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HTS-SQUID Magnetic Fatigue Detection in Stainless Steel 304

Laser ultrasonic solidification front detection experiment.

Historically, INL researchers have been involved in the development of nondestructive evaluation (NDE) technology for nuclear engineering applications. For example, characterization of structural materials used in radiological environments. In many cases, remote or standoff NDE investigations are advantageous. Years of experience in the area of noncontacting NDE analysis techniques has subsequently led INL researchers into the area of energy-intense process development and optimization. For example, a noncontacting technique for measuring the depth of a molten metal pool has been used to characterize the solidification front of molten titanium (m.p. 1600oC) in a plasma hearth re-melter1. INL's real-time method of locating and characterizing the shape of the solidification front permits an accurate assessment of the pool depth and the molten metal residence time. This is important in controlling the casting process, and thereby, material properties.

Sketch of molten pool solidification front measurement.

Voltage as a function of time showing reflections from the plasma gas, solidification front, a drilled hole, and the slug bottom.

Plasma arc and melt pool.

Noncontacting methods avoid problems such as contamination of the molten metal and deterioration of the probe that can occur if a measurement probe is immersed in molten metal. With the INL technique, a laser is used to generate ultrasonic waves at the surface of the molten metal pool. The ultrasonic waves propagate into the liquid metal and reflect from the solidification front and the boundaries of the solid plug. A second laser detects the reflected wave.

The properties of the solidification front are determined directly, in real time, by measuring the transit time of these reflected waves. This method can be used at the high temperatures needed for melting industrial materials when both the generation and detection laser beams can reach the molten surface and the detection beam can be recovered.

Citation

J. B. Walter, K. L. Telschow and R. E. Haun, "Laser acoustic molten metal depth sensing in titanium,"Advanced Sensors for Metals Processing, eds. B. W. Brusey, J. F. Bussière, M. Dubois, and A. Moreau (Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec, 1999) 265-274