A method for increasing the density of metal materials in additive manufacturing

Abstract

The present invention is a method for increasing the density of metal materials in additive manufacturing, including: screening out metal powders whose performance parameters meet the printing requirements; establishing a model of a part to be formed, performing slicing processing and path planning; determining a first range of the molten pool depth according to the relationship between the cut layer thickness, the actual powder spreading thickness and the molten pool depth; determining a second range of the molten pool depth according to the calculation formula between the overlap rate and the molten pool depth; taking the intersection of the two ranges of the molten pool depth to obtain a third range, that is, the final molten pool depth range; determining the scanning melting process parameter range from the third range; and performing selective melting forming. The present invention matches the control of powder performance with process parameters, determines the molten pool depth range according to the relationship between the cut layer thickness, the actual powder spreading thickness and the overlap rate and the molten pool depth, and then determines the scanning melting process parameters according to the molten pool depth range, thereby obtaining an additively manufactured metal component with a dense interior and a fine and uniform microstructure.

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of additive manufacturing technology, and in particular to a method for improving the density of metal materials produced by additive manufacturing. Background Art

[0002] The emergence of additive manufacturing technology has greatly expanded the freedom of metal material design and processing parts, and improved the ability to directly and quickly form complex structural metal components. The most widely used additive manufacturing technology is powder bed fusion technology, which mainly includes electron beam selective melting and laser selective melting.

[0003] The performance of additively manufactured components is closely related to the forming quality and microstructure, and is mainly affected by the raw material powder, printing parameters, equipment performance, etc. Any inappropriate parameter setting may lead to defects inside the formed part, and the existence of defects is not conducive to the mechanical properties of additively manufactured parts. First, the powder state, set layer thickness and actual layer thickness deviation will affect the number and volume fraction of unfused defects in the specimen. Secondly, in the process parameters, insufficient energy leads to insufficient interlayer bonding and produces unmelted defects. This defect mainly appears between layers or between adjacent scanning lines and has sharp edges. Large unmelted defects will contain unmelted or partially melted powder particles, while excessive energy will lead to highly unstable molten pools, causing serious spatter and instability in the forming process, resulting in pores in the molten pool. Surface tension dominates in liquid metal, and the final pores are spherical or elliptical. Higher porosity will lead to reduced or dispersed yield strength and elongation of the component, and will also promote the growth of fatigue cracks and reduce fatigue performance. Finally, the performance parameters of the equipment, such as spot size and voltage, are also important factors that determine the excellent and stable performance of the formed parts.

[0004] In the related technology, hot isostatic pressing process is used to control defects of formed parts and improve density. Although hot isostatic pressing can eliminate most internal defects, it will also cause new problems. For example, recrystallized grains in the holes will lead to reduced creep performance, and post-processing has the disadvantages of multiple processes, long cycle and high cost.

[0005] Therefore, it is necessary to improve one or more problems existing in the above-mentioned related technical solutions.

[0006] It should be noted that this section is intended to provide background or context for the technical solutions of the present invention stated in the claims. The description herein is not admitted to be prior art by inclusion in this section. Summary of the invention

[0007] The object of the present invention is to provide a method for increasing the density of metal materials in additive manufacturing, thereby solving one or more problems caused by the limitations and defects of the relevant technology at least to a certain extent.

[0008] The present invention provides a method for increasing the density of a metal material by additive manufacturing, comprising:

[0009] Detecting the performance parameters of the metal powder and selecting the metal powder whose performance parameters meet the printing requirements, wherein the performance parameters include: sphericity, hollow powder rate and bulk density;

[0010] Establish the model of the part to be formed, perform slicing and path planning, and determine the slice thickness and actual powder laying thickness;

[0011] According to the relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth, the first range of the molten pool depth is determined; wherein the relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth is as follows:

[0012]

[0013] In the formula, H is the depth of the molten pool, T is the actual powder thickness, and t is the cut layer thickness;

[0014] According to the calculation formula between the overlap rate and the molten pool depth, the second range of the molten pool depth is determined; wherein the calculation formula between the overlap rate and the molten pool depth is as follows:

[0015] H=T / (1-δ)

[0016] Where, δ is the overlap ratio;

[0017] Taking the intersection of the first range and the second range of the molten pool depth to obtain a third range, wherein the third range is a final molten pool depth range;

[0018] According to the calculation formula between the molten pool depth and the scanning melting process parameter, the scanning melting process parameter is determined from the third range; wherein the calculation formula between the molten pool depth and the scanning melting process parameter is as follows:

[0019]

[0020] Where L is the distance that heat is conducted from the high temperature end to the low temperature end, ε is the absorption efficiency of the energy source, P is the power of the energy source, K is the thermal conductivity of the metal powder, A is the heat conduction area from the metal powder to the surrounding area after melting, and T m is the melting point of the metal powder, T p is the powder bed temperature before the metal powder melts, ρ is the theoretical density of the metal powder, v is the scanning melting speed, D is the energy source spot diameter, C is the specific heat capacity of the metal powder, is the melting latent heat of the metal powder;

[0021] The scanning melting process parameters are used to perform selective melting forming.

[0022] In the present invention, the performance parameter range of the metal powder that meets the printing requirements is as follows:

[0023] The sphericity rate is ≥90%, the hollow powder rate is ≤1%, and the bulk density is greater than 55% of the theoretical density.

[0024] In the present invention, the melting distance d in the path planning is 0.05-0.2 mm, and the slice thickness t is 0.03 mm-0.3 mm.

[0025] In the present invention, the calculation formula for calculating the actual powder spreading thickness T according to the slice thickness is as follows:

[0026] T=t(1-k) / k+t

[0027] In the formula, k=ρ0 / ρ, ρ0 is the loose density of metal powder.

[0028] In the present invention, the overlap rate δ is any value between 40% and 75%.

[0029] In the present invention, the step of performing powder spreading and melting forming according to the process parameters corresponding to the calculated molten pool depth and overlap rate comprises:

[0030] The number of remelting layers n' during powder printing is calculated according to the following formula:

[0031]

[0032] Remelting layer number n'= , the number of remelting layers is selected from the range determined by the above formula.

[0033] In the present invention, the particle size distribution of the metal powder is as follows: the volume proportion of metal powder with a particle size of >45 μm and ≤150 μm is ≥90%, the volume proportion of metal powder with a particle size of ≤45 μm is ≤5%, and the volume proportion of metal powder with a particle size of >150 μm is ≤5%.

[0034] In the present invention, the metal powder includes at least one of titanium, aluminum, copper, tungsten, molybdenum, tantalum and niobium.

[0035] In the present invention, the method further comprises:

[0036] The density, melt pool depth and microstructure of the printed parts were tested.

[0037] In the present invention, the energy source is any one of laser, electron beam, plasma arc and electric arc.

[0038] The technical solution provided by the present invention includes the following beneficial effects:

[0039] In the present invention, from the perspective of raw materials and forming process, the control of powder properties is matched with process parameters, and the molten pool depth range is determined according to the relationship between the cut layer thickness, actual powder laying thickness and overlap rate and the molten pool depth. Then, the scanning melting process parameters are determined according to the molten pool depth range, and printing is performed using the scanning melting process parameters, thereby achieving the purpose of controlling the molten pool size and effective overlap of tiny molten pools, and obtaining additively manufactured metal components with dense interior and fine and uniform microstructure. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The accompanying drawings herein are incorporated into the specification and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the specification are used to explain the principles of the present disclosure. Obviously, the accompanying drawings described below are only some embodiments of the present disclosure, and for ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without creative work.

[0041] Figure 1 A schematic flow chart showing a method for increasing the density of metal materials by additive manufacturing in an exemplary embodiment of the present invention;

[0042] Figure 2 A schematic diagram showing the mechanism of the melting process in an exemplary embodiment of the present invention is shown;

[0043] Figure 3 A longitudinal schematic diagram showing different molten pool shapes in an exemplary embodiment of the present invention;

[0044] Figure 4 The figure shows the internal quality and microstructure of the formed sample in an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0045] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that the present invention will be more comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0046] In addition, the accompanying drawings are only schematic illustrations of embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings represent the same or similar parts, and their repeated descriptions will be omitted. Some of the block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0047] This example embodiment provides a method for increasing the density of metal materials in additive manufacturing, referring to Figure 1 As shown, the method comprises the following steps:

[0048] Step S101: detecting the performance parameters of metal powders, and selecting metal powders whose performance parameters meet the printing requirements, wherein the performance parameters include: sphericity, hollow powder rate and bulk density.

[0049] Step S102: Establish a model of the part to be formed, perform slicing and path planning, and determine the slice thickness and actual powder coating thickness.

[0050] Step S103: determining a first range of the molten pool depth according to a relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth.

[0051] The relationship between the slice thickness, the actual powder thickness and the molten pool depth is as follows:

[0052]

[0053] Where H is the molten pool depth, T is the actual powder thickness, and t is the cut layer thickness.

[0054] Step S104: determining a second range of the molten pool depth according to a calculation formula between the overlap ratio and the molten pool depth.

[0055] The calculation formula between the overlap rate and the molten pool depth is as follows:

[0056] H=T / (1-δ)

[0057] Where δ is the overlap ratio.

[0058] Step S105: taking the intersection of the first range and the second range of the molten pool depth to obtain a third range, wherein the third range is the final molten pool depth range.

[0059] Step S106: according to the calculation formula between the molten pool depth and the scanning melting process parameter, the scanning melting process parameter range is determined from the third range.

[0060] The calculation formula between the molten pool depth and the scanning melting process parameters is as follows:

[0061]

[0062] Where L is the distance that heat is conducted from the high temperature end to the low temperature end, ε is the absorption efficiency of the energy source, P is the power of the energy source, K is the thermal conductivity of the metal powder, A is the heat conduction area from the metal powder to the surrounding area after melting, and T m is the melting point of the metal powder, T p is the powder bed temperature before the metal powder melts, ρ is the theoretical density of the metal powder, v is the scanning melting speed, D is the energy source spot diameter, C is the specific heat capacity of the metal powder, is the melting latent heat of the metal powder;

[0063] Step S107: using the scanning melting process parameters to perform selective melting and forming.

[0064] In this embodiment, from the perspective of raw materials and forming process, the control of powder properties is matched with process parameters. The molten pool depth range is determined according to the relationship between the cut layer thickness, actual powder laying thickness and overlap rate and the molten pool depth. Then, the scanning melting process parameters are determined according to the molten pool depth range. Printing is performed using the scanning melting process parameters, thereby achieving the purpose of controlling the size of the molten pool and realizing effective overlap of tiny molten pools, thereby obtaining additively manufactured metal components with dense interior and fine and uniform microstructure.

[0065] The specific steps in the method of the present application are described in detail below.

[0066] In S101, the sphericity rate, hollow powder rate and bulk density of the metal powder are tested. In addition, the particle size distribution of the powder can also be tested to screen the metal powder that meets the requirements. The metal powder can be titanium and titanium alloy powder, copper and copper alloy powder, tungsten, molybdenum, tantalum, niobium metal or other alloy powder, intermetallic compound powder, etc., but is not limited thereto.

[0067] The metal powder has a sphericity of ≥90%, a hollow powder rate of ≤1%, good fluidity, and a bulk density ρ0 greater than 55% of the theoretical density ρ of the alloy. The powder particle size should have a coarse-fine ratio of different particle sizes. By controlling the properties of the powder, the density can be improved during the forming process. A higher sphericity, bulk density, good fluidity, and a suitable particle size distribution are conducive to powder spreading, obtaining a higher powder bed packing density, and reducing defect formation during the forming process.

[0068] In S102, a model of the part to be formed is established using 3D modeling software; slicing software is used to perform slicing along the z-axis direction according to a certain layer thickness, and path planning is performed to determine the layer thickness and the actual powder laying thickness, so as to obtain layer-by-layer 2D cross-sectional data of the 3D part and import it into the forming equipment control unit.

[0069] Specifically, the slice thickness t obtained after slicing is 0.03 mm to 0.3 mm, and the melting distance d in path planning is 0.05 to 0.2 mm.

[0070] In S103, a first range of the molten pool depth is determined according to a relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth.

[0071] First, the actual powder layer thickness is calculated based on the slice thickness. The actual powder layer thickness is T=t(1-k) / k+t, where k=ρ0 / ρ, and ρ is the theoretical density of the alloy. The powder shrinks in volume during the melting process, and the layer thickness after solidification is lower than the powder layer thickness before melting, resulting in the actual powder layer thickness of the next layer being greater than the set powder layer thickness.

[0072] Then, the first range of the molten pool depth is calculated based on the slice thickness and the actual powder laying thickness.

[0073] In S104, a second range of the molten pool depth is determined according to a calculation formula between the overlap rate and the molten pool depth. Generally, the overlap rate δ is in the range of 40-75%, and a dense formed part can be easily obtained within this overlap rate range. The second range of the molten pool depth can be deduced based on the overlap rate range.

[0074] In S105, the intersection of the first range and the second range calculated in steps S103 and S104 is taken to obtain a third range, ie, a final molten pool depth range, which helps to improve density.

[0075] In S106, the scanning melting process parameter is determined from the third range according to a calculation formula between the molten pool depth and the scanning melting process parameter.

[0076] It should be understood that the energy input by the light source when the metal powder is melted includes the energy that has been radiated during the emission process of the energy source and the energy that enters the object from the surrounding radiation. The stored energy is the energy absorbed by the material from heating. The latent heat is the heat generated when the material melts after absorbing heat during the process of changing from solid to liquid, resulting in an increase in heat, and the heat will be conducted to the surrounding area with lower temperature. Therefore, the energy transfer process has the following formula:

[0077]

[0078] In the formula, Q 输入 is the energy input by the energy source, that is, the energy required for powder melting; L is the distance that heat is conducted from the high temperature end to the low temperature end (mm), L=(0.8~2) T; ε is the absorption efficiency of the energy source (%), P is the power of the energy source (W), K is the thermal conductivity of the metal powder (W / m•K), and A is the heat conduction area from the metal powder to the surrounding area after melting (mm 2 ), A=πvD / 4; T m is the melting point of the metal powder (°C), T p is the powder bed temperature before the metal powder melts (°C), ρ is the theoretical density of the metal powder (g / cm 3 ), v is the scanning melting speed (mm / s), D is the energy source spot diameter (mm), C is the specific heat capacity of the metal powder (J / kg•K), is the latent heat of fusion of metal powder.

[0079] The above energy transfer formula can be used to derive the calculation formula between the molten pool depth and the scanning melting process parameters. The scanning melting process parameters include: P, T p , v, D, etc.

[0080] It can be seen that the molten pool depth is mainly related to the energy source power, scanning melting speed, energy source spot diameter and actual powder thickness parameters during melting. The molten pool depth increases with the increase of melting power, and increases with the decrease of scanning melting speed and energy source spot diameter. The actual powder thickness has little effect on the molten pool depth.

[0081] Generally, the appropriate overlap rate plays an important role in improving the density of the formed part and regulating the microstructure. A too small overlap rate makes the layers only contact or have pores, and the powder is subjected to the heat-affected zone (the heat-affected zone refers to the area outside the melting zone affected by the heat of the melting zone) to form a micro-sintering zone, and the interior is mostly unmelted powder particles. An excessively large overlap rate means that the overlap rate of the same melting position is high, which will lead to excessive concentration of heat. On the one hand, the surface of the formed part is prone to bulging, and thermal deformation is serious, which is not conducive to the powder spreading and forming quality of the next layer. Defects will appear inside and will be further magnified as the melting continues. In severe cases, it cannot be formed; on the other hand, if there are volatile elements in the alloy, some lightweight elements will be burned at high temperatures, and there will be problems of uneven organization and composition that do not meet the requirements inside the formed part; in addition, too high an overlap rate will also lead to reduced forming efficiency.

[0082] Please refer to Figure 2 , the number of remelting layers is determined by the following formula:

[0083]

[0084] Remelting layer number n'= .

[0085] The depth of the molten pool determines the number of remelting layers of the solidified layer. To ensure that the adjacent layers are tightly connected, an appropriate number of remelting layers should be selected. Too few layers will reduce the overlap rate, limit the bonding between layers, and produce unfused defects. More melting layers will increase the disturbance of the molten pool, increase spatter, and form pores at the bottom. A deeper molten pool causes the solidified metal to remelt, which in turn affects its microstructure.

[0086] The more times of remelting, the smaller and more uniform the grain size. Under the disturbance of the molten pool, it is equivalent to breaking the solidified grains, increasing the number of crystal nuclei and reducing the size. The number of remelting is the number of times the solidified layer is reheated by the molten pool. Usually, the number of remelting m is an integer in the range of 1≤m≤3.

[0087] like Figure 3 As shown in the figure, n represents the number of layers of the current melt layer, and W represents the width of the melt pool. The melt pool sizes of different process parameters are different. The wide and shallow melt pool (S1) is smaller in size, with less overlap between layers, resulting in pores and incompletely melted areas in the parts, and it is impossible to obtain dense parts; the deep and narrow melt pool (S3) produces more liquid metal, and the surface tension forms a gas cavity at the bottom. After the energy source leaves, the liquid metal collapses under the action of gravity and surface tension, forming a hole defect at the bottom; the appropriate melt pool (S2) size allows good bonding between the scanned layers, and the density of the formed part is higher.

[0088] By calculating the molten pool depth and overlap rate under different parameters, the number of remelting layers and the number of remelting times are determined. When the limited requirements of overlap rate and remelting are met, the process parameter range for densification forming is obtained.

[0089] In addition, the vacuum degree during printing should be ≤2×10 -2 Pa, the vacuum degree after backfilling with helium should be (1.0~3.0)×10 -1 Pa; the size of the substrate used for printing can be 80~250mm, and the material can be stainless steel, pure molybdenum, etc.; the energy source can be laser, electron beam, plasma arc, electric arc, etc.

[0090] The densification process parameters are preliminarily determined parameters, such as current, scanning melting speed, etc., and the powder is spread layer by layer and melted under the action of the energy source until the entire part is printed. The size of the molten pool is controlled by matching the process parameters to ensure effective overlap between adjacent layers and the number of remelting between layers, thereby achieving density improvement and microstructure regulation.

[0091] It is important to note that, in the process of determining the scanning melting process parameters by the third range using the calculation formula between the molten pool depth and the scanning melting process parameters, multiple sets of process parameter combinations can be listed, and the corresponding molten pool depth can be calculated using the multiple sets of process parameters. The calculated molten pool depth can be compared with the previous intersection, and the combination of process parameters corresponding to the molten pool depth that falls into the intersection can be used as the scanning melting process parameters used for the final scanning print.

[0092] In S107, the scanning melting process parameters obtained in S106 are used to perform powder printing. The density, molten pool depth and microstructure of the printed parts are tested, and the density of the obtained printed parts is 99.50-99.99%.

[0093] Specifically, the density of the formed part can be tested by the drainage method, and the density is the ratio of the alloy test density to the theoretical density. Under a certain forming process, the metal powder melts into liquid metal and flows to fill the residual pores, and forms a high-density solid part after solidification.

[0094] Actual measurement of the molten pool depth: The molten pool depth can be determined by metallographic method. The longitudinal section of the formed part is polished and corroded. Affected by the cyclic heating and remelting times, the bottom microstructure undergoes phase transformation, and the last layer of melted structure is not subjected to subsequent cyclic heat treatment and directly enters the cooling stage. The microstructure shows the original structure characteristics after melting. Therefore, the surface molten pool depth is measured. After measuring at least 3 molten pool depths, the average value is taken as the actual molten pool depth H' of the formed part. The error between the theoretically calculated molten pool depth H and the actually measured molten pool depth H' should be less than 10%.

[0095] The microstructure of the formed part is fine and uniform. The microstructure is disturbed by the molten pool during multiple remelting and subjected to long-term high temperature. The microstructure is broken into fine crystal nuclei and forms a uniform fine-grained structure after cooling. Through the verification of density and molten pool depth, the densification forming process window of metal materials is finally obtained.

[0096] Based on the above implementation steps, the steps and effects of this application are explained using a specific embodiment below.

[0097] Step 1: Select TiAl alloy metal powder with a spherical rate of 95%, a hollow powder rate of 0.2%, and a powder bulk density of 2.35g / cm 3 . Generally, the particle size distribution of metal powder can be: the volume proportion of metal powder with a particle size of >45μm and ≤150μm is ≥90%, the volume proportion of metal powder with a particle size of ≤45μm is ≤5%, and the volume proportion of metal powder with a particle size of >150μm is ≤5%. In this embodiment, the different particle sizes are <45μm, 45~75μm, 75~90μm, 90~106μm, 106~120μm, 120~150μm, >150μm, and the corresponding volume percentages are 2%, 11%, 13%, 14%, 36%, 23%, and 1%, respectively.

[0098] Step 2: Set the slice thickness t to 0.09 mm and the melting distance d = 0.1 mm.

[0099] Step 3: Calculate the actual powder thickness T = 153 μm according to the formula T = t (1-k) / k + t, where k = ρ0 / ρ, ρ = 4 g / cm 3 .

[0100] According to the relationship between the slice thickness, the actual powder thickness and the molten pool depth, the first range of the molten pool depth is determined, and the results are as follows:

[0101] 261μm≤H≤441μm

[0102] According to the calculation formula between the overlap rate (40-75%) and the molten pool depth, the second range of the molten pool depth is determined as follows:

[0103] 255μm≤H≤612μm

[0104] According to the calculation formula between the molten pool depth and the scanning melting process parameters, the scanning melting process parameters can be determined as follows:

[0105]

[0106] When the melting current I is 16.5 mA and the scanning melting speed v is 3.5 m / s, H=281 μm is calculated, which meets the intersection interval of the molten pool depth in the first range and the second range. Therefore, this parameter can be determined as the scanning melting process parameter.

[0107] in, The value is 0.9, the melting current I is 16.5mA, and the scanning melting speed v is 3.5m / s. =1460℃, =1150℃, =T=153μm, D=0.2mm, the others are material properties, substituting the above values ​​into the formula, we get H=281μm. P=UI, P is the energy source power, and U is the energy source voltage value.

[0108] I and v in the above calculation process are the process parameters to be used during printing.

[0109] Through calculation, it is determined that the number of remelting layers is 1.5 (the number of remelting layers can be a non-integer) and the number of remelting times is 1, which meets the requirements.

[0110] Step 4: Load TiAl alloy metal powder into the forming equipment, the substrate size is 175×175×15mm 3 , wait until the vacuum degree reaches 1.9×10 -2 Pa, fill with helium until the vacuum degree is 1.45×10 -1 Pa, turn on the high voltage, set the determined process parameters, the melting current I is 16.5mA, the scanning melting speed v is 3.5m / s, preheat the substrate to 1100℃ and start printing, use the electron beam to selectively melt the TiAl alloy metal powder spread on the substrate, the forming substrate descends, and then the powder is spread, preheated, and melted. Repeat this process until the entire part is melted and formed.

[0111] The density of the sample was tested by drainage method, and the density value was 3.99g / cm 3, the density is 99.75%, and the theoretical density is 4.0g / cm 3 .

[0112] The internal forming quality and microstructure of the specimens were analyzed, such as Figure 4 As shown, the internal forming quality is good, there are no unfused holes, and the overlap is good. The microstructure shows a fine dual-state structure composed of equiaxed γ and α2 / γ lamellae. The deeper molten pool causes the solidified metal to remelt, increases the wettability of the material at the bottom of the molten pool, inhibits the shrinkage effect of the metal powder during melting and solidification, ensures good metallurgical bonding between layers, and improves the density of the alloy. At the same time, it changes the local thermal gradient and solidification rate, so that the metal is repeatedly heated to the vicinity of the α single-phase region and then quickly cooled to the α+γ dual-phase region. Nucleation and phase precipitation occur during heating and cooling, respectively. After multiple cycles, a uniform and fine microstructure is obtained. Because the last layer of metal powder directly enters the solidification and cooling stage after melting, it is not remelted, and the structure retains the original state, which is significantly different from the bottom structure. Multiple measurements are made along the organizational boundary line, and the average value is calculated. Finally, the molten pool depth is 257μm, which is 9% error from the actual calculated result, which meets the requirements.

[0113] In summary, the beneficial effects of this application are as follows:

[0114] (1) The present invention calculates the molten pool depth and overlap rate under different parameters through formulas, preliminarily determines the process window for densification forming, and finally optimizes the process parameters through experimental verification. This can greatly shorten the process development cycle, obtain the densification forming process window more quickly, reduce process development costs, and improve efficiency.

[0115] (2) The present invention proposes a reasonable and effective molten pool depth and overlap rate, which can well control the remelting times and the number of remelting layers of the solidified layer. It can not only make the layers closely connected, control internal pores and unmelted defects, and improve the density, but also change the local thermal gradient and solidification rate, and adjust the microstructure of the parts. A certain number of remelting times can obtain parts with fine and uniform structure.

[0116] It should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like that may appear in the above description indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the embodiments of the present invention.

[0117] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0118] In the embodiments of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, or a detachable connection or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0119] In the embodiments of the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0120] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0121] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses or adaptations of the present invention, which follow the general principles of the present invention and include common knowledge or customary techniques in the art that are not disclosed by the present invention. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present invention are indicated by the appended claims.

Claims

1. A method for increasing the density of metal materials in additive manufacturing, characterized in that: include: Detecting the performance parameters of the metal powder and selecting the metal powder whose performance parameters meet the printing requirements, wherein the performance parameters include: sphericity, hollow powder rate and bulk density; Establish the model of the part to be formed, perform slicing and path planning, and determine the slice thickness and actual powder laying thickness; According to the relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth, the first range of the molten pool depth is determined; wherein the relationship between the slice thickness, the actual powder spreading thickness and the molten pool depth is as follows: In the formula, H is the depth of the molten pool, T is the actual powder thickness, and t is the cut layer thickness; According to the calculation formula between the overlap rate and the molten pool depth, the second range of the molten pool depth is determined; wherein the calculation formula between the overlap rate and the molten pool depth is as follows: H=T / (1-δ) Where, δ is the overlap ratio; Taking the intersection of the first range and the second range of the molten pool depth to obtain a third range, wherein the third range is a final molten pool depth range; According to the calculation formula between the molten pool depth and the scanning melting process parameter, the scanning melting process parameter range is determined by the third range; wherein the calculation formula between the molten pool depth and the scanning melting process parameter is as follows: Where L is the distance that heat is conducted from the high temperature end to the low temperature end, ε is the absorption efficiency of the energy source, P is the power of the energy source, K is the thermal conductivity of the metal powder, A is the heat conduction area of ​​the metal powder after melting to conduct heat to the surrounding area, and T m is the melting point of the metal powder, T p is the powder bed temperature before the metal powder melts, ρ is the theoretical density of the metal powder, v is the scanning melting speed, D is the energy source spot diameter, C is the specific heat capacity of the metal powder, is the melting latent heat of the metal powder; The scanning melting process parameters are used to perform selective melting forming.

2. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The performance parameter range of metal powder that meets printing requirements is as follows: The sphericity rate is ≥90%, the hollow powder rate is ≤1%, and the bulk density is greater than 55% of the theoretical density.

3. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The melting distance d in path planning is 0.05~0.2mm, and the slice thickness t is 0.03mm~0.3mm.

4. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The formula for calculating the actual powder thickness T based on the slice thickness is as follows: T=t(1-k) / k+t In the formula, k=ρ0 / ρ, ρ0 is the loose density of metal powder.

5. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The overlap rate δ is any value between 40% and 75%.

6. The method for increasing the density of metal materials by additive manufacturing according to claim 4, characterized in that: The number of remelting layers n' during powder printing is calculated according to the following formula: Remelting layer number n'= , the number of remelting layers is selected from the range determined by the above formula.

7. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The particle size distribution of the metal powder is as follows: the volume proportion of metal powder with a particle size greater than 45 μm and less than or equal to 150 μm is greater than or equal to 90%, the volume proportion of metal powder with a particle size less than or equal to 45 μm is less than or equal to 5%, and the volume proportion of metal powder with a particle size greater than 150 μm is less than or equal to 5%.

8. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The metal powder includes at least one of titanium, aluminum, copper, tungsten, molybdenum, tantalum and niobium.

9. The method for increasing the density of metal materials by additive manufacturing according to claim 1, characterized in that: The method further comprises: The density, melt pool depth and microstructure of the printed parts were tested.

10. The method for increasing the density of metal materials produced by additive manufacturing according to any one of claims 1 to 9, characterized in that: The energy source is any one of laser, electron beam, plasma arc and electric arc.

Images