Studies of laser-based, solid freeform fabrication and coating processes using nanoscale and functionally-graded materials.

摘要

The purpose of this body of investigation is to examine the role of nanoscale and functionally-graded materials on the laser processing and performance of freeform parts and coatings. Both laser experiments and thermal transport models were utilized to achieve the end goal with applications targeted to dies, molds and light-weight aluminum structures. Specifically a computer-numerical-controlled, high-power CO2 laser system with the aid of computer-aided-design models was used to study: (1) nanoscale material additive manufacturing (NAM) process where Ni-nanoparticles are dispersed in H13 steel molten pool in layer-by-layer fashion to produce three-dimensional gear molds; (2) laser-cladding based freeform fabrication (LBFF) process where shaped beam and novel quasi co-axial powder delivery system were used to produce functionally graded H13/Ni-Cr/TiC mold inserts; and (3) laser sintering (LS) of nanocrystalline diamond powders on aluminum alloy substrate to form thick diamond-like carbon coatings for enhanced wear resistance.; In the NAM process, AISI H13 steel micro-powder (70-100 mum), the standard material in the industry for dies and molds, was blended with Ni nano-powder (70-150 nm) in a volumetric ratio of 4:1 and then laser melted under conditions such that only H13 powder was melted and solidified. With the aid of CAD/CAM models and layer-by-layer addition process, gear-shaped molds were fabricated, characterized and tested. Scanning electron microscopy, surface profilometry, Rockwell and Vicker's hardness tests, corrosion test and injection-molding test using polystyrene were used to evaluate the performance of Ni/H13 molds. Results showed that nanoparticle dispersion has distinct improvements on the functional capability of H13 steel molds to produce precision plastic parts; this is attributed to the role of nanoparticles in enhancing mechanical, chemical and tribological properties.; In the LBFF process, a hollow square-shaped, functionally-graded mold (FGM) insert was designed and built with additive layers of H13 steel, Ni/Cr alloy and TiC using circular and rectangular beam profiles. Finite element numerical methods were applied to determine temperature fields and thermal gradients. The cooling rates were estimated and correlated with secondary dendrite arm spacing. Analysis and characterization of FGM insert revealed nearly full density mold with excellent integrity, favorable microstructures, strong interfaces and high hardness. Strength and dimensional stability of molds were tested in a thermal fatigue environment and compared with baseline H13 steel. Improved strain tolerance, better crack resistance and higher oxidation resistance were the primary benefits of FGM mold.; In LS, nanocrystalline diamond powders (4-8 nm) were sprayed on 6061 aluminum alloy substrates to a nominal thickness of 25 mum by an electrostatic spray method and then laser sintered to consolidate and transform the diamond powder to diamond-like carbon (DLC) for a nominal thickness of 10 mum. Raman spectroscopy and X-ray diffraction analysis confirmed the presence of DLC coatings. Microhardness tests showed an average hardness of 2250 kg/mm 2 (some regions had a hardness of 9000 kg/mm2) indicative of DLC. Fracture toughness and surface roughness were well within the typical ranges of DLC. Scanning electron microscope analysis revealed a near dense, fairly uniform coating with a flaw-free interface. Scratch tests indicated the ability of DLC coating to carry high loads without delamination. One-dimensional thermal energy transport models were formulated based on laser energy absorption, thermal properties of diamond and aluminum, heat conduction and convection and solved using finite element ANSYS code. Results guide to a hypothesis that laser sintering of nano-diamond powder takes place in solid state around 800 K followed by densification and phase transition to DLC and coating/substrate interface heating to nearly the melting temperatur