Material Properties and Structure of Al-Mg-Si Alloy Thin-Walled Profiles with Different Alloy Compositions and Aging Processes

Abstract

Thin-walled Al-Mg-Si alloy profiles with different compositions and aging states were prepared using the heating and extrusion process. The properties and structure of the profiles were then investigated using a metallographic microscope, scanning electron microscope, projection electron microscope, and universal testing machine. The results show that the yield strength and tensile strength of the profile increases with the increase in total Mg + Si content, and ductility is reduced. If the total Mg + Si content is too high or too low, the crush performance of the material would decrease. Compared with the under-aged and near-peak-aged states, the three types of AI-Mg-Si alloy thin-walled profiles at the over-aged state have better effective energy absorption during crushing and higher bending angle; however, the tensile strength of the profile is optimal at the near-peak-aged state. The effects of alloy composition and aging process on material strength and crushing energy absorption are mainly attributed to the grain structure and differences in precipitation. For coarse grain structures, the grain boundary precipitate free zones are wider, which decreases the profile ductility. Simultaneously, an increase in primary strengthening phases in the grains would increase the profile strength.

1. Introduction

Aluminum alloys are lightweight materials with the highest percentage of application and usage in the automotive light weighting revolution [1,2]. In recent years, the application of aluminum alloys in automobiles has been continuously promoted [3,4,5,6] with continuous development and advancement in the preparation and processing of aluminum alloy materials. Due to the addition of different alloying elements to aluminum, the resulting aluminum alloys have different properties and microstructures, and different series of aluminum alloys are applied to different parts of the vehicle according to requirements [7,8,9]. Thin-walled material profiles have excellent energy-absorbing properties due to their ability to undergo stable axial deformation; such profiles are often used as safety components in automobiles, such as crash beams, crash boxes, and longitudinal crash beams, and these components are often made of 6xxx-series aluminum alloys [10,11]. As crash-resistant components of automobiles, aluminum alloy thin-walled components’ crashworthiness is critical to the safe operation of automobiles.

To study the energy-absorption mechanisms of 6xxx-series aluminum alloy thin-walled beams and enhance the crushing energy absorption and crashworthiness of aluminum alloy thin-walled profiles, scholars globally have conducted numerous studies. For example, Xiong Zhang et al. [12] investigated the effect of thickness gradients on the energy-absorption efficiency and crush capacity of AA6061-O aluminum alloy thin-walled profiles. They found that the introduction of a thickness gradient in the cross-section can increase the energy-absorption efficiency by 30–35% without increasing the initial peak force. Yamazaki et al. [13] investigated the crashworthiness of rectangular tubes using finite element software and found that increasing the wall thickness and decreasing the width can improve the energy-absorption efficiency of rectangular tubes. Wang et al. [14] showed that the introduction of suitably placed cutout holes would result in uniform folding deformation of thin-walled beams, decreasing the peak load while increasing the average load, thus improving the energy-absorption performance. Wu et al. [15] showed that the crashworthiness of thin-walled beams increases with the increase in the number of cells, and the peak load and energy-absorption properties are improved. Qingwu Cheng et al. [16] investigated the crushing behavior of 6061-T6 alloy rectangular tubes and found that by locally introducing discontinuous through-holes, the total energy absorption can be increased by 26.6–74.6%. Nikkhah et al. [17] showed that the crush performance of Al alloy square tubes could be effectively improved by introducing discontinuous through-holes, and that square and rectangular through-holes are the ideal shapes for through-holes. M. Kathiresan [18] investigated the effects of the shape, size, and number of cutout holes on the crush load of thin-walled beams by performing a combination of experiments and finite element simulations. It was pointed out that the introduction of through-holes can reduce the peak load, and the peak load gradually decreases with the increase in the hole size of the openings. Hongtu Sun et al. [19] found that increasing the number of concave corners in polygonal tube cross-sections increased the energy-absorbing properties of 606 aluminum alloy rectangular thin-walled tubes, and the maximum compressive crushing stress increased by 11.6% in comparison with thin-walled rectangular tubes of the same material. Christopher P. Kohar et al. [20] optimized the cross-sectional dimensions of a 6063-T6 profile using the finite element method, with the optimized profile having 26.7% higher total energy absorption than the initial profile.

Hartmann et al. [21] studied the mechanical properties, microstructures, and static axial crushing properties of rectangular thin-walled parts of three aluminum alloys, including the 6063, 6061, and 6110 alloys, established a finite-element simulation model for the static crushing of rectangular thin-walled aluminum alloy parts based on the characterization of the material nanostructure, and successfully predicted the load-deformation and measured curves of energy absorption. Nguyen-Hieu Hoang et al. [22] used Deform V12 software to compare the effects of different quenching and cooling methods on axial compression and crushing energy absorption of rectangular thin-walled tubes of 6060 aluminum alloy. They found that quenching and cooling rates have a small effect on the compression and crushing energy absorption of under-aged alloys and a large effect on the energy absorption of peak-aged alloys. ARNOLD et al. [23] used DIGIMU 12 simulation software to investigate the effects of different aging states on the crush performance of thin-walled beams made of 6061 aluminum alloy. They found that mechanical properties of the material are a major factor affecting the energy-absorption performance of thin-walled aluminum alloy beams.

Thus far, research conducted on 6xxx-series aluminum alloy thin-walled profiles mainly focuses on two aspects: material structure and material preparation parameters. However, material preparation parameters, such as material composition and heat treatment design, mainly rely on finite element simulation software. Results from finite element simulation deviate from the actual results, and the finite element results cannot necessarily provide a guide for the industrial application of 6xxx-series aluminum alloy thin-walled profiles. Based on the considerations above, in this study, thin-walled profiles of Al-Mg-Si alloys with different compositions and aging states were prepared using the heating extrusion process. This study focuses on the effects of alloy composition and aging process on the crushing properties and tensile properties of Al-Mg-Si alloy thin-walled profiles, revealing the mechanisms of change of microstructures and precipitates and providing theoretical guidance in material composition design and process optimization for 6xxx-series aluminum alloy thin-walled profiles.

2. Materials and Methodology

2.1. Material Selection and Preparation Process of Thin-Walled Profiles

According to the different stoichiometric ratios of Mg and Si contents relative to Mg₂Si, in this study, three Al-Mg-Si cast rods with different primary alloy compositions are designed: 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) cast rods. The Mg and Si contents of the 6xxx-1(1.6Mg/Si) cast rod approximately satisfy the stoichiometric ratio of Mg₂Si, the Si content of 6xxx-2(1.33Mg/Si) cast rods exceeds the stoichiometric ratio of Mg₂Si, and 6xxx-3(1.14Mg/Si) cast rod contents are comparable Mg and Si contents. The elemental contents of Fe, Mn, Cr, and Cu of the three cast rod alloys are approximated as Fe ≈ 0.18%, Mn ≈ 0.15%, Cr ≈ 0.01%, and Cu ≈ 0.20%, and the specific chemical compositions of the three cast rod alloys are shown in

Table 1. Chemical compositions of the three cast rod alloys (wt. %).

Serial Number	Si	Fe	Cu	Mn	Mg	Cr	Al	Mg/Si	Mg + Si
6xxx-1(1.6Mg/Si)	0.55	0.17	0.20	0.14	0.90	0.01	Bal.	1.60	1.45
6xxx-2(1.33Mg/Si)	0.81	0.18	0.21	0.15	1.08	0.01	Bal.	1.33	1.85
6xxx-3(1.14Mg/Si)	0.59	0.19	0.20	0.15	0.67	0.01	Bal.	1.14	1.26

.

The preparation process of the thin-walled profiles is as follows: first, the homogenized Al-Mg-Si cast rod is heated to 520 °C and then is extruded through an extruder into a double-cell rectangular thin-walled profile as shown in Figure 1, where the extrusion speed is 4 ± 0.2 m/min and the extrusion outlet temperature is 550 ± 5 °C. After completing the extrusion, the thin-walled profile is quenched by water at the outlet, after which the thin-walled profile is moved into the aging furnace for artificial aging at 175 °C/10 h (under-aged), 175 °C/16 h (near-peak-aged), and 205 °C/6 h (over-aged), respectively. The thin-walled profiles corresponding to the 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) alloy cast rods are designated as the 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) profiles, respectively.

In aging treatment, the determination of heating time and heating temperature mainly depends on the following two aspects:

(1)

A large number of engineering experiments. Our team has conducted a great number of engineering experiments on the 7xxx series thin-walled aluminum alloys. It was found that with under 175 °C/10 h (under-aging), 175 °C/16 h (near-peak aging), and 205 °C/6 h (over-aging), the changes in crushing performance of 7xxx thin-walled aluminum alloy were most obvious. Therefore, 6xxx-series wall aluminum alloy also adopts the same failure system;

(2)

A large number of literature references [24,25,26]. Maurício et al. [11] studied AA6351 aluminum alloy and found that the fatigue strength of AA6351 aluminum alloy reaches its peak at 170 °C and an aging time of 6 h. Iswanto et al. [27] artificially aged A350a aluminum alloy for 2 h at 175 °C and 205 °C, respectively. The results showed that artificial aging treatment at 175 °C can improve the fatigue life and performance of aluminum alloys.

2.2. Method of Measurement

The tensile properties of thin-walled Al-Mg-Si alloy profiles were tested using the WDW-300S universal testing machine (Virtual Expo, Marseille, France), in which the tensile specimens were cut out along the extrusion direction with a thickness of 2.8 mm. The dimensions of the tensile specimen are shown in Figure 2. Subsequently, the fracture surface of the tensile specimen was manually sawed off, and the tensile fracture micromorphology was observed using a Prisma E scanning electron microscope (Tescan Brno, Brno-Kohoutovice, Czech Republic).

The bending properties of thin-walled Al-Mg-Si alloy profiles were tested using the WD-20B universal testing machine (Instron Limited, High Wycombe, UK). The bending specimens were 60 mm in length, 30 mm in width, and 2.8 mm in thickness.

The WAW-100FC (Shenzhen Wance Testing Machine Co., Shenzhen, China) single-space hydraulic universal testing machine was utilized to test the crush performance of the thin-walled Al-Mg-Si alloy profiles, with a downward speed of the press of 100 mm/min and a termination displacement of 200 mm. Figure 3 shows the schematic of the crush-performance test.

Thereafter, metallographic tests were performed on the thin-walled Al-Mg-Si alloy profiles. Figure 4 shows the location of metallographic sampling. First, the metallographic samples were sanded and polished in order. Second, a reagent with a volume ratio of HF:HCL:HNO₃:H₂O = 1:1.5:2.5:95 was chosen to corrode the sample, with a corrosion time of 30 S. Third, after the corrosion process, the sample was rinsed with water. Finally, the rinsed samples were placed under the VHX-7000 metallographic microscope (Japan Keyence Co., Akishima-shi, Japan) for observation.

The number and size of particles in the second phase of the Al-Mg-Si alloy thin-walled profiles were observed using a Prisma E scanning electron microscope, and the second-phase particles were analyzed by energy spectroscopy using the Oxford EDS-type energy spectrometer, which is a component of the scanning electron microscope.

Transmission samples with a length of ~5 μm and thickness of ~45 nm were prepared using the focused ion beam technique, and the microstructures of the Al-Mg-Si alloy thin-walled profiles were observed using a Tecnai G2F20 transmission electron microscope (Thermo Fisher Scientific, Middlesex County, MA, USA) at an accelerating voltage of 200 KV.

3. Results

3.1. Crush-Performance Analysis

3.1.1. Effect of Alloy Composition on Crushing Properties

Figure 5 shows the macro-morphology of the three Al-Mg-Si alloy thin-walled profiles in the near-peak-aged state after axial crushing. Different profile specimens show different severities of cracking, with the most severe cracking occurring in the 6xxx-2(1.33Mg/Si) and 6xxx-3(1.14Mg/Si) alloy profiles.

Figure 6 shows the displacement-load curves and crushing energy-absorption curves during axial crushing of the three Al-Mg-Si alloy thin-walled profiles in the near-peak-aged state. Figure 6a shows the displacement-load curve. The peak load of the 6xxx-2(1.33Mg/Si) profile was the largest at approximately 452 KN, and the peak load of the 6xxx-3(1.14Mg/Si) profile was the smallest at approximately 384 KN. This is because all three profiles have different extents of cracking, among which the 6xxx-2(1.33Mg/Si) profile is the most severely cracked, such that the load of the 6xxx-2(1.33Mg/Si) profile decreased rapidly after the peak load. Figure 6b shows the crushing energy-absorption curves, and the three profiles are ranked in the order of crushing energy absorption: 6xxx-1(1.6Mg/Si) > 6xxx-3(1.14Mg/Si) > 6xxx-2(1.33Mg/Si). Note that in practice, material buildup as a result of crack formation in energy-absorbing components can damage adjacent automotive components, thereby losing the energy absorption and protection capacities. Therefore, energy absorption due to material buildup is ineffective and cannot be considered in the total energy absorption of the profile. In the load-displacement curves and energy-absorption curves, curves of the profiles after cracking are marked with dashed lines, and only the solid line portion shall be considered in the evaluation of the final crush performance.

3.1.2. Effect of Aging Process on Crushing Properties

Figure 7 shows the displacement-load curves and crushing energy-absorption curves during axial crushing of the three Al-Mg-Si alloy thin-walled profiles in different aging states. The displacement value and duration at the first peak of the three specimen profiles of the displacement-load curve are close to each other, with only the peak size being different, after which the load peak gradually becomes disordered. In addition, all profiles have a high initial peak load, which reflects the ultimate-limit of structural load-bearing capacity of the thin-walled profiles. As shown in

Table 2. Axial crushing properties of the three types of Al-Mg-Si alloy thin-walled profiles in different aging states.

Profile Number	Aging Process	Peak Load (KN)	Total Energy Absorption during Crushing (KJ)	Effective Energy Absorption during Crushing (KJ)	Cracked/Not Cracked
6xxx-1(1.6Mg/Si)	175 °C/10 h	362.5	31.02	22.21	Yes
	175 °C/16 h	402.75	30.9	16.55	Yes
	205 °C/6 h	380.3	30.61	30.61	No
6xxx-2(1.33Mg/Si)	175 °C/10 h	425	18.93	9.55	Yes
	175 °C/16 h	449.95	16.17	5.28	Yes
	205 °C/6 h	397.75	21.88	21.88	No
6xxx-3(1.14Mg/Si)	175 °C/10 h	312.45	25.43	13.82	Yes
	175 °C/16 h	383.2	27.26	10.87	Yes
	205 °C/6 h	358.85	28.19	28.19	No

, the peak loads of profiles 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) in the near-peak-aged state are 402.75, 449.95, and 383.2 KN, respectively, which are greater than values in the under-aged and over-aged states. The peak load change pattern of the profiles is the same as the change pattern of the mechanical properties, from which it can be presumed that mechanical properties are important factors affecting the peak load. In addition, the aging process has a significant effect on the final energy absorption of the profiles. If crack formation is not considered, the total crushing energy absorptions of the profiles are similar for the three aging processes. However, if only the effective crushing energy absorption is calculated, the under-aged and near-peaked-aged states would have significantly lower values than the over-aged state. Compared with profiles in the under-aged and near-peak-aged states, the effective crushing energy absorptions of profile 6xxx-1(1.6Mg/Si) in the over-aged state are higher by 35% and 81.8%, respectively; the effective crushing energy absorptions of profile 6xxx-2(1.33Mg/Si) in the over-aged state are higher by 129.1% and 314.3%, respectively; and the effective crushing energy absorptions of profile 6xxx-3(1.14Mg/Si) in the over-aged state are higher by 47.4% and 87.2%, respectively.

From Figure 7 and Table 2, the surface of profile of 6xxx-1(1.6Mg/Si) in the over-aged state is undamaged, and no crack formation is observed; the surface of profile 6xxx-3(1.14Mg/Si) in the over-aged state cracked slightly, but it does not have a significant effect on the applied load, whereas for profile 6xxx-2(1.33Mg/Si), cracks formed along the direction of the plastic hinge during compressive deformation for all aging states, which leads to a significant reduction in the applied load. In summary, in the over-aged state, profile 6xxx-1(1.6Mg/Si) has the best crush performance and absorbs the highest amount of energy during deformation.

3.2. Metallographic Analysis

Figure 8 shows the metallographic structure of the three Al-Mg-Si alloy thin-walled profiles in the near-peak-aged state. The structures of the three profiles primarily comprise equiaxed grains, in which the 6xxx-2(1.33Mg/Si) profile has the largest grain size, the 6xxx-1(1.6Mg/Si) profile has the smallest grain size, and the 6xxx-3(1.14Mg/Si) profile has a grain size between the two. As shown in

Table 3. Average grain size in the near-peak-aged state of the three kinds of Al-Mg-Si alloy thin-walled profiles.

Profile Number	6xxx-1(1.6Mg/Si)	6xxx-2(1.33Mg/Si)	6xxx-3(1.14Mg/Si)
Average grain size (µm)	63.6	154.9	92.1

, the three profile types have average grain sizes of 63.6, 154.9, and 92.1 μm, respectively. Compared to profile 6xxx-1(1.6Mg/Si), the average grain sizes of profiles 6xxx-2(1.33Mg/Si) and 6xxx-3(1.14Mg/Si) are higher by 143.5% and 44.8%, respectively. The aging process has no effect on the grain structure of the profile [29], from which it can be concluded that an increase in the total Mg + Si content increases the grain size.

In addition, material ductility is inversely proportional to grain size, where the smaller the grain size, the more ductile the material [30,31]. Therefore, the 6xxx-1(1.6Mg/Si) and 6xxx-3(1.14Mg/Si) profiles showed good crushing properties with only minor crack formations on the surface during compressive deformation, while the 6xxx-2(1.33Mg/Si) profile has a large grain size and poorer ductility, and hence it is more susceptible to cracking during the deformation process.

3.3. Tensile Property Analysis

3.3.1. Effects of Alloy Composition on Tensile Properties

Table 4. Tensile properties of the three Al-Mg-Si alloy thin-walled profiles (at near-peak-aged state).

Profile Number	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
6xxx-1(1.6Mg/Si)	294	305	11.1
6xxx-2(1.33Mg/Si)	355	367	8.2
6xxx-3(1.14Mg/Si)	273	288	11.2

shows the tensile properties of the three Al-Mg-Si alloy thin-walled profiles in the near-peak-aged state. With the same aging process, the tensile properties differ significantly between profiles with different primary alloy compositions, of which the yield and tensile strengths of the 6xxx-2(1.33Mg/Si) profile are the largest at 367 MPa and 355 MPa, respectively; the yield and tensile strengths of the 6xxx-3(1.14Mg/Si) profile are smallest at 288 MPa and 273 MPa, respectively. The Mg content of the 6xxx-1(1.6Mg/Si) and 6xxx-2(1.33Mg/Si) profiles are roughly the same, with a relatively large difference in Si content. Therefore, an appropriate increase in the Si content could increase the strength of the profile. The Si content of the 6xxx-1(1.6Mg/Si) and 6xxx-3(1.14Mg/Si) profiles are roughly the same, with a relatively large difference in the Mg content. Therefore, an appropriate increase in Mg content can also increase the strength of the profile. In addition, note that an increase in Si content leads to a significant decrease in profile elongation, whereas the Mg content has a slight effect on elongation.

3.3.2. Effects of Aging Process on Tensile Properties

Table 5. Tensile properties of the three Al-Mg-Si alloy thin-walled profiles with different aging states.

Profile Number	Aging Process	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
6xxx-1	175 °C/10 h	252	270	15.3
	175 °C/16 h	294	305	11.1
	205 °C/6 h	273	287	14.6
6xxx-2	175 °C/10 h	342	355	11.7
	175 °C/16 h	355	367	8.2
	205 °C/6 h	302	317	11.1
6xxx-3	175 °C/10 h	232	272	13.3
	175 °C/16 h	273	288	11.2
	205 °C/6 h	258	272	13.7

shows the tensile properties of the three Al-Mg-Si alloy thin-walled profiles in different aging states. With different aging processes and treatments, the tensile properties of the three profiles differ significantly. After near-peak-aging processing, both the yield and tensile strengths are at the maximum, and the elongation during fracture is at the minimum. Compared with profiles in the near-peak-aged state, the tensile strength of the 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) profiles in the under-aged state decreased by 11.4, 3.3, and 5.6%, respectively; the yield strengths decreased by 14.3, 3.7, and 15%, respectively; and elongation after fracture increased by 37.8, 42.7, and 18.8%, respectively. The tensile strengths of the 6xxx-1(1.6Mg/Si), 6xxx-2(1.33Mg/Si), and 6xxx-3(1.14Mg/Si) profiles in the over-aged state decreased by 5.9, 13.6, and 5.6%, respectively; the yield strengths decreased by 7.1, 14.9, and 5.5%, respectively; and the elongation after fracture increased by 31.5, 35.4, and 22.3%, respectively.

3.3.3. Effect of Aging Process on Tensile Fracture Morphology

Figure 9 shows the tensile fracture morphology of the three Al-Mg-Si alloy thin-walled profiles with different aging states. The fracture mode of the three profiles under different aging states is a combination of intergranular fracture and dimpled transgranular fracture, and a large number of dimples are observed on the tensile fracture surface, which suggests that a ductile fracture occurred in all three profiles. Figure 9a,d,g show the tensile fracture surface of the three profiles in the under-aged state, Figure 9b,e,h show the tensile fracture surface of the three profiles in the near-peak-aged state, and Figure 9c,f,i show the tensile fracture surface of the three profiles in the over-aged state. As the 6xxx-2(1.33Mg/Si) profile has the highest strength, it has the lowest elongation after fracture. Compared with the other two profiles, the 6xxx-2(1.33Mg/Si) profile in all three aging states has the lowest number of dimples on the tensile fracture surface and the highest proportion of intergranular fracture modes, with the largest intergranular section. As shown in Figure 9a,g, profiles 6xxx-1(1.6Mg/Si) and 6xxx-3(1.14Mg/Si) in the under-aged state have similar numbers of dimples on the fracture surface, and the dimples are similar in size and depth; however, the 6xxx-3(1.14Mg/Si) profile has a larger intergranular section, resulting in a slightly lower elongation after fracture for that profile. In addition, the number of dimples in the over-aged state is higher for profile 6xxx-1(1.6Mg/Si) than for profile 6xxx-3(1.14Mg/Si), and the proportion of intergranular fracture modes of profile 6xxx-1(1.6Mg/Si) is lower than profile 6xxx-3(1.14Mg/Si), resulting in a higher elongation after fracture. By comparing the tensile fracture surface morphology of the three profiles in the under-aged, near-peak-aged, and over-aged states, the intergranular fracture sections can be found in the tensile fracture surfaces of all profiles, but the proportions vary. Compared with profiles in the under-aged and over-aged states, profiles in the near-peak-aged state have the least number of dimples and the highest proportion of intergranular fracture sections, whereas the over-aged and under-aged profiles have comparable numbers of dimples with similar proportions of intergranular fracture sections. This indicates that ductility is better for profiles in the under-aged and over-aged states compared to the near-peak-aged state.

In summary, the fracture surface morphologies of the three profiles in different aging states are different, but the difference is not significant, which is consistent with the results of elongation after fracture obtained from the experiments.

3.4. Bending-Performance Analysis

The bending results of the three profiles are all compared using an equivalent 2 mm bending angle, which is calculated as follows.

$${\alpha }_2={\alpha }_1·\left(\sqrt{\frac{d_1}{2}}\right)$$

Here, ${\alpha }_2$ is the equivalent 2 mm thick bending angle, ${\alpha }_1$ is the measured bending angle, and d₁ is the material thickness.

Figure 10 shows the bending macro-morphology of the three Al-Mg-Si alloy thin-walled profiles at different aging states. The bending effect of the 6xxx-2(1.33Mg/Si) profile in the near-peak-aged state is the worst, where cracks are visible to the naked eye and have penetrated through the surface of the profile. The three profiles have small cracks in the under-aged state, but the cracks are not connected. The surfaces of profiles in the over-aged state have some deformation texture, but macroscopic cracks cannot be observed with the naked eye.

Table 6. Bending properties of the three Al-Mg-Si alloy thin-walled profiles in different aging states.

Profile Number	Aging Process	Specimen Thickness d1 (mm)	Measured Bending Angle α1 (°)	2 mm Equivalent Bending Angle α2 (°)
6xxx-1(1.6Mg/Si)	175 °C/10 h	2.66	96	81
	175 °C/16 h	2.66	91	77
	205 °C/6 h	2.65	83	102
6xxx-2(1.33Mg/Si)	175 °C/10 h	2.65	51	59
	175 °C/16 h	2.65	36	45
	205 °C/6 h	2.66	60	69
6xxx-3(1.14Mg/Si)	175 °C/10 h	2.64	46	57
	175 °C/16 h	2.64	47	54
	205 °C/6 h	2.66	48	60

shows the bending properties of the three Al-Mg-Si alloy thin-walled profiles in different aging states, and the equivalent bending angle of the three profiles in the over-aged state is the largest, indicating that profiles in the over-aged state have better toughness. Compared to profiles in the near-peak-aged state, the equivalent bending angles of profile 6xxx-1(1.6Mg/Si) in the under-aged and over-aged states are higher by 5.1% and 32.4%, respectively; the equivalent bending angles of profile 6xxx-2(1.33Mg/Si) in the under-aged and over-aged states are higher by 31.1% and 53.3%, respectively; and the equivalent bending angles of profile 6xxx-3(1.14Mg/Si) in the under-aged and over-aged states are higher by 5.5% and 11.1%, respectively.

3.5. Second-Phase Particle Precipitation

Figure 11 shows the SEM images of the second-phase particles of the three Al-Mg-Si alloy thin-walled profiles in different aging states.

Table 7. Chemical composition of the second-phase particles (wt%) of the three Al-Mg-Si alloy thin-walled profiles.

Placement	Al	Mg	Cu	Fe	Mn	Si
a1	88.03	0.69	0.49	6.66	1.23	2.88
a2	86.50	0.69	0.38	7.07	1.96	3.40
d1	67.82	--	--	21.83	4.22	6.12
d2	83.09	0.43	--	11.38	1.29	3.81
f1	65.07	--	1.01	19.70	7.22	6.99
f2	74.35	0.36	0.63	14.17	5.08	5.41
g1	76.29	--	--	20.23	0.91	6.47
g2	76.51	--	--	16.52	0.86	5.74

and

Table 8. Area fractions of the second phase for the three Al-Mg-Si alloy thin-walled profiles.

Profile Number	Aging Process	Area Fraction of Second Phase (%)
6xxx-1(1.6Mg/Si)	175 °C/10 h	1.74
	175 °C/16 h	1.66
	205 °C/6 h	1.71
6xxx-2(1.33Mg/Si)	175 °C/10 h	2.02
	175 °C/16 h	1.85
	205 °C/6 h	1.96
6xxx-3(1.14Mg/Si)	175 °C/10 h	1.58
	175 °C/16 h	1.45
	205 °C/6 h	1.48

, respectively, show the chemical composition and area fraction of second-phase particles of the different alloy thin-walled profiles. There are some undissolved second-phase particles in all three types of Al-Mg-Si alloy thin-walled profiles, of which the coarser second-phase particles mainly comprise elements such as Fe and Si. In addition, the alloy composition and aging process have almost no effect on the area fraction of the second phase, and the area fraction of the second phase of the three profiles with different aging processes are similar, with the 6xxx-2(1.33Mg/Si) profile having a higher number of primary alloying elements having a slightly higher area fraction of the second phase. Therefore, it can be concluded that differences in the properties of the three profiles are independent of second-phase particles.

Different aging processes of the profiles result in the precipitation of different types, sizes, quantities, and distributions of strengthening phases. Usually, two types of precipitates can be observed in the Al-Mg-Si alloy matrix after near-peak aging treatment: (1) a fine needle-shaped precipitate, which is distributed across in the matrix and (2) a point-like precipitate. Dislocations require a higher amount of energy to bypass the fine precipitation phases, thereby increasing the strength of profiles in the near-peak-aged state. Compared with profiles in the near-peak-aged state, precipitation phases of profiles in the over-aged state undergo significant coarsening, and dislocations require less energy to bypass the coarsened strengthening phases than finer strengthening phases, reducing the profile strength.

Figure 12 shows the TEM images of intragranular precipitation of the three Al-Mg-Si alloy thin-walled profiles in different aging states. Numerous spherical and needle-shaped precipitates are distributed within the grains of all three profiles, most of which are the ${\beta }^{″}$ phase. The needle-shaped phase is the primary strengthening phase of the Al-Mg-Si alloys; it improves the profile strength by hindering dislocation motions and has the best strengthening effects, while the coarse spherical precipitation phase is the Q phase, and its strengthening effect is weak.

A two-by-two comparison of the three profiles reveals that profile 6xxx-2(1.33Mg/Si) has the largest number of ${\beta }^{″}$ phases, profile 6xxx-1(1.6Mg/Si) has the next largest number of ${\beta }^{″}$ phases, and profile 6xxx-3(1.14Mg/Si) has the lowest number of ${\beta }^{″}$ phases. Relevant literature shows that the ${\beta }^{″}$ phase is the best particle for hindering dislocations in Al-Mg-Si alloys [11,26]. Among the three profiles, the 6xxx-2(1.33Mg/Si) profile has the highest total Mg + Si content. Because the increase in total Mg + Si content promotes the precipitation of strengthening phases in the profile after aging treatment, the 6xxx-2(1.33Mg/Si) profile would have the largest number of ${\beta }^{″}$ phases and the highest strength, but the lowest elongation. By comparing the precipitation between the three aging states, it can be found that compared with the under-aged and over-aged states, the amount of ${\beta }^{″}$ phase precipitation of the three profiles in the near-peak-aged state is significantly higher; therefore, the profile strength is significantly improved, but with some reduction in ductility.

Figure 13 shows the images of grain boundary precipitation of the 6xxx-1(1.6Mg/Si) profile in different aging states. The width of the precipitate free zone (PFZ) at the grain boundary in the near-peak-aged state is the widest at 128.8 nm, the PFZ in the under-aged state is the next widest at 118.6 nm, and the PFZ in the over-aged state is the narrowest at 111.9 nm. In the near-peak-aged state, large numbers of strengthening phases precipitated from the profile crystal, resulting in the solute atoms being largely consumed and reducing the solubility of solute atoms near the grain boundaries. This leads to the formation of a solute atom “barren zone” on both sides of the grain boundary, that is, the formation of a wider PFZ. Compared to the near-peak-aged state, the number of intragranular strengthening phases are lower for the over-aged and under-aged states, fewer solute atoms are consumed, and the solute atom solubility increases on both sides of the grain boundaries, thereby decreasing the PFZ width. Note that an increase in PFZ width weakens the grain boundary strength and increases the difference between intragranular strength and grain boundary strength, which in turn is unfavorable for ductility.

In summary, the effects of profile composition and aging process on strength and crushing energy absorption are mainly attributed to differences in grain structure and precipitation phases. Coarse grain structures and wide grain boundary PFZs decrease the ductility of the profile. Simultaneously, the number of primary strengthening phases in the grain increases, thereby increasing the profile strength, which in turn results in the profile being unable to achieve a better strength–ductility match, reducing the crush performance. For instance, in the near-peak-aged state, the 6xxx-2(1.33Mg/Si) profile has the highest number of primary strengthening phases in the crystal, which give it the highest strength; however, the profile grains are coarse, and grain boundary PFZs are wide, which reduces the ability of coordinated deformation between grains during compressive deformation and results in the profile being prone to plugging dislocations at the grain boundaries, leading to microcrack formation and eventual fracture, as well as reducing the ductility. Consequently, the 6xxx-2(1.33Mg/Si) profile in the near-peak-aged state fails to achieve a good strength–ductility match and has the lowest crush performance, where severe crack formation occurs during the crush deformation. In the over-aged state, the number of primary strengthening phases in the 6xxx-1(1.6Mg/Si) profile grains is lower and coarser compared to profiles in the near-peak-aged state, which results in decreased strength; however, the reduced width of the grain boundary PFZ and the finer grain size improves ductility and reduces the strength difference between the grain and grain boundaries, and the resulting profile can achieve a better strength–ductility match. Consequently, the aging state of the 6xxx-1(1.6Mg/Si) profile has excellent crushing properties, with no cracking or only a few minor cracks forming with no further expansion during the crushing deformation process.

4. Conclusions

(1)

Compared with the 6xxx-1(1.6Mg/Si) profile, the 6xxx-2(1.33Mg/Si) and 6xxx-3(1.14Mg/Si) profiles have the most severe cracking. The crushing energy absorption of the three Al-Mg-Si alloy thin-walled profiles arranged in order from highest to lowest are: 6xxx-1(1.6Mg/Si) > 6xxx-3(1.14Mg/Si) > 6xxx-2(1.33Mg/Si), indicating that if the Mg + Si contents are too high or too low, the crush performance of the alloy thin-walled profiles will decrease.

(2)

As the total Mg + Si content increases from 1.26% to 1.89%, the yield strength and tensile strength of the profiles increase from 273 MPa and 288 MPa to 355 MPa and 367 MPa, respectively, and the elongation decreases from 11.2% to 8.2%. This this is because the increase in the total Mg + Si content increases the number of intragranular ${\beta }^{″}$ phases in Al-Mg-Si alloy thin-walled profiles, and large numbers of ${\beta }^{″}$ phases hinder grain dislocation, increasing the profile strength. However, with the increase in the total Mg + Si content, the grain size is coarser and the grain boundary PFZs are wider, thereby reducing ductility.

(3)

Compared with the under-aged and near-peak-aged states, the three Al-Mg-Si alloy thin-walled profiles in the over-aged state have increased bending angles and effective crushing energy absorption, suggesting that profiles in the over-aged state have better toughness and crush performance.

(2)

The yield strength and tensile strength of the three Al-Mg-Si alloy thin-walled profiles increased significantly by near-peak aging treatment. This is because of the significant increase of ${\beta }^{″}$ phase precipitation in the near-peak-aged state of the three profiles.