R. Freeman, R. J. Scudamore, G. Ng, TWI (Yorkshire) Ltd., Rotherham, United Kingdom
Direct Laser Deposition (DLD), an additive manufacturing technique where powder is delivered into a molten pool created by a laser, was used to create deposits that were graded in terms of material chemistry. Functionally Graded Materials (FGM) were generated in a layered manner to demonstrate the practical feasibility of the technique, and determine the mechanical properties across the graded areas. The work was carried out using a Trumpf DMD505 laser deposition system and specialist deposition nozzles. The DMD505 comprises a fully integrated five-axis, CNC controlled gantry, a powder feed system, and a 2.0kW CO2 laser. The trials comprised of two tasks. Initially, tool steel and mild steel were graded and evaluated. The second, more detailed task, graded from titanium 6-4 alloy to titanium 6-2-4-2 alloy. Deposition parameters were required for all combinations of materials. An emphasis was placed on the optimisation of metallurgical quality, before the mechanical testing was carried out. The grading of the titanium alloys was achieved to a level of metallurgical quality, in terms of solidification defects and porosity. Hardness, tensile strength and ductility were evaluated at differing deposit chemistries, with properties approaching that of wrought material, and no unpredictable drop-off in mechanical behaviour. Demonstration deposits were created as rectangular blocks, and cylindrical tubes.
Summary: Additive manufacturing techniques rely on the build up of material to create a required shape. These processes frequently compete against techniques that rely on the removal of material. Direct Metal Deposition (DMD) using lasers, Shaped Metal Deposition (SMD) using TIG sources and wire deposition using electron beam systems, are examples of additive manufacturing techniques that have been used for a number of years. The layered nature of these techniques allows parts to be created in a more flexible manner than with more conventional processes. The potential of additive manufacturing techniques to create a part that is of functionally graded material (FGM), presents a significant opportunity. Material costs can be reduced, compared to conventional additive manufacturing techniques, by placing expensive materials where they are required and using inexpensive structural material elsewhere within the part. This report details the results of an initial study at TWI, into the manufacture of FGM, using laser DMD. The work entailed the procedure development of the deposition processes, as well as characterisation of the deposits, and an evaluation of resulting mechanical properties, including hardness, tensile strength, and ductility.
