Heat Treatments and Critical Quenching Rates in Additively Manufactured Al–Si–Mg Alloys

Abstract

Laser powder-bed fusion (LPBF) has significantly gained in importance and has become one of the major fabrication techniques within metal additive manufacturing. The fast cooling rates achieved in LPBF due to a relatively small melt pool on a much larger component or substrate, acting as heat sink, result in fine-grained microstructures and high oversaturation of alloying elements in the α-aluminum. Al–Si–Mg alloys thus can be effectively precipitation hardened. Moreover, the solidified material undergoes an intrinsic heat treatment, whilst the layers above are irradiated and the elevated temperature in the built chamber starts the clustering process of alloying elements directly after a scan track is fabricated. These silicon–magnesium clusters were observed with atom probe tomography in as-built samples. Similar beneficial clustering behavior at higher temperatures is known from the direct-aging approach in cast samples, whereby the artificial aging is performed immediately after solution annealing and quenching. Transferring this approach to LPBF samples as a possible post-heat treatment revealed that even after direct aging, the outstanding hardness of the as-built condition could, at best, be met, but for most instances it was significantly lower. Our investigations showed that LPBF Al–Si–Mg exhibited a high dependency on the quenching rate, which is significantly more pronounced than in cast reference samples, requiring two to three times higher quenching rate after solution annealing to yield similar hardness results. This suggests that due to the finer microstructure and the shorter diffusion path in Al–Si–Mg fabricated by LPBF, it is more challenging to achieve a metastable oversaturation necessary for precipitation hardening. This may be especially problematic in larger components.

Tensile and compressive behaviour of additively manufactured AlSi10Mg samples

“…This is a result which originates from the same principle, mainly the rapid cooling of the melt to a solid, which forms the fine cellular structure, followed by the rapid quenching of the solidified material from solidification temperature to the process preheating temperature. These quenching rates cannot be reproduced by means of heat treatments and, therefore, lead to a maximum of dissolved alloying elements in the oversaturated α-Al crystals [38]. This, in turn, benefits the solid solution hardening, but more importantly, the possible precipitation hardening of these alloys, whereby the artificial aging of the just solidified and quenched material starts instantaneously during the process, due to the common process temperatures of around 200 °C being in the regime of artificial aging temperatures of Al-Si-Mg alloys.…”

Hot Isostatic Pressing of Aluminum–Silicon Alloys Fabricated by Laser Powder-Bed Fusion

“…A recently published study by our research group [22] showed that the critical cooling rate necessary to achieve appreciable age-hardening for Laser-powder bed fused Al-Si-Mg ranged between 11 to 15 K/s under vacuum condition, whereas a quenching rate of 5 K/s was sufficient for cast reference material.…”

Effect of Microstructure on the Dimensional Stability of Extruded Pure Aluminum

“…[22,23]. In recent years, annealing heat treatment has attracted wide attention due to its advantages of easy operation, mature technology and low cost [24][25][26]. For the most pure Al alloys, the recovery process starts at 95 • C~150 • C, while the recrystallization process starts at 200 • C~300 • C [27].…”