Effect of heat treatment on microstructure and properties of additively manufactured aluminum bronze-steel bimetallic structures

To achieve improved microstructure and mechanical properties, the bypass current metal inert gas welding (BC-MIG) with forced cooling was firstly utilized in the additive manufacturing of aluminum bronze-steel bi-metallic structures. Microstructure and mechanical properties, including tension strength, residual stresses, and hardness distribution, were studied with and without post-heat treatment. The results show that a sound metallurgical bonding interface was formed without any defect due to the low dilution rate from bypass coupled arc and high molten solidification rate from forced cooling, and the diffusion zone thickness was below 1.5 μm. Moreover, the as-fabricated microstructure displayed significant hierarchical heterogeneity due to uneven deformation of grains. After heat treatment, the sample exhibited reoriented grains and homogenized microstructure. Concurrently, dislocation density and residual stress reduced and achieved a uniform distribution recrystallization within the aluminum bronze alloy region and dislocation climb and slip phenomena within the steel portion. Consequently, the heat-treated specimen in the welding direction exhibits a 36.3% decrease in YS (249 MPa) and an 11% decrease in UTS (493 MPa), while the EL (65%) experiences a significant increase of 58.5%. In the building direction, the heat-treated specimen shows a 17.8% decrease in YS (124.6 MPa) and an 2.88% decrease in UTS (508.8 MPa), while the EL (55.8%) experiences an increase of 26.8%.