WO2020034093A1 - 3d打印方法 - Google Patents

3d打印方法 Download PDF

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Publication number
WO2020034093A1
WO2020034093A1 PCT/CN2018/100450 CN2018100450W WO2020034093A1 WO 2020034093 A1 WO2020034093 A1 WO 2020034093A1 CN 2018100450 W CN2018100450 W CN 2018100450W WO 2020034093 A1 WO2020034093 A1 WO 2020034093A1
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Prior art keywords
heat treatment
printing
laser
printing method
scanning
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PCT/CN2018/100450
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English (en)
French (fr)
Inventor
李长鹏
周忠娇
陈国锋
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西门子（中国）有限公司
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Priority to PCT/CN2018/100450 priority Critical patent/WO2020034093A1/zh
Publication of WO2020034093A1 publication Critical patent/WO2020034093A1/zh

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C67/00—Shaping techniques not covered by groups B29C39/00 - B29C65/00, B29C70/00 or B29C73/00

Definitions

  • the invention relates to the field of additive manufacturing, in particular to a 3D printing method.
  • Additive Manufacturing is one of the important 3D printing technologies. Additive Manufacturing can quickly produce pre-designed CAD models, and can produce complex structural parts in a short time.
  • the selective laser melting (Selected Laser Melting, SLM) process is a kind of additive manufacturing (Additive Manufacturing) technology, which can quickly manufacture the same parts as the CAD model by laser sintering.
  • SLM Select Laser Melting
  • the selective laser melting process has been widely used. Different from the traditional material removal mechanism, additive manufacturing is based on the completely opposite material addition manufacturing philosophy. Among them, selective laser melting uses a high-power laser to melt metal powder, and layer by layer through 3D CAD input. Ground build components / components so that components with complex internal channels can be successfully manufactured.
  • additive manufacturing still needs to face many technical challenges, hindering its widespread application in the field of industrial manufacturing, especially for those critical components that strictly consider mechanical performance (components with critical performance).
  • Technical challenges faced by additive manufacturing include, but are not limited to, residual stress caused by rapid cooling after powder melting. Correlation of mechanical property orientation due to columnar crystal structures, as well as the requirements and performance position dependence of complex components (such as thin-walled structures).
  • the invention provides a 3D printing method, which includes the following steps: performing laser scanning on a metal powder to perform 3D printing; and performing a local stress relief step on a print, wherein the local stress relief step includes any one or any of the following Item: First heat treatment; ultrasonic vibration, wherein the printing step decomposes the metal powder into a powder matrix, and laser scans the powder matrix until the powder matrix is sintered from bottom to top into a predetermined shape. The print is then subjected to a second heat treatment on the print.
  • both the printing step and the first heat treatment step are performed in a selective laser melting apparatus, wherein a smaller laser power density and a faster scanning speed than the printing step are used when performing the heat treatment step.
  • the first heat treatment step uses an additional laser source other than a laser source parallel to the selective laser melting device, wherein the additional laser source is a CO2 laser source or an electron beam energy source.
  • the first heat treatment step uses the electron beam energy source, wherein the current value ranges from 3 mA to 15 mA, the scan rate is 2000 mm / s to 14000 mm / s, the heating time is 1 s to 20 s, and the acceleration voltage is 60 KV. .
  • the first heat treatment step uses a CO2 laser source, wherein the beam diameter is 0.2 mm to 1.0 mm, the laser power is 30 W to 150 W, the scanning rate is 0.5 mm / s to 500 mm / s, and the scanning interval is 0.2 mm to 0.8mm.
  • the laser forming scanning step and the first heat treatment step are performed separately.
  • the laser forming scanning step is a rotation scanning.
  • the second heat treatment step is performed in a high-temperature furnace, wherein the temperature ranges from 980 ° C to 1200 ° C, the aging treatment ranges from 710 ° C to 770 ° C for 8 to 12 hours, and the furnace is cooled to 620 ° C to 660 ° C. It lasts 8h to 12h.
  • the frequency of the ultrasonic vibration ranges from 10000hz to 100,000hz.
  • the print obtained by using the 3D printing method has an equiaxed crystal structure, or a mixed structure of columnar crystals and equiaxed crystals.
  • the present invention relates the present invention to residual stress, heat treatment setting and microstructure after heat treatment. Based on theoretical calculations and experimental verifications, the present invention can predict and adjust the crystal structure by selecting the residual stress level at a specific location and selecting appropriate heat treatment conditions (temperature and holding time) to obtain different crystal structures, including pure columnar crystals, pure isometric Crystals, and mixed materials of columnar crystals and equiaxed crystals to obtain different mechanical properties such as anisotropy. Among them, the anisotropic mechanical properties include fatigue properties and creep properties, and the present invention can optimize all mechanical properties of the entire additive manufacturing element. The invention can also formulate a printing strategy according to the force analysis of the component, the failure mechanism, and different performance requirements.
  • FIG. 1 is a schematic structural diagram of a selective laser melting device
  • FIG. 2 is a schematic sectional structural view of a sealing gland of a gas turbine
  • FIG. 3 is a schematic diagram of a crystal structure of a columnar crystal
  • FIG. 4 is a schematic diagram of a crystal structure of an equiaxed crystal
  • FIG. 5 is a schematic diagram of a creep property curve of a columnar crystal and an equiaxed crystal
  • FIG. 6 is a schematic diagram of the fatigue performance curves of columnar crystals and equiaxed crystals
  • FIG. 8 is a schematic diagram of the correspondence between recrystallization time and residual stress.
  • Residual stress has been regarded as an unfavorable factor and negative effect in additive manufacturing.
  • the present invention uses residual stress as the driving force for the adjustment of crystal particles, and controls the residual stress of materials in different regions of the integrally formed print through a local stress relief step. Then, the partial area in the print is completely or partially converted from columnar crystals to equiaxed crystals, thereby improving the microstructure and corresponding mechanical properties of the additive manufacturing element.
  • the present invention predicts adjustment of position and orientation-dependent mechanical properties through a local residual stress and local stress release step.
  • FIG. 1 is a schematic diagram of a selective laser melting apparatus.
  • the selective laser melting apparatus 100 includes a laser source 110, a mirror scanner 120, a prism 130, a powder feeding cylinder 140, a forming cylinder 150 and a recovery cylinder 160.
  • the laser source 110 is disposed above the selective laser melting device 100 and serves as a heating source for the metal powder, that is, the metal powder is melted for 3D printing.
  • the powder feeding cylinder 140 has a first piston (not shown) that can move up and down.
  • a spare metal powder is placed in the cavity space above the first piston of the powder feeding cylinder 140 and follows the first piston.
  • the metal powder is fed into the molding cylinder 150 from the powder feeding cylinder 140 by moving up and down.
  • a 3D printed part placing table 154 is provided in the molding cylinder 150.
  • a 3D printed part C is clamped above the placing table 154, and a second piston 152 is fixed below the placing table 154.
  • the second piston 152 and the placing table 154 Vertical setting. During the 3D printing process, the second piston 152 moves from top to bottom to form a printing space in the molding cylinder 220.
  • the laser source 110 for laser scanning should be set above the molding cylinder 150 of the selective laser melting equipment.
  • the mirror scanner 120 adjusts the position of the laser by adjusting the angle of a prism 130.
  • the adjustment of the prism 130 determines the area where the laser melts the metal powder.
  • the powder feeding cylinder 140 further includes a roller (not shown).
  • the metal powder P is stacked on the upper surface of the first piston.
  • the first piston moves vertically from bottom to top to transfer the metal powder to the upper portion of the powder feeding cylinder 140.
  • the roller can roll on the metal powder P to send the metal powder P into the forming cylinder 150. Therefore, the laser scanning of the metal powder is continuously performed, the metal powder is decomposed into a powder matrix, and the laser scanning of the powder matrix is continued until the powder matrix is sintered from bottom to top into a print C having a predetermined shape.
  • the selective laser melting apparatus 100 further includes a recovery cylinder 160 for recovering the used metal powder in the forming cylinder 150.
  • the invention provides a 3D printing method, which includes the steps of: performing laser scanning on a metal powder to perform 3D printing; and performing a local stress relief step on a print, wherein the printing step decomposes the metal powder into a powder matrix, and Laser scanning the powder substrate until the powder substrate is sintered from bottom to top into a print having a predetermined shape; and then, performing a second heat treatment on the print.
  • the residual stress is the force exerted by the bottom layer material on the upper layer material in the printed part caused during the 3D printing process.
  • a laser source is required to perform laser scanning on a metal powder in 3D printing, a rapid temperature change in the 3D printing device will be caused at this time, and thus the material will be deformed when the temperature is changed at a variable speed. Due to the deformation of the underlying material, the subsequent material is pulled, so when printing the subsequent material, it is necessary to overcome the force exerted by the underlying material. Similar tensile forces cause deformation of the print, and even internal cracks.
  • the 3D printed part is printed vertically from bottom to top and integrally formed. The present invention selectively reduces the local residual stress of the printed part by applying a local stress release step to different regions or local regions of the integrally formed printed part.
  • a laser source is used to decompose the metal powder into a powder matrix, and the powder matrix is laser-scanned until the powder matrix is sintered from bottom to top into a print with a preset shape.
  • the present invention performs a second heat treatment on the print, thereby transforming the material of the local area with a large residual stress in the print from the columnar crystals to the equiaxed crystals.
  • anisotropy is also an issue that must be considered in 3D printing.
  • the materials after 3D printing are usually columnar crystals.
  • Figure 3 shows the crystal structure of columnar crystals.
  • the anisotropy of columnar crystals is relatively high, because the grain size is in two directions. Specifically, the grain size of the columnar crystals in the length direction (Z direction) is relatively long, and although it has a high creep strength, it also causes a relatively short fatigue life. At the same time, the width of the columnar crystals in the horizontal direction is relatively narrow. Although the fatigue performance is high, the creep performance is also poor, so it is difficult to regulate the columnar crystals. In addition, the scattered crystals (fine grains) around the columnar crystals will further reduce the molding.
  • FIG 4 shows the crystal structure of the equiaxed crystal.
  • the anisotropy of the equiaxed crystal is significantly reduced, and the different structures have similar structures and mechanical properties.
  • the equiaxed grains can easily adjust the grain size and obtain appropriate fatigue and creep comprehensive properties.
  • the number of scattered crystals near the grain boundary is greatly reduced, which is beneficial to further improve the creep performance of the molded part.
  • the creep characteristics include two indexes of creep rupture life and creep deformation rate.
  • a continuous stress is applied at a high temperature (for example, 650 degrees Celsius or even 900 degrees Celsius) to test the fracture time of the 3D printed material to obtain the creep fracture life, or to test the deformation rate of the 3D printed material to obtain the creep deformation rate.
  • Creep properties can exhibit significantly different mechanical properties in the direction of construction in additive manufacturing prints. The structural direction is brought about by the columnar particle structure. Generally, the traditional method will select the design limit through the minimum mechanical properties, and take into account the complex shapes and uncontrollable anisotropic mechanical properties of additive manufacturing elements.
  • the equiaxed crystals have lower anisotropy and comprehensive properties, and the columnar crystals have better long-axis creep properties.
  • the short-axis fatigue performance is good.
  • the columnar crystals can be changed into equiaxed crystals by local adjustment, and the specific design requirements for installation in different regions can obtain the best comprehensive performance by adjusting the grain shape and size.
  • the stress releasing step includes a first heat treatment step or an ultrasonic vibration step.
  • changing the scanning method of forming in 3D printing to rotary scanning can also improve residual stress.
  • the present invention can arbitrarily select and configure the first heat treatment step, the ultrasonic vibration step, and the rotation scanning method. Among them, the present invention may choose one of the above stress relief steps, or may choose two or more of the above stress relief steps.
  • a first heat treatment step is adopted to release the local residual stress of the print.
  • the 3D printing is preferably performed in the selective laser melting apparatus 100 shown in FIG. 1.
  • the order of the printing step and the first heat treatment step may be different.
  • the present invention may perform the first heat treatment step first, and then perform the printing step.
  • the present invention may also perform the printing step first, and then perform the first heat treatment step.
  • the present invention can also perform the heat treatment and printing steps simultaneously or separately.
  • the printing step is performed in a 3D printing device, and the heat treatment step may be performed in the 3D printing step or in another device.
  • 3D printing is to print components with complex structures layer by layer, each layer has a different metal powder, and a heat treatment step can be set during the 3D printing process of each layer.
  • the first heat treatment step may use the laser source 110 in the selective laser melting apparatus 100, and may use other additional laser sources.
  • the heat treatment step uses an additional energy source other than a laser source parallel to the selective laser melting device, wherein the additional laser source is a CO2 laser source or an electron beam energy source.
  • the second heat treatment step is used to convert the columnar crystals in the region of the print with larger internal stress into the axis-shaped crystals, so that there are both columnar crystals and axis-shaped crystals in an integrally formed print, among which
  • the columnar crystal part can achieve high mechanical properties in a specific direction (good creep performance in the long axis direction and good fatigue performance in the short axis direction), while the equiaxed crystal part has lower anisotropy and comprehensive mechanical properties.
  • the invention establishes a relationship between the residual stress, the heat treatment setting and the microstructure after the heat treatment, wherein the heat treatment setting includes temperature and holding time. As shown in FIG.
  • the abscissa is the residual stress and the ordinate is the recrystallization time, which fully illustrates that the present invention can predict and adjust the crystal microstructure through the stress level related to the operating position and the appropriate selection of pre-heat treatment conditions.
  • FIG. 2 shows a schematic cross-sectional structure diagram of a sealing gland of a gas turbine.
  • the sealing gland 20 is used to block hot air from the turbine disk, and also to protect the turbine when cold air is used to cool the turbine disk.
  • the side wall 22 a of the sealing gland 20 has a first air hole 222 and a second air hole 223, wherein a space is formed between the first air hole 222 and the second air hole 223.
  • the sealing gland 20 further provides a cooling channel 226, in which both ends of the cooling channel 226 communicate with the first air hole 222 and the second air hole, respectively.
  • the cooling channel 226 extends along the longitudinal direction of the sealing gland 20.
  • the sealing gland 20 also has a fastener 228 on the side wall 22 a.
  • the fastener 228 is used to fix the sealing gland 20 to the turbine disk 143.
  • the sealing gland 22a is subjected to centrifugal force parallel to the Z direction, so it is necessary to keep the columnar crystal structure (the long axis direction is parallel to the Z direction) as much as possible to ensure that the sealing gland 20 in the Z direction has better creep performance and lower creep. Deformation.
  • the fastener 228 needs to withstand greater external forces in other directions, so it needs lower anisotropy and excellent comprehensive mechanical properties that comprehensively consider fatigue and creep.
  • the fastener 228 should retain a relatively large residual stress compared to other parts of the sealing gland 20 so that During the printing process, the material of the fastener 228 is transformed from a columnar crystal to an axial crystal by using the residual stress as a driving force, so that it has better strength and better fatigue performance, and is not easy to break.
  • laser scanning is performed on the metal powder in the selective laser melting apparatus 100 shown in FIG. 1 to start 3D printing, and printing is performed from the bottom V 1 horizontal line of the sealing cover 20 in a bottom-up order.
  • laser scanning is performed on the metal powder to decompose the metal powder into a powder matrix, and continue to perform laser scanning on the powder matrix until the powder matrix is sintered into a part of the sealing cover 20 below the horizontal line V 2 in a predetermined shape from bottom to top.
  • the laser source 110 performs a first heat treatment step on the portion of the sealing cover plate 20 below the horizontal line V 2 to reduce the residual stress of the material in this portion.
  • the laser light source 110 is used in the selective laser melting apparatus 100 to print portions between the horizontal lines V 2 and V 3 of the sealing cover plate 20, respectively. Specifically, 3D printing is continued, and the portion between the horizontal lines V 2 and V 3 of the sealing cover plate 20 is printed and sintered in a predetermined shape, and the horizontal lines V 2 and V of the sealing cover plate 20 are continued using the laser source 110.
  • the first region 224 except for the fastener 228 between 3 performs a first heat treatment step to reduce the residual stress of the material in the first region 224, but by adjusting the first heat treatment process, the residual stress in this region is higher than V 2 The location.
  • the integrated sealing gland 20 is sent to a high-temperature furnace to perform a second heat treatment step. Since the fastener 228 has a large residual stress, it is easier to transform into an equiaxed crystal at the same temperature, and below V2 And V3 and above maintain columnar crystals (major axis parallel to the Z direction).
  • the sealing gland of the gas turbine printed by the 3D printing method provided by the present invention has two different types of materials.
  • the material of the fastener 228 in the printed sealing gland 20 is a shaft crystal, V 2 and V.
  • the first region 224 other than the fastener 228 between 3 includes a mixture of columnar crystals and equiaxed crystals, and the other parts of the sealing gland 20 are columnar crystals.
  • the fastener 228 therefore has better comprehensive mechanical properties than other parts in the sealing gland 20 and is less likely to break.
  • the print can also have a material having both columnar crystals and axial crystals.
  • the second region 224a region of the first region 224 shown in FIG. 4 may be There are both columnar and axial crystals, so the mechanical properties of this part of material are between columnar crystals and equiaxed crystals.
  • the fatigue performance and creep performance are opposite, some applications require a balance between fatigue performance and creep performance, and neither performance can be too bad. Therefore, materials with both columnar and equiaxed crystals need to be applied.
  • the columnar grain structure can be transformed into an equiaxial grain structure during the recrystallization process. Therefore, the materials in the equiaxed crystal structure also show significantly different mechanical properties compared to the columnar crystal structure. Comparing the columnar crystals shown in FIG. 3 and the equiaxed crystals shown in FIG. 4, these recrystallized samples have an adjustable grain size after performing a pre-heat treatment procedure, and show an isotropic crystal structure ( isotropic grain shape). Compared with the width of columnar crystals, the relatively long grain size can greatly improve the creep characteristics and slightly reduce the fatigue performance.
  • the step of locally releasing stress in the present invention may further include a rotation scanning step, that is, the laser scanning step in 3D printing is a rotation scanning.
  • Figure 7 shows different printing strategies for 3D printing. Among them, P1 and P2 are both scanned in parallel, and the residual stress in the parallel scan is much. Specifically, P1 is scanned along the X direction, and P2 is scanned along the Y direction. Among them, P3, P4, P5, and P6 are rotary scanning. The difference is that the scanning direction of each section is different. This can reduce the temperature gradient and the temperature is cumulative, so stress can be released locally.
  • P3 is a combination of parallel and vertical scanning, which is equivalent to first performing P1 scanning along the X direction on the first layer of material, and then performing P2 scanning along the Y direction on the second layer of material.
  • the present invention may also rotate, for example, 67 degrees each time and not overlap.
  • FIG. 5 is a schematic diagram of the creep performance curves of columnar crystals and equiaxed crystals at 650 ° C.
  • the abscissa is the creep deformation amount and the ordinate is the creep rate.
  • the creep deformation amount is expressed by the creep deformation percentage. Creep rate is expressed as the percentage of creep deformation within one hour.
  • the curves S 1 and S 2 are respectively the creep performance curves of two columnar crystals
  • the curves S 3 and S 4 are the creep performance curves of the equiaxed crystal.
  • the curve S 1 and the curve S 2 have the highest creep deformation amount and creep rate.
  • the creep rates of the curves S 1 and S 2 are very different.
  • the lowest creep speed of the curve S 1 is 0.1, and the lowest creep speed of the curve S 2 is 0.02, which indicates that the columnar crystal material has the greatest anisotropy.
  • the equiaxed crystals represented by the curves S 3 and S 4 have similar overall creep properties, low creep rate and creep deformation, and anisotropy is not obvious.
  • FIG. 6 is a schematic diagram of the fatigue performance curves of columnar crystals and equiaxed crystals at 650 ° C.
  • the abscissas are columnar crystals or equiaxed crystals printed perpendicular to or parallel to the Z direction, and the ordinates are low cycle fatigue cycles.
  • the vertical line of the abscissa is the boundary line, and the left of the vertical line is the columnar crystals A 11 , A 21 , A 31 , A 12 , A 22 , A 32 that have not been subjected to the recrystallization treatment of the present invention, and the vertical line is to the right
  • the equiaxed crystals B 11 , B 21 , B 31 , B 12 , B 22 , and B 32 which have undergone the recrystallization treatment of the present invention.
  • the columnar crystals A 11 , A 21 , A 31 and the equiaxed crystals B 11 , B 21 , B 31 are printed perpendicular to the Z direction, that is, the length of the print is printed in the horizontal direction.
  • the columnar crystals A 12 , A 22 , A 32 and the equiaxed crystals B 12 , B 22 , B 32 are printed parallel to the Z direction, that is, the length of the print is printed in the vertical direction.
  • the unit of the vertical coordinate is low cycle fatigue / week. 6
  • columnar crystals A 11, A 21, A 31 is printed along the length of the horizontal printing, high fatigue properties, the length of the horizontal member and the printing of the print columnar crystals A 11, A 21, A 31
  • the fatigue properties of the columnar crystals A 12 , A 22 , and A 32 printed in a vertical direction to the length of the print are very different.
  • the equiaxed crystals B 11 , B 21 , B 31 , B 12 , B 22 , B 32 have little difference in fatigue performance when printed in the horizontal or vertical direction, and have low anisotropy.
  • the metal powder used in step S1 is Inconel 718
  • the size of the metal powder ranges from 15 mm to 53 mm.
  • the parameters of 3D printing melting laser powder are: hatch distance is 0.11mm, speed is 960mm / s, power is 285W.
  • the first heat treatment step uses the electron beam laser source, wherein a current value ranges from 3 mA to 15 mA, a scan rate is 2000 mm / s to 14000 mm / s, a heating time is 1 s to 20 s, and an acceleration voltage is 60 KV.
  • the first heat treatment step uses a CO 2 laser source, wherein the beam diameter is 0.2 mm to 1.0 mm, the laser power is 30 W to 150 W, the scanning rate is 0.5 mm / s to 500 mm / s, and the scanning interval is 0.2 mm to 0.8mm.
  • the second heat treatment step is performed in a high-temperature furnace, wherein the temperature ranges from 980 ° C to 1200 ° C, the aging treatment ranges from 710 ° C to 770 ° C for 8 to 12 hours, and the furnace is cooled to 620 ° C to 660 ° C for continuous 8h to 12h.
  • the frequency of the ultrasonic vibration ranges from 10000hz to 100,000hz.
  • the invention establishes a relationship between residual stress, heat treatment settings (including temperature and holding time) and the microstructure after heat treatment. Based on theoretical calculations and experimental verifications, the present invention can predict and adjust the crystal structure through the residual stress levels at specific locations and select appropriate heat treatment conditions to obtain different crystal structures, including pure columnar crystals, pure equiaxed crystals, and columnar crystals. Equiaxed crystal mixed materials to obtain different comprehensive mechanical properties such as anisotropy. Among them, the anisotropic mechanical properties include fatigue properties and creep properties, and the present invention can optimize all mechanical properties of the entire additive manufacturing element. The invention can also formulate a printing strategy according to the force analysis of the component, the failure mechanism, and different performance requirements.

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Abstract

一种3D打印方法，其中，其包括如下步骤：对金属粉末执行激光扫描执行3D打印；对打印件进行局部释放应力步骤，其中，所述局部释放应力步骤包括以下任一项或任两项：第一热处理；超声波震动，其中，所述打印步骤将金属粉末分解为粉末基体，并且，对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件，然后，对所述打印件执行第二热处理。该方法能够选择性的把局部柱状晶结构全部或者部分地通过再结晶转化为等轴晶，从而调控和改善增材制造元件的微结构和相应的综合机械性能。

Description

3D打印方法 技术领域
本发明涉及增材制造领域，尤其涉及一种3D打印方法。
背景技术
增材制造工艺(Additive Manufacturing)是重要的3D打印技术之一，增材制造工艺能够快速地将预先设计的CAD模型制造出来，而且能够在较短的时间内制造出结构复杂的零部件。选择性激光熔化(Selected Laser Melting，SLM)工艺是增材制造(Additive manufacturing)技术的一种，其通过激光烧结的方式可快速地将与CAD模型相同的零部件制造出来。目前选择性激光熔化工艺得到了广泛的应用。和传统材料去除机制不同，增材制造是基于完全相反的材料增加制造理念(materials incremental manufacturing philosophy)，其中，选择性激光熔化利用高功率激光熔化金属粉末，并通过3D CAD输入来一层一层地建立部件/元件，这样可以成功制造出具有复杂内部沟道的元件。
除了上述的优点，增材制造仍然需要面对了很多技术挑战，阻碍了其在工业制造领域的广泛应用，尤其是针对那些对机械性能严格考虑的关键元件(components with critical concerns of mechanical performance)。增材制造所面对的技术挑战包括但不限于由于粉末熔化以后快速冷却带来的剩余应力residual stress)。由于柱状晶体结构(columnar grain structures)导致的机械性能取向相关性，以及复杂元件(例如薄壁结构)的需求和性能位置相关性等。
发明内容
本发明提供了3D打印方法，其中，其包括如下步骤：对金属粉末执行激光扫描执行3D打印；对打印件进行局部释放应力步骤，其中，所述局部释放应力步骤包括以下任一项或任两项：第一热处理；超声波震动，其中，所述打印步骤将金属粉末分解为粉末基体，并且，对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件，然后，对所述打印件执行第二热处理。
进一步地，所述打印步骤和所述第一热处理步骤都在选择性激光熔化设备中执行，其中，在执行热处理步骤时使用比所述打印步骤更小的激光功率密度和更快的扫描速度。
进一步地，所述第一热处理步骤采用平行于所述选择性激光熔化设备的激光源之外的额外激光源，其中，所述额外激光源是CO2激光源或者电子束能量源。
进一步地，所述第一热处理步骤采用所述电子束能量源，其中，电流取值范围为3mA～15mA，扫描速率为2000mm/s～14000mm/s，加热时间为1s～20s，加速电压为60KV。
进一步地，所述第一热处理步骤采用CO2激光源，其中，光束直径为0.2mm～1.0mm，激光功率为30W～150W，扫描速率为0.5mm/s～500mm/s，扫描间隔为0.2mm～0.8mm。
进一步地，所述激光成形扫描步骤和所述第一热处理步骤分别执行。
进一步地，所述激光成形扫描步骤为旋转扫描。
进一步地，所述第二热处理步骤在高温炉中进行，其中，温度取值范围为980℃～1200℃，老化处理为710℃～770℃持续8h～12h，炉内冷却至620℃～660℃持续8h～12h。
进一步地，所述超声波震动的频率取值范围为10000hz～100000hz。
进一步地，采用所述3D打印方法得到的打印件具有等轴晶结构，或者柱状晶和等轴晶混合结构。
本发明将本发明在剩余应力，热处理设定和热处理后组织结构联系起来。基于理论计算和实验验证，本发明可以预测并通过特定位置剩余应力等级和选择适当的热处理条件(温度和保温时间)来调整晶体结构，从而得到不同的晶体结构，包括纯柱状晶，纯等轴晶，以及柱状晶和等轴晶的混合材料，以得到各向异性等不同的机械性能。其中，各向异性机械性能包括疲劳性能、蠕变性能，本发明能够优化整个增材制造元件的全部机械性能。本发明还可以根据部件的受力分析，失效机制，以及不同性能要求来制定打印策略。
附图说明
图1是选择性激光熔化设备的结构示意图；
图2是燃气轮机的密封压盖的剖面结构示意图；
图3是柱状晶的晶体结构示意图；
图4是等轴晶的晶体结构示意图；
图5是柱状晶和等轴晶的蠕变性能曲线示意图；
图6是柱状晶和等轴晶的疲劳性能曲线示意图；
图7是3D打印的不同扫描策略示意图；
图8是再结晶时间和剩余应力的对应示意图。
具体实施方式
以下结合附图，对本发明的具体实施方式进行说明。
剩余应力一直被视为增材制造中的不利因素和负面效应，然而，本发明用剩余应力充当晶体颗粒的调整驱动力，通过局部释放应力步骤控制一体成型的打印件中不同区域材料的剩余应力，再将该打印件中的局部区域全部或者部分地从柱状晶转化为等轴晶，从而改善增材制造元件的微结构和相应的机械性能。本发明通过局部剩余应力和局部释放应力步骤来预测调整依赖于位置和方向的机械性能。
图1是选择性激光熔化设备的示意图。如图1所示，选择性激光熔化设备100包括一个激光源110、一个镜面扫描器120、一个棱镜130、一个送粉缸140、一成型缸150和一个回收缸160。其中，激光源110设置于选择性激光融化设备100上方，充当金属粉末的加热源，即融化金属粉末来进行3D打印。
其中，送粉缸140下部有一个能够上下移动的第一活塞(未示出)，在送粉缸140的第一活塞上面的腔体空间放置了备用的金属粉末，并随着第一活塞的上下移动从送粉缸140将金属粉末送入成型缸150。在成型缸150中设置有一个3D打印件放置台154，放置台154上方夹持有一个3D打印件C，放置台154下方固定有一个第二活塞152，其中，第二活塞152和放置台154垂直设置。在3D打印过程中，第二活塞152自上而下移动，以在成型缸220中形成打印空间。激光扫描的激光源110应设置于选择性激光融化设备的成型缸150的上方，镜面扫描器120通过调整一个棱镜130的角度调整激光的位置，通过棱镜130的调节来决定激光融化哪个区域的金属粉末。送粉缸140还包括一个滚轮(未示出)，金属粉末P堆设于第一活塞的上表面，第一活塞垂直地自下而上移动传递金属粉末至送粉缸140上部。滚轮可在金属粉末P 上滚动，以将金属粉末P送至成型缸150中。从而持续对金属粉末执行激光扫描，将金属粉末分解为粉末基体，继续对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件C。
此外，选择性激光熔化设备100还包括一个回收缸160，回收缸160用于回收成型缸150中的使用过的金属粉末。
本发明提供了一种3D打印方法，其中包括如下步骤：对金属粉末执行激光扫描执行3D打印；对打印件进行局部释放应力步骤，其中，所述打印步骤将金属粉末分解为粉末基体，并且，对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件；然后，对所述打印件执行第二热处理。
其中，剩余应力是在3D打印过程中导致的打印件中底层材料对上层材料具有的作用力。具体地，由于3D打印中需要采用激光源对金属粉末进行激光扫描，此时会引起3D打印设备中的温度快速变化，因此材料会在温度变速变化时产生形变。底层材料由于形变拉住了后续材料，因此当进行后续材料的打印时需要克服底层材料施加的作用力，类似的拉扯力导致打印件变形，甚至内部出现裂纹等。另外，3D打印件是自下而上垂直打印并一体成型的，本发明通过对一体成型的打印件的不同区域或者局部区域施加局部释放应力步骤，选择性地减小打印件的局部剩余应力。
在3D打印过程中，用激光源将金属粉末分解为粉末基体，并且，对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件。打印件打印完成以后，本发明对打印件执行第二热处理，从而将打印件中剩余应力大的局部区域材料从柱状晶转化为等轴晶。
具体地，各向异性也是3D打印中不得不考虑的一个问题。3D打印以后的材料通常为柱状晶。图3示出了柱状晶的晶体结构，如图所示，柱状晶的各向异性比较高，这是因为晶粒尺寸在两个方向。具体地，柱状晶的长度方向(Z方向)晶粒尺寸比较长，虽然具有较高的蠕变强度，但同时造成其疲劳寿命也比较短。同时，柱状晶水平方向宽度比较窄，虽然疲劳性能较高，但蠕变性能也比较差，因此很难对柱状晶做调控；另外柱状晶周围的散晶(细小晶粒)也会进一步降低成型件蠕变性能。图4示出了等轴晶的晶体结构，如图所示，等轴晶的各向异性明显降低，不同方向具有相近的组织结构和力学性能。另外等轴晶粒可以方便调控晶粒尺寸，得到合适的疲劳和蠕变综合 性能。同时在柱状晶向等轴晶转变过程中，晶界附近的散晶数量大大减少，有利于进一步改善成型件的蠕变性能。
具体地，蠕变特性包括蠕变断裂寿命和蠕变变形速率两个指标。在3D打印过程中在高温情况(例如650摄氏度甚至900摄氏度)下施加持续性应力测试3D打印材料断裂时间以获得蠕变断裂寿命，或者测试3D打印材料的变形速率以获得蠕变变形速率。蠕变特性能够展现增材制造打印件中对于构造方向的显著不同机械性能。其中构造方向是由柱状颗粒结构带来的，通常传统方法会通过最小机械性能来选择设计极限，并考虑到增材制造元件的复杂形状和不可控各向异性机械性能。
而在3D打印实际应用中，有些打印区域需要等轴晶，有些打印区域需要柱状晶，其中等轴晶的具有较低各向异性和综合性能，而柱状晶的长轴方向蠕变性能较好而短轴方向疲劳性能好。本发明可以通过局部调整柱状晶变成等轴晶，在不同区域安装具体设计要求通过调整晶粒形状和尺寸得到最佳的综合性能。
所述释放应力步骤包括：第一热处理步骤或者超声波震动步骤。此外，改变3D打印中的成形扫描方式为旋转扫描，也能够改善剩余应力。本发明可以根据具体应用场景，任意选择和配置第一热处理步骤、超声波震动步骤以及旋转扫描方式。其中，本发明可以选择以上一种释放应力步骤，也可以选择以上两种或者多种释放应力步骤。
根据本发明一个优选实施例，采用第一热处理步骤来释放打印件的局部剩余应力。其中，所述3D打印优选地是在图1所示的选择性激光熔化设备100中执行的。需要说明的是，所述打印步骤和所述第一热处理步骤次序可以不同。具体地，本发明可以先执行第一热处理步骤，再执行打印步骤。本发明也可以先执行打印步骤，再执行第一热处理步骤。本发明也可以同时或者分别执行热处理和打印步骤。另外，打印步骤是在3D打印设备中进行的，热处理步骤既可以在3D打印步骤中执行也可以也在另外的设备中执行。此外，3D打印是一层一层地打印结构复杂的元件的，每一层具有不同的金属粉末，每一层的3D打印过程中都可以设置热处理步骤。
根据本发明一个优选实施例，具体地，当所述打印步骤和所述第一热处理步骤都在如图1所示的选择性激光熔化设备100中执行时，其中，在执行热处理步骤时使用比所述打印步骤更小的激光功率密度和更快的扫描速度。 具体地，第一热处理步骤可以采用选择性激光熔化设备100中的激光源110，又可以采用其他额外激光源。此外，如果所述热处理步骤采用平行于所述选择性激光熔化设备的激光源之外的额外能量源，其中，所述额外激光源是CO2激光源或者电子束能量源。
进一步地，第二热处理步骤用于将打印件中内应力较大的区域中的柱状晶转化为轴状晶，使得在一个一体成型的打印件中既有柱状晶又有轴状晶，其中的柱状晶部分可以实现特定方向具有较高的力学性能(长轴方向蠕变性能较好而短轴方向疲劳性能好)，而等轴晶部分具有较低的各向异性和综合力学性能，。本发明在剩余应力，热处理设定和热处理后组织结构建立了联系，其中，热处理设定包括温度和保温时间。如图8所示，其横坐标为剩余应力，其纵坐标为再结晶时间，这充分说明本发明可以通过操作位置相关的应力水平和适当选择预热处理条件来预测和调整晶体微结构，这样我们可以获得不同的晶体结构，包括全柱状晶体、全等轴晶体以及柱状晶体和等轴晶体的混合，以得到不同的机械性能例如各向异性机械性能的平衡，包括用于优化增材制造性能的疲劳特性和蠕动特性。
下面结合一个具体实施例对本发明进行说明，图2示出了燃气轮机的密封压盖的剖面结构示意图。密封压盖20用于将热空气阻隔在涡轮盘之外，还用于在冷空气用来冷却涡轮盘时保护涡轮。如图2所示，密封压盖20的侧壁22a上具有一个第一空气孔222和一个第二空气孔223，其中，所述第一空气孔222和第二空气孔223中间具有一个空间。密封压盖20还提供了一个冷却通道226，其中冷却通道226的两端分别和第一空气孔222和第二空气孔贯通。并且，冷却通道226沿着密封压盖20的长度方向延伸。密封压盖20还具有一个紧固件228，所述紧固件228在侧壁22a上，紧固件228用于将密封压盖20固定于涡轮盘143。密封压盖22a受到平行于Z方向的离心力，所以需要尽量保持柱状晶结构(长轴方向平行于Z方向)而保证在Z方向密封压盖20具有更好的蠕变性能和更低的蠕变变形量。而相对于密封压盖20的其他部分，紧固件228需要承受较大的其它方向交变外力，所以需要较低的各向异性和综合考虑疲劳和蠕变的优良综合力学性能。
因此，如果利用本发明提供的3D打印方法打印如图2所示的燃气轮机的密封压盖20，其中，紧固件228应当较所述密封压盖20的其他部分保留比较大的剩余应力，以便在打印过程中利用剩余应力作为驱动力将所述紧固件228 的材料从柱状晶转变为轴状晶，从而具有更好的强度和更好的疲劳性能，不易断裂。
具体地，首先在如图1所示的选择性激光熔化设备100中对金属粉末执行激光扫描开始执行3D打印，从密封盖板20的底部V 1水平线开始按照自下而上的顺序进行打印。持续对金属粉末执行激光扫描，从而将金属粉末分解为粉末基体，对粉末基体继续进行激光扫描直至粉末基体自下而上地按照预定形状烧结为密封盖板20于水平线V 2以下的部分，采用激光源110对密封盖板20于水平线V 2以下的部分执行第一热处理步骤，减小该部分材料的剩余应力。
然后，在选择性激光熔化设备100中采用激光源110分别对密封盖板20的水平线V 2和V 3之间的部分进行打印。具体地，继续执行3D打印，将密封盖板20的水平线V 2和V 3之间的部分进行打印按照预设形状烧结成型，同时继续利用激光源110对密封盖板20的水平线V 2和V 3之间除紧固件228以外的部分第一区域224执行第一热处理步骤，减小第一区域224材料的剩余应力，但通过调节第一热处理工艺，使该区域剩余应力高于V 2以下的部位。
同理，继续自下而上对密封压盖20水平线V 3以上的部分执行3D打印，对这部分执行第一热处理步骤，减小材料的剩余应力。
最后，将一体成型的密封压盖20送入高温炉执行第二热处理步骤，由于紧固件228具有较大的剩余应力，因此在同样的温度下更易于转变为等轴晶，而在V2以下和V3以上部分保持柱状晶(长轴平行于Z方向)。
因此，采用本发明提供的3D打印方法打印燃气轮机的密封压盖具有两种不同类型的材料，其中，打印完成的密封压盖20中的紧固件228的材料为轴状晶，V 2和V 3之间除紧固件228以外的部分第一区域224包含柱状晶体和等轴晶体的混合，密封压盖20的其他部分材料为柱状晶。紧固件228因此较密封压盖20中的其他部分具有更好的综合力学性能，不易断裂。
需要说明的是，执行本发明的打印方法，还可以使得打印件具有既有柱状晶又有轴状晶的材料，例如在如图4所示的第一区域224中的第二区域224a区域可以既有柱状晶又有轴状晶，因此这部分材料机械性能介于柱状晶和等轴晶。虽然疲劳性能和蠕变性能是对立的，但是有些应用场合需要疲劳性能和蠕变性能的平衡，两个性能都不能太差，因此需要应用既有柱状晶又有等轴晶的材料。
原则上柱状晶体结构(columnar grain structure)能够在再结晶过程中被转化为等轴晶体结构(equiaxial grain structure)。因此等轴晶体结构中的材料和仍然是柱状晶体结构相比也显示出了显著不同的机械性能。对比图3所示的柱状晶和图4所示的等轴晶，这些再结晶的样品在执行了预热处理程序后具有可调整的晶粒尺寸，并显示出了各向同性的晶体结构(isotropic grain shape)。与柱状晶体的宽度相比，相对长的晶粒尺寸能够极大地改善蠕变特性，并轻微减少疲劳性能。
进一步地，本发明的局部释放应力步骤还可以包括旋转扫描步骤，即，在3D打印中所述激光扫描步骤为旋转扫描。图7示出了3D打印的不同打印策略。其中，P1和P2都是平行扫描，平行扫描的剩余应力很多。具体的，P1是沿着X方向扫描的，P2是沿着Y方向扫描的。其中，P3、P4、P5和P6是旋转扫描，区别在于每个区间的扫描方向不一样，这样可以把温度梯度减小，温度是累加的，因此可以在局部释放应力。例如，P3是平行和垂直方向扫描的组合，相当于首先在第一层材料执行P1的沿着X方向扫描，然后在第二层材料执行P2的沿着Y方向扫描。可选地，本发明还可以例如每次旋转67度，并且不重合。
图5是柱状晶和等轴晶在650℃的蠕变性能曲线示意图，横坐标为蠕变变形量，纵坐标为蠕变速率，其中，蠕变变形量是由蠕变变形百分比来表示的，蠕变速率是由一个小时以内的蠕变变形百分比来表示的。具体的，曲线S 1和曲线S 2分别为两个柱状晶体的蠕变性能曲线，曲线S 3和S 4为等轴晶体的蠕变性能曲线。如图5所示，曲线S 1和曲线S 2的蠕变变形量以及蠕变速率最高。并且，曲线S 1和曲线S 2的蠕变速率相差巨大，曲线S 1的最低蠕变速率为0.1，曲线S 2的最低蠕变速率为0.02，这说明柱状晶材料的各向异性最大。反观曲线S 3和S 4所代表的等轴晶，其整体蠕变性能很接近，蠕变速率和蠕变变形量都很低，各向异性不明显。
图6是柱状晶和等轴晶在650℃的疲劳性能曲线示意图，横坐标为垂直或者平行于Z方向打印的柱状晶或者等轴晶，纵坐标为低周疲劳周期。其中，横坐标的竖线为分界线，竖线以左为并未经过本发明再结晶处理的柱状晶A 11、A 21、A 31、A 12、A 22、A 32，竖线以右为经过本发明再结晶处理的等轴晶B 11、B 21、B 31、B 12、B 22、B 32。具体的，柱状晶A 11、A 21、A 31和等轴晶B 11、B 21、B 31为垂直于Z方向打印的，即打印件长度沿水平向打印的。柱状晶A 12、 A 22、A 32和等轴晶B 12、B 22、B 32为平行于Z方向打印的，即打印件长度沿垂直方向打印的。竖坐标的单位是低周疲劳/周。如图6所示，柱状晶A 11、A 21、A 31是打印件长度沿水平向打印的，其疲劳性能较高，并且打印件长度沿水平打印的柱状晶A 11、A 21、A 31和打印件长度沿垂直方向打印的柱状晶A 12、A 22、A 32的疲劳性能差异很大。反观等轴晶B 11、B 21、B 31、B 12、B 22、B 32不论是打印件长度沿水平还是垂直方向打印其疲劳性能相差不大，具有较低的各向异性。其中，示例性地，如果步骤S1采用的金属粉末为Inconel 718，金属粉末的尺寸取值范围为15mm～53mm。3D打印熔化激光粉末的参数为：舱口距离(hatch distance)为0.11mm，速度960mm/s，功率285W。
其中，所述第一热处理步骤采用所述电子束激光源，其中，电流取值范围为3mA～15mA，扫描速率为2000mm/s～14000mm/s，加热时间为1s～20s，加速电压为60KV。
其中，所述第一热处理步骤采用CO 2激光源，其中，光束直径为0.2mm～1.0mm，激光功率为30W～150W，扫描速率为0.5mm/s～500mm/s，扫描间隔为0.2mm～0.8mm。
其中，所述第二热处理步骤在高温炉中进行，其中，温度取值范围为980℃～1200℃，老化处理为710℃～770℃持续8h～12h，炉内冷却至620℃～660℃持续8h～12h。
其中，所述超声波震动的频率取值范围为10000hz～100000hz。
本发明在剩余应力，热处理设定(包括温度和保温时间)和热处理后组织结构建立了联系。基于理论计算和实验验证，本发明可以预测并通过特定位置剩余应力等级和选择适当的热处理条件来调整晶体结构，从而得到不同的晶体结构，包括纯柱状晶，纯等轴晶，以及柱状晶和等轴晶的混合材料，以得到各向异性等不同的综合机械性能。其中，各向异性机械性能包括疲劳性能、蠕变性能，本发明能够优化整个增材制造元件的全部机械性能。本发明还可以根据部件的受力分析，失效机制，以及不同性能要求来制定打印策略。
尽管本发明的内容已经通过上述优选实施例作了详细介绍，但应当认识到上述的描述不应被认为是对本发明的限制。在本领域技术人员阅读了上述内容后，对于本发明的多种修改和替代都将是显而易见的。因此，本发明的保护范围应由所附的权利要求来限定。此外，不应将权利要求中的任何附图 标记视为限制所涉及的权利要求；“包括”一词不排除其它权利要求或说明书中未列出的装置或步骤；“第一”、“第二”等词语仅用来表示名称，而并不表示任何特定的顺序。

Claims (10)

  1. 3D打印方法，其特征在于，其包括如下步骤：
    对金属粉末执行激光扫描执行3D打印；
    对打印件进行局部释放应力步骤，
    其中，所述局部释放应力步骤包括以下任一项或任两项：
    第一热处理；
    超声波震动，
    其中，所述打印步骤将金属粉末分解为粉末基体，并且，对所述粉末基体进行激光扫描直至使所述粉末基体自下而上地烧结为预设形状的打印件，
    然后，对所述打印件执行第二热处理。
  2. 根据权利要求1所述的3D打印方法，其特征在于，所述打印步骤和所述第一热处理步骤都在选择性激光熔化设备中执行，其中，在执行热处理步骤时使用比所述打印步骤更小的激光功率密度和更快的扫描速度。
  3. 根据权利要求根据权利要求2所述的3D打印方法，其特征在于，所述第一热处理步骤采用平行于所述选择性激光熔化设备的激光源之外的额外激光源，其中，所述额外激光源是CO 2激光源或者电子束能量源。
  4. 根据权利要求3所述的3D打印方法，其特征在于，所述第一热处理步骤采用所述电子束能量源，其中，电流取值范围为3mA～15mA，扫描速率为2000mm/s～14000mm/s，加热时间为1s～20s，加速电压为60KV。
  5. 根据权利要求3所述的3D打印方法，其特征在于，所述第一热处理步骤采用CO 2激光源，其中，光束直径为0.2mm～1.0mm，激光功率为30W～150W，扫描速率为0.5mm/s～500mm/s，扫描间隔为0.2mm～0.8mm。
  6. 根据权利要求1所述的3D打印方法，其特征在于，所述激光扫描步骤和所述第一热处理步骤分别执行。
  7. 根据权利要求1所述的3D打印方法，其特征在于，所述激光扫描步骤为旋转扫描。
  8. 根据权利要求1所述的3D打印方法，其特征在于，所述第二热处理步骤在高温炉中进行，其中，温度取值范围为980℃～1200℃，老化处理为710℃～770℃持续8h～12h，炉内冷却至620℃～660℃持续8h～12h。
  9. 根据权利要求1所述的3D打印方法，其特征在于，所述超声波震动 的频率取值范围为10000hz～100000hz。
  10. 根据权利要求1所述的3D打印方法，其特征在于，采用所述3D打印方法得到的打印件具有等轴晶结构，或者柱状晶和等轴晶混合结构。
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