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Surface pore defects are always formed during directed energy deposition processes , which may stem from entrapped gas bubbles .
 Surface, pore, defects, directed energy deposition processes, gas
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:
 Surface, pore, defects, directed energy deposition processes, gas