Understanding the Effect of Rotation Speed on Aluminum/Steel Friction Stir Welding Joint Properties

Summary

The aluminum-steel composite structure is an effective method to achieve lightweight vehicles.

To facilitate the industrial application of aluminum-steel friction stir welding technology, it is necessary to conduct a friction stir welding process test of dissimilar materials made of aluminum and steel.

In this study, Q235 cold rolled steel with a thickness of 2.8 mm and 5A06 aluminum alloy with a thickness of 2.9 mm were butt welded using filler wire friction stir welding. The joint’s macro morphology, microstructure, micro hardness, and fracture morphology were analyzed.

The results show that:

The concave depth and IMC layer thickness of the C-shaped interface increase with an increase in rotation speed.

A large number of Al3Ni particles are dispersed within the weld.

At a rotation speed of 420 r/min, the interface IMC layer is a FeAl phase with a thickness of 1.3 μm.

The joint primarily fractures in the nugget zone.

The fracture mode is ductile, 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, exhibiting a step characteristic.

Preface

Energy conservation, emission reduction, and green development have become the guiding principles for the development of all nations.

In terms of transportation, there is a growing emphasis on developing and applying lightweight body structures for vehicles.

Steel offers excellent economy, high strength, and toughness, while aluminum alloy boasts high specific strength and stiffness, with only 1/3 of the weight of steel.

Thus, replacing steel with aluminum alloy in some structures can significantly reduce the weight of vehicles and achieve greater fuel efficiency.

However, due to significant differences in their physical and chemical properties, effectively connecting aluminum alloy and steel has become a new technical challenge.

When using traditional fusion welding methods to weld aluminum and steel, the joint is prone to have an excessively thick intermetallic compound layer (IMC), which can compromise the joint’s performance.

Friction stir welding (FSW) is a solid-state welding technology that offers several advantages, including high efficiency, low heat input, and minimal deformation.

This process relies on a rigid mixing head that moves along the welding direction at high speed. The base metal reaches the plastic state through the heat generated by the friction between the mixing head and the base metal.

The strong stirring effect of the mixing needle causes the metal in the plastic state to undergo dynamic recrystallization, resulting in a joint connection.

Currently, many scholars at home and abroad have conducted experiments on friction stir welding of aluminum and steel. For example, M. DEHGHANI obtained a friction stir welded butt joint of 3003 aluminum alloy and low carbon steel and analyzed the effect of heat input on joint strength and the interface IMC layer.

Tanaka et al. conducted a study on the strength trends of joints at different rotating speeds, while keeping other welding parameters fixed. They discovered that the joint between aluminum and steel can achieve the highest strength with high rotating speed and low welding speed.

Wang Xijing and colleagues identified three different connection methods for friction stir welding joint positions: mechanical connection, metallurgical bonding, and aluminum diffusion to steel.

However, traditional friction stir welding techniques are prone to producing defects such as holes and interface cracks when welding dissimilar metal materials that are tough, hard, and brittle.

To address this issue, Xu Huibin and others invented a new type of Al-based welding wire suitable for difficult-to-weld materials. This wire fills holes and improves the composition of the intermetallic compound (IMC) layer.

Gao Pengyu found that using Al-5Si (wt.%) welding wire resolves the interface crack defect in the joint and reduces the thickness of the IMC layer at the interface.

Li Moyang further explains the necessity of using Al Si Cu Ni welding wire to fill joints.

Revised version:

It was observed that cold rolled Q235 steel and 5A06 aluminum alloy, which have poor toughness and high hardness, resulted in obvious cracks and holes at the joint when welded by friction stir welding without adding the welding wire.

Furthermore, there was no noticeable metallurgical bonding at the interface, and the resulting tensile strength was only 23.5 MPa. However, adding the welding wire effectively filled the weld seam and improved the composition of the IMC layer, which significantly improved the joint’s mechanical properties.

Therefore, it is crucial to optimize the process parameters applicable to the friction stir welding technology of aluminum and steel filler wire to improve the joint’s mechanical properties.

In this study, friction stir welding tests with filler wire were conducted for 5A06 aluminum alloy (2.9 mm thickness) and Q235 cold rolled steel (2.8 mm thickness) by varying the rotation speed of the stirring head.

The microstructures and mechanical properties of the joints were compared under different rotation speeds, and the optimal process parameters for friction stir welding of 5A06 aluminum alloy and Q235 cold rolled steel were determined. These findings provide theoretical guidance for the industrial production of aluminum steel welding.

1. Test materials and methods

The test materials used are Q235 cold rolled steel and 5A06 aluminum alloy, both with dimensions of 100 mm × 50 mm × 2.8 mm and 100 mm × 50 mm × 2.9 mm respectively. Their main chemical compositions are presented in Table 1 and Table 2.

The welding wire employed is a novel type of Al-based welding wire, designed, smelted, and processed by the laboratory. The primary chemical composition is detailed in Table 3.

The mixing head’s shaft shoulder diameter is 15 mm, and the mixing needle is a conical boss with a 4 mm diameter at the end, 5 mm at the bottom, and a length of 2.6 mm.

Figure 1 shows the schematic diagram of wire-filled friction stir welding, with Q235 steel placed on the forward side and 5A06 aluminum alloy 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 from the surface of the base metal and welding wire. Then, use a cotton swab dipped in absolute ethanol to remove impurities such as oil stains from the surface.

After completing the butt joint, fix the process parameters, including 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 dissimilar metals (aluminum and steel) was investigated under different rotation speeds (210 r/min, 420 r/min, 660 r/min), as illustrated in Fig. 2.

To test the mechanical properties of the joint and observe the micro morphology, the specimen was cut to 110 mm using a wire EDM machine.

Perform at least three tensile tests on the MTS E43.104 universal mechanical property testing machine for the joints obtained under different rotating speeds. Set the tensile rate to 1.0 mm/min.

After fracture, use the Zeiss Sigma/HD scanning electron microscope (SEM) to photograph the interface microstructure of the joint. Additionally, use EDS to analyze the composition and element distribution of the intermetallic compound layer at the interface.

Analyze the phase of the fracture using the PANalytical Empyrean Series 2 X-ray diffractometer (XRD).

After replacing the bending test fixture, conduct a three-point bending test with a loading rate of 1 mm/min.

To characterize the microhardness of the cross section of the welded joint, use the HVS-1000Z microhardness tester. The test position should be 1.5 mm from the top of the joint.

Make dots from the interface as the center to the base metal on both sides. The distance between each point should be 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 novel process in which, under proper process parameters, the addition of filler wire can not only fill hole defects but also enhance the microstructure of the nugget zone.

Figure 3 displays the surface morphology of the joint at varying rotation speeds.

As the rotation speed increases, more volume flash accumulates on the aluminum side of the weld, causing the joint interface to step up, enlarging the size of the Hook defect, and causing the steel particles in the weld to gradually distribute to the bottom.

At a rotation speed of 210 r/min, the particles are mainly concentrated in the near interface area and the bottom of the nugget area, resulting in a small and smooth Hook.

At a rotation speed of 420 r/min, the particles are primarily distributed in the near interface and nugget zones, causing the Hook size to increase and become sharper.

When the rotation speed is 660 r/min, the particles mainly exist in the upper and middle of the near interface area, with steel particles concentrated primarily at the bottom of the nugget area, causing a larger Hook size.

The morphology of the joint’s cross-section at different rotation speeds is depicted in Figure 4.

To elucidate the impact of the rotation speed on the bending degree of the C-shaped structure, the concave depth is defined, as illustrated in Figure 4.

As the rotation speed increases, the concave size of the joint enlarges from 0.36 mm to 1.06 mm.

It is apparent from the analysis that the heat input and joint temperature increase as the rotation speed rises. Additionally, the cutting effect of the mixing head on the interface becomes more pronounced, leading to an increase in the degree of interface bending. Strengthening the degree of mechanical engagement is favorable to the mechanical properties of the joint.

The metal in the weld nugget region is more fully reacted and has a higher degree of plasticization.

Fragmented particles are dispersed throughout the weld nugget area, which has a certain strengthening effect on the weld by dispersion.

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 contribute to mechanical occlusion.

Figure 5 displays the microstructure of the middle interface of a friction stir welded butt joint between 5A06 aluminum alloy and Q235 cold rolled steel at varying rotating speeds.

The EDS analysis outcomes of each point in Figure 5 are detailed in Table 4.

Figure 6 illustrates the line scanning results of the joint at different rotating speeds.

As observed in Figure 5a, when the rotational speed is 210 r/min, steel chips remain unpeeled at the interface, and a compound layer develops at the interface.

Based on Figure 6a, the thickness of the compound layer is approximately 1 μm, and a small amount of Ni is dissolved within it.

Table 4 identifies the compound layer as the FeAl3 phase.

In Figure 5b, when the rotation speed is 420 r/min, the interface transition layer is firmly bonded without noticeable gaps.

Combining with Figure 6b, it is apparent that the three elements Al, Fe, and Ni on the transition layer have diffused noticeably, and the thickness of the diffusion layer is about 1.3 μm.

According to Table 4, the atomic ratio of Al element and Fe element in the compound layer is equal, and the interface layer can be recognized as the FeAl phase, with Al3Ni particles adhering to the interface.

The weld contains numerous micron-sized particles dispersed throughout it.

Based on prior analysis, the micron-sized particles consist 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 depicted in Figure 5c, there is a crack defect about 1.5 m wide at the interface when the rotation speed is 660 r/min.

The compound layer at the interface is distributed intermittently, and the weld zone still contains a large number of fine particles.

Table 4 shows that the compound layer product is Fe2Al5 phase, and according to Figure 6c, its thickness is about 3.7 μm.

The analysis indicates that the formation of the intermetallic compound layer is necessary to establish a connection between aluminum and steel. However, the thickness and composition of the compound layer significantly affect the mechanical properties of the joint.

If the bonding layer is excessively thick, the compound layer may crack under the influence of residual stress after welding, which could deteriorate the mechanical properties of the joint.

The compound layer can be classified into two types: Fe-rich FeAl phase and Fe3Al phase, and Al-rich FeAl3 and Fe2Al5 phase.

From a toughness standpoint, the Fe-rich phase exhibits superior toughness.

During friction stir welding, the rotational speed significantly impacts the joint’s heat input.

If the rotational speed is too low, the heat input is insufficient.

Ideally, the intermetallic compound layer formed by the joint should be thin.

However, it must remain within a specific range. When the rotational speed is too high, the heat input of the joint increases, leading to a thicker intermetallic compound layer at the interface. This causes cracks to occur, significantly decreasing the joint’s mechanical properties.

2.3 Mechanical property analysis of joint

The results of the tensile strength and bending angle of the joints obtained at different rotation speeds are presented in Fig. 7. It can be observed that the tensile strength and bending angle initially increase, then decrease.

The average tensile strength for the joint with a rotation speed of 420 r/min is significantly higher (240.3 MPa) than that of the joint with a rotation speed of 210 r/min (210.1 MPa) and the joint with a rotation speed of 660 r/min (161.4 MPa).

The joint exhibits the best bending performance at a rotation speed of 420 r/min, where the positive bending angle (19.3°) and the back bending angle (13.4°) are the largest.

Based on the analysis in Section 2.1 and Section 2.2, it can be concluded that a suitable rotation speed can provide sufficient heat input, which enhances the plastic fluidity of the joint and thus improves its quality. Additionally, the heat input is a crucial factor that determines the type and thickness of interfacial compounds, which in turn affect the mechanical properties of the joints.

Notably, the joint interface at a rotation speed of 420 r/min exhibits a layer of Fe-rich FeAl phase with a thickness of 1.3 μm, which effectively improves the mechanical properties of the joint.

Fig.7   Tensile strength and bending angle of joint at different rotational speeds

Figure 8 shows the microhardness distribution of joints obtained at different rotation speeds.

There is a significant difference in hardness between the two sides of the friction stir welded joint of aluminum and steel.

The microhardness of the base metal on the steel side is noticeably higher than that of the weld zone. The hardness curve exhibits a “step” characteristic.

On the aluminum alloy side, the microhardness of the thermo mechanical affected zone (TMAZ) is approximately 90 HV.

At the interface of the joint, the hardness reaches a peak value and gradually decreases to 150 HV on the steel side.

The strong stirring effect of the stirring head causes dynamic recrystallization in the TMAZ on the steel side.

After cooling, the grains become smaller, and the microhardness is greater than that of the steel base metal.

However, there are a significant 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 displays the SEM image and XRD test results of the fracture morphology obtained at a rotating speed of 420 r/min.

As depicted in Fig. 9a and Fig. 9b, the fracture exhibits typical morphology, mainly occurring in the nugget area, with a small portion along the interface. The fracture surface showcases evident aggregation dimples and tear edge characteristics. The dimples’ size is small, and the pit-like dimples contain broken welding wire particles and steel chips, indicating obvious ductile fracture mode.

Furthermore, as shown in Fig. 9c, besides a significant amount of Al matrix, the joint fracture surface also comprises (Fe, Ni) solid solutions, AlFe3Si0.5 phase, and AlNi phase.

This observation suggests that numerous fine particles are combined with the aluminum alloy embedded in the weld, indicating good metallurgical bonding. Such a combination plays a crucial role in particle strengthening, further improving the joint’s comprehensive mechanical properties.

Fig.9   Fracture and XRD analysis at 420 r/min

3. Conclusion

(1) As the rotation speed increases, the brightness of the flash on the aluminum side of the weld also increases, while the level of oxidation on the steel side also rises. Additionally, 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 ultimately to 1.06 mm at 660 r/min.

(2) The thickness of the IMC layer at the interface between the aluminum and steel increases with the rotation speed. Specifically, when the rotation speed is 210 r/min, the thickness of the IMC layer at the interface is 1.0 μm, and it is composed of the FeAl3 phase.

At 420 r/min, the thickness of the IMC layer at the interface is 1.3 μm, with the FeAl phase being dominated by the elements of Al, Fe, and Ni. However, when the rotation speed reaches 660 r/min, the interface produces crack defects.

Revised version:

The thickness of the IMC layer is 3.7 μm, and it mainly consists of the Fe2Al5 phase. This phase can significantly reduce the mechanical properties of the joint.

3）The average tensile strength and bending angle of the joint increase initially and then decrease with an increase in rotation speed.

The optimal mechanical properties of the joint are achieved at a rotation speed of 420 r/min, with an average tensile strength of 240.3 MPa, a positive bending angle of 19.3 °, and a back bending angle of 13.4 °.

4）The fracture mainly occurs in the nugget zone, with numerous dimples on the fracture surface.

The fracture surface is mainly composed of an Al matrix and also contains (Fe, Ni) solid solution, AlNi phase, and AlFe3Si0.5 phase.

The fracture type is ductile fracture.