Ni-base superalloy composition and method for SLM processing such Ni-base superalloy composition

Abstract

A Ni-base superalloy composition to be used for powder-based additive manufacturing (AM) technology, such as selective laser melting (SLM) or electron beam melting (EBM). The cracking susceptibility during an AM process is considerably reduced by controlling the amount of elements, especially Hf, that form low-melting eutectics.

Claims

1. A nickel-base superalloy composition comprising a /-microstructure with a chemical composition of (in wt. %): 9.5 W, 9.2 Co, 8.1 Cr, 5.6 Al, 3.2 Ta, 2.4 Hf, 0.7 Ti, 0.5 Mo, 0.075 C, 0.015 Zr, 0.015 B, and balance Ni, wherein cracking susceptibility is reduced by controlling an amount of elements that form low-melting eutectics, and wherein the elements that form low-melting eutectics include at least one of Hf, B, and Zr.

2. The nickel-base superalloy composition as in claim 1, the elements that form low-melting eutectics are not bound in precipitates, and form low-melting eutectics.

3. The nickel-base superalloy composition as in claim 2, said first elements include Hf in the range 1.2 wt-%<Hf<5 wt-%.

4. The nickel-base superalloy composition as in claim 3, said first elements include Hf in the range 1.6 wt-%<Hf<3.5 wt-%.

5. The nickel-base superalloy composition as in claim 4 said first elements include Hf in the range 1.7 wt-%<Hf<2.8 wt-%.

6. The nickel-base superalloy composition as in claim 2, wherein Hf/C ratio >1.91.

7. The nickel-base superalloy composition as in claim 6, wherein C >0.01 wt-% for grain boundary strengthening.

8. The nickel-base superalloy composition as claim 7, wherein 0.01 wt-%<C<0.2 wt-%.

9. A nickel-base superalloy composition-comprising a /-microstructure with a chemical composition of (in wt. %): 10.0 W, 10.0 Co, 8.4 Cr, 5.5 Al, 3.0 Ta, 2.4 Hf, 1.0 Ti, 0.7 Mo, 0.15 C, 0.05 Zr, 0.015 B, and balance Ni, wherein cracking susceptibility is reduced by controlling an amount of elements that form low-melting eutectics, and wherein the elements that form low-melting eutectics include at least one of Hf, B, and Zr.

10. The nickel-base superalloy composition as in claim 9, wherein said first elements include Hf in the range 1.2 wt-%<Hf<5 wt-%.

11. The nickel-base superalloy composition as in claim 10 said first elements include Hf in the range 1.6 wt-%<Hf<3.5 wt-%.

12. The nickel-base superalloy composition as in claim 11, wherein said first elements Hf in the range 1.7 wt-%<Hf<2.8 wt-%.

13. The nickel-base superalloy composition as in claim 11, wherein Hf/C ratio >1.91.

14. The nickel-base superalloy composition as in claim 9, wherein 0.01 wt-%<C<0.2 wt-%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

(2) FIG. 1 shows in comparison microsections of three different SLM-processed alloys in as-built state (using same processing parameters & conditions) with (a) being related to standard MarM247, (b) being related to standard CM247LC, and (c) being related to an improved alloy composition;

(3) FIG. 2 shows an SEM micrograph with EDX map of an alloy showing evidence for backfilling of cracks by a Hf rich melt;

(4) FIG. 3 shows a micrograph of a side surface of an SLM processed part made out of (a) standard CM247LC and (b) optimized alloy (both samples are processed under equal conditions); while the standard composition (a) results in a large number of cracks open to the surface, a crack-free surface is obtained with the optimized alloy composition (b);

(5) FIG. 4 shows the reduction of hot cracking (cracking density) by an optimized alloy composition in comparison to standard alloys; and

(6) FIG. 5 shows the reduction of hot cracking with increasing Hf/C ratio of the alloy.

DETAILED DESCRIPTION

(7) Embodiments of the invention relate especially to a Ni-base superalloy composition to be used for powder bed-based additive manufacturing (AM) technology, such as selective laser melting (SLM) or electron beam melting (EBM). However, the claimed Ni-base superalloy could also improve weldability in other AM technologies such as laser metal deposition (LMD) or laser metal forming (LMF) (blown powder methods).

(8) In general, according to embodiments of the invention the cracking susceptibility during AM processes can be considerably reduced by controlling the amount of elements that form low-melting eutectics.

(9) Hot cracking occurs when the volume shrinkage between two solidification fronts, caused by solidification and thermal strains, cannot be compensated by fluid flow from the main melt pool. This fluid flow (backfilling) strongly depends on the permeability of the dendrite network, which is influenced by the last stage solidification behavior.

(10) Embodiments of the present invention intend to increase the ability of the alloy for this backfilling process and thus decrease the amount of hot cracks during AM/SLM processing.

(11) Embodiments of the present invention achieve this by increasing the amount of free (i.e. not bound in precipitates) elements that form low-melting eutectics, especially Hf. This increases the volume fraction of liquid that is present until the last stage of solidification and thus results in a larger dendrite separation and higher permeability.

(12) Hot cracks, which might start to form due to the presence of essential elements such as Zr can thus be backfilled and closed directly during solidification.

(13) Hf is a very strong carbide and oxide former. Hf carbides and oxides are formed from the melt very early in the solidification and a lot of the Hf is thus fixed in carbides/oxides before the critical phase of the solidification.

(14) In order to reduce hot cracking, an alloy composition is thus proposed that contains a minimum amount of 1.2 wt % Hf, comprises C and has a Hf [at %]/C [at %] ratio >1.55.

(15) Especially, said alloy composition is a modified version of the commercial available CM247LC alloy (nominal composition (in wt.-%): 9.5 W, 9.2 Co, 8.1 Cr, 5.6 Al, 3.2 Ta, 1.4 Hf, 0.7 Ti, 0.5 Mo, 0.075 C, 0.015 Zr, 0.015 B, and the balance Ni) with a higher Hf content (2.4 wt.-% instead of 1.4 wt.-%). The Hf [at %]/C [at %] ratio for that nominal chemical composition is 1.3, for the modified composition that ratio is 2.2.

(16) According to an additional embodiment of the invention said Ni-base superalloy is a modified version (especially with a higher amount of Hf) of MarM247, which has a nominal composition of (in wt.-%): 10.0 W, 10.0 Co, 8.4 Cr, 5.5 Al, 3.0 Ta, 1.5 Hf, 1.0 Ti, 0.7 Mo, 0.15 C, 0.05 Zr, 0.015 B, and the balance Ni. The Hf [at-%]/C [at-%] ratio is only 0.67 for the MarM247 alloy with a nominal composition.

(17) Alloys that fulfil these requirements show a sufficient volume of terminal liquid to allow backfilling of emerging hot cracks and thus show very low hot cracking susceptibility during SLM processing.

(18) To prevent binding of free Hf in oxides, the SLM process must be additionally performed under protective atmosphere with O2 <1%, in an embodiment <0.6% and more particularly 0,4%, and the O2 content in the powder must be <800 ppm, in an embodiment <500 ppm, and more particularly <300 ppm. For grain boundary strengthening, the C content must be >0.01 wt %.

(19) FIG. 1 shows three microsections from different alloys processed by SLM. While the two standard alloys (a) and (b) are very susceptible to hot cracking, the experimental alloy according to an embodiment of the present invention does not show any hot cracking. All three alloys were fabricated using identical process parameters/conditions.

(20) FIG. 2 shows a scanning electron microscope image with the EDX map insert exhibiting a Hf rich intergranular area which originated from the backfilling of an emerging hot crack by the Hf rich terminal liquid.

(21) FIG. 3 shows a micrograph of a side surface of an SLM processed part made out of (a) standard CM247LC and (b) optimized alloy according to an embodiment of the present invention (both samples are processed under equal conditions). As can be seen, SLM fabricated parts from alloys according to embodiments of the present invention (b) do not show cracks open to the surface, which is important as these cracks are especially difficult to remove and might decrease fatigue properties. The standard CM247LC alloy (a), on the other hand, shows numerous surface cracks when processed under the same conditions.

(22) FIG. 4 shows the results from a quantitative crack analysis for different alloys processed by SLM using identical conditions. An optimized alloy composition according to embodiments of the invention is compared to two standard alloys. The experimental (optimized) alloy according to embodiments of the present invention shows a considerably decreased hot cracking susceptibility.

(23) FIG. 5 shows the reduction of the hot cracking susceptibility with increasing Hf/C fraction in a range of the ratio from zero to 2.2, where the crack density almost vanishes.

(24) Addition of Hf to cast alloys to improve the castability is state of the art. However, for cast material, the addition of Hf has some severe limitations: First, Hf segregates strongly during solidification and forms eutectic structures with very low solidification temperature. This strongly increases the likelihood of incipient melting during subsequent heat treatment. Second, Hf is very reactive and can strongly react with the mould used in investment casting.

(25) Thus, the Hf content is typically limited to 1.5% in cast alloys. However, these limitations are not present for the SLM process, because the rapid solidification that takes place limits the Hf segregation and the size of the low melting eutectic structures. These very small segregations of size smaller than some hundred nm are homogenized already during heat up and incipient melting is thus not an issue. The high reactivity of Hf in the melt is no issue due to the direct generation of parts from the powder bed by SLM.

(26) This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.