Abstract
Residual stresses (RS) in hot forging severely degrade the machining accuracy and stability of super alloy parts. This is the main reason for deformation during subsequent mechanical machining. RS need recognition, as well as the microstructure and properties achieved by forging. In this study, a simulation and experimental research on the single-pass compression of GH4169 are presented. RS variations with forging temperature, loading speed, and cooling speed are established by finite element (FE) simulation. Based on the FE results, an experiment is conducted at a temperature of 1020 ℃, loading speed of 25 mm/s, and press amount of 16 mm, immediately followed by water cooling. A new layer-stripping method is put forward for the high-efficiency measurement and correction of interior RS. Compared with the traditional strain gauge layer-stripping method, the measurement efficiency of the new layer-stripping method is increased by 10 times. Meanwhile, grain photographs are collected and grain size evolution is summarized; thus, the RS is characterized and evaluated from the angle of grains. It is demonstrated that the RS level rises with the increase in forging temperature, loading speed, and cooling speed, while the cooling method influences both the stress value and distribution. Compressive RS changes to tensile, while the average grain size reduces from the surfaces to the center. In the compressive regions, stress values share the same rules as grain size, while, in the tensile regions, they are contrary. The RS levels are divided according to the grain degree standard. According to the residual stress and grain distribution law of the blank, the optimal position of the part in the blank can be determined. Compared with the center position of the part in the blank, the residual stress of the part is reduced by 70%. The results provide useful strategies for the better design of forging technology, qualification examinations, and subsequent mechanical machining.
Highlights
Nickel-based super alloy GH4169 (GB/T14992) has achieved widespread use in the aerospace industry due to the specific high strength and antioxidant property at high temperatures
In order to effectively control the generation of blank residual stress, this section describes the reduction of the residual stress in parts by optimizing the forging process parameters and the positions of the part in the blank
From the research results listed in the previous section, it can be seen that a low forging temperature, convective heat transfer coefficient, and loading speed are recommended to achieve stable parts with uniform and low-level Residual stresses (RS)
Summary
Nickel-based super alloy GH4169 (GB/T14992) has achieved widespread use in the aerospace industry due to the specific high strength and antioxidant property at high temperatures. Ma et al [18] predicted the variations in flow stress and grain size in the multistage heavy forging of Inconel 718 They built microstructure models considering both the strain hardening effect and dynamic softening effect. Tang et al [23] presented an internal state variable material model that can be used to predict the flow behavior during the dynamic regime They modeled the microstructure evolution of nickelbased super alloys based on the experimental results [24]. Results indicated that thermal stress varied only with deformation temperature and strain rate, while the stress distribution changed with the grain size evolution. Combining the trends of RS and grain size, the optimum workpiece machining sequence and position can be realized
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