Shot peening is one of the surface strengthening processes, and its process is relatively simple compared to other surface modification processes, but the use effect is very significant.
It has been used in various fields such as aerospace, locomotives and automobiles etc.
The principle of shot peening is to use the ejected projectile to strongly impact the material and form small pits on the surface, which causes plastic deformation on the surface of the material and thereby generates residual stress on the metal surface.
The compressed crystal grains under the surface layer have to be restored to their original shape, which will produce a uniform residual compressive stress layer to strengthen the surface of the material.
After shot peening, the structure of the surface layer of the material changes, the grains are refined, the dislocation density and the lattice distortion increase, and finally a high residual compressive stress is formed.
The existence of residual stress on the surface of the material will significantly improve the fatigue resistance and fatigue life of the material, improve the strength and hardness of the material surface, as well as resistance to stress corrosion and high-temperature oxidation and other properties.
I. Materials for testing
The test used 2A14 aluminum alloy barrel-shaped parts, its material has high strength, good hot-competence, good machinability, good electric welding and welding seam performance.
The specific composition is shown in Table 1.
Table 1 2A14 aluminum alloy chemical composition
The 2A14 aluminum alloy barrel-shaped parts were divided into four groups (see in 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
Using SP1200 G4 pneumatic shot blasting machine and the working principle is shown in Figure 2.
The glass fiber reinforced plastic shot is adsorbed to the high-pressure nozzle under negative pressure, and the shot is sprayed to the surface of the part under the action of high pressure.
Shot blasting pellets are made of glass pellets with the specification of AGB70, which meet the AMS 2431/6 standard, and their appearance is shown in Figure 3.
Fig.2 Shot peening treatment
Fig.3 Glass pellets
The verification of shot peening strength adopts the self-made tooling as shown in Figure 4.
The standard base for verifying the ALMEN test piece is fixed in the self-made tooling by screws, and the ALMEN standard test piece is fixed on the standard base.
ALMEN standard test piece meets the requirements of SAE J 442 and AMS 2431/2 documents.
The number of tests meets the minimum requirement (4 times).
Fig.4 Homemade work fixture
II. Test method
1. Selection of shot peening pressure and shot flow
During shot peening, in the presence of a certain pressure airflow, the projectile forms a regular projectile flow with a certain kinetic energy to act on the surface of the material.
The ejection speed and impact force of the projectile is determined by the air pressure, and the degree of plastic deformation of the material is determined by the strength of the shot peening on the surface of the material.
Through the verification of the ALMEN test piece, the saturation curve is drawn, the saturation point is determined, and the corresponding shot peening strength can be determined.
When determining the airflow pressure value, try to consider using a lower airflow pressure to reduce the wear on the surface of the material.
The projectile flow rate is the number of projectiles ejected by the nozzle per unit time and the flow rate is related to the airflow pressure.
The low airflow pressure should correspond to the lower flow rate.
For this part, the airflow pressure is selected to be 0.5×105Pa and the projectile flow rate is 3kg/min.
After the projectile flow and air pressure are determined, different shot peening strengths can be obtained by adjusting the up and down movement speed of the spray gun.
When the up and down movement speed of the spray gun is adjusted to 300mm/min, 600mm/min and 900mm/min, the parts can be obtained respectively.
The shot peening strength is three fixed values: 0.35mm (A), 0.31mm (A) and 0.27mm (A).
2. Determination of shot peening time and detection of coverage
The shot peening time is determined by the saturation time of the ALMEN test piece, but the time to reach 100% coverage on the surface of the part can be used for reference according to the saturation time of the test piece.
The Avrami equation is based on random statistics for the average coverage Assuming that the arrival speed of the particles is consistent, the equation is
In the formula,
- C is the coverage (%);
- n is the Avramy index;
- r is the radius of the dent;
- R is the average speed of forming the dent;
- t is the time required to form the dent.
According to the Avrami equation, it can be observed that the coverage rate is getting closer and closer to 100%, but theoretically, it is impossible to reach 100%.
The time required for the final 10% coverage is 1.5 times the time required for the initial 90% coverage.
The shot peening time required to reach the last 1% of 100% coverage will account for about 20% of the total time, and the shot peening time required to reach the last 2% coverage will be close to 40% of the total time.
In the case of 99% coverage, 85% of the positions were hit twice or more, and 50% of the positions were hit 5 or more times.
Generally speaking, if the coverage rate reaches 98%, it can be said to be equal to 100% coverage.
If it wants to achieve 100% coverage, it may cause excessive shot peening.
98% coverage control will significantly save shot peening time.
According to the above formula, the radius of the pit is the radius of the projectile, the average speed of forming the pit is approximately the jet velocity, and the time to reach 100% coverage is 20 minutes.
The surface coverage is measured by the fluorescence method.
Before shot peening, it needs to apply a layer of the fluorescent agent on the surface of the part and irradiate it under a black light to ensure that the surface of the part is completely covered with a layer of the fluorescent agent, and then the parts are shot-peened. After shot peening.
After blasting, the pellets are irradiated under a black light, and if there is no fluorescence, or little or no fluorescence, the coverage is judged to be 100%.
The specific process is shown in Figure 5.
(a) Effect of fluorescent coating on the surface of parts
(b) Parts before shot peening
(c) The part effect after shot blasting
Figure 5 Process of coverage testing by fluorescence method.
Selecting a part and further inspecting its surface post-blasting topography, as shown in Figure 6.
As can be seen in Figures 6a and 6b, the pellet craters are evenly distributed over the surface of the part.
There was no surface that was not injected, consistent with the fluorescence coverage test indicates good surface coverage.
After magnification, as shown in Figure 6c, no cracks appeared on the surface, and a more dense, reinforced layer was formed.
Fig. 6 Surface morphology after shot peening of aluminum barrel
III. Surface roughness analysis
A diamond stylus with a tip curvature radius of about 2μm is used to slowly slide along the measured surface.
The up and down displacement of the diamond stylus is converted into an electrical signal by an electrical length sensor.
After amplification, filtering and calculation, the display meter indicates the surface roughness value and it uses Ra to evaluate the roughness of the contour surface.
The surface roughness of 2A14 aluminum alloy was tested with a roughness meter, and the surface roughness before and after shot peening was measured, as shown in Table 2.
When the surface roughness value of the non-shot peened part is small, the surface roughness value starts to increase after shot peening.
This is because the surface hardness of the part before shot peening is not too high, the surface of the part is relatively uniform, and the impact energy generated by the projectile on the surface of the part It is uneven, resulting in the formation of larger pits on the relatively flat surface of the material, resulting in an increase in the surface roughness value.
But when the surface roughness value of the part being shot is large, the surface of the part itself is inhomogeneous and uneven.
When a projectile strikes the surface of a part at a uniform velocity, it causes a plastic deformation of the surface, which instead flattens an otherwise rough and uneven surface.
Table 2 The effect of shot peening process on the surface roughness of 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 strength 0.35mm (A)]||2.20||2.60||3.30||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 strength 0.31mm (A)]||1.96||2.10||2.80||4.96|
|Surface roughness value before shot peening Ra/μm||0.35||1.75||2.30||7.00|
|Surface roughness value after shot peening Ra/μm[Shot peening strength 0.27mm (A)]||1.65||1.85||2.50||4.85|
It can be seen from Table 2 that under different shot peening strengths, the higher the strength produced by the surface, the greater the impact of its relatively low strength on the surface, but the general trend of the impact on the surface roughness is the same.
The actual effect of shot peening on the surface of the part mainly depends on the energy transmission of the projectile on the surface of the part, and the energy mainly depends on the mass and speed of the projectile.
Figure 7 shows the schematic diagram of the force and acceleration direction of the 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);
- Cx is the drag coefficient;
- vG is the output air velocity;
- pG is the nozzle output air density (kg/mm3);
- vt is the projectile velocity in the nozzle output airflow (m/s);
- dis the diameter of the projectile (mm).
The differential equation of the projectile particle:
In the formula,
- t is the time (s) that the projectile is sprayed to the processed surface through the nozzle;
- p is the density of the projectile.
According to the thermodynamic formula:
In the formula,
- p0 and ρ0 are the density under standard atmospheric pressure and standard atmospheric pressure respectively;
- Pand ρG 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 movement of the projectile is:
Where c is the integral constant, when the boundary conditions t=0 and the projectile velocity v=0, c=1/vG, so
From the above derivation formula, it can be seen that the impact of different shot peening process parameters on the surface performance can be attributed to:
the kinetic energy of the projectile is related to the velocity of the projectile output by The nozzle, the time for the projectile to reach the surface of the part, the density and time of the projectile.
For greater control over the roughness of the part’s surface, i.e. to change the condition of the part’s surface, which is also necessary to adjust the pellet size of the projectile.
It can reflect not only the microscopic collection of shape characteristics of the part surface after blasting but also the maximum height of the surface pit profile, as well as the control of uneven surfaces.
The effect on the surface roughness of the part is not only related to the strength of the shot, but also to the size of the shot particles and the surface coverage have a corresponding relationship.
(1) There are surfaces that cannot be sprayed, which indicates that the surface coverage is good without cracks and a relatively dense strengthening layer is formed.
(2) Different shot peening strengths of the same type of projectile can change the surface roughness of the part within a certain range.
When the surface roughness value is Ra=0.30~0.65μm, the surface roughness can be increased to Ra=2.2μm.
When the surface roughness Ra=1.40~1.75μm, the surface roughness after shot blasting will be almost the same as the surface roughness of the part, around Ra=1.6μm.
When the surface roughness value Ra=2.8~7.1μm, the surface roughness can be reduced to Ra=2.3~6.1μm.
(3) The impact of different shot peening process parameters on the performance of the surface layer is derived from the differential equation of the projectile particles, which can be attributed to the kinetic energy of the projectile and the velocity of the airflow output from the nozzle, the time for the projectile to reach the surface of the part, the density of the projectile and the time.
The higher the strength, the greater the impact on the surface compared to the low strength, but the overall trend of the impact on the surface roughness is the same.
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