Microstructure and Fracture Toughness of an Aluminum-Steel Impact Weld and Effect of Thermal Exposure

Welding of aluminum alloys to steel is increasingly important in manufacturing; however, the use of fusion welding is difficult because of disparate melting points and the possibility of intermetallic compound (IMC) formation. Here, an impact welding technique, vaporizing foil actuator welding, was utilized to produce solid-state joints between AA1100-O and 1018 mild steel. The relationship between weld processing conditions, microstructure, and mechanical properties was investigated. For this purpose, the welds were annealed between 300 °C and 600 °C and a combination of optical and scanning electron microscopy, along with image analysis was performed to characterize the weld microstructure and monitor IMC growth. Wedge testing was applied to understand the effect of annealing on the weld fracture toughness. A numerical model incorporating the Fick’s laws of diffusion, grain boundary diffusion, and grain growth kinetics was also developed to simulate the IMC growth. The heterogeneity in the original microstructure caused persistent differences in IMC growth, as initial IMC seemed to increase nucleation and growth. Simulation results indicated short circuit diffusion to be the major contributor to IMC growth since it is consistently faster then experimental IMC growths compared with the computational results that used lattice diffusion only. Wedge testing reveals increased weld toughness for modest anneals of 300 °C, possibly due to homogeneity at the weld interface while avoiding IMC growth.