CN113020624A - Heat treatment method of laser stereo-forming TC4 titanium alloy - Google Patents
Heat treatment method of laser stereo-forming TC4 titanium alloy Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
- C22F1/18—High-melting or refractory metals or alloys based thereon
- C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon
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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
- B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
- B22F3/24—After-treatment of workpieces or articles
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- 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
- B33Y10/00—Processes of additive manufacturing
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- 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
- B33Y40/00—Auxiliary operations or equipment, e.g. for material handling
- B33Y40/20—Post-treatment, e.g. curing, coating or polishing
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- 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
- B33Y80/00—Products made by additive manufacturing
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D11/00—Process control or regulation for heat treatments
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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
- B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
- B22F3/24—After-treatment of workpieces or articles
- B22F2003/248—Thermal after-treatment
Abstract
A heat treatment method of a laser stereo-form TC4 titanium alloy is characterized in that a laser stereo-form TC4 titanium alloy is used as a research object, a heat treatment test is carried out on the laser stereo-form TC4 titanium alloy, and an optimal heat treatment process is obtained. Has important theoretical significance and engineering application value for guiding TC4 titanium alloy laser three-dimensional forming and post-processing technology thereof. Wherein, the study is specially used for the thermal processing technology of the laser stereo-morphism titanium alloy, which comprises optimizing the thermal processing technology to improve the microstructure; the thermal deformation of the material obtained by adopting the optimized heat treatment process is an important content of the invention, and has important theoretical significance and engineering application value.
Description
Technical Field
The invention belongs to the field of additive manufacturing and metal heat treatment, and particularly relates to a heat treatment method of a TC4 titanium alloy in a laser three-dimensional forming mode.
Background
The titanium alloy is a metal with wide application, has the advantages of high specific strength, good corrosion resistance, good high-temperature mechanical property and the like, and is widely concerned in the field of aerospace. The TC4 titanium alloy has the characteristics of high melting point, high melting state activity, larger deformation resistance and the like, can enable the tensile strength to reach 1173MPa through heat treatment, and is one of the most widely applied titanium alloys at present. The Laser Solid Forming (LSF) additive manufacturing technology of metal parts is a technology which can realize the die-free, rapid and full-compact near-net Forming of high-performance complex-structure compact metal parts. The titanium alloy part produced by utilizing the laser three-dimensional forming technology can meet the strict requirements of low cost, short period, high performance and high flexibility, conforms to the advanced airplane design principle, and provides a new technology for the national defense preparation of the titanium alloy.
The laser three-dimensional forming has the characteristic of layer-by-layer deposition and directional solidification, coarse columnar dendritic crystals are arranged on a microstructure, acicular martensite is arranged inside the dendritic crystals, and the difference is great compared with TC4 titanium alloy prepared by the traditional technology. The effective heat treatment can change the microstructure of the metal material, and the mechanical property of the titanium alloy can be improved by changing the microstructure of the laser three-dimensional forming TC4 titanium alloy.
Commercially pure titanium, TC4 titanium alloys, and the like, produced by American AeroMet corporation (cycles S, Archella F. laser forming titanium compositions from powder Technology [ J ]. Materials Technology,2000,15(1):8-12), using laser stereoforming techniques, have mechanical properties that meet AMS standards, but do not involve studies on the optimization of the modification of subsequent treatments. Research results of southern China's university of science also show that the mechanical properties of the laser three-dimensional forming titanium alloy reach the ASTMB381-2006a forging standard (Yangmongqiang, Wangdui, Wuweihui. research progress of the selective laser melting direct forming technology of metal parts [ J ]. Chinese laser 2011,6:60-70.), but the research is only on the basic properties of the laser three-dimensional forming titanium alloy and does not relate to the content of post-treatment modification. Lewis et Al (Lewis G K, Schlienger E.practical compositions and capabilities for laser assisted direct metal displacement [ J ] Materials & Design,2000,21(4): 417-. Research of Zhang Shuangyin (Zhang Shuangyin, research of structure and mechanical property of laser rapid prototyping TC4 titanium alloy, 2006.) and the like also finds that when the sample is stretched along the deposition direction, the tensile strength of the TC4 titanium alloy sample deposited under the three-dimensional laser form can reach 1200MPa, which is far higher than the standard of 895MPa forgings, but the elongation rate is very high. After the solution aging treatment, the tensile strength can be reduced to 1035MPa, the elongation can be improved to 13.5%, but due to the particularity of the microstructure of the laser three-dimensional forming titanium alloy, the traditional hot working process of the titanium alloy is not suitable for the laser three-dimensional forming titanium alloy, and the hot working process of the laser three-dimensional forming titanium alloy needs to be studied more deeply.
Disclosure of Invention
The technical problem solved by the invention is as follows: the structure, the performance and the like of a laser three-dimensional forming piece are different from those of the traditional casting and forging piece, the thermal deformation behavior of the laser three-dimensional forming piece is lack of comprehensive and deep research, in order to improve the mechanical property of the traditional laser three-dimensional forming titanium alloy and meet the higher performance requirement of the TC4 titanium alloy for aerospace, the invention provides a heat treatment method of the laser three-dimensional forming TC4 titanium alloy, the TC4 titanium alloy is prepared by adopting a laser three-dimensional forming technology, the comprehensive mechanical property of the titanium alloy is regulated and controlled by a solid solution aging heat treatment process, and the matched heat treatment method is summarized aiming at the microscopic structure of the laser three-dimensional forming TC4 titanium alloy.
The technical scheme of the invention is as follows: a heat treatment method of a laser three-dimensional forming TC4 titanium alloy comprises the following steps:
step 1: preparing a TC4 titanium alloy forging substrate and TC4 titanium alloy spherical powder respectively;
step 2: laser stereoforming test: placing TC4 titanium alloy spherical powder in a test instrument, determining laser three-dimensional forming process parameters, and performing the forming process under the argon atmosphere; during testing, a testing instrument continuously and uniformly sprays TC4 titanium alloy spherical powder to one surface of a TC4 titanium alloy forging substrate; when the powder thickness meets the requirement, stopping the test; obtaining a laser three-dimensional forming TC4 titanium alloy;
and step 3: carrying out heat treatment on the TC4 titanium alloy obtained in the step 2 in the laser stereo-morphology, and comprising the following steps:
step 3.1: determining heat treatment parameters, including the following parts:
step 3.1.1: testing the phase transformation point of the laser stereo-morphology TC4 titanium alloy by using a differential thermal analysis method to obtain the phase transformation temperature;
step 3.1.2: determining that the solid solution temperature of the laser stereo-forming TC4 titanium alloy changes above 800 ℃ according to the phase transition temperature, wherein the heat preservation time is determined by a formula 1: t ═ 5-8) + AD (1), where: t-holding time; a-temperature-keeping time coefficient/min mm < -1 >, D-effective thickness/mm of the workpiece, and the adopted solid solution temperature-keeping time is 0.5 h.
Step 3.1.3: the aging treatment of the laser three-dimensional forming TC4 titanium alloy is to promote the decomposition and transformation of metastable beta phase after the aging treatment is carried out and the maintenance is carried out for a period of time at a higher temperature after the solution treatment, so as to generate the strengthening effect, therefore, the aging temperature is selected to be between 500 and 600 ℃, and the heat preservation is carried out for 3 to 12 hours.
Step 3.1.4: solid solution temperature (the solid solution temperature of the laser stereo-morphology TC4 titanium alloy is determined to be above 800 ℃ according to the phase transition temperature)
Step 3.2: carrying out a plurality of groups of heat treatment tests, obtaining the heat-treated laser three-dimensional forming TC4 titanium alloy through the tests, observing the microstructure and the mechanical property of the titanium alloy under different heat treatment parameters, and obtaining the most available heat treatment process parameters of the laser three-dimensional forming TC4 titanium alloy;
a plurality of tests are divided into two groups AB, wherein the group A changes the solid solution temperature and keeps the aging temperature unchanged, and the group B changes the aging temperature and keeps the solid solution temperature unchanged; the variation of the solid solution temperature is 25 ℃, and the variation of the aging temperature is 50 ℃;
and 4, step 4: and cutting a metallographic sample of the laser three-dimensional forming TC4 titanium alloy, observing the microstructure of the metallographic sample, and carrying out microhardness test on the metallographic sample to obtain a microhardness change curve along with heat treatment parameters, thereby finally obtaining the optimal heat treatment parameters of the laser three-dimensional forming TC4 titanium alloy.
The further technical scheme of the invention is as follows: the thickness of the powder in the step 2 is required to be more than or equal to 10 mm.
The further technical scheme of the invention is as follows: the TC4 titanium alloy forging base plate in the step 1 is a forging annealing state TC4 titanium alloy plate.
The further technical scheme of the invention is as follows: the TC4 titanium alloy powder in the step 1 is TC4 titanium alloy powder prepared by a plasma rotating electrode, and the particle size of the powder is 80-120 mu m.
The further technical scheme of the invention is as follows: the laser three-dimensional forming process parameters in the step 2 are as follows: the power is 1500W, the diameter of a facula is 2mm, the layer thickness is 0.5mm, the Z-axis stroke is 2.5mm, and the powder feeding speed is 10 g/min.
The further technical scheme of the invention is as follows: the heat treatment test in the step 3.2 is to perform a heat treatment test on the cut TC4 titanium alloy block, and after the heat treatment test, the microstructures of the three surfaces XY, YZ and XZ are observed.
The further technical scheme of the invention is as follows: the process parameters of the differential thermal analysis method in the step 3.1.1 are as follows: the reference sample is Al2O3 powder, the heating rate is 20 ℃/min, and the protection argon flow is 50 ml/min.
The further technical scheme of the invention is as follows: the metallographic specimen of the TC4 titanium alloy in the laser three-dimensional forming form is cut in the step 4, and the preparation method of the metallographic specimen comprises the following steps: and (3) grinding and polishing the metallographic specimen, then corroding the metallographic specimen by using a corrosive liquid (HF: HNO 3: H2O: 10: 5: 85), wiping and cleaning the metallographic specimen by using alcohol, and drying the metallographic specimen by using a blower to ensure that the cross section has no dirt or water stain.
The further technical scheme of the invention is as follows: the microhardness change curve of the laser stereo-morphology TC4 titanium alloy along with the heat treatment parameters obtained in the step 4 is characterized in that the preparation method and the test parameters of the microhardness sample are as follows: the Vickers hardness was measured using a DHV-1000Z hardness machine. And (3) grinding the surface to be measured of the hot-compressed sample by using metallographic abrasive paper until no obvious scratch appears, and ensuring that the upper surface and the lower surface of the sample are parallel. The load used was 1kg/mm2, dwell time 15 s. And measuring the hardness values of the test surface in three directions, wherein 10 test points are taken as each test surface, and the average value is taken as the microhardness value of the test surface.
Effects of the invention
The invention has the technical effects that: aiming at the current situation that the structure, the performance and the like of a laser three-dimensional forming part are different from those of the traditional casting and forging piece to influence the use performance of the laser three-dimensional forming part, the phase change point of the laser three-dimensional forming state TC4 titanium alloy is determined through differential scanning calorimetry, the heat treatment parameters of the laser three-dimensional forming state TC4 titanium alloy are determined, the optimal heat treatment process parameters are determined according to the microstructure and the hardness distribution of the laser three-dimensional forming state TC4 titanium alloy after heat treatment, and the aim of modifying the laser three-dimensional forming state TC4 titanium alloy is further achieved.
The invention researches the thermal processing technology (including optimizing the thermal processing technology to improve the microstructure and thermally deforming the material obtained by the optimized thermal processing technology) specially used for the laser three-dimensional forming titanium alloy, is an important content of the invention and has important theoretical significance and engineering application value.
Drawings
FIG. 1 is a DSC graph of a laser three-dimensional morphology TC4 titanium alloy
FIG. 2 is a schematic view showing a sampling position of a heat-treated sample
FIG. 3 is a schematic view of the different-direction laser stereolithography TC4 titanium alloy microstructure and the non-heat-treated different-direction laser stereolithography TC4 titanium alloy microstructure at different solution temperatures of group A in example 1; wherein (1) is a TC4 titanium alloy microstructure with laser stereo-formation shapes in different directions, (a) a YZ plane; (b) an XZ plane; (c) the XY plane (2) is a TC4 titanium alloy microstructure formed by laser three-dimensionally in different directions at different solid solution temperatures; (a)800 ℃; (b) 825 deg.C; (c)850 ℃; (d)875 DEG C
FIG. 4 is a laser stereoformed TC4 titanium alloy microstructure from different directions at different aging temperatures for group B of example 2; (a)500 ℃; (b)550 ℃; (c)600 ℃; (d)650 deg.C
FIG. 5 is a graph of microhardness as a function of heat treatment parameters, (a) microhardness as a function of solution temperature; (b) curve of apparent hardness with aging temperature
FIG. 6 is a graph of the true stress and true strain of a sample under optimum heat treatment process parameters
FIG. 7 is a diagram of IPF before and after heat-compression after heat-treatment of a laser stereo-morphic TC4 titanium alloy (a) No. 2 heat-treatment sample; (b) heat-treated sample No. 8; (c) thermally compressing the sample under heat treatment parameter No. 2; (d) thermally compressed sample under No. 8 heat treatment parameters
FIG. 8 is a heat treatment specimen No. 2 showing the crystal grain size distribution ratio (a) before and after heat compression after heat treatment of a laser stereomorphic TC4 titanium alloy; (b) heat-treated sample No. 8; (c) thermally compressing the sample under heat treatment parameters No. 2; (d) thermally compressed sample under heat treatment parameter No. 8
FIG. 9 is a heat treatment specimen No. 2 of misorientation distribution (a) before and after heat compression of a titanium alloy after heat treatment in a laser stereomorphic TC 4; (b) heat-treated sample No. 8; (c) thermally compressing the sample under heat treatment parameters No. 2; (d) thermally compressed sample under heat treatment parameter No. 8
Detailed Description
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like, indicate orientations and positional relationships based on those shown in the drawings, and are used only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be considered as limiting the present invention.
Referring to fig. 1-9, the invention takes the laser stereo-morphology TC4 titanium alloy as a research object, and performs a heat treatment test on the laser stereo-morphology TC4 titanium alloy to obtain an optimal heat treatment process. Has important theoretical significance and engineering application value for guiding TC4 titanium alloy laser three-dimensional forming and post-processing technology. Wherein, the study is specially used for the thermal processing technology of the laser stereo-morphism titanium alloy, which comprises optimizing the thermal processing technology to improve the microstructure; the thermal deformation of the material obtained by adopting the optimized thermal treatment process is an important content of the invention, and has important theoretical significance and engineering application value.
In order to achieve the above object, the present invention provides a heat treatment method for a laser stereo-morphism TC4 titanium alloy. The technical scheme of the invention comprises the following steps:
(1) preparing a TC4 titanium alloy forging substrate (150mm multiplied by 50mm multiplied by 10mm) and TC4 titanium alloy spherical powder, wherein the TC4 titanium alloy forging substrate is a TC4 titanium alloy plate in a forging annealing state, the TC4 titanium alloy powder is TC4 titanium alloy powder prepared by a plasma rotating electrode, and the powder granularity is 80-120 mu m;
(2) carrying out a laser three-dimensional forming test: setting laser three-dimensional forming process parameters, depositing path forms and selecting protective gas;
(3) determining heat treatment parameters: the heat treatment parameters are solid solution aging parameters, and a laser stereo-forming TC4 titanium alloy phase change point is tested by using a differential thermal analysis method to determine the heat treatment parameters;
(4) heat treatment test and determination of optimum heat treatment parameters: the heat treatment test comprises the steps of firstly analyzing microstructures in different directions of the laser three-dimensional forming TC4 titanium alloy at different solid solution temperatures, determining an optimal solid solution temperature parameter based on a structure analysis condition, then analyzing the microstructures in different directions of the titanium alloy at different aging temperatures, researching a change rule of hardness along with the solid solution temperature and the aging temperature, researching a corresponding relation between the microstructure evolution and the hardness change, and obtaining the solid solution aging parameter corresponding to the fact that the microstructure and the performance of the laser three-dimensional forming TC4 titanium alloy are obviously improved;
thermal deformation behavior of the laser stereo-forming shape TC4 titanium alloy under the optimal heat treatment parameters: carrying out a thermal compression deformation test on the TC4 titanium alloy under the optimal heat treatment parameters, selecting thermal compression process parameters, and determining the optimal heat treatment parameters with the best thermal deformation performance of the TC4 titanium alloy in the laser three-dimensional forming form;
the present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the spirit of the invention. All falling within the scope of the present invention.
Example one
The processing method of the embodiment comprises the following steps:
(1) preparing a TC4 titanium alloy forging substrate and TC4 titanium alloy spherical powder:
the test materials are a TC4 titanium alloy forging base plate (150mm multiplied by 50mm multiplied by 10mm) and TC4 titanium alloy spherical powder, the base plate is a TC4 titanium alloy plate in a forging annealing state, and the granularity of the TC4 powder is 80-120 mu m. The specific components of the TC4 titanium alloy powder are shown in Table 1, the main component is titanium, vanadium is an element with stable beta phase, aluminum is an element with stable alpha phase, and the addition of vanadium and aluminum ensures that the TC4 titanium alloy has excellent comprehensive performance. The original powder particles appeared to be uniformly spherical.
Table 1 is original TC4 powder chemistry (wt.%);
(2) laser stereoforming test:
the laser stereolithography test equipment is from the solidification technology national key laboratory, and the system is composed of PRC4000CO2The laser, DPSF-2 type high precision adjustable automatic powder feeder, coaxial powder feeding nozzle mechanism, the laser wavelength is 10.6 μm. To make the texture more uniform, a braided path deposition was performed with the as-deposited TC4 titanium alloy having dimensions of 45mm by 85mm by 15 mm. The laser three-dimensional forming process parameters are as follows: the power is 1500W, the diameter of a light spot is 2mm, the thickness of a layer is 0.5mm, the Z-axis stroke is 2.5mm, the powder feeding speed is 10g/min, and the whole forming process is carried out in an argon atmosphere.
(3) Determining heat treatment parameters:
in order to determine the heat treatment parameters, the phase transformation point of the laser stereo-morphology TC4 titanium alloy is tested by using a differential thermal analysis method. The phase transition temperature is determined by Differential thermal analysis, in which a sample to be measured and another reference sample are heated under the same conditions by means of a Differential thermal analysis device, and the state of a substance is determined according to the change relationship between the temperature difference and the temperature or time (DSC). Differential thermal analysis process parameters: reference sample is Al2O3Powder, the heating rate is 20 ℃/min, and the protective argon flow is 50 ml/min. Description of the drawings fig. 1 is a DSC curve of a laser stereomorphic TC4 titanium alloy measured by differential thermal analysis, from which curve: for the TC4 titanium alloy, the alpha-phase to beta-phase transition is an endothermic reaction, and the DSC curve decreases and reaches an extreme value when the temperature is 897 ℃, which indicates that the phase transition temperature is around 900 ℃.
According to the size of the sample, the solid solution heat preservation time is 0.5 h. The aging treatment is to promote the decomposition and transformation of metastable beta phase after the solid solution treatment and the holding at a higher temperature for a period of time, so as to generate the strengthening effect, and the performance can change along with the change of the holding time. The aging temperature of the titanium alloy is generally selected to be 500-600 ℃, and the heat preservation is carried out for 3-12 h.
A10 mm by 10mm cube was cut out from a bulk of TC4 titanium alloy in the as-deposited state, and the microstructure of each of three surfaces (XY, YZ, XZ) was observed as shown in the drawing, which is schematically illustrated in FIG. 2. And dividing the samples into eight equal parts, dividing the samples into two groups AB and carrying out solid solution aging treatment. The aging temperature of the A group is set to 550 ℃, and the solid solution temperature is changed. The aging temperature of the B group is taken as a variable, the solid solution temperature is fixed at 800 ℃, and the specific parameters of the heat treatment are shown in Table 2.
(4) Heat treatment test and determination of optimum heat treatment parameters:
a group A heat treatment test is carried out, the solid solution temperature is 800 ℃, 825 ℃, 850 ℃, 875 ℃, the solid solution heat preservation time is 0.5h, the aging temperature is 550 ℃, and the solid solution heat preservation time is 3 h. The attached figure 3 shows that the laser three-dimensional forming TC4 titanium alloy microstructures in different directions under different group A solid solution temperatures and the laser three-dimensional forming TC4 titanium alloy microstructures in different directions without heat treatment.
TABLE 2 solid solution aging treatment parameters
The results of FIG. 3 show that: the structure becomes more uniform after heat treatment. Solution treatment non-diffusion shear due to rapid cooling forms α' lath martensite, and during subsequent aging these metastable phases transform into stable and dispersed secondary α phases, which exhibit a basket structure. After the solid solution aging treatment, the grains grow to different degrees compared with the microstructure in a deposition state, along with the increase of the solid solution temperature, the coarsening of the grains is more obvious, the grain size is increased, and the grains are compared with the microstructure in the deposition state. After the YZ plane solution aging treatment, the alpha-phase crystal boundary is clearer, the XZ plane structure is more uniform, and the content of the XY plane alpha-phase is increased. Along with the increase of the solid solution temperature, the crystal grains are gradually coarsened, and the phase boundary is clearer after corrosion.
Example two
(1) The steps (2) and (3) are also based on the steps (1), (2) and (3) of the embodiment 1, and are not described again here.
(4) Heat treatment test and determination of optimum heat treatment parameters:
the heat treatment test of group B is carried out, the solid solution temperature is 800 ℃, the solid solution heat preservation time is 0.5h, the aging temperature is 500 ℃, 550 ℃, 600 ℃, 650 ℃, and the aging heat preservation time is 3 h. FIG. 4 is a schematic diagram of a laser stereolithography TC4 titanium alloy microstructure in different directions at different aging temperatures of group B and a laser stereolithography TC4 titanium alloy microstructure in different directions without heat treatment. FIG. 5 is a graph showing the variation of microhardness of a laser stereoformed TC4 titanium alloy with heat treatment parameters.
The results of FIG. 4 show that: the structure is alpha/alpha' lath martensite, the beta phase is retained between alpha lath grain boundaries, when the aging temperature is 500 ℃, the volume fraction occupied by the residual beta phase and the secondary alpha phase is higher, the secondary alpha phase is slender, the orientation is disordered, and the state of fine dispersion is presented. Along with the increase of the aging temperature, the precipitation of the secondary alpha phase is gradually reduced, the arrangement direction becomes single, and the trend of merging and coarsening appears. This is mainly due to the fact that in a fully equilibrated state, an increase in the ageing temperature reduces the nucleation rate of the secondary alpha, and the driving force for the transformation of the metastable beta phase to the secondary alpha is reduced. The coarsening of the crystal grains is obvious along with the increase of the aging temperature. The crystal grains are gradually combined and coarsened along with the increase of the aging temperature, and the phase boundary is clearer.
The results of FIG. 5 show that: the hardness in the YZ plane of the as-deposited state was 412.1HV, the XZ was 393.2HV, and the XY was 384.1HV, and the microhardness of the sample was significantly reduced after the solution aging treatment compared to the as-deposited state, since the lower the solution temperature (825 ℃ C.), the higher the aging temperature (650 ℃ C.), the more significant the decrease in microhardness was obtained after the solution aging treatment to obtain a basket structure. The microhardness rises with the rise of the solid solution temperature, and the hardness mainly plays a role in strengthening secondary alpha. The microhardness is reduced along with the increase of the aging temperature, mainly because the volume fraction of the secondary alpha is reduced due to the increase of the aging temperature, the dispersed and distributed small secondary alpha gradually disappears, and the dispersion strengthening effect is weakened. From the above-mentioned structural analysis, the YZ plane is a coarse regular bright and dark alternate columnar crystal structure and has the highest hardness, the XZ plane is an irregular columnar crystal structure and has a medium hardness, and the XY plane is a coarse isometric structure and has the lowest hardness.
The solid solution aging treatment is carried out on the TC4 titanium alloy in the laser three-dimensional form, the microstructure and the mechanical property of the titanium alloy are improved, and the analysis shows that the solid solution aging parameters No. 2 and No. 8 have the most obvious improvement on the structure property of the TC4 titanium alloy in the deposition state. The initial optimal solid solution aging parameters are as follows: the solid solution temperature is 825 ℃, the solid solution heat preservation time is 0.5h, the aging temperature is 550 ℃, and the aging heat preservation time is 3 h; the solid solution temperature is 800 ℃, the solid solution heat preservation time is 0.5h, the aging temperature is 650 ℃, and the aging heat preservation time is 3 h.
(5) Thermal deformation behavior of the laser stereo-forming shape TC4 titanium alloy under the optimal heat treatment parameters:
and selecting the sample under the two heat treatment parameters of No. 2 and No. 8 to carry out a thermal compression deformation test. The parameters of safe deformation zone in the hot working diagram (temperature 800 deg.C, strain rate 0.1 s) are selected as the hot compression parameters -160% deformation and 5min holding time) were subjected to a thermal compression test. FIG. 6 is a graph showing the true stress and true strain curves of a sample under optimum heat treatment process parameters; FIG. 7 is an IPF diagram of a laser stereo morphism TC4 titanium alloy before and after heat treatment and heat compression; the figure illustrates 8 is the grain size distribution ratio before and after thermal compression after the thermal treatment of the laser stereo-morphism TC4 titanium alloy; FIG. 9 is the orientation difference distribution before and after heat treatment of the laser stereo-morphism TC4 titanium alloy;
the results of the accompanying drawings, descriptions 6, 7, 8, 9, show that:
the true stress true strain curve shows that the peak stress of the No. 8 sample is the largest, the No. 2 sample is the smallest, the deposition state is centered, and the fact that the strength of the No. 8 heat treatment sample is the highest and the deformation resistance of the No. 2 heat treatment sample is small is indicated, so that the method is suitable for thermoplastic processing.
A YZ plane IPF diagram of a sample under No. 2 solid solution aging parameters (solid solution temperature: 825 ℃, heat preservation time: 0.5h, aging temperature: 550 ℃, heat preservation time: 3h) shows that the primary alpha phase grows and coarsens to form cross-distributed lath-shaped martensite, the average grain size is 1.06 mu m, and the orientations tend to be consistent. When the sample YZ plane IPF diagram shows that alpha-phase plate strips are also coarsened under the No. 8 solid solution aging parameters (the solid solution temperature is 800 ℃, the heat preservation time is 0.5h, the aging temperature is 650 ℃, and the heat preservation time is 3h), the average grain size is 0.70 mu m, and the complex difference of orientation distribution is large. The structure of an IPF (in-situ plasma) diagram of a large deformation zone of a sample subjected to thermal compression under the No. 2 solid solution aging parameter is uniform isometric crystals, which shows that the martensite laths are recrystallized after large deformation, the grains are finer and more uniform, and the average grain size is reduced to 0.36 mu m. The IPF diagram of the sample in the large deformation zone after hot compression under the No. 8 solid solution aging parameter shows that the crystal grains are finer, the average crystal grain size is 0.32 mu m, the fine crystal anisotropy is small, the strength is high, the toughness is good, the grain boundary area is large, and dislocation and intercrystalline slippage can be effectively prevented.
The accompanying drawings illustrate the proportion of the grain size distribution before and after thermal compression after the heat treatment of the laser stereomorphic TC4 titanium alloy in FIG. 8, it can be seen that the grains are mainly small-sized grains, the sizes are intensively distributed in the range of 0.05-3 μm, for the heat treatment sample pictures (a), (b), there are a few large-sized grains, which are caused by the abnormal growth of a few grains during aging, for the thermal compression sample pictures (c), (d), the grains are recrystallized due to large deformation and heat input, and the grains are distributed uniformly and finely.
The accompanying drawings illustrate the misorientation distribution of the laser three-dimensional forming TC4 titanium alloy after heat treatment and before and after heat compression, and for heat treatment sample graphs (a) and (b), the misorientation distribution mainly has large-angle grain boundaries, reaches more than ninety percent, has three obvious peaks and is a typical martensite structure misorientation distribution. For the hot-compressed samples, the high angle grain boundary fraction was reduced, and the graph (c) shows that the high angle grain boundary fraction was 73% for the hot-compressed samples under heat treatment parameter No. 2, indicating that recrystallization occurred and that the low angle grain boundaries were continuously transitioning to the high angle grain boundaries, indicating that continuous dynamic recrystallization occurred. In the graph (d), the sample was thermally compressed under heat treatment parameter No. 8, the proportion of large angle grain boundaries was 83%, the degree of recrystallization was reduced, and the large and small angle grain boundary transitions were not sufficiently continuous, as compared with the thermally compressed sample under heat treatment parameter No. 2.
In summary, the thermal deformation capability of the TC4 titanium alloy in the laser three-dimensional form is improved to some extent by the No. 2 thermal treatment process and the No. 8 thermal treatment process. The heat treatment parameter No. 2 is smaller in deformation resistance and more favorable for thermal deformation than the heat treatment parameter No. 8. In terms of microstructure, the alloy martensite lath structure under the heat treatment parameter No. 2 is more uniform and more consistent in orientation compared with the heat treatment parameter No. 8. In the thermal deformation process under the same thermal compression parameters, the No. 2 thermal treatment sample is subjected to continuous dynamic recrystallization, and isometric crystals in a large deformation area are fine and uniform. Therefore, the alloy has better thermal deformation performance under the heat treatment parameters No. 2 (the solid solution temperature is 825 ℃, the heat preservation time is 0.5h, the aging temperature is 550 ℃, and the heat preservation time is 3 h).
The results of example 1 and example 2 show that: the thermal deformation capability of the TC4 titanium alloy in the laser three-dimensional form is improved to a certain extent by the No. 2 solid solution aging parameter (solid solution temperature: 825 ℃, heat preservation time: 0.5h, aging temperature: 550 ℃, heat preservation time: 3h) and the No. 8 solid solution aging parameter (solid solution temperature: 800 ℃, heat preservation time: 0.5h, aging temperature: 650 ℃, heat preservation time: 3 h). However, in the thermal deformation process under the same thermal compression parameters, the No. 2 heat treatment sample is subjected to continuous dynamic recrystallization, and isometric crystals in a large deformation area are fine and uniform. Therefore, the alloy has better thermal deformation performance under the heat treatment parameters No. 2 (solid solution temperature: 825 ℃, heat preservation time: 0.5h, aging temperature: 550 ℃, heat preservation time: 3 h).
The above are merely preferred embodiments of the present invention, but the scope of the present invention should not be limited thereby; therefore, all the equivalent changes and modifications made in the claims of the present invention should be covered by the scope of the present invention.
Claims (9)
1. A heat treatment method for a TC4 titanium alloy in a laser three-dimensional forming mode is characterized by comprising the following steps:
step 1: preparing a TC4 titanium alloy forging substrate and TC4 titanium alloy spherical powder respectively;
step 2: laser stereoforming test: placing TC4 titanium alloy spherical powder in a test instrument, determining laser three-dimensional forming process parameters, and performing the forming process in an argon atmosphere; during testing, a test instrument continuously and uniformly sprays TC4 titanium alloy spherical powder to one surface of a TC4 titanium alloy forging substrate; when the powder thickness meets the requirement, stopping the test; obtaining a laser three-dimensional forming TC4 titanium alloy;
and step 3: carrying out heat treatment on the TC4 titanium alloy obtained in the step 2 in the laser stereo-morphology, and comprising the following steps:
step 3.1: determining heat treatment parameters, including the following parts:
step 3.1.1: testing the phase transformation point of the laser stereo-morphology TC4 titanium alloy by using a differential thermal analysis method to obtain the phase transformation temperature;
step 3.1.2: determining that the solid solution temperature of the laser stereo-forming TC4 titanium alloy changes above 800 ℃ according to the phase transition temperature, wherein the heat preservation time is determined by a formula 1: t ═ 5-8) + AD (1), where: t-holding time; a-temperature-keeping time coefficient/min mm < -1 >, D-effective thickness/mm of the workpiece, and the adopted solid solution temperature-keeping time is 0.5 h.
Step 3.1.3: the aging treatment of the laser three-dimensional forming TC4 titanium alloy is to promote the decomposition and transformation of metastable beta phase after the aging treatment is carried out and the maintenance is carried out for a period of time at a higher temperature after the solution treatment, so as to generate the strengthening effect, therefore, the aging temperature is selected to be between 500 and 600 ℃, and the heat preservation is carried out for 3 to 12 hours.
Step 3.1.4: solid solution temperature (the solid solution temperature of the laser stereo-morphology TC4 titanium alloy is determined to be above 800 ℃ according to the phase transition temperature)
Step 3.2: carrying out a plurality of groups of heat treatment tests, obtaining the heat-treated laser three-dimensional forming TC4 titanium alloy through the tests, observing the microstructure and the mechanical property of the titanium alloy under different heat treatment parameters, and obtaining the most available heat treatment process parameters of the laser three-dimensional forming TC4 titanium alloy;
a plurality of tests are divided into two groups AB, wherein the group A changes the solid solution temperature and keeps the aging temperature unchanged, and the group B changes the aging temperature and keeps the solid solution temperature unchanged; the variation of the solid solution temperature is 25 ℃, and the variation of the aging temperature is 50 ℃;
and 4, step 4: and cutting a metallographic sample of the laser three-dimensional forming TC4 titanium alloy, observing the microstructure of the metallographic sample, and carrying out microhardness test on the metallographic sample to obtain a microhardness change curve along with heat treatment parameters, thereby finally obtaining the optimal heat treatment parameters of the laser three-dimensional forming TC4 titanium alloy.
2. The method for heat-treating the TC4 titanium alloy in the laser three-dimensional form according to claim 1, wherein the thickness of the powder in the step 2 is required to be 10mm or more.
3. The heat treatment method for the laser three-dimensional morphological TC4 titanium alloy, as claimed in claim 1, wherein the TC4 titanium alloy forging substrate in the step 1 is a forged and annealed TC4 titanium alloy plate.
4. The method for heat-treating the TC4 titanium alloy in the laser stereomorphic form according to claim 1, wherein the TC4 titanium alloy powder in the step 1 is TC4 titanium alloy powder prepared by a plasma rotating electrode, and the powder particle size is 80-120 μm.
5. The heat treatment method of the laser stereolithography TC4 titanium alloy according to claim 1, wherein the laser stereolithography process parameters in step 2 are: the power is 1500W, the diameter of a facula is 2mm, the layer thickness is 0.5mm, the Z-axis stroke is 2.5mm, and the powder feeding speed is 10 g/min.
6. The method for heat-treating a TC4 titanium alloy in accordance with claim 1, wherein the heat treatment test in step 3.2 is a heat treatment test of a cut TC4 titanium alloy block, and the microstructure of the three XY, YZ and XZ surfaces of the titanium alloy block is observed after the heat treatment test.
7. The method for heat-treating the TC4 titanium alloy in the laser stereomorphic form according to claim 1, wherein the process parameters of the differential thermal analysis in the step 3.1.1 are as follows: the reference sample is Al2O3 powder, the heating rate is 20 ℃/min, and the protective argon flow is 50 ml/min.
8. The method for heat-treating the laser stereo-form TC4 titanium alloy as claimed in claim 1, wherein the metallographic specimen of the laser stereo-form TC4 titanium alloy is cut in the step 4 and is prepared by the following steps: and (3) grinding and polishing the metallographic specimen, then corroding the metallographic specimen by using a corrosive liquid (HF: HNO 3: H2O: 10: 5: 85), wiping and cleaning the metallographic specimen by using alcohol, and drying the metallographic specimen by using a blower to ensure that the cross section has no dirt or water stain.
9. The heat treatment method of the laser stereo-morphology TC4 titanium alloy, as claimed in claim 1, wherein the variation curve of the microhardness of the laser stereo-morphology TC4 titanium alloy obtained in the step 4 along with the heat treatment parameters is as follows: the Vickers hardness was measured using a DHV-1000Z hardness machine. And (3) grinding the surface to be measured of the hot-compressed sample by using metallographic abrasive paper until no obvious scratch exists, and ensuring that the upper surface and the lower surface of the sample are parallel. The load used was 1kg/mm2, dwell time 15 s. And measuring the hardness values of the test surface in three directions, wherein each test surface takes 10 test points, and the average value is taken as the microhardness value of the test surface.
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