A method for forming a three-dimensional article through successive fusion of parts of a powder bed, which parts correspond to successive cross sections of the three-dimensional article, said method comprising the steps of: providing a model of said three dimensional article; applying a first powder layer on a work table; directing a first energy beam from a first energy beam source over said work table, said directing of said first energy beam causing said first powder layer to fuse in first selected locations according to said model, so as to form a first part of the first cross section of said three-dimensional article; and directing a second energy beam from a second energy beam source over said work table, said directing of said second energy beam causing said first powder layer to fuse in second selected locations according to said model, so as to form a second part of the first cross section of said three-dimensional article, wherein said first and second locations of said first powder layer are at least partially overlapping each other in a first overlapping zone, said method further comprising the steps of fusing said first and second locations of said first powder layer simultaneously via said first and second energy beam from said first and second energy beam source, respectively, overlapping a spot of said first energy beam at least partially with a spot of said second energy beam during at least one occasion of said fusing of said first overlapping zone, maintaining the sum of power of the first and second beam in said first overlapping zone at a predetermined value, wherein the individual power of each of the first and the second beams in said first overlapping zone is half that of each of the first and the second beams outside said first overlapping zone or said predetermined value is equal to the individual power of at least one of the first or the second beam outside said first overlapping zone. providing a model of said three dimensional article; applying a first powder layer on a work table; directing a first energy beam from a first energy beam source over said work table, said directing of said first energy beam causing said first powder layer to fuse in first selected locations according to said model, so as to form a first part of the first cross section of said three-dimensional article; and directing a second energy beam from a second energy beam source over said work table, said directing of said second energy beam causing said first powder layer to fuse in second selected locations according to said model, so as to form a second part of the first cross section of said three-dimensional article, wherein said first and second locations of said first powder layer are at least partially overlapping each other in a first overlapping zone, said method further comprising the steps of fusing said first and second locations of said first powder layer simultaneously via said first and second energy beam from said first and second energy beam source, respectively, overlapping a spot of said first energy beam at least partially with a spot of said second energy beam during at least one occasion of said fusing of said first overlapping zone, maintaining the sum of power of the first and second beam in said first overlapping zone at a predetermined value, wherein the individual power of each of the first and the second beams in said first overlapping zone is half that of each of the first and the second beams outside said first overlapping zone or said predetermined value is equal to the individual power of at least one of the first or the second beam outside said first overlapping zone.