Laser-based directed energy deposition of novel Sc/Zr-modified Al-Mg alloys: columnar-to-equiaxed transition and aging hardening behavior

doi:10.1016/j.jmst.2020.08.003

Abstract

Abstract:

The control of grain morphology is important in laser additive manufacturing (LAM), as grain morphology further affects the hot cracking resistance, anisotropy, and strength-ductility synergy of materials. To develop a solidification-control solution and achieve columnar-to-equiaxed transition (CET) in Al-based alloys during LAM, Sc-and-Zr-modified Al-Mg alloys were processed via directed energy deposition (DED). CET was achieved by introducing high potent primary Al₃(Sc,Zr) nucleation sites ahead of the solidification interface. Furthermore, the relationship between the solidification control parameters and precipitation behavior of primary Al₃(Sc,Zr) nucleation sites was established using the time-dependent nucleation theory. Then, the CET was studied according to the Hunt criterion. The results indicated that coarse columnar grain structure was still obtained at the inner region of the molten pool at low Sc/Zr contents owing to the effective suppression of the precipitation of the primary Al₃(Sc,Zr) nucleation sites via rapid solidification during DED. In addition, the relatively low melt temperature at the fusion boundary unavoidably promoted the precipitation of primary Al₃(Sc,Zr) nucleation sites, which resulted in a fine equiaxed grains band at the edge of the molten pool. As the Sc/Zr content increased, the solidification cooling rate was not sufficient to suppress the precipitation of the primary Al₃(Sc,Zr) nucleation sites, and a fully equiaxed grain structure was obtained. Furthermore, the effect of the layer-by-layer manufacturing process on the subsequent precipitation strengthening of secondary Al₃(Sc,Zr) precipitates was discussed. Both the remelting and subsequent aging during thermal cycling should be considered to achieve greater precipitation strengthening.

Key words: Al-Mg-Sc-Zr alloys, Additive manufacturing, Directed energy deposition, Columnar-to-equiaxed transition, Phase precipitation

Zihong Wang, Xin Lin, Yao Tang, Nan Kang, Xuehao Gao, Shuoqing Shi, Weidong Huang. Laser-based directed energy deposition of novel Sc/Zr-modified Al-Mg alloys: columnar-to-equiaxed transition and aging hardening behavior[J]. J. Mater. Sci. Technol., 2021, 69: 168-179.

Figures/Tables 16

Fig. 1. Secondary electron (SE) image showing the morphology of the Al-Mg-Sc-Zr alloy powders (a); particles size distribution of the powders (b); schematics of DED process (c), scanning strategies of single-track/multi-layer deposits (d) and bulk deposits (e). The sample coordinate system is indicated and Z is the deposition direction.

Table 1 Chemical compositions of the three types of powder materials (wt.%).

Alloys	Mg	Mn	Sc	Zr	Fe	Si	Al
0.25Sc-0.12 Zr alloy	4.82	0.68	0.25	0.12	0.19	0.03	Bal.
0.35Sc-0.16 Zr alloy	4.87	0.67	0.35	0.16	0.17	0.03	Bal.
0.50Sc-0.21 Zr alloy	4.86	0.68	0.50	0.21	0.20	0.02	Bal.

Table 2 Thermal physical parameters used in thermal simulation [14].

Property	Unit	T = 100 °C	T =635 °C	T =900 °C	T = 1400 °C
Specific heat capacity	J/(mol K)	25.6	31.6	Constant 29.4
Thermal conductivity	W/(m K)	129.5	86.9	98.6	117.2
Density	kg/m3	2675

Fig. 2. Polarized light micrographs showing the grain structure of bottom-middle area (a), and top layer (b); EBSD map at the equiaxed-to-columnar transition region (c); {001} pole figures of the selected CG region (d) and FG region (e); <001> is parallel to the deposition direction (Z).

Fig. 3. Backscattered electron (BSE) image showing the distribution characteristic of the second phase particles at the equiaxed-to-columnar transition region (a); EDS results of point A (b), point B (c) and point C (d).

Fig. 4. SEM image showing the FG-to-CG transition region, and the interface was marked by a microhardness indentation (a). EPMA mapping images showing the distribution of Al, Mg, Sc, Zr, Mn, Fe, and Si elements over the FG-to-CG transition region (b-h).

Fig. 5. STEM-EDS mapping results of the Sc, Zr, Mg, Mn, Fe, and Si for the second phase particles (a); BF-TEM image showing the Al3(Sc,Zr) precipitate within the grain (b); SADP image taken along $[\bar{1}12]$ showing the 110 type super-lattice reflections of L12 phase structure (c).

Fig. 6. Polarized light micrographs showing the grain structure of the top layer and middle area of block deposits. (a, d) 0.25Sc-0.12 Zr alloy; (b, e) 0.35Sc-0.16 Zr alloy; (c, f) 0.50Sc-0.21 Zr alloy.

Fig. 7. BSE images showing the characteristics of the particles at the fusion boundary and the inner region of molten pool for 0.25Sc-0.12 Zr alloy (a), 0.35Sc-0.16 Zr alloy (b) and 0.50Sc-0.21 Zr alloy (c), respectively; area fractions of the particles at the fusion boundary and the inner region of molten pool (d).

Fig. 8. Polarized light micrograph showing the matrix of microhardness indentations in single-track/multi-layer deposits (0.35Sc-0.16 Zr alloy) (a); microhardness contour maps under as built (b) and after aging conditions (c); average microhardness results along the deposition direction (d). The indentations at FG region were marked as the red rectangles in (a).

Fig. 9. Microhardness results of block deposits under as built and after aging conditions (a); evolution of the microhardness with the total Sc/Zr content (b).

Fig. 10. Equilibrium phase diagram of Al-3.3Mg-0.68Mn-xSc (wt.%) alloy system calculated by Thermo-Calc software with TTAL7 database (a); the predicted critical condition required to suppress the nucleation of primary Al3Sc phase (b). The equivalent Sc contents were marked by the red dotted lines.

Table 3 Physical properties used in the prediction model of primary Al3Sc phase [20,28,30].

Physical properties	Value
⍴ (Al₃Sc)	3030 kg/m³
a	5 × 10^-10 m
S_m (Al₃Sc)	42.2 J/(mol K)

Fig. 11. Simulated temperature field (a) and temperature-time dependency of the marked 13 different positions at the center line of the molten pool (b); evolution of solidification cooling rate with the depths of the molten pool (c). The liquids temperature of primary Al3Sc phase was marked as the black dot line.

Fig. 12. Simulated temperature field of molten pool (a) and the evolution of temperature gradient (G) and solidification velocity (V) with the depths of the molten pool (b).

Fig. 13. BF-TEM image showing the in-situ precipitated secondary Al3(Sc,Zr) in α-Al matrix of the block deposits for the 0.35Sc-0.16 Zr alloy (a); simulated thermal history of points at heights of 0 and 4 mm in the block deposits during DED (b).

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