*2.1. Investment Casting and Chemical Composition of Alloy 718 Casting Heats*

Investment casting moulds were manufactured, incorporating 20 test samples as flat plates of 150 × 50 × 10 mm in each mould. Moulds were covered with a ceramic shell composed by three primary layers and five backup layers, dewaxed, preheated, poured, and later demoulded, as described in Figure 1.

Casting samples were cast at industrial facilities under vacuum conditions. The mould preheating temperature was always 1150 ◦C, whereas the molten material was poured at 1450 ◦C.

The chemical composition of the different castings is shown in Table 1. The content of each alloying element was determined in cast samples using the following analytic techniques. C and S contents were measured by combustion and infrared absorption; N by inert gas fusion and thermal conductivity; O by inert gas fusion and infrared absorption; and finally Si, Mn, P, Fe, Cr, Mo, Ti, Al, and Nb contents were determined by spark atomic emission spectrometry. Co and B contents were not measured in casted samples, but they were taken from the chemical composition of the ingots employed as raw material for castings averaging 0.11 wt % and 0.002 wt %, respectively.

(**a**) Mould in wax (**b**) Mould with shell (**c**) Mould without shell

**Figure 1.** Investment casting moulds in different manufacturing steps.

**Table 1.** Chemical composition of alloy 718 casting heats in weight percentage.


Note that mould O (low Si content) was manufactured by using high purity ingots as raw material. In mould P (high Si content), Si was intentionally added during vacuum melting, and chemically adjusted ingots (28 Kg in total) were manufactured in a first melting step. Mould E corresponded to conventional chemical composition and casting process, whereas the cooling rate during solidification was reduced in moulds N and NP by incorporating 1 ceramic blanket over cast parts. The cooling rates between 800 and 500 ◦C were determined and they were 0.52 ◦C/s and 1.65 ◦C/s for the moulds with blanket (ref. N/NP) and without blanket (ref. O/E/P), respectively.

Once parts were shot blasted, they were submitted to a heat treatment process, according to GKN V.AC:9922 standard and comprising HIP and solution annealing thermal cycles. An additional solubilisation pre-HIP treatment consisting of solubilisation at 1052 ◦C for 2 h followed by air cooling was applied to slowly cooled mould NP. The goal of this treatment was to dissolve Laves phases before HIP treatment.
