Method for additive manufacturing

An apparatus for forming a three-dimensional article through successive fusion of parts of a metal powder bed, which parts corresponds to successive cross sections of the three-dimensional article, the apparatus comprising:

• a build chamber;
• a working table onto which layers of metal powder are to be placed;
• at least one high energy beam source;
• and
• at least one control unit comprising a processor and computer program instructions that, when executed by the processor of the at least one control unit, cause the apparatus to:
• distribute a first metal powder layer on a work table inside the build chamber,
• direct at least one high energy beam from the at least one high energy beam source over the work table causing the first metal powder layer to fuse in selected locations to form a first cross section of the three-dimensional article,
• distribute a second metal powder layer on the work table,
• direct at least one high energy beam over the work table causing the second metal powder layer to fuse in selected locations to form a second cross section of the three-dimensional article, wherein the second layer is bonded to the first layer,
• release a predefined concentration of at least one gas from the first metal powder layer into the build chamber when fusing the first metal powder layer and the second metal powder layer,
• release a first supplementary gas into the build chamber, which first supplementary gas comprising hydrogen, is capable of reacting chemically with or being absorbed by the finished three-dimensional article, and
• heat the finished three-dimensional article to a predetermined temperature at which a predefined concentration of the first supplementary gas which had reacted chemically with or being absorbed by the finished three-dimensional article is released from the finished three-dimensional article,
• wherein the releasing of the predefined concentration of the first supplementary gas which had reacted chemically with or being absorbed by the three dimensional article is performed by holding the finished three-dimensional article at a predetermined temperature interval for a predefined time interval in the build chamber when a second supplementary gas is introduced into the build chamber, the second supplementary gas is free from H2.