The present invention relates to a method of surface smoothening of additive manufactured metal components. Currently, additively manufactured (AM) metallic components are dominantly made by powder bed fusion methods such as for example (a) selective laser melting, (b) electron beam melting, and (c) binder jetting process.

All the above mentioned processes form an undesirable surface finish i.e. un-melted powder particles attached to the surface causing high surface roughness, waviness inherited from layer by layer process, and sub-surface porosity depending on the alloy solidification behavior, power density, scan parameters, scan orientation etc.

Fig 1a, shows an additively manufactured component with different geometries using Co-Cr-Ni alloy in Trumpf Tru-Print 1000 Selective laser melting machine. Fig 1b, shows a scanning electron micro-graph of an AM side surface parallel to build direction. The micrograph reveals a high surface roughness, with values of Sz ~ 100 pm, and Sa ~ 8 pm. The measurements were made by using Mountains map software from Digital Serve version 7.4.8495. The following steps were implemented to measure Sa and Sz values following the ISO standard 25178. (1) levelling , (2) Form removal was done by applying a polynomial of 3° this is a F-operator which gives you a SF-surface), (3) A filter (robust gaussian) of 8 micron to separate roughness from waviness.

The current invention will aim to treat the above mentioned undesirable surface quality issues of the additive manufactured components by post-treating with pulsed plasma polishing in a unique configuration.

The inventive treatment was shown to reduce the AM surface roughness from Sz ~ 100 um, and Sa ~ 8 pm down to Sz < 20 pm, and Sa< 1.5 pm, without using any abrasive chemical, and or mechanical polishing medium.

In the proposed inventive configuration, the part to be treated is made as cathode and a high density pulsed plasma operating in the vicinity of the target surface causes severe and controlled surface erosion in self sputtered mode. In contrast to the state of the art grinding and polishing methods, the inventive method does not use any external abrasive chemical, and or mechanical media and is there by free from any possible contamination for sensitive applications for example medical implants. In the proposed method, complicated external profiles, and internal open features could be treated.

The high roughness (Sz ~ 100 pm) of AM component surfaces is not desirable in several structural and functional applications. It leads to poor accuracy and unpleasant aesthetics. Higher roughness and surface cracks causes increased stress concentration, which is an lead to premature end of life of the component. The roughness as well increases the coefficient of friction, which is bad for sliding contacts.

To overcome the above-mentioned undesirable effects of poor quality surfaces fabricated by additive manufactured components, post treatments are necessary.

State of the art post processes are primarily classified into:

(a) Grinding, polishing, drag finishing, brushing, vibratory finishing: In all these processes the skin is eroded mechanically using an abrasive medium. One draw backs is that the surface is thereby contaminated by the abrasive medium.

In some cases, like manual grinding and polishing, complicated surface profiles could not be treated well.

(b) Electrochemical polishing: The surface to be treated is electrochemically eroded by forming a galvanic cell, where the work piece acts as anode. One draw backs is that the process is not an environmental friendly process. In addition the parts are exposed to a corrosive medium. Any residual or entrapped electrolyte from the process might cause corrosion related issues that are not desired.

(c) Ion etching: Parts to be treated can be treated by ion etching in a chamber comprising ion sources. The process is clean and relatively complicated surface profiles can be treated. But the drawback is their low erosion rates. This can be for example done with the configuration as shown in Fig.6a in an Oerlikon Balzers Innova machine using 2 ion guns with a filament current of 200 A each. The measured substrate current was 25 A with an Ar gas flow of 100 SCCM, at a substrate voltage of 400 V. The measured etch rate was 1 pm per hour for the Nickel based alloys. This is primarily because of relatively low ion density. This configuration also has an issue of redeposition.

In the present invention, the above mentioned issues in the ion etching were addressed in a unique configuration as described below, where the part to be treated is made as a cathode, and the surface finish is achieved in sputtering mode in a high density pulsed plasma with a current density between 1 A/cm ² and 5 A/cm ² . The pulsed plasma is operated to keep the component surface temperature less than 300°C.

In the inventive method, a high rate plasma polishing/ etching of AM surface is performed in a dense localized ( 50 cm ² or less) pulsed plasma pocket that is dynamically swept over the AM component surface to be treated.

For a high rate plasma polishing/etching, following parameters are preferred: (a) A high plasma density eg: cathode current density between 1A/cm ² and 5 A/cm ² , (b) A high kinetic energy of the incident ions, between 600 eV and 1000 eV, (c) a minimal redeposition, and (d) a non-stationary plasma pocket ( Race track) to avoid component heating, and localized pitting (excessive material removal) of the component surface to be treated.

In one embodiment according to the inventive method, the component to be treated is configured as cathode, plasma is created by a glow discharge of an inert gas and a high rate ion polishing / etching is achieved in a self-sputtered mode schematically shown in Fig.2.

Ar (and/or other noble gases like Kr, Xe ,Ne or a mixture thereof) plasma is created by applying voltage between the cathode and anode similar to a sputtering process. The component to be treated is configured as cathode. Component polishing /etching is achieved by sputter erosion, and the eroded particles are transported to the adjacent anode enabled by the magnetic, and electro-magnetic forces, to avoid the re deposition. In the proposed configuration, for example a high plasma density with a current density between 1A /cm ² and 3 A/cm ² is achieved , by applying voltage of 800 V in an Ar gas pressure between 0.1 Pa and 0.3 Pa by confining the race track to small size ~ 30 cm ² .

When the component is immersed in such a high density plasma, the cathode/component surface undergo sputter erosion (as shown in Fig.3a). For a Nickel base super alloy, a measured sputter rate of 15 pm/h is achieved in the proposed configuration.

However, during the sputter erosion, the component temperature also raises rapidly due to collision cascade in the sub-surface region (as shown in Fig. 3b). To avoid this, the sputter eroded zone is continuously moved by dynamically moving the magnetic field. This results in a dynamic motion of the race track, schematically shown in Fig. 2. The dynamic motion of the magnetic field is manipulated by energizing and deenergizing the electro-magnetic array that was arranged inside carousel holder. In addition, a pulsed power supply is needed. An additional air / water cooling is provided to the chamber holder to keep the surface temperature of the component being treated less than 300°C.

In summary, by controlling the applied voltage, Ar gas pressure, confining the race track to smaller size, and a controlled dynamic motion of race track, a sputter erosion rate between 5 pm and 20 pm/h is achieved based on the geometry while keeping the component temperature lower than 300°C, preferably even lower than 200°C.

An AM surface has a typical surface roughness between Rz ~ 40 pm and 100 pm.

In the proposed configuration, Rz value is reduced from 100 pm to 10 pm in less than 10 hrs.

The above mentioned configuration is proved in the following experiment.

The test pieces from different locations and different orientations of AM build (Fig.1) is extracted and made as a part of the cathode as schematically shown in Fig. 4. A pulsed glow discharge of Ar is achieved with a discharge voltage of 800 V, in an Ar partial pressure of 0.2 Pa, and a peak power of 45 kW, and a frequency of 100 Hz, race track size of 30 cm ² . The race track is rotated at an RPM of 5 Hz. This results in a cathode discharge current density of 1.5 A/cm ² .

Evolution of surface topography and scanning electron micrography are presented in Fig. 5. The SEM, and confocal microscopic images, shows that after 11 Hrs of treatment, a smooth surface is achieved, and the surface is free from droplets, cracks, and pores. A surface roughness of Sz < 20 pm, and Sa < 2 pm is achieved from the as-printed value of Sz 60-160 pm.

The erosion rates in the proposed configuration was measured as 15 pm/h while a typical ion etching with an external ion sources of 400 A is measured as 1 pm/ hour, as shown in Fig. 6.

In-summary, the proposed configuration proves to be a high rate sputter polishing of AM surface with erosion rates of 15 pm/h while keeping the surface temperature lower than 200 °C.

Furthermore, in the proposed configuration the erosion rates can be amplified by using reactive gasses that form volatile compounds of the surface to be treated. This could for example be Cl and/or F.

In the proposed method, interior features can also be treated by a hallow cathode effect. However these features need to be open .

Even though, the proposed configuration is schematically shown in a batch type system, experts in the field realize that the process can be realized in a continues or semi-continues configuration in an in-line coater configuration.

In addition to above-mentioned, improved surface topography further improved AM quality is realized by

(a) Relieving tensile stresses, and inducing a favorable surface compressive stress

(b) Homogenizing or annealing (c) Surface chemistry modification for corrosion resistance. For example Nitriding, Oxidizing, and Oxy- nitriding can be performed, if reactive gases such as nitride and/or oxide are used.

(d) Surface hardening by Plasma nitriding, if nitride is added to the working gas

The invention is now described in detail with the help of an example and with the help of the figures. The figures show the following:

Fig 1(a) shows additively manufactured components with different geometries, and build layout of Co-Cr-Ni alloy in Trumpf Tru Print 1000 selective laser melting. Fig 1(b) shows Scanning electron micro-graph of AM side surface parallel to build direction.

Fig 2 shows a proposed high rate plasma polishing chamber with a self-sputtered configurations.

Fig 3 (a) shows a snap shot the cathode with a glow discharge,

Fig 3 (b) shows the schematic representation of sputtered erosion Fig 4 (a) shows test pieces from different build locations embedded in a plate, such plate being normally used as target for sputtering. This assembly creates an embedded target which is then configured as cathode.

Fig 4 (b) shows the target of figure 4 (a) subjected to sputter erosion.

Fig: 5 (a) shows the evolution of surface topology of AM surface, where the surface is examined in confocal microscope and a scanning electron microscope. Figure 5 (b) shows the evolution of surface topology of AM surface, where the variation in surface roughness as a function of surface treatment time is shown . Annotations correspond to different surface orientations in the build as shown in Fig.1 (a)

Fig. 6 (a) shows the ion etching configuration according to state of the art. Shown are 1. Chamber, 2a, 3a Ion source cathode, 2b, 3b ion source anode. Plasma is ignited at the pocket source.

Fig. 6 (b) shows the Inventive sputter erosion configuration where the component to be treated acts as cathode in a pulsed plasma with a high current density.

Fig. 6 (c) shows the comparison of the current density, and ion etch rates of both the process.

Shown in Figure 1 are additively manufactured parts with Co-Cr-Ni alloy using selective laser melting. Fig.1a gives an overview of build layout, whereas figure 1b is a scanning electron micrograph of AM surface parallel to build direction.

Shown in figure 2 is the proposed high rate ion polishing chamber with a self-sputtered configurations. The different components are: l .lon chamber, 2. Component holder, 3. Example component to be treated, 4. Spatially controllable Electro-magnet array, 5.Anode, 6. Localized plasma, 7. Feed-through for the electric connections of the magnetic array. Arrows near pos.6 indicates direction of the plasma pocket movement

Figure 3 a shows a snap shot of target / cathode with a glow discharge and figure 3 b indicates a schematic representation of sputtered erosion.

In figure 4 can be seen an example configuration of proposed idea. Figure 4a shows test pieces from different build locations are embedded in the target. Embedded target is configured as cathode. Figure 4b shows a target is subjected to sputter erosion.

In figure 5 the evolution of surface topology of an AM surface can be seen. Figure 5a shows surface examination in confocal microscope, and Scanning electron microscope. Figure 5b shows the variation in surface roughness as a function of surface treatment time.

Figure 6 gives a comparison of a typical ion etching configuration with the proposed sputter erosion (a) Ion etching: 1. Chamber, 2a, 3a Ion source cathode, 2b, 3b ion source anode. Plasma is ignited at the pocket source (b) Inventive sputter erosion configuration. The bottom graph of figure 6 compares the current density, and ion etch rates of both the process.

The test pieces from different locations of AM build (Fig.1 ) is extracted and made as a part of the cathode as shown in Fig. 4. A pulsed glow discharge of Ar is achieved with a discharge voltage of 800 V, resulting a peak power of 45 kW, in an Ar partial pressure of 0.2 Pa, and a Peak power of 45 kW, and a frequency of 100 Hz, and a pulse length of 30 ms, with a race track size of 30 cm ² , and the race track rotated at an RPM of 5 Hz, and an average power of 4.5 kW/ cathode. This results in a cathode discharge current density of 1.5 A/cm ² .

Evolution of surface topography and scanning electron micrography are presented in Fig. 5. The SEM, and confocal microscopic images, shows that after 11 Hrs of treatment, a smooth surface is achieved, and the surface is free from droplets, cracks, and pores. A surface roughness of Sz < 20 pm, and Sa < 2 pm is achieved from the as-printed value of Sz 60-160 pm. Experts in the field also realize that the time can be further reduced by modifying the parameters especially the average power.

The erosion rates in the proposed configuration was measured as about 15 pm/ hour~ while the rate for an ion etching method with a dedicated ion sources of 400 A using 100 SCCM Ar, at a substrate potential of 400 V was measured as 1 pm/ hour, as shown in Fig. 6.

A key feature in the proposed configuration is a high current density of 1.5 A/cm ² on the surface to be treated by confining the plasma without using any expensive plasma sources. A pulsed power, and continues race track motion keep the surface Temp lower than 200°C. In-summary, the proposed configuration proves to be a high rate sputter polishing of AM surfaces with erosion rates potentially as high as 15 pm / hour while keeping the surface temperature lower than 200°C

Furthermore, in the proposed configuration the erosion rates can be amplified by using the reactive gases that forms a volatile compounds of the surface to be treated. Such compounds could be for example Cl and/or F.

In the proposed method, interior features can also be treated by a hallow cathode effect. However these features must be exposed to surface.

Even though, the proposed configuration is schematically shown in a batch type system, experts in the field realize that it can be realized in a continues or semi- continues configuration.

In the present description a method for surface smoothening of compounds, in particular of addiditve manufactured compounds was disclosed, the method comprising the steps of:

- providing sputtering equipment with a sputtering vacuum chamber with a first cathode to be used as sputter target, an a first anode to receive sputtered particles characterized in that at least part of the cathode is formed by the compounds to be smoothened

- establishing a vacuum in the sputtering vacuum chamber

- introducing a working gas preferably Argon into the sputtering vacuum chamber

- applying electrical power to sputtering target, thereby starting the sputtering characterized in that at least part of the first cathode is formed by the compounds to be smoothened and while particles are sputtered form the surface of the compound the first anode reduces redeposition of sputtered particles onto the compounds to be smoothened.

In this method two or more cathodes comprising compounds to be smoothened can be used and/or two or more anodes can be used. Preferably the power applied onto the cathode is operated in such a way that a high density with a current density between 1 A/cm ² and 5 A/cm ² is created while the plasma is pulsed in order to keep the component surface temperature less than 300°C. Preferably means to establish movable localized magnetic fields are used to localize and making the plasma pocket move the along the surface of the cathode thereby allowing compound during the time where they are not exposed to the plasma at least partially to cool down.

In particular preferably the means to establish movably localized magnetic fields comprise at least one electromagnet and/or preferably comprise at least one spatially controllable array of electromagnets.