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DOI: 10.1016/j.jmrt.2019.09.070
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Available online 13 October 2019
Improvement of the mechanical properties of Al–Mg–Si alloys with nano-scale precipitates after repetitive continuous extrusion forming and T8 tempering
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Jiamin Hu, Wengang Zhang, Dingfa Fu, Jie Teng, Hui Zhang
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College of Materials Science and Engineering, Hunan University, Changsha 410082, China
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Abstract

Repetitive continuous extrusion forming (R-Conform) followed by T8 tempering treatment was found to be a viable approach for obtaining superior strength with high ductility in an Al–Mg–Si alloy. The evolution of mechanical properties and microstructure of the Al–Mg–Si alloy, and its mutual relationship during this novel approach were investigated. The results showed that a homogeneous and refined microstructure was formed during the R-Conform process, which improved the mechanical properties of the alloy. Concurrently, the R-Conform process caused effective dynamic strain redissolution of β″ precipitates, and the precipitation kinetics were affected by the subsequent T8 tempering process. The refined (sub)grains and induced high densities of the dislocations accelerated the formation of numerous dispersive nano-scale β″ precipitates during an additional T8 tempering treatment at a low artificial aging temperature (120°C) and a short aging time (3h). Therefore, remarkable improvements in both the tensile strength and ductility were achieved by 7 R-Conform passes and T8 tempering.

Keywords:
Al–Mg–Si alloy
Severe plastic deformation
Repetitive Conform process
Microstructure
Mechanical properties
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1Introduction

Al–Mg–Si alloys are lightweight with high strength, good formability, desirable corrosion resistance and excellent electrical conductivity and thus have been widely used in the structural, automotive, aerospace and conducting fields [1–4]. In the development of new generations of lightweight structures and technologies, numerous attempts have been made to improve the strength of these alloys while maintaining their formability. However, improving both the strength and ductility of these alloys presents a considerable challenge. For instance, severe plastic deformation (SPD) has resulted in significant grain refinement and considerable strength improvement, but the ductility generally has remained limited [5–7].

During the artificial aging of heat-treatable Al–Mg–Si alloys, a supersaturated solid solution decomposes into fine precipitates that strengthen the alloys by acting as obstacles to the motion of dislocations [8]. Especially, the formation of nano-scale and dispersive precipitates increases both the strength and ductility of the alloys [3,7,9]. Thus, the introduction of intermediate metastable precipitates, especially nano-scale precipitates in ultrafine-grained microstructures prepared by SPD, should be an effective means of further improving the mechanical properties of alloys comprehensively [7,10]. Nageswara rao and Jayaganthan [11] effectively used warm rolling, cryogenic rolling and aging to obtain desired Al–Mg–Si alloys with high strength and ductility. Sauvage et al. [12] developed a high density of nanosized precipitates in an Al–Mg–Si alloy and improved the mechanical properties of the alloy through equal channel angular pressing (EACP) and subsequently aging. Murashkin et al. [13] found that artificial aging led to the increase of both strength and ductility in all ECAP with parallel channels (ECAP-PC) processed Al–Mg–Si alloys. However, these SPD methods have mostly been implemented at the laboratory scale. Hence, it is of vital importance to develop an effective approach for large scale industrial application.

Continuous extrusion forming (Conform) is a well-developed forming process that has been extensively applied in the efficient continuous industrial production of Al alloys and other metals. The Conform process offers numerous advantages, such as the absence of feedstock pre-heating, a high extrusion ratio and production efficiency, energy saving and unlimited product length [14,15]. Many continuous SPD techniques based on the principle of the Conform process have been developed, such as ECAP-Conform [16–18], repetitive Conform (R-Conform) process [15,19,20], and accumulative continuous extrusion forming (ACEF) [21–24]. The formation of an intense internal shear band (IISB) in the plastic deformation zone during the Conform process has been reported, and a continuous in-line solution treatment has been developed [25–27]. Thus, solid solution treatment and the SPD process, which are conventionally carried out separately, could be performed synchronously using Conform process. The synchronous solid solution treatment and SPD process could be used to produce refined microstructures and high densities of dislocations, which would affect the subsequent tempering treatment. Therefore, R-Conform followed by T8 tempering treatment is explored in this study.

2Experimental procedures

The main chemical compositions of the adopted Al–Mg–Si alloy were Mg 0.66, Si 0.51, Fe 0.269, Cu 0.031 and the remainder Al (wt.%). The as-received Al–Mg–Si alloy was a commercial continuous casting and rolling (CSR) rod with a diameter of 9.5mm. The CSR rod was first pre-aged at 175°C for 8h to obtain an abundance of precipitates. The pre-aged CSR rod was fed into the entrance of a roll-shoe gap on a LJ300 Conform machine to provide starting materials for the R-Conform process [14,18,19]. The Conform wheel revolving speed was 15rpm (the maximum strain rate was approximately 30s−1). After each Conform pass, the temperature of the extruded rod at extrusion die exit was approximately 460°C. The extruded rod was then water-cooled in a sink approximately 2m away from the extrusion die exit. The duration time before cooling was about 9.5s. During the R-Conform process, the material was immediately fed in the same direction after repeating the aforementioned Conform deformation and R-Conform process seven times. Finally, the processed Al–Mg–Si alloy rods from different R-Conform passes were cold drawn into wires with diameters of 2.97mm that were then aged at 120, 155 and 175°C for various time (up to 8h): this process is henceforth referred to as T8 tempering.

Vickers microhardness (HV) was measured on the plane normal to longitudinal direction of the Al–Mg–Si samples. Five indentations were performed on per sample by using 500 gf load and 15s dwelling time. Tensile tests were conducted at room temperature with constant strain rate of 10−3s−1 using a computer-controlled INSTRON 3382 universal testing machine. The fracture surfaces of the tensile samples were examined by a FEI QUANTA 200 environmental scanning electron microscopy (SEM). The microstructures of the processed alloys were examined by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD analysis was performed on FEI QUANTA 200 SEM equipped with EBSD detectors. The TEM foils were examined using a FEI Tecnai G2 F20 TEM, which were prepared by twin-jet electro-polishing machine at −20°C using a solution of 30% nitric acid and 70% methanol.

3Results and discussion3.1Evolution of microstructure and mechanical properties of Al–Mg–Si alloys during R-Conform process

Fig. 1 shows EBSD micrographs of the pre-aged CSR specimen and the processed Al–Mg–Si alloys after different R-Conform passes. In the pre-aged CSR alloy, fibrous microstructures with average grain size of 259μm were observed. After one R-Conform pass, the fibrous structures were eliminated, and the microstructure was characterized by a high fraction of equiaxed grains with a heterogeneous size distribution. Both refined and coarse grains were observed. By increasing the number of R-Conform passes, more extensive grain refinement occurred, and a homogeneous microstructure was gradually formed. However, increasing the number of passes beyond 4 R-Conform passes resulted in only slight changes in the microstructure. The evolution of the grain morphology was consistent with our previously reported results from high-speed Conform processing [20]. In addition, Su et al. [22] and Wang et al. [23] found that the grain size of pure Al, Al-1Si and Al-0.2Er gradually decreased under the action of continuous dynamic recrystallization (cDRX) during ACEF process at 300°C. Sakai et al. [28] concluded that the high stacking fault energy (SFE) in aluminum alloy caused dynamic recovery (DRV) and cDRX during high-temperature plastic deformation (above 0.5 times the melting temperature), which can be considered to be a strain-induced continuous reaction. In this study, the occurrence of cDRX also led to the grain refinement of Al–Mg–Si alloys during the R-Conform process. However, heterogeneous deformation [26,27] resulted in the formation of both fine and coarse grains in the initial stage of the R-Conform process (Fig. 1(b)). Then, increasing the number of R-Conform passes improved the homogeneity of plastic deformation and resulted in homogeneous recrystallized grains (Fig. 1(c)). However, continuous in-line solution treatment could also be achieved at high temperature (460°C) and by severe plastic deformation, resulting in grain growth [29]. Consequently, the grain size increased slightly after approximately 7 R-Conform passes (Fig. 1(d)), which was attributed to the balance between grain refinement and grain growth.

Fig. 1.

EBSD micrographs of the pre-aged CSR specimen (a) and the processed Al–Mg–Si alloys after different R-Conform passes: (b) 1 R-Conform pass; (c) 4 R-Conform passes; (d) 7 R-Conform passes.

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The TEM micrographs of the pre-aged CSR specimen and the processed Al–Mg–Si alloys after different R-Conform passes are shown in Fig. 2. In the pre-aged CSR Al–Mg–Si alloy, the pre-aging treatment resulted in many uniform distributed needle β″ precipitates with an average length of 100nm and few dislocations, as shown in Fig. 2(a). The β″ precipitates were verified by the selected area electron diffraction pattern in Fig. 2(a). The dislocation density gradually increased with the number of R-Conform passes. Tangled dislocation lines and even cell structures can be observed in Fig. 2(c) and (d). The accumulation of SPD caused rearrangement and annihilation of the dislocations, leading to the formation of cellular substructures. Dynamic strain redissolution [20] and continuous in-line solution [25] occurred concurrently during the R-Conform process under the special thermo-mechanical conditions. The initially large precipitates (Fig. 2(a)) gradually fragmented and dissolved into the Al matrix, thus fewer and finer precipitates can be seen in Fig. 2(b)–(d). The R-Conform process offers the advantages of high temperature and high deformation strain for a short processing time (of only a few seconds) that can dissolve precipitates in alloys in contrast to conventional solid solution treatment, which requires a high temperature and long processing times.

Fig. 2.

TEM micrographs of the pre-aged CSR specimen (a) and the processed Al–Mg–Si alloys after different R-Conform passes: (b) 1 R-Conform pass; (c) 4 R-Conform passes; (d) 7 R-Conform passes.

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Fig. 3 shows the tensile properties of the pre-aged CSR specimen and processed Al–Mg–Si alloys after different R-Conform passes. After one R-Conform pass, the ultimate tensile strength clearly decreased (from 254.7 to 167.5MPa) and the elongation to failure (ductility) significantly increased (from 11.5 to 25.7%). Subsequently, both the tensile strength and elongation increased after 4 R-Conform passes. Further increasing the number of R-Conform passes slightly increased the tensile strength and elongation. After 7 R-Conform passes, the ductility was high, reaching 26.3% (an increase of 128% compared with the initial ductility of 11.5%), and the tensile strength was 179.2MPa. The variation in the mechanical properties was closely related to the evolution of microstructure during the R-Conform process. As shown in Figs. 1 and 2, the improvement in the mechanical properties could be attributed to the synthetic effects of precipitate dissolution, grain refinement and the increase in the number of dislocations. Compared with the results in Ref. [20], there was a clearer drop in the tensile strength after one R-Conform pass because of the pre-aging treatment before R-Conform process in this study.

Fig. 3.

The tensile properties of the pre-aged CSR specimen and the processed Al–Mg–Si alloys after different R-Conform passes.

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3.2Evolution of mechanical properties of Al–Mg–Si alloys during T8 tempering

The Vickers microhardness variations of Al–Mg–Si alloys that were aged at different temperatures and times during T8 tempering are shown in Fig. 4. Before artificial aging treatment, the Al–Mg–Si alloys were deformed by cold drawing to further increase the hardness of the alloys. After drawing, the microhardnesses of the R-Conform-processed alloys were higher than that of the initial sample (92.7HV). Further artificial aging treatment affected the mechanical and microstructural properties of the age-hardenable Al–Mg–Si alloys. When aged at 120 and 155°C, the hardness first increased up to a peak and then gradually decreased as the aging time was increased further. At a higher temperature of 175°C, the hardness generally decreased with the aging time. The hardness aging at 120°C was higher than at 155 and 175°C. As the number of R-Conform passes increased, the peak aging hardness (at 120°C) was also enhanced. The optimized hardness increased by 18% (from 95.6 to 112.8HV) after 7 R-Conform passes and T8 tempering compared with the initial sample, which was only treated by T8 tempering.

Fig. 4.

Variations of Vickers hardness with different aging time for Al–Mg–Si alloys processed after: (a) direct drawing; (b) 1R-Conform pass and drawing; (c) 4 R-Conform passes and drawing; (d) 7 R-Conform passes and drawing; (e) peak aging Vickers hardness under different R-Conform passes and drawing.

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Based on the hardness results in Fig. 4, the peak aging conditions at 120°C were selected for the tensile tests, as shown in Fig. 5(a) and (b). It can be clearly seen that in the absence of the Conform process, the tensile strength and elongation of initial Al–Mg–Si alloy after T8 tempering were 338.5MPa and 5%, respectively. After 1 R-Conform pass and T8 tempering, both the tensile strength and elongation decreased slightly. After 4 R-Conform passes and T8 tempering, both the tensile strength and elongation were significantly improved at 344MPa and 6.9%, respectively. Furthermore, as the number of R-Conform passes increased, the tensile strength remained stable, and the elongation increased to 7.6% after 7 R-Conform passes and T8 tempering. Compared with the initial Al–Mg–Si alloy after direct T8 tempering, there was a remarkable improvement in the elongation (52%) and a slight increase in the tensile strength (from 338.5 to 345MPa). In addition, the fracture toughness of the specimen, i.e. the area under the stress–strain curve (Fig. 5(a)), increased with the number of R-Conform passes. A 51.6% improvement in the fracture toughness was obtained after 7 R-Conform passes and T8 tempering (23.24MPa) over that of the initial Al–Mg–Si alloy after direct T8 tempering (15.34MPa). Meanwhile, a significant improvement in the ductility was verified from SEM fracture micrographs that were taken after tensile testing, as shown in Fig. 5(c) and (d). Some quasi-cleavage fracture characteristics were observed in the Al–Mg–Si alloy after direct T8 tempering. After 7 R-Conform passes and T8 tempering, the fracture micrographs were covered with mass equiaxed and homogeneous dimples, showing that the microstructure became more homogeneous during the R-Conform process. The R-Conform- and T8-temper-processed Al–Mg–Si alloy exhibited increased resistance to crack formation.

Fig. 5.

(a) Engineering stress–engineering strain curve and (b) tensile properties of the Al–Mg–Si alloys after different R-Conform passes and T8 temper processing; microcosmic fracture features of the Al–Mg–Si alloys processed by: (c) direct T8 temper and (d) 7 R-Conform passes and T8 temper processing.

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It has been demonstrated that a novel processing route produced an alloy with high strength (345MPa) and high elongation (7.6%) after 7 R-Conform passes and T8 tempering (Fig. 5(a)). No alloy elements were added in this approach; however, the mechanical properties were comprehensively improved over those of alloys processed by conventional extrusion-drawing (CED) process with the addition of 3% AlB2[2] and continuous casting and hot-rolled (CCR) with the addition of Sr [30]. In addition, the Conform and cold drawing processes are both mature industrial technologies. Hence, this novel processing approach may be much easier to incorporate into industrial production than other conventional methods, including shearing/cooling rolling process (SCR)-on-line solution-aging heat treatment plus cold drawing (AHCD) and SCR-cold drawing and aging heat treatment (CDAH) [31], semi-solid continuous casting-extrusion and on-line solution process (CCES)-artificial aging and cold drawing (AACD) and CCES-cold drawing and artificial aging (CDAA) [32]. Recently, Ji et al. [25] combined horizontal continuous casting (HCC) with the Conform process to prepare Al–Mg–Si alloy wires with significant improved in strength and ductility. In the present study, even further improvements in both the strength and ductility were achieved. The processing route of 7 R-Conform passes, cold drawing and 120°C/3h (T8 tempering) effectively improved the mechanical properties of Al–Mg–Si alloys comprehensively.

3.3Evolution of microstructure of Al–Mg–Si alloys during T8 tempering

The TEM microstructures of the Al–Mg–Si alloys after different R-Conform passes and T8 tempering are shown in Fig. 6. The cold-drawing elongated the grains and introduced a large number of substructures and dislocations into the alloy, which effectively impeded dislocation motion and increased the alloy strength. The subsequent artificial aging treatment (120°C) slightly affected the substructures because of the low aging temperature and the short aging time. The Al–Mg–Si alloy after direct T8 tempering without Conform processing exhibited coarse subgrains with an average length of approximately 2μm and an average width of approximately 0.5μm (Fig. 6(a)). As the R-Conform deformation accumulated, the subgrain size clearly decreased. After 7 R-Conform passes and T8 tempering, the subgrains were refined to average lengths of ˜1μm and average widths of ˜0.2μm (Fig. 6(g)). Slight recovery can occur during artificial aging via the reduction of the dislocation density and internal stresses. The low artificial aging temperature and short aging time used in this study resulted in a much weaker recovery effect. Thus, many dislocations remained in the Al–Mg–Si alloy. Besides, the dislocation density increased with the increasing number of R-Conform passes (Fig. 6(d), (f) and (h)), which can be attributed to the accumulation deformation during R-Conform process (Fig. 2).

Fig. 6.

TEM micrographs of the Al–Mg–Si alloy processed by: (a) & (b) direct T8 tempering; (c) & (d) 1 R-Conform pass and T8 temper processing; (e) & (f) 4 R-Conform passes and T8 temper processing; (g) & (h) 7 R-Conform passes and T8 temper processing.

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The schematic in Fig. 7 better illustrates the (sub)grains evolutions in the Al–Mg–Si alloys during R-Conform and T8 tempering. The initial CSR Al–Mg–Si alloy was full of large and elongated fibrous (sub)grains. Without the Conform deformation, the (sub)grains were more elongated after direct T8 tempering, and the average length increased and the width decreased (Fig. 7(b)). During the Conform process, the IISB in the plastic zone caused the fragmentation of coarse elongated (sub)grains and then produced exquiaxed and refined (sub)grains. As the number of R-Conform passes increased, the microstructure became more homogeneous and the (sub)grains were further refined (Fig. 7(c)). Additional cold drawing compressed these (sub)grains in the radial direction and continuously stretched the (sub)grains in the axial direction, thereby re-forming the elongated microstructure. The homogeneous microstructure (i.e., the refined and equiaxed (sub)grains) that formed during the R-Conform process significantly refined and uniformly distributed the (sub)grains in the Al–Mg–Si alloys after subsequent T8 tempering (Fig. 7(d)).

Fig. 7.

Schematic illustration of the refined grain and subgrain developments during R-Conform and T8 tempering process.

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The nano-scale precipitates in the processed Al–Mg–Si alloys are shown in Fig. 8. These images were all obtained along the <001> zone axis of the Al matrix, which was verified by the diffraction patterns shown in Fig. 8(e) and (f). The presence of the β″ (Mg5Si6) phases and the corresponding hkl indices were marked in the selected area electron diffraction patterns of Fig. 8(e) and (f). As shown in Fig. 8, the β″ phases were clearly visible aligned along the [100]Al and [010]Al directions of the Al matrix. The crystal structure of the β″ phases is monoclinic with lattice parameters of a=1.516nm, b=0.405nm, c=0.674nm, β=105.3°. And its orientation relationship with Al is (010)β//{001}Al, [100]β//<230>Al, [001]β//<310>Al[33–35]. After direct T8 tempering, many coherent needle-shaped β″ precipitates with average lengths of ˜28nm formed in the Al matrix (Fig. 8(a)). And the effective particle spacing and the density of these nano-precipitates, as estimated from the TEM images, were approximately 5.244nm and 1.59×1023m−3, respectively. In the R-Conform- and T8-tempering-processed alloys (Fig. 8(b)–(d)), the β″ precipitates were significantly refined and homogeneously distributed throughout the matrix. As the number of R-Conform passes increased, the number of nano-scale precipitates increased, the average size decreased, and the morphology became increasingly dispersed and uniform. After 7 R-Conform passes and T8 tempering, the average length of the needle-like nano-scale β″ precipitates was approximately 2.5nm and the effective particle spacing was approximately 2.66nm. These precipitates nucleated and were distributed uniformly in the alloy. The density of the precipitates was significantly higher at approximately 8.96×1023m−3. A different morphology was observed that was reported by Ji et al. [25] and Hu et al. [20], which could be attributed to the combined effects of dislocations, the resulting shear of the precipitates, the low aging temperature and the short aging time during R-Conform and T8 tempering process.

Fig. 8.

TEM micrographs of precipitates of the Al–Mg–Si alloys processed by: (a) direct T8 tempering; (b) 1 R-Conform pass and T8 temper processing; (c) 4 R-Conform passes and T8 temper processing; (d) 7 R-Conform passes and T8 temper processing; (e) diffraction pattern in the beam direction of [100] of (a); (f) diffraction pattern in the beam direction of [100] of (d).

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It is well known that Al–Mg–Si alloys are precipitate hardening alloys. During artificial aging treatment, the following precipitation sequence has been reported: αAlGP zonesββ'β[10,36,37]. The R-Conform process with special deformation thermo-mechanical conditions (a high deformation strain rate and temperature) can provide a significant driving force for the fragmentation and dissolution of precipitates [20,25]. Thus the R-Conform process affects the kinetics of the subsequent aging process in providing more nucleation sites than in alloys without Conform deformation processing. The solution aggregation state should significantly affect the precipitation process during artificial aging. Moreover, during artificial aging, long times and high temperatures result in precipitate coarsening and a decreased density, which consequently decreases both the strength and ductility of the alloy [9]. In this study, for a low artificial aging temperature (120°C) and a short aging time (3h) during T8 tempering, numerous dispersive nano-scale precipitates formed in the R-Conform-processed Al–Mg–Si alloy. Hence, both the strength and ductility improved (Fig. 5(a)). Murashkin et al. [13] obtained many fine needle-type metastable β″ second-phase precipitates in the grain interior of Al–Mg–Si alloys using ECAP-PC and artificial aging at 130°C. Sauvage et al. [37] also found that most of the nano-scale needle-shaped precipitates were nucleated in a 6101 Al alloy after 20 turns high pressure torsion (HPT) followed by aging at 130°C.

The mechanical properties of Al–Mg–Si alloys were improved noticeably by T8 tempering, as shown in Fig. 5(a) and (b). The main strengthening mechanisms in aluminum alloys are solution strengthening, grain boundary strengthening, dislocations strengthening and precipitation strengthening [38]. During R-Conform deformation, a high degree of dissolution of precipitates, grain refinement, an increased number of dislocations and an increased amount of a solid solution comprehensively improved the mechanical properties of processed Al–Mg–Si alloys. In addition, after T8 tempering, there were high densities of dislocations, refined (sub)grains and numerous fine dispersive nano-scale precipitates in the Al–Mg–Si alloy. The high densities of nano-scale precipitates significantly enhanced the mechanical behavior of the Al–Mg–Si alloys via precipitate-strengthening functions. Precipitate strengthening depends on the effective interparticle spacing, L', i.e., τp∝1/L'[39], (L'=r/f, where r is the average diameter of the precipitates, and f is the volume fraction of the precipitates). Hence, the increase in strength due to precipitation can be written as σp=kf/r. For a fixed f, an increase in the number of nucleation sites should decrease the interparticle spacing (L') or particle size (r)[9]. Thus, in this study, a finer and higher density of nano-scale precipitates (Fig. 8) led to effective strengthening. High densities of dislocations and refined (sub)grains also increase the tensile strength. However, high densities of nano-scale precipitates and the refined microstructure that was observed in this study could also have contributed to the relatively high observed elongation [9,12,40,41]. More homogeneous microstructures could further improve the ductility of processed Al–Mg–Si alloys. Therefore, the developed novel processing route of R-Conform and T8 tempering may have practical application for obtaining superior strength and high ductility for Al–Mg–Si alloys or other heat-treatable aluminum alloys in industrial production.

4Conclusions

R-Conform followed by T8 tempering treatment was performed on the pre-aged CSR Al-0.66Mg-0.51Si alloy in this study. The relationship between microstructural features and mechanical properties were investigated. The main results can be summarized as below:

  • 1

    The R-Conform process effectively refined and homogenized microstructures. Increasing the number of R-Conform passes resulted in further refined and equiaxed grains, enhanced β″ precipitate dissolution and increased the number of dislocations. After 7 R-Conform passes, there was a 128% increase in the ductility (from an initial value of 11.5–26.3%), and the tensile strength decreased to 179.2MPa.

  • 2

    The effective dynamic strain redissolution of β″ precipitates during the R-Conform process provided more nucleation sites and affected precipitation kinetics during the subsequent artificial aging process. After 7 R-Conform passes and T8 tempering, numerous dispersive nano-scale precipitates were distributed uniformly in the Al–Mg–Si alloys.

  • 3

    Compared with a sample after T8 tempering without Conform processing, more refined and homogeneous (sub)grains were obtained with an elongated morphology after 7 R-Conform passes and T8 tempering. There were high densities of dislocations in the processed Al–Mg–Si alloys.

  • 4

    With the combined effects of (sub)grain refinement, high densities of dislocations and large number of nano-scale precipitates, coexisting superior strength (345MPa) and high ductility (7.6%) were obtained after 7 R-Conform passes and T8 tempering.

Conflict of interest

The author declares no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant number 51671083, 51774124) and the Doctoral Program of the Ministry of Education (grant number 20130161110007).

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