Aluminum steel composite structure is one of the effective ways to achieve lightweight vehicle.
In order to guide the industrial application of aluminum steel friction stir welding technology, it is necessary to carry out the friction stir welding process test of aluminum steel dissimilar materials.
Q235 cold rolled steel with a thickness of 2.8 mm and 5A06 aluminum alloy dissimilar metal with a thickness of 2.9 mm were butt welded with filler wire friction stir welding, and the macro morphology, microstructure, microstructure, micro hardness and fracture morphology of the joint were analyzed.
The results show that:
The concave depth and IMC layer thickness of the C-shaped interface increase with the increase of rotation speed;
A large number of Al3Ni particles are dispersed in the weld;
When the rotation speed is 420 r/min, the interface IMC layer is a FeAl phase with a thickness of 1.3 μm.
The joint is mainly fractured in the nugget zone.
The fracture mode is ductile fracture, with an average tensile strength of 240.3 MPa, a positive bending angle of 19.3 °and a back bending angle of 13.4 °;
The microhardness distribution of the joint is asymmetric, showing a step characteristic.
Energy conservation, emission reduction and green development have become the development concept of all countries.
In terms of transport vehicles, the lightweight of body structure will be vigorously developed and applied.
Steel has the advantages of good economy, high strength and high toughness, and aluminum alloy has the advantages of high specific strength and stiffness, and the quality is only 1/3 of steel.
Therefore, the use of aluminum alloy instead of steel in some structures can effectively achieve lightweight vehicles.
Aluminum alloy and steel have great differences in physical and chemical properties, so it becomes a new technical problem to effectively connect them.
When the traditional fusion welding method is used to weld aluminum and steel, the joint is prone to have an excessively thick intermetallic compound layer (IMC), which deteriorates the joint performance.
Friction stir welding (FSW), as a solid state welding technology, has the advantages of high efficiency, low heat input and small deformation.
This technology relies on the rigid mixing head moving along the welding direction under high-speed rotation, and the base metal reaches the plastic state through the heat generated by the friction between the mixing head and the base metal.
Due to the strong stirring effect of the mixing needle, the metal in the plastic state undergoes dynamic recrystallization, thus realizing the joint connection.
At present, scholars at home and abroad have carried out a lot of experiments on friction stir welding of aluminum and steel.
M. DEHGHANI obtained the friction stir welded butt joint of 3003 aluminum alloy and low carbon steel, and analyzed the effect of heat input on the joint strength and the interface IMC layer.
Tanaka et al. studied the strength trend of joints at different rotating speeds under the condition of fixing other welding parameters, and found that the joint of aluminum and steel can obtain the highest strength under the condition of high rotating speed and low welding speed.
Wang Xijing and others found that the connection methods of different friction stir welding joint positions are divided into mechanical connection, metallurgical bonding and aluminum diffusion to steel.
However, the above-mentioned traditional friction stir welding technology is prone to appear holes, interface cracks and other defects when welding dissimilar metal materials with poor toughness and hard and brittle.
To solve this problem, Xu Huibin and others invented a new type of Al-based welding wire suitable for difficult-to-weld materials, which plays the role of filling holes and improving the composition of the IMC layer.
Gao Pengyu found that the filling of Al-5Si (wt.%) welding wire solved the interface crack defect in the joint and reduced the thickness of the IMC layer at the interface.
Li Moyang elaborated on the necessity of filling Al Si Cu Ni welding wire.
He found that when the welding wire was not added, the cold rolled Q235 steel and 5A06 aluminum alloy with poor toughness and high hardness were welded by friction stir welding, and obvious cracks and holes appeared in the joint.
There was no obvious metallurgical bonding at the interface, and the tensile strength was only 23.5 MPa;
After welding wire is added, the weld seam is effectively filled and the IMC layer composition is improved, and the mechanical properties of the joint are significantly improved.
Therefore, it is of great significance to further optimize the process parameters applicable to the friction stir welding technology of aluminum and steel filler wire for improving the mechanical properties of the joint.
In this paper, taking the rotation speed of the stirring head as a variable, the friction stir welding test with filler wire was carried out for 5A06 aluminum alloy with a thickness of 2.9 mm and Q235 cold rolled steel with a thickness of 2.8 mm.
The microstructures and mechanical properties of the joints under different rotation speeds were compared, and the optimal process parameters for friction stir welding of 5A06 aluminum alloy and Q235 cold rolled steel were obtained, providing theoretical guidance for the industrial production of aluminum steel welding.
1. Test materials and methods
The test materials are Q235 cold rolled steel and 5A06 aluminum alloy, with dimensions of 100 mm×50 mm×2.8 mm and 100 mm×50 mm×2.9 mm respectively.
The main chemical compositions are shown in Table 1 and Table 2.
The welding wire is a new type of Al based welding wire designed, smelted and processed by the laboratory itself.
The main chemical composition is shown in Table 3.
The diameter of the shaft shoulder of the mixing head is 15 mm, the mixing needle is conical boss, the diameter of the end is 4 mm, the diameter of the bottom is 5 mm, and the needle length is 2.6 mm.
The schematic diagram of wire filled friction stir welding is shown in Fig. 1, where Q235 steel is placed on the forward side and 5A06 aluminum alloy is placed on the backward side.
Table 1 Chemical composition of 5A06 aluminum alloy （wt.%）
Table 2 Chemical composition of Q235 cold-rolled steel （wt.%）
Table 3 Chemical composition of filler wire （wt.%）
Fig.1 Schematic diagram of wire-filler friction stir welding
Before welding, use sandpaper to remove the oxide film on the surface of base metal and welding wire, and use cotton swab dipped in absolute ethanol to remove impurities such as oil stain on the surface.
After the butt joint is completed, fix the process parameters: welding speed (44 mm/min), mixing head inclination (3 °), offset (0.5 mm), and press in (0.3 mm).
The effect of rotation speed on the microstructure and mechanical properties of friction stir welded joints of aluminum and steel dissimilar metals was investigated under different rotation speeds (210 r/min, 420 r/min, 660 r/min).
As shown in Fig. 2, the specimen was cut to 110 mm with a wire EDM machine to test the mechanical properties of the joint and observe the micro morphology.
Carry out at least three tensile tests on the MTS E43.104 universal mechanical property testing machine for the joints obtained under different rotating speeds, and set the tensile rate as 1.0 mm/min.
After fracture, Zeiss Sigma/HD scanning electron microscope (SEM) was used to photograph the interface microstructure of the joint, and EDS was used to analyze the composition and element distribution of the intermetallic compound layer at the interface.
The phase of the fracture was analyzed by PANalytical Empyrean Series 2 X-ray diffractometer (XRD).
After replacing the bending test fixture, conduct three-point bending test with a loading rate of 1 mm/min.
HVS-1000Z microhardness tester is used to characterize the microhardness of the cross section of the welded joint. The test position is 1.5 mm from the top of the joint.
Dots are made from the interface as the center to the base metal on both sides. The distance between each point is 0.25 mm.
Fig.2 Schematic diagram of tensile, bending and metallographic specimens
2. Test results and analysis
2.1 Weld formation
Friction stir welding with filler wire is a new process of friction stir welding. Under proper process parameters, the addition of filler wire can not only fill the hole defects, but also improve the microstructure of the nugget zone.
The surface morphology of the joint at different rotation speeds is shown in Fig. 3.
With the increase of rotation speed, more volume flash is accumulated on the aluminum side of the weld, the joint interface is stepped, the size of the Hook defect increases, and the steel particles in the weld are gradually distributed to the bottom.
When the rotation speed is 210 r/min, the particles are mainly distributed in the near interface area and the bottom of the nugget area, and the Hook hook is small and smooth;
When the rotation speed is 420 r/min, the particles are mainly distributed in the near interface zone and nugget zone, and the hook size becomes larger and sharper;
When the rotation speed is 660 r/min, the particles are mainly distributed in the upper and middle of the near interface area, the steel particles are mainly concentrated in the bottom of the nugget area, and the hook size is larger.
The cross-section morphology of the joint at different rotation speeds is shown in Fig. 4.
In order to clarify the influence of rotation speed on the bending degree of C-shaped structure, the concave depth is defined as shown in Fig. 4.
With the increase of rotation speed, the concave size of the joint increases from 0.36 mm to 1.06 mm.
It can be seen from the analysis that, with the increase of rotation speed, the heat input increases, the joint temperature increases, and at the same time, the cutting effect of the mixing head on the interface is more obvious, the degree of interface bending increases, and strengthening the degree of mechanical engagement is beneficial to the mechanical properties of the joint.
The metal in the weld nugget area has a higher degree of plasticization and a more complete reaction.
The fragmented particles are distributed in the weld nugget area as dispersion, which has a certain dispersion strengthening effect on the weld.
Fig.3 Surface morphology of the joint at different rotational speeds
Fig.4 Cross-sectional shape of the joint at different rotational speeds
2.2 Microstructure of joint
In conclusion, the C-shaped interface can play a role in mechanical occlusion.
Fig. 5 shows the microstructure of the middle interface of friction stir welded butt joint between 5A06 aluminum alloy and Q235 cold rolled steel at different rotating speeds.
The EDS analysis results of each point in Fig. 5 are shown in Table 4.
Fig. 6 shows the line scanning results of the joint under different rotating speeds.
As shown in Fig. 5a, when the rotation speed is 210 r/min, there are steel chips not peeled at the interface, and a compound layer is generated at the interface.
According to Fig. 6a, its thickness is about 1 μm, and a small amount of Ni is dissolved in the compound layer.
It can be seen from Table 4 that the compound layer is identified as FeAl3 phase.
As shown in Fig. 5b, when the rotation speed is 420 r/min, the interface transition layer is tightly bound without obvious gaps.
Combining with Fig. 6b, it can be seen that the three elements Al, Fe and Ni on the transition layer have obviously diffused, and the thickness of the diffusion layer is about 1.3 μm.
It can be seen from Table 4 that the atomic ratio of Al element and Fe element in the compound layer is equivalent, and the product of the interface layer can be identified as FeAl phase, with Al3Ni particles attached to the interface.
There are many micron sized particles dispersed in the weld.
Based on the previous analysis, the micron sized particles are composed of the original structure of the broken welding wire and steel chips.
Fig.5 Microstructure of joints at different rotational speeds
Table 4 Chemical compositions analyzed by EDS energy spectrum for each point in Figure 5 （at.%）
Fig.6 Line scan results at different rotation speeds
As shown in Fig. 5c, when the rotation speed is 660 r/min, there is a crack defect about 1.5 m wide at the interface.
The compound layer at the interface is distributed intermittently, and the weld zone still contains a large number of fine particles.
It can be seen from Table 4 that the compound layer product is Fe2Al5 phase.
According to Fig. 6c, its thickness is about 3.7 μm.
The analysis shows that the formation of intermetallic compound layer is a necessary condition to realize the connection between aluminum and steel, but the thickness and composition of the compound layer have a great influence on the mechanical properties of the joint.
If the bonding layer is too thick, it is easy to crack in the compound layer under the effect of residual stress after welding, which will deteriorate the mechanical properties of the joint;
The compound layer is mainly divided into two types, one is Fe rich FeAl phase and Fe3Al phase, the other is Al rich FeAl3 and Fe2Al5 phase.
From the perspective of toughness, the Fe rich phase has better toughness.
In the process of friction stir welding, the rotation speed has a great influence on the heat input of the joint.
If the rotation speed is too small, the heat input is insufficient.
The thinner the compound layer formed by the joint, while the thinner the intermetallic compound layer is, the better.
It needs to be kept within a certain range. However, if the rotation speed is too large, the heat input of the joint increases, and the thicker the intermetallic compound layer generated at the interface, the cracks are easy to occur, which greatly reduces the mechanical properties of the joint.
2.3 Mechanical property analysis of joint
The tensile strength and bending angle of the joint obtained at different rotation speeds are shown in Fig. 7.
The tensile strength and bending angle increase first and then decrease.
The average tensile strength (240.3 MPa) of the joint with a rotation speed of 420 r/min is significantly higher than that of the joint with a rotation speed of 210 r/min (210.1 MPa) and that of the joint with a rotation speed of 660 r/min (161.4 MPa).
When the rotation speed is 420 r/min, the positive bending angle (19.3 °) and the back bending angle (13.4 °) of the joint are the largest, that is, the bending performance is the best.
According to the analysis results in Section 2.1 and Section 2.2, when the rotation speed can provide sufficient heat input, the plastic fluidity of the joint is better, which effectively improves the joint quality;
Moreover, the heat input directly determines the type and thickness of interfacial compounds, thus affecting the mechanical properties of joints.
When the rotation speed is 420 r/min, there is a layer of Fe rich FeAl phase with a thickness of 1.3 μm at the joint interface, which effectively improves the mechanical properties of the joint.
Fig.7 Tensile strength and bending angle of joint at different rotational speeds
The microhardness distribution of joints obtained at different rotation speeds is shown in Fig. 8.
The hardness difference between the two sides of the friction stir welded joint of aluminum and steel is great.
The microhardness of the base metal on the steel side is obviously greater than that of the weld zone. The hardness curve is characterized by a “step”.
The microhardness of the TMAZ on the aluminum alloy side is about 90 HV.
The hardness of the joint at the interface reaches a peak value and gradually decreases to 150 HV on the steel side.
Dynamic recrystallization occurs in the thermo mechanical affected zone at the steel side under the strong stirring effect of the stirring head.
After cooling, the grains become smaller and the microhardness is greater than that of the steel base metal.
However, there are a large number of stripped steel chips and broken welding wire structures in the weld, which enhance the microhardness of the nugget zone.
Fig.8 Microhardness distribution of joints at different rotational speeds
2.4 Fracture analysis
Fig. 9 shows the SEM image and XRD test results of the fracture morphology obtained at the rotating speed of 420 r/min.
As shown in Fig. 9a and Fig. 9b, the fracture is a typical fracture morphology.
The fracture location is mainly in the nugget area, and a small part is along the interface.
The fracture surface has obvious aggregation dimple and tear edge characteristics, the dimple size is small, and the dimple like pit contains broken welding wire particles and steel chips, and the fracture mode is obvious ductile fracture mode.
It can be seen from Fig. 9c that in addition to a large amount of Al matrix, the fracture surface of the joint also contains (Fe, Ni) solid solution, AlFe3Si0.5 phase and AlNi phase, indicating that a large number of fine particles are combined with the aluminum alloy embedded in the weld, which has good metallurgical bonding, plays a role in particle strengthening, and further improves the comprehensive mechanical properties of the joint.
Fig.9 Fracture and XRD analysis at 420 r/min
(1) With the increase of the rotation speed, the flash on the aluminum side of the weld increases, and the oxidation degree on the steel side increases.
The concave depth of the C-shaped interface increases from 0.36 mm at 210 r/min to 0.78 mm at 420 r/min, and finally to 1.06 mm at 660 r/min.
(2) The IMC layer thickness of the interface increases with the increase of the rotation speed.
When the rotation speed is 210 r/min, the thickness of IMC layer at the interface between aluminum and steel is 1.0 μm, which is FeAl3 phase;
When the rotation speed is 420 r/min, the thickness of IMC layer at the interface between aluminum and steel is 1.3 μm, and the FeAl phase is dominated by Al, Fe, Ni elements;
When the rotation speed is 660 r/min, the interface produces crack defects.
The IMC layer thickness is 3.7 μm, and its composition is mainly Fe2Al5 phase, which deteriorates the mechanical properties of the joint.
3) With the increase of rotation speed, the average tensile strength and bending angle of the joint increase first and then decrease.
The mechanical properties of the joint are the best under the rotating speed of 420 r/min, the average tensile strength is 240.3 MPa, the positive bending angle is 19.3 °, and the back bending angle is 13.4 °.
(4) The fracture is mainly located in the nugget zone, with a large number of dimples on the fracture surface.
The fracture surface is mainly composed of Al matrix, and also contains (Fe, Ni) solid solution, AlNi phase and AlFe3Si0.5 phase.
The fracture form is ductile fracture.