Introduction

As to machine components in operation, one should be aware that their surfaces are subjected to the strongest loads as well as to environmental influences in mutual contact. In order to provide an adequate lifetime of a machine component, a suitable material, an adequate construction and processing, particularly surface processing, should be selected. Presuming that the material and construction chosen are the right ones, correct material processing, particularly of a surface layer, should be provided to ensure an adequate lifetime of a machine component.

In surfaces of machine components being most stressed, all attention should be paid to treatment processes, with which the size and variation of residual stresses can be influenced. If the influence of mechanical stresses on material wear or development of stress corrosion is to be reduced, then a suitable size and variation of residual stresses in a surface layer of a machine component shall be provided [1].

Highly exacting machine components for the most exacting applications resulted in the early 1960s in the development of a new scientific discipline, i.e. Surface Integrity [2]. Figure 1 shows basic knowledge about surface integrity. Surface integrity takes into account influences of manufacturing processes and machining conditions on the quality of a surface and a thin surface layer with particular regard to a microstructure, hardness variation, i.e. microhardness, and residual stresses as well as influences on material fatigue and corrosion.

The scientific discipline of surface integrity describes only surface properties after different mechanical and thermal treatment processes. Professionals started wondering what is to be done to ensure adequate quality of individual machine components and what is to be taken into account when assessing the quality of a surface or of a thin surface layer to fulfil operating requirements.

In order to provide favourable total stresses in a material during loading of a machine component, experts on manufacturing technologies should be well acquainted with influences of manufacturing conditions on microstructure changes and, in turn, variations of microhardness and residual stresses in the surface layer with individual machining processes. As to material fatigue from the viewpoint of residual stresses, suitable surface heat-treatment processes such as surface hardening, case hardening, nitriding etc., can influence the occurrence of compressive residual stresses. In surface-hardening and case-hardening processes, it should be taken into account that the quenching process may produce high internal stresses during heat treatment, which are due to temperature stresses occurring during cooling, i.e. quenching, which are, in a certain time interval, accompanied also by stresses due to phase transformations. High internal stresses during quenching may lead to high residual stresses that are related to volume changes and/or distortion of machine components. [3].

Control of volume changes and distortion of machine components and knowledge of residual stresses in them are becoming increasingly important in practical applications to high-tech products, particularly from the viewpoint of more adequate operating conditions and economic reasons. Cold working hardening is a hardening process occurring under cold conditions; consequently, these processes are easier to control and the size and variation of residual stresses can be efficiently predicted.

Cold working hardening of materials proceeds at a temperature below their recrystallization annealing temperature. To this group of surface treatment processes belong shot peening, deep rolling, roto peening or flap peening, and laser shock processing. Here also belongs laser shock processing, although in the interaction between laser light and workpiece surface in the thin surface layer, evaporation of material or absorber can be observed; which, however, depends on the preparation of a machine component, i.e. workpiece material or the type of absorbent applied to the machine-component surface [4].

Experimental procedure and results

The aluminium alloy 7075 was precipitation-hardened and experienced a minimum degree of cold deformation ranging between 1% and 3%. Precipitation hardening of the alloy was achieved by homogenizing at a temperature of 475ºC, which was followed by ageing at different temperatures. Only the S170 steel balls with a diameter of 0.5 mm were used in shot peening surface treatment on all specimens [5]. The process was performed on a number of specimens with different Almen intensities, which was achieved by selecting various working air pressures determining particle travel speed and kinetic energy. These different air pressure values were also used to obtain different mass flows of the steel balls. Upon impact, the particles' kinetic energy causes plastic deformation of the specimen surface. The level of plastic deformation is determined through surface hardness modifications and the hardness profile of the thin surface layer. The changes in the hardness profile and residual stresses of the shot-peened layer depend on dislocation density after surface treatment. The treatment utilizing the particles' kinetic energy guarantees longer life cycle of mechanical parts, which depends on the density of the dislocations occurring after treatment. The overlap of the indentations made by individual balls on the specimen surface is defined by the particle mass flow and travel speed.

Figure 2 shows residual stress profiles in aluminium alloy after shot peening treatment with the intensity of 12A and with coverage of 100%. Stress values on the surface were relatively low, i.e. between -90 MPa and -150 MPa. These values were gradually increased to the depth of 250 µm, except in specimens at ageing temperature of 195°C, where the residual stresses remain almost constant to the threshold depth of 1 mm. This is also the depth to which residual stresses were measured. The highest compressive residual stress values were obtained at the quenched specimen, i.e. -335 MPa. The compressive residual stresses in quenched specimens and artificially aged specimens with ageing temperatures 145°C and 170°C have very similar profile distributions. The values of maximal residual stresses at all three specimens were achieved in depth of 260 µm, i.e. -220 MPa at temperature 170°C, -275 MPa at temperature 145°C and -335 MPa at quenched state of the alloy.

Figure 3 illustrates residual stress profiles in aluminium alloy following the shot peening treatment with the intensity of 12A and with coverage of 200%.

Residual stress values on the surface were relatively low, i.e. between -75 MPa and -150 MPa. These values were gradually increased to the depth between 250 µm and 300 µm. The highest compressive residual stress values of the artificial aged aluminium alloy were obtained at the ageing temperature of 145°C, i.e. -295 MPa. The highest compressive residual stresses changed with the ageing temperature. Thus, the maximum residual stress values were obtained at the ageing temperature of 170°C, i.e. -235 MPa, while, at the temperature of 195°C, this value was only -180 MPa. The alloy that was only in the quenched state, i.e. in the soft state, obtained rather high maximum compressive residual stress values after treatment. At a depth of 310 µm, this value was -365 MPa. The value of residual stresses at a depth of 1 mm depended on the state of aluminium alloy. The residual stress value for a specimen in the quenched state amounts to -240 MPa at this depth, while at the ageing temperature of 145°C, this value is -210 MPa.

It is important to note that residual stresses obtained in greater depths were still of compressive nature. It can be stated that higher ageing temperatures result in smaller residual stress values.

The comparison of the ageing conditions shows that residual stress profiles are related to the growth, density and size of precipitates, resulting from the ageing of aluminium alloy.

Figure 4 shows the measured residual stresses as a function of depth for the specimens shot peened with different Almen intensities (8A and 12A) and different ageing temperatures (145°C, 170°C and 195°C). The highest residual stresses were reached at a depth of 260 µm with a value of -295 MPa at specimen aged at temperature 145°C and treated with Almen intensity of 12A. When compared to the same ageing temperature with lower Almen intensity 8A, it can be concluded that the highest residual stresses are located at the same depth but with a lower value of 260 MPa. With changed ageing temperature to 170°C, the residual stresses are moved towards the tensile zone, but still remain in the compressive area. The minimum effect on residual stresses is shown at ageing temperature 195°C. At this temperature, the highest residual stresses are just below the surface in the thin surface layer with a value of -121 MPa and they remain almost constant. At a greater depth, i.e. 965 µm, the value of residual stresses depends on the state and peening intensity of the aluminium alloy. It can be stated that the higher ageing temperature leads to lower values of residual stresses due to the softening of the material.

The basic objective of shot peening treatment of machine components is obtaining a higher resistance of an aluminium alloy to fatigue. The fatigue study was conducted on the 7075 aluminium alloy in the ageing state at different temperatures and was different at the two Almen intensities, i.e., 8A and 12A [6]. The first ageing temperature selected was 145°C. This temperature enables the highest hardness. Then, two higher temperatures were selected that provide the aged microstructure and lower hardness values, i.e. 170°C and 195°C. Bending fatigue tests were conducted using a resonance fatigue-testing device Cracktonic II, made by Swiss manufacturer Rumul AG. Dynamic fatigue tests were performed at a maximum bending stress of ?max = 375 MPa and a stress ratio of R = 0.1. Excitation of specimens was conducted on the principle of static preload, performed with a servomotor and spring, and with dynamic resonance excitation with a pair of electromagnets and oscillating mass.

Figure 5 represents the results of fatigue testing conducted under various ageing conditions (145°C, 170°C and 195°C) and at different intensities of surface SP treatment (8A and 12A) where the coverage was set to 200%.

The diagram with log scale shows the obtained cycles to failure of specimen with regard to amplitude bending stress to which individual specimens were subjected. It can be seen that ageing temperature has major influence on fatigue behaviour. It can also be observed that the more intense peening treatment, i.e. 12A represents a further improvement of the fatigue behaviour for this aluminium alloy as compared to the same ageing temperature or to different ones resulting in extending the fatigue life. Material fatigue is a process of the occurrence of progressive cumulative and permanent microstructural damage on machine components in operating conditions under dynamic loads with recurring or cyclic stress. Microstructural material damage is followed by the initiation of fatigue cracks that propagate and eventually result in material failure. The life cycle of a machine component of the given type and state of material depends on the number of cycles to failure.

Figure 6 shows a microhardness variation of aluminium alloy at different shot peening conditions. A microhardness variation in the surface layer after shot peening indicates a degree and depth of material hardening, which affects the operation of a machine component. The hardening results indicate that the material microhardness increased in all the specimens with a similar variation with reference to peening conditions. Microhardness of aluminium alloy in the soft state amounts to 175 HV0.2 and in the hardened state at 8A to 188 HV0.2 and at 12A to 195 HV0.2 respectively. The microhardness at the surface is a bit lower than the maximum value, which is due to surface relaxation. From the microhardness variation, it can usually be inferred what the depth of the layer hardened with individual peening parameters is. The hardened-layer depth is very difficult to establish particularly due to a very low loading force in microhardness measurements. From the hardness variation, it can be inferred that the hardened-layer depth depends on the shot peening conditions and that it amounts between 230 µm at Almen intensity 8A to around 260 µm with Almen intensity 12A respectively.

Conclusions

The work hardening of specimen surface by means of steel spheres has favourable effects on the operating life of the material by increasing the number of cycles to failure at the same stress amplitude values, which is especially evident at this coverage degree. Due to the shot peening treatment of the surface, the introduced compressive residual stresses reduce stress concentration, significantly increasing the number of cycles to failure in treated specimens.

Accounting for the residual stress distribution, the improvement in fatigue behaviour due to peening can be approximately estimated considering the residual stress field as a mean stress superposed on the stress field due to external loading.

After shot peening, microhardness in the thin surface layer increases. The depth of the hardened layer is mutually dependent on peening conditions, whereas microhardness depends on the alloy concerned and peening conditions.

References

1. Bach F.W., Laarmann A., Wenz T.: Modern Surface Technology, WILEY-VCH Verlag, Weinheim, 2004.
2. Field M., Kahles J.F.: Review of Surface Integrity of Machined Component, Ann. CIRP, Vol. 20 (No. 1), 1970, 107-108.
3. Field M., Kahles J.F., Cammet J.T.: Review of Measuring Methods for Surface Integrity, Ann. CIRP, Vol. 21 (No. 2), 1971, 219-237.
4. Schulze V.: Modern Mechanical Surface Treatment, States, Stability, Effects, WILEY-VCH Verlag, Weinheim, 2006.
5. Zagar S., Grum J.: Surface Integrity after Mechanical Hardening of Various Aluminium Alloys, Journal of Mechanical Engineering 57(4), 2011, 334-344.
6. Zagar S., Grum J.: Residual Stress, Fatigue and Electrical Conductivity Analysis after Shot Peening of Aluminium Alloy AlZn5.5MgCu, International Journal of Microstructure and Materials Properties, Vol. 8, No. 6, 2013, 447-461.

Zagar Sebastjan
E-mail: sebastjan.zagar@fs.uni-lj.si

Grum Janez
E-mail: janez.grum@fs.uni-lj.si

Faculty of Mechanical Engineering
Askerceva 6, 1000 Ljubljana, Slovenia