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 (K) to a lower target temperature (T1) of below 30° C. and subsequently warmed in a warming phase (W) to an upper target temperature (T2).

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 (T1) is in the range from 50° C. to 195° C., preferably in the range from 100° C. to 185° C.

5. The method as claimed in claim 1, wherein the upper target temperature (T2) is in the range from 20° C. to 40° C.

6. The method as claimed in claim 1, wherein the workpiece goes through a hold phase (H) in which the workpiece is held for a time of at least 30 s at the lower target temperature (T1) after the cooling phase (K) and before the warming phase (W).

7. The method as claimed in claim 6, wherein the cooling phase (K) and/or the hold phase (H) and/or the warming phase (W) is interrupted and the workpiece is warmed by at least 10K to an intermediate temperature (T3, T4) during an intermediate warming phase (A1, A2) and subsequently cooled again.

8. The method as claimed in claim 7, wherein the workpiece is warmed by at least 50K during the intermediate warming phase (A1, A2).

9. The method as claimed in claim 1, wherein the cooling of the workpiece in the cooling phase (K) and/or the warming of the workpiece in the warming phase (W) and/or at least one intermediate warming phase (A1, A2) is carried out at a rate of from 1.5 K/min to 10 K/min.

10. The method as claimed in claim 1, wherein the cooling phase (K) and/or the warming phase (W) and/or at least one intermediate warming phase (A1, A2) is interrupted for a prescribed period of time during which the workpiece is essentially held at an attained temperature (T3).

Description

BRIEF DESCRIPTION OF THE DRAWING

[0027] A working example of the invention will be described in more detail with the aid of the drawing. The single drawing (FIG. 1) 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

[0028] 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 ΔT/Δt 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 intro-duction 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.

[0029] 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, ΔT/Δt 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 T5 where T.sub.1<T.sub.5<T.sub.2.

[0030] 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.