Process Fundamentals, Microstructure Control, and Niche Applications of MELD (a.k.a. Additive Friction Stir Deposition)

M.E.J. Perry, D. Garcia, R.J. Griffiths, J.K. Yoder, W.D. Hartley, H.Z. Yu
Virginia Tech,
United States

Keywords: metal additive manufacturing, dynamic recrystallization, material flow, in situ monitoring, temperature evolution


Over the past three years, the Yu Research Group at Virginia Tech has focused on studying the fundamental manufacturing science underlying a solid-state additive manufacturing technique developed by MELD Manufacturing Inc. The MELD process, also known as Additive Friction Stir Deposition (AFSD), uses friction heating to additively deposit material without melting which can result in fully-dense metals in the as-printed state with equiaxed, fine grains and wrought-like mechanical properties. This poster will highlight some of the most impactful research we have published so far including process fundamentals (e.g., temperature, material flow, distortion), dynamic phase and microstructure evolution, and applications for large-scale cladding, repair, and materials recycling. We have established an in situ monitoring platform for MELD/AFSD, which provides real-time information on temperature field of the printed material, materials flow, applied force and torque, and substrate temperature. Based on that, the relationship between the thermal characteristics (e.g., peak temperature, exposure time, heating/cooling rate) and processing variables has been quantitatively established. Using tracer-based X-ray computed tomography in dissimilar aluminum printing, we have discovered unique 3D features forming at the interface with significant macroscopic material mixing. From these observations, the mechanisms for interfacial material flow and interface morphology formation have been elucidated. Smaller tracer volumes have shown drastic mesoscopic shape evolution from millimeter-scale cylinders to long, curved micro-ribbons accompanied by simultaneous grain refinement. The lower bound of strain was estimated based on extrusion, torsion, and shear-thinning factors. We have compared the process-microstructure linkages and dynamic microstructure evolution mechanisms between materials of high and medium stacking fault energy, namely aluminum and copper. The intrinsic thermomechanical properties and strain development through tool-material interactions have been identified as the key factors for final microstructures in MELD/AFSD. We have developed an economical recipe for printing high-strength aluminum using MELD/AFSD. The printed AA7075 is shown to exhibit wrought-like mechanical properties, with yield strength of 477 MPa, ultimate tensile strength of 541 MPa, and elongation of 8.2%. Without adding nanoparticles, such values are notably higher than those published for fusion-based additive manufacturing. A parametric study using a Ti-6Al-4V alloy has shown that the microstructure and therefore properties can be controlled predictably by changing process parameters. Furthermore, due to high strain rates, transformed structures are produced with prior beta grain sizes an order of magnitude smaller than what one would expect from traditional processing routes. We have employed additive friction stir deposition for additive repair of structural components, showing that deep holes and grooves are readily filled with good interface bonding and mixing. Together with industrial collaborators from Ford Motor Company, our research group has developed a recipe for selective-area cladding on thin automotive sheet metals without local buckling. This work also allows for a first assessment of residual stress levels for this emerging process. These substantial insights help to better understand and control this promising technology and promote multiple pathways for the widespread implementation of MELD/AFSD.