Ultrasonic impact enhances ferrous and nonferrous casting physical properties. (Case History).

The use of ultrasonics to impart compressive stresses into metallic parts was originally invented in Russia to reduce welding stresses and deformations, introduce compressive stresses, increase corrosion-fatigue strength of welded joints, and enhance fatigue resistance at subzero temperatures. The logical progression for applications of ultrasonic impact technology (UIT) led to the treatment of gears, shafts, wheels, brake components, and other cast or machined components subjected to severe cyclical loading during use. For cast components, Applied Ultrasonics, Birmingham, Alabama, has developed Esonics UIT.

In the case of gears, UIT testing has been performed to treat the teeth and roots of bevel gear teeth. The results of these tests show fatigue life enhancement of up to 15 times that of typical shot peening, while also improving surface finish in the treated areas and increasing the surface hardness.

The altered surface properties reduce the surface fatigue normally caused by micro surface defects while increasing resistance to contact fatigue. Surface hardness levels increased from 5456 Rockwell Hardness after carburization to 59-61 Rockwell Hardness after UIT. Surface finish improved from Ra 0.28-0.38 to Ra 0.15-0.18 microns after UIT treatment.

The principle of UIT is based on instrumental conversions of harmonic oscillations of an. acoustically tuned body into resonant impulses of ultrasonic frequency. The acoustically tuned body is brought to resonance by energizing an ultrasonic transducer. The energy generated from these high frequency impulses is imparted to the surface to be treated through the contact of specially designed steel pins. These transfer pins are free to move axially between the resonant body and the treated surface.

Depending on the desired effects from the treatment, a combination of different frequencies and displacement amplitude is applied. The frequencies range from 27-55 KHz, with displacement amplitude of the resonant body from 22-50 microns. The controlled action of the application allows one to define the exact combination of effects, including surface finish properties, enhanced corrosion resistance properties and compressive stress depth.

The UIT technology can be applied to a wide variety of metals, including steel, iron, aluminum, inconel, titanium, stainless steel and bronze. The technology is rapidly being accepted as a technique for enhancing fatigue properties of cast, machined or welded components that are subject to fatigue failures during use or need enhanced strength and hardness properties. The mechanics of applying the technology allow integration with machining operations that include lathes, milling machines or robotic systems.

Following is a look at specific test results by cast metal:

* aluminum--sealed surface defects and sub-surface porosity, increased surface hardness and yielded strengths equal to forged components; increased hardness 38% at the surface and 23% at a depth of more than 2mm;

* bronze--improved surface finish, increased surface hardness and decreased corrosion failures in weld-repaired areas; hardness increased 20% at a depth of more than 1.5mm;

* gray iron--brake drums demonstrated a 5-7 times increase in life before heat checking; hardness increased by 10% at a depth of 1.5 mm on drums and 37% at the surface and 12% at a depth of more than 1.5mm for rotors;

* ductile iron-yokes demonstrated up to 7-9 times greater life.

Select No. 001 at www.moderncasting.com/info