Functionally graded materials (FGMs) consisting of Ni-tung (nickel-tungsten carbide) powders with different concentrations of tungsten carbide particles are successfully deposited by a laser-based direct metal deposition (LBDMD) process on 4140 steel substrates. The slurry erosion behaviour of the Ni-tung FGMs is studied at different impingement angles. The slurry erosion tests are performed at Southern Methodist University's Center for Laser-Aided Manufacturing using a centrifugal-force-driven erosiontesting machine. For the purpose of comparison, Ni-tung 40 depositions and 4140 steel samples are also tested. The results indicate that the LBDMD process is able to deposit defect-free Ni-tung FGMs with uniform distribution of tungsten carbide particles in a nickelbased matrix. The slurry erosion resistance of Ni-tung FGMs as observed to be much better than that of the Ni-tung 40 and 4140 steels. The superior slurry erosion resistance of Ni-tung FGMs is attributed to the presence of large amounts of very hard tungsten carbide particles. The material removal rate (MRR) values by erosion decrease with a decrease in the impingement angle, except at a 45 impingement angle on 4140 steel. The relationship among the MRRs, the craters' depth of penetration, the areas of the craters formed, the average surface roughness values, and the impingement angles are established for Ni-tung FGMs, Ni-tung 40, and 4140 steels. The surface profiles of the eroded samples are analysed by measuring the depth of penetration of the craters formed by the slurry jet using a needleshaped probe and a linear scale with a digital readout. The damaged surfaces are characterized by scanning electron microscopy to investigate the possible application of a material failure model, called damage initiation and damage propagation, to the case of the impingement of a mixture of solid and liquid particles on the Ni-tung FGMs, Ni-tung 40, and 4140 steels. The potentiodynamic polarization curves are generated for the three tested materials and to discover the susceptibility of the material in an erosive environment.
The inherent limitation of most solid freeform fabrication (SFF) is deposition in the form of layers. The set-up rather than the geometry and the material composition of the part becomes more important in the process planning. For a functionally graded material (FGM), the desired composition variation is of infinitesimal order; however, the finite size of the deposition head and the molten pool allows for a quantized volume addition. Such artificial imposition of the process for the desired geometric morphology and the functional gradience of materials limit the accuracy of the part. The frequent variation in the material composition is yet another issue associated with the fabrication of FGMs. The suitability of a field can be attributed to the desired material distribution of a part. Different features of the field are identified and used as the input for process planning. The mathematical morphing of the material gradience allows a smooth variation of the material composition across the geometry of the part during deposition. The paper describes a framework for FGM representation using maxel, process planning, and implementation of the fabrication of geometries, and the control of the material composition. The experimental results for the suggested approach are described.
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