Combined fabricating method for gradient nanostructure in surface layer of metal workpiece

Abstract

Provided is a combined fabricating method for gradient nanostructure in the surface layer of a metal workpiece. A plastic deformation layer in great depth is induced by laser shock peening, then the surface of the metal workpiece is nanocrystallized by surface mechanical attrition treatment, and finally a gradient nanostructure is obtained in the surface layer of the metal workpiece with desirable layer thickness and optimized micro-structure distribution.

Claims

1. A combined fabricating method for a gradient nanostructure in a surface of a metal workpiece, the method comprising: performing a laser shock peening (LSP) treatment on the surface of the metal workpiece to induce a plastic deformation layer on the surface of the metal workpiece; after performing the LSP treatment, then performing a surface mechanical attrition treatment (SMAT) on the surface of the metal workpiece to nanocrystallize the surface of the metal workpiece; and after performing the SMAT, finally obtaining the gradient nanostructure in the surface of the metal workpiece with a desirable layer thickness and an optimized micro-structure distribution, wherein the method further comprises: (1) prior to performing the LSP treatment, burnishing, polishing, and performing a first cleaning of the surface of the metal workpiece; (2) prior to performing the LSP treatment, determining a pulse width, a pulse energy, a frequency, a spot diameter, and a number of coverage layers of LSP related with a material of the metal workpiece, and attaching an absorption layer to the surface of the metal workpiece, wherein the performing of the LSP treatment comprises performing a massive overlapped LSP treatment on areas of the surface of the metal workpiece; (3) removing the absorption layer on the surface of the metal workpiece after the areas of the surface of the metal workpiece are treated by massive overlapped LSP, and performing a second cleaning of the surface of the metal workpiece; and (4) determining a vibration frequency, a ball diameter, and a treatment time of SMAT related with the material of the metal workpiece, wherein the SMAT on the surface of the metal workpiece is performed based on the vibration frequency, the ball diameter, and the treatment time of SMAT related with the material of the metal workpiece.

2. The according to claim 1, the wherein the material of the metal workpiece is a metal material used in the aeronautic and astronautic industry, automobile and ship-building industries, and chemical industry, and wherein the material of the metal workpiece comprises aluminum alloy, titanium alloy, magnesium alloy, stainless steel, or nickel-based superalloy.

3. The method according to claim 1, wherein in the surface of the metal workpiece, after the obtaining of the gradient nanostructure, a thickness of a nanostructured layer is 30˜50, a thickness of a submicron structural layer is 80˜200 and a thickness of an entire grain refined layer is 800˜1,300 μm.

4. The method according to claim 1, wherein, during the LSP treatment, the pulse width is 8˜30 ns, the pulse energy is 2˜15 J, the frequency is 1 Hz, the spot diameter is 2˜3 mm, an overlap ratio of a spot in a transverse direction and a longitudinal direction is 50%, a number of coverage layers is 1˜3, a confinement layer is water film formed in 1˜2 mm thickness by deionized water flow, a peak pressure of the pulses is P1, a pressure at edges of the spot is P2, and $2{\sigma }_H⩽P_1⩽2.5{\sigma }_H,P_2⩾{\sigma }_H,\mathrm{where},{\sigma }_H={\sigma }_Y^{\mathrm{dyn}}\frac{\left(1-v\right)}{\left(1-2v\right)},$ σ.sub.Y.sup.dyn is dynamic yield strength, and ν is the Poisson's ratio of the material, and wherein an entire spot area subjected to the LSP treatment has dynamic plastic deformation, while the metal workpiece in a central area of the spot has no macroscopic deformation.

5. The method according to claim 1, wherein the vibration frequency is 50 Hz, the ball diameter is 6˜8 mm, and the treatment time is 5˜60 min.

6. The method according to claim 2, wherein the burnishing, polishing, and performing of the first cleaning of the surface of the metal workpiece comprises: burnishing and polishing the surface of the metal workpiece with abrasive paper of 500 #, 800 #, 1000 #, 1600 #, 2000 #, and 1500 #, sequentially; and then cleaning the surface of the metal workpiece by ultrasonic cleaning with deionized water.

Description

DESCRIPTION OF DRAWINGS

(1) To describe the embodiments of the present application and the technical solutions in the prior art clearly, hereunder the accompanying drawings necessary for describing the embodiments or the prior art will be introduced briefly.

(2) FIG. 1 is a schematic diagram of sample size in two embodiments;

(3) FIG. 2 is a schematic diagram of gradient change of the grain size of AM50 magnesium alloy along the depth direction;

(4) FIG. 3A-3B show TEM images of the surface layer of AM50 magnesium alloy: FIG. 3(a) shows a TEM image of the surface layer of a sample subjected to LSP; FIG. 3(b) shows a TEM image of the surface layer of a sample treated by LSP first and then by SMAT;

(5) FIG. 4A-4B show TEM images of the surface layer of H62 brass: FIG. 4(a) shows a TEM image of the surface layer of a sample subjected to LSP; FIG. 4(b) shows a TEM image of the surface layer of a sample treated by LSP first and then by SMAT.

EMBODIMENTS

(6) Hereunder the embodiments of the present invention will be detailed in embodiments with reference to the accompanying drawings, but the present invention is not limited to those embodiments.

Embodiment 1

(7) An example of preparing gradient nanostructure in the surface layer of magnesium alloy with the method described above, including the following steps:

(8) (1) Two AM50 magnesium alloy samples in 30 mm×50 mm×5 mm dimensions are selected for comparison test, the two samples are denoted as sample 1 and sample 2 respectively, and the treated area A is shown in FIG. 1.

(9) (2) LSP treatment is carried out in the area A determined in the step (1) for the sample 1 and sample 2, wherein, the parameters of LSP are as follows: the spot is in a circular shape in 3 mm diameter, the pulse width is 10 ns, the pulse energy is 12 J, the overlap ratio in the transverse direction and longitudinal direction is 50%, and the number of coverage layers is 1.

(10) (3) The sample 2 is treated by SMAT, wherein, the parameters of SMAT are as follows: the vibration frequency of the system is 50 Hz, the ball diameter is 8 mm, and the treatment time is 30 min.

(11) As shown in FIG. 3, wherein, FIG. 3(a) shows a TEM image of the surface layer of the sample 1, and FIG. 3(b) shows a TEM image of the surface layer of the sample 2. In the sample 1, the average grain size is about 50 nm, the gradient nanostructure are in about 30 μm depth, and the compressive residual stress layer is in about 1 mm depth. In the sample 2, the average grain size is about 20 nm, the gradient nanostructure are in about 50 μm depth, and the compressive residual stress layer is in about 1 mm depth. The depth of the compressive residual stress layer in the sample 1 and the depth of the compressive residual stress layer in the sample 2 are close to each other, owing to the fact that the depth of the compressive residual stress layer induced by SMAT is smaller than the depth of the compressive residual stress layer induced by LSP. However, the refined grains obtained by LSP are only induced by the force effect created by laser shock, while SMAT not only utilizes the force effect but also utilizes the heat effect, and thereby provides temperature required for recrystallization, and the grain size in the surface of the sample is smaller as a result.

Embodiment 2

(12) An example of preparing gradient nanostructure in the surface layer of copper alloy with the method described above, including the following steps:

(13) (1) Two H62 brass samples in 30 mm×50 mm×5 mm dimensions are selected for comparison test, the two samples are denoted as sample 1 and sample 2 respectively, and the treated area A is shown in FIG. 1.

(14) (2) LSP treatment is carried out in the area A determined in the step (1) for the sample 1 and sample 2, wherein, the parameters of LSP are as follows: the spot is in a circular shape in 3 mm diameter, the pulse width is 10 ns, the pulse energy is 6 J, the overlap ratio in the transverse direction and longitudinal direction is 50%, and the number of coverage layers is 1.

(15) (3) The sample 2 is treated by SMAT, wherein, the parameters of the surface mechanical attrition are as follows: the vibration frequency of the system is 50 Hz, the ball diameter is 8 mm, and the treatment time is 15 min.

(16) As shown in FIG. 4, wherein, FIG. 4(a) shows a TEM image of the surface layer of the sample 1, and FIG. 4(b) shows a TEM image of the surface layer of the sample 2. In the sample 1, the average grain size is about 15 nm, the gradient nanostructure are in about 50 μm depth, and the compressive residual stress layer is in about 1 mm depth. In the sample 2, the average grain size is about 10 nm, the gradient nano structure are in about 60 μm depth, and the compressive residual stress layer is in about 1 mm depth.

(17) Both of the embodiments demonstrate: compared with sole LSP treatment, the nanometer grain size in the surface layer of the metal material prepared with the method provided in the present invention is obviously decreased, and the depth of the gradient nanostructure is increased effectively. Owing to the fact that the compressive residual stress layer caused by LSP has greater depth, the depth values of the compressive residual stress layers in the sample 1 and the sample 2 in the two embodiments are close to each other and almost the same.