Ultrasonic Impact Method to Increase Fatigue Strength of Welded Joints
Introduction
Ultrasonic impact methods have been developed and refined over the last few decades to improve welded joint fatigue strength. They are commonly used in the aerospace, automotive and industrial fabrication industries for high-strength fatigue-resistant welded joints. Ultrasonic impact (or ultrasonic fatigue) methods are also known as sound-assisted welding, which uses a specific combination of vibratory force, pressure and heat to create mechanical welding. This process improves joint integrity and produces a superior welded joint with better fatigue-resistance than traditional welding processes.
Theory
The theory behind ultrasonic impact processing is based on the principles of plastic deformation and ultrasonic driven plastic deformation. In the process, a high-frequency mechanical vibration is applied in the vicinity of a welded joint with a force that is low enough to introduce martensitic transformation and hardened case depth independent of the joint structure, but still high enough to promote plastic deformation. The resulting plastic deformation of the welded joint improves the fatigue strength and fatigue life of the joint.
The plasticity in welded joints is caused by the changes in microstructure which shortens the welding cycle, helps to reduce the heat input and removes the need to reheat the welds prior to welding. The microstructural changes cause welds to become denser, increase the hardness, and decrease the fracture toughness at the same time. The shortened welding cycles and reduced heat input also help to reduce distortion and increase the fatigue life of welded joint.
Methods
There are several types of ultrasonic impact methods available, including conventional vibratory processing, oscillating or rotating inserts, and ultrasonic-assisted mechanical welding. In these processes, the welds are created using a combination of vibratory force, pressure, and heat and the welds also gain improved fatigue strength.
On the conventional vibratory processing method, a vibration is applied to the welded joint in a specific frequency and amplitude which will cause the plates to move elastically. As the welded joint approaches its maximum load, plastic deformation occurs and the joint will start to deform plastically. This plastic deformation serves to increase fatigue strength and fatigue life of welded joint.
In the oscillating or rotating inserts process, a vibratory force is applied to a welded joint through the use of an oscillating or rotating insert. This vibratory force causes a small degree of plastic deformation in the welded joint, which helps to increase its fatigue strength and fatigue life. The most commonly used oscillating insert is made of stainless steel and can be used to create both lap joint and butt joints.
In the ultrasonic-assisted mechanical welding process, a vibratory force is applied to the welded joint through the use of an acoustic tool tip. This vibratory force produces plastic deformation in the welded joint, which improves the fatigue strength and fatigue life of the joint. The acoustic tool tip is designed to optimize the energy transfer and to reduce welding time without compromising joint integrity.
Advantages
Ultrasonic impact methods offer a number of advantages over conventional welding processes. The reduced welding time helps to reduce heat input to the welded joint, resulting in a lower risk of overheating and excessive distortion. The size and shape of the welded joint are well controlled, since the vibratory force is applied directly to the joint. The welded joint gains improved fatigue strength and fatigue life, since the plastic deformation introduced by the vibratory force improves its integrity. Finally, ultrasonic impact methods are relatively simple and reliable, with minimal setup and equipment requirements.
Conclusion
Ultrasonic impact methods offer a number of advantages over conventional welding processes for improving the fatigue strength of welded joint. The plastic deformation introduced by the vibratory force helps to improve the integrity of the welded joint, resulting in improved fatigue life and fatigue strength. The lower heat input also reduces the risk of overheating and excessive distortion. Finally, the ultrasonic impact methods are relatively simple and reliable, making them an attractive choice for many industries.
