CN108480638B - Three-section type selective laser melting combined preheating system - Google Patents
Three-section type selective laser melting combined preheating system Download PDFInfo
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
- B22F10/20—Direct sintering or melting
- B22F10/28—Powder bed fusion, e.g. selective laser melting [SLM] or electron beam melting [EBM]
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
- B22F10/80—Data acquisition or data processing
- B22F10/85—Data acquisition or data processing for controlling or regulating additive manufacturing processes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F12/00—Apparatus or devices specially adapted for additive manufacturing; Auxiliary means for additive manufacturing; Combinations of additive manufacturing apparatus or devices with other processing apparatus or devices
- B22F12/10—Auxiliary heating means
- B22F12/17—Auxiliary heating means to heat the build chamber or platform
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F12/00—Apparatus or devices specially adapted for additive manufacturing; Auxiliary means for additive manufacturing; Combinations of additive manufacturing apparatus or devices with other processing apparatus or devices
- B22F12/20—Cooling means
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F12/00—Apparatus or devices specially adapted for additive manufacturing; Auxiliary means for additive manufacturing; Combinations of additive manufacturing apparatus or devices with other processing apparatus or devices
- B22F12/30—Platforms or substrates
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F12/00—Apparatus or devices specially adapted for additive manufacturing; Auxiliary means for additive manufacturing; Combinations of additive manufacturing apparatus or devices with other processing apparatus or devices
- B22F12/50—Means for feeding of material, e.g. heads
- B22F12/52—Hoppers
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F12/00—Apparatus or devices specially adapted for additive manufacturing; Auxiliary means for additive manufacturing; Combinations of additive manufacturing apparatus or devices with other processing apparatus or devices
- B22F12/50—Means for feeding of material, e.g. heads
- B22F12/55—Two or more means for feeding material
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B33—ADDITIVE MANUFACTURING TECHNOLOGY
- B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
- B33Y30/00—Apparatus for additive manufacturing; Details thereof or accessories therefor
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/25—Process efficiency
Abstract
The invention belongs to the technical field of selective laser melting additive manufacturing, and particularly discloses a three-section selective laser melting combined preheating system, which comprises a preheating section, a soaking section and a multi-stage heating section which are all arranged in a working cavity, wherein the preheating section is arranged above the inside of the working cavity and is used for storing and preheating powder; the soaking section is positioned below the preheating section and is used for soaking the powder falling from the preheating section, sending the soaked powder into the multi-stage heating section, and carrying out selective laser melting forming on the soaked powder in the multi-stage heating section; the multistage heating section is positioned at the side of the soaking section and is used for heating soaked powder and heating the laser formed part in a gradient way to form a temperature gradient from top to bottom. The invention obviously reduces the heat loss of the powder in the process of moving in the working cavity, realizes the sectional high-temperature preheating of the metal powder, and has the advantages of good preheating effect, strong applicability and the like.
Description
Technical Field
The invention belongs to the technical field of selective laser melting additive manufacturing, and particularly relates to a three-section selective laser melting combined preheating system.
Background
Selective laser melt forming technology (Selective Laser Melting, SLM) is an important component of rapid forming (Rapid Prototyping, RP) technology, and has been greatly developed in recent years. Compared with the traditional equal material manufacturing (casting, forging and welding) and material reduction manufacturing (machining), the SLM technology belongs to the field of material increase manufacturing, has the advantages of simple process, high dimensional accuracy of formed parts, good surface roughness, high density and the like, plays a great role in the manufacturing process of precise and complex parts, and is a direct metal part manufacturing technology. The specific technological process of the selective laser melting forming technology is as follows: firstly, designing a three-dimensional model of a part by using three-dimensional design software; slicing the three-dimensional model of the part into a two-dimensional laser scanning track through corresponding slicing software; then spreading a layer of metal powder with the same thickness as the model slice on the forming substrate, and carrying out zone-selection melting on a powder bed area by using a laser beam according to the laser scanning track determined before, and cooling and solidifying the metal powder to form a deposition layer; after the scanning is completed, the substrate is lowered, and the lowering height is equal to the slice thickness of the model; repeating the above process until the whole part is formed.
The focusing spot diameter of the laser beam used for processing the parts by the SLM technology is extremely small (generally 0.1-0.5 mm), the stacking layer thickness of the powder is also small (0.02-0.2 mm), the laser beam with high energy density rapidly scans and sinters the metal powder on a powder bed, monolayer processing is completed within a few seconds, and an ultrahigh temperature gradient is brought to a deposition layer (molten pool). The crystal growth in the part is strong in orientation due to the ultrahigh temperature gradient, so that the performance of the part in the orientation direction is greatly improved, but strong internal stress is brought to the part, and the cracking tendency of the part is increased. In the complex and fine structural parts which are difficult to process by the conventional method, particularly in the processing process of high-performance and complex structural metal components which have important significance in the fields of aerospace and the like, the phenomenon of part cracking caused by overlarge internal stress sometimes occurs, so that the yield of the parts is reduced intangibly, and the manufacturing cost is increased.
At present, the internal stress of a part is reduced by mainly preheating a powder bed to heat the formed part, and a plurality of patents related to a method for preheating an SLM and related equipment are disclosed in the industry, but no related patent is available for a cooling system in an SLM forming cavity.
CN103100713a discloses a method for heating powder by a powder feeding barrel in the equipment, the method heats by selectively laser selectively melting metal powder in a powder feeding barrel of a powder feeding barrel preheating device of SLM equipment under the action of high-frequency or medium-frequency magnetic field generated in an alternating magnetic field, eddy current loss and hysteresis loss which is the action of a conductor internal magnetic field formed by the metal powder, which causes self-heating of the conductor, and collecting powder temperature information by a temperature sensor for feedback control, thereby achieving the purpose of preheating powder. However, the powder movement period in the powder laying work using the powder feeding cylinder is long, and the powder is completely in the low-temperature circulating inert protective gas atmosphere in the SLM equipment in the process of forming the forming cylinder after discharging the cylinder, so that the heat loss caused by heat convection is extremely large. And the amount of powder paved by single-layer processing is small (hundreds of grams), and the small amount of powder is extremely easy to generate heat loss in the paving process, so that the heating effect is greatly reduced in actual processing.
CN105855544a discloses a method of preheating powder by preheating a device forming cylinder. The method achieves the aim of preheating powder by heating a forming cylinder by adopting an electromagnetic induction technology in the heating of the SLM forming part. However, the monolayer processing time of SLM is extremely short (tens of seconds to minutes), and it is very difficult to heat the fed powder to a desired temperature in such a short time. In addition, the skin effect of electromagnetic induction heating is concentrated, and uneven heating is easy to cause, so that the problem of how to effectively prevent the skin effect and prevent secondary internal stress caused by uneven heating in a short time is not neglected.
CN205888083U discloses a method of preheating a powder bed by a directional heating laser. The method heats the powder of the processing plane by outputting laser through a directional heating laser arranged on the same side of the processing laser, but a substrate or a solidified part below the powder can be used as an effective heat radiation body, so that the effective heating efficiency of the method is greatly reduced, and the method cannot be well applied to actual production.
Further, CN205888085U, CN103338881a et al also discloses preheating of powder by modification of a forming cylinder or other parts. Although these patent methods solve the problem of preheating the powder to some extent, there is no effective solution to the problem of how to maintain the powder temperature to a steady state throughout the process. This is mainly due to the unique process flow in SLM processing: firstly, the powder enters a powder storage hopper after pretreatment; then, releasing a certain amount of powder into a forming chamber, paving the powder on a substrate through a scraping plate, and enabling the powder to be in contact with a processing platform, the scraping plate and inert protective gas in the process to be in a uniformly dispersed state, wherein heat dissipation is extremely high; after that, the substrate is lowered after laser sintering and forming in a very short time, and the powder laying process of the next layer is started. These unique processes allow a single-to-one component to be modified to heat the powder with a significantly compromised and greater heat dissipation. Therefore, how to overcome the practical problems existing in each link of the process flow and design a high-temperature powder preheating system suitable for the SLM equipment has great practical significance on the development of the SLM process and the reduction of the cracking tendency of the processed workpiece.
Disclosure of Invention
Aiming at the defects or improvement demands of the prior art, the invention provides a combined preheating system for forming an SLM (selective laser melting) by adopting a three-section heating mode to heat powder, and the sectional heating of metal powder is realized by a powder storage hopper (heating section), a workbench (soaking section) and a forming cylinder (heat preservation section), so that the metal powder in the SLM processing process is stably preheated, the internal stress of SLM processing parts is reduced or eliminated, and the cracking tendency of processing workpieces is reduced; in addition, the cooling device is arranged below the powder bed substrate, so that the stability of the heat flow direction can be ensured, the heat flow disorder is avoided, the temperature gradient is controlled, a technical foundation is provided for SLM molding directional solidification and even single crystal alloy, and the method is particularly suitable for metal components and directional solidification components with high precision, large cracking tendency, large internal stress and complex structure.
In order to achieve the above purpose, the invention provides a three-section type selective laser melting combined preheating system, which comprises a preheating section, a soaking section and a multi-stage heating section which are all arranged in a working cavity, wherein the preheating section is arranged above the inside of the working cavity and is used for storing powder and preheating the powder; the soaking section is positioned below the preheating section and is used for soaking the powder falling from the preheating section, and sending the soaked powder into the multi-stage heating section, wherein the soaked powder is subjected to selective laser melting forming in the multi-stage heating section; the multistage heating section is positioned at the side of the soaking section and is used for heating soaked powder and heating the laser formed part in a gradient way so as to form a temperature gradient from top to bottom.
As a further preferable mode, the preheating section comprises a first powder falling hopper and a second powder falling hopper which are arranged above the inside of the working cavity and are symmetrically arranged left and right; the first powder falling hopper and the second powder falling hopper are identical in structure and comprise powder hoppers and blanking plugboards which are positioned below the powder hoppers and used for opening or closing the powder hoppers.
As a further preferable mode, the heat insulation material layer, the induction coil and the electromagnetic shielding layer are sequentially arranged outside the powder hopper.
As a further preferable mode, the soaking section comprises a first working platform and a second working platform which are respectively arranged below the first powder falling hopper and the second powder falling hopper, and heating rods are embedded in the first working platform and the second working platform to heat powder.
As a further preferable mode, a scraper is arranged between the first powder falling hopper and the first working platform or between the second powder falling hopper and the second working platform.
As a further preferable mode, the first working platform is further provided with a first powder falling port, the second working platform is further provided with a second powder falling port, powder falling onto the first working platform from the first powder falling hopper is firstly sent to the multi-stage heating section through reciprocating motion of the scraping plate, redundant powder is sent to the second powder falling port so as to finish one-layer powder sending of the part to be melted and formed by laser, then powder falling onto the second working platform from the second powder falling hopper is sent to the multi-stage heating section, redundant powder is sent to the first powder falling port, and then powder sending of the next layer of part to be melted and formed by laser is finished.
As a further preferable mode, the multistage heating section comprises a forming cavity and a substrate, the forming cavity is arranged in the middle of the lower portion of the inner portion of the working cavity, the substrate is arranged in the forming cavity and moves up and down under the action of the lifting mechanism, a plurality of heating rods which are sequentially arranged from top to bottom are arranged on two sides of the forming cavity, and the heating rods are selectively opened according to the positions of the substrate so as to realize gradient heating.
As a further preferable mode, a cooling circulation device is further arranged in the multi-stage heating section so as to ensure that the heat flow direction is stable and avoid heat flow disorder, the cooling circulation device is arranged below the base plate and is a brass supporting plate with a built-in cooling runner, a heating plate is arranged between the brass supporting plate and the base plate, and a heat insulation plate is arranged below the brass supporting plate.
As a further preferred feature, a laser is provided above the interior of the working chamber to provide a laser beam to effect laser melt-forming of the powder, the laser being located directly above the forming chamber; preferably, a gas preheating circulation device is further arranged in the working cavity, the gas preheating circulation device comprises a gas heat exchanger and a fan, the fan is connected with the gas heat exchanger and used for sending inert gas into the gas heat exchanger, the gas heat exchanger is respectively connected with an air outlet and an air inlet on the working cavity through pipelines, the pipelines connected with the air inlet of the gas heat exchanger are paved along the soaking section and the multi-stage heating section, the gas heat exchanger collects high-temperature inert gas from the air outlet on one hand and preheats the inert gas sent by the fan by utilizing the high-temperature inert gas, and on the other hand, the preheated inert gas is sent into the working cavity through the air inlet after being preheated again through the soaking section and the multi-stage heating section.
As a further preferred aspect, the combined preheating system further includes a temperature feedback control device including a central PID control module, a preheating section control module, a soaking section control module, a multi-stage heating section control module, and an atmosphere preheating control module, wherein the central PID control module distributes heating tasks of each module according to a set heating temperature, the preheating section control module controls the preheating section to heat the powder to the preheating temperature according to the distributed tasks, the soaking section control module controls the soaking section to heat the powder according to the distributed tasks, and the multi-stage heating section control module controls the multi-stage heating section to heat the powder to the using temperature according to the distributed tasks.
In general, compared with the prior art, the above technical solution conceived by the present invention mainly has the following technical advantages:
1. the invention realizes the combined heating of electromagnetic induction heating and heating flat plate heating by a three-section combined preheating system, and obviously reduces the heat loss of powder in the moving process of the powder in the working cavity by the powder storage hopper (heating section), the workbench (soaking section) and the forming cylinder (heat preservation section), thereby realizing the high-temperature preheating of the metal powder.
2. The invention adopts the powder falling hopper to store and fall out the process, adopts the electromagnetic induction heating mode to heat, and can quickly carry out feedback adjustment according to the feedback information of the workbench and the forming cylinder by the built-in measuring thermocouple;
3. the two powder falling hoppers are symmetrically arranged in the working cavity, and can heat and spread powder in two directions, so that compared with SLM equipment adopting unidirectional powder spreading, the powder falling hopper has the advantages of shortening the working stroke in multiple, reducing heat loss and improving heating efficiency;
4. the working platform of the invention uses the embedded heating rod (or the resistance wire) to soak the falling powder, and the working platform does not need larger heat load because the powder is preheated in the powder storage hopper to be close to the use temperature, and the working platform is matched with the electromagnetic induction heating used by the powder falling hopper in height, thereby avoiding huge matched coils and shielding layers caused by the electromagnetic induction heating on a large plane.
5. The forming cylinder is heated by adopting the method of embedding the heating rod (or the resistance wire), compared with electromagnetic induction heating used by the powder falling hopper, the forming cylinder is matched with the base plate in height, the embedded heating rod is arranged on the four walls of the cylinder body, and the forming cylinder wall can open or close a corresponding heating area through the control system according to the processing stroke, so that the heating efficiency is effectively improved; in addition, a cooling system is arranged below the base plate, so that heat can be rapidly led out downwards, and a controllable temperature gradient is formed in the cylinder body.
6. The invention adopts the high-low collocation of the heating mode, the heating load is reasonably distributed, the main heating load is concentrated on the powder storage hopper, the whole system cost is reduced, the process flow is smoother, the thermal stability is better, the accurate temperature control can be realized, the unstable phenomenon caused by heating by using a single heat source and the skin effect caused by independently adopting induction heating are effectively relieved, the internal stress and cracking tendency of SLM processing workpieces caused by repeated thermal circulation are reduced, a stable basis is provided for accurately controlling the SLM processing technology, and a cooling system is introduced, so that a controllable temperature gradient can be formed in a cylinder body, and a basis is provided for forming directional solidification components and even single crystal components.
7. The atmosphere circulation device effectively utilizes the high-temperature gas flowing out of the circulation system and the heat dissipated by other heating systems to preheat the gas, thereby greatly improving the heat efficiency.
8. Compared with a common two-stage feedback closed loop, the system adopts a soaking section in a three-stage heating system as a buffer section, so that oscillation feedback caused by abrupt change of the ambient temperature or the set temperature in the two-stage feedback closed loop is avoided, and the stability of the heating system and the improvement of the heat distribution efficiency are better promoted; the cooling system adopts independent control, does not consider a feedback closed loop, and can be independently opened and closed.
Drawings
FIG. 1 is a schematic diagram of a three-stage selective laser melting combined preheating system according to the present invention;
fig. 2a and b are schematic views of a powder hopper according to the present invention, wherein a is a closed state of the powder hopper, and b is an open state of the powder hopper;
FIG. 3 is a schematic view of the structure of the table and the molding cavity according to the present invention;
FIG. 4 is a schematic view of the structure of an atmosphere preheating circulation device according to the present invention;
fig. 5 is a schematic diagram of a temperature feedback control device of a three-stage selective laser melting combined preheating system according to the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
As shown in fig. 1, the three-stage selective laser melting combined preheating system provided by the embodiment of the invention comprises a preheating stage, a soaking stage and a multi-stage heating stage, wherein the preheating stage, the soaking stage and the multi-stage heating stage are all arranged in a working cavity 9, and the preheating stage is arranged above the inside of the working cavity and is used for storing and preheating powder; the soaking section is positioned below the preheating section and is used for soaking the powder falling from the preheating section and sending the soaked powder into the multi-stage heating section, and the soaked powder is subjected to selective laser melting forming in the multi-stage heating section; the multistage heating section is positioned at the side of the soaking section and is used for heating soaked powder and heating the laser formed part in a gradient way so as to form a temperature gradient from top to bottom.
As shown in fig. 1 and 2, the preheating section comprises a first powder falling hopper 1 and a second powder falling hopper 2, wherein the first powder falling hopper 1 and the second powder falling hopper 2 are arranged above the inside of the working cavity and are symmetrically arranged left and right; the first powder falling hopper 1 and the second powder falling hopper 2 have the same structure and both comprise a powder hopper and a blanking plugboard 25 which is positioned below the powder hopper and used for opening or closing the powder hopper. Compared with the process flow using a powder feeding barrel to perform the related process flow, the powder is directly fallen from the powder feeding barrel and paved to the powder bed through the powder paving scraping plate, the working stroke of rotary falling through the powder feeding barrel is omitted, the heat loss caused by contact heat exchange between the powder and the gas in the working cavity in the process is reduced, and the heating efficiency is improved.
As shown in fig. 2, the heat insulation material layer 22, the induction coil 21 and the electromagnetic shielding layer 23 are sequentially arranged outside the powder hopper, and the powder hopper can be prevented from being ablated due to the fact that the induction coil is directly contacted with the powder hopper by the aid of the heat insulation material layer, and the induction coil heats the powder hopper through magnetic induction by means of induction heating, so that powder in the powder hopper is heated. The induction coil can be replaced by a resistance wire, a silicon molybdenum rod, a silicon carbon rod or other heating components, and the facility needs to react rapidly in the invention, so that the induction coil is adopted, and the heat insulation material layer comprises a series of refractory materials including but not limited to refractory bricks, refractory clay, alumina ceramics, alumina fibers and the like. Wherein, the induction coil surrounds the powder falling hopper and heats the powder in the powder falling hopper; the heat insulation material layer is positioned on the surface of the powder falling hopper, plays a role in heat insulation and preservation, and prevents heat loss from affecting the strength of other mechanical mechanisms; the electromagnetic shielding layer is positioned at the outermost layer of the powder falling hopper and shields the electromagnetic field generated by the induction coil to prevent the electromagnetic field from affecting other electronic equipment. The blanking insert 25 is located below the heat insulating material layer 22, the induction coil 21 and the electromagnetic shielding layer 23, and the hopper can be opened or closed by the horizontal movement of the blanking insert 25, wherein the blanking insert 25 is in the closed state in fig. 2a and the blanking insert is in the open state in fig. 2 b.
For the soaking section, the powder homogenizing section comprises a first working platform 12 and a second working platform 11 which are respectively arranged below the first powder falling hopper 1 and the second powder falling hopper 2, and heating rods are embedded in the first working platform 12 and the second working platform 11 to heat powder. The working platform with the embedded heating rod is characterized in that the embedded heat source adopts a silicon-molybdenum rod, a silicon-carbon rod and the like due to the longer service life and the convenient replacement. The heating flat plate can also be arranged below the working platform, and in the powder laying process, the heating flat plate continuously soaks the powder, so that heat loss is prevented.
As shown in fig. 1, a scraper 4 is arranged between the first powder falling hopper 1 and the first working platform 12 or between the second powder falling hopper 2 and the second working platform 11, and after powder falls out of the powder falling hopper, the powder is bidirectionally paved through the scraper, and redundant powder is fallen out; the first working platform 12 is also provided with a first powder falling port 14, the second working platform 11 is also provided with a second powder falling port 13, and through the reciprocating motion of the scraping plate 4, for example, the powder falling onto the first working platform 12 from left to right and then from right to left is firstly sent to the multi-stage heating section by the first powder falling hopper 1, and the redundant powder is sent to the second powder falling port 13, so that one-layer powder sending of the part to be laser melted and formed is completed, then the powder falling onto the second working platform 11 by the second powder falling hopper 2 is sent to the multi-stage heating section, and the redundant powder is sent to the first powder falling port 14, so that the next-layer powder sending of the part to be laser melted and formed is completed. Specifically, the scraper 4 is located below the powder falling hopper, and the blanking plugboard is opened by mechanical movement in the movement process, for example, in the movement process of the scraper, the blanking plugboard is opened by collision with the blanking plugboard, powder falls under the action of gravity, and then the scraper 4 moves reversely, so that the fallen powder is paved on the powder bed. The working platform is provided with an infrared temperature sensor for measuring the temperature of the powder, the measured signal is connected with the powder falling hopper and the forming cylinder through the control system for feedback adjustment, and the infrared temperature sensor with higher sensitivity and higher accuracy is selected because the soaking time of the powder on the platform is shorter, and the temperature information is required to be quickly and accurately fed back to the powder falling hopper and the forming cylinder with a large amount of powder aggregation for feedback control of the temperature.
For the multistage heating section, as shown in fig. 3, the multistage heating section comprises a forming cavity (i.e. a forming cylinder) and a substrate 8, wherein the forming cavity is arranged in the middle of the lower part of the inner part of the working cavity, receives powder scraped by a scraping plate and heats the powder, the substrate 8 is arranged in the forming cavity, the substrate 8 can move up and down under the action of a lifting mechanism 15, a plurality of heating rods which are sequentially arranged from top to bottom are arranged on two sides of the forming cavity, and the heating rods are selectively opened according to the position of the substrate 8 so as to realize gradient heating, namely, each heating rod in the forming cavity can be independently controlled and can be selectively opened, and meanwhile, the heating power of each heating rod can be adjusted, so that a heating area in the forming cavity can be controlled to adjust the heating area. For example, when a first layer of powder on a substrate is formed by laser, a heating rod in a forming cavity at the same height as the first layer is required to be started to heat the layer of powder to a use temperature, then the substrate is lowered by one layer thickness after laser forming, and then a second layer of powder is paved, at the moment, the heating rod in the forming cavity at the same height as the second layer is required to be started to heat the second layer of powder to the use temperature, meanwhile, the heating rod in the forming cavity at the same height as the first layer is required to be started to heat the first layer formed by laser, then the next layer is formed sequentially, the heating temperature is reduced sequentially from top to bottom, gradient heating is realized, for example, 5 layers of laser formed parts are all achieved, the last procedure is that the 5 th layer (positioned at the uppermost) is formed by laser, the 1 st layer is positioned at the lowermost, the heating rods corresponding to the 5 th layer to the 1 st layer are all started, wherein the heating temperature of the 5 th layer at the uppermost is the highest, the heating temperature of the 1 st layer at the lowermost is the lowest, and the heating temperatures of all the layers from top to bottom are 500 ℃, 400 ℃, 300 ℃, 200 ℃ and 100 ℃ in sequence, so that gradient heating is realized, a heat flow direction from top to bottom is formed, a temperature demand basis is provided for directional solidification or single crystal forming, the stability of the heat flow direction can be ensured, the heat flow disorder is avoided, and the gradient distribution of the temperature is further ensured through the arrangement of a cooling circulation device below the substrate. The laser 3 is arranged above the inside of the working cavity, and emits high-energy laser beams to sinter laid powder in the processing process, so that laser melting forming of the powder is realized, and the laser 3 is positioned right above the forming cavity, and between the first powder falling hopper and the second powder falling hopper. The inner wall of the forming cylinder can be provided with a heating flat plate, and the heating and heat preservation can be continuously carried out in the powder processing process, so that the whole temperature of the powder bed is maintained.
Specifically, the cooling circulation device is provided below the base plate 8, the cooling circulation device is a brass supporting plate 31 with a built-in cooling flow channel, a heating plate 32 is arranged between the brass supporting plate 31 and the base plate 8, and a heat insulating plate 33 is arranged below the brass supporting plate 31. Wherein the heat insulating plate and the heat insulating layer 36 are made of a series of refractory materials including, but not limited to, refractory bricks, refractory clay, alumina ceramics, alumina fibers, etc.; the cooling medium in the brass supporting plate with the built-in cooling runner is not limited to cooling water; the heat source of the heating plate can be a resistance wire, a silicon molybdenum rod, a silicon carbon rod or other heating components; the substrate is made of corresponding materials according to the processing powder.
Further, as shown in fig. 4, the working chamber is further provided with a gas preheating circulation device 5, the gas preheating circulation device 5 includes a gas heat exchanger 51 and a fan 52, the fan 52 is connected with the gas heat exchanger 51 and is used for sending inert gas into the gas heat exchanger 51, the gas heat exchanger 51 is respectively connected with an air outlet 54 and an air inlet 55 on the working chamber 9 through pipelines, the pipelines connected with the air inlet 55 of the gas heat exchanger 51 are laid along a heat equalizing section and a multi-stage heating section, the gas heat exchanger 51 collects high-temperature inert gas from the air outlet 54 on one hand and preheats the inert gas sent by the fan by utilizing the high-temperature inert gas, and on the other hand, the preheated inert gas is sent into the working chamber 9 through the air inlet 55 after being preheated again through the heat equalizing section and the multi-stage heating section. In particular, the gas preheating circulation means 5 blow in the circulating preheating gas during the process, which gas can also heat the powder in the forming cavity. Specifically, inert gas is introduced into the working cavity, a pressure relief opening is formed in the working cavity, the air pressure in the working cavity is not more than a safety set value, and the fan continuously supplements and blows the inert gas to ensure the stability of the air pressure in the working cavity; the gas outlet and the gas inlet are positioned at the front side and the rear side of the working cavity, wherein the gas inlet is provided with an electric heating wire and a temperature feedback device, and the gas is blown into the upper part of the forming cavity to play a role in removing smoke dust generated in the laser processing sintering process, and meanwhile, the preheated gas can play a role in preserving heat of powder. In addition, the gas preheating circulation device is provided with a heat insulating layer 36 along the outside of the pipeline laid in the soaking section and the multistage heating section, and heat dissipation is prevented as a whole.
The gas preheating circulation device 5 works as follows:
1) The blower blows inert gas into the gas heat exchanger, the gas heat exchanger collects high-temperature gas at the gas outlet, and the gas heat exchanger preheats the blown gas by utilizing the residual temperature, which is a section of preheating;
2) The inert gas is blown to the lower part of the forming cavity after being preheated, and the waste heat of the processing platform and the forming cavity is utilized to carry out secondary preheating on the gas;
3) Then the gas is blown to the gas inlet, heated to the using temperature under the action of the heating wire and the temperature feedback device, and blown into the working cavity;
4) The gas in the working cavity flows back to the gas heat exchanger through the gas outlet, completes heat exchange with the newly blown gas, and completes circulation under the action of the fan.
As shown in fig. 5, the combined preheating system further includes a temperature feedback control device, where the temperature feedback control device includes a central PID control module 41, a preheating section control module 42, a soaking section control module 43, a multi-stage heating section control module 44, and an atmosphere preheating control module 45, where the central PID control module 41 distributes heating tasks of each module according to the set heating temperature, and the preheating section control module 42 controls the preheating section to heat the powder to the preheating temperature according to the distributed tasks, specifically, adopts induction heating to rapidly heat the powder to the preheating temperature; the soaking section control module 43 controls the soaking section to heat powder according to the distributed tasks, and plays a role in heat buffering while carrying out soaking work, so that larger heat shock is avoided; the multi-stage heating stage control module 44 controls the multi-stage heating stage to heat the powder to the use temperature according to the distributed tasks and feeds back to the central PID control module 41 to complete the closed loop. The combined preheating system of the invention is also provided with a supporting frame 16, and the supporting frame 16 is positioned below the whole combined preheating system and plays a role of bearing and stabilizing equipment. In the heating process, due to the existence of the soaking section control module, when the required preheating temperature changes or other reasons cause the powder temperature to change, the preheating section control module 2 rapidly reacts to adjust the heating power, and then the soaking section control module plays a role in heat buffering, so that the phenomenon that the power of the multi-stage heating section control module fluctuates greatly due to overlarge temperature change is avoided, and a heating system is stabilized. When the multi-stage heating section control module is started up, corresponding heat input can be adjusted, the heat flow direction is ensured, the temperature gradient is stabilized, and the central PID control module can automatically adjust and start or close corresponding heating areas of the multi-stage heating section according to the movement distance of the substrate in the multi-stage heating section control module, so that redundant heat loss is avoided. In addition, the atmosphere preheating control module 45 also can automatically adjust the heating power according to the feedback information received by the central PID control module and the magnitude of the air inflow, so as to ensure the thermal stability in the working cavity.
The three-section type selective laser melting combined preheating system specifically comprises the following working processes:
(1) The whole Selective Laser Melting (SLM) system is provided with a pressure working cavity, and inert gas is filled in the working cavity for protection after the processing is started;
(2) Firstly, adding required metal powder into a first powder falling hopper and a second powder falling hopper, closing a working cavity to be in a closed state, starting a gas preheating circulation device to replace inert gas, starting a powder combination preheating system of a three-section type selective laser melting SLM, performing heat load distribution on the three-section type preheating system by a PID system according to real-time temperature feedback, and determining whether to start a cooling system according to process parameters; after preheating is finished, the scraping plate moves towards the first powder falling hopper, the inserting plate on the first powder falling hopper is opened, powder required by one-time processing is fallen, then the scraping plate moves towards the second powder falling hopper in the reverse direction, in the process, the scraping plate lays the powder on the substrate, after that, the laser emits laser beams to sinter the laid powder according to a preset pattern layer, and after sintering is finished, the substrate descends by a preset height under the drive of the transmission shaft, so that a first stroke is finished; then the scraping plate moves towards the second powder falling hopper, the plugboard on the second powder falling hopper is opened, the powder required by one-time processing is fallen, the scraping plate moves towards the first powder falling hopper again and lays powder on the substrate, after that, the laser emits laser beams to sinter the laid powder according to a preset pattern layer, and after sintering is completed, the substrate descends by a preset height under the drive of the transmission shaft, and a second stroke is completed; and then, alternately carrying out the first stroke and the second stroke until the part is finally formed, and collecting the excessive metal powder through the first powder falling port and the second powder falling port.
In the step (2), the processed powder is preheated by electromagnetic induction through a three-section preheating system, namely a first powder falling hopper and a second powder falling hopper, soaked on a working platform embedded with a heating rod after falling, then enters a forming cylinder body (namely a forming cavity) for heating, and finally sintered and formed. The wall surface of the molding cylinder is provided with multi-stage heating, and corresponding heating areas are opened along with the descending of the substrate, so that the heat utilization rate is maximized. In the whole process of the step (2), powder is subjected to preheating, soaking and heating three-section heating, and meanwhile, preheating gas is used for assisting in heat preservation, so that the heating efficiency of the powder is always maintained at a higher level, high-temperature powder preheating can be realized, and whether a cooling system below a substrate is started or not depends on the requirements of process parameters.
In the process of the step (2), the first powder falling hopper, the second powder falling hopper, the working platform and the forming cylinder body are all provided with temperature measuring devices (including but not limited to thermocouples, infrared thermometers and the like) and are subjected to feedback adjustment through a PID control system. The electromagnetic heating system of the first powder falling hopper and the second powder falling hopper can quickly change heat input according to feedback data of other thermocouples and infrared thermometers, and balance the heating system, so that temperature change caused by processing parameter change can be effectively balanced; the working platform receives rapid reaction of the first powder falling hopper and the second powder falling hopper, is used as a buffer port, stabilizes self heat input and transmits information to the forming cylinder body; the molding cylinder body not only adjusts multistage heating according to the internal lifting height, but also receives thermal information of the working platform, finally heats powder to the working temperature, and transmits information to the PID control system when the heating power changes greatly, so that feedback closed loop is completed. Compared with a common secondary feedback closed loop, the system uses the soaking section (i.e. the working platform) in the three-section heating system as a buffer section, so that oscillation feedback easily occurring in the secondary feedback closed loop is avoided, and the stability and heat distribution of the heating system are better promoted. In addition, the under-substrate cooling system is independent of the feedback closed loop and is selectively turned on according to the requirements of the process parameters.
The system is except for the three-stage preheating system, the gas preheating circulation device adopts bipolar preheating, namely, the inlet gas is preheated by the high-temperature gas at the circulation outlet of the three-stage preheating device, then the inlet gas is heated to the using temperature by the heating system and then blown into the gas circulation device by the fan, so that heat dissipation is effectively utilized, and the heat efficiency is improved.
The invention can form a top-down temperature gradient in the forming cylinder aiming at special temperature gradient requirements in the forming process of directional solidification superalloy and monocrystal superalloy, and simultaneously leads out heat through the cooling circulation device at the bottom of the base plate in the cylinder body, ensures the direction of heat flow to be stable from top to bottom, and avoids the phenomenon of heat flow disorder in the forming cylinder caused by waste heat brought by laser beams in laser processing. Because the temperature gradient is distributed in the forming cylinder, if the heating load is distributed in the forming cylinder, the huge load in the heating process is very unfavorable for temperature control, and the three-stage heating system of the invention transfers the main heating load into the powder falling hopper, and the temperature of powder is uniform through the heating platform, so that the temperature gradient control in the forming cylinder is finally realized.
The whole SLM process is characterized in that the time of one stroke is short, so that falling powder is required to be quickly heated to the use temperature in a short time, in order to reduce uneven temperature distribution in the heating process and stabilize a heating system, the three-stage heating system achieves the aim through a powder falling hopper (preheating section), a heating platform (soaking section) and a forming cylinder (heating section), wherein the powder hopper is used as a powder storage device and can store the powder for a long time before processing, the main heating load is occupied, the electromagnetic induction can generate skin effect to cause uneven temperature distribution due to the adoption of electromagnetic heating of the powder hopper, the uniform temperature can be achieved through the uniform heating section of the heating platform, and finally the powder is heated to the use temperature through the heating section of the forming cylinder. The distribution of heating load can effectively reduce the huge load of single-section heating, and reduce the oscillation of the whole heating system from the temperature distribution (such as single-section or double-section heating is adopted singly, huge temperature concentration is necessarily brought in a process window of the SLM, the equipment structure and uniform temperature are both unfavorable, the powder heat conductivity coefficient is not high, the advantage of uniform temperature can be reflected by adopting multi-section heating, and when the temperature gradient or the process requirement temperature is changed, the increased heating load can be rapidly and uniformly distributed, and the system is stabilized); meanwhile, on the control system, a three-section heating design is adopted, the middle soaking section receives the input and output of the front end and the rear end, oscillation feedback in the control system can be effectively reduced, oscillation of the whole heating system can be reduced from the control system, if single-section heating is adopted, the use temperature is 200 ℃, the initial maximum lifting power of the system is assumed, the actual power is far higher than the stable power at 200 ℃, the use temperature is far higher than 200 ℃ in a short period, the power is greatly reduced by the system, the temperature is lower than 200 ℃ in a short period, the whole use temperature can oscillate greatly in a section near 200 ℃, the temperature is finally stabilized to 200 ℃, and when the three-section heating is adopted, the temperature can steadily rise according to 100-150-200 ℃, finally oscillate in a small section near 200 ℃, and finally stabilize to 200 ℃, so that the three-section heating system can be more accurately controlled.
The invention combines the unique processing technology of the SLM technology, combines the characteristics of each flow of the powder in the working cavity, adopts a brand new mode to heat the powder in sections, reduces the burden of a single heat source, reduces a great amount of heat loss in the processing process, ensures the temperature of the powder through the balanced heating process, and can realize higher powder preheating temperature; and meanwhile, the control of the temperature gradient in the cylinder body can be realized. According to the invention, the soaking section in the three-section heating system is used as the buffer section, so that oscillation feedback caused by abrupt change of the ambient temperature or the set temperature in the secondary feedback closed loop is avoided, the stability of the heating system and the improvement of the heat distribution efficiency are better promoted, the intelligent and modularized of the SLM heating system are truly realized, and the accurate high-temperature powder preheating can be realized.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (8)
1. The three-section type selective laser melting combined preheating system is characterized by comprising a preheating section, a soaking section and a multi-stage heating section which are all arranged in a working cavity (9), wherein the preheating section is arranged above the inside of the working cavity and is used for storing and preheating powder; the soaking section is positioned below the preheating section and is used for soaking the powder falling from the preheating section, and sending the soaked powder into the multi-stage heating section, wherein the soaked powder is subjected to selective laser melting forming in the multi-stage heating section; the multi-stage heating section is positioned at the side of the soaking section and is used for heating soaked powder and heating the laser formed part in a gradient manner so as to form a temperature gradient from top to bottom;
the multistage heating section comprises a forming cavity and a substrate (8), the forming cavity is arranged in the middle of the lower part of the inside of the working cavity, the substrate (8) is arranged in the forming cavity and moves up and down under the action of a lifting mechanism (15), a plurality of heating rods which are sequentially arranged from top to bottom are arranged on two sides of the forming cavity, and the heating rods are selectively opened according to the position of the substrate (8) so as to realize gradient heating;
the multi-stage heating section is internally provided with a cooling circulation device so as to ensure the stability of the heat flow direction and avoid heat flow disturbance, the cooling circulation device is arranged below the base plate (8), the cooling circulation device is a brass supporting plate (31) with a built-in cooling flow channel, a heating plate (32) is arranged between the brass supporting plate (31) and the base plate (8), and a heat insulation plate (33) is arranged below the brass supporting plate (31);
a laser (3) is arranged above the inside of the working cavity to provide a laser beam to achieve laser melt forming of the powder, the laser (3) being located directly above the forming cavity.
2. The three-section type selective laser melting combined preheating system according to claim 1, wherein the preheating section comprises a first powder falling hopper (1) and a second powder falling hopper (2), and the first powder falling hopper (1) and the second powder falling hopper (2) are arranged above the inside of the working cavity and are symmetrically arranged on the left and right; the first powder falling hopper (1) and the second powder falling hopper (2) are identical in structure and comprise powder hoppers and a blanking plugboard (25) which is positioned below the powder hoppers and used for opening or closing the powder hoppers.
3. The three-section type selective laser melting combined preheating system according to claim 2, wherein a heat insulation material layer (22), an induction coil (21) and an electromagnetic shielding layer (23) are sequentially arranged outside the powder hopper.
4. The three-section type selective laser melting combined preheating system according to claim 1, wherein the soaking section comprises a first working platform (12) and a second working platform (11) which are respectively arranged below the first powder falling hopper (1) and the second powder falling hopper (2), and heating rods are embedded in the first working platform (12) and the second working platform (11) so as to heat powder.
5. The three-section type selective laser melting combined preheating system according to claim 4, wherein a scraper (4) is arranged between the first powder falling hopper (1) and the first working platform (12) or between the second powder falling hopper (2) and the second working platform (11).
6. The three-stage selective laser melting combined preheating system as claimed in claim 5, wherein a first powder falling port (14) is further arranged on the first working platform (12), a second powder falling port (13) is further arranged on the second working platform (11), powder falling onto the first working platform (12) from the first powder falling hopper (1) is firstly sent to the multi-stage heating stage through the back-and-forth motion of the scraping plate (4), and excessive powder is sent to the second powder falling port (13) so as to finish one-layer powder sending of the part to be laser melted and formed, and then the powder falling onto the second working platform (11) from the second powder falling hopper (2) is sent to the multi-stage heating stage, and excessive powder is sent to the first powder falling port (14) so as to finish the next-layer powder sending of the part to be laser melted and formed.
7. The three-stage selective laser melting combined preheating system as claimed in claim 1, wherein a gas preheating circulation device (5) is further arranged in the working chamber, the gas preheating circulation device (5) comprises a gas heat exchanger (51) and a fan (52), the fan (52) is connected with the gas heat exchanger (51) and is used for sending inert gas into the gas heat exchanger (51), the gas heat exchanger (51) is respectively connected with an air outlet (54) and an air inlet (55) on the working chamber (9) through pipelines, the pipelines connected with the gas heat exchanger (51) and the air inlet (55) are laid along the heat equalizing section and the multi-stage heating section, the gas heat exchanger (51) collects high-temperature inert gas from the air outlet (54) and utilizes the high-temperature inert gas to preheat the inert gas sent by the fan, and the preheated inert gas is sent into the working chamber (9) through the air inlet (55) after being preheated again through the heat equalizing section and the multi-stage heating section.
8. The three-stage selective laser melting combined preheating system as claimed in any one of claims 1 to 7, further comprising a temperature feedback control device including a central PID control module (41), a preheating stage control module (42), a soaking stage control module (43), a multi-stage heating stage control module (44) and an atmosphere preheating control module (45), wherein the central PID control module (41) distributes heating tasks of the respective modules according to the set heating temperatures, the preheating stage control module (42) controls the preheating stage to heat the powder to the preheating temperature according to the distributed tasks, the soaking stage control module (43) controls the soaking stage to heat the powder according to the distributed tasks, and the multi-stage heating stage control module (44) controls the multi-stage heating stage to heat the powder to the use temperatures according to the distributed tasks.
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| CN206286554U (en) * | 2016-11-04 | 2017-06-30 | 上海航天精密机械研究所 | A kind of selective laser fusing former automatic charging device for preheating |
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