Method for finishing a workpiece made by additive manufacturing

Abstract

According to the invention, a metal workpiece made by additive manufacturing is subjected, following the additive manufacturing process, to a cold treatment in which the workpiece is cooled to a lower target temperature of less than minus 30? C. in a cooling phase and is then heated up to an upper target temperature in a heating phase. The cold treatment significantly improves the properties of the workpiece in respect of the mechanical quality thereof.

Claims

1. A method for after-treating a metallic workpiece produced by means of additive manufacturing, wherein the workpiece is, after it has been manufactured, subjected to a cold treatment in which the workpiece is cooled in a cooling phase to a lower target temperature of below ?30? C., held at the lower target temperature during a hold phase, and subsequently warmed in a warming phase to an upper target temperature; wherein the cooling phase or the hold phase is interrupted by a first intermediate warming phase in which the workpiece is warmed to a first intermediate temperature and subsequently cooled; wherein the cooling phase or the hold phase is interrupted by a second intermediate warming phase in which the workpiece is warmed to a second intermediate temperature and subsequently cooled, wherein the second intermediate temperature is different than the first intermediate temperature; and wherein the cooling phase is interrupted by the first intermediate warming phase and the hold phase is interrupted by the second intermediate warming phase.

2. The method as claimed in claim 1, wherein the workpiece has been manufactured by a powder bed process, a powder spraying process or in an additive manufacturing process by means of supply of wire.

3. The method as claimed in claim 1, wherein the cold treatment is used in addition to a heat treatment of the manufactured workpiece.

4. The method as claimed in claim 1, wherein the lower target temperature is in the range from ?50? C. to ?195? C.

5. The method as claimed in claim 1, wherein the upper target temperature is in the range from 20? C. to 40? C.

6. The method as claimed in claim 1, wherein the workpiece is held for a time of at least 30 seconds at the lower target temperature during the hold phase.

7. The method as claimed in claim 1, wherein the workpiece is warmed by at least 10? C. during the first intermediate warming phase and/or the second intermediate warming phase.

8. The method as claimed in claim 1, wherein the cooling of the workpiece in the cooling phase and/or the warming of the workpiece in the warming phase and/or at least one of the first and second intermediate warming phases is carried out at a rate of from 1.5? C./min to 10? C./min.

9. A method for after-treating a metallic workpiece produced by means of additive manufacturing, wherein the workpiece is, after it has been manufactured, subjected to a cold treatment in which the workpiece is cooled in a cooling phase to a lower target temperature of below ?30? C., held at the lower target temperature during a hold phase, and subsequently warmed in a warming phase to an upper target temperature; wherein the cooling phase or the hold phase is interrupted by a first intermediate warming phase in which the workpiece is warmed to a first intermediate temperature and subsequently cooled; wherein the cooling phase or the hold phase is interrupted by a second intermediate warming phase in which the workpiece is warmed to a second intermediate temperature and subsequently cooled, wherein the second intermediate temperature is different than the first intermediate temperature; and wherein the cooling phase and/or the warming phase and/or at least one of the first and second intermediate warming phases is interrupted for a prescribed period of time during which the workpiece is essentially held at an attained temperature.

10. The method as claimed in claim 7, wherein the workpiece is warmed by at least 50? C. during the first intermediate warming phase and/or the second intermediate warming phase.

11. The method as claimed in claim 1, wherein a heating rate of the workpiece and a cooling rate of the workpiece during the first intermediate warming phase is different than a heating rate of the workpiece and a cooling rate of the workpiece during the second intermediate warming phase.

12. The method as claimed in claim 1, wherein a cooling rate of the workpiece during the cooling phase is different than a heating rate of the workpiece and a cooling rate of the workpiece during the first intermediate warming phase.

13. The method as claimed in claim 12, wherein the cooling rate of the workpiece during the cooling phase is lower than the heating rate of the workpiece and the cooling rate of the workpiece during the first intermediate warming phase.

14. The method as claimed in claim 12, wherein the heating rate of the workpiece during the first intermediate warming phase and the cooling rate of the workpiece during the first intermediate warming phase are equal.

15. The method as claimed in claim 4, wherein the lower target temperature is in the range from ?100? C. to ?185? C.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) A working example of the invention will be described in more detail with the aid of the drawing. The single drawing (the figure) schematically shows, in a temperature (T)-time (t) graph, the course of the temperature of a workpiece produced by means of additive manufacturing during the after-treatment by a method according to the invention.

DETAILED DESCRIPTION

(2) A workpiece which has been produced by means of additive manufacturing and is present, for example, at ambient temperature or, owing to a preceding heat treatment, at a temperature above the ambient temperature, is introduced into a cold chamber and the latter is subsequently closed. The temperature of the atmosphere in the interior of the cold chamber is slowly decreased, for example at a rate ?T/?t in the range from 1 K/min to 10 K/min, by introduction of a coolant. To produce the coolant, nitrogen gas is mixed, for example in a separate vessel, with liquid nitrogen or nitrogen gas having a temperature close to the boiling point of nitrogen (196? C.) in such an amount that the resulting cold nitrogen gas has a prescribed temperature. The coolant is gradually introduced into the cold chamber and thus reduces the temperature of the atmosphere in the interior of the cold chamber and thus the temperature of the workpiece during a cooling phase K to a lower target temperature T.sub.1 of, for example, ?150? C. After the cooling phase K, the workpiece is maintained at the lower target temperature T.sub.1 for a period of, for example, from 1 minute to 100 minutes (hold phase H). After the hold phase H, the workpiece is gradually warmed, i.e. at a warming rate which is comparable to the cooling rate in the cooling phase K, to an upper target temperature T.sub.2 (warming phase W) by introduction of a warm gas (for example nitrogen) having a temperature higher than the temperature in the interior of the cold chamber. T.sub.2 corresponds, for example, to ambient temperature. After the cold treatment, the additively produced workpiece is taken from the cold chamber and can be passed to further treatment or processing steps. The gas which is taken off simultaneously from the cold chamber during introduction of the coolant or warm gas is discharged into the surroundings or passed to a further use.

(3) During the cooling phase K, the introduction of the coolant can be stopped one or more times and the cooling of the workpiece can thereby be slowed or held at a prescribed temperature. Likewise, during the warming phase W the introduction of warm gas can be interrupted one or more times and the warming rate can in this way be slowed or the workpiece can be held at a prescribed temperature. In these intermediate hold phases, ?T/?t is thus<<1 K/min. In the working example shown in the drawing, an intermediate hold phase is carried out in each case during the cooling phase K at the temperatures T.sub.3 where T.sub.1<T.sub.3<T.sub.2 and during the warming phase W at a temperature T.sub.5 where T.sub.1<T.sub.5<T.sub.2.

(4) In order to dissipate stresses in the treated material which occur as a result of the cold treatment, it is advantageous to temporarily increase the temperature of the workpiece in the cold chamber during the cooling phase K and/or the hold phase H. In these intermediate warming phases A.sub.1, A.sub.2, the temperature of the workpiece is increased to an intermediate temperature T.sub.3, T.sub.4. The intermediate temperature is, for example, from 10K to 50K above the temperature of the workpiece at the beginning of the intermediate warming phase A.sub.1, A.sub.2, but below the initial temperature before commencement of the cold treatment. In the working example shown here, a first intermediate warming phase A.sub.1 to the value T.sub.3 is carried out during the cooling phase K and a second warming phase A.sub.2, after which the lower target temperature T.sub.1 has already been attained, is carried out to a value T.sub.4, where in the working example T.sub.3>T.sub.4, but this is not absolutely necessary for the purposes of the invention. The warming phase A.sub.2 can be followed by a further intermediate warming phase (not shown here), or the workpiece remains at the lower target temperature T.sub.1 for a certain period of time.

(5) In some embodiments, the heating and cooling rates of the workpiece during the cooling phase K, the warming phase W, the first intermediate warming phase A.sub.1, and/or the second intermediate warming phase A.sub.2 may differ from each other. Referring to FIG. 1, the rate of change in the temperature of the workpiece (?T/?t) at various points in time during the after-treatment method disclosed herein can be visually compared by a comparison of the slope of the line at the corresponding points in time on the graph of FIG. 1.

(6) During the first intermediate warming phase A.sub.1 and the second intermediate warming phase A.sub.2, the temperature of the workpiece is raised to a corresponding intermediate temperature T.sub.3, T.sub.4 before being allowed to cool to the initial temperature at the beginning of the respective intermediate warming phase A.sub.1, A.sub.2. As illustrated in FIG. 1, the rate at which the temperature of the workpiece increases during the first intermediate warming phase A.sub.1 is the same as the rate at which the temperature of the workpiece decreases during the first intermediate warming phase A.sub.1. Thus, the heating rate of the workpiece during the first intermediate warming phase A.sub.1 and the cooling rate of the workpiece during the first intermediate warming phase A.sub.1 are equal. Similarly, the heating rate of the workpiece during the second intermediate warming phase A.sub.2 and the cooling rate of the workpiece during the second intermediate warming phase A.sub.2 are equal.

(7) In some embodiments, a heating rate of the workpiece and/or a cooling rate of the workpiece during the first intermediate warming phase A.sub.1 may be different than a heating rate of the workpiece and/or a cooling rate of the workpiece during the second intermediate warming phase A.sub.2. For example, as illustrated in FIG. 1, the slopes of the line of the temperature-time graph during heating and cooling portions of the second intermediate warming phase A.sub.2 are steeper than the slopes of the line during heating and cooling portions of the first intermediate warming phase A.sub.1. Thus, the heating and cooling rates during the second intermediate warming phase A.sub.2 are greater than the heating and cooling rates during the first intermediate warming phase A.sub.1.

(8) With continued reference to FIG. 1, the heating rate of the workpiece and the cooling rate of the workpiece during the first intermediate warming phase A.sub.1 are different than the cooling rate of the workpiece during the cooling phase K. The slopes of the line of the temperature-time graph during heating and cooling portions of the first intermediate warming phase A.sub.1 are steeper than slopes of the line during the cooling phase K. Thus, the cooling rate of the workpiece during the cooling phase K is lower than the heating rate of the workpiece and the cooling rate of the workpiece during the first intermediate warming phase A.sub.1.