Directed energy deposition (DED) not only inherits the high degree of freedom of the additive manufacturing technology, but also features a flexible material deployment. It can flexibly switch material types during the manufacturing process and precisely adjust the proportion of dissimilar powders, which enables high-efficiency manufacturing of large-scale multi-material parts. However, although the entire multi-material part can be easily fabricated using the DED process, the cost is relatively high when fabricating regular parts. A promising proposal is to manufacture regular parts using the traditional processes and fabricate complex parts using the DED process. By this, the purpose for improving efficiency and reducing manufacturing costs can be achieved while maintaining part performance. In this study, A131 EH36 steel is deposited on a commercial AISI 1045 steel using the DED process to verify the feasibility of a bimetallic structure by the hybrid DED and conventional processes as well as to reveal the interfacical binding mechanism. Furthermore, the effect of heat treatment on microstructure and mechanical properties of the bimetallic structure is also investigated. This research aims to explore a new way to improve the DED efficiency, reduce the costs, and provide theoretical and data supports for making full use of the performances of multiple materials.

The materials used in this study are the A131 EH36 powder and the AISI 1045 steel sheet. DED is used to deposit A131 EH36 on the AISI 1045 substrate. Quenching and tempering are performed to study the effect of heat treatment. Metallographic microscope and scanning electron microscope (SEM) are used for microstructural observation and fracture morphology observation. The elements are detected by an energy dispersive spectrometer. The porosity is determined using the image processing software (ImageJ). A Vickers microhardness tester is used to measure microhardness on the as-built and heat-treated samples. Quasi-static uniaxial tensile tests are conducted on a universal testing machine. The cutting experiment is done on an ULG-100 ultra-precision turning system equipped with a dynamometer. The surface roughness and groove morphology are measured and obtained using a laser confocal microscope.

A ~0.5 mm wide interface region with good metallurgical quality is obtained in the A131 EH36/AISI 1045 bimetallic structure (Fig. 5). The microstructure of the interfacical region in the as-built sample includes refinement zones, coarsening zones, dual heat-affected zones, and heat-affected zones (Fig. 6). Although the morphologies are different, they interfit with each other and are replaced by homogenized structures after heat treatment. The average hardness of as-built A131 EH36 is (297.1±20.1)HV, higher than (182.0±11.7) HV of AISI 1045. The hardness in the interfacical region increases gradually along the building direction due to the excellent interfacial fusion (Fig. 8). The inhomogeneous microstructure of the DED A131 EH36 steel causes the hardness to vary between 262 HV and 308 HV. However, it becomes uniform and decreases by ~37.2% to (186.5±6.0)HV after heat treatment. Since the tensile strength of the A131 EH36 steel is up to (970.5±10.9) MPa, the as-built strength of the bimetallic structure is close to that of the AISI 1045 steel (lower one, Fig. 10). After heat treatment, the strength of the A131 EH36 steel decreases significantly and is lower than that of the AISI 1045. Therefore, the tensile strength and yield strength of the bimetallic structure become close to those of the A131 EH36 steel, reaching (671.3±5.6) MPa and(572.8±8.4) MPa, respectively. Both the as-built and heat-treated bimetallic structures show ductile fractures with the fracture positions far away from the interfacial region (Fig. 9). During the cutting process, the maximum and average cutting forces in the F_X and F_Z directions decreases by 64.1% and 61.1%, respectively, when cutting from A131 EH36 to AISI 1045 (Fig. 14). In addition, the surface roughness after ultra-precision machining is reduced from (111.8±13.6) nm in the AISI 1045 to (107.0±10.4) nm in the A131 EH36 regions (Fig. 15).

In the present study, the A131 EH36/AISI 1045 bimetallic structure is successfully fabricated by the hybrid DED and conventional processes. At the interface of the bimetallic structure, a transition zone of about 0.5 mm wide with good metallurgical quality is obtained without large cracks and unfused defects. The interface consists of microstructural refinement zones, coarsening zones, dual heat-affected zones, and heat-affected zones. The hardness in the interfacial region increases gradually along the building direction. The tensile strength, yield strength, and elongation of the as-built bimetallic structure are (629.0±1.1) MPa, (471.4±9.2)MPa, and 17.9%, respectively, which increase slightly to (671.3±5.6) MPa, (572.8±8.4)MPa and 22.1% after heat treatment. The as-built bimetallic structure is easier to cut than the heat-treated counterpart. When cutting from the AISI 1045 to the A131 EH36 regions, the cutting force decreases significantly with the maximum reduction of 64.1%. In addition, the surface roughness of the ultra-precision machining face decreases from(111.8±13.6) nm in the AISI 1045 region to (107.0±10.4) nm in the A131 EH36 region.