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In this paper, an automatic WAAM technology is proposed to realize the gradient additive remanufacturing of ultra-large hot forging dies. Firstly, a vertical additive manufacturing strategy and a normal additive manufacturing strategy are proposed to meet different additive manufacturing demands. Secondly, the basic principle of layering design of ultra-large hot forging dies is developed, and the wear resistance of Ni-based, Co-based, and Fe-based alloys at room temperature and high temperature is analyzed. The Co-based alloy has the best high-temperature wear resistance, which can be used on the surface of the hot forging die to strengthen the die. In order to control the forming quality of additive manufacturing, the relationship between welding parameters and weld shape was discussed, and the reverse system of welding process parameters was built. Finally, a typical aviation ultra-large hot forging die is selected as the research object. According to different stress and temperature distribution in different regions of the ultra-large hot forging die in service, materials with different properties are used in corresponding regions to improve the service life of the die, reduce the remanufacturing costs, and improve the remanufacturing efficiency. The experimental results show that the service life of the hot forging dies repaired by the automatic gradient function WAAM technology is significantly increased, the material is reduced by more than 50%, and the production efficiency is increased by more than 50%.

Numerical simulations and experiments were utilized to study the removal of oxide scale formed on 300 M steel special-shaped hot forging surfaces during heating at elevated temperature by a high-pressure water descaling process. Specifically, the experimental setup of the special-shaped hot forging was designed and manufactured according to the descaling parameters and simulation results obtained from the hot rolling process. The force states of three typical hot forging surfaces impinged by high-pressure water jets were analyzed. Moreover, the mechanism of the high-pressure water descaling process was proposed based on the research results. The numerical simulations and experimental results revealed that the velocity distribution of the high-pressure water jets is relatively different in various areas of the special-shaped hot forging surfaces. Therefore, the descaling performance is synergistically influenced by the velocity of the high-pressure water jet and the shape of the special-shaped hot forging. Given a certain spray pressure, the value of impact force Fi plays a significant role in the descaling of the typical hot forging. The larger the value of Fi on the typical hot forging surface, the easier it is to remove the oxide scale, and vice versa. Accordingly, the difficulty of removing the oxide scale formed on the 300 M steel special-shaped hot forging surfaces during heating at elevated temperature by a high-pressure water descaling process is in the following order: plane surface < convex surface < concave surface. Additionally, only the inner-layer FeO of the oxide scale remained after the high-pressure water descaling process due to the appearance of FeO-Fe2SiO4 eutectic in the FeO layer.

Multi-stage hot forging is widely used to produce complex forged products in inFdustry. Flash after the forging should always be minimized for the material saving, and it is important to determine the preform shape minimizing the flash. The process parameters in multi-stage hot forging such as the billet temperature and the stroke have also an influence on the product quality as well as the flash, but they are still determined by a trial-and-error method. In this paper, multi-objective design optimization in three-dimensional multi-stage hot forging is numerically performed so as to minimize both the flash and the distribution of equivalent strain. The preform shape and the process parameters such as the billet temperature, the die temperature and the stroke are optimized. The three-dimensional numerical simulation is computationally so expensive that sequential approximate optimization that response surface is repeatedly constructed and optimized is adopted to identify the pareto-frontier between the flash and the distribution of equivalent strain. It is found from the numerical result that 60% reduction of flash can be achieved, compared to the conventional preform shape. In addition, the total forging energy considering the multi-stage hot forging is reduced. It is confirmed through the numerical result that the proposed approach is valid to determine the optimal preform shape and process parameters in multi-stage hot forging.

This paper provides a detailed analysis of the operation of representative forging tools (in the context of using various surface engineering techniques) used in the process of the hot forging of nickel–chromium steel elements. The influence of the microstructure and hardness of the material on the durability of the tools is also discussed, which is important for understanding the mechanisms of their wear. The research showed that the standard tools used in the process (only after nitriding) as a reference point worked for the shortest period, making an average of about 1400 forgings. In turn, the tools coated with the CrAlSiN coating allowed for the production of the largest number of forgings, reaching 2400 pieces, with uniform wear. In comparison, the tools with the CrAlBN coating made 1900 forgings. Three-dimensional scanning analysis showed that CrAlSiN- and CrAlBN-coated tools have lower volumetric wear, around 41–43 mm3, compared to 59 mm3 for nitrided tools. For a better comparison of tool life, the authors proposed the Z-factor, as the material loss to the number of forgings. The CrAlSiN coating showed the lowest material loss, despite a slightly higher Z-factor value compared to the CrAlBN coating. The use of hybrid coatings such as CrAlSiN and CrAlBN significantly reduces tool wear while increasing service life compared to tools that are nitrided only.

The article presents a comprehensive review of key issues and challenges related to enhancing the durability of hot forging tools. It discusses modern strategies aimed at increasing tool life, including modifications to tool materials, heat treatment, surface engineering, tool and die design, die geometry, tribological conditions, and lubrication. The review is based on extensive literature data, including recent publications and the authors’ own research, which has been implemented under industrial conditions at the modern forging facility in Forge Plant “Glinik” (Poland). The study introduces original design and technological solutions, such as an innovative concept for manufacturing forging dies from alloy structural steels with welded impressions, replacing traditional hot-work tool steel dies. It also proposes a zonal hardfacing approach, which involves applying welds with different chemical compositions to specific surface zones of the die impressions, selected according to the dominant wear mechanisms in each zone. General guidelines for selecting hardfacing material compositions are also provided. Additionally, the article presents technological processes for die production and regeneration. The importance and application of computer simulations of forging processes are emphasized, particularly in predicting wear mechanisms and intensity, as well as in optimizing tool and forging geometry.

In this article, we present research results on the closing conditions of internal metallurgical discontinuities during the hot elongation operation of a Zr-1%Nb alloy ingot using physical and numerical modeling. Research on the influence of thermal and deformation parameters of elongation operations on the rheological behavior of a Zr-1% Nb alloy was conducted using the Gleeble 3800 metallurgical process simulator. Modeling of the influence of thermal–mechanical parameters of hot elongation operations in combinations of rhombic and flat anvils on the closure of metallurgical discontinuities was performed with the help of the FORGE®NxT 2.1 program. Based on the results of the research, recommendations were made regarding forging elongation technology and the geometry of working tools in order to ensure the closure of metallurgical discontinuities during hot elongation operations of Zr-1% Nb alloy ingots.

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.

The study investigates the influence of post-weld heat treatment (PWHT) on the microstructure and hardness of hardfacing layers applied to hot forging tools. The research focuses on three tool steels (55NiCrMoV7, X37CrMoV5-1, and a modified X38CrMoV5-3) and uses robotized gas metal arc welding (GMAW) with DO015 filler material. It examines the structural and mechanical differences in the hardfaced layers before and after heat treatment involving quenching and tempering. The findings reveal that PWHT significantly improves microstructural homogeneity and hardness distribution, especially in the heat-affected zone (HAZ), mitigating the risk of crack initiation and tool failure. The study shows that untempered as-welded layers exhibit microstructural inhomogeneity and extreme hardness gradients, which negatively impact tool durability. PWHT leads to tempered martensite formation, grain refinement, and a more stable hardness profile across the joint. These improvements are critical for extending the service life of forging tools. The results underscore the importance of customizing PWHT parameters according to the specific material and application to optimize tool performance.

This work aimed to evaluate whether the established scientific knowledge for machining homogeneous and isotropic materials remains valid for machining additively manufactured parts. The machinability of thin-walled structures produced through two different variants of wire and arc additive manufacturing (WAAM) was studied, namely conventional MIG deposition and the innovative hot forging variant (HF-WAAM). Cutting operations were carried out varying the undeformed chip thickness (UCT) and the cutting speed, using a tool rake angle of 25°. A systematic comparison was made between the existing theoretical principles and the obtained practical results of the orthogonal cutting process, where the relation between the material properties (hardness, grain size, yield strength) and important machining outcomes (cutting forces, specific cutting energy, friction, shear stress, chip formation and surface roughness) is addressed. Additionally, high-speed camera records were used to evaluate the generated shear angle and chip formation process during the experimental tests. The machinability indicators shown that, through the appropriate selection of the cutting parameters, machining forces and energy consumption can be reduced up to 12%, when machining the mechanical improved additive manufactured material. Therefore, it has been confirmed the feasibility of machining such materials following the traditional machining principles, without compromising the surface quality requirements.

The surface of the hot forging die is prone to failure due to severe high heat and cyclical load during service. The remanufacturing of a forging die by wire and arc additive manufacturing technology could greatly improve the performance of the die and prolong its service life. In this paper, the remanufactured zone of the die was divided into three additive deposition layers with gradient mechanical properties based on temperature and stress analysis, namely, the transition layer, intermediate layer and strengthen layer, according to the thermal and stress conditions. A wire and arc additive manufacturing experiment with gradient material was conducted on 5CrNiMo hot forging die steel. The microstructure and mechanical properties of the gradient additive layer were characterized and analyzed. The results indicated that the microstructure of the gradient additive layer presents a transformation process of bainite-to-martensite from bottom to top. The hardness and strength of the transition layer, intermediate layer and strengthen layer exhibit a rational gradient increasing trend, while the impact toughness shows a decreasing trend. The wear loss and cumulative friction dissipated energy of the strengthen layer are considerably lower than those of the other layers. The mechanical properties of the gradient additive layer are shown as a gradient increase in hardness, strength, and wear resistance at high temperature, which is conducive to enhancing the overall pressure bearing capacity and preventing the failure of plastic deformation on the surface of the remanufactured hot forging die.