CN113584363A

CN113584363A - Surface strengthening method of 2024 aluminum alloy

Info

Publication number: CN113584363A
Application number: CN202110862048.3A
Authority: CN
Inventors: 董瑞峰; 李晨晖; 刘�东; 张晓阳; 赵宇宏; 侯华
Original assignee: North University of China
Current assignee: North University of China
Priority date: 2021-07-29
Filing date: 2021-07-29
Publication date: 2021-11-02

Abstract

The invention provides a method for strengthening the surface of 2024 aluminum alloy, relates to the technical field of shot peening, and particularly relates to a shot peening process method for strengthening the aluminum alloy. According to the invention, a large amount of crystal defects and strain are generated on the surface of the 2024 aluminum alloy by controlling the shot blasting flow, shot blasting distance, moving speed and shot blasting jet pressure, so that the surface stress of the material is increased, and then stress relief annealing is carried out, so that the size and the shape are stabilized, and the high strength and the hardness are maintained, thereby improving the mechanical property of the 2024 aluminum alloy. The invention adopts the double-nozzle opposite-spraying type shot blasting machine, greatly improves the shot blasting efficiency, has higher coverage rate, can directly finish spraying treatment on the prepared tensile sample, and has less damage to materials. The sample after shot blasting is further heat-treated, and vacancies are transferred to grain boundaries, dislocations, or bonded to interstitial atoms to disappear, so that the vacancy concentration is significantly reduced. After shot blasting and heat treatment, the microscopic strain of the alloy surface is greatly increased, and the yield strength, the tensile strength and the elongation are obviously improved.

Description

Surface strengthening method of 2024 aluminum alloy

Technical Field

The invention relates to the technical field of shot peening, in particular to a shot peening method for strengthening mechanical properties of aluminum alloy.

Background

2024 aluminum alloy is a heat-treatable strengthened hard aluminum alloy, has the advantages of high strength, low specific gravity, good heat resistance and fatigue strength, etc., is a typical high-strength structural aluminum alloy with excellent comprehensive performance, and has very wide application in the fields of aviation and aerospace, such as aircraft frameworks, rib beams, bulkheads, propeller elements, aerospace satellites and other spacecrafts, and some commercial and civil products, such as rivets, truck hubs and the like.

Until now, shot peening has been used as an effective means for improving the fatigue strength and other properties of materials. Xuyansong et al have studied the effect of shot peening on fatigue performance of 2024 aluminum alloys and have shown that the median fatigue life of the test specimens after shot peening is 1.53 to 2.55 times the median fatigue life of the test specimens without shot peening, but it is still difficult to meet the performance requirements of some special industries on 2024 aluminum alloy parts.

Shot peening can significantly improve the fatigue resistance of the material, in most cases due to the formation of a residual compressive stress field. The invention obtains a preferable shot blasting and heat treatment process for further strengthening the mechanical property of the 2024 aluminum alloy by analyzing and comparing the changes of the mechanical property of the alloy before and after shot blasting and under different shot blasting conditions.

Disclosure of Invention

The invention aims to provide a preferable shot blasting method and a heat treatment process, which are applied to the surface treatment of 2024 aluminum alloy to improve the mechanical property of 2024 aluminum alloy.

A surface strengthening method for strengthening mechanical property of 2024 aluminum alloy is characterized by comprising the following steps:

1) the experimental material is selected to be a 2024 aluminum alloy thick plate, and the chemical composition (wt%) of the alloy is as follows: cu 4.59; mg 1.44; mn 0.76; si 0.11; fe 0.22; 0.013 Zn; ti 0.044; and Al is the rest.

2) The 2024 aluminum alloy thick plate is prepared into a tensile sample along the rolling direction, and the sample is ground and polished by silicon carbide abrasive paper before the test and is cleaned by alcohol.

3) The prepared tensile sample was placed on a stage of a shot blasting machine to be shot-blasted. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 9-11 kg/min; shot blasting distance: 490-510 mm; moving speed: 2.0-4.0 m/min; shot blasting pressure: 0.4-0.6 MPa. The shot is carbon steel shot, the grain diameter is 3.18mm, and the shot blasting time is 30 minutes.

4) Grinding and polishing the sample subjected to shot blasting by using silicon carbide abrasive paper, cleaning the sample by using alcohol, then putting the sample into a sodium carbonate solution, heating the solution to 60-80 ℃, preserving heat, oscillating the solution for 30min, and then filtering, washing and drying the solution to constant weight.

5) And (3) putting the cleaned sample into a resistance furnace, heating to 80-120 ℃, preserving heat for 2 hours, and then air-cooling to obtain the shot blasting heat treatment aluminum alloy piece.

The invention prepares a tensile sample for a rolled 2024 aluminum alloy plate by a warp cutting machine, and before shot blasting, the sample is ground and polished by silicon carbide abrasive paper and is cleaned by alcohol. Fixing a prepared tensile sample 4 on a clamp 5 of a shot blasting device, putting screened shots into a shot box 1, accelerating the shots to be sprayed out from a nozzle through an air compressor 2, driving the tensile sample fixed on the clamp to move at 3.0m/min by a motor above a base 6, and uniformly spraying the shots on the tensile sample. The tensile sample can be sprayed on the front surface, and the side surface of the tensile sample can also be fixed and sprayed on the side surface, and the shot blasting coverage rate reaches 90%. According to the invention, double-sided single nozzle is adopted for opposite spraying, the stress generated in the shot blasting process is uniform, the efficiency is high, the final property of the sample is superior to that of the sample subjected to face-to-face shot blasting, and the specific heat treatment process and parameters aiming at the shot blasting 2024 aluminum alloy sample are combined, so that the strength and the toughness of the sample can be kept at high performance, the elongation can be greatly improved, and the workpiece requirements of special industries are met.

Compared with the prior art, the invention has the following beneficial effects:

the shot blasting surface strengthening treatment method can achieve better surface treatment effect by controlling the grain diameter, hardness, shot spraying pressure, shot flow, clamp moving speed and shot blasting time of the shots. Compared with the conventional shot blasting method, the shot blasting coverage rate of the invention is higher. The prepared tensile sample can be directly sprayed, and the material damage is small. Meanwhile, the double-nozzle opposite-spraying shot blasting machine is adopted, so that the shot blasting efficiency can be greatly improved.

The sample after shot blasting is further subjected to heat treatment, i.e., stress relief annealing, and vacancies are transferred to grain boundaries, dislocations, or bound to interstitial atoms to disappear, so that the vacancy concentration is significantly reduced. The internal stress in the sample is eliminated to a great extent while the cold deformation hardening effect is kept, the deformation or cracking of the workpiece can be avoided, and the corrosion resistance of the workpiece is improved. After shot blasting and heat treatment, the yield strength, tensile strength and elongation of the material are all obviously improved.

Drawings

In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the following drawings are taken for illustration:

FIG. 1 is a schematic view of a shot-blasting apparatus according to the method of the present invention;

FIG. 2 is a drawing of the dimensions of a tensile specimen;

FIG. 3 is a metallographic structure diagram of a shot-peened sample and a non-shot-peened sample near a surface layer, wherein FIG. 3a is a cross-sectional metallographic diagram of a sample near a shot-peened surface (in the direction of an arrow) in example 2, and FIG. 3b is a metallographic diagram of a sample which has not been subjected to shot-peening;

FIG. 4 is an X-ray diffraction pattern of the surface layer of the sample before and after shot blasting in example 2;

FIG. 5 is a scanning electron micrograph of a fracture after stretching of shot and non-shot samples, wherein FIG. 5a is a scanning electron micrograph of a fracture after stretching of a shot sample of example 2, and FIG. 5b is an enlarged micrograph thereof; FIG. 5c is a scanning electron micrograph of a fracture in the non-shot-peened tensile specimen, and FIG. 5d is an enlarged micrograph thereof.

In the figure: 1-pill box, 2-air compressor, 3-water tank, 4-tensile sample, 5-clamp, 6-motor, 7-clamping end.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail with reference to the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The technical solution of the present invention is described in detail below with reference to the embodiments and the drawings, but the scope of protection is not limited thereto.

The surface shot peening strengthening treatment is carried out by adopting 2024 aluminum alloy, and the process and the steps are as follows:

(1) a2024 aluminum alloy thick plate was selected as the test material, and the chemical composition (wt%) of the alloy is shown in Table 1:

TABLE 1 chemical composition of 2024 aluminum alloy used in this experiment

(2) The 2024 aluminum alloy thick plate is prepared into a tensile sample along the rolling direction, and the specific dimensions are as shown in figure 1: a is 6 mm; b is 15 mm; r is more than or equal to 12 mm; l is₀＝53.6mm(5.65√S₀)L_c＝70mm(≥L₀+1.5√S₀)；L_t140 mm. Before the test, the sample was ground and polished with silicon carbide abrasive paper and washed with alcohol.

(3) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 9-11 kg/min; shot blasting distance: 490-510 mm; moving speed: 2.0-4.0 m/min; the injection pressure is 0.4-0.6 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

(4) Grinding and polishing the sample subjected to shot blasting by using silicon carbide abrasive paper, cleaning the sample by using alcohol, then putting the sample into a sodium carbonate solution, heating the solution to 60-80 ℃, preserving heat, oscillating the solution for 30min, and then filtering, washing and drying the solution to constant weight.

(5) And (3) putting the cleaned sample into a resistance furnace, heating to 80-120 ℃, preserving heat for 2 hours, and then air-cooling to obtain the shot blasting heat treatment aluminum alloy piece.

Example 1

(1) The 2024 aluminum alloy thick plate is prepared into a tensile sample along the rolling direction, and the sample is ground and polished by silicon carbide abrasive paper before the test and is cleaned by alcohol.

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.4 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 2

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 3

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.6 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 4

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 9 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 5

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 11 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 6

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 490 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 7

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 510 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 8

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 2.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 9

(2) And (3) placing the prepared tensile sample on a clamp of a shot blasting device for shot blasting. The shot blasting parameters are as follows: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 4.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot with a particle size of 3.18mm and shot blasting time of 30 min.

Example 10

(2) And (3) putting the sample into the sodium carbonate melt, heating to 60 ℃, preserving heat, shaking for 30min, filtering, washing and drying to constant weight.

(3) And (3) putting the cleaned sample into a resistance furnace, heating to 100 ℃, preserving heat for 2 hours, and air cooling.

Example 11

(3) Grinding and polishing the sample subjected to shot blasting by using silicon carbide abrasive paper, cleaning the sample by using alcohol, then putting the sample into a sodium carbonate solution, heating the solution to 60 ℃, preserving heat, oscillating the solution for 30min, and then filtering, washing and drying the solution to constant weight.

(4) And (3) putting the cleaned sample into a resistance furnace, heating to 100 ℃, preserving heat for 2 hours, and then cooling in air.

Example 12

(4) And (3) putting the cleaned sample into a resistance furnace, heating to 80 ℃, preserving heat for 2 hours, and then cooling in air.

Example 13

(4) And (3) putting the cleaned sample into a resistance furnace, heating to 120 ℃, preserving heat for 2 hours, and then cooling in air.

Comparative example 1

(4) And (3) putting the cleaned sample into a resistance furnace, heating to 200 ℃, preserving heat for 2 hours, and then cooling in air.

The observation, detection and analysis of the 2024 aluminum alloy after shot blasting are as follows:

(1) the tensile test was carried out on the heat-treated test specimens before and after the shot blasting on an Instron1195 electronic tensile testing machine at a tensile rate of 1.0mm/min and at room temperature. The tensile test results are shown in Table 2, and the mechanical properties of the material after shot blasting are improved. By comparison, example 11 parameters: shot blasting angle: 90 degrees, spraying by a single nozzle on two sides; flow rate: 10 kg/min; shot blasting distance: 500 mm; moving speed: 3.0 m/min; the injection pressure is 0.5 MPa; the shot is carbon steel shot, the grain diameter of the shot is 3.18mm, the shot blasting time is 30min, the cleaning sample is placed in a resistance furnace to be heated to 100 ℃, the temperature is kept for 2 hours, then air cooling is carried out, and the yield strength, the tensile strength and the elongation of the material are improved to the maximum, namely 376.8MPa, 487.2MPa and 22.64 percent respectively.

TABLE 2 tensile test results

(2) And (3) analyzing the metallographic structure of the alloy before and after shot blasting by using an OLYMPUS GX51 Optical Microscope (OM). The observation results are shown in fig. 3a and 3b of fig. 3. FIG. 3a is a cross-sectional metallographic view taken close to the shot surface (in the direction of the arrow) in example 2, and it can be seen that the grains of the alloy are finer as the shot surface layer is closer. FIG. 3b is a gold phase diagram of a sample which has not been subjected to shot blasting, and shows that the structure near the surface or the structure inside has coarse crystal grains, and the size is substantially the same and no fine grain region is found.

(3) The phase composition of the alloy was analyzed using a DX-2700X-ray diffractometer (XRD) and the change in the microscopic strain of the surface layer before and after shot blasting of the alloy was analyzed. FIG. 4 shows X-ray diffraction patterns of the surface layers of the samples before and after shot blasting (example 2). After the sample is subjected to shot blasting treatment, the intensity of a diffraction peak is gradually reduced, the full width at half maximum is gradually increased, and a sharp diffraction peak in a diffraction pattern is obviously widened. The reduction of the diffraction peak intensity and the increase of the full width at half maximum indicate that a large amount of crystal defects and strain are generated on the surface of the alloy, and the surface stress of the material is increased. Calculating the full width at half maximum of the diffraction peak according to the X-ray diffraction spectrum, and calculating the microscopic strain of the deformation surface before and after shot blasting by using the formulas (1) and (2). The microscopic strain of the surface after shot blasting increased from 0.2181% (± 0.01084) to 0.5769% (± 0.00215).

FW(S)^D＝FWHM^D-FW(I)^D (2)

In the formula:

-micro-straining; FWHM-diffraction PeakHalf height and width; FW (I) -Instrument Width; d is a deconvolution parameter; taking 1 in the text; theta-diffraction angle;

(4) fracture microstructure and morphology of the tensile specimens were analyzed using a Vega Tescan type Scanning Electron Microscope (SEM). FIG. 5a is a scanning electron micrograph of a fracture in the shot-peening specimen (example 2) after stretching, and FIG. 5b is an enlarged micrograph thereof. From the scanning electron microscope photo of the tensile fracture, the fracture morphology is a ductile fracture, pits with different sizes are distributed, and the pits are connected in a large area on the fracture in a net shape. From FIG. 5b, it can be seen that there are second phase particles in the dimple, and the phase may be Al as judged by the combination of the energy spectrum and XRD analysis₂CuMg, and the second phase may be a crack source that promotes crack propagation. It was also found that the second phase was broken due to the fact that the dislocations were difficult to cut, bypass, or climb during the stretching by the mechanism of the second phase, and thus stress concentration was generated at the phase interface to promote the breakage of the second phase. FIG. 5c is a scanning electron micrograph of a fracture in an unapprozened tensile specimen, and FIG. 5d is an enlarged micrograph thereof, and it can be found that the surface of the unapprozened fracture has a certain dimple but is small and shallow compared to FIG. 5a, which may be ductile fracture with partial quasi-cleavage fracture. It was found that the test specimens were stretched after shot blasting, and the distribution of the tough pits in the fracture was large and deep, so that the strength and elongation of the test specimens after shot blasting were improved as compared with those of the test specimens without shot blasting.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. A surface strengthening method of 2024 aluminum alloy is characterized by comprising the following steps:

1) preparing a product sample from the 2024 aluminum alloy thick plate along the rolling direction, grinding and polishing the product sample by using silicon carbide abrasive paper, and cleaning the product sample by using alcohol;

2) placing the sample on a clamp of a shot blasting device, and driving the clamp to move through a motor to perform shot blasting treatment;

3) grinding and polishing the sample subjected to shot blasting by using silicon carbide abrasive paper, cleaning the sample by using alcohol, then putting the sample into a sodium carbonate solution, heating the solution to 60-80 ℃, preserving heat, vibrating the solution for 30min, and then filtering, washing and drying the solution to constant weight.

4) And (3) putting the cleaned sample into a resistance furnace, heating to 80-120 ℃, preserving heat for 2 hours, and then air-cooling to obtain the shot blasting heat treatment aluminum alloy piece.

2. The method of claim 1, wherein the aluminum alloy 2024 comprises: the shot blasting treatment is double-sided single nozzle opposite blasting, and the shot blasting angle is 90 degrees.

3. The method of claim 1, wherein the aluminum alloy 2024 comprises: the shot flow of the shot blasting is 9-11 kg/min.

4. The method of claim 1, wherein the aluminum alloy 2024 comprises: the distance of the shot blasting is 490-510 mm.

5. The method of claim 1, wherein the aluminum alloy 2024 comprises: the distance of the shot blasting is 490-510 mm.

6. The method of claim 1, wherein the aluminum alloy 2024 comprises: the moving speed of the sample in the shot blasting process is 2.0-4.0 m/min.

7. The method of claim 1, wherein the aluminum alloy 2024 comprises: the shot used for shot blasting is carbon steel shot with the grain diameter of 3.18 mm.

8. The method of claim 1, wherein the aluminum alloy 2024 comprises: the pressure of the shot blasting treatment is 0.4-0.6MPa, and the shot blasting time is 30 min.

9. The method of claim 1, wherein the aluminum alloy 2024 comprises: the 2024 aluminum alloy comprises, by mass, Cu 4.59, Mg 1.44, Mn 0.76, Si 0.11, Fe 0.22, Zn 0.013, Ti0.044 and the balance of Al.

10. A 2024 aluminium alloy material produced by the strengthening method of any one of claims 1 to 9.