Additive Manufacturing (AM) is a modern manufacturing concept actively developed in leading post-industrial countries since last decades of XX-th century. The principle is to make objects by the sequential delivery of material, for example, by fusing or projecting powders, adding liquid polymer or deposing composite. AM is supposed to complete a range of subtractive manufacturing techniques, based on removal of primary materials (conventional mechanic technologies as milling, grinding, drilling, etc.).
Selective Laser Melting (SLM) is a powder-based and laser assisted additive manufacturing technology capable to produce parts layer-by-layer from a 3D CAD model. Nowadays SLM is used in various industrial domains including aerospace, automotive, electronic, chemical and biomedical, as well as other high-tech areas. Industrial interest is focussed on manufacturing of fully functional objects with high geometrical complexity and excellent mechanical properties. Properties of the manufactured parts depend strongly on each single laser-melted track and each single layer, as well as the strength of the connections between them.
SLM comprises the following physical phenomena: absorption and scattering of laser radiation, heat transfer, phase transformation, fluid flow within the molten pool caused by surface-tension gradient, evaporation and emission of material, and chemical reactions. The SLM process is also defined by a large number of parameters including the processing parameters such as laser power, scanning speed, scan line spacing (hatch distance), thickness of layer, scanning strategy, working atmosphere, temperature of powder bed, and material-based input parameters. The nature of the effective heat source produced by laser irradiation of a powder layer considerably differs from the case of laser irradiation of an opaque metallic body. During laser treatment, only a part of radiation is absorbed by the particles at the outer surface of the loose powder layer. The rest of the radiation penetrates through the pores containing gases and interacts with the underlying particles. Further, the heat distribution into the powder layer is done by the usual heat transfer mechanisms. The essential operation at SLM is the laser beam scanning over the surface of a thin powder layer previously deposited on a substrate or previously remelted layer, which is a substrate for the following layer. Each cross-section (layer) of the part is sequentially filled with elongated tracks of melted powder. Line-by-line, a laser beam melts the material along a row of powder particles, thereby forming a molten pool. Under the effect of surface tension, the molten pool takes the shape of a circular or segmental cylinder. Fragmentation of the remelted tracks is a well-known drawback of SLM referred to as the “balling” effect. The features of the tracks’ instability depend of laser power, scanning speed, powder layer thickness, substrate material, physical properties and granulomorphometry of the powder used. The properties of a part produced by SLM technology depend strongly on the properties of each single track and each single layer, as well as the strength of the connections between them.
The analysis of the mechanical properties of the samples manufactured by SLM technique from Inconel 625, SS grade 316L and Co212-F powder has shown their excellent mechanical strength equivalent to the wrought materials. Based on the obtained results, several 3D models and functional prototypes with a complex geometry were fabricated from metal powders for different industrial applications: from aerospace and automotive to biomedical and chemical.
Among the dynamics methods, employing kinetic energy,Cold Gas Dynamic Sprayingis the most promising to realize both thick functional coatings and 3D objects. This spraying technique is called “cold”, since the temperature of the gas jet projecting the powder particles is relatively low (several hundreds centigrade compare to more that thousand centigrade typical for thermal spraying) that is favourable to preserve initial physical-chemical properties of the projected powder, for example, inherited nanostructure. It allows, in particular, getting specific and superior properties of the final product or surface.
The ability to produce billets of different shapes and sizes was shown. A few samples of axisymmetric configurations with a size of several cm have been successfully sprayed. Pictures below show the shapes generated proving that cold spray can be used for rapid prototyping and production of high value parts. The parts shown were sprayed at a deposition efficiency of 80%, powder feed rate of 3 kg/hr, and spray beam size of 7 mm. The time to spray these shapes took 4 minutes.