WO2017098417A1

WO2017098417A1 - Laser diode device for additive manufacturing

Info

Publication number: WO2017098417A1
Application number: PCT/IB2016/057409
Authority: WO
Inventors: Fabio FERRARIO; Paolo Calefati
Original assignee: Prima Electro S.P.A.; 3D New Technologies S.R.L.
Priority date: 2015-12-10
Filing date: 2016-12-07
Publication date: 2017-06-15
Also published as: ITUB20156894A1

Abstract

Laser-diode device for additive manufacturing (50), comprising: - an electronic control unit (52) arranged to receive an input signal (55a) representative of a manufacturing target of a part to be fabricated and to send a first control signal (55b) towards a light source (56) in order to control the power thereof, wherein the light source (56) emits a laser beam (56a) towards a laser scanner (60) adapted to focus the laser beam (56a) towards a powder bed from which the part will be obtained.

Description

LASER DIODE DEVICE FOR ADDITIVE MANUFACTURING

DESCRIPTION

The present invention relates to a laser-diode device for additive manufacturing.

More specifically, the present invention relates to a device in accordance with the preamble of claim 1, as well as to a system for the execution of an additive manufacturing process. The term additive manufacturing relates to a process wherein three-dimensional design data are used for fabricating a component by progressively depositing multiple layers of material. Additive manufacturing is a professional production technique clearly different from all conventional material removal methods: instead of producing a semifinished product by starting from a solid block, components are fabricated layer by layer starting from materials available as a thin powder. Many different types of materials can be used, particularly metals, plastics or composite components.

The process starts with the deposition of a thin layer of powder material onto a manufacturing platform (bed). A laser beam is then used to melt the powder exactly in predefined locations according to the component's design data. The platform is then lowered, a subsequent layer of powder is applied, and the material is melted again so that it will bind to the underlying layer in the predefined locations.

The penetration of the laser beam and the absorption by the powder bed are defined by the interaction between the laser beam and the powder bed, particularly by the thermal properties and temperature of the powder bed.

The thermal properties of the material include density, thermal conductivity, heat capacity and enthalpy capacity. Thermal conductivity is not a constant value, but changes with temperature. In particular, in an additive manufacturing technique called selective laser sintering/melting, thermal conductivity affects the process to a large extent.

The actual thermal conductivity of the powder depends on the thermal conductivity of the solid part and of the gaseous part that is formed during the process. It has been demonstrated that the actual thermal conductivity of a powder is essentially independent of the material, but depends on the size and morphology of the particles, on the vacuum fraction created between the powder bed and the laser beam, and also on the thermal conductivity of the gaseous environment. It is therefore very important to be able to control the temperature range close to the melting point.

In addition to the above, it should be reminded that the quality of sintered parts largely depends on the choice of the process parameters, such as laser power, laser scanning speed on the powder bed, laser beam shape, and material used.

A homogeneous temperature of the powder bed may lead to better fabrication in terms of structure, surface and mechanical properties of the final product. It is therefore very important to control the size and temperature of the melted pool and the most important parameter for temperature control which is a function of the laser power. In fact, laser power and temperature are closely related and, since temperature may change very rapidly, it is important for the quality of the process to check it very frequently, e.g. every 100 μβ.

In order to monitor these characteristics of the melted pool, several sensors can be used, such as, for example, CMOS and ToF (Time Of Flight) sensors for checking the size of the melted pool, and a pyrometer or photodiode for checking the temperature of the melted bath. In particular, the melted bath must be monitored (as regards its temperature and shape) in real time and continuously.

Figure 1 shows a graph of the properties of a sintered part as a function of the scanning speed and power of the laser beam.

Five operating areas can be identified: in a first area 2, continuous objects are obtained, which however feature high roughness and a low aspect ratio (see Figure 2a); in a second area 4, a partially melted semifinished product is obtained, which comprises multiple beads attached to one another (see Figure 2b); in a third area 6, the powder is not perfectly melted (see Figure 2c), in a fourth area 8, no changes occur in the starting material, whereas in a fifth area 10 a continuous part is obtained, which is rounded with good surface quality and a good aspect ratio (see Figure 2d).

The different thermal properties of the material are therefore created by the different temperature generated by the motion of the laser beam used in the sintering process.

The temperature of the melted bath is related to laser power and, the speed being equal, affects the properties of the semifinished product obtained.

In known additive manufacturing systems, the optical chain comprises a laser and a scanning device connected thereto in series to direct the laser beam towards the powder bed. Such systems cannot ensure a quick response to temperature variations because of the overall dimensions of the devices involved and of the slow communication between such devices. Furthermore, in the systems known in the art there is a tight relationship between industrially machinery, set of process parameters and powder materials in use. Therefore, manufacturers can guarantee specific performance levels in their processes only in combination with the use of predefined materials and powders.

On the one hand, machinery manufacturers assemble laser devices and numerical-control machines without using any control sensors to monitor the process.

On the other hand, powder material producers are not concerned with any aspects related to process control and laser beam characteristics.

This leads to non-automated additive manufacturing processes, wherein the operator's experience still plays a decisive role in the fabrication success, with no real-time control over the process.

It is therefore an object of the present invention to provide a laser-diode device for additive manufacturing which allows accurate control over the temperature of the melted pool in the powder bed during the production process and fast reactions to any variations thereof, while also ensuring a precise processing of the material being sintered by monitoring the process in real time, so as to automatically obtain components free from manufacturing defects. Some embodiments of the present invention concern a laser-diode device for additive manufacturing which can overcome the drawbacks of the prior art.

In one embodiment, the laser-diode device for additive manufacturing comprises an electronic control unit arranged to receive an input signal representative of a manufacturing target of a part to be fabricated and to send a first control signal towards a light source in order to control the power thereof. The light source emits a laser beam towards a laser scanner adapted to focus said laser beam towards a powder bed from which the part will be obtained.

In another embodiment, the device further comprises a dimension sensor and a temperature sensor respectively measuring the shape and the temperature of a melted pool created by the laser beam in the powder bed, and sending in real time respective measurement signals to the control unit. The control unit modifies the first control signal as a function of the measurement signals, so as to request the source to provide a new power output that will allow achieving the desired manufacturing target.

In another embodiment, the sensors are adapted to receive a reflected beam coming from the melted pool and having a wavelength which is different from the wavelength of the laser beam.

In another embodiment, the laser scanner moves the laser beam over the powder bed on the basis of position and scanning speed parameters depending on the input signal, and the parameters are received by the scanner through a second control signal coming from the control unit.

In another embodiment, the device further comprises lenses adapted to modify the qualitative characteristics of the laser beam.

In another embodiment, the device further comprises two dichroic mirrors adapted to allow the laser beam to be reflected towards the scanner and the reflected beam to be transferred towards the sensors.

In another embodiment, the sensors are located in proximity to both the source and the laser scanner.

Further features and advantages of the invention will become apparent in the light of the following detailed description, provided by way of non-limiting example with reference to the annexed drawings, wherein:

Figure 1 , as already described, shows a graph of the properties of a sintered part as a function of the scanning speed and power of the laser beam;

Figure 2, as already described, shows different semifinished parts obtained in function of different combinations of scanning speed and power of the laser beam of Figure 1; and

Figure 3 shows a diagram of a laser-diode device for additive manufacturing according to the present invention.

In brief, the invention relates to a laser-diode device for additive manufacturing which can control the shape and the power of the laser beam as a function of the temperature of the melted pool in the powder bed during the manufacturing process, which temperature is measured at predetermined intervals, preferably every 100 μβ, for keeping it at a constant value.

Figure 3 shows a diagram of a laser-diode device for additive manufacturing 50 according to the present invention.

This device comprises an electronic control unit 52 comprising, in a per se known manner, memory means 54, which unit is arranged to receive an input signal 55a and to send a first control signal 55b to a light source 56, preferably a high-power laser diode.

The input signal 55a is a numerical control signal representative of a desired manufacturing target (laser power, scanning speed, geometry of the part to be fabricated, manufacturing path, etc.), and comes from the numerical control of the machine in which the device shown in the drawing has been installed.

The first control signal 55b is a signal adapted to control the power of the light source 56 on the basis of a desired power value included in the input signal 55a.

The light source 56 is adapted to emit a laser beam 56a, which crosses lenses 57 that adjust the quality thereof, and is then reflected through two dichroic mirrors 58 and finally sent to a laser scanner 60 adapted to focus said laser beam 56a towards an underlying powder bed, not shown in the drawing.

The laser scanner 60 moves the laser beam 56a over the powder bed according to position and scanning speed parameters (which depend on the desired manufacturing target as expressed by the input signal 55a) received from the control unit 52 through a second control signal 55c and determined in a manner known to those skilled in the art on the basis of the input signal 55a.

The laser-diode device 50 further comprises a dimension sensor 62, preferably a CMOS or ToF sensor, for controlling the dimension of the melted pool created by the laser beam 56a in the powder bed during the process, and a pyrometer or photodiode 64 for controlling in real time the temperature of said powder bed. The sensor 62 and the pyrometer 64 are adapted to measure the shape and the temperature, respectively, of the melted pool at predefined time intervals, e.g. every 100 μβ, and to send in real time respective measurement signals 62a and 64a to the control unit 52, which in turn will modify the first control signal 55b in order to obtain a modified laser beam 56a.

The sensors 62 and 64 receive as an input a reflected beam 56b coming from the melted pool, which goes back through the scanner 60 after the manufacturing process and which has a wavelength (e.g. 200-600 nm) which is different from the wavelength of the initial laser beam 56a (e.g. 1096 nm). The use of dichroic mirrors 58 allows the laser beam 56a (e.g. 1096 nm) to be completely reflected towards the scanner 60, while the reflected beam 56b going back from the melted pool (e.g. 300-600 nm) is completely transferred to and analyzed by the sensors 62 and 64.

The dichroic mirrors 58 are transparent to the radiation that goes back after the process, while they completely reflect the radiation of the laser 56, which is useful for the manufacturing process.

In particular, the control unit 52 is arranged to analyze the measurement signals 62a and 64a and for requesting the source 56 to provide a new power output, so as to obtain in real time the desired powder bed temperature. The desired temperature depends on the desired manufacturing target.

The device 50 of the present invention provides real-time feedback control over the temperature and dimensions of the melted pool in the powder bed while processing the powder bed, because the sensors 62 and 64 are located in proximity to both the laser 52 and the laser scanner 60.

The solution of the present invention is both compact and economical, while also being flexible to use and providing a real-time closed loop control.

Of course, without prejudice to the principle of the invention, the embodiments and the implementation details may be extensively varied from those described and illustrated herein by way of non-limiting example, without however departing from the protection scope of the present invention as set out in the appended claims.

Claims

1. Laser-diode device for additive manufacturing (50), comprising:

- an electronic control unit (52) arranged to receive an input signal (55a) representative of a manufacturing target of a part to be fabricated and to send a first control signal (55b) towards a light source (56) in order to control the power thereof, said light source (56) emitting a laser beam (56a) towards a laser scanner (60) adapted to focus said laser beam (56a) towards a powder bed from which the part will be obtained.

2. Device according to any one of the preceding claims, further comprising a dimension sensor (62) and a temperature sensor (64), said dimension sensor (62) and temperature sensor (64) respectively measuring the shape and the temperature of a melted pool created by the laser beam (56a) in the powder bed, and sending in real time respective measurement signals (62a; 64a) to the control unit (52), the control unit (52) modifying the first control signal (55b) as a function of said measurement signals (62a; 64a), so as to request the source (56) to provide a new power output that will allow achieving the desired manufacturing target.

3. Device according to claim 2, wherein said sensors (62, 64) are adapted to receive a reflected beam (56b) coming from the melted pool and having a wavelength which is different from the wavelength of the laser beam (56a).

4. Device according to any one of the preceding claims, wherein the laser scanner (60) moves the laser beam (56a) over the powder bed on the basis of position and scanning speed parameters depending on said input signal (55a), said parameters being received by the scanner (60) through a second control signal (55c) coming from the control unit (52).

5. Device according to claim 1, further comprising lenses (57) adapted to modify qualitative characteristics of the laser beam (56a).

6. Device according to claim 3, further comprising two dichroic mirrors (58) adapted to allow the laser beam (56a) to be reflected towards the scanner (60) and the reflected beam (56b) to be transferred towards the sensors (62;64).

7. Device according to any one of claims 2 to 8, wherein the sensors (62; 64) are located in proximity to both the source (52) and the laser scanner (60).