a method and apparatus for forming an object by additive layer manufacturing . the method comprises : a ) applying , by a heat source , heat to a portion of a surface of a workpiece sufficient to melt said portion ; b ) adding material to the melted portion and moving the heat source relative to the workpiece whereby progressively to form a layer of material on the workpiece ; c ) cooling the formed layer to bring at least part of the layer to a state of crystallisation , there producing a modified workpiece ; d ) peening , using a plurality of independently controllable impact treatment devices , the modified work - piece so as to plastically deform the cooled at least part of the layer ; and repeating steps a ) to d ) as required whereby to form the object .