Regionalization of microstructure and mechanical properties of Ti6Al4V transition area fabricated by WAAM-LMD hybrid additive manufacturing

Abstract

Wire arc additive manufacturing (WAAM) and laser melting deposition (LMD) are two typical additive manufacturing technologies that fabricate large components and small complex structures. However, the composite manufacturing process of these two technologies is rarely reported. WAAM-LMD hybrid additive manufacturing is a feasible method to satisfy the dual requirements of production efficiency and structural accuracy in today's aerospace field. In this study, a 10-mm-thick wall structure was manufactured by this process. The microstructure in different areas was characterized using optical microscopy, scanning electron microscopy and electron backscattered diffraction to elucidate the formation mechanism of the interfacial microstructure. Meanwhile, the tensile properties were tested in different directions and regions of the component. The results indicate that the combined effect of WAAM and LMD areas lacks obvious defects. The average grain size of the bonding region is closer to the bottom of the LMD region and smaller than that of the WAAM region. Clear grain preference orientations and variant orientations were observed from the bottom to the top of the transition zone, which is affected by different thermal histories between the arc and the laser. In addition, equiaxed crystals and columnar crystals coexist in the remelting zone under the combined action of two factors: the epitaxial growth with columnar crystals and the high cooling rate of the molten pool. The properties of tensile specimens in different directions have few differences. However, the elongation obviously changes in the binding zone. Furthermore, the cleavage fracture gradually transforms into ductile fracture from the bottom to the top in the transition area.