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Here , we report the direct observation and quantification of melt pool variation during the laser powder bed fusion ( LPBF ) additive manufacturing process under constant input energy density by in-situ high-speed high-energy x-ray imaging .
 melt pool, laser powder bed fusion, LPBF, additive manufacturing process, energy density, in-situ, x-ray imaging
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:
 melt pool, laser powder bed fusion, LPBF, additive manufacturing process, energy density, in-situ, x-ray imaging