Condition Optimized Peening - COP; A Novel Fine-Peening Technology
In traditional peening treatment, it has been well recognized that increase in coverage is the most important factor for the improvement of fatigue strength, due to resultant higher compressive residual stresses, and that it is desirable to achieve a higher flow rate of impact particles. However, extreme flow rates of peening media have resulted in other technical problems, such as collision between the peening particles or erosive wear.
Recent technical trends have shown that fine particle peening becomes an attractive method of surface treatment since it is possible to create higher compressive residual stresses at selective layers near the surface without increasing surface roughness. In terms of fundamental performance, the strength, and especially the yield stress, of fine peening particles is higher than larger ones but a strong limitation to their use is the risk of clogging during blasting. Therefore, strict control of particle storage and the use of special gas arrangements for particle propulsion are usually necessary to obtain a satisfactory fine particle peening. It has been proven possible to develop a system of peening with fine particles using conventional pressurized air and a special particle storage device.
IKK Shot Co. Ltd. has collaborated with Itoh Kikoh Co. Ltd. and the tribology department of Meijo University in order to develop a peening apparatus based on a novel concept: Condition Optimized Peening, or COP. The original concept of COP is to be able to shot-peen with very fine particles -down to less than 10 µm- and controlled flow rates using conventional pressurized air. Furthermore, a new type of media was developed, consisting of particles down to 10 µm and, when needed, coated with micrometer-sized fine powders, thus allowing selective surface coating for the desired surface modification.
The new concept for the peening apparatus consists of a mechanism in which agglomerated particles can be separated and where it is possible to control the flow rate and the projection area. Although the basic system of the particle ejection is a direct air injection type similar to a conventional shot peening apparatus as shown in Figure 1, it is different in that the impact particles are fluidized in the tank with rubber balls and the nozzle has a double-walled construction.
In conventional fine shot peening, it is possible to blast particles smaller than 100 µm. However, agglomeration of the particles is frequent and damage to the material surface can result with such agglomerative impact. Thanks to the rubber balls mixed with the fine peening particles and the general agitation in the tank, such a problem can be solved. In addition, the flow rate and the projection speed of fine particles cannot be controlled separately with conventional fine shot peening systems. With the COP system, the fine particles go through the inside nozzle and the compressed air needed for particle acceleration goes through the outside nozzle, thus leading to separate control and allowing high speed projection with low flow rate of fine particles. With the COP system, it is then possible to avoid collision between impact particles, to reduce and to control surface roughness or damage and to form precisely controlled micro-dimples on the material surface.
The first interest of COP as explained above is to allow a perfectly controlled air peening process without the usual disadvantages, even with very fine particles. But new applications for the COP system can also be developed:
1. Surface texturing with micro-dimples for reduction of friction resistance
In recent years, the effect of friction reduction by a texturing treatment (through which a special groove design is formed on the material surface) has attracted a lot of attention. The reduction of friction in machine elements has improved not only the energy efficiency and the precision of the machine system, but also reduced wear. Many different technologies have been tested for micro-texturing, matter-removal processes, such as micro-machining or laser ablation, or press forming processes.
However micro-dimple formation by collision of fine particles is expected to be a much better method in terms of productivity and cost compared to the above-mentioned processes. The COP treatment enables a strict control of the diameters and the pattern of the micro-dimples, and the material surface with controlled micro-dimples thus shows a superior friction performance. The surface with COP-created micro-dimples shows a lower friction coefficient than even the mirror surface reference, as shown in Figure 2.
In addition, if the fine particles are coated with selected micro-powders, the combination of a well-controlled micro-dimple pattern and some material transfer on the surface can improve the friction characteristics even more.
2. Fabrication of a functionally-graded surface for wear resistance
The hardening of a material surface is recognized as an effective method to improve wear resistance. Even if some hardening is generated on the material surface by conventional shot peening, the hardening level depends on the peened material hardness and peening efficiency with the limit of surface degradation, especially a higher surface roughness. With the COP process, it is possible both to increase significantly the surface hardness and to reduce/control the material roughness, as shown in Figure 3.
Additionally, some carbide-coated fine particles were used in the special arrangement described above and a subsequent heat treatment was applied to the COPed material. It was then shown that carbide particle deposition and transfer into the material led to a significant increase of the surface hardness, adjustable by peening parameters and the heat treatment conditions. Such a combination has been shown to increase the wear resistance of the treated material (Figure 4).
The presented COP treatment method provides new possibilities for fine particle peening with an exclusive device. COP treatment can achieve the hardening and the creation of compressive residual stress that is the usual effect of conventional shot peening under better conditions. Moreover, it can also allow controlled surface modifications with possible functional additions and demonstrated benefits for various machine elements.
IKK SHOT Co., Ltd.
412-4, Nu-no-wari, Minami-Shibata Machi
Tokai, Aichi 476-0001, Japan