Patent ID: 12188101

DETAILED DESCRIPTION

Additive manufacturing has many advantages, including reduced manufacturing time, reduced costs and reduced overheads compared with moulded parts, injected parts or parts machined from a billet.

However, parts created by additive manufacturing feature some drawbacks: during solidification, residual tensile stress develops with each new layer, and can cause cracking.

The invention seeks to remedy these disadvantages with a manufacturing device as shown inFIG.2, which comprises:a first nozzle100for the deposition of powder102on the substrate80,a head104emitting a laser beam106, anda second nozzle108for the compression of the bead110after it has been created and before it is covered by the subsequent bead.

As shown inFIG.1, the function of the nozzle100could be integrated in the laser head104. In this case, the device would feature a laser head104configured to deposit powder102on the substrate80, as well as the nozzle108.

The method according to the invention comprises, after each step whereby a layer or a bead is created, a step whereby the bead110is compressed.

Preferably, the compression of the bead110is achieved by shot peening or by gas-cooling of the bead, after the passage of the beam106on the layer to provide a layer-by-layer or a bead-by-bead treatment, and to apply compression stresses, or to achieve a required microstructure. Shot peening or cooling have different effects depending on the temperature of the substrate, the positioning of the head104that emits the beam106, etc. For example, this temperature can be managed by identifying the surface isotherms of each bead110.

In the specific case of the bead being compressed when the bead is at ambient temperature, the working distance L between the layer and the head104can be of approximately 150 mm. This head104can have a diameter of 6 mm and the shot peening can be performed by depositing particles with a diameter of approximately 100 μm at a pressure of 0.2-0.8 MPa.

In the specific case of compression being performed when the bead is at high temperature (for example of around 300° C.), shot peening can be conducted at a pressure of 0.6 MPa with shots of 1.0 mm. These shot peening operations can be followed by a treatment of the microshot peening type, which is performed at a pressure of 0.6 MPa with shots of 0.1 mm.

The application of the invention on a steel bead featuring high hardness (600-1000 HV) achieves a surface stress of approximately −350 to −500 MPa, a maximum compression stress of around −400 to −2000 MPa, a maximum stress depth of around 5 to 20 μm, and a compression depth ranging from 50 to 100 μm.

For shot peening operations, a guided microshot peening nozzle can be used, using a fine powder with a particle size of 10 to 100 μm. The impact surface can be of a few square millimetres and the affected depth can range between 50 to 150 μm.

This is compatible with direct laser deposition methods. With the direct laser deposition method, fused layers have a thickness of around 200 to 500 μm. The fused powders have the same grain size; it is possible to consider using the same powders to avoid contaminating the parts. Shot peening works on the same scale as the abovementioned additive manufacturing method.

For stress-related aspects, depth stress modifications can be modulated. It is also possible to use the cooling effect of the carrier gas to change stress values and limit oxidation.

As mentioned above, compression can also be introduced by means of a carrier gas, without using a medium such as microshot peening, in order to temper the bead of matter and introduce residual stresses therein. The expelled gas can be a neutral gas or a reagent gas. Preferably, the flow is sufficient to accelerate the cooling of the bead faster than by conduction through the support.

The microshot peening or gas flow emitting nozzle108must follow the head104to impact the hardened bead with a slight delay that is determined, for instance, based on the distance d between the nozzle and the head, d being a factor of the cooling temperature of the bead and the temperature that is suitable for the compression of the bead. In fact, the guiding of the orientation of the shot peening nozzle is preferably differentiated from the guiding of the projection nozzle.

The emitting head104and the second nozzle108, and even the first nozzle100, are preferably supported by a shared robotic arm.

FIGS.3to4each represent two embodiments of the device according to the invention. InFIG.3, the arm120is rotationally mobile about an axis122, for example a vertical axis. The head104is centred on the axis122and the outputs of the microshot peening nozzle are located on a circumference centred on the axis122. The arm is moved along a plane that comprises the axis122, such as the plane of the drawing, and the nozzle108located downstream from the beam106, with respect to the travelling direction of the arm, is used to compress the beam.

As shown inFIG.4, the arm120supports the shot peening nozzle108and the beam-emitting head104, the distance between them being changeable by moving the nozzle in translation with respect to the arm. The arm is longitudinally and rotationally movable both in translation and in rotation about the axis122of the head104.

If the shot peening particles are of the same nature as the powder particles, there is a risk of a greater loss of powder. One solution resides in the use of a powder with a coarser particle size, so that the particles can be retrieved by sieving, or using a powder of a different material, such as ceramic, that can be retrieved by magnetic separation.