Vibration strengthening is to force the workpiece to be subjected to external cyclic load, and the exciting force comes from the eccentric portion of the exciter. This is a forced vibration problem with a multi-degree of freedom, damped system. In order to facilitate the analysis, we simplified the system into a single-degree-of-freedom, forced vibration of a damped system for analysis. The mechanical model is shown in Figure 1.
Its dynamic equation is:
It can be seen that the magnitude of the exciting force increases as the eccentricity e and the rotational speed ω2 increase. Therefore, in practical applications, by adjusting the eccentricity and the rotational speed of the exciter, a stress can be applied to the workpiece of the metal material, and the metal material can generate dislocation motion under the action of the alternating stress.
When the alternating stress increases from zero to the peak, as the applied dynamic stress increases, the dislocation of the metal material is excited, and new dislocations are continuously released and accumulated in front of the obstacle. The increasing dislocation plug-in stress field tends to shift the dislocations of adjacent grains. The plugging of the original stress field is first opened, and the stress concentration is released.
During the process of changing the stress from the maximum value to zero, the equilibrium state of the dislocation cluster is destroyed, and a large number of dislocations are greatly increased due to the intersection with other dislocations during the movement. With the alternating dynamic stress, the above process is repeated, the internal stress peak decreases while the dislocations continue to proliferate, and the increasing dislocation density is beneficial to the improvement of the fatigue strength of the material.
Fatigue failure is divided into three stages: crack initiation, crack propagation and transient fracture. The fatigue life of metal materials is mainly composed of two parts: crack initiation life and crack growth life.
Crack initiation is always formed at the highest stress and weakest part. After the vibration treatment, the high internal stress is reduced, the distribution is homogenized, and the influence of stress concentration is reduced. At the same time, the increase of dislocation density makes the slip zone slip more difficult. , thereby increasing the crack initiation life. The dislocation configuration change and dislocation density increase of the material increase the slip motion resistance, increase the energy required for crack propagation, and increase the crack propagation life, thereby improving the fatigue strength of the material and strengthening the material properties.
3, vibration aging characteristics
In the fields of machinery manufacturing, aviation, chemical equipment, power machinery, etc., various parts made of steel, cast iron, non-ferrous alloys and other materials have successfully adopted vibration aging. The reason why vibration aging is widely recognized in all aspects is that it has the following characteristics:
â—Low investment: Compared with thermal aging, it does not require a large aging furnace, which saves floor space and expensive equipment investment. Large castings and welded parts in modern industry, such as the use of thermal aging to eliminate stress, need to build large aging furnaces, not only expensive, low utilization, and the furnace temperature is difficult to uniform, the stress relief effect is very poor. These problems can be completely avoided by using vibration aging.
Therefore, at present, large-scale welded parts of bridges, ships, chemical equipment and heavy-duty castings weighing several tons to several tens of tons, which are several meters to several tens of meters long, are often subjected to vibration aging.
â— Short production cycle: natural aging requires long-term placement for several months. Thermal aging also takes tens of hours to complete, and vibration aging is generally only required to vibrate for tens of minutes. Moreover, the vibration aging is not limited by the site, which can reduce the round-trip transportation of the workpiece before and after aging. If the vibration equipment is placed on the machining production line, not only the production arrangement is more compact, but also the stress generated during the machining process can be eliminated.
â—Easy to use: The vibration equipment is small in size, light in weight and easy to carry. Since the vibration treatment is not limited by the site, the vibration device can be carried to the site, so the process is simpler to use and more adaptable than the heat aging.
â—Energy saving and cost reduction: aging treatment at the resonance frequency of the workpiece, the energy consumption is extremely small. It has been proved that a mechanical vibration exciter with a power of 0.25 to 1 hp can vibrate a workpiece of less than 150 tons, so the energy consumption is roughly calculated to be 3 to 5% of the heat aging, and the cost is only 8 to 10 of the thermal aging. %.
â— Others: Vibration aging is easy to operate and easy to mechanize automation. It can avoid the defects of warpage, oxidation, decarburization and hardness reduction of metal parts during thermal aging, and is the only method that can perform secondary aging.
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