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

Abstract

A Ni-based 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 Ni-based superalloy composition comprising: a γ/γ′-microstructure, responsive to a heat treatment, that renders the Ni-based superalloy composition suitable for additive manufacturing (AM) of three dimensional articles with a γ/γ′-microstructure; and first elements selected from a group of elements that are known to form low-melting eutectics, wherein an amount of each of the first elements is controlled to a level selected to reduce risk of crack formation during AM, and wherein the Ni-based superalloy composition is a modified version of CM247LC 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 the balance Ni.

2. A Ni-based superalloy composition comprising: a γ/γ′-microstructure, responsive to a heat treatment, that renders the Ni-based superalloy composition suitable for additive manufacturing (AM) of three dimensional articles with a γ/γ′-microstructure; and first elements selected from a group of elements that are known to form low-melting eutectics, wherein an amount of each of the first elements is controlled to a level selected to reduce risk of crack formation during AM, and wherein the Ni-based superalloy composition is a modified version of MarM247 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 the balance Ni.

3. A method for SLM processing a Ni-based superalloy composition, the method comprising: providing the Ni-based superalloy composition configured to form a γ/γ′-microstructure after a heat treatment and used for additive manufacturing of three dimensional articles with a γ/γ′-microstructure, wherein the amount of elements that form low-melting eutectics is controlled; using the Ni-based superalloy composition in powder bed-based additive manufacturing technology and the Ni-based superalloy composition comprises first elements not bound in precipitates forming low-melting eutectics, wherein the first elements are Hf in an amount of at least 1.2 wt %; providing protective atmosphere with O2 less than 1% with an O2 content in the powder being less than 800 ppm performing the powder bed-based additive manufacturing technology.

4. The method according to claim 3, wherein the protective atmosphere comprises O2 less than 0.6% with the O2 content in the powder being less than 500 ppm.

5. The method according to claim 4, wherein the protective atmosphere comprises O2 less than 0.4% with the O2 content in the powder being less than 300 ppm.

6. The Ni-base superalloy composition according to claim 1, wherein a ratio of Hf [at-%]/C [at-%] is greater than 1.3 and less than 2.3.

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-based 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-based 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 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 intends 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 achieves 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-based 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.