CN114289855B - Welding method for improving friction stir welding seam tissue asymmetry - Google Patents
Welding method for improving friction stir welding seam tissue asymmetry Download PDFInfo
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Abstract
A welding method for improving friction stir weld tissue asymmetry, comprising the steps of: (1) Fixing two alloy plates to be welded on a workbench of a friction stir welding system; (2) Inserting the rotating stirring pin into a region to be welded to perform first friction stir welding; (3) And maintaining the rotation and insertion state of the stirring pin, and performing friction stir welding of the second pass on the welding bead formed in the first pass along the direction opposite to the first pass. The two friction stir welding processes a welding seam through the reciprocating two friction stir welding, so that the structure of the welding seam is more uniform, the structural asymmetry of the cross section of the welding seam is eliminated, and the overall performance of the welding seam is obviously improved.
Description
Technical Field
The invention relates to friction stir welding, in particular to a welding method for improving structural asymmetry of friction stir welding seams.
Background
Friction Stir Welding (FSW) is a novel solid phase welding technology, can effectively avoid the defects of air holes, slag inclusion, looseness, large deformation and the like generated in the fusion welding process, has the advantages of small welding deformation, few defects, high joint quality, no environmental pollution and the like, is an energy-saving and environment-friendly green solid phase connection technology, has received general attention from scientists around the world since the advent, has also received rapid development, and has been widely applied to the fields of aerospace, high-speed rail, new energy automobiles, pressure vessels and the like. However, in the friction stir butt welding process, because the stress states of the advancing side and the retreating side of the stirring pin are different, the material on the advancing side is subjected to a stretching effect, and the material on the retreating side is subjected to an extrusion effect, so that onion-shaped structures are generated in the friction stir welding seam, and the non-uniformity and the non-symmetry of the structures are generated on the cross section of the welding seam, so that the performance of the welding seam is non-uniform, the overall service performance of the welding seam is reduced, and the application of welding pieces is severely limited.
The structural asymmetry and tunnel defect of the friction stir butt weld are the main problems puzzling the friction stir welding technology, and scientific researchers have conducted a great deal of research work around the structure and defect of the friction stir weld, aiming at improving the structure of the weld and improving the overall performance of the weld. Han Peipei and the like study the influence of welding speed and stirring pin rotating speed on the weld joint forming quality, and the results show that uniform and fine equiaxial grains can be formed in a weld core area under the proper welding speed and stirring pin rotating speed, the surface forming is good, the weld joint is free from flash and defect, and the obtained weld joint has excellent performance. Liu Zhenlei and the like find that the tunnel defect of welding can be eliminated by adjusting the process parameters when researching the influence of the process parameters on the forming rule of the aluminum-lithium alloy friction stir welding joint, but the onion ring structure of the welding joint still exists. Wang Hongduo and the like apply an electromagnetic field to the friction stir welding area, and utilize the electromagnetic field to influence the plastic metal flow and dislocation movement in the plastic deformation metal of the area so as to effectively refine crystal grains of a weld joint structure, thereby improving the mechanical property of the weld joint. Wang Dongsheng, pulse current and ultrasonic vibration are applied in the friction stir welding process to promote the softening and plastic flow of materials in the welding process so as to strengthen the formability of the welding seam and the uniformity of the welding seam structure. Guo Weijiang and the like are respectively processed into saw-tooth shapes on the butt joint surfaces of the two materials, and before welding, the butt joint surfaces of the two saw-tooth shapes are mutually meshed and butted together, and are fixed and welded. Chen Zheng and the like exert current action in the friction stir welding of the magnesium alloy thick plate, and obtain grain refinement action of a nugget region and a thermo-mechanical influence region, thereby improving the strength of the welding line. The ultrasonic field is acted on the friction stir welding of the aluminum alloy in the greedy field solving and the like, the welding acting force and the welding temperature are reduced, the grains of a welding core area are thinned, and the theta phase of second phase large particles of the welding area is reduced and distributed sparsely, so that the mechanical property of the welding seam is improved compared with the mechanical property of the welding seam when the ultrasonic field is not applied. The outer static shaft shoulder is also arranged on the stirring pin in the greedy request and the like, and the friction stir welding is carried out on 2219-T6, so that a weld joint with no defects and good surface quality is obtained, and the average tensile strength reaches 72% of the base metal.
The existing friction stir welding methods and process parameter adjustment improve the structure and mechanical properties of the welding seam to different degrees, but the symmetry and uniformity of the cross section of the welding seam are not improved obviously, especially the symmetry of the cross section structure of the welding seam is basically unchanged, so that the welding seam is easy to break on the advancing side of the friction stir welding. In the study of Bai Yafeng, although pulse current is applied during friction stir welding, the structure of the weld nugget region is obviously thinned along with the increase of the applied current, so that the mechanical property is obviously improved (the tensile strength and the elongation after break are respectively improved by 80.53MPa and 0.75%, and the tensile strength of the joint reaches 80.84% of that of a base material), the structure on the advancing side and the structure on the retreating side of friction stir welding show obvious asymmetry, the compactness and the uniformity of the structure on the advancing side are poorer than those on the retreating side, and the macroscopic morphology of the fracture of the friction stir welding joint of different pulse current carrying is shown in fig. 1, (a) 0A, (b) 400A and (c) 600A, and the tensile fracture of the welding joint is clearly seen on the advancing side of friction stir welding no matter whether the current is applied during friction stir welding.
Disclosure of Invention
The technical task of the invention is to provide a welding method for improving the structural asymmetry of friction stir welding seams, aiming at overcoming the defects of the prior art, and further refining and homogenizing the structure of the welding seams.
A welding method for improving friction stir weld tissue asymmetry, comprising the steps of:
(1) Fixing two alloy plates to be welded on a workbench of a friction stir welding system;
(2) Inserting the rotating stirring pin into a region to be welded to perform first friction stir welding;
(3) And maintaining the rotation and insertion state of the stirring pin, and performing friction stir welding of the second pass on the welding bead formed in the first pass along the direction opposite to the first pass.
Further, in the step (1), the two to-be-welded Jin Banjian are the same or different alloys.
Further, in the step (1), the two alloy plate materials to be welded are one or two of iron alloy, magnesium alloy, copper alloy, aluminum alloy and zinc alloy.
Further, the stirring pin in the step (2) is a conical threaded stirring pin.
Further, the rotation speeds of the first-pass stirring pin and the second-pass stirring pin in the steps (2) and (3) are the same, and the welding rates are also the same.
Further, before the step (1), the method further comprises the following operations: cleaning and polishing the surface to be welded of the alloy plate, cleaning and drying by using absolute alcohol, wherein the thickness of the processed surface to be welded Jin Banjian is 2-8 mm.
Further, when the two to-be-welded alloy plate materials are magnesium alloys, the rotation speed of the stirring pin for the first-pass and the second-pass friction stir welding in the steps (2) and (3) is 700-1300 r/min, and the welding speed is 30-150 mm/min.
Compared with the prior art, the invention has the beneficial effects that:
friction stir welding causes structural asymmetry of the advancing and retreating sides of the weld bead cross section due to the completely different stress conditions on both sides of the pin and the metal flow conditions. According to the two-pass friction stir welding, the travelling directions of the stirring pins are opposite, so that the advancing side and the retreating side of the welding area can be interchanged, stress and flowing state of materials are fundamentally changed between the two passes on the two sides of the stirring pins, and the part on the advancing side is positioned on the retreating side during the second-pass friction stir welding during the first-pass friction stir welding, so that the advancing side which is easy to generate defects or insufficient in metal plasticization and flowing can be filled and extruded, the tissues are further homogenized and refined, meanwhile, the metal originally positioned on the retreating side is positioned on the advancing side during the second-pass processing, the tissue is not affected, and the preparation is made for avoiding the defects on the advancing side during the second-pass processing due to the sufficient plasticization and extrusion of the metal during the first-pass processing. Because of the exchange of the advancing side and the retreating side between the two times, the tissue asymmetry generated by the friction stir welding of the single time is well eliminated. The invention processes a welding seam through reciprocating two times of friction stir welding, so that the structure of the welding seam is more uniform, the structure asymmetry of the cross section of the welding seam is eliminated, and the overall performance of the welding seam is obviously improved.
Drawings
Fig. 1 shows the macroscopic morphology of the fracture of a friction stir welding joint with different pulse current carrying characteristics:
FIG. 2 is a schematic illustration of a friction stir welding process according to the present invention;
FIG. 3 is a diagram of a macroscopic structure after a first friction stir welding of the Mg-4Si-4Zn alloy of the example;
FIG. 4 is a diagram of a macroscopic structure after a second pass friction stir welding of the Mg-4Si-4Zn alloy of the example;
FIG. 5 shows the mechanical properties of the weld after friction stir welding of the Mg-4Si-4Zn alloy of the example.
Detailed Description
The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Examples
Chemically stable Mg can appear in the magnesium alloy structure containing silicon 2 Si phase can obviously improve the heat resistance of magnesium alloy, but Mg 2 The Si phase itself is hard and brittle, being a typical facet phase with regular geometry, and is very detrimental to the room temperature mechanical properties of the alloy. For Mg-4Si-4Zn alloy, the casting structure is coarse Mg 2 Si primary phase + primary alpha-Mg phase + eutectic (Mg with complex Chinese character shape 2 Si+alpha-Mg), in this casting structure, primary Mg 2 Coarse Si phase, distinct edge angle and Mg in eutectic 2 The Si phase is in a complex Chinese character shape, and both phases have serious adverse effects on the organization and the performance.
Pure Mg, mg-10.5Si intermediate alloy and pure Zn are proportioned according to the components of Mg-4Si-4Zn magnesium alloy, and mixed gas protection (CO) is carried out at 780 DEG C 2 ：SF 6 =100: 0.5 And (3) smelting under the condition, after the smelting is finished, reducing the furnace temperature to 750 ℃, preserving the heat for 20 minutes, skimming slag, and pouring the alloy melt into a graphite die to obtain the magnesium alloy plate-shaped casting.
By adopting the plate, a welding method for improving the structural asymmetry of the friction stir welding seam is carried out, as shown in fig. 2, and comprises the following steps:
(1) Milling and leveling butt joint parts to be welded of the cast magnesium alloy plates 1 and 2 to a thickness of 6mm, cleaning with absolute alcohol, drying, butt joint the parts together, and fixing the parts on a workbench of a friction stir welding machine;
the working parameters of the friction stir welding machine are set as follows: the rotation speed of the stirring pin is 800r/min, the running speed of the stirring pin is 100mm/min, and a conical threaded stirring pin (the geometric dimension is that the diameter of the lower bottom is 5mm, the diameter of the upper bottom is 8mm, the height is 5mm, and the diameter of the shaft shoulder is 15 mm) is adopted;
(2) Inserting a stirring pin 3 rotating at a high speed into one end of a butt joint line of two alloy plates to be welded, walking along the butt joint line according to a preset speed, and completing first friction stir welding until the stirring pin moves to the tail end of the butt joint line as shown by an arrow A;
(3) Maintaining the rotation and insertion state of the stirring pin, and performing friction stir welding of the second pass on the welding bead formed in the first pass along the direction opposite to the first pass by adopting the same rotation speed and the same walking speed as those of the first pass as shown by an arrow B.
After the first pass of friction stir welding (i.e., single pass friction stir welding), the obtained weld structure is shown in FIG. 3, and it can be seen that the weld nugget region of the weld is relatively refined and uniform in structure on the advancing side (the center line of the weld nugget is about right), and the original coarse Mg 2 Si phase and Chinese character shape Mg 2 The Si phase is broken by the stirring pin rotating at high speed and uniformly distributed in the welding line, but a plurality of layers of hook-shaped uneven structures exist at the left side of the center line of the welding core, the hook-shaped areas are mainly composed of original structures, the middle of the plurality of layers of hooks is a structure refined by stirring friction, and particularly, the left upper corner of the welding core has obvious structure left-right asymmetry phenomenon at the middle upper part of the welding line.
As shown in FIG. 4, the weld joint structure obtained after the second-pass friction stir welding can be seen, the uneven structure in FIG. 3 can be completely disappeared by the reciprocating double-pass friction stir welding, the original structure in the weld joint can be effectively thinned and homogenized, meanwhile, due to the change of the positions of the advancing side and the retreating side, the structure state that the structure is not compact and even has defects caused by stress and metal flowing on the advancing side of the first pass can be effectively filled and extruded, the defects and the structure are not compact in the weld joint can be effectively reduced, and the uniformity of the weld joint performance is effectively improved.
In this embodiment, as shown in fig. 5, the actual measurement results of the mechanical properties of the weld joints obtained by the first-pass friction stir welding (single pass friction stir welding) and the second-pass friction stir welding (reciprocating friction stir welding) are 195.3MPa and 8.7% respectively, the tensile strength and the elongation of the first-pass friction stir welding weld joint are 213.2MPa and 11.48% respectively, the tensile strength and the elongation of the original cast magnesium alloy plate are 115.6MPa, and the elongation is 2.64%, and it can be seen that the tensile strength and the elongation of the reciprocating friction stir welding joint of the present invention are significantly improved by 9.2% and 32.0% respectively compared with the single-pass friction stir welding. From the performance data, the welding seam obtained by the reciprocating friction stir welding can improve the tensile strength and the elongation of the material at the same time.
The technical scheme of the invention is explained in the technical scheme, the protection scope of the invention cannot be limited by the technical scheme, and any changes and modifications to the technical scheme according to the technical substance of the invention belong to the protection scope of the technical scheme of the invention.
Claims (6)
1. A welding method for improving the structural asymmetry of a friction stir weld, comprising the steps of:
(1) The two to-be-welded joints Jin Banjian are butt-jointed and fixed on a workbench of the friction stir welding system;
(2) Inserting the rotating stirring pin into a region to be welded to perform first friction stir welding;
(3) Maintaining the rotation and insertion state of the stirring pin, and performing friction stir welding of a second pass on the welding bead formed in the first pass along the direction opposite to the first pass;
the rotation speeds of the first-pass stirring pin and the second-pass stirring pin in the steps (2) and (3) are the same, and the welding rates are also the same.
2. The welding method for improving structural asymmetry of a friction stir weld according to claim 1, wherein the two members to be welded Jin Banjian in step (1) are of the same or different type.
3. The welding method for improving the structural asymmetry of a friction stir weld according to claim 1, wherein the two sheet metal parts to be welded in step (1) are one or two of iron alloy, magnesium alloy, copper alloy, aluminum alloy and zinc alloy.
4. The welding method for improving tissue asymmetry of a friction stir weld according to claim 1, wherein the pin in step (2) is a tapered threaded pin.
5. The welding method for improving structural asymmetry of a friction stir weld according to claim 1, further comprising, prior to step (1), the steps of: cleaning and polishing the surface to be welded of the alloy plate, cleaning and drying by using absolute alcohol, wherein the thickness of the processed surface to be welded Jin Banjian is 2-8 mm.
6. The welding method for improving the structural asymmetry of a friction stir welding seam according to claim 1, wherein when the two alloy plate materials to be welded are magnesium alloys, the rotation speed of a stirring pin for the friction stir welding in the first pass and the second pass in the steps (2) and (3) is 700-1300 r/min, and the welding speed is 30-150 mm/min.
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