Patent Description:
The term additive manufacturing refers to a process wherein three-dimensional design data are used for manufacturing a component by progressively laying out multiple layers of material.

Additive manufacturing is a production technique that is clearly distinct from conventional methods based on material removal: instead of producing a semifinished product by starting from a solid block or by filling a mould in a single step, as is typical in foundries, components are built layer by layer starting from materials available as fine powder. Different types of materials can be used, in particular metals, plastics or composite components.

The process is started by laying a thin layer of powder material onto a work platform (bed).

A laser beam is then used in order to melt the powder exactly in predefined locations according to the component design data. The platform is then lowered and another layer of powder is applied, and the material is melted again in order to bind it to the underlying layer in the predefined locations.

<CIT> describes an additive manufacturing apparatus for building objects by layerwise consolidation of material. The apparatus comprises a build chamber containing a working area, a plurality of high energy beams or consolidating material deposited in the working area in layers and an optical unit for controlling transmission of the high energy beams onto material in the working area. The optical unit comprises a plurality of independently controllable optical elements, each optical element controlling transmission of at least one of the high energy beams onto the material in the working area, the optical unit being movable in the build chamber.

<FIG> shows an apparatus for additive manufacturing <NUM> according to the prior art.

Such apparatus comprises a laser source, associated optics for transmitting a beam, and scanner optics, designated as a whole by reference numeral <NUM>, which are adapted to emit a laser beam <NUM> directed towards a powder bed <NUM>.

The powder bed <NUM> is fed by a powder dispenser piston 6a, which feeds the powder, in a feed area <NUM>, onto a platform 6b. The dispenser piston 6a moves vertically upwards along a direction A as the powder is used.

A doctor blade <NUM> moves transversally relative to the platform 6b in a direction B parallel to the plane in which the powder bed <NUM> lies, thus moving the powder from the feed area <NUM> towards a work area <NUM>, wherein the laser beam <NUM> progressively creates a product <NUM> by melting the powder layer just laid by the doctor blade <NUM>. In the work area <NUM> there are also a platform 6b', whereon the powder brought by the doctor blade <NUM> is laid, and a support piston 6a', which lowers vertically in a direction C as the product <NUM> takes shape and increases in size.

In the work area <NUM> an emission opening and an opposite suction opening (not shown in the figure) are advantageously present, which are arranged transversal to the powder bed <NUM> and parallel to the plane in which a powder bed lies, for introducing a blade of a predefined gas, e.g. argon, and for sucking it in, respectively. The gas is used for cleaning the work area <NUM> from the vapours produced by evaporation of the powder; such vapours must not, in fact, be allowed to re-condense on the product <NUM>, because this would lead to processing defects.

The apparatus of <FIG> is a static system that cannot easily grow in size for manufacturing big parts; as the dimensions of the product <NUM> increase, the dimensions of the emission opening and suction opening should also increase accordingly, but, if an excessively large gas blade is emitted, the gas will produce turbulences on the surface of the powder bed <NUM> that will not allow for optimal processing, since they will impair the uniformity and homogeneity of the powder bed <NUM> (dune effect). An increase in the size of the product <NUM> would necessarily require higher speed values of the gas blade between the openings in order to clean the work area <NUM>, resulting in a higher Reynolds number and in the presence of turbulent motion. Thus, a part of the powder on the bed <NUM> would be dragged away by the gas blade, resulting in excessive material waste. The presence of turbulence in the gas blade would inevitably imply the generation of vortices and the loss of planarity in the previously laid powder bed, with adverse consequences on the manufactured part.

In order to maintain a laminar gas blade (e.g. Re < <NUM>), the blade speed should be decreased, resulting in adverse effects in terms of productivity of the system and contamination of the generated part.

Moreover, in the apparatus of <FIG> it is necessary, due to the fact that the laser source <NUM> is in a fixed position, that the doctor blade <NUM> completes the deposition of the powder bed <NUM> onto the platform 6b' before the source <NUM> can be turned on and production of the product <NUM> can be started. Therefore, there are many intervals between one step and the next, which limit the productivity of the system because it is necessary to wait for the completion of the laying of a new powder bed before starting a new processing step.

Likewise, a damaged component will cause a long downtime.

Penetration and absorption of the laser beam in the powder bed are defined by the interaction between the laser beam itself and the powder bed, in particular by the energy absorption properties and the temperature of the powder bed.

The absorption properties of the material include density, thermal conductivity, specific heat and emissivity. These properties do not have constant values, but change with the temperature of the material. In particular, according to an additive manufacturing technique called selective laser sintering/melting, thermal capacity (the product of specific heat by the temperature difference between ambient temperature and melting temperature) can widely affect the process.

The actual thermal capacity of the powder depends on its apparent density, which takes into account the gaps between one grain and the neighbouring ones, and on the temperature difference between the process start temperature and the melting temperature. While specific heat and apparent density are predefined as a function of the type of powder material in use and cannot be changed by the process, it is very important to be able to control the temperature range close to the melting point.

In addition to the above, it must be reminded that the quality of the manufactured parts is strongly dependent on the choice of the process parameters, such as laser power, laser scanning speed on the powder bed, shape of the laser beam, and material in use.

In the field of direct laser manufacturing or selective laser melting, one very important parameter is process speed, i.e. productivity. By optimizing this parameter it is possible to expand the application range of laser machines to spheres normally covered by foundries, while however offering several advantages, such as high spatial resolution, capillary process control, and capability of pre-processing the powder bed and post-processing the melted material.

Such advantages add to all the traditional advantages of additive manufacturing, such as the possibility of creating structures that cannot be manufactured in foundries.

Today, the above-described apparatuses for additive manufacturing allow the execution of a process that is articulated as follows:.

The vapours produced by evaporation of elements of the material are sucked in by an inert gas blade tangential to the powder bed <NUM>, produced by a suction opening in a fixed position. A detailed analysis of the total length of an additive manufacturing process allows identifying four times:.

An apparatus for additive manufacturing as described above requires the consecutive execution of all the process steps, and its duration cannot therefore be reduced.

It is therefore one object of the present invention to propose an apparatus for additive manufacturing which allows reducing the total processing time and eliminating any air turbulences that may develop when manufacturing big parts, thus increasing overall productivity.

It is a further object of the present invention to propose an innovative method of additive manufacturing.

These and other objects are achieved through an apparatus for additive manufacturing having the features set out in claim <NUM> and through a method as defined in claim <NUM>.

Particular embodiments of the invention are set out in dependent claims, the contents of which are intended to be an integral part of the present description.

Further features and advantages of the invention will be illustrated in the following detailed description, which is provided merely by way of non-limiting example with reference to the annexed drawings, wherein:.

<FIG> shows an apparatus for additive manufacturing <NUM> according to the present invention.

It comprises a powder bed <NUM> laid horizontally on a platform <NUM>, preferably rectangular in shape, supported by a piston <NUM> adapted to move, in a per se known manner, vertically along a vertical direction Z, so as to move said platform <NUM> along the direction Z.

The apparatus <NUM> further comprises a scanner head <NUM>. A fixed laser source is connected to it or integrated into it in a per se known manner, which is adapted to emit a laser beam 108a directed towards the powder bed <NUM>.

A first doctor blade <NUM> and a second doctor blade <NUM> are arranged on the powder bed <NUM>, opposite to each other at a constant and predefined distance.

Between the two doctor blades <NUM>, <NUM> a work area <NUM> is defined, wherein the desired product is manufactured; therefore, the laser beam 108a is specifically directed into said work area <NUM>.

Both doctor blades <NUM> and <NUM> are adapted to move in the same direction X and to slide along the entire platform <NUM> (on the powder bed <NUM>).

According to the invention, the powder bed <NUM> is laid out by the first doctor blade <NUM> (because it precedes the second doctor blades <NUM> in the direction of motion X). Alternatively, the doctor blades <NUM>, <NUM> may move in a direction opposite to the direction X, in which case the powder bed <NUM> will be laid out by the second doctor blade <NUM>.

The doctor blades <NUM>, <NUM> are provided with an emission opening 110a and a suction opening 112a, respectively, which are adapted to output and take in a blade of processing gas, such as, for example, argon or nitrogen. In this manner, delivery and suction of the processing gas will occur on opposite sides, and the work area <NUM> will always be properly covered with gas, thus eliminating the notorious "dune effect".

In addition, sensor means (not shown in the figure) are provided on the suction opening 112a for measuring the turbulence in the work area <NUM>. It will thus be possible to maintain, by means of an electronic control unit <NUM> controlling the gas speed, a laminar flow between the two openings 110a, 112a.

Moreover, the short distance between the two openings 110a, 112a allows using high speed values (Re≈<NUM>) while still maintaining a laminar flow, thereby increasing the productivity of the system and the quality of the manufactured parts. If the openings were kept at a fixed distance equal to the size of the powder bed <NUM>, on the contrary, it would be necessary to adopt much lower speeds to maintain a laminar flow.

The active control over the laminarity of the gas blade ensures planarity of the powder bed throughout the processing effected by the laser on the deposited layer. This makes it possible to attain growth uniformity along the axis z of the part, thus keeping the powder bed planar, while also ensuring high purity of the manufactured part by avoiding any inclusion caused by residues or carbonized particles, which can be effectively removed by a high-speed flow. The laser beam 108a follows the doctor blade <NUM>, <NUM> that is laying the powder layer to be melted, and laser melting occurs during the deposition of the powder onto the powder bed <NUM>, resulting in higher efficiency of the whole process.

The first doctor blade <NUM>, which precedes the second doctor blade <NUM> along the direction of motion X, blows said blade of nitrogen/argon, while the second doctor blades <NUM> sucks it in; when the first doctor blade <NUM> reaches a terminal transversal edge of the platform <NUM>, the motion of the doctor blades <NUM> and <NUM> is reversed and the two doctor blades start moving in a direction opposite to the first direction X. As soon as the direction of motion X is reversed, also the roles of the openings are exchanged, so that there will be, respectively, an emission opening 112a on the second doctor blade <NUM> and a suction opening 110a on the first doctor blade <NUM>. As an alternative, the roles of the opening may not be reversed.

Proximity sensors, preferably of the capacitive or optical type, are associated with the doctor blades <NUM>, <NUM> for controlling the distance between the doctor blades <NUM>, <NUM> and the powder bed.

A control unit <NUM> is connected to the apparatus <NUM> in order to control the movements of the doctor blades <NUM>, <NUM> and of the source <NUM> and to control the operation of the apparatus <NUM>.

<FIG> shows a variant of the apparatus of <FIG>, wherein the doctor blades <NUM>, <NUM> are secured to the base of a portal <NUM> comprising columns <NUM> and transversal elements <NUM> fixed between the columns <NUM>. The scanner head <NUM> that directs the beam 108a is positioned in a housing <NUM> defined by said transversal elements <NUM>.

The method of additive manufacturing according to the present invention is therefore based on the use of the apparatus <NUM> and comprises the steps of:.

Thanks to the use of the two doctor blades <NUM>, <NUM>, the apparatus of the present invention allows reducing the time necessary for laying out the bed of material to be melted, because the laying step overlaps to a large extent with the melting step, as opposed to such steps occurring sequentially as in the reference case.

The time required for positioning the laser beam 108a can be divided into two distinct times: the time necessary for moving the beam 108a during the processing, when the source is on, and the positioning time, when the source connected to the scanner head <NUM> is off.

The laser source <NUM> is only active in the work area 108a, while the doctor blades <NUM>, <NUM> are moving and laying out the material. In this manner, the process time is only determined by the melting speed, without being affected by the time necessary for depositing the material onto the powder bed <NUM>.

The melting time is inversely proportional to the laser power; therefore, a further reduction in the process time can be easily obtained by using more powerful lasers.

The apparatus for additive manufacturing equipped with two doctor blades as described herein allows reducing the fourth process time, i.e. the time necessary for resetting the system for processing the next layer.

The symmetrical arrangement of the two doctor blades <NUM>, <NUM> makes them usable in both directions, thus completely zeroing the reset time and making it possible to lay out the powder bed <NUM> in both directions.

The configuration with two symmetrical, self-moving doctor blades <NUM>, <NUM> also allows accommodating the inert-gas ventilation system for evacuating the melting vapours, which might contaminate, by re-condensing, both the powder and the surface just melted.

This function is provided by the fixed emission and suction openings, together with the delimited work area <NUM>: the ventilation system moves along with the doctor blades <NUM>, <NUM> due to the mutual vicinity of the two doctor blades <NUM>, <NUM>, and requires much less coverage gas while at the same time ensuring higher uniformity with a lower gas flow rate. This also has a positive effect on the heating and melting of the powder and on the next cooling of the melted layer, contributing to removing less heat by convection and improving in this way the thermal balance of the area where laser processing occurs.

Thanks to the use of the two openings 110a, 112a positioned at a fixed distance, shorter than the size of the platform <NUM>, a laminar gas blade can be maintained.

It is thus possible to adopt higher gas blade speeds, thereby advantageously preserving the purity of the powder and of the manufactured part, while also considerably shortening the production times. Furthermore, due to the sensor means used for measuring the turbulence of the gas blade, it is possible to verify the presence of a laminar flow in the work area <NUM>, thus ensuring that a homogeneous product will be obtained.

Claim 1:
Apparatus for additive manufacturing (<NUM>), comprising:
- a platform (<NUM>) adapted to receive a powder bed (<NUM>) that is laid thereon;
- a fixed laser source (<NUM>) adapted to emit a laser beam (108a) towards the powder bed (<NUM>);
- a first doctor blade (<NUM>) and a second doctor blade (<NUM>) opposite to the first doctor blade (<NUM>) and located at a predetermined distance from said first doctor blade (<NUM>), said doctor blades (<NUM>, <NUM>) being adapted to move in the same direction (X), so as to slide along the whole platform (<NUM>) and define a work area (<NUM>), into which the laser beam (108a) is directed in order to manufacture a product (<NUM>);
wherein said first doctor blade (<NUM>) is adapted to lay out said powder bed (<NUM>) and is provided with an emission opening (110a) adapted to produce a blade of a predetermined gas directed towards the powder bed, and the second doctor blade (<NUM>) is provided with a suction opening (112a) for sucking in said gas by each deposited layer, said suction opening (112a) being provided with sensor means adapted to measure the turbulence of the gas flow in said work area (<NUM>), so as to maintain, by means of a control unit (<NUM>), a laminar flow between said emission opening (110a) and said suction opening (112a).