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Hot forging dies are subjected to periodic thermal stress and often fail in the forms of thermal fatigue, wear, plastic deformation, and fracture. A gradient multi-material wire arc additive remanufacturing method for hot forging dies was proposed to extend the service life of hot forging dies and reduce total production costs. The properties of multi-material gradient interfaces play a critical role in determining the overall performance of the final products. In this study, the remanufacturing zone of a hot forging die was divided into three deposition layers: the transition layer, the intermediate layer, and the strengthening layer. Experiments of wire arc additive manufacturing with gradient material were conducted on a 5CrNiMo hot forging die steel. The microstructure, microhardness, bonding strength, and impact property of gradient interfaces were characterized and analyzed. The results revealed that the gradient additive layers and their interfaces were defect-free and that the gradient interfaces had obtained a high-strength metallurgical bonding. The microstructure of the gradient additive layers presented a gradient transformation process of bainite-to-martensite from the bottom to the top layer. The microhardness gradually increased from the substrate layer to the surface-strengthening layer, forming a three-level gradient in the range of 100 HV. The impact toughness values of the three interfaces were 46.15 J/cm2, 54.96 J/cm2, and 22.53 J/cm2, and the impact fracture morphology ranged from ductile fracture to quasi-cleavage fracture. The mechanical properties of the gradient interfaces showed a gradient increase in hardness and strength, and a gradient decrease in toughness. The practical application of hot forging die remanufactured by the proposed method had an increase of 37.5% in average lifespan, which provided scientific support for the engineering application of the gradient multi-material wire arc additive remanufacturing of hot forging dies.

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The microstructure of the parts manufactured by Wire Arc Additive Manufacture (WAAM) is dominated by columnar grains with large size, [1][2][3][4][5] which seriously affects the service performance and life of the parts. [2,[5][6][7] There are two kinds of viable approaches to improve the microstructure of the welds produced by Additive Manufacturing (AM): adjusting the welding process parameters [8][9][10][11][12][13] and introducing external auxiliary means. [14][15][16][17][18][19][20][21] The formation of "AMþ" hybrid AM technology by introducing external auxiliary means to assist the AM has shown good effect and prospect in solving the problem, especially the introduction of specific kinds of energy field such as the mechanical force-energy field, [14][15][16] the ultrasonic field, [17,18] the laser-energy field, [19] and the electromagnetic field. [20,21] However, the mechanical force-energy field-assisted AM technology has attracted much more attention of researchers in the researching field, for the reason of owning the following advantages: a remarkable effect, simple equipments, lower cost, and strong reproducibility, as well as a great potential in academic research and engineering application. [15,16] The interlayer rolling is the most commonly used WAAM technique assisted by mechanical forceenergy field in current researches. [1,2,[4][5][6]16,[22][23][24][25][26][27][28][29][30] The technique is equipped with heavy equipment to generate continuous rolling static pressure up to thousands of newtons. The large static pressure can drive the roller to introduce continuous and uniform plastic deformation in the additive layers which can improve the microstructure and mechanical properties of the additive layers. [16,30] Many researchers [1,2,5,16,25,26] have used this kind of technique to process different metal additive layers, studied the influence on the microstructure evolution of the additive layers under the process, and finally, revealed the influence mechanism of the microstructure evolution of the additive layers on the mechanical properties under the process.Interlayer hammer peening is another novel WAAM technique proposed in recent years, [15] which need not be assembled with heavy equipment to provide greater continuous static pressure but can combine with industrial robots well, so as to achieve a higher processing freedom. [15,31] This technique provides some new thoughts and directions for exploring and expanding the new style of "AMþ" hybrid AM technology. Different from the interlayer rolling technique, the interlayer hammer peening technique has the following characteristics: 1) the acting force is instantaneous impact force with the very short acting time, and the instantaneous contact force is great with high plastic