Search

A20X (Al-Cu-Ag-Mg-TiB2) is a precipitation hardening alloy, recently developed for additive manufacturing processing. Printed parts of A20X alloy are usually post-processed with a long T7 heat treatment for improved mechanical properties with respect to its as-built counterparts. However, in the present investigation, it was demonstrated that T7 might not be the best suitable heat treatment available for A20X alloy. A detailed microstructural characterization of A20X samples processed with laser powder bed fusion and post-processed with T7 was carried out. Microstructural features were analysed in terms of grain size, precipitate size, phase quantification, dislocation density and width of the precipitate free zones. After the analysis, a simple and rapid heat treatment was proposed which significantly improved the mechanical properties. The yield strength (YS), ultimate tensile strength (UTS) and elongation to fracture (e) for the T7 heat treatment were 370 ± 9 MPa, 435 ± 13 MPa and 7.3 ± 0.3 % respectively. With the proposed heat treatment, an increment of 7.1 % in YS, 6.3 % in UTS and 45 % in e was witnessed. This exceptional improvement in the mechanical behaviour has been associated with the absence of grain boundary cracking in the proposed heat treatment.

The Ti-6Al-2Sn-4Zr-6Mo alloy was recently successfully processed using the laser powder bed fusion technology. This material is characterized by a very low strength in the as-built state, due to the presence of α” martensite. In this work, the transformations taking place at different temperatures in this alloy were investigated with the aim of achieving the necessary knowledge to customize the mechanical properties via heat treatments. Martensite decomposition, residual stress relaxation and the β-transus temperature were identified at progressively increasing temperatures. Then, different annealing treatments, corresponding to the temperatures identified, were conducted. For each condition, dissimilar α + β microstructures were obtained, varying in terms of morphology (lamellar, bi-lamellar) and size of the α laths (0.5–2.2 µm). The corresponding tensile properties were also significantly different. In particular, outstanding strength values were achieved when the residual stress was retained. For higher temperatures, a slight drop in strength was coupled with an outstanding improvement in terms of ductility. Overall, the most balanced combination of tensile properties was obtained for an annealing temperature of 875 °C. In this condition, the bi-lamellar microstructure grants a reduction in the slip length due to the presence of secondary α laths.

Definition of the main process parameters, laser power (P), scanning speed (v), hatching distance (hd) and scanning strategy useful for producing dense samples, is fundamental to develop novel alloy compositions for Laser Powder Bed Fusion (LPBF). The present work has two aims: on one side, to verify the processability of a new AlSi10Mg + 4Cu alloy of mixed powders by LBPF; on the other side, on the basis of the experimental analysis, to define a method for processing new alloys. Producing Single Scan Tracks (SSTs), samples with unidirectional scanning strategy and samples with 67° rotated scanning strategy, the proper P-v-hd combinations were identified reaching a final porosity lower than 1.5%. A scenario of hardness and build-up rate vs. energy density is given, to adopt the main process parameters suitable to maximize mechanical properties or productivity. According to the novel method, P-v-hd combinations can be defined through the production and characterization of SSTs and samples with 67° rotated scanning strategy. Through two production steps dense samples can be then obtained, allowing the development of new compositions saving time and reducing costs related to the powder usage. Keywords: Laser Powder Bed Fusion (LPBF), Single scan tracks (SSTs), Hatching distance (hd), Overlap, Process parameters, Hardness, Build-up rate