PSI - Issue 24

Jacopo De Nisi et al. / Procedia Structural Integrity 24 (2019) 541–558 Paolo Folgarait et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Observed microstructure in groups 0, A1, and B1 and solid parts at x500 shows a mixture of columnar and equiaxial grains with homogeneously dispersed precipitates (apparent density increasing with ageing and holding time). For groups A2, B2, and C (with solution annealing before ageing) a rather fully equiaxial microstructure is predominant. In all 3D printed samples, a higher equiaxial/columnar grains ratio is appreciable in position E with respect any other quadrant (E being the central quadrant in the building platform). No other differences have been found among quadrants (equivalent results within experimental errors). For all samples the apparent precipitates’ density increases with incremental ageing temperature till a critical value (laying in the range 480÷550°C), then decreases (see Table 5 and Table 9). This phenomenon is clear in case group C (H1025), where the coarsening mechanism of dispersed precipitates becomes increasingly effective rapidly reducing strength and hardness. Also ductile properties are reduced with ageing temperature. The effect of the incremental holding time in 3D printed samples (from 1 to 4 hours) on the apparent precipitates’ density seems to be slighter or negligible. At any ageing temperature in the typical treatment range and below (T < 450°C), independently on the presence of a previous solution annealing treatment, mechanical properties are always higher in 3D printed samples than solid parts. This behavior is rapidly reverted from a critical temperature on, whose value has been graphically estimated (in the present test conditions) to lay between 525°C and 535°C. For solid parts the drop temperature is beyond 550°C. 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