Shot peening is a relatively simple surface strengthening process compared to other surface modification methods, but its effects are significant. It is used in a variety of industries, including aerospace, locomotives, automobiles, and others.
The principle behind shot peening involves using projectiles to impact the material, creating small holes in the surface and causing plastic deformation. This results in residual stress on the metal surface. The compressed crystal grains beneath the surface must be restored to their original shape, which creates a layer of uniform residual compressive stress that strengthens the surface of the material.
As a result of shot peening, the surface layer of the material undergoes structural changes. The grains become finer, the dislocation density and lattice distortion increase, and a high residual compressive stress is formed. This residual stress significantly improves the fatigue strength and fatigue life of the material, as well as its strength, hardness, stress corrosion resistance and high temperature oxidation properties.
I. Testing Materials
The test used barrel-shaped parts made from 2A14 aluminum alloy, a material known for its high strength, good heat resistance, good machinability, and good electric welding and welding seam performance. The specific composition of this material is shown in Table 1.
Table 1 Chemical composition of aluminum alloy 2A14
| Element | Yes | Ass | mg | Zn | Mn | You | No | Al |
|---|---|---|---|---|---|---|---|---|
| Ingredient | 0.6-1.2 | 3.9-4.8 | 0.4-0.8 | ≤0.3 | 0.4-1.0 | ≤0.15 | ≤0.1 | others |
The 2A14 aluminum alloy barrel-shaped parts were divided into four groups (see Figure 1),
- the 1st group of surface roughness values: Ra = 0.30-0.65 μm;
- the 2nd group of surface roughness values: Ra = 2.20-4.71 μm;
- the 3rd group of surface roughness values: Ra = 6.5-7.1 μm;
- the 4th group of surface roughness values: Ra = 1.40-1.75 μm.
(a) The 1st group
(b) The 2nd group
(c) The 3rd group
（d）The 4th group
Fig.1 Parts before shot peening
The test was carried out using a SP1200 G4 pneumatic blasting machine, and its working principle is shown in Figure 2. The glass fiber reinforced plastic shot was absorbed into the high-pressure nozzle under negative pressure, and then the shot was was driven to the surface of the part under high pressure.
The blasting pellets used in the test were made from glass pellets with specification AGB70 and met the AMS 2431/6 standard. Its appearance is shown in Figure 3.
Fig.2 Shot peening treatment
Fig.3 Glass pellets
The shot peening resistance was checked using self-made tooling, which is represented in Figure 4. The standard base for checking the ALMEN test piece was firmly fixed to the self-made tooling with screws, and the ALMEN standard test piece was fixed to the standard base.
The ALMEN standard test part complied with the requirements of SAE J 442 and AMS 2431/2 documents. A minimum of four tests were performed to meet the requirement.
Fig.4 Homemade work device
II. Test method
1. Selection of shot peening pressure and shot flow
During the shot peening process, projectiles are propelled over the surface of the material with a certain kinetic energy, forming a regular flow under a specific air pressure. The speed and impact force of the projectiles are determined by air pressure, while the degree of plastic deformation of the material is determined by the shot peening force.
The saturation curve is plotted and the saturation point is determined by checking the ALMEN specimen, allowing the corresponding shot peening resistance to be determined. When determining airflow pressure, it is advisable to use a lower pressure to reduce wear on the surface of the material.
Projectile flow rate, which is the number of projectiles ejected per unit time, is related to airflow pressure. A low airflow pressure should correspond to a lower flow rate. In this case, an airflow pressure of 0.5×105Pa was selected, resulting in a projectile flow rate of 3kg/min.
By adjusting the up and down movement speed of the spray gun, different shot peening forces can be achieved. With the gun movement speed adjusted to 300mm/min, 600mm/min and 900mm/min, parts with shot peening resistances of 0.35mm (A), 0.31mm (A) and 0.27mm (A) were obtained. , respectively.
2. Determination of shot peening time and coverage detection
The shot peening time is determined by the saturation time of the ALMEN test piece. However, the time required to reach 100% coverage on the surface of the part can be used as a reference based on the saturation time of the specimen.
The Avrami equation, which relies on random statistics for average coverage, assumes that particle arrival speed is consistent. The equation is as follows:
In the formula,
- C is coverage (%);
- n is the Avramy index;
- r is the radius of the tooth;
- R is the average speed of dent formation;
- t is the time needed to form the tooth.
According to Avrami's equation, the coverage rate approaches 100% but it is theoretically impossible to reach 100%. The time required to reach the final 10% coverage is 1.5 times longer than the time required for the initial 90% coverage. The shot peening time required to achieve the last 1% of coverage will account for approximately 20% of the total time, and the time required for the last 2% of coverage will account for almost 40% of the total time. In the case of 99% coverage, 85% of positions were hit at least twice and 50% were hit five or more times.
Typically, if the coverage rate reaches 98%, it is considered equal to 100% coverage. However, achieving 100% coverage can result in excessive shot peening. Controlling the coverage rate at 98% will significantly reduce shot peening time.
The Avrami equation states that the radius of the well is equal to the radius of the projectile and the average speed of well formation is approximately the speed of the jet. The time required to achieve 100% coverage is 20 minutes.
Surface coverage is measured using the fluorescence method. Before shot peening, a layer of fluorescent agent is applied to the surface of the part and it is illuminated under a black light to ensure complete coverage. Then, the parts are blasted. After shot peening, the pieces are illuminated again under black light and, if there is little or no fluorescence, coverage is considered 100%. The specific process is illustrated in Figure 5.
(a) Effect of fluorescent coating on the surface of parts
(b) Parts before shot peening
(c) The partial effect after blasting
Figure 5 Coverage test process using the fluorescence method.
After selecting a part, its surface topography after blasting was further inspected, as shown in Figure 6. Figures 6a and 6b show that the pellet craters are evenly distributed across the surface of the part, which indicates that no surface was lost, consistent with the fluorescence coverage test results. After enlargement, as shown in Figure 6c, there were no cracks on the surface and a denser reinforced layer was formed.
(The)
(B)
(w)
Fig. 6 Surface morphology after shot peening of the aluminum barrel
III. S surface roughness analysis
A diamond tip with tip curvature radius of approximately 2 μm is used to measure surface roughness. The up and down movement of the pen is converted into an electrical signal by an electrical length sensor. After amplification, filtering and calculation, the surface roughness value is displayed on a meter and evaluated using the Ra value.
The surface roughness of 2A14 aluminum alloy was tested using a roughness meter, and the roughness before and after shot peening was measured, as shown in Table 2. When the surface roughness value of the non-shot peening part is low, it begins increasing after shot peening. This is because the surface hardness of the part is not very high, the surface is relatively uniform, and the impact energy generated by the projectiles is uneven, leading to the formation of larger holes on the relatively flat surface, causing an increase in the surface roughness value.
However, when the surface roughness value of the shot blasted part is high, the surface is already heterogeneous and irregular. The uniform velocity of the projectiles causes plastic deformation of the surface, which actually smoothes the rough, uneven surface.
Table 2 Effect of the shot peening process on the surface roughness of the aluminum alloy
| Surface roughness value before shot peening Ra/μm | 0.35 | 1.47 | 2.60 | 6.70 |
|---|---|---|---|---|
| Surface roughness value after shot peening Ra/μm (Shot peening resistance 0.35 mm (A)) | 2.20 | 2.60 | 3:30 p.m. | 5.67 |
| Surface roughness value before shot peening Ra/μm | 0.55 | 1.78 | 2.20 | 6.60 |
| Surface roughness value after shot peening Ra/μm (Shot peening resistance 0.31 mm (A)) | 1.96 | 2.10 | 2.80 | 4.96 |
| Surface roughness value before shot peening Ra/μm | 0.35 | 1.75 | 2:30 p.m. | 7:00 |
| Surface roughness value after shot peening Ra/μm (Shot peening resistance 0.27 mm (A)) | 1.65 | 1.85 | 2.50 | 4.85 |
Table 2 shows that, under different shot peening resistances, the greater the resistance produced by the surface, the greater the impact on its relatively low resistance surface. However, the general trend of impact on surface roughness is consistent.
The actual impact of shot peening on the part surface depends mainly on the energy transmission of the projectiles to the surface, which is mainly determined by the mass and velocity of the projectiles.
Figure 7 shows a schematic diagram of the direction of force and acceleration of projectile particles.
Figure 7 Force and direction of acceleration of the projectile particle
According to Newton's second law, the differential equation of a projectile can be described as:
F is the drag force received by the projectile particles, which can be expressed as
In the formula,
- M is the mass of the projectile (kg);
- C x is the drag coefficient;
- v G is the exit air speed;
- p G is the density of the air leaving the nozzle (kg/mm3);
- v t is the velocity of the projectile in the air flow leaving the nozzle (m/s);
- d is the diameter of the projectile (mm).
The projectile particle differential equation:
In the formula,
- t is the time(s) that the projectile is sprayed onto the processed surface through the nozzle;
- p is the density of the projectile.
According to the thermodynamic formula:
In the formula,
- p 0 and ρ 0 are the density under standard atmospheric pressure and standard atmospheric pressure, respectively;
- P and ρ G is the density under working pressure and working pressure, respectively.
The mass of the projectile can be ignored and the final differential equation for the projectile's motion is:
Where c is the integral constant, when the boundary conditions t =0 and the projectile velocity v =0, c =1/v G then
From the formula derived above, it can be deduced that the impact of various shot peening process parameters on surface performance can be attributed to:
- The kinetic energy of the projectile, which is related to the speed at which the projectile exits the nozzle, the time it takes for the projectile to reach the surface of the part, and the density and frequency of the projectiles.
- To have greater control over the surface roughness of the part, it is necessary to adjust the projectile speed and the size of the fired particles.
- The surface roughness of the part after shot blasting reflects not only the microscopic collection of surface shape characteristics, but also the maximum height of the surface well profile and the control of irregular surfaces.
- The effect of the shot blasting process on the surface roughness is not only determined by the strength of the shot, but also by the size of the shot particles and the surface coverage, which have a corresponding relationship.
4. Conclusion
(1) There are certain surfaces that cannot be sprayed, which suggests that the surface coverage is good and free from cracks, forming a relatively dense reinforcing layer.
(2) The shot peening force of the same type of projectile can change the surface roughness of the workpiece within a specific range. For example:
- When the surface roughness value is between Ra=0.30-0.65μm, the surface roughness can increase to Ra=2.2μm.
- When the surface roughness value is between Ra = 1.40-1.75μm, the surface roughness after sandblasting will remain around Ra = 1.6μm, which is similar to the original surface roughness.
- When the surface roughness value is between Ra=2.8-7.1μm, the surface roughness can decrease to Ra=2.3-6.1μm.
(3) The effect of various shot peening process parameters on surface layer performance is derived from the projectile particle differential equation and can be attributed to:
- The kinetic energy of the projectile and the speed of the airflow from the nozzle.
- The time it takes for the projectile to reach the surface of the part.
- The density and frequency of projectiles.
The stronger the shot peening process, the greater impact it has on the surface compared to weaker processes, but the general trend of impact on surface roughness remains unchanged.
