Integrated Control of Melt Pool Geometry and Microstructure in Laser Powder Bed Fusion of AlSi10Mg
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ALSI10MG is extensively used in casting thin wall components and complex geometries. It is a hypo-eutectic alloy that consists of 10 pct Si, 0.35 pct Mg, 0.16 pct Fe, and 0.01 pct Ti.[1] The high fluidity, low shrinkage, and smaller solidification interval (~ 40 K) when compared to other aluminum alloys reduces the susceptibility of AlSi10Mg to hot cracking or hot tearing, making AlSi10Mg favorable for casting. For the same reasons, AlSi10Mg is also the major aluminum alloy supported for laser melting processes.[2] This material is used in the aerospace and automobile industries among others.[3] The equilibrium phase diagram for AlSi10Mg is shown in Figure 1. During solidification, a-Al nucleates first and continues to grow and during this process the concentration of silicon increases in the liquid and at a critical undercooling b phase nucleates. The a phase continues to grow in between the b phase resulting in a
SNEHA P. NARRA, LUKE SCIME, and JACK BEUTH are with the NextManufacturing Center, Mechanical Engineering Dept., Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted January 31, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
eutectic a + b matrix.[4] However, as shown in Figure 2, the structure of rapidly solidified AlSi10Mg is different from the equilibrium solidification structure observed in cast parts, which consists of primary aluminum and (Al + Si) eutectic.[5] Cast Al-Si has a coarser microstructure with secondary dendrite arm spacing on the order of tens of microns and the ultimate tensile strength increases with a decrease in the secondary dendrite arm spacing.[6] In laser melted AlSi10Mg, silicon is segregated at the aluminum cell boundaries. This fine structure in AlSi10Mg is the result of the high cooling rates in laser melting processes.[7,8] Solidification conditions which control the size scale of structures formed at solidification can be modified in real time in AM. For instance, AlMangour et al.[10] reported an increase in grain size of the TiC/316 L stainless steel nanocomposites with an increase in volumetric laser intensity resulting in a decrease in the cooling rate. Similarly, Raghavan et al.[11] were able to vary grain size and primary dendrite arm spacing in Inconel 718 via an electron beam melting process. Using heat transfer and phase field modeling, Ghosh et al.[12] demonstrated a change in primary arm spacing with cooling rate during laser deposition of Ni-Nb alloys. Because of rapid solidification, a fine solidification structure is observed in additively manufactured components.[13–15] Due to the fine microstructure, static mechanical properties are comparable to (or better than)
II.
Fig. 1—Equilibrium pseudo-binary phase diagram for AlSi10Mg.
Fig. 2—Rapidly solidified AlSi10Mg microstructure from a laser powder bed fusion process.[3,9]
the as-cast properties of AlSi10Mg,[7,9,16,17] and an improvement in mechanical properties of laser melted AlSi10Mg can be expected with a decrease in cell spacing. I
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