Preparation of fine-grain aluminum alloy by spinning method and its corrosion resistance test

Due to the lack of shear resistance, the implanted materials in the aluminum alloy will lead to obvious wear. The emergence of fine-grained aluminum alloy materials can perfectly solve the above problems. Therefore, in this paper, the fine-grain aluminum alloy material is prepared by the spinning method, and the corrosion resistance test is carried out. The experimental results show that there is no significant difference in weight loss and surface morphology between aluminum alloy and fine-grain aluminum alloy immersed in normal saline and D-Hand’s solution for 30 days. The experimental results show that the fine-grained aluminum alloy has the basic conditions for in vivo implantation, which lays a foundation for the development of a new generation of in vivo implantation materials.


Introduction
At present, the most widely used implant materials in the low-end market are aluminum alloy materials.Due to the lack of shear resistance of the material, there is obvious wear after implantation in the body.Fine-grain aluminum alloy material can greatly improve its shear resistance, so it is possible to solve the wear problem.At present, the methods for preparing fine-grained metal materials mainly include the high-pressure torsion method, multi-directional forging method, cumulative stacking rolling method, equal channel angular extrusion method, and power spinning method [1][2][3] .The first four methods are not suitable for large-scale production, so this paper uses the spinning method to prepare fine-grain aluminum alloy materials [4][5] .The corrosion resistance of the prepared fine-grained aluminum alloy is tested to explore the possibility of the material as a new generation of in vivo implant material [6] .

Preparation of fine-grain aluminum alloy by spinning method
Spinning is a plastic extrusion-forming process originating from rolling [7][8] .The processing method is to install the blank on the core mold, clamp it with a fixture, or use a tail top.The spindle drives the core mold and the blank to rotate together, while the spinning wheel compresses the blank and feeds it axially and radially to prepare the rotating parts, as shown in Figure 1.
①Spinning wheel 1 ②Spinning wheel 2 ③Tail top ④Blank Figure 1.Spinning Forming Process.In the spinning process, the spinning is divided into ordinary spinning and strong spinning according to the difference in the wall thickness reduction rate of the blank [9][10] .Among them, ordinary spinning refers to the spinning processing method in which only the outer diameter of the blank changes significantly during the spinning process, while the wall thickness of the blank does not change basically or the wall thickness reduction rate of the blank is small.The spinning processing method in which the shape of the blank changes and the wall thickness reduction rate of the blank also changes significantly during the power spinning.
In this paper, based on the spinning method of "strong spinning-heat treatment-re-strong spinningre-heat treatment-final ordinary spinning", according to the actual spinning experience and the influence trend of spinning process parameters on the forming quality of spinning products, it is preliminarily proposed to prepare fine-grain aluminum alloy by 8 passes of spinning.To improve the plasticity of the aluminum alloy blank and prevent the occurrence of work hardening, two heat treatments are carried out during the processing.The feed rate of the first 5 passes is 0.6 mm/r, the spindle speed is 110 r/min, and the total thinning rate is about 64 %.The internal grains of the metal are elongated and densely arranged by the large thinning amount of the first 5 passes.The feed rate of the last three passes is 0.4 mm/r, and the spindle speed is 110 r/min.The small thinning rate increases the total thinning rate to about 67 %, and the thinning rate of the last three passes increases slightly.This ordinary spinning is beneficial to refine the elongated grains.The specific spinning scheme is shown in Table 1.
Table 1 The fine-grained aluminum alloy products prepared according to the spinning scheme are shown in Figure 2.  The grain size of the aluminum alloy product is less than 3000 nm by metallographic examination.The metallographic spectrum is shown in Figure 3.

Corrosion resistance test
In this paper, normal saline and D-Hand's solutions are used as corrosion solution media, and aluminum alloy materials are used as a control group to test the corrosion resistance of fine-grained aluminum alloy materials.6 pieces of aluminum alloy and 6 pieces of fine-grained aluminum alloy are prepared.The samples are polished step by step by using 600 mesh, 1200 mesh, and 2000 mesh sandpaper and then polished.Then ultrasonic cleaning is carried out with 75 % ethanol and acetone for 15 min.After natural drying, the original weight weighing records of the metal materials in the test group and the control group are recorded.The treated metal materials are placed in D-Hand's and normal saline, respectively, and placed in a 37°C constant temperature incubator for immersion corrosion for 30 days.After the immersion corrosion is completed, the samples are naturally dried and weighed again.

Weight loss analysis during immersion corrosion
The weight loss of the material in the immersion corrosion experiment is an important index to test the corrosion resistance of the material.The weight loss can directly reflect the corrosion resistance of the metal material.Table 2 shows the weight and weight loss of aluminum alloy samples and fine-grained aluminum alloy samples in normal saline and D-Hand's solution at each test stage.It can be seen from Table 2 that the weight of aluminum alloy samples and fine-grained aluminum alloy samples in normal saline decreases by about 0.1 mg.The weight of aluminum alloy samples and fine-grain aluminum alloy samples increases by about 0.1-0.2mg in D-Hand's solution.There is no significant difference in the weight loss of the two samples in the above corrosive solution.

Morphology analysis after corrosion
Figure 4 shows the surface morphology of aluminum alloy samples and fine-grained aluminum alloy samples after immersion corrosion in normal saline and D-Hand's solutions for 30 days.4 that the surface morphology of aluminum alloy and fine-grained aluminum alloy samples does not change significantly after soaking in normal saline and D-Hand's solutions for 30 days, and no obvious corrosion and rust are observed.Therefore, we will observe the surface morphology of the two metal materials by scanning electron microscopy.Because the volume of the sample subjected to immersion corrosion is very small, the grinding and polishing are carried out manually, which is difficult.The grinding and polishing have certain inhomogeneity and leave some scratches on the surface, but it does not affect the subsequent analysis and meets the experimental requirements.
As shown in Figure 5 and Figure 6, the surface morphology of the aluminum alloy sample and the fine-grained aluminum alloy sample after 30 days of immersion corrosion in normal saline and D-Hand's solutions is observed by scanning electron microscopy.From Figure 5 and Figure 6, no obvious corrosion marks (corrosion pits, corrosion cracks) are observed on the surface of aluminum alloy and fine-grain aluminum alloy.However, it is observed that smaller grains are dispersed on the surface of both samples (shown in the red circle in the figure).Through research, this small grain is NaCl, which is the product of Na + and Cl -combined with the chemical bond on the surface of the sample.This phenomenon is because the aluminum alloy has a certain surface biological activity, and the material can form a stable chemical bond on its surface.

Conclusion
In this paper, the fine-grain aluminum alloy is prepared by the method of 'power spinning-heat treatmentpower spinning-heat treatment-finally ordinary spinning', and the corrosion resistance is tested.The results of the immersion corrosion test show that there is no significant difference in weight loss between aluminum alloy and fine-grained aluminum alloy immersed in saline and D-Hand's solutions for 30 days.The surface macroscopic morphology does not change significantly compared with that before corrosion.No obvious corrosion pits and corrosion cracks are observed on the surface of the two materials by scanning electron microscopy.The experimental results show that the fine-grained aluminum alloy has the basic conditions for in vivo implantation, which lays a foundation for the development of a new generation of in vivo implantation materials.
alloy saline corrosion for 30 days b) Aluminum alloy D-Hand's corrosion for 30 days c) Corrosion of fine-grain aluminum alloy in normal saline for 30 days d) Corrosion of fine-grain aluminum alloy in D-Hand's for 30 days.

Figure 4 .
Figure 4. Macroscopic surface morphology of sample after corrosion.It can be seen from Figure4that the surface morphology of aluminum alloy and fine-grained aluminum alloy samples does not change significantly after soaking in normal saline and D-Hand's solutions for 30 days, and no obvious corrosion and rust are observed.Therefore, we will observe the surface morphology of the two metal materials by scanning electron microscopy.Because the volume of the sample subjected to immersion corrosion is very small, the grinding and polishing are carried out manually, which is difficult.The grinding and polishing have certain inhomogeneity and leave some scratches on the surface, but it does not affect the subsequent analysis and meets the experimental requirements.As shown in Figure5and Figure6, the surface morphology of the aluminum alloy sample and the fine-grained aluminum alloy sample after 30 days of immersion corrosion in normal saline and D-Hand's solutions is observed by scanning electron microscopy.

Figure 5 .Figure 6 .
Figure 5.The surface morphology of saline immersion corrosion for 30 days.

Table 2 .
Quality Changes of Samples Before and After Immersion Corrosion.