Microstructure and corrosion properties of 5A06 aluminum matrix surface composite fabricated by friction stir processing

1 Introduction

Friction stir processing (FSP), which is based on the basic principles of friction stir welding (FSW) [1], has attracted a lot of attention as a new solid-state processing technique for microstructural modification and preparation of composites. During FSP, the material undergoes severe plastic deformation (SPD) and mixing, and thermal exposure takes place in the material resulting in a significant microstructural change. The microstructure of aluminum alloys is improved owing to the intense plastic deformation by FSP [2]. Moreover, FSP has also been successfully used to obtain the metal matrix composites (MMCs) and the surface composite [3, 4]. A main method to improve the performance of the surface composite is to obtain the ultrafine-grained (UFG) structure. Interestingly, FSP is an effective technique to refine grain for most aluminum alloys [5]. Mishra et al. [6] reported that Al-4Mg-1Zr aluminum alloy achieved the UFG microstructures with minimal grains sized at 0.7 μm by FSP. Recently, the studies of Ma et al. [7] suggested that the 7075 aluminum alloy also obtained obvious UFG microstructure by FSP with water cooling. Previously, Su et al. [8] produced an UFG 7075 aluminum alloy of 100∼200 nm by FSP with active cooling using a mixture of water. The minimal grain size was 0.8 μm. A low-temperature superplasticity of 350∼540% was achieved at 200∼350°C. In short, these studies suggest that FSP can cause UFG structure produced in aluminum alloys, similar to equal channel angular pressing (ECAP) [9] and high-pressure torsion (HPT) [10].

In this paper, a novel aluminum matrix surface composite added with Al-based amorphous was produced by FSP, at air cooling. A low rotational speed of the tool is used. In the experiment, the amorphous alloy, Al_84.2Ni₁₀La_2.1, is chosen as an auxiliary material. However, at present, the study on the corrosion properties in the surface of the materials processed or fabricated by FSP is still few. So the purpose of this work was to study the changes of microstructure, mechanical properties, and corrosion properties in aluminum matrix surface composites by FSP.

2 Experimental

The test materials are 5A06 aluminum alloys and Al_84.2Ni₁₀La_2.1 amorphous with a crystalline characteristic. The thin Al-Ni-La amorphous strip is used as an auxiliary material. The nominal chemical composition of 5A06 aluminum alloy in weight percentage is 0.10 Cu, 5.8∼6.8 Mg, 0.5∼0.8 Mn, 0.02∼0.10 Ti, and balance Al. The dimension of the test plate is 120 mm×60 mm. The thickness of the test plate is 15 mm. The thickness of the Al-Ni-La amorphous strip is 65 μm. The schematic of FSP is described in Figure 1.

Figure 1

The schematic of FSP.

The oxide film at the friction stir-processed region of the base metal was removed by the H₂SO₄ solution corrosion and the mechanical polish before FSP. A groove, which is 0.5 mm in width and 1.0 mm in depth, was prepared at the edge of the pin in the advancing side (AS). Then, the amorphous strip was embedded in the test plate before processing. These workpieces were fixed at an operation table. At this time, a stir tool with its columnar-shaped shoulder (18 mm) and screwed pin (4 mm) was made to penetrate into the test plate until the head face of the shoulder reached 0.5 mm under the upper surface. The rotational speed of the stir tool was R=500∼900 r/min, and the travel speed was V=200∼300 mm/min along the center line. In addition, a tool tilt forward angle of 2.5° was used. An air cooling technique was applied. After processing, a surface composite with 6 mm depth was obtained.

A series of specimens were cut from the surface composite along the longitudinal direction by a lining cutting machine. These specimens were made into metallographic samples. Then, these samples were etched using a mixed solution 1.0% HF+1.5% HCl+2.5% HNO₃+95% H₂O. The microstructure of the surface composite was observed, and analysis was done with a scanning electron microscope (SEM). Vickers hardness tests were conducted on the cross-sectional plane using Vickers indenter with a 25-g loading and a load time of 5 s. The chemical composition of the surface composite was measured by electron probe microanalysis (EPMA). Fine microstructure characterizations of the surface composite were performed by a transmission electron microscope (TEM). Some thin slice samples were cut from the surface composite and abraded into the samples of 50 μm thickness and, then, were prepared as thin-film samples by electrolyzing corrosion. The test conditions were 150 kV voltage and 200 mA current.

The corrosion behaviors of the as-received, surface composites added with the amorphous strip and the composites with no amorphous strip samples were evaluated using a dynamic potential scanning technique. The test equipment is the advanced electrochemical system of PARSTAT 2273 type. The surface sample for corrosion testing was cut from the processed surface region. The depth was 5∼6 mm. The samples were mounted in epoxy to expose only one surface for the electrochemical tests. The polarization behavior was studied in a 3.5% NaCl solution at a potential scanning speed of 3 mV/s. A saturated calomel camel electrode (SCE) was used as the reference electrode, and a platinum wire was used as the counter electrode. All electrochemical tests were carried out at room temperature.

3 Results and discussion

3.1 Microstructure

Figure 2A shows a typical microstructure of the surface composite. An obvious “sandwich structure” can be observed. The sandwich structure includes the bright structure and the narrow gray structure (see Figure 2B). The gray structure is similar to the base metal that is away from this region. However, it is obvious that the bright structure is different from the base metal. It may be a new structure formed after FSP. Therefore, the composition analysis of the narrow bright structure is quite necessary to further show how a structure transition is caused during FSP.

Figure 2

SEM micrographs of surface composite. (A) Sandwich structure of surface composite. (B) Fine characteristic of sandwich structure.

The EPMA instrument was used in this experiment. The test results indicate that the elements Al and Mg, similar to the base metal, constitute the gray structure (see Table 1). On the contrary, the bright structure is composed of the elements Al, Mg, Ni, and La. Consequently, the bright structure is a new structure whose main element is derived from the original amorphous strip. Moreover, the element Mg found in the bright structure indicates that the original amorphous strip could experience a large structural transition during FSP. Therefore, it is also a favorable evidence that the amorphous strip might produce a crystallization process. At this time, the element diffusion could occur between the base metal and the original amorphous strip. As a result, an obvious diffusion of the element Mg happened.

Table 1

The EPMA results in the sandwich structure region of the surface composite.

Element	Bright structure		Gray structure
	wt. %	At. %	wt. %	At. %
Al	71.64	83.60	94.61	94.06
Mg	8.84	11.45	5.39	5.94
La	17.80	4.03	–	–
Ni	1.72	0.92	–	–

The typical TEM microstructures and electron diffraction patterns of the surface composite are shown in Figures 3A and B. The surface composite mainly shows a novel irregular structure different from the common-grained structure (see Figure 3A). It can be seen that the structure is composed of a large number of fine bulk structures. Moreover, there are visible boundaries among the most bulk structures. Only a small quantity of the fine bulk structures has no visible boundaries. It is accepted that these fine bulk structures exhibit a structure character the same as the UFG. Their average grain size reaches up to 90∼400 nm. These UFG structures have an obvious change of crystal plane via the electron diffraction analysis. The α-Al phase and α-Al amorphous structure mainly constitutes the UFG structures (see Figure 3C). The zone axis of these UFG structures is B=[110]. It is obvious that the increase in the mechanical properties of the aluminum matrix surface composite is related to the existence of the UFG structures.

Figure 3

The TEM analysis of the surface composite. (A) TEM image of UFG structure. (B) Selected area of electron diffraction pattern. (C) Schematic index diagram of the panel.

3.2 Microhardness

The test location of the hardness is the sandwich structure region of the surface composite (see Figure 2B). The test results are demonstrated in Figure 4. The average hardness of the bright structure is about HV105, slightly higher than that of the gray structure, which is about HV90. So the average hardness of the bright structure is only 17% higher than that of the gray structure. If the bright structure still has a small original Al-Ni-La amorphous, the hardness should be two to three times that of the test results. However, it does not happen in this study. Therefore, by combining the microstructure analysis and EPMA, the original amorphous strip has been transformed into a new structure during the FSP. As far as we know, FSP has a special condition that includes an elevated temperature and a severely plastic deformation. Under these conditions, the Al-Ni-La amorphous strip may generate a crystallization process. Moreover, the new structure may also have an obvious influence on the hardness of the adjacent metal. Interestingly, the average hardness of the gray structure is about 12% higher than that of the base metal (about HV80).

Figure 4

Microhardness distribution of the surface composite and the base metal.

3.3 Analysis of electrochemical tests

In order to evaluate the corrosion resistance of the as-received surface composites added with the amorphous strip and with no amorphous strip, corrosive immersion tests were carried out. Figure 5 shows the potentiodynamic polarization curves of these specimens in a 3.5% NaCl solution at a scanning rate of 3 mV/s. The results indicated that the as-received sample has the highest resistance to corrosion in a salt solution. Obviously, after FSP, the corrosion properties in the surface layer of the aluminum alloy are low. However, the surface composite added with the amorphous strip had lower icorr, corrosion current density, and a higher passivation current (making the intersection points of the tangents of the upper and lower sides of the curves, the corresponding X axial values were the corrosion current density) than the surface composite not added with an amorphous strip. In this test, the rotational speed and travel speed of the two samples are both R=900 r/min and V=40 mm/min. This means that the amorphous added on the surface of aluminum has an important effect in the enhancement of the corrosion properties. The TEM analysis indicated that a large number of UFG structures (or nanocrystalline) could be observed in the aluminum matrix surface composite. As a result, the high density of the grain boundaries was formed, which was beneficial to the formation of the passive oxide film [11]. A thin passive film could restrict the movement of metal ions from the metal surface to the solution, thus, minimizing corrosion.

Figure 5

Polarization curves of the different specimens in a 3.5% NaCl solution. Scanning rate: 3 mV/s.

With regard to the changes of icorr, corrosion current density, and passivation current after FSP, it may be complicated. As is known, the surface layer during FSP should experience a SPD. The changes of the structure occurred in this region, such as phase constituents, dislocations and composition of the oxide film, etc. Further studies on the effect of amorphous on surface composite would be reported in the future.

4 Conclusions

The surface composite shows an obvious sandwich structure. It is considered to be a combination between the base metal and the amorphous strip via the FSP. TEM analysis indicates that the surface composite mainly shows a novel irregular structure different from the common-grained structure, which is composed of a large number of the fine bulk structures. These fine bulk structures show a character of UFG structures, wherein the grain size reaches up to 90∼400 nm. Moreover, the α-Al phase and the α-Al amorphous structure constitute the UFG structures. The hardness of the surface composite has shown an increase to a certain extent. The results indicated that the as-received sample has the highest resistance to corrosion in a salt solution. The surface composite added with the amorphous strip had lower icorr, corrosion current density, and higher passivation current than the surface composite not added with an amorphous strip. The amorphous added on the surface of aluminum could have an important effect on the enhancement of the corrosion properties.