Ultrasonic Additive Manufacturing (UAM) or ultrasonic consolidation is a solid state welding process in which thin foil layers are ultrasonically welded on top of one another and computer numerical control machined to create a final part. The substrate is preheated and significant normal forces are applied to aid in the joining process. The exact mechanism for joining is not fully understood, though it is believed to be similar to forge welding, with significant plastic flow of material and no melting. As this process is completely solid state, it offers many benefits over traditional arc welding processes. This includes allowing for complex shapes and designs, having a significantly lower process temperature, allowing for embedded materials and channels, and joining material combinations that are otherwise difficult or impossible.
Through mechanical testing it was found the strength of the consolidated interfaces was significantly below base metal strength, having 14% of the ultimate tensile strength and 47.9% of the ultimate shear strength. The individual foils, despite initially being in the fully strain hardened H-18 condition, were strain hardened and had an ultimate tensile strength of 117% of the base metal. Examining the fracture surfaces of the tensile tests revealed only 66% of the interface was properly bonded, likely the cause of the low tensile and shear strengths of the interface. Through focused ion beam machining and imaging a continuous oxide layer was found in many areas, even in areas with apparent good bonding.
Thermocouples were embedded during the UAM process to record the heating and cooling curves of the process. Maximum temperatures of 174C, 109C above the preheat temperature, were recorded. This is well below the melting temperature of Aluminum 3003 and melting or liquation was not expected. Vertically aligned thermocouples all reported heating and cooling at the same time, indicating every interface beneath the sonotrode was experiencing local and simultaneous heating. Thermal diffusion calculations revealed there should have been a time delay of at least 0.5 seconds between the time of the peak temperatures for the top and bottom thermocouple. Based upon the maximum temperatures recorded, it was calculated that only 2.9% of the system power was converted to heat at the topmost interface, while up to 27% was absorbed in the lower interfaces. An ultrasonic stress-strain field was proposed to quantify the interaction volume beneath the sonotrode.
Attempts at improving the quality of UAM builds through heat treating and the addition of a brazing filler layer (Braze 717) met with limited success. Heat treating at 343C for 2 hours increased the linear weld density from 62.7% to 80.6%, but decreased the hardness from 75HV to 62HV. The braze interlayer material flowed in and filled the channels left from the interaction with the sonotrode on the previous pass, but resulted in mechanical bonding only. Using energy dispersive X-ray spectroscopy in a scanning electron microscope, no diffusion across the interface was observed. Heat treating the braze builds to 371C for 1 hour resulted in delamination of the bonds, confirming they were not metallurgically bonded.