Measurement of the Degree of Recrystallization in Aluminum Sheet Using Ultrasonics

Some of the benefits of NDE are reduced failures, reduction in process development time, more efficient designs, and better performance. NDE techniques have the potential to offer robust and economical means for insuring that the target specifications of a part's bulk properties, dimensional and surface properties are achieved at critical stages of the manufacturing process. It is desirable to move the inspection of the part upstream in the manufacturing process so any rejectable defect can be detected before further value is added to the part. Ultrasonic techniques are useful and versatile methods for evaluating microstructure and mechanical properties as well as detecting microscopic and macroscopic discontinuities in solids.

Advantages of Ultrasonic Testing

1. Ultrasonic waves can propagate deep into a part to give information of the parts interior. Unlike X-ray, ultrasonic techniques do not generate hazardous ionizing radiation.
2. It has high sensitivity, enabling the detection of small discontinuities.
3. It can be used to determine the position and size of a discontinuity.
4. It is fast and can be easily automated.
5. Only one side of the part needs to be accessed.

Application of Ultrasonics includev :

1. Flaw Detection - Flaw detection in the interior of a part is the main application for ultrasonic testing. An ultrasonic wave propagating in a material will be reflected from a macroscopic discontinuity such as a crack or from a field of microscopic discontinuities such as fine porosity.
2. Thickness Measurements - The thickness of a part can be determined using ultrasound. Thicknesses down to .001"+- 0.0001" can be measured.
3. Determination of Elastic Constants - The velocity of an ultrasonic wave depends on the stiffness and density of the material. By measuring the ultrasonic velocity, the stiffness of a material can be determined. The stiffness of small samples or parts that are not amenable to tensile testing can be measured. The elastic anisotropy of a part can be determines by propagating ultrasonic waves in different directions.
4. Determination of Metallurgical Structure - As an ultrasonic wave propagates in a metal, the wave is attenuated due to grain boundary scattering and second phase particles. The amount of attenuation depends on the size of the grains so that the grain size in a part can be inferred by measuring the attenuation.

The microstructure and mechanical properties of the material effect the ultrasonic velocity and attenuation of an ultrasonic wave. Determining the mechanical properties and metallurgical structures of material is a growing application for ultrasonics.

Elastic Constant Measurements

The velocity of ultrasonic waves in a solid depends on the density of the material and its elastic constants. Ultrasonics can be used to determine the elastic constants of small samples that would be impossible to test using a tensile testing machine. One application involved determining the elastic constants and Debye Temperature of an Al₃Sc polycrystalline intermetallic material. A run to produce this intermetallic resulted in a button ingot roughly 1 / 8 inch thick and 1 / 2 inch in diameter. A sample of this geometry could not be easily tested using a tensile testing machine. However, the elastic constants were easily determined by measuring the density of the material and then measuring the longitudinal and shear wave velocity in the ingot. The elastic constants were higher than both aluminum and scandium indicating that an intermetallic phase was formed.

Metallurgical Structure Determination

The degree of recrystallization of hot rolled aluminum determines the texture of the cold rolled material and dictates the type and degree of earing in drawn and ironed products such as beverage cans. The texture of hot rolled material varies with the degree of recrystallization which can be from 0% (Figure 1) to close to 100% (Figure 2). Since recrystallized material exhibits different texture components than unrecrystallized material, and thus different elastic constants, measuring elastic wave speeds has the potential for determining the degree of recrystallization. Because of the slight anisotropy of a textured material, the speed of ultrasonic waves will depend on the direction they propagate with respect to the rolling direction (Figure 3). Ultrasonic techniques can be used to measure the degree of recrystallization for process monitoring. Electromagnetic acoustic transducers (Figure 4) were used to generate horizontally polarized shear waves which were propagated at 0 and 45 degrees with respect to the rolling direction. Figure 5 shows the degree of recrystallization vs the change in velocity, (V(45)-V(0))/V(0). This indicated that measuring the ultrasonic wave speed could be a good predictor for determining the degree of recrystallization in hot rolled sheet.

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