Research status of plastic flow of hottest frictio

Research status of plastic flow of friction stir welding materials

friction stir weld (FSW) is a new solid-phase connection technology, which was invented in the Welding Institute (TWI) in the UK in 1991 and obtained worldwide patent protection. The emergence of this new connection technology overcomes the defects of traditional fusion welding, makes it easier to realize the welding process of difficult welding materials such as aluminum alloy, and has less energy consumption and no pollution to the environment. Therefore, FSW is known as "the second revolution in the world welding history" [1]

the principle of FSW is very simple. The mixing head (tool) composed of a shoulder and a mixing needle (PIN) is inserted into the butt joint of the welded plate. The mixing head rotates and advances, so that the metal on the advancing side (as) and retreating side (RS) produces plastic flow, forms a welded joint, and completes the process of solid-phase connection

since the advent of FSW, a large number of scholars have done a lot of research on the joint structure and mechanical properties of FSW, but due to the invisibility of materials, the plastic flow of FSW weld metal in the welding process is still in the exploratory stage. At present, the plastic flow of materials is mainly studied through experiments and numerical simulation. A. P. reynolds[2] analyzed the current research on the plastic flow of FSW, and pointed out that the FSW flow field can be studied from three aspects: the material surface in contact with the shaft shoulder, the surrounding of the stirring needle and the bottom of the stirring needle. At present, the main problems in the research of material plastic flow in FSW are: what is the mechanism of material flow, what is the reason for the periodic change of FSW microstructure, and what is the relationship between them

experimental research

for using experiments to study the rheological behavior of materials in FSW, some tracer materials are mainly used to track the initial and final position of weld metal. These tracer materials mainly include copper foil, aluminum foil, tungsten wire, etc. However, there are differences in energy efficiency, environmental protection and product quality between tracer materials and base metal due to their mechanical properties, which often affect the flow of base metal in FSW. Therefore, in some experiments, we do not add any tracer materials, but simply observe the microstructure

in the early research, colligan[3] applied steel ball tracking technology to study the flow of materials in aluminum alloy FSW through X-ray detection and observation of slices. Colligan pointed out that not all the materials around the mixing head will stir. Most of the materials on the backward side of the mixing needle have only been simply squeezed, and precipitation has occurred behind the mixing needle

with the continuous progress of research means and technology, although the research on the plastic flow of materials in the FSW process is not very perfect, some consensus has been reached in some aspects. For example, the material flow on the forward side and the backward side in FSW is asymmetric, and the material will precipitate "layer by layer" behind the mixing head, and finally form a banded structure

r.m. leal et al. [4] used two different stirring heads respectively for AA 5182-h111 and AA 6016-t4 aluminum alloys to conduct friction stir welding under two different process parameters. Through the observation of the microstructure in the longitudinal, transverse and parallel directions with the backing plate, the material flow under the influence of two different stirring heads was given. Figure 1 and table 1 respectively show the macro morphology of the stirring head and the process parameters of two membrane materials: bipolar membrane electrodialysis membrane, high-performance lithium battery diaphragm, high-pressure reverse osmosis composite membrane material and high selective nanofiltration membrane material. Among them, type (a) mixing head has a conical cavity with an angle of 8 ° on the shaft shoulder, and type (b) mixing head shaft shoulder is in a vortex shape. (a) Type a mixing head uses the process parameters of the first series (WS1), and type B mixing head uses the process parameters of the second series (WS2)

the results show that the application of (a) type stirring head can obtain a relatively smooth welding surface, but it will make the plate thinner in thickness. The stirring needle plays a major role in the flow of material, forming an "onion ring" structure in the nugget area; (b) Type a stirring head will not cause changes in thickness, but the obtained welding surface is not as smooth as that of type (a) stirring head. The shaft shoulder plays a key role in the formation of welding structure. The amount of material flowing from the forward side to the backward side is significantly more than that of type (a) stirring head

like R.M. leal et al., K. Kumar, Satish v. KAILAS et al. [5] also studied the geometric dimensions of the mixing head and the effects of shaft shoulder and mixing on material flow. In order to avoid the influence of tracer materials on the flow of metal materials in FSW, no tracer materials were used in the research process, and only the microstructure of the weld was observed. The welding material used in the experiment is 7020-t6 aluminum alloy. The length of the stirring needle is 4.2mm, the upper diameter is 6mm, the lower diameter is 4mm, and the shaft shoulder diameter is 20mm. The material of the stirring head is H13, and the hardness is 55HRC. The process parameters are the rotating speed of the stirring head 1400r/min, the welding speed 80mm/min, and the inclination angle of the stirring head is 2 °. In order to study the role of mixing needle and shaft shoulder in the flow of weld material, K. Kumar, Satish v. KAILAS and others made the bottom backing plate have an inclined angle, so that the mixing head can be continuously inserted into the welded material from shallow to deep

a welding chamber will be formed when the stirring needle is inserted into the welding material. With the increasing interaction between the stirring head and the welding material, the axial pressure will continue to increase. Through the observation of microstructure, it can be seen that when the axial pressure reaches 7.4kn, the welding defects disappear (as shown in Figure 2); When the pressure reaches 8.1kn, the material flow on the forward side and the backward side loses symmetry, and more material flows from the backward side to the forward side. On the surface of the material, the shaft shoulder plays a major role in the material flow, making the material transfer from the forward side to the backward side; In the area where the stirring needle affects the material flow, due to the effect of a resistance on the retreating side, the material moves upward, resulting in the formation of flash. However, with the continuous action of the stirring head and the welding material, the shaft shoulder presses the upward flowing material back into the welding chamber

to sum up, K. Kumar, Satish v. KAILAS and others proposed to reasonably design the shape of the stirring needle to maximize the flow of metal, and reasonably design the shape of the shaft shoulder to maximize the return of upward flowing materials to the welding chamber

in many experiments, tracer materials are used to track the initial and final position of weld metal to study the rheological behavior of materials in FSW. Copper foil is a tracer material with high application frequency, mainly because of its high melting point, good plasticity and color that can be distinguished from the base metal

Ke Liming of Nanchang University of Aeronautics and Astronautics and others [6] believed that the surface morphology of the stirring needle had an important influence on the plastic flow of the weld metal in the thickness direction. The stirring heads with three kinds of stirring needles with left thread, right thread and smooth surface were used for experiments, and 0.02mm copper foil was used as the tracer material. The copper foil was inlaid with LF6 and LY12 alternately, In order to better observe the plastic flow of weld metal in the thickness direction

after welding, cut the specimen perpendicular to the welding direction and observe the shape of the weld section. Through observation, Ke Liming et al. Concluded that the cross-section morphology of the weld obtained with the threaded cylindrical stirring needle has obvious "onion ring" characteristics, and the direction of the stirring needle thread affects the position of the "onion ring" and the migration direction of the weld plastic metal: the left thread moves the center of the "onion ring" downward, and the metal around the "onion ring" upward; The right thread moves the center of the "onion ring" upward, and the metal around the "onion ring" moves downward; When the surface of the stirring needle is smooth, the weld metal is mainly translational on the cross section

g. buffa et al. [7] used copper foil to study the metal flow of T-type FSW joint. The experimental device is shown in Figure 3. A 0.1mm thick copper foil with purity of 99.95% is inserted at the contact between the upper and lower plates. The final position of the copper foil after welding is shown in Figure 4. From the results of the experiment, it can be seen that the final position of the copper foil is painted with a paint with high illumination on the gauge part (that is, the part between the two gauge lines) (if the color of the sample is light, the gauge length is fully divided into three areas a, B and C with black pigment. The metal in areas a and B is under pressure and flows to the fillet area of the fixture; the metal in the middle is under vertical downward pressure and flows downward; the metal in area C flows upward to the left, and then flows to the forward side.

Huang Yongde, Xing Li et al. [8] The LY12 aluminum alloy was inlaid with copper foil as the tracer material. The LY12 aluminum alloy and copper foil were inlaid alternately and in the form of "meter" respectively. The distribution of the tracer material on the cross section and horizontal section of the welded joint after welding was observed, and the flow pattern of plastic metal in friction stir spot welding joint was preliminarily discussed

the results show that on the cross-section of the solder joint, the width of the plastic deformation zone gradually decreases from the solder joint surface downward, and the plastic deformation zone of the plastic metal on both sides of the keyhole is basically symmetrical; In the upper part of the solder joint, the plastic metal is mainly affected by the shaft shoulder. Under the action of the friction of the shaft shoulder and the shear force between the materials, it moves along the rotation direction of the stirring needle. With the increase of the distance from the surface of the solder joint, the movement trend of the plastic metal along the rotation direction of the stirring needle gradually decreases; Under the downward pressure of the stirring needle thread, the plastic metal around the stirring needle moves spirally to the bottom of the solder joint. After moving to the bottom of the solder joint, it moves from the periphery of the stirring needle to the upper part of the solder joint due to the obstruction of the bottom plate and the extrusion of the unplasticized metal

Zhao Xudong et al. [9] welded LF2 and LY12 two different aluminum alloys with FSW, embedded copper foil at the thickness of 8mm and 2mm respectively, and observed the distribution of tracer material in the sample and the flow of weld metal after welding. The observation results are shown in Figure 5

through the analysis of the experimental results, it is shown that the flow field of FSW is asymmetric to the weld centerline, and the metal flow mode on the backward side and the forward side is also different. The flow of weld metal is affected by the shaft shoulder and the stirring needle. At the bottom of the weld, the flow mode of the material is controlled by the rotary extrusion of the stirring needle. When the stirring needle moves forward, a small amount of metal near the forward side moves along the welding direction, while the vast majority of metal materials are squeezed to the rear of the stirring needle with the forward movement of the stirring needle, and flow in the opposite direction of the welding. The farthest moving distance will not exceed the position of a stirring needle diameter; The material on the backward side only flows backward, and the metal on the forward side will not cross the centerline into the backward side, and a small part of the metal on the backward side will cross the weld centerline into the forward side. At the top of the weld, the flow mode of the material is controlled by the rotary friction of the shaft shoulder. Most of the metal on the forward side flows forward and terminates on the forward side. A small part of the metal flows backward no more than a distance of the radius of the mixing needle. The metal on the backward side flows backward around the centerline of the weld and enters the forward side until the front of the mixing head. Affected by the pressure and temperature field distribution of the shaft shoulder of the mixing head, the resistance of the weld metal flowing forward is greater than that of the backward flow, and most of the metal flows backward, and the longest distance of the forward flow is less than that of the backward flow

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