Part 2: Shot peening of an aluminium alloy

The aluminium alloy used in the research was ENAW 6082-T651, which was treated by shot peening. The alloy was homogenized and precipitation-hardened at a temperature of 160°C for 10 hours. The alloy contains, in addition to silicon (0.7-1.3 %), magnesium (0.6-1.2 %), and manganese (0.4-1.0 %), also iron (0.52 %). After precipitation hardening the aluminium alloy with magnesium and silicon forms a uniformly distributed secondary phase in the form of a chemical compound Mg2Si, which considerably improves alloy strength.

After precipitation annealing the alloy given shows a tensile strength Rm of 348 MPa, a yield stress Rp0.2 of 320 MPa, and elongation A of 13.5 %. The purpose of surface treading by shot peening is to produce microplastic deformation in the thin surface layer, and to increase, compressive residual stresses, which reduce material fatigue. 

In shot peening with S170 shot having a hardness of 56HRC, different operating air pressures (p = 1.6, 4.0, 8.0 bar) were chosen. They provide an adequate kinetic energy of particles hitting a specimen and a mass flow with a specimen travel speeds (Vy = 2.8, 3.2, 3.4, and 3.6 m/min), a nozzle distance being the same in all cases (y = 4 mm).

Specimen surface roughness is given as a mean arithmetic roughness Ra. With the original specimen material, Ra was 0.85 ?m, but after shot peening it increased considerably due to the kinetic energy of round steel shot, i.e. Ra = 6.38-13.93 ?m, depending on treating conditions.

Figure 1 shows macrographs of a surface after shot peening. The left image shows the specimen centre and the right one the specimen edge. A comparison of the two surfaces shows that the most explicit indentations due to the particles hitting the specimen surface are obtained in the case when the travel speed of the table with the specimens is the highest and so is the Almen intensity. It is surprising, however, that the particle indentations at the 6082/7 specimen surface deviate as though shot would be hitting the specimen surface at a certain angle. Consequently, the roughness appearance at this surface is less conspicuous although a quantitative value of the mean arithmetic value of the roughness between specimen 6082/7 (Ra = 6.38 ?m) and specimen 6082/3 (Ra = 6.82 ?m) is quite similar, which is confirmed by a small difference between the Almen intensities of the two specimens.

After cold deformation hardening by shot peening with quenched shot, a specimen microhardeness of 90 HV0.2, i.e. in the precipitation-hardened state, increased to 108-116 HV0.2 at the surface. The through-depth specimen microhardness decreases with a favourable microhardness variation gradient and is almost independent of shot-peening conditions. In all cases in a depth of 0.24 mm a substrate microhardness of 90 HV0.2 is obtained. Figure 2 shows a microhardness variation in the thin surface layer (z = 0.33 mm) in a precipitation-hardened specimen and in the other specimens that were additionally treated by shot peening under different conditions stated in the legend. The differences in microhardness variations after treating under different conditions are relatively small and range between minimum and maximum, where ? HV0.2 amounts to only 10 daN/mm2. Greater deviations in a microhardness variation occur only in an untreated specimen and are relatively strong, which is attributed to a very small load in measuring of microhradness.  Differences in microhardness occur due to relatively small indentations with reference to the type and size of individual grains.

Measurements of residual stresses were performed using a device manufactured by Vishay RS200 with an air turbine to achieve high drilling speeds [1]. Hole drilling with a drill of 1.6 mm in diameter was performed in the middle of a measuring rosette, through which strain was determined because of specimen relaxation close to the rosette. The signal received from measuring rosette was amplified using a computer card AT-MIO-16XE-50, through a program package National Instruments LabVIEW. The data obtained were processed with a program package RESTRESS for WINDOWS 1.07. A program package Microsoft Excel was used to elaborate diagrams of residual stresses. Strain measurement at the measuring resistance rosette was performed in increments with a drill depth of 0.1 mm. The method of determining residual stresses by the Hole Drilling Strain Gage method is standardized in ASTM E 837-01E [2].

Figure 3 shows a variation of minimum principal residual stresses in dependence of the surface distance. It is interesting that a specimen after precipitation hardening prior to shot peening shows extremely low absolute values, i.e. in the range from –15 MPa to +20 MPa.

The residual stress variations in individual specimens are very similar, i.e.:
-a value of the compressive residual stress at the surface is low, i.e. it ranges between 15 and 90 MPa, which is closely related to surface relaxation;
-a maximum  value of the residual stress ranging between –140 MPa do –235 MPa, depending on the shot peening conditions, is found in a depth of around 0.45 mm;
-gradients of residual stresses at the surface and in the subsurface are very similar;
-difference in the maximum residual stresses at the surface is 140 daN/mm2, and in the subsurface it ranges between –140 and –230 MPa.

There is an important relationship between residual stress variations and individual shot peening conditions. A higher air pressure is followed also by a higher kinetic energy of shot, which, when hitting a surface, provides a lower variation of compressive residual stresses. With lower mass flow rates residual-stresses are somewhat higher, which means that compressive residual stresses are lower. It can be confirmed that a through-depth residual-stress variation is in quite a good agreement with the Almen intensity. It has turned out that with reference to numerous variations of shot peening conditions they obey the Almen intensity well.

Conclusions

In the discussion proposed contributions of researchers in the field of shot peening, which is particularly suitable for dynamically loaded components and those exposed to fatigue during their operation that may result in their failure, were underlined. Shot peening is very suitable due to its simplicity, handiness, accessibility, and treatment rate, which confirm particular advantages of individual applications in comparison to heat treatment where the components are subjected to high temperatures and rapid cooling processes and difficult to control.

References

1 Tech note TN 503-5, Measurement of Rsidual Stresses by the Hole-Drilling Strain Gage Method, Vishay Measurements Group, 1993.
2 Standard Test Method for Determining Residual Stresses by the Hole Drilling Strain Gage method, ASTM E 837-01, 694-703, 1995.

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