Experimental Study of Reaming Sizes on Fatigue Life of Cold-Expanded 7050-T7451 Aluminum Alloy

Abstract

The split-sleeve cold expansion technology is widely used in the aerospace industry, particularly for fastening holes, to enhance the fatigue life of components. However, to ensure proper assembly and improve surface integrity, reaming of the cold-expanded holes is necessary. This study investigates the effects of cold expansion and reaming processes on the fatigue performance of 7050-T7451 aluminum alloy. Fatigue tests, residual stress measurements, and microstructural analyses of the hole edges were conducted on specimens with four different hole diameters after cold expansion and reaming. It was found that the depth of reaming significantly affects fatigue life. During the cold expansion process, the compressive residual stress formed around the hole effectively improves fatigue performance. The experiments demonstrated that reaming by 0.2 mm to 0.4 mm helps eliminate minor defects, thereby improving fatigue life. However, reaming beyond 0.5 mm may lead to stress relief and the removal of dense grains at the hole edges, reducing fatigue life. Therefore, determining the optimal reaming size is crucial for enhancing the reliability of aerospace fasteners.

1. Introduction

With the advancement of the aerospace industry, the study of the reliability of flight vehicles has become highly significant. Most connection methods for flight vehicles are mechanical, such as riveted and bolted connections. The most critical parts of the aircraft involve mechanical connections like rivets and bolts [1,2,3]. Parts that need frequent replacement, like wings and landing gear, are inevitably subjected to cyclic loads such as mechanical vibrations and centrifugal forces [4,5]. Fatigue cracks can develop in fastening holes with stress concentrations, and fatigue damage originating from these holes is the primary form of fatigue damage in aircraft [6,7]. The 7XXX aluminum alloy of the Al-Zn-Mg-Cu is widely used in the aerospace industry due to its excellent mechanical properties and is a key material in the study of metal fatigue properties [8]. Cold expansion is now widely used in the aircraft industry, particularly for fastening holes [9,10,11]. In this technique, an oversized ball or mandrel is forced through the fastening holes to locally yield the material, inducing a tangential compressive residual stress field around the holes. This counteracts external loads and reduces stress concentration at the hole edges, improving fatigue resistance [12]. It is generally recognized that cold expansion is the most effective way to improve fatigue resistance. Compressive residual stresses introduced during cold expansion are widely recognized for extending the fatigue life of hole structures [13,14,15]. Cold expansion methods have been developed over the last decade, with the main techniques being direct cold expansion and split-sleeve cold expansion. The open-seam cold expansion method significantly reduces damage to the hole wall, maintains the correct radial expansion, and allows access from either side [16,17]. Cold expansion of split-sleeve has been widely used to extend the lifespan of fasteners in aircraft [18,19,20,21]. Cold expansion of the split-sleeve leaves a ridge in the hole wall after expansion due to cracks in the split-sleeve [19]. The plastic deformation is not axisymmetrically distributed around the hole, and cracking may occur, usually near the crack in the sleeve [22]. In order to avoid material defects caused by the convex ridge after cold expansion and to ensure proper assembly of the fastener, the fastening holes are usually treated with a reaming process [23].

The principle of cold expansion to improve fatigue life is relatively complex, and many scholars have studied the factors and mechanisms involved. Yan et al. [24] conducted experimental studies showing that the cold expansion of split-sleeve resulted in a 1.5 to 3 times increase in the lifespan of TC4 titanium alloys. Li et al. [25] investigated residual stresses around the holes of 7050 aluminum alloy plates at different expansion rates, finding that maximum residual stresses occurred at a 4% expansion rate. Wang et al. [26] found that cold expansion can form gradient fine grains on the hole surface, inhibiting fatigue cracking. Yang et al. [27] found that cold expansion causes a hardened layer to form around the hole, with hardness decreasing up to 450 μm from the hole. Tang et al. [28] proposed a two-scale modeling approach combining the macro-scale cyclic ontology model with crystal plasticity theory, finding that residual stress and the plastic layer together determine the onset. The cold-expanded holes need to undergo a reaming process before they can be put into service. Reaming removes minor defects on the hole edges caused by cold expansion and adapts the fasteners accordingly. Liu et al. [29] analyzed through simulation that the fatigue life of cold-expanded holes increases with greater reaming depth. Alexandre et al. [30] analyzed the change in the stress state of the hole during reaming. However, there is a lack of research on the experimental study of the fatigue life of reamed holes after cold expansion, the changes in surface integrity, and the reasons for decreased fatigue resistance due to oversize reaming.

This paper investigates the 7050-T7451 alloy to study the effects of the reaming process on cold-expanded plates with holes. Four different diameters of cold-expanded holes were reamed to varying depths. Fatigue tests were conducted to assess the impact of different reaming sizes on the fatigue life of these holes. Scanning electron microscope (SEM) analysis examined the microscopic fracture morphology of specimens with varying fatigue lives under different conditions. Electron Backscatter Diffraction (EBSD) analyzed local deformation in reamed and unreamed holes after cold expansion, aiming to reveal the relationship between reaming behavior and fatigue life at the microscopic level. Residual stress before and after reaming was measured using X-ray diffraction. The study identified variations in fatigue life at different reaming depths, the effect of reaming on crystal refinement after cold expansion, and changes in residual stress distribution. These findings provide important guidance for the installation of fasteners in aircraft.

2. Materials and Methods

2.1. Experimental Materials

The experimental material selected for this study is 7050-T7451 aluminum alloy, part of the Al-Zn-Mg-Cu series, widely used in aerospace applications [8]. Its chemical composition and main mechanical properties are shown in

Table 1. Chemical composition of 7050-T7451 aluminum alloy.

Elements	Si	Zn	Mg	Cu	Fe	Mn	Cr	Zr	Al
Wt.%	0.03	6.02	2.31	2.04	0.11	0.003	0.009	0.1	Rest

and

Table 2. Mechanical properties of 7050-T7451 aluminum alloy.

Density $\mathsf{\rho }$ $(\mathbf{k}\mathbf{g}/{\mathbf{m}}^3$)	Young’s Modulus E (Gpa)	Poisson’s Radio $\mathsf{\nu }$	Tensile Strength ${\mathit{\sigma }}_{\mathit{b}}$ (MPa)	Yield Strength ${\mathit{\sigma }}_{\mathit{s}}$ (MPa)	Elongation A (%)	Fracture Toughness ${\mathit{K}}_{\mathit{I}\mathit{C}}$ (MPa·m1/2)
2830	70.3	0.33	564.7	479.4	13.3	30

. The tensile properties, determined by an INSTRON 8801 (Instron, Norwood, MA, USA) electronic universal testing machine, are illustrated in Figure 1. The alloy is supplied as a 10 mm thick sheet and is designed according to the Chinese standard HB/Z 170-2005 [31] for the cold expansion process of aerospace metal parts. Fatigue specimens with holes are shown in Figure 2, featuring four hole diameters: 4.3 mm, 5.3 mm, 7.18 mm, and 11.1 mm. A split-sleeve from a domestic company was used in this experiment. The sleeve material is 301 stainless steel, with localized material 12Cr7Ni7 as per GB/T 3280-2015 [32]. The thickness of the split-sleeve used for cold expansion is 0.152 mm for Figure 2a,b, 0.203 mm for Figure 2c, and 0.256 mm for Figure 2d. The cold expansion technique described in this paper involves the cold expansion of a split-sleeve; the cold expansion process and cold-expanded specimens as shown in Figure 3. The split-sleeve is secured in the hole, with a mandrel positioned at the center. The mandrel is swiftly pulled through the end of the hole to generate residual stresses. After removing the mandrel and split-sleeve, the hole is reamed at a machining center to achieve the required diameter for the experiment.

2.2. Fatigue Test

To compare the effects of different reaming depths on the fatigue life of cold-expanded 7050-T7451 aluminum alloy plates, various specimens were cold-expanded with approximately 4% expansion. The expansion E was calculated as follows:

$$E_r={\displaystyle \frac{\left(D_0+2d\right)-D}{D}}$$

(1)

In the equation, $D_0$ is the diameter of the cold-expanded mandrel, $D$ is the diameter of the initial hole before expansion, and d is the thickness of the split-sleeve. The extrusion rate is calculated in

Table 3. The size of the cold expansion tool and cold expansion rate.

Initial Diameter (mm)	Mandrel Diameter (mm)	Split-Sleeve Thickness (mm)	Expansion Ratio ${\mathbf{E}}_{\mathbf{r}}$ (%)
4.3	4.17	0.152	4.04
5.3	5.21	0.152	4.03
7.18	7.06	0.203	3.98
11.1	11.04	0.253	4.01

.

The specimens of each size were reamed with five different final hole diameters and each reaming depth was equipped with four experiments. The fatigue tests were carried out on INSTRON 8801. The uniaxial fatigue tests were performed with constant load amplitude, sinusoidal loading Stress ratio R = 0.1, stress amplitude 108 MPa, and loading frequency 50 Hz. The experimental matrices for fatigue tests are given in

Table 4. Parameters of fatigue specimens with different initial and final hole diameters.

Specimen	Quantity (PCS)	Initial Diameter (mm)	Expansion Ratio ${\mathit{E}}_{\mathit{r}}$ (%)	Final Diameter (mm)	Load Spectra
					Stress Amplitude (MPa)	Stress Ratio R
A1-01	4	4.3	4.04	4.6	108	0.1
A1-02	4	4.3	4.04	4.7	108	0.1
A1-03	4	4.3	4.04	5.0	108	0.1
A1-04	4	4.3	4.04	5.2	108	0.1
A1-05	4	4.3	4.04	5.4	108	0.1
A2-01	4	5.3	4.03	5.4	108	0.1
A2-02	4	5.3	4.03	5.9	108	0.1
A2-03	4	5.3	4.03	6.1	108	0.1
A2-04	4	5.3	4.03	6.3	108	0.1
A2-05	4	5.3	4.03	6.5	108	0.1
A3-01	4	7.18	3.98	7.6	108	0.1
A3-02	4	7.18	3.98	7.8	108	0.1
A3-03	4	7.18	3.98	8.0	108	0.1
A3-04	4	7.18	3.98	8.2	108	0.1
A3-05	4	7.18	3.98	8.4	108	0.1
A4-01	4	11.1	4.01	11.7	108	0.1
A4-02	4	11.1	4.01	12.0	108	0.1
A4-03	4	11.1	4.01	12.3	108	0.1
A4-04	4	11.1	4.01	12.6	108	0.1
A4-05	4	11.1	4.01	12.9	108	0.1
A1-00	4	4.3	0%	4.3	108	0.1
A2-00	4	5.3	0%	5.3	108	0.1
A3-00	4	7.18	0%	7.18	108	0.1
A4-00	4	11.1	0%	11.1	108	0.1

.

2.3. Characterization

An Electron Backscatter Diffraction (EBSD) system was used to observe the microstructure near the pore walls of specimens after cold expansion without reaming, as well as those cold-expanded and then reamed to 5.0 mm. The EBSD samples were prepared by vibratory polishing with silica suspension, and the data were analyzed using HKL-Channel 5 software. A scanning electron microscope (SEM, SU5000, HITACHI, Hitachi, Japan) was used to analyze the fracture after the fatigue test. X-ray diffraction was employed to measure eight points along the radial direction starting from the hole edge, with intervals of 0.5 mm, using a Cr-Kα X-ray source (wavelength λ = 2.291 Å).

3. Results and Discussion

3.1. Fatigue Life Behavior

In the study by Alexandre et al. [30], the reaming process of cold-expanded holes was simulated. However, the modeling strategy did not consider the thermal effects during reaming, focusing solely on the impact of material removal on the stress state of the part. As a result, there is still a deviation from real experimental conditions. This paper investigates the relationship between reaming size and fatigue life through extensive fatigue experiments. The cold expansion specimen after fatigue testing is shown in Figure 4.

Figure 5 shows the fatigue life of four different sizes at various final hole diameters. Initially, the fatigue life increases with the reamed size, consistent with the research results of Liu et al. [28]. The trend was similar for specimens with initial hole diameters ranging from 4.3 mm to 11.1 mm. The specimen with an initial hole diameter of 4.3 mm has the highest fatigue life when reamed to 4.8 mm to 4.9 mm; the specimen with an initial hole diameter of 5.3 mm has the highest fatigue life when reamed to 5.7 mm; the specimen with an initial hole diameter of 7.18 mm has the highest fatigue life when reamed to 7.6 mm; and the specimen with an initial hole diameter of 11.1 mm has the highest fatigue life when reamed to 11.8 mm. However, as the reamed hole diameter continues to grow, the fatigue life then decreases. This may be because the reaming process removes the fine-grain hardened layer formed by cold expansion, reducing the material’s strength at the hole edge and making fatigue cracks more likely to propagate. Additionally, if the reaming size is too large, it can damage the hole edge plastic layer, reducing constraint in the deeper elastic deformation layer. This weakens the residual stress field’s strength, diminishing its positive effect on fatigue life.

In Figure 6, the fatigue life of the reamed specimen with the optimal reamed dimension is compared to that of the untreated specimen. It can be observed that the fatigue life of the properly reamed cold-expanded specimen is approximately 4.4 to 8.7 times higher than that of the untreated specimen.

3.2. Observation of Port Morphology

A specimen with an initial hole diameter of 4.3 mm was selected, and microelectron microscopy fracture analysis was performed on the uncold-expanded specimen, the specimen reamed to 4.8 mm after cold expansion, and the specimen reamed to 5.0 mm after cold expansion.

Figure 7 illustrates the typical fracture morphology of the uncold-expanded specimen, which has a fatigue life of 26,500 cycles. The crack initiation point, indicated by an arrow in Figure 7a, shows that the specimen failed in a multi-source fatigue mode. The fatigue crack originated from machining marks and spread from the middle surface of the hole wall. In Figure 7b, the crack initiation zone is clearly visible, along with the crack extension and transient fracture zones. The fatigue expansion zone, shown in Figure 7c, is relatively flat with numerous fatigue bands spaced about 3.14 μm apart. Compared to the crack extension zone, the transient fracture zone is rugged and irregular. As the fatigue crack grows, the effective bearing area decreases, leading to specimen failure after reaching tensile strength. This accounts for about three-fourths of the fracture surface, with numerous plastic deformation features visible in this region, as shown in Figure 7d.

Figure 8 shows the fracture morphology of a specimen cold-expanded and reamed to 4.8 mm, with a fatigue life of 213,400 cycles. The crack extension zone is larger than in uncold-expanded specimens due to residual stresses. The crack originated at the hole entrance, where residual stress reinforcement is minimal, consistent with most studies [4,27]. Tire indentation marks, shown in Figure 8c, indicate effective involvement of compressive residual stresses in the fatigue process. The fatigue strip spacing is about 1.64 μm. The area of the transient fracture zone is compressed, with numerous plastic deformation dimples, as shown in Figure 8d.

Figure 9 depicts the fracture morphology of a specimen reamed to 5.0 mm, with a fatigue life of 90,100 cycles. Compared to the 4.8 mm specimen, this specimen shows more crack initiation sources, not originating at the hole entrance. The expansion zone is shorter, and the transient fracture zone is larger, covering more than one-third of the area, with an average fatigue band spacing of about 2.04 μm. The fatigue expansion rate is higher for the 5.0 mm specimen than for the 4.8 mm specimen, but still lower than that of uncold-expanded specimens. This suggests that larger reaming sizes increase fatigue crack growth rates, highlighting the need to carefully consider reaming depths.

3.3. EBSD Analysis

In this analysis, we examined two specimens. One was cold-expanded with an initial hole diameter of 4.3 mm without reaming, and the other was cold-expanded with an initial hole diameter of 4.3 mm and then reamed to 5.0 mm. Both specimens were analyzed using EBSD. Figure 10a,d show the inverse pole figures (IPF) before and after reaming. Figure 10a indicates that strong crystal refinement formed in the hole wall after cold expansion, with a layer of about 20 μm. It is widely believed that these refined crystals can prevent crack propagation, as cracks must traverse longer paths through the grains, reducing the rate of crack extension [33]. However, this phenomenon disappears after reaming, as shown in Figure 10d, where the crystal refinement layer has been removed. Since the depth of the reaming is much greater than that of the crystal refinement layer, it also removes the ridges of the cold-expanded holes, giving them a more regular shape.

The plastic deformation of the hole is evaluated by the kernel average misorientation (KAM) shown in Figure 10b,e. Significant plastic deformation is observed around the hole after cold expansion, with a deformation layer of about 360 μm. As the hole wall expands outward, the inner material layer deforms more, reaching the yield strength first, while the deeper layer does not reach yield strength but is restrained by the plastic layer, forming residual stresses [25]. In the specimen reamed to 5.0 mm, only a thin layer of deformation remains, which is due to the reaming process, confirming that excessive reaming harms the plastic deformation layer, reducing the constraint on residual stresses and thereby diminishing their beneficial effect on fatigue life.

The crystal orientation and grain boundary diagrams are shown in Figure 10c,f. Orientation deviations between 2 and 15° are indicated by black lines, and those above 15° by red lines. The crystal refinement at the hole edges is evident before reaming, but not after. Excessive reaming destroys the crystal refinement layer formed by cold expansion, reducing the fatigue strength of the cold-expanded holes, similar to the findings of Dang et al. [34].

Proper evaluation of the depth of the crystal refinement crystal layer and plastic deformation layer formed around the hole during the cold expansion process is crucial for determining the optimal reaming amount. Wang et al. [26] found that cold expansion of 7075-T6 aluminum alloy with a 4% expansion resulted in a plastic layer about 1200 μm thick. In contrast, Wang et al. [4] observed that cold expansion of TC4 titanium alloy with a 2% expansion formed a much thinner fine-grained layer. Other studies [35,36] observed varying depths of the plastic deformation layer depending on the material and expansion amount. Accurately predicting this thickness is key to guiding reaming dimensions.

3.4. Residual Stress Analysis

Figure 11 shows the distribution of residual stresses along the radial direction for the cold-expanded specimen with an initial hole diameter of 7.18 mm for the unreamed and reamed to 8.4 mm specimens. It can be observed that the intensity of compressive residual stresses in the plate reamed to 8.4 mm is lower than that of the unreamed specimen. The maximum compressive residual stress of the unreamed specimen is 252 MPa, while the maximum residual stress of the specimen reamed to 8.4 mm is only 150 MPa.

In the study by Liu et al. [29], it was also found by finite element simulation that the residual compressive stress decreases with the increase in reaming amount. The reason for the decrease in residual stress may be that the reaming size is too large, which reduces or even completely disappears the plastic zone, resulting in the loss of constraints on the residual stress in the plastic zone. The measurement of the residual stresses also confirms the conjecture that the high reaming depth in the fatigue test leads to a reduction in fatigue life. After the residual stresses generated by cold expansion are released, the fatigue gain effect of the residual stresses on the fastening holes is reduced, resulting in the reduction of the fatigue life of the over-reamed specimens with holes.

4. Conclusions

This study investigated the effect of the reaming process on cold-expanded holes by reaming cold-expanded plates with four different hole diameters. By analyzing fatigue life, fracture morphology, microstructure, and residual stress under various reaming conditions, the following conclusions were drawn:

• Fatigue testing after approximately 4% cold expansion of plates with four different sizes of perforations showed that fatigue life first increases and then decreases with greater reaming. This conclusion applies to apertures ranging from 4.3 mm to 11.1 mm. The maximum fatigue life was about 4.4 to 8.7 times higher than that of the uncold-expanded plate. Excessive reaming results in a significantly smaller crack extension zone in the fatigue specimen, weakening its ability to withstand fatigue.
• Cold expansion forms a dense grain structure around the hole, creating a crystal refinement layer of approximately 20 μm. Because the height of the convex ridge is much greater than its depth, the reaming process removes this crystal refinement layer.
• A plastic deformation layer forms at the hole wall, constraining residual stresses in the deeper layers. The reaming process disrupts this plastic layer, releasing the residual stresses. The extent of this effect depends on the reaming depth. Proper assessment of the plastic deformation layer’s depth, formed by cold expansion, is crucial in determining the optimal reaming amount.
• Reaming the cold-expanded plate reduces radial residual stresses around the hole by up to 102 MPa compared to the unreamed plate, due to the loss of stress restraint by the innermost plastic layer. This reduction in residual stresses also decreases the fatigue resistance of the cold-expanded split-sleeve.