A method suitable for improving the microstructure uniformity of AZ series magnesium alloy by arc additive manufacturing

Abstract

The application discloses a method suitable for improving the structure uniformity of an arc additive manufacturing AZ series magnesium alloy, and the method comprises the following steps: 1, selecting a hot-rolled AZ31 plate as a substrate; 2, performing three-dimensional modeling and slicing processing according to a target magnesium alloy workpiece, and designing a deposition path; 3, preheating the substrate, adjusting the distance between an electrode and the substrate and inputting a protective gas; and 4, performing arc additive manufacturing in an inner layer rotary deposition mode according to the deposition path to obtain the AZ series magnesium alloy workpiece. The application takes plasma arc welding as a heat source, has the advantages of fast preparation rate, accurate heat input control, good arc stiffness, effectively reduced spatter, good workpiece formability, and the like; the rotary deposition mode is adopted to effectively control heat cycle, avoid abnormal structure growth, and simultaneously adopt an auxiliary interlayer hammering deformation means to optimize a deposition interface, so that the obtained AZ series magnesium alloy workpiece has the advantages of uniform structure, small grain, few defects, excellent strength and plasticity, and the like, and the process is simple and easy to popularize.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material additive manufacturing, and particularly relates to a method for improving the microstructure uniformity of AZ magnesium alloy prepared by electric arc additive manufacturing. BACKGROUND

[0002] With the rapid development of manufacturing industry towards high performance and light weight, magnesium alloy, as the lightest metal structural material in current engineering application, has attracted extensive attention. Its low density, high specific strength, good shock absorption performance and many other advantages make it have great application potential in the fields of aerospace, automobile manufacturing, biological medicine and the like. However, the traditional magnesium alloy workpiece is mostly formed by casting, but this method has problems such as long development cycle, high mold cost, complex processing procedure and poor microstructure stability, and is difficult to meet the product demand of complex structure and excellent strength and plasticity. Under this background, the magnesium alloy additive manufacturing technology emerges as the times require. The use of metal additive manufacturing technology to replace the traditional casting technology to prepare magnesium alloy workpiece has become a new development trend, especially in the design and trial production stage of special components.

[0003] At present, there are mainly two kinds of magnesium alloy additive manufacturing technologies: selective laser melting technology (SLM) and electric arc additive manufacturing technology (WAAM), and the raw materials used in the two methods are powder and wire, respectively. In the SLM forming process, due to the physical characteristics of magnesium and its alloy such as high vapor pressure, low boiling point and low density, the metal powder is easily splashed, which causes a large amount of magnesium alloy powder to fly away from the powder bed, reduces the melting amount of the molten pool alloy, and causes serious waste of raw materials. In addition, the significant fluctuation of the molten pool melting amount reduces the stability of the SLM process, which causes the internal defects such as holes and cracks in the formed magnesium alloy workpiece, and deteriorates the quality of the workpiece. Based on the above reasons, the SLM forming technology is difficult to be widely applied in the field of magnesium alloy additive manufacturing. In comparison, the WAAM technology uses the following heat sources: tungsten inert gas arc welding (TIG) heat source, gas metal arc welding (MIG / MAG) heat source and plasma arc welding (PAW) heat source, and the required raw material is magnesium alloy wire. This method can effectively avoid the problems of low evaporation temperature, high vapor pressure and insufficient material utilization of magnesium alloy powder by using the arc heat to melt and accumulate the metal wire to manufacture the magnesium alloy workpiece, and is an ideal magnesium alloy additive manufacturing method.

[0004] In the research process of magnesium alloy electric arc additive manufacturing, AZ series magnesium alloy wire is often selected as the research object. The reason is that AZ series magnesium alloy has good corrosion resistance, casting performance and mechanical properties; at the same time, the AZ series alloy has good stability in the electric arc additive process, and the molten pool cooling speed is faster, which reduces the composition segregation defects, and is beneficial to improve the comprehensive performance of the material. But at present, the electric arc additive manufacturing of AZ series magnesium alloy mainly uses tungsten inert gas welding (TIG) heat source. The AZ series magnesium alloy workpiece prepared by the heat source has obvious front and rear two layers of wire cladding, deposition interface layer, the grain size and orientation near the interface are significantly different from the inner layer grains, and the interface has serious void and crack phenomenon, a large number of hard and brittle β-Mg 17 Al 12 Phase precipitates at the grain boundary, which leads to poor microstructure uniformity and poor comprehensive mechanical properties of the AZ series magnesium alloy bulk body prepared by electric arc wire melting additive manufacturing. Therefore, on the basis of the prior art, it is of great significance to study the additive manufacturing technology and process parameter range for optimizing the microstructure uniformity of the AZ series magnesium alloy workpiece prepared by electric arc additive manufacturing, which promotes the development and industrial application of AZ series magnesium alloy additive manufacturing technology. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a method for improving the microstructure uniformity of AZ series magnesium alloy prepared by electric arc additive manufacturing, which solves the problem of uneven microstructure and serious void defects near the adjacent deposition interface layer in the AZ series magnesium alloy workpiece prepared by electric arc additive manufacturing. The method uses plasma arc welding as the heat source, adopts rotary deposition method, and assists the plastic deformation of adjacent single layer solid sheet by hammering, which improves the melting and deposition rate of wire, deposition path accuracy, effectively controls the heat cycle, avoids abnormal organization growth, and optimizes the deposition interface. The AZ series magnesium alloy workpiece prepared by the method has no obvious adjacent deposition interface layer, uniform microstructure, small grain size and few defects.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is as follows: a method for improving the microstructure uniformity of AZ series magnesium alloy prepared by electric arc additive manufacturing, characterized in that the method uses AZ series magnesium alloy wire with a diameter of 1mm-2mm as raw material, and comprises the following steps:

[0007] Step one, select a hot-rolled AZ31 plate with a thickness of 20mm-30mm for polishing and degreasing, then fix the polished and degreased hot-rolled AZ31 plate in the electric arc additive manufacturing equipment as a substrate;

[0008] Step two, three-dimensional modeling is carried out according to the target magnesium alloy workpiece, then the model is sliced to obtain each slice data, the deposition path is designed for different parts of each slice according to the width range of single deposition molten pool and the overlap rate, and then the deposition path is imported into the electric arc additive manufacturing equipment;

[0009] Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1 to a temperature of 100℃~110℃ for 1h~2h. Use 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~10mm. At the same time, argon-helium mixed gas is introduced coaxially through the electrode for ionization and plasma formation. Argon protective gas is sprayed 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, arc additive manufacturing is performed using the inner layer rotational deposition method to melt the AZ-based magnesium alloy wire, obtaining the AZ-based magnesium alloy workpiece. The substrate heating is turned off, and the workpiece is removed after air cooling to room temperature. The arc additive manufacturing process is as follows: First, deposition is performed along an "I"-shaped straight path to complete the inner layer structure filling. Then, deposition is performed along the outer contour of the inner layer structure along a "U"-shaped path to complete the outer layer structure filling, obtaining a single-layer solid sheet. The single-layer solid sheet is cleaned using an electric grinder and the surface is hammered using an electric hammer. The above processes of inner layer structure filling, outer layer structure filling to obtain single-layer solid sheets, as well as cleaning and hammering processes, are repeated to allow each single-layer solid sheet to be stacked layer by layer, obtaining the AZ-based magnesium alloy workpiece. After each single-layer solid sheet is prepared and before preparing the next single-layer solid sheet, the starting deposition direction of the arc needs to be rotated by an angle α.

[0011] To address the technical problems of uneven microstructure and severe void defects near the interface of adjacent deposition layers in current arc additive manufacturing of AZ-based magnesium alloy workpieces, this invention employs an arc-wire additive manufacturing method. Using a high-energy-density plasma arc (PAW) as the heat source, and comprehensively considering the physical properties of AZ-based magnesium alloys, the wire melting and deposition process during PAW additive manufacturing, a spin deposition method is adopted. This is supplemented by hammering plastic deformation between adjacent single-layer solid sheets, effectively optimizing the manufacturing process. This results in AZ-based magnesium alloy workpieces with no obvious adjacent deposition interface layers, uniform microstructure, fine grains, and few defects, thus exhibiting excellent comprehensive mechanical properties.

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

[0013] The method for improving the uniformity of the AZ magnesium alloy in the electric arc additive manufacturing, characterized in that: the rotating angle of the initial deposition direction of the electric arc in the step four is 90°, and one cycle is completed by rotating four times.

[0014] The method for improving the uniformity of the AZ magnesium alloy in the electric arc additive manufacturing, characterized in that: in the electric arc additive manufacturing process in the step four, the molten pool width and the molten pool height are obtained by adjusting the process parameters, then the overlap rate is selected, and the other parameters are kept unchanged, and the electric arc additive manufacturing is performed by adjusting the current only.

[0015] The method for improving the uniformity of the AZ magnesium alloy in the electric arc additive manufacturing, characterized in that: the electric hammering device with the spherical hammering head is used to hammer the surface of the cleaned single-layer solid sheet layer in the step four, and the hammering force is 100N-300N.

[0016] The method for improving the uniformity of the AZ magnesium alloy in the electric arc additive manufacturing, characterized in that: the temperature of the single-layer solid sheet layer to be prepared is reduced to 0-5℃ different from the temperature of the substrate before the preparation of the next single-layer solid sheet layer in the step four. The temperature control avoids the stress generation, enables the AZ magnesium alloy workpiece to be successfully formed, and ensures that the AZ magnesium alloy workpiece has uniform structure and performance.

[0017] Compared with the prior art, the method has the following advantages:

[0018] 1. Compared with the existing arc additive manufacturing technology with tungsten inert gas (TIG) heat source and metal inert gas (MIG / MAG) heat source, the present application adopts plasma arc welding (PAW) heat source, utilizes the higher heat input characteristic of the PAW heat source to accelerate the melting and deposition rate of the AZ series magnesium alloy wire, improves the arc additive manufacturing efficiency, and the PAW heat source has good arc stiffness and strong directivity, can accurately deposit materials according to the preset path in the arc additive manufacturing process, reduces the splashing and waste of materials, effectively avoids the problems of unstable arc in the preparation process, molten pool overflow and collapse in the forming process, and the workpiece has good formability.

[0019] 2. In the present application, the arc additive manufacturing is carried out by adopting the inner layer rotation deposition method, the starting deposition direction of the arc is rotated after preparing a single layer solid sheet layer and before preparing the next single layer solid sheet layer, the heat cycle is effectively controlled, the abnormal growth of the organization is avoided, and the uniformity of the AZ series magnesium alloy workpiece organization is greatly improved.

[0020] 3. In the arc additive manufacturing process of the present application, the single layer solid sheet layer is subjected to plastic deformation by hammering, and then the next single layer solid sheet layer is deposited and prepared, the deposition interface is effectively optimized, the prepared AZ series magnesium alloy workpiece organization is fine and uniform equiaxed crystal, there is no front and rear two layer deposition interface zone, the pore defect size is small, and compared with the smelted magnesium alloy, the AZ series magnesium alloy has good comprehensive mechanical properties, effectively solves the problems of non-uniform organization, anisotropy and many pore defects of the periodic distribution of the front and rear two layer deposition interface layers in the existing arc additive manufacturing magnesium alloy workpiece.

[0021] 4. The arc additive manufacturing process of the present application greatly improves the uniformity of the AZ series magnesium alloy workpiece organization, the workpiece has excellent mechanical properties, and the process is simple, the additive manufacturing of the AZ series magnesium alloy can be realized by adjusting the current size in the process parameters during deposition, is easy to realize and easy to popularize.

[0022] The technical solutions of the present application are described in further detail below by means of the drawings and examples. DESCRIPTION OF DRAWINGS

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

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

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

[0026] Figure 4The image shows a physical picture and a cross-sectional view of the AZ91 magnesium alloy workpiece prepared in Example 4 of this invention.

[0027] Figure 5 Metallographic image of the top longitudinal section of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0028] Figure 6 Metallographic image of the longitudinal section at the middle position of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0029] Figure 7 Metallographic image of the bottom longitudinal section of the AZ91 magnesium alloy workpiece prepared in Example 4 of the present invention.

[0030] Figure 8 This is a metallographic image of the interlayer interface region between two adjacent layers in the AZ91 magnesium alloy workpiece prepared in Comparative Example 2 of this invention.

[0031] Explanation of reference numerals in the attached figures

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

[0033] 5—Protective gas; 6—Electric arc; 7—Plasma flame. Detailed Implementation

[0034] like Figure 1 As shown, the principle of the electric arc additive manufacturing of AZ-based magnesium alloys of the present invention is as follows: using a tungsten electrode 1 as a cathode, electrons are emitted from the surface to form an electric arc 6, which is ejected from the nozzle 2. At the same time, an argon-helium mixed gas is introduced into the internal channel of the nozzle 2, which is coaxial with the electrode, to form a compressed gas flow, which is used to constrain the tungsten electrode 1 to form an electric arc. 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 high-energy-density plasma arc. The magnesium alloy wire is melted to prepare a single-layer solid sheet. Argon protective gas 5 is introduced into the outer channel of the nozzle 2 to isolate air and prevent oxidation of the molten pool. 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 together.

[0035] Example 1

[0036] This embodiment uses plasma arc welding (PAW) as the heat source and AZ31 magnesium alloy wire with a diameter of 1.5 mm as the raw material. The AZ31 magnesium alloy wire contains 2.98% Al, 0.94% Zn, and 0.37% Mn by mass, with the balance being 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 = 150 mm × 150 mm × 30 mm), and then fix the hot-rolled AZ31 plate after grinding and degreasing in an arc additive manufacturing device as the substrate;

[0038] Step 2: According to the target AZ31 magnesium alloy workpiece (size: length × width × height = 110 mm × 100 mm × 80 mm), perform three-dimensional modeling through the computer software Autodesk Fusion 360, and then perform slicing processing on the established model to obtain the data of each sliced layer. Combining the width range and overlap rate of a single-pass deposition molten pool, design the deposition path for different parts of each sliced layer, and import it 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 to 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 to make the included angle between the AZ31 magnesium alloy wire reel and the upper surface of the substrate 30° ± 2°, and the distance between the tip of the wire and the upper surface of the substrate 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 mixed by argon and helium in a volume ratio of 70%:30%, into the electrode coaxially for ionization and plasma formation, and spray out argon shielding gas with a flow rate of 20 L / min from the periphery of the nozzle to isolate 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 along the "return" - shaped path, as Figure 3As shown, the outer layer structure filling is completed to obtain the first layer of single-layer solid sheet, the single-layer solid sheet is cleaned by using a steel brush and the surface is plastically deformed by using a hammering device with a spherical hammering head, the hammering force is 100 N, the above process of inner layer structure filling, outer layer structure filling to obtain the single-layer solid sheet and the cleaning and hammering process are repeated, so that the single-layer solid sheets are stacked layer by layer to obtain the AZ31 magnesium alloy workpiece, and the starting deposition direction of the electric arc needs to be rotated clockwise once by 90° relative to the prepared single-layer solid sheet after each single-layer solid sheet is prepared and before the next single-layer solid sheet is prepared, as shown in Figure 2 As shown, four single-layer solid sheets are deposited and prepared every four rotations to complete a cycle period.

[0041] The process parameters of the electric 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, the pulse base current is 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 pool height is 2.0 mm±0.5 mm, the argon-helium mixed gas flow rate is 2.5 L / min, and the argon protective gas flow rate is 20 L / min.

[0042] In the electric arc additive manufacturing process, the molten pool width and height are obtained by adjusting the process parameters, then the overlap rate is selected, and other parameters are kept unchanged, and only the current is adjusted for electric arc additive manufacturing when the deposition process appears to be insufficient or melt-through.

[0043] In the electric arc additive manufacturing process, after each single-layer solid sheet is deposited and prepared, the temperature of the prepared single-layer solid sheet is reduced to 0-5°C higher than the substrate temperature before the preparation of the next single-layer solid sheet.

[0044] It is detected that the AZ31 magnesium alloy workpiece prepared in the embodiment has a total of 27 deposited layers and an actual height of 85 mm, and the room temperature tensile properties along the X, Y and Z directions are close, and the average room temperature tensile properties are as follows: yield strength 110 MPa±7 MPa, tensile strength 230 MPa±7 MPa, and elongation 14%±2%.

[0045] Comparative Example 1

[0046] The difference between the present comparative example and example 1 is that a non-gas tungsten arc welding (TIG) heat source is used, and the process parameters for arc additive manufacturing in step four are as follows: a gun head travel speed of 220 mm / min, a wire feeding speed of 2600 mm / min, an arc voltage of 15 V, a fine-tuning base current of 120 A-140 A, a peak current of 150 A-180 A, an overlap rate of 45%±2%, a molten pool width of 6 mm±1 mm, a molten pool height of 5 mm±0.5 mm, no argon-helium mixed gas is introduced, an argon shielding gas flow rate is 20 L / min, and the single-layer solid piece layer is not subjected to surface hammering plastic deformation by using an electric hammering device.

[0047] It is detected that the room temperature tensile properties of the AZ31 magnesium alloy workpiece prepared in the present comparative example along the X, Y and Z directions are close, and the average room temperature tensile properties are as follows: a yield strength of 78 MPa±6 MPa, a tensile strength of 152 MPa±11 MPa, and an elongation of 7%±2%.

[0048] Example 2

[0049] In the present example, a plasma arc welding (PAW) is used as a heat source, and an AZ31 magnesium alloy wire with a diameter of 1 mm is used as a raw material, and 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, and the present example comprises the following steps:

[0050] Step one, a hot-rolled AZ31 plate with a thickness of 20 mm is polished and degreased (the size is 150 mm×150 mm×20 mm), and then the polished and degreased hot-rolled AZ31 plate is fixed in an arc additive manufacturing device as a substrate;

[0051] Step two, the same as example 1;

[0052] Step three, the substrate fixed in the arc additive manufacturing device in step one is preheated at a temperature of 100℃-110℃ for 1.5 h, and the upper surface of the substrate is used as a reference surface for arc additive manufacturing, the vertical distance between the electrode and the substrate is adjusted to 5 mm±1 mm, an AZ31 magnesium alloy wire disc is fixed in a synchronous wire feeding mechanism of the arc additive manufacturing device, and the wire feeding system is adjusted to make the AZ31 magnesium alloy wire disc and the upper surface of the substrate form an angle of 30°±2°, the distance between the wire tip and the upper surface of the substrate is 3 mm±0.4 mm, and at the same time, a flow rate of 2 L / min of argon-helium mixed gas mixed at a volume ratio of 70%:30% of argon and helium is introduced into the electrode coaxially for ionization and plasma formation, and a flow rate of 15 L / min of argon shielding gas is sprayed from the periphery of the nozzle to isolate air;

[0053] Step four, the same as example 1;

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

[0055] In the electric arc additive manufacturing process, first, the set molten pool width and height are obtained by adjusting the process parameters, then the overlap rate is selected, and other parameters are kept unchanged, and when the deposition process appears melting through or insufficient melting, only the electric arc additive manufacturing is adjusted by adjusting the current;

[0056] In the electric arc additive manufacturing process, after each layer of single-layer solid sheet is deposited, the temperature of the prepared single-layer solid sheet is reduced to 0-5℃ higher than the temperature of the substrate before the preparation of the next layer of single-layer solid sheet.

[0057] After detection, the total deposition of the AZ31 magnesium alloy workpiece prepared in this embodiment is 26 layers, the actual height is 83 mm, and the room temperature tensile properties along the X, Y and Z directions are close, and 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 embodiment, plasma arc welding (PAW) is used as a heat source, and AZ61 magnesium alloy wire with a diameter of 2 mm is used as a raw material, and 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, including the following steps:

[0060] Step one, select a hot-rolled AZ61 plate with a thickness of 25 mm for polishing and degreasing (size length x width x height is 150 mm x 150 mm x 25 mm), then fix the polished and degreased hot-rolled AZ61 plate in the electric arc additive manufacturing equipment as a 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 the data of each cut layer. Combining the width range and overlap rate of a single-pass deposition molten pool, deposition paths are designed for different parts of each cut 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 cut 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 - 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 reel 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 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 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 passed coaxially through 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 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, and make the AZ61 magnesium alloy wire first deposit 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 the single - layer solid sheet layers are 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 electric arc additive manufacturing are: gun head walking speed 180 mm / min, wire feeding speed 2000 mm / min, arc voltage 11 V, and the same is kept stable, fine-tuning pulse base current 60 A-100 A, pulse peak current 150 A-220 A, overlap rate 35%±3%, molten pool width 6 mm±0.5 mm, molten pool height 6.0 mm±0.5 mm, argon-helium mixed gas flow rate 4 L / min, argon protective gas flow rate 30 L / min;

[0065] In the electric arc additive manufacturing process, first, the set molten pool width and height are obtained by adjusting the process parameters, then the overlap rate is selected, and other parameters remain unchanged, when the deposition process appears melting through or insufficient melting, only by adjusting the current to carry out the electric arc additive manufacturing.

[0066] In the electric arc additive manufacturing process, after each layer of single-layer solid sheet is deposited, the temperature of the prepared single-layer solid sheet is reduced to 0-5℃ higher than the substrate temperature before the preparation of the next layer of single-layer solid sheet.

[0067] It is detected that the total deposition of the AZ61 magnesium alloy workpiece prepared in the embodiment is 28 layers, the actual height is 83 mm, and the room temperature tensile properties along 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 embodiment, plasma arc welding (PAW) is used as a heat source, and AZ91 magnesium alloy wire with a diameter of 1.8 mm is used as a raw material, and 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, including the following steps:

[0070] Step one, select a hot-rolled AZ91 plate with a thickness of 27 mm for polishing and degreasing (size length x width x height is 150 mm x 150 mm x 27 mm), then fix the polished and degreased hot-rolled AZ91 plate in the electric arc additive manufacturing equipment as a 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 the data of each sliced layer. Combining the width range and overlap 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 kind 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 plane 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 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 angle between the AZ91 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 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 coaxially introduced into 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 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, 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 AZ91 magnesium alloy wire first deposit 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 "square - return" 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 the single - layer solid sheet layers are 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 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 electric arc additive manufacturing are as follows: a gun head walking speed of 210 mm / min, a wire feeding speed of 2200 mm / min, an arc voltage of 10.2 V, a fine adjustment pulse base current of 80 A-150 A, a pulse peak current of 160 A-270 A, an overlap rate of 40%±3%, a molten pool width of 7 mm±0.8 mm, a molten pool height of 8.0 mm±0.6 mm, an argon-helium mixed gas flow rate of 3 L / min, and an argon protective gas flow rate of 26 L / min;

[0075] In the electric arc additive manufacturing process, the molten pool width and the molten pool height are first obtained by adjusting the process parameters, then the overlap rate is selected, and other parameters are kept unchanged. When the deposition process appears melting through or insufficient melting, the electric arc additive manufacturing is only adjusted by adjusting the current.

[0076] In the electric arc additive manufacturing process, after each single-layer solid piece layer is deposited, the next single-layer solid piece layer is prepared when the temperature of the prepared single-layer solid piece layer is reduced to 0-5 ℃ higher than the temperature of the substrate.

[0077] It is detected that the total deposition of the prepared AZ91 magnesium alloy workpiece in the embodiment is 25 layers, the actual height is 85 mm, and the room temperature tensile properties along the X, Y and Z directions are close, and the average room temperature tensile properties are as follows: a yield strength of 189 MPa±7 MPa, a tensile strength of 290 MPa±7 MPa, and an elongation of 20%±2%.

[0078] Figure 4 The physical map and the cross-sectional view of the prepared AZ91 magnesium alloy workpiece in the embodiment are shown in FIGS. 1 and 2, Figure 5

[0079] Figure 7 The top, middle and bottom longitudinal section metallographic views of the prepared AZ91 magnesium alloy workpiece in the embodiment are shown in FIGS. 3-5, respectively. Figures 4 to 7 It can be seen that the deposited AZ91 magnesium alloy workpiece block is uniformly distributed from top to bottom, and there is no obvious front and rear two-layer deposition interface, and the average grain size is 41 μm±5 μm.

[0080] Comparative Example 2

[0081] The difference between the present comparative example and Example 4 is that a non-consumable electrode (TIG) welding heat source is used, and the process parameters of the electric arc additive manufacturing in step four are as follows: a gun head walking speed of 230 mm / min, a wire feeding speed of 2500 mm / min, an arc voltage of 18 V, a fine adjustment pulse base current of 130 A-150 A, a pulse peak current of 170 A-210 A, an overlap rate of 50%±4%, a molten pool width of 8 mm±2 mm, a molten pool height of 3.0 mm±0.5 mm, no argon-helium mixed gas is introduced, an argon protective gas flow rate of 25 L / min, and the single-layer solid piece layer is not subjected to surface plastic deformation by using an electric hammering device.​

[0082] It is detected that the tensile properties of the AZ91 magnesium alloy workpiece prepared in the pair of examples along the X, Y and Z directions are close to each other, and the average tensile properties at room temperature are: yield strength 153 MPa ± 5 MPa, tensile strength 240 MPa ± 11 MPa, and elongation 9% ± 2%.

[0083] Figure 8 The metallographic structure of the interfacial region between the adjacent two layers in the AZ91 magnesium alloy workpiece prepared in the pair of examples is shown in the figure from Figure 8 It can be seen that the interface layer appears between the single solid sheet layers deposited in the front and rear two layers of the AZ91 magnesium alloy workpiece.

[0084] It can be seen from the comparison between Example 1 and Comparative Example 1, and the comparison between Example 4 and Comparative Example 2 that the mechanical properties including the yield strength, the tensile strength and the elongation of the AZ31 magnesium alloy workpieces prepared in Example 1 and Example 4 are all significantly better than those of Comparative Example 1 and Comparative Example 2, respectively, which indicates that the high-energy-density plasma arc welding heat source is used in the present application, which avoids the problems such as unstable arc in the preparation process, overflow and collapse of the molten pool in the forming process, and improves the workpiece formability. In combination with the hammering plastic deformation of the single solid sheet layer and then the deposition of the next single solid sheet layer, the microstructure of the AZ magnesium alloy workpiece is fine and uniform equiaxed crystal, there is no interface region between the front and rear two layers, the size of the pore defects is small, and thus the workpiece has better comprehensive mechanical properties.

[0085] The above is only a preferred embodiment of the present application, and does not limit the present application in any way. Any simple modification, change and equivalent change made according to the technical essence of the present application to the above embodiment are still within the protection scope of the technical solution of the present application.

Claims

1. A method suitable for improving the microstructure homogeneity of AZ series magnesium alloys in electric arc additive manufacturing, characterized in that, This method uses AZ-based magnesium alloy wire with a diameter of 1mm to 2mm as raw material and includes the following steps: Step 1: Select a hot-rolled AZ31 plate with a thickness of 20mm~30mm, grind and degrease it, and then fix the ground and degreased hot-rolled AZ31 plate in the arc additive manufacturing equipment as a substrate. Step 2: Create a 3D model of the target magnesium alloy workpiece, then slice the model to obtain data for each slice. Combine the width range and overlap rate of the single-pass deposition pool, design deposition paths for different parts of each slice, and import them into the arc additive manufacturing equipment. Step 3: Preheat the substrate fixed in the arc additive manufacturing equipment in Step 1 to a temperature of 100℃~110℃ for 1h~2h. Use 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~10mm. At the same time, argon-helium mixed gas is introduced coaxially through the electrode for ionization and plasma formation. Argon protective gas is sprayed from the periphery of the nozzle to isolate the air. Step 4: Based on the deposition path imported into the arc additive manufacturing equipment in Step 2, arc additive manufacturing is performed using an inner-layer spin deposition method to melt the AZ-based magnesium alloy wire, obtaining an AZ-based magnesium alloy workpiece. The substrate heating is then turned off, and the workpiece is air-cooled to room temperature before being removed. The arc additive manufacturing process is as follows: First, deposition is performed along an "I"-shaped straight path to complete the inner layer structure filling. Then, deposition is performed along the outer contour of the inner layer structure along a "U"-shaped path to complete the outer layer structure filling, obtaining a single-layer solid sheet. An electric grinder is used to clean the single-layer solid sheet, and an electric hammer is used to hammer the surface. The process of inner-layer structure filling, outer-layer structure filling to obtain a single-layer solid sheet, and cleaning is repeated. The process involves hammering and stacking single-layer solid sheets to obtain AZ-based magnesium alloy workpieces. After each single-layer solid sheet is prepared and before preparing the next, the initial deposition direction of the electric arc needs to be rotated by an angle α. The electric arc additive manufacturing uses plasma arc welding as the heat source, and the process parameters are: torch travel speed 150mm / min~210mm / min, wire feed speed 1800mm / min~2300mm / min, pulse base current 60A~120A, pulse peak current 150A~220A, arc voltage 9V~11V, overlap rate 30%~40%, weld pool width 4mm~7mm, and weld height 2mm~8mm.

2. The method for improving the microstructure uniformity of AZ magnesium alloy in arc additive manufacturing according to claim 1, characterized in that, The argon-helium mixed gas mentioned in step three 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, while the flow rate of the argon protective gas is 15L / min~25L / min.

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

4. The method for improving the microstructure uniformity of AZ magnesium alloys in arc additive manufacturing according to claim 1, characterized in that, In the arc additive manufacturing process described in step four, the set melt pool width and height are first obtained by adjusting the process parameters. Then, the overlap rate is selected, and other parameters are kept unchanged. Arc additive manufacturing is carried out by adjusting the current only.

5. The method for improving the microstructure homogeneity of AZ magnesium alloys in arc additive manufacturing according to claim 1, characterized in that, The electric hammering device with a spherical hammering head is used to hammer the surface of the cleaned single-layer solid sheet layer in step four, and the hammering force is 100N-300N.

6. The method for improving the microstructure homogeneity of AZ magnesium alloys in arc additive manufacturing according to claim 1, characterized in that, In step four, the temperature of the prepared single-layer solid sheet layer is reduced to 0-5℃ different from the temperature of the substrate before the preparation of the next single-layer solid sheet layer.

Images