Powder Compositions for Additive Manufacturing

Abstract

To provide a further suitable composition for a specified AM process.SOLUTION: An Ni base alloy powder has a composition corresponding to a modified 738LC alloy powder. The composition lies outside of standard 738LC in which the concentration, form and size distribution of a gamma prime phase are different. The composition is different from the standard 738LC alloy powder so as to be used in additional product such as a powder floor fusion process or a direct metal deposition process based on a laser or an electron beam, e.g., compared with the standard 738LC alloy for producing a component having reduced cracks. The Ni base alloy powder can have a gamma prime phase quantitatively and / or qualitatively modified compared with the 738LC alloy powder. An Ni base alloy powder according to one embodiment comprises boron of 0.007 wt.% at most and comprises iron of 3.5 to 10 wt.%, more preferably 3.5 to 4 wt.%.SELECTED DRAWING: Figure 2

Description

Summary of the Invention

[0001] The present invention is directed to the printability of 738LC alloy powder, which is known for its non-printability and crack formation during laser-based powder bed processes. Currently, additive manufacturing (AM) users use a hot isostatic pressing (HIP) process to mitigate a certain amount of cracking. However, due to the nature of the process, not all cracks and imperfections created during the process can be healed by HIP.

[0002] The present invention aims to disclose a more suitable composition for a specific AM process. This can provide a more reliable solution to the AM community. One important application of the novel alloy powder of the present invention would be for the land-based turbine industry, as it can be used to develop novel designs for high temperature components.

[0003] The above mentioned objectives are met with a non-standard Ni-based alloy composition that can be printed with minimal to zero cracks. Herein, the standard 738LC alloy shall have the composition as set forth in Figure 1. As in standard 738LC, the gamma prime phase has been quantitatively and qualitatively modified. The gamma prime phase in one of the new alloys is finer and non-modal compared to the standard composition of the prior art where it is bimodal and angular morphology. In the second new alloy, the gamma prime phase retains its bimodal distribution, but the morphology has relatively smooth edges. This results in higher room temperature tensile properties than the standard 738LC composition. As mentioned above, the standard 738LC composition is prone to cracking during the laser-based powder bed fusion process. Several mechanisms can cause microcracking in such alloys, which may be due to solidification cracking due to precipitation of the gamma prime phase generating internal residual stresses or the formation of low melting point phases due to segregation or formation of certain types of grain boundaries known to generate large internal stresses, which in combination with elemental segregation can cause hot cracks. To alleviate these stresses and improve the printability of the alloy, approaches have been taken to reduce the gamma prime content and improve grain boundary properties. However, since the gamma prime phase is also known to contribute to high temperature strength and stability, the reduction of this phase must be supplemented with other strengthening techniques. This requirement was met by increasing the solid solution strengtheners, which have high temperature resistance and atomic size.

[0004] The present invention will be described in detail below with reference to examples. [Brief description of the drawings]

[0005] [Figure 1] 1 is a table showing the composition of standard 738LC powder. [Diagram 2] 1 is a table showing a composition of a first embodiment of the present invention. [Diagram 3] 4 is a table showing a composition of a second embodiment of the present invention. [Figure 4]Room temperature properties are compared with standard casting and heat treated 738LC alloy. [Diagram 5] The modified gamma prime phase of a) 738-Mod1 and b) 738-Mod2 compared to [Figure 5c] Showing modified gamma prime phase: c) Cast &HT [Figure 6] FIG. 1 illustrates material properties of standard and non-standard 738LC alloy powder compositions. [Figure 7] FIG. 1 shows the horizontal and vertical microstructure of a member. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] As can be seen from Fig. 2, according to the first embodiment, the boron content is kept as low as economically possible, but below 0.007wt%, preferably below 0.005wt%, more preferably below 0.001wt% (compared to between 0.007wt% and 0.012wt% in the standard powder). In addition to this, iron is introduced in an amount between 3.5wt% and 10wt%, preferably between 3.5wt% and 4wt% (compared to no iron in the standard powder or at most 0.05wt%). The nitrogen content may be up to 0.08wt%, preferably up to 0.06wt%.

[0007] According to a second embodiment of the invention, the aluminum and titanium contents are reduced to less than 3 wt%, preferably less than 2 wt%, as can be seen in figure 3. The molybdenum content is increased to be more than 8.5 wt%, preferably between 8 and 9 wt%.

[0008] FIG. 4 shows the material properties of standard and non-standard 738LC alloy powder compositions additively manufactured in the X, Y, and Z directions at room temperature.

[0009] ID stands for alloy powder. Direction refers to the direction of the room.

[0010] UTS means ultimate tensile strength. Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or Ftu in the equation, is the capacity of a material or structure to withstand a load that tends to elongate, as opposed to compressive strength, which withstands a load that tends to reduce in size. In other words, tensile strength resists tension (pulling apart), whereas compressive strength resists compression (pushing together). Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. In the study of the strength of a material, tensile strength, compressive strength, and shear strength can be analyzed independently. Some materials break very sharply without plastic deformation, known as brittle fracture. Others, which are more ductile, including most metals, experience some plastic deformation and possible necking before fracture. UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point on the stress-strain curve is the UTS. It is an intensive property and therefore its value does not depend on the size of the test specimen. However, it depends on other factors such as the preparation of the specimen, the presence or absence of surface defects, as well as the test environment and the temperature of the material.

[0011] Generally, the measurement method for measuring UTS is well known and involves taking a small sample with a constant cross-sectional area and then pulling it in a tensometer until the sample breaks at a constant strain (change in gauge length divided by the initial gauge length). When testing some metals, indentation hardness correlates linearly with tensile strength. This important relationship allows economically important non-destructive testing of bulk metal deliveries using lightweight, portable equipment such as handheld Rockwell hardness testers. This practical correlation helps extend quality assurance in the metalworking industry well beyond the laboratory and universal testing machines.

[0012] YS stands for yield strength. The yield point is the point on the stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Yield strength or yield stress is a material property defined as the stress at which a material begins to deform plastically, and the yield point is the point at which nonlinear (elastic + plastic) deformation begins. Before the yield point, the material deforms elastically and returns to its original shape when the applied stress is removed. Above the yield point, some of the deformation will be permanent and irreversible. The yield point determines the limit of the performance of a machine member, since it represents the upper limit of the force that can be applied without permanent deformation. In structural engineering, this is a brittle failure mode that usually does not cause catastrophic or ultimate failure, unless accelerated buckling occurs. Yield strength is an important material property that is utilized in many fundamental techniques of material processing: reshaping the material with pressure (such as forging, rolling, pressing, bending, extrusion, or hydroforming), separating the material by cutting (such as machining) or shearing, and joining the members together rigidly with fasteners. The yield load may be taken as the load applied to the center of the carriage spring to straighten its leaves. The offset yield point (or proof stress) is the stress at which 0.2% plastic deformation occurs.

[0013] Methods for measuring YS are well known. It may be done, for example, by taking a small specimen with a fixed cross-sectional area and then pulling it with a controlled, gradually increasing force until the specimen changes shape or breaks. This is called a tensile test. The longitudinal and / or transverse strains are recorded using mechanical or optical extensometers. The yield behavior can also be simulated using virtual tests (on a computer model of the material), especially in cases where the macroscopic yielding is governed by the microstructure of the material under study. Indentation hardness correlates approximately linearly with the tensile strength of most steels, and measurements of one material cannot be used as a measure to measure the strength of another. Hardness testing is therefore not only an economical substitute for tensile testing, but may also provide local variations in yield strength due to, for example, welding or forming operations. In critical situations, however, tensile testing is performed to eliminate ambiguity.

[0014] El stands for elongation and is measured according to ASTM E139 (Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials), the measurement principle is "Under load after break" (https: / / www.astm.org / Standards / E139.htm).

[0015] As can be seen in Figure 5, the present invention involves modifying the gamma prime phase both quantitatively and qualitatively. Therefore, it was necessary to investigate the high temperature properties. Figure 6 of the embodiment shows the high temperature properties for both Figure 2 and the non-standard composition as described in Figure 3.

[0016] In both modifications, the properties are comparable to the cast and HT standard 738LC alloy. It is noted that due to the use of a non-optimized heat treatment for the AM material, in one case the elongation is shown to be lower than that obtained for the cast material. This is evidenced by the etched microstructure as seen in Figure 6 for the non-standard composition. From Figure 7 it can be seen that the morphology of the grain structure is elongated even after the aging treatment (perpendicular to the assembly direction) due to the epitaxial grain growth that occurs during additive manufacturing. This leads to the observation that no recrystallization of the grain structure occurs, to obtain maximum material properties, based on the standard heat treatment cycle of AMS 5410, which includes the following steps: Solution annealing at a temperature of 2050°F + / - 10°F for a holding time of 2 hours Aged at 1550°F + / - 10°F for 24 hours

[0017] A Ni-based alloy powder has been disclosed having a composition corresponding to a modified 738LC alloy powder, the composition being different from that of the standard 738LC alloy powder such that it can be used in additive manufacturing, such as laser or electron beam based powder bed fusion processes or direct metal deposition processes, to produce parts with reduced cracking, preferably minimized cracking, and most preferably zero crack density, as compared to the standard 738LC alloy.

[0018] The Ni-based alloy powder may have a quantitatively and / or qualitatively modified gamma prime phase compared to the 738LC alloy powder.

[0019] The Ni-based alloy powder according to one embodiment contains at most 0.007 wt% boron, preferably less than that, and it contains between 3.5-10 wt%, more preferably 3.5-4 wt% iron.

[0020] The Ni-based alloy powder may also contain an aluminum content less than 3 wt.% and a molybdenum content in an amount greater than 8.5 wt.%, preferably equal to 9 wt.%.

[0021] The qualitatively modified gamma prime phase is preferably modified in such a way that the crack density must be reduced by at least 10%, preferably by at least 25%, particularly preferably by at least 50%, in particular with the same process parameters.

[0022] A 3D printed component is disclosed that includes a material composition based on the Ni-based alloy powder as described above.

[0023] The 3D printed component may have an essentially elongated grain structure morphology.

[0024] Figure 5 - Modified gamma prime phase in a) 738-Mod1 and b) 738-Mod2 compared to c) cast &HT, SEM images (20,000x magnification)

[0025] Figure 7 - Horizontal and vertical microstructure

Claims

1. % chromium, 8-9 wt. % cobalt, 1.75-2.25 wt. % aluminum, 1.75-2.25 wt. % titanium, 2.4-2.8 wt. % tungsten, 1.5-2 wt. % tantalum, 8.5-9.5 wt. % molybdenum, 0.6-1.1 wt. % niobium, 0.09-0.13 wt. % carbon, 0.03-0.08 wt % zirconium, 0.007-0.012 wt % boron, up to 0.05 wt % iron, up to 0.1 wt % copper, up to 0.02 wt % manganese, up to 0.03 wt % silicon, up to 0.015 wt % phosphorus, up to 0.006 wt % sulfur, up to 0.03 wt % oxygen, up to 0.08 wt % nitrogen, up to 0.15 wt % T.A.O, and the balance nickel.

2. The Ni-based alloy powder according to claim 1, characterized in that the molybdenum content is 9 wt%.

3. 3D printed parts comprising a material composition according to claim 1 or 2.

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