Due to various setting parameters, the shot peening process is a rather complex one. The quality of a shot-peened surface largely depends on the type and properties of the material, on the type of preliminary surface treatments, and on the depth of the shot-peened layer. The effectiveness of various shot peening conditions in industrial applications is assessed by means of the Almen intensity. The Almen intensity classifies different shot peening conditions into shot peening rates, which, however, do not provide a direct comparison with microstructure, microhardness and resulting residual stresses in the machine component. Shot peening also results in changes in surface microgeometry, which depends on the size of particles and the intensity of particle impact on the surface of a machine component. A shot-peened surface is described with standard surface roughness parameters.

Cold microplastic deformation of the material‘s surface layer results in increased density of dislocations, which further results in higher material hardness and occurrence of compressive residual stresses in the thin surface layer. The scientific discipline dealing with the effects of various processes on the surface condition is called Surface Integrity. Therefore, a selection of shot-peening conditions is required that would enable the kinetic energy of particles (of a given diameter and made of suitable material) to create a quality surface and desired resistance of material to fatigue. If the shot-peening parameters are too harsh, the surface may be damaged. However, despite higher hardness and higher compressive residual stresses, the resistance of material under dynamic load is proportionally no better through time.

Shot peening experiments with hard steel particles were conducted on two types of aluminium alloy. The first aluminium alloy was 2007-T351. Its chemical composition in weight percentage (wt%) was as follows: Al-4.0Cu-1.2Pb-1.1Mg-0.8Mn-0.8Zn-0.8Si-0.8Fe-0.2Ti-0.2Ni-0.2Bi-02.Sn-0.1Cr; its mechanical properties were the following: Rm=370 MPa, Rp0.2=240 MPa and A=9%. The hardness value of the alloy in delivered temper was 109 HV0.2. The temper of delivered alloy was T351, which means that the alloy was homogenized at a temperature of 495ºC and then hardened through natural ageing at the ambient temperature. In addition to aluminium, the alloy also contained 4% of copper, forming the first secondary phase Al2Cu. It also contained magnesium and silicon, forming the second secondary phase Mg2Si. Both phases contribute to better mechanical properties of the alloy. An alloy with precipitated phases Al2Cu and Mg2Si results in rather high hardness and tensile strength values.

The second aluminium alloy was 6082-T651. Its chemical composition in weight percentage (wt%) was as follows: Al-1.1Si-1.0Mg-0.8Mn-0.5Fe-0.25Cr-0.2Zn-0.1Cu-0.1Ti; its mechanical properties were the following: Rm=310 MPa, Rp0.2=260 MPa and A=13.5%. The hardness value of the alloy in delivered temper was 89 HV0.2. The temper of delivered alloy was T651, which means that the alloy was homogenized at a temperature of 540ºC and then hardened through artificial ageing at a temperature of 160ºC for 10 hours. The alloy contained very little copper, but some magnesium and silicon, forming secondary phase Mg2Si, reinforcing the solid solution after the ageing process.

Specimens for both types of aluminium alloy were cut from a round extruded rod of 40 mm in diameter, which was then cut into discs of 10 mm in diameter. Cutting was performed with a machine cutter for the preparation of the specimens for a metallographic examination. The process was performed under mild conditions and in the presence of a cooling agent. The shot-peening process for both alloys was performed with hardened steel particles of 430 µm in diameter. Their specification was S170, while their hardness value was 56 HRC. The process was conducted at different air pressures and mass flows that correspond to Almen intensities of 10A, 12A, 21A and 28A. The objective of the study was to determine the effect of influential parameters of shot peening on surface integrity.

Microhardness variations show the intensity of the shot-peened surface layer, having a decisive effect on the behaviour of a machine component in operating conditions and under dynamic load. Higher rates of cold plastic deformation of material in the thin surface layer result in higher density of dislocations in the soft matrix material. Higher density of dislocations and the synergistic effect of the type, density and size of precipitates in the soft matrix significantly affect the changes in microhardness variations in the thin surface layer.

Figure 1 illustrates microhardness variations for the first aluminium alloy ENAW 2007, measured in individual specimens treated with different Almen intensities; namely 10A, 12A, 21A and 28A. It was established that the highest microhardness value is obtained on the surface, while the microhardness variations slightly decrease with the depth, as expected. In specimens treated with intensities 10A and 12A, the obtained microhardness values on the surface were lower, while the microhardness variations did not decrease with the depth to such an extent as was observed in specimens treated with intensities 21A and 28A. Differences in the obtained microhardness are significant and amount up to 20 HV0.2, representing 20% of the microhardness value of the alloy in delivered temper. Based on microhardness variations, the depth of hardening can be determined for individual surface treatment parameters. In specimens treated with intensities 10A and 12A, the obtained depth of hardening was between 260 µm and 320 µm, while a significantly higher depth of hardening, i.e. up to 390 µm, was obtained at higher intensities.

The following can be inferred from microhardness variations:
• the measured microhardness variations are similar in terms of different treatment intensities, and differ in absolute values along the depth of hardening;
• the obtained depth of hardening also depends on treatment conditions; at the intensity of 10A, the depth of the hardened layer was about 260 µm, while at the intensity of 28A, this depth was about 390 µm;
• the obtained microhardness variations at treatment intensities of 21A and 28A were significantly higher than in surfaces treated with intensities of 10A and 12A.

Figure 2 illustrates minimum residual stresses for alloy ENAW 2007 on the surface of specimens treated with hard steel particles at various intensities. Residual stresses were measured by drilling a blind hole, defined in standard ASTM E 837-01. Strains measured during an incremental drilling on individual elements of the 3-element resistance rosette CEA-06-062-UM helped obtain data required for the calculation of the size of main deformations, as well as for the calculation of the main residual stresses.

In the specimen from aluminium alloy ENAW 2007, the residual stress variations were minimal due to careful mechanical preparation of the specimen, i.e. due to the cutting up, grinding and polishing of the specimen. The residual stresses were of compressive nature. It was established that the minimum residual stress was obtained on the surface, the stress being about -50 MPa. Specimens treated with 10A and 12A show a very similar residual stress variation. The highest compressive residual stresses obtained in these specimens were that of about -295 MPa at the depth of about 250 µm. In the specimen treated with 21A, the highest residual stress was -340 MPa at the depth of 270 µm. In specimens treated with the intensity of 28A, the increase in compressive residual stress was negligible, i.e. -360 MPa at the depth of about 290 µm. What was surprising were the residual stress variations from the surface into the depth, up to the maximum compressive residual stress. Higher residual stress values under the surface at the depth of 270 µm confirm that stresses on the surface are decreased by relaxation. In part, these lower residual stress values on the surface may be attributed to system error in the measuring of residual stresses. Compressive residual stresses become tensile in all shot-peening conditions, with a gradient similar to the one on the surface, i.e. 1.05 MPa/µm. Transition from compressive to tensile stress occurs between 650 µm and 710 µm. The residual stress gradient is almost independent of the conditions or the intensity of treatment of individual specimens. The only exception is the residual stress variation in specimen with 10A, which was subjected to the mildest conditions, where after the transition to tensile stress, these stresses become almost constant at the value of 20 MPa and up to the threshold depth of 950 µm.

Figure 3 illustrates minimum residual stress variations for aluminium alloy ENAW 6082. In the delivered temper of the specimen after it was cut, only low compressive residual stresses were introduced, i.e. -15 MPa at the depth of 150 µm. At the depth of 950 µm, their value is +10 MPa. The lowest value of residual stress variation can be observed in specimen 12A. The calculated main residual stress is of compressive nature. Its highest value is approximately -115 MPa at the depth of 450 µm. With depth, the residual stresses become tensile, with the exception of specimen 12A, which remains in the compressive area to the threshold depth of 950 µm. This is also the depth to which residual stresses were measured. In the specimen treated with the lowest intensity of 10A, the value of compressive residual stresses is higher when compared with the above-mentioned specimen, namely -167 MPa at the depth of 250 µm. Minimum main residual stress variations on the surface of this specimen turn to tensile area when they reach the highest value, and become tensile at the depth of 500 µm.

Compressive residual stresses introduced to specimens treated with the highest intensities have similar variations, which means that in specimen with intensity 21A, the value is -203 MPa, while the value in specimen with intensity 28A amounts to -180 MPa. When treating the surface with the highest intensity of 28A, the highest residual stress value in the first measuring position, which is at the depth of 33 µm, is just below the surface and amounts to -203 MPa. Afterwards, this value decreases slowly and steadily. The transition from compressive to tensile stress in both specimens treated under the harshest conditions occurs at the same depth, i.e. at about 550 µm.

In order to provide the optimum properties of the hardened surface layer with cold deformation, the influential parameters of the shot peening process need to be adjusted, including the operating air pressure that creates the necessary kinetic energy of the particles that impact on the specimen surface. The shot peening process enables a uniform hardening of the surface throughout the specimen with the objective of improving the resistance of material to surface fatigue of machine components in operating conditions and under dynamic load. The optimization of the shot peening parameters could enable controlled sizes and microhardness or residual stress variations. Therefore, studies of the effect of surface shot peening parameters for individual materials are essential.

Based on microhardness and residual stress measurements conducted for both alloy types, the following can be inferred:
• Microhardness variations for alloy ENAW 2007-T351 correspond to the shot peening rate, defined with the Almen intensity. The difference in the specimen hardness between the lowest intensity (138 HV0.2) and the highest intensity (153 HV0.2) was only 15 HV0.2.
• Residual stress variations for alloy ENAW 2007-T351 are very similar and almost independent of the intensity of treatment of individual specimens. Residual stresses just below the surface are low compressive stresses that increase to the value of -370 MPa at the depth of about 300 µm.
• Residual stress variations for alloy ENAW 6082-T651 are very favourable at the highest treatment intensities (21A and 28A).

The highest residual stress values between -203 MPa (21A) and -198 MPa (28A) are obtained on the surface. With depth, these stress values decrease with a small gradient and become tensile at the depth of 550 µm.

Comparison of results between both aluminium alloys, ENAW 2007-T351 and ENAW 6082-T651, show that higher compressive residual stresses are obtained in the latter, and that stress variations from surface to depth are rather favourable in the second alloy.

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