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Shot peening post-treatment was conducted at SUS304 bar-plate rotary friction welding joint with fatigue property deterioration from welding flash. along the shotting intensity increasing, non-synchronous deformation of the plate and flash resulted in non-monotonically evolution of local microstructure, residual stresses, and make the fatigue life increased to flash-removed joint level.
In-situ thermal management (ISTM) is one of the most prospective approaches to alleviating heat accumulation and achieving progressive forming for arc-based additive manufacturing. The aim of this study was to investigate the influences of thermal history, i.e., the cooling rate and the intrinsic heat treatment (IHT), of ISTM on as-built property heterogeneity of plasma arc additively manufactured Inconel 625 thin-wall deposit. The thermal history is obtained from numerical simulation. Results show that the variation of the thermal history of ISTM results in the bottom-up varied cooling rate and IHT, which together lead to the location-heterogeneous primary dendrite arm spacing, interdendritic phases and grain size. IHT of each layer is quantified by temperature-time integration (TTI), and Gaussian distributed TTI (above 600℃) indicates the middle region of the deposit with ISTM is subjected to the most intense IHT. Compared with the counterpart of conventional interlayer cooling strategy, the intensified high-temperature IHT of ISTM contributes to significant mitigation of micro-segregation, i.e., the brittle-hard interdendritic phases reduce 61.9%. Furthermore, a weighted value of cooling rate and TTI is innovatively introduced to depict the heterogeneity of mechanical property, which presents an exponential growth of ultimate tensile strength with the weighted value. Moreover, the layer-by-layer decreased heat input and the high-temperature preheating between layers of ISTM contribute to apparent residual stress release effect along the building direction.
Vacuum hot-compression bonding (VHCB) is a highly competitive solid-state bonding technology. To expand the application prospects of VHCB in high-entropy alloys (HEAs), the interfacial bonding behavior of CoCrFeMnNi HEA VHCB joints was investigated, and its intense dependence on the strain rate was clarified. The microstructural characterization results suggested that excellent interfacial bonding was subject to interfacial grain boundary (IGB) migration and dynamically recrystallized (DRXed) grains, and exhibited different bonding behaviors at various strain rates. Interfacial residual nanoscale IOPs facilitated the nucleation of DRXed grains through particle-stimulated nucleation effect; however, their pinning effect on IGB migration also negatively affected interfacial bonding. In addition, this study focused on the prominent contribution of twin boundaries to IGB migration and revealed the evolution of interfacial microtexture. The tensile test results indicated that the ratio of joint elongation to base material elongation (REL) exhibited similar evolution characteristics to the interfacial DRX fraction (XDRX). A new interfacial bonding quality prediction model was developed by describing the quantitative relationship between REL and XDRX, and the accuracy of the model prediction was verified by comparing it with the tested values.
Vacuum hot-compression bonding (VHCB) is an advanced solid-state bonding technique. To expand its application in high-entropy alloys (HEAs), a new strategy to enhance the interfacial bonding quality of CoCrFeMnNi HEA VHCB joints by surface shot peening (SP) was proposed, and the effects of SP treatment on the interfacial bonding behavior and mechanical properties of VHCB joints were investigated. The results revealed that SP treatment introduced a nanocrystalline layer and plastic deformation layer with non-uniform strain on the specimen bonding surface, anda hierarchical grain size gradient fine microstructure was formed by a static recrystallization process under the thermal activation condition before VHCB. During the bonding process, the fine microstructure of the SP-treated layer was retained and provided a large number of grain boundaries and dislocations, which facilitated atomic diffusion and plastic flow in the interfacial region and promoted the closure of interfacial voids and the dissolution of interfacial oxide particles. Additionally, interfacial crystallographic and tensile test analyses indicated that the SP treatment induced a multiple interfacial grain boundary (IGB) migration mechanism in the VHCB joints, which dramatically improved the IGB migration level and ultimately led to a complete recovery of the mechanical properties of the CoCrFeMnNi HEA VHCB joints.
The sub-grains coarsening and low melting point eutectic are the internal causes of cracking of single crystal super-alloys in welding. Aiming to overcome two internal causes, in this paper, the effect of ultrasonic pulse arc welding on the grains, sub-grains and the precipitations of single crystal super-alloys (IN738) has been comprehensively studied, and the microstructure evolution of IN738 in ultrasonic pulse arc welding has been systematically revealed. The results show that (1) the combined effect of ultrasonic vibration and electromagnetic oscillation not only realizes the synchronous refinement of grains and sub-grains in WMZ (weld metal zone), and the polycrystalline transformation of single crystal super-alloys. A novel 3D schematic model of sub-grains is established to evaluate the size and orientation of sub-grains in 3D direction. (2) The ultrasonic vibration and electromagnetic field induced by ultrasonic pulse current can effectively reduce the microstructural segregation of welds. With the increase of ultrasonic pulse frequency, the size and volume fraction of harmful precipitations (η + δ eutectic, mixed eutectic and MC carbides) decrease greatly, especially the morphology and distribution characteristics of γ′ in HAZ (heat affected zone). Thanks to the above outstanding advantages, ultrasonic pulse arc welding has great application value in single crystal super-alloys.
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