Microstructure and mechanical properties of a recycled aluminum alloy fabricated by consolidation of small pieces

ABSTRACT

The microstructure and mechanical properties of recycled AA6063 aluminum alloy rods fabricated by hot extrusion of forging compacts formed by upsetting compacts of small AA6063 aluminum alloy pieces with different Fe contents before and after T6 heat treatment were studied. In the as-extruded state and with the higher extrusion ratio of 25:1, the microstructure consists of fully recrystallized grains, but the high iron content regions exhibit clearly finer equiaxed grains due to the stronger grain boundary pinning and nucleation effects of a higher number density of Fe-rich phase particles. Increasing the extrusion ratio increases both the strength and ductility of the as-extruded material. The reason is that increasing extrusion ratio enhances the bonding strength of the prior boundaries between the strips formed by deformation of the small pieces. In the T6 heat-treated state, increasing the extrusion ratio from 9:1 to 25:1 clearly increases the fraction of fine and equiaxed grains. The enhancement of the bonding strength and the refinement of the microstructure with the increase of extrusion ratio cause the yield strength and ultimate tensile strength to increase from 174 and 221 MPa to 213 and 237 MPa, respectively, and the elongation to fracture to decrease from 22.1 to 15.4%. Solid-state recycled samples all exhibit a ductile fracture behavior despite formation of microcracks at the prior boundaries.

KEYWORDS:

• Aluminum alloy
• solid state recycling
• hot extrusion
• microstructure
• mechanical properties

Introduction

Aluminum alloys are widely used in aerospace, construction, transportation and other fields because of their advantages of low density, corrosion resistance and excellent forming and processing properties. The amount of their consumption has become second largest among metallic materials after steels, and the proportion of use has gradually increased every year [1]. However, China’s bauxite resources are scarce, and the proportion of alumina imported each year is as high as 30% ∼50% [2], and the total output of raw aluminum is seriously insufficient. Therefore, from economic consideration and energy saving and environmental protection considerations, recycling of waste aluminum alloys is very important [3]. In 2019, the recovery of recycled aluminum raw materials in China was 6.07 million tons, accounting for 83.7% of the supply of recycled aluminum raw materials [4], and China has become a major recycling country of waste aluminum alloys.

The waste aluminum alloys can be recycled using two methods, namely the traditional remelting method and solid-state recycling (SSR) method [5]. However, the traditional remelting method not only destroys the original microstructure of the material, but also consumes a substantial amount of energy. At the same time, the melting aggravates the oxidation of aluminum alloy and reduces the recovery rate and produces aluminum ash with environmental pollution risk [6,7]. SSR is to destroy the surface oxide film of waste aluminum and establish a new interface metallurgical bonding effect by means of plastic deformation such as hot extrusion and high pressure torsion [8,9]. Stern [10] invented the SSR process by hot extrusion of compacts of aluminum alloy chips in 1945, and SSR has the advantages of low energy consumption, high recovery rate and no impact on the environment compared to the traditional remelting method. SSR has become the key direction of waste aluminum alloy recovery research [11].

Guley et al. [12] extruded pin-shaped AA1050 and AA6060 aluminum alloy chips into aluminum alloy rods, and found that the mechanical properties of the as-extruded rods were between those of AA1050 and AA6060 alloys. Schikorra et al. [13] mixed AA6060, AA6082 and AA7075 aluminum alloy chips, and then compacted and hot extruded. The mechanical properties of the extruded profiles are 5–10% lower than those of profiles produced by hot extrusion of the as-cast billets. Tekkaya et al. [14] studied the effect of different chip shapes of AA6060 aluminum alloy on the microstructure and mechanical properties of aluminum alloy prepared by hot extrusion. The results show that the chips are tightly bonded without obvious defects. If the pressure, strain and temperature exceed the critical value, different types of chips have no effect on the microstructure and mechanical properties of the prepared aluminum alloy. Guley et al. [15] mixed pin-shaped 1050 aluminum alloy with a diameter of 3∼5 mm and a length of 10∼40 mm with 6060 aluminum alloy turning chips, then cold compressed into billets, and finally hot extruded into aluminum alloy profiles. They found that not only aluminum chips can be directly recovered by hot extrusion, but also aluminum scrap can be directly recovered by hot extrusion. Guley et al. [16] found that through the flat die, the extrusion ratio of 10 cannot guarantee the effective consolidation of the chips, and at a higher extrusion ratio, it is found that the extruded profile has excellent strength and ductility. Fogagnolo et al. [17] found that when the extrusion ratio is low (ER = 6.25), hot pressing and hot extrusion are needed to obtain a material with better consolidation effect of aluminum alloy chips. Chen et al. [18] found that when the extrusion ratio increases within a certain range, the performance of the consolidated sample will be significantly improved due to grain refinement. However, when the extrusion ratio exceeds a certain value, due to the large extrusion ratio, the temperature in the material increases, resulting in the coarsening of recrystallized grains, and the performance of the consolidated sample decreases.

So far, the research on SSR of waste aluminum alloys mainly focuses on thermomechanical consolidation of chips or fragments with thickness smaller than 0.5 mm. This work is to study the effects of extrusion ratio on the microstructure and mechanical properties of rods fabricated by hot extrusion of forging compacts formed by upsetting compacts of small AA6063 aluminum pieces with sizes smaller than 20 mm, before and after T6 heat treatment.

Experimental procedure

The experimental materials are small pieces with sizes smaller than 20 mm and produced by cutting four scrap AA6063 aluminum alloy profiles, as shown in Figure 1(a). The compositions of the scrap profiles are shown in Table 1. The small pieces from the four scrap profiles were mixed with a mass ratio of 1:1:1:1 to produce a small piece blend (Figure 1(b)). The small piece blend was unidirectionally die-pressed under a pressure of 800 MPa and holding time of 5 min. to produce small piece compacts of Φ52 mm × 60 mm at room temperature (Figure 1(c)). The small piece compacts were heated to 500°C, held for 5 min. and upset to form forging compacts of Φ57 mm × 50 mm (Figure 1(d)). The forging compacts were heated to 500^oC again and hot-extruded into rods (Figure 1(e,f)) with extrusion ratios 9:1 and 25:1, respectively. The diameters of the as-extruded rods are 18 and 10 mm, respectively. The composition of the 9:1 rod is shown in Table 2. A part of each rod was subjected to T6 heat treatment (solution treatment at 530°C for 1 h, water quenching, and artificial aging at 170°C for 8 h). The samples are referred to as 9:1, 25:1, 9-T6 and 25-T6 samples corresponding to extrusion ratios of 9:1 and 25:1 and before and after T6 heat treatment, respectively. The density of the samples was measured using the Archimedes method. As shown in Table 3, the small piece compact had a high relative density of 98.5%, and after upsetting, the relative density of forging compact reached 99.5%. After hot extrusion, the as-extruded rod had a high relative density of 99.8%.

Figure 1. Images of experimental materials at different steps: (a) small pieces cut from the four scrap AA6063 profiles, (b) blend of the small pieces from different scrap profiles, (c) a small piece compact, (d) a forging compact, (e) 9:1 rod, (f) 25:1 rod.

Table 1. Compositions of four scrap AA6063 aluminum alloy profiles (wt.%).

Profile	Si	Mg	Fe	Al
1	0.40	0.54	0.14	Bal
2	0.41	0.52	0.13	Bal
3	0.44	0.54	0.15	Bal
4	0.45	0.58	0.54	Bal

Table 2. Composition of the 9:1 rod (wt.%).

Element	Si	Mg	Fe	Cu	Al
Content	0.42	0.53	0.21	0.03	Bal

Table 3. Relative densities of the small piece compact, forging compact and as-extruded rod.

Sample	Relative density /%
Small piece compact	98.5
Forging compact	99.5
As-extruded rod	99.8

The metallographic surfaces along the extrusion direction were prepared by sandpaper grinding, polishing and etching with the Keller reagent (2.5% HNO₃, 1.5% HCl and 1.0% HF). The microstructure of the samples was examined by an Olympus dsx-500 optical metallographic microscope, a scanning electron microscope (SEM) (JMS-7001F of Japan Electronics Co) in combination with an energy dispersive spectrometer, electron backscatter diffraction (EBSD) detector and an X-ray diffractometer (XRD) (Rigaku Smart Lab) with a Cu Kα1 radiation source, a scanning rate of 4 (°)/min, and the scanning range of 20°∼90°. The hardness test was carried out using a Vickers hardness tester, with a test load of 100 gf and a holding pressure of 15 s. The tensile tests were carried out using a Shimadzu AG-XPLUS 100 KN universal material testing machine at a strain rate of 5 × 10⁻⁴ s⁻¹. The dogbone-shaped tensile test specimens with a rectangular section of 3 × 2 mm and a gauge length of 10 mm were cut from the as-extruded and T6 heat-treated rods along the extrusion direction. For each sample, three specimens were cut and tested. After testing, the fracture surfaces and longitudinal sections of selected tensile test specimens were examined by SEM by JSM-6510A of Japan Electronics Co.

Results and discussion

Microstructure of the small pieces

As shown in Figure 2, the small pieces all have an equiaxed grain structure, and the grain sizes of the small pieces from No. 4 scrap profile are clearly smaller. As shown in Figure 3, the small pieces from No. 4 scrap profile clearly have a higher number density of second-phase particles distributed in the matrix in bands. In order to further study the morphology and composition of the second-phase particles, SEM observation and EDS spectrum analysis were carried out, as shown in Figure 4. The results show that the second phase particles mainly contain Al, Si and Fe, showing that they are likely an Fe-rich phase (AlFeSi). Since the Fe content in the small pieces from No. 4 scrap profile is relatively high (0.54 wt% vs. 0.13–0.15 wt%), it is understandable that the microstructure of the small pieces from it has a clearly higher number density of the Fe-rich phase particles. It is likely that these second particles are more effective in pinning the boundaries of the recrystallized grains during extrusion of the profile, resulting in smaller sizes of the grains.

Figure 2. SEM images of the microstructure of small pieces from the four scrap profiles: (a) 1, (b) 2, (c) 3, (d) 4.

Figure 3. Optical microscopy images of the microstructure of small pieces from the four scrap profiles: (a) 1, (b) 2, (c) 3, (d) 4.

Figure 4. SEM images and EDS spectra of the second phase particles in the microstructure of small pieces form the four scrap profiles: (a) and (b) 1, (c) and (d) 2, (e) and (f) 3, and (g) and (h) 4.

Microstructure of consolidated samples

The XRD analysis of the samples (Figure 5) shows that they all exhibit a high degree of texture [19]. With the extrusion ratio of 9:1, it is Al{220} texture, while with the extrusion of 25:1, the texture changes into Al{200} texture. After T6 heat treatment, the degree of texture becomes higher.

Figure 5. XRD patterns of the as-extruded and heat-treated samples.

From the metallographic observation by optical microscopy (Figure 6), it can be seen that the small pieces are deformed into regular long strips along the extrusion direction and the prior boundaries between the strips are clearly visible under the optical microscope. The prior boundaries are decorated by dark particles which were identified as Al₂O₃ particles based on the EDS analysis which show that they are O rich but not rich in any other alloying elements (Figure 7).

Figure 6. Optical microscopy images of the microstructure of the four samples: (a) 9:1, (b) 25:1, (c) 9-T6, (d) 25-T6.

Figure 7. SEM image and EDS elemental maps of the microstructure of two samples: (a) 9:1, (b) 25:1.

By performing multiple EDS small area analysis and marking by making indents with a Vickers microhardness tester, regions with a clearly higher Fe content (I region) and regions with a normal Fe content (II region) were identified, as shown in Figure 8 and Tables 4 and 5. It can be seen from the table that the Fe content in the I region is higher than that in the II region. Compared with the composition of the raw materials, the I region is a Fe-rich region (Fe-R), corresponding to the small pieces of high Fe content from No. 4 scrap profile, and the II region is a Fe-low region (Fe-L), corresponding to the small pieces of normal Fe content from other scrap profiles. In order to study the grain size, grain boundary orientation angle distribution and recrystallization distribution in different regions, EBSD microstructure characterization was performed in different regions.

Figure 8. SEM images of the microstructure of I and II regions in the as-extruded samples: (a) 9:1, (b) 25:1.

Table 4. Compositions of different types of regions in the 9:1 sample (wt.%), determined by EDS area analysis.

Region	Si	Mg	Fe	Al
I	0.31	0.63	0.45	Bal
II	0.33	0.63	<0.20	Bal

Note: The minimum value of EDS surface scanning element content statistics is 0.20%.

Table 5. Compositions of different types of regions in the 25:1 sample (wt.%), determined by EDS area analysis.

Region	Si	Mg	Fe	Al
I	0.36	0.71	0.44	Bal
II	0.36	0.66	<0.20	Bal

Note: The minimum value of EDS surface scanning element content statistics is 0.20%.

From the EBSD analysis of the as-extruded samples (Figure 9), it can be seen that with the extrusion ratio of 9:1, the microstructure of the material is composed of fibrous deformed grains and recrystallized and equiaxed grains. The reason is that the degree of plastic deformation is small, and the deformation energy stored in the alloy is small. Dynamic recrystallization can only occur in regions with a large degree of deformation. The size of Al grains ranges from 7 to 367 μm, and the average grain size is 46 μm. As the extrusion ratio increases to 25:1, severe plastic deformation during extrusion causes full recrystallization to occur, and almost all grains are recrystallized and equiaxed grains. At the same time, it has been observed that the grains in the Fe-rich regions (Fe-R) are significantly finer than those in the Fe-low regions (Fe-L). The sizes of Al grains in Fe-L regions range from 5 to 183 μm, and the average grain size is 36 μm. In contrast, the sizes of the Al grains in the Fe-R regions range from 3 to 155 μm, and the average grain size is 10 μm. This shows that the higher number density of Fe-rich phase particles increases the nucleation rate of recrystallization, causing formation of a larger number of grains and smaller grain sizes.

Figure 9. EBSD band contrast and IPF contrast images of the microstructure of as-extruded samples: (a) and (b) 9:1, (c) and (d) 25:1.

However, after the T6 heat treatment, these microstructural differences between as-extruded samples fabricated with extrusion ratios of 9:1 and 25:1 and those between Fe-R regions and Fe-L regions largely vanished. As shown in Figure 10, the microstructure of 9-T6 sample consists of a large fraction of coarse blocky elongated grains and a small fraction of equiaxed grains, while the microstructure of the 25-T6 sample consists of mostly equiaxed grains. The prior boundaries are still highly visible after T6 heat treatment.

Figure 10. EBSD band contrast and IPF contrast images of T6 heat-treated samples: (a) and (b) 9-T6, (c) and (d) 25-T6.

Through the statistical analysis of the grain size of the EBSD microstructure of the rod after T6 heat treatment (Figure 11), it can be seen that as the extrusion ratio increases from 9:1 to 25:1, the ratio of the grain aspect ratio in the range of 1∼2 increases significantly (55% to 71%), and the average grain size is refined from 78 to 52 μm. It shows that with the increase of plastic deformation, the degree of dynamic recrystallization increases, the number of elongated grains decreases significantly, the number of fine equiaxed grains increases, and the grains are obviously refined.

Figure 11. The distributions of aspect ratios of the Al grains of T6 heat-treated samples: (a) 9-T6, (b) 25-T6.

Figure 12 shows the recrystallization distribution of grains in the four samples. The blue, yellow and red regions represent recrystallized grains, substructure grains and deformed grains, respectively. When the extrusion ratio is 9:1, the extruded alloy is mainly deformed grains, the content is about 30%, the recrystallized grain content is 63%, and the substructure grain content is 7%, because the uneven plastic deformation causes the local area inside the metal to accumulate high enough dislocations to form substructures. When the extrusion ratio is increased to 25:1, the content of deformed grains in the extruded alloy is significantly reduced to 5%, and the content of substructure grains and recrystallized grains is increased to 8% and 87%, respectively. After T6 heat treatment of the extruded sample, the high-temperature solid solution treatment makes the fibrous deformed grains of the extruded state recrystallize, and new equiaxed grains are formed by nucleation and growth. The deformed grains basically disappear, and the content of substructure grains and recrystallized grains increases significantly. The content of substructure grains and recrystallized grains of the alloy with extrusion ratio of 9:1 is 19% and 81%, respectively, and the content of substructure grains and recrystallized grains of the alloy with extrusion ratio of 25:1 is 12% and 88%, respectively. EDS area analysis and markings with indentations show that the Fe-R regions still exist in the heat-treated samples, as shown in Figure 13 and Tables 6 and 7. These findings are consistent with previous observation in studying thermomechanical processing of alloys [20–22].

Figure 12. Recrystallization distribution of grains in the four samples: (a) 9:1, (b) 25:1, (c) 9-T6, (d) 25-T6, (e) histograms showing the fractions of different types of grains in the samples.

Figure 13. SEM images of the microstructure of different types of regions of T6 heat-treated samples: (a) 9-T6, (b) 25-T6.

Table 6. Compositions of different types of regions in the 9-T6 sample (wt.%), determined by EDS area analysis.

Region	Si	Mg	Fe	Al
Fe-R	0.37	0.67	0.42	Bal
Fe-L	0.32	0.63	<0.20	Bal

Note: The minimum value of EDS surface scanning element content statistics is 0.20%.

Table 7. Compositions of different types of regions in the 25-T6 sample (wt.%), determined by EDS area analysis.

Region	Si	Mg	Fe	Al
Fe-R	0.31	0.63	0.50	Bal
Fe-L	0.31	0.62	<0.20	Bal

Note: The minimum value of EDS surface scanning element content statistics is 0.20%.

Mechanical properties and fracture behavior

As shown in Figure 14, the average Vickers hardness of the as-extruded samples are 36 HV, and the extrusion ratio does not have any effect. After T6 heat treatment, the average Vickers hardness of the 9-T6 sample is 84 HV, being significantly higher than that of the as-extruded sample, confirming the age-hardening effect of the alloy. The average microhardness of the 25-T6 sample is 87 HV, slightly higher than that of the 9-T6 sample, showing that the increase of extrusion ratio causes an improvement of the strength of the recycled material.

Figure 14. Microhardness values of the samples.

Figure 15 shows the engineering stress-engineering strain curves of the specimens cut from the samples. The average tensile properties of the samples are shown in Table 8. It can be seen that in the as-extruded state, when the extrusion ratio is increased from 9:1 to 25:1, the yield strength (YS) and ultimate tensile strength (UTS) of the alloy are significantly increased from 67 and 118 MPa to 75 and 139 MPa, respectively, while the elongation to fracture is also increased from 11.4% to 24.3%, showing that increasing the extrusion ratio improves both the strength and ductility. In addition, the engineering stress-engineering strain curves of the as-extruded samples show serration which is likely caused by dynamic strain aging associated with the interactions between mobile dislocations and Mg solute atoms during plastic deformation [23].

Figure 15. Engineering stress-engineering strain curves of the samples.

Table 8. Tensile properties of the samples.

Sample	YS/MPa	UTS/MPa	EL/%
9:1	67 ± 0.5	118 ± 4.0	11.4 ± 0.1
25:1	75 ± 9.3	139 ± 4.0	24.3 ± 4.0
9-T6	174 ± 10.2	221 ± 7.9	22.1 ± 3.7
25-T6	213 ± 2.7	237 ± 2.9	15.4 ± 1.2

After T6 heat treatment, the YS and UTS of the recycled alloy are significantly improved, reflecting the strong age-hardening ability of the AA6063 alloy. When the extrusion ratio increases from 9:1 to 25:1, the YS and UTS of the alloy increase significantly from 174 and 221 MPa to 213 and 237 MPa, respectively, while the elongation to fracture clearly decreases from 22.1% to 15.4%. It should be noted that 15% elongation still shows that the heat-treated alloy has very good ductility. As shown above, the 25-T6 sample has a much larger fraction of the fine equiaxed grains than the 9-T6 sample, this significant strength improvement with increasing extrusion ratio can be attributed to grain refinement which enhances grain boundary strengthening based on the well-known Hall–Petch relationship.

Figure 16 shows the fracture surfaces of the tensile test specimens cut from the samples. It can be seen that all samples undergo severe plastic deformation and necking prior to fracture, as evidenced by the large number density of dimples on the fracture surfaces. Macroscopically, it shows ductile fracture, and microscopically, it is a process of micropore formation and coalescence, and in the last stage, tearing [24]. The fracture surfaces of all samples show a small number density of deep and relatively large ditches which are the results of formation of secondary microcracks, as shown in Figure 17. The prior boundaries between the small pieces are likely to have a clearly lower bonding strength than the grain boundaries. When the tensile test specimen is being plastic deformed during tensile testing, the prior boundaries, grain boundaries and second phase particle/matrix interfaces all hinders moving dislocations, resulting in dislocation pile-up and stress concentration [25]. With the further increase of the applied stress, the stress concentration at the prior boundaries is sufficiently high to cause microcracks to form at them first, since they are relatively weaker than grain boundaries and second phase/particle/matrix interfaces. Once the microcracks form, the stress concentration at the prior boundaries is relieved. Thanks to the high ductility of the matrix which means a very high amount of energy is required to grow the microcracks into cracks and cause fracture of the specimen, the microcracks are very much contained in the material as cavities. Similar observation was also made by Ibrahim et al. [26] in the tensile-tested specimens from the AA6061 aluminum alloy samples fabricated by hot extrusion of chip compacts. In other words, at the point of microcrack formation at the prior boundaries, the samples would not fracture, and can still allow further plastic deformation. With continued plastic deformation, the stress concentrations at the grain boundaries and second phase particle/matrix interfaces would become sufficiently large to cause a large number density of cavities to form. At this point, necking occurs and the specimens would fail soon after a small amount of plastic deformation.

Figure 16. SEM images of the fracture surfaces of the tensile test specimens cut from the samples: (a) and (b) 9:1, (c) and (d) 25:1, (e) and (f) 9-T6, (g) and (h) 25-T6.

Figure 17. Longitudinal sections below the fracture surfaces of the tensile test specimens cut from the samples: (a) 9:1, (b) 25:1, (c) 9-T6, (d) 25-T6.

From Figure 16(a–d), it can be seen that with the extrusion ratio increasing from 9:1 to 25:1, the ditches become relatively shallow and small. This indicates that increasing the extrusion ratio helps to enhance the bonding at the prior boundaries and enhance their deformation coordination ability, leading to an increase in the strength and ductility of the as-extruded samples. It is interesting and surprising to note that ductility of the 9-T6 sample is significantly higher than that of the 9:1 sample, even though the heat-treated sample has a significantly higher strength. This shows that with a lower flow stress which reflects a lower resistance of the matrix to dislocation slips, dislocation pile-up at prior boundaries can more easily occur. The lower strength of matrix is not favorable to suppressing microcrack growth. For these reasons, the samples can more easily fracture at a smaller strain, leading to a lower ductility.

The clear decrease of ductility with increasing extrusion ratio for the heat-treated samples may be associated with the increase of the flow stress which is already at high level for the AA6063 alloy. From Figure 16(e–h), it can be seen that the bonding across prior boundaries increases with the increase of extrusion ratio, but it is not enough to offset the negative effect on ductility caused by the decrease of deformation space related to the increase of flow stress and the decrease of grains size. It can also be seen that the number of relatively large dimples with sizes of 20–50 μm on the fracture surface of 25-T6 sample is clearly larger than that on the fracture surface of 9-T6 sample. This shows that the higher flow stress of the 25-T6 sample can more effectively initiate formation of cavities at some grain boundaries and second-phase particle/matrix interfaces. With a high level of flow stress, the tolerance of grain boundaries and second-phase particle/matrix interfaces to the stress concentration becomes more limited. Once the dislocation pile-up reaches a certain level, a large number density of cavities would form, and at this point, the material is very close to fracture. As shown above, the 25-T6 sample fails at a smaller strain, leading to a decrease of ductility.

Conclusions

In this study, AA6063 aluminum alloy rods were fabricated by hot extrusion of forging compacts formed by upsetting compacts pf small pieces with sizes smaller than 20 mm and from four scrap AA6063 profiles with different Fe contents. The effects of extrusion ratio on the microstructure and mechanical properties of the rods before and after T6 heat treatment were studied. The following conclusions are obtained:

1. In the as-extruded state, with the increase of extrusion ratio from 9:1 to 25:1, the level of recrystallization in the microstructure significantly increases, and with a high extrusion ratio of 25:1, the recrystallized grains in the high Fe content (0.54 wt%) regions are clearly finer than regions with normal Fe content (0.14 wt%) due to the stronger grain boundary nucleation and grain boundary pinning effect of a higher number density of Fe-rich phase particles in these regions.

2. After T6 heat treatment, the microstructural differences between the high Fe content regions and normal Fe content regions largely disappear. The fraction of equiaxed grains in the heat-treated sample fabricated with a higher extrusion ratio of 25:1 is significantly higher than that in the heat-treated sample fabricated with a lower extrusion ratio of 9:1.

3. In the as-extruded state, when the extrusion ratio increased from 9:1 to 25:1, both the strength and ductility improve, and the reason is that bonding strength of the prior boundaries increases with increasing extrusion ratio. In the T6 heat-treated state, with increasing the extrusion ratio from 9:1 to 25:1, the YS increases from 174 to 213 MPa, the UTS increases from 221 to 237 MPa, but the elongation to fracture decreases from 22.1% to 15.4%.

4. The microcracks first form at the relatively weak prior boundaries during plastic deformation of the solid state recycled AA6063 aluminum alloy samples due to stress concentration, but they are not sufficient to cause the material to fail due to the high intrinsic ductility of the matrix. The failure of the material requires further deformation which induces formation of cavities at grain boundaries and second-phase particle/matrix interfaces, leading to a salient ductile fracture behavior.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is supported by the Ministry of Science and Technology of the People’s Republic of China, with the 111 Project [B16009].