Additive Manufacturing of AlSi10Mg and Ti6Al4V Lightweight Alloys via Laser Powder Bed Fusion: A Review of Heat Treatments Effects

Abstract

Laser powder bed fusion (L-PBF) is an additive manufacturing technology that is gaining increasing interest in aerospace, automotive and biomedical applications due to the possibility of processing lightweight alloys such as AlSi10Mg and Ti6Al4V. Both these alloys have microstructures and mechanical properties that are strictly related to the type of heat treatment applied after the L-PBF process. The present review aimed to summarize the state of the art in terms of the microstructural morphology and consequent mechanical performance of these materials after different heat treatments. While optimization of the post-process heat treatment is key to obtaining excellent mechanical properties, the first requirement is to manufacture high quality and fully dense samples. Therefore, effects induced by the L-PBF process parameters and build platform temperatures were also summarized. In addition, effects induced by stress relief, annealing, solution, artificial and direct aging, hot isostatic pressing, and mixed heat treatments were reviewed for AlSi10Mg and Ti6AlV samples, highlighting variations in microstructure and corrosion resistance and consequent fracture mechanisms.

Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process

“…In opposition, "buy-to-fly" of AM of metals approaches a ratio of 1:1 due to its high material utilization in AM, the short production cycle and minimal use of tool machines [9][10][11]. As a matter of fact, AM technologies allow researchers to obtain a 3D physical object through the successive addition of material (layer-by-layer) following a CAD (Computer-Aided Design) project, which is converted into an .stl file before the manufacturing process [12,13]. Despite these advantages, Froes et al [1] affirmed that only 8.2% of the aerospace industry has adopted rapid-prototyping technology, while Gibbons et al [14] underlined that the certification and qualification of the Laser Powder-Bed Fusion (L-PBF) process, which is an AM technology, do not differ from the conventional manufacturing process in the aerospace field.…”

“…L-PBF is a technology wherein a laser source scans and melts a powder bed that is deposited by a roller or recoater on the build platform [12]. To obtain a fully dense and high-quality sample, the following full melting criterion proposed by Tang et al [15] must be satisfied:…”

“…Ti6Al4V-ELI (Extra-Low Interstitial) is an α + β alloy at room temperature that contains Al and V elements as αand β-stabilizers, respectively. In addition, the O, N and C atoms (in a small amount) stabilize the hcp (hexagonal closed packed) α-phase, while Fe stabilizes the bcc (body-centered cubic) β-phase [12,19,20]. Due to the high cooling rate (up to 10 8 K/s, [12]) that characterizes the L-PBF technology, the as-built microstructure of the Ti6Al4V-ELI sample is totally or partially formed by α -martensite contained within columnar β-grains arranged parallel to the build direction [21,22].…”

“…In addition, the O, N and C atoms (in a small amount) stabilize the hcp (hexagonal closed packed) α-phase, while Fe stabilizes the bcc (body-centered cubic) β-phase [12,19,20]. Due to the high cooling rate (up to 10 8 K/s, [12]) that characterizes the L-PBF technology, the as-built microstructure of the Ti6Al4V-ELI sample is totally or partially formed by α -martensite contained within columnar β-grains arranged parallel to the build direction [21,22]. Generally, the UTS (Ultimate Tensile Strength) and YS (Yield Strength) values of as-built Ti6Al4V-ELI samples reach values higher 1.1 and 1.0 GPa, respectively, which decrease when the temperature of the pre-heated build platform or the percentage of decomposed α -martensite are increased [12,[23][24][25].…”

“…Due to the high cooling rate (up to 10 8 K/s, [12]) that characterizes the L-PBF technology, the as-built microstructure of the Ti6Al4V-ELI sample is totally or partially formed by α -martensite contained within columnar β-grains arranged parallel to the build direction [21,22]. Generally, the UTS (Ultimate Tensile Strength) and YS (Yield Strength) values of as-built Ti6Al4V-ELI samples reach values higher 1.1 and 1.0 GPa, respectively, which decrease when the temperature of the pre-heated build platform or the percentage of decomposed α -martensite are increased [12,[23][24][25]. As a matter of fact, there is α → α decomposition and a subsequent loss in mechanical properties of the as-built sample if the build platform is kept at high temperature (T > 400 • C, the temperature at which the martensite decomposes [26][27][28]) and/or due to the different values of energy density used E = P/vht, J/mm 3 .…”