Patent Application
20240042551
This application claims the benefit of priority from a previously filed application in Luxembourg on Aug. 4, 2022, with application number 502626. This earlier application serves as the priority application for the present filing and provides the foundation for the invention disclosed herein.
The present invention relates to a method of post-manufacturing treatment of a 3D-printed product. More particularly, the present invention relates to post-manufacturing treatment by laser shock peening applied on a gear manufactured by means of additive manufacturing. Even more particularly, the invention relates to post-manufacturing laser shock peening of a gear having size between 1 mm to 10 mm.
The second aspect of the present invention relates to a gear having a size between 1 mm and 10 mm, so called mesoscopic gear, being treated by laser shock peening to provide increased lifetime service.
Gears are used in various industrial and technological applications to permit power transmission from one rotating or translating element to another. Each gear generally includes an array of gear teeth that mesh with the gear teeth of another gear so that the rotation or translation of the first gear can be transmitted to the second. The forces on the gear teeth over time may cause failure of the gear, i.e. end of life of the gear. Therefore, there is a demand to improve the quality of the gear and extend service life.
3D printing is a known method for component manufacturing made of wax or any suitable thermoplastic, such as a plastic toy. However, the 3D printing method for mechanical industry or construction is not yet well established. The mechanical components require long-term reliability and mechanical resilience, which, due to the nature of the method, cannot always be guaranteed. 3D-printing is also known as additive manufacturing. The main limitation of a product built by the additive manufacturing is its inadequate mechanical properties compared to conventionally produced products. The main limitation is the result of tensile residual stresses (TRS), increased surface roughness, and lower part density.
Laser shock peening (LSP) is a process that uses high-intensity laser pulses to generate deep compressive residual stresses. Standard LSP can be performed with laminar water flow over the surface of the component. The surface is also covered with an absorption layer that both absorbs the laser pulse and protects the underlying substrate from heat effects. The absorbent coating local to the laser impingement becomes plasma that is constrained by the component and water layer. A pressure pulse is thus created which propagates as a shock wave deep into the material and generates compressive residual stresses.
There exists a macroscopic gear defined as the gear having the size more than 10 mm. Microscopic gears are having the size less than 1 mm.
There is a demand for a gear printed on a 3D printer from certain type of material, such as stainless steel, wherein the size of the gear is between 1-10 mm, so called mesoscopic gear.
Thus, the object of the present invention is to provide a method for improving service life of the gear having outside diameter between 1 mm and 10 mm, wherein the gear is made of stainless steel by a method of additive manufacturing.
The above mentioned problem is solved by the present invention. In a first aspect, a method for improving surface integrity, therefor improving service life, of an additive manufactured gear having size from 1 mm to 10 mm is provided. The method comprises the steps of:
The above mentioned method provides an improvement in service life. In particular, the application of laser shock peening imparts the compressive residual stresses in the root, fillet radius, and space between the two teeth of the gears. Microstructure, residual stresses and surface roughness have been improved in terms of average surface roughness Ra and peak material volume Vmp. Particular improvement is connected to fatigue of the gear. Fatigue is defined as a process of progressive localized plastic deformation occurring in a material subjected to cyclic stresses and strains at high stress concentration locations that may culminate in cracks or complete fracture after a sufficient number of fluctuations.
Laser shock peening is performed without affecting the geometry of the gear tooth's face width, top land, and pitch. Furthermore, improved residual stresses, improved surface roughness and no alteration in the dimensional mesoscopic-geometry is observed, which due to the size of the gear is advantageous.
Better surface morphology, in particular, low level of applied energy of the laser beam ranging from 200 mJ to 1 J eliminates the need for high-power lasers, thus making the processing cost effective and suitable to be implemented by the state of the art laser device. Synergetic effect provided by the combination of energy and density of the laser beam as mentioned above provides the advantage of not using tape during LSP.
The size of the gear is hereby referred to distance between two teeth on opposite sides of the gear, i.e. outer diameter of the gear. There is no limit to the width of the gear, which can be more than 10 mm.
Once the gear is produced by the additive manufacturing process, it is immersed in a water pool for LSP processing. The gear is placed from 5 cm to 10 cm below the water surface. This distance is empirically determined for the laser shock peening treatment given the laser parameter.
The place of applying the laser shock peening to the gear is specific. They are a root along a fillet radius and space between two teeth. It is at these points that the gear is most stressed and requires reinforcement. Due to the laser parameters set as mentioned above, the laser beam is applied directly to the specific surface of the gear without any protective layer nor coating. The size of the spot corresponds to the gap between two neighbouring teeth. Furthermore, it is empirically observed that the spots of the beam, when laser shock peening is applied, is at least 90% to achieve improvement on the surface mechanical properties.
An alternative method of use an additive manufactured gear is provided. The gear is made of stainless steel having size from 1 mm to 10 mm comprising plurality of teeth. The method comprises:
The mesoscopic gear is made of made of stainless steel by a method of additive manufacturing. The stainless steel can be austenitic 316L stainless steel, which provides corrosion resistance, ductility, and biocompatibility. The austenitic steel 316L has low strength and wear resistance, limiting its high-performance applicability. However, due to the application of laser shock peening, the steel is plastically compressed and surface properties are improved. In another embodiment, the steel can be ferritic stainless steel or martensitic stainless steel.
The gear can be pre-characterized on residual stresses, surface roughness, scanning electron microscopy, micro-geometry. The gear manufactured by additive method is usually fragile and does not achieve the material properties of macroscopic gears. Hence, such a gear needs to be further treated so that it acquires the properties that are characteristic of macroscopic gears. In the next step, the gear is at least partially immersed in the water pool, so that the part to be treated by the laser shock peening is immersed in a depth of 5-10 cm below the water surface. Depending on the size of the gear, only the part of the gear that needs to be treated with LSP can be immersed in the water pool with no flowing water around it, thereby static water can be used. In flowing water the cavitation is prevalent phenomenon which may lead to detrimental effect on the interacting surface. The application of immersed water is used because of the static condition of the water, liquid replenishment occurs immediately before the second pulse interacts with the surface, which protects the interacting surface from any detrimental thermal effects. This gives scope for pulse overlap LSP without the need to use any protective layer nor coating. It is not necessary to submerge the entire gear under the water surface. Preferably, a robotic arm can be used to submerge the gear below the water surface to place the gear at a precisely positioned distance below the water surface. It is also advantageous to use the robotic arm in terms of automation and speed of treatment of the laser shock peening. The gear is placed from 5 cm to 10 cm below the water surface. This distance is empirically determined for the laser shock peening treatment given the laser parameter.
It is advantageous to use a robotic arm for immersing the gear. The water can be de-ionised water and does not require any special condition. In the next step, the laser is set to the empirically observed parameter, which provides sufficient compression stress below the surface of the gear, while the gear preserve its structural integrity without any observable damage. The laser is set to: laser pulse having energy between 200 mJ and 1 J, setting pulse duration of the beam to value between 10 ns and 15 ns, setting repetition rate from 1 Hz-15 Hz, and setting density of the laser beam is between 1,7 GW/cm2 and 8,49 GW/cm2. In a preferred embodiment, the laser beam can be calibrated with diagnostic system, such as photodiode, energy meter, CCD camera for laser profile. Preferably, the robotic arm can be turned on and connect with the laser source so that laser can be controlled from the robotic arm. The laser beam is than focused to specific part of the mesoscopic gear, in particular to a root along a fillet radius and space between two teeth of the gear. The above-mentioned setting introduces to the stainless steel compressive pressure via laser shock peening. The gear is provided without protective taping nor coating so that the laser beam is applied directly on the surface of the gear. The laser beam is provided with the spot size, which is substantially about the size of the gap 103, resp. 203, between two neighbouring teeth 101 and 102. The irradiated spots are overlapping for at least 90% of the area during the laser shock peening application.
In a preferred embodiment, the method can further comprise aligning the laser beam path from a laser source to the gear.
In another preferred embodiment, the method can further comprise calibrating diagnostics of the laser source, preferably calibrating photodiode, energy meter, CCD camera for laser profile.
In another preferred embodiment, the method can further comprise controlling the laser beam for laser shock peening by the robotic arm.
In another preferred embodiment, the method can comprises the step of providing the gear and applying the laser shock peening on a helical gear and applying the laser shock peening on the helical gear. Helical gear can be a species of the general gear, wherein the teeth are cut at an angle to the hole (axis) rather than straight and parallel to the hole like the teeth of a spur gear. Applying the laser beam to the area between the teeth can be problematic with the helical gears. In particular, the problematic part of the application lies in the need to change the position of the beam with respect to the changing angle at the tooth root depending on the position. At the same time, at least 90% overlap must be maintained. It is especially advantageous to combine the embodiment with an adaptive laser beam alignment.
In another aspect of the present invention, an additive manufactured gear made of stainless steel is provided. The additive manufactured gear made of stainless steel having size from 1 mm to 10 mm comprising plurality of teeth. Pressure angle in relation to the gear teeth can be within the range known to the skilled person in the art. Residual stress introduced in the gear by laser shock peening is negative. Arithmetic mean height on the gear surface can be up to 40 μm. Number of grains having size less than 50 μm can be greater than the number of grains having size greater than 50 μm in any selected area treated by laser shock peening.
Pressure angle in relation to gear teeth, also known as the angle of obliquity, is the angle between the tooth face and the gear wheel tangent. It is more precisely the angle at a pitch point between the line of pressure (which is normal to the tooth surface) and the plane tangent to the pitch surface.
Compressive residual stress introduced in the gear by laser shock peening is negative. The compressive residual stress refers to a negative residual stress. The compression is introduced into the material, nearby it's surface, via the laser shock peening. The compression re-distributes the pressure over the surface of the root of gear. Since the residual stresses can be also distributed over the gear surface, the surface layer properties produced by the laser shock peening directly affects part performance, including fatigue strength, wear behaviour, and chemical resistance. In a preferred embodiment, a method for measuring residual stress induced by laser shock peening and during the application of laser shock peening on the mesoscopic gear and/or before and after the application of laser peening on the mesoscopic gear is disclosed. The preferred method for residual stress monitoring and/or measurement leverages X-ray diffraction as an indirect means of quantifying strain within the material. The interaction between X-ray beams and the crystal lattice is harnessed in accordance with Bragg's Law, allowing for the precise determination of the diffraction peak's location, denoted as θ. The changes in the θ are small (in the range of tenths of a degree). If the 2θ angles are plotted against the (sin2ψ), where ψ is an angle made between sample surface normal and lattice plane normal, the relationship is linear. From the fitted line, its slope is evaluated. If there is a rising tendency the measured stress is in compressive region. For the case of line being horizontal, the stress is zero and if the line is going down the stress is positive, i.e tensile. Preferably, during the measurement process, variations in the θ angle are observed. This distinct correlation serves as an indicator of strain magnitude, specifically highlighting regions characterized by compressive stress. Notably, when this observed correlation is established, the resultant measurement values are conventionally considered negative, aligning with established practices in stress analysis. Since the residual stresses on the sample can be tested before laser shock treatment with X-ray diffractometry on the same location and measured in the same way after laser shock treatment, a skilled person can deduce by comparing that the negative stresses were introduced by the laser shock treatment.
In a preferred embodiment, the compressive residual stress can range from −430 MPa to −100 MPa.
In another preferred embodiment, peak material volume can be from 1,55 μm3/μm2 to 1,62 μm3/μm2; and core material volume is from 24,6 μm3/μm2 to 25,1 μm3/μm2; core void volume is from 41 μm3/μm2 to 48 μm3/μm2; and valley void volume is from 1,63 μm3/μm2 to 2,21 μm3/μm2.
In a preferred embodiment, the gear can be a helical gear or a spur gear.
The helical gear can be provided with the residual stress ranging from −430 MPa to −230 MPa. In another preferred embodiment of the helical gear embodiment, average surface roughness can be at least 10.
The spur gear can have residual stress introduced by LSP from −300 MPa to −100 MPa.
In an example, a mesoscopic spur gear having the size shown in table 1 has been
manufactured by means of additive manufacturing. The selective laser melting has been used as an additive manufacturing technique; the details of parameters used to print the gears are presented in table 2. The gear was designed with module 0.8 and number of teeth as 10 for both spur and helical geometry. While the pressure angle for both the geometries was chosen a 20° and helix angle for helical gear was 20°.
Laser Shock Peening (LSP) was applied done using a solid state diode pumped laser system with a laser source was a Nd:YAG—a pulsed laser having pulse duration of upto 15 ns, operating at 1064 nm. The beam spatial energy distribution is “top-hat” and the pulse shape is near—Gaussian. Round laser spots of 1 mm diameter were used with a laser energy per pulse in the range of 200 mJ to 1 J, whereas the laser device has capacity of maximum energy as 3 J. The ratio of spot size and energy per pulse was chosen such as to keep a power density for the present work in the range of 1.7 to 8.49 GW/cm2. The advantage in using lower energies per pulse (for a given power density) is observed as to secure that the profile of the gears will not differ from the original and at the same time to impart compressive residual stresses.
The laser shock peening was applied to mesoscopic spur and helical gear in the filler area and the root, the spot size of the laser beam i.e. 1 mm is selected in such a way that it covers the root and fillet gap and does not affect the flank surface of the gears as schematically shown on
The pressure created at the surface of the part was estimated to maximum of 5 GPa using the energy in the range of 200 mJ to 1 J, with pulse frequency of 15 Hz and the overlap of 90% was used for spot size of 1 mm without a protective ablative layer.
Residual stresses measurements were done with XRD measurements, the cathode source used for the measurement was chromium (Cr) radiation (Kα=2.29103 Å) at 40 kV and 40 mA. To analyse the manufactured gears with SLM Face centred structured (220) diffraction peak was used.
In a second example, a mesoscopic helical gear having the size shown in table 1 has been manufactured by means of additive manufacturing.
Laser Shock Peening (LSP) was applied done using the laser device. The laser source was a Nd:YAG. The beam spatial energy distribution is “top-hat” and the pulse shape is near—Gaussian. Round laser spots were used with a laser energy per pulse of either. The ratio of spot size and energy per pulse was chosen such as to keep a constant power density.
The pressure created at the surface of the part was estimated to maximum of 5 GPa using the energy in the range of 200 mJ to 1 J, with Pulse frequency of 15 Hz and the overlap of 90% was used for spot size of 1 mm without a protective ablative layer.
Residual stresses measurements were done with XRD measurements, the cathode source used for the measurement was chromium (Cr) radiation (Kα=2.29103 Å) at 40 kV and 40 mA. To analyse the manufactured gears with SLM Face centred structured (220) diffraction peak was used.
An another aspect of the present invention can be an additive manufactured gear made of stainless steel having size from 1 mm to 10 mm comprising plurality of teeth, wherein
In a preferred embodiment, the present invention pertains to test methods for the determination of grain size. The preferred method can include the comparison procedure, planimetric (or Jeffries) procedure, and intercept procedures. For the purposes of the preferred test methods, the term “grain” refers to an individual crystal possessing consistent atomic configuration throughout a polycrystalline material. Notably, a grain may encompass twinned regions or sub-grains. The three steps for grain size estimation can comprises the comparison procedure, that eliminates the need for direct counting of grains, intercepts, or intersections. This step entails contrasting the grain structure with a sequence of graded images. Such images can be presented through mediums like wall charts, clear plastic overlays, or an eyepiece reticle. The planimetric step can necessitate a direct count of the number of grains within a known area. The intercept step can involve directly counting the number of grains intercepted by a test line or the number of intersections between grain boundaries and a test line. This count is measured per unit length of the test line and is utilized in calculating the mean lineal intercept length. Preferably, the method of grain size determination can be utilized by computer software, such as EDAX APEX 2.5 software and EBSD map were evaluated with EDAX OIM 8.6 software, wherein grain size diameter were evaluated using weighing by grain area.
In a preferred embodiment, the measurement of surface roughness can be achieved using contact profilometry, a well-established technique involving the use of a stylus or diamond-tipped probe. The stylus can be systematically dragged across the metallic surface, and its vertical displacement along the scan path can be recorded. The resulting height variations can calculate the roughness parameters Ra (average surface roughness). In another embodiment, a contact profilometry method can be used for its exceptional accuracy and versatility in surface roughness measurement. In another embodiment, the invention employs non-contact optical profilometry techniques to measure surface topography without physical contact with the metallic surface. Techniques like white light interferometry and confocal microscopy can be used to generate high-resolution 3D images, offering precise measurement of roughness parameters. In another preferred embodiment, an Atomic Force Microscopy (AFM), a powerful nanoscale imaging technique, can be used for surface roughness evaluation. The AFM can utilize a sharp cantilever with a nanoscale tip to scan the metallic surface, detecting forces between the tip and the surface. This allows for ultra-high-resolution 3D imaging, enabling the measurement of roughness parameters at the nanometer scale. The AFM technique is particularly advantageous when assessing nanostructured metallic surfaces.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein the residual stress ranges from −430 MPa to −100 MPa.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein peak material volume is from 1,55 μm3/μm2 to 1,62 μm3/μm2; and core material volume is from 24,6 μm3/μm2 to 25,1 μm3/μm2; core void volume is from 41 μm3/μm2 to 48 μm3/μm2; and valley void volume is from 1,63 μm3/μm2 to 2,21 μm3/μm2.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein the gear is a helical gear.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein the residual stress ranges from −430 MPa to −230 MPa.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein average surface roughness is at least 10.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein the gear is a spur gear.
Preferably, the additive manufactured gear according the embodiment above is provided, wherein the residual stress ranges from −300 MPa to −100 MPa.
The above mentioned embodiment can be embodied in a method of use the gear.
| Number | Date | Country | Kind |
|---|---|---|---|
| LU502626 | Aug 2022 | LU | national |
