Method suitable for improving structure uniformity of AZ-series magnesium alloy manufactured through electric arc additive

Abstract

The invention discloses a method suitable for improving the structure uniformity of an AZ-series magnesium alloy manufactured through an electric arc additive. The method comprises the steps that firstly, a hot-rolled AZ31 plate is selected as a base plate; 2, performing three-dimensional modeling and slicing treatment according to a target magnesium alloy workpiece, and designing a deposition path; 3, preheating the substrate, adjusting the distance between the electrode and the substrate, and introducing protective gas; and fourthly, according to the deposition path, electric arc additive manufacturing is conducted in an inner layer rotating deposition mode, and the AZ series magnesium alloy workpiece is obtained. Plasma arc welding is used as a heat source, the preparation speed is high, heat input control is accurate, the arc stiffness is good, splashing is effectively reduced, and the workpiece formability is good; a rotary deposition mode is adopted, thermal circulation is effectively controlled, abnormal structure growth is avoided, meanwhile, the means of interlayer hammering deformation is assisted, a deposition interface is optimized, the obtained AZ series magnesium alloy workpiece is uniform in structure, fine in grain, few in defect and excellent in strength and plasticity, and the process is simple and easy to popularize.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive manufacturing of metal materials, and in particular relates to a method for improving the uniformity of the structure of AZ series magnesium alloys manufactured by electric arc additive manufacturing. Background Art

[0002] With the rapid development of the manufacturing industry towards high performance and lightweight, magnesium alloy, as the lightest metal structural material in current engineering applications, has received widespread attention. Its many advantages, such as low density, high specific strength, and good shock absorption performance, make it extremely potential for application in aerospace, automobile manufacturing, biomedicine and other fields. However, traditional magnesium alloy workpieces are mostly formed by casting, but this method has problems such as long development cycle, high mold cost, complex processing procedures and poor organizational stability, which makes it difficult to meet the needs of products with complex structures and excellent strength and plasticity. Against this background, magnesium alloy additive manufacturing technology came into being. Using metal additive manufacturing technology instead of traditional casting technology to prepare magnesium alloy workpieces has become a new development trend, especially in the design and trial production stage of special components.

[0003] At present, there are two main types of magnesium alloy additive manufacturing technologies: selective laser melting (SLM) and arc additive manufacturing (WAAM). The raw materials used in the two methods are powder and wire, respectively. In the SLM forming process, due to the physical properties of magnesium and its alloys, such as high vapor pressure, low boiling point and low density, it is easy to cause violent splashing of metal powder, resulting in a large amount of magnesium alloy powder flying off the powder bed, reducing the melting amount of the molten pool alloy, and causing serious waste of raw materials; in addition, the fluctuation of the melting amount of the molten pool significantly reduces the stability of the SLM process, resulting in defects such as holes and cracks inside the formed magnesium alloy workpiece, which deteriorates the quality of the workpiece. For the above reasons, SLM forming technology is difficult to be widely used in the field of magnesium alloy additive manufacturing. In comparison, the heat sources used in WAAM technology are mainly: tungsten inert gas arc welding (TIG) heat source, metal inert gas welding (MIG / MAG) heat source and plasma arc welding (PAW) heat source, and the required raw materials are magnesium alloy wire. This method uses arc heat to melt and deposit metal wire to manufacture magnesium alloy workpieces. This method can effectively avoid the problems of low evaporation temperature, high vapor pressure and insufficient material utilization of magnesium alloy powder, and is an ideal magnesium alloy additive manufacturing method.

[0004] In the research process of magnesium alloy arc additive manufacturing, AZ series magnesium alloy wires are often selected as the research object. The reason is that AZ series magnesium alloys have good corrosion resistance, casting properties and mechanical properties; at the same time, AZ series alloys have better melt pool stability during the arc additive process, and the melt pool cooling speed is relatively fast, reducing the composition segregation defect, which is beneficial to improving the comprehensive properties of the material. However, at present, the AZ series magnesium alloy arc additive manufacturing mainly uses tungsten inert gas arc welding (TIG) heat source. The AZ series magnesium alloy workpieces prepared with this heat source often have obvious cladding and deposition interface layers between the front and back layers of wires. The grain size and orientation near the interface are significantly different from those of the inner layer grains, and there are serious void and crack phenomena at the interface. A large number of hard and brittle β-Mg 17 Al 12 phases precipitate at the grain boundaries, resulting in poor microstructure uniformity and poor comprehensive mechanical properties of the AZ series magnesium alloy blocks prepared by arc wire additive manufacturing. Therefore, on the basis of the existing technology, researching and optimizing the additive manufacturing technology and process parameter range for improving the microstructure uniformity of AZ series magnesium alloy workpieces prepared by arc additive manufacturing is of great significance for promoting the development and industrial application of AZ series magnesium alloy additive manufacturing technology. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method suitable for improving the microstructure uniformity of AZ series magnesium alloys manufactured by arc additive manufacturing in view of the above-mentioned deficiencies of the prior art. This method uses plasma arc welding as the heat source, adopts a rotating deposition method, and is assisted by the means of hammering plastic deformation between adjacent single-layer solid slices, improves the melting, deposition rate and deposition path accuracy of the wire, effectively controls the thermal cycle, avoids abnormal grain growth, and optimizes the deposition interface, obtaining an AZ series magnesium alloy workpiece without obvious adjacent deposition interface layers, with uniform microstructure, fine grains and few defects, solving the problem of uneven microstructure and serious void defects near the interface of adjacent deposition layers in the current AZ series magnesium alloy workpieces manufactured by arc additive manufacturing.

[0006] To solve the above technical problem, the technical solution adopted by the present invention is: a method suitable for improving the microstructure uniformity of AZ series magnesium alloys manufactured by arc additive manufacturing, characterized in that the method uses AZ series magnesium alloy wires with a diameter of 1 mm to 2 mm as raw materials, and includes the following steps:

[0007] Step 1: Select a hot-rolled AZ31 plate with a thickness of 20 mm to 30 mm for grinding and degreasing, and then fix the ground and degreased hot-rolled AZ31 plate in the arc additive manufacturing equipment as the substrate;

[0008] Step 2: Perform three-dimensional modeling according to the target magnesium alloy workpiece, then perform slicing processing on the established model to obtain each slice layer data, design the deposition path for different parts of each slice layer in combination with the width range and overlapping rate of a single-pass deposition melt pool, and import it into the arc additive manufacturing equipment;

[0009] Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1. The preheating temperature is 100°C to 110°C, and the time is 1h to 2h. Take the upper surface of the substrate as the reference surface for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to 5mm to 10mm. At the same time, introduce a mixed gas of argon and helium coaxially into the electrode for ionization and plasma formation, and eject argon protective gas from the periphery of the nozzle to isolate the air;

[0010] Step 4: According to the deposition path imported into the arc additive manufacturing equipment in Step 2, perform arc additive manufacturing in an inner layer rotation deposition manner to melt the AZ series magnesium alloy wire and obtain an AZ series magnesium alloy workpiece. Turn off the substrate heating, air-cool to room temperature, and then take it out. The process of arc additive manufacturing is as follows: First, deposit along the "I"-shaped straight path to complete the filling of the inner layer structure, and then deposit along the outer contour of the inner layer structure in a "return"-shaped path to complete the filling of the outer layer structure, obtaining a single-layer solid sheet. Use an electric grinder to clean the single-layer solid sheet and use an electric hammer to hammer the surface. Repeat the processes of filling the inner layer structure, filling the outer layer structure to obtain a single-layer solid sheet, as well as the cleaning and hammering processes, so that the single-layer solid sheets are stacked layer by layer to obtain an AZ series magnesium alloy workpiece. And before preparing the next single-layer solid sheet after each preparation of a single-layer solid sheet, rotate the starting deposition direction of the arc by an α angle.

[0011] Aiming at the technical problems of uneven organization and serious void defects near the interface of adjacent deposition layers in the current arc additive manufacturing of AZ series magnesium alloy workpieces, the present invention adopts an arc wire melting additive manufacturing method, uses a high-energy density plasma arc (PAW) as the heat source, comprehensively considers the physical properties of the AZ series magnesium alloy, the melting and deposition processes of the wire during the PAW additive process, adopts a rotation deposition method, and assists with the means of hammering plastic deformation between adjacent single-layer solid sheets, effectively optimizing the preparation process, obtaining an AZ series magnesium alloy workpiece with no obvious adjacent deposition interface layer, uniform organization, fine grains, and few defects, and enabling it to have good comprehensive mechanical properties.

[0012] The above method for improving the microstructure uniformity of AZ series magnesium alloys by arc additive manufacturing is characterized in that in step three, the argon-helium mixed gas is composed of argon and helium mixed at a volume ratio of 70%:30%, and the flow rate of the argon-helium mixed gas is 2 L / min to 4 L / min, and the flow rate of the argon shielding gas is 15 L / min to 25 L / min; in step four, plasma arc welding is used as the heat source for arc additive manufacturing, and the process parameters of arc additive manufacturing are: the gun head travel speed is 150 mm / min to 210 mm / min, the wire feeding speed is 1800 mm / min to 2300 mm / min, the pulse base current is 60 A to 120 A, the pulse peak current is 150 A to 220 A, the arc voltage is 9 V to 11 V, the overlap ratio is 30% to 40%, the molten pool width is 4 mm to 7 mm, and the molten pool height is 2 mm to 8 mm. Using these process parameters for arc additive manufacturing, the AZ series magnesium alloy has good formability and microstructure uniformity, and also has good comprehensive mechanical properties.

[0013] The above method for improving the microstructure uniformity of AZ series magnesium alloys by arc additive manufacturing is characterized in that in step four, the starting deposition direction of the arc rotates by an α angle of 90°, and one cycle is completed every four rotations. By controlling the rotation angle and the number of rotations of the starting deposition direction of the arc, the influence of the arc thermal cycle on the microstructure of the AZ series magnesium alloy workpiece is effectively controlled, and abnormal grain growth is avoided.

[0014] The above method for improving the microstructure uniformity of AZ series magnesium alloys by arc additive manufacturing is characterized in that during the arc additive manufacturing process in step four, first, the set molten pool width and height are obtained by adjusting the process parameters, then the overlap ratio is selected, and while keeping other parameters unchanged, arc additive manufacturing is carried out only by adjusting the current.

[0015] The above method for improving the microstructure uniformity of AZ series magnesium alloys by arc additive manufacturing is characterized in that in step four, an electric hammering device with a spherical hammer head is used to hammer the surface of the cleaned single-layer solid slice, and the hammering force is 100 N to 300 N.

[0016] The above method for improving the microstructure uniformity of AZ series magnesium alloys by arc additive manufacturing is characterized in that in step four, the preparation of the next single-layer solid slice is carried out when the temperature of the to-be-prepared single-layer solid slice drops to a difference of 0 to 5 °C from the substrate temperature. By controlling this temperature, stress generation is avoided, the AZ series magnesium alloy workpiece is successfully formed, and at the same time, the AZ series magnesium alloy workpiece has uniform microstructure and properties.

[0017] The present invention has the following advantages compared with the prior art:

[0018] 1. Compared with the existing arc additive manufacturing technologies using tungsten inert gas welding (TIG) heat source and metal inert gas welding (MIG / MAG) heat source, the present invention adopts plasma arc welding (PAW) heat source. By utilizing the characteristic of higher heat input of PAW heat source, the melting and deposition rates of AZ series magnesium alloy wires are increased, improving the efficiency of arc additive manufacturing. Moreover, the PAW heat source has good arc stiffness and strong directivity, and can accurately deposit materials along the pre-set path during arc additive manufacturing, reducing material spatter and waste, effectively avoiding problems such as unstable arc during the preparation process, overflow and collapse of the molten pool during the forming process, and resulting in good workpiece formability.

[0019] 2. In the present invention, an inner layer rotating deposition method is adopted for arc additive manufacturing. By rotating the starting deposition direction of the arc after preparing a single-layer solid sheet layer and before preparing the next single-layer solid sheet layer, the thermal cycle is effectively controlled, avoiding abnormal grain growth and greatly improving the uniformity of the microstructure of AZ series magnesium alloy workpieces.

[0020] 3. During the arc additive manufacturing process of the present invention, after performing hammering plastic deformation on a single-layer solid sheet layer and then depositing the next single-layer solid sheet layer, the deposition interface is effectively optimized, resulting in a microstructure of the prepared AZ series magnesium alloy workpiece being composed of fine and uniform equiaxed grains, without a deposition interface area between the front and back two layers, and the pore defect size is small. Therefore, compared with the cast magnesium alloy, it has good comprehensive mechanical properties, effectively solving the problems of uneven microstructure, anisotropy, and many pore defects in the periodic distribution of the deposition interface layer between the front and back two layers inside the existing arc additive manufacturing magnesium alloy workpieces.

[0021] 4. The arc additive manufacturing process of the present invention greatly improves the microstructure uniformity of AZ series magnesium alloy workpieces. The workpieces have excellent mechanical properties, and the process is simple. During deposition, the additive manufacturing of AZ series magnesium alloy can be achieved only by adjusting the current magnitude in the process parameters, which is easy to implement and easy to promote.

[0022] The technical solutions of the present invention will be further described in detail below with reference to the drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the process principle of arc additive manufacturing of AZ series magnesium alloy according to the present invention.

[0024] Figure 2 It is a schematic diagram of the deposition path of the inner layer structure filling during the process of arc additive manufacturing of AZ series magnesium alloy according to the present invention.

[0025] Figure 3 It is a schematic diagram of the deposition path of the outer layer structure filling during the process of arc additive manufacturing of AZ series magnesium alloy according to the present invention.

[0026] Figure 4This is a physical diagram and a sectional view of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0027] Figure 5 This is a longitudinal sectional metallographic diagram of the top position of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0028] Figure 6 This is a longitudinal sectional metallographic diagram of the middle position of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0029] Figure 7 This is a longitudinal sectional metallographic diagram of the bottom position of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0030] Figure 8 This is a metallographic diagram of the adjacent two-layer interface region that appears in the AZ91 magnesium alloy workpiece prepared in Comparative Example 2 of the present invention.

[0031] Explanation of reference numerals

[0032] 1 - Tungsten electrode; 2 - Nozzle; 3 - Cooling water; 4 - Plasma gas;

[0033] 5 - Shielding gas; 6 - Arc; 7 - Plasma flame. Detailed implementation manners

[0034] As Figure 1 shown, the principle of the arc additive manufacturing of AZ series magnesium alloys in the present invention is as follows: Using the tungsten electrode 1 as the cathode, electrons are emitted from the surface to form an arc 6, which is ejected from the nozzle 2. At the same time, an argon-helium mixed gas is introduced into the inner channel of the nozzle 2 coaxially with the electrode to form a compressed air flow, which is used to confine the tungsten electrode 1 to form an arc, and the argon-helium mixed gas is ionized to form a plasma gas 4, which is ejected from the nozzle 2 to form a plasma flame 7, thereby forming a plasma arc with a high energy density. The magnesium alloy wire is melted to prepare a single-layer solid sheet layer. An argon shielding gas 5 is introduced into the outer peripheral channel of the nozzle 2 to isolate the air and prevent the molten pool from oxidizing, and cooling water 3 is introduced into the nozzle 2 to control the temperature and prevent the metal droplets formed by the melting of the magnesium alloy wire from sticking.

[0035] Example 1

[0036] In this example, plasma arc welding (PAW) is used as the heat source, and an AZ31 magnesium alloy wire with a diameter of 1.5 mm is used as the raw material. The mass fraction of Al in the AZ31 magnesium alloy wire is 2.98%, the mass fraction of Zn is 0.94%, the mass fraction of Mn is 0.37%, and the balance is Mg. The steps include:

[0037] Step 1: Select a hot-rolled AZ31 plate with a thickness of 30 mm for grinding and degreasing (size: length × width × height is 150 mm × 150 mm × 30 mm), and then fix the ground and degreased hot-rolled AZ31 plate in an arc additive manufacturing device as the substrate;

[0038] Step 2: Perform 3D modeling on the target AZ31 magnesium alloy workpiece (size: length × width × height is 110 mm × 100 mm × 80 mm) through the computer software Autodesk Fusion 360, and then perform slicing processing on the established model to obtain the data of each sliced layer. Combine the width range and overlap rate of a single-pass deposition molten pool, design deposition paths for different parts of each sliced layer, and import them into the arc additive manufacturing device. Since the target AZ31 magnesium alloy workpiece is a block structure and the cross-sections at each height are exactly the same, only one sliced layer scanning path needs to be planned;

[0039] Step 3: Preheat the substrate fixed in the arc additive manufacturing device in Step 1. The preheating temperature is 100°C - 110°C, and the time is 1 h. Take the upper surface of the substrate as the reference surface for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to 7 mm ± 1 mm, fix the AZ31 magnesium alloy wire reel in the synchronous wire feeding mechanism of the arc additive manufacturing device, and adjust the wire outlet of the wire feeding system so that the angle between the AZ31 magnesium alloy wire reel and the upper surface of the substrate is 30° ± 2°, and the distance between the tip of the wire and the upper surface of the substrate is 3 mm ± 0.4 mm. At the same time, introduce an argon-helium mixed gas with a flow rate of 2.5 L / min, which is a mixture of argon and helium in a volume ratio of 70%:30%, into the electrode coaxially for ionization and plasma formation, and spray argon protective gas with a flow rate of 20 L / min from the periphery of the nozzle to isolate the air;

[0040] Step 4: According to the deposition path imported into the arc additive manufacturing device in Step 2, perform arc additive manufacturing using the inner layer rotation deposition method to melt the AZ31 magnesium alloy wire and obtain the AZ31 magnesium alloy workpiece. Turn off the substrate heating, air-cool to room temperature, and then take it out; the process of the arc additive manufacturing is as follows: Plasma arc welding is used as the heat source, start the wire feeding system, and make the AZ31 magnesium alloy wire deposit first along the "I"-shaped straight path, as Figure 2 shown, to complete the filling of the inner layer structure, and then deposit along the outer contour of the inner layer structure in a "return"-shaped path, as Figure 3As shown, the outer structure filling is completed to obtain the first single-layer solid sheet layer. An electric grinding machine is used to clean the single-layer solid sheet layer with a steel brush, and an electric hammering device with a spherical hammer head is used to perform hammering plastic deformation on the surface. The hammering force is 100 N. Repeat the processes of inner structure filling, outer structure filling to obtain the single-layer solid sheet layer, as well as the cleaning and hammering processes, so that the single-layer solid sheet layers are stacked layer by layer to obtain an AZ31 magnesium alloy workpiece. And before preparing the next single-layer solid sheet layer after each preparation of a single-layer solid sheet layer, the starting deposition direction of the arc needs to be rotated clockwise once relative to the prepared single-layer solid sheet layer, and the rotation angle is 90°. As Figure 2 shown, every four rotations, that is, depositing and preparing four single-layer solid sheet layers completes a cycle;

[0041] The process parameters of the arc additive manufacturing are as follows: the gun head walking speed is 150 mm / min, the wire feeding speed is 1800 mm / min, the arc voltage is 9.6 V, and it remains stable accordingly. The pulse base current is finely adjusted to be 60 A - 100 A, the pulse peak current is 150 A - 220 A, the overlap rate is 30% ± 3%, the molten pool width is 4 mm ± 0.5 mm, the molten height is 2.0 mm ± 0.5 mm, the flow rate of the argon-helium mixed gas is 2.5 L / min, and the flow rate of the argon protection gas is 20 L / min;

[0042] During the arc additive manufacturing process, first, the set molten pool width and molten height are obtained by adjusting the process parameters, then the overlap rate is selected, and other parameters are kept unchanged. When melt-through or insufficient melting occurs during the deposition process, only the current is adjusted for arc additive manufacturing;

[0043] During the arc additive manufacturing process, after each deposition and preparation of a single-layer solid sheet layer, the next single-layer solid sheet layer is prepared when the temperature of the single-layer solid sheet layer to be prepared drops to 0 - 5 °C higher than the substrate temperature.

[0044] After testing, the AZ31 magnesium alloy workpiece prepared in this example is deposited 27 layers in total, with an actual height of 85 mm. The room temperature tensile properties in the X, Y, and Z directions are close, and the average room temperature tensile properties are: yield strength 110 MPa ± 7 MPa, tensile strength 230 MPa ± 7 MPa, and elongation 14% ± 2%.

[0045] Comparative Example 1

[0046] The differences between this comparative example and Example 1 are as follows: A non-consumable (TIG) welding heat source is adopted. The process parameters of arc additive manufacturing in Step 4 are as follows: the traveling speed of the gun head is 220 mm / min, the wire feeding speed is 2600 mm / min, the arc voltage is 15 V, the fine-tuned base current is 120 A - 140 A, the peak current is 150 A - 180 A, the overlap rate is 45% ± 2%, the molten pool width is 6 mm ± 1 mm, the molten height is 5 mm ± 0.5 mm. An argon-helium mixed gas is not introduced, the argon protection gas flow rate is 20 L / min, and the single-layer solid sheet is not hammered on the surface by an electric hammering device for plastic deformation.

[0047] After testing, the room-temperature tensile properties of the AZ31 magnesium alloy workpiece prepared in this comparative example are similar in the X, Y, and Z directions. The average room-temperature tensile properties are as follows: the yield strength is 78 MPa ± 6 MPa, the tensile strength is 152 MPa ± 11 MPa, and the elongation is 7% ± 2%.

[0048] Example 2

[0049] In this example, plasma arc welding (PAW) is used as the heat source, and AZ31 magnesium alloy wire with a diameter of 1 mm is used as the raw material. The mass fraction of Al in the AZ31 magnesium alloy wire is 2.78%, the mass fraction of Zn is 0.95%, the mass fraction of Mn is 0.41%, and the balance is Mg. The following steps are included:

[0050] Step 1: Select a hot-rolled AZ31 plate with a thickness of 20 mm for grinding and degreasing (the size is length × width × height: 150 mm × 150 mm × 20 mm), and then fix the ground and degreased hot-rolled AZ31 plate in the arc additive manufacturing equipment as the substrate.

[0051] Step 2: The same as in Example 1.

[0052] Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1. The preheating temperature is 100°C - 110°C, and the time is 1.5 h. Taking the upper surface of the substrate as the reference surface for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to 5 mm ± 1 mm. Fix the AZ31 magnesium alloy wire spool in the synchronous wire feeding mechanism of the arc additive manufacturing equipment, and adjust the wire outlet of the wire feeding system so that the angle between the AZ31 magnesium alloy wire spool and the upper surface of the substrate is 30° ± 2°, and the distance between the tip of the wire and the upper surface of the substrate is 3 mm ± 0.4 mm. At the same time, an argon-helium mixed gas with a flow rate of 2 L / min, which is composed of argon and helium in a volume ratio of 70%:30%, is introduced coaxially through the electrode for ionization and plasma formation, and an argon protection gas with a flow rate of 15 L / min is ejected from the periphery of the nozzle to isolate the air.

[0053] Step 4: The same as in Example 1.

[0054] The process parameters of the arc additive manufacturing are as follows: the gun head traveling speed is 170 mm / min, the wire feeding speed is 2300 mm / min, the arc voltage is 10 V, and they are kept stable. The pulse base current is finely adjusted from 60 A to 100 A, the pulse peak current is from 150 A to 220 A, the overlapping rate is 30% ± 3%, the molten pool width is 5 mm ± 0.5 mm, the molten height is 4.0 mm ± 0.5 mm, the flow rate of the argon-helium mixed gas is 4 L / min, and the flow rate of the argon shielding gas is 25 L / min;

[0055] During the arc additive manufacturing process, first, the set molten pool width and molten height are obtained by adjusting the process parameters, then the overlapping rate is selected, and other parameters are kept unchanged. When melt-through or insufficient melting occurs during the deposition process, only the current is adjusted for arc additive manufacturing;

[0056] During the arc additive manufacturing process, after each single-layer solid sheet is deposited and prepared, the next single-layer solid sheet is prepared when the temperature of the to-be-prepared single-layer solid sheet drops to 0 - 5 °C higher than the substrate temperature.

[0057] After testing, the AZ31 magnesium alloy workpiece prepared in this example is deposited with a total of 26 layers, the actual height is 83 mm, and the room-temperature tensile properties in the X, Y, and Z directions are close. The average room-temperature tensile properties are: yield strength 101 MPa ± 3 MPa, tensile strength 232 MPa ± 6 MPa, and elongation 12% ± 2%.

[0058] Example 3

[0059] In this example, plasma arc welding (PAW) is used as the heat source, and AZ61 magnesium alloy wire with a diameter of 2 mm is used as the raw material. The mass fraction of Al in the AZ61 magnesium alloy wire is 5.78%, the mass fraction of Zn is 0.89%, the mass fraction of Mn is 0.45%, and the balance is Mg. It includes the following steps:

[0060] Step 1: Select a hot-rolled AZ61 plate with a thickness of 25 mm for grinding and degreasing (the size is length × width × height of 150 mm × 150 mm × 25 mm), and then fix the ground and degreased hot-rolled AZ61 plate in the arc additive manufacturing equipment as the substrate;

[0061] Step 2: Based on the target AZ61 magnesium alloy workpiece (with dimensions of length × width × height being 110 mm × 100 mm × 80 mm), three-dimensional modeling is carried out using the computer software Autodesk Fusion 360. Then, the established model is sliced to obtain data of each sliced layer. Combining the width range and overlapping rate of a single-pass deposition molten pool, deposition paths are designed for different parts of each sliced layer and imported into the arc additive manufacturing equipment. Since the target AZ61 magnesium alloy workpiece is a block structure and the cross-sections at each height are exactly the same, only one type of sliced layer scanning path needs to be planned;

[0062] Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1. The preheating temperature is 100°C to 110°C, and the time is 2 h. Take the upper surface of the substrate as the reference plane for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to be 10 mm ± 1 mm. Fix the AZ61 magnesium alloy wire coil in the synchronous wire feeding mechanism of the arc additive manufacturing equipment, and adjust the wire outlet of the wire feeding system so that the included angle between the AZ61 magnesium alloy wire coil and the upper surface of the substrate is 30° ± 2°, and the distance between the tip of the wire and the upper surface of the substrate is 10 mm ± 0.4 mm. At the same time, an argon-helium mixed gas with a flow rate of 4 L / min, composed of argon and helium, is introduced coaxially into the electrode for ionization and plasma formation, and argon shielding gas with a flow rate of 30 L / min is ejected from the periphery of the nozzle to isolate the air;

[0063] Step 4: According to the deposition path imported into the arc additive manufacturing equipment in Step 2, arc additive manufacturing is carried out using the inner layer rotation deposition method to melt the AZ61 magnesium alloy wire and obtain the AZ61 magnesium alloy workpiece. Turn off the substrate heating, air-cool to room temperature and then take it out; The process of the arc additive manufacturing is as follows: Plasma arc welding is used as the heat source. Start the wire feeding system so that the AZ61 magnesium alloy wire is first deposited along the "I"-shaped straight path, as Figure 2 shown, to complete the filling of the inner layer structure. Then, deposit along the outer contour of the inner layer structure in a "return"-shaped path, as Figure 3 shown, to complete the filling of the outer layer structure and obtain the first single-layer solid sheet layer. Use an electric grinding machine to clean the single-layer solid sheet layer with a steel brush and use an electric hammering device to perform hammering plastic deformation on the surface. The hammering force is 300 N. Repeat the processes of filling the inner layer structure, filling the outer layer structure to obtain the single-layer solid sheet layer, as well as the cleaning and hammering processes, so that each single-layer solid sheet layer is stacked layer by layer to obtain the AZ61 magnesium alloy workpiece. And before preparing the next single-layer solid sheet layer after each preparation of a single-layer solid sheet layer, the starting deposition direction of the arc needs to be rotated clockwise once relative to the already prepared single-layer solid sheet layer, and the rotation angle is 90°, as Figure 2 shown. Every four rotations, that is, after depositing and preparing four single-layer solid sheet layers, a cycle period is completed;

[0064] The process parameters of the arc additive manufacturing are as follows: the gun head walking speed is 180 mm / min, the wire feeding speed is 2000 mm / min, the arc voltage is 11 V, and it remains stable accordingly. The pulse base current is finely adjusted to be 60 A - 100 A, the pulse peak current is 150 A - 220 A, the overlap rate is 35% ± 3%, the molten pool width is 6 mm ± 0.5 mm, the molten height is 6.0 mm ± 0.5 mm, the flow rate of the argon-helium mixed gas is 4 L / min, and the flow rate of the argon shielding gas is 30 L / min;

[0065] During the arc additive manufacturing process, first, the set molten pool width and molten height are obtained by adjusting the process parameters, and then the overlap rate is selected while keeping other parameters unchanged. When melt-through or insufficient melting occurs during the deposition process, the arc additive manufacturing is carried out only by adjusting the current;

[0066] During the arc additive manufacturing process, after each single-layer solid slice is deposited and prepared, the next single-layer solid slice is prepared when the temperature of the to-be-prepared single-layer solid slice drops to 0 - 5 °C higher than the substrate temperature.

[0067] After testing, the AZ61 magnesium alloy workpiece prepared in this example was deposited in a total of 28 layers, with an actual height of 83 mm. The room-temperature tensile properties in the X, Y, and Z directions are close, and the average room-temperature tensile properties are: yield strength 152 MPa ± 4 MPa, tensile strength 257 MPa ± 6 MPa, and elongation 16% ± 2%.

[0068] Example 4

[0069] In this example, plasma arc welding (PAW) is used as the heat source, and an AZ91 magnesium alloy wire with a diameter of 1.8 mm is used as the raw material. The mass fraction of Al in the AZ91 magnesium alloy wire is 8.92%, the mass fraction of Zn is 0.93%, the mass fraction of Mn is 0.43%, and the balance is Mg. It includes the following steps:

[0070] Step 1: Select a hot-rolled AZ91 plate with a thickness of 27 mm for grinding and degreasing (size length × width × height is 150 mm × 150 mm × 27 mm), and then fix the ground and degreased hot-rolled AZ91 plate in the arc additive manufacturing equipment as the substrate;

[0071] Step 2: Based on the target AZ91 magnesium alloy workpiece (with dimensions of length × width × height being 110 mm × 100 mm × 80 mm), three-dimensional modeling is carried out using the computer software Autodesk Fusion 360. Then, the established model is sliced to obtain data of each sliced layer. Combining the width range and overlapping rate of a single-pass deposition molten pool, deposition paths are designed for different parts of each sliced layer and imported into the arc additive manufacturing equipment. Since the target AZ91 magnesium alloy workpiece is a block structure and the cross-sections at each height are exactly the same, only one type of sliced layer scanning path needs to be planned;

[0072] Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1. The preheating temperature is 100°C to 110°C, and the time is 1.8 h. Taking the upper surface of the substrate as the reference surface for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to be 6 mm ± 1 mm. Fix the AZ91 magnesium alloy wire spool in the synchronous wire feeding mechanism of the arc additive manufacturing equipment, and adjust the wire outlet of the wire feeding system so that the included angle between the AZ91 magnesium alloy wire spool and the upper surface of the substrate is 30° ± 2°, and the distance between the tip of the wire and the upper surface of the substrate is 3.5 mm ± 0.4 mm. At the same time, an argon-helium mixed gas with a flow rate of 3 L / min, composed of argon and helium, is passed coaxially through the electrode for ionization and plasma formation, and argon shielding gas with a flow rate of 26 L / min is ejected from the periphery of the nozzle to isolate the air;

[0073] Step 4: According to the deposition path imported into the arc additive manufacturing equipment in Step 2, arc additive manufacturing is carried out using the inner layer rotation deposition method to melt the AZ91 magnesium alloy wire and obtain the AZ91 magnesium alloy workpiece. Turn off the substrate heating, and take it out after air cooling to room temperature. The process of the arc additive manufacturing is as follows: Plasma arc welding is used as the heat source. Start the wire feeding system, and make the AZ91 magnesium alloy wire deposit first along the "I"-shaped straight path, as Figure 2 shown, to complete the filling of the inner layer structure. Then, deposit along the outer contour of the inner layer structure in a "return" shape path, as Figure 3 shown, to complete the filling of the outer layer structure and obtain the first single-layer solid sheet layer. Use an electric grinding machine to clean the single-layer solid sheet layer with a steel brush and use an electric hammering device to perform hammering plastic deformation on the surface. The hammering force is 100 N. Repeat the processes of filling the inner layer structure, filling the outer layer structure to obtain the single-layer solid sheet layer, as well as the cleaning and hammering processes, so that each single-layer solid sheet layer is stacked layer by layer to obtain the AZ91 magnesium alloy workpiece. And before preparing the next single-layer solid sheet layer after each preparation of a single-layer solid sheet layer, the starting deposition direction of the arc needs to be rotated clockwise once relative to the already prepared single-layer solid sheet layer, and the rotation angle is 90°, as Figure 2 shown. Every four rotations, that is, after depositing and preparing four single-layer solid sheet layers, a cycle period is completed;

[0074] The process parameters of the arc additive manufacturing are as follows: the torch travel speed is 210 mm / min, the wire feeding speed is 2200 mm / min, the arc voltage is 10.2 V, and they are kept stable. The pulse base current is finely adjusted from 80 A to 150 A, the pulse peak current is from 160 A to 270 A, the overlapping rate is 40% ± 3%, the molten pool width is 7 mm ± 0.8 mm, the molten height is 8.0 mm ± 0.6 mm, the flow rate of the argon-helium mixed gas is 3 L / min, and the flow rate of the argon shielding gas is 26 L / min;

[0075] During the arc additive manufacturing process, first, the set molten pool width and molten height are obtained by adjusting the process parameters, then the overlapping rate is selected, and other parameters are kept unchanged. When melt-through or insufficient melting occurs during the deposition process, the arc additive manufacturing is carried out only by adjusting the current;

[0076] During the arc additive manufacturing process, after each single-layer solid sheet is deposited and prepared, the next single-layer solid sheet is prepared when the temperature of the single-layer solid sheet to be prepared drops to 0 - 5 °C higher than the substrate temperature.

[0077] After testing, the AZ91 magnesium alloy workpiece prepared in this example is deposited in a total of 25 layers, and the actual height is 85 mm. The room-temperature tensile properties in the X, Y, and Z directions are close, and the average room-temperature tensile properties are: yield strength 189 MPa ± 7 MPa, tensile strength 290 MPa ± 7 MPa, and elongation 20% ± 2%.

[0078] Figure 4 This is the physical drawing and sectional view of the AZ91 magnesium alloy workpiece prepared in this example, Figure 5 ~

[0079] Figure 7 This is the longitudinal sectional metallographic diagram of the top, middle, and bottom positions of the AZ91 magnesium alloy workpiece prepared in this example. It can be seen that the deposited AZ91 magnesium alloy workpiece block is evenly distributed from top to bottom, there is no obvious deposition interface between the front and back two layers, and the average grain size is 41 μm ± 5 μm. Figures 4 to 7 It can be seen that the deposited AZ91 magnesium alloy workpiece block is evenly distributed from top to bottom, there is no obvious deposition interface between the front and back two layers, and the average grain size is 41 μm ± 5 μm.

[0080] Comparative Example 2

[0081] The difference between this comparative example and Example 4 is that: a non-consumable electrode (TIG) welding heat source is used. The process parameters of the arc additive manufacturing in step 4 are as follows: the torch travel speed is 230 mm / min, the wire feeding speed is 2500 mm / min, the arc voltage is 18 V, the pulse base current is finely adjusted from 130 A to 150 A, the pulse peak current is from 170 A to 210 A, the overlapping rate is 50% ± 4%, the molten pool width is 8 mm ± 2 mm, the molten height is 3.0 mm ± 0.5 mm, the argon-helium mixed gas is not introduced, the flow rate of the argon shielding gas is 25 L / min, and the surface of the single-layer solid sheet is not subjected to hammering plastic deformation by an electric hammering device.

[0082] After detection, the room-temperature tensile properties of the AZ91 magnesium alloy workpieces prepared in this comparative example are similar in the X, Y, and Z directions. The average room-temperature tensile properties are as follows: yield strength 153 MPa ± 5 MPa, tensile strength 240 MPa ± 11 MPa, and elongation 9% ± 2%.

[0083] Figure 8 This is the metallographic diagram of the interlayer interface area between two adjacent layers in the AZ91 magnesium alloy workpiece prepared in this comparative example. From Figure 8 it can be seen that an interface layer appears between the single-layer solid lamellae deposited in the front and back layers of this AZ91 magnesium alloy workpiece.

[0084] By comparing Example 1 with Comparative Example 1 and Example 4 with Comparative Example 2, it can be known that the mechanical properties of the AZ31 magnesium alloy workpieces prepared in Example 1 and Example 4, including yield strength, tensile strength, and elongation, are significantly better than those of Comparative Example 1 and Comparative Example 2 respectively. This shows that the high-energy-density plasma arc welding heat source adopted in the present invention avoids problems such as unstable arc during the preparation process, overflow and collapse of the molten pool during the forming process, improves the formability of the workpiece. Combining the preparation by hammering the single-layer solid lamella into plastic deformation and then depositing the next single-layer solid lamella makes the structure of the AZ series magnesium alloy workpiece composed of fine and uniform equiaxed grains, without the deposition interface area between the front and back layers, and the pore defect size is small, thus having better comprehensive mechanical properties.

[0085] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent variations made to the above embodiments according to the technical essence of the invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for improving the uniformity of the structure of AZ magnesium alloys manufactured by arc additive manufacturing, characterized in that: The method uses AZ magnesium alloy wire with a diameter of 1 mm to 2 mm as a raw material, and comprises the following steps: Step 1: Select a hot-rolled AZ31 plate with a thickness of 20 mm to 30 mm for grinding and degreasing, and then fix the hot-rolled AZ31 plate after grinding and degreasing in an arc additive manufacturing device as a substrate; Step 2: 3D modeling is performed according to the target magnesium alloy workpiece, and then the established model is sliced ​​to obtain the data of each slice. According to the width range and overlap rate of the single-pass deposition molten pool, the deposition path is designed for different parts of each slice, and then imported into the arc additive manufacturing equipment; Step 3: preheat the substrate fixed in the arc additive manufacturing equipment in step 1 at a temperature of 100° C. to 110° C. for 1 h to 2 h, and use the upper surface of the substrate as a reference surface for arc additive manufacturing, adjust the vertical distance between the electrode and the substrate to 5 mm to 10 mm, and coaxially introduce argon-helium mixed gas into the electrode for ionization and plasma formation, and spray argon shielding gas from the periphery of the nozzle to isolate the air; Step 4: According to the deposition path introduced into the arc additive manufacturing equipment in step 2, arc additive manufacturing is performed by inner layer rotation deposition, so that the AZ series magnesium alloy wire fuses to obtain an AZ series magnesium alloy workpiece, the substrate heating is turned off, and the workpiece is taken out after air cooling to room temperature; the process of arc additive manufacturing is: first, deposition is performed according to an "I"-shaped straight line path to complete the inner layer structure filling, and then the outer edge deposition is performed along the outer contour of the inner layer structure according to a "U"-shaped path to complete the outer layer structure filling to obtain a single-layer solid sheet, and the single-layer solid sheet is cleaned by an electric grinder and hammered on the surface with an electric hammer, and the above inner layer structure filling, outer layer structure filling to obtain a single-layer solid sheet process and cleaning and hammering process are repeated, so that each single-layer solid sheet is stacked layer by layer to obtain an AZ series magnesium alloy workpiece, and after each single-layer solid sheet is prepared and before the next single-layer solid sheet is prepared, the starting deposition direction of the arc needs to be rotated by an angle α.

2. A method for improving the uniformity of the structure of AZ magnesium alloys manufactured by arc additive manufacturing according to claim 1, characterized in that: The argon-helium mixed gas described in step 3 is formed by mixing argon and helium in a volume ratio of 70%:30%, and the flow rate of the argon-helium mixed gas is 2L / min~4L / min, and the flow rate of the argon shielding gas is 15L / min~25L / min; the arc additive manufacturing described in step 4 uses plasma arc welding as a heat source, and the process parameters of the arc additive manufacturing are: gun head travel speed 150mm / min~210mm / min, wire feeding speed 1800mm / min~2300mm / min, pulse base current 60A~120A, pulse peak current 150A~220A, arc voltage 9V~11V, overlap rate 30%~40%, molten pool molten width 4mm~7mm, molten height 2mm~8mm.

3. The method for improving the uniformity of the structure of AZ magnesium alloy produced by arc additive manufacturing according to claim 1, characterized in that: In step 4, the initial deposition direction of the arc is rotated by an angle α of 90°, and one cycle is completed every four rotations.

4. The method for improving the uniformity of the structure of AZ magnesium alloy produced by arc additive manufacturing according to claim 1, characterized in that: In the arc additive manufacturing process described in step 4, the set molten pool width and height are first obtained by adjusting the process parameters, and then the overlap rate is selected, and other parameters are kept unchanged, and arc additive manufacturing is performed only by adjusting the current.

5. The method for improving the uniformity of the structure of AZ magnesium alloy produced by arc additive manufacturing according to claim 1, characterized in that: In step 4, an electric hammering device with a spherical hammering head is used to hammer the surface of the cleaned single-layer solid sheet layer, and the hammering force is 100N to 300N.

6. The method for improving the microstructure uniformity of AZ series magnesium alloy manufactured by arc additive manufacturing according to claim 1, characterized in that: In step 4, when the temperature of the single-layer solid sheet to be prepared drops to a temperature difference of 0 to 5° C. from the substrate temperature, the next single-layer solid sheet is prepared.

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