Methods of manufacturing of molybdenum and molybdenum-based structures by electron beam additive manufacturing, particularly structures for nuclear components
Methodologies and manufacturing processes to manufacture components by electron beam melting additive manufacturing, particularly components of molybdenum or a molybdenum-based alloy and particularly of complex nuclear component geometries. Input parameters are provided for controlling electron beam melting additive manufacturing equipment, such as electron beam melting machines. The input parameters relate to various process steps, including build set-up, initial thermal treatment, initial layering of powder, pre-consolidation thermal treatment, consolidation, post-consolidation thermal treatment, indexing of layers, and post-build thermal treatment. The methodologies and manufacturing processes allow manufacture of components of molybdenum having a purity of ≥99.0% and a density of ≥99.75%. Metallographic cross-sections of the manufactured molybdenum components were porosity-free and crack-free.
1. A method to manufacture a component by electron beam melting additive manufacturing, the method comprising:
a build set-up step including seating a start plate on a powder bed within a reaction zone of a vacuum chamber of an electron beam melting additive manufacturing machine, wherein the start plate has a composition consisting of molybdenum having a purity of 99.0% or higher and wherein the powder bed includes a first powder having a composition consisting of molybdenum having a purity of 99.0% or higher;
an initial thermal treatment step including positioning the start plate at a plate offset distance that is based on thermal expansion of the start plate during the initial thermal treatment step, heating the start plate to a first temperature that is sufficient to sinter the powder bed and is equal to or greater than 1040° C. and equal to or less than 1350° C., holding the start plate at the first temperature, and sintering a portion of the powder bed on which the start plate is seated;
covering at least a portion of a build surface of the start plate with a base layer of the first powder;
a pre-consolidation step including a first heating step in which the base layer is heated to a first preheat temperature and a second heating step in which the base layer is heated to a second preheat temperature, wherein the second preheat temperature is higher than the first preheat temperature, and wherein the first preheat temperature agglomerates the first powder of the base layer and the second preheat temperature densifies a portion of the agglomerated first powder;
a consolidation step including sintering a current layer of the component from the densified portion of the agglomerated first powder, wherein a rastering pattern used for sintering during the consolidation step includes an angular rotation that is non-90 degrees and that is based on a prime number or based on an integer multiple of a prime number; and
a post-build step including a cool-down step.
2. The method according to claim 1 , further comprising, after the consolidation step and before the post-build step:
a post-consolidation step including returning the powder bed to the second preheat temperature.
3. The method according to claim 2 , further comprising, after the post-consolidation step and before the post-build step:
an indexing step including moving a position of the start plate by a distance corresponding to thickness of a subsequent layer;
a replenishment step in which the current layer of the component is covered with a layer of the first powder; and
performing the pre-consolidation step and the consolidation step.
4. The method according to claim 3 , wherein the post-consolidation step, the indexing step, the replenishment step, the pre-consolidation step, and the consolidation step are repeated a plurality of times until the component is in a final form.
5. The method according to claim 1 , wherein the plate offset distance is equal to or greater than 0.1 mm and equal to or less than 0.3 mm.
6. The method according to claim 5 , wherein an area of the densified portion of the agglomerated first powder has a boundary enclosing an area in which the layer of the component is to be formed,
wherein the component has a composition consisting of molybdenum having a purity of equal to or greater than 99.0% and a density of equal to or greater than 99.75%, and
wherein the component is crack-free and porosity-free.
7. The method according to claim 1 , wherein an area of the densified portion of the agglomerated first powder has a boundary enclosing an area in which the layer of the component is to be formed.
8. The method according to claim 1 , wherein the cool-down step includes:
removing heat input to the component; and
(a) maintaining a vacuum atmosphere in the vacuum chamber for 20±2 minutes followed by backfilling the vacuum chamber with an inert gas to atmospheric pressure or (b) cooling the component to 100° C. under an inert gas.
9. The method according to claim 8 , wherein the inert gas is helium.
10. The method according to claim 1 , further comprising a geometry input step including entering a geometry of the component into a controller of the electron beam melting additive manufacturing machine.
11. The method according to claim 10 , wherein the geometry of the component is a geometry on a layer-by-layer basis.
12. The method according to claim 11 , wherein the geometry of the component is embodied in a computer-aided design (CAD) model or an Additive Manufacturing File (AMF) file or a stereolithography contour (STL) file.
13. The method according to claim 1 , wherein the base layer has a thickness of 40 to 70 microns.
14. The method according to claim 1 , wherein the component has a composition consisting of molybdenum having a purity of equal to or greater than 99.0% and a density of equal to or greater than 99.75%.
15. The method as in claim 14 , wherein the component is crack-free.
16. The method as in claim 14 , wherein the component is porosity-free.
17. The method as in claim 14 , wherein the component is crack-free and porosity-free.
18. The method according to claim 1 , wherein the component is crack-free.
19. The method according to claim 1 , wherein the component is porosity-free.
20. The method according to claim 1 , wherein the component is crack-free and porosity-free.
21. The method according to claim 1 , wherein the component is a nuclear component.
22. A component for a nuclear reactor manufactured by the method according to claim 1 .
23. The method according to claim 1 , wherein the base layer has a thickness of 45 to 55 microns.
24. The method according to claim 1 , wherein the first heating step of the pre-consolidation step includes a first plurality of parameters including current of 20 mA to 40 mA, a maximum number of sweeps of 300, a number of sweeps at maximum current of 50, a beam speed of 20,000 mm/s, a focus offset of 375 mA, and a line offset of 1 mm, and
wherein the second heating step of the pre-consolidation step includes a second plurality of parameters including current of 40 mA to 45 mA, a maximum number of sweeps of 30, a number of sweeps at maximum current of 1, a beam speed of 16,100 mm/s, a focus offset of 375 mA, and a line offset of 1 mm.
25. The method according to claim 24 , wherein the consolidation step includes a third plurality of parameters including focus offset, beam current, speed function, current compensation, and turning point function,
wherein focus offset is 5 mA to 30 mA,
wherein beam current is 2 mA to 30 mA,
wherein a value for current compensation results in a ratio of ref scan line to beam current of 0.5 to 1.35, and
wherein turning point function is defined by:
v
B
e
a
m
=
v
input
*
[
1
+
PEF
*
e
-
v
input
(
(
EF
1
*
l
0.1
)
-
EF
2
*
v
input
)
]
where Pre-Exponent Factor (PEF) is 0.5 to 1.3 and Exponential Factor I (EF 1 ) is 0.0002 to 0.001.
26. The method according to claim 1 , wherein the consolidation step includes a third plurality of parameters including focus offset, beam current, speed function, current compensation, and turning point function,
wherein focus offset is 5 mA to 30 mA,
wherein beam current is 2 mA to 30 mA,
wherein a value for current compensation results in a ratio of ref scan line to beam current of 0.5 to 1.35, and
wherein turning point function is defined by:
v
B
e
a
m
=
v
input
*
[
1
+
PEF
*
e
-
v
input
(
(
EF
1
*
l
0.1
)
-
EF
2
*
v
input
)
]
where Pre-Exponent Factor (PEF) is 0.5 to 1.3 and Exponential Factor I (EF 1 ) is 0.0002 to 0.001.