ISOTROPIC, CRACK-FREE STEEL DESIGN USING AN ADDITIVE MANUFACTURING METHOD

Abstract

The present invention relates to a metal powder for use within an additive manufacturing process, the powder comprising steel particles, wherein the steel particles comprise, in a proportion by weight greater than or equal to 0.01% by weight and less than or equal to 5% by weight, carbonitrides (C,N) and/or carbides (C) and/or nitrides (N) selected from the group consisting of titanium, zirconium or mixtures thereof. Furthermore, the present invention relates to a method for producing a steel powder suitable for use within an additive manufacturing process and to the use of the steel powder according to the invention in an additive manufacturing process.

Claims

1. A metal powder for use within an additive manufacturing process, characterized in that said powder comprises steel particles, said steel particles comprising, at a weight fraction greater than or equal to 0.01 wt% and less than or equal to 5 wt%, carbon nitrides (C,N) and/or carbides (C) and/or nitrides (N) selected from the group consisting of titanium, zirconium or mixtures thereof

2. Metal powder according to claim 1, wherein the metal powder comprises particles having an average particle diameter (D50) obtained via dynamic laser light scattering of greater than or equal to 10 nm and less than or equal to 100 μm.

3. Metal powder according to any one of the preceding claims, wherein the titanium and/or zirconium weight fraction in the metal powder is greater than or equal to 0.01 wt% and less than or equal to 2.0 wt%.

4. Metal powder according to any one of the preceding claims, wherein the metal powder has a carbon content of greater than or equal to 0.25 wt% and less than or equal to 4 wt%.

5. Metal powder according to any one of the preceding claims, wherein the steel particles comprise a base composition for tool steel according to 1.2344 H13.

6. Process for the preparation of a steel powder for use within an additive manufacturing process, characterized in that the process comprises at least the steps of: a) preparing a melt of steel, titanium and/or zirconium in an inert gas atmosphere; b) dropping the melt through a nozzle; and c) atomizing and cooling the drops in a nitrogen stream to obtain a powder.

7. Process according to claim 6, wherein the inert gas atmosphere in process step a) is an argon atmosphere.

8. Process according to claim 6 or 7, wherein in process step a) the carbon content of the melt is adjusted to greater than or equal to 0.2 wt% and less than or equal to 4 wt% by adding a carbon source.

9. Use of a metal powder according to any one of claims 1-4 in an additive manufacturing process.

10. Use according to claim 9, wherein the metal powder is used in a laser sintering process, wherein the powder is preheated to a temperature of greater than or equal to 100° C. and less than or equal to 500° C. by means of a cladding heater prior to laser sintering.

Description

[0025] They show:

[0026] FIG. 1 the morphological image of a steel part produced via laser sintering obtained from a prior art steel powder; FIG. 2 the morphological image of a steel part produced via laser sintering, wherein the pow-der bed was heated before the actual laser sintering; FIG. 3 the morphological image of a steel part produced via laser sintering obtained from a steel powder according to the invention;

[0027] FIG. 4 a transverse section of a metal particle according to the invention with finely distributed cubic titanium nitrides; FIG. 5 a transverse section of a metal particle according to the invention with finely dispersed cubic titanium nitrides; FIG. 6 transmission electron micrographs of a Titanium nitride particle in a steel matrix ac-cording to the invention.

[0028] FIG. 1 shows a workpiece produced by a prior art laser sintering process. The workpiece was made from H13 tool steel using a powder bed process and the H13 steel powder had no other additives. The powder bed was heated to a temperature of 200° C. before sintering. In a microscopic image, the workpiece shows anisotropy in the joint and a strong tendency to crack.

[0029] FIG. 2 shows a workpiece produced by a laser sintering process. The workpiece was made of H13 tool steel by a powder bedding process. The H13 steel powder did not contain any additives other than the usual alloying constituents. In contrast to FIG. 1, the powder bed was heated to a temperature of 400° C. before laser sintering. The workpiece shows anisotropy in a photograph, but this is less than the anisotropy shown in FIG. 1. The cracking tendency is reduced compared to the prior art, but cracks still show. In addition, a more or less large isotropy of the domains is still evident.

[0030] FIG. 3 shows a section through a workpiece produced by a laser sintering process. The powder bed was heated to a temperature of 200° C. before sintering. The workpiece was produced from tool steel H13 with a titanium addition of 0.8% by weight by means of a powder bed process. By adding titanium to steel, a fine grain is produced so that the micro-crack susceptibility of the sintered steel can be completely avoided. The result is a homogeneous workpiece with small, isotropic domains, which exhibits significantly improved mechanical properties, in particular compared with FIG. 1.

[0031] FIGS. 4-6 show the heterogeneous fine grain of a medium carbon CrMoV tool steel with a titanium content of 0.9 wt% and a nitrogen content of 0.05 wt%. The chemical composition of the steel is given by:

TABLE-US-00002 Element Atomic weight Weight-% Mol-% Fe 55.8 87.6 84.6 Cr 51.9 5.1 5.9 Mo 98.9 3.5 2.2 V 50.9 1.0 1.3 Mn 54.9 0.3 0.4 Si 28.0 0.8 1.9 C 12.0 0.3 2.0 Ti 47.8 0.9 1.1 N 14.0 0.05 0.2 P 30.9 0.01 0.02 S 32.0 0.003 0.005

[0032] FIG. 4 shows a cross-section of a metal particle according to the invention with finely divided cubic titanium nitrides.

[0033] FIG. 5 shows a transverse section of a metal particle according to the invention with finely divided cubic titanium nitrides. The crystal highlighted by the arrow has an extension of about 290 nm.

[0034] FIG. 6 shows a transmission electron micrograph of a titanium nitride particle in the steel matrix. The lattice parameter difference of the nucleating agent to the matrix is below about 10% and thus produces an effective grain refinement.