A method of modifying the structure of workpiece (1) is provided. The method comprises a first step of causing relative movement between a power beam and the workpiece (1) so that a region (3) of the workpiece (1) is melted and the melted material displaced to form a projection (2) at a first location in the region (3) and a hole (4) at a different location in the region. The melted material is then allowed to at least partially solidify after which the first step is repeated one or more times, with the region corresponding to each repeat intersecting the region (3) of the first step.