The extension of Ultrasonic Additive Manufacturing (UAM) to dissimilar materials allows for increased application in the aerospace, automotive, electrical and power generation industries. The benefit of UAM over standard ultrasonic welding is the ability to form complex geometries, such as, honeycomb structure and internal channels and also embed wires and sensors to create smart materials. UAM has had limited success bonding dissimilar materials and thus Very High Power Ultrasonic Additive Manufacturing (VHP UAM), which increases the amplitude (from 26μm to 52μm) and normal force (from 2.5kN to 33kN), has been introduced to address this deficiency.
Al3003 and Cu110 dissimilar VHP UAM builds were heat treated at 350°C for ten minutes. A measure of maximum push-pin force revealed an improvement in the heat treated condition (from 23% to 49%) for all geometries. Intermetallic phase formation was noticed using the scanning electron microscope (SEM) backscatter detector. X-ray diffraction (XRD) was utilized to characterize the intermetallic layers through peak phase analysis. Al2Cu, AlCu and Al4Cu9 were found on the fracture surface of a heat treated build. It was determined that fracture occurred between the AlCu and Al4Cu9 intermetallic layers. High resolution SEM and fractal analysis were used to verify these findings.
Surface modification was evaluated as a method for improving bonding between dissimilar aluminum and copper welds. The copper foils were rolled with the sonotrode prior to welding, which increased the surface roughness from 0.175 Ra μm to 1.170 Ra μm and then placed face down before welding. The maximum force during push-pin testing showed inconclusive results. Load versus displacement curves were analyzed and it was evident that modified structures exhibited a more energetic failure compared to as-welded builds.
A hypothesis was created to explain this phenomenon. It was expected that the peak load is a function of metallurgical bonding, while mechanical interlock requires more displacement for failure and is responsible for a more energetic failure. This indicates that surface modification led to an increase in mechanical interlock. This finding was supported by SEM fracture surface analysis where the amount of metallurgical bonding for the as-welded and surface modified builds was similar at 7.5% and 8.8% respectively. The surface modified builds, however, displayed 9% more flow morphology than the as-welded sample, indicating increased mechanical interlock.
In collaboration with researchers from the Mechanical Engineering department at The Ohio State University, linear weld density (LWD) and area weld density (AWD) were correlated to both ultimate shear strength (USS) and ultimate transverse tensile strength (UTTS). It was found that no correlation between LWD and mechanical properties existed. AWD yielded a correlation between USS and percent bonded area, however no correlation was found for UTTS. Based on these findings, a new method was devised, SEM fracture surface analysis, to analyze the fracture surfaces in depth and correlate the findings to mechanical strength. The initial findings of this method indicate a correlation between ductile fracture (expected to indicate metallurgical bonding) and both USS and UTTS.