A magnesium alloy current-assisted vacuum brazing fixture and brazing method

Abstract

This invention relates to the field of magnesium alloy brazing technology, specifically to a current-assisted vacuum brazing fixture and method for magnesium alloys. A MgAlAg alloy solder was designed and matched, and the main welding process parameters were defined: applied pressure of 10-50 kPa, temperature of approximately 450°C, and holding time of 3-5 minutes, ensuring that the vacuum brazing strength of the magnesium alloy is not less than 65 MPa. This invention provides the current-assisted vacuum brazing process steps for magnesium alloys, which can be applied to radar slotted waveguide antennas, and improves the shear strength of the welded joint by 90% compared to existing methods.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy brazing technology, specifically to a magnesium alloy current-assisted vacuum brazing fixture and brazing method. Background Technology

[0002] Magnesium alloys are the lightest metallic structural materials used in practical applications. They possess advantages such as low density and strong electromagnetic shielding capabilities, making them highly promising and significant in the fields of aerospace radar electronics. They can significantly reduce the weight of space-based radars while simultaneously improving their electrical performance. Slotted waveguide antennas are the core structure of space-based radars, composed of multiple complex thin-walled layers. Their maximum dimensions are approximately 1 meter, with a minimum wall thickness of about 0.5 mm, and they require extremely high precision, with a dimensional accuracy of 0.05 mm. Current technology uses aluminum alloys through vacuum brazing. Using magnesium alloys to manufacture slotted waveguide antennas can reduce weight by approximately 30%, which is significant. However, the core technological challenge lies in the vacuum brazing technology of magnesium alloys.

[0003] Existing brazing filler alloys for magnesium alloys generally have high melting points, resulting in brazing temperatures typically approaching 500°C. This can easily cause softening and excessive erosion of the magnesium alloy matrix, making it unsuitable for brazing thin-walled, precision magnesium alloy microwave components. Furthermore, traditional vacuum brazing heats the parts through thermal radiation and conduction, resulting in slow heating rates. For complex components, prolonged high-temperature periods can lead to the formation of numerous brittle phases at the brazing interface when applied to magnesium alloys, ultimately hindering the achievement of high-performance welds.

[0004] Based on the aforementioned characteristics of magnesium alloy brazing, compared to traditional aluminum alloys, magnesium alloys suffer from bottlenecks such as stubborn oxide films on their surfaces and easy volatilization of magnesium during vacuum brazing. As a result, the joint strength obtained by traditional magnesium alloy brazing techniques suitable for slot waveguide antennas does not exceed 40 MPa, which cannot meet the application requirements of slot waveguide antennas. Therefore, there is an urgent need to develop new magnesium alloy solders and new vacuum brazing methods to achieve good vacuum brazing of magnesium alloys and significantly improve the vacuum brazing strength of magnesium alloys to over 65 MPa, thus meeting the application requirements of magnesium alloy slot waveguide antennas.

[0005] In view of the above-mentioned defects, the inventors of this invention have finally obtained this invention after a long period of research and practice. Summary of the Invention

[0006] The purpose of this invention is to solve the problems that the stubborn oxide film on the surface of magnesium alloys and the easy volatilization of magnesium during vacuum brazing result in the joint strength obtained by traditional magnesium alloy brazing technology suitable for slot waveguide antennas not exceeding 40MPa, which cannot meet the application requirements of slot waveguide antennas. The invention provides a magnesium alloy current-assisted vacuum brazing fixture and brazing method.

[0007] To achieve the above objectives, this invention discloses a magnesium alloy current-assisted vacuum brazing fixture, comprising a graphite ring, a graphite lower pressure head, and a graphite upper pressure head. The graphite upper pressure head and the graphite lower pressure head are cylindrical structures. The graphite ring has blind holes. The graphite lower pressure head, the weldment, and the graphite upper pressure head are stacked sequentially from bottom to top and placed inside the graphite ring. The height center plane of the weldment is coplanar with the height center plane of the graphite ring.

[0008] The graphite ring has an inner diameter of R1, an outer diameter of R1+20mm, and a height of 2H. The blind hole is located at the height H of the graphite ring and has a diameter of 3mm and a depth of 8mm.

[0009] The diameter of both the lower and upper graphite indenters is R, and the height is H+10mm. The inner diameter of the graphite ring is R1=1.01R+0.2mm.

[0010] This invention also discloses a current-assisted vacuum brazing method for magnesium alloys, comprising the following steps:

[0011] S1, process two magnesium alloy parts to be welded, each with a diameter of R and a height of H / 2;

[0012] S2, a welding intermediate layer is prepared using MgAlAg alloy, with a thickness of 0.05-0.15mm, a diameter of R, and an alloy melting point of T0;

[0013] S3, sand the two surfaces of the magnesium alloy to be welded and the two surfaces of the intermediate layer to be welded with sandpaper until the roughness is below Ra1.6, clean the sanded surface with alcohol and blow dry;

[0014] S4, place the first magnesium alloy part to be welded with the welding surface facing up, place the middle layer on top of it, and place the second magnesium alloy part with the welding surface facing down on top of the middle layer to form a three-layer structure.

[0015] S5, sequentially insert the graphite lower pressure head, the three-layer structure, and the graphite upper pressure head into the inner diameter of the graphite ring, and adjust the height so that the welding intermediate layer is located on the axis of the blind hole;

[0016] S6. Place the assembled workpiece from step S5 into a current-assisted vacuum brazing furnace, insert the thermocouple into the blind hole, close the furnace door, and set the brazing parameters for brazing.

[0017] In step S1, the magnesium alloy is either AZ31 magnesium alloy or ZK61 magnesium alloy.

[0018] In step S2, the MgAlAg alloy composition is Mg-(21-24wt.%)Al-(9.7-6.7wt.%)Ag, and the melting point T0 is 400-420℃.

[0019] In step S7, the vacuum degree of the vacuum brazing furnace is 0.1 to 0.01 Pa, the applied pressure is 10 to 50 kPa, the heating rate is 50 to 150 °C / min, the holding temperature is T0 to 30 °C to T0 °C, the holding time is 3 to 5 min, and the cooling rate is 30 to 50 °C / min.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. The pulsed current used in the current-assisted vacuum brazing of this invention acts on the interfacial gap between the base material / oxide film / solder / oxide film / base material before the brazing filler melts, and discharges to form plasma, which bombards and rapidly breaks up the oxide film, significantly improving the film breaking speed and ensuring that the interfacial reaction proceeds basically synchronously. In addition, the excessive brittle phase can be avoided by controlling the metallurgical reaction time.

[0022] 2. The low-temperature short-time welding in the current-assisted vacuum brazing process of this invention can reduce the performance loss of magnesium alloy base material. The lower the temperature and the shorter the time of the solder in the melting state, the more controlled its spreading and overflow, which can reduce the spreading on the side wall of the base material at the welding interface and ensure that the overflow height does not exceed 0.3mm. This side wall is the electromagnetic wave transmission interface, so it can reduce electromagnetic loss and improve electrical performance. Low-temperature short-time welding can significantly improve welding production efficiency, reduce welding energy consumption, and significantly reduce welding costs.

[0023] 3. The novel Mg-(21-24)Al-(9.7-6.7)Ag (wt.%) solder designed in this invention has a melting point of only about 400-420℃, which is significantly lower than that of conventional vacuum brazing solders, further reducing welding stress. Simultaneously, this solder can react with the magnesium alloy matrix to form a ternary eutectic, Mg... 17 Al 12 Phases such as AgMg3 can form good interfacial metallurgical bonding;

[0024] 4. This invention employs a novel Mg-(21-24)Al-(9.7-6.7)Ag (wt.%) solder, which has multiple reaction mechanisms with the magnesium alloy matrix. This enables reliable current-assisted vacuum brazing of magnesium alloys under pressures of 10-50 kPa, thereby meeting the welding requirements of complex thin-walled slotted waveguide antennas and preventing deformation of thin-walled structures during high-pressure welding, which would affect the product's 0.05 mm precision control requirements.

[0025] 5. This invention establishes a quantitative relationship between thermocouple temperature readings and welding interface temperature through extensive simulation and measurement data, accurately obtaining the magnesium alloy interface reaction temperature. This solves the problem of high requirements for temperature control and spreading / spreading control in low-melting-point magnesium alloy welding, and also provides a technical reference for temperature control in the same industry. Under the tooling conditions used in this invention, the measured temperature of the thermocouple is about 50°C lower than the temperature of the welding intermediate layer. The ideal melting temperature of the welding intermediate layer is T0+20°C to T0+50°C. Therefore, the welding temperature displayed by the thermocouple needs to be controlled to be T0-30°C to T0°C.

[0026] 6. This invention achieves a magnesium alloy vacuum brazed joint with a strength of 65-77 MPa, which is more than 90% higher than the existing technology, meeting the needs of engineering applications and possessing value for the promotion and application of space-based radar. Attached Figure Description

[0027] Figure 1 Schematic diagram of current-assisted vacuum brazing of magnesium alloy;

[0028] Figure 2 Schematic diagram of the temperature field along the axis of the blind hole;

[0029] Figure 3 Setting the temperature field curve for current-assisted vacuum brazing thermocouples of magnesium alloys;

[0030] Figure 4 A schematic diagram of the process for current-assisted vacuum brazing of magnesium alloys.

[0031] The numbers in the image represent:

[0032] 1-Graphite ring; 2-Graphite lower pressure head; 3-Graphite upper pressure head; 4-Thermocouple blind hole; 5-Magnesium alloy part; 6-MgAlAg alloy intermediate layer. Detailed Implementation

[0033] The above-mentioned and other technical features and advantages of the present invention will be described in more detail below with reference to the accompanying drawings.

[0034] Example 1

[0035] In this embodiment, the welding objects are two magnesium alloys with a diameter of 30mm and a height of 10mm.

[0036] The welding fixture consists of a graphite ring 1, a graphite lower pressure head 2, and a graphite upper pressure head 3. The graphite ring 1 has an inner diameter R1 = 30.5 mm, an outer diameter of 50.5 mm, and a height of 40 mm. A blind hole 4 with a diameter of 3 mm and a depth of 8 mm is designed at a height of 20 mm. The graphite lower pressure head 2 and the graphite upper pressure head 3 are cylindrical structures with a diameter of 30 mm and a height of 20 mm.

[0037] A current-assisted vacuum brazing method for magnesium alloys includes the following steps:

[0038] Step S1: Machining two AZ31 magnesium alloy parts 5 with a diameter of 30mm and a height of 10mm to be welded;

[0039] Step S2: Prepare welding intermediate layer 6 with a thickness of 0.15 mm and a diameter of 30 mm using Mg-22.4Al-8.3Ag (wt.%) alloy. The alloy melting point is 406℃.

[0040] Step S3: Sand the two surfaces of the magnesium alloy to be welded and the intermediate layer of the weld with sandpaper until the roughness is below Ra1.6. Clean the sanded surfaces with alcohol and blow them dry.

[0041] Step S4: Place the first magnesium alloy part to be welded with the welding surface facing up, place the middle layer on top of it, and place the second magnesium alloy part with the welding surface facing down on top of the middle layer to form a three-layer structure.

[0042] Step S5: Sequentially insert the two graphite lower pressure heads, the three-layer structure, and the graphite upper pressure head into the inner diameter of the graphite ring, and adjust the height so that the welding intermediate layer 5 is located on the axis of the blind hole 4.

[0043] Step S6: Place the assembled workpiece from step S5 into the current-assisted vacuum brazing furnace, insert the thermocouple into the blind hole 4, close the furnace door, and insert the thermocouple into the bottom of the blind hole 4. This achieves the design goal of a minimum distance of 2mm between the thermocouple tip and the intermediate layer 6, ensuring that the actual temperature T2 of the intermediate layer can be accurately obtained using the formula T2 = T1 + 50℃, where T1 is the temperature measured by the thermocouple.

[0044] Step S7: Set the process parameters for the current-assisted vacuum brazing furnace, including vacuum level, applied pressure, heating rate, holding temperature, holding time, and cooling rate. The heating rate is 100℃ / min, the holding time is 5min, the cooling rate is 40℃ / min, and the holding temperature is 406℃. Further calculations show that the intermediate layer temperature is 456℃, which ensures the intermediate layer melts while preventing the magnesium alloy base material from overheating. The applied pressure is 50KPa, which prevents the molten solder from being excessively squeezed out and avoids deformation of the magnesium alloy parts. The vacuum level is 0.01Pa to ensure that the magnesium alloy does not volatilize during welding.

[0045] The brazed joint obtained in this embodiment has a shear strength of approximately 74 MPa and a weld sidewall overflow height of 0.2 mm.

[0046] Example 2

[0047] In this embodiment, the welding objects are two magnesium alloys with a diameter of 400mm and a height of 20mm.

[0048] The welding fixture consists of a graphite ring 1, a graphite lower pressure head 2, and a graphite upper pressure head 3. The graphite ring 1 has an inner diameter R1 = 30.5 mm, an outer diameter of 50.5 mm, and a height of 40 mm. A blind hole 4 with a diameter of 3 mm and a depth of 8 mm is designed at a height of 20 mm. The graphite lower pressure head 2 and the graphite upper pressure head 3 are cylindrical structures with a diameter of 30 mm and a height of 20 mm.

[0049] A current-assisted vacuum brazing method for magnesium alloys includes the following steps:

[0050] Step S1: Machining two ZK61 magnesium alloy parts 5 with a diameter of 400mm and a height of 20mm to be welded;

[0051] Step S2: Prepare the welding intermediate layer 6 using Mg-24Al-6.7Ag (wt.%) alloy, with a thickness of 0.05 mm, a diameter of 400 mm, and an alloy melting point of 418℃;

[0052] Step S3: Sand the two surfaces of the magnesium alloy to be welded and the intermediate layer of the weld with sandpaper until the roughness is below Ra1.6. Clean the sanded surfaces with alcohol and blow them dry.

[0053] Step S4: Place the first magnesium alloy part to be welded with the welding surface facing up, place the middle layer on top of it, and place the second magnesium alloy part with the welding surface facing down on top of the middle layer to form a three-layer structure.

[0054] Step S5: Sequentially insert the two graphite lower pressure heads, the three-layer structure, and the graphite upper pressure head into the inner diameter of the graphite ring, and adjust the height so that the welding intermediate layer 5 is located on the axis of the blind hole 4.

[0055] Step S6: Place the assembled workpiece from step S5 into the current-assisted vacuum brazing furnace, insert the thermocouple into the blind hole 4, close the furnace door, and insert the thermocouple into the bottom of the blind hole 4. This achieves the design goal of a minimum distance of 2mm between the thermocouple tip and the intermediate layer 6, ensuring that the actual temperature T2 of the intermediate layer can be accurately obtained using the formula T2 = T1 + 50℃, where T1 is the temperature measured by the thermocouple.

[0056] Step S7: Set the process parameters for the current-assisted vacuum brazing furnace, including vacuum level, applied pressure, heating rate, holding temperature, holding time, and cooling rate. The heating rate is 60℃ / min, the holding time is 3min, the cooling rate is 30℃ / min, and the holding temperature is 388℃. Further calculations show that the intermediate layer temperature is 438℃, which ensures the intermediate layer melts while preventing the magnesium alloy base material from overheating. The applied pressure is 10KPa, which prevents the molten solder from being excessively squeezed out and avoids deformation of the magnesium alloy parts. The vacuum level is 0.1Pa to ensure that the magnesium alloy does not volatilize during welding.

[0057] The brazed joint obtained in this embodiment has a shear strength of approximately 74 MPa and a weld sidewall overflow height of 0.2 mm.

[0058] Example 3

[0059] In this embodiment, the objects to be welded are two magnesium alloys with a diameter of 100mm and a height of 15mm.

[0060] The welding fixture consists of a graphite ring 1, a graphite lower pressure head 2, and a graphite upper pressure head 3. The graphite ring 1 has an inner diameter R1 = 30.5 mm, an outer diameter of 50.5 mm, and a height of 40 mm. A blind hole 4 with a diameter of 3 mm and a depth of 8 mm is designed at a height of 20 mm. The graphite lower pressure head 2 and the graphite upper pressure head 3 are cylindrical structures with a diameter of 30 mm and a height of 20 mm.

[0061] A current-assisted vacuum brazing method for magnesium alloys includes the following steps:

[0062] Step S1: Machining two AZ31 magnesium alloy parts 5 with a diameter of 100mm and a height of 15mm to be welded;

[0063] Step S2: Prepare welding intermediate layer 6 using Mg-22Al-7Ag (wt.%) alloy, with a thickness of 0.15 mm, a diameter of 30 mm, and an alloy melting point of 413℃;

[0064] Step S3: Sand the two surfaces of the magnesium alloy to be welded and the intermediate layer of the weld with sandpaper until the roughness is below Ra1.6. Clean the sanded surfaces with alcohol and blow them dry.

[0065] Step S4: Place the first magnesium alloy part to be welded with the welding surface facing up, place the middle layer on top of it, and place the second magnesium alloy part with the welding surface facing down on top of the middle layer to form a three-layer structure.

[0066] Step S5: Sequentially insert the two graphite lower pressure heads, the three-layer structure, and the graphite upper pressure head into the inner diameter of the graphite ring, and adjust the height so that the welding intermediate layer 5 is located on the axis of the blind hole 4.

[0067] Step S6: Place the assembled workpiece from step S5 into the current-assisted vacuum brazing furnace, insert the thermocouple into the blind hole 4, close the furnace door, and insert the thermocouple into the bottom of the blind hole 4. This achieves the design goal of a minimum distance of 2mm between the thermocouple tip and the intermediate layer 6, ensuring that the actual temperature T2 of the intermediate layer can be accurately obtained using the formula T2 = T1 + 50℃, where T1 is the temperature measured by the thermocouple.

[0068] Step S7: Set the process parameters of the current-assisted vacuum brazing furnace, including vacuum degree, applied pressure, heating rate, holding temperature, holding time, and cooling rate. The heating rate is 150℃ / min, the holding time is 5min, the cooling rate is 50℃ / min, and the holding temperature is 400℃. Further calculations show that the intermediate layer temperature is 450℃, which ensures the intermediate layer melts while preventing the magnesium alloy base material from overheating. The applied pressure is 30KPa, which prevents the molten solder from being excessively squeezed out and avoids deformation of the magnesium alloy parts. The vacuum degree is 0.05Pa to ensure that the magnesium alloy does not volatilize during welding.

[0069] The brazed joint obtained in this embodiment has a shear strength of approximately 70 MPa and a weld overflow height of 0.25 mm on the sidewall.

[0070] Example 4

[0071] This embodiment is basically the same as Embodiment 1, except that the holding temperature is 436℃, and the intermediate layer temperature can be further calculated to be 486℃. Due to the excessively high welding temperature, the interfacial metallurgical reaction intensifies, forming a large number of coarse and brittle phases, significantly reducing weldability. Simultaneously, the excessively high welding temperature intensifies the flow of molten solder, increasing the overflow height of the weld sidewall.

[0072] The brazed joint obtained in this embodiment has a shear strength of approximately 45 MPa and a weld overflow height of 0.25 mm on the sidewall, which is insufficient to meet the antenna welding requirements.

[0073] Example 5

[0074] This embodiment is basically the same as embodiment 2, except that the heat preservation temperature is 358℃, and the intermediate layer temperature can be further calculated to be 408℃. Since the intermediate layer welding temperature is lower than its melting point, except for the connection formed by partial discharge, most of the interface does not form a metallurgical reaction, resulting in poor interface connection.

[0075] The shear strength of the brazed joint obtained in this embodiment is approximately 8 MPa.

[0076] Example 5

[0077] This embodiment is basically the same as embodiment 3, except that the applied pressure is 100 kPa. Under excessive pressure, the molten solder is severely squeezed out, the overflow height on the sidewall increases significantly, and the shear strength of the joint also decreases slightly due to insufficient remaining solder in the weld.

[0078] The brazed joint obtained in this embodiment has a shear strength of approximately 60 MPa and a weld sidewall overflow height of 0.5 mm. Neither the strength nor the overflow height meets the requirements for antenna welding.

[0079] The above description is merely a preferred embodiment of the present invention and is illustrative rather than restrictive. Those skilled in the art will understand that many changes, modifications, and even equivalents can be made within the spirit and scope defined by the claims of the present invention, all of which will fall within the protection scope of the present invention.

Claims

1. A method for current-assisted vacuum brazing of magnesium alloys, employing a magnesium alloy current-assisted vacuum brazing fixture comprising a graphite ring, a graphite lower pressure head, and a graphite upper pressure head, wherein the graphite ring has a height of 2H, and a blind hole is provided at the height H of the graphite ring, characterized in that... Includes the following steps: S1, process two magnesium alloy parts to be welded, each with a diameter of R and a height of H / 2; S2, a welding intermediate layer is prepared using MgAlAg alloy, with a thickness of 0.05-0.15mm, a diameter of R, and an alloy melting point of T0; S3, sand the two surfaces of the magnesium alloy to be welded and the two surfaces of the intermediate layer to be welded with sandpaper until the roughness is below Ra1.6, clean the sanded surface with alcohol and blow dry; S4, place the first magnesium alloy part to be welded with the welding surface facing up, place the middle layer on top of it, and place the second magnesium alloy part with the welding surface facing down on top of the middle layer to form a three-layer structure. S5, sequentially insert the graphite lower pressure head, the three-layer structure, and the graphite upper pressure head into the inner diameter of the graphite ring, and adjust the height so that the welding intermediate layer is located on the axis of the blind hole; S6. Place the assembled workpiece in step S5 into the current-assisted vacuum brazing furnace, insert the thermocouple into the blind hole, close the furnace door, and set the brazing parameters to perform brazing. In step S2, the MgAlAg alloy composition is Mg-(21-24 wt.%)Al-(9.7-6.7 wt.%)Ag, and the melting point T0 is 400-420℃.

2. The current-assisted vacuum brazing method for magnesium alloys as described in claim 1, characterized in that, In step S1, the magnesium alloy is either AZ31 magnesium alloy or ZK61 magnesium alloy.

3. The current-assisted vacuum brazing method for magnesium alloys as described in claim 1, characterized in that, In step S6, the vacuum degree of the vacuum brazing furnace is 0.1~0.01Pa, the applied pressure is 10~50KPa, the heating rate is 50-150℃ / min, the holding temperature is T0-30℃~T0℃, the holding time is 3~5min, and the cooling rate is 30-50℃ / min.

Images