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This article reports the use of in situ X-ray imaging to directly probe the morphological evolution of the liquid-vapor interface during laser melting as a function of ambient pressure and oxygen partial pressure under LPBF conditions in 316 L steel , Ti-64 , aluminum 6061 , and Nickel 400 .
 in situ, imaging, probe, evolution, interface, laser, as, pressure, oxygen, pressure, LPBF, steel, aluminum, Nickel
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 in situ, imaging, probe, evolution, interface, laser, as, pressure, oxygen, pressure, LPBF, steel, aluminum, Nickel