Novel hard alloy cutter for cutting titanium alloy and preparation method of novel hard alloy cutter

Abstract

The invention relates to the technical field of metal powder manufacturing products, and discloses a novel hard alloy cutter for titanium alloy cutting machining and a preparation method of the novel hard alloy cutter. A base body of the hard alloy cutter is composed of Co, Ni and Fe; wherein the molar percentage of each element is 5-35%, and the component ratio of Co to Ni to Fe is (3-6): (2-4): (1-5); the preparation method comprises the steps that Co powder, Ni powder, Fe powder and Al powder or Ti powder or Cr powder are adopted, the selected four kinds of element powder are mixed according to the molar ratio of all the elements being 5%-35%, and binding phase powder is obtained; mixing the binding phase powder with WC powder, and adding a grain inhibitor for mixing to obtain composite powder; and the composite powder is ground, dried, pressed and sintered, and then the hard alloy cutter base body is obtained. The medium / high-entropy alloy binder hard alloy with a small number of principal elements is combined with the grain inhibitor to prepare the hard alloy cutter base body, the service performance of the hard alloy cutter is improved, and the machining efficiency and precision of titanium alloy products are optimized.

Description

Technical Field

[0001] The invention relates to the technical field of metal powder manufacturing products, and in particular to a new type of cemented carbide tool for titanium alloy cutting and a preparation method thereof. Background Art

[0002] Titanium alloy is a high-strength, lightweight material with excellent mechanical properties and corrosion resistance, and is widely used in aerospace, automotive, biomedical and other fields. However, due to the high strength, low thermal conductivity, high chemical reactivity and low elastic modulus of titanium alloy, the processing difficulty of titanium alloy remains high, making high-speed and high-precision processing of titanium alloy a difficult problem.

[0003] Currently, commercial cemented carbide tools are widely used in the cutting of titanium alloys, but there are still problems such as low processing efficiency, easy peeling, and low chip removal rate. With the rapid development of the manufacturing industry, the proportion of titanium alloy applications has increased year by year, and the requirements for titanium alloy cutting efficiency and cutting accuracy in industrial applications are also constantly increasing, which has also put forward higher performance requirements for cemented carbide tools for cutting titanium alloys.

[0004] In order to improve the cutting performance of cemented carbide tools, the current optimization solutions for cemented carbide tools are mainly focused on the tool surface coating structure, micro-texture design, and tool geometry optimization. These solutions effectively improve the tool cutting force to a certain extent, and optimize the thermal management and chip removal capabilities during the cutting process. However, due to the limited strength and high-temperature mechanical properties of the tool base material, it is still difficult to meet the requirements of efficient and precise processing when facing difficult-to-process materials such as titanium alloys. Summary of the invention

[0005] The present invention aims to provide a new type of cemented carbide tool for titanium alloy cutting and a preparation method thereof, so as to solve the problem that cemented carbide tools in the prior art are difficult to meet the high-precision and high-efficiency processing requirements for titanium alloy materials.

[0006] To achieve the above purpose, the present invention adopts the following technical scheme, a new type of cemented carbide tool for titanium alloy cutting and its preparation method, the scheme adopts a combination of medium / high entropy alloy binder cemented carbide with a small number of principal elements and a grain growth inhibitor to obtain a new type of cemented carbide tool, improve the service performance of the cemented carbide tool, and optimize the processing efficiency and precision of titanium alloy products. Among them, the cemented carbide tool includes a matrix of the cemented carbide tool composed of the following elements: Co, Ni, Fe; wherein the molar percentage of each element is 5% to 35%, and the mass ratio of the components is Co: Ni: Fe = 3 to 6: 2 to 4: 1 to 5; and is doped with a grain inhibitor.

[0007] Furthermore, the Al element is included, the particle size of which is selected to be 1 to 4 μm, and the mass ratio of the Al element is 2 to 13 wt.%.

[0008] Furthermore, it also includes Ti element, the particle size of which is selected to be 1-4 μm, and the mass ratio of which is 3-24 wt.%.

[0009] Furthermore, it also includes Cr element, the particle size of which is selected to be 1 to 4 μm, and the component mass ratio is 3 to 21 wt.%.

[0010] Another object of the present invention is a method for preparing the above-mentioned cemented carbide tool, comprising the following steps:

[0011] Step 1, preparation of bonding phase powder: using three elements of Co powder, Ni powder and Fe powder as bonding phase, the mass ratio of which is Co:Ni:Fe=3-6:2-4:1-5; and adding Al powder or Ti powder or Cr powder, the particle size of which is selected to be 1-4 μm; mixing the selected four element powders according to the molar ratio of each element of 5% to 35% to obtain bonding phase powder;

[0012] Step 2, composite powder preparation: select WC powder with a particle size of 0.5 to 1.2 μm, mix the WC powder with a binder phase powder; and add a grain inhibitor for mixing, wherein the grain inhibitor accounts for 0.2 to 1.2 wt.%, to obtain a composite powder;

[0013] Step 3, preparation of cemented carbide powder: putting the composite powder into a ball mill, adding paraffin, protective atmosphere and alcohol, grinding according to the set grinding parameters, and drying after grinding to obtain cemented carbide powder;

[0014] Step 4, sample forming: pressing the cemented carbide powder into a compact, and sintering the compact according to a sintering method to obtain a cemented carbide tool matrix.

[0015] Furthermore, in step 1, the particle sizes of Co powder, Ni powder and Fe powder are 1.5-3.8 μm, and the mass ratio of the components is Co:Ni:Fe=5:4:1 wt.%.

[0016] Furthermore, in step 2, the grain inhibitor is one or more of Cr3C2, VC, and TaC; the proportion of the grain inhibitor is set to 0.4 to 0.9 wt.%.

[0017] Furthermore, in step 3, the proportion of paraffin added is 2wt.%; the alcohol is the ball milling medium, and the ball-to-material ratio is 2:1-8:1; the grinding parameters are set as: mill speed 50-120r / min, grinding time 12-48h; when drying, the drying temperature is set to 60-100°C, and the drying time is 40-80min.

[0018] Furthermore, in step 4, the sintering method is to use a sintering temperature of 1350-1470° C. and low-pressure sintering for 1 hour.

[0019] Furthermore, the protective atmosphere is nitrogen or argon; in step 3, it also includes granulating and sieving after drying to obtain cemented carbide powder.

[0020] The principles and advantages of this solution are:

[0021] The performance of the tool matrix is ​​the primary prerequisite for the high-quality development of the tool. The traditional cemented carbide tool matrix is ​​mainly composed of Co as the bonding phase and WC as the hard phase. It has excellent hardness, wear resistance and elastic modulus. However, with the manufacturing industry's increasing requirements for tool material performance, traditional cemented carbide tool matrix materials are gradually unable to meet the requirements.

[0022] At present, medium / high entropy alloys are used as cemented carbide binders, which show better hardness, fracture toughness, corrosion resistance and high-temperature oxidation resistance than Co binder cemented carbide. Medium / high entropy alloys are a type of multi-principal alloys, composed of five or more elements in almost equal percentages. These alloy structures usually have obvious lattice distortion and solid solution strengthening characteristics, so they have unique structures and excellent high-temperature mechanical properties. In order to further improve the performance of medium / high entropy alloys, the existing technology often adopts the method of adding elements with corresponding properties and adjusting the corresponding element ratio to achieve this. However, if the number of principal elements is too large, it will lead to more serious lattice distortion and element supersaturation segregation of medium / high entropy alloys, making their toughness lower and difficult to meet the strength and toughness requirements required for high-temperature cutting. However, if the number of principal elements is too small, the diffusion hysteresis effect of medium / high entropy alloys will be weakened, and the abnormal growth of WC grains will increase, resulting in a decrease in the hardness and high-temperature strength of the cemented carbide, and it will be difficult to meet the performance requirements. Therefore, in the preparation of cemented carbide, at least four or more elements will be used to ensure the performance requirements. For example, the patent with publication number CN104372230A discloses a solution containing at least five elements such as Ni, Fe, Co, Cr, and Al to improve the performance of cemented carbide.

[0023] In addition, since the main components of medium / high entropy alloys are solid-dissolved with each other, the interactions between atoms are complex, and the lattice distortion is serious, the alloy structure has a strong diffusion hysteresis effect. The intensity of this effect is directly affected by the number of components and the physical and chemical properties of the components. Under this effect, the dissolution and precipitation process of the components will be inhibited. Therefore, in the preparation process of medium / high entropy alloy binder cemented carbide, it is generally not necessary to use grain inhibitors, which will increase the process and cost, and the addition of grain inhibitors may also affect other properties of the alloy, so generally no additional grain inhibitors are added.

[0024] In the prior art, in order to improve the comprehensive ability of cemented carbide, the general way is to further add elements with corresponding excellent performance, without considering the reverse reduction of synthetic elements. However, this scheme breaks away from the inherent thinking of increasing the number of principal alloy preparation methods, but adopts the method of reducing the number of principal elements, and uses 3 to 4 kinds of critical point minor principal elements as constituent elements to prepare the cemented carbide matrix. The reason is that we found that after adding too many principal elements, it will cause serious lattice distortion and element supersaturation segregation of medium / high entropy alloys, making their toughness lower and difficult to meet the strength and toughness requirements required for high-temperature cutting. Therefore, 5 or more elements are often selected. The reason why this scheme can achieve 3 to 4 minor principal elements is that this scheme combines effective composition mass ratio parameters for configuration, and adds grain inhibitors that are generally not used in the preparation process to prepare ultrafine-grained high-performance cemented carbide tool substrates, that is, without increasing too many principal elements, high-performance service requirements can be met. At the same time, by reasonably controlling the dosage ratio of grain inhibitors, it will not affect the solid solution between the main components of medium / high entropy alloys, thereby achieving the inhibitory effect itself, and can also inhibit the abnormal growth of WC grains, thereby improving the performance of cemented carbide, breaking away from the technical prejudice that grain inhibitors are not needed, effectively improving the high-temperature service performance of cemented carbide tools, and improving the processing efficiency and precision of titanium alloy products. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a schematic flow chart of a novel cemented carbide tool for titanium alloy cutting and a preparation method thereof.

[0026] Figure 2 The present invention discloses a novel cemented carbide tool for titanium alloy cutting and a preparation method thereof, and shows the microstructure morphology and particle size distribution structure of cemented carbide with medium / high speed binder having different Cr3C2 contents.

[0027] Figure 3 The present invention discloses a new type of cemented carbide tool for titanium alloy cutting and a preparation method thereof, and a bending strength and hardness curve diagram of WC-CoNiFe with different Cr3C2 contents. DETAILED DESCRIPTION

[0028] The following is further described in detail through specific implementation methods:

[0029] Example 1

[0030] As attached Figure 1As shown, this scheme uses a medium / high entropy alloy with a low number of principal elements (≤4) as a binder, and dopes a grain inhibitor to prepare an ultrafine-grained high-performance cemented carbide tool matrix to improve the high-temperature service performance of the cemented carbide tool. In this embodiment, the matrix of the cemented carbide tool is composed of the following elements: Co, Ni, Fe, and Al powder is added; wherein the molar percentage of each element is 5% to 35%, and the mass ratio of the components is Co: Ni: Fe = 3 to 6: 2 to 4: 1 to 5; and is doped with a grain inhibitor.

[0031] Its preparation method is:

[0032] S1, preparation of bonding phase powder:

[0033] Co powder, Ni powder and Fe powder are selected as the bonding phase, and the mass ratio of the components is Co:Ni:Fe=3-6:2-4:1-5. Co powder, Ni powder and Fe powder are mixed, and Al powder is added, and the particle size is selected to be 1-4 μm, accounting for 2-13wt.%. After mixing, the molar ratio of the four element powders is 5%-35%, and the bonding phase powder is obtained.

[0034] In this embodiment, 1.5-3.8 μm Co powder, Ni powder and Fe powder are selected, and the Co powder, Ni powder and Fe powder are mixed in a ratio of Co:Ni:Fe=5:4:1wt.%, and the selected element powders are pre-mixed evenly using a three-dimensional mixer to obtain CoNiFe bonding phase powder.

[0035] S2, composite powder preparation:

[0036] Fine-grained WC powder is selected, and its particle size range is 0.5-1.2 μm. The WC powder is mixed with a bonding phase powder to obtain a WC-CoNiFe cemented carbide powder. In this embodiment, a WC powder with a particle size of 0.8 μm is selected, and the bonding phase powder accounts for 6-10 wt.%. Subsequently, a grain inhibitor accounting for 0.2-1.2 wt.% is added to the obtained WC-CoNiFe cemented carbide powder for mixing. In this embodiment, the grain inhibitor is one or more of Cr3C2, VC, and TaC, and the grain inhibitor ratio is preferably set to 0.4-0.9 wt.%.

[0037] Specifically, in this embodiment, the proportion of Cr3C2 grain inhibitor added to WC-CoNiFe cemented carbide powder is: 99.4wt.% of WC-CoNiFe cemented carbide powder and 0.6wt.% of Cr3C2 grain inhibitor are mixed to obtain a composite powder.

[0038] S3, cemented carbide powder preparation:

[0039] The obtained composite powder is put into a ball mill, and paraffin, protective atmosphere and alcohol are added, and the powder is ground according to the set grinding parameters, and the powder is dried after grinding to obtain cemented carbide powder.

[0040] In this embodiment, the composite powder is placed in a drum ball mill, 2wt.% paraffin is added, and a protective atmosphere is filled, wherein the protective atmosphere can be nitrogen or argon. In this embodiment, nitrogen is selected to fill the protective atmosphere. Alcohol is the ball milling medium, and its ball-to-material ratio is 2:1 to 8:1. In this embodiment, the ball-to-material ratio is preferably set to 4:1 to 6:1.

[0041] The grinding parameters are set as follows: mill speed 50-120 r / min, grinding time 12-48 h; preferably, mill speed 70-90 r / min, grinding time 24-36 h. In this embodiment, the ball-to-material ratio is set to 4:1, the mill speed is 80 r / min, and the grinding time is 24 h.

[0042] The wet abrasive after ball milling is dried in an electric hot air drying oven at a drying temperature of 60 to 100° C. for a drying time of 40 to 80 min. In this embodiment, the wet abrasive after ball milling is dried in an electric hot air drying oven at a drying temperature of 80° C. for 60 min, granulated, and sieved through a 600-mesh sieve to obtain a medium / high entropy binder cemented carbide powder.

[0043] S4, sample forming:

[0044] The obtained cemented carbide powder is pressed into a compact, and the compact is sintered according to a sintering method to obtain a cemented carbide tool matrix.

[0045] The sintering method is to use a sintering temperature of 1350-1470°C, preferably 1410-1450°C, and low pressure sintering for 1 hour. In this embodiment, the compact is sintered at a sintering temperature of 1430°C for 1 hour to obtain a new medium / high entropy binder cemented carbide tool matrix, which is labeled as sample C for comparison. The matrix microstructure and grain size are shown in the attached Figure 2 (c) shows the bending strength and hardness. Figure 3 As shown, the hardness is 91.25HRA and the fracture toughness is 11.18MPa·m 1 / 2 , the flexural strength is 3883.2MPa.

[0046] Example 2

[0047] Different from Example 1, in this embodiment, the matrix of the cemented carbide tool uses three elements of Co, Ni, and Fe, and Ti powder is added. During the preparation process, the Ti powder accounts for 3-24wt.%. The preparation process is the same as that of Example 1 to obtain a cemented carbide tool matrix.

[0048] Example 3

[0049] Different from Example 1, in this embodiment, the matrix of the cemented carbide tool uses three elements of Co, Ni and Fe, and Cr powder is added. During the preparation process, the Cr powder accounts for 3-21wt.%. The preparation process is the same as that of Example 1 to obtain a cemented carbide tool matrix.

[0050] Comparative Example 1

[0051] The difference between this comparative example and Example 1 is that in the preparation step S2, the proportion of the added Cr3C2 grain inhibitor is: WC-CoNiFe cemented carbide powder 100wt.%, Cr3C2 grain inhibitor 0wt.%, that is, no grain inhibitor is added to obtain a composite powder. Finally, a new medium / high entropy binder cemented carbide tool matrix is ​​obtained, which is marked as sample A. The matrix microstructure and grain size are shown in the attached Figure 2 (a) shows the bending strength and hardness. Figure 3 As shown, the grain size is 1.28 μm, the hardness is 90.1 HRA, and the fracture toughness is 17.8 MPa·m 1 / 2 , the flexural strength is 3533.86MPa.

[0052] Comparative Example 2

[0053] The difference between this comparative example and Example 1 is that in the preparation step S2, the proportion of the added Cr3C2 grain inhibitor is: WC-CoNiFe cemented carbide powder 99.8wt.%, Cr3C2 grain inhibitor 0.2wt.% mixed to obtain a composite powder. Finally, a new medium / high entropy binder cemented carbide tool matrix is ​​obtained, which is marked as sample B. The matrix microstructure and grain size are shown in the attached Figure 2 (b) shows the bending strength and hardness. Figure 3 shown.

[0054] Comparative Example 3

[0055] The difference between this comparative example and Example 1 is that in the preparation step S2, the proportion of the added Cr3C2 grain inhibitor is: 99.1wt.% of WC-CoNiFe cemented carbide powder and 0.9wt.% of Cr3C2 grain inhibitor are mixed to obtain a composite powder. Finally, a new medium / high entropy binder cemented carbide tool matrix is ​​obtained, which is marked as sample D. The matrix microstructure and grain size are shown in the attached Figure 2 (d) as shown, the bending strength and hardness are as shown in the attached Figure 3 shown.

[0056] Comparative Example 4

[0057] The difference between this comparative example and Example 1 is that in the preparation step S2, the proportion of the added Cr3C2 grain inhibitor is: WC-CoNiFe cemented carbide powder 98wt.%, Cr3C2 grain inhibitor, and agent 2wt.% are mixed to obtain a composite powder. Finally, a new medium / high entropy binder cemented carbide tool matrix is ​​obtained, which is marked as sample E. The matrix microstructure and grain size are shown in the attached Figure 2 (e) as shown, the bending strength and hardness are as shown in the attached Figure 3 shown.

[0058] From the attached Figure 2 To Attachment Figure 3 From the microstructure and grain size, it can be seen that sample C with 0.6wt.% Cr3C2 grain inhibitor addition has the best comprehensive mechanical properties, with a hardness of 91.25HRA and a fracture toughness of 11.18MPa·m 1 / 2 , the flexural strength is 3883.2MPa.

[0059] At the same time, the new medium / high entropy binder cemented carbide tool substrates prepared in Example 1 and Comparative Examples 1 to 4 and the current mainstream GY83 cemented carbide tool were used to cut 42CrMo alloy, respectively, and the processing parameters were: feed F was 0.25mm / min, cutting depth was 2mm, and water-soluble cutting fluid was used. (Note: In order to intuitively verify the performance of the cemented carbide tool substrate prepared by the present invention, the tool coating will not be constructed in the service performance test, and the tool substrate will be directly used for testing). The service performance test data is shown in Table 1:

[0060] Table 1

[0061] Time / cutting length GY83 A B C D E 1h / 15mm intact intact intact intact intact chipping edge 2h / 30mm intact Slight chipping intact intact intact / 3h / 45mm chipping edge / intact intact intact / 4h / 60mm / / chipping edge intact intact / 5h / 75mm / / / Slight chipping Slight chipping /

[0062] From the above test results, it can be seen that in this scheme, the performance of the new medium / high entropy binder cemented carbide tool matrix prepared after adding grain inhibitors is better than that of the cemented carbide tool prepared without adding grain inhibitors, and the added grain inhibitor accounts for 0.2~1.2wt.%, preferably 0.4~0.9wt.%, thereby preparing an ultrafine-grained high-performance cemented carbide tool matrix and improving the high-temperature service performance of the cemented carbide tool.

[0063] In this embodiment, the method of reducing the number of principal elements is adopted, and 3 to 4 kinds of minor principal elements are used as constituent elements to prepare the cemented carbide matrix, jumping out of the inherent preparation method of the number of principal element alloys, and combining the configured component mass ratio parameters to avoid the problem of too many principal elements in the medium / high entropy alloy, resulting in serious lattice distortion and element supersaturation segregation, and causing low toughness; at the same time, it also overcomes the problem that the number of principal elements is small, the diffusion hysteresis effect of the medium / high entropy alloy is weakened, and the abnormal growth of WC grains increases, which reduces the hardness and high-temperature strength of the cemented carbide. In addition, in the preparation process, grain inhibitors that are generally not added are used to break the inherent cognitive limitations of the industry, and by reasonably controlling the dosage ratio of grain inhibitors, ultrafine-grained high-performance cemented carbide tool substrates are prepared, effectively improving the high-temperature service performance of cemented carbide tools, and optimizing the processing efficiency and precision of titanium alloy products.

[0064] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A new type of cemented carbide tool for titanium alloy cutting, characterized in that: The matrix of the cemented carbide tool is composed of the following elements: Co, Ni, Fe; the molar percentage of each element is 5% to 35%, the mass ratio of the components is Co:Ni:Fe=3 to 6:2 to 4:1 to 5; and the matrix is ​​doped with a grain inhibitor.

2. The new cemented carbide tool for titanium alloy cutting according to claim 1, characterized in that: It also includes Al element, the particle size of which is selected to be 1 to 4 μm, and the mass ratio of the component is 2 to 13 wt.%.

3. The new cemented carbide tool for titanium alloy cutting according to claim 1, characterized in that: It also includes Ti element, the particle size of which is selected to be 1 to 4 μm, and the mass ratio of which is 3 to 24 wt.%.

4. The new cemented carbide tool for titanium alloy cutting according to claim 1, characterized in that: It also includes Cr element, the particle size of which is selected to be 1 to 4 μm, and the mass ratio of the component is 3 to 21 wt.%.

5. A method for preparing a new type of cemented carbide tool for titanium alloy cutting, characterized in that: A new type of cemented carbide tool for titanium alloy cutting as claimed in any one of claims 1 to 4 comprises the following steps: Step 1, preparation of bonding phase powder: using three elements of Co powder, Ni powder and Fe powder as bonding phase, the mass ratio of which is Co:Ni:Fe=3-6:2-4:1-5; and adding Al powder or Ti powder or Cr powder, the particle size of which is selected to be 1-4 μm; mixing the selected four element powders according to the molar ratio of each element of 5% to 35% to obtain bonding phase powder; Step 2, composite powder preparation: select WC powder with a particle size of 0.5 to 1.2 μm, mix the WC powder with a binder phase powder; and add a grain inhibitor for mixing, wherein the grain inhibitor accounts for 0.2 to 1.2 wt.%, to obtain a composite powder; Step 3, preparation of cemented carbide powder: putting the composite powder into a ball mill, adding paraffin, protective atmosphere and alcohol, grinding according to the set grinding parameters, and drying after grinding to obtain cemented carbide powder; Step 4, sample forming: pressing the cemented carbide powder into a compact, and sintering the compact according to a sintering method to obtain a cemented carbide tool matrix.

6. The method for preparing a new type of cemented carbide tool for titanium alloy cutting according to claim 5, characterized in that: In step 1, the particle sizes of Co powder, Ni powder and Fe powder are 1.5-3.8 μm, and the mass ratio of the components is Co:Ni:Fe=5:4:1wt.%.

7. The method for preparing a new type of cemented carbide tool for titanium alloy cutting according to claim 5, characterized in that: In step 2, the grain inhibitor is one or more of Cr3C2, VC, and TaC; the proportion of the grain inhibitor is set to 0.4-0.9wt.%.

8. The method for preparing a new type of cemented carbide tool for titanium alloy cutting according to claim 5, characterized in that: In step 3, the proportion of paraffin added is 2wt.%; the alcohol is the ball milling medium, and the ball-to-material ratio is 2:1-8:1; the grinding parameters are set as: mill speed 50-120r / min, grinding time 12-48h; when drying, the drying temperature is set to 60-100°C, and the drying time is 40-80min.

9. The method for preparing a new type of cemented carbide tool for titanium alloy cutting according to claim 5, characterized in that: In step 4, the sintering method is to use a sintering temperature of 1350-1470° C. and low-pressure sintering for 1 hour.

10. The method for preparing a new type of cemented carbide tool for titanium alloy cutting according to claim 5, characterized in that: The protective atmosphere is nitrogen or argon; in step 3, it also includes granulating and sieving after drying to obtain cemented carbide powder.

Images