Experimental study on microstructure and properties of friction stir welding of magnesium-aluminum dissimilar alloy
1. Introduction of friction stir welding technology
Friction stir welding is a novel welding technology invented by the British Welding Research Institute (TWI) in 1991. Its principle is that the high-speed rotating special geometry of the mixing head moves into the workpiece and moves in the welding direction, under the high-speed rotation of the mixing head. The friction head generates frictional heat with the contact part to form a plastic softening layer around the metal, and the cavity formed by the softening layer metal filling the stirring needle is formed by the rotation of the stirring head, and the stirring head and the stirring needle are arranged on the stirring head. A solid phase welding method for material connection under stirring and extrusion.
Friction stir welding principle
2. Research background of magnesium-aluminum dissimilar metal friction stir welding
Friction stir welding is a new type of joining technology. It was originally applied to the connection of the same alloy aluminum alloy. During the welding process, the friction heat generated by the welding workpiece and the mixing needle and the shoulder can not melt the welding workpiece. The welded parts are brought to a thermoplastic state and cannot be brought into a molten state.
Based on the above characteristics, this welding method has the characteristics of high efficiency, energy saving, environmental protection, and is known as “green connection technologyâ€.
Compared with the traditional welding method, friction stir welding can present the following advantages (the research background focuses on why friction stir welding of magnesium/aluminum dissimilar alloys is used, compared to the advantages and disadvantages of other welding methods)
1) In terms of welding consumables, less consumables are consumed. Since the mixing head is a non-consumable material, there is no need for protective gas during the welding process and no need for wire filling consumables, so a large part of the cost is saved.
2) In terms of welding operation, it is easy to operate, easy to automate, and has a short welding cycle. Taking the same kind of aluminum alloy friction stir welding and tungsten argon arc welding as an example, the friction stir welding can shorten the welding time by half.
3) In metallurgy, friction stir welding is a solid phase welding method, the heat input is small, the metal is not melted, and the solidification defects such as pores, shrinkage cavities, looseness and composition segregation generated during solidification are avoided.
4) In terms of environmental protection, the friction stir welding process is safe, no splash, smokeless dust, no radiation, and low pollution.
3. Effect of process parameters on microstructure and properties of interface alloyed magnesium/aluminum friction stir welding
In the experiment, a method of alloying the interface with an alloying element which can form an intermediate transition layer well with the magnesium/aluminum alloy is added to the weld to suppress the generation of the brittle phase and improve the strength of the welded joint.
In this paper, the butt welding of AZ31 magnesium alloy and 5052 aluminum alloy friction stir welding is taken as the research object. The method of adding elements to the interface is adopted, and the following contents are mainly studied:
1) Study the effect of interfacial alloying Cu on tissue changes.
2) Study the effect of adding Cu element on the properties of welded joints under different process conditions.
3) Preliminary discussion on the connection mechanism of Mg/Al dissimilar alloy friction stir welding.
Orthogonal experimental factor level table
Orthogonal experimental design
Main factors affecting the interface alloyed magnesium/aluminum dissimilar alloy welded joints: rotation speed of the stirring head (r/min), offset of the stirring head, feed rate of the stirring head (mm/min), thickness of copper foil (mm, copper The foil thickness is indicated by n. The specific parameters are as shown in the above table.
Orthogonal experimental results visual analysis table
The tensile strength of the welded joint obtained in the above table No. 3 is the lowest, the average tensile strength of the joint is 13.45MPa, and the fracture position is on the aluminum side of the welded joint. The tensile joint strength of the welded joint obtained in the No. 5 experiment is the highest, and the fracture position is on the aluminum side of the welded joint. The average tensile strength of the joint was 92.15 MPa, which corresponds to 38.4% of the strength of the AZ31 magnesium alloy.
According to the factors affecting the interface alloying magnesium/aluminum friction stir welding, the following factors are: feed rate>copper foil thickness>rotation speed>offset, single-factor influence experiment, and the feed rate and copper foil thickness to interface alloyed magnesium / The effect of aluminum friction stir welding, that is, the influence of different feed rates on the interface alloying magnesium/aluminum friction stir welding and the effect of adding different copper foil thickness on the interfacial alloying magnesium/aluminum friction stir welding. The welding parameters are shown in the table.
Selected welding parameters
Corresponding to the macroscopic appearance of the welded joint of the above table
1. Effect of feed rate on interface alloying magnesium/aluminum friction stir welding
Effect of feed rate on interface alloying magnesium/aluminum friction stir welding: No.1 experiment, No.2 experiment, No.3 experiment, tensile strength is 82.7MPa, 85.7MPa, 69.8MPa in sequence; No.4 experiment, No.5 experiment, Experiment No. 6 and tensile strength were 62.4 MPa, 53.8 MPa, and 45.3 MPa in sequence; No. 7 experiment, No. 8 experiment, No. 9 experiment, tensile strength was 79.8 MPa, 73.2 MPa, and 55.7 MPa, respectively. The speed does not have much effect on the post-weld strength (see the summary of the tensile strength curve in the figure below for details).
2. Effect of copper foil thickness on friction stir welding of magnesium/aluminum
Effect of copper foil thickness on friction stir welding of magnesium/aluminum: tensile strength of No.1 experiment, No.4 experiment and No.7 experiment were 82.7MPa, 62.4MPa, 79.8MPa in order; No.2 experiment, No.5 experiment, No.8 experiment pull The tensile strengths were 85.7 MPa, 53.8 MPa, and 73.2 MPa, respectively; the tensile strengths of the No. 3 experiment, No. 6 experiment, and No. 9 experiment were 69.8 MPa, 45.3 MPa, and 55.7 MPa, respectively. Obviously, as the thickness of the added copper foil increases, the tensile strength of the welded joint decreases in turn (see the summary of the tensile strength curve in the figure below for details).
Tensile strength curve summary chart
The following figure is the experimental number 2, 5, 8 is the experimental microhardness curve, the three curves change basically the same, the changes on both sides are gentle, the middle is "M" shape, indicating that there is copper element in the middle of the weld nugget, possibly producing the phase Al2Cu and MgCu2 reduces the hardness. The middle curve of the hardness curve is because the heat generation during the friction stir welding process is very large, which may cause some metals in the weld nugget to melt. During the metal cooling process, metallurgical reaction of magnesium and aluminum may produce brittleness with high hardness. The Mg17Al12 phase is flat on both sides because the metallurgical reaction occurs on both sides, and the intermetallic compound formed is small, and the hardness value is close to that of the base metal. From the hardness curve, it is also possible to reflect a hard metal phase in the middle, thereby reducing the mechanical properties of the weld.
　 Hardness distribution in the center of the weld
Photograph of experimental tissue No. 2 (ν=100mm/min)
The three photographs of Figure (E) and Figure (F) and Figure (J) are photographs of the microstructure of the weld nugget. Figure (E) is a photo of the weld nugget area on the magnesium side, and Figure (F) is the weld pair. Photograph of the weld nugget of the interface, Fig. (J) is a photo of the weld nugget area on the aluminum side.
The microstructure of the weld nugget area is obviously smaller and more uniform than that of the base material area. The microstructure of the weld nugget area is simultaneously subjected to thermal cycling and mechanical agitation, and the crystal grains undergo dynamic recrystallization. At the same time, the mechanical action of the agitating needle is also achieved. Broken, so the grain in this area is even and small.
Photograph of experimental tissue No. 5 (ν=200mm/min)
Photograph of experimental tissue No. 8 (ν=300mm/min)
Experimental XRD pattern of welded joint area of ​​2, 5 and 8 welded joints
The X-ray diffraction patterns obtained in the weld joint areas of the welded joints of Experiments 2, 5, and 8 were analyzed from the diffraction peaks in the figure to the diffraction peaks of Al3Mg2, Mg17Al12, AlCu, Al2Cu, and MgCu2.
4. Effect of interface Cu alloying on weld performance
The welding material is AZ31 magnesium alloy (thickness 2mm), 5052 aluminum alloy (thickness 2mm), Cu foil thickness is 0.01, welding method butt welding, when assembling, 5052 aluminum alloy is placed on the advancing side, AZ31 magnesium alloy is placed on the retreating side And fix the base metal with a clamp.
The shape of the agitating head is cylindrical, the root diameter is 4 mm, the length is 1.85 mm, and the shoulder diameter is 10 mm. The welding process parameters are: welding feed rate 100mm/min, welding rotation speed is 1200r/min, and the mixing head inclination angle is 2°. The stirring needle is at the center of the magnesium-aluminum rolled sheet. The process in which the Cu foil is not added is referred to as M1, and the process in which the pure copper foil is used as the intermediate interlayer is referred to as M2.
From the macroscopic morphology of the joint, the surface of M1 has cracks and peeling. The M2 weld surface has no obvious wiring and obvious defects (as shown in the figure below).
Surface soldering morphology
Connector fracture photo
Tensile test shows that the 5052 aluminum alloy and AZ31 magnesium alloy welded joints are brittle when they are used as tensile specimens, and have almost no tensile strength. Because of the obvious defects at the interface of the two dissimilar alloys, the connection method is simple. Mechanically connected, so the welded joint has a low tensile strength.
Only at the feed rate of 100mm/min, the surface speed of the weld surface of 1000r/min is better; the maximum tensile strength can reach 32.7MPa. From the M1 diagram, the fracture occurred on the aluminum alloy side of the joint center, and the cleavage plane in the fracture photograph was a brittle fracture.
When the copper foil is added between the 5052 aluminum alloy and the AZ31 magnesium alloy, the tensile strength of the joint can reach 86.2 MPa. From the macroscopic morphology of the joint fracture, the fracture occurs on the magnesium alloy side of the joint as shown in Fig. M2.
The reason why the welded joint causes the fracture to occur is that during the friction stir welding process, the metal melts at a higher welding temperature. As the temperature decreases, Mg and Al react in a eutectic reaction to form intermetallic compounds Al3Mg2 and Al12Mg17. The origin of crack initiation.
The copper foil is added to form intermetallic compounds such as AlCu, Al2Cu and MgCu2 by elemental diffusion, which improves the mechanical properties of the joint and improves the mechanical properties of the welded joint.
The microhardness distribution curve of the welded joints of M1 and M2 is shown in the figure above. The hardness distribution of the welded joint of 5052 aluminum alloy and AZ31 magnesium alloy is approximately 'M' shape. The hardness distribution of the welded joint of copper foil added to the butt joint of magnesium-aluminum dissimilar alloy is approximately 'Λ' shape. Both welded joints are intermediate stirring needle area. The hardness is significantly higher than the hardness on both sides.
There is no significant change in the hardness of the material in the region where the shoulder is applied, and the area where the stirring needle acts is significantly higher than the area where the shoulder is applied. This is because the two dissimilar alloys Al and Mg in the weld nugget react to form intermetallic compounds Al3Mg2 and Al12Mg17, so that the hardness of the stirring zone is increased.
5. Effect of interface Cu alloying on microstructure change of weld
Microstructure picture of friction stir welding of M1 sample
(c) is the microstructure of the affected zone of the magnesium side heat engine. Due to the shearing force of the shoulder, the heat generated by the friction causes some grains to recrystallize dynamically, and relatively fine grains appear.
(d) is the microstructure of the weld nugget zone. The weld nugget zone is stirred by the stirring needle, undergoes strong plastic deformation, and dynamic recrystallization occurs, thereby forming a fine grained equiaxed crystal structure. Therefore, the grain core area is significantly thinner than other areas.
It can be seen in (e) that there is a clear tunnel-type defect at the junction of the 5052 aluminum alloy and the AZ31 magnesium alloy, which is about 14.5 μm.
In (f), it can be seen that the magnesium alloy and the aluminum alloy are intertwined, and the boundaries between the magnesium alloy and the aluminum alloy are blurred.
Microstructure picture at the interface of M2 sample
(a) and (b) are the magnesium alloy base material zone and the heat-affected zone, respectively. It can be seen that the grains in the heat-affected zone are significantly deformed and have a small amount of recrystallized grains, which is the same as the corresponding regional structure of M1.
(c) is the microstructure of the heat affected zone, and the grain size is smaller than that of the heat affected zone.
(d) and (f) can be seen that there is no obvious defect in the weld nugget zone. Copper has good thermal conductivity. Copper and aluminum undergo eutectic reaction to form eutectic Al2Cu. Copper and magnesium undergo eutectic reaction to form eutectic MgCu2. The crystals of Al2Cu and MgCu2 have good plasticity. After adding copper foil as the intermediate interlayer on the butt joints from (d) and (f), the defects on the mating surface of magnesium and aluminum are eliminated.
M1 joint XRD diagram
X-ray diffraction analysis showed that the diffraction peaks of Al3Mg2 and Al12Mg17 were detected in the welded joint of M1 as shown in (a). Two intermetallic compounds were formed at the interface of magnesium-aluminum alloy during welding, and the brittle phase Al12Mg17 seriously affected the welded joint. Mechanical properties and hardness.
M2 joint XRD diagram
The Al2Mg2, Al12Mg17, AlCu, Al2Cu, MgCu2 and other phases were detected in the welded joint of M2, and the diffraction peaks thereof are shown in Fig. (b). Comparing the results of XRD analysis under the two welding conditions, it can be seen that under the condition of M1, the strongest peaks of Al3Mg2 and Al12Mg17 are higher than the strongest peak of the corresponding intermetallic compound under M2, indicating that the addition of Cu foil reduces harmful. Formation of intermetallic compounds.
6. Test conclusion
(1) 5052 Aluminium alloy and magnesium alloy AZ31 are butt welded. The welding joint has obvious defects in the welded joint structure under the parameters of rotating speed of 1200r/min feed rate of 100mm/min. A 0.1 mm copper foil was added as an intermediate interlayer on the welded butt joint to obtain a better welded joint structure.
(2) 5052 Aluminium alloy and magnesium alloy AZ31 have obvious defects in the microstructure of butt welded joints. After adding copper foil, good microstructure of welded joints is obtained.
(3) The hardness distribution of 5052 aluminum alloy and magnesium alloy AZ31 welded joint is approximately 'M' shape, and the hardness distribution of the welded joint of copper foil is approximately 'Λ'.
(4) 5052 AZ31 welded joints of aluminum alloy and magnesium alloy have been analyzed for the presence of Al3Mg2 and Al12Mg17 intermetallic compounds, which is one of the main reasons for the low mechanical properties of the joints; the addition of copper foil changes the type and quantity of intermetallic compounds at the joints. The tensile strength of the joint has also been improved to a maximum of 86.2 MPa.
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