Investigation on effect of high-efficiency solid solution and hot stamping process on microstructure evolution and mechanical properties of high-strength aluminum alloy

Aluminum alloy has become an important lightweight material in the automotive industry due to its excellent performance. The development of hot stamping technology has effectively promoted the application of high-strength aluminum alloys in auto parts. However, aluminum alloy hot stamping requires a long time for solid solution and aging heat treatment, which hinders its mass production application in the existing mature hot stamping production line. Therefore, this paper studies the effect of a high-efficiency solid solution and hot stamping process on the microstructure and mechanical property strengthening mechanism of high-strength aluminum alloy. The contact heating and hot stamping experiment device was designed. It was found that the temperature rise rate of the sheet during the contact heating process could reach 44.51 °C/s. The experiment results show that contact heating treatment with a solution temperature of 480 °C and a solution time of 20s can achieve the high-efficient solution treatment of the sheet metal. The high-temperature deformation behavior of 7075-T6 at different temperatures (300-450 °C) and different strain rates (0.01∼1/s) under the condition of high-efficiency solid solution rapid heating was studied.


Introduction
Although aluminum alloy has the advantages of high specific strength, corrosion resistance, and recyclability, its formability is poor at room temperature due to high deformation resistance and obvious springback.To effectively enhance formability and reduce springback in high-strength aluminum alloy sheets, researchers proposed the use of hot forming and quenching technology [1].Zheng et al. [2] investigated the effects of transferring aluminum alloy sheets from a heating furnace to the die stage on the mechanical properties of the final part during the hot stamping.Their analysis focused on the temperature-time-properties correlation and the reason for the decline in material performance.Omer et al. [3] conducted a study on the hot stamping process of AA7075 aluminum alloy and highlighted that a cooling rate no less than 56 o C/s is required when quenching the aluminum alloy after solid solution to achieve an optimal solid solution effect.
The hot stamping process enables the use of high-strength aluminum alloy sheets for complexstructured automobile parts.Nevertheless, the lengthy solid solution and aging heat treatment of aluminum alloy pose a hindrance to its mass production application in the current mature hot stamping production line.The processes of hot stamping and quenching occur simultaneously, whereas the solid solution and artificial aging treatments require extensive time, with the aging process particularly requiring several hours or more.The incongruity between the processes makes synchronization challenging, while the lengthy heat treatment results in high energy consumption, contradicting the "double carbon" objective.
During the aluminum alloy hot stamping process, a heating furnace is utilized for solution treatment, with the sheet being heated primarily through radiation heat transfer.A smooth aluminum alloy sheet has an emissivity of approximately 0.09, while a sheet with a rough surface has an emissivity of around 0.28.Due to the lower emissivity, the heating rate during the radiation heating is significantly slowed.To enhance the heating rate of sheet metal, Liu et al. [4] investigated the impact of lubricant spraying on the radiation heating of 6061 and 7075 aluminum alloy sheets.They discovered that applying lubricant enhanced the surface condition, resulting in reduced time required to heat up the sheet during solution treatment.They indicated that spraying boron nitride (BN) or graphite lubricant can lessen the solid solution time to less than 50% of the initial value.Xu et al. [5] employed electric pulses to conduct solution treatment on 7075 aluminum alloy.Their findings indicate that the supersaturation was less than that of traditional solution treatment, despite taking only 220 ms, resulting in smaller second-phase particles and grain sizes after aging.Furthermore, the mechanical properties of the samples are superior to those of regular T6 aluminum alloys.Shao et al. [6] utilized a jet flame arranged in a matrix pattern to perform the solid solution treatment on a thin sheet of AA6082 with a thickness of 1.5 mm.Their experimentation observed that the sheet's heating rate exceeded 10 o C/s.However, they also found that the temperature uniformity was inadequate, and proposed that the configuration of the flame nozzles be arranged reasonably to enhance the temperature distribution across the sheet.Our research indicates that using the contact heating process on a 7075 sheet with a thickness of 2 mm, can achieve solid solution temperature (480 o C) within 15 to 20 seconds [7].
The contact heating method offers high heating efficiency and shape flexibility, making it a lowcost and adaptable option for rapidly heating sheet metals.However, the impact of the increased heating velocity on high-temperature formability of the sheet warrants further investigation.Additionally, the efficacy of short-term heating in ensuring a fully solid-solution of the aluminum alloy sheet and its consequent effects on the material's final properties deserves scrutiny.

Experimental Material
The 7075-T6 aluminum alloy, with a thickness of 2 mm, was utilized in this study.The chemical composition of the material can be found in Table 2.1.The sheet was cut into 300 × 150 mm specimens to facilitate experiments.

High-temperature Tensile Test
The Gleeble-3500 thermal simulation testing machine was used to simulate the temperature history of the contact heating and hot stamping process.High-temperature tensile experiments were then conducted on the heated samples at different strain rates and temperatures.The aim was to analyze the material's behavior during high-temperature deformation under rapid heating conditions.Figure 1 High-temperature tensile experiment flow chart To replicate the pressure of the contact heating device on the sheet during sample heating, a constant pressure of approximately 564 N (determined by the pressure of the contact heating device) is applied to the sample via the testing machine's chuck, directed toward the center of the sample.However, due to the sheet specimen's limited stiffness and susceptibility to instability and warping when pressure is applied at both ends, an additional fixture was developed to prevent this occurrence, as illustrated in Figure 2a. Figure 2b displays the actual fixture used.A heat-insulating cloth was placed between the clamp and the sample to hinder heat transfer and decrease friction.It is important to avoid clamping the clamp too tightly as it may lead to excessive force, which can ultimately impact the accuracy of the later tensile experiment.

Contact heating-hot stamping-aging Test
The designed contact heating device is shown in Figure 3a and 3b.The experimental device's main body is comprised of two thick metal heating blocks made of QRO 90 Supreme hot work die steel.This material possesses high thermal conductivity, which improves temperature uniformity in contact heating devices.Insulation material surrounds the heating block, and a pressure-resistant insulation board is placed underneath to minimize heat loss and enhance the temperature uniformity within the heating block.The heating rods and K-type thermocouples are methodically positioned inside the block to facilitate temperature adjustment feedback.The total surface area of the heating block is 380 × 290 square millimeters.However, in high-temperature conditions, heat dissipates quickly in the surrounding area of the device, resulting in lower temperature in that area compared to the central area.As a result, the sheet should be placed in the middle area of the heating block when undergoing contact heating.Figure 3c shows the temperature distribution of the surface when the heating temperature is set to 480 oC.Research was conducted on the heating process of aluminum alloy sheets at various pressures (100 kN, 500 kN, 1000 kN) and under different heating temperatures of the contact heating device.The rate of the aluminum alloy sheet heating during contact heating was calculated.
Then, the contact heating-hot stamping experiments, as depicted in Figure 4, were carried out through the following steps: (a) Heating the contact heating apparatus up to the desired temperature of 480 o C and holding it steady for a period to obtain a uniform temperature distribution across the apparatus.(b) Laying the aluminum alloy sheet onto the heated apparatus and close it while simultaneously exerting pressure for 20 seconds to complete solution treatment of the sheet.(c) Transfer the heated aluminum alloy sheet to the cold U-shaped die, then form, quench, and hold for 20 seconds.(d) Transfer the formed parts to a radiation heating furnace for artificial aging treatment at 120 o C/24 h.This will precipitate the second-phase particles in the material to strengthen the parts.
Finally, samples for microstructure observation were processed from the bottom of the formed Ushaped part.The microstructure was then observed using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), while the recrystallization of materials was analyzed through SEM's backscatter diffraction (Electron Backscatter Diffraction, EBSD) function.

Plastic deformation behavior of 7075 under high-temperature conditions
The true stress-strain curves for 7075 aluminum alloy were obtained by stretched it at different temperatures and different strain rates after rapid heating.The obtained curves are shown in Figure 6.As the plastic strain increases during the deformation process, the material's true stress increment is negligible.Of course, when the deformation rate of the material varies, there are differences in the effects of work hardening and dynamic recovery.When the strain rate is high, the work-hardening effect is noticeably stronger.However, when the material is deformed at a higher temperature (450 o C), even if the deformation rate is high (1 s -1 ), the work-hardening effect is still mostly counterbalanced by the dynamic recovery.
After comparing these results to our prior work under slow heating conditions, we discovered that, even when the deformation temperature and strain rate remain constant, the flow stress of the samples subjected to rapid heating conditions increases to a certain extent.Moreover, the lower the deformation temperature, the greater the rise in flow stress.However, it is evident that both slow and fast heating samples show consistent effects of work hardening and dynamic recovery on the flow stress curve of the material when deformed at varying temperatures and strain rates.

Effect of contact heating process on microstructure and strengthening mechanism of 7075 aluminum alloy
Figure 7 depicts the distribution of recrystallized structure, sub-grain and deformation structure in various regions of the samples and the as-supplied samples.The supplied 7075-T6 aluminum alloy displays a relatively complete degree of recrystallization, and the majority of the grains within its microstructure are recrystallized.Nonetheless, for the materials after contact heating, a substantial proportion of sub-grain and deformed grains are evident in the microstructure.Especially concerning the microstructure of the surface area after contact heating, there is a significant proportion of deformed grains, resulting in a substantial reduction in grain size.Due to the brief contact heating solution treatment time, the deformed grains do not have sufficient time to transform into recrystallized grains, resulting in a large number of deformed grains.These observation suggest that the contact heating process induces considerable plastic deformation in the metal sheet.
After applying contact heating and hot stamping to the sample, we selected 5 positions from the surface of the sample to the center, and observed the microstructure using an optical microscope.The corresponding positions' grain size was counted.In comparison to the supplied sample's microstructure (with an average grain size is 10.85 μm), the material's grain size decreased after contact heating process.The grain size in surface area of the sample was refined.Most obviously, there is a decrease in the average grain size to 6.28 μm in this area.Additionally, the grain refinement effect is lessened in the central area of the sample's thickness direction to yield an average grain size of 8.85 μm.
The statistical results of the grain sizes clearly demonstrate that the contact heating process refines the material grains, resulting in an enhanced grain boundary strengthening effect.Calculations reveal that after contact heating and hot stamping, the sample's grain boundary strengthening effect provides a strength ranging from 40.34 to 47.89 MPa.In comparison, the supplied sample's grain boundary strengthening effect is only 36.43 MPa.Through calculations, it was determined that the contact heating-hot stamping sample has a dislocation density of about 1.904×10 13 m -2 , contributing to a mechanical property enhancement of 20.54 MPa through dislocation strengthening.Similarly, for the supplied T6 state sample, the dislocation density was calculated to be around 1.349×10 13 m -2 , with a dislocation strengthening contribution of 17.29 MPa.After contact heating and hot stamping, the dislocation density in the sample increased slightly increased.However, its enhancement of the dislocation strengthening effect was limited.The occurrence of this phenomenon is believed to be due to the recovery of the contactheated sample during the longer artificial aging process, resulting in a decrease in the dislocation density in the material.

Conclusion
The high-efficiency solid solution treatment for aluminum alloy sheets during the hot stamping process was achieved using the contact heating method.Experimental verification confirmed the feasibility of high-efficiency solid solution.The study investigated the high-temperature deformation behavior of aluminum alloy under rapid heating conditions.Additionally, the study examined the impact of the contact heating and hot stamping process on the microstructure and mechanical properties of the material.
The study examined the high-temperature deformation behavior of the 7075 aluminum alloy under rapid heating conditions at various temperatures (300-450 o C) and strain rates (0.01~1 s -1 ).It was determined that at lower deformation temperature (300 o C), the material's work hardening dominates, resulting in a significant increase in true stress with increasing plastic strain.Conversely, at higher deformation temperature (≥400 o C), the material's work hardening and dynamic recovery are essentially in equilibrium.The increase in true stress with plastic strain is minimal under these conditions.
A contact heating-hot stamping experimental device was developed to analyze the appropriate contact heating time for achieving efficient solution treatment of 7075 aluminum alloy sheets.Based on the results of mechanical property testing and microstructure observations, the treatment at 480 o C/20 s complete efficient solid solution process.Parts produced using the contact heating process exhibited higher strength than the supplied T6 material.The analysis demonstrates that the values of grain boundary and dislocation strengthening are 40.34 to 47.89 MPa and 20.54 MPa, respectively.These strengthening effects are superior to the supplied T6 state material, resulting in higher final strength of the parts than the T6 state material strength.The utilization of contact heating, in lieu of a radiant furnace, not only reduces the aluminum alloy solution treatment time but also enhances the final part's performance.
Figure 1 illustrates the test process.The experimental steps included: Heating the sample to 460 o C at a rate of 44.51 o C per second.Continue to heat at a rate of 5 o C per second until the temperature reaches 480 o C. Hold the temperature for 6 seconds to maintain consistency with the contact heating solution treatment process.Following this, proceed to cool the specimens using an air blowing system at a 3 cooling rate of 20 o C/s until they reach the specified tensile test temperatures, including 300 o C, 350 o C, 400 o C, 450 o C.After each cooling stage, stabilize the temperature of the specimens for 2 seconds before conducting any further testing.Finally, the high-temperature tensile experiments should be conducted at stretching rates of 0.01 s -1 , 0.1 s -1 , and 1 s -1 until the sample fractures and then allowed to cool to room temperature.

Figure 3
Figure 3 Physical picture of the contact heating module: (a) upper heating block; (b) lower heating block; (c) surface temperature distribution of the contact heating device

Figure 4
Figure 4 Contact heating-hot stamping experiment flow chart 3. Experimental Results and Analysis 3.1.Analysis of Contact Heating Temperature Rise Process From the temperature rise curve obtained for the aluminum alloy sheet at a heating temperature of 480 o C, Figure 5 displays the change in the calculated heating rate in response to pressure.It is noticeable that an increase in pressure results in an acceleration of the heating rate.At a pressure of 1000 kN, the heating rate can reach up to 44.51 o C/s.The temperature rise curve measurement results indicate that when the contact heating temperature is adjusted to the solid solution temperature of 7075 aluminum alloy (480 o C) and exposed to varying pressures, the aluminum alloy sheets can attain near the target temperature (480±5 o C) within 15 to 20 seconds.

Figure 5
Figure 5 The heating rate of the aluminum alloy sheet changes with pressure when the heating temperature is 480 o C

Figure 6
Figure 6 True stress-plastic strain curves of 7075 aluminum alloy after rapid heating at different temperatures and different strain rates: (a) 300 o C; (b) 350 o C; (c) 400 o C; (d) 450 o C Throughout the process of high-temperature plastic deformation, both the work hardening and dynamic recovery of the material have a simultaneous impact on the flow stress change with plastic strain.At a lower temperature of 300 o C, work hardening dominates, resulting in a significant increases in the material's true stress as plastic strain increase.However, at a higher deformation temperature (≥400 o C), work hardening and dynamic recovery of the material are in a balanced state.As the plastic strain increases during the deformation process, the material's true stress increment is negligible.Of course, when the deformation rate of the material varies, there are differences in the effects of work hardening and dynamic recovery.When the strain rate is high, the work-hardening effect is noticeably stronger.However, when the material is deformed at a higher temperature (450

Figure 7
Figure 7 EBSD observation results of contact heating-hot stamping sample and T6 state sample: recrystallization diagram, KAM diagram, orientation difference angle frequency distribution histogram of (a-c) contact heating-hot stamping sample surface and (d-f) middle area and (g-i) T6 state sample.Through calculations, it was determined that the contact heating-hot stamping sample has a dislocation density of about 1.904×10 13 m -2 , contributing to a mechanical property enhancement of 20.54 MPa through dislocation strengthening.Similarly, for the supplied T6 state sample, the dislocation density was calculated to be around 1.349×10 13 m -2 , with a dislocation strengthening contribution of 17.29 MPa.After contact heating and hot stamping, the dislocation density in the