CN113210808A

CN113210808A - Electric arc additive manufacturing method of magnesium alloy

Info

Publication number: CN113210808A
Application number: CN202010055609.4A
Authority: CN
Inventors: 申俊琦; 毕极; 甄亚辉; 胡绳荪
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2020-01-17
Filing date: 2020-01-17
Publication date: 2021-08-06

Abstract

The invention discloses an electric arc additive manufacturing method of magnesium alloy, which takes cold metal transition welding as welding equipment, uses a cold metal transition welding gun to provide a heat source for the additive manufacturing process, the welding gun is vertical to the surface of a workpiece, and the additional swinging mode adopts triangular swinging. According to the technical scheme of the invention, the CMT welding process is additionally provided with the swinging single-channel multi-layer additive manufacturing thick-walled wall, the surface is very flat, the forming precision of two sides is higher, obvious cracks and air holes are not found, the fracture appearance of the tensile piece is shown, the fracture is mainly formed by the dimple, and the fracture form is shown to be ductile fracture.

Description

Electric arc additive manufacturing method of magnesium alloy

Technical Field

The invention belongs to the technical field of magnesium alloy additive manufacturing, and particularly relates to an electric arc additive manufacturing method of a magnesium alloy, belonging to a welding method of CMT welding process with additional swinging.

Background

The magnesium alloy is taken as the lightest commercial metal structure material at present, has the advantages of low density, high specific strength and specific stiffness, good damping and vibration damping performance, easy processing and forming, renewable utilization and the like, is considered to be one of lightweight materials with the greatest development prospect, and has been widely applied to the fields of automobiles, 3C, aerospace, national defense and military industry and the like.

The magnesium alloy has a crystal structure of hexagonal close packing, has poor plastic processing performance at normal temperature, and has good liquid formability. At present, the magnesium alloy in engineering application mainly takes die castings as a main part (accounting for more than 90 percent), and the quality and yield stability of the magnesium alloy die castings are poor, and the rejection rate is high, so that the price of a magnesium alloy product is high, and the popularization and application of the magnesium alloy product and the development of a new product are restricted.

The electric arc additive manufacturing technology can process a shape structure which is difficult to realize by the traditional technology, and meanwhile, the technology can prepare a high-density formed part, so that the casting defect caused by the traditional casting technology is overcome to a certain extent, a feasible way is provided for the development of the magnesium alloy manufacturing technology, and the magnesium alloy manufacturing technology can enter the field of high-performance structures.

The Cold Metal Transfer (CMT) welding technology is a welding process developed by the Fronius company, and has the characteristics of high deposition rate, low heat input, accurate arc length control and no splashing. The CMT welding technology mainly adopts external mechanical back-drawing force to promote short circuit transition of molten drops, improves the waveforms of voltage and current, ensures that the current is almost zero during short circuit transition, and greatly limits welding heat input. Aiming at a series of weldability problems of coarse grains, oxidation evaporation, thermal cracks, thermal stress and the like in the welding process of the magnesium alloy, the control of heat input in the welding process is particularly critical. Lower CMT, more precise heat input control helps to suppress the above-mentioned problems of magnesium alloys during welding, and mechanical withdrawal control of the wire also helps to form a stable, splash-free welding process. Therefore, the CMT technology has good application prospect in the aspect of magnesium alloy arc additive manufacturing. Currently, research on magnesium alloy electric arc additive manufacturing mainly adopts MIG (metal inert gas) and TIG (tungsten inert gas) welding processes, and no literature on CMT (magnesium alloy) additive manufacturing is searched at present.

Disclosure of Invention

The invention aims to overcome the defects of the prior art, provides an electric arc additive manufacturing method of magnesium alloy, belongs to a method for adding swing to a CMT welding process, and can obtain a magnesium alloy thick-wall component with excellent quality.

The technical purpose of the invention is realized by the following technical scheme:

an electric arc additive manufacturing method of magnesium alloy uses a cold metal transition welding gun to provide a heat source for the additive manufacturing process, the welding gun is vertical to the surface of a workpiece, the vertical direction is the deposition direction of additive manufacturing, additive manufacturing is carried out on one layer from left to right by CMT, after reaching a right side cut-off, additive manufacturing is carried out on the next layer from right to left, and after reaching a left side cut-off, additive manufacturing is carried out on the next layer from left to right, so that the whole additive manufacturing is realized; in each layer of additive manufacturing, the CMT welding gun is swung out from a starting position by taking a central line of the layer of additive manufacturing as a symmetry axis, runs to an amplitude vertex at a certain angle deviated from the central line of the additive manufacturing, runs to the position of the central line of the additive manufacturing to form an isosceles triangle of a CMT running track, swings out in the opposite direction at the same angle, runs to the amplitude vertex, runs to the position of the central line of the additive manufacturing to form an isosceles triangle of the CMT running track again, and repeats the steps until the end position of the layer of additive manufacturing is reached, the additive manufacturing of the cost layer is completed, and the additive manufacturing of the next layer is performed.

Furthermore, the certain angle from the additive manufacturing centre line is 30-60 degrees, preferably 45 degrees.

And the dry elongation of the welding gun is 10-18mm, preferably 12-15 mm, the wire feeding speed is 8-14m/min, preferably 10-13m/min, the welding gun walking speed is 10-90cm/min, preferably 30-70cm/min, the shielding gas of the cold metal transition welding gun adopts one of nitrogen, helium and argon, and the airflow of the shielding gas of the cold metal transition welding gun adopts 12-25L/min, preferably 15-20L/min.

Moreover, triangular swing is adopted when outward swinging is carried out, the frequency is 1-5Hz, and the amplitude is 1-15mm, preferably 6-8 mm; the residence time of the CMT at the peak position of the amplitude is 0.1 to 0.5s, preferably 0.2 to 0.3 s; the residence time of the CMT at the intersection of the running track and the additive manufacturing centerline is 0.1-0.5 s, preferably 0.2-0.3 s.

The electric arc additive manufacturing method of the magnesium alloy takes cold metal transition welding as welding equipment, uses a cold metal transition welding gun to provide a heat source for the additive manufacturing process, the welding gun is vertical to the surface of a workpiece, and the additional swinging mode adopts triangular swinging. According to the technical scheme of the invention, the CMT welding process is additionally provided with the swinging single-channel multi-layer additive manufacturing thick-walled wall, the surface is very flat, the forming precision of two sides is higher, obvious cracks and air holes are not found, the fracture appearance of the tensile piece is shown, the fracture is mainly formed by the dimple, and the fracture form is shown to be ductile fracture.

Drawings

FIG. 1 is a photograph of a CMT welding process with the addition of a swinging single pass multi-layer additive manufacturing thick wall forming (1).

FIG. 2 is a photograph (2) of a CMT welding process with the addition of a swinging single-pass multi-layer additive manufacturing thick wall forming.

FIG. 3 is a photograph of a cross-section of a CMT welding process with the addition of a swinging single pass multi-layer additive manufacturing thick wall forming member of the present invention.

FIG. 4 is a photograph of the fracture morphology of the tensile member of the CMT welding process of the present invention with the addition of a swinging single pass multi-layer additive manufacturing thick wall forming member.

FIG. 5 is a schematic diagram (1) of the CMT welding process of the present invention with the addition of a swinging single pass multi-layer additive manufacturing process.

FIG. 6 is a schematic diagram of the structure of a sample according to the present invention.

FIG. 7 is a schematic diagram (2) of the CMT welding process of the present invention with the addition of a swinging single pass multi-layer additive manufacturing process.

Detailed Description

The technical solution of the present invention is further illustrated by the following specific examples.

The experimental base material is magnesium alloy AZ91D, the specification of a test piece is 250 multiplied by 150 multiplied by 6mm, and a welding wire which is made of magnesium alloy AZ91D and has the diameter of 1.2mm is selected. A single-layer multi-channel additive manufacturing test is carried out on the magnesium alloy with the thickness of 6mm by adopting a direct current CMT process. And obtaining an ideal single-layer multi-channel additive manufacturing layer by respectively changing the CMT process parameters, the advancing speed and the swinging parameters. The experimental equipment selected the CMT welder as the CMT Advanced 4000 type welder from the Fonce corporation.

The method mainly comprises the following steps:

1) preparation before welding

Before surfacing, removing an oxide film on a magnesium plate by using a steel wire brush until the metallic luster is exposed, cleaning oil stains and dirt on the surface of a welding position within the range of about 30-40mm by using alcohol, and welding within 2 hours after removing the oxide film so as to avoid generating a new oxide film.

2) Set up the experiment platform

As shown in fig. 5, a fixture is used on a worktable to fix a substrate, the deposition direction of additive manufacturing is vertical to the substrate, additive manufacturing of a first layer is performed from left to right by CMT, after the first layer reaches a right-side cut-off position, additive manufacturing of a next layer is performed from right to left, and after the first layer reaches a left-side cut-off position, additive manufacturing of the next layer is performed from left to right, so that the whole additive manufacturing (i.e., the deposition process) is realized. After additive manufacturing, the vertical and horizontal tension members are taken at the positions shown in fig. 5, respectively, according to the structure and dimensions shown in fig. 6. As shown in fig. 7, the triangular swing of the CMT torch is schematically illustrated as viewed from the CMT torch position downward (i.e., from the top view) during the additive manufacturing process shown in fig. 5, resulting in the trajectory shown in fig. 7. Specifically, in fig. 7, the triangular graph is a motion trajectory of the CMT welding gun in the additive manufacturing of one layer, the horizontal direction is a central line of each layer of additive manufacturing, in each layer of additive manufacturing, the CMT welding gun performs outward swinging by taking the central line of the layer of additive manufacturing as a symmetry axis from a starting position, moves to an amplitude vertex at a certain angle away from the central line of additive manufacturing, then moves to a central line position of additive manufacturing to form an isosceles triangle of a CMT running trajectory, performs outward swinging in the opposite direction at the same angle, moves to the central line position of additive manufacturing after moving to the amplitude vertex to form an isosceles triangle of the CMT running trajectory, and repeats such steps as to reach an end position of the layer of additive manufacturing, completing the additive manufacturing of the layer, and then performing the additive manufacturing of the next layer. In fig. 7, the deviation angle is 45 degrees.

3) And setting welding parameters. Adopting a direct current CMT process, setting the wire feeding speed to be 12m/min, the dry elongation to be 15mm, the welding gun walking speed to be 60cm/min, selecting 99.999 percent pure argon as protective gas and setting the gas flow to be 15L/min. The swing mode of the external swing adopts triangular swing, the frequency is 5Hz, the amplitude is 8mm, the residence time of the left side and the right side is 0.2s (namely the vertex positions of the upper triangle and the lower triangle in the figure 7 and the vertex position of the amplitude), and the residence time of the middle is 0.2s (namely the intersection point of the CMT running track and the additive manufacturing central line). The interlayer retention temperature is 3min, then the height of a welding gun is adjusted to ensure that the dry elongation is 15mm, in order to achieve the effect of relatively flat surface, a reciprocating welding method is adopted to carry out reciprocating welding on 12 layers in total, and the size of the finally obtained thick-wall is 115 multiplied by 22 multiplied by 46 mm.

4) A cross section crystal sample is prepared through a component, three micro mechanical tension pieces are respectively taken at the longitudinal position of the transverse position, the tension pieces are sequentially polished by 600-mesh, 1000-mesh, 1500-mesh, 2000-mesh and 3000-mesh abrasive paper, so that the surface of each tension piece has no obvious defects such as cracks, and then a tension test is carried out, and the fracture morphology is observed.

5) Fig. 1 and 2 are graphs for single-pass multi-layer additive manufacturing forming of a magnesium alloy material CMT with an external swing arc, so that a thick wall is good in forming and smooth in surface, and fig. 3 is a cross section of the thick wall component, so that forming accuracy of two sides is high, and obvious air holes and cracks are not found. FIG. 4 shows the fracture morphology of the tensile member, and it can be seen that the fracture is dominated by the dimple, indicating that the fracture mode is ductile fracture. A tensile test is carried out through an Instron MicroTester 5848 micro-force material testing machine, the tensile speed is 0.3mm/s, the tensile strength of the thick-wall component in the horizontal direction and the tensile strength of the thick-wall component in the vertical direction are 253.0MPa and 249.3MPa respectively, no obvious anisotropy exists, the elongation is 24-27%, and the method is proved to be capable of obtaining the additive manufacturing component with excellent mechanical property and good plasticity.

The additive manufacturing of the magnesium alloy can be realized by adjusting the process parameters according to the content of the invention, and the performance basically consistent with the invention is shown by tests. The invention has been described in an illustrative manner, and it is to be understood that any simple variations, modifications or other equivalent changes which can be made by one skilled in the art without departing from the spirit of the invention fall within the scope of the invention.

Claims

1. An electric arc additive manufacturing method of magnesium alloy is characterized in that a cold metal transition welding gun is used for providing a heat source for the additive manufacturing process, the welding gun is vertical to the surface of a workpiece, the vertical direction is the deposition direction of additive manufacturing, additive manufacturing is carried out on one layer from left to right by CMT, after the next layer reaches a right side cut-off position, additive manufacturing is carried out on the next layer from right to left, and after the left side cut-off position is reached, additive manufacturing is carried out on the next layer from left to right, so that the whole additive manufacturing is realized; in each layer of additive manufacturing, a CMT welding gun performs outward swinging by taking a central line of the layer of additive manufacturing as a symmetry axis from a starting position, runs to an amplitude vertex at a certain angle deviated from the central line of the additive manufacturing, then runs to the position of the central line of the additive manufacturing to form an isosceles triangle of a CMT running track, then performs outward swinging in the opposite direction at the same angle, runs to the amplitude vertex, then runs to the position of the central line of the additive manufacturing to form an isosceles triangle of the CMT running track, and repeats the steps until reaching the end position of the layer of additive manufacturing, completes the additive manufacturing of the layer, and then performs the additive manufacturing of the next layer, wherein: when the material is swung outwards, triangular swing is adopted, a certain angle deviating from the central line of material increase manufacturing is 30-60 degrees, the frequency is 1-5Hz, and the amplitude is 1-15 mm; the residence time of the CMT at the position of the amplitude vertex is 0.1-0.5 s, and the residence time of the CMT at the position of the intersection point of the running track and the additive manufacturing central line is 0.1-0.5 s; the dry elongation of the welding gun is 10-18mm, the wire feeding speed is 8-14m/min, the walking speed of the welding gun is 10-90cm/min, and the airflow of the shielding gas of the cold metal transition welding gun is 12-25L/min.

2. The arc additive manufacturing method of magnesium alloy according to claim 1, wherein the outward swinging is performed by a triangular swinging, and the certain angle from the additive manufacturing center line is 45 degrees.

3. The electric arc additive manufacturing method of the magnesium alloy according to claim 1, wherein the swinging is performed by triangular swinging, the frequency is 1-5Hz, and the amplitude is 6-8 mm; the residence time of the CMT at the position of the amplitude peak is 0.2-0.3 s; the residence time of the CMT at the intersection point of the running track and the additive manufacturing central line is 0.2-0.3 s.

4. The electric arc additive manufacturing method of the magnesium alloy according to claim 1, wherein the dry extension of a welding gun is 12-15 mm, the wire feeding speed is 10-13m/min, the walking speed of the welding gun is 30-70cm/min, and the flow of shielding gas of a cold metal transition welding gun is 15-20L/min.

5. The arc additive manufacturing method of magnesium alloy according to claim 1, wherein the shielding gas of the cold metal transition welding torch is one of nitrogen, helium and argon.