Vibratory stress relief Dimensional stability of welded components through vibration

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Jebens GmbH

FAULHABER Drive Systems

To reduce internal stresses in metallic structures, vibratory stress relief is suitable.

In components made of metallic materials, internal stresses arise during manufacturing, processing, and deformation, which are unevenly developed and distributed. During further processing, these can lead to warping and cracking of the component, and together with operational loads, even to its failure. Traditionally, stress relief annealing is considered the method of choice to reduce internal stresses in metallic structures. Equally proven for decades, but significantly less known and used, is vibration stress relief—a method that is considerably faster, more cost-effective, and environmentally friendly. In light of increasing demands for environmental protection, resource conservation, and cost structure, vibration stress relief is gaining importance for dimensional stabilization of welded components before, during, or after mechanical processing, with a focus on sustainability and cost-effectiveness.

As a recognized expert in large heavy flame-cut parts and complex assemblies, Jebens GmbH, based in Korntal-Münchingen (Germany), uses this method out of conviction and advocates for its broad acceptance. Introduced in the USA about 100 years ago and distributed in Germany for 50 years by VSR-Industrietechnik GmbH—where it has also been significantly improved—vibration stress relief works with targeted vibrations in component areas with existing internal stresses. Through the targeted application of load from the imbalance motor in areas with a high proportion of internal stress, the yield point of the material is locally exceeded. This results in local relaxation and reduction of internal stresses.

The stress relief process

At the start of the process, it's important to consider the correct positioning of the workpiece in addition to the material and weight of the component. Then, the required minimum acceleration in the component of ten meters per square second and the maximum speed of up to 6,000 revolutions per minute are determined and set at the natural resonances. The modal hammer generates a force impulse several times by striking the component firmly. In vibration stress relief, two vibration sensors measure and monitor the acceleration induced in the component by the impulse. A triaxial accelerometer measures the acceleration of the workpiece in the X, Y, and Z axes. A monoaxial sensor measures how much acceleration reaches a point on the sheet metal farthest from the motor. The relaxation process occurs in five cycles with up to ten frequencies, during which the modal hammer determines the natural frequencies of the component. Once the frequency of the component changes, the automatic control adjusts the imbalance and rotation speed accordingly using modal analysis. The respective relaxation state of the component is determined based on the frequency band. After the five cycles are completed, a final measurement is conducted. All frequencies are recorded in a protocol, and the relaxation state is depicted in a diagram. For large components, as is common at Jebens, the motor must be repositioned several times to ensure the relaxation of the entire component. All processing frequencies are automatically determined, executed, and documented alphanumerically and graphically in the vibration protocol.

According to VSR-Industrietechnik, modal analysis provides scientifically valid proof that internal stresses in the component have been reduced to the extent that distortion can be excluded. The low vibration amplitude also excludes cracking in the components. The company, headquartered in Duisburg, has sold around 150 systems for vibration stress relief to date. Renowned industrial companies such as ThyssenKrupp, Audi, Ford, Caterpillar, Liebherr, and Sennebogen have been using this method in their own production for decades or require their suppliers to apply it. At Jebens, many demanding industrial customers also rely on vibration stress relief for components for press brakes with up to 100 tons of part weight, forging manipulators with up to 40 tons of part weight, or presses with up to 80 tons of part weight.

The advantages

There are numerous advantages to this method: While an average stress relief annealing emits around 6.6 tons of CO₂, CO₂ emissions with vibration stress relief are only 0.03 tons. The energy consumption of 32,400 kWh for stress relief annealing contrasts with just 8 kWh for vibration stress relief. The time required is also significantly lower: a furnace run typically takes 72 hours, while vibration stress relief takes only about eight hours for large components. Additionally, vibration stress relief does not create a scale layer on the component that would need to be removed by sandblasting, saving costly rework and being more gentle on the material. Subsequent straightening is generally not needed either, as no distortion occurs with vibration stress relief. Since any component size between 100 kilograms and 200 tons, including extreme component geometries, can be treated on-site, transport to a suitable annealing furnace is unnecessary. Despite the higher personnel effort during treatment, vibration stress relief is a considerably more cost-effective solution to reduce internal stresses due to the substantial savings in energy, rework, transport, and time. Jebens also favored the method because the component dimensions produced there are often so large that complete annealing is not possible, or a suitable furnace can only be reached with great effort. Lastly, for tempered components, annealing is not feasible, as no significant stress reduction would occur at the permissible annealing temperature for these components.

The comparison test

Nevertheless, there are also customers at Jebens who do not yet allow the process. Unlike stress relief annealing, there is no established factor in the FKM guideline for calculating the strength of machine components as a design guideline for proving fatigue strength. Therefore, Jebens recently conducted a highly regarded comparative experiment on deformation during milling of stress-relief annealed and vibration-stress-relieved components. The aim of this experiment was to draw conclusions about the stress state in the component. For this purpose, Jebens cut two plates with a thickness of 110 millimeters to 800 x 800 millimeters (approx. 4,3 inches to 31,5 x 31,5 inches). Then, the company prepared a weld seam preparation for a V 110 seam on one side and tack-welded the components by hand. They were then welded with a welding robot. Both welds were carried out with the same parameters, which were chosen to create the highest possible internal stresses in the weld area. One set of plates was then stress-relief annealed, and the other was vibration-stress-relieved. Next, milling in the weld area was intended to create a change in the internal stresses in the components and lead to deformation of the workpieces. Since internal stress, unlike deformation, cannot be measured in a component, three measurement grooves were milled into the back of each plate set. In these grooves, the respective flatness deviations were to be measured by laser after each milling pass. An initial measurement was taken as a zero measurement in this state.

On the top side of the weld seam, a wide milling path was milled out to completely remove the weld material and heat-affected zone. Three millimeters were added per milling pass. In total, three milling passes were conducted with a chip thickness of three millimeters. The first two passes were milled with coolant, and the third without. Each milling pass was performed using the same parameters. After each pass, the flatness of the measurement grooves was measured and plotted in a diagram where each path was represented separately. The unified scaling of the diagrams allowed for a direct comparison of the results: the deflection in the vibration-stress-relieved component was only one-third as large as in the stress-relief-annealed component. From Jebens' perspective, this suggests that the internal stresses are lower in the vibration-stress-relieved component than in the stress-relief-annealed one. This practical result confirms a theoretical study that VSR conducted several years ago with RWTH Aachen University (Germany). The Aachen researchers demonstrated through FEM calculations that vibration stress relief surpasses the efficiency of stress relief annealing. This conclusion was also confirmed by users like Audi, Ford, and ThyssenKrupp in their own practical investigations, and they subsequently adopted the method. Therefore, from the perspective of Jebens and VSR-Industrietechnik, vibration stress relief is as reliable as conventional stress relief annealing, yet it outshines it in most cases with its array of advantages.