Selective laser melting of high-strength Al-Mg-Si-Sc-Zr alloy: crack suppression and multiple strengthening mechanisms

Aluminum and its alloys are one of the most widely used structural materials after steel, and selective laser melting (SLM) is one of the most widely developed additive manufacturing methods. However, SLM parts often have metallurgical defects such as cracking, spheroidization and pores. Only a limited number of metals are suitable for printing parts with satisfactory density, required microstructure and strength. In order to develop high-strength aluminum alloys for selective laser melting (SLM) additive manufacturing, Li Ruidi and others from Central South University designed and manufactured a series of Al-Mg(-Si)-Sc-Zr alloys. In the absence of silicon, the developed Al-xMg-0.2Sc-0.1Zr (x=1.5, 3.0 and 6.0wt%) alloys are prone to hot cracks. The addition of 1.3wt% Si to the Al-6Mg-0.2Sc-0.1Zr alloy effectively suppresses the hot cracks in the SLM process and greatly refines the grains.

Figure 1. The effect of Mg and Si elements on solidification cracks and phases of SLM printed samples. The numbers (a~d) show the optical micrographs of SLM printed samples under 75 J/mm3 VED. (A1) and (a2): alloy #1 containing 1.5 wt% Mg; (b1) and (b2): alloy #2 containing 3.0 wt% Mg; (c1) and (c2): alloy containing 6.0 wt% Mg #3 alloy; (d1) and (d2): #4 alloy containing 6.0 wt% Mg + 1.3 wt% Si. The construction direction is from bottom to top.

Figure 2 EBSD chart shows the grain size and morphology of printed samples with different compositions. (a) No. 1 alloy 1.5 wt% Mg; (b) No. 2 alloy 3.0 wt% Mg; (c) No. 3 alloy 6.0 wt% Mg; (d) No. 4 alloy, 6.0 wt% Mg + 1.3 wt% Si. The inverse pole figure (IPF) is used to indicate the crystal orientation of aluminum. The construction direction is from bottom to top.

Through further refinement of the alloy composition, a new type of alloy Al-8.0Mg-1.3Si-0.5Mn-0.5Sc-0.3Zr was designed. This new alloy shows a significantly refined microstructure, including submicron cell bodies and coherent Al3(Sc, Zr) nanoparticles (2~15nm) existing in the unit cell and intercrystalline Al-Mg2Si eutectic (Mg2Si diameter is 10 ~100nm).

Figure 3 The microstructure of casting alloy #5 after aging at 360°C for 8 hours. (A) Bright field TEM image of cell body; (b) taken from (a) bright field TEM image showing Al3(Sc, Zr) nanoprecipitation; (C) HRTEM image (a) showing 9R phase and FCC α-Al matrix , Embed SAD pattern to confirm 9R phase. The phase boundary of 9R with CTB or ITB is marked with a red dotted line; (d, e, f) The HRTEM image of the three boxes in Figure (c) shows the gap between 9R and the FCC α-Al matrix containing a distorted 9R structure The diffusion zone.

Among aluminum alloys, solid solution strengthening, grain boundary strengthening, work hardening and precipitation/dispersion hardening are well-proven strengthening mechanisms. The #5 alloy sample formed high-density stacking faults and a unique 9R phase, and its tensile strength and elongation reached 497mpa and 11%, respectively. After calculating the strengthening effects of each strengthening mechanism, the author believes that the high-density stacking defects (SFs) and 9R long-period ordered stacking (LPSO) phase strengthening mechanisms provide additional strengthening. After aging treatment, the tensile strength reaches 550mpa, and the plasticity is between 8% and 17%.

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Selective laser melting of high-strength Al-Mg-Si-Sc-Zr alloy: crack suppression and multiple strengthening mechanisms

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