Microstructure and Properties of Dissoluble LA141-0.5Cu Magnesium Alloy Wires Applied to Oil and Gas Resource Exploitation

Abstract

Mg-Li-based dissoluble metal is a promising material for preparing dissoluble magnesium alloy wires. However, there are few reports on the development of Mg-Li dissoluble magnesium alloy wires so far. In this paper, the mechanical properties and dissoluble properties of as-drawn and annealed LA141-0.5Cu wires were investigated in detail. It was found that the tensile strength of the LA141-0.5Cu wires decreased from 160 MPa to 127 MPa and the elongation increased from 17% to 22% after annealing. The difference in corrosion rates (93 °C/3% KCl solution) between the as-drawn wires and annealed wires is not significant, with values of 5.1 mg·cm⁻²·h⁻¹ and 4.5 mg·cm⁻²·h⁻¹, respectively. This can be explained as follows: after annealing, the number of dislocations in the wire decreases, the strength decreases, and the plasticity increases. The reason why the wires have a significant corrosion rate is that there is a large potential difference between the Cu-containing second phase and the magnesium matrix, which forms galvanic corrosion. The decrease in dislocation density after annealing leads to a slight reduction in the corrosion rate of the wires. This work provides a qualified material for fabricating temporary blocking knots for the exploitation of unconventional oil and gas resources.

1. Introduction

Shale oil and gas are important components of energy at present. However, shale oil and gas reservoirs are difficult to exploit, requiring horizontal well exploration and fracturing. Fracturing tools and temporary plugging tools are required during the exploration and fracturing process of horizontal wells. Dissoluble magnesium alloy fracturing tools have gradually been replacing drillable tools. Temporary plugging material is a very important type of material in horizontal well fracturing technology. It can temporarily plug boreholes or cracks, allowing the pressure of the fracturing fluid to act on other parts, causing more cracks to be formed. And the presence of more cracks allows more oil and gas to flow out of the cracks and into the casing, increasing oil and gas production.

Degradable fiber temporary plugging agents are a new type of plugging tool [1]. The fibers are easy to bend and form into various shapes, which can be filled into the micropores or small pores of the cracks, making the temporary plugging layer denser [2]. In addition, the fiber density is low, and it is easy to fill the cracks and form a temporary blocking layer after entering the gaps. However, the scarcity of synthetic products, complex synthesis processes, and high costs of certain materials in degradable fibers have limited their application. In addition to cost factors, degradable fibers also have the following disadvantages in practical applications: low pressure-bearing capacity, poor heat resistance and chemical stability, and easy aggregation and blockage of pipelines during construction. Therefore, finding a suitable material to weave knots and overcome the various drawbacks of polymer knots is an urgent task that the academic community needs to face.

Magnesium alloys have the characteristics of high specific strength, good thermal conductivity, good thermal stability, good mechanical properties, and easy corrosion [3,4,5,6,7,8]. In addition, magnesium resources are widely distributed in nature, with abundant reserves and low cost. Adding some chemically active alloying elements to magnesium alloys can cause corrosion and dissolution under certain conditions, making them suitable as dissoluble materials [9,10,11]. The application of dissoluble magnesium alloys in shale oil and gas extraction mainly includes dissoluble bridge plugs and dissoluble fracturing balls. So far, the studies on magnesium alloys for hydraulic fracturing have focused on Mg-Al [12,13], Mg-Zn [14], and Mg-RE [15,16] alloy systems. Most of the reported studies on dissoluble magnesium alloys for hydraulic fracturing have focused on high strength, with relatively little investigation of plasticity. For example, Zhong et al. [17] found that in Mg-2Gd-0.5 (Cu/Ni) alloys, adding Ni instead of Cu in trace amounts is more effective in refining grains, improving the texture strength of the basal plane, and promoting the formation of the LPSO phase, resulting in better strength. However, the addition of Ni is not conducive to improving the work hardening rate and elongation of the alloy. In addition, Ma et al. [18] investigated the effect of Ni on the mechanical properties and corrosion behavior of MgGd1Nix alloy. The results indicate that the LPSO phase containing Ni not only improves the compressive strength of the alloy but also accelerates the alloy dissolution process under galvanic corrosion. Liu et al. [12] investigated the role of Cu in AZ91 alloy. The results showed that AZ91-RE-3Cu alloy had the highest strength, with a yield strength and maximum compressive strength of 244 and 405 MPa, respectively. However, so far the reports on dissoluble magnesium alloys have mostly focused on the extruded rods, with almost no relevant reports on dissoluble magnesium alloy wires. In the field of dissoluble magnesium alloy wire, only related literature on a portion of biodegradable magnesium alloy wire is available for reference. The reason for this status is that magnesium alloys have a hexagonal structure with poor plasticity, which makes the preparation of dissoluble magnesium alloy wires extremely difficult. As mentioned earlier, if magnesium alloy wire is used to weave knots, it will overcome the disadvantages of low strength and poor thermal and chemical stability of polymer knots. At the same time, due to the good solubility of dissoluble magnesium alloys, it can also overcome the problem of poor solubility of polymer knots. However, designing high-plasticity dissoluble magnesium alloys and preparing ultrafine dissoluble magnesium alloy wires are indeed challenges faced in both academia and industry.

The Mg-Li alloy system is a representative system of high-plasticity magnesium alloys. This alloy exhibits ultra-high plasticity due to its BCC lattice structure. For example, Ji et al. [19] increased the plasticity of Mg-Li-Al alloy to over 40% by regulating the precipitation phase at the grain boundaries. In summary, Mg-Li alloy may be an ideal material for developing ultrafine magnesium alloy wires. However, so far there have been no reports in the academic community on dissoluble Mg-Li alloy wires. Therefore, based on this idea, this article intends to develop a novel dissoluble Mg-Li alloy and prepare ultrafine wire materials through Cu microalloying. At the same time, in order to further enhance the plasticity of the wire, annealing treatment is carried out on the wire, and the microstructure, mechanical, and corrosion properties of the wire in two states are studied, especially the plasticization and dissolution.

2. Experimental Procedure

In this paper, the alloy composition Mg-14Li-0.8Al-0.5Cu was smelted by using pure Mg (99.99 wt.%), pure Li (99.999 wt.%), pure Al (99.99 wt.%), and Mg-Cu (25 wt.%). A vacuum induction furnace was used to melt the alloy, and the alloys were cast into the metal mold to obtain the required ϕ 90 × 100 mm ingot. After alloy melting, homogenization treatment was carried out at a heat treatment temperature of 230 °C for 5 h, followed by air cooling. Before extrusion, the ingot and mold were preheated to 220 °C. The extrusion temperature was 220 °C, and the extrusion ratio was 24:1. Then the extruded alloy rod could be processed into a round rod with a diameter of 6 mm. Before drawing, the alloys were annealed at 200 °C/10 min, followed by cold drawing. The deformation amount per pass was 10%~15%; when the deformation was 55% to 70%, the alloy wires were annealed at 200 °C/10 min. The final deformation was 75%, and finally alloy wires with a diameter of 1 mm were obtained.

XRD tests were carried out (Smart Lab, Nippon Science, Tokyo, Japan). The microstructure was observed by scanning electron microscope (SEM, Zeiss supra55, Carl Zeiss AG, Oberkochen, German). The texture of wires was collected and analyzed by X-ray diffractometer (Smart Lab, Nippon Science, Japan). The precipitated phase of the sample was observed by transmission electron microscope (TEM, Talosf200x, Thermo Fisher Scientific, Waltham, MA, USA). Tensile specimens with gauge dimensions of 1 mm in diameter and 25 cm in length were prepared. Then the CMT6104 testing machine (MTS Systems Corporation (China) Limited, Shenzhen, China) was employed to conduct the tensile test and the initial rate was 1 mm/min.

To evaluate corrosion behaviors, the weight loss of the samples immersed in the solution for 1 h at 93 °C was measured. The composition of the solution was 3% KCl. Hank’s solution was also used for the weight loss test to obtain a comparison of the degradation rate among the different materials. The sample with a length of 35 mm was used for the measurement of weight loss. Before the measurement, the samples were ultrasonically cleaned with alcohol. Then the samples were immersed in a solution of 200 g/L Cr₂O₃ + 10 g/L AgNO₃ to remove the corrosion products, rinsed with distilled water and ethanol, and finally dried in warm flowing air.

Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were analyzed using a CS310H electrochemical testing system (Wuhan Corrtest Instrument Co., Ltd., Wuhan, China) in 3% KCl solutions at 93 °C. The impedance spectrum test parameters were as follows: the DC potential was selected as a relative open circuit, the potential was set to 0 V, and the AC amplitude was set to 10 mV. The polarization curve setting parameters were as follows: the initial potential was set to −0.3~−0.5 V, the termination potential was set to 1 V, the potential type was selected as a relative open circuit, the scanning frequency was set to 0.5 mV/s, and the sampling frequency was set to 2. Scanning Kelvin Probe Force Microscopy (SKPFM, SPI4000/SPA300HV, NSK LTD., Tokyo, Japan) was used to measure the Volta potential of the matrix and other phases.

3. Results

3.1. Microstructure

The XRD images of wires are shown in Figure 1. The XRD results showed that after drawing, there were α-Mg and β-Li phases in the LA141-0.5Cu alloy, and the peak value of α-Mg decreased after annealing. Due to the fact that the intensity of XRD diffraction lines is directly proportional to the content of alloy elements, it is generally difficult to display the diffraction peaks of certain phases in the spectrum when their content is less than 5%. No obvious MgLi2Al and AlLi phase peaks were observed in LA141-0.5Cu alloy, and there may be overlapping peaks of AlLi and α-Mg phases between 2θ angles of 55 and 60°. There may be overlapping peaks of AlCuMg and Al2Cu phases between 2θ angles of 40 and 45°. An AlCuMg phase peak appears in the annealed LA141-0.5Cu alloy. Due to the low content of Al and Cu alloy elements, the peaks of the Mg-Li-Al-Cu second phase are few and the intensity is low, making it difficult to accurately determine the second phase. Further analysis of the alloy phase composition is needed.

The SEM images of each wire are shown in Figure 2. The second phase is uniformly dispersed on the alloy matrix after drawing and annealing, and the content of the annealed second phase slightly increases, as shown in Figure 2a,f. From Figure 2b,g, it can be observed that the size of the second phase particles in the LA141-0.5Cu alloy in the drawn state varies, with the larger phase appearing irregular and the smaller phase appearing granular. It is preliminarily speculated that there are two different phases, and the second phase is more complex after annealing at 200 °C. The EDS images of the sheets are shown in Figure 2c–e,h–j. The solid solubility of Cu in Mg-Li alloys is extremely low, almost insoluble in magnesium alloys [20]. EDS results show that the second phase is the AlCuMg phase, and in some areas where Cu element is distributed, there is no distribution of Al element. Therefore, there are other second phases containing Cu. Due to the inability of EDS to identify the Li element, it is necessary to further determine the type of second phase in the alloy through TEM.

The TEM images of the drawn wires are shown in Figure 3. It is shown in Figure 3a that the main phases are α-Mg and β-Li and the white phase is the matrix phase, and according to the diffraction pattern calibration results, it is confirmed that the matrix phase is the β-Li phase. There are elongated white phases in the alloy, as shown in Figure 3b. The diffraction pattern of the elongated white phase was calibrated to determine that it is the LiMg phase (Li_0.81Mg_0.19). Diffraction pattern calibration was performed on the block-shaped black phase to determine that it is the AlCuLi phase (Li_12.21Cu_4.21Al_23.58). as shown in Figure 3c. Figure 3d is the high-resolution transmission image of the dashed area in Figure 3c; from the high-resolution image, it can be seen that the AlCuLi phase is coherent with the matrix.

Figure 4 shows the TEM images of annealed LA141-0.5Cu wires, indicating an increase in white color compared to the drawn wires, and the presence of both micro and nano phases. By calibrating the diffraction pattern of the elongated black phase shown in Figure 4b, it can be determined that the elongated black phase is the AlCuLi phase (Li_12.21Cu_4.21Al_23.58). In addition to elongated black phases, annealed alloys also contain blocky black phases. The large block black phase diffraction pattern matches well with the AlCuLi phase (Li_12.21Cu_4.21Al_23.58) diffraction pattern, as shown in Figure 4c. Therefore, the annealed elongated black phase and the blocky black phase are the same phase.

3.2. Mechanical Properties

Figure 5a shows the tensile stress–strain curves of drawn and annealed LA141-0.5Cu wires. The yield strength of the drawn wires alloy is 151 MPa, the tensile strength is 160 MPa, and the elongation is 17.7%. The yield strength of annealed wires is 104 MPa, the tensile strength is 127 MPa, and the elongation is 22.4%. After annealing, the strength of the alloy decreases and the plasticity increases. Figure 5b,c show the room temperature tensile fracture SEM images of LA141-0.5Cu alloy. It can be seen that the alloy exhibits obvious ductile fracture characteristics. In the drawn wires, the dimples are small and uniform, with shallow dimples and a few tearing edges present. After annealing, the number of second-phase particles slightly increases, the number and depth of tough dimples increase, and the tearing edges become longer.

3.3. Corrosion Properties

The weight loss experiment was conducted at 93 °C/3% KCl solution, and Figure 6a shows the corrosion results. The corrosion rate of drawn wires is 5.10 mg/cm²/h, and the corrosion rate of annealed wires is 4.54 mg/cm²/h. The corrosion resistance of the alloy after annealing is improved compared to the drawn wires. Figure 6b,c show the corrosion surface morphology of the drawn and annealed LA141-0.5Cu wires. From the corrosion surface SEM images, it can be seen that the corrosion of the drawn wire is more severe.

Initial electrochemical corrosion experiments were performed on the drawn and annealed alloys separately, and the polarization curve of LA141-0.5Cu alloy was obtained, as shown in Figure 7. It can be seen that the corrosion current density of LA141-0.5Cu alloy in the drawn state is about 5.07 A/cm², and the corrosion current density in the annealed state is about 4.01 A/cm² (Figure 7a). After annealing, the corrosion current density decreases, thus improving the corrosion resistance of the alloy. From the Nyquist plot in Figure 7b, it can be seen that the capacitance arc diameter of the annealed and drawn alloys is not significantly different, indicating that the corrosion rate of the alloys is not significantly different. The low-frequency impedance modulus can be used to evaluate corrosion resistance, and the smaller the polarization resistance, the worse the corrosion resistance of the alloy. From Figure 7d, it can be seen that the impedance values of the drawn and annealed alloys are not significantly different at low frequencies, and the annealed alloy is slightly higher than the drawn alloy, indicating that the annealed alloy has better corrosion resistance.

4. Discussion

It can be seen from Figure 5a that the mechanical properties of the drawn and annealed wires are completely different. As shown in Figure 5b,c, after adding alloying elements, a large number of second-phase particles appear in LA141-0.5Cu alloy. According to dislocation theory, there are dislocation loops around the second-phase particles, which are in an equilibrium state in the absence of external forces. During the deformation process, when the external force is large enough, the dislocation loop will restart and move towards the second-phase particles, causing them to separate from the matrix and form micro voids. A large number of voids will grow and connect to form tough dimples [21]. Due to the uniform and fine second-phase particles in LA141-0.5Cu alloy, the toughness dimples in LA141-0.5Cu alloy are relatively small. According to transmission electron microscopy, the second phase in LA141-0.5Cu alloy is mostly distributed within the crystal, reducing the segregation of alloy elements at grain boundaries, reducing intergranular fracture of the alloy, and improving the plasticity of the alloy to a certain extent. After annealing, the stress in the wire material is released and dislocations are reduced, resulting in increased plasticity and decreased strength.

In the electrochemical impedance spectroscopy (Figure 7), it can be seen that an inductive arc appears in the LA141-0.5Cu wires, indicating that the corrosion product film of the alloy has been damaged. The drawn LA141-0.5Cu wires can be completely dissolved in a 93 °C/3wt% KCl solution for about 6 h. Based on this, it is speculated that the corrosion process of the alloy is as follows: After corrosion, the surface of the alloy rapidly corrodes, forming a corrosion product film. Under the action of Cl⁻, the corrosion product film is quickly destroyed, and the exposed Mg continues to corrode. The Cu-containing second phase in the alloy undergoes galvanic corrosion with the substrate, further promoting the corrosion of Mg-Li alloys.

The second phase is an important factor affecting the corrosion of magnesium alloys. In LA141-0.5Cu wires, there is a potential difference between the Cu-containing second phase and α-Mg; as shown in Figure 8, the potential of the second phase is higher than the potential of α-Mg. And the potential of α-Mg is higher than the potential of β-Li [22], which can form an electric couple with the matrix, leading to galvanic corrosion in the alloy. The solid solubility of Cu in Mg and Li is extremely low, almost insoluble in Mg-Li alloys, and exists in the form of a second phase in Mg-Li alloys. In previous microstructural studies, it was found that the vast majority of the second phase in Mg-Li-Al-Cu alloys is the Cu-containing phase. The electrode potential of the Cu-containing phase is high, and even with trace addition, the corrosion rate will sharply increase [23]. In LA141-0.5Cu wires, the potential difference between the Cu-containing phase and the matrix is as high as 153 mV (Figure 8). The presence of a large amount of Cu-containing second phase greatly promotes the corrosion of the alloy and significantly reduces the corrosion resistance of LA141-0.5Cu wires.

The dislocation density can be roughly calculated using XRD results, and it is related to the diffraction peak width. The grain size and lattice distortion caused by defects such as dislocations and stacking faults can all affect the diffraction peak width of XRD. Here, the Williamson–Hall formula is used to calculate dislocation density [24]. The formula for calculating the diffraction peak width related to grain size is Equation (1), the formula for calculating the diffraction peak width related to lattice distortion is Equation (2), and the formula for calculating the total diffraction peak width is Equation (3):

ΔK_L = (kλ)/(Lcosθ)

(1)

ΔK_ε = cεtanθ

(2)

ΔK_tot = ΔK_L + ΔK_ε

(3)

In the formula, λ is the wavelength of the incident wave (λ = 0.15406 nm), L is the average grain size, θ is the diffraction angle, and k is a constant with a value of 0.9 [25]. ε is the full width at half maximum of the strain distribution, and the value of ε is related to the crystal plane. The formula here assumes that the strain distribution is isotropic and does not consider the influence of anisotropy, so the ε value of any diffraction crystal plane is the same. The value c is a constant, and its value is related to the distribution of lattice distortion in the grain. Here, the value of c is set to 2. At this point, Δ K_tot = (k λ)/(Lcos θ) + c ε tan θ = 0.135414/(Lcos θ) + ε 2tan θ. Using 2tan θ as the horizontal axis and Δ Ktot as the vertical axis, the fitting results are shown in Figure 9, where 1-1 represents the drawn state and 1-2 represents the annealed state. It was clearly observed that the slope ε decreased after annealing, and the fitting results showed that the as-drawn ε was 0.06059 and the as-annealed ε was 0.03345.

The calculation formula for dislocation density is shown in Equation (4) [24]:

ρ = Bε²/b²

(4)

Here, ρ is the dislocation density, B is a constant related to the elastic modulus and dislocation configuration of the material, and b is the Bernoulli vector modulus [24]. This formula assumes the isotropy of continuum mechanics and elasticity, without considering the anisotropy of elasticity on different crystal planes. After annealing, the value of ε decreases, which leads to a decrease in dislocation density. The ε of drawn wires is 1.8 times that of the annealed wires; therefore the dislocation density in the drawn wires is 1.8 times that of the annealed wires. This is the reason why the corrosion rate decreases after annealing [26].

5. Conclusions

The microstructure and properties of the drawn and annealed LA141-0.5Cu wires were systematically investigated. The following main conclusions can be drawn:

(1)

The tensile strength of Mg-14Li-0.8Al-0.5Cu alloy in the drawn state is 160 MPa, the yield strength is 151 MPa, and the elongation is 17.7%. The tensile strength of the annealed alloy is 127 MPa, the yield strength is 104 MPa, and the elongation is 22.4%. Although the plasticity of the annealed alloy is increased by 5% compared to the drawn alloy, the yield strength is reduced by 47 MPa and the tensile strength is reduced by 32 MPa. Overall, the drawn alloy balances plasticity and strength.

(2)

The main reasons for the high plasticity are as follows: In the alloy, the Li content reaches 14 wt. %. Based on the transmission results and previous research, the matrix transforms from the conventional HCP structure of the α-Mg phase to the BCC structure of the β-Li phase. Compared with closely packed hexagonal magnesium alloy, the addition of Li reduces the axial ratio of the body-centered cubic magnesium alloy, increases the symmetry of the crystal structure, and reduces the stacking fault energy, increasing the possibility of more slip systems starting.

(3)

The corrosion rate of the drawn Mg-14Li-0.8Al-0.5Cu alloy is 5.10 mg/cm²/h, and the corrosion rate of the annealed alloy is 4.54 mg/cm²/h. Annealing has little effect on the corrosion rate of alloys. The reasons affecting the corrosion rate are second phases and dislocation. The second phase in the alloy forms galvanic corrosion with the substrate, promoting substrate dissolution. SKPFM results showed that the potential difference between the second phase and the substrate was approximately 153 mV, causing galvanic corrosion of the alloy. The second phase acted as the cathodic electrode and the substrate as the anodic electrode, accelerating the dissolution of the substrate. Dislocations can also affect the corrosion of magnesium alloys, manifested as a decrease in dislocation density and corrosion rate. After annealing, the dislocation density of Mg-14Li-0.8Al-0.5Cu alloy decreases and its corrosion resistance improves.

(4)

The mechanical properties of the alloy meet the requirements for subsequent weaving as a metal knot, and the corrosion resistance of the alloy allows the woven knot to be used as a temporary plugging agent for shale oil and gas extraction.