Metal additive manufacturing method based on surface layering

Abstract

The application belongs to the technical field of additive manufacturing, and relates to a metal additive manufacturing method based on curved surface layering, which comprises the following steps: 1) reading an STL file of a part to be printed, and establishing a spatial topological relation between triangular facets in the STL file of the part to be printed; 2) generating a curved surface separated from a model of the part to be printed based on the triangular facets on the curved surface of the model of the part to be printed according to the spatial topological relation between the triangular facets; wherein the model of the part to be printed is generated through the STL file of the part to be printed; 3) offsetting the curved surface separated from the model of the part to be printed to intersect with the model of the part to be printed, and obtaining a contour line for curved surface printing; 4) generating an internal filling path for curved surface printing in the contour line for curved surface printing; and 5) completing an additive manufacturing process by using a laser based on the contour line and the internal filling path. The application can eliminate the step effect and spheroidization problem occurring in the forming process, and improve the forming quality.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of additive manufacturing technology, and relates to a metal additive and subtractive manufacturing method, in particular to a metal additive manufacturing method based on curved surface layering. BACKGROUND

[0002] SLM (selective laser melting) is the most mainstream metal additive manufacturing technology at present, and its principle is to use metal powder to completely melt under the heat action of a laser beam and solidify after cooling to form a shape, which has the advantages of wide selection of forming materials, realization of complex shapes and internal features, etc. Since the current SLM main technology adopts the principle of plane layering manufacturing, this method has the advantages of simple implementation and fast layering speed, but the non-straight arm surface of the formed part inevitably has a stepped structure, which is called the step effect. The step effect reduces the surface quality of the formed part, causes the loss of small features on the part surface, increases the surface roughness of the part, and increases the model surface roughness. At the same time, when processing a single layer, the forming machine can only accumulate materials on the plane, and when the cross-sectional size is large, the forming path needs to be frequently reversed, which will reduce the adhesion between the formed layers, thereby weakening the interlayer strength of the formed part. At the same time, the SLM (selective laser melting) is prone to spheroidization and other phenomena. Spheroidization leads to the formation of pores in the metal part, reduces the mechanical properties of the formed part, and increases the surface roughness. The powder spreading scraper generates a large friction force with the previous layer during the powder spreading process, not only damaging the surface quality of the metal, but also hindering the movement of the scraper, ultimately leading to the failure of the formed part.

[0003] The generation of the step effect is closely related to the layering thickness, and the forming precision can be improved by reducing the layering thickness, but the forming time and cost increase significantly with the reduction of the layering thickness, and the forming efficiency is low. The current adaptive plane layering algorithm can be used to reduce the step effect in the forming process. This algorithm automatically adjusts the layer thickness for layering according to the change of the shape of the part, so as to achieve the purpose of improving the precision without reducing the efficiency. The above two methods still use the plane layering algorithm, and the step effect cannot be completely avoided. The spheroidization effect is related to the laser power, scanning speed, powder thickness, and other forming parameters. Reducing the scanning speed or reducing the powder thickness can alleviate the spheroidization effect, but the forming efficiency will be affected. SUMMARY

[0004] In order to solve the above technical problems in the background art, the present application provides a metal additive manufacturing method based on curved surface layering, which can eliminate the step effect and spheroidization problem in the forming process and improve the forming quality.

[0005] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0006] A metal additive manufacturing method based on curved surface layering, characterized in that the metal additive manufacturing method based on curved surface layering comprises the following steps:

[0007] 1) reading an STL file of a part to be printed, and establishing a spatial topological relationship between triangular facets in the STL file of the part to be printed;

[0008] 2) generating a curved surface separated from a model of the part to be printed based on triangular facets on the curved surface of the model of the part to be printed according to the spatial topological relationship between the triangular facets established in step 1); wherein the model of the part to be printed is generated through the STL file of the part to be printed;

[0009] 3) offsetting the curved surface separated from the model of the part to be printed obtained in step 2) to intersect the model of the part to be printed, and obtaining a contour line for curved surface printing;

[0010] 4) generating an internal filling path for curved surface printing in the contour line for curved surface printing obtained in step 3);

[0011] 5) completing an additive manufacturing process by using a laser based on the contour line and the internal filling path.

[0012] The step 1) of reading the STL file of the part to be printed and establishing the spatial topological relationship between the triangular facets in the STL file of the part to be printed specifically comprises:

[0013] reading the STL file of the part to be printed, and saving all the triangular facets in the STL file in a matrix format of n rows and 13 columns, wherein n is the number of triangular facets in the STL file, the first column of the matrix is the index of the triangular facet in the matrix, and columns 2-13 save the three-dimensional coordinates and normal vectors of the three vertices of the triangular facet;

[0014] randomly reading a triangular facet in the matrix, and judging whether the remaining triangular facets have two same vertices as the read triangular facet; if there is a same vertex, recording the row index of the triangular facet adjacent to the read triangular facet;

[0015] iterating all the triangular facets, and repeating the above process to establish the spatial topological relationship between the triangular facets in the STL file of the part to be printed.

[0016] The step 2) of generating the curved surface separated from the model of the part to be printed based on the triangular facets on the curved surface of the model of the part to be printed according to the spatial topological relationship between the triangular facets established in step 1) specifically comprises:

[0017] selecting an initial triangular facet from the STL file of the part to be printed, and the angle between the normal vector of the initial triangular facet and the Z axis is within the range of 0-90°;

[0018] Starting from the initial triangular facet, it is judged whether the angle between the normal vector of the triangular facet adjacent to the initial triangular facet and the Z-axis is within the range of 0-90°, if yes, the triangular facet adjacent to the initial triangular facet is taken as a new starting point, and the above steps are repeated until all the triangular facets with the angle between the normal vector and the Z-axis within the range of 0-90° are extracted;

[0019] The curved surface separated from the model of the part to be printed is generated by using all the triangular facets with the angle between the normal vector and the Z-axis within the range of 0-90°.

[0020] The curved surface separated from the model of the part to be printed obtained in step 2) is offset to intersect with the model of the part to be printed to obtain the contour line of the curved surface printing, and the specific process includes the following steps.

[0021] The curved surface separated from the model of the part to be printed obtained in step 2) is offset in the negative direction of the Z-axis in sequence, and the offset distance is the layer thickness of the curved surface each time. After each offset is completed, the intersection line between the triangular facets in the curved surface separated from the model of the part to be printed and the triangular facets in the model of the part to be printed is determined by intersecting the curved surface separated from the model of the part to be printed with the model of the part to be printed, and then the contour line of the curved surface printing is obtained based on the intersection line.

[0022] The internal filling path of the curved surface printing is generated in the contour line of the curved surface printing obtained in step 3), and the specific process includes the following steps.

[0023] The step 4) includes the following steps.

[0024] 4.1) Calculate the maximum value X max and the minimum value X min of the curved surface separated from the model of the part to be printed in the X direction, and the maximum value Y max and the minimum value Y min of the curved surface separated from the model of the part to be printed in the Y direction.

[0025] 4.2) Project the curved surface separated from the model of the part to be printed onto the XOY plane, and calculate the minimum envelope rectangle of the projected curved surface separated from the model of the part to be printed. Take the vertex P0 of the diagonal line of the envelope rectangle as a point on the initial offset plane, and take the unit vector of the diagonal line of the envelope rectangle as the normal vector m of the offset plane. According to the point P0 and the normal vector m, the reference offset plane S is defined.

[0026] 4.3) input the fill angle θ, rotate the reference offset plane S around the Z axis by θ to obtain a new offset plane s1, calculate the normal vector m1 of the offset plane s1, define a new offset plane s1 with the normal vector m1 and the point P0, offset s1 along the normal vector m1, and each time the offset distance is the fill spacing h, after t times of offset, calculate the new coordinates p1 of the point P0 on the offset s1, p1 = p0 + m1 * t * h, define the new offset plane s2 after offset according to the coordinates p1 and the normal vector m1, calculate the intersection line of the offset plane s2 and the curved surface separated from the model of the part to be printed, and obtain the inner layer fill path of the curved surface printing.

[0027] The metal additive manufacturing method based on curved surface layering described above further comprises the following steps after step 5):

[0028] 6) using a femtosecond laser to subtract the spheroidization area generated in the additive manufacturing process.

[0029] The spheroidization area refers to an area in which the line energy density of the metal powder after absorbing laser energy in the additive manufacturing process is lower than the standard line energy density value.

[0030] The calculation method of the line energy density of the metal powder after absorbing laser energy in the additive manufacturing process is as follows:

[0031] Ei = p / v;

[0032] Wherein:

[0033] p is the power of the laser used in the additive manufacturing process;

[0034] v is the scanning speed of the laser used in the additive manufacturing process;

[0035] Ei is the line energy density of the metal powder after absorbing laser energy in the forming process. The advantages of the present application are:

[0036] The present application provides a metal additive manufacturing method based on curved surface layering, which completes the additive manufacturing process along the curved surface layering path, thereby reducing the step effect. In addition, for the area prone to spheroidization in the additive manufacturing process, a subtractive laser (such as a femtosecond laser or other short pulse laser) is used for ablation to remove the spheroidization part of the printing layer, thereby further improving the forming quality. DETAILED DESCRIPTION

[0037] The present application provides a metal additive manufacturing method based on curved surface layering, which first reads the STL file of the part to be printed, and establishes a spatial topological relationship for the triangular facets in the STL file of the part to be printed, which facilitates subsequent rapid extraction of triangular facets with the same features.

[0038] Next, according to the spatial topological relationship of the triangular facets established in the last step, the triangular facets in the model surface of the part to be printed are quickly separated, the separated triangular facets of the surface are regenerated into a model surface separated from the model of the part to be printed (generated by the STL file of the part to be printed),

[0039] and the surface offset from the model of the part to be printed intersects with the model of the part to be printed, to obtain the contour line for surface printing, and finally the internal filling path for surface printing is generated in the contour line for surface printing.

[0040] In the forming process of each layer, first, the metal powder is melted along the curved layering path by using a selective laser, the step effect is eliminated, and the additive manufacturing process is completed. Then, the linear energy density of the metal powder after absorbing the laser energy in the additive manufacturing process is calculated in the forming process, so as to determine the area prone to spheroidization, and the area prone to spheroidization is ablated and trimmed along the curved layering path by using a high-power femtosecond laser, the spheroidized part of the material on the surface of the forming layer is eliminated, and the subtractive manufacturing process is completed, so as to further improve the forming quality.

[0041] The method of the present application is as follows:

[0042] 1) Read the STL file of the part to be printed, save all the triangular facets in the STL file of the part to be printed in the form of a matrix with n rows and 13 columns, n is the number of triangular facets in the STL file, the first column of the matrix is the index of the triangular facet in the matrix, and columns 2-13 save the three-dimensional coordinates and normal vector of the three vertices of the triangular facet. Randomly read any triangular facet in the matrix, find the adjacent triangular facet by judging whether the remaining triangular facets and the read triangular facet have two same vertices, if there are same vertices, record the row index of the triangular facet adjacent to the read triangular facet. By traversing all the triangular facets, record the triangular facets adjacent thereto, and establish the spatial topological relationship between the triangular facets in the STL file of the part to be printed.

[0043] 2) Select an initial triangular facet from the STL file of the part to be printed, the angle between the normal vector of the initial triangular facet and the Z axis is within the range of 0-90°, take the selected triangular facet as the starting point, judge whether the angle between the normal vector of the triangular facet adjacent to the initial triangular facet and the Z axis is within the range of 0-90°, if yes, take the triangular facet adjacent to the initial triangular facet as a new starting point, repeat the above steps until all the triangular facets with the angle between the normal vector and the Z axis within the range of 0-90° are extracted, and generate a surface separated from the model of the part to be printed by using all the triangular facets with the angle between the normal vector and the Z axis within the range of 0-90°.

[0044] 3) the obtained surface separated from the model of the part to be printed is offset in the negative direction of the Z axis in turn, and the offset distance is set as the layering thickness of the surface, after the offset is completed, the intersection of the surface separated from the model of the part to be printed and the model of the part to be printed is obtained. Because the surface separated from the model of the part to be printed and the model of the part to be printed are both composed of a plurality of triangular facets, the intersection lines of the triangular facets in the surface separated from the model of the part to be printed and the triangular facets in the model of the part to be printed are calculated one by one, and the intersection lines are sequentially connected at the head and tail to obtain the overall layering contour line of the surface separated from the model of the part to be printed and the model of the part to be printed.

[0045] 4) the surface separated from the model is offset in an arbitrary direction in turn by using a plane perpendicular to the XOY plane, and the offset distance is the filling line interval distance, the intersection line of the offset plane and the surface separated from the model is determined, and the intersection line is the internal filling path of the surface forming layer, and the method specifically comprises the following steps:

[0046] 4.1. the maximum value X of the surface separated from the model of the part to be printed in the X direction is calculated max and the minimum value X min , the maximum value Y of the surface separated from the model of the part to be printed in the Y direction is calculated max and the minimum value Y min .

[0047] 4.2. the surface separated from the model of the part to be printed is projected onto the XOY plane, and the minimum envelope rectangle of the projection of the surface separated from the model of the part to be printed is calculated, the vertex P0 of the diagonal line of the envelope rectangle is taken as a point on the initial offset plane, and the unit vector of the diagonal line of the envelope rectangle is taken as the normal vector m of the offset plane, and the reference offset plane S is defined according to the point P0 and the normal vector m.

[0048] 4.3. the filling angle θ is input, the reference offset plane S is rotated by θ degrees around the Z axis to obtain a new offset plane s1, the normal vector m1 of the offset plane s1 is calculated, the new offset plane s1 is defined according to the normal vector m1 and the point P0, the s1 is offset along the direction of the normal vector m1, the offset distance is the filling interval h each time, after the offset t times, the new coordinates p1=p0+m1*t*h of the point P0 on the offset s1 are calculated, the new plane s2 after the offset is defined according to the coordinates p1 and the normal vector m1, the intersection line of the plane s2 and the surface separated from the model of the part to be printed is calculated, and thus the internal layer filling path of the surface is obtained.

[0049] 5) the metal powder is melted along the surface layering path by using a selective laser, and in the process, the size of the linear energy density Ei of the metal powder after absorbing the laser energy in the forming process determines whether spheroidization is generated in the forming process, Ei=p / v, p is the power of the laser used in the additive manufacturing process, and v is the scanning speed of the laser used in the additive manufacturing process, and the region with the linear energy density lower than the standard value is recorded.

[0050] 6) After the additive manufacturing is completed, when a molding area with a lower standard line energy density value exists, a femtosecond laser is turned on to ablate the area and remove the spheroidization area.

[0051] It should be noted that the above subtractive laser can be a femtosecond laser, and can also be a nanosecond laser or other short pulse laser.

Claims

1. A metal additive manufacturing method based on curved surface layering, characterized by: The metal additive manufacturing method based on curved surface layering includes the following steps: 1) Read the STL file of the part to be printed and establish the spatial topological relationship between the triangular facets in the STL file of the part to be printed; 2) establishing a spatial topological relationship between the triangular facets obtained in step 1), and generating a curved surface separate from the model of the part to be printed based on the triangular facets on the curved surface of the model of the part to be printed; wherein the model of the part to be printed is generated using an STL file of the part to be printed; 3) intersecting the curved surface offset obtained in step 2) and the model of the part to be printed with the model of the part to be printed to obtain a contour line of the curved surface printing; 4) generating an internal filling path for the curved surface printing within the contour line of the curved surface printing obtained in step 3); 5) completing an additive manufacturing process using a laser based on the contour line and the internal filling path; The step 1) reads the STL file of the part to be printed and establishes the spatial topological relationship between the triangular facets in the STL file of the part to be printed, specifically including: Read the STL file of the part to be printed and save all the triangles in the STL file in a matrix format of n rows and 13 columns, where n is the number of triangles in the STL file, the first column of the matrix is ​​the index of the triangle in the matrix, and columns 2-13 store the 3D coordinates and normal vectors of the three vertices of the triangle; Randomly read any triangle in the matrix and determine whether the remaining triangles and the read triangle have two common vertices. If they do, record the row index of the triangle adjacent to the read triangle. Traverse all triangular facets and repeat the above process to establish the spatial topological relationship between the triangular facets in the STL file of the part to be printed; The step 2) establishing a spatial topological relationship between the triangular facets obtained in step 1), and generating a curved surface separated from the model of the part to be printed based on the triangular facets on the curved surface of the model of the part to be printed, specifically includes: Select an initial triangle from the STL file of the part to be printed, where the angle between the normal vector of the initial triangle and the Z axis is in the range of 0-90°; Taking the initial triangle as the starting point, determine whether the angle between the normal vector of the triangle adjacent to the initial triangle and the Z axis is within the range of 0-90°. If so, use the triangle adjacent to the initial triangle as the new starting point and repeat the above steps until all triangles whose normal vectors have an angle with the Z axis within the range of 0-90° are extracted. The extracted triangular facets whose normal vectors have an angle with the Z axis within the range of 0-90° are used to generate a surface separated from the model of the part to be printed.

2. The metal additive manufacturing method based on curved surface layering according to claim 1, characterized in that: The step 3) intersects the curved surface offset separated from the model of the part to be printed obtained in step 2) with the model of the part to be printed to obtain a contour line of the curved surface printing, specifically comprising: The surface separated from the model of the part to be printed obtained in step 2) is offset in sequence along the negative direction of the Z axis, and each offset distance is the thickness of the surface layer. After each offset is completed, the surface separated from the model of the part to be printed is intersected with the model of the part to be printed to determine the intersection line of the triangular facets in the surface separated from the model of the part to be printed and the triangular facets in the model of the part to be printed, and then the contour line of the surface printing is obtained based on the intersection line.

3. The metal additive manufacturing method based on curved surface layering according to claim 2, characterized in that: The step 4) generates an internal fill path for the curved surface printing within the contour line of the curved surface printing obtained in step 3), specifically comprising: using an offset plane perpendicular to the XOY plane to sequentially offset in any direction at a fill line interval, and determining an intersection line of the surface separating the offset plane and the model of the part to be printed, wherein the intersection line is the inner layer fill path for the curved surface printing.

4. The metal additive manufacturing method based on curved surface layering according to claim 3, characterized in that: The step 4) comprises: 4.1) Calculate the maximum value X in the X direction of the surface separated from the model of the part to be printed max and the minimum value X min , the maximum value Y in the Y direction max and the minimum value Y min ; 4.2) Project the surface separated from the model of the part to be printed onto the XOY plane, and calculate the minimum enveloping rectangle of the projection of the surface separated from the model of the part to be printed. Take the vertex P0 of the diagonal of the enveloping rectangle as a point on the initial offset plane, and take the unit vector of the diagonal of the enveloping rectangle as the normal vector m of the offset plane. Define the reference offset plane S based on point P0 and the normal vector m; 4.3) Input the fill angle θ, rotate the base offset plane S around the Z axis by angle θ to obtain a new offset plane s1, calculate the normal vector m1 of the offset plane s1, define the new offset plane s1 with the normal vector m1 and point P0, offset s1 along the direction of the normal vector m1, and each offset distance is the fill spacing h. After offsetting t times, calculate the new coordinates p1 = p0 + m1 * t * h of point P0 on s1 after offset. Define the new offset offset plane s2 according to the coordinates p1 and the normal vector m1, calculate the intersection line between the offset plane s2 and the surface separated from the model of the part to be printed, and obtain the inner layer fill path of the surface printing.

5. The metal additive manufacturing method based on curved surface layering according to any one of claims 1 to 4, characterized in that: The metal additive manufacturing method based on curved surface layering further includes, after step 5): 6) Use femtosecond laser to reduce the area where spheroidization occurs during additive manufacturing.

6. The metal additive manufacturing method based on curved surface layering according to claim 5, characterized in that: The area where spheroidization occurs is an area where the linear energy density of the metal powder after absorbing laser energy during the additive manufacturing process is lower than the standard linear energy density value.

7. The metal additive manufacturing method based on curved surface layering according to claim 6, characterized in that: The calculation method for the linear energy density of metal powder after absorbing laser energy in the additive manufacturing process is: Ei=p / v; in: p is the power of the laser used in the additive manufacturing process; v is the scanning speed of the laser used in the additive manufacturing process; Ei is the linear energy density after the metal powder absorbs the laser energy during the forming process.