Method for additive manufacturing

The invention relates to a method for additive manufacturing, particularly additive manufacturing using metal powders.

Starting in the late 1980s a large number of additive manufacturing processes are now available and widely spread. The main differences between the different additive manufacturing processes relate to the way layers are deposited to obtain parts and in the materials that are used.

Especially, in additive manufacturing methods using metal powders several methods are common meanwhile.

For the processing of metal powders the nowadays used methods are, electron beam melting (EBM), selective laser melting (SLM), selective heat sintering (SHS), selective laser sintering (SLS), the direct metal laser sintering (DMLS).

In the selective laser sintering (SLS) a high power laser fuses a powder of metal (or ceramics or glass) into a mass that has a desired three-dimensional shape. In this case the laser selectively fuses powdered material by scanning cross-sections generated from a description of the part on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of powder material is applied on top and the process is repeated until the part is completed. As the density of the finished part depends on the power of the laser and not on the laser duration normally pulsed lasers are used. In these processes normally the powder material is preheated below the melting point of the powder to make it easier for the laser to reach the temperature above the melting point.

With selective laser sintering parts from metals can be made including steel alloys, titanium, and alloy mixtures. In the method the physical process can be a full melting, a partial melting or a liquid phase sintering. The direct metal laser sintering (DMLS) uses a laser which is directed to a bed of powdered metal wherein the material is melted or welded together to create the solid structure. A device for direct metal laser sintering uses a laser wherein a built chamber area uses a material dispensing platform and a built platform along with a recoater plate used to move new powder over the built platform. This technology fuses metal powder into a solid part by local melting using the focus laser beam. Parts are additively built up layer-by-layer typically using layers which are for example about 20 μηι thick. The used material is stainless steel, tool steel, cobalt chromium, inconel, aluminum or titanium, low alloyed steel, super alloys, copper, and more.

One other method to consolidate metal powders into a solid mass is the so-called electron beam melting which uses an electron beam as a heat source. This is a technique similar to selective laser sintering (sis). In contrast to sintering techniques electron beam melting as well as selective laser sintering fully melts the metal powder in the area where the beam impacts the powder layer.

In the selective laser melting (slm) thin layers of atomized fine metal powders are selectively melted after they have been evenly distributed using a coating mechanism onto a substrate plate using metal that is fastened to an indexing table that moves in the vertical axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, for example argon or nitrogen at oxygen levels below 1000 volume parts per million. Once each layer has been distributed, the part is layerwise produced by selectively melting the powder. This is accomplished with a high power laser beam.

All these above mentioned techniques have in common, that the fusing or melting step is conducted in a closed process chamber which is purged with a process gas, for example an inert gas in order to remove air. The oxygen contained in the air would oxidize the metallic powder when the laser melts the powder to form the 3-dimensional part. For example the oxygen content inside the process chamber is after purging and during the additive manufacturing process at a level of about 0,1 % by volume oxygen.

A lower oxygen content would be beneficial but reducing the oxygen content from 0,1 % by vol. to for example 0,01 or 0,001 % by vol. would take too much time and would be uneconomical. The EP 2 050 526 A1 discloses an atmosphere for sintering, annealing and hardening comprising silane or borane in a furnace atmosphere comprising nitrogen, hydrogen, argon or any mixture of these gases with an addition of gaseous hydride such as silane or borane.

Metal sintering is defined in the document as a thermal treatment of a metal powder or mixture of metal powders at an enhanced temperature for the purpose of increasing its strength by bonding together the particles. During sintering atomic diffusion takes place and the powder particles are welded together. The sintering operation has normally to be carried out under a protective atmosphere in order to prevent an oxidation and to promote the reduction of surface oxides. By adding a gaseous hydride to the furnace atmosphere any oxygen or water vapor in the atmosphere shall react with the gaseous hydride which should make it possible to produce an atmosphere with an extremely low dew-point and low partial pressure of oxygen. The amount of the gaseous hydride which is added to the furnace atmosphere is preferably between 0,00001 % and 2%, preferred between 0,001 % to 0,05%.

The EP 2 050 528 A1 discloses atmospheres for metal atomization including silane or borane wherein in the metal atomization process the raw material is melted and then the liquid metal is broken into fine particles. This is achieved by atomizing a flow of the liquid metal by an atomization agent such as water or gas. The atomization agent strikes the liquid metal at a high velocity whereby the molten metal is disintegrated into fine droplets which solidify thereafter. During the production process fine particles absorb oxygen from the atomization agent which might react with alloying elements and might cause problems in the production of special alloyed metal powder. Instead of using water or compressed air as an atomization agent it is known to atomize a molten metal with an inert gas, for example nitrogen or argon. In that document a method for producing a metal powder by atomizing a molten metal by means of an atomization gas is disclosed which is characterized in that the atomization gas comprises a gaseous hydride such as silane or mono silane.

It is an object of the present invention to provide a method for additive manufacturing of a part wherein the time for purging the process chamber is reduced and the purity of the gas in the process chamber is increased and improvements to the quality of the part are expected, e. g. lower oxygen content, better bonding etc.

This object is achieved by a method for additive manufacturing, wherein a part is built by selectively sintering or melting a material and building the part additively layer by layer using a heat source sintering or melting the material, wherein the process is conducted in a process chamber and wherein the process chamber is purged with a purging gas, and which is characterized in that the purging gas is enriched with a gaseous hydride and/or nitric oxide (NO).

Gaseous hydrides such as silane are known to have a very high reactivity with oxygen containing substances or compounds. For example, at room temperature silane or mono silane undergoes a spontaneous reaction with oxygen as well as with air: SiH ₄ + 2 0 ₂ -> Si0 ₂ + 2H ₂ 0

By the inventive addition of a gaseous hydride to the purging gas the oxygen content of the gas atmosphere in the process chamber is extremely reduced. A similar effect can be achieved by adding nitric oxide (nitrogen monoxide; NO) to the purging gas. The NO will react with any oxygen in the purging gas or in the gas atmosphere within the process chamber.

In the additive manufacturing of parts or components from metal powders, namely by melting or sintering with a high energy laser beam, the respective process chamber is purged with a purging gas before the additive manufacturing process starts. By the inventive addition of NO or a gaseous hydride like silane this purging time is significantly reduced. According to an embodiment of the invention the oxygen content of the purging gas and/or of the gas atmosphere in the process chamber is determined and the amount of the gaseous hydride or NO added to the purging gas depends on the determined oxygen content. The gaseous hydride and/or the NO can be added to the purging gas continuously or intermittently. For example, the process chamber is initially purged with the purging gas only and the gaseous hydride or NO is added only after a certain time or when the oxygen content of the gas atmosphere in the process chamber falls below a pre- defined threshold. The purging can be processed continuously but the adding of the gaseous hydride or NO can be carried out sequentially depending on the oxygen content of the purging gas and / or of the gas atmosphere in the process chamber.

The oxygen content of the purging gas can vary over the sometimes for hours or days lasting production process for example depending on the used powders. In general, oxygen can be undesirably introduced into the system by the processed powder itself or by even smallest leaks in the system.

According to a preferred embodiment silanes or boranes are used as gaseous hydrides. Silanes are chemical compounds of silicon and hydrogen. The lowest silanes, mono silane with a chemical formula SihU and disilane with the chemical formula Si2H ₆ are gaseous and are particular suitable for the invention since they are gaseous at room temperature. Boranes are chemical compounds of boron and hydrogen. The two lowest members of the boron hydrid group are mono borane or simply borane BH ₃ and di borane B2H6. All these compounds are known to be very reactive with oxygen and air. Thus, boranes are also suitable for reducing the oxygen content of the purging gas and/or of the gas atmosphere in the process chamber.

In particular, it is possible to use a mixture of silanes and boranes.

Silanes or boranes react with oxygen to a solid reaction product (Si0 ₂ , B0 ₂ ). Preferably, it should be avoided that these solid reaction products come into contact with the metal powder or if so only in amounts which are so small that they do not deteriorate the chemical or physical properties of the finished part.

According to another embodiment a portion of the purging gas is withdrawn from the process chamber, recirculated and re-introduced into the process chamber. The purging gas may be enriched with the gaseous hydride and/or with nitric oxide inside or outside the process chamber. In a preferred embodiment the gaseous hydride is added to the purging gas outside the process chamber so that the gaseous hydride and oxygen do not react inside the process chamber. In a preferred embodiment the purging gas is enriched with the gaseous hydride outside the process chamber and the enriched purging gas is passed through a filter element prior to entering the process chamber. The filter element is designed to retain solid particles, in particular the solid reaction products of the gaseous hydride with oxygen.

The invention is especially useful for additive manufacturing of metallic parts, and in particular for additive manufacturing of metallic parts by melting or sintering a metal powder. In order to improve the mixing efficiency and to avoid that reaction products get into the process chamber, it is preferred to mix the purging gas with the gaseous hydride and/or with the nitric oxide in a separate reaction chamber.

Further, the reaction between oxygen and the gaseous hydride forms solid particles and water vapour. For example, silane (SiH4) reacts with oxygen to Si02 and H20. Thus, the purging gas is preferably be dried before entering the process chamber in order to reduce the water (vapour) content of the purging gas. In that case the purging gas may be passed through a device for drying of the purging gas, in particular through water vapor absorbing means.

The inventive method is preferably used to purge the process chamber before the actual additive manufacturing process is started. Only when the oxygen content of the gas atmosphere in the process chamber is below a pre-defined threshold the additve manufacturing process is started, that is powder is provided and selectively melted.

The gaseous hydride and/or the nitric oxide are preferably not only used to remove the final ppm of oxygen from the gas atmosphere in the process chamber. According to a preferred embodiment the focus of the invention is to reduce the total purging time and thus the gaseous hydride and/or nitric oxide is added already at the beginning of the purging process. Thereby, the total purging time until a certain maximum oxygen content is achieved can be considerably reduced.

In order to monitor and control the oxygen level in the process chamber while the gaseous hydride is used, the purging gas enriched with the gaseous hydride is preferably subjected to a standard lambda probe prior to being introduced into the process chamber. The lambda probe will heat up the gaseous hydride-purging gas- mixture and accelerate the reaction between oxygen and the gaseous hydride. No heat is needed to react the gaseous hydride with oxygen. But as described above enhanced temperature can accelerate the reaction. Therefore, it is preferred to heat up the enriched purging gas and/or the process chamber atmosphere, respectively. For example, the gas may be heated up to a temperature of at least 100 °C, of more than 250 °C or to more than 400 °C.

In another embodiment an external control unit with a chemical cell is provided. The chemcial cell is not heated and does not suffer cross sensitivity. Thus, the oxygen content monitored by the chemical cell will be more precise than the oxygen content monitored by a lambda probe.

According to a preferred embodiment a sample of the gas atmosphere in the process chamber is passed to the chemical cell for analysis. The sample could be, for example, between 0,5 l/min and 3 l/min. The analysis is carried out outside of the process chamber, but it could also be integrated into the whole additive manufacturing system. Based on the result of the analysis the amount of gaseous hydride added to the purging gas is controlled.

The invention shows several advantages over the prior art technologies:

• The oxygen content in the process chamber will decrease much faster.

· The inert gas consumption (used for purging) can be reduced.

• The additive manufacturing process can be run under lower oxygen

concentrations resulting in improved mechanical properties of the manufactured end product. The invention is better understood by way of a drawing. Figure 1 schematically shows a preferred embodiment.

Figure 1 schematically shows a process chamber 1 for additive manufacturing of a metallic part. The process chamber 1 is provided with a purging gas pipe 7 for introducing a purging gas into the process chamber 1 . The purging gas pipe 7 is designed as a recirculation circuit which allows to withdraw purging gas from the process chamber 1 , recirculate the purging gas and re-introduce the purging gas into the process chamber 1 . The purging gas may be recirculated by means of a fan 8. In a preferred embodiment an inert gas, in particular nitrogen or argon, is used as the purging gas. Before starting the additive manufacturing process the oxygen content of the atmosphere in the process chamber 1 has to be lowered below a certain threshold in order to avoid undesired oxidation.

In a first embodiment the purging gas is enriched with a gaseous hydride, preferably with a silane or borane before the production process / additive manufacturing process in the process chamber 1 starts so that the initial oxygen content of the purging gas is as low as possible. Instead of or in addition to the gaseous hydride NO (nitric oxide) can alos be added. NO reacts with oxygen to N02 which could subsequently be removed by molecular sieves.

It is in particular preferable to add the gaseous hydride and/or NO to the purging gas already at the start of the purging process. Thereby, the inventive addition of a gaseous hydride and/or NO to the purging gas which is recirculated through the process chamber 1 allows to considerably reduce the purging time.

If, during the production of the part, a lowering of a meanwhile increased oxygen content is necessary, the purging gas is again enriched with the gaseous hydride and/or with nitric oxide. In these cases it is preferred to have a high flow rate or a high circulation rate of the purging gas for example caused by a fan 8 so that the very fine solid products of the reaction between the gaseous hydride and the oxygen do not sediment in the process chamber 1 .

Further, it is preferred that a filter 2 for fine solid particles is provided within the recirculation circuit 3. The gaseous hydride reacts with any oxygen present in the purging gas or in the gas atmosphere of the process chamber 1 to fine solid particles, such as for example Si02. The filter 2 retains these solid particles and prevents that the solid particles enter the process chamber 1 .

The gaseous hydride and/or NO may be directly introduced into the process chamber 1 or mixed with the purging gas outside of the process chamber 1 . In another embodiment in which it is possible to have a sequential enrichment of the purging gas with gaseous hydrides as well as a continuous enrichment with gaseous hydrides, the gaseous hydrides 6 are fed to and mixed with the purging gas outside the processing chamber 1 , for example in a separate reaction chamber 4 followed downstream by the filter element 2 or in the portion of the purging gas pipe 7 which leads to the processing chamber 1 or away from the processing chamber.

In this case it is more preferred to have a mixing apparatus 5 inside the reaction chamber 4 to improve the mixing of the gaseous hydride and the purging gas and to achieve a good distribution of the gaseous hydride within the purging gas.

After purging and during the additive manufacturing process the oxygen content of the gas atmosphere in the process chamber 1 shall normally be below a certain threshold, for example below a level of around 0,1 % 0 ₂ (1000 ppm). The invention allows to lower the oxygen content to lower levels within a reduced purging time. The oxygen content of the gas atmosphere may be reduced to 0,1 % by volume to 0,01 % by vol. or 0,001 % by vol.

According to the invention the gaseous hydride is mixed with the purging gas in an amount for example 0,5% with 99,5% by vol of the purging gas. It is further preferred to have a purging gas drying in the flow circuit. Such a dryer 9 may be a water vapor absorbing media. The inventive method is useful to decrease the oxygen and water vapor content of the purging gas which leads to a higher quality of the produced end product. Further, the processing times are shortened as the 0 ₂ -content in the process chamber 1 will decrease much faster than with inert purging gas only. It is further advantageous that no temperature increase is needed to react the gaseous hydride with oxygen. Thus, no heating means have to be provided but may be provided to accelerate the purging process and the oxygen reduction time.

According to another example of the present invention a mixture of 99,5 vol-% argon and 0,5 vol-% SiH4 is prepared and used as enriched purging gas for purging a process chamber used for additive manufacturing of a metal part. Prior to starting the actual additve manufacturing process, the process chamber is filled with air. Thus, the process chamber has to be purged in order to reduce the oxygen content in the chamber to a concentration of below 1000 vpm.

The process chamber may have a volume between 50 liters and 500 liters, for example 150 liters. A recirculation circuit is provided. Between 1 l/min and 5 l/min gas is withdrawn from the process chamber, recirculated and re-introduced into the process chamber. The oygen content of the recirculated gas is monitored using a chemical cell.

The enriched purging gas (comprising argon and silane) is first passed through a small heater or reactor and heated up to a temperature between 200 °C and 300 °C. The heated enriched purging gas is introduced into the recirculated gas stream and reacts with the oxygen present. The reaction forms sand (Si02) and water vapour (H20) as reaction products. The oxygen content of the recirculated gas is reduced.

In order to remove the sand and the water vapour the recirculation circuit is provided with a filter, for example a 10 micrometer filter, and a water absorbing medium. The recirculated gas is then re-introduced into the process chamber. The chemical cell monitors the oxygen content of the recirulated gas and controls the amount of enriched purging gas added to the recirculated gas. The chemical cell is preferably provided upstream of the point where the enriched purging gas is added.