranges. *Indicates this study. dependent of laser parameters and scan strategies. The solidification and growth behavior of the baseline Al7075 and Al6061 alloys implies that an un- dercooled region exists ahead of the so- lidification front. In order to promote equiaxed grain growth, one or more nu- cleation events are required to occur in the available undercooled region ahead of the local solidification front, to be- gin the growth of new grains that block the continued growth of detrimental columnar structures. Due to the solid- ification velocities and associated ther- mal gradients encountered in additive processes, the likelihood of a homoge- nous nucleation event occurring in this region is low. The authors instead lever- age heterogeneous nucleation to en- sure a highly active nucleation site is available at minimal amounts of under- cooling, eliminating the need for signifi- cant parameter manipulation to control the final microstructure. The process by which this is accomplished is termed nanofunctionalization . For nanofunctionalization, first, nucleants that promote growth of the target phase are identified. In the case of aluminum, this is the fcc-alpha alu- minum structure. A large suite of po- tential candidate compounds were an- alyzed to determine which crystal struc- tures might be capable of promoting heterogeneous nucleation for the tar- get phase. For fcc-alpha aluminum, the Al 3 Zr phase was identified as an ideal candidate based on its thermodynam- ic stability and low crystallographic mismatch with the primary alumi- num phase (Fig. 1). After identification of the target composition, nanoparti- cle nucleants were then assembled on the surface of the baseline alloy pow- der (Al7075 or Al6061) to ensure uni- form distribution of the nucleant phase at the scale of individual powder par- ticles, which ranged from 15 to 60 µm, as opposed to a stochastic mixture nu- cleant and alloy particles. These func- tionalized alloy powders were then processed in conventional AM equip- ment, in this case a Concept Laser M2 system. While this process was demon- strated in its entirety using the Al7075 and Al6061 systems in a laser powder bed process, the general approach can be applied to almost any alloy system and any powder based additive pro- cess (such as LENS, SLM, and EBM). In comparison to the aforementioned lo- calized parameter control methods, an alloy and machine agnostic nanofunc- tionalization approach is an ideal meth- od to rapidly expand available alloy systems for AM using currently avail- able laser parameter sets. The initial efforts on AM of nano- functionalized Al7075 and Al6061 were recently presented by Martin et al. [1] with the main results summarized in Figs. 1 and 4. By controlling the mi- crostructure during solidification, hot cracking is completely eliminated and the resulting strength falls within the bounds of conventional wrought Al7075-T6 plate. It was also reported that while the improvement in strength over equivalently processed additive AlSi10Mg alloys was substantial, ad- ditional gains could be made in both strengthandductility to reach theupper bounds of wrought Al 7075-T6 proper- ties. To retain consistency in comparing the performance of AlSi10Mg and nano- functionalized Al7075, equivalent laser parameters were used for both alloys in this initial study. As a result, signifi- cant vaporization of high vapor pres- sure elements in Al7075 (Zn and Mg) was observed, resulting in residual po- rosity and a decrease in strength com- pared to the wrought baseline (Fig. 5).