Additive manufacturing of three-dimensional articles

1. A method for forming a three-dimensional article through successively depositing individual layers of powder material that are fused together so as to form the article, said method comprising the steps of:

providing at least one electron beam source emitting an electron beam for at least one of heating or fusing said powder material, where said electron beam source comprises a cathode, an anode, and a grid between said cathode and anode;

controlling the electron beam source in a first mode when said formation of said three dimensional article is in a first process step; and

controlling said electron beam source in a second mode when said formation of said three dimensional article is in a second process step,

wherein:

the second process step comprises one of the group of: preheating the surface prior to applying a new powder layer onto said surface, preheating a new powder layer, preheating a powder distribution, and post heat treatment of an already fused powder layer;

an electron beam current from said electron beam source is controlled in a feed-forward mode in said first mode;

the first process step involves fusing said powder material;

a predetermined grid potential provided to the grid is changed, during fusion of said powder material in the first process step, from a first predetermined grid potential to a second predetermined grid potential different from the first predetermined grid potential;

said electron beam current is controlled in a feed-back mode in said second mode; and

said feed-back mode comprises measuring said electron beam current.

2. The method according to claim 1 , further comprising the steps of:

applying a predetermined accelerator voltage between said cathode and said anode;

applying a predetermined number of different grid voltages between said grid and said cathode for producing a corresponding predetermined number of electron beam currents; and

at least one of creating or updating a look-up table or mathematical function during said second mode wherein said look-up table or mathematical function defines a relationship between a desired electron beam current and an applied grid voltage.

3. The method according to claim 2 , further comprising the steps of:

comparing a first actual electron beam current with a first desired electron beam current; and

updating at least one of the look-up table or mathematical function if the difference between said first actual and first desired electron beam currents is greater than a predetermined value.

4. The method according to claim 3 , wherein the updating is performed after fusing a predetermined number of layers of the three-dimensional article.

5. The method according to claim 3 , wherein the updating is performed between the fusion process of each layer.

6. The method according to claim 1 , wherein said predetermined grid voltage is changed from the first predetermined grid voltage to the second predetermined grid voltage during fusion of at least one individual scan line for changing said electron beam current in said scan line from a first value to a second desired value.