CN107150165A - A kind of method that mixing yoghurt prevention aluminum alloy lapped welds grain boundary liquation crackle - Google Patents

Abstract

A method for preventing liquefaction cracks at the grain boundary of aluminum alloy lap welding by friction stir processing. Due to the action of frictional heat and severe plastic deformation, the fibrillar grains undergo complete dynamic recrystallization to form fine equiaxed grains, and the eutectic phase with low melting point at the grain boundary is dispersed and distributed, which improves the resistance to liquefaction and cracking of the grain boundary. ability. The friction stir processing head is mainly composed of three parts: clamping handle, shaft shoulder and stirring needle. Stirring head material: martensitic stainless steel, or tool steel. The parameters of the friction stir processing are that the rotation speed of the stirring head is 500-2000r/min, the traveling speed is 20-100mm/min, and the inclination angle of the stirring needle is 0-5°. The four processing surfaces in the thickness direction of the plate are respectively subjected to friction stir treatment in order to obtain lap welded plates with good crack resistance.

Description

A method of friction stir processing to prevent liquefaction cracks in aluminum alloy lap welding grain boundaries

technical field

The invention relates to the technical field of material processing, and relates to a method for preventing liquefaction cracks in aluminum alloy lap welding grain boundaries through friction stir processing.

Background technique

Due to the advantages of low density, high strength, good plasticity, corrosion resistance and easy forming, aluminum alloy has been widely used in aerospace, transportation, machinery manufacturing and other fields. Especially in recent years, the concept of lightweight and environmentally friendly high-speed trains and automobiles has been proposed, and various lightweight aluminum alloy welded structural parts have been widely used. However, aluminum alloy fusion welding is prone to defects such as pores, cracks and deformation. At present, the friction stir welding process for thin aluminum alloy is relatively mature, which avoids the common defects of fusion welding. However, the friction stir welding of thick plate aluminum alloy is still in the engineering test stage. In many cases, the traditional welding method (such as MIG welding) is still tested for thick plate aluminum alloy. It can be seen that the crack defect of thick plate aluminum alloy fusion welding safety hazards worthy of attention.

Hot crack is a very common crack type in the welding process of aluminum alloy. Hot crack is divided into crystallization crack and liquefaction crack. Crystalline crack generally occurs in the weld zone, while liquefaction crack is easy to appear in the heat-affected zone near the weld. Liquefaction cracks are easy to appear in the thick plate aluminum alloy during welding. This is because the aluminum alloy thick plate is generally prepared by rolling forming process, so the grains of the aluminum alloy thick plate are elongated and fibrous when viewed from the rolling direction or the transverse direction. grains, and the low-melting eutectic phase that exists due to the pre-sequence heat treatment is distributed on the grain boundary. When welding medium and high-strength aluminum alloy thick plates, low-melting eutectics occur at the grain boundary at a specific position in the heat-affected zone near the seam The phenomenon of melting, the so-called "grain boundary liquefaction". In the later stage of weld metal solidification, when the welding stress generated by solidification shrinkage increases to a certain extent, grain boundary liquefaction cracks will be formed at the liquefaction of grain boundaries. However, the fibrous grains arranged elongated along the rolling direction have a weak ability to inhibit the propagation of liquefaction cracks at grain boundaries, and the cracks can easily propagate along the grain in the elongated direction of the grains.

The existing methods for eliminating defects in welded joints mainly include pre-weld heat treatment and post-weld heat treatment. Chinese patent 200780043508.8 discloses a method and device for heat treatment of welds, mainly through induction heating butt joint post-weld heat treatment to reduce the probability of welding cracks; Chinese patent 201210421407.2 discloses a method for welding aluminum alloy thick plates , mainly through cleaning process, preheating process and welding process to reduce welding deformation and improve product quality. But above-mentioned method technology is loaded down with trivial details, reduces production efficiency.

Friction stir processing (FSP) is a method for material microstructure modification and fabrication. Its principle is mainly to use the stirring head to make severe plastic deformation, mixing and crushing of the materials in the processing area, so as to realize the densification, homogenization and refinement of the processing area. In particular, the ultra-fine-grained aluminum alloy prepared by friction stir processing has greatly improved its strength, plasticity and material ductility. The microstructure of the material is modified by stirring and friction, and the densification, homogenization and refinement of the microstructure of the material processing area can be realized through the severe plastic deformation, mixing, crushing and thermal exposure of the material processing area. Therefore, the present invention pre-treats the aluminum alloy lap joints to make the structure grains finer, and eliminate the directionality of the grains, reduce the probability of cracks in the joints, and improve production efficiency.

Contents of the invention

The invention provides a method for preventing liquefaction cracks at the grain boundary of aluminum alloy lap welding by friction stir processing. Complete dynamic recrystallization occurs in the stirred zone, forming fine equiaxed grains, which improves the resistance to welding cracking in this part.

The technical scheme that the present invention adopts is as follows:

A method for preventing liquefaction cracks at aluminum alloy lap welding grain boundaries by friction stir processing, comprising the steps of:

Step 1: Select the plate to be processed, clamp and fix the lead-in plate, lead-out plate and plate to be processed on the friction stir processing machine tool; due to the corresponding vibration of the plate during the friction stir processing process, multiple clamping The device installs it. During the installation process, the protection work of this position should be done well to prevent scratches or damage to the clamping part of the plate.

Step 2: using a mechanical method to polish the friction stir processing position of the plate to expose the metallic luster; to polish the friction stir processing position of the plate. Prevent oil stains and oxides on the surface of the plate from being brought into the plate during friction stir processing.

Step 3: Select the corresponding stirring head according to the depth to be processed in the thickness direction of the plate. The selection standard is that L is slightly larger than H, L is the length of the stirring needle, and H is the depth to be processed in the thickness direction of the plate; the selected stirring head It is a conical stirring head, wherein the length of the stirring needle ranges from 4-20mm, and the diameter of the stirring needle ranges from 3-15mm.

Step 4: Install the friction stir processing head, fix the lead-in plate, the lead-out plate and the test plate to be processed by the fixture. In order to prevent unsatisfactory processing effect due to instability at the beginning of friction stir processing, the lead-in plate is added; in order to prevent the keyhole from being left after friction stir processing, the lead-out plate is added; the processed test plate will eventually form a good friction stir processing area. In order to ensure the smooth progress of the friction stir processing, the movement track of the plate and the friction stir processing head is on a straight line, and the process parameters of the friction stir processing are set as the rotation speed of the stirring head is 500-2000r/min, and the travel speed is 20-100mm /min, the inclination angle of the stirring needle is 0-5°, so that the stirring head rotates at high speed and inserts into the test plate to be processed, and travels at a constant speed along the pretreatment direction of the test plate; according to different series of aluminum alloy plates, set the process of friction stir processing parameter.

Step 5: During friction stir processing, the stirring head is cooled by side-blowing compressed gas from the side axis;

Step 6: After friction stir processing on one side of the test plate, rotate the test plate 90 degrees and fix it with a fixture, and continue to perform friction stir processing on the adjacent surface according to step 4 until all four sides in the thickness direction of the plate are processed. After the friction stir processing on one side of the test plate in the thickness direction, after the processed part is naturally cooled, the test plate is rotated 90 degrees and fixed with a fixture, and the adjacent surface is continued to perform friction stir processing according to step 4. The welding sequence is end to end, that is, The starting position of the friction stir processing of the adjacent surface is close to the ending position of the previous processed surface.

In the present invention, the aluminum alloy plate is processed by using the friction stir processing equipment. First, the lead-in plate, the lead-out plate and the plate to be processed are clamped and fixed on the friction stir processing machine tool, and the position of the plate friction stir processing is polished to expose Metallic luster. Then, select the corresponding stirring head according to the depth requirements to be processed in the thickness direction of the plate, install the friction stir processing head, and set the process parameters of the friction stir processing as the rotation number of the stirring head is 500-2000r/min, and the travel speed is 20 -100mm/min, the inclination angle of the stirring needle is 0-5°, so that the stirring head rotates at high speed and inserts into the test plate to be processed, and travels at a constant speed along the pretreatment direction of the test plate. Method to cool the stirring head. After friction stir processing on one side of the test plate, rotate the test plate 90 degrees and fix it with a fixture, and continue to perform friction stir processing on the adjacent surface according to step 4 until all four sides in the thickness direction of the plate are processed. In this way, the layered structure existing along the rolling direction becomes dispersed fine equiaxed grains, and the cracking resistance of the welded part is improved.

The invention processes the fibrous crystal grains of the lap joint to form fine equiaxed crystals, and breaks the layered inclusions into a diffuse distribution state, thereby improving the resistance to welding cracking. In the present invention, only one stirring and friction processing is performed on the joint area, and the efficiency is far greater than that of heat treatment. The method mainly consumes a stirring head, and for processing an aluminum alloy with a stirring head made of alloy steel, the consumption frequency is very small, the stirring head can be used many times, and the cost is low.

Description of drawings

Figure 1 Schematic diagram of lap fillet weld welding.

Fig. 2 Schematic diagram of microstructure of aluminum alloy sheet rolled or short transverse direction.

Fig. 3 Schematic diagram of welding crack formation in aluminum alloy sheet.

Fig. 4 Schematic diagram of the microstructure of welded joints after friction stir processing.

Fig. 5 is a schematic front view of a friction stir processed plate according to the present invention.

Fig. 6 is a top view of the friction stir processed plate of the present invention.

Figure 7 is a schematic diagram of a stirring head.

Fig. 8 is a schematic diagram of the friction stir processing sequence of the present invention.

Fig. 9 is a schematic diagram of lap welding after friction stir processing according to the present invention.

Fig. 10 Macrophotograph of the plate after friction stir processing in the thickness direction.

Fig. 11 Macroscopic photographs of friction stir processed joints before fillet weld welding.

Fig. 12 Macroscopic photographs of non-friction stir machined joints before fillet weld welding.

Fig. 13 Microscopic photographs of friction stir processed joints before fillet weld welding.

Fig. 14 Microscopic photographs of non-friction stir machined joints before fillet weld welding.

detailed description

Specific embodiments of the present invention will be described in detail below in conjunction with technical solutions and accompanying drawings.

For 20mm and 9mm thick 7N01-T4 aluminum alloy plates, MIG fillet welding is performed, of which the 20mm plate is the reinforcing plate and the 9mm plate is the bottom plate. During the inspection of MIG fillet weld joints, it was found that there were crack defects in the heat-affected zone of the 20mm thick reinforcing plate near the weld. It is considered that the main reason is that the eutectic MgZn ₂ phase with low melting point is distributed on the grain boundary of this position, and there is a large welding tensile stress at this position. So the 20mm thick 7N01-T4 aluminum alloy plate was subjected to friction stir processing and MIG fillet welding. The surface of the 7N01-T4 aluminum alloy plate is mechanically polished to produce a metallic luster, and the workpiece is fixed on the friction stir processing equipment. During friction stir processing, the stirring head is cooled by side-blowing compressed gas from the side shaft. The technological parameters of the friction stir processing are that the rotation speed of the stirring head is 1000r/min, the traveling speed is 50mm/min, the inclination angle of the stirring needle is 2.5°, the length of the stirring needle is 4.2mm, and the diameter of the shaft shoulder is 12mm. Finally, no hot cracks were found in the fillet welds after friction stir treatment.

As mentioned above, similar technical methods can be derived in combination with the solutions given in the drawings and embodiments. Friction stir processing can be used to prevent the generation of thermal cracks for aluminum alloy thick plates that need to be welded lap joints and T-shaped joints. Any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention are still valid. It belongs to the scope of the technical solutions of the present invention.

Claims (2)

1. A method for friction stir processing to prevent liquefaction cracks in aluminum alloy lap welding grain boundaries, is characterized in that comprising the steps: Step 1: Select the plate to be processed, and clamp and fix the plate on the friction stir processing machine tool; Step 2: Use a mechanical method to polish the friction stir processing position of the plate to reveal the metallic luster; Step 3: Select the corresponding stirring head according to the depth to be processed in the thickness direction of the plate. The selection standard is that L is greater than H, L is the length of the stirring needle, and H is the depth to be processed in the thickness direction of the plate; Step 4: Install the friction stir processing head, fix the lead-in plate, the lead-out plate and the test plate to be processed by the fixture; make the movement track of the plate and the friction stir processing head on a straight line, and set the process parameters of the friction stir processing as The rotation speed of the stirring head is 500-2000r/min, the traveling speed is 20-100mm/min, the inclination angle of the stirring needle is 0-5°, so that the stirring head rotates at a high speed and inserts into the test plate to be treated, and the speed is uniform along the direction of the test plate pretreatment Marching; according to different series of aluminum alloy plates, set the process parameters of friction stir processing; Step 5: During friction stir processing, the stirring head is cooled by side-blowing compressed gas from the side axis; Step 6: After friction stir processing on one side of the test plate, rotate the test plate 90 degrees and fix it with a fixture, and continue to perform friction stir processing on the adjacent surface according to step 4 until all four sides in the thickness direction of the plate are processed; the test plate After friction stir processing on one side in the thickness direction, after the processed part is naturally cooled, rotate the test plate 90 degrees and fix it with a fixture, and continue to perform friction stir processing on the adjacent surface according to step 4. The welding sequence is end to end, that is, opposite phase The starting position of the friction stir processing of the adjacent surface is close to the ending position of the previous processing surface. 2. The method according to claim 1, characterized in that: the stirring head is a conical stirring head, wherein the length of the stirring needle ranges from 4-20mm, and the diameter of the stirring needle ranges from 3-15mm.