The influence of pre-strain levels on the microstructure and performance of aircraft 7b04 aluminum alloy skin

The study focuses on the 7B04 alloy sheet. It investigates the effects of different pre-strain levels on the microstructure, mechanical properties, and fatigue behavior of the alloy through the use of optical microscopy, scanning electron microscopy, and DIC (Digital Image Correlation) system. The experimental results demonstrate that the yield strength and tensile strength of the alloy increase with the increase in pre-strain levels. At the same time, the elongation decreases with the increase in tensile deformation. In the non-uniform region, the plastic deformation capacity index of the material (denoted as ψ) follows the order ψ (non-uniform region (DIC value)) > ψ (original gauge length value) > ψ (uniform region (DIC value)). It is recommended to control the maximum local strain below ψ (uniform region (DIC value)) during production to avoid deformation instability or concentration. Furthermore, the thickness of the material decreases linearly with increasing tensile deformation, with each 4% increase in tensile deformation resulting in a 1.2% reduction in thickness. The fatigue life of the alloy increases gradually with the increase in pre-strain levels in the direction perpendicular to the fiber. At the same time, it decreases in the direction parallel to the fiber. Different deformation levels and loading directions exhibit similar fatigue fracture characteristics. Within the range of 12% pre-strain levels, the microstructure of the material remains continuous and intact without noticeable cracks at grain boundaries or phase boundaries. At the macroscopic level, when the pre-strain level exceeds 6%, distinct slip bands appear on the sheet, which is correlated with the micro-scale grain slip. This is accompanied by deformation bands and micro-cracks in the microstructure. The findings of this study are of significant importance for understanding the influence of pre-strain levels on the mechanical properties and fatigue behavior of the 7B04 alloy. The study provides valuable references for engineering practices in related fields, thereby ensuring the integrity and usability of manufactured components.


Introduction
The 7B04 aluminum alloy belongs to the Al-Zn-Mg-Cu series of ultra-high-strength aluminum alloys.It exhibits excellent strength-toughness combination, good corrosion resistance, and fatigue resistance.Additionally, it possesses favorable thermal processing and welding properties, making it widely utilized as a structural material in aerospace, transportation, and military applications [1][2] .In aerospace equipment, the selection of aluminum alloy materials for different components requires precise material selection criteria.
Skin is an important component of aircraft structures, characterized by its large area and thin-walled structure.It serves as a vital component in shaping the aerodynamic profile of the aircraft and functions as a primary load-bearing structure.Therefore, the 7B04 aluminum alloy is widely used in aircraft skin structures.Stretch forming is one of the most commonly used processes for manufacturing skin components [3][4] .Figure 1 illustrates a schematic diagram of convex die stretch forming for skin components.Typically, the sheet material, in an annealed state, is clamped at both ends by the grips of the stretch-forming machine while the forming die rises to make contact with the sheet, generating nonuniform planar tensile strain.This process ensures the conformity between the sheet material and the forming die.Due to the non-uniform tensile stress imposed on the material during the stretch-forming process, the fibers of the material experience uneven amounts of tensile deformation.However, there is limited research on the influence of pre-strain levels during the forming process on the geometric characteristics, mechanical properties, microstructure, and surface micro-topography of the raw material.Therefore, this study focuses on analyzing the changes in the microstructure and properties of 7B04 alloy thin sheets under different pre-strain levels.The aim is to provide a theoretical basis and practical guidance for the skin-forming process.

Experimental materials and apparatus
The material used in this experiment is a 7B04 alloy sheet provided by the company.The sheet is in the annealed state (O temper) with a thickness of 1.5 mm.The chemical composition of the alloy is shown in Table 1.
Table 1 Bal.The stretch-forming process of the experimental material was conducted on the skin stretch-forming machine at Shenyang Aircraft Industry (Group) Co., Ltd.The stretching direction during forming was perpendicular to the material's fiber direction.The pre-strain levels used were 2%, 4%, 6%, 8%, 10%, and 12%, respectively.

Experimental method
On pre-strained specimens, tension specimens were extracted in two directions: perpendicular and parallel to the fibers.They are in accordance with the GB/T228.1-2010standard for room temperature tensile testing.The yield strength, tensile strength, and elongation at fracture of the specimens were measured.The geometric shape and dimensions of the tensile specimens at room temperature are shown in Figure 2.During the room temperature tensile test, a newly developed three-dimensional Digital Image Correlation (DIC) system was employed to detect the micro-scale stress distribution of the specimens.The experiment was conducted using the SUNS-890 servo-hydraulic fatigue testing machine from Sanstar Corporation.The tests were carried out at room temperature with a maximum stress of 400 MPa and a frequency of 10 Hz. Figure 3 illustrates the geometric shape and dimensions of the roomtemperature fatigue specimens.
Tensile samples were taken along the longitudinal section from materials subjected to different prestrain deformations.The metallographic preparation was carried out following the GB/T 3246.1 standard.The longitudinal section of the samples was observed for grain structure using a Zeiss Axio Vert.A1 optical microscope.The distributions of second phases, slip lines, and fatigue fracture surfaces were analyzed using the Thermo Scientific Apreo2C field emission scanning electron microscope.

The influence of pre-strain deformation on microstructure
In this section, we will discuss the impact of pre-strain deformation on the metallographic structure and morphology of the second phase of the samples based on the experimental results.Figure 4 shows the metallographic structure of the 7B04 alloy after different pre-strain deformations, with the length direction of the photograph parallel to the stretching direction.From Figure 4, it can be observed that when the deformation reaches 12%, no apparent cracks are observed at the grain boundaries.Furthermore, with increasing unidirectional pre-strain deformation, the external load continuously increases, and the material begins to undergo plastic deformation.The grains start to elongate gradually along the direction of the pre-strain.However, due to the hindrance of grain boundaries and the influence of different grain orientations, the deformation is not uniform.Some grains experience larger deformation, while others experience smaller deformation.When the deformation reaches 4% (as shown in Figure 4(b)), partial grains are observed to elongate, forming an elongated strip-like shape.When the tensile deformation exceeds 10%, some grains exhibit noticeable refinement and take on an approximately equiaxed elliptical shape.
Under scanning electron microscopy, the second phase particles of the samples under different deformation levels were observed, as shown in Figure 5.With increasing pre-strain deformation, no apparent micro-cracks were formed at the interface between the second phase and the matrix.The morphology and size of the second phase particles underwent significant changes, gradually transitioning from irregular shapes (Figure 5 The experimental results indicate that during room temperature pre-strain deformation of 7B04 aluminum alloy skin, the grain size and second phase in the alloy undergo significant changes with increasing deformation.Initially, the grains undergo elongation, and as the deformation reaches a certain level, grain refinement occurs.This is because, during the pre-strain process, the strain induces dislocation slip and rearrangement within the grains, thereby promoting grain boundary motion and grain refinement.The second phase particles are subjected to stretching, deformation, and even fracture as the deformation increases, leading to their reorientation and causing refinement of the second phase particles [5] .As a result, they become locally distributed more intensively.

The influence of pre-strain deformation on surface topography
In this section, the influence of pre-strain deformation on the surface morphology of sheet metal alloys was investigated.Different levels of pre-strain deformation were applied to the specimens, and the effects on surface morphology were analyzed.The results indicate that the magnitude of pre-strain deformation significantly affects the surface morphology.As the level of pre-strain deformation increases, noticeable changes occur in the surface morphology.
After undergoing different levels of pre-strain deformation, the 7B04 alloy specimens exhibit distinct surface characteristics.When subjected to pre-strain deformation below 6%, the surface appears relatively smooth with minimal roughness.However, as the pre-strain deformation reaches or exceeds 6%, significant changes in surface morphology are observed.The surface roughness intensifies, irregularities become more prominent, and visible slip bands appear, forming distinct line patterns.Macroscopically, these slip bands exhibit extremely fine wave-like patterns on the surface of the sheet metal, with depths of only a few micrometers.Moreover, the slip bands extend from one end of the sheet to the other in a back-and-forth overlapping manner, covering the entire sheet densely.Microscopic examination of the slip bands in the 7B04 alloy with different deformation levels is depicted in Figure 6.At a deformation level of 2% (Figure 6(a, b)), no slip bands are observed, and the microstructure does not exhibit significant features such as micro-cracks.However, as the deformation level increases to 6% and beyond, prominent macroscopic slip bands (as shown in the black circle) become visible.Between adjacent slip bands, micro-deformation bands with a vertical orientation can be observed in the microstructure (Figure 6(c) and (d)).No micro-cracks are present within the internal structure.At a deformation level of 10% (Figure 6(e) and (f)), the contours of the deformation bands become distinct, with larger spacing between the patterns.Micro-cracks, as the white arrow points, appear between the deformation bands, with the longest crack reaching up to 3 μm.
Figure 7 illustrates the formation mechanism of micro-scale deformation bands and macro-scale slip bands during the pre-deformation process of sheet metal.As shown in Figure 7 In polycrystalline materials, as depicted in Figure 7(b), during the pre-tensile deformation, the sameoriented slip systems and deformation bands in different grains connect with each other; this can lead to the division of the sheet metal into multiple regions, forming macroscopically visible slip band patterns [6-7]   .The experimental results indicate that the pre-tensile deformation has a significant influence on the surface morphology of the sheet metal.As the deformation amount increases, macroscopic slip bands appear on the surface of the sheet, accompanied by the gradual occurrence of deformation bands and micro-cracks in the alloy's microstructure.This suggests that the pre-tensile deformation of the sheet metal is approaching the material's elongation limit [8] .In practical production, it is important to control the occurrence of macroscopic slip bands to ensure the integrity of the microstructure of the components.

The influence of pre-tensile deformation on mechanical properties
During the pre-tensioning of the 7B04 alloy, the tensile direction is perpendicular to the material's fiber direction.However, during the tensile mechanical property testing, two scenarios are considered: perpendicular and parallel to the fiber direction.Figure 8 shows the variation of strength parameters of the 7B04 aluminum alloy under different pre-tension deformation amounts.
From the graph, it can be observed that with an increase in pre-tension deformation amount, the tensile strength and yield strength of the material exhibit an approximately linear increase.The yield strength shows a greater increase compared to the tensile strength, leading to an increased yield-totensile strength ratio.When tested perpendicular to the fiber direction, the yield strength increases from 330 MPa to 410 MPa.When tested parallel to the fiber direction, the yield strength increases from 370 MPa to 450 MPa.Although the yield strength trends are consistent for both directions, the yield strength parallel to the fiber direction is approximately 40 MPa higher than that perpendicular to the fiber direction.As shown in Figure 9, the post-fracture elongation of the 7B04 alloy under different pre-deformation amounts is presented.With an increase in pre-tension deformation level, the elongation of the material decreases, and the decreasing trend is evident.Furthermore, the samples stretched perpendicular to the fiber direction exhibit significantly higher elongation compared to the samples stretched parallel to the fiber direction, which is opposite to the observed strength pattern mentioned earlier.When the predeformation amount of the skin reaches 12%, the elongation of the material can decrease from 20% to 13%.During the pre-tension forming process of aluminum alloy skins, the sheets undergo plastic deformation, leading to a rearrangement of the crystal structure within the material.This rearrangement results in the formation of additional dislocations and grain boundaries.The interaction between dislocations and grain boundaries increases the resistance to deformation, causing an increase in deformation resistance and achieving the effect of work hardening [9] , thereby enhancing the strength of the material.Furthermore, due to the preferential grain orientation resulting from pre-tension forming, the material exhibits anisotropy, resulting in different strengths parallel and perpendicular to the fiber direction [10] .On the other hand, excessive pre-tension deformation can cause grain boundary sliding, changes in grain orientation, and fracture of grain boundaries.It can limit the material's ability for plastic deformation and result in a decrease in elongation rate [11] .
The elongation rate obtained through room temperature tensile tests, as indicated by the values in Figure 9.It refers to the original gauge length elongation and does not reflect the local plastic deformation capacity of the material prior to fracture.Therefore, the DIC (Digital Image Correlation) method is employed to measure the local strain of the tensile specimen prior to fracture [12] .Figure 10 illustrates the DIC-measured local strain of a 7B04 alloy specimen with a pre-deformation amount of 12%.
The tensile specimens oriented perpendicular and parallel to the fiber direction exhibit non-uniform plastic deformation before fracture.The maximum strain values in the necking region reach 27.8% and 18.0%, respectively, while the strain values in the uniform deformation region are 18.2% and 12.6%.By comparing the test results in Figure 8 and Figure 9, it can be observed that ψ (non-uniform region based on DIC measurements) > ψ (original gauge length measurements) > ψ (uniform region based on DIC measurements).The parameter ψ (non-uniform region, measured by DIC) obtained through the DIC method reflects the material's ultimate plastic deformation capability.In actual production processes, the strain in localized regions of the sheet should not exceed this value.On the other hand, ψ (uniform region, measured by DIC) represents the material's plastic deformation capability before significant necking occurs.In practical production processes, it is desirable to prevent deformation instability or concentration by controlling the maximum strain in localized regions of the sheet within this value [13] .

The influence of pre-tensile deformation on thickness
Due to the constant overall volume of the sheet before and after pre-strain, the thickness of the sheet inevitably decreases as it is stretched and elongated.Figure 11 shows the influence of pre-strain on the thickness of the specimens.The thickness of the material decreases linearly with the increase in prestrain applied to the skin.As the pre-strain increases, the specimens become progressively thinner.For the 1.5 specimens, when the applied strain increases by 4%, its thickness decreases by approximately 0.03 mm, accounting for 1.2% of the total thickness.Therefore, it is necessary to consider the effect of deformation-induced thinning during the selection of materials for skinning processes.

The influence of pre-tensile deformation on material fatigue life
The focus of this study is to investigate the influence of pre-strain on the fatigue life of skin materials under fixed cyclic loading.The experimental results are presented in Table 2, where the maximum stress level for cyclic loading is chosen as 80% of the tensile strength of the specimens.In general, it can be observed that as the pre-strain increases from 2% to 10%, the fatigue life under the load perpendicular to the fiber direction gradually increases.In contrast, the fatigue life under the load parallel to the fiber direction decreases instead.7B04-10%-# 1.308 7848 23449 * Load perpendicular to the fiber direction # Load parallel to the fiber direction Further investigation was conducted on the fatigue fracture surfaces of the alloy subjected to different pre-strain levels.The fractured surfaces of the specimens were observed, as shown in Figures 12 and 13.They represent the fatigue fracture surfaces of specimens with a pre-strain of 2% loaded along the perpendicular fiber direction and a pre-strain of 10% loaded along the parallel fiber direction, respectively.Despite the difference in pre-strain levels and fatigue loading directions, both surfaces exhibited typical features of fatigue fracture [14] .The fatigue initiation zone was located at the edge of the specimen where stress concentration occurred, displaying a point source characteristic with a smooth and fine appearance.Clear fatigue striations were visible, exhibiting a beach-like pattern.Secondary crack features were formed during fatigue crack propagation, and mixed features of trans-granular and intergranular fracture were observed in the overload region [15][16][17] .It is worth noting that although both fatigue fracture surfaces exhibit features of trans-granular fracture and dimples, there are differences in morphology, size, and distribution.When the pre-strain is 2%, the crystalline shape and reflective facets of the trans-granular fracture are more prominent, with a larger area.There are more dimples, which are larger and more evenly distributed.When the pre-strain is 10%, there are fewer dimples, they are smaller in size, and their distribution is uneven, appearing only between adjacent trans-granular fractures.At lower pre-strain levels, the alloy exhibits lower yield strength and better ductility.However, as the pre-strain increases, the alloy's yield strength increases while ductility significantly decreases.The higher pre-strain leads to the accumulation of dislocations, causing stress concentration and promoting secondary cracking [18] .This can result in a significant reduction in the number of dimples and a more pronounced trans-granular fracture.

Figure 1 .
Figure 1.Schematic Diagram of Convex Die Stretch Forming for Skin Components.

Figure 2 .
Figure 2. Geometric Shape and Dimensions of the Room Temperature Tensile Specimen.

Figure 3 .
Figure 3. Geometric Shape and Dimensions of the Room Temperature Fatigue Specimens.
(a), in a single crystal material, different regions within a grain experience different shearing stresses due to the applied pretensile deformation, resulting in different slip systems.The regions within adjacent slip systems are divided by deformation bands with different orientations.Ultimately, the grain is divided into multiple regions constrained by different deformation bands (A, B, and C represent different deformation bands).

Figure 7 .
Figure 7.The Schematic Diagram of the Formation Mechanism of Micro-scale Deformation Bands and Macro-scale Slip Bands during the Pre-deformation Process of Sheet Metal.(a) Formation Mechanism of Micro-scale Deformation Bands in a Single Crystal.(b) Schematic Diagram of the Formation of Macro-scale Slip Bands in Polycrystalline Material.

Figure 8 .
Figure 8.The Influence of Pre-tension Deformation on the Strength of 7B04 Alloy.

Figure 9 .
Figure 9.The Influence of Pre-tension Deformation on Elongation of 7B04 Alloy.

9 Figure 10 .
Figure 10.The DIC Test Results for the 7B04 Alloy with a Pre-strain of 12%.(a) Tensile Test Performed Perpendicular to the Fiber Direction.(b) Tensile Test Performed Parallel to the Fiber Direction.

Figure 11 .
Figure 11.The Effect of Pre-tension Deformation on the Thickness of Specimens.

Figure 12 .
Figure 12.The Fatigue Fracture Surface of the Specimen Loaded in Tension along the Perpendicular Fiber Direction with a Pre-strain of 2%.(a) Fatigue Initiation Zone; (b) Fatigue Striations; (c) Transgranular Fracture Characteristics; (d) Dimpled Feature.

Figure 13 .
Figure 13.The Fatigue Fracture Surface of the Specimen Loaded in Tension along the Parallel Fiber Direction with a Pre-strain of 10%.(a) Fatigue Initiation Zone; (b) Fatigue Striations; (c) Trans-granular Fracture Characteristics; (d) Dimpled Feature.

Table 2 .
Fatigue Life of Specimens at Different Pre-tensile Deformation.