CN113042749B - Method for eliminating formation defect of melting near surface layer of laser powder bed in real time - Google Patents
Method for eliminating formation defect of melting near surface layer of laser powder bed in real time Download PDFInfo
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Abstract
The invention provides a method for eliminating a fused near-surface layer forming defect of a laser powder bed in real time, which comprises the steps of monitoring the distance h between the fused forming defect of the laser powder bed and the surface of a deposition layer in real time through an external acquisition system; then judging the position of the defect, and immediately carrying out laser remelting when the thickness of h is larger than that of N deposition layers; and when the thickness of the deposition layer H is less than or equal to N, performing powder paving and laser powder bed fusion forming, and repeating the steps until the accumulated printing height H reaches a specified height, and performing laser remelting on the surface of the deposition layer. Compared with the prior forming technology, the invention can not only reduce the surface roughness of the formed part, but also eliminate the pore defect in time, greatly improves the forming efficiency compared with the forming mode of remelting layer by layer and continuously depositing multiple layers and then remelting laser, and gains more abundant response time for the defect on-line feedback adjustment.
Description
The technical field is as follows:
the invention belongs to the technical field of additive manufacturing, and particularly relates to a method for eliminating a laser powder bed melting near-surface layer forming defect in real time.
Background art:
additive manufacturing is an advanced rapid prototyping technology, and the near-net prototyping of a complex part is realized in a layer-by-layer deposition manner through a computer-aided technology. Compared with the traditional manufacturing technology, the additive manufacturing technology effectively improves the forming efficiency and the material utilization rate, and is expected to realize free manufacturing. Laser Powder Bed Fusion (LPBF) is a typical metal 3D printing technique, and a high-energy Laser beam is used to directly melt metal Powder, so that a formed part has excellent mechanical properties and dimensional accuracy. However, the laser powder bed fusion forming process is extremely complex and extreme in conditions, manufacturing defects such as spatters, pores and cracks are easily formed, and the mechanical properties of parts are seriously affected. Finding effective defect suppression methods is currently a major and difficult point of research.
Studies have shown that the quality of laser powder bed fusion forming is heavily dependent on the forming process. The generation of defects can be effectively reduced by optimizing the forming process, but the LPBF forming process is various in types, different in influence degree on forming quality and difficult to avoid forming defects. At present, the formation defects are mainly ameliorated by post-treatment. However, in some special cases, post-processing not only does not completely eliminate the forming defects, but also adds additional forming time and cost. The real-time elimination of the forming defects is more beneficial to improving the forming quality and the forming efficiency, and based on the method, the invention provides a method for eliminating the forming defects of the melting near-surface layer of the laser powder bed in real time so as to solve the problems.
The invention content is as follows:
the invention aims to provide a method for eliminating the defect of the molten near-surface layer forming of a laser powder bed in real time aiming at the defects of the prior art, so as to realize the real-time elimination of the internal defect of the molten laser powder bed.
The invention adopts the following technical scheme:
a method for eliminating the defect of the formation of a molten near-surface layer of a laser powder bed in real time comprises the following steps:
s1, monitoring the distance h between the fusion forming defect of the laser powder bed and the surface of the deposition layer in real time;
s2, judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, specifically,
when h is larger than N deposition layer thicknesses, immediately carrying out laser remelting; judging the condition of eliminating the defects of the remelted deposition layer, and if the defect elimination meets the requirement, performing powder paving and laser powder bed fusion forming; on the contrary, the remelting process is optimized to carry out laser remelting again until the defects are eliminated and the requirements are met;
when h is less than or equal to N deposition layer thicknesses, carrying out laser remelting; or powder spreading and laser powder bed fusion forming are carried out until h is larger than the thickness of N deposition layers, and then laser remelting is carried out;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
s3, repeating S1 and S2 until the accumulated printing height H reaches the designated height, and stopping the fusion forming of the laser powder bed;
and S4, performing laser remelting on the surface of the deposited layer.
Furthermore, the thickness of the single powder deposition layer is 30 μm, and the depth of a molten pool formed by laser remelting is 100 μm; and the number N of the deposited layers is 2, namely when h is greater than 60 mu m, laser remelting is immediately carried out, and when h is less than or equal to 60 mu m, powder paving and laser powder bed fusion forming are carried out.
Further, in step S1, the external collection system is used to monitor the distance h between the laser powder bed fusion forming defect and the deposition layer surface, and the external collection system and the laser operate synchronously.
Further, in S1, Ti-6.5Al-3.5Mo-l.5Zr-0.3Si (TC11) titanium alloy powder prepared by gas atomization is selected as a forming material; the particle size of the powder is 15-53 μm.
Further, in S1, the process parameters of the laser powder bed fusion forming include: the laser power range is 150-200W, the scanning speed is 1.25-1.45 m/s, and the scanning distance is 80-90 μm.
Further, in S2, the number N of the deposition layers is 2, namely when h is greater than 60 μm, laser remelting is immediately carried out; and when h is less than or equal to 60 mu m, powder paving and laser powder bed fusion forming are carried out.
Further, laser remelting refers to a process of directly scanning the upper surface of a deposition layer by laser in a melting and forming process of a laser powder bed, and parameters of laser remelting include: the laser power range is 180-200W, the scanning speed range is 1.25-1.45 m/s, and the scanning interval range is 80-90 μm.
Furthermore, the laser scanning mode is always consistent and is bidirectional scanning; the laser powder bed fusion forming is the same as the scanning path of the laser remelting.
Further, the laser wavelength of the laser is 1064-1100 nm, and the diameter of a laser spot is 80 μm; the whole process of selective laser melting and forming is carried out in the atmosphere of protective gas, and the protective gas adopts argon with the purity of 99.99 percent.
The invention has the beneficial effects that:
compared with the prior forming technology, the method can not only reduce the surface roughness of the formed part, but also eliminate the defect of the pore in time. Compared with a forming mode of remelting layer by layer and continuously depositing multiple layers and then remelting laser, the forming efficiency is greatly improved, and meanwhile, more abundant response time is won for defect on-line feedback adjustment.
Description of the drawings:
FIG. 1 is a surface topography map of LPBF continuous deposition of 5 layers without laser remelting;
FIG. 2 is a cross-sectional profile of an LPBF with 5 layers deposited continuously without laser remelting;
FIG. 3 is a schematic view of a laser remelting formation process for every 1 deposition layer;
FIG. 4 is a surface topography map of a laser remelting of every 1 deposition layer;
FIG. 5 is a cross-sectional profile of a laser remelting of every 1 deposition layer;
FIG. 6 is a schematic view of a laser remelting formation process for every 2 deposited layers;
FIG. 7 is a cross-sectional profile of a laser remelting shot of every 2 deposits;
FIG. 8 is a schematic view of a laser remelting formation process for every 3 deposited layers;
FIG. 9 is a cross-sectional profile of a laser remelting of every 3 deposited layers;
FIG. 10 is a flow chart of the method of the present invention.
The specific implementation mode is as follows:
in order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The invention provides a method for eliminating a laser powder bed melting near-surface layer forming defect in real time, which comprises the following steps:
monitoring the distance h between a fusion forming defect of a laser powder bed and the surface of a deposition layer through an external acquisition system, and synchronously operating the external acquisition system and a laser, namely, the acquisition system can synchronously obtain the distance h between the defect and the surface of the deposition layer when the forming defect appears; the external acquisition system is assumed and does not belong to the key point of the invention, and the related working principle is not elaborated in detail.
Specifically, Ti-6.5Al-3.5Mo-l.5Zr-0.3Si (TC11) titanium alloy powder prepared by gas atomization is selected as a forming material; the particle size range of the powder is 15-53 mu m; the thickness of the single powder deposition layer is 30 μm; the technological parameters of the laser powder bed fusion forming include: the laser power range is 150-200W, the scanning speed is 1.25-1.45 m/s, and the scanning distance is 80-90 μm.
Step two, judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, specifically,
when h is larger than N deposition layers, immediately carrying out laser remelting, judging the condition of eliminating defects of the remelted deposition layers, and if the defect elimination meets the requirement, carrying out powder paving and laser powder bed fusion forming; on the contrary, the remelting process is optimized to carry out laser remelting again until the defect elimination meets the requirement;
when h is less than or equal to N deposition layer thicknesses, carrying out laser remelting; or powder spreading and laser powder bed fusion forming are carried out until h is larger than the thickness of N deposition layers, and then laser remelting is carried out;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
the laser remelting refers to a process of directly scanning the upper surface of a deposition layer by laser in the melting and forming process of a laser powder bed, a deeper molten pool can be formed by remelting with higher laser energy density, the elimination of internal pores is facilitated, and the remelting laser power, the scanning speed and the scanning interval can fluctuate within a certain range.
The improper remelting process can not only not completely eliminate the existing defects in the deposition layer, but also can possibly generate new forming defects in the laser remelting process; therefore, the elimination condition of the deposited layer defect after remelting needs to be judged in time, and when the elimination condition of the forming defect is not ideal, the remelting process is optimized in time to ensure the effective elimination of the deposited layer defect. The defect elimination requirement is set according to the actual working condition requirement of the formed part.
In the invention, the parameters of laser remelting can be selected as follows: the laser power range is 180-200W, the scanning speed range is 1.25-1.45 m/s, and the scanning interval range is 80-90 μm. The powder spreading thickness is kept unchanged at 30 mu m all the time, and the parameters of the laser powder bed fusion forming process are kept consistent with those of the steps.
Taking the TC11 titanium alloy as an object of study, the thickness of a single powder deposition layer was considered to be 30 μm, regardless of the thickness variation caused by melting and solidification of metal powder particles during the laser powder bed fusion forming process. In the melting and forming process of the TC11 titanium alloy laser powder bed, when single remelting is carried out at the laser power of 180W, the effective action depth of a molten pool is about 100 mu m, and the single remelting can effectively eliminate all defects within the range of 3 deposition layers away from the surface of a part at most. Therefore, according to the effect of TC11 titanium alloy laser remelting, when the number N of the deposition layers is 2, the internal pores of the part can be effectively eliminated, namely when h is greater than 60 micrometers, laser remelting is immediately carried out; and when h is less than or equal to 60 mu m, powder paving and laser powder bed fusion forming are carried out. And when the forming defects are not completely eliminated, optimizing the remelting process, and carrying out laser remelting again until the deposition layer defects are completely eliminated.
Because the acquisition system can monitor the distance h between the forming defect and the deposition layer in real time, and the thickness of a single deposition layer is considered to be kept unchanged at 30 mu m, when h is larger than 60 mu m, the forming defect is positioned in the 3 rd deposition layer far away from the surface of the deposition layer, and the internal defect can be completely eliminated by immediately carrying out laser remelting. Therefore, in the present invention, the distribution range of h is 0 to 90 μm.
Step three, repeating the step one and the step two until the accumulated printing height H reaches a designated height, and stopping the fusion forming of the laser powder bed;
and fourthly, carrying out laser remelting based on the surface of the deposition layer, wherein the parameters of the laser remelting are consistent with the remelting process with a better remelting effect in the second step.
In each step of the invention, the laser scanning mode is always kept consistent and is bidirectional scanning; the laser powder bed fusion forming is the same as the scanning path of the laser remelting.
Other process parameters in the forming process of the invention comprise: the laser wavelength of the fiber laser is 1064-1100 nm, and the diameter of a laser spot is 80 mu m; in order to prevent oxidation, the selective laser melting formation is carried out in a protective gas atmosphere throughout, and argon gas with a purity of 99.99% is used as the protective gas.
After the printing of the parts is finished, the parts are separated from the substrate by a wire cutting technology. And observing the surface appearance of the part by using a Keynshi VK-100 laser microscope. Before observation, an ultrasonic cleaner was used to remove excess sticky powder from the surface of the sample. And performing line cutting in the middle of the part perpendicular to the forming direction to obtain the section of the part. And (3) carrying out inlaying, coarse grinding and fine grinding on the divided parts until the section is bright and has no obvious scratch, and observing the appearance and the pores of the section by using a Keynes VK-100 laser microscope.
Example 1
The embodiment provides a method for eliminating a molten near-surface layer forming defect of a laser powder bed in real time, which comprises the following specific implementation steps of:
step 1: monitoring the distance h between the fusion forming defect of the laser powder bed and the surface of the deposition layer in real time through an external acquisition system;
step 2: judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, and specifically comprising the following steps:
1) when h is larger than N deposition layers, immediately carrying out laser remelting, judging the condition of eliminating defects of the remelted deposition layers, and if the defect elimination meets the requirement, carrying out powder paving and laser powder bed fusion forming; on the contrary, the remelting process is optimized to carry out laser remelting again until the defects are eliminated and the requirements are met;
2) when the h is less than or equal to the thickness of N deposition layers, powder paving and laser powder bed fusion forming are carried out;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
and step 3: repeating the step 1 and the step 2 until the accumulated printing height H reaches a designated height, and stopping the fusion forming of the laser powder bed;
and 4, step 4: and carrying out laser remelting based on the surface of the deposited layer.
The forming material is Ti-6.5Al-3.5Mo-l.5Zr-0.3Si (TC11) titanium alloy, the particle size range of the powder is 15-53 mu m, and the thickness of the powder layer is always kept at 30 mu m.
In this embodiment, the number N of deposition layers is 0 in step 2, that is, every 1 deposition layer is subjected to laser remelting. In order to verify the defect elimination capability of the embodiment, a lower laser power is selected for LPBF forming, each deposition layer has defects, and each deposition layer is subjected to laser remelting once, wherein the specific forming process is shown in fig. 3. The forming process parameters of the LPBF comprise: the laser power is 90W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning; four different laser remelting processes are selected: 1) the laser power is 150W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning; 2) the laser power is 180W, the scanning speed is 1.25m/s, the scanning distance is 90 mu m, and the scanning strategy is bidirectional scanning; 3) the laser power is 200W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning; 4) the laser power is 180W, the scanning speed is 1.05m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning.
In the whole forming process, the technological parameters of LPBF forming and laser remelting in the steps 1-3 are always kept consistent, the accumulated LPBF deposition layer number is 5, and the laser remelting times are 5.
Fig. 1 and 2 show the results of experiments in which 5 layers were deposited continuously by LPBF without laser remelting. As can be seen from the surface topography, there are obvious poor lap joints between the deposited tracks, the surface is rugged, and the surface roughness is higher to 23.26 μm. It can be seen from the cross-sectional profile that there is an obvious poor metallurgical bonding phenomenon between the deposition layer and the substrate, and the deposition channel carries unmelted spherical powder particles.
In this embodiment, the number N of LPBF deposition layers is selected to be 0, and laser remelting is performed on every 1 deposition layer, and the experimental results after remelting are shown in fig. 4 and 5. Wherein, fig. 4 is a surface topography after remelting, and fig. 5 is a section topography after remelting. Compared with surface topography maps before and after remelting, the surface after remelting is smoother, and the surface roughness is obviously reduced; the remelted surface roughness is decreased with the increase of the energy density of the remelting laser, that is, when the remelting laser power is 180W and the scanning speed is 1.05m/s, the remelted surface roughness is 11.35 μm at the lowest, as shown in fig. 4 (d). When the remelting laser power is 150W and the scanning speed is 1.25m/s, the number of pores in the remelted deposition layer is obviously reduced, the size of the pores is obviously reduced, but a small amount of pores remain, as shown in fig. 5(a), mainly caused by insufficient input of the remelting laser energy; when the remelting laser power is 180W and 200W and the scanning speed is 1.25m/s, the remelting laser energy input is reasonable, the section after remelting has no obvious pores, and the density of the part is high, as shown in fig. 5(b) and (c); when the remelting laser power is 180W and the scanning speed is 1.05m/s, no obvious large-size pores exist in the remelted deposition layer, but a small number of small-size circular pores exist in the bottom area of the deposition layer, as shown in FIG. 5(d), the main reason is that the excessively high laser energy input is easy to form deeper key-holes in the forming process, and the improper closure of the key-holes forms spherical pores. Therefore, the defect of the deposited layer cannot be completely eliminated by excessively high or excessively low input of the remelting laser energy, the internal pores cannot be completely eliminated by only once laser remelting, and the remelting process needs to be optimized in time to completely eliminate the internal defects. And each deposition layer is remelted by a reasonable remelting process, so that the internal pores of the deposition layers are completely eliminated, and the surface forming quality is obviously improved.
Example 2
The embodiment provides a method for eliminating a laser powder bed melting near-surface layer forming defect in real time, which comprises the following specific implementation steps:
step 1: monitoring the distance h between the fusion forming defect of the laser powder bed and the surface of the deposition layer in real time through an external acquisition system;
step 2: judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, and specifically comprising the following steps:
1) when h is larger than N deposition layers, immediately carrying out laser remelting, judging the condition of eliminating defects of the remelted deposition layers, and if the defect elimination meets the requirement, carrying out powder paving and laser powder bed fusion forming; on the contrary, the remelting process is optimized to carry out laser remelting again until the defects are eliminated and the requirements are met;
2) when the h is less than or equal to the thickness of N deposition layers, powder paving and laser powder bed fusion forming are carried out;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
and step 3: repeating the step 1 and the step 2 until the accumulated printing height H reaches a designated height, and stopping the fusion forming of the laser powder bed;
and 4, step 4: and carrying out laser remelting based on the surface of the deposited layer.
The forming material is Ti-6.5Al-3.5Mo-l.5Zr-0.3Si (TC11) titanium alloy, the particle size range of the powder is 15-53 mu m, and the thickness of the powder layer is always kept at 30 mu m.
In this embodiment, the number N of deposition layers in step 2 is 1, i.e., when h is greater than 30 μm, laser remelting is immediately performed; when h is less than or equal to 30 mu m, powder paving and laser powder bed fusion forming are carried out. Neglecting the variation of the thickness of the deposition layer in the melting and forming process of the TC11 titanium alloy laser powder bed, the thickness of the single deposition layer is kept unchanged at 30 mu m. In order to verify the defect elimination capability of the embodiment, a lower laser power is selected for LPBF forming, each deposition layer has defects, that is, every 2 deposition layers need to be subjected to laser remelting, and the specific forming process is shown in fig. 6. The forming process parameters of the LPBF comprise: the laser power is 90W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning; the laser remelting process parameters comprise: the laser power is 180W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning.
In the whole forming process, the technological parameters of LPBF forming and laser remelting in the steps 1-3 are always kept consistent, the accumulated LPBF deposition layer number is 4, and the laser remelting frequency is 2.
In this embodiment, the number N of LPBF deposition layers is selected to be 1, each LPBF deposition layer has a defect, and each 2 deposition layers need to be laser-reflowed once, and the simulation result is shown in fig. 7. Wherein fig. 7(a) and 7(c) are cross-sectional views before remelting, and fig. 7(b) and 7(d) are cross-sectional views after remelting. In the figure, the laser action region is within the dotted line, and the powder outside the dotted line is always kept in the original state.
Based on a discrete element method and computational fluid mechanics, a multi-layer multi-channel LPBF simulation model with mesoscopic scale is built. Considering the random accumulation of powder, the action of surface tension, Marangoni force and counter-pressure force, a Gaussian heat source model is selected, and the tracking of the free surface is realized based on a VOF method. The LPBF forming process is divided into two stages: in the powder laying stage and the laser and powder interaction stage, two simulation models are respectively created: a random powder bed stacking model and a thermal fluid simulation model. The motion condition of the powder particles in the powder paving process is calculated through a discrete element method, and the size and spatial distribution information of the effective powder particles is obtained, so that a random powder bed stacking model is established. And (3) introducing the powder bed model into the heat flow transmission model, constructing a mesoscale thermal fluid simulation model, and setting initial conditions and boundary conditions of a calculation domain. And solving a control equation to obtain the calculation results of the phase field, the temperature field and the speed field. And visualizing the calculation result through post-processing software such as ParaView and the like to obtain the profile morphology diagrams before and after remelting.
Due to insufficient laser energy input during LPBF formation, significant unfused voids form between deposited layers. Remelting every 2 deposition layers at a time with higher laser power, wherein the remelted cross section has no obvious pores. Compared with embodiment 1, this embodiment remelts every 2 sedimentary layers, has not only eliminated the internal porosity of sedimentary layer completely, has obviously promoted shaping efficiency moreover.
Example 2
The embodiment provides a method for eliminating a laser powder bed melting near-surface layer forming defect in real time, which comprises the following specific implementation steps:
step 1: monitoring the distance h between the fusion forming defect of the laser powder bed and the surface of the deposition layer in real time through an external acquisition system;
and 2, step: judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, and specifically comprising the following steps:
1) when h is larger than N deposition layers, immediately carrying out laser remelting, judging the condition of eliminating defects of the remelted deposition layers, and if the defect elimination meets the requirement, carrying out powder paving and laser powder bed fusion forming; on the contrary, the remelting process is optimized to carry out laser remelting again until the defects are eliminated and the requirements are met;
2) when the h is less than or equal to the thickness of N deposition layers, powder paving and laser powder bed fusion forming are carried out;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
and 3, step 3: repeating the step 1 and the step 2 until the accumulated printing height H reaches a designated height, and stopping the fusion forming of the laser powder bed;
and 4, step 4: and performing laser remelting on the surface of the deposited layer.
The forming material is Ti-6.5Al-3.5Mo-l.5Zr-0.3Si (TC11) titanium alloy, the particle size range of the powder is 15-53 mu m, and the thickness of the powder layer is always kept at 30 mu m.
In this embodiment, the number N of the deposition layers in step 2 is 2, that is, when h is greater than 60 μm, laser remelting is immediately performed; and when h is less than or equal to 60 mu m, powder paving and laser powder bed fusion forming are carried out. Neglecting the variation of the thickness of the deposition layer in the melting and forming process of the TC11 titanium alloy laser powder bed, the thickness of the single deposition layer is kept unchanged at 30 mu m. In order to verify the defect elimination capability of the embodiment, a lower laser power is selected for LPBF forming, each deposition layer has defects, that is, every 3 deposition layers need to be subjected to laser remelting, and a specific forming process is shown in fig. 8. The forming process parameters of the LPBF include: the laser power is 90W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning; the laser remelting process parameters comprise: the laser power is 180W, the scanning speed is 1.25m/s, the scanning interval is 90 μm, and the scanning strategy is bidirectional scanning.
In the whole forming process, the technological parameters of LPBF forming and laser remelting in the steps 1-3 are always kept consistent, the accumulated LPBF deposition layer number is 6, and the laser remelting frequency is 2.
In this embodiment, the number N of LPBF deposition layers is selected to be 2, each LPBF deposition layer has a defect, and each 3 deposition layers need to be laser-reflowed once, and the simulation result is shown in fig. 9. Fig. 9(a) and 9(c) are cross-sectional views before remelting, and fig. 9(b) and 9(d) are cross-sectional views after remelting. In the figure, the laser action region is within the dotted line, and the powder outside the dotted line is always kept in the original state. The simulation model and the calculation method in this embodiment are the same as those in embodiment 2. The simulation result shows that: remelting every 3 deposition layers at a time with higher laser power, wherein the remelted section has no obvious pores. Compared with the embodiment 1 and the embodiment 2, the remelting is carried out on every 3 deposition layers, so that the internal pores of the deposition layers are completely eliminated, and the forming efficiency is improved again.
In the above three embodiments, in order to verify the defect elimination capability of the present invention, the power of the LPBF forming laser is low, and each deposition layer has defects, so that the laser remelting frequency is high, and the remelting frequency is high. However, in the actual LPBF forming process, the forming process parameters are better, the defect generation rate is lower, and therefore, the required remelting times are greatly reduced. In addition, the maximum effective molten pool depth is about 100 μm in the forming process of the TC11 titanium alloy LPBF, when the value of the LPBF deposition layer number N is larger than 2, the action area of the molten pool can not completely cover the previous deposition layer in the laser remelting process, and the internal defects of the deposition layer can not be completely eliminated. Therefore, when the remelting process is kept unchanged and the value range of N is 0-2, the embodiment of the invention can realize the real-time elimination of the forming defects. When the number N of LPBF deposition layers is 2, the forming efficiency is the highest.
The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention, it should be noted that, for those skilled in the art, several modifications and decorations without departing from the principle of the present invention should be regarded as the protection scope of the present invention.
Claims (8)
1. A method for eliminating the defect of the formation of a molten near-surface layer of a laser powder bed in real time is characterized by comprising the following steps:
s1, monitoring the distance h between the fusion forming defect of the laser powder bed and the surface of the deposition layer in real time;
s2, judging the position of the defect according to the distance h between the forming defect and the surface of the deposition layer, specifically,
when h is larger than N deposition layer thicknesses, immediately carrying out laser remelting;
when the thickness of the deposition layers h is less than or equal to N, carrying out laser remelting, or carrying out powder paving and laser powder bed fusion forming;
the depth of a molten pool formed by laser remelting is not less than the accumulated thickness of the N +1 deposition layers;
s3, repeating S1 and S2 until the accumulated printing height H reaches the designated height, and stopping the fusion forming of the laser powder bed;
s4, performing laser remelting on the surface of the deposited layer;
the thickness of a single powder deposition layer is 30 mu m, and the depth of a molten pool formed by laser remelting is 100 mu m; and the number N of the deposited layers is 2, namely when h is greater than 60 mu m, laser remelting is immediately carried out, and when h is less than or equal to 60 mu m, powder paving and laser powder bed fusion forming are carried out.
2. The method for eliminating the laser powder bed melting near-surface layer forming defect in real time as claimed in claim 1, wherein in S1, the distance h between the laser powder bed melting forming defect and the surface of the deposition layer is monitored by an external acquisition system, and the external acquisition system and the laser operate synchronously.
3. The method for eliminating the laser powder bed melting near-surface layer forming defect in real time as claimed in claim 1, wherein in S1, Ti-6.5Al-3.5Mo-l.5Zr-0.3Si titanium alloy powder prepared by gas atomization is selected as a forming material; the particle size of the powder is 15-53 μm.
4. The method for eliminating the defect of the molten near-surface layer forming of the laser powder bed according to claim 1, wherein in S2, when h is larger than N deposition layers, laser remelting is carried out, the condition of eliminating the defect of the remelted deposition layer is judged, and if the defect elimination meets the requirement, powder paving and the molten forming of the laser powder bed are carried out; and conversely, optimizing the remelting process to perform laser remelting again until the defect elimination meets the requirement.
5. The method for eliminating the defect of the laser powder bed melting near-surface layer forming in real time according to claim 1, wherein the process parameters of the laser powder bed melting forming comprise: the laser power range is 150-200W, the scanning speed is 1.25-1.45 m/s, and the scanning distance is 80-90 μm.
6. The method for eliminating the defect of the formation of the molten near-surface layer of the laser powder bed in real time as claimed in claim 1, wherein the laser remelting refers to a process of directly performing laser scanning on the upper surface of a deposition layer in the process of the formation of the molten laser powder bed, and parameters of the laser remelting include: the laser power range is 180-200W, the scanning speed range is 1.25-1.45 m/s, and the scanning interval range is 80-90 μm.
7. The method for eliminating the formation defect of the melting near-surface layer of the laser powder bed in real time according to claim 1, wherein the laser scanning mode is always consistent and is bidirectional scanning; the laser powder bed fusion forming is the same as the scanning path of the laser remelting.
8. The method for eliminating the formation defect of the molten near-surface layer of the laser powder bed in real time as claimed in claim 1, wherein the laser wavelength of the laser is 1064-1100 nm, and the diameter of a laser spot is 80 μm; the whole process of selective laser melting and forming is carried out in the atmosphere of protective gas, and the protective gas adopts argon with the purity of 99.99 percent.
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